Selective detection of multicarboxylate anions based on "turn on" electron transfer by self-assembled molecular rectangles

Two new large molecular rectangles (4 and 5) were obtained by the reaction of two different dinuclear arene ruthenium complexes [Ru ₂(arene) ₂(OOOO) ₂Cl ₂] (arene=p-cymene; OOOO=2,5-dihydroxy-1,4-benzoquinonato (2), 6,11-dihydroxy-5,12-naphthacene dionato (3)) with the unsymmetrical amide NN (N-[4-(pyridin-4-ylethynyl)phenyl] isonicotinamide) donor ligand 1 in methanol in the presence of AgO ₃SCF ₃, forming tetranuclear cations of the general formula [Ru ₄(arene) ₄(NN) ₂(OOOO) ₂] ⁴⁺. Both rectangles were isolated in good yields as triflate salts and were characterized by multinuclear NMR, ESI-MS, UV/Vis, and photoluminescence spectroscopy. The crystal structure of 5 was determined by X-ray diffraction. Luminescent rectangle 5 was used for anion sensing with an amide ligand as a hydrogen-bond donor and an arene-ruthenium acceptor as a signaling unit. Rectangle 5 strongly bound multicarboxylate anions, such as oxalate, tartrate, and citrate, in UV/Vis titration experiments in 1:1 ratios, in contrast to monoanions, such as F ^-, Cl ^-, NO ₃ ^-, PF ₆ ^-, CH ₃COO ^-, and C ₆H ₅COO ^-. The fluorescence titration experiment showed a large fluorescence enhancement of 5 upon binding to multicarboxylate anions, which could be attributed to blocking of the photoinduced electron transfer process from the arene-ruthenium moiety to the amidic donor in 5; this was likely to be a result of hydrogen bonding between the ligand and the anion. On the other hand, rectangle 5 was not selective towards any other anions. To the naked eye, multicarboxylate anions in a solution of 5 in methanol appear greenish upon irradiation with UV light.