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.