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 molecul ar architecture defines a coordinative 'cleft,' a potential binding site fo r metals and anionic species. Their coordination properties toward Zn(II), Cd(II) and Pb(II) were studied by means of potentiometric, microcalorimetri c 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 th e crystal structure of the [ZnL2](2+) cation. On the other hand, the analys is 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 t o a stiffening of the ligands. Both L1 and L2, in their protonated forms, b ehave as multifunctional receptors for the nucleotide anions at neutral or slightly acidic pH, giving 1:1 complexes. Binding of diphosphate, triphosph ate, 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 b etween the polyphosphate chain of nucleotides and the polyammonium groups o f L1 and L2, whereas the adenine moiety shows charge-dipole interactions wi th the ammonium groups and x-stacking with the heteroaromatic units of the receptors. Copyright (C) 2001 John Wiley & Sons, Ltd.