Solid state synthesis, structure and optical limiting properties of seleno cuboidal clusters [M₃Se₄X₃(diphosphine)₃] ⁺ (M=Mo, W; X=Cl, Br)

The trinuclear complexes [M₃(μ₃-Se)(μ-Se)₃X₃(dppe) ₃]PF₆ [M=Mo, X=Cl (1), Br (2); M=W, X=Br (3)] have been prepared by both solid state and solution syntheses. A structural study of 1 confirms the presence of an incomplete cuboidal Mo₃Se₄ core. The electrochemical, linear optical and optical limiting properties of 1-3 and the previously reported [M₃(μ₃-Q)(μ-Q)₃X₃(L ₂)₃]PF₆ (M=Mo, Q=Se, X=Cl, L₂=dmpe (4); M=Mo, Q=Se, X=Br, L₂=dmpe (5); M=W, Q=Se, X=Br, L₂=dmpe (6); M=Mo, Q=S, X=Cl, L₂=dppe (7); M=Mo, Q=S, X=Br, L₂=dppe (8); M=W, Q=S, X=Br, L₂=dppe (9)) have been examined. Two different kinds of electrochemical behavior are proposed for the trinuclear [M₃Q₄X₃(diphosphine)₃] ⁺ complexes upon reduction: (i) M^IV₃↔M^IVM₂ ^III→M^III₃ for all complexes except 5 and 6 (ii) M^IV₃→M^IV₂M ^III→M^IVM₂^III→M ^III₃ for 5 and 6. The ease of reduction within the [M₃Q₄Br₃(dppe)₃]⁺ series follows the trend Mo₃S₄ > Mo₃Se₄ > W₃Se₄≈W₃S₄, while the ease of reduction for the analogous dmpe derivatives follows the trend Mo₃S₄≈Mo₃Se₄ > W₃Se₄≈W₃S₄. A quasi-reversible oxidation process at half-wave potential close to approximately 1.2 V vs. Ag|AgCl has been identified for the first time in [W₃Q₄] cluster compounds. Linear optical absorption spectra are broadly similar, with intense bands at high energy and weaker bands at lower energy. The longest wavelength absorption maximum is red-shifted in this series of complexes upon replacing chloride by bromide, sulfur by selenium, and tungsten by molybdenum. Complexes 4-6 have weak absorption from 500 to 800 nm. Optical limiting in 1-9 has been assessed by open-aperture Z-scan at 523 nm using 40 ns pulses, all nine clusters exhibiting optical limiting (values of the excited state cross-sections σ_eff are larger than those of the ground-state cross-sections σ₀) at fluences approximately 100 mJcm^-2.