SYNTHESES, STRUCTURES, AND ISOMERIZATION PROCESSES OF DIALKYLALUMINUM SILYLAMIDO COMPLEXES

Reactions of R3SiNH2 with R'3Al in refluxing hexane afford a family of dimeric aluminum silylamides, [R'2AlN(H)SiR3]2 (R' = Me, R3 = Ph3, Et3; R' = i-Bu, R3 = Ph3, R3 = t-Bu2H). [Me2AlN(H)SiMe3]2 is prepared from [Cl2AlN(H)SiMe3]2 and dimethylmagnesium. X-ray crystallography of [Me2AlN(H)SiEt3]2 and [Me2AlN(H)SiPh3]2 indicates planar Al2N2 ring frameworks with the silyl groups oriented trans about the ring. A distorted-tetrahedral geometry at nitrogen places the silyl groups nearly in the plane of the Al2N2 ring, while the N-H bond vector is nearly orthogonal to the Al2N2 plane. The degree of distortion depends on the steric bulk of SiR3. All dimers undergo a cis-trans isomerization process in benzene. This process is fast for methylaluminum compounds but slow for isobutylaluminum ones: the methylaluminum amides attain equilibrium solution isomer distributions of ca. 50/50, while the isobutyl compounds are strongly biased toward the cis or trans isomer and do not attain equilibrium. Isomerization of the methylaluminum compounds is catalyzed by less than one equiv of 4-methylpyridine, while hindered pyridines have no effect. Greater amounts of 4-methylpyridine lead to adduct formation; the adduct (i-Bu)2(4-Me-py)AlN(H)SiPh3 is isolated. A mechanism with rate-determining attack of base at aluminum is discussed. Crystal data for [Me2AlN(H)SiPh3]2: M(r) = 662.96, triclinic, P1BAR, a = 8.821 (2) angstrom, b = 9.435 (3) angstrom, c = 13.342 (4) angstrom, alpha = 70.97 (3)-degrees, beta = 74.23 (2)-degrees, gamma = 66.02 (2)-degrees, Z = 1, R = 0.041, R(w) = 0.042. Crystal data for [Me2AlN(H)SiEt3]2: M(r) = 374.6, monoclinic, P2(1)/n, a = 7.095 (3) angstrom, b = 11.452 (5) angstrom, c = 15.467 (5) angstrom, beta = 96.83 (3)-degrees, Z = 2, R = 0.055, R(w) = 0.052.