|Investigation of Valence Tautomerism in Cobalt-Dioxolene Complexes Using Kβ X-ray Emission Spectroscopy|
|Full Author List||Winnie Liang, Dennis Nordlund, Tsu-Chien Weng, Dimosthenis Sokaras, Cortlandt Pierpont, Kelly Gaffney|
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 [CoIII(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.
[i] (a) Moss, J. R., J. Mol. Cat. A: Chem., 1996, 107, 169-174. (b) Costas, M., et al., Chem. Rev., 2004, 104, 939-986. (c) Johannes, H. et al., Nature communications, 2012, 3, 720.
[ii] (a) Allan, L. E. et al., Inorg. Chem., 2007, 46, 8963-8970. (b) Buchachenko, A. L., Pure Appl. Chem., 2000, 72, 2243-2258. (c) Swart, M., Int. J. Quant. Chem., 2013, 113, 2-7.