Elucidating Proton-Coupled Electron Transfer Mechanisms of Metal Hydrides with Free Energy- and Pressure-Dependent Kinetics

Abstract

Proton-coupled electron transfer (PCET) was studied in a series of tungsten hydride complexes with pendant pyridyl arms ([(PyCH2Cp)­WH­(CO)3], PyCH2Cp = pyridyl­methyl­cyclo­pentadienyl), triggered by laser flash-generated RuIII-tris-bipyridine oxidants, in acetonitrile solution. The free energy dependence of the rate constant and the kinetic isotope effects (KIEs) showed that the PCET mechanism could be switched between concerted and the two stepwise PCET mechanisms (electron-first or proton-first) in a predictable fashion. Straightforward and general guidelines for how the relative rates of the different mechanisms depend on oxidant and base are presented. The rate of the concerted reaction should depend symmetrically on changes in oxidant and base strength, that is on the overall ΔG 0 PCET, and we argue that an “asynchronous” behavior would not be consistent with a model where the electron and proton tunnel from a common transition state. The observed rate constants and KIEs were examined as a function of hydrostatic pressure (1–2000 bar) and were found to exhibit qualitatively different dependence on pressure for different PCET mechanisms. This is discussed in terms of different volume profiles of the PCET mechanisms as well as enhanced proton tunneling for the concerted mechanism. The results allowed for assignment of the main mechanism operating in the different cases, which is one of the critical questions in PCET research. They also show how the rate of a PCET reaction will be affected very differently by changes of oxidant and base strength, depending on which mechanism dominates. This is of fundamental interest as well as of practical importance for rational design of, for example, catalysts for fuel cells and solar fuel formation, which operate in steps of PCET reactions. The mechanistic richness shown by this system illustrates that the specific mechanism is not intrinsic to a specific synthetic catalyst or enzyme active site but depends on the reaction conditions.

Applications of Proton-Coupled Electron Transfer in Organic Synthesis

“…The free energy relationship between the rate constants and BDFE eff have been applied to reveal the detailed mechanism in several PCET processes. [6,[13][14][38][39][43][44] Examples of HAT and PCET reactions are shown in Scheme 1.…”

“…Recently, a series of publications have experimentally revealed the fundamental rules on how to switch the mechanism among these three by controlling the driving force and other conditions of the reaction. [6,[13][14] These guidelines and rules may guide the organic chemists to design new redox reactions more rationally and efficiently, to make the reaction conditions much milder and greener in future. As shown in Figure 3, the first two pathways are sequential and generate distinct but short-lived intermediates.…”

“…Proton-coupled electron transfer (PCET) reactions, in which an electron coupled with a proton transfer from donors to acceptors in a stepwise or concerted way, [1][2][3][4][5][6] are a sort of fundamental reactions in a various of areas, including organic chemistry, [7][8][9] inorganic chemistry, [10][11] bioorganic chemistry, [12] organometallic and material chemistry [6,[13][14][15][16][17][18][19] , and so on. Most recently, the applications of PCET in organic synthesis have attracted attentions from organic chemists.…”

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

“…Kinetic isotope effect, the change of rate that occurs upon isotopic substitution, is a widely used tool for elucidating a reaction mechanism, particularly in the study of the hydrogen (H + , H − , H•) transfer reactions and the role of hydrogen tunnelling (see -6,55]. Therefore, the analysis of KIE reveals also the fundamental features of PCET reactions [4,6,[9][10][11][12][13][14]47,[56][57][58][59]. KIE can be used to distinguish between concerted and stepwise PCET reactions.…”

Bonding Energetics of Palladium Amido/Aryloxide Complexes in DMSO: Implications for Palladium‐Mediated Aniline Activation

“…1]. S uch combinational operation of experimental pK a sa nd other energetic values,l ike the DG het (M À H) values of metal hydrides,t or egulate reactivity has well been represented in proton-coupled electron transfer, [21] H 2 activation or evolution [22] and CO 2 reductions, [23] but little was addressed for MÀ Xs ystems due to lack of the necessary DG het (MÀX) data. In sharp contrast to the abundant pK a values accumulated over the last century, [24] almost no experimental DG het (M À X) values were reported in the past.…”