Untangling ancillary ligand donation versus locus of oxidation effects on metal nitride reactivity

Abstract

We detail the relative role of ancillary ligand electron-donating ability in comparison to the locus of oxidation (either metal or ligand) on the electrophilic reactivity of a series of oxidized Mn salen nitride complexes. The electron-donating ability of the ancillary salen ligand was tuned via the para-phenolate substituent (R = CF₃, H, tBu, OⁱPr, NMe₂, NEt₂) in order to have minimal effect on the geometry at the metal center. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we have demonstrated that metal-based oxidation to [Mn^VI(Sal^R)N]⁺ occurs for R = CF₃, H, tBu, OⁱPr, while ligand radical formation to [Mn^V(Sal^R)N]⁺˙ occurs with the more electron-donating substituents R = NMe₂, NEt₂. We next investigated the reactivity of the electrophilic nitride with triarylphosphines to form a Mn^IV phosphoraneiminato adduct and determined that the rate of reaction decreases as the electron-donating ability of the salen para-phenolate substituent is increased. Using a Hammett plot, we find a break in the Hammett relation between R = OⁱPr and R = NMe₂, without a change in mechanism, consistent with the locus of oxidation exhibiting a dominant effect on nitride reactivity, and not the overall donating ability of the ancillary salen ligand. This work differentiates between the subtle and interconnected effects of ancillary ligand electron-donating ability, and locus of oxidation, on electrophilic nitride reactivity.