ISMSC 2007 - Università degli Studi di Pavia

Nitroxide Radicals as Probes for Exploring the Binding Properties of
Cucurbit[n]uril Hosts
Elisabetta Mezzina, Elisabetta Mileo, Gian Franco Pedulli, Marco Lucarini
Department of Organic Chemistry “A. Mangini”, University of Bologna, Via S. Giacomo 11,
I-40126 Bologna, Italy
In the last few years we have shown that the formation of a host-guest complex can be studied
very conveniently by EPR spectroscopy, by using an appropriate radical probe.[1] Actually,
benzyl tert-butyl nitroxide and related dialkyl nitroxides were found to be suitable probes to
investigate host-guest interactions in cyclodextrins, calixarenes, and protected nanoparticles.
Evidence for the formation of paramagnetic complexes between these radicals and the host
systems was provided by large spectral changes due to the different environment experienced
by the radical guest, and to conformational changes occurring upon complexation. In most
cases, the EPR spectra also showed a strong linewidth dependence on temperature, indicating
that the lifetime of nitroxides in the associated and free form is comparable to the EPR
timescale; this enabled us to measure the rate constants for the association and dissociation
processes.
In the present communication we show that the use of EPR spectroscopy can be extended to
the study of paramagnetic supramolecular complexes formed by cucurbiturils (CBn) in
solution.[2] Evidence for the formation of a complex between nitroxide radicals and
cucurbit[n]uril (n=7,8) in water solution was provided by large changes of nitrogen hyperfine
splitting, due to the different polar environment experienced by the radical when included. In the
presence of alkali cations the EPR spectra of benzyl tert-butyl nitroxide were characterized by
new signals due to radical hosted in the CB cavity in which one metal cation is in close contact
with the nitroxidic oxygen. The EPR spectra showed selective line broadening effects due to the
metal exchange between bulk water and the coordination complex; analysis of the EPR line
width variations allowed us to measure for the first time the corresponding kinetic rate
constants. By using NMR spectroscopy it was shown that this behavior is not peculiar to
X ≡ Guest
M + =Li + , Na + , K + , Cs +
M +
+ M +
+ X + M + X
+ M +
+ X + M +
M + +
X
M +
M + M +
nitroxides but also to the related
carbonyl compounds. These data
allowed us to quantify the template
effect and to reach the conclusion that
in the presence of guest having a
coordinating lone pair the formation of
ternary metal-guest-CB complexes
must be taken into account when
discussing the complexation behavior of
cucurbituril derivatives in the presence
of salts.[3]
[1] P. Franchi, M. Lucarini and G. F. Pedulli, Curr. Org. Chem. 2004, 8, 1831. P. Franchi, M.
Lucarini, E. Mezzina and G. F. Pedulli, J. Am. Chem. Soc. 2004, 126, 4343. P. Franchi, M.
Lucarini, G. F. Pedulli and D. Sciotto, Angew. Chem. Int. Ed. 2000, 39, 263. M. Lucarini, P.
Franchi, G. F. Pedulli, C. Gentilini, S. Polizzi, P. Pengo, P. Scrimin and L. Pasquato, J. Am.
Chem. Soc. 2005, 127, 16384.
[2] For a recent review on cucurbiturils see: J. Lagona, P. Mukhopadhyay, S. Chakrabarti and L.
Isaacs, Angew. Chem., Int. Ed. 2005, 44, 4844.
[3] E. Mezzina, F. Cruciani, G. F. Pedulli and M. Lucarini, Chem. Eur. J. 2007, in press.
M +
M +
X
M +
X
+ M +
PSA 95
PSA 96
Supramolecular complexes of polythiophene derivatives with different types
of metal cations
Elena V. Lukovskaya a , Alla A. Bobylyova a , Olga A. Fedorova a ,Yury V. Fedorov b , Emely
Marmois c , Gediminas Jonusauskas c , Sergey V. Kardashev a , Anton L. Maksimov a , Y. Didane d ,
Hugues Brisset d , Frederic Fages d , Artem A. Mizerev a , Aleksander V. Anisimov a
a Department of Chemistry, M. V. Lomonosov Moscow State University, Leninskie Gory,
119992 Moscow, Russia
b Photochemistry Center of RAS, Novatorov str., 7a,119421 Moscow, Russia
Centre de Physique Moleculare Optique et Hertzienne U.M.R. 5798 Universite Bordeaux 1,
Bordeaux, France
d Universite de la Mediterranee, Faculte des Sciences de Luminy, GCOM2, Marseille, France
A very promising approach to electrochemically active compounds with various covalently
attached functional groups is to use the electrochemically active compounds bearing molecular
recognizing groups in which very selective host-guest interaction modulate, switch, and amplify
the electron transport properties. In the present research we synthesized the polythiophene
derivatives with crown-containing styryl and vinylpyridyne groups in 2 and 3 position of
thiophene ring. The complex formation of prepared compounds was studied with Mg 2+ and Ba 2+
cations by applying of NMR- and optical spectroscopy as well as ESI MASS spectrometry and
electrochemical studies. It was found that addition of Ba 2+ cations causes the formation of only
intermolecular sandwich complexes in case of 2-substituted derivatives. On the contrsry in the
presence of Ba 2+ cations 3-substituted derivatives form only itramolecular sandwich complexes
of high stability. Formation of complexes causes changes in optical spectra; electrochemical
response depends on the type of complexes formed by interaction of poythiophene derivatives
with metal cations.
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
S
S
n
CH 2
Ba 2+
N
S
S
S
S
S
n = 0, 1
Acknowledgements. The research was done by financial support of INTAS 03-51-4696
and program of Russian Academy of Sciences 07-03-00724.
S
n
n
2Ba 2+
S
n
S
S
S
S
Ba 2+
R
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
S
N