Metallo-Supramolecular Structures by Self-Assembly through Weak Interactions in Mixed Ligand Metal Complexes of Adenine and Malonate

The reactions of sodium malonate with Co-II, Cu-II, or Zn-II salts in the presence of adenine afforded the compounds [Co(7H-ade-N9)(2)(H2O)(4)][Co(kappa(2)-mal)(H2O)(2)]center dot 4H(2)O (1), (1)(infinity)[Cu(mu-mal)(7H-ade-N9)(mu(2)-H2O)] (2), and [Zn-2(mu-mal)(2)(mu-7H-ade-N3,N9)(2)(H2O)(2)] center dot 2H(2)O (3), all of which have been characterized by elemental analyses, mass spectrometry, FT-IR, electronic and EPR spectroscopy, magnetic measurements, and single-crystal X-ray diffraction. Complex 1 is an ion-pair product managed by charge-assisted noncovalent interactions. Complex 2 is formed by binuclear [Cu(mal)(Hade)(H2O)](2) units bridged by malonate ligands, and 1D coordination polymeric chains are formed. Complex 3 is a centrosymmetric dimer, in which two Zn-II ions are bridged by two adenine ligands coordinated by N3 and N9 atoms from a pair of head-to-tail Hade ligands to give a mu-N3,N9 coordination mode. The metallo-supramolecular crystal structures of 1-3 are essentially maintained by hydrogen-bonding and pi,pi-stacking interactions involving the adenine, the carboxylate groups, and water molecules of coordination and crystallization (in compounds 1 and 3). In these structures, the nature of the malonate coordination is dependent on the metal, whereas the hydrogen bonding and pi-stacking capabilities of the adenine ligand influence the interactions observed in forming the three-dimensional structures. The magnetic properties of 2 were investigated in the temperature range of 2-300 K. Overall, weak antiferromagnetic behavior occurs (J = -3 cm(-1)) with a susceptibility maximum at 50 K, with the exchange pathway provided by aqua bridges [Cu-Cu separation of 3.3522(5) angstrom]. Very weak ferromagnetic interactions are also observed through the malonate ligand in an uncommon anti-skew noncoplanar coordination mode, with a copper-copper separation of 5.3828(5) angstrom. Magnetostructural comparisons with analogous compounds with oxo-bridged Cu(II) binuclear complexes are also provided. Spectroscopic and thermal stability data for all complexes are described in detail.