Supramolecular Assembly of Protonated Xanthine Alkaloids in Their Perchlorate Salts

摘要

Single crystal X-ray structures of new perchlorate salts of a series of four xanthine alkaloids, for example, xanthine (xt), caffeine (cf), theophylline (tp), and theobrornine (tb) are reported. The treatment of xanthine derivatives with 70% of HClO4 afforded their salts Hxt center dot ClO4 center dot 2(H2O), 1; Htp center dot ClO4, 2; Htb center dot ClO4, 3; and Hcf center dot ClO4 center dot H2O, 4. Xanthine form dimers via N(9)-H center dot center dot center dot O-C(6) hydrogen bond in the crystal structure. Of the two possible resonance structures of the protonated imidazoles, one of the structures was found to be major in xanthine, theobromine, and caffeine salts, while both structures are of equal contribution in theophylline salts. These observations also have been verified by using the structural information available in the Cambridge Structural Database. Interestingly, xanthine 1 is present as a dihydrate and 4 as a monohydrate, whereas 2 and 3 are crystallized without any water. Thus, proton transfer from the strong acid HClO4 to the weakly basic imidazole ring of xanthines occurs which leads to the build up of H-bonded supramolecular networks.