SYNTHESIS AND CHARACTERIZATION OF AN AZA-CAGE, BASICITY BEHAVIOR AND CRYSTAL-STRUCTURE OF ITS DIPROTONATED SPECIES

The synthesis and characterization of the macrobicycle methyl-1,4,7,10 -tetraazabicyclo[5.5.5]-heptadecane (L) is reported. The proton-transf er properties of L in aqueous solution have been investigated by poten tiometry (25-degrees-C, I = 0.15 mol dm-3) and NMR spectroscopy (H-1 a nd C-13). L behaves as a fairly strong base in the first protonation s tep (log K1 = 11.55), and as a moderate base in the second step (log K 2 = 6.94). H-1 and C-13 resonances have been assigned for both [HL]+ a nd [H-2L]2+ species. NMR experiments also indicate the stepwise proton ation sites. Crystals of [H-2L]-[ClO4]2 are orthorhombic, space group Pca2(1), with a = 13.130(1), b = 16.185(4), c = 20.320(6) angstrom, an d Z = 8; final R value of 0.086 (R(w) = 0.079) for 2383 unique observe d reflections with I > 3sigma(I). Two independent [H-2L]2+ cations are present in the unit cell; they differ from each other in the conforma tion of the hydrocarbon chain connecting the two unmethylated nitrogen atoms. In both cases the overall cage conformation is due to the pres ence of two internal hydrogen bonds, connecting the two bridgehead nit rogens and the two methylated nitrogens.