Photosensitized samarium(iii) and erbium(iii) complexes of planar N,N-donor heterocyclic bases: crystal structures and evaluation of biological activity

Abstract

Samarium(III) and erbium(III) complexes, namely [Sm(dpq)(DMF)₂(H₂O)Cl₃] (1), [Sm(dppz)(DMF)₂(H₂O)Cl₃] (2), [Er(dpq)(DMF)₂Cl₃] (3) and [Er(dppz)₂Cl₃] (4) (dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq in 1 and 3), dipyrido[3,2-a:2′,3′-c]phenazine (dppz in 2 and 4), N,N′-dimethylformamide (DMF) and water (H₂O)), have been synthesized and structurally characterized. The X-ray crystal structures of complexes 1–4 show discrete mononuclear Ln(III)-based structures. Sm(III) in [Sm(dpq)(DMF)₂(H₂O)Cl₃] (1) and [Sm(dppz)(DMF)₂(H₂O)Cl₃] (2) adopts an eight-coordinated distorted square antiprism structure with a bidentate N,N-donor dpq/dppz ligand, three Cl⁻ anions, two DMF molecules and one water molecule. The Er(III) complexes, [Er(dpq)(DMF)₂Cl₃] (3) and [Er(dppz)₂Cl₃] (4), show a seven-coordinated monocapped octahedral structure where Er(III) is coordinated to a bidentate dpq/dppz ligand, two DMF molecules and three Cl⁻ anions. The crystal lattices of the complexes show intermolecular π–π stacking interactions between the planar dpq and dppz ligands. Considering the planarity and photosensitizing ability of the coordinated dpq and dppz ligands, the complexes were studied for their binding interaction with DNA and proteins and their photo-induced DNA cleavage activity. They display significant binding propensity to CT-DNA (K_b ∼ 10⁴ M⁻¹) in the order: 2, 4 (dppz) > 1, 3 (dpq). Complexes 1–4 bind DNA through groove binding and partial intercalation. All the complexes also show binding propensity (K_BSA ∼ 10⁵ M⁻¹) to bovine serum albumin (BSA) protein. Complexes 1–4 efficiently cleave supercoiled (SC) ds-DNA to its nicked circular (NC) form upon exposure to UV-A light of 365 nm via formation of singlet oxygen (¹O₂) and a hydroxyl radical (HO˙) as reactive oxygen species in a photoredox pathway.