화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.131, No.25, 8895-8902, 2009
A Five-Center Redox System: Molecular Coupling of Two Noninnocent Imino-o-benzoquinonato-Ruthenium Functions through a pi Acceptor Bridge
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(mu-tppz)Ru(Q)Cl](n) where Q(o) 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,Ru-III,Ru-IV, Q(o,center dot-,2-), and tppz(o,center dot-,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(mu-tppz)Ru(Q)Cl](PF6)(2), 2(PF6)(2), the only configuration obtained. The combined investigations reveal that 2(2+) is best described as [Cl(Q(center dot-))Ru-III(mu-tppz(o))Ru-III(Q(center dot-))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,Ru-III-bonded quinones, implying redox-induced electron transfer. Reduction takes place stepwise at the metal centers yielding iminosemiquinone complexes of Ru-III,Ru-II as evident from radical complex EPR spectra with small Ru-99,Ru-101 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(center dot-))Ru-II(mu-tppz(center dot-))Ru-II(Q(center dot-))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.