N2 hydrogenation promoted by a side-on bound hafnocene dinitrogen complex

Reduction of the hafnocene diiodide (eta(5)-C5Me4H)(2)HfI2 with excess sodium amalgam under 1 atm of dinitrogen yielded the hafnocene dinitrogen complex [(eta(5)-C5Me4H)(2)Hf](2)(mu(2),eta(2):eta(2)-N-2). X-ray diffraction has established a strongly activated, side-on bound, bridging N-2 ligand with an N-N bond length of 1.423(11) angstrom. Exposure of the dinitrogen compound to 1 atm of dihydrogen at 23 degrees C resulted in rapid hydrogenation of the N-2 ligand, affording the hydrido hafnocene diazenido complex [(eta(5)-C5Me4H)(2)HfH](2)(mu(2),eta(2):eta(2)-N2H2). Confirmation of N-2 hydrogenation has been provided by NMR and IR spectroscopic studies and X-ray diffraction. Pseudo-first-order observed rate constants for H-2 addition have been measured by monitoring the disappearance of a principally LMCT band centered at 886 nm [(eta(5)-C5Me4H)(2)Hf](2)(mu(2),eta(2):eta(2)-N-2) 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 [(eta(5)-C5Me4H)(2)HfH](2)(mu(2),eta(2):eta(2)-N2H2), which results in cyclopentadienyl methyl group cyclometalation rather than N-N bond scission.