Computational and Experimental Studies on the Thermolysis Mechanism of Zirconium and Hafnium Tetraalkyl Complexes. Difference between Titanium and Zirconium Complexes
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- 作者：Yun-Dong Wu ; Zhi-Hui Peng ; Kyle W. K. Chan ; Xiaozhan Liu ; Albert A. Tuinman ; and Ziling Xue
- 刊名：Organometallics
- 出版年：1999
- 出版时间：May 24, 1999
- 年：1999
- 卷：18
- 期：11
- 页码：2081 - 2090
- 全文大小：188K
- 年卷期：v.18,no.11(May 24, 1999)
- ISSN：1520-6041
文摘
The first reaction step in the thermolysis of zirconium and hafnium tetraalkyl complexeshas been studied with ab initio molecular orbital calculations in comparison with that ofthe titanium tetraalkyl complexes (Wu, Y.-D.; Peng Z.-H.; Xue, Z. J. Am. Chem. Soc. 1996,118, 9772). Several clear differences in geometry and reactivity between TiR4 and ZrR4 (HfR4)are predicted: (1) While TiMe4 is in a staggered conformation, ZrMe4 and HfMe4 are predictedto be in an eclipsed conformation; (2) the activation energy for the unimolecular methaneelimination through intramolecular hydrogen abstraction is in the order TiMe4 
ZrMe4 <HfMe4; (3) the activation energy for the bimolecular methane elimination through intermolecular hydrogen abstraction for the three systems is much lower than that of theunimolecular mechanism and is in the order ZrMe4 < HfMe4 < TiMe4; (4) unimolecular
-hydrogen abstraction for Ti(n-Pr)Me3 and Ti(CH2CMe3)4 is more favorable than 
-hydrogenabstraction. However, the opposite is predicted for the Zr and Hf complexes. Chemical vapordeposition of ZrC from Zr(CH2CMe3)4 and Zr(CD2CMe3)4 has been studied. The major volatileproducts are neopentane and isobutene, which are in a ratio of about 2.3 in both reactions.In the case of Zr(CD2CMe3)4, the molar ratios of neopentane-d2/neopentane-d3 and isobutene-d2/isobutene-d0 are about 4.9 and 1.52, respectively. These support a mechanism in which-hydrogen abstraction is the first step of thermolysis.
ZrMe4 <HfMe4; (3) the activation energy for the bimolecular methane elimination through intermolecular hydrogen abstraction for the three systems is much lower than that of theunimolecular mechanism and is in the order ZrMe4 < HfMe4 < TiMe4; (4) unimolecular-hydrogen abstraction for Ti(n-Pr)Me3 and Ti(CH2CMe3)4 is more favorable than -hydrogenabstraction. However, the opposite is predicted for the Zr and Hf complexes. Chemical vapordeposition of ZrC from Zr(CH2CMe3)4 and Zr(CD2CMe3)4 has been studied. The major volatileproducts are neopentane and isobutene, which are in a ratio of about 2.3 in both reactions.In the case of Zr(CD2CMe3)4, the molar ratios of neopentane-d2/neopentane-d3 and isobutene-d2/isobutene-d0 are about 4.9 and 1.52, respectively. These support a mechanism in which-hydrogen abstraction is the first step of thermolysis.