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The synthesis of zirconium, hafnium, and tantalum amide silyl complexes are described. The novel reactions of silyl ligands, gamma-hydrogen abstraction to give metallaheterocyclic moiety were investigated.

The theoretical study on magnetostructural correlation in unsymmetrical Cu(II) binuclear complexes bridged by [C2O2(NH)2](super 2-) was performed using the broken symmetry approach within the framework of density functional theory (DFT). The calculated results show that the magnetic coupling interaction decreases with the changes of the coordination environment around the Cu atom from a planar structure.

The reaction mechanisms of the (NHC)Ni(0)-catalyzed aldehyde-alkyne reductive couplings with silanes as reducing agent have been theoretically investigated with the aid of DFT calculations. The impacts of N-heterocyclic carbene (NHC) ligands and silanes on the reversal of regioselectivity and the rate-limiting step alteration were rationalized. It is found that the steric effects play a dominant role. The reversal of the regioselectivity is found to be related to the switching of the steric effect, from the aldehyde phenyl hindrance with the adjacent alkyne substituent to the NHC ligand hindrance with the adjacent alkyne substituent, when the NHC ligand employed is changed from small to large. The rate-limiting step alteration caused by using bulkier silanes is due to the generated strong steric effect, which makes the σ-bond metathesis transition state relatively high in enthalpic energy, thus with the entropy penalty making the metathesis step rate-limiting instead of the oxidative cyclization step. [ABSTRACT FROM AUTHOR]

New transition metal silyl amide complexes (Me2N)3Ta[N(SiMe3)2](SiPh2-But) (1) and (Me2N)M[N(SiMe33)]2(SiPh2- But) (M = Zr, 2a, and Hf, 2b) were found to undergo γ-H abstraction by the silyl ligands to give metaJlaheterocyclic complexes (Me2N)3Ta(NSiMe2SiMe2CH2) (3) and {(Me2N)[(Me3Si)2N](Me2N)[(Me3Si)2N]M(NSiMe3CH2)}2 (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)2SiPh2 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 M(NSiMe3SiMe2CH2) 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](SiPh2Bt)(1), (Me2N)Zr[N(SiMe3)2]2Cl (5a), and {(Me2N)[(Me3-Si)2N]M(NSiMe3SiMe2CH2)}2 (M = Zr, 4a, and Hf, 4b) are also reported. [ABSTRACT FROM AUTHOR]