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

Abstract

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 η 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 generation of complexes bearing the uncommon η 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′-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 characterized 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 monitoring 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.

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

“…However, these new species bear exclusively endo -η 4 -Cp*H, with the special ring proton oriented toward the rhodium­(I) center. These findings, along with very recent time-resolved spectroscopic studies that have identified the transient hydride supported by bpy, [Cp*Rh­(bpy)­H] + , and allowed for its kinetic characterization, strongly support that transient [Cp*Rh] hydride complexes are involved in this chemistry . These hydrides form by protonation of the reduced forms and undergo proton-hydride tautomerization that results in the transfer of the proton to the Cp* ring.…”

“…Based on the most recent kinetic data, protonation of the Cp* ring via proton-hydride tautomerism may not contribute significantly to catalysis under conditions involving strong acid (Reaction 1a). 13 Previous work has demonstrated that with an attached bipyridyl (bpy) ligand, the catalyst readily forms dihydrogen gas. 4,8 Efforts to further optimize the catalytic activity of this complex have been stymied by an inability to isolate the reactive Rh(III)H intermediate supported by bpy.…”

“…In light of the relevance of catalyst design to myriad issues in sustainability science, significant experimental and computational work has focused on identifying the key intermediates involved in hydrogen evolution with molecular catalysts. − One notable catalyst for hydrogen generation, originally reported by Kölle and Grützel in 1987, is built on the [Cp*Rh] motif, where Cp* = η 5 -pentamethylcyclopentadienyl . Catalysts in this family are supported by additional bidentate diimine ligands, such as 2,2′-bipyridyl (bpy) or 1,10-phenanthroline (phen), and are effective for hydrogen generation under both electrochemical and photochemical conditions.…”

“…Catalysts in this family are supported by additional bidentate diimine ligands, such as 2,2′-bipyridyl (bpy) or 1,10-phenanthroline (phen), and are effective for hydrogen generation under both electrochemical and photochemical conditions. [Cp*Rh] complexes have also found utility in the catalytic conversion of NAD­(P)+ to NAD­(P)H under bioelectrocatalytic conditions. , Synthetic manipulation and handling of [Cp*Rh] is straightforward and modular, , facilitating derivatization efforts, , incorporation into more elaborate constructs and devices, , and detailed mechanistic work …”

“…Summarizing the consensus catalytic cycle , for these [Cp*Rh] complexes that is relevant to work in strong acid, four key species are believed to be involved in the catalytic cycle and these are shown in Figure . The main cycle begins with a Rh­(I) species, which is protonated to generate the hydride Rh­(III)H (Reaction 1).…”