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Methods Used to Determine Hydricity in Water , DMSO , and Acetonitrile

The free energy of hydride donation (hydricity) for [HNi(DHMPE)2][BF4] (DHMPE = 1 ,2 -b i s (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. ■ INTRODUCTION Transition metal hydrides play a significant role in synthetic fuel forming (energy storage) and fuel cell (energy utilization) reactions. The rational design of energy efficient homogeneous catalysts for these reactions requires an understanding of the major bond making and breaking steps in order to avoid energetically inaccessible intermediates or thermodynamic sinks in the catalytic cycle. Transition metal hydrides are key redox catalyst intermediates in these reactions. As a result, the hydricities, or hydride donor strengths, (eq 1, ΔGH−) for many transition metal complexes in acetonitrile have been determined. These quantitative determinations have demonstrated exceptional value in the design and optimization of catalysts for many reactions including proton reduction, hydrogen oxidation, formate oxidation, and hydrogenation reactions, but thus far their utility has been limited to reactions in acetonitrile. There is experimental evidence that metal hydride transfer to substrates such as CO2 is strongly solvent dependent, 50−53 but a dearth of quantitative measurements to rationalize the reactivity patterns. For example, the hydricities of only two hydride donors have been measured in two different solvents. We sought to improve the understanding of solvation effects by measuring the hydricity of a transition metal hydride in acetonitrile, dimethyl sulfoxide, and water to examine the effect of both solvent polarity and hydrogen bonding. To accomplish this study, we synthesized a new nickel(II) bis(diphosphine) complex soluble in all three of our solvents of interest. Along with the Ni(II) complex, we independently synthesized and isolated the diamagnetic Ni(0) and Ni(II) hydride complexes, and utilized NMR spectroscopy to measure the equilibrium constants required to complete the thermodynamic cycles. The hydricity values presented herein were determined versus the hydricity of hydrogen (or free energy for heterolytic cleavage, ΔGH2) under standard state conditions. Measuring hydricity versus ΔGH2 is convenient for establishing the thermodynamic requirements for generating the metal hydride from hydrogen and its subsequent stability toward protonation. This work represents the first time ΔGH− has been experimentally determined for a transition metal hydride donor in three different solvents. Our values were compared to the hydricity of formate and revealed a narrowing in the range of hydricity values as the solvent polarity increases. However, the values did not track in a linear fashion. As a result, the nickel hydride becomes a much stronger hydride donor relative to formate in water. This is a compelling effect since it indicates hydride transfer from a transition metal to CO2 or other substrates can become more thermodynamically favorable under benign aqueous conditions than in organic solvents. Our analysis highlights the effect of solvation on the hydricity of our complex relative to that of formate, since the latter’s value is known in all three of our solvents of interest. However, the trend of increasing hydride donor strength that corresponds with increasing solvent polarity is likely to have an impact on the broad scope of synthetic reactions that involve formal transfer of a hydride. Additionally, our experimentally determined thermochemical data are essential as benchmarks for improving solvation models in the computational determination and prediction of hydricity. Received: July 29, 2015 Published: October 14, 2015 Article