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Crown Ether–Ammonium Complexes: Binding Mechanisms and Solvent Effects
Author(s) -
Rüdiger Volker,
Schneider HansJörg,
Solov'ev Vitally P.,
Kazachenko Vladimir P.,
Raevsky Oleg A.
Publication year - 1999
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/(sici)1099-0690(199908)1999:8<1847::aid-ejoc1847>3.0.co;2-q
Subject(s) - chemistry , hydrogen bond , solvent , 18 crown 6 , solvation , enthalpy , crown ether , tetramethylammonium , inorganic chemistry , calorimetry , ammonium , molecule , crystallography , ion , organic chemistry , thermodynamics , physics
Thermodynamics of 18‐crown‐6 complexes with ammonium cations (NH 4 , MeNH 3 , Me 2 NH 2 , Me 3 NH, Me 4 N, Et 4 N, PhNH 3 , and PhCH 2 NH 3 ) in methanol were determined by titration calorimetry. The results show strong contributions from entropy terms counteracting the enthalpy of complexation, and a linear decrease of the complexation free energy Δ G with the number of available N–H hydrogen bonds. In several cases formation of relatively strong complexes containing two ammonium ions per crown unit was observed. Tetramethylammonium ions show no detectable association with the crown ether, demonstrating the absence of significant Coulomb‐type interaction between the partial charges at the crown ether oxygen and the N + –C–H atoms. Ammonium ions bind to aza crown ethers with almost equal affinity as to the all ‐oxygen anologs only, if methyl groups at the nitrogen atoms force the lone pairs into equatorial position. Molecular mechanics calculations (CHARMm) of corresponding gas‐phase complexes yield geometries and energies in agreement with this, with energetically equally good conformations of an essentially undistorted D 3d crown accepting either 3 linear hydrogen bonds, or 6 bifurcated bonds from the primary ammonium cations. Complexation equilibria were measured with PhNH 3 , and PhCH 2 NH 3 in water, 2‐propanol, tert ‐butyl alcohol, n ‐octanol, DMF, DMSO, pyridine, HMPT and acetone mostly by calorimetry, in some cases by potentiometry. The observed association constants varied by factors of up to 1000; the solvent effects can be described generally as a linear function of the hydrogen bond accepting power of the solvent molecules, in line with the mechanisms derived above. The lg K and Δ H values of the complexation of the PhNH 3 or PHCH 2 NH 3 cation with 18‐crown‐6 ligand are compared with a large range of available solvent properties. The best correlations ( R ≈ 0.9) for lg K (or Δ G ) are obtained with values characterizing the electron donor capacity of the solvent ( C a , β*, DN) for lg K , as found earlier for complexes between K + and 18C6.