The coupling of long-range electron transfer to proton transport over multiple sites plays a vital role in many biological and chemical processes. Recently the concerted proton-coupled electron transfer (PCET) reaction in a molecule with a hydrogen-bond relay inserted between the proton donor and acceptor sites was studied electrochemically. The standard rate constants and kinetic isotope effects (KIEs) were measured experimentally for this double proton transfer system and a related single proton transfer system. In the present paper, these systems are studied theoretically using vibronically nonadiabatic rate constant expressions for electrochemical PCET. Application of this approach to proton relays requires the calculation of multidimensional proton vibrational wave functions and the incorporation of multiple proton donor-acceptor motions. The decrease in proton donor-acceptor distances due to thermal fluctuations and the contributions from excited electron-proton vibronic states play important roles in these systems. The calculated KIEs and the ratio of the standard rate constants for the single and double proton transfer systems are in agreement with the experimental data. The calculations indicate that the standard PCET rate constant is lower for the double proton transfer system because of the smaller overlap integral between the ground state reduced and oxidized proton vibrational wave functions, resulting in greater contributions from excited electron-proton vibronic states with higher free energy barriers. The theory predicts that this rate constant may be increased by modifying the molecule in a manner that decreases the equilibrium proton donor-acceptor distances or alters the molecular thermal motions to facilitate the concurrent decrease of these distances. These insights may guide the design of more efficient catalysts for energy conversion devices.

The design of molecular electrocatalysts for H 2 oxidation and production is important for the development of alternative renewable energy sources that are abundant, inexpensive, and environmentally benign. Recently, nickel-based molecular electrocatalysts with pendant amines that act as proton relays for the nickel center were shown to effectively catalyze H 2 oxidation and production. We developed a quantum mechanical approach for studying protoncoupled electron transfer processes in these types of molecular electrocatalysts. This theoretical approach is applied to a nickelbased catalyst in which phosphorous atoms are directly bonded to the nickel center, and nitrogen atoms of the ligand rings act as proton relays. The catalytic step of interest involves electron transfer between the nickel complex and the electrode as well as intramolecular proton transfer between the nickel and nitrogen atoms. This process can occur sequentially, with either the electron or proton transferring first, or concertedly, with the electron and proton transferring simultaneously without a stable intermediate. The electrochemical rate constants are calculated as functions of overpotential for the concerted electron-proton transfer reaction and the two electron transfer reactions in the sequential mechanisms. Our calculations illustrate that the concerted electron-proton transfer standard rate constant will increase as the equilibrium distance between the nickel and nitrogen atoms decreases and as the pendant amines become more flexible to facilitate the contraction of this distance with a lower energy penalty. This approach identifies the favored mechanisms under various experimental conditions and provides insight into the impact of substituents on the nitrogen and phosphorous atoms.hydrogen evolution | heterogeneous catalysis | PCET E nvironmental and economic concerns about the use of fossil fuels have led to the development of new technologies that are more environmentally friendly but are also cost-effective alternatives to nonrenewable resources. An important example is the oxidation and production of H 2 for use in hydrogen-based fuel cells and functional storage devices for the energy harvested from solar, wind, and other environmentally benign processes (1). While efficient methods have been developed for H 2 oxidation and production, platinum catalysts are neither abundant enough nor cost effective enough for mass production and large-scale use (2). On the other hand, H 2 oxidation and production occur naturally in the hydrogenase class of enzymes. These biological systems could serve as the key to the design of effective synthetic catalysts because the catalytic center of the enzyme is comprised of iron and/or nickel, both of which are highly abundant and inexpensive metals.The presence of an amine ligand in the second coordination sphere is thought to contribute significantly to the high catalytic activity of the [FeFe] hydrogenase enzymes. These pendant amines may assist in the heterolytic cleavage of H 2 by facilitati...

The design of electrocatalysts for the oxidation and production of H2 is important for the development of alternative energy sources. This Article focuses on the [Ni(P2 RN2 R′)2]2+ electrocatalysts, where P2 RN2 R′ denotes 1,5-diaza-3,7-diphosphacyclooctane ligands with substituent groups R and R′ covalently bound to the phosphorus and nitrogen atoms, respectively. Theoretical methods are used to investigate the mechanism of the step in the catalytic cycle corresponding to [HNiII(P2N2)2]+ – e– → [NiI(P2HN2)(P2N2)]2+ for H2 oxidation and the reverse reaction for H2 production. This step involves electron transfer (ET) between the Ni complex and the electrode as well as proton transfer (PT) between the Ni and the N. The sequential mechanisms, PT–ET and ET–PT, are investigated for the following (R,R′) substituents: (Me,Me), (Ph,Ph), and (Ph,Bz), where Me, Ph, and Bz denote methyl, phenyl, and benzyl substituents. Density functional theory is used to calculate reduction potentials, pK a values, and PT pathways, and the inner- and outer-sphere reorganization energies for electrochemical ET are calculated within the framework of Marcus theory. For the (Ph,Ph) and (Ph,Bz) systems, the sequential PT–ET mechanism for H2 production would require surmounting a large free energy barrier for the initial PT step, followed by thermodynamically favorable ET. The sequential ET–PT mechanism for these systems would require a moderate initial applied overpotential, followed by a PT reaction with a relatively low free energy barrier. Consistent with experimental data, the calculated overpotential required for the initial reduction in the ET–PT mechanism is lower for the (Ph,Bz) system than for the (Ph,Ph) system for H2 production. The concerted mechanism, in which the electron and proton transfer simultaneously without a stable intermediate, may be thermodynamically favorable and is a direction of future research.

The nickel-based P2(Ph)N2(Bn) electrocatalysts comprised of a nickel atom and two 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane ligands catalyze H2 production in acetonitrile. Recent electrochemical experiments revealed a linear dependence of the Ni(II/I) reduction potential on pH with a slope of 57 mV/pH unit, implicating a proton-coupled electron transfer (PCET) process with the same number of electrons and protons transferred. The combined theoretical and experimental studies herein provide an explanation for this pH dependence in the context of the overall proposed catalytic mechanism. In the proposed mechanisms, the catalytic cycle begins with a series of intermolecular proton transfers from an acid to the pendant amine ligand and electrochemical electron transfers to the nickel center to produce the doubly protonated Ni(0) species, a precursor to H2 evolution. The calculated Ni(II/I) reduction potentials of the doubly protonated species are in excellent agreement with the experimentally observed reduction potential in the presence of strong acid, suggesting that the catalytically active species leading to the peak observed in these cyclic voltammetry (CV) experiments is doubly protonated. The Ni(I/0) reduction potential was found to be slightly more positive than the Ni(II/I) reduction potential, indicating that the Ni(I/0) reduction occurs spontaneously after the Ni(II/I) reduction, as implied by the experimental observation of a single CV peak. These results suggest that the PCET process observed in the CV experiments is a two-electron/two-proton process corresponding to an initial double protonation followed by two reductions. On the basis of the experimental and theoretical data, the complete thermodynamic scheme and the Pourbaix diagram were generated for this catalyst. The Pourbaix diagram, which identifies the most thermodynamically stable species at each reduction potential and pH value, illustrates that this catalyst undergoes different types of PCET processes for various pH ranges. These thermodynamic insights will aid in the design of more effective molecular catalysts for H2 production.

The combination (AIM-ME) of atomic layer deposition in metal–organic frameworks (MOFs) and metal exchange (ME) is introduced as a technique to install dispersed metal atoms into the mesoporous MOF, NU-1000. Zn-AIM, which contains four Zn atoms per Zr6 node, has been synthesized through AIM and further characterized through density functional calculations to provide insight into the possible structure. Zn-AIM was then subjected to modification via transmetalation to yield uniform porous materials that present nonstructural Cu, Co, or Ni atoms.