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

Abstract

The reaction of (NH₄)₂Mo^VIS₄ with the complexes ctc-Ru^II(L)₂Cl₂ (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 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^VIS₄^2- unit get hydroxylated and the molybdenum ion is reduced from the starting Mo^VI in MoS₄^2- to Mo^IV in the final product 2. The cis–trans–cis (with respect to sulfurs, pyridine and azo nitrogens respectively) configuration of the RuL₂S₂ 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 ∨ (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 (γ₁=3798–4022 cm^-1) and (γ₂=5752–6614 cm^-1) within the t₂ 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 γ₂ values.