posted on 2024-02-09, 07:13authored bySerap Mert, Özden Erdebil
Squaramides (SQs), which are very popular for their H-bonding
ability,
have attracted great interest due to their wide range of applications
such as asymmetric synthesis, pharmacology, and anion transportation.
In this study, aliphatic symmetric SQs based on cis/trans-1,2-diaminocyclohexane (DACH) substituted
with cyclic tertiary amines, synthesized in four steps under simple
reaction conditions, were investigated for the first time for their
ability to bind Cl–, Br–, and
I– anions. The changes in cis/trans geometric isomers and the cyclic ring (pyrrolidine
vs piperidine) were found to have a combined effect on the degree
of anion binding. The spectroscopic titrations of the SQs with TBA-Cl,
TBA-Br, and TBA-I in the range of 0.2 to 20.0 equiv were monitored
by 1H NMR, and the analyses of the magnitude of chemical
shift differences in the NH peaks of the SQs in course of titration
were performed by DynaFit and BindFit programs for the calculation
of their Ka values. All symmetric SQs I–IV were found to selectively bind Cl– anion more strongly than Br– anion
to varying degrees depending on the SQ derivatives. Especially, SQ IV, which has a symmetric trans-DACH and
a pyrrolidine ring, was found to have the highest Cl– anion-binding ability compared to the other SQs. However, the SQs
did not show any change in the chemical shift of the NH proton in 1H NMR upon successive addition of TBA-I, indicating that they
do not interact with I– anion. The stoichiometries
of the complexation behavior of SQs I–IV toward Cl– and Br– anions were
also analyzed by Job plots.