Title

Cation exchange kinetics and mechanisms of macrocyclic polyether cation complexes and chiral recognition by triazole-crown ligands

Date of Award

1993

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Committee Member

Luis Echegoyen, Committee Chair

Abstract

Chapter 1. Cation exchange kinetics and mechanisms as well as dynamic properties of the sodium complexes of a series of amide and ester containing lariat ethers have been studied by $\sp{23}$Na and $\sp1$H NMR. This is the first kinetic and mechanistic study of nitrogen pivot lariat ether-cation complexes. These synthetic ion carriers have systematic structural variations including crown cavity sizes, sidearm functional groups and lipophilicity of the sidearms. Three mechanisms, a unimolecular one and bimolecular ones involving either two cations or two ligands, have been found to be operative for the different systems studied. It has been observed that the sidearm functional groups play an important role in determining the cation exchange mechanisms. Activation parameters, $\rm\Delta G\sp\ne,\ \Delta H\sp\ne$ and $\rm\Delta S\sp\ne$ have been measured and correlated with the structural features of the complexes. The hypotheses about the transition state for the cation dissociation step have been proposed based on the experimental results. Slow ligand dynamic processes were found to coexist with the cation exchange process. Separation of these two types of kinetic processes depends on the complex structures. Some conformational rearrangements of the ligands associated with cation binding have also been observed.Chapter 2. Structure and cation exchange properties of the cation complexes of a cylindrical macrocycle, named cistulyne, have been studied via crystallography and NMR spectroscopy. Although the free ligand has a channel-like structure, the cylinder is pronouncedly distorted upon complexation. It has been found that an intermolecularly dissociative mechanism is operative for the cation exchange process of mononuclear cistulate.Chapter 3. Chiral recognition of organic ammonium salts by three new chiral triazole-18-crown-6 ligands has been studied. The triazole-crowns contain two methyl substituents on the chiral macrocycles and two of the ligands also contain lipophilic sidearms connected to the triazole ring. Binding constants (K) were measured by the $\sp1$H NMR titration technique in CDCl$\sb3.$ Ligands 3.2b and 3.2c exhibited chiral recognition for the enantiomers of (1-(1-naphthyl) ethyl) ammonium cation, and for the enantiomers of (l-phenylethyl) ammonium. Both chiral hosts recognize preferentially R over S enantiomers. The recognition mechanism has been investigated via $\sp{13}$C and $\sp1$H longitudinal relaxation time for ligand 3.2c in the presence and absence of the enantiomeric guests.

Keywords

Chemistry, Organic; Chemistry, Physical

Link to Full Text

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