Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes

Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by & eta;5-pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the gen-eration of complexes bearing the uncommon & eta;4-pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron-and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2 & PRIME;-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)]+, an elusive hydride complex that has been spectroscopically and kinetically character-ized here. Tautomerization of the hydride leads to the clean formation of [(Cp*H) Rh(bpy)]+. Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic moni-toring of the second proton transfer event reveals that both the hydride and related Cp*H complex can be involved in further reactivity, showing that [(Cp*H)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands.

Automated summary

Protonation reactions of organometallic complexes often generate reactive metal hydrides. However, some organometallic species with Cp* ligands have been found to undergo ligand-centered protonation, leading to the generation of complexes with Cp*H ligands. This study investigates the kinetics and atomistic details of the protonation steps using time-resolved pulse radiolysis and stopped-flow spectroscopy. The results reveal the formation of an elusive hydride complex, Cp*Rh(H)(bpy), via the initial protonation, which further undergoes tautomerization to form Cp*H Rh(bpy). The spectroscopic monitoring of the reaction also shows that both the hydride and Cp*H complex can participate in further reactivity. The mechanistic roles of these intermediates provide insights for the design of catalytic systems using non-innocent cyclopentadienyl-type ligands.