Convergence of spectroscopic and kinetic electron transfer parameters for mixed-valence binuclear dipyridylamide ruthenium ammine complexes

A series of binuclear ruthenium(II,III) pentaammine complexes bridged by 4-pyridyl isonicotinamide (iso-apy) and methyl,4-pyridyl isonicotinamide (iso-mapy), and their mononuclear congeners, were studied by spectroscopic and kinetic techniques. The amide functionality provides asymmetry between the electronic environments of the metal ions bound to the aminopyridine (apy) and pyridine carbonyl (iso) ends. The resulting difference is observed in the charge transfer spectra and the electrochemical properties of the mononuclear and binuclear complexes. The mixed-valence binuclear ruthenium(II,III) complexes exhibit bands in the NIR region assigned to intervalence charge transfer transitions between the metal centers (MMCT). The MMCT band for [(NH3)5Ru-iso-apy-Ru(NH3)5]5+ has λmax = 809 nm and ɛmax = 50 M−1 cm−1 and [(NH3)5Ru-iso-mapy-Ru(NH3)5]5+ has λmax = 743 nm and ɛmax = 20 M−1 cm−1. Methylation of the amide nitrogen increases the energy of the MMCT transition while decreasing the electronic coupling between the two metal centers. The electronic coupling constants (HMM′) for the mixed-valence complexes were evaluated from the metal-to-ligand and metal-to-metal charge transfer spectra using both Hush's model and the CNS method. The spectroscopy shows that electronic coupling is more efficient when the bridging ligand can adopt a more planar configuration. The kinetics of the spontaneous (thermal) intramolecular ET reactions in the binuclear ruthenium pentaammine systems of iso-mapy and iso-apy were studied using pulse radiolysis transient absorption spectroscopy. An ET rate constant of 2.7 × 106 s−1 was obtained for [(NH3)5Ru-iso-mapy-Ru(NH3)5]5+, while only a lower limit for the thermal reaction rate constant could be obtained for [(NH3)5Ru-iso-apy-Ru(NH3)5]5+. The estimated ET rate constants calculated by Hush's model are slightly faster than those directly obtained through kinetic measurements.