Chiral and Achiral Macrocyclic Copper(II) Complexes: Synthesis, Characterization, and Comparative Binding Studies with Calf‐Thymus DNA

The new chiral macrocyclic complexes [1,2‐bis(1 H ‐benzimidazol‐2‐yl)‐1‐(1,8‐dihydro‐1,3,5,8,10,12‐hexaazacyclotetradecane)‐2‐hydroxyethanolate] copper(II) and ‐nickel(II) perchlorate, 3 and 4 , respectively, were synthesized by the reaction of 1,2‐bis(1 H ‐benzimidazol‐2‐yl)ethane‐1,2‐diol ( L ) and (1,8‐dihydro‐1,3,5,8,10,12‐hexaazacyclotetradecane)copper(II) and ‐nickel(II) diperchlorate complexes, 1 and 2 , respectively. All complexes were characterized by various spectroscopic techniques. Molar‐conductance measurements showed that all of the complexes are ionic in nature. In complexes 3 and 4 , the metal center is encapsulated by the ligand L in a pentacoordinated environment. The optical‐rotation values ([ α ] D ) of 3 and 4 at 25° indicate that the complexes are chiral. Absorption‐ and fluorescence‐spectral studies, cyclic voltammetry, and viscosity measurements have been carried out to assess the comparative binding of complexes 1 and 3 with calf thymus (CT)‐DNA. Analysis of the results suggests that the new chiral complex 3 binds to CT‐DNA through a partial intercalation mode that is different from the binding mode of parent achiral complex 1 . The complexes 1 and 3 bind to CT‐DNA with binding constants K b of 2.7×10 4 and 6.6×10 4   M −1 , respectively. Circular‐dichroism (CD) studies have been further employed to ascertain the binding mode of complex 3 , which is consistent with the other spectral studies.