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.

Recent developments in first-row transition metal complex-catalyzed CO2 hydrogenation

“…The solvation effect significantly influences the free energy of hydride transfer by stabilizing the charged species generated upon hydride transfer from HNi(dmpe) 2 + with differential solvent polarity dependence. 38 Bertrand and co-workers reported copper-hydride and classical Lewis pair dyad induced CO 2 activation. The mechanistic pathway involves the insertion of CO 2 into the Cu-hydride bond, leading to the formation of copper formate.…”

“…The solvation effect significantly influences the free energy of hydride transfer by stabilizing the charged species generated upon hydride transfer from HNi(dmpe) 2 + with differential solvent polarity dependence. 38…”

Pre-Equilibrium Reaction Mechanism as a Strategy to Enhance Rate and Lower Overpotential in Electrocatalysis

“…Contributions from a number of research groups have demonstrated that we can use catalyst design for thermochemical control of reaction chemistry to achieve selectivity for C–H bond formation over H 2 evolution. − However, that approach does not necessarily produce fast rates for C–H bond formation with CO 2 . Approaches to enhancing the reaction rate for C–H bond formation with CO 2 are needed, and have primarily used reaction conditions, rather than catalyst design to achieve improvements: successful examples of this approach include stabilization of transition states for hydride transfer to CO 2 by choice of the solvent, − use of hydride-transfer mediators, or additions of base or alcohol. , …”

Pre-equilibrium Reaction Mechanism as a Strategy to Enhance Rate and Lower Overpotential in Electrocatalysis

“…9,10,11,12,13,14,15,16 However, that approach does not produce fast rates for C-H bond formation with CO2. 17 Approaches to kinetic enhancement of reaction rate of C-H bond formation with CO2 are needed, and have primarily used reaction conditions, rather than catalyst design to achieve improvements: successful examples of this approach include stabilization of transition states for hydride transfer to CO2 by choice of solvent, 18,19,20,21,22,23,24,25 use of hydride transfer mediators, 26 or additions of base or alcohol. 27,28 In the area of catalyst design elegant studies have provided mechanistic insights into catalyst design that achieves faster hydride formation rates.…”