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(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,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(mu-tppz)Ru(Q)Cl](PF(6))(2), 2(PF(6))(2), the only configuration obtained. The combined investigations reveal that 2(2+) is best described as [Cl(Q(*-))Ru(III)(mu-tppz(o))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 (99,101)Ru 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)(mu-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.

Bridge‐Localized HOMO‐Binding Character of Divinylanthracene‐Bridged Dinuclear Ruthenium Carbonyl Complexes: Spectroscopic, Spectroelectrochemical, and Computational Studies

“…EPR spectroscopy was employed to analyze the nature of the singly occupied molecular orbitals (SOMOs) in the singly oxidized complexes,17, 22 which is propitious to understanding the intrinsic properties of the mixed‐valence states. Previous experimental results have indicated that, for a metal‐dominated contribution to the SOMO, the radical cations displayed a higher isotropic g value 17.…”

Dinuclear Palladium Complexes with Two Ligand‐Centered Radicals and a Single Bridging Ligand: Subtle Tuning of Magnetic Properties

“…Strikingly, the synthesis of dinuclear complexes with one redox-active ligand on each metal is ill-explored and hence little is known about magnetic exchange phenomena for such species. [9] Especially in case of only as ingle bridging ligand, subtle variations potentially influence the overall magnetic properties of the system. Depending on the relative orientation and/ord istance between the two spin carriers, diradicals can exist in at riplet (ferromagnetic coupling) or singlet (antiferromagnetic coupling) ground state due to spin-exchange coupling (J).…”

“…This strategy would permit the use of redox‐active metals and metalloids, which greatly widens the synthetic scope compared to systems where the diradical character is purely metal‐based. Strikingly, the synthesis of dinuclear complexes with one redox‐active ligand on each metal is ill‐explored and hence little is known about magnetic exchange phenomena for such species 9…”

Mixed valency in ligand-bridged diruthenium frameworks: divergences and perspectives

“…Structural, spectroscopic and magnetic studies showed that the uncoupled semiquinone ancillary ligands stabilized the tppz-bridged diruthenium( ii ) complex 10 n ( n = 0) in the triplet ( S = 1, J = −3.07 cm −1 ) ground state. 51 Stepwise oxidations (O1, O2) and reductions (R1, R2) of complex 10 yielded the calculated K c values of 10 2 and 10 4 , respectively, for the intermediate states ( Fig. 10 ).…”