The Intrinsic Barrier for NN Bond Cleavage or Formation Mediated by the CpCr(dmpe) Fragment Is Insurmountable

We report the synthesis and characterization of a series of related chromium dinitrogen and nitrido complexes supported by the Cp*Cr­(dmpe) fragment. Reduction of Cp*­(dmpe)­CrCl (1, Cp* = η5-C5Me5, dmpe = 1,2-bis­(dimethylphosphino)­ethane) with sodium metal under a nitrogen atmosphere yielded Cr­(I) dinitrogen complex {Cp*­(dmpe)­Cr}2(μ-N2) (2). Reaction of the latter with 1 or 2 equiv of HBArF4 (HBArF 4 = [H­(OEt2)2]­[(B­(3,5-(CF3)2C6H3)4]) at room temperature did not effect protonation but rather gave mixed-valent [{Cp*­(dmpe)­Cr}2(μ-N2)]­BArF 4 (3) and Cr­(II) dinitrogen complex [{Cp*­(dmpe)­Cr}2(μ-N2)]­(BArF 4)2 (4), apparent products of oxidation by H+. 4 can be prepared independently by the reaction of [Cp*­(dmpe)­CrCl] with 1 equiv of NaBArF 4 under N2. The Cr­(IV) nitride Cp*­(dmpe)­CrN (5) was synthesized by the reaction of 1 with an excess of NaN3. The cationic Cr­(V) nitride [Cp*­(dmpe)­CrN]­BArF 4 (6) is the product of oxidation of 5 with 1 equiv of FcBArF 4. DFT calculations were performed to suggest that the dinitrogen complexes are the thermodynamic products of the relevant interconversion processes. However, no NN bond cleavage or nitride coupling was observed under any circumstances.