Abstract
Polyoxometalates (POMs) are a large group of anionic polynuclear metal-oxo clusters with discrete and chemically modifiable structures. In most aqueous POM solutions, numerous, and often highly negatively charged, species of different nuclearities are formed. It is rather difficult to determine the dominant POM species or their combination, which is responsible for the specific POM activity, during a particular application. Thus, the identification of all individual speciation profiles is essential for the successful implementation of POMs in solution applications. This review article summarizes species that are present in isopoly- and heteropolyvanadates, -niobates, -molybdates and -tungstates aqueous solutions and covers their stability and transformations. The ion-distribution diagrams over a wide pH range are presented in a comprehensive manner. These diagrams are intended for the targeted use of POMs, and in a clear form shows species that are in equilibrium at the given pH value. Thus, the data accumulated in this review can serve as both a starting point and a complete reference material for determining the composition of POM solutions. Some examples are highlighted where the POM speciation studies led to a detailed understanding of their role in applications. In doing so, we aim to motivate the POM community for more speciation studies and to make the subject more comprehensible, both for synthetic POM chemists and for scientists with different backgrounds interested in applying POMs in biological, medical, electrochemical, supramolecular and nanochemistry fields, or as homogeneous catalysts and other water-soluble materials.
Highlights
POMs are generally characterized in the solid state prior to dissolution, and this structural information is used as the framework on which the solution chemistry is developed
The POM solution equilibria presented in details can and should be used to interpret the results obtained in POM solution application for fundamental vision and full understanding of the POM nature
An exceptional example for a detailed species analysis is an investigation of acidified orthotungstate solution using electrospray-ionization massspectrometry (ESI-MS), 183W-Nuclear magnetic resonance (NMR) and Raman spectroscopy.[19]
Summary
The rapidly growing number of POMs application in solution, especially their catalytic[7] and biological ones,[8,9,10,11] entails the need for a deeper understanding and analysis of the fundamental relationship between POM’s structural behavior in the solid state and in solution. This is currently an acute drawback, often leading to an incorrect interpretation and erroneous determination of structure–activity relationships. The POM solution equilibria presented in details can and should be used to interpret the results obtained in POM solution application for fundamental vision and full understanding of the POM nature
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