Preparation and characterization of dimeric molybdenum(III)–ethylenediaminetetra-acetate complexes

Abstract

The potassium salt of the complex µ-acetato-µ-ethylenediaminetetra-acetato(4–)NN′OO′O″O‴-di-µ-hydroxo-di-molybdate(III), [Mo₂(O₂CMe)(OH)₂(edta)]^–, (1), has been prepared by the Zn–Hg reduction of the molybdenum(V) dimer [Mo₂O₄(edta)]^2–, in acetate buffer, pH 5. At pH values in the range 3–4, with and without acetate, a water-insoluble molybdenum(III) product, (2), is obtained whose analysis is consistent with its formulation as [Mo₂(OH)₂(H₂O)₂(edta)]. The bisthiocyanato-complex [Mo₂(OH)₂(NCS)₂(edta)]^2–(3) has been isolated following treatment of (1) with thiocyanate. All three complexes are diamagnetic which is consistent with the presence of metal–metal bonding. From a comparison of solution and diffuse reflectance u.v.–visible spectra, it is concluded that complexes (2) and (3), like complex (1), have a di-µ-hydroxo-structure. Complexes (1) and (3) give reversible spectrophotometric changes in the pH range 4–12, and pH titrations indicate one end-point corresponding to the formation of µ-oxo-µ-hydroxo-species pK_a 7.73 for (1) at 0 °C and 7.29 for (3) at ca. 20 °C. Effects resulting from the addition of the acids p-toluenesulphonic acid, H₃PO₄, and H₂SO₄ to (1) are considered. Analogues of complex (1) with formate replacing acetate, (4), and hedta (N-hydroxyethylethylenediaminetriacetate) replacing edta, (5), have also been prepared.