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, (η⁵-C₅Me₅)(η⁵-C ₉H₅-1-R¹-3-R²)Zr (R¹ = Me, ⁱPr, ^tBu; R² = Me) to one atmosphere of dinitrogen resulted in N₂ coordination. X-ray diffraction and NMR spectroscopy establish that the resulting dimeric dinitrogen compounds contain an unusual μ₂,η²-bridging indenyl ring and a weakly activated N₂ ligand. N₂ coordination from the isolable zirconium sandwich compounds is extremely sensitive to the number and size of the indenyl subsituents. Compounds bearing two [ⁱPr] or three methyl substituents are stable as η⁹ sandwich compounds for weeks under dinitrogen likely due to the inability to dimerize through a two-atom N ₂ bridge. Performing the reduction of (η⁵-C ₅Me₅)(η⁵-C₉H₅-1- R¹-3-R²)ZrCl₂ (R¹ = ⁱPr, _tBu; R₂ = Me; R¹ = R₂ = SiMe ₃) under an N₂ 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 Brønsted acids were used to evaluate the relative activation of the bridging dinitrogen ligands.