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

Abstract

Nitrogen K-edge X-ray absorption spectra (XAS) were obtained for 19 transition metal complexes bearing bipyridine, ethylenediamine, ammine, and nitride ligands. Time-dependent density functional theory (TDDFT) and DFT/restricted open configuration interaction singles (DFT/ROCIS) calculations were found to predict relative N K-edge XAS peak energies with good fidelity to experiment. The average difference (|ΔE|) between experimental and linear corrected calculated energies were found to be 0.55 ± 0.05 eV and 0.46 ± 0.04 eV, respectively, using the B3LYP hybrid density functional and scalar relativistically recontracted ZORA-def2-TZVP(-f) basis set. Deconvolution of these global correlations into individual N-donor ligand classes gave improved agreement between experiment and theory with |ΔE| less than 0.4 eV for all ligand classes in the case of DFT/ROCIS. In addition, calibration method-dependent values for the N 1s → 2p radial dipole integral of 25.4 ± 1.7 and 26.8 ± 1.9 are obtained, affording means to estimate the nitrogen 2p character in unfilled frontier molecular orbitals. For the complexes studied, nitrogen covalency values correlate well to those calculated by hybrid DFT with an R² = 0.92 ± 0.01. Additionally, as a test case, a well-characterized PNP ligand framework (PNP = N[2-P(CHMe₂)₂-4-methylphenyl]₂¹⁻) coordinated to Ni^II is investigated for its ability to act as a redox non-innocent ligand. Upon oxidation of (PNP)NiCl with [FeCp₂](OTf) to its radical cation, [(PNP)NiCl](OTf) (OTf = triflate), a new low-energy feature emerges in the N K-edge XAS spectra. This feature is assigned as N 1s to a PNP-localized acceptor orbital exhibiting 27 ± 2% N 2p aminyl radical character, obtained using the aforementioned nitrogen covalency calibration. Combined, these data showcase a direct spectroscopic means of identifying redox-active N-donor ligands and also estimating nitrogen 2p covalency of frontier molecular orbitals in transition metal complexes.