Chelate-controlled synthesis of racemic ansa-zirconocenes

The reaction of Zr{PhN(CH2)(3)NPh}Cl-2(THF)(2) (5) with lithium ansa-bis-indenyl reagents Li-2[XBI]-(Et2O) (XBI = (1-indenyl)(2)SiMe2 (SBI, 7a), (2-methyl-1-indenyl)(2)SiMe2 (MSBI, 7b), (2-methyl-4,5-benz-1-indenyl)(2)SiMe2 (MBSBI, 7c), (2-methyl-4-phenyl-1-indenyl)(2)SiMe2 (MPSBI, 7d), and 1,2-(1-indenyl)(2)ethane (EBI, 7e)) affords rac-(XBI)Zr{PhN(CH2)(3)NPh} (8a-e) in high yield. The meso isomers were not detected by H-1 NMR. X-ray crystallographic studies show that the Zr{PhN(CH2)(3)NPh) rings in 5, 8a, 8c, and (C5H5)(2)-Zr{PhN(CH2)(3)NPh} (10) adopt twist conformations that position the N-Ph groups on opposite sides of the N-Zr-N plane. This conformation complements the metallocene structures of rac-8a-e but would destabilize the corresponding meso isomers. It is proposed that the Zr{PhN(CH2)(3)NPh} ring adopts a similar twist conformation in the stereodetermining transition state for addition of the second indenyl ring in these reactions, which leads to a preference for rac products. The results of metallocene syntheses from other Zr amide precursors support this proposal. 8a-e are converted to the corresponding rac-(XBI)ZrCl2 complexes (9a-e) by reaction with HCl.