Racemic and enantiomerically pure phenyl α-nitronyl nitroxide radicals: Influence of chirality on solution and solid state properties

Article abstract of DOI:10.1039/b106239p

Two chiral phenyl α-nitronyl nitroxides substituted in the aromatic ring with a lactate moiety have been prepared in their enantiopure and racemic forms. The X-ray crystal structures of the racemic methyl ester and both the racemic and enantiopure carboxy

Full text of DOI:10.1039/b106239p

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Racemic and enantiomerically pure phenyl a-nitronyl nitroxide
radicals: influence of chirality on solution and solid state properties{
Maria Minguet,a David B. Amabilino,*a Jose´ Vidal-Gancedo,a Klaus Wurstb and
Jaume Veciana*a
aInstitut de Cie`ncia de Materials de Barcelona (CSIC), Campus Universitari, 08193
Bellaterra, Catalonia, Spain. Tel: 34 93 580 1853; Fax: 34 93 580 5729;
E-mail: vecianaj@icmab.es; amabilino@icmab.es
bInstitut fu¨r Allgemeine Anorganische und Theoretische Chemie, Universita¨t Innsbru¨ck,
A-6020, Innrain 52a, Austria
Received 13th July 2001, Accepted 12th November 2001
First published as an Advance Article on the web 15th January 2002
Two chiral phenyl a-nitronyl nitroxides{ substituted in the aromatic ring with a lactate moiety have been
prepared in their enantiopure and racemic forms. The X-ray crystal structures of the racemic methyl ester and
both the racemic and enantiopure carboxylic acid have been solved. The ester forms chains of molecules linked
by weak hydrogen bonds, while the racemic acid also forms chains, but linked by strong hydrogen bonds
between the acid OH group and one of the nitroxide oxygen atoms. In stark contrast, the racemic acid forms
cyclic hydrogen bonded dimers by virtue of the same interaction. In addition, these dimers are observed in
solution state for both enantiopure and racemic acids as witnessed by EPR spectroscopy. The ground state of
the cyclic aggregates appears to be a triplet with a very weak magnetic interaction. Circular dichroism
spectroscopy of the chiral compound reveals additional evidence for dimer formation, while LDI–TOF mass
spectrometry indicates the presence of chains in amorphous films of the compounds. Magnetic susceptibility
measurements of the solids reveal antiferromagnetic interactions in all cases: in the racemic ester and
enantiopure acid they are weak, while in the racemic acid they are strong as a result interactions between cyclic
dimers.
lactate followed by condensation of the resulting chiral
Introduction
aldehydes 1 with 2,3-bis(hydroxylammonio)-2,3-dimethylbu-
tane sulfate10 (2) and oxidation of the resulting products 3 with
sodium periodate (according to Ullman’s procedure11) to give
While the use of the influence of chirality on the properties of
several types of molecular materials is well established,1,2 the
consequences for magnetic materials are less well known,
though the potential for magneto-optical phenomena is a
particularly interesting area.3 Some interesting examples of
chiral coordination compounds incorporating paramagnetic
metal ions have been reported,4 and chiral organic radicals
have been prepared.5 Our interest lies in this second group of
materials, specifically chiral phenyl a-nitronyl nitroxide{
radicals.6 These compounds, part of the extensive family of
a-nitronyl nitroxides7 so popular among those studying
molecular magnetism,8 are prepared with the aim of establish-
ing some of the effects of stereochemistry in molecular mag-
netic materials. Here we report the solution (EPR and circular
dichroism) and solid state (structure and magnetic suscept-
ibility) characteristics of the radicals (R)-4MLNN, (RS)-
4MLNN, (R)-4LNN, and (RS)-4LNN (Scheme 1).
Results and discussion
Synthesis
The radicals were synthesised (Scheme 1) by the Mitsunobu
reaction9 of 4-hydroxybenzaldehyde with (S)- or (RS)-methyl
{Electronic supplementary information (ESI) available: figures show-
ing alternative views of the crystal structures and the shortest distances
between SOMOs in the crystals. See http://www.rsc.org/suppdata/jm/
b1/b106239p/
{The IUPAC name for these phenyl a-nitronyl nitroxides is 2-phenyl-4,5-
dihydro-4,4,5,5-tetramethyl-3-oxido-1H-imidazol-3-ium-1-oxyl.
Scheme 1
570
J. Mater. Chem., 2002, 12, 570–578
This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b106239p

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the esters (R)-4MLNN and (RS)-4MLNN. These compounds
were saponified with sodium hydroxide in aqueous ethanol to
give the corresponding carboxylic acids (R)-4LNN and (RS)-
4LNN.
Solid state structures
The X-ray crystal structure of (RS)-4MLNN (space group
P21/c, Table 1) reveals molecules with a pseudo-eclipsed
conformation,12 in which the torsion angles (i) between the
phenyl and imidazolyl rings (APNN) and (ii) of the NCCN unit
in the imidazolyl ring (TIM) are opposite giving the molecule a
relatively planar general form when compared with the
alternative pseudo-anti conformation.12 The R enantiomer in
the racemic crystals has PM chirality and the S has MP, the
torsion angles APNN and TIM being 13.6 and 19.6u respectively
(Fig. 1). The molecules pack together in the form of homo-
chiral chains which run along the crystallographic b axis
(Fig. 2) in which the molecules are linked through three [Csp3–
Two act between methyl groups
12–14
…
H
O] hydrogen bonds.
attached to the imidazolyl ring of one molecule and one of the
NO groups of the next, and the other to the carbonyl group of
the first molecule from a different hydrogen atom in the related
methyl group of the second molecule (See Table 2 for distances
and angles). These undulating chains come together in an
antiparallel fashion to give a three dimensional structure
without the formation of other significant non-covalent
interactions. Despite numerous attempts, (R)-4MLNN, the
enantiopure form of this compound, could not be crystallised
to give X-ray quality crystals, preventing a comparison of the
conformations in the two modifications.
The enantiopure lactic acid-derived radical (R)-4LNN
crystallises in the P212121 space group with just one molecule
in the asymmetric unit which has a pseudo-eclipsed PM
conformation15 (Fig. 3), the same gross form as the corre-
sponding enantiomer in crystals of (RS)-4MLNN. Unusual in
the molecule is the difference between the two N–O bond
Fig. 1 Views of the enantiomeric molecules of 4MLNN in the crystal
structure of the racemate and a representation of the molecule as
viewed down the long axis from the imidazolyl end of the molecule,
showing the pseudo-eclipsed arrangement of the rings.
neighbouring one. This hydrogen bond is aided by the existence
of two hydrogen bonds formed by the carbonyl group and
hydrogen atoms of two different methyl groups attached to the
imidazolyl ring (Table 2), and leads to the formation of non-
covalent chains which run along the crystallographic c axis.
These chains aggregate along the b axis direction in an
˚
lengths (1.276(4) and 1.301(4) A), an effect which is caused by
the formation of a strong intermolecular hydrogen bond
between the acidic hydrogen atom of one molecule and the
oxygen atom associated with the longer N–O group of a
…
antiparallel fashion by virtue of a [Csp2–H O] hydrogen bond
Table 1 Crystallographic data for (RS)-4MLNN, (RS)-4LNN and (R)-4LNN
Compound
(RS)-4MLNN
(RS)-4LNN
(R)-4LNN
Formula
M
C17H23N2O5
335.37
C16H21N2O5
321.35
C16H21N2O5
321.35
Colour, habit
Crystal dimensions/mm
T/K
Dark blue prisms
0.7 6 0.4 6 0.25
218(2)
Dark blue prisms
0.2 6 0.2 6 0.15
218(2)
Dark blue plates
0.5 6 0.3 6 0.045
218(2)
˚
Radiation (l/A)
MoKa (0.71073)
MoKa (0.71073)
MoKa (0.71073)
System
Space group
Monoclinic
P21/c
8.307(3)
10.366(3)
20.439(4)
Triclinic
¯
P1
Orthorhombic
P212121
6.910(2)
11.910(3)
19.481(5)
90
90
90
1603.2(7)
4
1.331
˚
a/A
8.5129(5)
9.4113(7)
10.4820(8)
85.847(3)
76.415(4)
79.503(4)
802.25(10)
2
˚
B/A
˚
c/A
a/u
b/u
c/u
97.11(2)
3
˚
V/A
1746.5(9)
4
1.275
716
Z
Dc/g/cm3
F(000)
1.330
342
684
h range/u
2.8–23.5
2564
2098
224
0
2.0–23.5
2354
1957
217
0
2.7–20.5
1207
1048
213
0
Measured reflections
Observed reflections, (I w 2s(I))
Parameters
Restrictions
R (I w 2s(I))
wR2 (I w 2s(I))
R (all data)
0.0445
0.1100
0.0570
0.1161
0.0430
0.1045
0.0542
0.1099
0.0322
0.0730
0.0413
0.0767
wR2 (all data)
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Fig. 3 Views of (R)-4LNN in its crystal structure and a representation
of the molecule as viewed down the long axis (see Fig. 1 for key).
corrugated sheets that pile up along the a axis (Fig. 4). The chains
stack in a parallel manner with the formation of several [Csp3–
O] hydrogen bonds (Table 2) and a [Csp3–H p] interaction of
…
…
H
the methyl group attached to the stereogenic centre and the
…
…
˚
benzene ring (shortest H C distance 2.893 A, C C distance
˚
3.369 A) (Fig. 4).
The racemate of the lactic acid-derived radical 4LNN
¯
crystallises in the triclinic space group P1. The centrosymme-
Fig. 2 The hydrogen bonded chains formed in the crystal structure of
…
(RS)-4MLNN. Hydrogen bonds: a) CMe22–H O4LC; b) CMe22–
… …
O2–N; c) CMe32–H O1–N.
H
trically-related R and S enantiomers have pseudo-eclipsed PM
and MP conformations, respectively (Fig. 5). Therefore, once
again the R enantiomer in this racemate has the same gross
conformation of the two ring system (angle APNN) as in the
other compounds reported here. The density of the racemate is
practically the same as that of the enantiopure compound, in
between the ‘‘free’ N–O group and a hydrogen at the
3-position of the aromatic ring in a neighbouring chain as well
…
as a [Csp3–H O] hydrogen bond from one of the imidazolyl
groups to the ether oxygen atom in every second molecule of
the chain (Table 2). These aggregated chains form highly
Table 2 Hydrogen bond distances and geometries found in crystals of (RS)-4MLNN, (RS)-4LNN and (R)-4LNN
H-bonda
[X–H–O] angle/u
…
˚
[H O] distance/A
˚
[X–H–O] distance/A
Radical
3.671(3)i
3.779(4)j
3.452(3)j
…
(RS)-4MLNN
C
CMe32–H O1–N
Me22–H O2–N
2.73
2.86
2.63
163
159
143
…
…
CMe22–H O4LC
b
2.664(2)k
3.532(3)l
3.470(3)m
3.606(3)m
3.465(3)n
3.594(3)o
3.476(3)p
3.696(3)q
3.723(3)r
…
(RS)-4LNN
O3–H O1–N
1.73
2.59
2.53
2.69
2.64
2.77
2.55
2.73
2.79
168
164
172
154
147
143
159
172
161
Im
c
…
22–H O2–N
d
C
Me
…
Cmeta6-H O2–N
d
…
C*10–H O2–N
…
d
Cortho9–H O3(H)CO–
Im
d
…
…
C
C
22–H O5–Ph
Me
Im
e
31–H O4LC
Me
c
…
C*CMe12–H O5–Ph
d
…
C*CMe12–H O1–N
f
2.619(4)s
3.579(5)t
3.546(5)t
3.210(5)u
3.753(5)u
3.512(5)v
3.599(5)v
3.496(5)w
3.369(5)v
…
(R)-4LNN
O3–H O2–N
C
C
1.63
2.67
2.74
2.33
2.78
2.62
2.74
2.66
2.55
159
156
141
155
177
153
148
144
142
Im
Im
f
f
…
…
22–H O4LC
Me
32–H O4LC
Me
g
…
C6–H O2–N
Im
g
…
…
…
C
C
C
22–H O5Ph
Me
Me
Me
Im
Im
h
32–H O4LC
h
31–H O4LC
h
…
C*CMe12–H O1–N
C
Im
h
…
32–H O5Ph
Me
aImCMe signifies the methyl groups attached to the imidazolyl ring, and C*CMe that attached to the stereogenic centre. bIntra-dimer hydrogen
bonds which link R and S sheets ‘‘face-to-face’. cInter-dimer weak hydrogen bonds (cyclic dimer) which link R and S sheets ‘‘back-to-back’.
e
f
dIntra-homochiral sheet hydrogen bonds. Weak hydrogen bonds between sheets ‘‘face-to-face’. Indicates intra-chain hydrogen bond. gIndicates
intra-sheet hydrogen bond. hIndicates intra-stack hydrogen bond. Symmetry operations: 2x, 0.5 1 y, 0.5 2 z; 2x, 20.5 1 y, 0.5 2 z; k1 2 x,
i
j
1 2 y, 2 2 z; 2x, 2y, 2 2 z; m1 1 x, y, z; n21 1 x, y, z; ox, y, 21 1 z; p2x, 1 2 y, 2 2 z; q1 2 x, 2y, 3 2 z; x, y, 11 1 z; 20.5 2 x, 1 2 y,
l
r
s
20.5 1 z; 20.5 2 x, 1 2 y, 0.5 1 z; u2x, 0.5 1 y, 0.5 2 z; 0.5 2 x, 1 2 y, 0.5 1 z; w1 1 x, y, z.
t
v
572
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…
Fig. 4 The parallel-stacked hydrogen bonded chains formed along the a axis in the crystal structure of (R)-4LNN. Hydrogen bonds: a) O3–H O2–
…
…
…
…
…
…
N; b) CMe12–H O1–N; c) CMe22–H O4LC; d) CMe31–H O4LC; e) CMe32–H O4LC; f) CMe32–H O4LC: g) CMe32–H O5Ph.
contradiction to Wallach’s rule.16 In contrast to the enantio-
pure compound that forms hydrogen bonded chains, (RS)-
4LNN crystallises to form cyclic dimers (Fig. 6) held together
by the same type of hydrogen bond seen in the former, between
the acid group and one of the NO moieties. In this compound,
the hydrogen bonds (Table 2) are somewhat longer than in the
enantiopure compounds (a fact which might partially explain
the contradiction to Wallach’s rule), but a significant difference
the acid-assembled dimers (the donor and acceptor of the
hydrogen atom being one of the methyl groups attached to the
imidazolyl ring and the ‘‘free’ nitroxyl (aminoxyl) group,
respectively) and a [Csp3–H O] bond between the methyl
group attached to the chiral group and the phenoxy oxygen
atom.
The observation of a cyclic dimer in the case of the racemic
compound and chains for the enantiopure is interesting, and
contrasts with the related 3-substituted radicals (R)-3LNN and
(RS)-3LNN,6 in which both racemate and enantiopure
compound form chains in the crystal. This result indicates,
perhaps, that the cyclic dimer of (RS)-4LNN is a thermo-
dynamically preferred aggregate, but that the kinetics of crystal
growth lead to preferential formation of chains in the
enantiopure sample. In order to confirm this supposition we
performed mass spectral, circular dichroism and electron
paramagnetic resonance studies on the radicals.
…
˚
in the N–O bond lengths (1.279(2) and 1.298(2) A) is observed
here too.
The three dimensional structure within the crystals is
characterised by the presence of homochiral sheets in the b
plane in which the molecules are linked together in parallel
…
manner by a series of weak hydrogen bonds of the [Csp3–H O]
…
and [Csp2–H O] varieties (Fig. 7 and Table 2). These sheets are
joined in alternating manner by virtue of the aforementioned
cyclic dimers on one side of the sheet, and on the other by the
formation of further hydrogen bonds: A cyclic motif between
Fig. 5 Views of the enantiomeric molecules of 4LNN in the crystal
structure of the racemate and a representation of the molecules as
viewed down their long axis.
Fig. 6 Two views of the cyclic dimers formed by (RS)-4LNN in its
crystals.
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Fig. 7 The homochiral sheets formed by (RS)-4LNN in its crystals.
…
…
Hydrogen bonds: a) C6–H O2–N; b) C10–H O2–N; c) C9–
… … …
(H)O3–C; d) CMe22–H O5Ph; e) CMe12–H O1–N.
H
Fig. 8 LDI-TOF mass spectra of (R)-4MLNN (top) and (R)-4LNN
(bottom).
Circular dichroism
Circular dichroism (CD) can also be a very sensitive tool for the
study of aggregation behaviour of chiral molecules.19 The CD
spectrum of the two chiral compounds reported here reveal
significant Cotton effects between 200 and 350 nm (Fig. 9), but
present no significant optical activity in the visible part of the
electromagnetic spectrum (unlike the corresponding 3-sub-
stituted analogs6,20). This situation probably reflects the large
distance and lack of conjugation between the stereogenic centre
and the ONCNO unit which is responsible for absorption in
this area. The methyl ester (R)-4MLNN dissolved in dichloro-
methane presents a weak negative Cotton effect at 365 nm, and
a large positive one centred at 275 nm, which are both assigned
(on the basis of ab initio calculations)6b to pAp* transitions, of
the ONCNO and phenyl chromophores, respectively. In
Mass spectrometry
The laser desorption ionisation time-of-flight mass spectrum
(LDI-TOF MS) of the methyl esters 4MLNN produced ions with
charge to mass ratios consistent with the molecular ions (m/z
335.2), although the most intense peaks correspond to the loss of
oxygen atoms from the compound (Fig. 8), as also witnessed in
electron impact mass spectra (EI-MS) of related compounds.17
No significant signals are observed at higher masses in either
technique. In contrast, the spectra of both the enantiomerically
pure and racemic modifications of 4LNN indicate the presence of
aggregates in the samples evaporated from dichloromethane
solutions. In particular, batches of peaks corresponding to
dimeric and trimeric species are observed, in which alkali metal
ion adducts and loss of oxygen atoms and carboxylate groups
complicate the spectra (Fig. 8). While the spectra provide no
quantitative information regarding the linear or cyclic nature of
the assemblies, the spectra are indicative of the tendency of the
molecules to form aggregates.
Electrospray mass spectrometry (ES-MS) can often provide
useful information concerning the formation of ‘‘self-assembled’
aggregates in solution.18 ES mass spectra of racemic and
enantiopure 4LNN from toluene solutions of the radicals
reveal peaks corresponding to the protonated molecular ion
([M 1 H]1, 100%) and also to the dimer ([2M 1 H]1, 20%),
giving clear evidence for its formation. An extremely weak ion
(v2%) was also observed corresponding to the ([3M 1 H]1 ion.
Fig. 9 CD spectra of (R)-4MLNN in CH2Cl2 and (R)-4LNN in both
CH2Cl2 and EtOH.
574
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Fig. 10 Full fieldEPRsignalof(R)-4LNNat160 KinCH2Cl2–toluene1 : 1.
Fig. 12 Temperature dependence of the product of intensity and
temperature of the half field EPR signal displayed by (R)-4LNN.
contrast, in the same solvent the corresponding acid (R)-4LNN
displays a strong negative Cotton effect at 290 nm, and a weak
positive one at 325 nm. However, when the same compound is
dissolved in ethanol, the spectrum becomes remarkably similar
to that of the methyl ester. Therefore, the CD spectrum of the
chiral acid in dichloromethane gives strong qualitative evidence
for the formation of aggregates in solution, which are
destroyed upon addition of a polar and protic solvent, such
as ethanol.
giving supporting evidence for the notion of a cyclic rather than
linear aggregate.
The observation of a half field signal in frozen solutions of
the radicals provides the opportunity for studying the magnetic
properties of the cyclic dimer by following the intensity of the
half-field signal with temperature. In the case that the unpaired
electrons in the two radical units of the cyclic dimers behave
independently magnetically, the intensity should obey the
Curie law, while ferromagnetic or antiferromagnetic interac-
tions between them should lead to positive and negative
deviations from it, respectively. The plot of the intensity–
temperature product versus temperature (Fig. 12) for the half-
field signal for (R)-4LNN shows a gentle rise at very low
temperatures indicative of a triplet ground state. Fitting of the
curve to the Bleaney–Bowers equation indicates a small
ferromagnetic interaction between the spins of J/k ~ 14.5
(¡2 K). While there is much error in the absolute magnitude of
this small interaction (arising from difficulty in integration of
the EPR spectra), it is clear from the measurements that the
triplet is the ground state.
Electron paramagnetic resonance
The electron paramagnetic resonance (EPR) spectra of
solutions of both the racemic and enantiomerically pure
4LNN also reveal the presence of aggregates. While dilute
solutions of all radicals in dichloromethane–toluene (1 : 1)
show the five line pattern with hyperfine splitting patterns
typical of the phenyl a-nitronyl nitroxide radicals,21 concen-
trated solutions of the acids present strong dipolar signals
(Fig. 10) as well as half-field signals at low temperature
(Fig. 11), strongly indicative of species with spin greater than
½. No signal was observed at M field in either case.
The zero field splitting parameters of both the racemic and
enantiomerically pure radical 4LNN, obtained by simulation of
the dipolar signal in frozen solution (Fig. 11), are very similar
(D’ ~ 28 G, E’ ~ 0 G). This parameter can be used to calculate
the mean effective distance between radical centres (r) using the
equation D’ ~ 27887/r3, which is valid for the point dipole
approximation. The resulting mean effective distance is
Magnetic properties of the solids
Magnetic susceptibility measurements of ground crystalline
samples of the radicals reveal that the exchange interactions
(J/k, using the following effective Heisenberg Hamilto-
nian:H ~ 22JSASB) in all the materials are antiferromagnetic,
but by varying degrees (Fig. 13).
The crystals of (RS)-4MLNN have very weak interactions. In
accord with the solid state structure, the susceptibility data
were fitted using a one-dimensional antiferromagnetic Heisen-
berg chain model with molecular field (correlation 99.9%),
giving J/k of 20.65 K for the predominant interaction. The
weakness of the interaction is not surprising given the remote
nature of the singly occupied molecular orbitals (SOMOs)22—
˚
approximately 10.0 A, a value very similar to the distance
between the a carbon atoms in the dimer present in the crystals
˚
of (RS)-4LNN (9.67 A), giving strong support for the same
structure in solution for both modifications. Also, it should be
pointed out that the EPR spectrum of the corresponding 3LNN
does not indicate the formation of aggregates in solution,6
˚
the nearest NO–ON distance is 4.15 A—and their quasi-
perpendicular orientation (see Supplementary Information{).
The SOMOs in crystals of (R)-4LNN form chains resulting
from contact between a-nitronyl nitroxide moieties in adjacent
structural stacks of the molecules. The closest NO–ON distance
˚
is relatively large (4.07 A), and the overlap lies between
orthogonal and parallel. Thus, fitting of the magnetic data to a
one-dimensional antiferromagnetic Heisenberg chain model
(correlation 99.9%) gave the expected weak exchange interac-
tion (J/k of 22.0 K).
In contrast, crystals of (RS)-4LNN present very strong
antiferromagnetic interactions. These interactions surely arise
from the close approach between the oxygen atoms bearing the
spin in between the non-covalent dimers. The distance between
˚
the oxygen atoms is a mere 3.43 A, and their p-orbital overlap
is appreciable (see Supplementary Information{). The mag-
netic data were fitted to the Bleaney–Bowers equation
(correlation 99.7%), giving an exchange coupling (J/k) of
Fig. 11 Half field signal EPR signal of (R)-4LNN at 4.6 K in CH2Cl2–
toluene 1 : 1 (bottom) and simulation of the signal with D’ ~ 28 G and
E’ ~ 0 G (top).
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Experimental
Materials and methods
The following products were purchased commercially (supplier
as indicated) and were used without further purification:
4-Hydroxybenzaldehyde (Aldrich, 98%), (S)-methyl lactate
(Fluka, 97%), (RS)-methyl lactate (Fluka, 97%), triphenylphos-
phine (Merck, 98%), diethyl azodicarboxylate (Aldrich, 85%).
2,3-Bis(hydroxylammonio)-2,3-dimethylbutane sulfate (2) was
prepared according to a literature method.10 Thin-layer
chromatography (TLC) was performed on aluminium plates
coated with Merck Silica gel 60 F254. Developed plates were air-
dried and scrutinised under UV lamp. Silica gel 60 (35–
70 mesh, SDS) was used for column chromatography. Melting
points were determined by differential scanning calorimetry
(DSC) using a Perkin Elmer DSC 7 instrument. LDI-TOF-MS
were obtained using a Kratos Kompact Maldi 2 K-probe
(Kratos Analytical) operating with pulsed extraction of the
ions in linear high power mode. The samples were deposited
directly onto a non-polished stainless steel sample plate from
dichloromethane solution. 1H and 13C NMR spectra were
recorded on a Bruker ARX 300 spectrometer (using the
deuterated solvent as lock and tetramethylsilane as internal
reference). Polarimetry was performed using a Dr. Kernchen
Optik 1 ElectronikPropol polarimeter in a 1 cm cell.25 Circu-
lar dichroism spectra were recorded on a JASCO-715 spectro-
meter. Magnetic susceptibility measurements were obtained
with a Quantum Design SQUID magnetometer.
Fig. 13 Temperature dependence of the magnetic susceptibility (x)
(bottom) and magnetic susceptibility (x) 6 temperature product (top)
of the three crystalline radicals reported here.
The EPR spectra were obtained using a Bruker ESP 300E
spectrometer on solutions of radicals dissolved in 1 : 1
dichloromethane–toluene which were degassed with a flow of
argon, according to a previously described method.21 The
hyperfine splitting parameters were obtained by simulation of
spectra obtained in dilute solution. The intensity of the half
field signal of the radicals was determined using the peak-
picking method (which gave considerably less error than
integration of the signals). During the recording of the spectra,
special care was taken not to saturate the signal.
229.6 K. The relatively large magnitude of the antiferromag-
netic coupling between dimers makes it practically impossible
to observe the very weak ferromagnetic couplings within each
dimer which were inferred in the solution state experiments.
Conclusions
We have confirmed that the presence of a stereogenic centre
appended to the phenyl a-nitronyl nitroxide radicals leads to
the preferential crystallisation of one of the four possible gross
conformational isomers for a given enantiomer. In the case of
the (R)-lactate group attached at the 4-position of the phenyl
ring attached to the a-nitronyl nitroxide, the three crystal
structures all have molecules of the same gross conformation of
the component phenyl and imidazolyl rings, that is the pseudo-
eclipsed PM.
The modification of the acid derivatives has a very profound
effect on the crystal packing and consequently on the magnetic
properties of the compounds. While the enantiomerically pure
radical 4LNN forms chains in its crystals and presents very
weak antiferromagnetic interactions, the racemic modification
crystallises as non-covalent cyclic dimers which interact with
each other giving rise to strong antiferromagnetic interactions
in spite of the very weak ferromagnetic interaction in operation
within dimers.
Cyclic dimers held together by hydrogen bonds between the
carboxylic acid function and the nitrone present in the solid
state for the racemic compound are also observed in solution
for both the racemic and enantiomerically pure modifications
of radical 4LNN. While the dimerisation of the molecules is
possible by the carboxylic acid functions in principle,23 this
mode of aggregation has not been observed here, presumably
because of the greater strength of the nitrone–carboxylic
acid hydrogen bond.24 The temperature dependence of the half
field signal of the aggregate reveals a very small exchange
coupling between the free electrons, with the triplet being the
ground state.
Preparation of (R)-methyl 2-(4-formylphenoxy)propionate
((R)-1). 4-Hydroxybenzaldehyde (400 mg, 3.28 mmol), (S)-
methyl lactate (416 mL, 3.94 mmol) and triphenylphosphine
(1.031 g, 3.94 mmol) were dissolved in dry THF (30 mL) with
stirring under an atmosphere of argon, and the mixture was
cooled in an ice bath. To this mixture, a solution of diethyl
azodicarboxylate (DEAD, 685 mL, 3.94 mmol) in dry THF
(5 mL) was added dropwise over a period of 30 minutes at 0 uC,
and the mixture was allowed to warm up to room temperature
with stirring overnight. After addition of water (10 mL), the
THF was removed in vacuo, and the residue was partitioned
between dichloromethane (50 mL) and water (50 mL), and the
aqueous phase was extracted once more with the same solvent.
The combined organic phases were dried over Na2SO4, filtered,
and stripped of solvent. The residue was subject to column
chromatography (SiO2, hexane–CH2Cl2, 1 : 10), giving the
product as a clear oil (581 mg, 82%). [a]546 ~ 146 deg cm2 g21
(c ~ 0.02 M, in CH2Cl2); IR (neat): n ~ 2995, 2957, 2846,
2740, 1757 (n C(O)OCH3), 1702 (n C(O)H), 1592, 1484, 1451,
1
1260, 1136, 789 cm21; H NMR (300 MHz, CDCl3): d ~ 1.67
(d, J ~ 6.9 Hz, 3H, CH3CH), 3.78 (s, 3H, OCH3), 4.87 (q,
J ~ 6.9 Hz, 1H, CH3CH), 6.97 (d, J ~ 8.6 Hz, 2H, H-3 H-5),
7.83 (d, J ~ 8.6 Hz, 2H, H-2 H-6), 9.89 (s, 1H, CHO) ppm; 13
C
NMR (75 MHz, CDCl3); d ~ 30.9 (CH3CH), 52.5 (OCH3),
72.5 (CH3CH), 115.1 (C3,C5), 130.6 (C1), 132.0 (C2,C6), 162.4
(C4), 171.4 (COO), 190.7 (CHO) ppm.
Preparation of (RS)-methyl 2-(4-formylphenoxy)propionate
((RS)-1). The racemic compound was prepared in 70% yield
576
J. Mater. Chem., 2002, 12, 570–578

View Article Online
using the same procedure as that for the enantiomerically pure
compound, employing (RS)-methyl lactate as the starting
material, and gave identical analytical data with the exception
that it presents no optical activity.
but using (RS)-methyl 2-[4-(4,5-dihydro-4,4,5,5-tetramethyl-3-
oxido-1H-imidazol-3-ium-1-oxyl-2-yl)phenoxy]propionate((RS)-
4MLNN) as starting material, the racemic modification was
isolated as a blue solid in 94% yield. Mp: 145 uC. Calculated for
C16H21O5N2 (%) N 8.72, C 59.80, H 6.59. Found (%) 8.32,
59.23, 6.40; UV–vis (CH2Cl2): lmax (e/dm3 mol21 cm21) ~ 281
(14000), 365 (9300), 618 nm (600); LDI-TOF MS: m/z ~ 360.0
[M 1 K]1, 344.0 [M 1 Na]1, 329.0 [M 1 Li]1, 321.0 [M]1,
307.1 [M 2 CH2]1; EPR (CH2Cl2 : toluene 1 : 1): g factor,
2.0065; aN, 7.67; aortho, 0.50; ameta, 0.21; aMe, 0.19.
Preparation of (R)-methyl 2-[4-(4,5-dihydro-4,4,5,5-tetramethyl-
3-oxido-1H-imidazol-3-ium-1-oxyl-2-yl)phenoxy]propionate ((R)-
4MLNN). (R)-Methyl 2-(4-formylphenoxy)propionate ((R)-1,
144 mg, 0.69 mmol), 2,3-bis(hydroxylammonio)-2,3-dimethyl-
butane sulfate (2, 161 mg, 0.69 mmol) and NaHCO3 (55 mg,
0.69 mmol) were combined in methanol (15 mL) and were stirred
at room temperature for 24 hours. After the addition of aqueous
NaHCO3 (55 mg, 0.69 mmol in 15 mL), a precipitate formed.
Without further purification, this solid was dissolved in CH2Cl2
(20 mL) and aqueous NaIO4 (140 mg, 0.69 mmol in 10 mL) was
added, and the mixture was stirred for 30 minutes at 0 uC. The
organic phase was separated, and the aqueous phase was
extracted further with CH2Cl2 (2 6 20 ml). The combined
organic layers were dried over Na2SO4, filtered and stripped of
solvent. The residue was subjected to column chromatography
(SiO2, AcOEt–CH2Cl2 1 : 12), giving the product as a blue
amorphous solid (12%) Mp: 104uC; UV–vis (CH2Cl2): lmax
(e/L mol21 cm21) ~ 280 (15000), 365 (10000), 618 nm (660);
CD (CH2Cl2): lmax (De/L mol21 cm21) ~ 270 (11.2); IR (KBr):
n ~ 3444, 2987, 2945, 1757 (n C(O)OCH3), 1609, 1454, 1363
(n NO), 1250, 1131, 835, 632, 540 cm21; LDI-TOF-MS:
m/z ~ 335.3 ([M]1) 321.2 [M 2 CH2]1; EPR (CH2Cl2–toluene
1 : 1): g factor, 2.0065; aN, 7.65; aortho, 0.43; ameta, 0.21; aMe, 0.21.
X-Ray diffraction analyses
The X-ray crystal structures were solved by direct methods using
SHELXS-8626 and refined with anisotropic displacement para-
meters by using SHELXL-97.27 Diffractometer Bruker P4, Scan
type v. Of the hydrogen atoms, only those of the hydroxy groups
of (R)-4LNN and (RS)-4LNN were refined as regular atoms. The
data have been deposited with the Cambridge Crystallographic
Data Centre. CCDC reference numbers 170508–170510. See
http://www.rsc.org/suppdata/jm/b1/b106239p/ for crystallo-
graphic files in .cif or other electronic format.
Acknowledgement
This work was supported by grants from the DGES, Spain
(Proyecto nu MAT2000-1388-C03-01), the 3MD Network of
the TMR program of the EU (Contract ERBFMRXCT
980181), an Accio´n Integrada Hispano-Austriaco (HU1999-
0015), and the Fundacio´n Ramo´n Areces. M. M. thanks the
Fundacio´n Ramo´n Areces for a PhD fellowship.
Preparationof(RS)-methyl2-[4-(4,5-dihydro-4,4,5,5-tetramethyl-
3-oxido-1H-imidazol-3-ium-1-oxyl-2-yl)phenoxy]propionate ((RS)-
4MLNN). Using the same procedure as for (R)-4MLNN but
using (RS)-methyl lactate as starting material, the racemic
modification was isolated as a blue solid in 30% yield. Mp
141 uC. Calculated for C16H21O5N2 (%) N 8.35, C 60.88, H
6.91. Found (%) 8.32, 60.77, 6.85; UV–vis (CH2Cl2): lmax
(e/dm3 mol21 cm21) ~ 280 (15000), 365 (10000), 618 nm (660);
IR (KBr): n ~ 2988, 2960, 2929, 1754 (n C(O)OCH3), 1606,
1571, 1451, 1390, 1365 (n NO), 1203, 1131, 1096, 841, 619,
542 cm21; LDI-TOF MS: m/z ~ 357.9 [M 1 Na]1, 334.9
[M]1, 320.9 [M 2 CH2]1, 305.0 [M 2 CH2–O]1; EPR
(CH2Cl2–toluene 1 : 1): g factor, 2.0065; aN, 7.67; aortho, 0.45;
ameta, 0.22; aMe, 0.20.
References
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4
Preparation of (R)-2-[4-(4,5-dihydro-4,4,5,5-tetramethyl-
3-oxido-1H-imidazol-3-ium-1-oxyl-2-yl)phenoxy]propionic acid
((R)-4LNN). (R)-Methyl 2-[4-(4,5-dihydro-4,4,5,5-tetramethyl-3-
oxido-1H-imidazol-3-ium-1-oxyl-2-yl)phenoxy]propionate ((R)-
4MLNN, 100 mg, 0.3 mmol) was dissolved in
a water
(10 mL)–ethanol (10 mL) mixture with NaOH (1 g, 25 mmol)
at ambient temperature and the mixture was stirred for 3 h.
After acidifying with HCl (aq, 10%), the product was extracted
with CH2Cl2 (3 6 20 mL). The combined organic layers were
dried over Na2SO4, filtered and stripped of solvent. The residue
was crystallised from CH2Cl2–hexane as dark blue needles
(97%). Mp 152 uC. Calculated for C16H21O5N2 (%) N 8.72, C
59.80, H 6.59. Found (%) 8.37, 59.14, 6.42; UV–vis (CH2Cl2):
lmax (e/dm3 mol21 cm21) ~ 281 (15000), 367 (10000), 617 nm
(620); CD (CH2Cl2): lmax (De) ~ 290 (20.69); IR (KBr):
n ~ 3440, 2984, 2503 (n C(O)OH), 1741(n C(O)OH), 1604,
1488, 1346 (n NO), 1298, 1256, 1134, 1087, 833 cm21; LDI-
TOF MS: m/z ~ 360.0 [M 1 K]1, 344.0 [M 1 Na]1, 329.0
[M 1 Li]1, 321.0 [M]1, 307.1 [M 2 CH2]1, 291.0 [M 2 CH2–
O]1; EPR (CH2Cl2–toluene 1 : 1): g factor, 2.0065; aN, 7.67;
aortho, 0.46; ameta, 0.20; aMe, 0.20.
5
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oxido-1H-imidazol-3-ium-1-oxyl-2-yl)phenoxy]propionic acid
((RS)-4LNN). Using the same procedure as for (R)-4LNN
J. Mater. Chem., 2002, 12, 570–578
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6
7
20 In the compounds related to (R)-3LNN, a significant Cotton
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220 . It is interesting to note that a lactic acid derivative, 2-(4-
chlorophenoxy)propionic acid, crystallises to form dimers in the
racemate; (d) C. H. L. Kennard, G. Smith and A. H. White, Acta
Crystallogr., Sect. B, 1982, 38, 868–875; (e) S. Raghunathan,
K. Chandrasekhar and V. Pattabi, Acta Crystallogr., Sect. B,
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lises as a chain; (f) H. O. Sørensen, A. Collet and S. Larsen, Acta
Crystallogr., Sect. C, 1999, 55, 953–956.
24 In the structures reported so far of nitronyl nitroxide derivatives
bearing carboxylic acid functionalities, the acidic hydrogen atom
participates in a hydrogen bond with the nitrone: (a) K. Inoue and
H. Iwamura, Chem. Phys. Lett., 1993, 305, 367–384; (b) C. Ba¨tz,
P. Amman, H. J. Deriseroth and L. Dulog, Liebigs Ann. Chem.,
1994, 739–740; (c) O. Fe´lix, M. W. Hosseini, A. D. Cian, J. Fischer,
L. Catala and P. Turek, Tetrahedron Lett., 1999, 40, 2943–2946;
(d) C. Stroh, F. M. Romero, N. Kyritsakas, L. Catala, P. Turek and
R. Ziessel, J. Mater. Chem., 1999, 9, 875–882.
25 Polarimetry of the nitronyl nitroxide radicals reported here was
not possible on account of the high optical absorption : optical
rotatory power ratio that these compounds possess.
15 M. Minguet, D. B. Amabilino, J. Vidal-Gancedo, K. Wurst and
J. Veciana, Mol. Cryst. Liq. Cryst., 1999, 334, 347–358.
16 (a) O. Wallach, Liebigs Ann. Chem., 1895, 286, 90–143; (b) for a
26 G. M. Sheldrick, University of Go¨ttingen, Germany, 1990.
27 G. M. Sheldrick, University of Go¨ttingen, Germany, 1997.
578
J. Mater. Chem., 2002, 12, 570–578