Electronic Effects of Para-phenolate Substitution in an Extended Series of Neutral and Oxidized Chromium Salen Nitrides

Abstract

High-valent transition-metal nitrides have recently emerged as versatile platforms for N-atom transfer, and their reactivity remains sensitive to subtle electronic perturbations. Cr salen (where "salen" represents N2O2 bis-phenolate bis-Schiff-base ligands) nitrides offer a rare platform in which both the metal center and the redox-active salen ligand are both susceptible to one-electron oxidation, enabling systematic evaluation of how salen ligand substituent effects can change the electronic structure, and ultimately nitride reactivity. Herein, we evaluate a series of CrNSal R complexes to better understand how changing the ancillary ligand donating ability via the para-phenolate substituent (R = NO2, CF3, H, tBu, OMe, OiPr, NMe2) dictates overall electronic structure. For electron-donating R = OMe and R = OiPr, designed to probe the threshold for switching the oxidation locus from metal to ligand, one-electron oxidation results in a metal-centered Cr(VI) nitride, extending the window for metal-based oxidation beyond the previously established R = tBu derivative. In contrast, the R = NO₂-substituted analogue, while not promoting bimolecular nitride coupling as observed in Mn congeners, nonetheless renders the Cr≡N unit more electrophilic than previously reported R = CF₃-substituted systems. Together, these results expand the electronic landscape of Cr-salen-nitrides and demonstrate how fine-tuning the donating ability of the ancillary salen ligand can be used to systematically manipulate the electronic structure at the nitride.