Synthesis, crystal structure and nuclease activity of bis(dipyridoquinoxaline)copper(I)perchlorate

Abstract

Copper(I) complex [Cu(dpq)₂](ClO₄) (1), where dpq is dipyrido-[3,2-d:2',3'-f]-quinoxaline, is prepared and structurally characterized. The complex crystallizes in the orthorhombic space group Pnna with the unit cell parameters a = 7.7797(9) A, b = 20.185(2) A, c = 16.593(2) Å, V = 2605.6(5) ų, Z = 4. The structure was refined to R = 0.0713 for 2525 reflections with 1127 having 1 > 2σ(1) and 192 parameters. The cationic complex has the copper(I) center bonded to two N,N-donor dpq ligands giving a distorted tetrahedral coordination geometry with a Cu-N distance of 2.05 Å. The crystal structure displays extensive π-π stacking interactions involving the planar aromatic rings of dpq. Such an interaction gives rise to the formation of chains and the perchlorate anions are located in-between the chains. The complex is diamagnetic and exhibits a metal to ligand charge transfer band at 442 nm (ε, 6000 μ⁻¹ cm⁻¹) in MeCN and a perchlorate stretch at 1088 cm⁻¹ in the IR spectrum in KBr phase. It shows a quasi-reversible cyclic voltammetric response at 0.17 V (ΔE^P = 120 mV) at 50 mV s⁻¹ in DMF for the Cu(II)/Cu(I) couple. Complex 1 is a chemical nuclease. It shows efficient nuclease activity on treatment with supercoiled DNA in presence of hydrogen peroxide. The nuclease activity of 1 is found to be better than [Cu(phen)₂]⁺ under similar reaction conditions. Mechanistic studies have shown minor groove binding of the complex and the involvement of hydroxyl radicals in the DNA cleavage reactions.