Computational and Experimental Studies on the Thermolysis Mechanism of Zirconium and Hafnium Tetraalkyl Complexes. Difference between Titanium and Zirconium Complexes

Abstract

The first reaction step in the thermolysis of zirconium and hafnium tetraalkyl complexes has been studied with ab initio molecular orbital calculations in comparison with that of the 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 predicted to be in an eclipsed conformation; (2) the activation energy for the unimolecular methane elimination 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 the unimolecular mechanism and is in the order ZrMe4 < HfMe4 < TiMe4; (4) unimolecular α-hydrogen abstraction for Ti(n-Pr)Me3 and Ti(CH2CMe3)4 is more favorable than γ-hydrogen abstraction. However, the opposite is predicted for the Zr and Hf complexes. Chemical vapor deposition of ZrC from Zr(CH2CMe3)4 and Zr(CD2CMe3)4 has been studied. The major volatile products 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 γ-hydrogen abstraction is the first step of thermolysis.