Theoretical study of C-H and N-H σ-bond activation reactions by titinium(IV)-imido complex. Good understanding based on orbital interaction and theoretical proposal for N-H σ-bond activation of ammonia

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作者：
Ochi, Noriaki [1 ]
Nakao, Yoshihide [1 ]
Sato, Hirofumi [1 ]
Sakaki, Shigeyoshi [1 ,2 ]
机构：
[1] Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
[2] Fukui Institute for Fundamental Chemistry, Nishihiraki-cho, Takano, Sakyo-ku 610-8103, Japan
来源：
Journal of the American Chemical Society | 2007年 / 129卷 / 27期
关键词：
The C-H σ-bond activation of methane and the N-H σ-bond activation of ammonia by (Me3-SiO)2Ti(=NSiMe3) 1 were theoretically investigated with DFT; MP2 to MP4(SDQ); and CCSD(T) methods. The C-H σ-bond activation of methane takes place with an activation barrier (Ea) of 14.6 (21.5) kcal/mol and a reaction energy (ΔE) of -22.7 (-16.5) kcal/mol to afford (Me3SiO) 2Ti(Me){NH(SiMe3)}; where DFT-and MP4(SDQ)-calculated values are given without and in parentheses; respectively; hereafter. The electron population of the CH3 group increases; but the H atomic population decreases upon going to the transition state from the precursor complex; which indicates that the C-H σ-bond activation occurs in heterolytic manner unlike the oxidative addition. The Ti atomic population considerably increases upon going to the transition state from the precursor complex; which indicates that the charge transfer (CT) occurs from methane to Ti. These population changes are induced by the orbital interactions among the dπ-pπ bonding orbital of the Ti=NSiMe3 moiety; the Ti dz2 orbital and the C-H σ-bonding and σ*-antibonding orbitals of methane. The reverse regioselective C-H σ-bond activation which leads to formation of (Me3SiO) 2Ti(H)-{NMe(SiMe3)} takes place with a larger E a value and smaller exothermicity. The reasons are discussed in terms of Ti-H; Ti-CH3; Ti-NH3; N-H; and N-CH3 bond energies and orbital interactions in the transition state. The N-H σ-bond activation of ammonia takes place in a heterolytic manner with a larger E a value of 19.0 (27.9) kcal/mol and considerably larger exothermicity of -45.0 (-39.4) kcal/mol than those of the C-H σ-bond activation. The N-H σ-bond activation of ammonia by a Ti-alkylidyne complex; (PNP)Ti(≡; CSiMe3); 3; (PNP; N-[2-(PH2); 2-phenyl]2-]); was also investigated. This reaction takes place with a smaller Ea value of 7.5 (15.3) kcal/mol and larger exothermicity of -60.2 (-56.1) kcal/mol. These results lead us to predict that the N-H ≡-bond activation of ammonia can be achieved by these complexes. © 2007 American Chemical Society;
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页码：8615 / 8624
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