Electroreduction of CO2 to Formate with Low Overpotential using Cobalt Pyridine Thiolate Complexes.

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.

Metal-Metal Synergy in Well-Defined Surface Tantalum-Iridium Heterobimetallic Catalysts for H/D Exchange Reactions.

A novel heterobimetallic tantalum/iridium hydrido complex, [{Ta(CH2tBu)3}{IrH2(Cp*)}] 1, featuring a very short metal-metal bond, has been isolated through an original alkane elimination route from Ta(CHtBu)(CH2tBu)3 and Cp*IrH4. This molecular precursor has been used to synthesize well-defined silica-supported low-coordinate heterobimetallic hydrido species [≡SiOTa(CH2tBu)2{IrH2(Cp*)}], 5, and [≡SiOTa(CH2tBu)H{IrH2(Cp*)}], 6, using a surface organometallic chemistry (SOMC) approach. The SOMC methodology prevents undesired dimerization as encountered in solution and leading to a tetranuclear species [{Ta(CH2tBu)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, sub-atmospheric D2 pressure) without any additives.

Controlling Hydrogen Evolution during Photoreduction of CO2 to Formic Acid Using [Rh(R-bpy)(Cp*)Cl]+ Catalysts: A Structure-Activity Study.

The photochemical reduction of CO2 to formic acid catalyzed by a series of [Rh(4,4'-R-bpy)(Cp*)Cl]+ and [Rh(5,5'-COOH-bpy)(Cp*)Cl]+ complexes (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine, and R = OCH3, CH3, H, COOC2H5, CF3, NH2, or COOH) was studied to assess how modifications in the electronic structure of the catalyst affect its selectivity, defined as the HCOOH:H2 product ratio. A direct molecular-level influence of the functional group on the initial reaction rate for CO2 versus proton reduction reactions was established. Density functional theory computations elucidated for the first time the respective role of the [RhH] and [Cp*H] tautomers, recognizing rhodium hydride as the key player for both reactions. In particular, our calculations explain the observed tendency of electron-donating substituents to favor CO2 reduction by means of decreasing the hydricity of the Rh-H bond, resulting in a lower hydride transfer barrier toward formic acid production as compared to substituents with an electron-withdrawing nature that favor more strongly the reduction of protons to hydrogen.

Synthesis, Characterization, and DFT Analysis of Bis-Terpyridyl-Based Molecular Cobalt Complexes.

Terpyridine ligands are widely used in chemistry and material sciences owing to their ability to form stable molecular complexes with a large variety of metal ions. In that context, variations of the substituents on the terpyridine ligand allow modulation of the material properties. Applying the Stille cross-coupling reaction, we prepared with good yields a new series of terpyridine ligands possessing quinoline-type moieties in ortho, meta, and para positions and dimethylamino substituents at central or distal positions. The corresponding cobalt(II) complexes were synthesized and fully characterized by elemental analysis, single-crystal X-ray crystallography, mass spectrometry, and UV-vis, 1H NMR, and Fourier transform infrared (FT-IR) spectroscopy as well as by cyclic voltammetry (CV). Density functional theory (DFT) calculations were performed to investigate the electronic structure of all the Co(II) bis-terpyridyl molecular complexes. In this work, we show that terpyridine ligand functionalization allows tuning the redox potentials of the Co(III)/Co(II), Co(II)/Co(I), and Co(I)/Co(I) (tpy)•- couples over a 1 V range.

Molecular Level Characterization of the Structure and Interactions in Peptide-Functionalized Metal-Organic Frameworks.

We use density functional theory, newly parameterized molecular dynamics simulations, and last generation 15 N dynamic nuclear polarization surface enhanced solid-state NMR spectroscopy (DNP SENS) to understand graft-host interactions and effects imposed by the metal-organic framework (MOF) host on peptide conformations in a peptide-functionalized MOF. Focusing on two grafts typified by MIL-68-proline (-Pro) and MIL-68-glycine-proline (-Gly-Pro), we identified the most likely peptide conformations adopted in the functionalized hybrid frameworks. We found that hydrogen bond interactions between the graft and the surface hydroxyl groups of the MOF are essential in determining the peptides conformation(s). DNP SENS methodology shows unprecedented signal enhancements when applied to these peptide-functionalized MOFs. The calculated chemical shifts of selected MIL-68-NH-Pro and MIL-68-NH-Gly-Pro conformations are in a good agreement with the experimentally obtained 15 N NMR signals. The study shows that the conformations of peptides when grafted in a MOF host are unlikely to be freely distributed, and conformational selection is directed by strong host-guest interactions.

Octafluorodirhenate(III) Revisited: Solid-State Preparation, Characterization, and Multiconfigurational Quantum Chemical Calculations.

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.

Molecular and Electronic Structure of Re2Br4(PMe3)4.

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.

Connecting defects and amorphization in UiO-66 and MIL-140 metal­organic frameworks: a combined experimental and computational study.

The mechanism and products of the structural collapse of the metal­organic 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 metal­ligand bonding in each case. The amorphous products contain inorganic­organic 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.

Biphasic water splitting by osmocene.

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.

The ligand-based quintuple bond-shortening concept and some of its limitations.

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 Cr-Cr distance of 1.7056 (12) Å. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C-C bond (1.704 (4) Å). 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.

DFT and CASPT2 analysis of polymetallic uranium nitride and oxide complexes: how theory can help when X-ray analysis is inadequate.

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.

Structural, spectroscopic, and multiconfigurational quantum chemical investigations of the electron-rich metal-metal triple-bonded Tc(2)X(4)(PMe(3))(4) (X = Cl, Br) complexes.

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.

A bioinspired molybdenum-copper molecular catalyst for CO2 electroreduction.

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 one-electron 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.

Synthetic and computational assessment of a chiral metal-organic framework catalyst for predictive asymmetric transformation.

Understanding and controlling molecular recognition mechanisms at a chiral solid interface is a continuously addressed challenge in heterogeneous catalysis. Here, the molecular recognition of a chiral peptide-functionalized metal-organic framework (MOF) catalyst towards a pro-chiral substrate is evaluated experimentally and in silico. The MIL-101 metal-organic framework is used as a macroligand for hosting a Noyori-type chiral ruthenium molecular catalyst, namely (benzene)Ru@MIL-101-NH-Gly-Pro. Its catalytic perfomance toward the asymmetric transfer hydrogenation (ATH) of acetophenone into R- and S-phenylethanol are assessed. The excellent match between the experimentally obtained enantiomeric excesses and the computational outcomes provides a robust atomic-level rationale for the observed product selectivities. The unprecedented role of the MOF in confining the molecular Ru-catalyst and in determining the access of the prochiral substrate to the active site is revealed in terms of highly face-specific host-guest interactions. The predicted surface-specific face differentiation of the prochiral substrate is experimentally corroborated since a three-fold increase in enantiomeric excess is obtained with the heterogeneous MOF-based catalyst when compared to its homogeneous molecular counterpart.

The Ru-Hbpp water oxidation catalyst.

A thorough characterization of the Ru-Hbpp (in,in-{[Ru(II)(trpy)(H(2)O)](2)(mu-bpp)}(3+) (trpy is 2,2':6',2''-terpyridine, bpp is bis(2-pyridyl)-3,5-pyrazolate)) water oxidation catalyst has been carried out employing structural (single crystal X-ray), spectroscopic (UV-vis and NMR), kinetic, and electrochemical (cyclic voltammetry) analyses. The latter reveals the existence of five different oxidation states generated by sequential oxidation of an initial II,II state to an ultimate, formal IV,IV oxidation state. Each of these oxidation states has been characterized by UV-vis spectroscopy, and their relative stabilities are reported. The electron transfer kinetics for individual one-electron oxidation steps have been measured by means of stopped flow techniques at temperatures ranging from 10 to 40 degrees C and associated second-order rate constants and activation parameters (DeltaH() and DeltaS()) have been determined. Room-temperature rate constants for substitution of aqua ligands by MeCN as a function of oxidation state have been determined using UV-vis spectroscopy. Complete kinetic analysis has been carried out for the addition of 4 equiv of oxidant (Ce(IV)) to the initial Ru-Hbpp catalyst in its II,II oxidation state. Subsequent to reaching the formal oxidation state IV,IV, an intermediate species is formed prior to oxygen evolution. Intermediate formation and oxygen evolution are both much slower than the preceding ET processes, and both are first order with regard to the catalyst; rate constants and activation parameters are reported for these steps. Theoretical modeling at density functional and multireference second-order perturbation theory levels provides a microscopic mechanism for key steps in intermediate formation and oxygen evolution that are consistent with experimental kinetic data and also oxygen labeling experiments, monitored via mass spectrometry (MS), that unambiguously establish that oxygen-oxygen bond formation proceeds intramolecularly. Finally, the Ru-Hbpp complex has also been studied under catalytic conditions as a function of time by means of manometric measurements and MS, and potential deactivation pathways are discussed.

Understanding, controlling and programming cooperativity in self-assembled polynuclear complexes in solution.

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 (DeltaE(1-4)(Lu,Lu)>DeltaE(1-2)(Lu,Lu)), ii) of larger magnitude when Zn2+ replaces Lu3+ (DeltaE(1-2)(Zn,Lu)>DeltaE(1-2)(Lu,Lu) and iii) attractive between two triply charged cations held at some specific distance (DeltaE(1-3)(Lu,Lu)<0). The consequences of these trends are discussed for the design of polynuclear complexes in solution.

Theoretical study of the gas-phase chemiionization reactions La + O and La + O2.

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.

Nonuniform temperature dependence of the reactivity of disordered VO(x)/kappa-Al2O3(001) surfaces: a density functional theory based Monte Carlo study.

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.

A cobalt complex with a bioinspired molybdopterin-like ligand: a catalyst for hydrogen evolution.

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.

Vanadium oxides on aluminum oxide supports. 1. Surface termination and reducibility of vanadia films on alpha-Al2O3(0001).

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.