Two macrocycles of different molecular topology obtained by the same synthetic procedure. Their crystal structures and ligational properties

Abstract

The synthesis and characterization of the two macrobicycles 10-methyl-4,15-dioxo-1,7,10-triaza[5.5.5]cycloheptadecane (1) and 27,32-dimethyl-4,10,16,22-tetraoxa-1,7,13,19,27,32-hexaazatricyclo-[17.5.5.5^7,13]tetratriacontane (2) is reported. The basicity behaviour in aqueous solution have been studied by potentiometry (25°C, I = 0.15 mol dm⁻³) and NMR spectroscopy (¹H and ¹³C). Compound 1 behaves as diprotic base (log K₁ = 11.46(9), log K₂ = 5.40(9)), while compound 2 behaves as a pentaprotic base (log K₁ = 11.1(1), log K₂ = 10.85(9), log K₃ = 4.72(9), log K₄ = 2.76(9), log K₅ = 2.54(9)). NMR experiments indicate most of the stepwise protonation sites. Crystals of [H1][BPh₄] are monoclinic, space group P2₁/n, with a = 11.532(2), b = 20.538(2), c = 13.974(2) Å, β = 90.32(1)° and Z = 4; final R value of 0.085 (R_w = 0.080) for 1253 unique observed reflections with I < 4σ(I). The structure shows that the molecule adopts a fairly regular square pyramidal shape with N(1), N(2), O(1) and O(2) forming the basal plane and N(3) the vertex. Geometrical considerations indicate the methylated nitrogen N(3) as the protonation site. Crystals of [H₂2][BPh₄]₂ are triclinic, space group P 1, with a = 11.320(2), b = 12.576(3), c = 12.675(6) Å, α = 83.42(3), β = 73.61(2), γ = 75.68(2)° and Z = 1; final R value of 0.086 (R_w = 0.083) for 1058 unique observed reflections with I < 3σ(I). The macrotricycle has a cylindrical molecular shape, where the two bases are constituted by two N₃O macrocycles and the two O(CH₂-CH₂-)₂ chains representing the cylindrical wall.