Tri-, tetra-, and hexanuclear mixed-valence molybdenum clusters: structural diversity and catalysis of acetylene hydrogenation

Abstract

A series of novel cluster compounds comprising molybdenum in a low valence state was synthesized by means of a disproportionation of the dimeric compound [Mo⁺⁴₂Cl₄(OCH₃)₄(CH₃OH)₂] (1). The reaction of 1 with CH₃OH leads to the disproportionation of Mo⁺⁴ yielding an unusual mixed-valence cluster [Mo^+3.5₄Cl₄O₂(OCH₃)₆(CH₃OH)₄] (2). By exploring this synthetic approach further, tri-{[Mo₃Cl₃(OCH₃)₇(CH₃OH)₃] (3)}, tetra-{[Mo₄Cl₄(OCH₃)₁₀(CH₃OH)₂] (4), [Mo₄Cl₃O(OCH₃)₉(CH₃OH)₃] (5), [Mo₄Cl₂(OCH₃)₁₂(CH₃OH)₂] (6)}, and hexanuclear {[Mo₆Cl₄O₆(OCH₃)₁₀(CH₃OH)₂] (7)} molybdenum alkoxides were synthesized by the reaction of 1 with methanol and stoichiometric amounts of magnesium methoxide, thus providing a general facile access to the polynuclear methoxide complexes of a low-valence molybdenum. Due to the feasibility to adopt multiple oxidation states in a reversible manner and the documented competence of molybdenum alkoxide compounds to catalyze the reduction of inert molecules, including N₂, the synthesized compounds were expected to function as catalysts of small molecule substrates reduction/hydrogenation. Accordingly, the reduction of acetylene (C₂H₂) to an ethylene (C₂H₄) and ethane (C₂H₆) mixture, in methanol (with water additives) serving as a reaction medium and a proton donor, and using sodium or europium amalgams as reducing agents, was performed in the presence of 2. Preliminary kinetic studies evidently point to a catalytic function of molybdenum species derived from 2, thus establishing the observed reactivity as a rare example of non-precious metal-catalyzed acetylene hydrogenation, providing, in addition, a convenient model for further mechanistic studies.