Computational Insights into the Influence of Ligands on Hydrogen Generation with [Cp*Rh] Hydrides

Abstract

This work reports a computational investigation of the effect of ancillary ligands on the activity of an Rh catalyst for hydrogen evolution based on the [Cp*Rh] motif (Cp* = η5-pentamethylcyclopentadienyl). Specifically, we investigate why a bipyridyl (bpy) ligand leads to H2 generation but diphenylphosphino-based (dpp) ligands do not. We compare the full ligands to simplified models and systematically vary structural features to ascertain their effect on the reaction energy of each catalytic step. The calculations based on density functional theory show that the main effect on reactivity is the choice of linker atom, followed by its coordination. In particular, P stabilizes the intermediate Rh-hydride species by donating electron density to the Rh, thus inhibiting the reaction toward H2 generation. Conversely, N, a more electron-withdrawing center, favors H2 generation at the price of destabilizing the hydride intermediate, which cannot be isolated experimentally and makes determining the mechanism of this reaction more difficult. We also find that the steric effects of bulky substituents on the main ligand scaffold can lead to large effects on the reactivity, which may be challenging to fine-tune. On the other hand, structural features like the bite angle of the bidentate ligand have a much smaller impact on reactivity. Therefore, we propose that the choice of linker atom is key for the catalytic activity of this species, which can be further fine-tuned by a proper choice of electron-directing groups on the ligand scaffold.

Redox Properties of [Cp*Rh] Complexes Supported by Mono-substituted 2,2′-Bipyridyl Ligands

“…30,31,32,33,34 In principle, enhanced back-bonding from the metal center to the phosphorus donor atoms can be envisioned to result in diminished hydricity in the bisphosphine-supported systems and thus lowered reactivity. 35 Characterization of the bisphosphine analogues has served as a useful strategy for elucidation of such electronic influences over hydride generation, a process of relevance to catalytic H2 evolution and also other hydride-transfer reactions which can be carried out by [Cp*Rh] species. 36,37,38 In the course of our investigations on the redox properties of [Cp*Rh] complexes, we were surprised to find that little work has examined how the chemical or electrochemical properties of rhodium complexes can be modulated by singly-substituted, non-C2-symmetric 2,2′-bipyridyl ligands.…”