Di- and Tri-Cobalt Silicotungstates: Synthesis, Characterization, and Stability Studies

Di- and tricobalt silicotungstate complexes, K5Na4H4[{Na3(μ-OH2)2Co2(μ-OH)4} (Si2W18O66)]·37H2O (1) and K6Na3[Na­(H2O)­{Co­(H2O)3}2{Co­(H2O)2}­(Si2W18O66)]·22H2O (2), have been synthesized through reaction of cobalt chloride and [A-α-SiW9O34]10– in acidic buffer solution. They have been characterized by X-ray crystallography, elemental analysis, cyclic voltammetry, infrared, and UV–vis spectroscopy. In 1, two cobalt atoms as well as three sodium atoms are incorporated in the central pocket of the [Si2W18O66]16– polyanion. In 2, one cobalt atom and one sodium atom are incorporated in the pocket of [Si2W18O66]16–; two other cobalt atoms in this complex protrude outside the pocket and connect with WO6 units of other [Si2W18O66]16– polyanions to form a one-dimensional polymeric structure. The crucial parameters in the synthesis of these two compounds are discussed, and their stability in different buffer solutions is studied. The decomposition of 1 or 2 in heated potassium acetate buffer (pH 4.8, 1 M) yields K11[{Co2(H2O)8}­K­(Si2W18O66)]·17H2O (3) based on spectroscopic studies and an X-ray crystal structure.