Coordination-Driven Self-Assembly of Metallamacrocycles via a New Pt^II₂ Organometallic Building Block with 90° Geometry and Optical Sensing of Anions

A new PtII2 organometallic building block with 90° 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° building unit. The self-assembly of 5 with three flexible ditopic donors (La−Lc) afforded [2 + 2] metallamacrocycles (6a−c) [where La = 1,3-bis(4-pyridyl)isophthalamide; Lb = 1,3-bis(3-pyridyl)isophthalamide, and Lc = 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 La 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° 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−).