Benzoaza-15-crown-5 ethers: synthesis, structure, and complex formation with metal and ethylammonium ions

Abstract

Benzoaza-15-crown-5 ethers containing one or two nitrogen atoms in different positions of the macrocycle and bearing different substituents at these atoms were synthesized. The structures of azacrown ethers and their metal complexes were studied by X-ray diffraction. The stability constants of the complexes of azacrown ethers with Na⁺, Ca²⁺, Ba²⁺, Ag⁺, Pb²⁺, and EtNH₃ ⁺ ions were determined by ¹H NMR titration in MeCN-d₃. In free benzoazacrown ethers containing secondary nitrogen atoms bound to the benzene ring, as well as in N-acetyl derivatives, the N atoms are sp²-hybridized and have a planar geometry. The nitrogen lone pairs on the p orbitals are efficiently conjugated to the benzene ring or the carbonyl fragment of the acetyl group, which is unfavorable for the complex formation. In addition, the formation of complexes with benzoazacrown ethers containing secondary nitrogen atoms is hindered because the hydrogen atoms of the NH groups are directed to the center of the macrocyclic cavity. In benzoazacrown ethers bearing N-alkyl substituents or secondary nitrogen atoms distant from the benzene ring, the N atoms show a substantial contribution of the sp³-hybridized state and have a pronounced pyramidal configuration, which promotes the complex formation. The lead and calcium cations form the most stable complexes due to the high affinity of Pb²⁺ ions for O,N-containing ligands, a high charge density on these ions, and the better correspondence of the cavity size of the 15-membered macrocycles to the diameter of the Ca²⁺ ion. An increase in the stability of the complexes is observed mainly in going from monoazacrown ethers to diazacrown ethers containing identical substituents at the N atoms and in the following series of substituents: C(O)Me < H < Me < CH₂CO₂Et. In the case of the CH₂CO₂Et substituents, the carbonyl oxygen atom is also involved in the coordination to the cation. The characteristic features of the complexing ability of N-alkylbenzomonoaza-15-crown-5 ethers bearing the nitrogen atom conjugated to the benzene ring show that macro-cyclic ligands having this structure are promising as selective and efficient complexing agents for metal cations.