Designing Catalytic Systems Using Binary Solvent Mixtures: Impact of Mole Fraction of Water on Hydride Transfer.

Abstract

The free energy for hydride transfer reactions of transition metal hydrides is known to be influenced by solvent effects. The first-row transition metal hydride [HNi(dmpe)2][BF4] (dmpe = 1,2-bis(dimethylphosphino)ethane) has starkly different hydride transfer reactivities with CO2 in different solvents. A binary mixture of water and acetonitrile was used to tune the hydride transfer reactivity of HNi(dmpe)2+ with CO2 so that the free energy for this reaction approached zero. Various mole fractions of water were tested and a linear relationship between the hydride transfer free energy and solvent composition was established for 0-0.24 mole fraction of water. A deviation from linearity was found upon moving toward higher mole fractions of water. The tuning of the free energy for hydride transfer allowed HNi(dmpe)2+ to be used as a catalyst for the hydrogenation of CO2. The optimized catalyst conditions produced 58 turnovers at room temperature in 0.082 mole fraction of water using 60 atm of a 1:1 mixture of H2 to CO2 gas.