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

Abstract

Four coordination networks based on the {ε-PMo^V₈Mo^VI₄O₄₀(OH)₄Zn₄} Keggin unit (εZn) have been synthesized under hydrothermal conditions. (TBA)₃{PMo^V₈Mo^VI₄O₃₆(OH)₄Zn₄}[C₆H₄(COO)₂]₂ (ε(isop)₂2) is a 2D material with monomeric εZn units connected via 1,3 benzenedicarboxylate (isop) linkers and tetrabutylammonium (TBA) counter-cations lying between the planes. In (TPA)₃{PMo^V₈Mo^VI₄O₃₇(OH)₃Zn₄}[C₆H₃(COO)₃] (TPA[ε(trim)]_∞∞), 1D inorganic chains formed by the connection of εZn 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){PMo^V₈Mo^VI₄O₄₀Zn₄}(C₇H₄N₂)₂(C₇H₅N₂)₂·12H₂O (ε(bim)₄4) is a molecular material with monomeric εZn POMs bound to terminal benzimidazole (bim) ligands. Finally, (TBA)(C₁₀H₁₀N₄)₂(HPO₃){PMo^V₈Mo^VI₄O₄₀Zn₄}₂(C₁₀H₉N₄)₃(C₁₀H₈N₄) (ε₂2(pazo)₄4) is a 1D compound with dimeric (εZn)₂ POMs connected by HPO₃²⁻ ions and terminal para-azobipyridine (pazo) ligands. In this compound an unusual bond cleavage of the central NN 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 ε₂2(pazo)₄4 and ε(isop)₂2 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.