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

Abstract

The C-H sigma-bond activation of methane and the N-H sigma-bond activation of ammonia by (Me3SiO)2Ti(=NSiMe3) 1 were theoretically investigated with DFT, MP2 to MP4(SDQ), and CCSD(T) methods. The C-H sigma-bond activation of methane takes place with an activation barrier (Ea) of 14.6 (21.5) kcal/mol and a reaction energy (DeltaE) 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 sigma-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 d(pi)-p(pi) bonding orbital of the Ti=NSiMe3 moiety, the Ti d(z2) orbital and the C-H sigma-bonding and sigma*-antibonding orbitals of methane. The reverse regioselective C-H sigma-bond activation which leads to formation of (Me3SiO)2Ti(H)[NMe(SiMe3)] takes place with a larger Ea 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 sigma-bond activation of ammonia takes place in a heterolytic manner with a larger Ea 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 sigma-bond activation. The N-H sigma-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 E(a) 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 sigma-bond activation of ammonia can be achieved by these complexes.