Electron delocalization in mixed-valence butadienediyl-bridged diruthenium complexes

We report electrochemical and spectroelectrochemical investigations on the butadienediyl-bridged diruthenium complexes [{Ru(PPh3h(CO)CI} 2(Il-C4H4)] (1), [{Ru(PEt3h(CO)Clh(~l-C4H4)] (2), and [{Ru(PPh3h (CO)CI(NCsH4COOEt-4) h(p-C4H4)] (3). All these complexes are oxidized in two consecutive one-electron steps separated by 315 to 680 mV, depending on the coligands. The first oxidation is a chemically and electrochemically reversible process whereas the second va ries from nearly reversible to irreversible at room temperature. We have genera ted and investigated the mixedvalence monocations and observed CO band shifts of ca 25 cm I and the appea rance of new bands in the visible regime at ca 720 to 800 and 430 to 450 nm. The lowerenergy band which tails into the near infrared has been assigned as a charge-resonance (or intervalence chargetransfer) absorption and used to estimate the electronic coupling parameter HAB' Our investigations point to valence delocalization for 2 +, and nearly delocalized behavior for 1 + and 3 +. Even the complex with the smallest potential splitting is, however, fully delocalized on the longer ESR timescale, as is evident from the coupling pattern of the solu tion spectrum. Overall IR band shifts on full oxidation and the hyperfine splittings for 1+ argue for charge and spin delocalization onto the Presented at the 3rd Chianti Electrochemistry Meeting, July 3.9.2004, Certosa di Pontignano, Italy J. Maurer' B. Sarkar Inst itut für Anorganische Chem ie, Universitiit Stuttgart, Pfaffenwaldring 55, 70569 Stutlgart, Germany S. Zä li s J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, Prague, Czech Republic Presenl address: R. F. Winter ((g) Inst itut für Anorganische Chemie, U niversitiit Regensburg, Universitiitsstraße 31, 93040 Regensburg, Germany E-mai l: [email protected] Fax: + 49941 9434488 bridging C4H4 ligand . This issue has a lso been addressed by quantum chemical calculations employing DFT methods. Geometry optimizations at each oxidation level reveal inversion of the CC bond pattern from a shortIongshort to a longshortIong alteration and a bis(carbenic) structure at the dication stage. All spectroscopic features such as IR band shifts, average gva lues and g-tensor anisotropies are fully reproduced by the calculations.