Homogeneous ligand-centered hydrogen evolution and hydrogen oxidation : exploiting redox non-innocence to drive catalysis.

Hydrogen is a promising carbon-free fuel / energy carrier and is an essential building block for many industrial and agricultural processes. Rising energy demands have ignited interest in the development of carbon-free and carbon neutral energy sources. In this context, hydrogen is an attractive candidate—being energy-dense, carbon-free—and easily accessible through a two-electron reduction of water. Accordingly, many electrochemical homogeneous catalyst systems have been studied, with a focus on understanding the mechanism of hydrogen evolution proceeding through metal-hydride intermediates. However, there has been a renaissance in hydrogen evolution reaction (HER) catalyst design, with many groups implicating ligand redox non-innocence as a crucial driving force for catalysis rather than metal-hydride formation. In this dissertation, using characterization techniques including, cyclic voltammetry, controlled potential coulometry, UV-visible spectroscopy, 1H NMR, cyclic voltammetry modeling, x-ray crystallography, kinetic isotope effect studies, and density functional theory, we investigate ligand-centered electrocatalysts, which function without the generation of metal-hydride intermediates, for the production and oxidation of dihydrogen. Chapter three expands upon the previous work in the Grapperhaus Lab, and focus on ReL3 (L = diphenylphosphinobenzenethiolate). ReL3 reduces acids to H2 in dichloromethane with an overpotential of 0.708 V and a turnover frequency (TOF) of 32 s-1, and also oxidizes H2 in the presence of base with an overpotential of 0.970 V and a TOF of 4 s-1. The mechanism is supported by kinetic isotope effect (KIE) studies and density functional theory calculations (DFT). Chapters four and five will build on Chapter three, aiming to develop sustainable approaches for ligand-centered catalysis. The non-transition metal complex, ZnL1, the metal-free complex, H2L1, and the transition metal complex, CuL1 (L1 = diacetyl-bis(N4-methyl-3-thiosemicarbazonato)), function as electrocatalysts for hydrogen evolution (ZnL1, H2L1 and CuL1) and hydrogen oxidation (ZnL1 and H2L1). H2L1and ZnL1 display TOF’s of 1,320 s-1 and 1,170 s-1 at overpotentials of 1.43 and 0.756 V, respectively, while the CuL1 complex demonstrates a TOF of 10,000 s-1. H2L1 and ZnL1 also display TOF values for H2 oxidation of 32 s-1 and 72 s-1 at overpotentials of 0.328 and 0.315 V, respectively. Mechanisms for the HER were modeled using digital simulations and are further supported by DFT calculations. ReL3, ZnL1, H2L1, and CuL1 represent a fundamentally new class of electrocatalysts. Contrary to traditional molecular electrocatalysts that employ a metal-hydride as the key mechanistic intermediate, this approach facilitates H2 evolution through ligand-centered proton and electron-transfer events resulting in the evolution of H2 through either ligand-centered H● radical coupling or ligand-centered hydride proton coupling