Proton-coupled electron transfer : from basic principles to small molecule activation

Abstract

Proton-coupled electron transfer (PCET) is the basic mechanism for bioenergetic conversion. Hallmark examples of such reactivities include water oxidation which is coupled to photosynthesis and oxygen reduction which is coupled to ATP generation by respiration. These two complementary energy converstion schemes are intimately controlled by PCET in Nature. Despite the importance of PCET in such catalytic bond-making and bond-breaking reactions, the underlying mechanisms by which electron and proton transport couple to one another are not particularly not well understood. To address mechanistic issues surrounding the role of PCET in long range charge transport in biology and catalytic chemical transformations, we have undertaken a multifaceted investigation of PCET events at the molecular level. Two distinct lines of study have been embarked upon. The first revolves around the design, synthesis and photophysical interrogation of supramolecular D-[H+]-A PCET models. Such architectures are composed of two redox sites linked together in a lateral arrangement via a hydrogen-bonding interface.

(cont.) Comparative photophysical studies have shown that changes of the polarization within the protonic interface or the coupling between the redox sites and the supramolecular bridge dramatically attenuates the kinetics of charge transport along such paths. The area of major focus has centered on the development of discrete molecular systems capable of driving water <--> oxygen interconversion via PCET. These studies have taken advantage of Pacman and Hangman porphyrin architectures to drive oxygen activation. Both Pacman and Hangman motiefs are well suited to facilite oxygen reduction and oxidation chemistry. Comparative reactivity studies for the generation of H20 from oxygen reveals that it is necessary to dictate the delivery of protons and electrons to activated oxygen species in order to efficiently drive drive 0-0 bond activation.