Complexation studies of crown ethers with alkali metal cations in methanol
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Date
2004
Authors
Keng Yoon, Yeong
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Abstract
The alkylation reaction between dibenzo-15-crown-5 (DB15C5) and t-butyl
alcohol in trifluoroacetic acid yielded 2,3,8,9-(4’,4”-di-tert-butyl)-dibenzo-1,4,7,10,13-
pentaoxacyclopentadeca-2,8-diene (t2-DB15C5). The f2-DB18C6 (mixture of 4’,4”-diformyl
benzo18-crown-6 and 4’,5”-di-formyl benzo18-crown-6) was synthesized from
dibenzo-18-crown-6 (DB18C6) via a facile formylation process employing
hexamethylenetetramine and trifluoroacetic acid. Both compounds were obtained in
good yield ( 80% for t2-DB15C5 and 55-65% for f2-DB18C6). The structures were
identified using elemental analysis, IR, MS, 1H NMR and 13C NMR spectroscopy.
The complexation of DB15C5 with alkali metal cations as well as Sm3+, Nd3+ and Pr3+ in
methanol were investigated using 1H NMR spectroscopy and UV spectroscopy. A 1:1
stoichiometry complex was observed with Li+ and Na+ as well as Sm3+, Nd3+ and Pr3+
while K+ gave 2:1 (sandwich) complex. Both 1:1 and 2:1 (sandwich) complexes were
observed Cs+. The type of complex formed by DB15C5-Rb+ depends on the ratio of host
to guest used. At 1:1 host to guest ratio used, DB15C5-Rb+ showed a mixture of 1:1 and
2:1 sandwich complexes. Nevertheless, when we have excess of host DB15C5-Rb+ form
2:1 sandwich complex.
The complexation of t2-DB15C5 with alkali metal cations was also carried out in
methanol. A 1:1 stoichiometry complex was observed with Li+ and Na+ while K+, Rb+ and Cs+ all gave 2:1 (sandwich) complex. For 1:1 stoichiometry complexes, the t-butyl
substituent groups on the benzene ring of t2-DB15C5 increase the stability of the
complex by one fold compared to DB15C5. The association constants for 1:1
stoichiometry complexes were found in the order of Li+ Na+ for DB15C5 and t2-
DB15C5.
The complexation of DB18C6 and f2-DB18C6 with alkali metal cations were carried out
in the methanol-chloroform (8:2 v/v). DB18C6 gave 1:1 stoichiometry complexes with
Li+, Na+ and K+ while Cs+ gave both 1:1 and 2:1 (sandwich) complexes. The type of
complex formed by DB15C5-Rb+ depends on the ratio of host to guest used. DB18C6-
Rb+ gave 1:1 complex when we have excess guest. However, when we have excess host,
the majority of complex formed was 2:1 sandwich complex. The association constants
for 1:1 stoichiometry complexes were found in the order of Na+ K+ Li+ for DB18C6.
For f2-DB18C6, besides the ether oxygens, the carbonyl oxygens were also involved in
the complexation with alkali metal cations.
Description
Master
Keywords
Chemical science , Crown ethers , Methanol