Scrutinizing metal–ligand covalency and redox non-innocence via nitrogen K-edge X-ray absorption spectroscopy

Abstract

A series of nitrogen K-edge XAS data obtained for coordination complexes of diverse transition metals is used to calibrate computational pre-edge peak energies and to afford estimates of metal–ligand covalencies. The approach is extended to probe an inner-sphere aminyl radical ligand.

Direct Comparison of C–H Bond Amination Efficacy through Manipulation of Nitrogen-Valence Centered Redox: Imido versus Iminyl

“…In a N K-edge XAS calibration study of PNP-pincer supported Ni complexes, a similar low energy pre-edge feature was identified upon oxidation of the backbone amide to an aminyl donor. 46 This new pre-edge feature present at 396.4 eV was assigned, through corroboration with TDDFT calculations, as a transition from N 1s into the nitrogen localized aminyl-based SOMO on the PNP ligand framework. Hence, we assign the pre-edge features observed here at 394.5 and 394.8 eV as transitions into the nitrogen-localized iminyl radicals present in 6 and 2 , respectively.…”

Synthesis, characterization and C–H amination reactivity of nickel iminyl complexes

“…6b, pre-edge features at 398.0 and 400.9 eV are observed for all the complexes surveyed (2-5) that can be assigned as transitions into high-lying antibonding molecular orbitals of N 2p parentage. 40,47 Two low-lying pre-edge absorptions are present for both 3 and 4 at 395.4 eV, accompanied by a smaller side-band at 396.3 eV. These peak energies are consistent with those encountered for the redoxactive N-donor ligands in Fe-iminyl (394.5 and 394.8 eV), 40 Niaminyl (397.0 eV), 47 and Cu-nitrene 48 (doublet with peaks at 395.3 and 395.9 eV) complexes.…”

“…40,47 Two low-lying pre-edge absorptions are present for both 3 and 4 at 395.4 eV, accompanied by a smaller side-band at 396.3 eV. These peak energies are consistent with those encountered for the redoxactive N-donor ligands in Fe-iminyl (394.5 and 394.8 eV), 40 Niaminyl (397.0 eV), 47 and Cu-nitrene 48 (doublet with peaks at 395.3 and 395.9 eV) complexes. This tentative assignment of these transitions as N 1s into partially occupied, N-localized orbitals present in 3 and 4, respectively is further supported by time-dependent density functional theory (TDDFT) calculations (vide infra).…”

“…To afford some justication for selecting between BP86 and B3LYP results, TDDFT calculations 47,56,57 to model the N K-edge XAS obtained for 3 and 4 were carried out starting from either unrestricted single point solution. Spectra calculated using BP86 gave insufficient resolved peak structure to reliably correlate to the experimental data ( Fig.…”

The duality of electron localization and covalency in lanthanide and actinide metallocenes

“…Through a combination of ligand K-edge XAS and time-dependent density functional theory (TDDFT) calculations, Solomon and coworkers provided the rst demonstrations of this approach as a direct and quantitative probe of electronic structure and covalent bonding in transition metal complexes with M-Cl and M-S bonds. [65][66][67][68] Recently, the range of ligand chemistries was expanded to include systems incorporating M-O, [69][70][71] M-N, [72][73][74] and M-C bonds. 75,76 By acquiring XAS using a scanning transmission X-ray microscope (STXM), we are able to control saturation effects and overcome other challenges with photon attenuation that can preclude measurements with weakly penetrating incident radiation using so X-rays.…”