Recent progresses on synthesis, structures and applications of reduced polyoxometalates with low-nuclearity

Abstract

Polyoxometalate (POMs) are large group of discrete, mostly anionic polynuclear metal-oxo clusters amenable to a variety of chemical transformations. Because of high oxidation of the central metal, the original structure of POMs did not change after electron implantation. The electrons introduced during the reduction process can be immobilized on a certain metal or between the metal center and the titanium oxide metal ions. Compared with fully oxidized POMs, reduced POMs have different electron densities of oxygen atoms, so their coordination ability is different, which affects their physical and chemical properties. When synthesizing reduced POMs, some basic principle must be considered, such as whether the POMs system contains diversified structure points, whether the intermediate product has high enthalpy value, whether it has isomerization, whether the system can cover other metal ions should be considered, etc. According to the stability of reduced POMs, they can divided into three types: firstly each center metal contains only one end oxygen atom, second each center metal contains two end oxygen atoms, last the combination of the first two conditions. There are significant differences in the preparation methods of different types of reduced POMs. There are three kinds of common methods: photochemical reduction, control potential electrolysis and chemical oxidation reduction. Besides, hydrothermal synthesis, microwave synthesis and one pot synthesis were new approaches without specific reaction mechanism. With the injection of electrons in the system, different degrees of reduction and the way will be derived from new derivatives. The reduced POMs of various types of structures and corresponding typical examples are briefly introduced, and the application of reduced POMs in many aspects is discussed. In the field of traditional catalysis, reduced POMs in molecular devices, biological reagents and new application of energy materials has gradually begun to attract peoples attention. Chemists have long realized that the delocalized electrons of reduced POMs can produce magnetism, but the magnetic utilization work has just started, although there are a large number of heteropoly blue molecules. It has been reported, but only a few molecules have been studied as functional elements of magnet devices or spins. Scientists understanding of the biological activity of POMs is not very thorough, but research in this field is gradually deepening and systematic. Studies have shown that reduced POMs can exhibit higher biological activity relative to their oxidation states. There are many challenges in the study of reduced POMs from basic chemistry to molecular applications. However, the discovery and characterization of multi-molecular molecules has led to the refinement of POMs in the structural model, the clear relationship between structure and properties. The advantages are more obvious. Furthermore, the research on related materials will be more and more worthy of attention. It is believed that through unremitting efforts, the reduced POMs in future new material systems, such as molecular-based magnetic materials, nanotechnology, catalyst molecular-based energy storage devices and life. In areas such as learning, it will show its revolutionary potential and prospects.