Chromium Nitride Umpolung Tuned by the Locus of Oxidation

Oxidation of a series of CrV nitride salen complexes
(CrVNSalR) with different para-phenolate substituents
(R = CF3, tBu, NMe2) 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 CrVI nitrides ([CrVINSalR]+;
R = CF3, tBu) and a localized CrV nitride phenoxyl
radical for the more electron-donating NMe2 substituent
([CrVNSalNMe2]•+).
The facile nitride homocoupling observed for the MnVI analogues
was significantly attenuated for the CrVI 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 CrV ligand radical derivative ([CrVNSalNMe2]•+) in comparison to
the CrVI derivatives ([CrVINSalR]+; R = CF3, tBu). As a result, [CrVNSalNMe2]•+ reacts with
B­(C6F5)3, thus exhibiting similar
nucleophilic reactivity to the neutral CrV nitride derivatives.
In contrast, the CrVI derivatives ([CrVINSalR]+; R = CF3, tBu) act as
electrophiles, displaying facile reactivity with PPh3 and
no reaction with B­(C6F5)3. Thus,
while oxidation to the ligand radical does not change the reactivity
profile, metal-based oxidation to CrVI results in umpolung,
a switch from nucleophilic to electrophilic reactivity at the terminal
nitride.