Role of anions in the complex formation of crown ethers with ammonium ions chemically bonded on silica gel

Abstract

The complex formation of crown ethers with ammonium ions chemically bonded on silica gel has been chromatographically investigated. Although the selectivity in the complex formation of crown ethers is basically the same as in solution, a counter anion existing in the diffuse double layer of the silica gel also plays an important role, even when a polar solvent with hydrogenbond formation ability such as methanol, is used as a medium. The overall complex formation at the silica gel surface can be divided into three processes: (i) the dissociation of a counter anion from its ion-pair with the ammonium ion; (ii) complex formation of a crown ether with the ammonium ion; (iii) formation of an ion-pair between the anion and the ammonium ion complexed by the crown ether. This mechanism is evaluated from thermodynamic viewpoints including the solvation of anions, the ion-pair formation of anions with ammonium, etc.Much attention has been paid to the complex formation of polyethers with hard cations by a number of researchers in various branches of fundamental and applied chemistry since the discovery of crown ethers in 1967 by Pedersen.' One of the main interests is what is the thermodynamic origin of the selectivity in polyether complexation. To seek the answer to this question, a number of approaches have been attempted, including spe~trometry,~-~ ~alorimetry,',~ X-ray crystallo-graph^,^.* electrochemical methods,g-' ' computer simulation,12-15 etc.'Size-fit' theory is one of the most well known concepts used to explain the selectivity of macrocyclic compounds; i.e. the fit of the cavity size of a macrocycle to the crystalline size of a cation is thought to be the most important factor according to this concept. The size-fit theory is an idea based on the rigid structures of macrocycles, and does not necessarily govern the overall free energy of complex formation if conformational changes are taken into account. However, a suitable arrangement of donor atoms around a cation is necessary for stable bond formation; 'preorganization' of a ligand is a preferable term rather than ' size-fit '. ' Preorganization' results

Molecular recognition of organic ammonium ions in solution using synthetic receptors

“…The strength of the electrostatic interaction in solution, despite the solvation [ 788 – 789 ] of host and guest, influences the binding to an artificial receptor. Strongly co-ordinating counterions such as chloride generally lead to weaker binding constants upon recognition of the associated cation as compared to when large, soft and weakly co-ordinating counterions such as iodide (tetrafluoroborate, hexafluorophosphate or perchlorate) are employed [ 790 – 792 ].…”

Interpretation of Chromatographic Retention Based on Electrostatic Theories

“…65 In the separation sciences, polyethers have been mainly used for the separation of cations, while very little attention has been paid to the separation of polyethers themselves. It has been reported that the chromatographic separation of polyethers provides very useful information on the solution behaviors of polyethers [66][67][68][69][70] , which cannot be elucidated by other methods.…”

“…71 A thermodynamic study indicated the importance of the ion-pair formation of the ammonium ions with counteranions. 69 Though the complexation of polyethers is not governed by electrostatic potential, the distribution of counteranions is influenced by the potential. Thus, the overall retention of polyethers on this stationary phase is affected by electrostatic potentials by which the distribution of counter-anions is controlled.…”

Stationary-Phase Complexation of Crown Ethers with Polyammonium Ions Chemically Bonded on Polymer Resin

“…In contrast, studies on the complexation of crown ethers with organic cations have been relatively few. However, this does not necessarily mean that organic cations are less important than inorganic cations in crown ether chemistry; on the contrary, complexation with organic cations including the conjugate acids of amines and aminoacids is expected to become more important in biological, biochemical, medical, and pharmaceutical sciences.For investigating polyether complexation, we have developed efficient means based on separation techniques, such as cation-exchange chromatography 19-22 , anion-exchange chromatography 23,24 , and capillary electrophoresis. 25 These methods have permitted us to elucidate polyether chemistry, which is difficult to study with conventional spectrometric, electrochemical, and thermodynamic methods.…”

“…For investigating polyether complexation, we have developed efficient means based on separation techniques, such as cation-exchange chromatography 19-22 , anion-exchange chromatography 23,24 , and capillary electrophoresis. 25 These methods have permitted us to elucidate polyether chemistry, which is difficult to study with conventional spectrometric, electrochemical, and thermodynamic methods.…”