A Series of Octahedral First-Row Transition-Metal Ion Complexes Templated by Wells–Dawson Polyoxometalates: Synthesis, Crystal Structure, Spectroscopic, and Thermal Characterizations, and Electrochemical Properties

Abstract

Three Wells-Dawson polyoxotungstates-based hybrid compounds of general formula [M(C₃H₇NO)₆][{M(C₃H₇NO)₅}₂(μ-P₂W₁₈O₆₂)]· nC₃H₇NO· n'H₂O [with M = Mn^II (1), Fe^II (2), Co^II (3) ; n = 2, 2, 3 and n' = 0, 0, 1, respectively] were synthesized at room temperature by a facile method and characterized by IR and ¹H and ³¹P NMR spectroscopy studies, thermogravimetric analysis-differential scanning calorimetry thermal analyses, UV-vis, X-ray diffraction (XRD) powder and single-crystal XRD analyses, and cyclic voltammetry studies. From the X-ray study, it was established that the metal (M = Mn, Fe, Co) is located on an inversion center, being octahedrally coordinated to six dimethylformamide (DMF) molecules to form the complex cation [M(dmf)₆]²⁺. Also, in the dinuclear complex anion [{M(dmf)₅}₂(μ-P₂W₁₈O₆₂)]^2-, the M atoms are coordinated to five DMF molecules through the oxygen atoms, while the sixth coordination site is occupied by a terminal oxygen atom of the Wells-Dawson anion [P₂W₁₈O₆₂]^6- that plays the role of a bridging ligand. The crystal components are connected through numerous weak C-H···O hydrogen bonds to construct a three-dimensional network. The UV-vis shows the two characteristic absorption bands for the three compounds at 266-268 and 297 nm. These two strong bands are attributed to the charge-transfer absorption band of Ot-W and Ob/c-W, respectively. Cyclic voltammetry study of compounds (1), (2), and (3) reveals at least two reduction reversible peaks ascribed to a Wells-Dawson cluster.