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 η⁵-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 η⁴-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.

Keywords: catalysis; energy; pulse radiolysis; redox chemistry; stopped-flow.

Fig. 1.

Notable complexes bearing the η⁴-pentamethylcyclopentadiene (Cp*H) ligand.

Fig. 2.

Proposed catalytic cycle for hydrogen evolution starting with 1, where L = H₂O or MeCN.

Fig. 3.

Data from pulse radiolysis (PR) and benchtop UV-visible spectroscopy. Absorption spectrum of 1 in MeCN (solid gold line); spectrum of 2 obtained during PR after 1e^– reduction of 40 mM 1 in MeCN in the presence of 50 mM formate (green squares, measured at 10 μs after the e^– pulse); further reduction of 2 during PR leads to the formation of 3 (purple squares, measured at 1 ms after the e^– pulse). The solid gold and light-purple lines are benchtop UV-visible spectra of isolated 1 and 3, respectively. The Inset at the upper right shows the transient absorption kinetic trace monitored at 520 nm after PR, with a biexponential fit overlaid in blue.

Fig. 4.

(A) Stopped-flow UV-visible spectra for the protonation of 3 with [HNEt₃]⁺ to generate 4. Isosbestic points at 379 and 431 nm are denoted by *. (B) Observed rate constants monitored at 511 nm as a function of [HNEt₃]⁺ concentration. The linear dependence reveals a first-order dependence on [HNEt₃]⁺ and the overall second-order rate constant. (C) Stopped-flow UV-visible spectra for the tautomerization of 4 to generate 5. Isosbestic behavior at 370 nm is denoted by *. (D) Observed rate constants monitored at 399.8 nm as a function of [HNEt₃]⁺ concentration. The slope of zero reveals a zero-order dependence on [HNEt₃]⁺.

Fig. 5.

(A) Stopped-flow IR spectra for the tautomerization of 4 to generate 5. The signal at 1,920 cm⁻¹ is consistent with a Rh–H stretch. (B) Observed rate constants monitored at 1,920 cm⁻¹ as a function of [HNEt₃]⁺ concentration. The slope of zero reveals a zero-order dependence on [HNEt₃]⁺.

Fig. 6.

(A) Stopped-flow UV-visible spectra for the reaction between 5 and [PhNH₃]⁺, assigned to backtautomerization (k_-4) of 5 to generate 4, followed by fast protonation of 4 to regenerate 1 and dihydrogen. Isosbestic behavior at 326 nm is denoted by *. (B) Observed rate of disappearance of 5 monitored at 399.8 nm as a function of [PhNH₃]⁺ concentration. The slope of zero reveals a zero-order dependence on [PhNH₃]⁺. (C) Stopped-flow UV-visible spectra for the proposed protonation of 5 by [DMFH]⁺ to generate 6, followed by the fast elimination of dihydrogen to generate 1. Isosbestic behavior at 325 nm is denoted by *. (D) Observed rate constant monitored at 399.8 nm as a function of [DMFH]⁺ concentration. The linear dependence reveals a first-order dependence on [DMFH]⁺ and the overall second-order rate constant k₅ = 230 ± 10 M⁻¹ s⁻¹.