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(CH 3 ) 3 )X 2 ( 2 (X = Cl), 3 (X = Br)) (Tp‘ = hydrotris(3,5-dimethylpyrazolyl)borate), have been synthesized by the reaction of KTp‘ with Ta(CHC(CH 3 ) 3 )X 3 (thf) 2 in THF. One Cl ligand in 2 can be substituted with OR - (R = i -Pr, Me) or NMe 2 - . The values of 1 J C-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 1 J C-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(CH 2 C(CH 3 ) 3 )(NAr)Cl (Ar = 2,6- i -PrC 6 H 3 ( 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(η 2 -CH 2 NPh)(CH 2 CMe 3 )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.