AN INERT PROTON COORDINATED INSIDE A NEARLY TETRAHEDRAL TETRAAMINE CAVITY - SYNTHESIS AND X-RAY CRYSTAL-STRUCTURE OF THE AQUATRICHLOROZINCATE(II) SALT OF THE INSIDE MONOPROTONATED AMINE 1,4,8,11-TETRAAZATRICYCLO[6.6.2.2(4,11)]OCTADECANE

The reaction of the bicyclic tetraamine L1 . 3HBr (L1 = 1,4,7,10-tetraazabicyclo[5.5.3]pentadecane) with the ditosylate of 1,3-propanediol affords the inside protonated form of the tricyclic amine 1,4,8,11-tetraaza tricyclo[6.6.2.2(4.11)]octadecane (L). Monoprotonated L was isolated as a perchlorate, [HL]ClO4 (57%). The tricyclic structure of HL(+) was established by H-1 and C-13 NMR spectroscopy, which shows that the cation in solution has a time-averaged D-2d symmetry. The crystal structure of the salt [HL][Zn(H2O)Cl-3] . H2O has been solved by X-ray diffraction at T = 120 K, M(r) = 461.2, monoclinic, P2(1), a = 11.505(4), b = 8.186(4), c = 11.185(6) Angstrom, beta = 105.87(3)degrees, Z = 2, D-x = 1.52 g cm(-3), MoK alpha = 0.71073 Angstrom, mu = 16.2 cm(-1), F(000) = 484, R(F) = 0.0448 for 2411 reflections with I > 2 sigma(1) and wR(F-2) = 0.1032 for all 3244 unique reflections. In the HL(+) cation the acidic hydrogen atom is situated between the two bridgehead nitrogen atoms N(2) and N(4), the N(2)... H and the N(4)... H distances being 1.38(9) and 1.35(9) Angstrom, respectively. The distances from the hydrogen atom to the other two bridgehead nitrogen atoms N(1) and N(3) are 2.29(9) and 2.13(9) Angstrom, respectively. The distance between N(2) and N(4) is 2.636(7) Angstrom and that between N(1) and N(3) is 3.081(7) Angstrom, giving rise to a slightly elongated conformation of the 12-membered tetraaza ring. HL(+) is aprotic in the pH range 0-14 and it is unusually inert with respect to exchange of the proton bound to the four nitrogen atoms. From H-1 NMR measurements it is estimated that the rate constant for the dissociation of the proton in 1 M NaOD must be less than 2 x 10(-8) s(-1) at 25 degrees C.