Preparation and properties of new molybdenum–ethylenediaminetetra-acetato-complexes formed by the oxidation of the molybdenum(III,III) dimer [Mo2(O2CMe)(OH)2(edta)]–

Abstract

Oxidation of the molybdenum(III,III) dimer µ-acetato-µ-ethylenediaminetetra-acetato(4–)NN′OO′O″O‴-di-µ-hydroxodimolybdate(III), [Mo₂(O₂CMe)(OH)₂(edta)]^–, by the two-equivalent oxidant N₃^– yields a cyclic molybdenum(III,IV) mixed oxidation state tetramer, and the already well characterized molybdenum(V,V) dimer [Mo₂O₄-(edta)]^2–. It has been demonstrated that displacement of acetate and co-ordination of azide, as well as acid dissociation of one of the hydroxo-bridges (pK_a 7.73), precede oxidation. The ratio of products (up to 37% tetramer) can be accounted for by formation of an intermediate, possibly a Mo^IV,IV dimer, for which the two initial reactants, the Mo^III,III complex and N₃^–, compete. Controlled oxidation of a solution of the Mo^III,IV tetramer with O₂ yields the analogous Mo^IV,IV tetramer which has also been isolated. The reduction potential of the two tetramers has been determined in aqueous solution, pH 7.43, and is +0.07 V. Both tetramers react readily with oxidants, e.g.[Co(NH₃)₅Cl]²⁺(t½ <1 min). Oxidation of the Mo^III,III dimer with Mo^V and Mo^VI complexes, [Mo₂O₄(edta)]^2–(on heating) and [Mo₂O₆(edta)]^4– respectively, yields a polymeric molybdenum(IV) species.