Electronic Structure of Aqueous [Co(bpy)₃]^2+/3+ Electron Mediators

Abstract

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 Ledge 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 Ledge , 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.

Dynamic Stereoselection of Peptide Helicates and Their Selective Labeling of DNA Replication Foci in Cells**

“…For this reason, due to 15 N T 1 long relaxation times, the 15 N bipyridine resonances could not be observed in the 15 N‐ 1 H HMBC spectrum (data not shown). Based on the NMR experimental data, [29] and considering the similar coordination geometries of the Co III and Fe II trisbipyridyl complexes, we generated two reasonable starting point models of the Fe II isomers A and B , and assessed their stability by Molecular Dynamics (MD) simulations in explicit solvent and periodic boundary conditions (See the Supporting Information for details). During the simulations, the helical twist of the helicates, as well as the octahedral coordination geometry of the Fe II metal centers with the Bpy units were conserved.…”

Revealing the nature of electron correlation in transition metal complexes with symmetry breaking and chemical intuition

“…On the one hand, X-ray absorption spectra have been wellreproduced by linear combinations of simulated spectra from the LS (doublet) and HS (quartet) states, leading to the claim that Co(bpy) 2+ 3 is 57% HS and 43% LS, whereas Co(bpy) 3+ 3 is relatively more monoconfigurational (∼ 80% LS). 124 Taking these distributions and assuming the spectra were taken at room temperature implies spin gaps of roughly 0.01 and 0.03 eV for the Co(II) and Co(III) species, respectively, which indeed would imply SC. On the other hand, that the bpy ligand is between NH 3 and CO on the spectrochemical series suggests that the 10Dq should be relatively large due to π-backbonding, and therefore SSB is likely to be restored with a suitable level of dynamical correlation (and indeed it is with B3LYP).…”

Dynamic Stereoselection of Peptide Helicates and Their Selective Labeling of DNA Replication Foci in Cells**

“…Fort his reason, due to 15 NT 1 long relaxation times,t he 15 Nb ipyridine resonances could not be observed in the 15 N-1 HH MBC spectrum (data not shown). Based on the NMR experimental data, [29] and considering the similar coordination geometries of the Co III and Fe II trisbipyridyl complexes,w eg enerated two reasonable starting point models of the Fe II isomers A and B,a nd assessed their stability by Molecular Dynamics (MD) simulations in explicit solvent and periodic boundary conditions (See the Supporting Information for details). During the simulations,t he helical twist of the helicates,a swell as the octahedral coordination geometry of the Fe II metal centers with the Bpy units were conserved.…”