Binding of nucleobases to a dizinc macrocyclic complex. Supramolecular assembling of dinuclear clusters through N-H center dot center dot center dot O and C-H center dot center dot center dot O hydrogen bonding

The interaction of cytosine (CyH), thymine (TH) and uracil (UH) with the di nuclear Zn(II) complexes of ligand 1,4,7,16,19,22-hexaza-10,13,25,28-tetrao xacyclotriacontane (L) has been studied by means of potentiometric titratio ns (0.1 mol dm(-3) NMe4NO3, 298.1 K), and H-1 NMR measurements. The equilib rium constants for the formation of the ternary complexes with the nucleoba ses indicate that the [Zn2L](4+) complex displays similar binding features towards thymine and uracil. The dizinc complex forms stable 1:1 and 1:2 com plexes with both deprotonated thymine and uracil at neutral or slightly alk aline pH values. The constants for the addition of first and second substra te molecules are almost equal indicating that each nucleobase is coordinate d, almost independently, by one metal ion of [Zn2L](4+). This feature is co nfirmed by the crystal structures of [(ZnLT2)-L-2](2+) and [Zn2LU2](2+). In the [Zn(2)LA(2)](2+) (A(-) =T-, U-) complexes the two metal ions have almo st the same coordination environment and each Zn(II) is coordinated to thre e nitrogens of the macrocycle and to a deprotonated nitrogen of the nucleob ases. In the crystal packing the dinuclear clusters give rise to a supramol ecular architecture based on infinite pillars of [Zn(2)LA(2)](2+) (A(-) = T -, U-) assembled via hydrogen bonding. Cytosine forms only 1:1 adducts with the dizinc complex and the stability constants values suggest that the dep rotonated nucleobase bridges the two metal ions. (C) 2001 Published by Else vier Science B.V.