Influence of Oxidation State on Bonding Properties of [CeIII (PhC(Nt Bu) 2) 3] and [CeIV (PhC(Nt Bu) 2) 3] + [Al(OC4F9) 4] - Probed by High-Resolution X-Ray Absorption

Recently, the assumption that lanthanide bonds are completely ionic has been under scrutiny, especially regarding tetravalent species like Ce(IV), where meaningful covalent contributions to bonding were found.[1] Both oxidation state and ligand environment play an important role in tuning bond covalency in lanthanide complexes. Advanced high-resolution X-ray spectroscopic methods can be used to gain insight into such complex bonding situations, as previously shown in actinide research.[2] Here, a range of such methods is implemented to take a detailed look at the bonding properties of a pair of Cerium complexes, (CeIII(PhC(NtBu)2)3 & CeIV(PhC(NtBu)2)3][Al(OC4F9)4), which offer the opportunity to directly study the influence of the oxidation state via an identical ligand system around the Cerium central atom.
Ce L3-edge high resolution X-ray absorption near edge spectra (HR-XANES) show differences that go beyond a simple energy shift for the oxidation state, including the familiar double white line indicative of a mixed-valence ground state in CeIV compounds.[3] Pre-edge features corresponding to transitions into spin-forbidden 4f-dominated molecular orbitals are evaluated and findings are compared to finite difference method near edge structure (FDMNES) calculations based on theoretically optimized structure. ... mehrAnalysis of the valence-band resonant inelastic X-ray scattering spectra (VB-RIXS) is used to quantify and compare the contributions of 5d electron density of the Cerium atoms to the bond. Radiation-induced changes in both types of spectra are used to gain valuable insight into the way bonds evolve upon gradual change of the oxidation state in the bulk material. Insight goes beyond the expected change in bond length – to changes in contributing 5d electron density and electronic structure. Soft X-ray-based absorption spectroscopy is used to verify results pertaining to the Cerium central atom at the Ce M4,5-edge. The bonds are further explored via the N K-edge to gain insight and discuss covalent contributions to bonding from the ligand point of view.
The authors gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through the Collaborative Research Centre ″4f for Future” project A1 (CRC 1573, project number 471424360). We thank the Institute for Beam Physics and Technology (IBPT) for the operation of the storage ring, the Karlsruhe Research Accelerator (KARA). We thank the KIT Light Source for provision of beamtime.
References
[1] aT. Vitova, P. W. Roesky, S. Dehnen, Communications Chemistry 2022, 5, 12; bS. G. Minasian, E. R. Batista, C. H. Booth, D. L. Clark, J. M. Keith, S. A. Kozimor, W. W. Lukens, R. L. Martin, D. K. Shuh, S. C. E. Stieber, T. Tylisczcak, X.-d. Wen, Journal of the American Chemical Society 2017, 139, 18052-18064.
[2] T. Vitova, I. Pidchenko, D. Fellhauer, P. S. Bagus, Y. Joly, T. Pruessmann, S. Bahl, E. Gonzalez-Robles, J. Rothe, M. Altmaier, M. A. Denecke, H. Geckeis, Nature Communications 2017, 8, 16053.
[3] K. O. Kvashnina, Chemistry – A European Journal 2024, 30, e202400755.