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Binuclear Ruthenium Complexes of a Neutral Radical Bridging Ligand. A New “Spin” on Mixed Valency

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posted on 19.02.2016, 03:39 by Stephen D. J. McKinnon, Brian O. Patrick, A. B. P. Lever, Robin G. Hicks
The electronic structures of (LX)2Ru­(Vd)­Ru­(LX)2 complexes (Vd = 1,5-diisopropyl-3-(4,6-dimethyl-2-pyrimidinyl)-6-oxoverdazyl radical; LX = acac (acetylacetonate) or hfac (hexafluoroacetylacetonate)) in multiple charge states have been investigated experimentally and computationally. The main focus was to probe the consequences of the interplay between the ruthenium ions and the redox-active verdazyl ligand for possible mixed-valent behavior. Cyclic voltammetry studies reveal one reversible reduction and one reversible oxidation process for both complexes; in addition the acac-based derivative possesses a second reversible oxidation. Analysis of a collection of experimental (X-ray structures, EPR, electronic spectra) and computational (TD-DFT (PCM)) data reveal that the ruthenium ancillary ligands (acac vs hfac) have dramatic consequences for the electronic structures of the complexes in all charge states studied. In the hfac series, the neutral complex is best regarded as a binuclear Ru­(II) species bridged by a neutral radical ligand. Reduction to give the anionic complex takes place on the verdazyl ligand, whereas oxidation to the cation (a closed shell species) is shared between Vd and ruthenium. For the acac-based complexes, the neutral species is most accurately represented as a Ru­(II)/Ru­(III) mixed valent complex containing a bridging verdazyl anion, though some bis­(Ru­(II))-neutral radical character remains. The monocation complex contains a significant contribution from a “broken symmetry” singlet diradical structure, best represented as a bis-Ru­(III) system with an anionic ligand, with significant spin coupling of the two Ru­(III) centers via the Vd­(−1) ligand (calculated J = −218 cm–1). The dication, a spin doublet, consists of two Ru­(III) ions linked (and antiferromagnetically coupled) to the neutral radical ligand. Computed net σ- and π-back-donation, spin densities, and orbital populations are provided. Time dependent DFT is used to predict the optical spectra and assign experimental data.