Chelate-Controlled Synthesis of Racemic <i>a</i><i>nsa</i>-Zirconocenes

The reaction of Zr{PhN(CH2)3NPh}Cl2(THF)2 (5) with lithium ansa-bis-indenyl reagents Li2[XBI](Et2O) (XBI = (1-indenyl)2SiMe2 (SBI, 7a), (2-methyl-1-indenyl)2SiMe2 (MSBI, 7b), (2-methyl-4,5-benz-1-indenyl)2SiMe2 (MBSBI, 7c), (2-methyl-4-phenyl-1-indenyl)2SiMe2 (MPSBI, 7d), and 1,2-(1-indenyl)2ethane
(EBI, 7e)) affords rac-(XBI)Zr{PhN(CH2)3NPh} (8a−e) in high yield. The meso isomers were not detected
by 1H NMR. X-ray crystallographic studies show that the Zr{PhN(CH2)3NPh} rings in 5, 8a, 8c, and (C5H5)2Zr{PhN(CH2)3NPh} (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)3NPh} 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.