Dinitrogen functionalization with terminal alkynes, amines, and hydrazines promoted by [(η5-C5Me4H)2Zr]2(μ2,η2 η2-N2):: Observation of side-on and end-on diazenido complexes in the reduction of N2 to hydrazine

Functionalization of the N-2 ligand in the side-on bound dinitrogen complex, [(eta(5)-C5Me4H)(2)Zr](2-)(mu(2), eta(2), eta(2) -N-2), has been accomplished by addition of terminal alkynes to furnish acetylide zirconocene diazenido complexes, [(eta(5)-C5Me4H)(2)Zr(C CR)](2)(mu(2), eta(2), eta(2)-N2H2) (R = Bu-n, Bu-t, Ph). Characterization of [(eta(5)-C5Me4H)(2)Zr(C CCMe3)](2)(mu(2), eta(2), eta(2)-N2H2) by X-ray diffraction revealed a side-on bound diazenido ligand in the solid state, while variable-temperature H-1 and N-15 NMR studies established rapid interconversion between eta(1), eta(1) and eta(2), eta(2) hapticity of the [N2H2](2-) ligand in solution. Synthesis of alkyl, halide, and triflato zirconocene diazenido complexes, [(eta(5)-C5Me4H)(2)ZrX](2)(mu(2), eta(1), eta(1)-N2H2) (X = Cl, I, OTf, CH2Ph, CH2SiMe3), afforded eta(1),eta(1) coordination of the [N2H2](2-) fragment both in the solid state and in solution, demonstrating that sterically demanding, in some cases pi-donating, ligands can overcome the electronically preferred side-on bonding mode. Unlike [(eta(5)-C5Me4H)(2)ZrH](2)(mu(2),eta(2),eta(2)-N2H2), the acetylide and alkyl zirconocene diazeniclo complexes are thermally robust, resisting a-migration and N2 cleavage up to temperatures of 115 degrees C. Dinitrogen functionalization with [(eta(5)-C5Me4H)(2)Zr](2)(mu(2), eta(2),eta(2)-N-2) was also accomplished by addition of proton donors. Weak Bronsted acids such as water and ethanol yield hydrazine and (eta(5)-C5Me4H)(2)Zr(OH)(2) and (eta(5)-C5Me4H)(2)Zr(OEt)(2), respectively. Treatment of [(eta(5)-C5Me4H)(2)Zr](2)(mu(2),eta(2), eta(2)-N-2) 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 [(eta(5)-C5Me5)(2)Zr(eta(1)-N-2)](2)(mu(2),eta(1),eta(1)-N-2) 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.