Synthesis and characterization of an aza-cage, basicity behaviour and crystal structure of its diprotonated species

Abstract

The synthesis and characterization of the macrobicycle 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.5]-heptadecane (L) is reported. The proton-transfer properties of L in aqueous solution have been, investigated by potentiometry (25 °C, I= 0.15 mol dm^–3) and NMR spectroscopy (¹H and ¹³C). L behaves as a fairly strong base in the first protonation step (log K₁= 11.55), and as a moderate base in the second step (log K₂= 6.94). ¹H and ¹³C resonances have been assigned for both [HL]⁺and [H₂L]²⁺ species. NMR experiments also indicate the stepwise protonation sites. Crystals of [H₂L]-[ClO₄]₂ are orthorhombic, space group Pca2₁ with a= 13.130(1), b= 16.185(4), c= 20.320(6)Å, and Z= 8; final R value of 0.086 (R_w= 0.079) for 2383 unique observed reflections with I > 3σ(I). Two independent [H₂L]²⁺ cations are present in the unit cell; they differ from each other in the conformation of the hydrocarbon chain connecting the two unmethylated nitrogen atoms. In both cases the overall cage conformation is due to the presence of two internal hydrogen bonds, connecting the two bridgehead nitrogens and the two methylated nitrogens.