Halide-bridged arsine- and phosphine-capped diruthenium complexes, [(R3As)3Ru(µ-X)3Ru(AsR3)3]+ and [(R3P)3Ru(µ-X)3Ru(PR3)3]+ (X = Cl or Br), as precursors to confacial mixed-valence ruthenium ‘blues’: spectroelectrochemical studies spanning the binuclear oxidation states II,II, II,III and III,III

Abstract

A series of six tertiary-arsine-capped binuclear complexes, [L₃Ru(µ-X)₃RuL₃][CF₃SO₃] (L = AsMe₃, AsMe₂Ph or AsMePh₂; X = Cl or Br) together with a full range of purely PR₃-capped analogues and the mixed-ligand complex [(Ph₃P)(Me₃As)₂Ru(µ-Cl)₃Ru(AsMe₃)₂(PPh₃)][CF₃SO₃] have been characterised. The previously neglected arsine-capped compounds share the well defined electrochemical behaviour of their phosphine congeners. Stepwise reversible oxidations connect the Ru₂^II,II closed-shell d⁶d⁶ (=12-e) resting state with the d⁵d⁶ (11-e) and d⁵d⁵ (10-e) levels, and all the mixed-valence [L₃Ru(µ-X)₃RuL₃]²⁺ species can be characterised through electrogeneration in CH₂Cl₂ at –60 °C. Unexpectedly, the Ru₂^II,III arsine complexes strongly resemble the classical ruthenium ‘blues’ where L = NH₃ or H₂O. For such valence-delocalised systems the visible region ordinarily contains an intense σ → σ* band (the source of the intense blue colour) together with a much weaker, near-infrared δ_π* → σ* band. Bonding within the {RuX₃Ru}²⁺ core can then be monitored directly by ν_σ→σ*. The distinctly different spectral appearance of the more familiar PR₃-capped mixed-valence compounds has been a long-standing puzzle, but the twenty electrogenerated 11-e binuclear systems assembled here with various AsR₃ or PR₃ terminal ligands are all delocalised, and clearly belong within a continuum of electronic behaviour with steadily decreasing metal–metal interaction. In all, ν_σ→σ* declines over a considerable range from 17 000 to below 5000 cm^–1, with the ligands ranked as follows: L = NH₃ (and 1,4,7-trimethyl-1,4,7-triazacyclononane) > H₂O > Cl, Br (i.e. nonahalides) > AsR₃ > PR₃ and µ-Cl > µ-Br. These changes are well correlated with systematic trends in the g_∥ and g_⊥ components of the axial g tensor, and also with the gap between the stepwise oxidation potentials which shrinks from 1.2 to 0.45 V. For the PR₃ complexes the decrease in ν_σ→σ* is accompanied by progressive intensity transfer to the δ_π* → σ* band. The anticipated Ru · · · Ru separation is of the order of 2.9 and 3.0 Å for the mixed-valence AsMe₃/µ-Cl and PMe₃/µ-Cl systems respectively, markedly longer than the crystallographic value of 2.75 Å in [(NH₃)₃Ru(µ-Cl)₃Ru(NH₃)₃]²⁺. The geometric distinction between the AsR₃- and PR₃-capped dimers is an unexpected consequence of selective crowding between the substituent R groups and the {µ-X₃} array. The present Ru₂^II,III systems are electronically distinct from their PR₃-containing osmium counterparts, such as [(Et₃P)₃Os(µ-Cl)₃Os(PEt)₃]²⁺, which show still greater visible/near-infrared spectral deviations.