Functional materials containing luminescent octahedral halide clusters of molybdenum and tungsten: Diversity of synthetic approaches and binding modes

The history of the chemistry of halide octahedral cluster complexes began long ago in the 19th century, but to this day these compounds have not lost their topicality or even become more relevant. The unique combination of useful properties of cluster compounds, including bright red phosphorescence, photosensibilization of the singlet oxygen generation process, and high radiopacity, forecasts the possibility of using such complexes in practical oriented fields. To overcome the main issue of these compounds - low hydrolytic stability, which hinders their application in real conditions, researchers are actively applying the approach of creating functional cluster-containing materials by incorporating active component into the matrices of different nature (organic, inorganic, hybrid). This approach has demonstrated significant efficiency, resulting in the accumulation of a large amount of data, and according to the analysis of literature data, this area is currently under active development. Complexes can be bound in a variety of ways, determined by the presence or absence of functional groups in the matrix/cluster, as well as by chemical transformations that occur with the cluster during material preparation. The types of binding modes include strong (ionic and covalent) and weak (hydrogen electrostatic, supramolecular, hydrophobic, etc.) interactions and combinations thereof. In addition, matrix properties (oxygen permeability, solubility/insolubility, robustness, optical transparency, luminescent properties) can significantly influence or even enhance the properties of the resulting material. The efficient stabilization of the complexes, as well as the rich variety of possible matrices, provides a wide range of different applications for these materials, only some of which have been demonstrated to date. This review aims to classify the existing synthetic approaches for the preparation of functional cluster-containing materials within the nature of the bonding of cluster units to the matrix.