The complexation of alkali metal cations by 15-crown-5 and benzo-15-crown-5 in nonaqueous solvents: A multinuclear NMR study.
Creators: name: Briere, Kathleen Marie.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1990
Subjects: subject: Chemistry, Analytical.

description: $\sp{23}$Na and $\sp7$Li NMR have proven to be very useful techniques for studying the extent of complexation reaction of lithium and sodium cations with 15-crown-5 (15C5) and benzo-15-crown-5 (B15C5) in nitromethane (NM), acetonitrile (AN), pyridine (PY) and dimethylformamide (DMF). Although the formation constants (K$\sb{\rm F1})$ of (Na:15C5)$\sp+,$ (Na:B15C5)$\sp+,$ (Li:15C5)$\sp+$ and (Li:B15C5)$\sp+$ in NM were too large to be accurately measured (log K$\sb{\rm F1}$ $&gt;$ 4), that of (Na:B15C5)$\sp+$ in DMF and PY were determined from $\sp{23}$Na chemical shift and longitudinal relaxation rates: log K$\sb{\rm F1}$ = 2.9 $\pm$ 0.3 in PY and 1.2 $\pm$ 0.4 in DMF. A combined $\sp{23}$Na and $\sp7$Li NMR study for systems containing both lithium and sodium cations in the presence of B15C5 in NM revealed a selectivity for the complexation of lithium where the equilibrium constant, reflecting the ratio K$\sb{\rm FLi}$/K$\sb{\rm FNa},$ was determined: log K$\sb{\rm eq}$ = 1.9 $\pm$ 0.2. In the two-site chemical exchange of the alkali metal cations, two exchange mechanisms have accounted for the $\sp{23}$Na and $\sp7$Li dynamic NMR results: associative and dissociative exchange. The chemical exchange of sodium and lithium individually with B15C5, was shown, by $\sp{23}$Na and $\sp7$Li NMR respectively, to follow the associative pathway in NM whereas in AN the chemical exchange of sodium with B15C5 was shown to prefer the dissociative pathway. Kinetic studies of sodium-B15C5 in binary mixtures of AN-NM and DMF-NM have indicated that the change in observed mechanism is related to the solvent donicity where dissociative exchange is observed in high donicity solvents. An increase of the concentration of AN or DMF in the solvent mixture gives rise to an increase of the dissociative contribution to the exchange. The activation parameters of the dissociative exchange are highly solvent dependent indicating that the solvent, at the molecular level, must play an important role in this mechanism. However, the activation parameters for the associative exchange do not display such a dependence on the solvent; it was postulated that conformational rearrangements play the governing role in this mechanism. In studies with DMF-NM binary solvent mixtures, a model based on the direct participation of one DMF molecule in the coordination sphere of sodium in (Na:B15C5)$\sp+,$ has accounted for the observed $\sp{23}$Na chemical shift and longitudinal relaxation rate variations of (Na:B15C5)$\sp+$ as a function of the concentration of DMF in the solvent mixture. It was postulated that it is the formation of (Na:B15C5:DMF)$\sp+$ which is responsible for the observed increase of the rate of exchange as the concentration of DMF in the solvent increases. This direct participation of one solvent molecule in the dissociative exchange was qualitatively and quantitatively characterized. The application of 2-dimensional $\sp7$Li exchange NMR spectroscopy (EXSY), for the kinetic investigations of the exchange of the lithium cation in its solvated (NM) and complexed (B15C5) sites, is illustrated. Results obtained by this method are in agreement with the conventional 1D lineshape results. (Abstract shortened by UMI.)