Complexes of amino acids with a crown-ether derivative of 4-styrylpyridine. Monotopic or ditopic?

Abstract

An E-4-styrylpyridine derivative endowed with 18-crown-6 as a substituent (E-1) was prepared and evaluated in acetonitrile as a potential ditopic ligand for protonated amino acids. The interactions of E-1 with the protonated amino acid perchlorates, ClO₄⁻ H₃N⁺(CH₂)_nCOOH (n = 2, 5 and 10, A2, A5 and A10, respectively), were studied by optical methods, ¹H NMR and mass spectroscopy. Complex formation involves coordination of the ammonium ion at the crown ether moiety of E-1. The spectral changes were evaluated by comparison with results obtained on protonation of E-1 with HClO₄ and on association with ammonium perchlorate. Protonation by the protonated amino acid perchlorates was thwarted due to reversal of carboxyl/pyridinium pK_A order in acetonitrile relative to water. Evidence for ditopic hydrogen bonding complex formation was especially sought for A10 because its CH₂ chain is sufficiently long to bridge the distance between the crown ether and pyridyl N sites of E-1. Despite some subtle hints to the contrary, the absence of NOE interaction between the pyridyl protons of E-1 and the methylene protons of A10 indicates that the E-1·A10 complex is in the main monotopic, as is the case for A2 and A5. The photophysical and photochemical behaviour of the complexes change significantly on protonation by HClO₄. The optical response of E-1 on binding the amino acids as ammonium salts allows convenient monitoring of complex formation.