A phenyl α-nitronyl nitroxide with a forced chiral conformation

Article abstract of DOI:10.1007/s007060170146

A new phenyl α-nitronyl nitroxide bearing a hydroxyl group at position 2 of the aromatic ring and a chiral methyl lactate substituent at position 5 has been synthesized with the aim of combining the magnetic properties of this kind of radical with the opt

Full text of DOI:10.1007/s007060170146

Monatshefte fuÈr Chemie 132, 71±82 (2001)
A Phenyl a-Nitronyl Nitroxide with a Forced
Chiral Conformation
Maria Minguet1, David B. Amabilino1, Klaus Wurst2, and Jaume Veciana1;Ã
1
Á
Institut de Ciencia de Materials de Barcelona (CSIC), Campus Universitari, E-08193 Bellaterra,
Spain
È
Institut fuÈr Allgemeine Anorgische und Theoretische Chemie, Universitat Innsbruck, A-6020
2
Innsbruck, Austria
Summary. A new phenyl ꢀ-nitronyl nitroxide bearing a hydroxyl group at position 2 of the aromatic
ring and a chiral methyl lactate substituent at position 5 has been synthesized with the aim of
combining the magnetic properties of this kind of radical with the optical properties endowed by the
chiral group. The optically active compound forms intramolecular hydrogen bonds between the OH
group and one of the NO groups and shows a large torsion angle between the two rings when
compared with similar radicals with no substituent in this position. Therefore, the optical properties
are distinct. The optical and magnetic properties of the new radical in both solution and solid state
are presented.
Keywords. Circular dichroism; Chirality; EPR spectroscopy; Magnetic properties.
Introduction
Magnetic materials based on purely organic molecules have recently attracted
increasing interest as their properties can be subtly modi®ed by adequate selection
of substituents [1]. Since the discovery of the ®rst purely organic ferromagnet by
Kinoshita [2], the family of phenyl ꢀ-nitronyl nitroxides has been widely studied
[3]. Many groups [4] have focused on the possibility of designing ferromagnets
using crystal engineering tools, with varying degrees of success [5]. The intro-
duction of substituents on the phenyl ring which are able to form hydrogen bonds to
the NO group is a very interesting way to control the geometrical ordering of the
molecules of these kind of radicals, e.g. the hydroxyphenyl nitronyl nitroxides. A
very interesting case is that of radical 2 (the 2-hydroxyphenyl derivative, Fig. 1) [6]
which orders ferromagnetically in the solid state below 0.4 K because of the
establishment of a three-dimensional network of weak hydrogen bonds. Another
example is provided by radical 3 (the 2,5-bis-hydroxyphenyl derivative), which is
also a bulk ferromagnet below 0.5 K [7].
The combination of magnetic properties and optical activity is a subject of
great interest [8] because of potential magneto-optical effects of such compounds.
Ã
Corresponding author

72
M. Minguet et al.
Fig. 1. Related phenyl ꢀ-nitronyl nitroxide radicals (R)-1±4
Cross-effects have been predicted and subsequently measured between natural and
magnetically induced optical activity, so-called magneto-chiral dichroism (MChD)
in absorption or emission, and magneto-chiral birefringence (MChB) in refraction
[9], all in inorganic salts. These effects may take place when the purely organic
radical unit (ONCNO moiety) has a chiral surrounding. There are two main sources
of conformational chirality next to the radical unit: the angle APNN, which is the
torsion angle between the two rings, and TIM, which is the torsion angle of the
NCCN unit of the imidazolyl ring. These two angles, which can have negative (M)
or positive (P) chirality, give rise to four gross conformational stereoisomers. The
introduction of a chiral group on the phenyl ring of the radical to give compounds
such as (R)-4 has been shown to induce one sign preferentially in both torsion
angles in the crystal [10±12]. Thus, the chirality is transmitted from the stereogenic
center to the radical unit of the molecule. This fact could lead to interesting
magneto-optical properties in compounds of this type.
In (R)-4 and its analogues, the angle APNN is relatively small. Since the optical
and magnetic properties can depend on this angle, we wished to prepare a chiral
derivative in which this angle was forced to be large. This could be achieved by the
introduction of a substituent at position 2 of the benzene ring, such as a halogen
[13], which operates purely sterically, or with an OH group ± as in 2 [6] and 3 [7] ±
which, in addition to in¯uencing the angle sterically, also forms an intramolecular
hydrogen bond. We chose to prepare the molecule (R)-1 ((R)-methyl(3-(4,5-dihydro-
4,4,5,5-tetramethyl-3-oxido-1 H-imidazol-3-ium-1-oxyl)-4-hydroxyphenoxy)-2-pro-
pionate, Fig. 1), which is a chiral derivative of the bulk ferromagnet 3. In the
following sections we present the magnetic and optical properties of (R)-1, ®rst in
solution (EPR spectroscopy and solution CD spectroscopy) and then in the solid
state (magnetic susceptibility and solid state CD spectroscopy), correlating the latter
with the crystal structure as determined by X-ray analysis.
Results and Discussion
Synthesis
The chiral radical (R)-1 was prepared as follows. 2,5-Dihydroxy-benzaldehyde (5)
was condensed with (L)-methyl lactate using a Mitsunobu protocol [14]. The

A Chiral Phenyl ꢀ-Nitronyl Nitroxide
73
Scheme 1
intramolecular hydrogen bond between the hydroxyl group at position 2 of the
benzene ring and the formyl group inactivates the former, resulting in a selective
substitution of the hydroxyl group at position 5. The chiral aldehyde (R)-6 thus
obtained was used to synthesize the corresponding ꢀ-nitronyl nitroxide by
condensation with 2,3-bis-(hydroxylamino)-2,3-dimethylbutane in methanol and
subsequent oxidation of the condensation product (R)-7 with sodium periodate
following the classic procedure of Ullman et al. [15]. The resulting methyl ester of
the radical is a dark blue-violet solid. Crystals suitable for X-ray diffraction in the
form of small needles were obtained by slow evaporation from a dichloromethane/
hexane solution. Satisfactory elemental analyses as well as infrared and mass
spectra were obtained.
Characterization in solution
The chirality transmission from the stereogenic center to the radical unit can be
observed in the CD spectrum of (R)-1 in solution (Fig. 2), in which Cotton effects
Ã
arise at 350 nm (ascribable to the ꢁ ! ꢁ transition of the ONCNO chromophore)
Ã
and 280 nm (corresponding to the ꢁ ! ꢁ transition of the phenyl ring). When
comparing the CD spectra of (R)-1 with those of (R)-4 (the derivative without a
hydroxyl group), the same intensity is revealed for the Cotton effects along with a
Ã
hypochromic shift of that arising from the ꢁ ! ꢁ transition of the ONCNO
chromophore. This effect could result from the presence of the hydroxyl group that
prevents coplanarity of the two rings, thus impeding conjugation.
The EPR spectrum of a solution of (R)-1 (0.1 mM in CH2Cl2:toluene ˆ 1:1; see
Experimental section for spectrometer settings) at room temperature (Fig. 3) shows

74
M. Minguet et al.
Fig. 2. Circular dichroism spectra of (R)-1 and (R)-4 in CH2Cl2
the ®ve groups of lines corresponding to the coupling of the unpaired electron with
two equivalent nitrogen nuclei (I ˆ 1). Each of these ®ve groups of lines shows the
hyper®ne structure corresponding to the coupling of the electron with the hydrogen
nuclei of the four equivalent methyl groups and with the four aromatic protons. The
coupling constants obtained by simulation of the spectrum are aN ˆ 7:63 G,
aMe ˆ 0:18 G, aortho ˆ 0:53 G, ameta ˆ 0:18 G, and apara ˆ 0:53 G; the isotropic
Â
Lande factor is 2.0062. Coupling with the OH hydrogen atom was ruled out by
deuteration of this functionality. There is no evidence of aggregate formation by
Fig. 3. Room temperature EPR spectrum of (R)-1 (inset) and expansion of the central line (MI ˆ 0)

A Chiral Phenyl ꢀ-Nitronyl Nitroxide
75
lowering the temperature. Neither is there any hint that diastereomers (resulting
from the stereogenic centre and presence of M and P conformers) might be
observed in the spectra of (R)-1, unlike to those of other chiral radical-containing
systems [16].
The ability of (R)-1 to form intermolecular hydrogen bonds has been con®rmed
by LDI-TOF mass spectrometry of neat samples (Fig. 4), where peaks corre-
sponding to the dimeric aggregate are observed along with those arising from the
isolated radical. This phenomenon has been also observed with the radical 2, for
which also trimeric as well as tetrameric aggregates have been found, characteristic
of linear chains. The phenyl ꢀ-nitronyl nitroxides always show two losses of oxygen
(as observed previously by EI-MS studies [17]), accounting for the two peaks of
lower mass with respect to the molecular ions.
The observation of linear aggregates by LDI-MS and their absence in the
crystalline state can be ascribed to the kinetically governed crystallization of the
samples in the holder, leading to chains like that schematized in Fig. 5. These chains
are presumably not suf®ciently thermodynamically stable to be observed in solution
by EPR spectroscopy.
Characterization in the solid state
The radical (R)-1 crystallizes in the orthorhombic non-centrosymmetric space group
P212121. The asymmetric unit consists of one ꢀ-nitronyl nitroxide molecule. As a
consequence, all molecules present the same torsion angle between the planes of the
ONCNO moiety and the aromatic ring (APNN) of ‡37ꢀ. The heterocyclic ring
exhibits an NCCN torsion angle of ‡26ꢀ. Since both torsion angles are of the same
sign, the global conformation is the PP pseudo-anti one, which is less favoured in
the solid state [12]. As in the compounds 2 and 3, a strong intramolecular hydrogen
Ê
bond exists between one NO group and the OH moiety (HÁ Á ÁO distance: 1.72 A,
O±HÁ Á ÁO angle: 169.9ꢀ).
The molecules of (R)-1 pack forming chains along the b axis (Fig. 7) linked by
weak hydrogen bonds between the NO group of one molecule and three methyl
groups of the ®ve-membered ring of the next one (HÁ Á ÁO distances: 2.54, 2.69, and
ꢀ
Ê
2.77 A; C±HÁ Á ÁO angles: 152, 145, and 146 ), and the carbonyl moiety and one
methyl group of the ®ve-membered ring on the same neighbouring molecule (HÁ Á ÁO
ꢀ
Ê
distance: 2.65 A, C±HÁ Á ÁO angle: 155 ), the neighbours being related by a two-fold
screw axis. The chains are disposed in a corrugated plane (a), in which the chains
are linked by weak C±HÁ Á ÁO hydrogen bonds between the CH and methyl groups
Ê
with the oxygen atom of the OH group (HÁ Á ÁO distance: 2.77 A, C±HÁ Á ÁO angle:
144ꢀ) and the NO group with which the latter forms an intramolecular hydrogen
bond (HÁ Á ÁO distance: 2.66 , C±HÁ Á ÁO angle: 142ꢀ). This type of hydrogen bond has
been inferred from the proliferation of magnetic interactions in this type of radical,
for example in the case of 2 [6]. The shortest contacts between SOMOs (which are
localized on the planar ONCNO part of the molecule, Fig. 7) occurs within the
chains.
Ê
The shortest NOÁ Á ÁON distance found (5.04 A) is on the long side for magnetic
interactions, and the angle between the mean plane of the SOMOs (as de®ned by the
ONCNO atoms) is 18ꢀ; therefore, only weak magnetic interactions are expected.

76
M. Minguet et al.
Fig. 4. LDI-TOF mass spectrum of (R)-1 (top, monomer: m=z ˆ 351, dimer: m=z ˆ 702) and (R)-2
(bottom, monomer: m=z ˆ 249, dimer: m=z ˆ 498, trimer: m=z ˆ 747); the radicals ¯y together
with alkali metal ions of the holder (Li: 7, Na: 23, K: 39), thus accounting for peaks of larger mass
than the molecular ion in the spectrum

A Chiral Phenyl ꢀ-Nitronyl Nitroxide
77
Fig. 5. Representation of the possible hydrogen bonded linear chain of molecules observed in the
LDI-TOF MS of 2OHNN derivatives (R)-1 (R ˆ OCHMeCOOMe) and 2 (R ˆ H)
Fig. 6. Molecular views of the solid state structure of (R)-1 and a representation of the helicity of the
rings
Fig. 7. View of the sheets of molecules of (R)-1 along a crystallographic plane

78
M. Minguet et al.
Fig. 8. Solid state CD spectra of (R)-1 and (R)-4 in a KBr matrix
These short contacts are probably related with the observed magnetic behaviour in
the solid state.
The chiral induction from the stereogenic centre to the radical unit observed in
solution has also been proved by solid state CD spectroscopy [18]. The signs of
both Cotton effects have also been con®rmed by theoretical ab initio calculations
of the transitions [12].
The Cotton effect located between 290 and 330 nm in the solid state CD
Ã
spectrum of (R)-1 corresponds to the ꢁ ! ꢁ transition of the ONCNO chromo-
phore (Fig. 8). The band is located at higher energy than the corresponding band in
the CD spectrum of the closely related radical (R)-4 [12]. This effect could result
from the higher APNN angle, which is larger by 6ꢀ in the case of the radical (R)-1,
resulting in a lower conjugation of the two ꢁ systems. (R)-1 also exhibits two
signi®cant Cotton effects in the visible region, both positive, one centred at 450 nm
and another one at 560 nm. These Cotton effects, less intense in the case of (R)-4,
could prove very important in order to measure magneto-optical effects, since the
absorption of the radical is low at this point of the electromagnetic spectrum.
The magnetic susceptibility curve of the solid radical (R)-1 shows a deviation
from the Curie law as the temperature is lowered, indicating low-dimensional
antiferromagnetic interactions. This magnetic behaviour is completely reproduced by
a one-dimensional antiferromagnetic Heisenberg spin chain (S ˆ 1=2) model with
J=k ˆ 1:9 K (Fig. 9), supporting the hypothesis that the short contacts between
the NO groups are responsible for the observed antiferromagnetic interaction.
Conclusions
The new chiral nitronyl nitroxide reported possesses a sterogenic centre which
in¯uences the conformation of the molecule as a whole, at least in the crystal, in
which the intermolecular forces (hydrogen bonds) and packing forces in general are
clearly at play. We have observed a chirality transmission from the stereogenic

A Chiral Phenyl ꢀ-Nitronyl Nitroxide
79
Fig. 9. Temperature product of magnetic susceptibility (ꢂ Á T) vs. temperature for (R)-1; the solid
line represents calculated values of ꢂ Á T for a radical with a one-dimensional Heisenberg spin chain
(J=k ˆ 1:9 K) behaviour
centre to the radical unit by using solid state CD spectroscopy, and important Cotton
effects in the visible region have been observed. The presence of a strong
intramolecular hydrogen bond between one NO group and the OH group in position
2 of the aromatic ring forces the molecules to form chains in the solid state, since
only one NO group is free for interactions beyond the molecule, resulting in one-
dimensional magnetic interactions.
Experimental
Materials and Methods
Solvents were puri®ed when necessary according to literature methods [19]. In particular, THF was
dried over sodium/benzophenone ketyl and distilled. 2,5-Dihydroxybenzaldehyde (5) and diisopropyl
azodicarboxylate (DIAD, 95%) were purchased from Aldrich Chemical Company, triphenylphosphine
from Merck, and (S)-methyl lactate from Fluka. All reagents were used as received. Thin-layer
chromatography (TLC) was performed on aluminum plates coated with Merck Silica gel 60 F254
.
Developed plates were air-dried and scrutinized under a 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
the linear high power mode. 1H and 13C NMR spectra were recorded on a Bruker ARX 300
spectrometer using the deuterated solvent as lock substance and internal TMS as reference. EPR
spectra were recorded on samples degassed with a ¯ow of argon on a Bruker ESP 300E instrument
with the following settings for high resolution spectra: gain 8Â104, centre ®eld 3346.2 G, sweep width
5 G, modulation frequency 100.0 kHz, modulation amplitude 0.101 G, time conversion 20.48 ms, time
constant 20.48 ms, sweep time 20.972 s; microwave: frequency 9.396437 GHz, power 1.016 mW.
Circular dichroism spectra were recorded on a JASCO-715 spectrometer. The solid state spectra were
obtained using a method described in detail elsewhere [18]. Magnetic susceptibility measurements
were obtained with a Quantum Design SQUID magnetometer.

80
M. Minguet et al.
(R)-Methyl-(3-formyl-4-hydroxyphenoxy)-2-propionate ((R)-6; C11H12O5)
2,5-Dihydroxybenzaldehyde (5; 1.0 g, 7.24 mmol), (S)-methyl lactate (759 mm3, 7.96 mmol), and
triphenylphosphine (2.08 g, 7.96mmol) were dissolved in dry THF (30 cm3) with stirring under an
atmosphere of argon, and the mixture was cooled in an ice bath. To this mixture, a solution of
diisopropyl azodicarboxylate (DIAD, 1.65 cm3, 7.96mmol) in dry THF (5 cm3) was added dropwise
over a period of 30 min at 0ꢀC, and the mixture was stirred overnight while warming to room
temperature. After addition of water (10 cm3), the THF was removed in vacuo, and the residue was
partitioned between CH2Cl2 (50 cm3) and H2O (50 cm3), and the aqueous phase was extracted once
more with the same solvent. The combined organic phases were dried over Na2SO4, ®ltered, and
stripped of solvent. The residue was subject to column chromatography (SiO2, hexane:AcOEt ˆ 3:1)
giving the product as a clear oil (136 mg, 9%).
IR (neat): ꢃ ˆ 3309, 2990, 2955, 2850, 1752 (ꢃꢁCꢁO†OCH3)), 1659 (ꢃ(C(O)H)), 1622, 1589,
1
1484, 1450, 1271, 1135, 770cm
;
1H NMR (300 MHz, CDCl3): ꢄ ˆ 1:65 (d, J ˆ 6:9 Hz, 3H,
OCHCH3), 3.77 (s, 3H, COOCH3), 4.83 (q, J ˆ 6:9 Hz, 1H, OCHCH3), 6.34 (d, 1H, H-6, JH ;H
ˆ
4
6
1:6 Hz, 6.55 (dd, 1H, H-4, JH-3;H-4 ˆ 8:8 Hz, JH-4;H-6 ˆ 1:6 Hz), 7.45 (d, 1H, H-3, JH-3;H-4 ˆ 8:8 Hz),
9.72 (s, 1H, CHO), 11.43 (s, 1H, OH) ppm; 13C NMR (75 MHz, CDCl3): ꢄ ˆ 18:5 (CH3CH), 52.4
(OCH3), 73.9 (CH3CH), 118.1, 118.9, 126.4, 150.6, 156.8, 172.3 (COO), 195.9 (CHO) ppm.
(R)-Methyl-(3-(4,5-dihydro-4,4,5,5-tetramethyl-3-oxido-1H-imidazol-3-ium-1-oxyl)-4-
hydroxyphenoxy)-2-propionate ((R)-1; C17H23N2O6)
(R)-Methyl-(3-formyl-4-hydroxyphenoxy)-2-propionate (123 mg, 0.55 mmol) and 2,3-bis-(hydroxy-
lamino)-2,3-dimethylbutane [16]. (100 mg, 0.68 mmol) were dissolved in dry methanol (20 cm3) and
stirred at room temperature overnight. After addition of H2O (20 cm3), a precipitate was formed
which was ®ltered. Without further puri®cation this adduct was dissolved in CH2Cl2 (50 cm3) and
oxidized with NaIO4 (117 mg, 0.55 mmol) previously dissolved in 30 cm3 H2O. After stirring the two-
phase mixture for 30 min at 0ꢀC, the organic phase was separated, and the aqueous phase was
extracted with CH2Cl2 (2 Â 20 cm3). The combined organic phases were dried over Na2SO4, ®ltered,
and stripped of solvent. The residue was subject to column chromatography (SiO2, AcOEt:
CH2Cl2 ˆ 1 : 10), giving the product as a dark blue solid (19 mg, 10%).
M.p.: 114ꢀC; IR (KBr): ꢃ ˆ 3436 (Ph-OH), 2987, 2952, 1762 (ꢃꢁCꢁO†OCH3†) 1486, 1342
‡
‡
(ꢃꢁNO†) 1140, 1096, 871, 815, 770, 540 cm 1; LDI-TOF MS: m=z ˆ 351:3 [M] , 335.3 [M±O] ,
‡
321.3 ‰M 2O ‡ 2HŠ .
Crystal Structure of (R)-1
C17H23N2O6, Mr ˆ 351:37, orthorhombic, space group P212121 (no. 19), a ˆ 6:3348ꢁ4†,
3
1
b ˆ 12:342ꢁ1†, c ˆ 22:736ꢁ2† A, V ˆ 1777:6ꢁ2† A , Z ˆ 4, ꢅc ˆ 1:313 g Á cm 3, ꢆ ˆ 0:10 mm
,
Ê
Ê
Ê
Nonius ꢇ CCD, ꢈ ˆ 0:71073 A (MoKꢀ), crystal dimensions: 0:35  0:2  0:04 mm, T ˆ 218 K,
1644 unique re¯ections collected, 1525 re¯ections with I ꢂ 2ꢉꢁI†, data reduction with DENZO-SMN
[21], Âmax ˆ 20:0ꢀ, no absorption correction, structure solution with SHELXS86 [22], re®nement
on F2 with SHELXL93 [23], hydrogen atom of the hydroxyl group re®ned isotropically, hydrogen
atoms at carbon atoms calculated and re®ned in the riding model with isotropic displacement
parameters, 236 parameters re®ned, R-values (I ꢂ 2ꢉꢁI†): R1 ˆ 0:0401, wR2 ˆ 0:0950, R-values
(all data): R1 ˆ 0:0452, wR2 ˆ 0:1006. Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the Cambridge Crystallographic Data
Centre (CCDC-149616). Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (‡44) 1223 336-033; e-mail: deposit@
ccdc.cam.ac.uk).

A Chiral Phenyl ꢀ-Nitronyl Nitroxide
81
Acknowledgements
This work was supported by grants from the DGES, Spain (Proyecto No. PB96-0862-C0201), the
Â
  Â
Accion Integrada Hispano-Austrõaca (HU1999-0015), and the Fundacion Ramon Areces. M.
 Â
Minguet thanks the Fundacion Ramon Areces for a PhD fellowship. We are grateful to Dr. Carlos J.
Â
 Â
Gomez-Garcõa (Universitat de Valencia) for SQUID measurements and to Dr. Jose Vidal-Gancedo
(ICMAB-CSIC) for EPR measurements.
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Â
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Received June 23, 2000. Accepted (revised) September 18, 2000