Dinitrogen functionalization with terminal alkynes, amines, and hydrazines promoted by [(η5-C5Me4H) 2Zr]222, η2-N2): Observation of side-on and end-on diazenido complexes in the reduction of N2 to hydrazine

Wesley H. Bernskoetter, Jaime A. Pool, Emil Lobkovsky, Paul J. Chirik

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Abstract

Functionalization of the N2 ligand in the side-on bound dinitrogen complex, [(η5-C5Me4H) 2Zr]2-(μ22, η2-N2), has been accomplished by addition of terminal alkynes to furnish acetylide zirconocene diazenido complexes, [(η5-C5Me4H)2Zr(C≡CR)] 2222-N 2H2) (R = nBu, tBu, Ph). Characterization of [(η5- C5Me4H) 2Zr(C≡CCMe3)]22, η22-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 [N2H 2]2- ligand in solution. Synthesis of alkyl, halide, and triflato zirconocene diazenido complexes, [(η5-C 5Me4H)2ZrX]22, η11-N2H2) (X = Cl, I, OTf, CH2Ph, CH2SiMe3), afforded η11 coordination of the [N2H 2]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-N 2H2), 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-N 2) was also accomplished by addition of proton donors. Weak Bronsted acids such as water and ethanol yield hydrazine and (η5-C 5Me4H)2Zr(OH)2 and (η5-C5Me4H)2-Zr(OEt) 2, respectively. Treatment of [(η5-C 5Me4H)2Zr]22, η22-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-C 5Me5)2Zr(η1-N2)] 2211-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.

Original languageEnglish (US)
Pages (from-to)7901-7911
Number of pages11
JournalJournal of the American Chemical Society
Volume127
Issue number21
DOIs
StatePublished - Jun 1 2005
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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