A new class of sulfur bridged ruthenium–molybdenum complexes, (L) 2Ru II(μ-S) 2Mo IV(OH) 2 [L=NC 5H 4N=NC 6H 4(R), R=H, o-Me/Cl, m-Me/Cl]. Synthesis, spectroscopic and electron-transfer properties

Abstract

The reaction of (NH 4) 2Mo VIS 4 with the complexes ctc-Ru II(L) 2Cl 2 (1a–1e) [L=NC 5H 4N=NC 6H 4(R), R=H, o-Me/Cl, m-Me/Cl; ctc= cis–trans–cis with respect to chlorides, pyridine and azo nitrogens respectively] in MeOH–H 2O (1:1) resulted in a group of stable sulfur bridged ruthenium–molybdenum complexes of the type (L) 2Ru II(μ-S) 2Mo IV(OH) 2 (2a–2e). In complexes 2 the terminal MoS bonds of the Mo VIS 4 2− unit get hydroxylated and the molybdenum ion is reduced from the starting Mo VI in MoS 4 2− to Mo IV in the final product 2. The cis–trans–cis (with respect to sulfurs, pyridine and azo nitrogens respectively) configuration of the RuL 2S 2 fragment in 2 has been established by the 1H NMR spectroscopy. In dichloromethane solution the complexes 2 exhibit a strong dπ(Ru II)→Lπ* MLCT transition near 550 nm, a strong sulfur to molybdenum LMCT transition near 500 nm and intra ligand π–π* transition in the UV region. In dichloromethane solution the complexes display reversible Ru II⇌Ru III oxidation couples in the range 1.15–1.39 V, irreversible Mo IV→Mo V oxidations in the range 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-electron reduction) vs SCE respectively. The presence of trivalent ruthenium in the oxidized solutions 2 + is evidenced by the rhombic EPR spectra. The EPR spectra of the coulometrically oxidized species 2 + have been analyzed to furnish values of axial (Δ=4590–5132 cm −1) and rhombic ( ν=1776–2498 cm −1) distortion parameters as well as energies of the two expected ligand field transitions ( γ 1=3798–4022 cm −1) and ( γ 2=5752–6614 cm −1) within the t 2 shell. One of the ligand field transitions has been observed 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 γ 2 values.