SYNTHESIS AND CHARACTERIZATION OF THE MONOMERIC ALUMINUM MONOAMIDES (T-BU)2ALN(R)R'(R AND R' = BULKY ARYL, ALKYL, OR SILYL GROUPS)

The synthesis and characterization of the compounds (t-Bu)2AlNR2 (R = Mes, 1, and SiPh3, 2) and (t-Bu)2AlN(R')SiPh3 (R' = 2,6-i-Pr2C6H3 (Dipp), 3, and I-adamantyl (1-Ad), 4) are described. They were characterized by X-ray crystallography and H-1 NMR and IR spectroscopy. In the crystal phase, the structures of 1-4 are monomeric with essentially trigonal planar coordinations at aluminum and nitrogen and Al-N bond lengths that range from 1.82 to 1.89 angstrom. The variation in the Al-N bond lengths are a consequence of the steric interactions between the large groups at aluminum and nitrogen, different change separations across the Al-N bond, and, in some instances, weak Al-N pi-bonding. The structural data together with VT H-1 NMR studies of these and related compounds indicate that a decrease in the size of these groups affords considerably shorter Al-N distances and, in some cases, barriers to rotation around the Al-N bond that have a maximum value of ca. 9-10 kcal mol-1. The synthesis and characterization of [(t-Bu)2Al{N(H)CPh3}2(Li)], 5, are also described. Crystal data with Mo Kalpha (lambda = 0.710 69 angstrom) at 130 K: 1, C26H40AlN, a = 9.965(3) angstrom, b = 13.746(5) angstrom, c = 17.700(6) angstrom, orthorhombic, space group Pca2(1), Z = 4, R = 0.069; 2, C44H48AlNSi2, a = 12.623(4) angstrom, b = 16.880(3) angstrom, c = 18.600(2) angstrom, a = 93.45(1)-degrees, beta = 100.25(2)-degrees, gamma = 99.46(2)-degrees, triclinic, space group P1BAR, Z = 4, R = 0.051; 3, C38H50AlNSi, a = 10.413(4) angstrom, b = 10.585(6) angstrom, c = 18.281(6) angstrom, alpha = 97.75(4)-degrees, beta = 96.41(3)-degrees, gamma = 118.97(2)-degrees, triclinic, space group P1BAR, Z = 2, R = 0.050; 4, C36H48AlNSi, a = 10.372(3) angstrom, b = 17.957(8) angstrom, c = 19.184(8) angstrom, a = 113.87(3)-degrees, beta = 99.23(3)-degrees, gamma = 96.71(3)-degrees, triclinic, space group P1BAR, Z = 4, R = 0.045; 5, C46H50AlLiN2, a = 37.78(2) angstrom, b = 10.635(4) angstrom, c = 19.237(7) angstrom, beta = 98.19(3)-degrees, monoclinic, space group C2/c, Z = 8, R = 0.056.