Solid state synthesis, structure and optical limiting properties of seleno cuboidal clusters [M 3Se 4X 3(diphosphine) 3] + (M=Mo, W; X=Cl, Br)

Abstract

The trinuclear complexes [M 3(μ 3-Se)(μ-Se) 3X 3(dppe) 3]PF 6 [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 Mo 3Se 4 core. The electrochemical, linear optical and optical limiting properties of 1–3 and the previously reported [M 3(μ 3-Q)(μ-Q) 3X 3(L 2) 3]PF 6 (M=Mo, Q=Se, X=Cl, L 2=dmpe ( 4); M=Mo, Q=Se, X=Br, L 2=dmpe ( 5); M=W, Q=Se, X=Br, L 2=dmpe ( 6); M=Mo, Q=S, X=Cl, L 2=dppe ( 7); M=Mo, Q=S, X=Br, L 2=dppe ( 8); M=W, Q=S, X=Br, L 2=dppe ( 9)) have been examined. Two different kinds of electrochemical behavior are proposed for the trinuclear [M 3Q 4X 3(diphosphine) 3] + complexes upon reduction: (i) M IV 3↔M IVM 2 III→M III 3 for all complexes except 5 and 6 (ii) M IV 3→M IV 2M III→M IVM 2 III→M III 3 for 5 and 6. The ease of reduction within the [M 3Q 4Br 3(dppe) 3] + series follows the trend Mo 3S 4>Mo 3Se 4>W 3Se 4≈W 3S 4, while the ease of reduction for the analogous dmpe derivatives follows the trend Mo 3S 4≈Mo 3Se 4>W 3Se 4≈W 3S 4. 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 [W 3Q 4] 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 σ 0) at fluences approximately 100 mJ cm −2.