Additive-free selective oxidation of aromatic alcohols with molecular oxygen catalyzed by a mixed-valence polyoxovanadate-based metal–organic framework

Abstract

Selective oxidation of alcohols to aldehydes is an industrially significant chemical transformation. Herein, we report a mixed-valence polyoxovanadate-based metal–organic framework (MOF), (H₂bix)₅{[Cd(bix)₂][V^IV₈V^V₇O₃₆Cl]₂}·3H₂O (V-Cd-MOF), for catalyzing the additive-free oxidation of a series of aromatic alcohols with high selectivity and in nearly quantitative yield to the corresponding aldehydes with O₂ as the oxidant. Experimental results, corroborated with density functional theory calculations, indicate that it is the synergistic operation of the dual active sites of the V^IV–O–V^V building units in the polyoxovanadate cluster that is responsible for the excellent catalytic performance observed: on the one hand, the exposed and readily accessible reduced V^IV site is believed to activate O₂, resulting in a reactive oxygen species for the subsequent activation and breaking of the substrate's C_α–H bond. On the other hand, the V^V site coordinates with the alcoholic O atom to facilitate the cleavage of the O–H bond. The catalyst can be recycled by centrifugation and re-used at least five times with uncompromised performance. To our knowledge, V-Cd-MOF represents the first example of a polyoxometalate-based MOF catalyst for additive-free selective oxidation of alcohol to aldehyde with O₂ as an oxidant.