Computational and experimental studies on the thermolysis mechanism of zirconium and hafnium tetraalkyl complexes. Difference between titanium and zirconium complexes

The first reaction step in the thermolysis of zirconium and hafnium tetraal kyl complexes has been studied with ab initio molecular orbital calculation s in comparison with that of the titanium tetraalkyl complexes (Wu, Y.-D.; Peng Z.-H.; Xue, Z. J. Am. Chem. Sec. 1996, 118, 9772). Several clear diffe rences in geometry and reactivity between TiR4 and ZrR4 (HfR4) are predicte d: (1) While TiMe4 is in a staggered conformation, ZrMe4 and HfMe4 are pred icted to be in an eclipsed conformation; (2) the activation energy for the unimolecular methane elimination through intramolecular hydrogen abstractio n is in the order TiMe4 much less than ZrMe4 < HfMe4; (3) the activation en ergy for the bimolecular methane elimination through intermolecular hydroge n abstraction for the three systems is much lower than that of the unimolec ular mechanism and is in the order ZrMe4 < HfPMe4 < TiMe4; (4) unimolecular alpha-hydrogen abstraction for Ti(n-Pr)Me-3 and Ti(CH2CMe3)(4) is more fav orable than gamma-hydrogen abstraction. However, the opposite is predicted for the Zr and Hf complexes. Chemical vapor deposition of ZrC from Zr(CH2CM e3)(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 reacti ons. Zn the case of Zr(CD2CMe3)(4), the molar ratios of neopentane-d(2)/neo pentane-d(3) and isobutene-d2/isobutene-d(0) are about 4.9 and 1.52, respec tively. These support a mechanism in which gamma-hydrogen abstraction is th e first step of thermolysis.