Opposites Attract: Synthesis and Electrochemical Studies of Electron-Rich and Electron-Poor Rhodium Complexes for Hydrogen Evolution Catalysis

Abstract

Electrocatalysis represents an attractive route to coupling renewable energy sources such as wind or solar power with sustainable generation of chemicals. An attractive target chemical would be hydrogen gas because it can be used as a fuel that does not emit pollution (CO2). Progress toward this goal is hampered by a poor mechanistic understanding of how the electrocatalysts couple electrons with substrates to generate products. This problem is especially serious in the case of highly active catalysts that involve redox-active or proton-responsive ligands. Rhodium compounds featuring pentamethylcyclopentadienyl (Cp*) and diimine-type ligands are especially complex because they involve both of these modes of non-innocence. Changes in ligand substitution patterns are often used to improve the activity and stability of catalysts, but the consequences of such modifications are unknown in this class of catalysts. This limits the usefulness of these compounds and their incorporation into more elaborate energy-conversion systems. Here, we will discuss two specific cases that involve use of electron-donating and electron-withdrawing bipyridine variants.

Specifically, this thesis describes the synthesis and electrochemical properties of two novel rhodium compounds featuring pentamethylcyclopentadienyl (Cp*) and 4,4′-disubstituted 2,2′-bipyridine (bpy) ligands. The compounds were prepared with two disubstituted bipyridine derivatives, 4,4′-bis(tert-butyl)-2,2′-bipyridine (tBu-bpy) and 4,4′-bis(trifluoromethyl)-2,2′-bipyridine (CF3-bpy); these ligands are more electron-donating and electron-withdrawing, respectively, than the parent underivatized bpy system. Once synthesized these compounds were characterized using 1H, 13C{1H}, and 31P{1H} nuclear magnetic resonance, mass spectrometry, UV-visible spectroscopy and single-crystal X-ray diffraction.

Electrochemical studies with these complexes revealed that they are catalysts for hydrogen production. The catalytic activity is modulated by the choice of ligand. Compared to the parent bpy complex, the overpotential for hydrogen evolution is shifted to a smaller value for the [Cp*Rh(CF3-bpy)Cl]+(PF6)– complex, but shifted to a larger value for [Cp*Rh(tBu-bpy)Cl]+(PF6)–. Bulk electrolyses carried out with these complexes confirmed catalytic turnover and a high faradaic efficiency for hydrogen evolution in all cases. Notably, [(Cp*H)Rh(CF3-bpy)NCMe]+, a putative intermediate in the process of hydrogen evolution, was detected by 1H NMR following electrocatalytic H2 generation with [Cp*Rh(CF3-bpy)Cl]+(PF6)–. Few such [(Cp*H)Rh] complexes have been observed or reported in past work, and the observation of a species of this type therefore suggests a general role for such intermediates in hydrogen evolution with this class of catalysts.