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

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.