1.0 M HNO3 standard solution. Titrations were run by addition of
10 mL portions of standard 0.1 M NaOH, collecting 80–100 points
for each titration. On each addition of standard 0.1 M NaOH,
the UV-vis. spectrum of the solution was recorded. Prior to each
potentiometric titration, the standard electrochemical potential
(E◦) of the glass electrode was determined in CH3CN/water
mixture (9/1 v/v, 0.1 M TBA–PF6), by a titration experiment
according to the Gran method.17 Titration data (emf vs. mL of
NaOH) were processed with the Hyperquad package to determine
the equilibrium constants.11
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X-Ray crystallographic studies
Diffraction data were collected at room temperature by means
of an Enraf-Nonius CAD4 four-circle diffractometer, working
with graphite monochromated Mo-Ka X-radiation (l = 0.71073
˚
A). Crystal data for the receptor 1: C22H17F3N6O; M 428.42;
4 T. Steiner, Angew. Chem., Int. Ed., 2002, 41, 4876.
colourless; monoclinic, P21/a (no. 14); a = 8.751(3), b = 10.633(2),
3
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˚
˚
c = 21.676(6) A; V = 1983.2(9) A ; T = 293 K; Z = 4; rc =
1.468 g cm-3; mMo-Ka = 0.115 mm-1; 3915 measured reflections,
3517 unique reflections (Rint 0.0258), 2637 strong reflections [IO
> 2s(IO)]; refined parameters = 295; R1 and wR2 (strong data)
0.0737 and 0.1688; R1 and wR2 (all data) 0.0992 and 0.1916. Data
reductions (including intensity integration, background, Lorentz,
and polarization corrections) were performed with the WinGX
package.18 Absorption effects were evaluated with the psi-scan
method,19 and absorption correction was applied to the data (0.926
and 0.990 min and max transmission factor). Crystal structure was
solved by direct methods (SIR 97)20 and refined by full-matrix
least-squares procedures on F2 using all reflections (SHELXL
97).21 Anisotropic displacement parameters were refined for all
non hydrogen atoms. Hydrogens belonging to C atoms were placed
at calculated positions with the appropriate AFIX instructions and
refined using a riding model. Hydrogens of the urea group were
located in the final DF map and their position refined restraining
˚
the N–H distance to be 0.96 0.01 A.
13 T. Melton, J. Taylor and D. G. Wibberley, Chem. Commun. (London),
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15 L. S. Reddy, S. Basavoju, V. R. Vangala and A. Nangia, Cryst. Growth
Des., 2006, 6, 161–173.
Acknowledgements
This work was financially supported by the Italian Ministry
of Research (MIUR), project Supramolecular Devices (PRIN
2008HZJW2L_003).
16 C. S. Wilcox, Frontiers in Supramolecular Chemistry and Photochem-
istry, VCH Weinheim, Germany, 1991, 123-143.
Notes and references
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University of Go¨ttingen, Germany, 1997.
This journal is
The Royal Society of Chemistry 2011
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