Investigation of the electronic structures and chemical properties of cobalt T bis (o-dioxolene) valence tautomers using X-ray spectroscopies, magnetic susceptibility and theoretical calculations [electronic resource]
Keywords: Chemistry.

Redox active ligands have been invoked as a critical component of molecular catalysts utilizing 3d transition metal centers. Many of these metal complexes are known to catalyze multi-electron reactions or provide models for catalytic and enzymatic transformations. The redox activity of the ligands also increases spin multiplicity and presents an opportunity for spin state transitions to occur during a catalytic cycle, the importance of which remains unclear in 3d transition metal catalysis. For coordination complexes with redox active ligands, characterizing the spatial dependence of the electronic structure proves critical. In my thesis, I have utilized the atomic selectivity of X-ray spectroscopy to characterize changes in the spatial dependence of the electronic structure due to valence tautomeric transitions in [Co(3,5-DBQ)2(NˆN)], where 3,5-DBQ is either the semiquinonate (SQ-) or catecholate (Cat2-) form of 3,5-di-tert-butyl-1,2-benzoquinone, and NˆN corresponds to a variety of polypyridyl ligands such as 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), 5-nitro-1,10-phenanthroline (NO2-phen), 5-amino-1,10-phenanthroline (NH2-phen), N, N, N', N'-tetramethylmethylenediamine (tmmda), N, N, N', N'-tetramethylethylenediamine (tmeda), and N, N, N', N'-tetramethylpropylenediamine (tmpda). Specifically, I have made use of the sensitivity of 1s-3p X-ray emission spectroscopy (퐾b XES) to the oxidation and spin states of 3d transition metal ions to determine the cobalt-specific electronic structure of valence tautomers. A comparison of their 퐾b XES spectra with the spectra of cobalt coordination complexes with known oxidation and spin states demonstrates that the low temperature valence tautomer can be described as a low-spin Co(III) configuration and the high-temperature valence tautomer as a high-spin Co(II) configuration. This conclusion is further supported by my Co L-edge X-ray absorption spectroscopy (L-edge XAS) of the high-temperature valence tautomers and ligand-field atomic-multiplet calculations of the 퐾b XES and L-edge XAS spectra. The nature and strength of the magnetic exchange interaction between the cobalt center and SQ- in cobalt valence tautomers is discussed in view of the effective spin at the Co site from 퐾b XES and the molecular spin moment from magnetic susceptibility measurements. During my work, I have also collected evidence suggesting the presence of soft X-ray induced valence tautomerism and the possibility of hard X-ray induced valence tautomerism in these cobalt compounds. Finally, a qualitative comparison of the 1s-3p Resonant Inelastic X-ray Scattering (1s-3p RIXS) planes of Co(tmeda)(3,5-DBSQ)2 and Co(phen)(3,5-DBSQ)2 implicates the presence of metal-to-ligand charge transfer character in the ground state of Co complexes coordinated to aromatic nitrogen coligands—a conjecture that has since been reproduced and verified by charge-transfer multiplet calculations. Key words: valence tautomerism, magnetic exchange interaction, intramolecular electron transfer, spin crossover, 1s-3p X-ray Emission Spectroscopy (퐾b XES), L-edge X-ray Absorption Spectroscopy (L-edge XAS), ligand-field atomic-multiplet calculations (LFM), magnetic susceptibility, dioxolene (o-benzoquinone, semiquinone, catecholate), soft X-ray induced valence tautomerism, two-state thermodynamics, nitrogen coligand effects, K-edge X-ray Absorption Spectroscopy (K-edge XAS), 1s-3p resonance inelastic X-ray scattering (1s-3p RIXS), density functional theory, metal-ligand covalency.