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

The trinuclear complexes [M-3(mu(3)-Se)(mu-Se)(3)X-3(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 I confirms the presence of an incomplete cuboidal Mo3Se4 core. The electrochemical, linear optical and optical limiting properties of 1-3 and the previously reported [M-3(mu(3)-Q)(mu-Q)(3)X-3(L-2)(3)]PF6 (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(3)Q(4)X(3)(diphosphine)(3)](+) complexes upon reduction: (i) M-3(IV)<---->(MM2III)-M-IV-->M-3(III) for all complexes except 5 and 6 (ii) M-3(IV)-->(M2MIII)-M-IV-->(MM2III)-M-IV-->M-3(III) for 5 and 6. The ease of reduction within the [M(3)Q(4)Br(3)(dppe)(3)](+) series follows the trend Mo3S4 > Mo3Se4 > W3Se4 approximate to W3S4, while the ease of reduction for the analogous dmpe derivatives follows the trend Mo3S4 approximate to Mo3Se4 > W3Se4 approximate to 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 [W(3)Q(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 2 excited state cross-sections sigma(eff) are larger than those of the ground-state cross-sections sigma(0)) at fluences approximately 100 mJ cm(-2). (C) 2003 Elsevier Science B.V. All rights reserved.