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

Abstract

Using the [RuCl(mu-tppz)ClRu](2+) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine] platform for bridging two o-quinone/catecholate two-step redox systems (unsubstituted, Q(n), or 3,5-ditert-butyl-substituted, DTBQ(n)), we have obtained the stable complexes [(Q(center dot-))Ru(II)Cl(mu-tppz)ClRuII(Q(center dot-))] (1) and the structurally characterized [(DTBQ(center dot-))Ru(II)Cl(mu-tppz)ClRu(II)(DTBQ(center dot-))] (2). The compounds exhibit mostly quinone-ligand-based redox activity within a narrow potential range, high-intensity near-IR absorptions (lambda(max) approximate to 920 nm; epsilon > 50 000 M(-1) cm(-1)), and variable intra- and intermolecular spin-spin interactions. Density functional theory calculations, electron paramagnetic resonance (EPR), and spectro-electrochemical 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-phenyhminoquinone 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.