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

The interaction of cytosine (CyH), thymine (TH) and uracil (UH) with the dinuclear Zn(II) complexes of ligand 1,4,7,16,19,22-hexaza-10,13,25,28-tetraoxacyclotriacontane (L) has been studied by means of potentiometric titrations (0.1 mol dm−3 NMe4NO3, 298.1 K), and 1H NMR measurements. The equilibrium constants for the formation of the ternary complexes with the nucleobases indicate that the [Zn2L]4+ complex displays similar binding features towards thymine and uracil. The dizinc complex forms stable 1:1 and 1:2 complexes with both deprotonated thymine and uracil at neutral or slightly alkaline pH values. The constants for the addition of first and second substrate molecules are almost equal indicating that each nucleobase is coordinated, almost independently, by one metal ion of [Zn2L]4+. This feature is confirmed by the crystal structures of [Zn2LT2]2+ and [Zn2LU2]2+. In the [Zn2LA2]2+ (A−=T−, U−) complexes the two metal ions have almost the same coordination environment and each Zn(II) is coordinated to three nitrogens of the macrocycle and to a deprotonated nitrogen of the nucleobases. In the crystal packing the dinuclear clusters give rise to a supramolecular architecture based on infinite pillars of [Zn2LA2]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 deprotonated nucleobase bridges the two metal ions.