Computational Modeling of ansa-Zirconocene Amide Complexes

详细信息    查看全文

• 作者：Adam R. Dunn ; Lucas E. Sweet ; Dawn C. Wiser ; Matthew D. LoCoco ; and Richard F. Jordan
• 刊名：Organometallics
• 出版年：2004
• 出版时间：November 22, 2004
• 年：2004
• 卷：23
• 期：24
• 页码：5671 - 5680
• 全文大小：228K
• 年卷期：v.23,no.24(November 22, 2004)
• ISSN：1520-6041

文摘

The structures and relative energies of ansa-zirconocene bis-amide complexes were studiedby DFT computations. X-ray crystal structures and experimental trends in E_rac-meso valuesfor a series of zirconocenes with terminal and chelated bis-amide ligands were reproducedwith reasonable accuracy by B3LYP/LANL2DZ//B3LYP/LANL2MB. In the absence ofoverriding steric interactions, the most stable conformation of Zr{PhN(CH₂)₃NPh} rings inzirconocenes is the twist conformation. For (C₅H₅)₂Zr{PhN(CH₂)₃NPh} (5), the twistconformer is 2.7 kcal/mol more stable than the chair and 3.6 kcal/mol more stable than theboat. rac-(MBSBI)Zr{PhN(CH₂)₃NPh} (rac-3; MBSBI = Me₂Si(2-Me-4,5-benzindenyl)₂) is 5.7kcal/mol more stable than the meso isomer because the latter species must undergo acombination of distortions to accommodate the favored twist conformation of the Zr{PhN(CH₂)₃NPh} ring. These distortions include a significant ³-slip and an exaggerated anglebetween the C6 and C5 ring planes for one benzindenyl ligand and a subtle flattening of theZr{PhN(CH₂)₃NPh} ring. In contrast, rac-(MBSBI)Zr{PhN(CH₂)₂NPh} (rac-7) is only 2.4 kcal/mol more stable than the meso isomer. In this case, the Zr{PhN(CH₂)₂NPh} ring adopts anenvelope conformation in which one N-Ph ring lies in the N-Zr-N plane and stericinteractions on the crowded side of the meso metallocene unit are minimized. These resultsprovide insight into the origin of rac/meso selectivity in chelate-controlled syntheses of ansa-zirconocenes based on the reaction of Zr bis-amide compounds with lithium ansa-bis-indenyland cyclopentadienyl reagents.