Synthesis, electrochemical behavior and magnetic properties of polyradicals of the TTM series

Article abstract of DOI:10.1016/j.tet.2006.10.087

Condensation reactions between (4-amino-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl radical and a new hexafunctionalized radical of the TTM series, tris(3,5-dichloroformil-2,4,6-trichlorophenyl)methyl radical or its corresponding diamagnetic precu

Full text of DOI:10.1016/j.tet.2006.10.087

Tetrahedron 63 (2007) 708–716
Synthesis, electrochemical behavior and magnetic
properties of polyradicals of the TTM series
a
M. Castillo, E. Brillas, C. Rillo, M. D. Kuzmin and L. Julia
b
c
c
ꢀa,
*
aDepartament de Quꢀımica Orgaꢁnica Bioloꢁgica, Institut d’Investigacions Quꢀımiques I Ambientals de Barcelona (CSIC),
Jordi Girona 18-26, 08034 Barcelona, Spain
bDepartament de Quꢀımica Fꢀısica, Universitat de Barcelona, Martꢀı I Franqueꢁs 1-11, 08028 Barcelona, Spain
cInstituto de Ciencia de Materiales de Aragoꢀn, CSIC – Universidad de Zaragoza, c/Pedro Cerbuna 12, 50009 Zaragoza, Spain
Received 11 October 2006; revised 23 October 2006; accepted 29 October 2006
Available online 15 November 2006
Abstract—Condensation reactions between (4-amino-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl radical and a new hexafunction-
alized radical of the TTM series, tris(3,5-dichloroformil-2,4,6-trichlorophenyl)methyl radical or its corresponding diamagnetic precursor 6
yield macromolecular heptaradical 3 and hexaradical 4, respectively, with high magnetic purity. Both solid polyradicals show stability in
nitrogen at temperatures up to 300 ꢀC and in solution their electronic absorptivities do not decrease appreciably for weeks. EPR studies of
both polyradicals in solution suggest that unpaired electrons are exchange coupled, and at low temperatures they show weak dipolar spin–
spin couplings. Their electrochemical behavior was analyzed by cyclic voltammetry and differential pulse voltammetry. Heptaradical 3 is
reduced in seven-electron two-stage processes, and oxidized in seven-electron one-stage process.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
magnetic organic molecules. Radicals of the TTM [tris(2,4,
6-trichlorophenyl)methyl (1)] series are very stable carbon-
centered radicals whose persistence is conferred by the
presence of six aromatic chlorines surrounding the trivalent
carbon atom.4 These organic radicals are completely unasso-
ciated as crystalline solids with no appreciable decomposi-
tion both in solid and in solution. Consequence of this
stability is the feasibility to incorporate the amide function
by condensation of (4-amino-2,6-dichlorophenyl)bis(2,4,6-
trichlorophenyl)methyl radical (2), a functional radical of
the TTM series, with different acid chlorides without impair-
ment of the radical character of the molecule.4b
Nowadays many efforts are focused in the field of organic
magnetic molecules as materials with attractive potential ap-
plications in different areas such as magnetic imaging and
magneto-optics.1 One of the advantages of the use of organic
molecules instead of inorganic based materials lies in
the possibility of chemical modification onto the molecule
affecting their electronic properties and allowing the modu-
lation of the dipolar interactions between the different
magnetic centers. An important requirement to design and
prepare high-spin organic molecules is a strong p-conju-
gated coupling between unpaired electrons. One rational ap-
proach for it consists in dividing the high-spin molecule into
two components: the spin-containing (SC) fragment, which
provides the unpaired electron and the ferromagnetic cou-
pling (FC) for connecting the different radical centers ferro-
magnetically.2 Many of the SC fragments are based on
triarylmethyl radicals, which have been the object of in-
creasing interest in the last years and many high-spin species
based on them as building blocks have been reported.3,4 On
the other hand, the amide bond –N–CO– as a FC unit has
shown to be an exchange coupler between spin centers due
to the double bond character of the N–C bond allowing the
extension of p-conjugation between spin-bearing moieties.5
The stability is another essential requirement to consider in
Cl
NH2
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
C
C
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
1
2
This article describes the synthesis and magnetic properties
of two macromolecular-branched polyradicals 3 and 4 with
amide functions as linker for connecting several triphenyl-
methyl radicals of the TTM series. These polyradical species
are composed of a central triphenylmethyl radical or its dia-
magnetic precursor as the core of the dendrimer and three
* Corresponding author. Tel.: +34 93 4006107; fax: +34 93 2045904;
e-mail: ljbmoh@cid.csic.es
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.10.087

M. Castillo et al. / Tetrahedron 63 (2007) 708–716
709
pairs of peripheral triphenylmethyl radicals, each pair con-
nected to one of the phenyl groups of the core through amide
functions as linkers employing a meta-topology. This ar-
rangement of the radical centers in the macromolecule, char-
acterized by larger dimensions than that of linear systems,
guarantees the in-phase periodicity of the spin polarization
in the p-orbitals as a consequence of each radical center in
the macromolecule to make feasible the generation of high-
spin systems. Besides the magnetic properties of these or-
ganic molecules, radicals of the TTM series are redox units
and they are electrochemically reduced and oxidized in re-
versible processes. Chemically they behave as oxidants in the
presence of electron-rich species to give stable negatively
charged species, by electron transfer processes, which yield
the corresponding diamagnetic triphenylmethanes in the
presence of proton-donors. In addition, they participate as
electron-donors in front of oxidants to yield the stable carbo-
cations.4d Therefore, radicals of the TTM series are species
of fundamental importance in the study of single-electron
transfer reactions, as well as in the investigation of reaction
mechanisms. Consequently, the new polyradicals described
in this article will be the electroactive materials with several
redox units that can accept or transfer multiple number of
electrons under specific applied potentials. Therefore, we
also report the redox properties of these polyradicals.
known and simple synthetic route following the sequence
outlined in Scheme 1.
Dichloromethyl groups in tris(a,a,a0,a0,2,4,6-heptachloroxy-
lyl)methane (8) were introduced by alkylation of tris(2,4,6-tri-
chlorophenyl)methane (7) with chloroform in the presence of
AlCl3 by means of Friedel–Crafts reaction as reported.6 An
acid hydrolysis of hydrocarbon 8 with sulfuric acid gives 9
withsix carboxyaldehydes inall meta aromatic positions. Oxi-
dation of aldehydes to carboxylic acids with CrO3 yields tri-
phenylmethane 10, which affords 6 by treating with thionyl
chloride. Finally, radical5wasgeneratedbytreating6withtet-
rabutylammonium hydroxide to give the anion followed by
oxidation with chromium(VI) oxide. Condensation of an ex-
cess over 6:1 of radical 2 with 5 or 6 in boiling benzene and
in the presence of cesium carbonate takes place without im-
pairment of the radical character of the magnetic species, as
observed in the benzoylation of radical 2 with benzoyl chlor-
ide.4b Thus, the multiple magnetic character of 4 with six tri-
valent carbon atoms and of 3 with seven trivalent carbon
atoms is retained. To know more thoroughly the dipolar inter-
actions among electronic spins in simpler model systems with
only two unpaired electrons, the synthesis of the three new
biradical species, 11, 12 and 13, was performed. Biradicals
11 and 12 result from the monocondensation reaction between
Ar
Ar
Ar
C
Cl
C
Cl
Ar
Cl
Cl
Cl
HNOC
CONH
Cl
Cl
Cl
Cl
Cl
Cl
Ar
Ar
Ar
Cl
C
X
C
CONH
Cl
HNOC
Cl
C
Ar
Cl
Cl
Cl
CONH
CONH
Cl
Cl
C
Cl
Cl
C
Ar
Ar
Ar
Ar
Cl
Cl
Ar =
Cl
3: X =
4: X = H
2. Results and discussion
2.1. Synthesis and stability
amino radical 2 and bis(pentachlorophenyl)[2,4,6-trichloro-3,
5-bis(chlorocarbonyl)phenyl]methyl radical (14) and radical
5, respectively. The electronic exchange and dipolar interac-
tions in both biradicals 11 and 12 will confirm the amide func-
tion as a good linker of two SC fragments. In the same way,
biradical 13, product of condensation between 2,4,5,6-tetra-
chloroisophthaloyl dichloride and amino radical 2 in a 1:2
molar proportion, will give information of electronic spins be-
tween the two peripheral triphenylmethyl moieties in hexarad-
ical 4 and heptaradical 3 linked through two amide functions.
Heptaradical 3 and hexaradical 4 have been prepared by
multiple condensations of amino radical 2 with the hexa-
valent tris(3,5-dichloroformil-2,4,6-trichlorophenyl)methyl
radical (5) and its diamagnetic parent hydrocarbon 6, respec-
tively, without impairment of the radical character of all
trivalent carbon atoms. Radical 5 was generated by well-

710
M. Castillo et al. / Tetrahedron 63 (2007) 708–716
Cl
Cl
R
R
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
R
CH
CX
Cl
R
R
Cl
Cl
Cl
Cl
Cl
Cl
Cl
R
8: R = CHCl2 ; X = H
9: R = CHO ; X = H
10: R = CO2H ; X = H
7
6: R = COCl ; X = H
5: R = COCl ; X =
Scheme 1.
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
C
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
C
C
Cl
Cl
Cl
Cl
Cl
Cl
C
ClOC
CONH
Cl
Cl
HNOC
Cl
CONH
Cl
Cl
Cl
Cl
Cl
X
X
ClOC
COCl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
13
X
Cl
X
C
Cl
Cl
Cl
Cl
11: X = Cl
12: X = COCl
Cl
Cl
Cl
Cl
14
Polyradicals 3 and 4 are stable in air, either in solution or in
solid form. Their stability is mainly due to steric shielding of
thesixorthochlorineatomsaroundeachradicalcenter,which
forces torsional displacements from planarity of the aromatic
planes around the sp2 carbon atom. This effect justifies the
poor delocalization of the single electron into the aromatic
substituents, similar to all free radicals of the TTM series.4a,b
The electronic absorption spectra of 3 and 4 in CH2Cl2 solu-
tiondisplaythecharacteristicbandsoftheradicalcharacterof
themolecules, withabsorptivitiesapproximatelysevenandsix
times for 3 and 4, respectively, as high as those of monorad-
ical species of the same TTM series [l (3, dm3 molꢁ1 cmꢁ1),
3: 376 (191,500), 514 (8330), 557 (8230) nm; 4: 376
(168,500), 512 (5800), 553 (5880) nm] and their absorptivi-
ties do not decrease appreciably for weeks. In situ oxidation
with an excess of SbCl5 resulted in the generation of the sta-
ble and highly blue solutions of the multiple charged cations
193 and 298 K. The spectrum at 193 K showed a single
line centered at g¼2.0033 with a peak-to-peak line width
DHpp¼1.0 G. At high gain values the isotropic coupling
with the 13C (natural abundance, 1.07 %) nuclear spins of
the three bridgehead carbons and the six ortho-carbons ap-
peared on both sides of the central line at 9.75 and 13.0 G,
respectively. At 298 K, the pair of bands equidistant of the
central line and associated with the 13C nuclear spins of
a-carbon atom was well observed in the spectrum with a
coupling value of 29.4 G.
The spectrum of biradical 11 in deoxygenated CH2Cl2 solu-
tion (w10ꢁ3 M) and recorded at room temperature gave a
single and broad line (g¼2.0035, DHpp¼2.0 G). Isotropic
coupling with the naturally abundant 13C nuclei is only
appreciable at high gains with the a-carbon atom and its
value, a(a13C)w13 G, is approximately half the value found
in monoradical species of the TTM series, suggesting that
the electron-exchange interaction is larger than the hyperfine
interaction, (jJj>jaij). Figure 1 shows the evolution of the
spectra of biradical 11 in 2-methyltetrahydrofuran (MTHF)
at temperatures from 270 K down to 120 K. This effect is
due to the anisotropic dipolar electron–electron interaction
and at intermediate temperatures when the solution is not
3
(3) 564 (sh) (136,800), 668 (264,270) nm].
7+ [l (3) 568 (sh) (160,140), 670 (331,940) nm] and 46+ [l
2.2. Electron paramagnetic resonance
X-band EPR spectrum of the hexafunctional radical 5 was
recorded in deoxygenated CH2Cl2 solution (w10ꢁ3 M) at

M. Castillo et al. / Tetrahedron 63 (2007) 708–716
711
Figure 1. Left: a series of EPR spectra of a solution of biradical 11 in MTHF
from 270 K down to 120 K, showing the intensity dependence of the dipolar
electron–electron interaction. Right: (a) experimental spectrum of biradical
11 in MTHF at 120 K. (b) Simulated spectrum using the values of the zero-
field parameters D and E given in the text.
Figure 3. Left: EPR spectrum of (1) biradical 11 and (2) biradical 12 in
MTHF at 4 K. Right: (a) experimental and (b) simulated spectrum of birad-
ical 13 in MTHF at 120 K, using the zero-field parameters given in the text.
glassy yet, the central line showed hyperfine structure due to
the unresolved coupling of the six aromatic protons with
a value a(6H)¼1.25 G, very close to the coupling value in
most radicals of the TTM series. Most probably, this line
corresponds to some small impurities of monoradical. The
fine structure showed in the spectrum at 120 K (solid matrix)
provides an estimation of the zero-field parameters D and E
by spectral simulation (Fig. 1, right). This spectrum is
consistent with jD/hcj¼22.5ꢂ10ꢁ4 cmꢁ1 and jE/hcj¼0.6ꢂ
10ꢁ4 cmꢁ1 being gx¼gy¼2.0033 and gz¼2.0027, and using
a line width DHpp¼4.6 G. Confirmation of the triplet state
was provided by the presence of a doublet in the Dms¼ꢃ2
region of the spectrum, which obeyed the Curie’s law
(Iw1/T) between 4 and 23 K (Fig. 2), consistent with a triplet
ground state or a degeneracy of singlet and triplet states.
Within the magnetic dipole–dipole approximation, the
experimental value D can provide information about the
mean distance between unpaired electrons via the equation
R¼(0.65g2/D)1/3. In our case, the calculated distance is
may be attributed to some small monoradical impurities or
to other frozen conformations with weaker electron dipolar
interactions.
The spectrum of biradical 13 in MTHF at 120 K showed a
broad signal with small inflection, which suggests a much
poorer dipolar interaction between the electronic spins. De-
spite the lack of fine structure, we have been able to estimate
the zero-field parameters by spectral simulation. Figure 3
shows the experimental and the simulated spectra using
the values jD/hcj¼5.70ꢂ10ꢁ4 cmꢁ1 and jE/hcjw0.0 cmꢁ1
being gx¼gy¼2.0031 and gz¼2.0020, and using a line width
DHpp¼4.0 G. Confirmation of the triplet state was provided
by the presence of a doublet in the Dms¼ꢃ2 region of the
spectrum, which obeyed the Curie’s law (Iw1/T) between 4
and 8 K (Fig. 2), consistent with a triplet ground state or a
degeneracy of singlet and triplet states. The mean distance
between unpaired electrons via the equation R¼(0.65g2/
D)1/3 is Rw16.6 A and the distance estimated from molecular
˚
˚
˚
R¼10.5 A, which is close to 9.3 A, a value between the
centers of the two triphenylmethyl moieties estimated from
molecular modeling. This similarity between both values
suggests that spin densities are mainly confined in the central
carbon atoms of both moieties.
˚
modeling is 13.6 A.
The EPR spectra of MTHF solutions (w10ꢁ4 M) of heptarad-
ical 3 and hexaradical 4 at 295 K showed broad (DHppw4 G)
and single lines with g¼2.0030ꢃ5. At high gain, these broad
lines probably hamper the observation of satellite lines cor-
responding to the small couplings with 13C nuclear spins of
The spectrum of biradical 12 in MTHF recorded at 4 K
showed a multiplet very similar to that observed for biradical
11, as expected for very close molecular structures, with the
difference that the central signal is more intense. A direct
comparison of the Dms¼ꢃ1 for both spectra at 4 K is shown
in Figure 3. The two pairs of broad lines due to the aniso-
tropy of immobilized triplet species are practically at the
same positions, and the more intense central signal in 12
25000
0
0
0,1
0,2
0,3
1/T
Figure 2. Temperature dependence of the Dms¼ꢃ2 EPR signal intensity of
Figure 4. Up: EPR spectrum of heptaradical 3 in MTHF at 4 K; inset
represents the Dms¼ꢃ2 line. Down: EPR spectrum of hexaradical 4 in
MTHF at 4 K; inset represents the Dms¼ꢃ2 line.
the biradicals: 11 (-) and 13 (C). Solid lines are least-square fits to Curie’s
law.

712
M. Castillo et al. / Tetrahedron 63 (2007) 708–716
the a-carbon atoms in 3 and 4. Anyway these couplings are
much lower than those of the corresponding monoradicals of
the TTM series, strongly suggesting that the unpaired elec-
trons are exchange coupled. The EPR spectra of 3 and 4
(Fig. 4) recorded at 4 K showed in the Dms¼ꢃ1 region a
broad signal, DHpp¼19.0 and 18.0 G, respectively, without
fine structure, and in the Dms¼ꢃ2 region a small transition
characteristic of a triplet state. In both cases, it is difficult to
estimate the zero-field parameters, but they should be very
small suggesting low dipolar spin–spin couplings between
unpaired electrons.
corresponding data points. The best-fit values of q, the only
adjustable parameter in Eq. 1, are ꢁ1.2 K for the heptarad-
ical 3 and ꢁ0.7 K for the hexaradical 4. The small negative
values of q manifest the presence of a weak antiferromag-
netic interaction.
The initial susceptibility of the two samples was fitted to the
following formula:
c ¼ nNAm2B=kðT ꢁ qÞ þ c0
ð2Þ
The curves calculated using Eq. 2 as well as the experimental
points are shown in Figure 5b. Apart from the Curie–Weiss
term, which follows naturally from Eq. 1, Eq. 2 contains a
temperature-independent term c0. The values used in the fit
were c0¼0.064 (4) and 0.11 emu/mol (3). Those rather large
values account for the addition of all possible temperature-
independent contributions, i.e., the sample holder capsule
and cotton, typically >0 and the negative diamagnetic
sample contribution, but do not affect the low temperature
(T<100 K) analysis. In fact, as stated in what follows, the
initial susceptibility data analysis also agrees very well
with the expected behavior for 6 and 7 spin radicals with
s¼1/2.
2.3. Magnetic measurements
Magnetization was measured in static magnetic fields 0–5 T
using a SQUID magnetometer (Quantum Design model
MPMS-XL). The same device was employed in the measure-
ments of ac susceptibility in the temperature range 1.8–
300 K. The ac drive amplitude and frequency were 4.5 Oe
and 10 Hz, respectively. The low-temperature magnetization
curves were fitted to the following parametric expressions
obtained within the mean-field approximation for n spins
with s¼1/2 and g¼2:
M ¼ nNAmB tanh x; B ¼ mꢁB 1ðkTx ꢁ kq tanh xÞ
ð1Þ
The thermal variation of the effective magnetic moments per
radical in 3 and 4 is given by meff¼mB[3kT(cꢁc0)/nNA]1/2
and is shown in the insets of Figure 5b. In the range
40 K<T<100 K, meff is slightly temperature dependent
with mean values 1.70ꢃ0.03 and 1.76ꢃ0.02, for compounds
3 and 4, respectively. These quantities are quite close to the
M is the magnetization and B is the applied magnetic field
induction in tesla. Here n¼6 and 7 for 4 and 3, respectively,
x is a parameter running from zero to infinity. The curves
calculated using Eq. 1 at T¼1.8 (continuous line) and 5 K
(dotted line) are presented in Figure 5a along with the
Figure 5. (a) Data points corresponding to the magnetization of polyradicals, left: heptaradical 3 and right: hexaradical 4 in solid as a function of field strength at
1.8 (upper function) and 5 K (lower function). Continuous lines calculated as indicated in the text. (b) Data points corresponding to the susceptibility of poly-
radicals, left: heptaradical 3 and right: hexaradical 4 as a function of temperature. Continuous lines calculated as indicated in the text. Insets show effective
magnetic moments as a function of the temperature.

M. Castillo et al. / Tetrahedron 63 (2007) 708–716
713
theoretical value of 1.73mB for independent radical centers
and only decrease at very low temperatures. Such a behavior
is characteristic of weakly intermolecular antiferromagnetic
interaction.
recorded at 20 mV sꢁ1 displays two consecutive reduction
peaks with peak potentials Ep(R1)¼ꢁ0.57 V vs SCE and
Ep(R2)¼ꢁ0.76 V vs SCE and a relative intensity of 6–7:1.
These peak potentials can be taken as the standard
potentials for the first and second redox couples. The inten-
sity ratio between both redox peaks is indicative of the con-
tribution of six electrons in the first process and one electron
in the second one. Therefore, the first peak is assigned to the
reduction of the six external carbon-centered radicals and the
second one to the internal trivalent carbon atom. Electro-
chemical analysis is a powerful tool in determining the
extent of interactions between the electroactive groups in a
molecule. The appearance of two redox couples suggests
that the six trivalent carbon atoms in the periphery of the hep-
taradical 3 are electrochemically equivalent, each one being
reduced at the same applied potential without any apprecia-
ble columbic charge interaction. A similar behavior should
then be expected for the oxidation of heptaradical 3. How-
ever, CV and DPV measurements recorded at the same
scan rates revealed the existence of a quasi-reversible
seven-electron redox process with Eo¼1.02 V vs SCE and
with slight broadening of the oxidation and reduction peaks,
associated to the equilibrium 3ꢁ7eꢁ#37+. In this case, each
trivalent carbon atom of the molecule is oxidized at essen-
tially the same potential.
2.4. Cyclic voltammetry
Cyclic voltammograms for the reduction of a 10ꢁ3 M radical
5 solution in CH2Cl2 with 0.1 M tetrabutylammonium
perchlorate on Pt showed a quasi-reversible one-electron
redox pair corresponding to the process 5+eꢁ#5ꢁ, with
a standard potential E o¼0.16 V vs SCE (difference between
the anodic and cathodic peak potentials, EapꢁEpc¼0.15 V at
v¼100 mV sꢁ1). The strong electron-acceptor properties of
5 denoted by the positive value of its redox potential is
ascribed to the presence of six electron withdrawing
substituents in aromatic meta positions. That is feasible
because its oxidant power is much higher than that of
TTM radical (Eo¼ꢁ0.66 V versus SCE)7, but still lower
than that of the tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl
radical (Eo¼0.58 V versus SCE),7 the most oxidant radical
species in the TTM series so far. Figure 6a shows the cyclic
voltammograms for the reduction of heptaradical 3 under the
same conditions. Two quasi-reversible redox pairs, which are
more clearly separated at the lowest scan rate of 20 mV sꢁ1
,
can be observed, indicating that both the reduction states are
reasonably stable. At higher scan rates, however, both the
couples are partially overlapped. To determine accurately
the standard potential and the relative intensity for both pro-
cesses, the differential pulse voltammetry of heptaradical 3
was carried out. As is shown in Figure 6b, the voltammogram
2.5. Thermal analysis
Thermogravimetric analysis (TGA) of heptaradical 3 and
hexaradical 4 (both samples dried before analysis under
vacuum and heated at 150 ꢀC to remove solvent molecules)
showed a stability in nitrogen at temperatures up to 300 ꢀC
(Fig. 7). Above this temperature a loss of weight
(w10ꢁ12 %) occurs, corresponding to the liberation of ap-
proximately 12 chlorines. This result was corroborated by
elemental analysis of the product obtained after thermolysis
of heptaradical 3 at 300 ꢀC, which fitted quite well to the
formula C139H42Cl45N6O6 (molecular structure of 3 with 12
chlorines lost). Regardless of the different electronic struc-
ture of the core carbon atom, thermal diagrams of 3 and 4
are very similar, suggesting that dechlorination processes
should correspond most probably to the external triphenyl-
methyl parts. Therefore, a sixfold cyclodechlorination of
the six pairs of phenyls can take place to yield a macromol-
ecule with six fluorenyl systems. Loss of chlorine atoms in
both polyradicals is also confirmed by mass spectrometry
200
R1
150
100
50
R2
Icat
a
b
c
d
O2
0
-50
-100
O1
R1
40
30
20
10
0
%
100
R2
80
60
40
0.200 0.000 -0.200 -0.400 -0.600 -0.800 -1.000 -1.200
E / V vs. SCE
Figure 6. Up: cyclic voltammograms corresponding to the reduction of 3
(10ꢁ3 M) in CH2Cl2 with TBAP (0.1 M) on Pt at 25 ꢀC. Initial and final po-
tential 0 V; reversal potential ꢁ1.000 V; scan rate: (a) 200, (b) 100, (c) 50,
(d) 20 mV sꢁ1. Down: differential pulse voltammogram for the reduction of
3 under the same conditions. Initial potential 0 V; final potential ꢁ1.000 V;
100
300
500
°C
Figure 7. Thermogravimetric analysis of the heptaradical 3 (red line) and
hexaradical 4 (black line). Initial and final temperatures: 30.0–700.0 ꢀC;
scan rate: 10.0 ꢀC/min; nitrogen flow: 200.0 mL/min.
scan rate 20 mV sꢁ1
.

714
M. Castillo et al. / Tetrahedron 63 (2007) 708–716
(MALDI/TOF). A high number of peaks corresponding to
the loss of up to 10 chlorines appeared in the region of
the molecular peak, those with an even number of chlorines
being less intense. Further work to confirm the structure of
dechlorinated structure is now in progress.
platinum (Pt) disk with 0.093 cm2 area was used as the work-
ing electrode and a Pt wire as the counter electrode. The ref-
erence electrode was a saturated calomel electrode (SCE),
submerged in a salt bridge of the same electrolyte, which
was separated from the test solution by a Vycor membrane.
Solutions of monoradicals and polyradicals (w10ꢁ3 M) in
CH2Cl2 containing 0.1 M tetrabutylammonium perchlorate
as background electrolyte were studied by CV and DPV.
The volume of all test solutions was 50 mL. Electrochemical
measurements were performed under an Ar atmosphere and
at 25 ꢀC using an Eco Chemie Autolab PGSTAT100 poten-
tiostat–galvanostat controlled by a computer with GPES
software. Cyclic voltammograms of all solutions were re-
3. Conclusion
Radical TTM (1) has been easily functionalized in meta
positions with six chlorocarbonyl substituents enabling the
condensation with the amino function of radical 2. Thus,
polyradicals 3 and 4, composed by radicals of the TTM
series connected by amide bonds, have been prepared in
good yields. This synthetic strategy avoids the generation
of radical centers, always difficult, in the last stage of the
sequential preparation, yielding 3 and 4 with very good
agreement between magnetic susceptibility measurements
and the expected values for 6 and 7 spin radicals, respec-
tively. Analysis of the EPR spectra of them and of those of
different biradical models, suggests the presence of strong
electron–electron exchange and weak dipolar couplings.
Macromolecules 3 and 4 are very stable polyradicals that are
both cathodically reduced and anodically oxidized in elec-
trochemical processes having an intense coloration in the
three different redox states. Polyradical 3 is reduced in a
seven-electron two-stage processes and oxidized in a seven-
electron one-stage process.
corded at scan rates (v) ranging from 20 to 200 mV sꢁ1
while their differential pulse voltammograms were obtained
at 20 mV sꢁ1
,
.
4.1.1. Tris(3,5-diformyl-2,4,6-trichlorophenyl)methane
(9). A solution of tris(a,a,a0,a0,2,4,6-heptachloroxylyl)me-
thane6 (2.30 g) in fuming sulfuric acid (30 %, 50 mL) was
heated (95 ꢀC, 5 h). The resulting mixture was poured into
cracked ice/water, and the precipitate was filtered, washed
with water, and dried. The solid was treated with chloroform
at reflux for 2 h, the solution was evaporated and the residue
flash chromatographed (silica gel, CH2Cl2) to afford 9
1
(0.4 g, 25 %), mp 260–263 ꢀC. H NMR (CDCl3) d 10.32
(d, 6H), 7.68 (s, 1H); IR (KBr) 3397 (w), 2873 (w), 1712
(s), 1537 (s), 1350 (m), 1270 (w), 1217 (w), 1119 (w),
1040 (s), 994 (s), 970 (s), 816 (m), 735 (w), 703 (w) cmꢁ1
;
HRMS (m/z) for C25H7Cl9O6 (M+): calcd 721.738037, found
721.739964.
4. Experimental
4.1. General procedures
4.1.2. Tris(3,5-dicarboxy-2,4,6-trichlorophenyl)methane
(10). Sulfuric acid (7 mL) was added to a solution of 9
(0.096 g, 0.1 mmol) in acetone (18 mL) and, then, a solution
of chromium(VI) oxide (0.1 g, 1 mmol) in water (5 mL) was
slowly added. The mixture was stirred at room temperature
(4 h) and volatile parts were evaporated. The residue in ethyl
acetate was washed with water, dried over sodium sulfate
and evaporated, yielding 10 (0.124 g, quantitative yield).
IR (KBr): 3550–2100 (s), 1723 (s), 1620 (m), 1548 (s),
1400–1198 (m), 1116 (w), 1017 (w), 926 (w), 809 (w),
713 (w) cmꢁ1; HRMS (m/z) for C25H7Cl9O12 (M+): cacld
817.707525, found 817.708077.
€
Melting points were obtained by using a Kofler microscope
‘Reichert’ and are uncorrected. The IR spectra were re-
corded with a FTIR ‘Bomem–Michalson’ model MB-120
spectrophotometer. H NMR spectra were determined at
1
200 MHz with Varian Gemini 200HC spectrometer. The
electronic spectra were recorded with a Varian model Cary
300 Bio spectrophotometer. CHCl3, CCl4, and CH2Cl2
were dried over calcium chloride and distilled. THF was
dried over sodium and distilled before use. High-resolution
mass spectra were recorded with a VG Auto Spec ‘Micro-
mass Ins.’ and matrix-assisted laser-desorption ionization/
time-of-flight (MALDI/TOF) mass spectra were recorded
using 2,5-dihydroxybenzoic acid or dithranol as a matrix.
EPR spectra were recorded with a Varian E-109 spectrome-
ter working in the X band and using a Varian E-257 temper-
ature-controller to obtain spectra at temperatures as low as
130 K. A Bruker ESP 300 spectrometer with a Bruker ER
4112 HV continuous-flow liquid helium cryostat and an Ox-
ford Instruments temperature-controller system was used to
obtain EPR spectra at lower temperatures (4 K). Samples of
polyradicals in CH2Cl2 or 2-methyltetrahydrofuran were in-
troduced in quartz EPR tubes and degassed before being in-
serted into the EPR cavity. Handling of radicals in solution
was performed in the dark. Magnetic susceptibility data
for microcrystalline samples of polyradicals were measured
from 1.8 to 300 K with a SQUID magnetometer operating
with the field strength of 50 kOe. Cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) were carried
out in a standard thermostated three-electrode cell. A
4.1.3. Tris(3,5-dichloroformil-2,4,6-trichlorophenyl)me-
thane (6). A solution of 10 (0.065 g) in SOCl2 (3 mL) was
refluxed (22 h) and poured into water. The mixture was
treated with an excess of sodium hydrogen carbonate and ex-
tracted with chloroform. The organic solution was washed
with water, dried over sodium sulfate, and the solvent was
evaporated. The residue was flash chromatographed (silica
gel, hexane/CHCl3 3:1), to give 6 (0.046 g, 62 %), mp
300 ꢀC (dec). IR (KBr): 3540 (w), 2924 (w), 1779 (s),
1547 (s), 1374 (m), 1282 (w), 1220 (m), 1138 (m), 1095
(s), 1014 (s), 945 (m), 836 (s), 813 (m), 794 (m), 735 (w),
724 (w), 701 (w) cmꢁ1; HRMS (m/z) for C25H1Cl15O6
(M+): calcd 927.501254, found 927.503006.
4.1.4. Tris(3,5-dichloroformil-2,4,6-trichlorophenyl)-
methyl radical (5). An excess of aqueous solution of tetra-
butylammonium hydroxide (1.5 M, 1.2 mL) was added to
a solution of 6 (0.500 g, 0.53 mmol) in acetone (85 mL)

M. Castillo et al. / Tetrahedron 63 (2007) 708–716
715
and the mixture was stirred at room temperature (0.5 h).
Then crushed chromium(VI) oxide (0.540 g) was added
and the mixture was stirred (0.5 h) again. Acetone was elim-
inated at low pressure and the residue in chloroform, washed
with aqueous solution of hydrochloric acid and water, was
dried to give radical 5 (0.495 g, 98 %), mp 280–282 ꢀC. IR
(KBr): 1781 (s), 1525 (s), 1355 (m), 1107 (s), 1016 (s),
953 (w), 844 (s), 816 (m), 803 (m), 738 (w), 705
(w) cmꢁ1; UV–vis (CHCl3): lmax (3, dm3 molꢁ1 cmꢁ1) 384
(22,900), 510 (575), 555 (600) nm; HRMS (m/z) for
C25Cl15O6 (M+): calcd 928.490478, found 928.492935.
(MALDI/TOF) (m/z) for C40H7Cl22NO2 (M+): calcd
1312.30, found: 1312.30.
4.1.8. Biradical 12. A mixture of radical 5 (117 mg,
0.12 mmol), radical 2 (0.072 g, 0.13 mmol), cesium carbon-
ate (122 mg) and benzene (10 mL) was stirred at reflux in an
inert atmosphere (4 h). The reaction mixture was dried and
the residue in chloroform washed with HCl aqueous solu-
tion, was dried over sodium sulfate, and evaporated. The res-
idue was flash chromatographed (silica gel, hexane/CHCl3
1:1) to give biradical 12 (0.067 g, 37 %), mp >300 ꢀC
(dec). IR (KBr): 3385 (w), 3119 (w), 3063 (w), 1783 (s),
1702 (m), 1571 (s), 1551 (s), 1526 (s), 1371 (m), 1298
(w), 1183 (m), 1138 (m), 1049 (s), 1015 (s), 858 (m), 806
(s) cmꢁ1; MS (MALDI/TOF) (m/z) for C44H7Cl22NO6
(M+): calcd 1425.40, found: 1425.43.
4.1.5. Heptaradical 3. A mixture of (4-amino-2,6-dichloro-
phenyl)bis(2,4,6-trichlorophenyl)methyl radical (0.370 g,
0.69 mmol), radical 5 (0.101 g, 0.11 mmol) and cesium car-
bonate (0.378 g) in benzene (10 mL) was stirred at reflux
(4 h) under an argon atmosphere. The solution was dried
and the residue in chloroform was washed with HCl aqueous
solution, dried over sodium sulfate, and evaporated. The res-
idue was flash chromatographed (silica gel, hexane/CHCl3
1:4) to afford heptaradical 3 (0.357 g, 84 %), mp >320 ꢀC
(dec). IR (KBr): 3387 (w), 3294 (w), 3067 (w), 2928 (w),
1702 (m), 1678 (m), 1574 (s), 1553 (s), 1491 (m), 1373
(s), 1300 (m), 1236 (w), 1209 (w), 1182 (m), 1138 (m),
4.1.9. Biradical 13. A mixture of 2,4,5,6-tetrachloroiso-
phthaloyl dichloride (100 mg, 0.29 mmol), radical 2 radical
(315 mg, 0.59 mmol), sodium bicarbonate (250 mg), and
benzene (5 mL) was stirred at reflux in an inert atmosphere
(48 h). The benzene was distilled off and the residue was
treated with chloroform/water. The insoluble fraction, sepa-
rated by filtration, was characterized as biradical 13
(0.123 g, 31 %), mp¼374 ꢀC (dec). IR (KBr): 3397 (w),
3237 (w), 3142 (w), 3068 (w), 1663 (s), 1578 (s), 1555 (s),
1525 (m), 1498 (m), 1383 (s), 1370 (s), 1300 (m), 1238
(w), 1182 (m), 1136 (m), 1083 (w), 950 (w), 924 (w), 857
(s), 809 (s), 794 (s), 758 (w), 723 (w) cmꢁ1; UV–vis
(CHCl3): lmax (3, dm3 molꢁ1 cmꢁ1) 377 (60,500), 510
(2030), 554 (1980) nm; MS (MALDI/TOF) (m/z) for
C139H42Cl57N6O6 (M+): calcd 1335.57, found: 1335.60.
1084 (w), 951 (w), 927 (w), 858 (s), 810 (s), 796 (s) cmꢁ1
.
Anal. Calcd for C139H42Cl57N6O6: C, 42.7; H, 1.1; N, 2.1;
Cl, 51.6. Found: C, 42.7; H, 1.2; N, 2.1; Cl, 51.6; MS
(MALDI/TOF) (m/z) for C139H42Cl57N6O6 (M+): calcd
3912.38, found: 3914.80
4.1.6. Hexaradical 4. A mixture of (4-amino-2,6-dichloro-
phenyl)bis(2,4,6-trichlorophenyl)methyl radical (0.372 g,
0.69 mmol), methane 6 (0.102 g, 0.11 mmol) and cesium
carbonate (0.380 g) in benzene (10 mL) was stirred at reflux
(36 h) under an argon atmosphere. The reaction mixture was
dried and the residue in chloroform washed with HCl aque-
ous solution, was dried over sodium sulfate, and evaporated.
The residue was flash chromatographed (silica gel, CHCl3)
to afford hexaradical 4 (0.123 mg, 29 %), mp >313 ꢀC
(dec). IR (KBr): 3650 (w), 3389 (w), 3071 (w), 1684 (m),
1576 (s), 1557 (s), 1526 (m), 1494 (m), 1374 (s), 1293
(m), 1234 (w), 1184 (m), 1133 (w), 1082 (w), 948 (w),
925 (w), 857 (s), 811 (s), 796 (s) cmꢁ1; MS (MALDI/
TOF) (m/z) for C139H42Cl57N6O6 (M+): calcd 3913.38,
found 3914.77.
Acknowledgements
Financial support for this research from the CICYT (MEC,
Spain) through projects MAT05/1272 and CTQ2006-
15611-C02-02/BQU is gratefully acknowledged.
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