The Trans Effect in Electrocatalytic CO2 Reduction: Mechanistic Studies of Asymmetric Ruthenium Pyridyl-Carbene Catalysts.

A comprehensive mechanistic study of electrocatalytic CO2 reduction by ruthenium 2,2':6',2″-terpyridine (tpy) pyridyl-carbene catalysts reveals the importance of stereochemical control to locate the strongly donating N-heterocyclic carbene ligand trans to the site of CO2 activation. Computational studies were undertaken to predict the most stable isomer for a range of reasonable intermediates in CO2 reduction, suggesting that the ligand trans to the reaction site plays a key role in dictating the energetic profile of the catalytic reaction. A new isomer of [Ru(tpy)(Mebim-py)(NCCH3)]2+ (Mebim-py is 1-methylbenzimidazol-2-ylidene-3-(2'-pyridine)) and both isomers of the catalytic intermediate [Ru(tpy)(Mebim-py)(CO)]2+ were synthesized and characterized. Experimental studies demonstrate that both isomeric precatalysts facilitate electroreduction of CO2 to CO in 95/5 MeCN/H2O with high activity and high selectivity. Cyclic voltammetry, infrared spectroelectrochemistry, and NMR spectroscopy studies provide a detailed mechanistic picture demonstrating an essential isomerization step in which the N-trans catalyst converts in situ to the C-trans variant. Insight into molecular electrocatalyst design principles emerge from this study. First, the use of an asymmetric ligand that places a strongly electron-donating ligand trans to the site of CO2 binding and activation is critical to high activity. Second, stereochemical control to maintain the desired isomer structure during catalysis is critical to performance. Finally, pairing the strongly donating pyridyl-carbene ligand with the redox-active tpy ligand proves to be useful in boosting activity without sacrificing overpotential. These design principles are considered in the context of surface-immobilized electrocatalysis.

Photocatalytic CO2 Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO2, CO, or Proton.

The cobalt complexes CoIIL1(PF6)2 (1; L1 = 2,6-bis[2-(2,2'-bipyridin-6'-yl)ethyl]pyridine) and CoIIL2(PF6)2 (2; L2 = 2,6-bis[2-(4-methoxy-2,2'-bipyridin-6'-yl)ethyl]pyridine) were synthesized and used for photocatalytic CO2 reduction in acetonitrile. X-ray structures of complexes 1 and 2 reveal distorted trigonal-bipyramidal geometries with all nitrogen atoms of the ligand coordinated to the Co(II) center, in contrast to the common six-coordinate cobalt complexes with pentadentate polypyridine ligands, where a monodentate solvent completes the coordination sphere. Under electrochemical conditions, the catalytic current for CO2 reduction was observed near the Co(I/0) redox couple for both complexes 1 and 2 at E1/2 = -1.77 and -1.85 V versus Ag/AgNO3 (or -1.86 and -1.94 V vs Fc+/0), respectively. Under photochemical conditions with 2 as the catalyst, [Ru(bpy)3]2+ as a photosensitizer, tri- p-tolylamine (TTA) as a reversible quencher, and triethylamine (TEA) as a sacrificial electron donor, CO and H2 were produced under visible-light irradiation, despite the endergonic reduction of Co(I) to Co(0) by the photogenerated [Ru(bpy)3]+. However, bulk electrolysis in a wet CH3CN solution resulted in the generation of formate as the major product, indicating the facile production of Co(0) and [Co-H] n+ ( n = 1 and 0) under electrochemical conditions. The one-electron-reduced complex 2 reacts with CO to produce [Co0L2(CO)] with νCO = 1894 cm-1 together with [CoIIL2]2+ through a disproportionation reaction in acetonitrile, based on the spectroscopic and electrochemical data. Electrochemistry and time-resolved UV-vis spectroscopy indicate a slow CO binding rate with the [CoIL2]+ species, consistent with density functional theory calculations with CoL1 complexes, which predict a large structural change from trigonal-bipyramidal to distorted tetragonal geometry. The reduction of CO2 is much slower than the photochemical formation of [Ru(bpy)3]+ because of the large structural changes, spin flipping in the cobalt catalytic intermediates, and an uphill reaction for the reduction to Co(0) by the photoproduced [Ru(bpy)3]+.

Picolinamide-Based Iridium Catalysts for Dehydrogenation of Formic Acid in Water: Effect of Amide N Substituent on Activity and Stability.

To develop highly efficient catalysts for dehydrogenation of formic acid in water, we investigated several Cp*Ir catalysts with various amide ligands. The catalyst with an N-phenylpicolinamide ligand exhibited a TOF of 118 000 h-1 at 60 °C. A constant rate (TOF>35 000 h-1 ) was maintained for six hours, and a TON of 1 000 000 was achieved at 50 °C.

Iridium Complexes with Proton-Responsive Azole-Type Ligands as Effective Catalysts for CO2 Hydrogenation.

Pentamethylcyclopentadienyl iridium (Cp*Ir) complexes with bidentate ligands consisting of a pyridine ring and an electron-rich diazole ring were prepared. Their catalytic activity toward CO2 hydrogenation in 2.0 m KHCO3 aqueous solutions (pH 8.5) at 50 °C, under 1.0 MPa CO2 /H2 (1:1) have been reported as an alternative to photo- and electrochemical CO2 reduction. Bidentate ligands incorporating an electron-rich diazole ring improved the catalytic performance of the Ir complexes compared to the bipyridine ligand. Complexes 2, 4, and 6, possessing both a hydroxy group and an uncoordinated NH group, which are proton-responsive and capable of generating pendent bases in basic media, recorded high initial turnover frequency values of 1300, 1550, and 2000 h-1 , respectively. Spectroscopic and computational investigations revealed that the reversible deprotonation changes the electronic properties of the complexes and causes interactions between pendent base and substrate and/or solvent water molecules, resulting in high catalytic performance in basic media.

Erratum: First-Principles Approach to Calculating Energy Level Alignment at Aqueous Semiconductor Interfaces [Phys. Rev. Lett. 113, 176802 (2014)].

This corrects the article DOI: 10.1103/PhysRevLett.113.176802.

Hydricity, electrochemistry, and excited-state chemistry of Ir complexes for CO2 reduction.

We prepared electron-rich derivatives of [Ir(tpy)(ppy)Cl]+ with modification of the bidentate (ppy) or tridentate (tpy) ligands in an attempt to increase the reactivity for CO2 reduction and the ability to transfer hydrides (hydricity). Density functional theory (DFT) calculations reveal that complexes with dimethyl-substituted ppy have similar hydricities to the non-substituted parent complex, and photocatalytic CO2 reduction studies show selective CO formation. Substitution of tpy by bis(benzimidazole)-phenyl or -pyridine (L3 and L4, respectively) induces changes in the physical properties that are much more pronounced than from the addition of methyl groups to ppy. Theoretical data predict [Ir(L3)(ppy)(H)] as the strongest hydride donor among complexes studied in this work, but [Ir(L3)(ppy)(NCCH3)]+ cannot be reduced photochemically because the excited state reduction potential is only 0.52 V due to the negative ground state potential of -1.91 V. The excited state of [Ir(L4)(ppy)(NCCH3)]2+ is the strongest oxidant among complexes studied in this work and the singly-reduced species is formed readily upon photolysis in the presence of tertiary amines. Both [Ir(L3)(ppy)(NCCH3)]+ and [Ir(L4)(ppy)(NCCH3)]2+ exhibit electrocatalytic current for CO2 reduction. While a significantly greater overpotential is needed for the L3 complex, a small amount of formate (5-10%) generation in addition to CO was observed as predicted by the DFT calculations.

Efficient Hydrogen Storage and Production Using a Catalyst with an Imidazoline-Based, Proton-Responsive Ligand.

A series of new imidazoline-based iridium complexes has been developed for hydrogenation of CO2 and dehydrogenation of formic acid. One of the proton-responsive complexes bearing two -OH groups at ortho and para positions on a coordinating pyridine ring (3 b) can catalyze efficiently the chemical fixation of CO2 and release H2 under mild conditions in aqueous media without using organic additives/solvents. Notably, hydrogenation of CO2 can be efficiently carried out under CO2 and H2 at atmospheric pressure in basic water by 3 b, achieving a turnover frequency of 106 h-1 and a turnover number of 7280 at 25 °C, which are higher than ever reported. Moreover, highly efficient CO-free hydrogen production from formic acid in aqueous solution employing the same catalyst under mild conditions has been achieved, thus providing a promising potential H2 -storage system in water.

Noninnocent Proton-Responsive Ligand Facilitates Reductive Deprotonation and Hinders CO2 Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH3)](2+) (6DHBP = 6,6'-(OH)2bpy).

Ruthenium complexes with proton-responsive ligands [Ru(tpy)(nDHBP)(NCCH3)](CF3SO3)2 (tpy = 2,2':6',2″-terpyridine; nDHBP = n,n'-dihydroxy-2,2'-bipyridine, n = 4 or 6) were examined for reductive chemistry and as catalysts for CO2 reduction. Electrochemical reduction of [Ru(tpy)(nDHBP)(NCCH3)](2+) generates deprotonated species through interligand electron transfer in which the initially formed tpy radical anion reacts with a proton source to produce singly and doubly deprotonated complexes that are identical to those obtained by base titration. A third reduction (i.e., reduction of [Ru(tpy)(nDHBP-2H(+))](0)) triggers catalysis of CO2 reduction; however, the catalytic efficiency is strikingly lower than that of unsubstituted [Ru(tpy)(bpy)(NCCH3)](2+) (bpy = 2,2'-bipyridine). Cyclic voltammetry, bulk electrolysis, and spectroelectrochemical infrared experiments suggest the reactivity of CO2 at both the Ru center and the deprotonated quinone-type ligand. The Ru carbonyl formed by the intermediacy of a metallocarboxylic acid is stable against reduction, and mass spectrometry analysis of this product indicates the presence of two carbonates formed by the reaction of DHBP-2H(+) with CO2.

Striking Differences in Properties of Geometric Isomers of [Ir(tpy)(ppy)H](+): Experimental and Computational Studies of their Hydricities, Interaction with CO2, and Photochemistry.

We prepared two geometric isomers of [Ir(tpy)(ppy)H](+), previously proposed as a key intermediate in the photochemical reduction of CO2 to CO, and characterized their notably different ground- and excited-state interactions with CO2 and their hydricities using experimental and computational methods. Only one isomer, C-trans-[Ir(tpy)(ppy)H](+), reacts with CO2 to generate the formato complex in the ground state, consistent with its calculated hydricity. Under photocatalytic conditions in CH3CN/TEOA, a common reactive C-trans-[Ir(tpy)(ppy)](0) species, irrespective of the starting isomer or monodentate ligand (such as hydride or Cl), reacts with CO2 and produces CO with the same catalytic efficiency.

Mechanistic studies of hydrogen evolution in aqueous solution catalyzed by a tertpyridine-amine cobalt complex.

The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as Co(I) and Co(III)-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)](n+) (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)](n+) where L is the pentadentate DPA-Bpy ligand or [Co(OH2)](n+) as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while the water ligand is strongly coordinated to the metal center in the oxidation state 3+, one-electron reduction of the complex to form a Co(II) species results in weakening the Co-O bond. The further reduction to a Co(I) species leads to the loss of the aqua ligand and the formation of [Co(I)-VS)](+) (VS = vacant site). Interestingly, DFT calculations also predict the existence of a [Co(I)(κ(4)-L)(OH2)](+) species at least transiently, and its formation is consistent with the experimental Pourbaix diagram. Both electrochemical and kinetics results indicate that the Co(I) species must undergo some structural change prior to accepting the proton, and this transformation represents the rate-determining step (RDS) in the overall formation of [Co(III)-H](2+). We propose that this RDS may originate from the slow removal of a solvent ligand in the intermediate [Co(I)(κ(4)-L)(OH2)](+) in addition to the significant structural reorganization of the metal complex and surrounding solvent resulting in a high free energy of activation.

Push or Pull? Proton Responsive Ligand Effects in Rhenium Tricarbonyl CO2 Reduction Catalysts.

Proton responsive ligands offer control of catalytic reactions through modulation of pH-dependent properties, second coordination sphere stabilization of transition states, or by providing a local proton source for multiproton, multielectron reactions. Two fac-[Re(I)(α-diimine)(CO)3Cl] complexes with α-diimine = 4,4'- (or 6,6'-) dihydroxy-2,2'-bipyridine (4DHBP and 6DHBP) have been prepared and analyzed as electrocatalysts for the reduction of carbon dioxide. Consecutive electrochemical reduction of these complexes yields species identical to those obtained by chemical deprotonation. An energetically feasible mechanism for reductive deprotonation is proposed in which the bpy anion is doubly protonated followed by loss of H2 and 2H(+). Cyclic voltammetry reveals a two-electron, three-wave system owing to competing EEC and ECE pathways. The chemical step of the ECE pathway might be attributed to the reductive deprotonation but cannot be distinguished from chloride dissociation. The rate obtained by digital simulation is approximately 8 s(-1). Under CO2, these competing reactions generate a two-slope catalytic waveform with onset potential of -1.65 V vs Ag/AgCl. Reduction of CO2 to CO by the [Re(I)(4DHBP-2H(+))(CO)3](-) suggests the interaction of CO2 with the deprotonated species or a third reduction followed by catalysis. Conversely, the reduced form of [Re(6DHBP)(CO)3Cl] converts CO2 to CO with a single turnover.

CO2 Hydrogenation Catalyzed by Iridium Complexes with a Proton-Responsive Ligand.

The catalytic cycle for the production of formic acid by CO2 hydrogenation and the reverse reaction have received renewed attention because they are viewed as offering a viable scheme for hydrogen storage and release. In this Forum Article, CO2 hydrogenation catalyzed by iridium complexes bearing sophisticated N^N-bidentate ligands is reported. We describe how a ligand containing hydroxy groups as proton-responsive substituents enhances the catalytic performance by an electronic effect of the oxyanions and a pendent-base effect through secondary coordination sphere interactions. In particular, [(Cp*IrCl)2(TH2BPM)]Cl2 (Cp* = pentamethylcyclopentadienyl; TH2BPM = 4,4',6,6'-tetrahydroxy-2,2'-bipyrimidine) enormously promotes the catalytic hydrogenation of CO2 in basic water by these synergistic effects under atmospheric pressure and at room temperature. Additionally, newly designed complexes with azole-type ligands were applied to CO2 hydrogenation. The catalytic efficiencies of the azole-type complexes were much higher than that of the unsubstituted bipyridine complex [Cp*Ir(bpy)(OH2)]SO4. Furthermore, the introduction of one or more hydroxy groups into ligands such as 2-pyrazolyl-6-hydroxypyridine, 2-pyrazolyl-4,6-dihydroxypyrimidine, and 4-pyrazolyl-2,6-dihydroxypyrimidine enhanced the catalytic activity. It is clear that the incorporation of additional electron-donating functionalities into proton-responsive azole-type ligands is effective for promoting further enhanced hydrogenation of CO2.

Mechanism of water oxidation by [Ru(bda)(L)2]: the return of the "blue dimer".

We describe here a combined solution-surface-DFT calculations study for complexes of the type [Ru(bda)(L)2] including X-ray structure of intermediates and their reactivity, as well as pH-dependent electrochemistry and spectroelectrochemistry. These studies shed light on the mechanism of water oxidation by [Ru(bda)(L)2], revealing key features unavailable from solution studies with sacrificial oxidants.

First-principles approach to calculating energy level alignment at aqueous semiconductor interfaces.

A first-principles approach is demonstrated for calculating the relationship between an aqueous semiconductor interface structure and energy level alignment. The physical interface structure is sampled using density functional theory based molecular dynamics, yielding the interface electrostatic dipole. The GW approach from many-body perturbation theory is used to place the electronic band edge energies of the semiconductor relative to the occupied 1b1 energy level in water. The application to the specific cases of nonpolar (101¯0) facets of GaN and ZnO reveals a significant role for the structural motifs at the interface, including the degree of interface water dissociation and the dynamical fluctuations in the interface Zn-O and O-H bond orientations. These effects contribute up to 0.5 eV.

Theoretical modeling of low-energy electronic absorption bands in reduced cobaloximes.

The reduced Co(I) states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen-evolving catalytic activity of these species. In this work we analyze the low-energy electronic absorption bands of two cobaloxime systems experimentally and use a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task.

Tungsten carbide-nitride on graphene nanoplatelets as a durable hydrogen evolution electrocatalyst.

Alternatives to platinum-based catalysts are required to sustainably produce hydrogen from water at low overpotentials. Progress has been made in utilizing tungsten carbide-based catalysts, however, their performance is currently limited by the density and reactivity of active sites, and insufficient stability in acidic electrolytes. We report highly active graphene nanoplatelet-supported tungsten carbide-nitride nanocomposites prepared via an in situ solid-state approach. This nanocomposite catalyzes the hydrogen evolution reaction with very low overpotential and is stable operating for at least 300 h in harsh acidic conditions. The synthetic approach offers a great advantage in terms of structural control and kinetics improvement.

New water oxidation chemistry of a seven-coordinate ruthenium complex with a tetradentate polypyridyl ligand.

The mononuclear ruthenium(II) complex [Ru](2+) (Ru = Ru(dpp)(pic)2, where dpp is the tetradentate 2,9-dipyrid-2'-yl-1,10-phenanthroline ligand and pic is 4-picoline) reported by Thummel's group (Inorg. Chem. 2008, 47, 1835-1848) that contains no water molecule in its primary coordination shell is evaluated as a catalyst for water oxidation in artificial photosynthesis. A detailed theoretical characterization of the energetics, thermochemistry, and spectroscopic properties of intermediates allowed us to interpret new electrochemical and spectroscopic experimental data, and propose a mechanism for the water oxidation process that involves an unprecedented sequence of seven-coordinate ruthenium complexes as intermediates. This analysis provides insights into a mechanism that generates four electrons and four protons in the solution and a gas-phase oxygen molecule at different pH values. On the basis of the calculations and corroborated substantially by experiments, the catalytic cycle goes through [(2)Ru(III)](3+) and [(2)Ru(V)(O)](3+) to [(1)Ru(IV)(OOH)](3+) then [(2)Ru(III)(···(3)O2)](3+) at pH 0, and through [(3)Ru(IV)(O)](2+), [(2)Ru(V)(O)](3+), and [(1)Ru(IV)(OO)](2+) at pH 9 before reaching the same [(2)Ru(III)(···(3)O2)](3+) species, from which the liberation of the weakly bound O2 might require an additional oxidation to form [(3)Ru(IV)(O)](2+) to initiate further cycles involving all seven-coordinate species.

Formic acid dehydrogenation with bioinspired iridium complexes: a kinetic isotope effect study and mechanistic insight.

Highly efficient hydrogen generation from dehydrogenation of formic acid is achieved by using bioinspired iridium complexes that have hydroxyl groups at the ortho positions of the bipyridine or bipyrimidine ligand (i.e., OH in the second coordination sphere of the metal center). In particular, [Ir(Cp*)(TH4BPM)(H2 O)]SO4 (TH4BPM: 2,2',6,6'-tetrahydroxyl-4,4'-bipyrimidine; Cp*: pentamethylcyclopentadienyl) has a high turnover frequency of 39 500 h(-1) at 80 °C in a 1 M aqueous solution of HCO2 H/HCO2 Na and produces hydrogen and carbon dioxide without carbon monoxide contamination. The deuterium kinetic isotope effect study clearly indicates a different rate-determining step for complexes with hydroxyl groups at different positions of the ligands. The rate-limiting step is ß-hydrogen elimination from the iridium-formate intermediate for complexes with hydroxyl groups at ortho positions, owing to a proton relay (i.e., pendent-base effect), which lowers the energy barrier of hydrogen generation. In contrast, the reaction of iridium hydride with a proton to liberate hydrogen is demonstrated to be the rate-determining step for complexes that do not have hydroxyl groups at the ortho positions.

Computational investigation of structural and electronic properties of aqueous interfaces of GaN, ZnO, and a GaN/ZnO alloy.

The GaN/ZnO alloy functions as a visible-light photocatalyst for splitting water into hydrogen and oxygen. As a first step toward understanding the mechanism and energetics of water-splitting reactions, we investigate the microscopic structure of the aqueous interfaces of the GaN/ZnO alloy and compare them with the aqueous interfaces of pure GaN and ZnO. Specifically, we have studied the (101̄0) surface of GaN and ZnO and the (101̄0) and (12̄10) surfaces of the 1 : 1 GaN/ZnO alloy. The calculations are carried out using first-principles density functional theory based molecular dynamics (DFT-MD). The structure of water within a 3 Šdistance from the semiconductor surface is significantly altered by the acid/base chemistry of the aqueous interface. Water adsorption on all surfaces is substantially dissociative such that the surface anions (N or O) act as bases accepting protons from dissociated water molecules while the corresponding hydroxide ions bond with surface cations (Ga or Zn). Additionally, the hard-wall interface presented by the semiconductor imparts ripples in the density of water. Beyond a 3 Šdistance from the semiconductor surface, water exhibits a bulk-like hydrogen bond network and oxygen-oxygen radial distribution function. Taken together, these characteristics represent the resting (or "dark") state of the catalytic interface. The electronic structure analysis of the aqueous GaN/ZnO interface suggests that the photogenerated holes may get trapped on interface species other than the adsorbed OH(-) ions. This suggests additional dynamical steps in the water oxidation process.