Volume 8, Issue 3-4

We here report on triarylamine-derived styryl ruthenium complexes 1 – 3 where one ( 1 ), two ( 2 ) or three ( 3 ) vinyl ruthenium moieties are appended to a triphenylamine core. The near equivalency of the styryl ruthenium and the triarylamine redox systems leads to strong interactions between these moieties and strong mixing of the respective frontier orbitals. This results, inter alia , in the observation of two to four consecutive, reversible one-electron redox couples with potential splittings of 185–435 mV. The associated radical cations and higher oxidized forms show strong absorptions whose positions vary from deep in the near-infrared (NIR) to the border region between the Vis and NIR regimes as a function of the oxidation state. Extinction coefficients and oscillator strengths reach rather impressive values of up to 90 000 l mol -1  cm -1 and ≥1.0. Complexes 1 – 3 thus constitute polyelectrochromic dyes with two to three addressable and distinguishable states that can be reversibly interconverted by application of an appropriate potential. The electronic transitions underlying the intense low energy absorptions are assigned with the aid of time dependent-density functional theory (TD-DFT) and involve strongly delocalized molecular orbitals (MOs). Charge and spin delocalization in the (radical) cations are probed by electron paramagnetic resonance spectroscopy (EPR) and infrared (IR) spectroelectrochemistry.

In this work, electrochemical studies on the seven-coordinate iron complex (Fe III/II dapsox), a functional model for the superoxide dismutase enzyme (SOD mimetic), were performed in an aqueous solution over the pH range 1–12 (with and without buffers) to conceive the underlying heterogeneous proton-coupled electron transfer (PCET) processes of various complex species. The thermodynamics of the corresponding proton transfer, electron transfer, and PCET processes have been evaluated as well as the kinetics of related heterogeneous electrode reactions of all complex species in solution. Measurements in D 2 O have also been carried out for selected redox couples to assess a possible kinetic isotope effect (KIE). In the case of a two-proton one-electron transfer process in the absence of a buffer, the KIE was found to be 4.8, whereas in the case of a one-proton one-electron transfer process in buffered solutions, an inverse KIE was observed. The overall data suggest the operation of a concerted PCET mechanism for the two-proton one-electron transfer process, whereas involvement of an H-bonded precursor adduct formed between the buffer components, and the complex species within a one-proton one-electron transfer mechanism is feasible. These results shed light on both the enzymatic and the mimetic SOD mechanisms, speaking in favor of the two-proton one-electron transfer process as a desirable mechanism for efficient catalytic superoxide dismutation.