Chromium Nitride Umpolung Tuned by the Locus of Oxidation.

Oxidation of a series of Cr nitride salen complexes () with different -phenolate substituents (R = CF, tBu, NMe) was investigated to determine how the locus of oxidation (either metal or ligand) dictates reactivity at the nitride. -phenolate substituents were chosen to provide maximum variation in the electron-donating ability of the tetradentate ligand at a site remote from the metal coordination sphere. We show that one-electron oxidation affords Cr nitrides (; R = CF, tBu) and a localized Cr nitride phenoxyl radical for the more electron-donating NMe substituent (). The facile nitride homocoupling observed for the Mn analogues was significantly attenuated for the Cr complexes due to a smaller increase in nitride character in the M≡N π* orbitals for Cr relative to Mn. Upon oxidation, both the calculated nitride natural population analysis (NPA) charge and energy of molecular orbitals associated with the {Cr≡N} unit change to a lesser extent for the Cr ligand radical derivative () in comparison to the Cr derivatives (; R = CF, tBu). As a result, reacts with B(CF), thus exhibiting similar nucleophilic reactivity to the neutral Cr nitride derivatives. In contrast, the Cr derivatives (; R = CF, tBu) act as electrophiles, displaying facile reactivity with PPh and no reaction with B(CF). Thus, while oxidation to the ligand radical does not change the reactivity profile, metal-based oxidation to Cr results in umpolung, a switch from nucleophilic to electrophilic reactivity at the terminal nitride.