Syntheses, Structures And Properties Of Molybdenum-oxygen Clusters

Polyoxometalates(POMs) constitute a unique class of inorganic compounds due to their structural varieties and interesting properties in fields such as catalysis, medicine and materials science. This class of metal-oxygen clusters is formed by early transition metals of groups V and VI(V, Nb, Ta, Mo and W) in their highest oxidation states(e.g., V5+ and W6+). The existence of POMs has already been known for almost 200 years, but the first structural details were only revealed in the last century. From then on, the number of POMs was developed at an exponential rate.Recently, the modification of the surface of the POM has attracted more attention because of their intriguing structural versatilities and potential applications in areas of molecular devices, biochemistry and catalysis. In the field of modified POMs, capped POMs and POM supported metal complexes have been regarded as two important families, which have become of increasing interest and relevance to several disciplines due to their topological and electronic versatilities.The Keggin POM is a valuable building block since it can be directly used to prepare materials where the structural integrity of the Keggin POM can be maintained throughout construction processes(although with(VO)2+ addition or metal exchange). A large number of Keggin POM supported metal complexes have been synthesized, including Keggin POM mono-, bi- tri-, tetra-, penta- supported transition metal complexes. In Keggin POM supported metal complexes, linking modes of transition metal complexes with the central cluster can be divided into three situations:(I) terminal-oxygen atom of the cluster,(II) bridging-oxygen atom of the cluster and(III) both terminal- and bridging-oxygen atoms of the cluster.Our group have focused on POM supported transition metal complexes for years. As part of the continuing work in this system, In this paper, we report the syntheses, structures, and properties of 1-3 Keggin POM supported transition metal complexes formed via both terminal- and bridging-oxygen atoms, formulated as [As Mo8V6O42][Cu(2,2’-bpy)2]2[Cu(2,2’-bpy)]·4H2O(1) [PMo8V6O42][Cu(2,2’-bpy)2]2[Cu(2,2’-bpy)]·3H2O(2) and [PMo8V6O42][Cu(2,2’-bpy)2]2[Cu(2,2’-bpy)]·3.5H2O(3) and compound 4. Compounds 2 and 3 are polymorphs. Polymorphism, defined as the phenomenon of a chemical species with more than one crystal forms, is attracting much attention because different forms maybe result in significantly difference in terms of both their structures and physical and chemical properties of compounds. To date, several polymorphs based on different POMs have been reported in literatures. Compounds 2 and 3 are two new examples of polymorphs based on Mo-V POMs. Compound 4{Co(phen)3}2[Mo6O19][Mo8O26]·2H2O contains two crystallo graphically different polyoxomolybdates. Although the polyoxomolybdate compounds of each hexa- and octamolybdate are well-known, the compound constructed from both {Mo6O19}2- and {Mo8O26}4-clusters is little known. These compounds have been characterized by the elemental analyses, IR spectrum, UV-Vis spectrum, fluorescence, TG analysis, and the single crystal X-ray diffraction,and catalytic oxidation properties of some compounds.