SUPRAMOLECULAR CHEMISTRY
11
factors. First, the nitro group in the para position with
respect to the active urea should decrease the steric hin-
drance observed for the previous triads (L4-L9), allowing
for easier access of the anion to the pseudo-cavity of the
receptors and for bigger anion like benzoate. Moreover,
an electron- withdrawing nitro group in the para position
should influence in a positive way the coordination prop-
erties of the ligands, increasing the acidity of the urea NH
protons.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by Fondazione Banco di Sardegna.
References
The anion binding activity across this series is consist-
ent with the trends previously described for the other
receptors. The stability constants decrease from L13 to L15
because of the varying steric hindrance of the halogen
on the phenyl ring. By comparing the stability constant
of receptor L15 with that of receptor L3 (without substitu-
ents on the non-halogenated phenyl ring) and receptor L12
(with a nitro group in place of the tri-fluoromethyl unit),
it is possible to define the increasing anion affinity in the
order L3 <L15 <L12. This evidence is in agreement with the
lower acidity of the NH protons in the unsubstituted recep-
tor L3 compared to receptors L15 and L12. On the other
hand, between receptors L15 and L12, the lower ability of
receptor L15 to bind anions can be explained by taking into
account the electron-withdrawing nature of the CF3 group
with respect to the NO2 group. The same behaviour can be
found for the series L1-L10-L13 and L2-L11-L14.
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Conclusions
In conclusion, we have described herein the synthesis and
the anion binding properties of fifteen N-N′-diphenylurea
receptors substituted with electron-withdrawing groups
(namely nitro and trifluoromethyl) and halogens (chlorine
and iodine). We were able to obtain crystals suitable for sin-
gle crystal X-ray diffraction for nine receptors (including two
polymorphs and two solvates) and the 1:1 adduct of L6 with
benzoate. As expected, the classic urea 1-D chains were
observed in most of the structures. Only L6-BzO−, L8•DMSO
and L11•2DMSO•DMF adopted alternative supramolecular
synthons because of the presence of the anion guest or the
solvents that prevents the formation of the typical urea-
urea N–H⋅⋅⋅O tapes. Solution studies conducted by means of
1H-NMR spectroscopic titrations allowed us to calculate the
stability constant for the formation of the 1:1 adducts with
−
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Binding with Urea and Thiourea Derivatives. Coord. Chem.
The highest values of stability constants were obtained for
the receptors L10-L12 bearing the nitro group in the para
position with respect to the urea moiety.
Acknowledgements
The authors thank the financial support from Fondazione Banco
di Sardegna.