Article,

Intramolecular Valence and Spin Interaction in meso and rac Diastereomers of a p-Quinonoid-Bridged Diruthenium Complex

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Journal of the American Chemical Society, 130 (51): 17575--17583 (2008)
DOI: 10.1021/ja807043s

Abstract

The complexes meso- and rac-(acac)2Ru(m-L)Ru(acac)2n, 1 and 2, where L2- = 1,4-dioxido-2,3-bis(3,5-dimethylpyrazol-1-yl)benzene and acac- = 2,4-pentanedionato, were characterized structurally, magnetically, electrochem., and spectroscopically as well as spectroelectrochem. (UV-visible-NIR, EPR) in the accessible redox states (n = 0, +, -, 2-). Due to steric interference, the neutral compds. contain a severely twisted L2- bridging ligand with 43-48° dihedral angles between the planes of the hydroquinone dianion and those of the ortho positioned pyrazolyl substituents. The difference between meso and rac isomers is rather pronounced in terms of the redox potentials (easier oxidn. and redn. of the rac form 2) and with respect to the absorption spectra of the oxidized states. Susceptibility and EPR measurements confirm the RuIII(μ-L2-)RuIII configuration of the neutral species, showing J values of -37 and -21 cm-1 for the spin-spin interaction between the $\sim$7.75 \AA sepd. metal centers in 1 and 2, resp. Two-step redn. involves the metals and produces RuIIIRuII mixed-valent monoanions with comproportionation consts. of $\sim$104, with RuIII-type EPR signals, and with broad intervalence charge transfer bands at $\sim$1200-1500 nm absorption max., suggesting localized valence (class II). Oxidn. produces intense near-IR absorption at 892 (1+) or 1027 nm (2+) and narrow isotropic EPR spectra at g $\approx$ 2.005, signifying unprecedented spin localization at the p-semiquinone bridge. These results are not compatible with an (L2-)-bridged RuIVRuIII situation nor with an RuIII(μ-L·-)RuIII three-spin arrangement with up-down-up spin configuration in the ground state, which would result in metal-centered spin through antiferromagnetic coupling between the adjacent individual spins. Only the RuIII(μ-L·-)RuIII situation, with up-up-down spin configuration, leads to ligand-centered resulting spin through the strong antiferromagnetic coupling between the remote metal spins, an unusual situation which is favored here because of weakened metal-radical coupling resulting from the pyrazolyl/p-semiquinone twist. on SciFinder(R)

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