Cyanide-melt synthesis of reduced molybdenum selenide clusters

The tightly cross-linked Mo(3n)Se(3n+2) (n = 2, 3,... infinity) cluster compounds react with alkali metal cyanide or cyanide salt mixtures at temperatures of 450-675 degrees C to yield cyanide-terminated molybdenum chalcogenide clusters, [Mo6Se8(CN)6]n- (1n-) (n = 6, 7) and [Mo4Se4(CN)12](8-) (2(8-)). The process by which discrete 1(n-) clusters are excised from a CN-linked intermediate chain compound, K6Mo6Se8(CN)5 (3), was investigated, and the cubane cluster 2(7-) plays an essential role. An efficient one-step synthesis for Na8[2(8-)] 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 1(6-/7-), 1(7-/8-), and 1(8-/9-) redox couples with half-wave potentials of E(1/2) = -0.442, -0.876, and -1.369 V, respectively, versus SHE. Half-wave potentials (E(1/2)) 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 2(8-) compounds are K7Na[2(8-)].5H2O.MeOH, Cs7Na4[2(8-)]Cl3, Na8[2(8-)], and K4Na4[2(8-)].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[1(8-)].20H(2)O was isolated by reduction of 1(7-) with Zn in aqueous NaCN solution. Reduction potentials measured in basic NaCN solutions of 2(8-) also differ widely from previous reported values.