epsilon-Keggin-based coordination networks: Synthesis, structure and application toward green synthesis of polyoxometalate@graphene hybrids

Four coordination networks based on the {epsilon-(PMo8Mo4O40)-Mo-V-O-VI(OH)(4)Zn-4} Keggin unit (epsilon Zn) have been synthesized under hydrothermal conditions. (TBA)(3){(PMo8Mo4O36)-Mo-V-O-VI(OH)(4)Zn-4}[C6H4(COO)(2)](2) (epsilon(isop)(2)) is a 2D material with monomeric eZn units connected via 1,3 benzenedicarboxylate (isop) linkers and tetrabutylammonium (TBA) counter-cations lying between the planes. In (TPA)(3){(PMo8Mo4O37)-Mo-V-O-VI(OH)(3)Zn-4}[C6H3(COO)(3)] (TPA[epsilon(trim)]infinity), 1D inorganic chains formed by the connection of epsilon Zn POMs, via Zn-O bonds, are linked via 1,3,5 benzenetricarboxylate (trim) ligands into a 2D compound with tetrapropylammonium (TPA) cations as counter-cations. (TBA) {(PMo8Mo4O40Zn4)-Mo-V-O-VI}(C7H4N2)(2)(C7H5N2)(2)center dot 12H(2)O (epsilon(bim)(4)) is a molecular material with monomeric epsilon Zn POMs bound to terminal benzimidazole (bim) ligands. Finally, (TBA)(C10H10N4)(2)(HPO3) {PMo(8)(V)Mo(4)(VI)O(4)0Zn(4)}(2)(C10H9N4)(3)(C10H8N4) (epsilon(2)(pazo)(4)) is a 1D compound with dimeric (epsilon Zn)(2) POMs connected by HPO32- ions and terminal para-azobipyridine (pazo) ligands. In this compound an unusual bond cleavage of the central N=N bond of the pazo ligand is observed. We report also a green chemistry-type one-step synthesis method carried out in water at room temperature using epsilon(2)(pazo)(4) and epsilon(isop)(2) as reducing agent of graphite oxide (GO) to obtain graphene (G). The POM@G hybrids were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, powder X-ray diffraction, energy dispersive X-ray analysis, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and cyclic voltammetry.