X-Ray Spectroscopic and Computational Methods in Elucidating Transition Metal Oxidation States, Identifying Ligand Redox Non-Innocence, and Quantifying Metal-Ligand Covalency

Abstract

A comprehensive suite of X-ray absorption spectroscopies supported by computational predictions have been used to characterize the metal-ligand bonding in a variety of transition metal species. By quantifying Cu covalency through Cu L2,3-edge spectroscopy, complexes containing the formal Cu3+ oxidation state have been shown to contain predominantly ligand-centered frontier molecular orbitals, representing a physically more reduced Cu center and electrophilic ligand character, borne out in the reactivities observed. Cu K-edge XAS was similarly shown to not be diagnostic in determining the Cu3+ oxidation state. Cu 3d covalency values calculated by density functional theory, shown to well reproduce those determined experimentally, were then used to survey the validity of other Cu3+ assignments, which revealed the prevalence of ligand field inversion in high-valent transition metal complexes. By then directly probing the ligand using nitrogen K-edge X-ray absorption spectroscopy, quantification of N 2p participation in a series of bipyridine, ethylenediamine, ammine, and nitride complexes afforded a means of estimating nitrogen admixture in unfilled frontier molecular orbitals. Using the calibration developed, features directly attributable to a Ni-coordinated aminyl radical are observed as characteristic of ligand-based oxidation. Further, application of these methods was able to characterize the elusive alkyl iminyl intermediate implicated in Fe-catalyzed C−H bond functionalization reactions. The direct comparison of the series of aryl vs alkyl imido and iminyl redox isomers revealed that delocalization of radical nitrene spin-density throughout an adjacent aryl substituent results in lower C-H bond amination efficiencies. The electronic structure of dicopper nitrene species was next examined, detailing that upon molecular reduction, redistribution of electron density away from the bridging nitrene fragment further reduces the copper centers, resulting in a complex that contains radical nitrene character. However, delocalization of electron density away from the N 2p center again leads to a lower proclivity towards group transfer reactivity. Lastly, transition-metal fluorine bonding is explored by means of F K-edge X-ray absorption spectroscopy, using computationally-calibrated energies and covalencies to hypothesize for reactivity consequences.