Structural overview and evolution paths of lacunary polyoxometalates

Self-assembly is a natural way for primitive things to grow on earth. For long, mankind has been attracted by the mechanism of such assembly processes, in an attempt to manipulate the outcome by whatever chemical forces available. The study on molecular building blocks, with an emphasis on lacunary polyoxometalates (l-POMs), delivers a unique perspective on the formation of natural and synthetic phases (e.g., minerals and nanomachines). The tunability of structure, composition, and physicochemical property confers a competitive edge on l-POMs in the design and fabrication of materials with desired functions. In this contribution, we provide a structural overview of l-POMs that covers not only the classical members of polyoxotungstates, -molybdates, -vanadates, -niobates, and -tantalates, but also the recently emerged non-classical counterparts of polyoxotitanates, -ferrates, and -palladates also. The evolution paths of l-POMs in the course of isomerization, transformation, and fusion are summarized and proposed. The empirical rules that supervise structural switches are discussed as well, in an effort to mature the design of POM-based materials in a controlled manner.