Reactions of (H₂AlMes*)₂ (Mes* = 2,4,6-(t-Bu)₃C₆H₂) with H₂EAr (E = N, P, or As; Ar = aryl):  Characterization of the Ring Compounds (Mes*AlNPh)₂ and (Mes*AlEPh)₃ (E = P or As)

The reaction between sterically crowded, dimeric, arylalane (H₂AlMes*)₂ and some aryl amines, phosphines, or arsines are described. Treatment of (H₂AlMes*)₂ (Mes* = 2,4,6-(t-Bu)₃C₆H₂) with the amines H₂NPh and H₂NDipp (Dipp = 2,6-(i-Pr)₂C₆H₃) affords the monomeric bis(amino)alane products Mes*Al(NHPh)₂·Et₂O (1·Et₂O) and Mes*Al(NHDipp)₂ (2) which feature three-coordinate aluminum and nitrogen centers and short Al−N bonds. Heating of a 1:1 mixture of H₂AlMes* and aniline under carefully controlled conditions affords the dimer (Mes*AlNPh)₂, 3. Reaction of (Mes*AlH₂)₂ with H₂PPh or H₂AsPh at ca. 160 °C gives the trimeric, six-membered-ring compounds (Mes*AlPPh)₃·Et₂O (4·Et₂O) and (Mes*AlAsPh)₃·Et₂O (5·Et₂O) which are formal valence analogues of borazine. The structure of 3 features a planar Al₂N₂ core, which also extends to the ipso carbons of the aryl rings; the Al−N distance is 1.824(2) Å. The structures of 4 and 5 have six-membered rings of alternating aluminum and phosphorus or arsenic atoms arranged in a nonplanar boat conformation. Little or no delocalization of the phosphorus or arsenic lone pairs is evident since the average Al−P and Al−As distances are consistent with single bond lengths. Moreover, pyramidal coordination is observed at all the pnictide centers. The structures of 5 and 4 represent respectively the first and second examples of bonding between three-coordinate aluminum and arsenic or phosphorus, whereas 3 represents a unique instance of an Al₂N₂ ring with organic substituents. The compounds were characterized by ¹H, ¹³C, and ³¹P (4, only) NMR spectroscopy and by X-ray crystallography in the case of 1, 3, 4, and 5. Crystal data with Cu Kα (λ = 1.54178 Å) radiation at 130 K:  1, a = 10.255(3) Å, b = 16.733(5) Å, c = 19.267(8) Å, β = 95.09(3)°, V = 3293(2) ų, Z = 4, space group P2₁/c, 3070 (I > 2σ(I)) data, R = 0.061; 3, a = 8.750(1) Å, b = 9.326(2) Å, c = 14.025(3) Å, α = 101.04(2)°, β = 92.90(1)°, γ = 113.11(1)°, V = 1088.7(4) ų, Z = 1, space group P1̄, 2629 (I > 2σ(I)) data, R = 0.053; 4, a = 9.383(5) Å, b = 17.719(7) Å, c = 23.603(14) Å, α = 80.86(4)°, β = 82.50(5)°, γ = 75.61°(4), V = 3736(3) ų, Z = 2, space group P1̄, 5626 (I > 2σ(I)) data, R = 0.084; 5, a = 9.404(7) Å, b = 17.79(2) Å, c = 23.643(10) Å, α = 81.01(6)°, β = 83.04(5)°, γ = 75.68(7)°, V = 3771(5) ų, Z = 2, space group P1̄, 6363 (I > 2σ(I)) data, R = 0.063.