Cyanide-Melt Synthesis of Reduced Molybdenum Selenide Clusters

The tightly cross-linked Mo3nSe3n+2 (n = 2, 3, ...∞) cluster compounds react with alkali metal cyanide or cyanide salt mixtures at temperatures of 450−675 °C to yield cyanide-terminated molybdenum chalcogenide clusters, [Mo6Se8(CN)6]n- (1n-) (n = 6, 7) and [Mo4Se4(CN)12]8- (28-). The process by which discrete 1n- clusters are excised from a CN-linked intermediate chain compound, K6Mo6Se8(CN)5 (3), was investigated, and the cubane cluster 27- plays an essential role. An efficient one-step synthesis for Na8[28-] is presented. These clusters are stable in basic aqueous solutions. Cyclic voltammetric (CV) measurements in basic aqueous media show multiple reversible redox waves corresponding to 16-/7-, 17-/8-, and 18-/9- redox couples with half-wave potentials of E1/2 = −0.442, −0.876, and −1.369 V, respectively, versus SHE. Half-wave potentials (E1/2) for the [Mo4Se4(CN)12]6-/7- and [Mo4Se4(CN)12]7-/8- couples are 0.233 and −0.422 V, respectively, versus SHE. The 28- compounds are K7Na[28-]·5H2O·MeOH, Cs7Na4[28-]Cl3, Na8[28-], and K4Na4[28-]·12H2O. The products were characterized by X-ray crystallography, cyclic voltammetry, and UV−vis spectroscopy. Reduction potentials measured by voltammetry are consistent with conditions needed for isolating reduced species on a preparative scale but are much more negative than previously reported values. Na8[18-]·20H2O was isolated by reduction of 17- with Zn in aqueous NaCN solution. Reduction potentials measured in basic NaCN solutions of 28- also differ widely from previous reported values.