Low temperature pathways to Chevrel phases via reduced molybdenum sulfide cluster complexes

Abstract

<p>Molecular Mo[subscript]6S[subscript]8L[subscript]6 cluster complexes have been explored as potential intermediates to the Chevrel phases. An improved method of preparing the initial cluster compound, Na[subscript] 2yMo[subscript]6S[subscript] 8 + y(py)[subscript] x, was discovered by reacting Mo[subscript]6Cl[subscript]12 with higher stoichiometries of sodium hydrosulfide and sodium butoxide in butanol/pyridine. This compound can subsequently undergo ligand exchange reactions to form a variety of cluster complexes.;Single-crystal structure determinations are reported for the pyridine (py), piperidine (pip), and pyrrolidine (pyrr) complexes: triclinic Mo[subscript]6S[subscript]8(py)[subscript]6·1.65py, cubic Mo[subscript]6S[subscript]8(py)[subscript]6·2py, Mo[subscript]6S[subscript]8(pip)[subscript]6·7pip, and Mo[subscript]6S[subscript]8(pyrr)[subscript]6·1pyrr. Solubility of the piperidine complex allowed for two-dimensional NMR studies and the structure supports coordination via equatorial positions of the piperidine ring. Raman and XPS spectra give characteristic bands which reflect the presence of the Mo[subscript]6S[subscript]8 cluster unit. The nitrogen-donor complexes exhibit the weakest Mo-L bonding; consequently, these complexes were the best materials for deligation studies.;Deligation of the cluster complexes was explored under a variety of reaction conditions. Direct heating under dynamic vacuum led to incomplete deligation. Reactions with tin and lead metal resulted in the formation of the corresponding Chevrel phase compounds along with MoS[subscript]2. Thermolysis reactions in the presence of flowing ammonia gas resulted in further deligation than was evidenced for reactions explored in vacuo. Solution reactions with strong acids were studied in an attempt to protonate the ligands and thus make them better leaving groups. Preliminary observations indicate only limited success in this area.;A new preparative route has been developed for ternary molybdenum sulfides. Cation exchange reactions with Na[subscript] 2yMo[subscript]6S[subscript] 8 + y(MeOH)[subscript] x were used to form the cobalt, tin, lead, and holmium compounds. Raman and XPS spectra support the presence of the Mo[subscript]6S[subscript]8 cluster unit. High temperature heating of the tin molybdenum sulfide under flowing hydrogen gas produced the tin Chevrel phase. Preliminary hydrodesulfurization studies on the sodium and cobalt compounds showed that these materials possess larger surface areas and thiophene conversion percentages than Chevrel phases; however, these ternary molybdenum sulfides exhibit a lower HDS rate.</p>