Detection of paramagnetic pH-dependent inclusion complexes between β-cyclodextrin dimers and nitroxide radicals

Article abstract of DOI:10.1002/anie.200250526

Three's company: The construction of a three-component organic free radical has been obtained with symmetric nitroxides and β-cyclodextrin. Its formation can be reversibly controlled by changing pH or temperature (see scheme).

Full text of DOI:10.1002/anie.200250526

Communications
pH-Dependent Inclusion Complexes
of the unpaired electron with nitrogen and four equivalent
benzylic protons (see Table 1). In the presence of b-CD
(3.0 mm), additional signals assigned to the radical included in
the cavity of b-CD (1b), in equilibrium with the free
nitroxide, were observed (see Figure 1 and Figure 2).
Detection of Paramagnetic pH-Dependent
Inclusion Complexes between b-Cyclodextrin
Dimers and Nitroxide Radicals**
By increasing the absolute concentration of b-CD, the
ratio between included and free species varied linearly and, at
6 mm of b-CD, the dominant spectrum was that of the 1:1
inclusion complex 1b. When b-CD concentrations higher than
9 mm were used, new signals appeared in the EPR spectrum
(see Figure 1), which arise from a third radical species,
characterized by spectroscopic parameters similar to those of
the previous two species. The relative concentration of the
third species changed reversibly on changing the b-CD
concentration. This third species was identified as a second
inclusion complex having a 1:2 stoichiometry (1c). Its EPR
nitrogen hyperfine splitting (aN) is significantly smaller than
that of both the uncomplexed radical (1a) and the 1:1
complex (1b; see Table 1), which indicates that the nitroxide
group is located in a less polar environment[7] consistent with
a 1:2 complex in which the nitroxide group is completely
shielded from the aqueous solvent. The molar ratio [1c]/[1b]
of the two complexes, obtained from the EPR spectra,
increased linearly with increasing concentration of the
dissolved b-CD. When recording the EPR spectra in strongly
basic solutions (pH > 12.5) the signals of the 1:2 complex 1c
disappeared while those of the 1:1 complex remained (see
Supporting Information), similar to what had been found
previously with a pyrene-modified g-CD[8a] and with the 1:2
Paola Franchi, Marco Lucarini,* and
Gian Franco Pedulli
We have recently designed a series of paramagnetic probes
based on benzyl tert-butyl nitroxide to investigate, in aqueous
solution, several types of supramolecular reaction kinetics in
the submicrosecond time range by means of EPR spectros-
copy.[1–3] The EPR spectra of these probes show a significant
difference in the hyperfine splittings both at nitrogen, a(N),
induced by the less polar environment of the host component
with respect to the solution, and at the benzylic protons,
a(2Hb), because of conformational changes, so that the signals
of both the complexed and uncomplexed species are easily
distinguishable.
When benzyl tert-butyl nitroxide was investigated in
aqueous solution in the presence of b-cyclodextrins (b-CDs)
only one inclusion paramagnetic species was detected; this
was identified as the 1:1 complex, similar to what was found
by NMR spectroscopy for the parent amine compound.[1,4]
Since the tendency of CDs to form inclusion complexes with
guest molecules and simultaneously to self-associate to give
dimers can be exploited to produce supramolecular structures
at an higher level of organization,[5] we have been looking for
paramagnetic organic guests capable to form CD complexes
with a stoichiometry other than 1:1.[6] Herein we report the
first EPR detection and characterization of paramagnetic
inclusion complexes between symmetric dialkyl nitroxides
(1a–2a) and two molecules of b-cyclodextrin.
N
N
O
O
2a
1a
The EPR spectrum 1a, which is produced by oxidation of
dibenzyl amine (2.0 mm) with the magnesium salt of monop-
eroxyphthalic acid (1.0 mm) in water, is shown in Figure 1.
The spectrum is easily interpreted on the basis of the coupling
[*] Prof.M.Lucarini, Dr.P.Franchi, Prof.G.F.Pedulli
Department of Organic Chemistry “A.Mangini”
University of Bologna
Figure 1. EPR spectra of 1a in water at 295 K in the absence (a) and in
the presence (b), (c) of various amounts of b-CD.
Via S.Donato 15, 40127 Bologna (Italy)
Fax: (+39)051-244-064
E-mail: lucarini@alma.unibo.it
[**] Financial support from MURST (Research project “Free Radical
Processes in Chemistry and Biology: Synthesis, Mechanisms,
Applications”) and University of Bologna is gratefully acknowl-
edged.
K1
K2
G
+
G
+
G
1a-2a
β-CD
1b-2b
β-CD
1c-2c
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Figure 2. Schematic representation of the equilibria taking place in the
presence of b-cyclodextrin.
1842
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200250526
Angew. Chem. Int. Ed. 2003, 42, 1842 – 1845

Angewandte
Chemie
Table 1: Spectroscopic and thermodynamic parameters for CD complex formation.
wider rings of the cyclodextrins.
295[b]
This process also implies a loss of
entropy because of the reduced
freedom of molecular motion of
the complex which, in contrast to
the 1:1 complexation, is not com-
pensated by the desolvation of the
guest as this is already included in
the cavity of one CD molecule.
Thus, the measured thermodynamic
parameters are consistent with the
formation of a 1:2 complex.[8b]
System[a]
aHb
[mT]
aN
[mT]
DH
DS
DG295 [kJmolꢀ1
]
Keq
[kJmolꢀ1
]
[JKꢀ1 molꢀ1
]
[mꢀ1
]
1a (H2O)
1.081
1.007
0.796
0.958
0.923
1.649
1.601
1.559
1.584
1.595
1.682
1.620
1.565
1.602
1.616
1b (b-CD, m)
1c (b-CD, d)
1d (DM-b-CD)
1e (g-CD)
ꢀ13.5
ꢀ41.0
5.1
ꢀ14.6
ꢀ8.5
385
32
539
67
ꢀ108.9
ꢀ15.5
ꢀ10.3
2a (H2O)
2b (b-CD, m)
2c (b-CD, d)
2d (DM-b-CD)
2e (g-CD)
ꢀ18.0
ꢀ37.5
ꢀ2.7
ꢀ17.3
ꢀ9.9
1106
56
1503
257
ꢀ94.1
ꢀ18.0
ꢀ13.6
To gain a more detailed picture
of the 1:2 complex formation, sto-
[a] In parenthesis is reported the host species (m=monomer, d=dimer).[b] Keq =K1 or K2.
complex between 4-(dimethylamino) benzoni-
trile and b-CD.[8b] The lability of the 1:2 complex
at high pH values is attributed to the fact that in
basic media dissociation of the secondary CD
hydroxy group (pKa = 12.5) takes place to form
an anionic species. Since molecular modeling
studies have demonstrated that the driving force
responsible for the stabilization of the b-CD
dimer is the formation of intermolecular hydro-
gen bonds between the secondary hydroxy groups
of the two larger rings,[9] the anionic b-CD unit
can not associate to give dimers because of
Coulombic charge repulsion. This hypothesis
was confirmed by using as host 2,6-dimethylated
b-cyclodextrins (DM-b-CDs) in which some of
the hydroxylic hydrogen atoms responsible for
the intermolecular hydrogen bonding have been
replaced with methyl groups. Actually, the EPR
spectra of dibenzyl nitroxide in the presence of
0.1m of DM-b-CD (1d) did not show any signals
other than those of the 1:1 complex (see Support-
ing Information).
Figure 3. Van't Hoff plot for the temperature dependence of the equilibrium con-
*
stants, K1 and K2; & =1a and =2a.
The values for the K1 and K2 binding constants were
obtained by plotting the ratio between the concentrations of
the 1:1 complex and the free species ([b-CD] 0–8 mm) and the
ratio between the concentrations of the 1:2 and 1:1 complex
([b-CD] 8–15 mm) as a function of the cyclodextrin concen-
tration in water. The temperature dependence of these
equilibrium constants, reported in van't Hoff plots
(Figure 3), shows that K1 changes only slightly with temper-
ature, while K2 significantly decreases with increasing temper-
ature.
The first step, that is, the formation of the 1:1 complex, is
associated with a small negative DH8 and an entropy change,
DS8, close to zero (see Table 1). Although the entropy of
complexation of a guest by an empty host in the gas phase
would be negative by 120–200 Jmolꢀ1 Kꢀ1 the complexation in
solution is expected to have entropies of complexation much
closer to zero, because of the desolvation of the guest from
water molecules is entropically favorable.[10]
chastic dynamics (SD) simulations were performed in water
at constant temperature by using AMBER* force field.[11,12]
After the sample had been heated to 295 K, the different
complexes were equilibrated for 200 ps with time step of 1 fs
and the total simulation time was set to 15000 ps to achieve
full convergence (see Table 2).[13]
Consistent with the experimental evidence, the SD
simulations show that on average, the formations of the 1:1
and 1:2 complexes are energetically favorable by 50 and
101 kJmolꢀ1, respectively. While in the 1:1 complex only one
phenyl ring is located inside the torus, in the 1:2 complex the
nitroxide is totally embedded in the macrocylic rings (see
Table 2: Summary of data obtained by SD simulation at 295 K in water.
Equilibrium
Energy of reactants Energy of complex DE
[kJmolꢀ1
]
[kJmolꢀ1
]
[kJmolꢀ1
]
1a+b-CDÐ1b
2b-CDÐdimer
767 (116+651)
1302 (2”651)
1b+b-CDÐ1c 1368 (717+651)
717
1238
1267
1267
ꢀ50
ꢀ64
The formation of the 1:2 complex, however, is charac-
terized by largely negative enthalpy and entropy changes. The
negative DH value is the result of the formation of a large
number of intermolecular hydrogen bonds between the two
ꢀ101
ꢀ87
1a+dimerÐ1c 1354 (116+1238)
Angew. Chem. Int. Ed. 2003, 42, 1842 – 1845
www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1843

Communications
dimers can be exploited to produce paramagnetic supra-
molecular structures at an higher level of organization. The
formation of such supramolecular ordered polyradical assem-
blies can be useful for the preparation of functional molecular
magnetic materials.
Experimental Section
Radicals 1a and 2a were generated by mixing a methanol solution
(1 mL) containing the corresponding amine (0.2m) and a water
solution (1 mL) containing the Mg salt of monoperoxyphthalic acid
(0.1m) with a water solution (100 mL) containing variable amounts of
b-CD. The pH value of the solution was adjusted by adding NaOH.
for pH > 12 Oxone was used as the oxidant. Samples were transferred
in capillary tubes (1 mm i.d.), heated for 30–60 s at 808C and then
placed inside the thermostatted cavity of EPR spectrometer (Bruker
ESP300). The instrument settings were: microwave power 5.0 mW,
modulation amplitude 0.05 mT, modulation frequency 100 kHz, scan
time 180 s. The computed spectra were best-fitted to the experimental
ones by using a Monte Carlo minimization procedure.[1b,17]
Figure 4. Clustered molecular display.Dynamics of the 1:1 (a) and
1:2 (b) complexes of b-CD (gray) with dibenzyl nitroxide (black).The
drawings include only 500 structures that refer to the 15000 ps simula-
tion.Hydrogen atoms have been omitted for clarity.
Figure 4), in agreement with the decrease of aN observed
experimentally when passing from 1b to 1c.
Moreover, the SD calculated hcos2 qi values [14] for 1a and
1c, where q is the dihedral angle between the symmetry axis
of the 2pp orbital of nitrogen atom and the N-C-Hb plane in
the nitroxide, were 0.41 and 0.37, respectively. These values
are in agreement with the large reduction of aHb found
experimentally when passing from 1a to 1c. The computa-
tions indicate that in the 1:2 complex the only populated
conformers are those in which q is ꢀ308 and 908, while in
water the energy minimum in which the phenyl group is
eclipsed by the oxygen atom is also significantly populated
(see Supporting Information) and contributes to the value of
hcos2 qi.
SD simulations were carried out using the MacroModel 7.0
program. Extended nonbonded cutoff distances were set to 8 Š and
20 Š for the van der Waals and electrostatic interactions. The
generalized Born/surface area (GB/SA) solvation model was used
ꢀ
when modeling the solvent effect under MacroModel. All C H and
ꢀ
O H bond lengths were held fixed using the SHAKE algorithm.
Translational and rotational momentum were removed every 0.1 ps.
b-Cyclodextrin with a C7 symmetry was used as the starting host. All
the complexes were generated by docking the guest to the host in a
suitable orientation. In all cases the origin of a Cartesian reference
frame was placed on the center of mass of the CD and the z axis
aligned with the C7 symmetry axis of CD. No significant differences
were obtained by using different starting orientations.
To increase the life time of the paramagnetic 1:2 complex
we investigated the behavior of symmetric di-tert-alkyl nit-
roxides, because substitution of the b-hydrogen atoms with
alkyl groups increases considerably the life time of the
nitrogen containing species.[15] In the absence of b-hydrogen
atoms, however, the characterization of the complexed
species by EPR spectroscopy is not as straightforward as for
1a, since the changes with complexation of the only meas-
urable splitting constant, that is aN, are not sufficiently large to
clearly differentiate the spectra of the various species.
However, when using di-tert amyl nitroxide (2a) partial
resolution of the high-field EPR lines of the free and
complexed nitroxides is observed.[16] Actually, we were able
to distinguish two different signals, attributed to the radical in
water (2a) and to the 1:1 complex (2b) in the presence of
3.7 mm b-CD, and to the 1:1 and 1:2 complex (2c) at 16 mm b-
CD concentration (see Supporting Information). The assign-
ment of the signals to the different species was on the basis of
considerations similar to those reported above for dibenzyl
nitroxide. In this case, however, the increased radical life time
(several days) made it possible to switch reversibly from the
1:1 to 1:2 complex many times by successive acid–base
treatments without any EPR evidence of side reactions.
Summarizing, we have been able to construct the first
three-component organic free-radical complex which can be
reversibly formed by changing the pH value or temperature.
Simple chemical modifications of the radical guest can give
species able to self associate. The combination of this
property with the tendency of cyclodextrins to form inclusion
complexes and simultaneously to self-associate and form
Received: November 12, 2002 [Z50526]
Keywords: cyclodextrins · EPR spectroscopy ·
.
host–guest systems · nitroxides · radicals
[1] a) M. Lucarini, B. P. Roberts, Chem. Commun. 1996, 1577; b) M.
Lucarini, B. Luppi, G. F. Pedulli, B. P. Roberts, Chem. Eur. J.
1999, 5, 2048.
[2] P. Franchi, M. Lucarini, G. F. Pedulli, D. Sciotto, Angew. Chem.
2000, 112, 269; Angew. Chem. Int. Ed. 2000, 39, 263.
[3] G. Brigati, P. Franchi, M. Lucarini, G. F. Pedulli, L. Valgimigli,
Res. Chem. Intermed. 2002, 28, 131.
[4] M. Lucarini, E. Mezzina, G. F. Pedulli, Eur. J. Org. Chem. 2000,
3927.
ˇ ˇ
ˇ
ˇ
[5] a) T. Kraus, M. Budesínsky, I. Císarovµ, J. Zµvada, Angew.
Chem. 2002, 114, 1791; Angew. Chem. Int. Ed. 2002, 41, 1715;
b) D. T. Bong, T. D. Clark, J. R. Granja, M. R. Ghadiri, Angew.
Chem. 2001, 113, 1016; Angew. Chem. Int. Ed. 2001, 40, 988; c) S.
Makedonopoulou, I. M. Mavridis, K. Yannakopoulou, J.
Papaioannou, Chem. Commun. 1998, 2133; d) G. Gattuso, S.
Menzer, S. A. Nepogodiev, J. F. Stoddart, D. J. Williams, Angew.
Chem. 1997, 109, 1615; Angew. Chem. Int. Ed. Engl. 1997, 36,
1451.
[6] J. A. Crayston, J. N. Devine, J. C. Walton, Tetrahedron 2000, 56,
7829.
[7] B. R. Knauer, J. J. Napier, J. Am. Chem. Soc. 1976, 98, 4395, and
references therein.
[8] a) A. Ueno, I. Suzuki, T. Osa, J. Am. Chem. Soc. 1989, 111, 6391;
b) A. Nakamura, S. Sato, K. Hamasaki, A. Ueno, F. Toda, J.
Phys. Chem. 1995, 99, 10952.
1844
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 1842 – 1845

Angewandte
Chemie
[9] P. Bonnet, C. Jaime, L. Morin-Allory, J. Org. Chem. 2001, 66, 689.
[10] a) M. V. Rekharsy, Y. Inoue, Chem. Rev. 1998, 98, 1875; b) K.
Kano, T. Kitae, Y. Shimofuri, N. Tanaka, Y. Mineta, Chem. Eur.
J. 2000, 6, 2705.
[11] a) S. J. Weiner, P. A. Kollman, D. A. Case, U. C. Singh, C. Ghio,
G. Alagona, S. Profeta, P. Weiner, J. Am. Chem. Soc. 1984, 106,
765; b) S. J. Weiner, P. A. Kollman, D. T. Nguyen, D. A. Case, J.
Comput. Chem. 1986, 7, 230.
ꢀ
¼
[12] The N O bond was modeled by the C O bond because of their
similar geometries and the lack of corresponding parameters in
the AMBER* force field. F. Perez, C. Jaime, X. Sµnchez-Ruiz, J.
Org. Chem. 1995, 60, 3840.
[13] K. B. Lipkowitz, G. Pearl, R. Coner, M. A. Peterson, J. Am.
Chem. Soc. 1997, 119, 600.
[14] The magnitude of the splitting constant for the b-hydrogen
atoms (Hb) depends on the time-average value of cos2 q
according to the Heller-McConnell relationship:
a(H)b = 1N(B0 + B2hcos2 qi).
[15] Dibenzyl nitroxide persists at room temperature for only few
minutes because of a disproportionation reaction that leads to
the formation of nitrones and hydroxylamines.
[16] The first example of a persistent nitroxide–cyclodextrin complex
was reported in 1975. J. Martinie, J. Michon, A. Rassat, J. Am.
Chem. Soc. 1975, 97, 1818.
[17] B. Kirste, J. Magn. Reson. 1987, 73, 213.
Angew. Chem. Int. Ed. 2003, 42, 1842 – 1845
www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1845