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Electrocatalytic CO2 reduction to value-added products provides a viable alternative to the use of carbon sources derived from fossil fuels. Carrying out these transformations at reasonable energetic costs, for example, with low overpotential, remains a challenge. Molecular catalysts allow fine control of activity and selectivity via tuning of their coordination sphere and ligand set. Herein we investigate a series of cobalt(III) pyridine-thiolate complexes as electrocatalysts for CO2 reduction. The effect of the ligands and proton sources on activity was examined. We identified bipyridine bis(2-pyridinethiolato) cobalt(III) hexaflurophosphate as a highly selective catalyst for formate production operating at a low overpotential of 110 mV with a turnover frequency (TOF) of 10 s-1 . Electrokinetic analysis coupled with density functional theory (DFT) computations established the mechanistic pathway, highlighting the role of metal hydride intermediates. The catalysts deactivate via the formation of stable cobalt carbonyl complexes, but the active species could be regenerated upon oxidation and release of coordinated CO ligands.

In the recent years, significant progress has been made toward designing active and selective catalysts for electrochemical CO2 reduction, with particular interest focused on the two major C2 produ...

International audience; A novel heterobimetallic tantalum/iridium hydrido complex, [{Ta(CH2 t Bu)3}{IrH2(Cp*)}] 1, featuring a very short metal-metal bond, has been isolated through an original alkane elimination route from Ta(CH t Bu)(CH2 t Bu)3 and Cp*IrH4. This molecular precursor has been used to synthesize well-defined silica-supported low-coordinate heterobime-tallic hydrido species [≡SiOTa(CH2 t Bu)2{IrH2(Cp*)}] 5 and [≡SiOTa(CH2 t Bu)H{IrH2(Cp*)}] 6 using a surface organometallic chemistry approach (SOMC). The SOMC methodology prevents undesired dimerization as encountered in solution and leading to a tetranuclear species [{Ta(CH2 t Bu)2}(Cp*IrH)]2, 4. This approach therefore allows access to unique low-coordinate species not attainable in solution. These original supported Ta/Ir species exhibit drastically enhanced catalytic performances in H/D exchange reactions with respect to (i) monometallic analogues as well as (ii) homogeneous systems. In particular, material 6 promotes the H/D exchange between fluorobenzene and C6D6 or D2 as deuterium sources with excellent productivity (TON up to 1422; TOF up to 23.3 h-1) under mild conditions (25°C, subatmospheric D2 pressure) without any additives.

Electrocatalytic CO2 reduction to value-added products provides a viable alternative to the use of carbon sources derived from fossil fuels. Nevertheless, the ability to carry out these transformations at reasonable energetic costs, e.g. with low overpotential, remains a significant challenge. Molecular catalysts offer a great option in this context, as fine control of their activity and selectivity can be obtained via the tuning of their coordination sphere and ligand set. To this end, we investigated here a series of cheap cobalt(III) pyridine-thiolate complexes as electrocatalysts for CO2 reduction. The effect of the ligands and proton sources on activity was examined. We were able to identify [bipyridine-bis-(2-pyridinethiolato)-cobalt(III)-hexaflurophosphate] as a highly selective catalyst for formate production operating at a very low overpotential of 110 mV to achieve a TOF of 10 s-1. Detailed electrokinetic analysis coupled with density functional theory allowed establishing a mechanistic pathway for these catalysts, highlighting the role of key metal hydride intermediates. The catalysts deactivate via the formation of stable Co carbonyl complexes, but we demonstrated that the active species could be regenerated upon oxidation and release of coordinated CO ligands.

Electrocatalytic CO2 reduction to value-added products provides a viable alternative to the use of carbon sources derived from fossil fuels. Nevertheless, the ability to carry out these transformations at reasonable energetic costs, e.g. with low overpotential, remains a significant challenge. Molecular catalysts offer a great option in this context, as fine control of their activity and selectivity can be obtained via the tuning of their coordination sphere and ligand set. To this end, we investigated here a series of cheap cobalt(III) pyridine-thiolate complexes as electrocatalysts for CO2 reduction. The effect of the ligands and proton sources on activity was examined. We were able to identify [bipyridine-bis-(2-pyridinethiolato)-cobalt(III)-hexaflurophosphate] as a highly selective catalyst for formate production operating at a very low overpotential of 110 mV to achieve a TOF of 10 s-1. Detailed electrokinetic analysis coupled with density functional theory allowed establishing a mechanistic pathway for these catalysts, highlighting the role of key metal hydride intermediates. The catalysts deactivate via the formation of stable Co carbonyl complexes, but we demonstrated that the active species could be regenerated upon oxidation and release of coordinated CO ligands.

Electrocatalytic CO

Non‐noble metal molecular catalysts mediating the electrocatalytic reduction of carbon dioxide are still scarce. This work reports the electrochemical reduction of CO2 to formate catalyzed by the bimetallic complex [(bdt)MoVI(O)S2CuICN]2‐ (bdt = benzenedithiolate), a mimic of the active site of the Mo‐Cu carbon monoxide dehydrogenase enzyme (CODH2). Infrared spectroelectrochemical (IR‐SEC) studies coupled with density functional theory (DFT) computations revealed that the complex is only a pre‐catalyst, the active catalyst being generated upon reduction in the presence of CO2. We found that the two‐electron reduction of [(bdt)MoVI(O)S2CuICN]2‐ triggers the transfer of the oxo moiety to CO2 forming CO3 2‐and the complex [(bdt)MoIVS2CuICN]2– and that a further oneelectron reduction is needed to generate the active catalyst. Its protonation yields a reactive MoVH hydride intermediate which reacts with CO2 to produce formate. These findings are particularly relevant to the design of catalysts from metal oxo precursors.

Non‐noble metal molecular catalysts mediating the electrocatalytic reduction of carbon dioxide are still scarce. This work reports the electrochemical reduction of CO2 to formate catalyzed by the bimetallic complex [(bdt)MoVI(O)S2CuICN]2‐ (bdt = benzenedithiolate), a mimic of the active site of the Mo‐Cu carbon monoxide dehydrogenase enzyme (CODH2). Infrared spectroelectrochemical (IR‐SEC) studies coupled with density functional theory (DFT) computations revealed that the complex is only a pre‐catalyst, the active catalyst being generated upon reduction in the presence of CO2. We found that the two‐electron reduction of [(bdt)MoVI(O)S2CuICN]2‐ triggers the transfer of the oxo moiety to CO2 forming CO3 2‐and the complex [(bdt)MoIVS2CuICN]2– and that a further oneelectron reduction is needed to generate the active catalyst. Its protonation yields a reactive MoVH hydride intermediate which reacts with CO2 to produce formate. These findings are particularly relevant to the design of catalysts from metal oxo precursors.

Preparation of a series of terpyridyl ligands bearing different substituents recently led to the synthesis of new cobalt-bisterpyridyl complexes spanning over a wide range of redox potentials. In this work, we describe the catalytic properties of these complexes for the electroreduction of protons into hydrogen (hydrogen evolution reaction (HER)) in acetonitrile. The substituents of the ligands were found to greatly affect the catalytic performances of the systems, in terms of stability and overpotential. Interestingly, systems based on dimethylamino-terpyridine derivatives perform HER with high efficiency, low overpotential and excellent stability. Density functional theory calculations were used to provide insights into the reaction mechanism of HER catalyzed by these systems, highlighting the role of the ligand for proton activation.

The photochemical reduction of CO

Cobalt dithiolene complexes are a new class of H2-evolving catalysts. Here we describe the preparation, the structure and the catalytic activity of an original cobalt complex using a bioinspired ligand, a quinoxaline-pyran-fused dithiolene derivative (qpdt(2-)) that mimics the molybdopterin cofactor present in the active sites of formate dehydrogenases. This complex displays very good activity for electrochemical proton reduction under weak acid conditions in terms of turnover frequency, faradic yields and stability. Density functional theory calculations show that protonation of a nitrogen atom of the ligand decreases overpotentials by 520 mV and H2 formation proceeds via protonation of an intermediate Co-H hydride, with an adjacent S atom of the dithiolene ligand serving as a proton relay.

The molecular and electronic structure, as well as the electronic absorption spectrum of Tc2(O2CCH3)2Cl4 were studied by multiconfigurational quantum chemical methods. The computed ground state geometry is in excellent agreement with the experimental structure determined by single crystal X-ray diffraction (SCXRD). The total bond order (i.e., 3.20) is consistent with the presence of a moderately strong quadruple Tc–Tc bond and is the largest bond order reported so far for a multiple Tc–Tc bonded complex. Effective bond order analysis indicates stronger p and d bonds for Tc2(O2CCH3)2Cl4 (i.e., 1.71 for p and 0.59 for d) than for Tc2Cl8 2� (i.e., 1.68 for p and 0.47 for d). The electronic absorption spectrum was recorded in

Packing constraints and precise placement of functional groups are the reason that organic molecules in the crystalline state often display unusual physical or chemical properties not observed in solution. Here we report a single crystal to single crystal dimerization of a bromodiacetylene that involves unusually large atom displacements as well as the cleavage and formation of several bonds. Density functional theory computations support a mechanism in which the dimerization is initiated by a [2 + 1] photocycloaddition favoured by the nature of carbon carbon short contacts in the crystal structure. The reaction proceeded up to the theoretical degree of conversion without loss of crystallinity and it was also performed on a preparative scale with good yield. Moreover it represents the first synthetic pathway to (E) 12 dibromo 12 diethynylethenes which could serve as synthetic intermediates for the preparation of molecular carbon scaffolds. Our findings both extend the scope of single crystal to single crystal reactions and highlight their potential as a synthetic tool for complex transformations. © 2013 Macmillan Publishers Limited. All rights reserved.

The dinuclear rhenium(II) complex Re2Br4(PMe3)4 was prepared from the reduction of [Re2Br8](2-) with (n-Bu4N)BH4 in the presence of PMe3 in propanol. The complex was characterized by single-crystal X-ray diffraction (SCXRD) and UV-visible spectroscopy. It crystallizes in the monoclinic C2/c space group and is isostructural with its molybdenum and technetium analogues. The Re-Re distance (2.2521(3) Å) is slightly longer than the one in Re2Cl4(PMe3)4 (2.247(1) Å). The molecular and electronic structure of Re2X4(PMe3)4 (X = Cl, Br) were studied by multiconfigurational quantum chemical methods. The computed ground-state geometry is in excellent agreement with the experimental structure determined by SCXRD. The calculated total bond order (2.75) is consistent with the presence of an electron-rich triple bond and is similar to the one found for Re2Cl4(PMe3)4. The electronic absorption spectrum of Re2Br4(PMe3)4 was recorded in benzene and shows a series of low-intensity bands in the range 10 000-26 000 cm(-1). The absorption bands were assigned based on calculations of the excitation energies with the multireference wave functions followed by second-order perturbation theory using the CASSCF/CASPT2 method. Calculations predict that the lowest energy band corresponds to the δ* → σ* transition, while the next higher energy bands were attributed to the δ* → π*, δ → σ*, and δ → π* transitions.

A simple method for the high-yield preparation of (NH4)2[Re2F8]·2H2O has been developed that involves the reaction of (n-Bu4N)2[Re2Cl8] with molten ammonium bifluoride (NH4HF2). Using this method, the new salt [NH4]2[Re2F8]·2H2O was prepared in ∼90% yield. The product was characterized in solution by ultraviolet-visible light (UV-vis) and (19)F nuclear magnetic resonance ((19)F NMR) spectroscopies and in the solid-state by elemental analysis, powder X-ray diffraction (XRD), and infrared (IR) spectroscopy. Multiconfigurational CASSCF/CASPT2 quantum chemical calculations were performed to investigate the molecular and electronic structure, as well as the electronic absorption spectrum of the [Re2F8](2-) anion. The metal-metal bonding in the Re2(6+) unit was quantified in terms of effective bond order (EBO) and compared to that of its [Re2Cl8](2-) and [Re2Br8](2-) analogues.

N,N-Dimethylaminodiboranate complexes with praseodymium, samarium, erbium, and uranium, which are potential chemical vapor deposition precursors for the deposition of metal boride and oxide thin films, have been investigated by DFT guided by field-ionization mass spectros- copy experiments. The calculations indicate that the volatilities of these complexes are correlated with the M−H bond strengths as determined by Mayer bond order analysis. The geometries of the gas-phase monomeric, dimeric, and trimeric species seen in field-ionization mass spectroscopy experiments were identified using DFT calculations, and the relative stabilities of these oligomers were assessed to understand how the lanthanide aminodiboranates depolymerize to their respective volatile forms during sublimation.

Norrish-type-II reaction on a semiquinone radical: Stable semiquinone radicals serve as novel molecular platforms on which a Norrish-type-II photoreaction can be initiated. A detailed reaction scheme involving a 1,5-hydrogen transfer followed by a cyclization step that finally leads to a new CC bond formation could be verified. Transient absorption spectroscopy and DFT calculations trace convincingly the intermediates and transition states along the reaction path (see scheme). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The photochemical reactivity of osmocene in a biphasic water-organic solvent system has been investigated to probe its water splitting properties. The photoreduction of aqueous protons to hydrogen under anaerobic conditions induced by osmocene dissolved in 1,2-dichloroethane and the subsequent water splitting by the osmocenium metal-metal dimer formed during H 2 production were studied by electrochemical methods, UV-visible spectrometry, gas chromatography, and nuclear magnetic resonance spectroscopy. Density functional theory computations were used to validate the reaction pathways.

Recent studies of organouranium chemistry have provided unusual pairs of similar polymetallic molecules containing (N)(3-) and (O)(2-) ligands, namely [(C(5)Me(5))U(mu-I)(2)](3)(mu(3)-N), 1, and [(C(5)Me(5))U(mu-I)(2)](3)(mu(3)-O), 2, and chair and boat conformations of [(C(5)Me(5))(2)U(mu-N)U(mu-N(3))(C(5)Me(5))(2)](4), 3. These compounds were analyzed by density functional theory and multiconfigurational quantum chemical studies to differentiate nitride versus oxide in molecules for which the crystallographic data were not definitive and to provide insight into the electronic structure and unique chemical bonding of these polymetallic compounds. Calculations were also performed on [(C(5)Me(5))(2)UN(3)(mu-N(3))](3), 4, and [(C(6)F(5))(3)BNU(N[Me]Ph)(3)], 5, for comparison with 1 and 3. On the basis of these results, the complex, [(C(5)Me(5))U(mu(3)-E)](8), 6, for which only low-quality X-ray crystallographic data are available, was analyzed to predict if E is nitride or oxide.

The compounds Tc(2)Cl(4)(PMe(3))(4) and Tc(2)Br(4)(PMe(3))(4) were formed from the reaction between (n-Bu(4)N)(2)Tc(2)X(8) (X = Cl, Br) and trimethylphosphine. The Tc(II) dinuclear species were characterized by single-crystal XRD, UV-visible spectroscopy, and cyclic voltammetry techniques, and the results are compared to those obtained from density functional theory and multiconfigurational (CASSCF/CASPT2) quantum chemical studies. The compound Tc(2)Cl(4)(PMe(3))(4) crystallizes in the monoclinic space group C2/c [a = 17.9995(9) A, b = 9.1821(5) A, c = 17.0090(9) A, beta = 115.4530(10) degrees ] and is isostructural to M(2)Cl(4)(PMe(3))(4) (M = Re, Mo, W) and to Tc(2)Br(4)(PMe(3))(4). The metal-metal distance (2.1318(2) A) is similar to the one found in Tc(2)Br(4)(PMe(3))(4) (2.1316(5) A). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc(2)X(8)(2-) and Tc(2)X(4)(PMe(3))(4) (X = Cl, Br) indicate stronger pi bonds in the Tc(2)(4+) core than in Tc(2)(6+) core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10 000-26 000 cm(-1). Assignment of the bands as well as computing their excitation energies and intensities were performed at both TD-DFT and CASSCF/CASPT2 levels of theory. Calculations predict that the lowest energy band corresponds to the delta* --sigma* transition, the difference between calculated and experimental values being 228 cm(-1) for X = Cl and 866 cm(-1) for X = Br. The next bands are attributed to delta* --pi*, delta --sigma*, and delta --pi* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc(2)Br(4)(PMe(3))(4) exhibits more positive oxidation potentials than Tc(2)Cl(4)(PMe(3))(4.) This phenomenon is discussed and ascribed to stronger metal (d) to halide (d) back bonding in the bromo complex. Further analysis indicates that Tc(II) dinuclear species containing pi-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established.

Since the discovery of a formal quintuple bond in Ar'CrCrAr' (CrCr = 1.835 angstrom) by Power and co-workers in 2005, many efforts have been dedicated to isolating dichromium species featuring quintuple-bond character. In the present study we investigate the electronic configuration of several, recently synthesized dichromium species with ligands using nitrogen to coordinate the metal centers. The bimetallic bond distances of Power's compound and Cr-2-diazadiene (1) (CrCr = 1.803 angstrom) are compared to those found for Cr-2(mu-eta(2)-ArNC(R)NAr)(2) (2) (CrCr = 1.746 angstrom; R = H, Ar = 2,6-Et2C6H3), Cr-2(mu-eta(2)-(ArNC)-N-Xyl(H)NArXyl)(3) (3) (CrCr = 1.740(reduced)/1.817(neutral) angstrom; Ar-Xyl=2,6-C6H3-(CH3)(2)), Cr-2(mu-eta(2)-TippPyNMes)(2) (4) (CrCr = 1.749 angstrom; TippPyNMes = 6-(2,4,6-triisopropylphenyl)pyridin-2-yl (2,4,6-trimethylphenyl)-amide), and Cr-2(mu-eta(2)-DippNC(NMe2)N-Dipp)(2) (5) (CrCr = 1.729 angstrom, Dipp = 2,6-i-Pr2C6H3). We show that the correlation between the CrCr bond length and the effective bond order (EBO) is strongly affected by the nature of the ligand, as well as by the steric hindrance due to the ligand structure (e.g., the nature of the coordinating nitrogen). A linear correlation between the EBO and CrCr bond distance is established within the same group of ligands. As a result, the CrCr species based on the amidinate, aminopyridinate, and guanidinate ligands have bond patterns similar to the Ar'CrCrAr' compound. Unlike these latter species, the dichromium diazadiene complex is characterized by a different bonding pattern involving Cr-N pi interactions, resulting in a lower bond order associated with the short metal-metal bond distance. In this case the short CrCr distance is most probably the result of the constraints imposed by the diazadiene ligand, implying a Cr2N4 core with a closer CrCr interaction. (Less)

Deviations from statistical binding, that is cooperativity, in self-assembled polynuclear complexes partly result from intermetallic interactions DeltaE(M,M), whose magnitudes in solution depend on a balance between electrostatic repulsion and solvation energies. These two factors have been reconciled in a simple point-charge model, which suggests severe and counter-intuitive deviations from predictions based solely on the Coulomb law when considering the variation of DeltaE(M,M) with metallic charge and intermetallic separation in linear polynuclear helicates. To demonstrate this intriguing behaviour, the ten microscopic interactions that define the thermodynamic formation constants of some twenty-nine homometallic and heterometallic polynuclear triple-stranded helicates obtained from the coordination of the segmental ligands L1-L11 with Zn(2+) (a spherical d-block cation) and Lu(3+) (a spherical 4f-block cation), have been extracted by using the site binding model. As predicted, but in contrast with the simplistic coulombic approach, the apparent intramolecular intermetallic interactions in solution are found to be i) more repulsive at long distance (${{ m{Delta }}E_{{ m{1 - 4}}};{{ m{Lu}}{ m{,Lu}}} }$>${{ m{Delta }}E_{{ m{1 - 2}}};{{ m{Lu}}{ m{,Lu}}} }$), ii) of larger magnitude when Zn(2+) replaces Lu(3+) (${{ m{Delta }}E_{{ m{1 - 2}}};{{ m{Zn}}{ m{,Lu}}} }$>${{ m{Delta }}E_{{ m{1 - 2}}};{{ m{Lu}}{ m{,Lu}}} }$) and iii) attractive between two triply charged cations held at some specific distance (${{ m{Delta }}E_{{ m{1 - 3}}};{{ m{Lu}}{ m{,Lu}}} }$

The La + O and La + O 2 chemiionization reactions have been investigated with quantum chemical methods. For La + O 2(X (3)Sigma g) and La + O 2(a (1)Delta g), the chemiionization reaction La + O 2 --LaO 2 (+) + e (-) has been shown to be endothermic and does not contribute to the experimental chemielectron spectra. For the La + O 2(X (3)Sigma g) reaction conditions, chemielectrons are produced by La + O 2 --LaO + O, followed by La + O --LaO (+) + e (-). This is supported by the same chemielectron band, arising from La + O --LaO (+) + e (-), being observed from both the La + O( (3)P) and La + O 2(X (3)Sigma g) reaction conditions. For La + O 2(a (1)Delta g), a chemielectron band with higher electron kinetic energy than that obtained from La + O 2(X (3)Sigma g) is observed. This is attributed to production of O( (1)D) from the reaction La + O 2(a (1)Delta g) --LaO + O( (1)D), followed by chemiionization via the reaction La + O( (1)D) --LaO (+) + e (-). Potential energy curves are computed for a number of states of LaO, LaO* and LaO (+) to establish mechanisms for the observed La + O --LaO (+) + e (-) chemiionization reactions.

The atomic structure of a well-ordered silica film grown on a Mo(112) single crystal substrate is discussed in detail using the experimental and theoretical results available to date. New photoelec ...

Low-coverage vanadia species (monomers, dimers, trimers, and one-dimensional vanadia rows) as well as vanadium oxide films of varying thickness supported on the metastable κ-Al2O3(001) surface are investigated by density functional theory in combination with statistical thermodynamics. At low-vanadium chemical potentials and typical reducing conditions, species with V−O(3)−Al interface bonds are stable. These aggregates are partially reduced with vanadium in the VIII oxidation state. This correlates with defect formation energy values for the initial removal of lattice oxygen in the range of 1.3−2.7 eV. As the length of the polymeric species increases, the reduction energy decreases. We demonstrate that the support structure does affect the structure of the model catalyst and the lattice oxygen bond strength. On the α-Al2O3(0001) surface, the only stable low-coverage VOx species are dimers with V−O(2)−Al interface bonds and a defect formation energy of 2.8 eV. Reduction remains more facile for vanadia fil...

Bis(dithiolene)tungsten complexes, W(VI)O2 (L = dithiolene)2 and W(IV)O (L = dithiolene)2, which mimic the active site of formate dehydrogenases, have been characterized by cyclic voltammetry and controlled potential electrolysis in acetonitrile. They are shown to be able to catalyze the electroreduction of protons into hydrogen in acidic organic media, with good Faradaic yields (75-95%) and good activity (rate constants of 100 s(-1)), with relatively high overpotentials (700 mV). They also catalyze proton reduction into hydrogen upon visible light irradiation, in combination with [Ru(bipyridine)3](2+) as a photosensitizer and ascorbic acid as a sacrificial electron donor. On the basis of detailed DFT calculations, a reaction mechanism is proposed in which the starting W(VI)O2 (L = dithiolene)2 complex acts as a precatalyst and hydrogen is further formed from a key reduced W-hydroxo-hydride intermediate.

The mechanism and products of the structural collapse of the metal-organic frameworks (MOFs) UiO-66, MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal-ligand bonding in each case. The amorphous products contain inorganic-organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4 clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140 frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked., Comment: 24 pages, 7 figures, 2 tables

Die atomare Struktur eines geordneten Alumosilicatfilms auf einem Mo(112)-Substrat wurde durch hochauflosende Messungen bestimmt und mithilfe der Dichtefunktionaltheorie berechnet. Bei einem niedri ...

We have observed formation of one-dimensional silica structures of 0.5 nm in width on Mo(112) single crystal surface. Combination of high-resolution scanning tunneling microscopy, photoelectron and infrared spectroscopy, and density functional theory provides strong evidence for formation of paired rows of corner sharing [SiO4] tetrahedra chemisorbed on a metal substrate. (c) 2006 Elsevier B.V. All rights reserved.

The stability of ordered one- and two-dimensional silica structures formed on a Mo(112) surface as a function of silicon coverage and oxygen pressure (phase diagram) is derived from density functio ...

Review: [preparation, solid state and electronic structures, and spectroscopic properties of (Bu4N)2Tc2X8, Tc2(OAc)4X2, Tc2(OAc)2Cl4, cesium salts of Tc2X83-, and Tc2X4(PMe3)4 (X: Cl, Br); 38 refs.

The only operating mechanism in the oxidation of water to dioxygen catalyzed by the mononuclear cis-[RuII(bpy)2(H2O)2]2+ complex when treated with excess CeIV was unambiguously established. Theoretical calculations together with 18O-labeling experiments (see plot) revealed that it is the nucleophilic attack of water on a Ru=O group.

A method is suggested which allows truncation of the virtual space in Cholesky decomposition-based multiconfigurational perturbation theory (CD-CASPT2) calculations with systematic improvability of the results. The method is based on a modified version of the frozen natural orbital (FNO) approach used in coupled cluster theory. The idea is to exploit the near-linear dependence among the eigenvectors of the virtual-virtual block of the second-order Møller–Plesset density matrix. It is shown that FNO-CASPT2 recovers more than 95% of the full CD-CASPT2 correlation energy while requiring only a fraction of the total virtual space, especially when large atomic orbital basis sets are in use. Tests on various properties commonly investigated with CASPT2 demonstrate the reliability of the approach and the associated reduction in computational cost and storage demand of the calculations.

Periodic density functional theory (DFT) calculations concerning VO(x)/kappa-Al(2)O(3)(001) surfaces are used for parametrizing Monte Carlo simulations performed on a mesoscopic scale surface sample (41.95x48.80 nm(2)). New structural and chemical information are then obtained that are not accessible from DFT calculations: segregation, short- and long-range ordering, and the effect of the temperature on the reducibility. The reducibility of the surface is investigated for locating chemical potential regions where the catalyst is especially reactive for oxidation reactions. Comparison with the V(2)O(5)(001) surface at catalytic conditions (800 K, 1 bar) is performed. The reducibility exhibits an unexpectedly strong temperature dependence.

The atomic structure of the thin SiO2 film on a Mo(112) substrate has been determined based on a combination of density functional theory calculations and high-quality experimental data obtained from scanning tunneling microscopy, infrared reflection absorption spectroscopy, and x-ray photoelectron spectroscopy. The film consists of a honeycomblike, two-dimensional network of corner-sharing [SiO4] tetrahedra. One oxygen atom of each tetrahedron binds to the Mo(112) substrate and is located in a bridge position between Mo atoms located in rows protruding from the metal surface. The other three oxygen atoms form Si-O-Si bonds with the neighboring tetrahedra.

The atomic structure of the thin SiO2 film on a Mo112 substrate has been determined based on a combination of density functional theory calculations and high-quality experimental data obtained from scanning tunneling microscopy, infrared reflection absorption spectroscopy, and x-ray photoelectron spectroscopy. The film consists of a honeycomblike, two-dimensional network of corner-sharing SiO4 tetrahedra. One oxygen atom of each tetrahedron binds to the Mo112 substrate and is located in a bridge position between Mo atoms located in rows protruding from the metal surface. The other three oxygen atoms form Si-O-Si bonds with the neighboring tetrahedra.

Using density functional theory and statistical thermodynamics, we obtained the phase diagram of thin VnOm films of varying thickness (approximately 2-6 A, 1-6 vanadium layers) supported on alpha-Al2O3(0001). Depending on the temperature, oxygen pressure, and vanadium concentration, films with different thickness and termination may form. In ultrahigh vacuum (UHV), at room temperature and for low vanadium concentrations, an ultrathin (1 x 1) O=V-terminated film is most stable. As more vanadium is supplied, the thickest possible films form. Their structures and terminations correspond to previous findings for the (0001) surface of bulk V2O3 [Kresse et al., Surf. Sci. 2004, 555, 118]. The presence of surface vanadyl (O=V) groups is a prevalent feature. They are stable up to at least 800 K in UHV. Vanadyl oxygen atoms induce a V(2p) core-level shift of about 2 eV on the surface V atoms. The reducibility of the supported films is characterized by the energy of oxygen defect formation. For the stable structures, the results vary between 4.11 and 3.59 eV per 1/2O2. In contrast, oxygen removal from the V2O5(001) surface is much easier (1.93 eV). This provides a possible explanation for the lower catalytic activity of vanadium oxides supported on alumina compared to that of crystalline vanadia particles.

The [Cs((2 + x))][H(3)O((1 - x))]Tc(2)Br(8)·4.6H(2)O (x = 0.221) salt has been synthesized and characterized by single crystal XRD. Multi-configurational quantum chemical calculations on Tc(2)X(8)(n-) (X = Cl, Br; n = 2, 3) have been performed and indicate the π component in the Tc-Tc bond to be stronger for n = 3.

Density functional theory in combination with genetic algorithm is applied to determine the atomic models of p(1x2) and p(1x3) surface structures observed upon oxygen adsorption on a Mo(112) surface. The authors' simulations reveal an unusual flexibility of Mo(112) resulting in oxygen-induced reconstructions and lead to more stable structures than any suggested so far. Comparison of the stabilities of the predicted models shows that different p(1x2) and p(1x3) structures may coexist over a wide range of oxygen pressures. A pure p(1x2) structure can be obtained only in a narrow region of oxygen pressures. In contrast, a pure p(1x3) structure cannot exist as a stable phase. The results of simulations are fully supported by a multitude of experimental data obtained from low energy electron diffraction, x-ray photoelectron spectroscopy, and scanning tunneling microscopy.

Herein, the ligand-based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium-diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride-ate complex gave a quintuply bonded bimetallic complex with a CrCr distance of 1.7056 (12)angstrom. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane CC bond (1.704 (4)angstrom). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr-I dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by-product. The formation of this by-product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over-reduction takes place, and a chromium-arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.