STRONG COOPERATIVE PROTONATION AND COPPER(II)-ASSISTED HYDROLYSIS OF THE NOVEL X-RAY CHARACTERIZED POLYOXAMIDE CRYPTAND AZA-BICYCLO[8.8.8]HEXACOSAN-5,6,14,15,22,23-HEXONE

The bicyclic cryptand aza-bicyclo[8.8.8]hexacosan-5,6,14,15,22,23-hexo ne (2) bearing oxamide groups along the eight-atom bridges has been sy nthesized and its crystal structure solved by single-crystal X-ray ana lysis. 2 crystallizes in the monoclinic system, space group P2(1)/n, w ith a = 12.246(4) Angstrom, b = 14.421(2) Angstrom, c = 13.708(3) Angs trom, beta = 112.21(3)degrees and Z = 4. The structure consists of dis crete neutral macrobicyclic units. The two cycles share the two tertia ry amine nitrogen atoms which exhibit an endo-endo conformation. The t wo tertiary amine groups are linked by three identical branches formed by N,N'-bis(ethyl)oxamide. The protonation reactions of 2 and its com plex formation with copper(II) have been investigated by potentiometry in water and in a dmso-water (80:20 w/w) mixture as solvents. 2 acts as a bis-base through its two N-bridgehead and exhibits a strong coope rativity that favours the first protonation and makes difficult the se cond one (Delta pK = 5.2). An inward rotation of the oxamide groups to form hydrogen bonds accounts for this cooperativity. The interaction of 2 with copper(II) leads to several dinuclear complexes which differ ent proton contents. The anionic ones are unstable and undergo hydroly tic decomposition to yield the novel dinuclear copper(II) complex 16-h exaazacyclooctadecane-5,6,14,15-tetronato(4-)} dicopper(II) (3).