Thermodynamic Hydricity across Solvents: Subtle Electronic Effects and Striking Ligation Effects in Iridium Hydrides

Insights into the influence of solvent on hydride transfer emerge from a study of iridium complexes with the formula [Cp*Ir­(bpy-X)­H]⁺ (bpy-X = 4,4′-X₂-2,2′-bipyridine, X = H, Me, tBu, OMe, CO₂Me, and CF₃). Hydricity (or hydride donor ability) is found to be equally sensitive to bipyridine ligand electronic structure in both CH₃CN and H₂O. In contrast, hydride transfer is found to be more strongly influenced by subsequent chloride ion binding to the metal center in CH₃CN than in H₂O. With thermochemical parameters for six iridium complexes available in both CH₃CN and H₂O, a general approach to comparing thermodynamic parameters across solvents was developed. The free energy to transfer the free hydride ion from water to eight organic solvents (acetonitrile, methanol, ethanol, ethylene glycol, dimethyl sulfoxide, N-methylpyrrolidin-2-one, ethylene carbonate, and tetrahydrofuran) was estimated. An equation based on the solvent transfer free energies of the hydride ion and the organometallic species involved in hydride transfer was developed, enabling accurate and quantitative predictions of the change in hydricity moving between solvents.