A new class of sulfur bridged ruthenium-molybdenum complexes, (L)(2)Ru-II(mu-S)(2)Mo-IV(OH)(2) [L = NC5H4N=NC6H4(R), R = H, o-Me/Cl, m-Me/Cl]. Synthesis, spectroscopic and electron-transfer properties

The reaction of (NH4)(2)(MoS4)-S-VI with the complexes ctc-Ru-II(L)(2)Cl-2 (1a-1e) [L=NC5H4N=NC6H4 (R), R=H, o-Me/Cl, m-Me/Cl; ctc=cis-trans-cis with respect to chlorides, pyridine and azo nitrogens respectively] in MeOH-H2O (1:1) resulted in a group of stable sulfur bridged ruthenium-molybdenum com plexes of the type (L)(2)Ru-II(mu-S)(2)Mo-IV(OH)(2) (2a-2e), In complexes 2 the terminal Mo=S bonds of the (MoS42-)-S-VI unit get hydroxylated and the molybdenum ion is reduced from the starting Mo-VI in MoS42- to Mo-IV in th e final product 2, The cis-trans-cis (with respect to sulfurs, pyridine and azo nitrogens respectively) configuration of the RuL2S2 fragment in 2 has been established by the H-1 NMR spectroscopy. In dichloromethane solution t he complexes 2 exhibit a strong d pi(Ru-II)-->L pi* MLCT transition near 55 0 nm, a strong sulfur to molybdenum LMCT transition near 500 nm and intra l igand pi-pi* transition in the UV region. In dichloromethane solution the c omplexes display reversible Ru(II)reversible arrow Ru-III oxidation couples in the range 1.15-1.39 V, irreversible Mo-IV-->Mo-V oxidations in the rang e 1.68-1.71 V vs SCE. Four successive reversible ligand (-N=N-) reductions are observed for each complex in the ranges -0.37-->0.67 V (one-electron), -0.81-->-1.02 V (one-electron) and -1.48-->-1.76 V (simultaneous two-electr on reduction) vs SCE respectively. The presence of trivalent ruthenium in t he oxidized solutions 2(+) is evidenced by the rhombic EPR spectra. The EPR spectra of the coulometrically oxidized species 2+ have been analyzed to f urnish values of axial (Delta=4590-5132 cm(-1)) and rhombic (nu=1776-2498 c m(-1)) distortion parameters as well as energies of the two expected ligand field transitions (gamma(1)=3798-4022 cm(-1)) and (gamma(2)=5752-6614 cm(- 1)) within the t(2) shell. One of the ligand field transitions has been obs erved experimentally at 6173 cm(-1) and 6289 cm(-1) for the complexes 2b(+) and 2d(+) respectively by near-IR spectra which are close to the computed gamma(2) values. (C) 1999 Elsevier Science Ltd. All rights reserved.