Solvation Effects on Transition Metal Hydricity

The free energy of hydride donation (hydricity) for [HNi(DHMPE)2][BF4] (DHMPE = 1,2-bis(dihydroxymethylphosphino)ethane was experimentally determined versus the heterolytic cleavage energy of hydrogen in acetonitrile, dimethyl sulfoxide, and water to be 57.4, 55.5, and 30.0 kcal/mol, respectively. This work represents the first reported hydricity values for a transition metal hydride donor in three different solvents. A comparison between our values and the hydricity of hydrogen and formate reveals a narrowing in the range of values with increasing solvent polarity. The thermochemical values also reveal solvation effects that impact the overall thermodynamic favorability of hydride generation from hydrogen and transfer to carbon dioxide. The quantitative solvation effects described herein have important consequences to the design and reactivity of catalysts for transformations that have hydride transfer steps throughout synthetic chemistry.