Benzoaza-15-crown-5 ethers: synthesis,structure, and complex formation with metal and ethylammonium ions |
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Authors: | A I Vedernikov S N Dmitrieva L G Kuz’mina N A Kurchavov Yu A Strelenko J A K Howard S P Gromov |
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Institution: | (1) Tianjin Key Laboratory of Structure and Performance for Functional Molecule, College of Chemistry, Tianjin Normal University, Tianjin, 300387, People’s Republic of China |
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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. |
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