Redox-Rich Spin–Spin-Coupled Semiquinoneruthenium Dimers with Intense Near-IR Absorption

Using the [RuCl(μ-tppz)ClRu]2+ [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine] platform for bridging two o-quinone/catecholate two-step redox systems (unsubstituted, Qn, or 3,5- di-tert-butyl-substituted, DTBQn), we have obtained the stable complexes [(Q•–)RuIICl(μ-tppz)ClRuII(Q•–)] (1) and the structurally characterized [(DTBQ•–)RuIICl(μ-tppz)ClRuII(DTBQ•–)] (2). The compounds exhibit mostly quinone-ligand-based redox activity within a narrow potential range, high-intensity near-IR absorptions (λmax ≈ 920 nm; ε > 50 000 M–1 cm–1), and variable intra- and intermolecular spin–spin interactions. Density functional theory calculations, electron paramagnetic resonance (EPR), and spectroelectrochemical results (UV–vis–near-IR region) for three one-electron-reduction and two one-electron-oxidation processes were used to probe the electronic structures of the systems in the various accessible valence states. EPR spectroscopy of the singly charged doublet species showed semiquinone-type response for 1+, 2+, and 2–, while 1 exhibits more metal based spin, a consequence of the easier reduction of Q as compared to DTBQ. Comparison with the analogous redox series involving a more basic N-phenyliminoquinone ligand reveals significant differences related to the shifted redox potentials, different space requirements, and different interactions between the metals and the quinone-type ligands. As a result, the tppz bridge is reduced here only after full reduction of the terminal quinone ligands to their catecholate states.