Indenyl zirconium dinitrogen chemistry:: N2 coordination to an isolated zirconium sandwich and synthesis of side-on, end-on dinitrogen compounds

Exposure of the isolable zirconocene sandwich compounds, (eta(5)-C5Me5)(eta(5)-C9H5-1-R-1-3-R-2)Zr (R-1 = Me, Pr-i, Bu-t; R-2 = Me) to one atmosphere of dinitrogen resulted in N-2 coordination. X-ray diffraction and NMR spectroscopy establish that the resulting dimeric dinitrogen compounds contain an unusual mu(2),eta(2)-bridging indenyl ring and a weakly activated N-2 ligand. N-2 coordination from the isolable zirconium sandwich compounds is extremely sensitive to the number and size of the indenyl subsituents. Compounds bearing two [Pr-i] or three methyl substituents are stable as eta(9) sandwich compounds for weeks under dinitrogen likely due to the inability to dimerize through a two-atom N-2 bridge. Performing the reduction of (eta(5)-C5Me5)(eta(5)-C9H5-1-R-1-3-R-2)ZrCl2 (R-1 = Pr-i, Bu-t; R-2 = Me; R-1 = R-2 = SiMe3) under an N-2 atmosphere produced a different outcome; rare examples of side-on, end-on zirconium dinitrogen compounds were isolated and in one case, crystallographically characterized. Protonolysis studies with weak Bronsted acids were used to evaluate the relative activation of the bridging dinitrogen ligands.