Stanford University, PULSE Institute for Ultrafast Energy Science

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.[i]  The redox activity of the ligands can affect 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.[ii]  For coordination complexes with redox active ligands, characterizing the charge distribution at each metal and ligand component proves critical.  Here, we make use of the atomic selectivity of x-ray spectroscopy to track changes in electronic structure due to valence tautomeric transitions in [Co^III(3,5-DBSQ)(3,5-DBCat)(NˆN)], where 3,5-DBSQ = 3,5-di-tert-butyl-1,2-benzosemiquinone, 3,5-DBCat = 3,5-di-tert-butyl-1,2-benzocatecholate, and NˆN corresponds to a variety of polypyridyl ligands.

In the poster session, we will present our most recent Kβ x-ray emission spectra (XES), and compare that with the bulk-sensitive variable-temperature magnetic susceptibility measurements, on three cobalt-dioxolene complexes in the solid state.  We will also talk about our ongoing effort in utilizing soft x-ray absorption spectroscopy (XAS) at the N and O K-edges and the Co L-edge to track temperature-dependent changes in the charge distribution and metal-ligand covalency.  A next step in this project is to use resonant inelastic x-ray scattering (RIXS) spectroscopy at the Co K pre-edge, in combination with density functional theory (DFT) calculations, to characterize the ligand-to-metal charge transfer (LMCT) excitations of the complex.