pKa of alcohols dictates their reactivity with reduced uranium-substituted thiomolybdate clusters

Abstract

The uranium-substituted thiomolybdate cluster, (Cp*₃Mo₃S₄)UCp*, has been demonstrated as a model for water reduction by single uranium atoms supported on a molybdenum sulfide surface (U@MoS₂). In this study, the scope of O–H bond activation is expanded through the investigation of the reactivity of various alcohols with differing pK_a values for the –OH proton. The reaction of (Cp*₃Mo₃S₄)UCp* with stoichiometric amounts of methanol, phenol, 2,6-dichlorophenol, and nonafluoro-tert-butyl alcohol affords the corresponding mono-alkoxide species, (Cp*₃Mo₃S₄)Cp*U(OR), via a uranium-metalloligand cooperative activation of the O–H bond. This observed reactivity is analogous to the O–H bond activation reported by (Cp*₃Mo₃S₄)UCp* in the presence of water. However, addition of tert-butanol induces protonolysis of the Cp* ligand on uranium, resulting in the formation of a uranium tris-tert-butoxide cluster, (Cp*₃Mo₃S₄)U(O^tBu)₃. Independent synthesis of (Cp*₃Mo₃S₄)Cp*U(O^tBu) was possible via an alternative pathway, eliminating sterics as a justification for the observed discrepancy in reactivity. These results offer insight into the role the –OH proton pK_a plays in dictating the mechanism of O–H bond activation of alcohols by the uranium-substituted thiomolybdate cluster.