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

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 L-1 1-[bis(pyridine-2-ylmethyl)amino]methylpyrene and L-2 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 H-1 NMR, MS, IR and UV-Vis studies. X-Ray crystallographic data were obtained for fac-{Re(CO)(3)(L-1)}(BF4), C34H26BF4N4O3Re: monoclinic, P2(1)/c, a = 18.327(2) angstrom, alpha = 90.00 degrees, b = 14.1537(14) angstrom, beta = 96.263(6)degrees, c = 23.511(3) angstrom, gamma = 90.001, 6062.4(11) angstrom(3), 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)( L-1)}(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)(L-1)}(BF4) were undertaken and revealed that fac-{Re(CO)(3)(L-1)}(BF4) binding to fish sperm DNA (binding constant 1.5 +/- 0.2 x 10(5) 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.