Intramolecular valence and spin interaction in meso and rac diastereomers of a p-quinonoid-bridged diruthenium complex

Abstract

The complexes meso- and rac-[(acac)₂Ru(μ-L)Ru(acac)₂]ⁿ, 1 and 2, where L^2- = 1,4-dioxido-2,3-bis(3,5-dimethylpyrazol-1'-yl)benzene and acac^- = 2,4-pentanedionato, were characterized structurally, magnetically, electrochemically, and spectroscopically as well as spectroelectrochemically (UV-vis-NIR, EPR) in the accessible redox states (n = 0, +, -, 2-). Due to steric interference, the neutral compounds contain a severely twisted L^2- 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 oxidation and reduction of the rac form 2) and with respect to the absorption spectra of the oxidized states. Susceptibility and EPR measurements confirm the {Ru^III(μ-L^2-)Ru^III} configuration of the neutral species, showing J values of -37 and -21 cm^-1 for the spin-spin interaction between the ca. 7.75 Å separated metal centers in 1 and 2, respectively. Two-step reduction involves the metals and produces Ru^IIIRu^II mixed-valent monoanions with comproportionation constants of ca. 104, with Ru^III-type EPR signals, and with broad intervalence charge transfer bands at about 1200-1500 nm absorption maximum, suggesting localized valence (class II). Oxidation produces intense near-infrared absorption at 892 (1⁺) or 1027 nm (2⁺) and narrow isotropic EPR spectra at g ≈ 2.005, signifying unprecedented spin localization at the p-semiquinone bridge. These results are not compatible with an (L^2-)-bridged {Ru^IVRu^III} situation nor with an {Ru^III(μ-L^·-)Ru^III} 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 {Ru^III(µ-L^·-)Ru^III} 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.