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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

Supporting Info

Wesley H. Bernskoetter, Jaime A. Pool,^† Emil Lobkovsky, and Paul J. Chirik*

Contribution from the Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853

J. Am. Chem. Soc., 2005, 127 (21), pp 7901–7911

DOI: 10.1021/ja050387b

Publication Date (Web): May 7, 2005

†

Current address: Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, pc92@cornell.edu

Abstract

Functionalization of the N₂ ligand in the side-on bound dinitrogen complex, [(η⁵-C₅Me₄H)₂Zr]₂(μ₂,η²,η²-N₂), has been accomplished by addition of terminal alkynes to furnish acetylide zirconocene diazenido complexes, [(η⁵-C₅Me₄H)₂Zr(CCR)]₂(μ₂,η²,η²-N₂H₂) (R = ⁿBu, ^tBu, Ph). Characterization of [(η⁵-C₅Me₄H)₂Zr(CCCMe₃)]₂(μ₂,η²,η²-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 η¹,η¹ and η²,η² hapticity of the [N₂H₂]^2- ligand in solution. Synthesis of alkyl, halide, and triflato zirconocene diazenido complexes, [(η⁵-C₅Me₄H)₂ZrX]₂(μ₂,η¹,η¹-N₂H₂) (X = Cl, I, OTf, CH₂Ph, CH₂SiMe₃), afforded η¹,η¹ 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]₂(μ₂,η²,η²-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]₂(μ₂,η²,η²-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]₂(μ₂,η²,η²-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)]₂(μ₂,η¹,η¹-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.