DFT examination of rare α-SiMe_3 abstraction in Ta(NMe_2)_4[N(SiMe_3)_2]: formation of the imide compound Ta(=NSiMe_3)(NMe_2)_3 and its trapping to give guanidinate imides

摘要

A density functional theory (DFT) investigation on the formation of Ta(=NSiMe_3)(NMe_2)_3 (B) and Me_3SiNMe_2 (C) from Ta(NMe_2)_4[N(SiMe_3)_2] is reported. Three different ground-state minima are computed for Ta(NMe_2)_4[N(SiMe_3)_2], and of these only the stereoisomer based on a square pyramid (A2) with an apical N(SiMe_3)_2 group undergoes α-elimination to give Ta(=NSiMe_3)(NMe_2)_3 (B) and Me_3SiNMe_2 (C). The barrier computed for the concerted α-elimination is in agreement with the results from our earlier experimental study. The thermodynamics for the monomer-dimer equilibrium involving Ta(=NSiMe_3)(NMe_2)_3 (B) has been computationally evaluated, and the preference for the dimeric form of the compound is discussed relative to the Nugent imide derivative Ta(=NCMe_3)(NMe_2)_3, which exists as a monomer. The trapping of the intermediate B by the heterocumulene MeN=C=NMe has been modeled, and the mechanism involved in the formation of the guanidinate-based insertion products Ta(NSiMe_3)(NMe_2)_2[MeNC(NMe_2)NMe] (G1) and Ta(NSiMe_3)(NMe_2)[MeNC(NMe_2)NMe]_2 (I) is presented.