Coordination-Driven Self-Assembly of Metallamacrocycles via a New Pt2II Organometallic Building Block with 90° Geometry and Optical Sensing of Anions

A new Pt-2(II) organometallic building block with 900 geometry, 3,6-bis[trans-platinum(triethylphosphine)(2)(nitrate)(ethynyl)]carbazole (5), containing an ethynyl functionality is synthesized in reasonable yield by employing a Sonagashira coupling reaction. Multinuclear NMR and electrospray ionization mass spectrometry (ESI-MS) including a single-crystal X-ray diffraction study characterized this 90 degrees building unit. The self-assembly of 5 with three flexible ditopic donors (L-a-L-c) afforded [2 + 2] metallamacrocycles (6a-c) [where L-a = 1,3-bis(4-pyridyl)isophthalamide; L-b = 1,3-bis(3-pyridyl)isophthalamide, and L-c = 1,2-di(4-pyridyl)ethane]. All three [2 + 2] self-assembled macrocycles were characterized by various spectroscopic techniques, and the molecular structure of 6a was determined by the single-crystal X-ray diffraction method. The square-type macrocycle 6a was designed using amide-based ligand L-a to make it a suitable system composed of a fluorophore receptor combination. The amide functional group is the receptor site for the anions, which can bind through hydrogen-bonding interaction, whereas the carbazole-based 90 degrees building unit is the fluorophore unit. Macrocycle 6a was found to be fluorescent in nature and showed dramatic fluorescence enhancement in a DMF/H2O solvent mixture upon titration with P2O74-, which plays important roles in various biological functions such as energy transduction in organisms and controls metabolic processes by participating in various enzymatic reactions. Interestingly, the nonresponsive nature of the fluorescence intensity of 6a upon addition of the anions F-, ClO4-, and H2PO4- makes it a suitable fluorescent sensor for pyrophosphate anion (P2O74-).