Competition between π Donation and α-C−H Agostic Interactions in Complexes of the Type Tp‘Ta(CH-t-Bu)(X)(Y) (X = Halide; Y = Halide, NR2, OR; Tp‘ = Hydrotris(3,5-dimethylpyrazolyl)borate)

The compounds Tp‘Ta(CHC(CH3)3)X2 (2 (X = Cl), 3 (X = Br)) (Tp‘ = hydrotris(3,5-dimethylpyrazolyl)borate), have been synthesized by the reaction of KTp‘ with Ta(CHC(CH3)3)X3(thf)2 in THF. One Cl ligand in 2 can be substituted with OR- (R = i-Pr, Me) or NMe2-. The values of 1JC-H for the alkylidene carbon are uniformly low due to the α-H agostic interaction between the metal center and the alkylidene proton. The magnitude of 1JC-H in these compounds depends upon the π donor properties of the remaining ligands. Attempted synthesis of monoarylamide complexes by displacement of Cl- from 2 with N(H)Ar- results in proton transfer from N to C and formation of the imido complexes Tp‘Ta(CH2C(CH3)3)(NAr)Cl (Ar = 2,6-i-PrC6H3 (7), Ph (8)). When 2 reacts with KN(Me)Ph, proton transfer occurs from the Me group to the alkylidene carbon giving the imine complex Tp‘Ta(η2-CH2NPh)(CH2CMe3)Cl (9). A single-crystal X-ray diffraction study of 9 confirmed the structure and suggested that 9 is best described as an azametallacyclopropane complex.