Design, Synthesis, and Characterizations of a Series of Pt4 Macrocycles and Fluorescent Sensing of Fe3+/Cu2+/Ni2+ Through Metal Coordination

A Pt(II)(2) organometallic "clip" (1a) containing ethynyl functionality is synthesized. Multinuclear NMR and electrospray ionization mass spectrometry characterized this "clip", and the molecular structure was determined in an X-ray single-crystal diffraction study. A series of discrete molecular rectangles (2a-d) have been synthesized from this "clip" in combination with dipyridyl-based linear linkers (L(1-4)) by a metal-ligand coordination driven self-assembly approach [where L(1) = 4,4'-bipyridine, L(2) = trans-1,2-bis(4-pyridyl)ethylene, L(3) = N-(4-pyridyl)isonicotinamide, and L(4) = N,N'-bis(4-pyridylidene)ethylenediamine]. Rectangle 2d was designed using the imine-based ligand L(4) to make it a system composed of a fluorophore-receptor-fluorophore combination. The imine N(4) pocket is the receptor site, while the anthracene-based "clip" is the fluorophore. Complex 2d is fluorescent in nature and showed fluorescence quenching in solution upon the binding of hard transition metal ions (Fe(3+), Cu(2+), Ni(2+), and Mn(2+)) into the N(4) pocket. The nonresponsive nature of the fluorescence intensity upon the addition of soft metal ions (Zn(2+) and Cd(2+)) having d(10) configuration makes it a suitable sensor for transition metal ions. The fluorescence intensity of the Ni(2+) bound complex was regained when the metal was removed by a stronger chelating 2,2'-dipyridyl ligand.