Molecular recognition modes between adenine or adeniniun(1+) ion and binary MII(pdc) chelates (M=Co-Zn; pdc = pyridine-2,6-dicarboxylate(2-) ion)

Mixed ligand Mu-complexes (M=Co-Zn) with pyridine-2,6-dicarboxylate(2-) chelator (pdc) and adenine (Hade) have been synthesized and studied by X-ray diffraction and other spectral and thermal methods: [Cu(pdc)(H(N9)ade)(H2O)] (1), [Cu2(pdc)(2)(H2O)(2)(mu(2)-N3,N7-H(N9)ade)]center dot 3H(2)O(2), trans-[M(pdc)(H(N9) ade)(H2O)(2)]center dot nH(2)O for M=Co (3-L, 3-M, 3-H) or Zn (4-L, 4-H), where n is 0,1 or 3 for the 'lowest' (L), 'medium' (M) and 'highest' (H) hydrated forms, and the salt trans-[Ni(pdc)(H-2(N1,N9)ade)(H2O)(2)]Cl center dot 2H(2)O (5). In all the nine compounds, both neutral and cationic adenine exist as their most stable tautomer and the molecular recognition pattern between the metal-pdc chelates and the adenine or adeninium(1+) ligands involves the M-N7 bond in cooperation with an intra-molecular N6-H center dot center dot center dot O(coordinated carboxylate) interligand interaction. In addition the dinuclear copper(II) compound (2) has the Cu-N3 bond and the N9-H center dot center dot center dot O(coord. carboxylate) interaction. The structures of mononuclear ternary complexes proved that the molecular recognition pattern is the same irrespective of (a) the coordination geometry of the complex molecule, (b) the different hydrated forms of crystals with Co or Zn, and (c) the neutral of cationic form of the adenine ligand. These features are related to the mer-NO2 chelating ligand conformation (imposed by the planar rigidity of pdc) as a driving force for the observed metal binding mode. (C) 2013 Elsevier Inc. All rights reserved.