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European Journal of Inorganic Chemistry - Current Research Articles

Current research articles: Inorganic Chemistry

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European Journal of Inorganic Chemistry - published by Wiley-VCH

EurJIC is the fastest growing journal in inorganic chemistry. It publishes Full Papers, Short Communications, and Microreviews from the entire spectrum of inorganic, organometallic, bioinorganic, and solid-state chemistry.

Current articles of the journal:

(9,10-Dihydroxyoctadecyl)ammonium: A Structurally Unique Class of Clay Intercalable Surfactants

The structurally unique clay–intercalable (9,10-dihydroxyoctadecyl)trimethylammonium (OH-ODTMA) cationic surfactant was synthesized in two steps from commercially available oleylamine. Sodium montmorillonite was modified by a cation-exchange reaction by using three different octadecyltrimethylamonium ODTMA-based organic cations. Basal spacings, surfactant configuration, the interlamellar structure, and the thermal stability of the resulting organomontmorillonites were characterized by using X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and NMR spectroscopy techniques. Magic-angle spinning 13C NMR spectroscopy demonstrated the coexistence of ordered (all-trans) and disordered (gauche) alkyl chain conformations. Deconvolution of the NMR spectra indicated that the conformational ordering of the surfactant molecules within the gallery of montmorillonite strongly depended on their orientation, the presence of functional groups, and the temperature. Relative to that found in ODTMA-MT, the amount of gauche conformers was significantly higher in montmorillonite intercalated with the OH-ODTMA cation and higher than that in the synthetic precursor, the oleyltrimethylammonium cation. Changes in the ordering of the organic cations were confirmed by FTIR spectroscopy with a distinct frequency shift in the CH stretching and overtone bands. The novel alkylammonium surfactant functionalized with two OH groups in the middle of the backbone represents a prospective ingredient for organoclay–polyurethane nanocomposites. The structurally unique (9,10-dihydroxyoctadecyl)trimethylammonium (OH-ODTMA) surfactant is synthesized from oleylamine. The OH-ODTMA, ODTMA, and OLEYL-TMA amphiphiles are successfully intercalated within clay galleries. The amount of gauche conformers is significantly higher in the novel OH-ODTMA organoclay than in the ODTMA intercalate.

Posted on 22 May 2015 | 11:40 am

Bio-Inspired Formation of Silica Thin Films: From Solid Substrates to Cellular Interfaces

Silica formation in biological systems (biosilicification) has been intensively studied for synthesizing inorganic materials such as silica and titania under biocompatible conditions. Inspired by biosilicification, biocompatible strategies have been developed to fabricate silica thin films on the surfaces of solid substrates such as flat wafers and micro/nanoparticles. In the search for biological applications, these strategies have been applied to cellular surfaces to coat living cells with silica or titania thin films while maintaining cell viability, and it was found that the inorganic coat is beneficial for the protection of living cells and to control cell division. This Microreview focuses on bio-inspired approaches to the formation of silica thin films on solid substrates and cellular surfaces, and includes the bio-inspired formation of other inorganic thin films that have been formed by bio-inspired silicification. The current status and future prospects of this field are also discussed. The bio-inspired formation of silica thin films on the surfaces of flat solids, microparticles, nanoparticles, and living cells is reviewed.

Posted on 19 May 2015 | 4:40 pm

Binuclear Sulfide Niobium Clusters Coordinated by Diimine Ligands: Synthesis, Structure, Photocatalytic Activity and Optical Limiting Properties

Substitution reactions of binuclear niobium cluster anion [Nb2S4(NCS)8]4– with different types of pyridine-based diimine ligands have been carried out. In the case of 2,2?-bipyridine (bipy) and its derivatives, only monosubstituted complexes [Nb2S4(NCS)6L]2– have been isolated. In the case of 1,10-phenanthroline and 2-(2-pyridyl)benzimidazole (Hpbz), both mono- and disubstituted complexes, [Nb2S4(NCS)6L]2– and [Nb2S4(NCS)4L2], respectively, have been obtained. Crystal structures of (Et4N)2[Nb2S4(NCS)6(bipy)]·0.5bipy·0.7CH3CN and [Nb2S4(pbmz)2(NCS)4]·5DMF·0.8H2O have been determined. All compounds have been characterized by NMR, ESI mass spectrometry, IR, UV/Vis spectroscopy, and CV. TiO2 nanoparticles modified with (Et4N)2[Nb2S4(NCS)6(bipy)] show moderate photocatalytic activity in the reaction of H2 generation from water in the presence of Na2S/Na2SO3 as a sacrificial electron donor system. The linear and nonlinear optical properties of the complexes with bipyridine ligands have been studied; the complexes function as broad-band optical limiters. A series of complexes, [Nb2S4(NCS)6L]2– and [Nb2S4(NCS)4L2], where L is a bipyridine, phenanthroline or 2-(2-pyridyl)benzimidazole has been prepared. TiO2 nanoparticles modified with (Et4N)2[Nb2S4(NCS)6(bipy)] exhibit photocatalytic activity for H2 generation from water in the presence of a sacrificial electron-donor system. The complexes function as optical limiters.

Posted on 19 May 2015 | 4:40 pm

Catalytic Diamino-Sugar-Assisted Enantioselective Hydrogenation

Carbohydrate ligands have been studied in transition-metal-catalyzed hydrogenations in recent decades. Herein, we report the diastereoselective synthesis of trans-dichlorido(bisphosphane)ruthenium(II) complexes with four different methyl 2,3-diamino-4,6-O-benzylidene-2,3-dideoxy-?-D-hexopyranosides and their application as catalyst precursors in asymmetric hydrogenation reactions. Depending on the hexopyranose, an enantiomeric excess of up to 78?% was obtained. The Noyori-type hydrogenation catalyst precursor is modified with methyl 2,3-diamino-4,6-O-benzylidene-2,3-dideoxy-?-D-hexopyranoside ligands (glucose, mannose, gulose, and talose). The trans-dichloridoruthenium(II) complexes are diastereoselectively formed. Enantiomeric excesses of up to 78?% are obtained in enantioselective hydrogenation reactions under normal hydrogen pressure.

Posted on 19 May 2015 | 11:40 am

Synthesis and Characterization of Asymmetric 1,2-Dihydroxy-5,5?-bitetrazole and Selected Nitrogen-Rich Derivatives

A selective synthesis of asymmetric 1,2?-dihydroxy-5,5?-bitetrazole monohydrate (4·H2O) from inexpensive starting materials is presented. This bitetrazole derivative contains two N-oxides at the N1 and at the N2? position, which both increase the energetic performance. Various energetic salts, such as hydroxylammonium (5), tetraaminobitriazolium (10), and triaminotriazolotriazolium (11) were synthesized. Neutral 4 and hydroxylammonium salt 5 were compared in detail with other tetrazole mono- and di-N-oxides to gain a deeper insight into the performance increment of N-oxides in tetrazoles. Compounds 3–5, 9, and 11 were structurally characterized by using single-crystal X-ray diffraction. Additionally, all new compounds were characterized by using NMR, IR and Raman spectroscopy, mass spectrometry, elemental analysis as well as differential thermal analysis (DTA) measurements. The sensitivities of the compounds toward shock, friction, and electrostatic discharge were determined. Finally, the heats of formation were calculated and several detonation parameters were computed with the EXPLO5 v. 6.01 code. To study the performance increment of 5,5?-bitetrazole N-oxides a selective synthetic protocol toward asymmetric 1,2?-dihydroxy-5,5?-bitetrazole was established. The title compound completes the series of bitetrazole N-oxides, which represent an outstanding class of energetic materials.

Posted on 19 May 2015 | 11:40 am

Diiron Benzenedithiolate Complexes Relevant to the [FeFe] Hydrogenase Active Site

The reactions of the precursor complex [Fe2(CO)6(?-bdt)] F with PPh3, PPh2Me, PPh2H have been investigated. Treatment of F with the phosphine ligands yielded both mono- and disubstituted complexes [Fe2(CO)5(?-bdt)(PPh3)] (1), [Fe2(CO)4(?-bdt)(PPh3)2] (2), [Fe2(CO)5(?-bdt)(PPh2Me)] (3), [Fe2(CO)4(?-bdt)(PPh2Me)2] (4), [Fe2(CO)5(?-bdt)(PPh2H)] (5) and [Fe2(CO)4(?-bdt)(PPh2H)2] (6). Crystal structures have been reported for complexes 1–3. Complexes 1, 3 and 5 participate in electrocatalytic proton reduction in the presence of two distinct acids of varying strengths: HClO4 and CF3CO2H. Bioinspired hydrogenase model complexes (A–E) with ?-donating ligands are known. Complexes of the type B and C with monodentate phosphine ligands (PPh3, PPh2Me, PPh2H) have been synthesized. The monosubstituted complexes with phosphine ligands participate in electrocatalytic proton reduction in the presence of acids.

Posted on 19 May 2015 | 11:40 am

Paramagnetic ([5]Trovacenyl)phenylphosphane: Intramolecular Electronic Communication and Construction of Heterobimetallic Palladium and Chromium Complexes

Group 15 P-atom-bridged paramagnetic unsymmetric sandwich binuclear di([5]trovacenyl)phenylphosphane (2··) was prepared and characterized by mass spectroscopy, elemental analysis, and X-ray diffraction analysis (XRD). The investigation of 2·· by means of cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and magnetic susceptibility (SQUID) revealed weak intramolecular electronic communication. The coordination ability of 2·· with the transition metals palladium and chromium was explored to afford unprecedented heterobimetallic complexes 34· and 44·, respectively, which were characterized by mass spectroscopy, elemental analysis, and/or X-ray diffraction analysis. Group 15 P-bridged paramagnetic unsymmetric sandwich binuclear complex di([5]trovacenyl)phenylphosphane (2··) was synthesized and characterized by means of electrochemistry, electron paramagnetic resonance (EPR) spectroscopy, magnetic susceptometry, and X-ray diffraction analysis. It was applied as ligand to construct unprecedented heterobimetallic PdII/CrIV complexes.

Posted on 19 May 2015 | 11:30 am

On the Question of Site-Selective Ligand Exchange in Carboxylate-Substituted Metal Oxo Clusters

Reaction of [Ti4Zr4O6(OBu)4(OMc)16] (OMc = methacrylate) with acetylacetone (acacH) resulted in dissection of the cluster and formation of [Ti(OBu)2(acac)2] and the smaller cluster [Ti2Zr4O4(OMc)16]. In contrast, the same reaction with [Zr6O4(OH)4(OOCR)12]2·6RCOOH (R = Et, CH2CH=CH2) led to site-selective substitution of two carboxylate ligands and formation of isostructural [Zr6O4(OH)4(OOCR)12–x(acac)x]2·6RCOOH (x ? 1). The question of cluster degradation upon ligand exchange was probed for reactions of [Ti4Zr4O6(OBu)4(OMc)16] (OMc = methacrylate) and [Zr6O4(OH)4(OOCR)12]2·6RCOOH with acetylacetone (acacH). A [Ti(OBu)2(acac)2] unit was dissected from the bimetallic Ti/Zr cluster, while site-selective substitution of two carboxylate ligands occurred in the latter cluster.

Posted on 19 May 2015 | 11:30 am

Aqueous Synthesis and Visible-Light Photochromism of Metastable h-WO3 Hierarchical Nanostructures

A facile aqueous approach to the synthesis of metastable hexagonal tungsten oxide (h-WO3) hierarchical nanostructures at low temperature was investigated. The spherelike h-WO3 architectures are constructed from 1D nanorods. Polyvinyl pyrrolidone (PVP) in the reaction system facilitates the quick formation of h-WO3 nuclei of the metastable phase through electrostatic interactions between protonated PVP and tungstate anions, which is in accord with the rule of kinetics favoring the formation of the metastable form. PVP also acts as a soft template for the assembly of WO3 crystallites. Oriented growth of WO3 crystallites along the [001] direction resulted in the nanorod-assembled h-WO3 architectures. The h-WO3-rich samples all exhibited good recycled visible-light photochromic performance. Metastable hexagonal tungsten oxide (h-WO3) hierarchical nanostructures are prepared at low temperature. Growth of WO3 crystallites along the [001] direction results in the nanorod-assembled h-WO3 architectures. The h-WO3-rich samples all exhibit good recycled visible-light photochromic performance.

Posted on 19 May 2015 | 11:30 am

Structures, Electrochemical and Spectral Properties of a Series of [MnN(CN)3­(diimine)]– Complexes

The structures and the electrochemical and spectral properties of a series of discrete [PPh4][MnN(CN)3(diimine)] complexes {diimine = 2,2?-bipyridine [1(bpy)], 4,4?-dimethyl-2,2?-bipyridine [1(4-dmbpy)], 5,5?-dimethyl-2,2?-bipyridine [1(5-dmbpy)], 1,10-phenanthroline [1(phen)] and bipyrimidine [1(bpym)]} have been assessed. These complexes represent potential building blocks for the construction of coordination frameworks. The incorporation of diimine ligands with different ?-accepting capacities allows the systematic tuning of the metal-to-ligand charge-transfer (MLCT) band as well as the MnV/MnVI redox potential. The complexes exhibit solvatochromism, and the marked sensitivity of ?max of the MLCT band to polar solvents leads the way towards their potential application in chemical sensing. The crystal structures, redox activities and spectral properties of five discrete [PPh4][MnN(CN)3(diimine)] complexes are reported. Through the modulation of the metal-to-ligand charge-transfer (MLCT) band by variation of the ?-accepting capacity of the diimine ligand, the MnV/MnVI redox potential and solvatochromism can be tuned systematically.

Posted on 15 May 2015 | 10:43 am

Zero-Field Splitting in {MnIII3(?3-O)} Core Single-Molecule Magnets Investigated by Inelastic Neutron Scattering and High-Field Electron Paramagnetic Resonance Spectroscopy

The global zero-field splitting (ZFS) parameters of three, ferromagnetically coupled, ?3-?3-[XO4]– (X = Cl, Re) capped, manganese(III) oximate single-molecule magnets, [Mn3O(R-sao)3(2,4?-bipyridine)3XO4] [X = Cl, R = Me, Et; X = Re, R = Me; Me-sao = 2-hydroxyphenylethanone oximate(2–)], with crystallographic trigonal symmetry were determined by use of inelastic neutron scattering and high-field/high-frequency electron paramagnetic resonance spectroscopy. ReO4– (O···O ca. 1.7 Å) is larger than ClO4– (O···O ca. 1.4 Å), which allows more parallel alignment of the local ZFS tensors. However, this chemical modification results in concomitant distortions in the equatorial ligand plane. Consistent parametrization of all spectroscopic data was achieved, and effective spin-reversal barriers determined from alternating current susceptibility data were shown to be in good accordance with the energy barriers deduced from spectroscopy. Highly resolved inelastic neutron scattering spectroscopy and high-field electron paramagnetic resonance spectroscopy yield a uniquely accurate picture of the ground-state energetics of trinuclear MnIII–oximate complexes. Variations in the magnitude of the anisotropy barriers leading to single-molecule magnet behavior of these systems can be explained in terms of molecular structures.

Posted on 15 May 2015 | 10:43 am

Lithium-Filled Double-Deck-Layered Structure of the RELixCu2–yP2 (RE = La, Pr, Nd, Gd, Er; 0.82 ? x ? 1; 1.19 ? y ? 1.54) Series: Experimental and Theoretical Studies

Five rare-earth metal containing quaternary phosphides in the RELixCu2–yP2 (RE = La, Pr, Nd, Gd, Er; 0.82 ? x ? 1; 1.19 ? y ? 1.54) series were synthesized by using high-temperature synthetic methods, and they were characterized by single-crystal X-ray diffraction. The title compounds crystallize in the trigonal space group P$\bar {3}$m1 (Z = 1, Pearson code hP6) with four crystallographically independent atomic positions. The overall crystal structure can be described as a “Li-filled” CaAl2Si2-type structure, in which a partially occupied Li site is embedded within the “double-deck” layers formed by two anionic components. Tight-binding linear muffin-tin orbital calculations including density of states, crystal orbital Hamilton population, and electron localization function analyses provided rationales for the overall electronic structure and chemical bonding of the title phase. PrLi0.98(5)Cu1.54(1)P2 indicated an antiferromagnetic interaction of Pr at a relatively low temperature with a paramagnetic Curie temperature of –6.8 K and a thermal conductivity of 3.2 W?mK–1 at 323 K. Five rare-earth metal containing quaternary phosphide compounds show the “Li-filled” double-deck-layered structure, in which the Cu and P atoms consist of 2D puckered hexagonal double-deck layers.

Posted on 15 May 2015 | 10:43 am

Preparation and Structural Characteri­zation of Mono-Ru-Substituted ?2-Dawson-Type Phosphotungstate with a Carb­onyl Ligand and Other Ru(CO)-Substituted Heteropolytungstates

In our previous article (Dalton Trans. 2008, 6692–6698) we reported that hydrothermal reaction of a mono-lacunary Keggin-type silicotungstate, [SiW11O39]8–, and Ru(acac)3 (acac: acetylacetonate) for 5–24 h produced a mono-Ru-substituted Keggin-type silicotungstate with a terminal aqua ligand, [SiW11O39RuIII(H2O)]5–, which was further converted into a carbonyl (CO) derivative, [SiW11O39RuII(CO)]6–, by increasing the reaction time for several days. Here, reactions of Ru(acac)3 with other mono-lacunary heteropolytungstates for the formation of other mono-Ru(CO)-substituted heteropolytungstates, including Keggin derivatives, [GeW11O39RuII(CO)]6– and [PW11O39RuII(CO)]5–, and Dawson-type phosphotungstates, [?1-P2W17O61RuII(CO)]8– and [?2-P2W17O61RuII(CO)]8–, are presented. The desired ?2-isomer of Dawson-type phosphotungstate, [?2-P2W17O61RuII(CO)]8–, was produced in high purity and was fully characterized by CV, 31P NMR, 13C NMR, IR, UV/Vis, elemental analysis, mass spectroscopy, TG-DTA, TPD-MS, and single-crystal structure analysis. On the other hand, Keggin-type complexes, [GeW11O39RuII(CO)]6– and [PW11O39RuII(CO)]5–, were formed together with their aqua derivatives, [GeW11O39RuIII(H2O)]5– and [PW11O39RuIII(H2O)]4–, respectively, and the ?1-isomer of Dawson-type phosphotungstate, [?1-P2W17O61RuII(CO)]8–, was produced together with the ?2-derivative, as confirmed by CV, IR, mass spectroscopy, and 31P NMR spectroscopy. New mono-RuII(CO) substituted heteropolytungstates, including Keggin derivatives [GeW11O39RuII(CO)]6– and [PW11O39RuII(CO)]5– and Dawson-type phosphotungstates [?1-P2W17O61RuII(CO)]8– and [?2-P2W17O61RuII(CO)]8–, were prepared.

Posted on 13 May 2015 | 11:30 am

Possible DNA-Interacting Pathways for Metal-Based Compounds Exemplified with Copper Coordination Compounds

Since the fortuitous discovery of the anticancer properties of cisplatin and its successful introduction in chemotherapy in 1978, numerous cytotoxic, metal-based molecules have been increasingly reported in the literature. Due to the specific mechanism of action of cisplatin, DNA represents the main target for the design of potential metal-containing antitumor agents. In the present microreview, the different pathways in which a metal complex can interact/degrade DNA are presented and some illustrative examples of DNA-binding/DNA-cleaving copper coordination compounds are described. In this microreview, the main routes towards DNA damage mediated by metal complexes are described concisely. Emphasis is placed on the great potential of copper coordination compounds for which some illustrative examples are given.

Posted on 11 May 2015 | 12:20 pm

Synthesis of Iron Hydrides by Selective C–F/C–H Bond Activation in Fluoroaryl­imines and Their Applications in Catalytic Reduction Reactions

The reactions of Fe(PMe3)4 with different 2,6-diflurophenylarylimines 1–5 were explored. Fluoroarylimines 1–3, the aryl rings of which are substituted with electron-withdrawing groups, reacted with Fe(PMe3)4 to afford the C–H activation products 6–8. However, if the aryl rings of the fluoroarylimines were substituted with electron-donating groups, the iron hydrides 9 and 10 were obtained from the reactions of the fluoroarylimines with Fe(PMe3)4 through C–F bond activation. In a further study, silanes, especially triethoxysilane, were found to benefit the reactions and improve the yields of the hydridoiron complexes. The three-component reaction of Fe(PMe3)4, a fluoroarylimine, and a silane could also be utilized in reactions involving 2,6-(CH3)2C6H3–C(=NH)–2,6-F2C6H3 (13) and 2,6-F2C6H3–C(=NH)–C6F5 (16) to synthesize iron hydrides (15 and 18). The hydridoiron complexes could be utilized as efficient catalysts in the hydrosilylation of aldehydes and ketones. Furthermore, cinnamaldehydes were selectively reduced to the corresponding cinnamyl alcohols in high yields. The mechanism of the catalytic reduction reaction was studied extensively through operando IR spectroscopy. The selective activation of C–H/C–F bonds was realized in the reactions of Fe(PMe3)4 with fluorophenylimines by controlling the substituents of the fluorophenylimines. Iron hydrides were obtained through C–F bond activation with the assistance of silanes. These iron hydrides show excellent catalytic activity in the hydrosilylations of aldehydes, ketones, and ?,?-unsaturated aldehydes.

Posted on 11 May 2015 | 12:20 pm

Syntheses, Spectroscopy, and Structural Analyses of Dinuclear Chiral-at-Metal ?-Aqua-tetrakis[(R or S)-N-1-(Ar)ethyl­sal­icylaldiminato]di-?- or -?-nickel(II) Complexes

Enantiopure dinuclear ?-aqua-tetrakis[(R or S)-N-1-(Ar)ethylsalicylaldiminato]di-?- or -?-nickel(II) [Ar = C6H5 (R-1/S-1), p-MeOC6H4 (R-2/S-2), p-ClC6H4 (R-3/S-3), p-BrC6H4 (R-4)] complexes have been synthesized from the reaction between (R or S)-N-1-(Ar)ethylsalicylaldimine and nickel(II) acetate. Their CD spectra demonstrate chirality transfer from the ligands to the metal ions and a mirror-image relationship for the enantiomeric pair R-3/S-3 in thf. DSC analyses show that the complexes exhibit an irreversible dissociation to the mononuclear species, which in turn undergo decomposition. The 1H NMR spectra reveal the presence of four salicylaldiminates, one aqua ligand, and two molecules of methanol in each dinuclear complex. They also confirm the existence of a dinuclear-bridged aqua bis-octahedral NiII complex in solution. The IR spectra in CH2Cl2 show a very strong band at around 2306 cm–1 due to the ?O–H of the bridged aqua ligand. The X-ray structures of R-2, S-2, R-3, and S-3 confirm the formation of dinuclear compounds comprising two nickel ions, four salicylaldiminates, and one aqua ligand with two molecules of methanol (or water in S-2). Each nickel ion is surrounded by two N,O-chelating salicylaldiminates, a bridging salicyl-O atom from the neighboring nickel ion, and a bridging aqua ligand in a distorted octahedral polyhedron. Analyses of the absolute structures reveal a diastereomeric induction of the R or S ligand giving a ? or ? configuration at the nickel atoms in R-2/3 or S-2/3 that is independent of the ligand substituents. Dinuclear ?-aqua-tetrakis[(R or S)-N-1-(Ar)ethylsalicylaldiminato]di-?- or -?-nickel(II) complexes have been synthesized from enantiopure Schiff bases. Their X-ray structures show each Ni surrounded by two salicylaldiminates, one bridging salicyl-O, and an aqua ligand in a distorted octahedral with diastereoselective induction-at-Ni through R or S ligand chirality to give the ? or ? configuration.

Posted on 11 May 2015 | 12:20 pm

Masthead: Eur. J. Inorg. Chem. 14/2015

Posted on 8 May 2015 | 1:38 pm

Graphical Abstract: Eur. J. Inorg. Chem. 14/2015

Posted on 8 May 2015 | 1:38 pm

Facile Rearrangement of a Bis(N-heterocyclic carbene)borate Chelate Ligand and Access to [:GeX]+ Complexes (X = H, Cl) (Eur. J. Inorg. Chem. 14/2015)

The cover picture shows the unexpected rearrangement of the bis(N-heterocyclic carbene)borate chelating ligand induced by the “Kalium” (German for potassium) ion in toluene. This rearrangement affords a unique imidazolyl–NHC–borate potassium salt, which represents a new chelating bis(donor) ligand that is otherwise difficult to synthesize. This ligand allows access to a novel hydridogermyliumylideneborate complex. Details are discussed in the Short Communication by M. Driess et al. on p. 2377 ff. For more on the story behind the cover research, see the Cover Profile.

Posted on 8 May 2015 | 1:38 pm

Facile Rearrangement of a Bis(N-heterocyclic carbene)borate Chelate Ligand and Access to [:GeX]+ Complexes (X = H, Cl)

Invited for the cover of this issue is the group of Matthias Driess at the Technische Universität Berlin, Germany. The cover image shows the unexpected rearrangement of the bis(N-heterocyclic carbene)borate chelating ligand induced by the “Kalium” (German for potassium) ion in toluene. This rearrangement affords a unique imidazolyl–NHC–borate potassium salt, which represents a new chelating bis(donor) ligand, allowing access to a novel hydridogermyliumylideneborate complex. The rearrangement product represents a new type of bis(donor) ligand, which is otherwise difficult to synthesize...Read more about the story behind the cover in the Cover Profile and about the research itself on p. 2377 ff.

Posted on 8 May 2015 | 1:38 pm

Assessment of Catalysis by Arene-Ruthenium Complexes Containing Phosphane or NHC Groups bearing Pendant Conjugated Diene Systems

Two p-cymene-ruthenium complexes 1 and 2 were isolated in high yields by treating the [RuCl2(p-cymene)]2 dimer with new hybrid phosphane- or NHC-linked diene ligands. Both complexes were fully characterized by NMR spectroscopy, and the molecular structure of the ruthenium–p-cymene complex 1, containing the phosphane–diene ligand system, was determined by X-ray diffraction analysis. The catalytic activities of both compounds were probed in atom-transfer radical addition (ATRA) and polymerization (ATRP), in the cyclopropanation of olefins, in the ring-opening metathesis polymerization (ROMP) of norbornene, and in the synthesis of enol esters from hex-1-yne and 4-acetoxybenzoic acid. The synthesis of two arene-ruthenium complexes incorporating new hybrid phosphane- or NHC-linked diene ligands is described. Preliminary assessment of their catalytic performances in atom-transfer radical addition (ATRA) and polymerization (ATRP), olefin cyclopropanation, ring-opening metathesis polymerization (ROMP), and enol ester synthesis is reported.

Posted on 8 May 2015 | 12:10 pm

Raman Spectroscopy as a Superior Tool To Understand the Synthetic Pathway of ­Cu2FeSnS4 Nanoparticles

The quaternary chalcogenide Cu2FeSnS4 is important and interesting owing to its many advantageous characteristics for photovoltaic and optoelectronic applications, such as a suitable direct band gap (1.0–1.5 eV), low toxicity, and a relatively high abundance of the elements in the Earth's crust. To date, most of the reports on Cu2FeSnS4 have focused on the synthetic method and optical properties, and few studies have comprehensively and systematically reported the growth mechanism of Cu2FeSnS4 nanoparticles. Therefore, we have synthesized Cu2FeSnS4 nanocrystals by a simple hot-injection method and performed detailed characterizations. Raman spectroscopy is a characterization tool to differentiate the mixed-composition phases in the synthetic pathway of Cu2FeSnS4 nanocrystals, which cannot be efficiently distinguished by X-ray diffraction. A possible synthesis mechanism for Cu2FeSnS4 has been proposed. Few studies have systematically reported the growth mechanism of Cu2FeSnS4 nanoparticles. The XRD patterns of these quaternary compounds are difficult to interpret, as their structural similarity to the binary chalcogenides of the same elements results in overlapping diffraction peaks. We use Raman spectroscopy as a sensitive characterization technique to understand the pathway to Cu2FeSnS4.

Posted on 8 May 2015 | 12:10 pm

Ligand-Field Strength and Symmetry-Restricted Covalency in CuII Complexes – a Near-Edge X-ray Absorption Fine Structure Spectroscopy and Time-Dependent DFT Study

The low-lying empty orbitals of bis(acetylacetonato)copper have been probed by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the Cu L2,3 edges and time-dependent density functional theory (TDDFT) calculations within the Tamm–Dancoff approximation (TDA) coupled to the relativistic zeroth-order regular approximation including spin–orbit effects (SO-ZORA TDDFT-TDA). Both the relative positions of NEXAFS features and their linear dichroism are satisfactorily reproduced. Moreover, a comparison with literature data pertaining to two other square-planar CuII complexes ([CuCl4]2– and copper phthalocyanine) provided information about how metal-to-ligand charge-transfer transitions associated with excitations from CuII 2p orbitals to low-lying, ligand-based ?* molecular orbitals in a simplified one-electron picture may contribute to the CuII L2,3 edge intensity and, thus, weaken its believed relationship with the CuII–ligand symmetry-restricted covalency. The legitimacy of using the position of the CuII L3 edge to get information about the ligand-field strength and the possibility of a Cu–ligand symmetry-restricted covalency underestimation when using the CuII L3-edge intensity as a gauge are discussed.

Posted on 8 May 2015 | 12:10 pm

Ba5Tr2(Ge,Sn)2P8 and Ba5Tr5(Ga,Tt)2P12 (Tr = Al, Ga; Tt = Ge, Sn): Phosphides with Triel and Tetrel Mixing

A series of new multinary phosphide Zintl compounds, Ba5Ga2GeSnP8, Ba5Al2GeSnP8, Ba5Ga6GeP12, and Ba5Ga6SnP12, have been synthesized through high-temperature Pb or Sn flux reactions. The structures of the compounds, which have been determined by single-crystal X-ray diffraction, feature two different new structural types bearing various polyanion chain fragments. Ba5Tr2GeSnP8 (Tr = Al, Ga) crystallize in the monoclinic C2/m (No. 12) space group with cell parameters: a = 16.8211(10)/16.7586(21) Å, b = 6.0115(4)/6.0867(8) Å, c = 9.5873(6)/9.5983(12) Å, ? = 111.9270(10)/112.1880(10)° for the Al- and Ga-containing compounds, respectively. For Ba5Ga6TtP12 (Tt = Ge, Sn), the compounds belong to a different orthorhombic system [space group: Cmcm (No. 63)] with the cell dimensions: a = 10.5222(6)/10.5802(9) Å, b = 16.2717(10)/16.3529(14) Å, c = 13.9093(8)/13.9643(12) Å for Ba5Ga6GeP12 and Ba5Ga6SnP12. Despite the clear differences in the crystallographic parameters, these two types of structures are closely related through the [Tr2P4] and [Tr5P12] polyanion chains (Tr = triels) as well as the significant mixing between the triel and tetrel atoms. The close relationships between these phases and some previously reported analogues are also discussed in detail. Theoretical calculations based on DFT methods confirm the precise electron nature of these compounds, which are also consistent with results from crystallographic data and the optical absorption measurements. A series of multinary phosphide Zintl compounds, Ba5Ga2GeSnP8, Ba5Al2GeSnP8, Ba5Ga6GeP12, and Ba5Ga6SnP12, have been synthesized. The compounds feature two new structure types, which are closely related by the triel-P chains. Extensive mixing between the triel and tetrel elements determines the structure of these phases, which is understood by incorporation of the Zintl concept.

Posted on 8 May 2015 | 12:10 pm

A Disk-Like CoII3DyIII4 Coordination Cluster Exhibiting Single Molecule Magnet Behavior

The employment of (E)-4-(2-hydroxy-3-methoxybenzylideneamino)-2,3-dimethyl-1-phenyl-1,2-dihydropyrazol-5-one (HL) for the first time in 3d/4f chemistry, without any co-ligand, provides access to a disk-like heptanuclear CoII3DyIII4 coordination cluster displaying single molecule magnetism (SMM) behavior. The topology of this compound has hitherto not been observed in CoII/DyIII chemistry. The use of (E)-4-(2-hydroxy-3-methoxybenzylideneamino)-2,3-dimethyl-1-phenyl-1,2-dihydropyrazol-5-one (HL) in Co/Ln chemistry for the first time affords a disk-like heptanuclear CoII3DyIII4 coordination cluster displaying single molecule magnet (SMM) behavior. This topology is hitherto unobserved in CoII/DyIII chemistry.

Posted on 8 May 2015 | 12:10 pm

Dinuclear Zinc and Cobalt Complexes with Imidazolyl and N-Methylmidazolyl Units and Their Solution Speciation and Redox Properties

Herein, we report the synthesis of a new ligand, which is preorganised for the complexation of two metal ions and has two 1H-imidazolyl units in the outer ligand sphere. The zinc and cobalt complexes with the ligand were synthesised and fully characterised, and their solid-state structures were determined. The complexes show a highly symmetric [M2L2]2+ structural motif. The thermodynamic data, that is, the overall stability constants and redox potential of the Co2+/Co3+ couple, were determined. Potentiometric studies revealed the presence of mononuclear (below pH 7) and differently protonated dimeric species (above pH 7). The formation of distinct species was corroborated by ESI-MS measurements. Similar species were obtained for the corresponding zinc complexes. Additionally, we determined the dependence of the Co2+/Co3+ redox couple on the protonation state of the complex. The studies showed that there is a considerably strong thermodynamic coupling between the NH protons of the imidazolyl units and the oxidation state of the metal ion. Upon deprotonation of all four NH units, the potential of the redox couple shifted by ca. 1.1 V. We also synthesised the analogous ligand with N-methylimidazolyl units to investigate the influence of the NH protons on the properties of the complexes. The methylation of the NH unit has virtually no influence on the redox, magnetic and structural properties of the zinc and cobalt complexes. Dinuclear cobalt and zinc complexes based on a new ligand system with 1H-imidazolyl units are synthesised. The dependence of the species distribution and the redox properties on the protonation state of the ligand are examined. The potential of the Co2+/Co3+ redox couple shifts by ca. 1.1 V upon deprotonation of all of the imidazolyl units.

Posted on 6 May 2015 | 10:40 am

A Useful Route to Metal Complexes of Poorly Coordinating Sulfonamides

We report a synthetic approach towards metal complexes of the poorly coordinating sulfonamide 2-tosylaminomethylaniline (HLTs) that exploits both the good donor capacity of imine N atoms and the hydrolysis of imine ligands mediated by d-block metals. Zn2+, Cd2+ and Pd2+ ions appear to promote the efficient hydrolysis of a coordinated imine ligand (H2LSB) at room temperature, especially in a protic medium. The crystal structures of chiral R-H2LSB and achiral trans-Pd(LTs)2 allow us to compare some conformational changes of the sulfonamide residue after complexation. The direct coordination of 2-tosylaminomethylaniline to Zn2+, Cd2+ and Pd2+ ions is not possible. However, with a coordinating Schiff base derivative as a starting material, the Zn2+, Cd2+ and Pd2+ complexes can be obtained efficiently at room temperature.

Posted on 5 May 2015 | 8:30 am

Facile Synthesis of (110)-Plane-Exposed Au Microflowers as High Sensitive Hydrogen Peroxide Sensors

A flower-like Au microstructure with a dominantly exposed active (110) plane has been prepared by a simple, facile method at room temperature. Various experimental factors that can control the shape of the Au microflowers have been investigated and optimized. Furthermore, the glassy carbon electrode modified with these Au microflowers exhibits high electrochemical activity towards H2O2 reduction. The sensor shows a linear range from 10 ?M to 5.53 mM with a detection limit of 2 ?M, which can be attributed to the exposed (110) planes and 3D hierarchical structure of the Au microflowers. A 3D flower-like Au microstructure with a dominantly exposed active (110) plane was synthesized by a simple, facile method. This special structure exhibits excellent sensing capability towards H2O2 with a wide linear response range.

Posted on 5 May 2015 | 8:30 am

Insights on the UV/Vis, Fluorescence, and Cyclic Voltammetry Properties and the Molecular Structures of ZnII Tetraphenylporphyrin Complexes with Pseudohalide Axial Azido, Cyanato-N, Thiocyanato-N, and Cyanido Ligands

The syntheses, spectroscopic and photophysical properties, and molecular structures of azido- (1), cyanato-N- (2), thiocyanato-N- (3), and cyanido- (4) (meso-tetraphenylporphyrinato)zinc(II) complexes are reported. These species were prepared by using cryptand-222 to solubilize the pseudohalide salts in organic solvents. The UV/Vis spectra of these zinc metalloporphyrins are solvent-dependent and exhibit large redshifted Soret bands compared with those of the [Zn(Porph)L] derivatives in which Porph is a meso-porphyrinato ligand and L is a monodentate neutral axial ligand. The room-temperature fluorescence spectra of the zinc complexes 1–4 indicate that the Q bands are not very affected by the nature of the axial ligands, and their positions are very close to those of previously reported (meso-porphyrinato)zinc complexes. The quantum yields of the S1??So fluorescence of 1–4 range between 2.8 and 5.5?%, and their fluorescence lifetimes are the same (1.7 ns). Cyclic voltammetry investigations on 1–4 show that the characteristic potentials for the reduction and the two first oxidations of the porphyrin ring are not very affected by the nature of the axial ligand. A third irreversible oxidation of the porphyrin ring is observed. Additional anodic irreversible waves are observed for the thiocyanato-N (3) and cyanido (4) species. The solid-state molecular structures of 1–4 are the first examples of zinc porphyrin complexes with anionic ligands. The average equatorial zinc–pyrrole N atom (Zn–Np) distances for 1–4 range between 2.083(1) and 2.117(2) Å and are much longer than those of the related pentacoordinate zinc porphyrin complexes with monodentate neutral ligands. As a consequence, the displacement of the Zn2+ cation from the mean 24-atom plan of the porphyrin core is significant (ca. 0.5 Å), and the porphyrin core is very distorted. The crystal structures of 1–4 are stabilized by weak intermolecular ? interactions, C–H···Cg (Cg are the centroids of some six-membered phenyl rings and five-membered pyrrole rings). The molecular structure of 1 is further stabilized by weak intermolecular C–H···N hydrogen bonds between one carbon atom of cryptand-222 and the terminal nitrogen atom of the azido ligand. The [Zn(TPP)X]– (TPP = meso-tetraphenylporphyrinato and X = N3–, NOC–, NCS– or CN–) complex ions were characterized by X-ray diffraction. The effect of the nature of the pseudohalide axial ligand X on the redox and photophysical properties of these species in dichloromethane has also been studied by cyclic voltammetry, absorption, steady-state and time-resolved fluorescence techniques.

Posted on 1 May 2015 | 4:10 pm

Reactivity Diversification – Synthesis and Exchange Reactions of Cobalt and Iron 2-Alkenylpyridine/-pyrazine Complexes Obtained by Vinylic C(sp2)–H Activation

The reactivity of 2-alkenylpyridine derivatives with trimethylphosphane-supported iron– and cobalt–methyl adducts were investigated and provided a series of C,N-cyclometalated complexes through smooth vinyl C(sp2)–H activation. The reactions of Co(CH3)(PMe3)4 with 2-vinylpyridine, 2-(1-phenylvinyl)pyridine, and 2-vinylpyrazine provided dark green crystals of the five-membered metallacycles (?2-C,N-R1R2C=CH)Co(PMe3)3 (1: R1 = C5H4N, R2 = H; 2: R1 = C5H4N, R2 = Ph; 3: R1 = C4H3N2,R2 = H). The oxidative addition of 1–3 with iodomethane afforded the mer-trans trivalent cobalt complexes (?2-C,N-R1R2C=CH)Co(CH3)I(PMe3)2 (4: R1 = C5H4N, R2 = H; 5: R1 = C5H4N, R2 = Ph; 6: R1 = C4H3N2, R2 = H) in modest yields. The reaction of 2 with an additional equivalent of 2-(1-phenylvinyl)pyridine incorporated a second C,N metallacycle with an ?2-bonded alkenyl moiety to provide (?2-C,N-C5H4N–CH=CH)(?3,?2-C,C,N-C5H4N–CH=CH2)Co(PMe3) (7). No cyclometalation was observed with 8-vinylquinoline and Co(CH3)(PMe3)4, which afforded (?3,?2-C,C,N-C9H6N–CH=CH2)Co(CH3)(PMe3)2 (8) with retention of the Co–CH3 group; therefore, a suitable bite angle is required for C–H activation. No N coordination was observed with Fe(CH3)2(PMe3)4 and 8-vinylquinoline (reductive elimination of C2H6), which afforded a low-valent Fe(PMe3)3 moiety bound in an ?4-fashion with the exocyclic vinyl group and the ortho-carbon atoms to give (?4,?4-C,C,C,C-C9H6N–CH=CH2)Fe(PMe3)3 (9). In polar solvents, an equilibrium of cyclometalated 7,8-benzoquinoline exists between the mer-trans (90?%) and mer-cis (10?%) configurations of (?2-C,N-C13H6N)Fe(CH3)(PMe3)3 (10). At variance with the previously reported bicyclometalation of 2-(2-naphthalene-1-ylvinyl)pyridine with iron adducts, similar reactions with methylcobalt species provided the monocyclometalated ?2,?1-bound coordination motif (?3,?2-C,C,N-C5H4N–CH=CH–C10H6)Co(PMe3)2 (11). The reaction of 11 with carbon monoxide provided the monocarbonyl complex (?3,?2-C,C,C-C5H4N–CH=CH–C10H6)Co(CO)(PMe3)2 (12), accompanied by the release of the N-coordination site. The carbonylation of (?3-C,C,N-C5H4N–CH=C–C10H6)Fe(PMe3)3 resulted in the exchange of one PMe3 ligand with the retention of the coordination sites to generate (?3-C,C,N-C5H4N–CH=C–C10H6)Fe(CO)(PMe3)2 (13). All new compounds were identified by NMR spectroscopy and structurally characterized by single-crystal X-ray crystallography (with the exception of 10 and 13). The regioselective cyclometalation reactions of 2-alkenylpyridine/-pyrazine derivatives with Co(CH3)(PMe3)4 provide cobalt complexes by vinylic C(sp2)–H activation. These cobalt complexes smoothly add iodomethane but readily demetalate with carbon monoxide. Conversely, reductive elimination through the release of ethane is the dominant reaction with the same ligands and Fe(CH3)2(PMe3)4.

Posted on 30 April 2015 | 2:30 pm

Synthesis and Hydroboration of a Mixed-Donor Iminoboryl Complex of Platinum

The first iminoboryl complex with a non-phosphine donor ligand has been isolated and fully characterized. The structural and spectroscopic parameters of the PtII complex, which bears one phosphine and one N-heterocyclic carbene ligand, suggest that the Pt center backdonates less to the iminoboryl ligand than in conventional (iminoboryl)bis(phosphine) complexes, despite the presence of a strongly ?-donating NHC ligand. This iminoboryl complex was found to undergo facile hydroboration with catecholborane, leading to an N-borylated hydroboryl complex. The first iminoboryl complex with a non-phosphine donor ligand has been isolated and fully characterized. This iminoboryl complex was found to undergo facile hydroboration with catecholborane, leading to an N-borylated hydroboryl complex.

Posted on 30 April 2015 | 2:30 pm

Too Many Cooks Spoil the Broth – Variable Potencies of Oxidizing Mn Complexes of a Hexadentate Carboxylato Ligand

Given the hexadenticity of the monoanionic ligand in the procatalyst [Mn(tpena)(H2O)](ClO4) {tpena– = N,N,N?-tris(2-pyridylmethyl)ethylenediamine-N?-acetate}, it is perhaps surprising that this complex can catalyze the epoxidation of alkenes. When peracetic acid is used as terminal oxidant, the selectivity and rates of reactions are comparable with those reported for the manganese complexes of the commonly employed neutral tetradentate N4 ligands under analogous conditions. Cyclooctene conversion rates are similar when tert-butyl hydroperoxide (TBHP) is used; however, the selectivity is greatly diminished. In the absence of organic substrates, [MnII(tpena)]+ catalyzes water oxidation by TBHP (initial rate ca. 23 mmol/h when [Mn] = 0.1 mM, at room temp.). To explain the variations in the selectivity of catalytic epoxidations and the observation of competing water oxidation, we propose that several metal-based oxidants (the “cooks”) can be generated from [MnII(tpena)]+. These embody different potencies. The most powerful, and hence least selective, is proposed to be the isobaric isomer of [MnIV2(O)2(tpena)2]2+, namely an oxylic radical complex, [(tpena)MnIII(?2-O)MnIV(O·)(tpena)]2+. The formation of this species depends on the catalyst concentration, and it is favoured when TBHP is used as the terminal oxidant. The generation of the less potent [MnIV(O)(tpena)]+, which we propose as the direct oxidant in epoxidation reactions, is favoured in non-aqueous solutions when peracetic acid is used as the terminal oxidant. A manganese(II) complex of a hexadentate ligand catalyzes epoxidation. However, reaction conditions are critical since several metal-based oxidants with varying potencies can be generated. This observation is probably general for related catalysts, and consequences include decreased selectivity and competing water oxidation.

Posted on 29 April 2015 | 3:10 pm

Bi2WO6 Nanosheets Synthesized by a Hydrothermal Method: Photocatalytic Activity Driven by Visible Light and the Superhydrophobic Property with Water Adhesion

The Bi2WO6 nanosheets have been synthesized at different pH values by a hydrothermal method. Various characterization techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), UV/Vis absorption spectra and contact angle (CA) analysis, were employed to investigate the as-prepared products. The Bi2WO6 nanosheets prepared at pH = 9 (BW9) showed excellent photocatalytic performance by degradation of the methylene blue (MB) solution (10 mg/L) and the degradation rate was found to be 93?% under visible light for 3 h. Furthermore, the wettability of the as-synthesized Bi2WO6 film was studied by measuring the water CA. It was found that they show extraordinary superhydrophobicity and excellent adhesion of water, even without modification by low-surface-energy materials. Bi2WO6 nanosheets with excellent photocatalytic activity and the superhydrophobic property were prepared at pH = 9 by a hydrothermal method.

Posted on 29 April 2015 | 3:10 pm

Semiconducting BiOCuSe Thermoelectrics and Its Metallic Derivative Bi2YO4Cu2Se2

ZrCuSiAs-type mixed-anion oxychalcogenides and oxypnictides form a massive family of functional materials readily tailored to host diverse transport properties. Here we investigate semiconducting BiOCuSe, which has been considered as a promising candidate for intermediate-temperature thermoelectric applications, and its intergrowth derivative Bi2YO4Cu2Se2, by using X-ray absorption spectroscopy. Stronger anisotropy and bond covalency are revealed for the latter phase, and in contrast to the simple charge-balance scenario of [Bi3+O2–]+[Cu+Se2–]– in the former phase, mixed-valence Cu+/Cu2+ with a single coordination sphere is seen in the selenide block of Bi2YO4Cu2Se2, which gives rise to a metallic electrical conductivity. X-ray absorption spectroscopy at multiple absorption edges is successfully employed to elaborate the differences in chemical bonding and charge balance between the two layered oxyselenide compounds, semiconducting BiOCuSe and metallic Bi2YO4Cu2Se2, both being potential hosts for high-performance thermoelectrics.

Posted on 29 April 2015 | 3:10 pm

The Ion-Like Supersilylium Compound tBu3Si–F–Al[OC(CF3)3]3

The ion-like silylium compounds tBu3Si–F–Al[OC(CF3)3]3 and Me3Si–F–Al[OC(CF3)3]3 were prepared by degradation of the halonium salts [R3Si–X–SiR3][Al{OC(CF3)3}4] {1a(X): R = tBu, X = Br, I; 1b(X): R = Me, X = Br, I}. The bromonium and iodonium salts 1a(Br), 1a(I), 1b(Br), and 1b(I) were quantitatively obtained from R3SiX (R = Me, tBu) and [Ag][Al{OC(CF3)3}4] in dichloromethane at –50 °C. However, the related fluoronium and chloronium salts, [R3Si–X–SiR3][Al{OC(CF3)3}4] {1a(X): R = tBu, X = F, Cl; 1b(X): R = Me, X = F, Cl}, could not be generated under these conditions. Generally, at low temperatures (< –50 °C) the halonium salts 1a(Br), 1a(I), 1b(Br), and 1b(I) are stable compounds. However, at higher temperatures 1a(Br), 1a(I), 1b(Br), and 1b(I) undergo R3SiX (R = Me, tBu; X = Br, I) elimination to form the highly reactive silyl cations [R3Si]+ (R = Me, tBu). Two different decomposition pathways were observed in the thermolysis of halonium compounds 1a(Br), 1a(I), 1b(Br), and 1b(I): (1) the silylium cations [R3Si]+ reacted with dichloromethane, forming 1a(Cl) as well as 1b(Cl); (2) the silylium cations [R3Si]+ degraded the counteranion to give tBu3Si–F–Al[OC(CF3)3]3 and Me3Si–F–Al[OC(CF3)3]3 along with epoxide C4F8O. Both ion-like silylium compounds could be isolated, and single crystals of tBu3Si–F–Al[OC(CF3)3]3 (orthorhombic, Pnma) as well as Me3Si–F–Al[OC(CF3)3]3 (orthorhombic, P212121) were grown from dichloromethane at room temp. Supersilylium [tBu3Si]+ has higher Lewis acidity than [Me3Si]+, as demonstrated by the reaction of 1a(I) with Me3SiF. Thereby the fluoronium ion 1b(F), along with tBu3SiF and tBu3SiI, was formed. Access to the supersilylated and trimethylsilylated halonium salts [R3Si–X–SiR3]{Al[OC(CF3)3]4} (R = tBu, X = Br, I; R = Me, X = Br, I) was achieved. Degradation of these halonium salts results in the formation of the ion-like silylium compounds tBu3Si–F–Al[OC(CF3)3]3 and Me3Si–F–Al[OC(CF3)3]3.

Posted on 27 April 2015 | 10:21 pm

Carbonylation of Hydrido–Phosphanido-Bridged Dinuclear Platinum Complexes

The carbonylation of the solvento species [(PHCy2)(L)Pt(?-PCy2)(?-H)Pt(PHCy2){?P-P(OH)Cy2}][BF4]2(Pt–Pt) ([1-L]2+, L = CH2Cl2, thf, CH3CN) led to the formation of a singly bridged dicarbonyl PtI compound syn-[(PHCy2)(CO)Pt(?-PCy2)Pt{?P-P(OH)Cy2}(CO)]+(Pt–Pt) (2+). The reaction proceeded in two steps: 1) Substitution of L by CO to give the intermediate syn-[(PHCy2)(CO)Pt(?-PCy2)(?-H)Pt(PHCy2){?P-P(OH)Cy2}]2+(Pt–Pt) ([1-CO]2+) and 2) uptake of a second molecule of CO and elimination of [PH2Cy2]+ (as the BF4– salt) to give 2+. The dicarbonyl complex [2][BF4] was also synthesized starting from the dicyclohexylphosphanido-bridged complex [(PHCy2)Pt(?-PCy2){?2P,O-?-P(O)Cy2}Pt(PHCy2)](Pt–Pt) (5). In this case the first step consisted of the carbonylation of 5 to give [(PHCy2)2Pt(?-PCy2)Pt{?P-P(O)Cy2}(CO)](Pt–Pt) (6), which was transformed into 2+ by protonation with HBF4 under an atmosphere of CO. Prolonged reaction times for the carbonylation of [1-L]2+ in halogenated solvents resulted in the formation of the symmetrical dicarbonyl compound syn-[(CO)(PHCy2)Pt(?-PCy2)Pt(PHCy2)(CO)]+(Pt–Pt) (3+) along with Cy2P(H)OBF3. Complexes 6 and [1-L][BF4]2 were also formed by the carbonylation of 5 and [1-OMe2][BF4]2 in the solid state. The singly bridged dicarbonyl PtI compound 2+ can be obtained, both in solution and in the solid state, either by carbonylation of the solvento species [1-L]2+ or by carbonylation of 5 followed by the addition of HBF4.

Posted on 27 April 2015 | 10:21 pm

Template-Free Synthesis of Three-Dimensional Nanoporous Bulk Graphitic Carbon Nitride with Remarkably Enhanced Photocatalytic Activity and Good Separation Properties

A facile template-free method is presented for the fabrication of three-dimensional (3D) nanoporous bulk graphitic carbon nitride (g-C3N4) with an interconnected framework. Various techniques, namely, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, elemental analysis, specific surface area measurements (Brunauer–Emmett–Teller method), UV/Vis diffuse reflectance spectroscopy, time-resolved fluorescence decay spectroscopy, electron paramagnetic resonance spectroscopy, and photoelectrochemical measurements, were adopted to analyze the structures and physicochemical properties of the as-prpared samples. The results show that the nanoporous bulk g-C3N4 with a particle size of ca. 20 ?m exhibits a high specific surface area, which is ca. 30.9 times higher than that of the original material. The g-C3N4 with such a structure exhibits an improved adsorption capacity for the target pollutants and is readily separable from the photocatalysis reaction system. It is believed that a two-step protonation process plays a key role in the formation of the structure. Furthermore, this 3D nanoporous bulk g-C3N4 also shows more-efficient photogenerated carrier transfer and separation. As a result, the visible-light photocatalytic activity of g-C3N4 is significantly enhanced, and the degradation rates of methyl orange (MO) and rhodamine B (RhB) dyes over the nanoporous bulk g-C3N4 are ca. 5.0 and 22.3 times higher, respectively, than that over the original material. A facile template-free method is presented for the fabrication of nanoporous graphitic carbon nitride (g-C3N4) with a three-dimensional (3D) interconnected framework through a two-step protonation. The material exhibits excellent photocatalytic activity owing to its 3D nanoporous structure and highly efficient charge separation and transfer.

Posted on 27 April 2015 | 10:20 pm

Regioselective Cleavage of Electron-Rich Double Bonds in Dienes to Carbonyl Compounds with [Fe(OTf)2(mix-BPBP)] and a Combination of H2O2 and NaIO4

A method for the regioselective transformation of dienes to carbonyl compounds has been developed. Electron-rich olefins react selectively to yield valuable aldehydes and ketones. The method is based on the catalyst [Fe(OTf)2(mix-BPBP)] with an oxidant combination of H2O2 (1.0 equiv.) and NaIO4 (1.5 equiv.); it uses mild conditions and short reaction times, and it outperforms other olefin cleavage methodologies. The combination of an Fe-based catalyst, [Fe(OTf)2(mix-BPBP)], and the oxidants H2O2 and NaIO4 can discriminate between electronically different double bonds and oxidatively cleave the electron-rich bond in dienes to yield aldehydes and ketones in a regioselective manner. The reaction requires mild conditions (0–50 °C) and short reaction times (70 min).

Posted on 27 April 2015 | 9:50 pm

Dioxomolybdenum(VI) and -Tungsten(VI) Complexes with Multidentate Aminobisphenol Ligands as Catalysts for Olefin Epoxidation

The synthesis of four molybdenum and tungsten complexes bearing tetradentate tripodal amino bisphenolate ligands with either hydroxyethylene (1a) or hydroxyglycolene (1b) substituents is reported. The molybdenum dioxo complexes [MoO2L] (L = 2a, 2b) and tungsten complexes [WO2L] (3a, 3b) were synthesized using [MoO2(acac)2] and [W(eg)3] (eg = 1,2-ethanediolato, ethylene glycolate), respectively, as precursors. All complexes were characterized by spectroscopic means as well as by single-crystal X-ray diffraction analyses. The latter reveal, in all cases, hexacoordinate complexes in which the hydrogen atom of the hydroxy group is involved in hydrogen bonding with one of the metal oxo groups. In the case of the glycol substituent, the ether oxygen atom is coordinated to the metal whereas the hydroxy group remains uncoordinated. The complexes were tested as catalysts in the epoxidation of cyclooctene under eco-friendly conditions, using an aqueous solution of H2O2 as the oxidant and dimethyl carbonate (DMC) as solvent or neat conditions, as substitutes for chlorinated solvents. Molybdenum complexes 2a and 2b showed good catalytic activity using H2O2 without added solvent, and tungsten complexes 3a and 3b showed very high activity in the epoxidation of cyclooctene using H2O2 and DMC as solvents. Four new molybdenum and tungsten complexes bearing tetradentate tripodal amino bisphenolate ligands with either hydroxyethylene or hydroxyglycolene substituents were synthesized and found to catalyze olefin epoxidation reactions under eco-friendly conditions.

Posted on 27 April 2015 | 9:50 pm

Bis(phosphinimino)amide Supported Borondihydride and Heteroleptic Dihalo Compounds of Group 13

The reaction of tetraphenyldiphosphazane (Ph2P)2NH with mesityl azide 2,4,6-Me3C6H2-N3 affords a new [N,N?] chelating ligand, [HN(Ph2PN(2,4,6-Me3C6H2))2] (LH). The ligand can be easily deprotonated by using nBuLi or Li[N(SiMe3)2] in Et2O to yield [{N(Ph2PN(2,4,6-Me3C6H2))2}Li·OEt2] (1). The reaction of LH with AlMe3 and BH3·SMe2, respectively, gives the corresponding mononuclear complexes [{N(Ph2PN(2,4,6-Me3C6H2))2}AlMe2] (2) and a rare borondihydride [{N(Ph2PN(2,4,6-Me3C6H2))2}BH2] (3). Similarly, reaction of 1 with the trihalides, MX3, of group 13 elements afford the corresponding dihalo complexes, [{N(Ph2PN(2,4,6-Me3C6H2))2}MX2] [M = B, X = F (4); M = Al, X = Cl (5); M = Ga, X = Cl (6); M = In, X = Br (7)]. All the complexes reported in this work have been isolated in good yields and are expected to serve as useful synthons in a number of reactions. The solid-state structure of LH and 1–7 have been investigated by single-crystal X-ray structural analysis. Synthesis of a rare borondihydride complex has been made possible through the use of a relatively strong donor monoanionic bis(phosphinimino)amide ligand. Crucially, the ligand backbone is void of any reactive acidic hydrogen. The same ligand has also made feasible the synthesis of some heteroleptic dihalo derivatives of group 13 elements.

Posted on 27 April 2015 | 9:50 pm

Spin Crossover in cis Manganese(III) Quinolylsalicylaldiminates

Two new MnII/III redox pairs, [Mn(qsal-Cl)2]0/+ 10/+ and [Mn(qsal-Br)2]0/+ 20/+ have been synthesized employing the tridentate Schiff base ligands, 5-X–N-(8-quinolyl)salicylaldimine (Hqsal-X, X = Cl and Br). The neutral MnII complexes 1 and 2 were prepared from MnCl2 and Hqsal-Cl and Hqsal-Br while oxidation of 1 or 2 with AgOTf yields the cationic MnIII complexes, [Mn(qsal-Cl)2]OTf, 1+, and [Mn(qsal-Br)2]OTf, 2+. 1 and 2 have been characterized by single-crystal X-ray diffraction as CH2Cl2 solvates. The Mn centres adopt a strongly distorted octahedral geometry with cis O donors due to the pair of meridionally bound qsal-X ligands. Electrochemical studies indicate two reversible one-electron redox processes, Mn2+/3+ and Mn3+/4+. Spectroscopic studies show the LMCT bands move to lower wavelengths by ca. 30 nm while the ?C=N stretches are little changed in the cationic complexes. Variable temperature magnetic susceptibility measurements indicate that 1+ and 2+ undergo gradual half spin crossover, despite having cis O donors. DFT calculations reveal a small HS–LS gap in the MnIII systems consistent with spin crossover and provide insight into the ligand design necessary for spin crossover in cis-N4O2 MnIII compounds. We report the redox pairs [Mn(qsal-X)2]0/+ in which the MnIII cations exhibit spin crossover despite the cis O donors.

Posted on 27 April 2015 | 12:13 pm

Ruthenium Carboxylate Complexes as Efficient Catalysts for the Addition of Carboxylic Acids to Propargylic Alcohols

Ruthenium complexes [Ru(CO)2(PPh3)2(O2CR)2] – 3a (R = CH2OCH3), 3b (R = iPr), 3c (R = tBu), 3d (R = 2-cC4H3O), and 3e (R = Ph) – were synthesized by treatment of Ru(CO)3(PPh3)2 with the corresponding carboxylic acids. The molecular structures of the newly synthesized complexes in the solid state are discussed. Compounds 3a–e were successfully applied as catalysts in the addition of carboxylic acids to propargylic alcohols to give the corresponding ?-oxo esters in good to excellent yields even in air. The different carboxylate ligands do not have an influence on the productivities, because the carboxylates exchange rapidly under the applied reaction conditions, as was confirmed by 31P{1H} NMR spectroscopic studies. The addition of catalytic amounts of Na2CO3 resulted in an increase in ?-oxo ester formation. The reaction is tolerant to the use of versatile functional groups on the propargylic alcohols and carboxylic acids, revealing a broad substrate generality. In contrast to most other known catalytic systems, even sterically hindered substrates, including 2,4,6-trimethylbenzoic acid, 1,1-diphenylprop-2-yn-1-ol, or the biologically active steroid ethisterone, were successfully converted. Ruthenium carboxylate complexes were applied in the synthesis of diverse ?-oxo esters by the addition of carboxylic acids to propargylic alcohols. The catalytic system tolerates a broad range of functional groups and even facilitates the successful conversion of challenging substrates such as sterically demanding steroids.

Posted on 27 April 2015 | 12:13 pm

Syntheses of Ir4(CO)6(?5-C5Me4H)2 and Ir7(?3-CO)3(CO)12(?5-C5Me5) from Pentametallic Molybdenum-Iridium Cluster Precursors

Reaction of Mo2Ir3(?-CO)3(CO)6(?5-C5H5)2(?5-C5Me5) with Ir(CO)2(?5-C5Me4H) afforded the four-valence-electron-deficient butterfly cluster Ir4(CO)6(?5-C5Me4H)2; its stability has been rationalized with the aid of density functional theory calculations, which suggest that significant additional intracluster bonding alleviates the formal electron deficiency. Reaction of MoIr4(CO)10(?5-C5H5)(?5-C5Me5) with [N(PPh3)2][Ir(CO)4] afforded the capped octahedral cluster Ir7(?3-CO)3(CO)12(?5-C5Me5), which possesses three semi-face-capping CO ligands. These outcomes demonstrate that heterometallic clusters may serve as a potential source of new homometallic clusters following appropriate M–M? cleavage. Reactions of molybdenum-iridium clusters with iridium-containing reagents afforded new homometallic clusters. Ir7(?3-CO)3(CO)12(?5-C5Me5) possesses three semi-face-capping CO ligands, while Ir4(CO)6(?5-C5Me4H) is formally very electron-deficient; theoretical studies suggest stabilization by significant intracluster multiple Ir–Ir bonding.

Posted on 27 April 2015 | 12:13 pm

Tin Oxometallates from the Hydrolysis of SnMe22+ in the Presence of 2,6-Lutidine-?2,3-diol and Different Anions

The reaction of the dimethyltin(IV) cation with 2,6-lutidine-?2,3-diol (Lut, 2-hydroxymethyl-3-hydroxy-6-methylpyridine) has been investigated in ethanol/water mixtures (80:20 v/v) containing Cl– and NO3–, NO3– and OAc– or Cl– and OAc– ions in various molar ratios. From these reactions, several tin oxometallates were isolated and characterized by IR, Raman, 1H NMR, 13C NMR and 119Sn NMR spectroscopy as well as ESI-MS. The crystal structures of the compounds [Lut+H]Cl·H2O, [(SnMe2)4O2(Lut-H)2Cl2] (1), [SnMe2(Lut-H)Cl] (2), [(SnMe2)3O(Lut-H)2Cl]Cl (3), [(SnMe2)3O(Lut-H)2Cl2]·H2O (4·H2O) and [(SnMe2)4O2(Lut-H)2(NO3)2] (5) were determined by X-ray diffractometry. The structure of 2 contains dimeric [SnMe2(Lut-H)Cl]2 units based on a central Sn2O2 fragment, the Lut ligand is monodeprotonated, and both the tin atoms are hexacoordinate. The other metallates are all tri- or tetranuclear and show the dimethyltin(IV) unit in several penta- or hexacoordinate environments. The C–OH group of the Lut-H fragment, the Cl atoms, the O atoms of the nitrate group and the water molecules (for 4·H2O) are involved in hydrogen bonds, which modulate the interactions between the units and the packing in the crystal. The NMR and ESI-MS spectra show the existence of complex equilibria in solution. Tin oxometallates incorporating lutidine are synthesized in ethanol/water mixtures containing different anions in various molar ratios. The compounds are liberated from solutions containing several species and show the ability of the tin atom to coordinate to different anions, which are sometimes present in solution at lower concentrations than others.

Posted on 23 April 2015 | 12:40 pm

Synthesis of Binary Sb2E3 (E = S, Se) and Ternary Sb2(S,Se)3 Nanowires Using Tailor-Made Single-Source Precursors

Thermal decomposition of four single-source precursors of the type (Et2Sb)2E and Et3SbE (E = S, Se) at 170 °C in the presence of suitable capping agents yielded binary Sb2S3 and Sb2Se3 nanowires. In addition, simultaneous decomposition of (Et2Sb)2S and(Et2Sb)2Se gave the ternary phase Sb2(S,Se)3 with almost equal S and Se concentrations. The materials were characterized by XRD, REM, EDX, ED and HRTEM. Binary Sb2S3 and Sb2Se3 nanowires as well as ternary Sb2(S,Se)3 nanowires were synthesized by thermal decomposition of four novel single-source precursors of the type (Et2Sb)2E and Et3SbE (E = S, Se) in the presence of suitable capping agents and characterized by XRD, REM, EDX, ED and HRTEM.

Posted on 23 April 2015 | 12:40 pm

Heterocyclic Digallanates, Encapsulation of Lithium Cations in Organometallic Cages and C–H Bond Activation

Hydrogallation of the dialkynylbenzenes 1,3-(R2R?Si-C?C)2C6H4 (3: R = Me, R? = Ph; 4: R = Ph, R? = Me) with dichlorogallane, H-GaCl2, afforded the corresponding dialkenylbenzenes 5 and 6, with GaCl2 groups geminal to the silyl substituents. Compound 6 was characterised by crystal structure determination, which revealed intramolecular bridging of both gallium atoms by two chlorine atoms as a new structural motif. Treatment of 5 and 6 with six equivalents of ethyllithium gave unprecedented heterocyclic digallanates 7 and 8, in which two four-coordinate gallium atoms were bridged by two 1,3-dialkenylbenzene spacer ligands. The obtained large heterocycles encapsulated two lithium counterions in their molecular cavities. The lithium atoms were coordinated in a cryptand-like fashion and showed a variety of short contacts to aromatic rings, alkenyl groups, and ?-carbon atoms of terminal ethyl groups. They could be removed from the molecular cavity by coordination with chelating 1,2-dimethoxyethane ligands. Heating of digallanates 7 and 8 in toluene to more than 100 °C over a period of 5 h resulted in fascinating transformations with C–H bond activation and ortho-metallation. Five-membered GaC4 metallacycles were formed, which had a terminal R2R?Si-CH=CH- group resulting from the formal protolytic cleavage of an alkenyl–gallium bond by the ortho-hydrogen atom. C–H bond activation and ortho-metallation was achieved by heating novel macrocyclic phenylene centered dilithium di(galliumalkenyl) compounds in solution. The products contained five-membered GaC4 heterocycles. Various Li–? interactions resulted in fascinating molecular structures for the digallanate intermediates and the finally isolated metallacycles.

Posted on 23 April 2015 | 12:40 pm

Effects of Co/Ni Ratio on the Supercapacitive Properties of ?-Form Hydroxides

?-Form CoxNi1–x hydroxides with different Co/Ni ratios were synthesized by a chemical co-precipitation method under mild conditions. The effects of Co/Ni ratio on the structure, morphology, and supercapacitive properties of ?-form CoxNi1–x hydroxides were investigated in detail. ?-Form CoxNi1–x hydroxides had structures similar to those of hydrotalcite and ?-Co(OH)2, and they exhibited better electrochemical performance than ?-Co(OH)2 on testing by XRD, FTIR, and electrochemical performance (when x ? 0). The highest specific capacitance at 1 A?g–1 for ?-form CoxNi1–x hydroxides (for x = 0.2) is 1696.7 F?g–1. An asymmetric supercapacitor packaged with Co0.6Ni0.4 hydroxide and activated carbon as positive and negative material, respectively, had an energy density of 23.5 Wh?kg–1 at a power density of 412.5 W?kg–1. ?-Form CoxNi1–x hydroxides have structures similar to that of ?-Co(OH)2, with better electrochemical performance than pure ?-Co(OH)2. The highest specific capacitance at 1 A?g–1 for ?-form CoxNi1–x hydroxides is 1696.7 F?g–1. An asymmetric supercapacitor using Co0.6Ni0.4 hydroxide and activated carbon as electrode materials has an energy density of 23.5 Wh?kg–1 at a power density of 412.5 W?kg–1.

Posted on 23 April 2015 | 12:34 pm

Synthesis, Structure and Redox Properties of Asymmetric (Cyclopentadienyl)(ene-1,2-dithiolate)cobalt(III) Complexes Containing Phenyl, Pyridyl and Pyrazinyl Units

The compounds [(?5-C5H5)Co{SC(H)CRS}] [R = phenyl (1), pyridin-3-yl (2) or pyrazin-2-yl (3)] have been synthesized and characterized by elemental analysis, 1H NMR, mass spectrometry and X-ray crystallography. The variation in the UV/Vis and redox properties of these compounds alone and upon acidification has been investigated. In CH2Cl2 solution each compound undergoes a reversible one-electron reduction, and the EPR spectrum of each monoanion has been recorded. In the presence of a 5:1 excess of trifluoroacetic acid the one-electron reduction of both 2 and 3 occurs at a less negative potential. The information obtained has been interpreted with the aid of DFT calculations for [1]y, [2]y and [3]y (y = 0 or –1) and the monoprotonated forms [2H]z and [3H]z (z = +1 or 0), and this has provided insight into the nature of the redox-active orbitals in these complexes. The HOMOs and LUMOs of these species are delocalized and each involves contributions from cobalt, ene-1,2-dithiolate and R orbitals. The information from the experimental and theoretical investigations is used to suggest that, for the pyran ring-opened form of the molybdenum cofactor of oxygen-atom-transfer enzymes, the pterin may participate in the redox reactions involved in the catalytic cycle. Structural, electrochemical, spectroscopic and theoretical studies of [(?5-C5H5)Co(dithiolene)] complexes suggest that the pyran ring-opened form of the molybdenum cofactor of Mo- and W-containing oxygen-atom-transfer enzymes may participate in the redox reactions involved in the catalytic cycle.

Posted on 22 April 2015 | 12:50 pm

Unexpected Emission Properties of a 1,8-Naphthalimide Unit Covalently Appended to a Zn–Salophen

We report the synthesis, characterization, and binding properties of a Zn–salophen complex, 1a, functionalized with a 1,8-naphthalimide unit. Unexpectedly, the emission spectrum of 1a shows a remarkable quenching of the band assigned to the naphthalimide unit. To better understand this phenomenon, a supramolecular model system constituted by a symmetric Zn–salophen and a pyridyl derivative of 1,8-naphthalimide, 1b·2a, was investigated. We propose the existence of a photoinduced energy transfer process between the naphthalimide (donor) and the salophen (acceptor) units in 1b·2a. A similar process must be operative in the covalent receptor 1a. Nevertheless, the results deriving from steady-state fluorescence experiments do not rule out the occurrence of a photoinduced electron transfer process as alternative pathway for the quenching. We also describe the chemosensing properties of receptor 1a and the supramolecular system 1b·2a towards acetate. The nonsymmetrically substituted salophen receptor 1a only transduces the binding of the anion to the Zn metal center in significant spectroscopic changes in its absorption spectrum. On the other hand, we exploit the strong emission quenching experienced by the naphthalimide component in the supramolecular complex 1b·2a to detect anions (e.g. acetate) by means of a typical “turn-on” fluorescent indicator displacement assay. Fluorescence switching of 1,8-naphthalimide: A zinc–salophen complex quenches the emission of 1,8-naphthalimide both in the covalently linked and in the supramolecular assembly. The nonfluorescent 1:1 complex was exploited as an on-off-on receptor for anions according to the fluorescent indicator displacement assay approach.

Posted on 22 April 2015 | 12:50 pm

Simple Synthesis of Two-Dimensional Micro/Mesoporous Boron Nitride

Micro/mesoporous boron nitride (BN) nanosheets were synthesized through a simple one-pot procedure using cetyl trimethylammonium bromide (CTAB) and ammonia borane (AB) as a structure-directing agent and a BN precursor, respectively. The pyrolysis of the CTAB/AB hybrid under an ammonia atmosphere at 1150 °C produced a white BN powder. We investigated the effect of the CTAB/AB weight ratio on the morphology and physical properties of the prepared powders. The obtained products were characterized by using X-ray diffraction techniques, X-ray photoelectron spectroscopy, scanning electron microscopy, scanning transmission electron microscopy, transmission electron microscopy, and N2-sorption analysis. We found that increasing the weight ratio of CTAB/AB changed the morphology of the synthesized materials from densely packed porous structures to two-dimentional micro/mesoporous BN powders. The high surface area of 1401 cm2?g–1 and pore volume of 1.56 cm3?g–1 and micropore volume of 0.41 cm3?g–1 were obtained for a CTAB/AB weight ratio of 10:1. Hydrogen sorption analysis revealed that the obtained powder has a hydrogen-adsorption capacity of approximately 1 wt.-% at 77 K and 100 kPa. A facile one-pot synthesis route is presented to prepare micro/mesoporous boron nitride (BN) nanosheets by pyrolysis of cetyl trimethylammonium bromide (CTAB)/ammonia borane (AB) at 1150 °C. Increasing the weight ratio of CTAB/AB changed the morphology of the synthesized materials from densely packed porous structures to micro/mesoporous two-dimensional BN powder.

Posted on 22 April 2015 | 12:50 pm

Viral Nanotemplates Armed with Oxygenic Polyoxometalates for Hydrogen Peroxide Detoxification

Layer-by-layer (LbL) assembly strategies were exploited to decorate wild-type TMV (tobacco mosaic virus) 1D nanoscaffolds with a totally inorganic, multiredox, tetraruthenate complex belonging to the class of polyoxometalate catalysts. The hybrid capsids give rise to an entangled network of fibrils and ribbon-like nanoassemblies, whose functional activity was probed towards H2O2 dismutation in neutral water. Combined solid-state and surface characterization evidence, including Z-potential, electronic microscopy, thermogravimetry and XPS, delineate a favorable tunability of the nanohybrid material as a function of the added cationic binder. A polyoxometalate with oxygenic activity was anchored on the TMV (tobacco mosaic virus). The rod-like biogenic template enables the formation of catalytic nanoarrays for H2O2 dismutation.

Posted on 22 April 2015 | 12:40 pm

Mechanistic Links in the in-situ Formation of Dinuclear Manganese Catalysts, H2O2 Disproportionation, and Alkene Oxidation

The oxidation of substrates, such as alkenes, with H2O2 and the catalyst [MnIV2(?-O)3(tmtacn)2]2+ (1; tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) is promoted by the addition of carboxylic acids through the in situ formation of bis(carboxylato) complexes of the type [MnIII2(?-O)(?-RCO2)2(tmtacn)2]2+. The conversion of 1 to these complexes requires a complex series of redox reactions coupled with the overall exchange of ?-oxido ligands for ?-carboxylato ligands. Here, we show that the mechanism by which this conversion occurs holds implications with regard to the species that is directly engaged in the catalytic oxidation of alkenes. Through a combination of UV/Vis absorption, Raman, resonance Raman and electron paramagnetic resonance (EPR) spectroscopy, it is shown that the conversion proceeds by an autocatalytic mechanism and that the species that engages in the oxidation of organic substrates also catalyses H2O2 decomposition, and the former process is faster. The in situ formation of catalytically active species through the reduction of a precatalyst, H2O2 disproportionation and alkene oxidation are linked to a common active species.

Posted on 21 April 2015 | 12:20 pm

Ultra-Fast Microwave Synthesis of 3D Flower-Like Co9S8 Hierarchical Architectures for High-Performance Supercapacitor Applications

Ultra-fast microwave irradiation is used to synthesize 3D flower-like Co9S8 hierarchical architectures. Based on time-dependent SEM observations, a multistep splitting–growth mechanism is proposed to understand the formation of the observed structures. Moreover, the materials were used as electrodes to fabricate supercapacitors with high specific capacitances of 522, 499, 441, 436, and 397 F?g–1 at current densities of 0.5, 0.8,1.0, 1.5, and 2.0 A?g–1, respectively. The devices also show high charge–discharge reversibility, with an efficiency of 97.7?% after cycling 1000 times at a current density of 1.0 A?g–1. Ultra-fast microwave irradiation is used to synthesize 3D flower-like Co9S8 hierarchical architectures. The materials display both high discharge capacity and long cycle life under charge–discharge cycling for supercapacitors.

Posted on 20 April 2015 | 1:10 pm

Homogeneous Catalysis with AuIII: Insights into the Mechanism of the Alkoxylation of Alkynes

The mechanism of the AuIII-catalyzed nucleophilic addition of a small alkanol to an alkyne, which yields a ketal product, has been investigated computationally by using GGA (BP86), hybrid (PBE0) and double-hybrid (B2-PLYP-D3BJ) density functional theory with the large aug-cc-pVTZ (aug-cc-pVTZ-PP) basis sets of triple-? quality. The importance of hydrogen-bonding networks is highlighted in the mechanism. The nucleophilic attacks possess relatively lower activation barriers than the hydrogen migrations, and the rate-determining step is predicted to be the last hydrogen migration step. A thorough mechanistic study of gold(III)-catalyzed addition of methanol to propyne was carried out by using density functional theory. Each individual process is assisted by explicitly included solvent methanol, which also acts as the nucleophile. Results showed that the double nucleophilic attacks proceed fast over low activation barriers with the last hydrogen migration as the rate-determining step.

Posted on 17 April 2015 | 12:10 pm

Investigations Concerning [Cu4OX6L4] Cluster Formation of Copper(II) Chloride with Amine Ligands Related to Benzylamine

The ?4-oxido motif is well known in copper clusters and often forms spontaneously and unexpectedly; however, little is known about the reaction conditions that generate ?4-oxido clusters. This is surprising because these ?4-oxido copper clusters seem to play an important role in catalysis. Herein, a systematic investigation of ?4-oxido cluster formation is reported. The influence of several factors on cluster formation was studied, namely, the ligand environment, the central chalcogenide atom, and the bridging halido ligands. In this context, the crystal structures of the copper clusters and complexes [Cu4OCl6L24][CuL22Cl2] (L2 = phenethylamine), [CuL32Cl2] (L3 = N-methylbenzylamine), [Cu4OCl6L44] (L4 = N,N-dimethylbenzylamine), [CuL54Cl2] (L5 = cyclohexylamine), and [Cu4OCl6L64]·1.5[CuL62Cl2] (L6 = cyclohexanemethylamine) as well as that of HL4Cl are reported. Furthermore, the acetonitrile complexes [Cu(CH3CN)4][CuBr4], [Cu(CH3CN)Br], and [Cu(CH3CN)Br2] were obtained and characterized within attempts to prepare a ?4-sulfido unit. The reaction conditions that generate ?4-oxido cluster units are discussed in more detail. The ?4-oxido motif is well known in copper clusters and often forms spontaneously and unexpectedly. However, little is known about the reaction conditions that generate ?4-oxido clusters. Here, a systematic investigation of ?4-oxido cluster formation is reported. The effects of the ligand environment, central chalcogenide atom, and bridging halide ligands on cluster formation are investigated.

Posted on 17 April 2015 | 12:10 pm

Two Structure Types Based on Si6O15 Rings: Synthesis and Structural and Spectroscopic Characterisation of Cs1.86K1.14DySi6O15 and Cs1.6K1.4SmSi6O15

The silicate Cs1.86K1.14DySi6O15 represents a mixed tetrahedral-octahedral framework structure type based on roughly circular Si6O15 rings and isolated DyO6 octahedra. The silicate Cs1.6K1.4SmSi6O15 has a layered atomic arrangement built from corrugated Si6O15 layers containing four-, six- and eight-membered rings. The layers are connected by isolated SmO6 octahedra to form a mixed tetrahedral-octahedral framework. This structure shows a close structural relationship to ?-K3NdSi6O15 and a less close one to dehydrated elpidite (Na2ZrSi6O15). In both structures, Cs/K atoms occupy large voids. The silicates were obtained through high-temperature flux syntheses. Their crystal structures have been determined from single-crystal X-ray diffraction data. Cs1.86K1.14DySi6O15 crystallises in R32 (no. 155) with a = 13.896(2), c = 35.623(7) Å and V = 5957.2(17) Å3, whereas Cs1.6K1.4SmSi6O15 crystallises in Cmca (no. 64) with a = 14.474(3), b = 14.718(3), c = 15.231(3) Å and V = 3244.7(11) Å3. The Dy3+ and Sm3+ cations present in the silicates cause PL emission bands in the visible yellow-to-orange spectral range. Two (Cs,K)-REE3+ silicates (REE3+ = Dy or Sm) containing Si6O15 rings represent mixed tetrahedral-octahedral framework structure types. Their crystal structures were characterised by single-crystal X-ray diffraction and photoluminescence spectroscopy. The presence of REE3+ cations in the structures causes emission bands in the visible yellow-to-orange spectral range.

Posted on 16 April 2015 | 9:45 am

Improved Synthesis of a Zirconium(IV) Muconate Metal–Organic Framework: Characterization, Stability and Gas Sorption Properties

The formerly reported ZrIV trans,trans-muconate framework material (1) was synthesized by using an improved synthetic protocol. Instead of using the noncommercial Zr6-methacrylate oxido cluster as the metal source, the same MOF was successfully synthesized by using commercially available ZrCl4 as the Zr source. Small amounts of H2O (1-H2O-AS; AS = as-synthesized) or concd. aqueous HCl (1-HCl-AS) were used as additives in N,N-dimethylformamide (DMF) in order to enhance the crystallinity of the compounds. The guest molecules were removed from the pores by heating the as-synthesized compounds under vacuum, resulting in the empty-pore forms of the compounds. The as-synthesized and thermally activated compounds were characterized by X-ray powder diffraction (XRPD), Fourier transform infrared (FTIR), thermogravimetric (TG) and elemental analyses. Thermogravimetric analyses suggest that the two compounds are stable up to ca. 250 °C in an air atmosphere. As verified by the XRPD experiments, the 1-HCl compounds gradually lose their crystallinity when exposed to water and moisture. As confirmed by the N2 sorption analyses, the optimally activated 1-HCl compound displayed a BET surface area of 557 m2?g–1, which is lower than the isostructural UiO-66 (950 m2?g–1) but it is close to the literature value (705 m2?g–1). The moderate physiochemical stability paired with considerable porosity renders the material a promising candidate for gas storage and separation. The improved synthetic procedure, characterization, thermal and chemical stability and gas adsorption properties of the previously reported ZrIV muconate compound are presented.

Posted on 14 April 2015 | 11:50 am

An Iron-Based Photosensitizer with Extended Excited-State Lifetime: Photophysical and Photovoltaic Properties

Herein, we report a homoleptic iron complex bearing tridentate bis-carbene (CNC) ligands designed for sensitization of TiO2 photoanodes. Its excited state has been characterized by ultra-fast transient spectroscopy and time-dependent density functional theory (TD-DFT) computations, which reveal a record triplet metal-to-ligand charge-transfer (3MLCT) excited-state lifetime (16 ps). The new dye was efficiently chemisorbed on TiO2 and promoted electron injection and photocurrent generation in a dye-sensitized solar cell upon solar irradiation. A new iron(II) complex with a record 16 ps triplet metal-to-ligand charge-transfer (3MLCT) state lifetime has been designed and evaluated as a photosensitizer in a laboratory dye-sensitized solar cell (DSSC), which led to measurable photocurrent and power-conversion efficiency. Such an improvement of the photophysical properties of iron-based complexes opens the way to applications that go far beyond energy production.

Posted on 14 April 2015 | 11:50 am

Hg5AsS2I3 – A Narrow-Band-Gap 2D Layered Compound with Different Trapped I– Anions

A new layered compound, Hg5AsS2I3, has been synthesized in the quaternary system mercury/arsenic/chalcogen/halogen by a moderate-temperature solid-state reaction. The compound crystallizes in the space group C2/c of the monoclinic system, and its structure consists of ?1[Hg(1)2Hg(2)2As2/2S2] eight-membered-ring chains bridged by Hg atoms to form [Hg(1)2Hg(2)2Hg(3)As2/2S2] layers with inner- and interlayer electrostatic interactions between I and Hg. The optical properties were investigated by means of diffuse reflectance and FTIR spectra. The electronic band structures, along with density of states (DOS) calculations by DFT, indicate that the title compound is a semiconductor, and its optical absorption mainly leads to charge transitions from I-5p, As-4p and S-3p states to Hg-6s states, and both I(1) and I(2) atoms contribute to the top of the valence bands. The structure consists of ?1[Hg(1)2Hg(2)2As2/2S2] eight-membered-ring chains bridged by Hg atoms to form [Hg(1)2Hg(2)2Hg(3)As2/2S2] layers with inner- and interlayer electrostatic interactions between I and Hg.

Posted on 14 April 2015 | 11:50 am

Determining the Ligand Properties of N-Heterocyclic Carbenes from 77Se NMR Parameters

The 1JCSe coupling constants for a range of NHC–selenium adducts have been measured and used to establish a correlation with the ?-donor strength of the respective carbenes. For the subclass of amido-carbenes, the 1JCSe values revealed a high donor capacity, very much in contrast to what the DFT-calculated HOMO energies suggest. The 1JCH coupling constants for the C-2 atoms in azolium-type NHC precursors were more readily obtained and show the same trend as the 1JCSe coupling constants. In addition, the use of 77Se chemical shifts to determine ?-acidity has been extended to a broader range of derivatives, namely 1·Se–22·Se. The superior resolution of the ?(77Se) method in comparison with the Tolman electronic parameters derived from IR spectroscopy is demonstrated for the caffeine-derived bis-carbene 19. The electronic properties of a range of N-heterocyclic carbenes have been evaluated by using the 77Se NMR chemical shifts of NHC–Se adducts, to map the ?-acceptor character, and 1JCSe coupling constants, which provide insights into the ?-donor ability of the respective carbene. DFT calculations were conducted to rationalize the data. The NMR-derived parameters have been compared with TEP values.

Posted on 10 April 2015 | 10:30 am

A Bioinspired System for Light-Driven Water Oxidation with a Porphyrin Sensitizer and a Tetrametallic Molecular Catalyst

Inspired by natural photosynthesis, the aim of light-driven water splitting is to produce renewable fuels by exploiting solar radiation. Sustained hydrogen production is desirable in such systems, and the oxidation of water to oxygen is currently recognized as the bottleneck of the entire process. Therefore, solutions for this difficult task retain a fundamental interest. In this paper, we present a bioinspired, three-component system for water oxidation that comprises a tetracationic porphyrin ZnII complex as the photosensitizer, a tetraruthenium water-oxidation catalyst, and sodium persulfate as the electron acceptor. An in-depth photophysical study reveals the photogeneration of a pentacation radical of the porphyrin (quantum yield up to ? = 1.01) upon oxidative quenching of the triplet excited state by persulfate. Electron transfer from the water-oxidation catalyst to the pentacation radical (hole scavenging) is slow (bimolecular rate constant, k?<?4?×?107 M–1?s–1), and this is likely the main reason for the low efficiency of the system in photocatalytic tests for water oxidation. Perspectives for improvements of the system and for the development of a light-activated device for water splitting are discussed. The photoinduced events and the light-driven water-oxidation ability of a bioinspired, three-component catalyst/photosensitizer/acceptor molecular system are investigated.

Posted on 8 April 2015 | 12:30 pm

Facile Rearrangement of a Bis(N-heterocyclic carbene)borate Chelate Ligand and Access to [:GeX]+ Complexes (X = H, Cl)

The unexpected isomerization of the potassium bis(N-heterocyclic carbene)borate [BPh2(tBuNHC)2]K (2?) (tBuNHC = 3-tert-butylimidazole-2-ylidene) in toluene, affording the unique potassium imidazolyl–NHC–borate [BPh2(tBuIm)(tBuNHC)]K (2) (tBuIm = 3-tert-butyl-2-imidazolyl), is reported. The latter crystallizes as the centrosymmetric dimer 22. According to the results of DFT calculations, the rearrangement of 2? is basically triggered by the solvation of the K+ ion: while 2? is most stable in THF solutions, the formation of 2 is feasible in least coordinating toluene and drastically favored through dimerization to give 22. The latter reacts with GeCl2·dioxane to form solely the unprecedented chlorogermyliumylideneborate [BPh2(tBuIm)(tBuNHC)]GeCl (3). Treatment of 3 with KHB(sBu)3 furnishes the corresponding hydridogermyliumylideneborate complex [BPh2(tBuIm)(tBuNHC)]GeH (4) in 66?% yield. Flip the chelate: The unexpected solvent-dependent potassium-induced isomerization of a potassium bis(N-heterocyclic carbene)borate in toluene afforded a unique potassium imidazolyl–NHC–borate isomer, which was successfully utilized for the synthesis and isolation of unprecedented chloro- and hydridogermyliumylideneborate complexes.

Posted on 8 April 2015 | 12:30 pm

From Homonuclear Metal String Complexes to Heteronuclear Metal String Complexes

The study of metal string complexes with 1D transition-metal frameworks began in the early 1990s. As these complexes provide great insights into metal–metal multiple bonds and may have potential applications as molecular wires, this field of research has grown in the past 20 years. As such, the electronic structures of the simplest trinuclear complexes, the supporting ligand systems, and the single-molecular conductance of metal string complexes are discussed. This review introduces the development of this field and summarizes some important results in the newly designed heteronuclear metal string complexes (HMSCs). These molecules may be of great interest in studies of the nature of heterometallic electronic effects and molecular electronic applications. This microreview describes the development of metal string complexes, from homonuclear to heteronuclear ones, including their synthesis and physical properties.

Posted on 7 April 2015 | 10:10 am

Controlling the Activities of the Diiron Centre in Bacterial Monooxygenases: Lessons from Mutagenesis and Biodiversity

The soluble diiron monooxygenases (SDIMOs) are a diverse group of bacterial enzymes that possess a deeply buried binuclear iron centre that forms an oxo-diferryl (FeIV) intermediate that is capable of oxygenating a wide range of unfunctionalised hydrocarbons and other hydrophobic organic molecules, ranging in size from methane to diaromatics such as naphthalene and substituted biphenyls. In the environment, these enzymes are important in bioremediation of hydrocarbons and chlorinated hydrocarbons and in mitigation of the greenhouse effect due to methane, and also have numerous potential applications in synthetic organic chemistry. In this review we consider how both natural variations among the enzymes and an increasing body of evidence from mutagenesis studies cast light on how the protein controls the size of substrates that can access the active site, the precision of regio- and stereoselectivity and the oxidising power of the active centre. The soluble diiron monooxygenases (SDIMOs) are a diverse group of enzymes that use a diiron active centre to add oxygen to a wide range of organic molecules. Here we review how natural biodiversity and laboratory mutagenesis studies have revealed how the protein scaffold of SDIMOs controls the activity of the enzymes and the presentation of substrates to the diiron site.

Posted on 2 April 2015 | 2:20 pm

Photoluminescence Tuning and Water Detection of Yttrium Diazinedicarboxylate Materials through Lanthanide Doping

A series of yttrium(III)-based compounds with three diazinedicarboxylate ligands, namely, {[Y2(?-pmdc)2(?-ox)(H2O)6]·4H2O}n (1, pmdc = pyrimidine-4,6-dicarboxylato, ox = oxalato), {[Y2(?-pmdc)3(H2O)6]·2DMF·8H2O}n (2, DMF = N,N-dimethylformamide), {[Y2(?3-pmdc)2(?-ox)(H2O)4]·xH2O}n (3, x ? 4.66), {[Y(?4-pmdc)(NO3)(H2O)]·H2O}n (4), [Y2(?-pddc)3(H2O)6]·7H2O (5, pddc = pyridazine-3,6-dicarboxylato) and {[Y6(?4-pzdc)6(NO3)(H2O)16](NO3)5·2H2O}n (6, pzdc = pyrazine-2,5-dicarboxylato), have been prepared by different synthetic approaches. The crystal structures range from 0D to 3D architectures, and all of the compound show solvent-occupied volumes. The thermal behaviour of these compounds has been analysed, and the open structure of 3 shows a reversible uptake/release of water. Moreover, the yttrium matrix of 3 has been doped with Eu3+ and Tb3+ ions to investigate the photoluminescence response of the hybrid materials. In addition to a tuneable emission colour through the concentration of luminescent ions and the excitation wavelength, these hybrid materials could serve as potential photofluorescent sensors for water detection through the reversible dehydration/rehydration process. Six diazinedicarboxylate-based yttrium(III) coordination compounds are prepared through different synthetic approaches. Activation by lanthanide doping enables the modulation of the emission colour. Coupling the luminescence with reversible water capture/release opens up the way to a functional material that shows promise as a water sensor.

Posted on 2 April 2015 | 2:20 pm

Electronic Fine-Tuning of Oxygen Atom Transfer Reactivity of cis-Dioxomolybdenum(VI) Complexes with Thiosemicarbazone Ligands

A series of six cis-dioxomolybdenum(VI) complexes with thiosemicarbazone ligands was synthesized and characterized. The ligands were obtained by reacting ethyl thiosemicarbazide with salicylaldehydes substituted with a selection of electron-withdrawing and electron-donating groups. The crystal structures, IR, NMR spectroscopic data and oxygen atom transfer activities of the complexes revealed that the electronic effects of the substituents located in the para-position of the phenolate donor are transmitted through to the molybdenum center, as reflected by linear relationships between Hammett constants and key properties of the complexes, including the molybdenum–phenolate bond lengths and the coordination shift of the imine proton resonance. Compared with the unsubstituted catalyst, electron-withdrawing substituents increase the rate of oxygen atom transfer from dimethyl sulfoxide to triphenylphosphine, whereas electron-donating groups have the opposite effect. The highest rate enhancement was achieved through the introduction of a strongly electron-withdrawing NO2 substituent in the p-position of the phenolate donor. Thiosemicarbazone ligands (H2LR) derived from para-substituted salicylaldehydes allow the electronic properties and catalytic activities of cis-dioxo MoVI complexes of composition [MoO2(LR)MeOH] to be fine-tuned by electron-donating or electron-withdrawing groups R.

Posted on 2 April 2015 | 2:20 pm

Catalytic Models of Tyrosinase: Reactivity Differences between Systems Based on Mono- and Binucleating Ligands

A new tyrosinase model based on the binucleating ligand Lpy2 is synthesized and characterized. The ligand Lpy2 contains a combination of an imine and a pyridine function in the sidearms, which are bridged by a flexible alkyl spacer. As shown by UV/Vis and NMR spectroscopy, the Cu2Lpy2 complex catalyzed the conversion of the monophenol 2,4-di-tert-butylphenol (DTBP-H) into the o-quinone 3,5-di-tert-butylquinone (DTBQ) with a turnover number (TON) of 18. The dicopper complex of Lpy2 thus shows monophenolase activity that is comparable to that of the recently developed Lpy1 model of tyrosinase, which is based on a known mononucleating ligand (M. Rolff, J. Schottenheim, G. Peters, F. Tuczek, Angew. Chem. Int. Ed. 2010, 122, 6583). The electron-poor substrate 4-hydroxybenzoic acid methyl ester (MeBA-OH), in contrast, is converted by Cu2Lpy2 into the semiquinone. For both substrates, the oxygenation reactions were also conducted in a stoichiometric fashion to obtain information on the intermediates involved. For the substrate MeBA-OH, we detected a binuclear ?-catecholato copper(II) complex by high-resolution ESI mass spectrometry. These studies were complemented by investigations of deactivation mechanisms that could be invoked to explain the limitation of the TON. To this end, a bis-?-hydroxido Lpy2 dicopper(II) complex as well as a semiquinone Lpy2 complex were prepared. Both complexes may represent decay products of the catalyst. A tyrosinase model based on the binucleating ligand Lpy2 was developed and characterized. The ligand Lpy2 contains a combination of an imine and a pyridine function in the sidearms that are bridged by a flexible alkyl spacer. The Cu2Lpy2 complex catalyzes the conversion of monophenol DTBP-H into the o-quinone DTBQ (TON = 18). An electron-poor substrate is converted into the semiquinone.

Posted on 2 April 2015 | 2:20 pm

The Electrically Wired Molybdenum Domain of Human Sulfite Oxidase is Bioelectrocatalytically Active

We report electron transfer between the catalytic molybdenum cofactor (Moco) domain of human sulfite oxidase (hSO) and electrodes through a poly(vinylpyridine)-bound [osmium(N,N?-methyl-2,2?-biimidazole)3]2+/3+ complex as the electron-transfer mediator. The biocatalyst was immobilized in this low-potential redox polymer on a carbon electrode. Upon the addition of sulfite to the immobilized separate Moco domain, the generation of a significant catalytic current demonstrated that the catalytic center is effectively wired and active. The bioelectrocatalytic current of the wired separate catalytic domain reached 25?% of the signal of the wired full molybdoheme enzyme hSO, in which the heme b5 is involved in the electron-transfer pathway. This is the first report on a catalytically active wired molybdenum cofactor domain. The formal potential of this electrochemical mediator is between the potentials of the two cofactors of hSO, and as hSO can occupy several conformations in the polymer matrix, it is imaginable that electron transfer from the catalytic site to the electrode through the osmium center occurs for the hSO molecules in which the Moco domain is sufficiently accessible. The observation of catalytic oxidation currents at low potentials is favorable for applications in bioelectronic devices. Human sulfite oxidase and its catalytic molybdenum-cofactor-containing domain were wired to an electrode with poly(vinylpyridine)–[osmium(N,N?-methyl-2,2?-biimidazole)3]2+/3+, which mediated the electron transfer between either of the enzyme cofactors and the electrode, as demonstrated by electrocatalytic turnover currents.

Posted on 31 March 2015 | 10:10 am

Thymol Bromination – A Comparison between Enzymatic and Chemical Catalysis

The catalytic activity of the vanadium-dependent bromoperoxidase isolated from the brown alga Ascophyllum nodosum is compared with the activity of a cheap, commercially available V-catalyst precursor in the bromination of thymol. Organic solvents have been avoided to make the system appealing from a sustainable chemistry point of view. It is noteworthy that, notwithstanding the low solubility of the substrate, the thymol bromination reactions were performed in water, with a safe brominating source, under mild conditions, and with relatively inexpensive reagents. In this regard, the greenness of the systems was evaluated by the estimation of the E-factor value; the result is that the chemical reaction has a lower environmental impact than the enzymatic process, with an E-factor in the range of eco-friendly processes. Catalysis of thymol bromination by vanadium derivatives is directly compared to catalysis by a V-dependent bromoperoxidase. All reactions were performed under mild and sustainable conditions with relatively inexpensive reagents. Appealing results were obtained in terms of selectivity and sustainability.

Posted on 20 March 2015 | 12:10 pm

Synthesis, Characterization, and Stereoselective Oxidations of the Dinuclear Copper(II) Complex Derived from a Chiral Diamino-m-xylenetetra(benzimidazole) Ligand

Recent advances in dinuclear copper complexes as mimics of the catalytic centers of tyrosinase and catechol oxidase allowed the reproduction of the structural and mechanistic aspects of the enzymes. However, a challenging objective is the development of chiral complexes for bioinspired enantioselective oxidation reactions. Here, we report the synthesis and characterization of a dinuclear copper(II) complex with a new chiral diamino-m-xylenetetra(benzimidazole) ligand (L55Bu4), which has chiral centers at the four 2-methylbutyl substituents of the benzimidazole rings. The spectral characteristics, ligand binding properties, and reactivity of [CuII2L55Bu4]4+ in the catalytic oxidations of several biogenic catechols (L-/D-dopa, L-/D-DopaOMe, and L-/D-norepinephrine) and thioanisole are reported. The best discriminating properties are displayed towards the DopaOMe derivatives, for which the oxidation rate of the L enantiomer is approximately one order of magnitude larger than that of the opposite D isomer. A dinuclear copper(II) complex derived from a chiral hexadentate nitrogen ligand is reported as a new catalyst for asymmetric oxidations. For biogenic catechols as model substrates, the best enantioselectivity is obtained in the oxidation of the methyl esters of L-/D-Dopa, for which 70?%?ee is obtained in favor of the L enantiomer.

Posted on 9 March 2015 | 12:10 pm

Mononuclear Iron(III) Complexes as Functional Models of Catechol Oxidases and Catalases

Mononuclear amino triphenolate iron(III) complexes have been synthesized and characterized (UV/Vis spectroscopy, cyclic voltammetry, ESI-MS, 1H NMR spectroscopy, X-ray diffraction). These complexes act as biomimetic catalysts promoting the aerobic auto-oxidation of 3,5-di-tert-butylcatechol to the corresponding o-benzoquinone at room temperature and displaying catalase activity for the hydrogen peroxide dismutation with remarkable activity (TON up to 4000). Amino triphenolate iron(III) complexes promote effectively, at room temperature, the aerobic auto-oxidation of 3,5-di-tert-buthylcatechol to the corresponding o-benzoquinone (oxidase mimicking) and hydrogen peroxide dismutation (catalase mimicking).

Posted on 16 February 2015 | 4:50 pm

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