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Electronic Structure of Aqueous Co[bpy3] Electron Mediators

We report on the electronic structure of cobalt (II) tris-2,2’-bipyridine and cobalt (III) tris-2,2’-bipyridine in aqueous solution using resonant inelastic X-ray scattering (RIXS) spectroscopy at the Co L-edge and N K-edge resonances. Partial fluorescence yield X-ray absorption spectra at both edges were obtained by signal integration of the respective RIXS spectra. Experiments are complemented by calculations of the Xray absorption spectra for high and low spin configurations using density functional theory/restricted open shell configuration interaction singles and time-dependent density functional theory methods. We find that linear combinations of the simulated X-ray absorption spectra for different spin states reproduce the experimental spectra. Best agreement is obtained for measurements at the Co L-edge, for both samples. For cobalt (II) tris-2,2’-bipyridine our combined experimental and computational study reveals ~40% low-spin and ~60% high-spin state components. Much stronger low-spin character is found for cobalt (III) tris-2,2’bipyridine, ~80% low spin and ~20% high spin. Prominent energy-loss features in the Co RIXS spectra are indicative of d–d excitations and charge-transfer excitations due to strong mixing between metal and ligand orbitals in both complexes. Analysis of N 1s RIXS data reveals the emission from metal dominated orbitals in the valence region, supporting the strong metal–ligand mixing.