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Abstract
Metal sulfide clusters are attractive components for flow batteries owing to the abundance of their constituent atoms and their tunable size, solubility, and redox properties. We prepared an atomically precise cobalt sulfide cluster, Co6S8(PTA)6•4HCl (PTA = 1,3,5-triaza-7-phosphaadamantane) (1), in a single step using low-cost precursors and water solubilizing phosphine ligands. Compound 1 represents the first structurally characterized, water-soluble cobalt-sulfide cluster. Remarkably, cluster 1 undergoes two electrochemically reversible oxidations in aqueous solutions and is stable in air. The first redox process is chemically reversible during charge–discharge experiments using a static cell and aqueous solutions of NaCl. An aqueous cell comprising methyl viologen (MV2+) as the anolyte and an anion exchange membrane provides an operating Vcell = 0.63 V.Additionally, cluster 1 was used as a preformed metalloligand in the synthesis of a novel coordination solid. Heating a 1:2 mixture Co6S8(PTA)6•4HCl and CuI in a solution of acetonitrile/water under solvothermal conditions leads to the formation of dark black crystals. The resulting three-dimensional framework (2) contains three distinct cluster constituents (2[Co6S8(PTA)6•H+]•2[Cu4I4]•[Cu4I62-]) and exhibits reversible redox behavior in the solid state. Framework 2 was structurally characterized using single crystal X-ray diffraction and powder X-ray diffraction. The UV-Vis diffuse reflectance spectrum of framework 2 was measured and used to calculate the band gap (1.98 eV) based on the Kubelka-Munk function.