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)(3)Ta[N(SiMe3)(2)](SiPh2But) (1) and (Me2N)M[N(SiMe3)(2)](2)(SiPh2But) (M = Zr, 2a, and Hf, 2b) were found to undergo gamma-H abstraction by the silyl ligands to give metallaheterocyclic complexes (Me2N)(3)Ta(NSiMe3SiMe2CH2) (3) and {(Me2N)[(Me3Si)(2)N]M(NSiMe3SiMe2CH2)}(2) (M = Zr, 4a, and Hf, 4b), respectively. The conversion of 1 to 3 follows first-order kinetics with DeltaH(double dagger) = 23,6(1.6) kcal/mol and DeltaS(double dagger) = 3(5) eu between 288 and 313 K. The formation of 4a from (Me2N)Zr[N(SiMe3)(2)](2)Cl (5a) and Li(THF)(2)SiPh2Bu1 (6) involves the formation of the intermediate 2a, followed by gamma-H abstraction. Kinetic studies of these consecutive reactions, a second-order reaction to give 2a and then a first-order y-H abstraction to give 4a, were conducted by an analytical method and a numerical method. At 278 K, the rate constants k(1) and k(2) for the two consecufive reactions are 2.17(0.03) x 10(-3) M-1 s(-1) and 5.80(0.15) x 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 metal laheterocyclic moiety M(NSiMe3SiMe2CH2) have, to our knowledge, not been reported. In addition, gamma-H abstraction by a silyl ligand to give such a metallaheterocyclic moiety is new. Theoretical investigations of the gamma-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 gamma-H abstraction is entropically driven. X-ray crystal structures of (Me2N)(3)Ta[N(SiMe3)(2)](SiPh2But) (1), (Me2N)Zr[N(SiMe3)(2)](2)Cl (5a), and {(Me2N)[(Me3Si)(2)N]M(NSiMe3SiMe2CH2)}(2) (M = Zr, 4a, and Hf, 4b) are also reported.