Sandwich Complexes of the Heavier Alkaline Earth Metals Containing η⁵-β-Diketiminato Ligand Sets

Treatment of N-tert-butyl-4-(tert-butylimino)-2-penten-2-amine (LtBuH) with 0.5 equiv of M(N(SiMe3)2)2(THF)2 (M = Ca, Sr, Ba) in toluene at ambient temperature afforded Ca(η5-LtBu)2 (98%), Sr(η5-LtBu)2 (93%), and Ba(η5-LtBu)2 (93%). Crystal structure determinations of these complexes revealed bent sandwich structures in which the β-diketiminato ligands are coordinated in a η5-fashion to the metal centers. Similar treatment of N-isopropyl-4-(isopropylimino)-2-penten-2-amine (LiPrH) with 0.5 equiv of M(N(SiMe3)2)2(THF)2 led to the isolation of Ca(η2-LiPr)2 (72%), Sr(η5-LiPr)2 (80%), and a novel dinuclear barium complex [(η5-LiPr)Ba(μ-η5:η5-LiPr)(μ-η1:η1-LiPr)Ba(η5-LiPr)] (39%). Structural assignments for these complexes were based on spectral and analytical data as well as X-ray crystallography. [(η5-LiPr)Ba(μ-η5:η5-LiPr)(μ-η1:η1-LiPr)Ba(η5-LiPr)] exhibits three different bonding modes for the β-diketiminato ligands, including η5, μ-η1:η1, and μ-η5:η5. Variable-temperature NMR studies suggest that the dinuclear structure of [(η5-LiPr)Ba(μ-η5:η5-LiPr)(μ-η1:η1-LiPr)Ba(η5-LiPr)] persists in solution.