Pendant Phosphonium Outperforms Ammonium by Strengthening Local Electric Fields in a Ruthenium Hydrogenation Catalyst

Abstract

Pendant ammonium cations are known to enhance catalytic activity through the generation of local electric fields; however, there is little understanding of how the magnitude of these effects can be controlled through the cation identity. In this study, we investigate the impact of pendant pnictonium identity (ammonium versus phosphonium) incorporated into the ligand backbone of [Ru(triphos)(CH₃CN)₃]ⁿ⁺ complexes for CO₂ hydrogenation to MeOH. Kinetic analysis by operando ¹H NMR spectroscopy demonstrates that the phosphonium-based complex achieves significantly higher turnover frequency and selectivity for production of MeOH compared to its lighter ammonium analogue and the neutral bridgehead control complex. Mechanistic experiments reveal that MeOH is formed through an on-cycle pathway in which the substrate remains bound to the catalyst; however, once partially reduced intermediates dissociate, they accumulate as off-cycle by-products that are not further converted to MeOH. Computational studies demonstrated that phosphonium substitution lowers the free-energy barrier for rate-limiting hydride transfer to the bound formate ligand. Natural bond orbital (NBO) analysis and electrostatic potential calculations indicate that phosphonium exhibits a higher localization of charge than the ammonium analog, resulting in an increased electrostatic stabilization of negative charge in the transition state. These findings demonstrate that changing the identity of a pendant pnictonium group can enhance enhance cationic effects and improve catalytic performance.