Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes.

Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η5-pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η4-pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2'-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)]+, an elusive hydride complex that has been spectroscopically and kinetically characterized here. Tautomerization of the hydride leads to the clean formation of [(Cp*H)Rh(bpy)]+. Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic monitoring of the second proton transfer event reveals that both the hydride and related Cp*H complex can be involved in further reactivity, showing that [(Cp*H)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands.

Understanding the Role of Inter- and Intramolecular Promoters in Electro- and Photochemical CO2 Reduction Using Mn, Re, and Ru Catalysts.

Recycling of carbon dioxide to fuels and chemicals is a promising strategy for renewable energy storage. Carbon dioxide conversion can be achieved by (i) artificial photosynthesis using photoinduced electrons; (ii) electrolysis using electricity produced by photovoltaics; and (iii) thermal CO2 hydrogenation using renewable H2. The focus of our group's research is on molecular catalysts, in particular coordination complexes of transition metals (e.g., Mn, Re, and Ru), which offer versatile platforms for mechanistic studies of photo- and electrochemical CO2 reduction. The interactions of catalytic intermediates with Lewis or Brønsted acids, hydrogen-bonding moieties, solvents, cations, etc., that function as promoters or cofactors have become increasingly important for efficient catalysis. These interactions may have dramatic effects on selectivity and rates by stabilizing intermediates or lowering transition state barriers, but they are difficult to elucidate and challenging to predict. We have been carrying out experimental and theoretical studies of CO2 reduction using molecular catalysts toward addressing mechanisms of efficient CO2 reduction systems with emphasis on those containing intramolecular (or pendent) and intermolecular (solution phase) additives. This Account describes the identification of reaction intermediates produced during CO2 reduction in the presence of triethanolamine or ionic liquids, the benefits of hydrogen-bonding interactions among intermediates or cofactors, and the complications of pendent phenolic donors/phenoxide bases under electrochemical conditions.Triethanolamine (TEOA) is a common sacrificial electron donor for photosensitizer excited state reductive quenching and has a long history of use in photocatalytic CO2 reduction. It also functions as a Brønsted base in conjunction with more potent sacrificial electron donors, such as 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). Deprotonation of the BIH•+ cation radical promotes irreversible photoinduced electron transfer by preventing charge recombination. Despite its wide use, most research to date has not considered the broader reactions of TEOA, including its direct interaction with CO2 or its influence on catalytic intermediates. We found that in acetonitrile, TEOA captures CO2 in the form of a zwitterionic adduct without any metal catalyst. In the presence of ruthenium carbonyl catalysts bearing α-diimine ligands, it participates in metal hydride formation, accelerates hydride transfer to CO2 to form the bound formate intermediate, and assists in the dissociation of formate anion from the catalyst ( J. Am. Chem. Soc. 2020, 142, 2413-2428).Hydrogen bonding and acid/base promoters are understood to interact with key catalytic intermediates, such as the metallocarboxylate or metallocarboxylic acid during CO2 reduction. The former is a high energy species, and hydrogen-bonding or Lewis acid-stabilization are beneficial. We have found that imidazolium-based ionic liquid cations can stabilize the doubly reduced form of the [ReCl(bpy)(CO)3] (bpy = 2,2'-bipyridine) electrocatalyst through both hydrogen-bonding and π-π interactions, resulting in CO2 reduction occurring at a more positive potential with a higher catalytic current ( J. Phys. Chem. Lett. 2014, 5, 2033-2038). Hydrogen bonding interactions between Lewis basic methoxy groups in the second coordination sphere of a Mn-based catalyst and the OH group of the Mn-COOH intermediate in the presence of a Brønsted acid were also found to promote C-(OH) bond cleavage, enabling access to a low-energy protonation-first pathway for CO2 reduction ( J. Am. Chem. Soc. 2017, 139, 2604-2618).The kinetics of forming the metallocarboxylic acid can be enhanced by internal acids, and its proton-induced C-OH bond cleavage to the metallocarbonyl and H2O is often the rate-limiting step. Therefore, proton movement organized by pendent hydrogen-bonding networks may also accelerate this step. In contrast, during electrolysis, OH groups in the second coordination sphere are deprotonated to the oxyanions, which deter catalytic CO2 reduction by directly binding CO2 to form the carbonate or by making an M-O bond in competition with CO2 binding ( Inorg. Chem. 2016, 55, 4582-4594). Our results emphasize that detailed mechanistic research is critical in discovering the design principles for improved catalysts.

Structural and Electronic Influences on Rates of Tertpyridine-Amine CoIII -H Formation During Catalytic H2 Evolution in an Aqueous Environment.

In this work, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [CoII -Cl]+ species results in the formation of [CoII (κ4 -L)(OH2 )]2+ . Further reduction produces [CoI (κ4 -L)(OH2 )]+ , which undergoes a rate-limiting structural rearrangement to [CoI (κ5 -L)]+ before being protonated to form [CoIII -H]2+ . The rate of [CoIII -H]2+ formation is similar for all complexes in the series. Using E1/2 values of various Co species and pKa values of [CoIII -H]2+ estimated from PR experiments, we found that while the protonation of [CoIII -H]2+ is unfavorable, [CoII -H]+ reacts with protons to produce H2 . The catalytic activity for H2 evolution tracks the hydricity of the [CoII -H]+ intermediate.

Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO2 to Formate by Ruthenium Complexes.

A series of 4,4'-dimethyl-2,2'-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO2 to formate in acetonitrile (CH3CN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO2 from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer. Furthermore, TEOA showed the ability to capture CO2 from CH3CN solutions to form a zwitterionic alkylcarbonate adduct and was actively engaged in key catalytic steps such as metal-hydride formation, hydride transfer to CO2 to form the bound formate intermediate, and dissociation of formate ion product. Collectively, the data provide an overview of the transient intermediates of Ru(II) carbonyl complexes and emphasize the importance of considering the participation of TEOA when investigating and proposing catalytic pathways.

Molecular Catalysts with Intramolecular Re-O Bond for Electrochemical Reduction of Carbon Dioxide.

A new Re bipyridine-type complex, namely, fac-Re(pmbpy)(CO)3Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2'-bipyridine), 1, carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO2 has been characterized. Two isomers of 1, namely, 1-cis characterized by the proximity of Cl to OH and 1-trans, are identified. The interconversion between 1-cis and 1-trans is clarified by DFT calculations, which reveal two transition states. The energetically lower pathway displays a non-negligible barrier of 75.5 kJ mol-1. The 1e- electrochemical reduction of 1 affords the neutral intermediate 1-OPh, formally derived by reductive deprotonation and loss of Cl- from 1. 1-OPh, which exhibits an entropically favored intramolecular Re-O bond, has been isolated and characterized. The detailed electrochemical mechanism is demonstrated by combined chemical reactivity, spectroelectrochemistry, spectroscopic (IR and NMR), and computational (DFT) approaches. Comparison with previous Re and Mn derivatives carrying local proton sources highlights that the catalytic activity of Re complexes is more sensitive to the presence of local OH groups. Similar to Re-2OH (2OH = 4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine), 1 and Mn-1OH display a selective reduction of CO2 to CO. In the case of the Re bipyridine-type complex, the formation of a relatively stable Re-O bond and a preference for phenolate-based reactivity with CO2 slightly inhibit the electrocatalytic reduction of CO2 to CO, resulting in a low TON value of 9, even in the presence of phenol as a proton source.

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.

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]+.

Tetra- and Heptametallic Ru(II),Rh(III) Supramolecular Hydrogen Production Photocatalysts.

Supramolecular mixed metal complexes combining the trimetallic chromophore [{(bpy)2Ru(dpp)}2Ru(dpp)]6+ (Ru3) with [Rh(bpy)Cl2]+ or [RhCl2]+ catalytic fragments to form [{(bpy)2Ru(dpp)}2Ru(dpp)RhCl2(bpy)](PF6)7 (Ru3Rh) or [{(bpy)2Ru(dpp)}2Ru(dpp)]2RhCl2(PF6)13 (Ru3RhRu3) (bpy = 2,2'-bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine) catalyze the photochemical reduction of protons to H2. This first example of a heptametallic Ru,Rh photocatalyst produces over 300 turnovers of H2 upon photolysis of a solution of acetonitrile, water, triflic acid, and N,N-dimethylaniline as an electron donor. In contrast, the tetrametallic Ru3Rh produces only 40 turnovers of H2 due to differences in the excited state properties and nature of the catalysts upon reduction as ascertained from electrochemical data, transient absorption spectroscopy, and flash-quench experiments. While the lowest unoccupied molecular orbital of Ru3Rh is localized on a bridging ligand, it is Rh-centered in Ru3RhRu3 facilitating electron collection at Rh in the excited state and reductively quenched state. The Ru → Rh charge separated state of Ru3RhRu3 is endergonic with respect to the emissive Ru → dpp 3MLCT excited and cannot be formed by static electron transfer quenching of the 3MLCT state. Instead, a mechanism of subnanosecond charge separation from high lying states is proposed. Multiple reductions of Ru3 and Ru3Rh using sodium amalgam were carried out to compare UV-vis absorption spectra of reduced species and to evaluate the stability of highly reduced complexes. The Ru3 and Ru3Rh can be reduced by 10 and 13 electrons, respectively, to final states with all bridging ligands doubly reduced and all bpy ligands singly reduced.

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.

Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore-Imidazole-Phenol Complex: Stepwise Oxidation and Concerted Reduction.

Proton-coupled electron transfer (PCET) reactions were studied in acetonitrile for a Photosystem II (PSII)-inspired [Ru(bpy)2(phen-imidazole-Ph(OH)((t)Bu)2)](2+), in which Ru(III) generated by a flash-quench sequence oxidizes the appended phenol and the proton is transferred to the hydrogen-bonded imidazole base. In contrast to related systems, the donor and acceptor are strongly coupled, as indicated by the shift in the Ru(III/II) couple upon phenol oxidation, and intramolecular oxidation of the phenol by Ru(III) is energetically favorable by both stepwise and concerted pathways. The phenol oxidation occurs via a stepwise ET-PT mechanism with kET = 2.7 × 10(7) s(-1) and a kinetic isotope effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was characterized as adiabatic with λDA = 1.16 eV and 280 < HDA < 540 cm(-1) consistent with strong electronic coupling and slow solvent dynamics. Reduction of the phenoxyl radical by the quencher radical was examined as the analogue of the redox reaction between the PSII tyrosyl radical and the oxygen-evolving complex. In our PSII-inspired complex, the recombination reaction activation energy is <2 kcal mol(-1). The reaction is nonadiabatic (VPCET ≈ 22 cm(-1) (H) and 49 cm(-1) (D)) and concerted, and it exhibits an unexpected inverse KIE = 0.55 that is attributed to greater overlap of the reactant vibronic ground state with the OD vibronic states of the proton acceptor due to the smaller quantum spacing of the deuterium vibrational levels.

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.

RNA content in the nucleolus alters p53 acetylation via MYBBP1A.

A number of external and internal insults disrupt nucleolar structure, and the resulting nucleolar stress stabilizes and activates p53. We show here that nucleolar disruption induces acetylation and accumulation of p53 without phosphorylation. We identified three nucleolar proteins, MYBBP1A, RPL5, and RPL11, involved in p53 acetylation and accumulation. MYBBP1A was tethered to the nucleolus through nucleolar RNA. When rRNA transcription was suppressed by nucleolar stress, MYBBP1A translocated to the nucleoplasm and facilitated p53-p300 interaction to enhance p53 acetylation. We also found that RPL5 and RPL11 were required for rRNA export from the nucleolus. Depletion of RPL5 or RPL11 blocked rRNA export and counteracted reduction of nucleolar RNA levels caused by inhibition of rRNA transcription. As a result, RPL5 or RPL11 depletion inhibited MYBBP1A translocation and p53 activation. Our observations indicated that a dynamic equilibrium between RNA generation and export regulated nucleolar RNA content. Perturbation of this balance by nucleolar stress altered the nucleolar RNA content and modulated p53 activity.

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.

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.

Calculation of thermodynamic hydricities and the design of hydride donors for CO2 reduction.

We have developed a correlation between experimental and density functional theory-derived results of the hydride-donating power, or "hydricity", of various ruthenium, rhenium, and organic hydride donors. This approach utilizes the correlation between experimental hydricity values and their corresponding calculated free-energy differences between the hydride donors and their conjugate acceptors in acetonitrile, and leads to an extrapolated value of the absolute free energy of the hydride ion without the necessity to calculate it directly. We then use this correlation to predict, from density functional theory-calculated data, hydricity values of ruthenium and rhenium complexes that incorporate the pbnHH ligand-pbnHH = 1,5-dihydro-2-(2-pyridyl)-benzo[b]-1,5-naphthyridine-to model the function of NADPH. These visible light-generated, photocatalytic complexes produced by disproportionation of a protonated-photoreduced dimer of a metal-pbn complex may be valuable for use in reducing CO(2) to fuels such as methanol. The excited-state lifetime of photoexcited [Ru(bpy)(2)(pbnHH)](2+) is found to be about 70 ns, and this excited state can be reductively quenched by triethylamine or 1,4-diazabicyclo[2.2.2]octane to produce the one-electron-reduced [Ru(bpy)(2)(pbnHH)](+) species with half-life exceeding 50 µs, thus opening the door to new opportunities for hydride-transfer reactions leading to CO(2) reduction by producing a species with much increased hydricity.

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.

Interconversion of CO2 and formic acid by bio-inspired Ir complexes with pendent bases.

Recent investigations of the interconversion of CO2 and formic acid using Ru, Ir and Fe complexes are summarized in this review. During the past several years, both the reaction rates and catalyst stabilities have been significantly improved. Remarkably, the interconversion (i.e., reversibility) has also been achieved under mild conditions in environmentally benign water solvent by slightly changing the pH of the aqueous solution. Only a few catalysts seem to reflect a bio-inspired design such as the use of proton responsive ligands, ligands with pendent bases or acids for a second-coordination-sphere interaction, electroresponsive ligands, and/or ligands having a hydrogen bonding function with a solvent molecule or an added reagent. The most successful of these is an iridium dinuclear complex catalyst that at least has the first three of these characteristics associated with its bridging ligand. By utilizing an acid/base equilibrium for proton removal, the ligand becomes a strong electron donor, resulting in Ir(I) character with a vacant coordination site at each metal center in slightly basic solution. Complemented by DFT calculations, kinetic studies of the rates of formate production using a related family of Ir complexes with and without such functions on the ligand reveal that the rate-determining step for the CO2 hydrogenation is likely to be H2 addition through heterolytic cleavage involving a "proton relay" through the pendent base. The dehydrogenation of formic acid, owing to the proton responsive ligands changing character under slightly acidic pH conditions, is likely to occur by a mechanism with a different rate-determining step. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Kinetic and mechanistic studies of carbon-to-metal hydrogen atom transfer involving Os-centered radicals: evidence for tunneling.

We have investigated the kinetics of novel carbon-to-metal hydrogen atom transfer reactions, in which homolytic cleavage of a C-H bond is accomplished by a single metal-centered radical. Time-resolved IR spectroscopic measurements revealed efficient hydrogen atom transfer from xanthene, 9,10-dihydroanthracene, and 1,4-cyclohexadiene to Cp(CO)2Os(•) and (η(5)-(i)Pr4C5H)(CO)2Os(•) radicals, formed by photoinduced homolysis of the corresponding osmium dimers. The rate constants for hydrogen abstraction from these hydrocarbons are in the range 1.5 × 10(5) M(-1) s(-1) to 1.7 × 10(7) M(-1) s(-1) at 25 °C. For the first time, kinetic isotope effects for carbon-to-metal hydrogen atom transfer were determined. Large primary deuterium kinetic isotope effects of 13.4 ± 1.0 and 16.8 ± 1.4 were observed for the hydrogen abstraction from xanthene to form Cp(CO)2OsH and (η(5)-(i)Pr4C5H)(CO)2OsH, respectively, at 25 °C. Temperature-dependent measurements of the kinetic isotope effects over a 60 °C temperature range were carried out to obtain the difference in activation energies (E(D) - E(H)) and the pre-exponential factor ratio (A(H)/A(D)). For hydrogen atom transfer from xanthene to (η(5)-(i)Pr4C5H)(CO)2Os(•), the (E(D) - E(H)) = 3.3 ± 0.2 kcal mol(-1) and A(H)/A(D) = 0.06 ± 0.02 values suggest a quantum mechanical tunneling mechanism.

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.

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.