Thermodynamic and steady-state fluorescence emission studies on metal complexes of receptors containing benzene subunits

The thermodynamic properties of the Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ complexes of a family of N,N'-dibenzylated open-chain polyamines are described. For comparison, similar studies are reported for polyazacyclophane macrocyclic receptors containing an aromatic subunit linking the ends of a polyamine bridge. The metal complexes of the dibenzylated ligands show lower stability constants than those reported for related nonbenzylated open-chain polyamines. On the other hand, the stability constants of these complexes are clearly higher than those found for complexes of polyazacyclophane macrocycles containing a single para-substituted benzene spacer interrupting saturated polyamine bridges. All the studied complexes follow the Irving-Williams stability order. The crystal structure of [Cu(L7)(H2O)](ClO4)(2) (L7 = 1-benzyl-1,5,8,12-tetrazadodecane) shows a very strongly axially distorted square planar coordination geometry for Cu2+. Crystals of [Cu(L7)(H2O)](ClO4)(2) (C15H24Cl2CuN4O9) are orthorhombic, space group P2(1)2(1)2(1), With a = 7.586(1) Angstrom, b = 10.715(3) Angstrom, and c = 28.13(2) Angstrom, Z = 4, R-1 = 0.0572, and wR(2) = 0.1570. Steady-state fluorescence emission studies performed on the Cu2+ and Zn2+ complexes show that, while none of the Cu2+ complexes is emissive (CHEQ effect), fluorescence emission is observed for those Zn2+ complexes with all the nitrogen donors either protonated or coordinated to the metal ions (CHEF effect). The composition of the frontier molecular orbitals of the free-ligands and of the Cu2+ and Zn2+ complexes supports this behavior. The use of these water-soluble ligands as chemosensors by means of enhancement or quenching of the fluorescence emission is also discussed.