Synthesis of [3 sup2]Bis([2. sup4.3sup1]adamanzane), 1,4,8,11,15,18,22,25-Octaazapentacyclo-[20.6.2.2 sup4,25.2 sup11,18]hexatriacontane, and Crystal Structure of the Tetrachlorozincate Salt of the Tetraprotonated Octaamine

The reaction of [2 sup 4. 3 sup1]adamanzane, 1,4,7,10-tetraazabicyclo[5.5.3]pentadecane, with the bis(3-chloropropyl) derivative of [2 sup4. 3 sup1]adamanzane, 4,10-bis(3-chloropropyl)- 1,4,7,10-tetraazabicyclo [5.5.3]pentadecane, affords the dimeric octaamine [3 sup2]bis([2 sup4. 3 sup1]adamanzane), 1,4,8,11,15,18,22,25- octaazapentacyclo-[20.6.2.2 sup 4.25.2 sup11.18] hexatriacontane. The tetraprotonated form having two inside and two outside bound hydrogen ions was isolated as a tetrachlorozincate salt, {[3 sup2] bis(i sup+,i,i,o sup+-H sub2[2 sup4.3 sup1]adz} (ZnCl sub4)sub2 x 1.5H sub2 O in high yield (62%), and its crystal structure was solved by X-ray diffraction at T=120 K. The two halves of the cationic dimer are realted by a centre of symmetry. The acidic hydrogen atom attached to N(1) is oriented away from the cavity, whereas the hydrogen atom attached to N(2) as well as the lone pairs at N(3) and N(4) point towards the inside of the cage. The acidic hydrogen atom at N(2) is hydrogen-bonded to N(4), the H(2)...N(4) distance being 1.74(3) Å.From potentiometric measurements it is estimated that the tetraprotonated species has pK suba1= 4.23(3) and pK sub a2=5.43(12)(25 degrees C, 1 M NaCl), while the last two pK sub a values are assumed to be greater than 14.