Article
Breaking Bonds with Electrons and Protons. Models and Examples
Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d’Electrochimie Moléculaire, Unité Mixte de Recherche Université - CNRS N° 7591, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France
Acc. Chem. Res., Article ASAP
DOI: 10.1021/ar4001444
Abstract
Besides
its theoretical interest, the attention currently aroused by
proton-coupled electron transfers (PCET reactions) has two main motives.
One is a better understanding of biological processes in which PCET
reactions are involved, Photosystem II as well as a myriad of other
natural systems. The other is directed toward synthetic processes, many
of which are related to global energy challenges. Until recently, the
analyses of the mechanism and reactivity of PCET reactions have focused
on outersphere transfers, those in which no bond between heavy atoms
(all atoms with the exception of H) is concomitantly formed or broken.
Conversely, reactions in which electron transfer triggers the breaking
of a heavy-atom bond with no proton transfer have been extensively
analyzed, both theoretically and experimentally. In both cases,
strategies have been developed to distinguish between stepwise and
concerted pathways. In each case, kinetic models have been devised,
allowing the relation between activation and thermodynamic driving force
to be established by means of parameters pertaining to the initial and
final state. Although many natural and artificial processes include
electron transfer, proton transfer, and heavy-atom bond breaking
(/formation), no means were offered until recently to analyze the
mechanism of such reactions, notably to establish the degree of
concertedness of the three constitutive events. Likewise, no kinetic
models were available to describe reactions where the three events are
concerted. In this Account, we discuss the strategies to distinguish
stepwise, partially concerted (when two of the three events are
concerted), and totally concerted pathways in these reactions that
include electron transfer, proton transfer, and heavy-atom bond
breaking. These mechanism analysis methods are illustrated and validated
by three examples. First we describe the electrochemical cleavage of an
O–O bond in an aliphatic peroxide molecule with a pendant carboxylic
acid group that can serve as proton donor for electron transfer and bond
breaking. In the second example, we examine the breaking of one of the
C–O bonds of CO2 within a multistep process where the reduction of CO2
into CO is catalyzed by an electrogenerated iron(0) porphyrin in the
presence of various Brönsted acids. In this case, an intramolecular
electron transfer triggers proton transfer and bond cleavage. In the
first two examples, all three events are concerted. The third example
also involves catalysis. It describes the cleavage of a cobalt–carbon
bond in the reduction of chloroacetonitrile catalyzed by an
electrogenerated cobalt(I) porphyrin. It illustrates the rather common
case where the intermediate formed by the reaction of a transition metal
complex with the substrate has to be cleaved to close the catalytic
cycle. In the first two examples, all three events are concerted,
whereas, in the last case, a partially concerted pathway takes place:
proton transfer and bond-breaking (Co–C cleavage) are concerted after an
initial electron transfer step. The all-concerted cases require a model
that connects the kinetics to the thermodynamic driving force of the
reaction. Starting from previous models of outersphere electron
transfer, concerted proton-electron transfer, and concerted dissociative
electron transfer, we describe a model for all-concerted
proton–electron-bond breaking reactions. These pathways skip the
high-energy intermediates that occur in stepwise pathways, but could
introduce kinetic penalties. The all-concerted model allows one to
assess these penalties and the way in which they can be fought by the
supplement of driving force offered by concerted proton transfer.
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