Solvent influence on the thermodynamics for hydride transfer from bis(diphosphine) complexes of nickel

Abstract

The thermodynamic hydricity of a metal hydride can vary considerably between solvents. This parameter can be used to determine the favourability of a hydride-transfer reaction, such as the reaction between a metal hydride and CO2 to produce formate. Because the hydricities of these species do not vary consistently between solvents, reactions that are thermodynamically unfavourable in one solvent can be favourable in others. The hydricity of a water-soluble, bis-phosphine nickel hydride complex was compared to the hydricity of formate in water and in acetonitrile. Formate is a better hydride donor than [HNi(dmpe)2](+) by 7 kcal mol(-1) in acetonitrile, and no hydride transfer from [HNi(dmpe)2](+) to CO2 occurs in this solvent. The hydricity of [HNi(dmpe)2](+) is greatly improved in water relative to acetonitrile, in that reduction of CO2 to formate by [HNi(dmpe)2](+) was found to be thermodynamically downhill by 8 kcal mol(-1). Catalysis for the hydrogenation of CO2 was pursued, but the regeneration of [HNi(dmpe)2] under catalytic conditions was unfavourable. However, the present results demonstrate that the solvent dependence of thermodynamic parameters such as hydricity and acidity can be exploited in order to produce systems with balanced or favourable overall thermodynamics. This approach should be advantageous for the design of future water-soluble catalysts.

Transition Metal Hydride Catalysts for Sustainable Interconversion of CO2 and Formate: Thermodynamic and Mechanistic Considerations

“…Neither the Ni nor Rh systems have been examined in water at this time. The vast majority of hydricity measurements, in general, have been performed in acetonitrile solution, but some aqueous studies have been reported recently. ,,− Based on the small data set of available aqueous hydricities, switching from acetonitrile to water can significantly increase hydride donating abilities (lower Δ G H– ). Similar to acidity scales in organic versus aqueous solution, the range of accessible hydricities narrows in higher polarity solvents like water, but the hydricities do not change linearly due to different relative stabilizations of hydride species between the two solvents, different specific interactions with the solvent (e.g., hydrogen bonding), and the relative energies of the M-aqua versus M-acetonitrile complex (for cases where the parent metal complex is solvated).…”

Thermodynamic Hydricity across Solvents: Subtle Electronic Effects and Striking Ligation Effects in Iridium Hydrides

“…45,46 The small difference in hydricity relative to transition metal hydride complexes has been exploited for solvent-dependent CO 2 reduction reactions. 4,6,7,13,14 The small solvent dependent hydricity of formate has previously been attributed to Hbonding stabilization of HCO 2 − . 7 Equation 1 reveals that the CO 2 being a gas phase hydride acceptor is also an important, previously unrecognized factor.…”

“…2 The different relative hydricities of metal complexes and small molecules in different solvents have been exploited for significant advances in the field of CO 2 reduction, where a hydride transfer that is uphill in acetonitrile can become thermodynamically favorable in water. 4,6,7,13,14 A quantitative understanding of how thermodynamic hydricity will change across solvents and the factors that determine the relative hydricity of hydride donors in each solvent has been lacking. Deeper conceptual and quantitative insights into the influence of solvent on hydricity would provide more predictive power and new opportunities for applications in catalyst design.…”

Emerging Implications of the Concept of Hydricity in Energy‐Relevant Catalytic Processes

“…Thus, a simple change of solvent altered the mechanism through modulation of thermodynamic hydricity, and thereby, resulted in efficient aqueous‐phase CO 2 ‐hydrogenation catalysis in the presence of a mild bicarbonate base. Appel and co‐workers reported a similar study on how to drive the conversion of CO 2 to formate just by switching the solvent [14a] . The value of the formate anion (HCO 2 − ) in CH 3 CN was found to be 44 kcal mol −1 .…”

“…Very few Ni catalysts are known that are both very good hydride donors to CO 2 and efficient for easy catalytic liberation of CO 2 ‐reduction products. Notably, by applying a ligand‐tuning strategy, Appel and co‐workers developed moderately efficient Ni catalysts for aqueous‐phase CO 2 hydrogenation under mild‐base conditions, as discussed earlier [14a, 36] . However, Lu and co‐workers explored a different concept, in which they sought to stabilize the Ni−H complex in a more electron‐rich environment, yet allowing facile regeneration of Ni−H from H 2 and base.…”