Dinitrogen Functionalization with Terminal Alkynes, Amines, and Hydrazines Promoted by [(η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>Zr]<sub>2</sub>(μ<sub>2</sub>,η<sup>2</sup>,η<sup>2</sup>-N<sub>2</sub>): Observation of Side-On and End-On Diazenido Complexes in the Reduction of N<sub>2</sub> to Hydrazine
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posted on 2005-06-01, 00:00 authored by Wesley H. Bernskoetter, Jaime A. Pool, Emil Lobkovsky, Paul J. ChirikFunctionalization of the N<sub>2</sub> ligand in the side-on bound dinitrogen complex, [(η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>Zr]<sub>2</sub>(μ<sub>2</sub>,η<sup>2</sup>,η<sup>2</sup>-N<sub>2</sub>), has been accomplished by addition of terminal alkynes to furnish acetylide zirconocene
diazenido complexes, [(η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>Zr(C⋮CR)]<sub>2</sub>(μ<sub>2</sub>,η<sup>2</sup>,η<sup>2</sup>-N<sub>2</sub>H<sub>2</sub>) (R = <sup>n</sup>Bu, <sup>t</sup>Bu, Ph). Characterization of [(η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>Zr(C⋮CCMe<sub>3</sub>)]<sub>2</sub>(μ<sub>2</sub>,η<sup>2</sup>,η<sup>2</sup>-N<sub>2</sub>H<sub>2</sub>) by X-ray diffraction revealed a side-on bound diazenido ligand in
the solid state, while variable-temperature <sup>1</sup>H and <sup>15</sup>N NMR studies established rapid interconversion between
η<sup>1</sup>,η<sup>1</sup> and η<sup>2</sup>,η<sup>2</sup> hapticity of the [N<sub>2</sub>H<sub>2</sub>]<sup>2-</sup> ligand in solution. Synthesis of alkyl, halide, and triflato zirconocene
diazenido complexes, [(η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>ZrX]<sub>2</sub>(μ<sub>2</sub>,η<sup>1</sup>,η<sup>1</sup>-N<sub>2</sub>H<sub>2</sub>) (X = Cl, I, OTf, CH<sub>2</sub>Ph, CH<sub>2</sub>SiMe<sub>3</sub>), afforded η<sup>1</sup>,η<sup>1</sup>
coordination of the [N<sub>2</sub>H<sub>2</sub>]<sup>2-</sup> 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 [(η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>ZrH]<sub>2</sub>(μ<sub>2</sub>,η<sup>2</sup>,η<sup>2</sup>-N<sub>2</sub>H<sub>2</sub>), the acetylide and alkyl zirconocene diazenido complexes
are thermally robust, resisting α-migration and N<sub>2</sub> cleavage up to temperatures of 115 °C. Dinitrogen
functionalization with [(η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>Zr]<sub>2</sub>(μ<sub>2</sub>,η<sup>2</sup>,η<sup>2</sup>-N<sub>2</sub>) was also accomplished by addition of proton donors.
Weak Brønsted acids such as water and ethanol yield hydrazine and (η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>Zr(OH)<sub>2</sub> and (η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>Zr(OEt)<sub>2</sub>, respectively. Treatment of [(η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>H)<sub>2</sub>Zr]<sub>2</sub>(μ<sub>2</sub>,η<sup>2</sup>,η<sup>2</sup>-N<sub>2</sub>) with HNMe<sub>2</sub> or H<sub>2</sub>NNMe<sub>2</sub> furnished amido
or hydrazido zirconocene diazenido complexes that ultimately produce hydrazine upon protonation with
ethanol. These results contrast previous observations with [(η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>Zr(η<sup>1</sup>-N<sub>2</sub>)]<sub>2</sub>(μ<sub>2</sub>,η<sup>1</sup>,η<sup>1</sup>-N<sub>2</sub>) 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.
