Photoinduced ligand exchange and covalent DNA binding by two new dirhodium bis-amidato complexes

Two new dirhodium complexes, the head-to-tail (H,T) and head-to-head (H,H) isomers of cis-[Rh(2)(HNOCCH(3))(2)(CH(3)CN)(6)](2+), were synthesized, separated, and characterized following the reaction of Rh(2)(HNOCCH(3))(4) with trimethyloxonium tetrafluoroborate in CH(3)CN. The products were characterized by (1)H NMR spectroscopy, mass spectrometry, elemental analysis, and single crystal X-ray diffraction. Each bis-amidato isomer has a total of six CH(3)CN ligands, two along the internuclear Rh-Rh axis, CH(3)CN(ax), two in equatorial positions trans to the oxygen atoms of the bridging amidato groups, CH(3)CN(eq)(O), and two in equatorial positions trans to the amidato nitrogen atoms, CH(3)CN(eq)(N). When aqueous solutions of the complexes are irradiated with low energy light (λ(irr) ≥ 495 nm, 60 min), both types of CH(3)CN(eq) ligands undergo efficient ligand exchange with solvent H(2)O molecules to form monoaqua, followed by bis-aqua, adducts, releasing two CH(3)CN(eq) ligands in the process. The quantum yields, Φ(400nm), for the H,T and H,H isomers to form monoaqua adducts are 0.43 and 0.38, respectively, which are substantially greater than the 0.13 yield observed for cis-[Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+); importantly, no ligand exchange is observed when the complexes are kept in the dark. Finally, low energy excitation (λ(irr) ≥ 610 nm, 30 min) of the H,T isomer was shown to generate photoproducts that covalently bind to linearized DNA, making 2 a potential agent for photochemotherapy that does not require the formation of (1)O(2), as is typical of organic photodynamic therapy (PDT) agents.