ConspectusProton-coupled electron transfer
(PCET) is a
fundamental process
involved in all areas of chemistry, with relevance to biological transformations,
catalysis, and emergent energy storage and conversion technologies.
Early observations of PCET were reported by Meyer and co-workers in
1981 while investigating the proton dependence of reduction of a molecular
ruthenium oxo complex. Since that time, this conceptual framework
has grown to encompass an enormous scope of charge transfer and compensation
reactions. In this Account, we will discuss ongoing efforts in the
Matson Laboratory to understand the fundamental thermodynamics and
kinetics of PCET processes at the surface of a series of Lindqvist-type
polyoxovanadate clusters. This project aims to provide atomistic resolution
of net H atom uptake and transfer at the surfaces of transition-metal
oxide materials.First, we discuss our efforts aimed at understanding
PCET at metal
oxide surfaces using the Lindqvist-type polyoxovanadate-alkoxide (POV-alkoxide)
cluster [nBu4N]2[V6O13(TRIOLNO2)2]. These clusters reversibly bind H atom equivalents at bridging
oxide sites, mirroring the proposed uptake and release of e–/H+pairs at transition-metal oxide surfaces. Summarized
results include the measurement of bond dissociation free energies
of surface hydroxide moieties (BDFE(O–H)) as well as mechanistic
analyses that verify concerted proton electron transfer as the operative
pathway for PCET at the surface of POV-alkoxide clusters.Next,
we discuss net proton and H atom uptake at the surface of
reduced variants of the Lindqvist-type POV-alkoxide cluster, [V6O7(OR)12]n(R = Me, Et;n= −2, −1, 0, + 1).
In the case of these low-valent POV-alkoxide clusters, nucleophilic
bridging sites are kinetically inhibited by functionalization of the
cluster surface with organic ligands. This molecular modification
enables site-selectivity in proton and H atom uptake to terminal oxide
sites. The impact of reaction site and cluster electronics on reaction
driving force of PCET is explored, with core electron density playing
a critical role in dictating thermodynamics of H atom uptake and transfer.
Additional work described here contrasts the kinetics of PCET at terminal
oxide sites to the reactivity observed at bridging oxides in POV-alkoxide
clusters.Overall, this Account summarizes our foundational
knowledge regarding
the assessment of PCET reactivity at the surfaces of molecular metal
oxides. Drawing analogies between POV-alkoxide clusters and nanoscopic
metal oxide materials provide design principles for the advancement
of materials applications with atomic precision. These complexes are
additionally highlighted as tunable redox mediators in their own right;
our studies demonstrate how cluster surface reactivities can be optimized
by modifying electronic structure and surface functionalities.