Chromium Nitride Umpolung Tuned by the Locus of Oxidation.

Oxidation of a series of Cr V nitride salen complexes ( Cr V NSal R ) with different para -phenolate substituents (R = CF 3 , tBu, NMe 2 ) was investigated to determine how the locus of oxidation (either metal or ligand) dictates reactivity at the nitride. Para -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 VI nitrides ( [Cr VI NSal R ] + ; R = CF 3 , tBu) and a localized Cr V nitride phenoxyl radical for the more electron-donating NMe 2 substituent ( [Cr V NSal NMe2 ] •+ ). The facile nitride homocoupling observed for the Mn VI analogues was significantly attenuated for the Cr VI 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 V ligand radical derivative ( [Cr V NSal NMe2 ] •+ ) in comparison to the Cr VI derivatives ( [Cr VI NSal R ] + ; R = CF 3 , tBu). As a result, [Cr V NSal NMe2 ] •+ reacts with B(C 6 F 5 ) 3 , thus exhibiting similar nucleophilic reactivity to the neutral Cr V nitride derivatives. In contrast, the Cr VI derivatives ( [Cr VI NSal R ] + ; R = CF 3 , tBu) act as electrophiles, displaying facile reactivity with PPh 3 and no reaction with B(C 6 F 5 ) 3 . Thus, while oxidation to the ligand radical does not change the reactivity profile, metal-based oxidation to Cr VI results in umpolung, a switch from nucleophilic to electrophilic reactivity at the terminal nitride.