STERICALLY CROWDED ARYLOXIDE COMPOUNDS OF ALUMINUM - ELECTRONIC AND STERIC EFFECTS

The mononuclear, four-coordinate aluminum compounds AlR2(BHT)L and AlR(BHT)2L [R = Me, Et; BHT = 2,6-di-tert-butyl-4-methylphenoxide; L = 2-Mepy, 4-Mepy, 2,6-Me2py, CH3CN, N(C2H4)3CH, HNEt2, HN(i)Bu2, H2N(t)Bu, H2N(n)Bu, NH3, py.O] are obtained when AlR3 is treated with the correct molar equivalent of BHT-H and the appropriate Lewis base. The aluminum-methyl C-13 NMR chemical shift for the monoaryloxide compounds AlMe2(BHT)L is dependent primarily on the steric bulk of the Lewis base. The reaction of AlMe3 with pentafluorophenol yields [Me2Al(mu-OC6F5)]2. Cleavage of the phenoxide-bridged dimer by N(C2H4)3CH leads to the formation of the Lewis acid-base complex. The molecular structures of AlMe2(BHT)(2,6-Me2py) (4), AlEt2(BHT)(H2N(t)Bu) (16), AlEt2(BHT)(py.O) (19), AlMe(BHT)2(py.O).1/2CH2Cl2 (20), and AlMe2(OC6F5)[N(C2H4)3CH] (23) have been determined by X-ray crystallography. The relationship between Al-O bond distances, Al-O-C bond angles, and the presence of Al-O pi-bonding is discussed. Crystal data for 4: monoclinic, P2(1)/n, a = 9.994 (2) angstrom, b = 21.647 (4) angstrom, c = 10.822 (2) angstrom, beta = 93.74 (2) degrees (198 K), Z = 4, R = 0.058, R(w) = 0.066. Crystal data for 16: monoclinic, P2(1)/n, a = 9.025 (2) angstrom, b = 13.862 (2) angstrom, c = 19.339 (4) angstrom, beta = 96.60 (1) degrees (183 K), Z = 4, R = 0.060, R(w) = 0.068. Crystal data for 19: monoclinic, P2(1)/n, a = 9.630 (2) angstrom, b = 18.232 (4) angstrom, c = 13.980 (3) angstrom, beta = 101.62 (2) degrees (173 K), Z = 4, R = 0.055, R(w) = 0.055. Crystal data for 20: monoclinic, C2/c, a = 25.025 (4) angstrom, b = 16.877 (3) angstrom, c = 19.261 (5) angstrom, beta = 117.59 (2) degrees (193 K), Z = 8, R = 0.073, R(w) = 0.077. Crystal data for 23: triclinic, P1BAR, a = 6.9822 (6) angstrom, b = 9.4896 (8) angstrom, c = 12.7230 (9) angstrom, alpha = 93.140 (6) degrees, beta = 94.650 (6) degrees, = 102.580 (7) degrees (183 K), Z = 2, R = 0.039, R(w) = 0.044.