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 P2₁2₁2₁, with a = 7.586(1) Å, b = 10.715(3) Å, and c=28.13(2) Å, Z = 4, R₁ = 0.0572, and wR₂ = 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.