Ternary iron(II) complexes of phenanthroline bases showing DNA binding and cleavage activity

Ternary iron(II) complexes [Fe(L)B](PF6)2 (1-3), where L is formed from FeCl3 mediated cyclization of N,N'-(2hydroxypropane-1,3-diyl)bis(pyridine-2-aldimine) (L) and B is a phenanthroline base, viz. 1,10-phenanthroline (phen, 1), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq, 2) and dipyrido[3,2-a:2′,3′-c]phenazine (dppz, 3), are synthesized and complex 1 is structurally characterized by X-ray crystallography. The diamagnetic iron(II) complexes with L and B showing respective tetradentate and bidentate chelating modes in a FeN6 coordination geometry exhibit two metal-centered visible electronic bands near 520 and 570 nm in MeCN. The complexes bind to calf thymus DNA giving an order: phen (1) < dpq (2) < dppz (3). Control DNA cleavage experiments using supercoiled (SC) pUC19 DNA and distamycin-A suggest major groove binding for the dppz and minor groove binding for the other complexes. The redox active complexes cleave SC DNA to its nicked circular (NC) form with H2O2 involving hydroxyl radical as the cleavage active species. The complexes show photoinduced DNA cleavage activity at UV light of 365 nm (12 W) in absence of H2O2 via singlet oxygen (O2) mechanism. Complexes 2 and 3 having respective photoactive dpq and dppz ligands show relatively poor DNA cleavage activity at visible light.