Synthesis of [3(2)]Bis([2(4).3(1)]adamanzane), 1,4,8,11,15,18,22,25-octaazapentacyclo-[20.6.2.2(4,25).2(8,15).2(11,18)]hexatriacontane and crystal structure of the tetrachlorozincate salt of the tetraprotonated octaamine

The reaction of [2(4).3(1)]adamanzane, 1,4,7,10-tetraazabicyclo [5.5.3]pent adecane, with the bis(3-chloropropyl) derivative of [2(4).3(1)]adamanzane, 4,10-bis(3-chloropropyl)-1,4,7,10-tetraazabicyclo[ 5.5.3]pentadecane, affor ds the dimeric octaamine [3(2)]bis([2(4).3(1)]adamanzane), 1,4,8,11,15,18,2 2,25-octaazapentacyclo-[20.6.2.2(4,25).2(8,15).2(11,18)]hexatriacontane The tetraprotonated form having two inside and two outside bound hydrogen ions was isolated as a tetrachlorozincate salt, {[32(])bis(i(+),i,i,o(+)-H-2[2( 4).3(1)]adz)}(ZnCl4)(2). 1.5H(2)O in high yield (62%), and its crystal stru cture was solved by X-ray diffraction at T = 120 K. The two halves of the c ationic dimer are related by a centre of symmetry. The acidic hydrogen atom attached to N(1) is oriented away from the cavity, whereas the hydrogen at om attached to N(2) as well as the lone pairs at N(3) and N(4) point toward s the inside of the cage. The acidic hydrogen atom at N(2) is hydrogen-bond ed to N(4), the H(2) ... N(4) distance being 1.74(3) Angstrom. From potentiometric measurements it is estimated that the tetraprotonated s pecies has pK(a1) = 4.23(3) and pK(a2) = 5.43(12) (25 degrees C, 1 M NaCl), while the last two pK(a) values are assumed to be greater than 14.