Syntheses and Characterization of Tantalum Alkyl Imides and Amide Imides. DFT Studies of Unusual α-SiMe3 Abstraction by an Amide Ligand

Reaction of TaCl2(=NSiMe3) [N(SiMe3)(2)] (1) with alkylating reagents form the alkyl amide imide complexes TaR2(=NSiMe3) [N(SiMe3)(2)] (R = Me (2), CH2Ph (3)) and mixed amide imide compounds Ta(NR'(2))(2)(=NSiMe3)-[N(SiMe3)(2)] (R' = Me (4), Et (5)). The reaction of 2 and 0.5 equiv of O-2 leads to preferential oxygen insertion into one Ta-Me bond, yielding the alkoxy-bridged alkyl dimer Ta-2(mu-OMe)(2)Me-2(=NSiMe3)(2)[N(SiMe3)(2)](2) (6) as cis and trans isomers. Crystallization of the cis-6 and trans-6 mixture gave only crystals of trans-6. When the crystals of trans-6 were dissolved in benzene-d(6), conversion of trans-6 to cis-6 occurred until the trans-6 reversible arrow cis-6 equilibrium was reached with K-eq = 0.79(0.02) at 25.0(0.1) degrees C. Kinetic studies of the exchange gave the rate constants k = 0.018(0.002) min(-1) for the trans-6 -> cis-6 conversion and k' = 0.022(0.002) min(-1) for the reverse cis-6 trans-6 conversion at 25.0(0.1) degrees C. Complex 6 reacts with additional 02, forming the dialkoxy dimer Ta-2(mu-OMe)(2)(OMe)(2)(=NSiMe3)(2)[N(SiMe3)(2)](2) (7) as cis and trans isomers. Solid-state structures of 3 and trans-6 have been determined by X-ray diffraction analyses. The mixed amide imide compounds Ta(NR'(2))(2)(=NSiMe3) [N(SiMe3)(2)] (R' = Me (4), Et (5)) have also been prepared by salt metathesis reactions employing TaCl3[N(SiMe3)(2)](2) (8). The pathway from 8 to 4 and 5 eliminates Me3Si-NR'(2) (R' = Me, Et), converting the amide N(SiMe3)(2) ligand to the imide =NSiMe3 ligand. Such intramolecular imidation is rare. The mechanism of this process has been computationally probed, and a-elimination involving the mixed amide species TaCl2(NMe2)[N(SiMe3)(2)](2) (9) is discussed. Diffusion-ordered spectroscopy (DOSY) studies of 1-6 and 8 show that only the alkoxy-bridged cis-6 and trans-6 are dimers in benzene-d(6) solution at 25 degrees C.