Synthesis and Characterization of a Ruthenium-edta Dimer by Spectral and Electrochemical Techniques

Section I The synthesis of a ruthenium-edta dimer is described. The dimer is shown to be the one-electron oxidized product of two [Ru(III)edta(OH2)]- monomers. The dimer was shown to contain one unpaired spin by magnetic susceptibility and esr spectroscopy indicating spin pairing between the two ruthenium metal centers. A band was detected in the near infrared that analyzed for a Class III complex indicating that the dimer is truly delocalized and that the oxidation state on each ruthenium is 3.5. The band was much narrower than predicted for a delocalized, Class II complex and its position did not exhibit any solvent dependence. The optical spectrum indicated some similarity with other formally Ru(III)-Ru(IV) dimers indicating that the formation of this type of dimer is favored regardless of the type of auxiliary ligands attached to the ruthenium. The change in the optical spectrum upon raising the pH indicated a pKa of 10.32 for the dimer. This, taken in conjunction with the infrared, the raman and the nmr results indicated that the dimer was µ-oxo bridged at low pH and was, most likely, µ-oxo, µ-hydroxo bridged at higher pH. When an excess of Ce(IV) was added to a solution of the dimer the catalytic evolution of dioxygen was observed. The dimer also catalyzed the disproportionation of hydrogen peroxide. Section II describes the electrochemistry of Ru(III)edta and the dimer. Cyclic voltammetry, polarography and rotating ring-disk electrode (RRDE) voltammetry were used to characterize the oxidation and reduction of each of the two complexes. The electrochemical oxidation of Ru3Y produced the dimer after one Faraday per two moles of ruthenium. The catalytic evolution of dioxygen was also observed at a platinum electrode. The step-wise reduction of the dimer to produce Ru(II)edta is also described. The combined use of the RRDE with digital simulations of the mechanism of reduction yielded the rate constants for the rate of breakup of the intermediates. A spectro-electrochemical experiment also showed that the dimer can be oxidized further to produce a Ru(IV)-Ru(IV) dimer.