Greater than the Sum of Its Parts: Tuning Nickel for Uncommon Small Molecule Reactivity and Catalysis via Dative Bonds with Group 13 Lewis Acidic Metalloligands

Abstract

In transitioning to an energy infrastructure which is less reliant on fossil fuels and less deleterious to the environment, it will be critical to couple renewable energy sources to chemical reactions, like the multi-electron conversions of water to dihydrogen (H2) and of carbon dioxide (CO2) to liquid fuels, to allow for efficient energy storage and transport. Many of these essential chemical reactions require expensive metal catalysts to proceed; catalysts featuring multiple Earth-abundant metals are utilized in biological enzymes to facilitate these reactions, and offer underexplored possibilities in synthetic and industrial settings for replacing precious metals. Although inexpensive metals are often poor catalysts for challenging multi-electron processes, there are a multitude of possible metal-metal combinations, which may exhibit more desirable properties when paired together compared to those of their constituent metals. In this vein, an isostructural series of bimetallic complexes which feature a dative bond between Ni and a varied group 13 supporting metal has been systematically studied. The steric and electronic effects of larger group 13 supporting metals were found to poise Ni for the binding and activation of H2, with the Ni center rendered more electron-deficient due to stronger Ni→M dative bonds and more favorably positioned geometrically for small molecule binding. Pairing Ni with Ga was found to be optimal for catalyzing the hydrogenations of olefins to alkanes and of CO2 to formate, both of which often require precious metal catalysts and are challenging two-electron processes that a similarly-ligated mononuclear Ni center without a supporting metal is unable to mediate. By quantitatively comparing structure, redox properties, and the reactivity of key catalytic intermediates, the effects of the supporting metal on the properties of Ni and the catalytic activity of the Ni−M bimetallic complexes have been elucidated. Collectively, experimental and computational results demonstrate that modulating an active transition metal center via a direct interaction with a Lewis acidic supporting metal can be a powerful strategy for favorably altering the properties of inexpensive metals and promoting new reactivity paradigms in base-metal catalysis.