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 1-adamantyl (1-Ad), 4) are described. They were characterized by X-ray crystallography and ¹H 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 Å. 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 π-bonding. The structural data together with VT ¹H 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 Kα (λ = 0.710 69 Å) at 130 K: 1, C₂₆H₄₀A1N, a = 9.965(3) Å, b = 13.746(5) Å, c = 17.700(6) Å, orthorhombic, space group Pca2₁, Z = 4, R = 0.069; 2, C₄₄H₄₈AlNSi₂, a = 12.623(4) Å, b = 16.880(3) Å, c = 18.600(2) Å, α = 93.45(1)°, β = 100.25(2)°, γ = 99.46(2)°, triclinic, space group P1̅, Z = 4, R = 0.051; 3, C₃₈H₅₀AlNSi, a = 10.413(4) Å, b = 10.585(6) Å, c = 18.281(6) Å, a = 97.75(4)°, β = 96.41(3)°, γ = 118.97(2)°, triclinic, space group P1̅, Z = 2, R = 0.050; 4, C₃₆H₄₈AlNSi, a = 10.372(3) Å, b = 17.957(8) Å, c = 19.184(8) Å, a = 113.87(3)°, β = 99.23(3)°, γ = 96.71(3)°, triclinic, space group P1̅, Z = 4, R = 0.045; 5, C₄₆H₅₀AlLiN₂, a = 37.78(2) Å, b = 10.635(4) Å, c = 19.237(7) Å, β = 98.19(3)°, monoclinic, space group C2/c, Z = 8, R = 0.056.