Polyoxometalate Supported Transition Metal Complexes: Synthesis, Crystal Structures, and Supramolecular Chemistry

A new series of polymometalate (POM) supported transition metal complexes (TMCs), namely, [2-ampH](4)[{Zn(H2O)(5)} Mo7O24]center dot 9H(2)O (1), [3-ampH]4[{Zn(3-ampy)(H2O)(4)}Mo7O24]center dot 4H(2)O (2), [3-ampH](4)[{Co(3-ampy)(H2O)(4)}Mo7O24] 4H(2)O (3), [2,3-diampH](4)Co(H2O)(6)][Mo7O24]center dot 6H(2)O (4), and [2,3-diampH](2)[{Zn(2,3-diampH)(2)(H2O)(2)}Mo8O27]center dot 2H(2)O (5) have been synthesized (where amp = aminopyridine and champ = 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 1,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-O-b-Mo (O-b is bridging oxygen) bonds, and these chains, in turn, arc laterally in terlinked by [Mo8O27N4](6-) supported coordination complexes [(Zn(2,3-diampH)(2)(H2O2)(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 supra molecular tectons.