A Novel Crown Ether Generation Containing Different Heteroaromatic Cations:  Synthesis, Characterization, Solid-Phase 13C NMR, X-ray Crystal Structure, and Selective Amino Acid Recognition

The synthesis and structural characterization of a novel generation of crown ethers, 3, 5 and 6 containing pyrilium, thiopyrilium, and pyridinium subunits, respectively, are reported. The crown ether unit is potentially capable of forming host−guest complexes with inorganic and organic cations, while the heteroaromatic cationic unit is suitable to bind with anions. A variety of physicochemical methods including electrospray mass spectrometry, UV−vis spectroscopy, solution and solid-phase NMR, and X-ray crystallography were applied for structural characterization of the new crown ether derivatives. The 1H and 13C NMR studies indicate rapid rotation of the B9C3 unit about the C−C bond that connects the two units to each other. Single crystals for 3, 4, and 5 were successfully obtained, and their X-ray crystal structures were resolved. The perchlorate anion in 3 (orthorhombic, space group P212121) and 5 (orthorhombic, space group P21) is far from O+ and close to S+. The solid-phase structure of 3 and 5 show small deviation from planarity for the four aromatic rings, whereas two of the aromatic rings in 4 are out of heteroaromatic ring. Spectrophotometric studies in methanol solution revealed that the ligand 3 can be successfully applied to selective amino acid recognition.