Redox-rich spin-spin-coupled semiquinoneruthenium dimers with intense near-IR absorption

Abstract

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^·-)Ru^IICl(μ-tppz)ClRu^II(Q^·-)] (1) and the structurally characterized [(DTBQ^·-)Ru^IICl(μ-tppz)ClRu^II(DTBQ^·-)] (2). The compounds exhibit mostly quinone-ligand-based redox activity within a narrow potential range, high-intensity near-IR absorptions (λ_max ≈ 920 nm; e > 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.