Dinitrogen Functionalization with Terminal Alkynes, Amines, and Hydrazines Promoted by [(η5-C5Me4H)2Zr]2222-N2):  Observation of Side-On and End-On Diazenido Complexes in the Reduction of N2 to Hydrazine

Functionalization of the N2 ligand in the side-on bound dinitrogen complex, [(η5-C5Me4H)2Zr]2222-N2), has been accomplished by addition of terminal alkynes to furnish acetylide zirconocene diazenido complexes, [(η5-C5Me4H)2Zr(C⋮CR)]2222-N2H2) (R = nBu, tBu, Ph). Characterization of [(η5-C5Me4H)2Zr(C⋮CCMe3)]2222-N2H2) by X-ray diffraction revealed a side-on bound diazenido ligand in the solid state, while variable-temperature 1H and 15N NMR studies established rapid interconversion between η11 and η22 hapticity of the [N2H2]2- ligand in solution. Synthesis of alkyl, halide, and triflato zirconocene diazenido complexes, [(η5-C5Me4H)2ZrX]2211-N2H2) (X = Cl, I, OTf, CH2Ph, CH2SiMe3), afforded η11 coordination of the [N2H2]2- fragment both in the solid state and in solution, demonstrating that sterically demanding, in some cases π-donating, ligands can overcome the electronically preferred side-on bonding mode. Unlike [(η5-C5Me4H)2ZrH]2222-N2H2), the acetylide and alkyl zirconocene diazenido complexes are thermally robust, resisting α-migration and N2 cleavage up to temperatures of 115 °C. Dinitrogen functionalization with [(η5-C5Me4H)2Zr]2222-N2) was also accomplished by addition of proton donors. Weak Brønsted acids such as water and ethanol yield hydrazine and (η5-C5Me4H)2Zr(OH)2 and (η5-C5Me4H)2Zr(OEt)2, respectively. Treatment of [(η5-C5Me4H)2Zr]2222-N2) with HNMe2 or H2NNMe2 furnished amido or hydrazido zirconocene diazenido complexes that ultimately produce hydrazine upon protonation with ethanol. These results contrast previous observations with [(η5-C5Me5)2Zr(η1-N2)]2211-N2) where loss of free dinitrogen is observed upon treatment with weak acids. These studies highlight the importance of cyclopentadienyl substituents on transformations involving coordinated dinitrogen.