Synthesis, spectral and redox properties of tetraammine dioxolene ruthenium complexes

A series of species [Ru-III(NH3)(4)(Cat-R)](n+) have been synthesized where Cat-R is a catecholate dianion having the substituent R=CO2-, CO2H, OMe or H. These so-called parent species were characterized by their electronic s pectra, FTIR, mass spectrum, cyclic voltammetry and EPR. Controlled potenti al reduction yields [Ru-II(NH3)(4)(Cat-R)]((n-1)+) while controlled potenti al oxidation yields [Ru-II(NH3)(4)(Q-R)]((n+1)+) (Q-R=substituted quinone). Density Functional Theory (DFT) was primarily used to explore the electron ic structures of these complexes. Application of the INDO semi-empirical mo del proved less useful. Time dependent density functional response theory w as used to calculate the electronic spectra of the species with R=H. The el ectronic spectra of the closed shell species are well reproduced by the cal culations. The physical properties of these complexes indicate a charge del ocalized system reminiscent of a delocalized organic molecule. The simple v alence descriptions noted above are convenient to use but do not reflect th e actual electronic structure. The electronic spectra of the parent species are temperature dependent. The visible region charge transfer band shifts by about 1500 cm(-1) to higher energy in acidic media at liquid nitrogen te mperature. This is interpreted in terms of solvent effects rather than vale nce tautomerism. The electrochemical properties of [Ru-III(NH3)(4)(Cat-R)], in aqueous solution, reveal the first example of a reversible and stable R u-quinone species in that medium. The pK(a) values for several dioxolene sp ecies, with R=CO2-, are derived from a Pourbaix diagram.