Reversible Interfacial Hydride Transfer Quantifies Hydricity of Main Group Reagents

Hydride transfer is an essential elementary reaction across the chemical value chain, but there are limited methods available for quantifying thermodynamic hydricity (ΔGH−), particularly amongst main group reagents. Herein, we exploit facile H2 activation and reversible hydride transfer from a metal surface to a molecular reagent, the net hydrogen reduction reaction (HRR), to develop a potentiometric method for quantifying ΔGH− of main group reagents recalcitrant to conventional methods. HRR potentiometry is first validated with a benzimidazolium hydride donor and then applied to uncover the impact of the reaction environment on hydricity. Across benzimidazolium hydride donors, HRR equilibrium potentials are roughly invariant across solvents, indicating that the solvent dependence of its hydricity largely reflects the differential solvation of H− across media. For formate, HRR potentials and corresponding hydricities depend strongly on water content. For borohydrides, HRR potentiometry reveals that effective hydricity values are strongly influenced by Lewis acid-base adduct formation with the hydride acceptor but are minimally influenced by the counter cation. These studies highlight the power of HRR potentiometry to both quantify and uncover trends in hydricity across main group reagents.