the three guest methyl hydrogen atoms disordered over two differ-
ent orientations. We were intrigued in the choice between the two
space groups, however the comparison between the refinements
in the two space groups showed that: a) the final electron density
difference DF map in the P31 space group did not show residual
peaks to support the hypothesis of the disordered guest methyl
group; b) the refinement in the P3121 space group converged
at a higher R1 value (0.0616). On these basis we concluded
that the correct space group is P31with a pseudo-C2 symmetry.
In the 42 …PQ(TsO)2 structure, systematic h0l absences clearly
indicate the P21 space group even if the structure approximates
a P21/c space group. The hydrogen atoms were placed at their
7 (a) A. Arduini, E. Brindani, G. Giorgi, A. Pochini and A. Secchi, J. Org.
Chem., 2002, 67, 6188–6194; (b) A. Arduini, G. Giorgi, A. Pochini, A.
Secchi and F. Ugozzoli, J. Org. Chem., 2001, 66, 8302–8308.
8 For recent examples of positive allosteric effect on ion-pair binding,
see, for example: (a) K. Zhu, S. Li, F. Wang and F. Huang, J. Org.
Chem., 2009, 74, 1322–1328; (b) C. M. G. dos Santos, T. McCabe,
G. W. Watson, P. E. Kruger and T. Gunnlaugsson, J. Org. Chem., 2008,
73, 9235–9244; (c) J. L. Sessler, E. Tomat and V. M. Lynch, J. Am.
Chem. Soc., 2006, 128, 4184–4185; (d) L. Kovbasyuk and R. Kramer,
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9 L. Pescatori, A. Arduini, A. Pochini, A. Secchi, C. Massera and F.
Ugozzoli, CrystEngComm, 2009, 11, 239–241.
10 (a) S. Tsuzuki and A. Fujii, Phys. Chem. Chem. Phys., 2008, 10, 2584–
2594; (b) M. Nishio and Y. Umezawa, in Topics in Stereochemistry, eds.
S. E. Denmark and J. S. Siegel, John Wiley & Sons, Hoboken, NJ, 2006,
vol. 25, pp. 255–302.; (c) M. Nishio, CrystEngComm, 2004, 6, 130.
11 (a) N. Zacharias and D. A. Dougherty, Trends Pharmacol. Sci., 2002,
23, 281–287; (b) J. P. Gallivan and D. A. Dougherty, Proc. Natl. Acad.
Sci. U. S. A., 1999, 96, 9459; (c) J. C. Ma and D. A. Dougherty, Chem.
Rev., 1997, 97, 1303–1324.
12 (a) Intelligent Materials, eds. M. Shahinpoor and H.-J. Schneider,
Royal Society of Chemistry, Cambridge, 2007; (b) Supramolecular
Polymers, ed. A. Ciferri, Marcel-Dekker, New York, 2000; (c) C.
Rao, Supramolecular Organization and Materials Design, Cambridge
University Press, 2001.
13 For the recognition of N-alkyl pyridinium and N,N¢-dialkyl dipyri-
dinium salts by calix[n]arene derivatives in aqueous media, see, for
example: (a) R. Kaliappan, Y. Ling, A. E. Kaifer and V. Ramamurthy,
Langmuir, 2009, DOI: 10.1021/la900659r; (b) D.-S. Guo, K. Wang and
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T. Caruso, M. Mincolelli, F. Troisi, E. Vasca and P. Neri, Tetrahedron,
2008, 64, 5370–5378; (d) N. Korbakov, P. Timmerman, N. Lidich, B.
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A. Arduini and A. Pochini, Angew. Chem., Int. Ed., 2005, 44, 2913.
14 For a review on determination of binding constants using NMR data,
see: L. Fielding, Tetrahedron, 2000, 56, 6151–6170.
15 The interaction of the counterion with cations or anions in apolar or-
ganic solvents is mainly electrostatic in nature, but other contributions
may not be neglected. The evaluation of such interaction becomes
problematic when cations, such as the planar N-alkylpyridinium,
characterised both by non-isotropic charge distribution and important
stereoelectronic requirements are present in the pair. To the best of our
knowledge, there are only few systematic evaluation of the interacting
power of anions in ion pairing, see, for example: (a) K. Fumino, A.
Wulf and R. Ludwig, Angew. Chem., Int. Ed., 2008, 47, 3830; (b) R.
Bini, O. Bortolini, C. Chiappe, D. Pieraccini and T. Siciliano, J. Phys.
Chem. B, 2007, 111, 598–604.
16 The study of the interaction energy of tetramethylammonium ion pairs
having different counteranions with a host with low steric hindrance
can be employed as an additional experimental method to evaluate
such interactions, see , for example: (a) S. Bartoli and S. Roelens, J. Am.
Chem. Soc., 2002, 124, 8307–8315; (b) V. Bo¨hmer, A. Dalla Cort and
L. Mandolini, J. Org. Chem., 2001, 66, 1900–1902and Ref. 7a.
17 K. A. Connors, Binding Constants, Wiley, New York, 1987.
18 C. S. Wilcox, in Frontiers of Supramolecular Organic Chemistry and
Photochemistry, eds. H.-J. Schneider and H. Du¨rr, VCH, Weinheim,
1991, pp. 123–143.
19 The interaction of 1 with NMPCl and NMPI salts can be complicated
by the partial dissociation of the ion pairs that add further binding
equilibria to the 1:1 binding model. These effects and their mathe-
matical treatment have been very recently discussed by Roelens and
co-workers; see: S. Roelens, A. Vacca and F. Venturi, Chem. Eur. J.,
2009, 15, 2635–2644.
˚
calculated positions with the geometric constraint C–H 0.96 A
and refined “riding” on their corresponding carbon atoms with
relative isotropic atomic displacements. Geometric calculations
were performed with the PARST9742.
Acknowledgements
This work was partly supported by the Italian MIUR (Sistemi
Supramolecolari per la Costruzione di Macchine Molecolari,
Conversione dell’Energia, Sensing e Catalisi). We thank CIM
(Centro Interdipartimentale di Misure), G. Casnati of the Uni-
versity of Parma for NMR measurements and Dr. Stefano Sforza
of the University of Parma for a fruitful discussion on mass
measurements.
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