Zirconium, Hafnium, and Tantalum Amide Silyl Complexes: Their Preparation and Conversion to Metallaheterocyclic Complexes via γ-Hydrogen Abstraction by Silyl Ligands

New transition metal silyl amide complexes (Me2N)3Ta[N(SiMe3)2](SiPh2But) (1) and (Me2N)M[N(SiMe3)2]2(SiPh2But) (M = Zr, 2a, and Hf, 2b) were found to undergo γ-H abstraction by the silyl ligands to give metallaheterocyclic
complexes (3) and (M = Zr, 4a, and Hf,
4b), respectively. The conversion of 1 to 3 follows first-order kinetics with ΔH⧧ = 23.6(1.6) kcal/mol and ΔS⧧ =
3(5) eu between 288 and 313 K. The formation of 4a from (Me2N)Zr[N(SiMe3)2]2Cl (5a) and Li(THF)2SiPh2But (6)
involves the formation of the intermediate 2a, followed by γ-H abstraction. Kinetic studies of these consecutive
reactions, a second-order reaction to give 2a and then a first-order γ-H abstraction to give 4a, were conducted by
an analytical method and a numerical method. At 278 K, the rate constants k1 and k2 for the two consecutive
reactions are 2.17(0.03) × 10-3 M-1 s-1 and 5.80(0.15) × 10-5 s-1 by the analytical method. The current work is
a rare kinetic study of the A + B → C → D (+ E) consecutive reactions. Kinetic studies of the formation of a
metallaheterocyclic moiety have, to our knowledge, not been reported. In addition, γ-H
abstraction by a silyl ligand to give such a metallaheterocyclic moiety is new. Theoretical investigations of the γ-H
abstraction by silyl ligands have been conducted by density functional theory calculations at the Becke3LYP (B3LYP)
level, and they revealed that the formation of the metallacyclic complexes through γ-H abstraction is entropically
driven. X-ray crystal structures of (Me2N)3Ta[N(SiMe3)2](SiPh2But) (1), (Me2N)Zr[N(SiMe3)2]2Cl (5a), and (M = Zr, 4a, and Hf, 4b) are also reported.