Polyoxometalate-assisted synthesis of transition-metal cubane clusters as artificial mimics of the oxygen-evolving center of photosystem II

The {Mn4O5Ca} cluster of photosystem II is a well-known cubane-like transition-metal (TM) cluster that plays important roles in solar energy conversion. The artificial synthesis of cubane-like TM clusters and their interesting potential applications have been widely studied. Most of them were isolated by use of organic ligands, and therefore their potential applications were essentially restricted to organic solvents because of their limited stability in aqueous solution. In recent decades, lacunary polyoxo-metalates (POMs) composed of W-VI/Mo-VI centers have been used to replace organic ligands as stable and oxidative-resistant pure-inorganic multidentate ligands to construct water-soluble and water-stable molecular clusters. In the course of the mimicking of the {Mn4O5Ca} subsite of photosystem II, significant advances have already been made in POM-assisted artificial synthesis of TM cubanes. Compared with classic cubane-like TM clusters, these POM-based TM cubane clusters can not only dissolve in organic solvents by virtue of a simple phase-transfer reaction but also display excellent aqueous stability. Consequently, the synthesis of these pure-inorganic cubane clusters offers a great opportunity for expanding the applications of cubane-like TM clusters. This review focuses on the synthetic strategies for, structures of, and applications of these POM-based TM cubane clusters, particularly highlighting how POMs can be used as the inorganic ligands to construct cubane-like TM clusters. The purpose of this review is to give a representative and comprehensive overview of the work done so far in this emerging field. (C) 2016 Elsevier B.V. All rights reserved.