Transition metal incorporation into seleno-bridged cubane type clusters of molybdenum and tungsten. X-Ray crystal structures of the first [Mo3CuSe4] derivatives

Abstract

Trinuclear clusters of formula [M₃Se₄X₃(dmpe)₃]PF₆, M = Mo with X = Cl (1), M = Mo with X = Br (2) and M = W with X = Br (3) have been synthesized by excision of polymeric {M₃Se₇X₄}_x phases with dmpe in acetonitrile. Cu(I) is readily incorporated into the molybdenum selenide clusters in organic solvents to give [Mo₃CuSe₄Cl₄(dmpe)₃]PF₆ (4) and [Mo₃CuSe₄Br₄(dmpe)₃]PF₆ (5). These are the first reported complexes with [Mo₃CuSe₄] units. No reaction with Cu(I) has been observed for the tungsten cluster. In contrast with the aquo cuboidal seleno derivatives these trinuclear, 1–3, and tetranuclear, 4 and 5, clusters are air stable making easier their full characterization. X-Ray structural analysis of complexes 1 and 3 shows that the overall geometry of the [M₃Se₄] core is basically the same as for the [Mo₃S₄] units. Structures 1 and 3 do not present the weak dimerization through Se–Se bonds observed for the previously reported [M₃Se₄] cyano and thiocyanato cluster complexes. The addition of a Cu atom to [Mo₃Se₄] results in cluster compounds with single cube type structures. The six intermetallic distances within the [Mo₃Cu] tetrametallic unit in 4 are statistically the same. The Mo–Cu bond distance in 5 is 0.02 Å longer than the Mo–Mo bond length. Electrochemical reduction processes of compounds 1–5 have been investigated by cyclic voltammetry. Cu insertion into complexes 1 and 2 to give 4 and 5 results in an anodic shift potential of 0.25 V for the first reduction process.