Rhenium complexes of chromophore-appended dipicolylamineligands: syntheses, spectroscopic properties, DNA binding and X-ray crystal structure

Abstract

The syntheses of two chromophore-appended dipicolylamine-derived ligands and their reactivity with pentacarbonylchlororhenium have been studied. The resultant complexes each possess the fac-Re(CO)₃ core. The ligands L¹11-[bis(pyridine-2-ylmethyl)amino]methylpyrene and L²2 2-[bis(pyridine-2-ylmethyl)amino]methylquinoxaline were isolated via a one-pot reductive amination in moderate yield. The corresponding rhenium complexes were isolated in good yields and characterised by ¹H NMR, MS, IR and UV-Vis studies. X-Ray crystallographic data were obtained for fac-{Re(CO)₃(L¹1)}(BF₄), C₃₄H₂₆BF₄N₄O₃Re: monoclinic, P2(1)/c, a = 18.327(2) Å, α = 90.00°, b = 14.1537(14) Å, β = 96.263(6)°, c = 23.511(3) Å, γ = 90.00°, 6062.4(11) ų, Z = 8. The luminescence properties of the ligands and complexes were also investigated, with the emission attributed to the appended chromophore in each case. Isothermal titration calorimetry suggests that fac-{Re(CO)₃(L¹1)}(BF₄) self-aggregates cooperatively in aqueous solution, probably forming micelle-like aggregates with a cmc of 0.18 mM. Investigations into the DNA-binding properties of fac-{Re(CO)₃(L¹1)}(BF₄) were undertaken and revealed that fac-{Re(CO)₃(L¹1)}(BF₄) binding to fish sperm DNA (binding constant 1.5 ± 0.2 × 10⁵ M⁻¹, binding site size 3.2 ± 0.3 base pairs) is accompanied by changes in the UV-Vis spectrum as typically observed for pyrene-based intercalators while the calorimetrically determined binding enthalpy (−14 ± 2 kcal mol⁻¹) also agrees favourably with values as typically found for intercalators.