PNNL

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 periodic trends of pendant pnictonium groups (ammonium and 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 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 pnictonium-containing ligands induce stronger electronic effects within the ligand cavity, in the order of P(+) > N(+) > C, that results in an increased through-space electrostatic stabilization of negative charge in the transition state. These findings underscore the importance of periodic trends in tuning the ligand electronics as a strategy to enhance cationic effects and improve catalytic performance by employing heavier pnictogen elements.