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N₂ Hydrogenation Promoted by a Side-On Bound Hafnocene Dinitrogen Complex

Supporting Info

Wesley H. Bernskoetter, Andrea V. Olmos, Emil Lobkovsky, and Paul J. Chirik*

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

Organometallics, 2006, 25 (4), pp 1021–1027

DOI: 10.1021/om0509037

Publication Date (Web): December 24, 2005

To whom correspondence should be addressed. E-mail: pc92@cornell.edu.

Abstract

Reduction of the hafnocene diiodide (η⁵-C₅Me₄H)₂HfI₂ with excess sodium amalgam under 1 atm of dinitrogen yielded the hafnocene dinitrogen complex [(η⁵-C₅Me₄H)₂Hf]₂(μ₂,η²:η²-N₂). X-ray diffraction has established a strongly activated, side-on bound, bridging N₂ ligand with an N−N bond length of 1.423(11) Å. Exposure of the dinitrogen compound to 1 atm of dihydrogen at 23 °C resulted in rapid hydrogenation of the N₂ ligand, affording the hydrido hafnocene diazenido complex [(η⁵-C₅Me₄H)₂HfH]₂(μ₂,η²:η²-N₂H₂). Confirmation of N₂ hydrogenation has been provided by NMR and IR spectroscopic studies and X-ray diffraction. Pseudo-first-order observed rate constants for H₂ addition have been measured by monitoring the disappearance of a principally LMCT band centered at 886 nm [(η⁵-C₅Me₄H)₂Hf]₂(μ₂,η²:η²-N₂) as a function of time. The hafnocene complex hydrogenates dinitrogen approximately 4 times faster than the corresponding zirconocene compound. Divergent chemistry from zirconium is observed upon thermolysis of [(η⁵-C₅Me₄H)₂HfH]₂(μ₂,η²:η²-N₂H₂), which results in cyclopentadienyl methyl group cyclometalation rather than N−N bond scission.

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Published In Issue February 13, 2006
Received October 18, 2005

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Article

N₂ Hydrogenation Promoted by a Side-On Bound Hafnocene Dinitrogen Complex