Article,
Bioinspired Water Preorganization in Confined Space for Efficient Water Oxidation Catalysis in Metallosupramolecular Ruthenium Architectures
Acc. Chem. Res., 57 (10): 1538–1549 (May 2024)
DOI: 10.1021/acs.accounts.4c00148
Abstract
CONSPECTUS: Nature has established a sustainable way to maintain aerobic life on earth by
inventing one of the most sophisticated biological processes, namely, natural photosynthesis,
which delivers us with organic matter and molecular oxygen derived from the two abundant
resources sunlight and water. The thermodynamically demanding photosynthetic water
splitting is catalyzed by the oxygen-evolving complex in photosystem II (OEC-PSII), which
comprises a distorted tetramanganese−calcium cluster (CaMn4O5) as catalytic core. As an
ubiquitous concept for fine-tuning and regulating the reactivity of the active site of
metalloenzymes, the surrounding protein domain creates a sophisticated environment that
promotes substrate preorganization through secondary, noncovalent interactions such as
hydrogen bonding or electrostatic interactions. Based on the high-resolution X-ray structure of
PSII, several water channels were identified near the active site, which are filled with extensive
hydrogen-bonding networks of preorganized water molecules, connecting the OEC with the
protein surface. As an integral part of the outer coordination sphere of natural
metalloenzymes, these channels control the substrate and product delivery, carefully regulate the proton flow by promoting
pivotal proton-coupled electron transfer processes, and simultaneously stabilize short-lived oxidized intermediates, thus highlighting
the importance of an ordered water network for the remarkable efficiency of the natural OEC.
Transferring this concept from nature to the engineering of artificial metal catalysts for fuel production has fostered the fascinating
field of metallosupramolecular chemistry by generating defined cavities that conceptually mimic enzymatic pockets. However, the
application of supramolecular approaches to generate artificial water oxidation catalysts remained scarce prior to our initial reports,
since such molecular design strategies for efficient activation of substrate water molecules in confined nanoenvironments were
lacking. In this Account, we describe our research efforts on combining the state-of-the art Ru(bda) catalytic framework with
structurally programmed ditopic ligands to guide the water oxidation process in defined metallosupramolecular assemblies in spatial
proximity. We will elucidate the governing factors that control the quality of hydrogen-bonding water networks in multinuclear
cavities of varying sizes and geometries to obtain high-performance, state-of-the-art water oxidation catalysts. Pushing the boundaries
of artificial catalyst design, embedding a single catalytic Ru center into a well-defined molecular pocket enabled sophisticated water
preorganization in front of the active site through an encoded basic recognition site, resulting in high catalytic rates comparable to
those of the natural counterpart OEC-PSII.
To fully explore their potential for solar fuel devices, the suitability of our metallosupramolecular assemblies was demonstrated under
(electro)chemical and photocatalytic water oxidation conditions. In addition, testing the limits of structural diversity allowed the
fabrication of self-assembled linear coordination oligomers as novel photocatalytic materials and long-range ordered covalent organic
framework (COF) materials as recyclable and long-term stable solid-state materials for future applications.
