The need for benchmarking hydrogen evolution catalysts has increasingly been recognized. The influence of acid choice on activity is often reduced to the overpotential for catalysis. Through the study of a stable cobalt hydride complex, we demonstrate the influence of acid choice, beyond pK, on the kinetics of hydride formation. A linear free energy relationship between acid pK and second-order rate constants is observed for weaker acids. For stronger acids, however, further increases in pK do not correlate to increases in rate constants. Further, steric bulk around the acidic proton is shown to influence rate constants dramatically. Together, these observations reveal the complex factors dictating catalyst performance.

We describe a new strategy for enhancing the efficiency of electrocatalytic CO2 reduction with a homogeneous catalyst, using a room-temperature ionic liquid as both the solvent and electrolyte. The electrochemical behavior of fac-ReCl(2,2'-bipyridine)(CO)3 in neat 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][TCB]) was compared with that in acetonitrile containing 0.1 M [Bu4N][PF6]. Two separate one-electron reductions occur in acetonitrile (-1.74 and -2.11 V vs Fc(+/0)), with a modest catalytic current appearing at the second reduction wave under CO2. However, in [emim][TCB], a two-electron reduction wave appears at -1.66 V, resulting in a ∼0.45 V lower overpotential for catalytic reduction of CO2 to CO. Furthermore, the apparent CO2 reduction rate constant, kapp, in [emim][TCB] exceeds that in acetonitrile by over one order of magnitude (kapp = 4000 vs 100 M(-1) s(-1)) at 25 ± 3 °C. Supported by time-resolved infrared measurements, a mechanism is proposed in which an interaction between [emim](+) and the two-electron reduced catalyst results in rapid dissociation of chloride and a decrease in the activation energy for CO2 reduction.

Excited-state proton-coupled electron transfer (ES-PCET) is a promising avenue for solar fuel production and small molecule activation. Although less is known about ES-PCET reactions than related transformations involving exclusively thermal processes, ES-PCET holds promise as a simple and efficient reaction scheme for the formation of solar fuels, and it may provide access to new reactivity not accessible from ground electronic states. This review classifies ES-PCET into six categories based on the identity of the photoexcited reactant: excited-state H + /e − donors, excited-state H + /e − acceptors, photo-oxidants, photoreductants, photoacids, and photobases. A brief overview of each class of ES-PCET is presented. Recent advances and key discoveries within the six classes of ES-PCET are examined, and underexplored reaction systems and promising paths for future research are discussed.

A series of rhenium diimine carbonyl complexes was prepared and characterized in order to examine the influence of axial ligands on electronic structure. Systematic substitution of the axial carbonyl and acetonitrile ligands of [Re(deeb)(CO)(NCCH)] (deeb = 4,4'-diethylester-2,2'-bipyridine) with trimethylphosphine and chloride, respectively, gives rise to red-shifted absorbance features. These bathochromic shifts result from destabilization of the occupied d-orbitals involved in metal-to-ligand charge-transfer transitions. Time-Dependent Density Functional Theory identified the orbitals involved in each transition and provided support for the changes in orbital energies induced by ligand substitution.

Artificial photosynthesis relies on coupling light absorption with chemical fuel generation. A mechanistic study of visible light-driven H production from [Cp*Ir(bpy)H] (1) has revealed a new, highly efficient pathway for integrating light absorption with bond formation. The net reaction of 1 with a proton source produces H, but the rate of excited state quenching is surprisingly acid-independent and displays no observable deuterium kinetic isotopic effect. Time-resolved photoluminescence and labeling studies are consistent with diffusion-limited bimetallic self-quenching by electron transfer. Accordingly, the quantum yield of H release nearly reaches unity as the concentration of 1 increases. This unique pathway for photochemical H generation provides insight into transformations catalyzed by 1.