Quantifying the Steric Effect on Metal–Ligand Bonding in Fe Carbene Photosensitizers with Fe 2p3d Resonant Inelastic X-ray Scattering
Kristjan Kunnus, Meiyuan Guo, Elisa Biasin, Christopher B. Larsen, Charles J. Titus, Sang‐Jun Lee, Dennis Nordlund, Amy A. Cordones, Jens Uhlig, Kelly J. Gaffney
Abstract
Understanding the electronic structure and chemical bonding of transition metal complexes is important for improving the function of molecular photosensitizers and catalysts. We have utilized X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L3 edge to investigate the electronic structure of two Fe N-heterocyclic carbene complexes with similar chemical structures but different steric effects and contrasting excited-state dynamics: [Fe(bmip)2]2+ and [Fe(btbip)2]2+, bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)pyridine and btbip = 2,6-bis(3-tert-butyl-imidazole-1-ylidene)pyridine. In combination with charge transfer multiplet and ab initio calculations, we quantified how changes in Fe–carbene bond length due to steric effects modify the metal–ligand bonding, including σ/π donation and π back-donation. We find that σ donation is significantly stronger in [Fe(bmip)2]2+, whereas the π back-donation is similar in both complexes. The resulting stronger ligand field and nephelauxetic effect in [Fe(bmip)2]2+ lead to approximately 1 eV destabilization of the quintet metal-centered 5T2g excited state compared to [Fe(btbip)2]2+, providing an explanation for the absence of a photoinduced 5T2g population and a longer metal-to-ligand charge-transfer excited-state lifetime in [Fe(bmip)2]2+. This work demonstrates how combined modeling of XAS and RIXS spectra can be utilized to understand the electronic structure of transition metal complexes governed by correlated electrons and donation/back-donation interactions.