Synthesis, structure, and optical-limiting properties of heterobimetallic [M₃CuS₄] cuboidal clusters (M = Mo or W) with terminal phosphine ligands

Cubane-type clusters of formula [Mo₃CuS₄Cl₄(dmpe)₃] (PF₆) (4), [Mo₃CuS₄Br₄(dmpe)₃] (PF₆) (5), and [W₃CuS₄Br₄(dmpe)₃] (PF₆) (6) have been prepared by reacting the incomplete cuboidal trimers [Mo₃S₄Cl₃(dmpe)₃](PF₆) (1), [Mo₃S₄Br₃(dmpe)₃](PF₆) (2), and [W₃S₄Br₃(dmpe)₃](PF₆) (3), respectively, with CuX (X = Cl or Br) or the mononuclear copper complex [Cu(CH₃CN)₄]⁺ in THF. The reaction takes place without global changes in the metal oxidation states, and compounds 4-6 with a [M₃CuS₄]⁵⁺ core possess 16 e^- for metal-metal bonding. X-ray structural analysis of 4-6 revealed an effective C_3ν symmetry for the M₃Cu unit with the M-M distances being statistically the same for M = Mo or W. However, the M-Cu distance is 0.04 and 0.1 Å longer than the M-M bond length for Mo and W, respectively. There is no significant structural rearrangement of the ligand-metal bonding in proceeding from [M₃S₄X₃(dmpe)₃]⁺ to [M₃CuS₄X₄(dmpe)₃]⁺. The cyclic voltammograms of the [Mo₃CuS₄] cubane clusters show one quasi-reversible reduction process at E_1/2 = -0.31 V for 4 and at E_1/2 = -0.23 V for 5 and one irreversible reduction at -0.69 and -0.58 V for 4 and 5, respectively. The tungsten cluster 6 shows a unique quasi-reversible reduction wave at E_1/2 = -0.71 V. The incorporation of copper into the incomplete [M₃S₄] cuboidal complexes produces a decrease of the reduction potential for both molybdenum and tungsten. Absorption spectra of 1-6 are broadly similar; replacing Mo by W in proceeding from 2 to 3 or from 5 to 6 and replacing Br by Cl in proceeding from 2 to 1 or from 5 to 4 results in a blue shift of the three UV-visible absorption bands. All six clusters exhibit optical limiting, as measured by the Z-scan technique at 523 nm using 40 ns pulses. The power-limiting mechanism remains obscure, but under the conditions employed, threshold-limiting fluence decreases on replacing W by Mo on proceeding from 3 to 2 or 6 to 5 and on proceeding from tetranuclear cluster (4-6) to trinuclear precursor (1-3, respectively). For all six clusters, values of the excited-state cross section σ_eff are larger than those of the corresponding ground-state cross section σ_o; i.e., all clusters are efficient optical limiters.