Polyoxometalate supported transition metal complexes: synthesis, crystal structures, and supramolecular chemistry

Abstract

A new series of polyoxometalate (POM) supported transition metal complexes (TMCs), namely, [2-ampH]₄[{Zn(H₂O)₅}Mo₇O₂₄]·9H₂O (1), [3-ampH]₄[{Zn(3-ampy)(H₂O)₄}Mo₇O₂₄]·4H₂O (2), [3-ampH]₄[{Co(3-ampy)(H₂O)₄}Mo₇O₂₄]·4H₂O (3), [2,3-diampH]₄[Co(H₂O)₆][Mo₇O₂₄].6H₂O (4), and [2,3-diampH]₂[{Zn(2,3-diampH)₂(H₂O)₂}Mo₈O₂₇].2H₂O (5) have been synthesized (where amp = aminopyridine and diamp = diaminopyridine) from the corresponding aqueous sodium molybdate solution, by varying the transition metal salts and aminopyridine derivatives in an acidic medium. In compounds 1-4, the POM cluster is [Mo₇O₂₄]^6-, whereas in compound 5, it is [Mo₈O₂₇]^6-. In compound 1, the heptamolybdate moiety is functionalized by the zinc-aqua complex [Zn(H₂O)₅]²⁺ and the residual negative charges have been compensated by the protonated 2-aminopyridine molecules acting as cations. In the crystal structure of compound 1, a dimer of 2-aminopyridine has been identified as a supramolecular synthon. Compounds 2 and 3 are isomorphous, in which the POM cluster anion supports the zinc and cobalt coordination complexes, {Zn(3-ampy)(H₂O)₄}²⁺ and {Co(3-ampy)(H₂O)₄}²⁺, respectively. The overall negative charges of the resulting POM supported transition metal complexes in compounds 2 and 3 have been counter-balanced by the protonated 3-aminopyridine molecules. In compound 4, the anionic cluster unit [Mo7O24]6- exists as a discrete moiety, where the species {Co(H₂O)₄}²⁺ and protonated 2,3-diaminopyridine act as mere counter cations. Compound 5 is a two-dimensional (2-D) coordination polymer which is constructed by [Mo₈O₂₇]^6- anion and [Zn(2,3-diampH)₂(H₂O)₄]⁴⁺ cation moieties. In the crystal structure of 5, interestingly, a one-dimensional chain is formed by a condensation reaction among [Mo₈O₂₇]^6- anionic cluster units through the formation of Mo-Ob-Mo (Ob is bridging oxygen) bonds, and these chains, in turn, are laterally interlinked by [Mo₈O₂₇]^6- supported coordination complexes [{Zn(2,3-diampH)₂(H₂O)2}Mo₈O₂₇]^2- (as linker) resulting in the formation of a 2-D porous network having well-defined cavities. All compounds 1-5, that are isolated at an ambient temperature, are characterized by single crystal X-ray crystallography and are additionally characterized by elemental analyses, infrared, thermogravimetric analysis/mass spectral studies. The crystal structures analyses of compounds 2, 3, and 5 show the existence of supramolecular tectons.