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

Abstract

A copper(I) complex [Cu(dpq)₂](ClO₄) (<b>1</b>), where dpq is dipyrido-[3,2-<i>d</i>:2',3'-<i>f</i>]-quinoxaline, is prepared and structurally characterized. The complex crystallizes in the orthorhombic space group <i>P</i>nna with the unit cell parameters <i>a</i> =7.7797(9)Å, <i>b </i>= 20.185(2)Å, <i>c</i> = 16.593(2) Å, <i>V </i>= 2605.6(5) ų, Z = 4. The structure was refined to <i>R </i>= 0.0713 for 2525 reflections with 1127 having <i>I </i>&gt; 2σ(<i>I</i>) 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 2.05Å. The crystal structure displays extensive π-π<i> </i>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 <i>M-</i>¹<i> </i>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 <b>1</b> is a chemical nuclease. It shows efficient nuclease activity on treatment with supercoiled DNA in presence of hydrogen peroxide. The nuclease activity of <b>1</b> 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.