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 Ru2II,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 Ru2II,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 δπ* → σ* band. Bonding within the {RuX3Ru}2+ core can then be monitored directly by νσ→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, νσ→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 νσ→σ* is accompanied by progressive intensity transfer to the δπ* → 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 Ru2II,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.