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}Mo 7O24]•9H2O (1), [3-ampH] 4[{Zn(3-ampy)(H2O)4}Mo7O 24]•4H2O (2), [3-ampH]4[{Co(3-ampy)(H 2O)4}Mo7O24]•4H2O (3), [2,3-diampH]4[Co(H2O)6][Mo 7O24]•6H2O (4), and [2,3-diampH] 2[{Zn(2,3-diampH)2(H2O)2}Mo 8O27]•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)(H 2O)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(H 2O)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 [Mo8O 27]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. © 2010 American Chemical Society.