Synthesis of Chromium(IV) Nitrides Through High-Spin Tetrahedral Chromium(I) Intermediates

Reduction of (depe)₂CrCl₂ (depe = 1,2-bis(diethylphosphino)ethane) and (dep-benz)₂CrCl₂ (dep-benz = 1,2-bis(diethylphosphino)benzene) under 1 atm of N₂ furnished the dinitrogen complexes (depe)₂Cr(N₂)₂ and (dep-benz)₂Cr(N₂)₂, respectively. One-electron oxidation of these products with FcBArF₄ (Fc = ferrocenium, BArF₄ = B(3,5-(CF₃)₂C₆H₃)₄) yielded the unusual, high-spin tetrahedral complexes [(depe)₂Cr][BArF₄] and [(dep-benz)₂Cr][BArF₄] with concomitant loss of dinitrogen. Reaction of the chromium(I) derivatives with Ph₃CN₃ furnished rare examples of chromium(IV) nitrides as confirmed spectroscopically and by X-ray crystallography. While [(depe)₂Cr(≡N)][BArF₄] underwent association of isocyanides accompanied by partial ligand dissociation, neither chromium nitride was reactive toward H₂ or diphenylsilane under thermal or photochemical conditions. These results distinguish the unique properties of the chromium(IV) nitrides as compared to heavier group 6 congeners and demonstrate both the feasibility of nitride synthesis and the limitations of dinitrogen cleavage and subsequent N-H bond formation.