Anion-Cation Recognition Pattern, Thermal Stability and DFT-Calculations in the Crystal Structure of H(2)dap[Cd(HEDTA)(H2O)] Salt (H(2)dap = H-2(N3,N7)-2,6-Diaminopurinium Cation)

The proton transfer between equimolar amounts of [Cd(H(2)EDTA)(H2O)] and 2,6-diaminopurine (Hdap) yielded crystals of the out-of-sphere metal complex H-2(N3,N7)dap[Cd(HEDTA)(H2O)]H2O (1) that was studied by single-crystal X-ray diffraction, thermogravimetry, FT-IR spectroscopy, density functional theory (DFT) and quantum theory of "atoms-in-molecules" (QTAIM) methods. The crystal was mainly dominated by H-bonds, favored by the observed tautomer of the 2,6-diaminopurinium(1+) cation. Each chelate anion was H-bonded to three neighboring cations; two of them were also connected by a symmetry-related anti-parallel pi,pi-staking interaction. Our results are in clear contrast with that previously reported for H-2(N1,N9)ade [Cu(HEDTA)(H2O)]2H(2)O (EGOWIG in Cambridge Structural Database (CSD), Hade = adenine), in which H-bonds and pi,pi-stacking played relevant roles in the anion-cation interaction and the recognition between two pairs of ions, respectively. Factors contributing in such remarkable differences are discussed on the basis of the additional presence of the exocyclic 2-amino group in 2,6-diaminopurinium(1+) ion.