A five-center redox system: molecular coupling of two noninnocent imino-o-benzoquinonato-ruthenium functions through a π acceptor bridge

Abstract

Combining the concepts of noninnocent behavior of metal/ligand entities and the coupling of redox-active moieties via an electronically mediating bridge led to the synthesis and the structural, electrochemical, and spectroscopic characterization of [Cl(Q)Ru(μ-tppz)Ru(Q)Cl]ⁿ where Q° is 4,6-di-tert-butyl-N-phenyl-o-iminobenzoquinone and tppz^o is 2,3,5,6-tetrakis(2-pyridyl)pyrazine, the available oxidation states being Ru^II,III,IV, Q^o,·-,2-, and tppz^o,·-,2-. One-electron transfer steps between the n = (2-) and (4+) states were studied by cyclic voltammetry and by EPR, UV-vis-NIR spectroelectrochemistry for the structurally characterized anti isomer of [Cl(Q)Ru(μ-tppz)Ru(Q)Cl](PF₆)₂, 2(PF₆)₂, the only configuration obtained. The combined investigations reveal that 2²⁺ is best described as [Cl(Q^·-)Ru^III(μ-tppzo)Ru^III(Q^·-)Cl]²⁺ with antiferromagnetic coupling between the ruthenium(III) and the iminosemiquinone components at each end. A metal-based spin as evident from large g factor anisotropy (EPR) and an intense intervalence absorption band at 1850 nm in the near-infrared (NIR) suggest that oxidation occurs at both iminosemiquinones to yield two Ru^II,III-bonded quinones, implying redox-induced electron transfer. Reduction takes place stepwise at the metal centers yielding iminosemiquinone complexes of Ru^III,II as evident from radical complex EPR spectra with small ^99,101Ru hyperfine contributions. After complete metal reduction to ruthenium(II) the bridging ligand tppz is being reduced stepwise as apparent from typical NIR absorption bands around 1000 nm and from small g anisotropy of the monoanion [Cl(Q^·-)Ru^II(μ-tppz^·-)Ru^II(Q^·-)Cl]^-. A structure-based DFT calculation confirms the Ru-Cl character of the HOMO and the iminoquinone-dominated LUMO and illustrates the orbital interaction pattern of the five electron transfer active components in this new system.