Thermodynamic and Steady-State Fluorescence Emission Studies on Metal Complexes of Receptors Containing Benzene Subunits

The thermodynamic properties of the Co<sup>2+</sup>, Ni<sup>2+</sup>, Cu<sup>2+</sup>, Zn<sup>2+</sup>, Cd<sup>2+</sup>, and Pb<sup>2+</sup> 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(<b>L7</b>)(H<sub>2</sub>O)](ClO<sub>4</sub>)<sub>2</sub><b></b>(<b>L7</b> = 1-benzyl-1,5,8,12-tetrazadodecane) shows a very strongly axially distorted square planar coordination geometry for Cu<sup>2+</sup>. Crystals of [Cu(<b>L7</b>)(H<sub>2</sub>O)](ClO<sub>4</sub>)<sub>2</sub><b></b>(C<sub>15</sub>H<sub>24</sub>Cl<sub>2</sub>CuN<sub>4</sub>O<sub>9</sub>) are orthorhombic, space group <i>P</i>2<sub>1</sub>2<sub>1</sub>2<sub>1</sub>, with <i>a</i> = 7.586(1) Å, <i>b </i>= 10.715(3) Å, and <i>c</i> = 28.13(2) Å, <i>Z</i> = 4, R<sub>1</sub> = 0.0572, and wR<sub>2</sub> = 0.1570. Steady-state fluorescence emission studies performed on the Cu<sup>2+</sup> and Zn<sup>2+</sup> complexes show that, while none of the Cu<sup>2+</sup> complexes is emissive (CHEQ effect), fluorescence emission is observed for those Zn<sup>2+</sup> 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 Cu<sup>2+</sup> and Zn<sup>2+</sup> 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.