Cleft-like hexaamine ligands containing large heteroaromatic moieties as receptors for both anions and metal cations

Ligands L1 and L2 contain two ethylenediamine chains linked to the 2,9 and 6,6' positions of phenanthroline and bipyridyl, respectively. Their molecular architecture defines a coordinative 'cleft,' a potential binding site for metals and anionic species. Their coordination properties toward Zn(II), Cd(II) and Pb(II) were studied by means of potentiometric, microcalorimetric and UV-vis spectrophotometric measurements. In the [ML](2+) complexes (L = L1 or L2), the metal is enveloped inside the ligand cleft, as shown by the crystal structure of the [ZnL2](2+) cation. On the other hand, the analysis of the thermodynamic data for metal complexation reveals that in the [ML](2+) complexes some nitrogen donors are weakly bound, or not bound, to the metal, owing to the presence of a rigid heteroaromatic unit, which leads to a stiffening of the ligands. Both L1 and L2, in their protonated forms, behave as multifunctional receptors for the nucleotide anions at neutral or slightly acidic pH, giving 1:1 complexes. Binding of diphosphate, triphosphate, ATP and ADP was studied by means of potentiometry and H-1 and P-31 NMR spectroscopy. Charge-charge and hydrogen bonding interactions take place between the polyphosphate chain of nucleotides and the polyammonium groups of L1 and L2, whereas the adenine moiety shows charge-dipole interactions with the ammonium groups and x-stacking with the heteroaromatic units of the receptors. Copyright (C) 2001 John Wiley & Sons, Ltd.