Planar biphenyl-bridged biradicals as building blocks for the design of quantum magnets

Article abstract of DOI:10.1021/cg201224g

We have synthesized and investigated a new biphenyl-4,4′-bis(nitronyl nitroxide) radical with intermediately strong antiferromagnetic interactions. This organic biradical belongs to a family of materials that can be used as a building block for the design

Full text of DOI:10.1021/cg201224g

Article
pubs.acs.org/crystal
Planar Biphenyl-Bridged Biradicals as Building Blocks for the Design
of Quantum Magnets
Evgeny A. Mostovich,†,# Yulia Borozdina,† Volker Enkelmann,† Katarina Removic-Langer,‡ Bernd Wolf,‡
́
Michael Lang,*,‡ and Martin Baumgarten*,†
†Max-Planck-Institut fur Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany
̈
‡Physikalisches Institut, J.W. Goethe-Universitat, Max-von-Laue Strasse 1, D-60438 Frankfurt(M), Germany
̈
S
* Supporting Information
ABSTRACT: We have synthesized and investigated a new biphenyl-4,4′-
bis(nitronyl nitroxide) radical with intermediately strong antiferromagnetic
interactions. This organic biradical belongs to a family of materials that can
be used as a building block for the design of new quantum magnets. For
1
quantum magnetism, special attention has been paid to coupled S = /2
dimer compounds, which when placed in a magnetic field, can be used as
model systems for interacting boson gases. Short contacts between the
oxygen atoms of the nitronyl nitroxide units and the hydrogen atoms of the
benzene rings stabilize a surprisingly planar geometry of the biphenyl
spacer and are responsible for a small magnetic interdimer coupling. The strength of the antiferromagnetic intradimer coupling
constant J/kB = −14.0 0.9 K, fitting the experimental SQUID-data using an isolated-dimer model. The deviations from the
isolated-dimer model are attributed to a small interdimer coupling J′/kB, on the order of 1 K, consistent with the crystal structure.
one-dimensional arrays can be studied. The comparatively weak
intradimer interactions, the absence of any orbital contribution
to the magnetism, and the weak dimer−dimer interactions
always present through the hydrogen atoms11 (and references
cited therein) in the crystal structure make organic biradicals
very promising candidates for studying interacting quantum
particles under well-controlled conditions. In a previous study,
we investigated nitronyl-nitroxide-based biradicals, bridged by
tolane-derived linkers,12 and found a magnetic field-induced
quantum phase transition13 (unpublished results) at temper-
atures below 0.3 K and magnetic fields 7.8 T ≤ B ≤ 10.2 T
While these observations prove the use of nitronyl-nitroxide
biradicals for generating quantum magnets, extensions of this
concept should be explored. In particular, there is a need for
tuning the materials’ parameter in a way such that the B-induced
phase transition can be accessed by standard experimental con-
ditions, that is, not too low temperatures and magnetic fields
below about 10 T.
Here we describe the synthesis, X-ray structure, and magnetic
properties of 4,4′-bis(1-oxyl-3-oxy-4,4,5,5-tetramethyl-4H,5H-
imidazolin-2-yl)biphenyl in solution and in the solid state. The
strength of the antiferromagnetic intradimer coupling constant
is determined to be J/kB = −14.0 0.9 K. This value, together
with an interdimer coupling J′/kB on the order of 1 K, implies a
B-induced transition for field distinctly above 20 T, depending
on the topology of the magnetic couplings. Fields of this size in
INTRODUCTION
■
Organic biradicals have for a long time fascinated chemists and
physicists.1 It was the control of the electronic properties of
these molecules, plus their function as building blocks or com-
ponents for molecular electronics and molecular magnets, which
brought them into focus.2 Although biradicals with conjugated
p-phenylene bridging usually give a singlet ground state,3 there
are also some examples with m-phenylene bridges, which have
been found in the low spin ground state.4 In order to adjust the
character and size of the spin exchange energy (J) in biradicals,
the geometries, topologies, and heteroatom substitutions can be
modified to act as setscrews, allowing for a broad variation and
fine-tuning of the spin exchange energy.5 In order to further
handle such biradicals, they should be stable under ambient
conditions for in-depth characterization and supramolecular
ordering. In that respect, nitronyl and imino nitroxides are ex-
cellent candidates for adopting them as building blocks.6 They
have widely been used for the construction of new magnetic
materials with different prospective properties.6−8 Thus, a
promising synthetic approach would be to combine the nitronyl
nitroxide spins through a p-phenylene bridge, in order to obtain
a novel antiferromagnetically coupled biradical. These birad-
icals, properly designed, may serve as building blocks for new
quantum magnets for studying fundamental aspects of inter-
acting quantum particles.9 In fact, by exposing closely spaced
pairs of spin S = 1/2 entities (dimers) to a magnetic field, strong
enough to close the singlet−triplet gap of the isolated dimer, a
gas of delocalized bosonic excitations can be generated. Depending
on the dimensionality of the dimer−dimer interaction, phenomena
such as Bose−Einstein condensation of magnons9 for three-
dimensional systems or Luttinger-liquid behavior10 in quasi-
Received: April 25, 2011
Revised: November 4, 2011
Published: December 1, 2011
© 2011 American Chemical Society
54
dx.doi.org/10.1021/cg201224g | Cryst. Growth Des. 2012, 12, 54−59

Crystal Growth & Design
Article
purified by TLC (SiO2, CH2Cl2-MeOH, 40:1, Rf = 0.6) yielding blue-
green crystalline solid. Yield 50 mg (39%), UV/vis (λmax, lg ε) CHCl3:
601 (3.09), 383 (4.40), 324 (4.72) nm. Anal. Calcd for C26H32N4O4
(464.60): C, 67.22; H, 6.94; N, 12.06. Found C, 66.77; H, 6.98;
N, 12.12.
combination with the small magnetization density of the
material make an experimental investigation very difficult.
EXPERIMENTAL SECTION
■
All the reagents were used as received. 2,3-Bis(hydroxylamino)-2,3-
dimethylbutane14 and 4-bromo-(1,3-dioxalan-2-yl)benzene15 were
prepared according to the literature from commercially available 2,3-
dinitro-2,3-dimethylbutane. Electron paramagnetic resonance (EPR)
spectra were recorded in diluted and oxygen-free solutions of toluene,
with a concentration of 10−4 M unless otherwise stated, by using a Bruker
X-band spectrometer ESP300E, equipped with an NMR gaussmeter
(Bruker ER035), and a frequency counter (Bruker ER 041 XK). The
g-factor corrections were obtained by using either PNT (g = 2.0026)
or DPPH radicals (g = 2.0037) as EPR standards. UV/vis spectra were
recorded in toluene and chloroform solution with a Perkin-Elmer spec-
trometer (UV/vis/NIR Lambda 900) by using a 1 cm optical-path
RESULTS AND DISCUSSION
■
The preparation of nitronyl and imino nitroxide biradicals usually
follows the standard protocol via a dialdehyde precursor, which is
then condensed with 2,3-bis(hydroxylamino)-2,3-dimethylbutane
(BHA) to the bisimidazolidine moiety, followed by oxidation
with NaIO4 to the nitronyl nitroxide or imino nitroxide radical
(Scheme 1).16 The Kumada-Tamao method17 was successfully
a
Scheme 1. Synthetic Procedure for Biradical 4
1
quartz cell at room temperature. H and 13C NMR spectra were re-
corded on a Bruker AMX 250 spectrometer. Mass spectra were ob-
tained on a FD-MS, VG Instruments ZAB-2 mass spectrometer. C, H,
N elemental analyses were carried out by the Chemical Service Center
of the Novosibirsk Institute of Organic Chemistry (Varieo EL). Melt-
ing points were measured on a Buchi B-545 apparatus (uncorrected) using
̈
open-ended capillaries. The X-ray crystallographic data were collected
on a Nonius Kappa CCD diffractometer equipped with graphite
monochromator and with Mo−Kα (λ = 0.71073 Å) radiation. The
structures were solved by a direct method and refined by a full-matrix
least-squares procedure. The magnetic susceptibility was measured in
the temperature range between 2−200 K and in magnetic fields of
B = 0.1 T and B = 1 T, using a Quantum Design superconducting
quantum interference device (SQUID) magnetometer. The magnetization
data were taken at a fixed temperature of 10 K and fields up to 5 T.
Synthesis of Biphenyl-4,4′-dicarbaldehyde (2). The mixture
of 4-bromo-(1,3-dioxalan-2-yl)benzene 1 (4.0 g, 18.4 mmol) with mag-
nesium turnings (0.22 g, 9.2 mmol) was refluxed in dry THF for 24 h
until all magnesium was consumed. Then a solution of Ni(PPh3)2Cl2
(80 mg, 0.7 mol % of 1) in benzene (10 mL) was added and refluxing
was continued for 72 h. The resultant solution was mixed with saturated
NH4Cl (20 mL), vigorously stirred for 12 h at rt, and then extracted
with CH2Cl2 (3 × 40 mL). The combined organic phases were dried
under MgSO4, and the solvents were removed under reduced pressure.
The residual oil was purified chromatographically (SiO2, EtOAc/
pentane, 1:1). The main fraction containing 2 was combined and
evaporated, yielding white transparent crystals. Yield: 1.39 g (72%). 1H
NMR (CDCl3, 250.13 MHz) δ: 10.06 (s, 2H, −CHO), 7.99 (d, 4H,
Ar−H, 3JHH = 8.20 Hz), 7.76 (d, 4H, Ar−H, 3JHH = 8.20 Hz). MS-FD
(70 eV, DMSO): M-H 210.7 (calc. MW 210.1).
Synthesis of 2,2′-(Biphenyl-4,4′-diyl)bis(4,4,5,5-tetramethyl-
imidazolidine-1,3-diol) (3). A mixture of 2 (80 mg, 0.4 mmol) and
2,3-bis(hydroxylamino)-2,3-dimethylbutane (0.130 g, 0.9 mmol) in
toluene (10 mL) was heated near reflux under argon atmosphere for
3 h, and then the residue was filtered off, washed carefully with toluene,
and then dried in air, yielding white tranparent crystals. Yield: 175 mg
(100%), mp =180−183 °C. 1H NMR (DMSO-d6, 250.13 MHz) δ: 1.11
(s, 12H, CH3), 1.13 (s, 12H, CH3), 4.59 (s, 2H, N−CH-N), 7.42−7.76
(m, 8H, CH biph), 7.83 (s, 4H, N−OH). 13C NMR (DMSO-d6, 62.9
MHz) δ: 17.6 and 24.8 (8C, CH3), 66.5 (4C, C(CH3)2), 90.4 (2C, N−
a
(a) Ethylene glycol, toluene, reflux, 70%. (b) 1. Mg (0.5 equiv),
THF, reflux, 24 h; 2. Ni(PPh3)2Cl2 (0.7 mol %), reflux, 72 h, 72%.
(c) 2,3-Bis(hydroxylamino)-2,3-dimethylbutane, PhCH3, reflux, 100%.
(d) NaIO4, CH2Cl2−H2O, 39%.
utilized for the synthesis of 4,4′-biphenyldicarbaldehyde 2. In
this vein, double excess of 4-bromo-(1,3-dioxalan-2-yl)benzene
1 was reacted with 1 equiv of magnesium in the presence of
0.7 mol % of Ni(PPh3)2Cl2 in THF at reflux to promote the
homocoupling product 1. Quenching of the reaction mixture
with aqueous NH4Cl led to removal of the 1,3-dioxolane protect-
ing groups and formation of the target dialdehyde 2 in 72% yield.
Condensation of the dialdehyde 2 with BHA and oxidation of
the corresponding bis(imidazolidine) derivative 3 resulted in
the bis(nitronyl nitroxide) 4, which was isolated by thin layer
chromatography in 39% yield.
The UV−vis absorption spectra were recorded in toluene
and CHCl3 as shown in Figure 1. They contain two main features,
one band in the 300−400 nm range and one around 600 nm, as
found for many nitronyl nitroxide biradicals. The optical ab-
sorptions show maxima at 324 nm for the biphenyl moiety
(π−π* transition) with large extinction and distinct shoulder at
383 nm (ε = 18−25 × 103 M−1 cm−1), together with a weak
absorption in the visible region around 600 nm (ε = 900−1200
M−1 cm−1), the latter originating from the two nitronyl
CH−N), 126.3 and 129.5 (8C, CH, biph), 139.8 and 141.4 (2C, Cipso
,
biph). MS-FD (70 eV, DMSO): M-H 470.1 (calc. MW 470.6). Anal.
Calcd. for C26H38N4O4 (470.60) C, 66.36; H, 8.14; N, 11.91; Found C,
66.66; H, 8.19; N, 12.04.
Synthesis of 4,4′-Bis(1-oxyl-3-oxy-4,4,5,5-tetramethyl-
4H,5H-imidazolin-2-yl)biphenyl (4). The solution of 3 (125 mg,
0.27 mmol) in CH2Cl2−H2O mixture (50 mL, 1:1) was stirred
vigorously at room temperature for 10 min to form a suspension. A
solution of NaIO4 (115 mg, 0.54 mmol) in water (5 mL) was added
drop by drop to this mixture. After 5 min, the characteristic blue color
appeared, the mixture was stirred further for 30 min, and the organic
layer was separated, washed with water, dried under MgSO4, filtered
off, and evaporated under reduced pressure. The obtained solid was
55
dx.doi.org/10.1021/cg201224g | Cryst. Growth Des. 2012, 12, 54−59

Crystal Growth & Design
Article
and c = 11.4935(5) Å.18 The N−O bonds have lengths of 1.266
and 1.268 Å, which are typical for nitronyl nitroxides (Table 1).
Table 1. Selected Bond Lengths for Biradical 4
a
experimental
geometry optimization
O(1)−N(1)
N(2)−O(2)
C(7)−C(4)
C(1)-C′(1)
N(1)−C(7)
N(2)−C(7)
1.266
1.268
1.445
1.494
1.358
1.353
1.276
1.276
1.462
1.481
1.364
1.364
a
B3LYP, 6-31G*.
Figure 1. UV−vis absorption spectra of 4 with concentration of 2.1 ×
10−5 M (inset: 2.1 × 10−4 M) in CHCl3 and in toluene (dashed line)
solution.
The torsional angles between the benzene ring and the
ONCNO group of nitronyl nitroxide are 30.8° N(2)−C(7)−
C(4)−C(5) and 30.0° N(1)−C(7)−C(4)−C(3), respectively
(Table 2). Each radical molecule forms 12 short contacts and
nitroxides (n−π* transitions). The difference in the absorption
features between toluene and the slightly more polar chloroform
mainly consists of higher extinction for the chloroform solution
by roughly one-third compared to toluene, while nearly no shift
of the maxima or their partially resolved vibrational splitting
(shoulders) was observed.
Table 2. Selected Dihedral Angles for Biradical 4
a
experimental
geometry optimization
C(2)−C(1)−C′(1)−C′(6)
N(2)−C(7)−C(4)−C(5)
N(1)−C(7)−C(4)−C(3)
0.9
30.8
30.0
35
11
11
The ESR spectrum was recorded in toluene solution (Figure 2).
At room temperature, it is represented by well-resolved 9 lines
a
B3LYP, 6-31G*.
coordinates with six neighbored molecules. According to the
obtained X-ray structures (Figure 4), the oxygen atom O(2)
coordinates with two hydrogen atoms H(21) and H(61) of
biphenyl from another molecule with relatively short H-bond
contacts of 2.473 and 2.508 Å, respectively. These van der Waals
interactions lead to a surprisingly planar orientation of the two
neighboring biphenyl rings. The torsional angle between the
planes of the two benzene rings is <1°. An extended view of the
different short contact types in the crystal is shown in Figure 5a,b.
DFT Calculations. In order to understand the influence of
the bridging unit on the electronic properties and especially on
the exchange coupling, density functional theory (DFT) cal-
culations were performed. We thus carried out a geometry
optimization with the B3LYP hybrid function and 6-31G* basis
set, and we then applied the broken symmetry approach (BS)
for the singlet and triplet states with BLYP (to avoid Hartree−
Fock contamination) and 6-31G* basis set according to an
approximate projection technique as suggested by Yamaguchi.19
For the simplest case of two spins S = 1/2 this form of the spin
Hamiltonian (H = −2 × J12 × S1 × S2, here the exchange
parameter J will be negative in the case of antiferromagnetic
interaction) gives the exchange interaction as J = (E(BS) −
E(T))/(S2(T) − S2(BS)), where E(BS) is the energy of the
broken symmetry approach, E(T) is the energy of the triplet
state, and S2 are the eigenvalues of the spin operator for these
states. In the case of weak interaction without spin contamina-
tion S2(T) = 2 and S2(BS) = 1 the direct exchange yields J =
E(BS) − E(T).
Figure 2. ESR spectrum of 4 in toluene at rt (c = 10−4 M).
with a spacing of 0.37 mT (giso = 2.0062), which is exactly half
the hyperfine splitting found for a typical monoradical of nitronyl
nitroxide (aN = 0.75 mT). This demonstrates that in the limit
of EPR spectroscopy, we are in the range of strong exchange
coupling J, where J ≫ hyperfine splitting aN.
X-ray Structure of Biradical 4. Crystals for X-ray diffrac-
tion analysis of biradical 4 were obtained by slow evaporation
of a concentrated solution of biradical 4 from a mixture of
dichloromethane/hexane (1:3). The determined X-ray struc-
ture is shown in Figure 3.
Upon geometry optimization of 4, which led to a twisted
biphenyl unit (Table 2), we thus found Jintra/kB ∼ −18.8 K. In
order to look for the difference toward the X-ray crystal struc-
ture, we applied a single energy calculation for the triplet and
the singlet states of the crystal structure geometry obtaining
Jintra/kB ∼ −17.7 K, which is very close to the former value. The
structural difference, however, is quite large (Tables 1 and 2) since
the geometry optimization led to nearly planar arrangements of
Figure 3. ORTEP view of 4 showing 50% of probability displacement.
Hydrogens were removed for clarity.
In the solid state radical, 4 belongs to the P21/n space group
with unit cell parameters a = 10.2610(5) Å, b = 11.1001(5) Å,
56
dx.doi.org/10.1021/cg201224g | Cryst. Growth Des. 2012, 12, 54−59

Crystal Growth & Design
Article
Figure 4. Different short contacts in the crystal of 4 in (Å) − yellow and blue lines with only a few hydrogens of interest left for clarity.
Figure 5. Representation of the crystal structure of 4 illustrating the different van der Waals interactions: (a) the hydrogen bonds of the central
biphenyl unit and (b) the methyl proton interaction with the NOs.
the nitronyl nitroxide units with respect to the twisted biphenyl
spacer, quite opposite to the crystal structure, showing twisted
radical units and a planar backbone. Thus, both structures have
nearly identical overall degree of conjugation and hence result
in very close values for the exchange integrals.
Magnetization Measurements. The molar magnetic sus-
ceptibility χmol(T) of the biradical 4 is displayed in Figure 6 in
the temperature range 2 K ≤ T ≤ 200 K. The experimental data
are measured in a SQUID magnetometer employing a magnetic
field of 1 T. For the experiments, a high-quality, structurally
characterized single crystal with a mass of 1.48 mg was used.
The magnetic signal of the empty sample holder and the dia-
magnetic contribution of the biradical 4 were subtracted from
the experimental data. For the latter, a value of 1.767 × 10−3 cm3/
mol was used according to Kahn.20 The inset of Figure 6 shows
the effective magnetic moment neff = g × [S(S + 1)]1/2 as a
function of temperature. The theoretical value of 1.73 μB,
expected for a spin S = 1/2 system, is displayed in the inset as a
dashed line. Down to approximately 50 K, neff is temperature
independent and, within the experimental error of 6%, close
to the 1.73 μB. Below 50 K, the effective magnetic moment is
Figure 6. Molar magnetic susceptibility χmol(T) as a function of tempera-
ture. The open circles exhibit experimental data taken on a single crystal
at a magnetic field of 1 T. The diamagnetic contributions and the signal
of the empty sample holder were subtracted. The red line is a fit to the
experimental data according to a model for isolated dimers.21 Inset:
Effective magnetic moment, neff, as a function of temperature. The
dashed line indicates neff for a spin-1/2 system with a g-factor of 2.
continuously decreasing due to the antiferromagnetic intra-
dimer interaction.
57
dx.doi.org/10.1021/cg201224g | Cryst. Growth Des. 2012, 12, 54−59

Crystal Growth & Design
Article
The main panel of Figure 6 shows the molar magnetic sus-
ceptibility χmol(T). With decreasing temperature χmol(T) first
increases, exhibiting a Curie−Weiss-like behavior at high tem-
peratures. Around 17.5 K, before χmol(T) starts to decrease
exponentially with decreasing temperature, a broad maximum
shows up, indicating an antiferromagnetic intradimer inter-
action. The red solid line is a fit to the experimental data (open
symbols) by using a model for isolated dimers.21 This fit
describes the data reasonably well within the experimental error.
From this fitting procedure, the antiferromagnetic intradimer
CONCLUSION
■
In summary, we have synthesized and crystallized a new
nitronyl nitroxide biradical 4 with planar biphenyl spacer. Each
radical molecule has 12 short contacts with other molecules to
form an extended crystal network. Short contacts between
oxygen atoms of the nitroxide unit and hydrogen atoms of the
benzene rings stabilize a planar geometry of the biphenyl
spacer. DFT optimized geometry has shown that in the single
molecule the dihedral angle between two benzene rings is about
35°, while for the crystal structure it is less than 1°. Conversely,
the nitronyl nitroxide unit is less twisted with respect to the
biphenyl unit than in the X-ray structure.
The determination of the J/kB value by the broken-symmetry
approach has shown that such significant structure differences
had canceled each other out and led to an almost equivalent
value of the intradimer coupling constant, −18.8 and −17.7 K,
respectively. Susceptibility measurements, as a function of
temperature on a single crystal of 4 using a SQUID magne-
tometer, yield a value of antiferromagnetic intradimer coupling
constant of J/kB = −14.0 K 0.9 K, about a factor of 3 larger as
compared to the nitronyl nitroxide systems studied previously11
(note the different conventions for J used in these publica-
tions). As a result of weak dimer−dimer interactions, which
always exist in these systems through magnetic exchange via
H-atoms, a magnetic field-induced ordered state has been
observed at low temperatures in those tolane-bridged nitronyl
nitroxide biradicals. This demonstrates the usefulness of these
biradicals for generating new families of quantum magnets
enabling to address fundamental aspects of quantum magnet-
ism. In fact, indications for weak interdimer couplings J′/kB on
the order of 1 K have also been observed in the present nitronyl
nitroxide biradical 4. This implies a field-induced order state at
low temperatures and fields in excess of 20 T.
coupling constant J/kB = −14.0 K
0.9 K (based on the
Hamiltonian used for the DFT calculations) is determined.
Deviations of the experimental χmol(T) data from the cal-
culated magnetic susceptibility for isolated dimers are likely due
to a magnetic interdimer coupling J′. As has already been men-
tioned, each dimer has 12 short contacts and coordinates with
six other molecules via several H-bonds. As a consequence, a
magnetic interdimer coupling J′ in the range 1−4 K can be
expected for the present material 4, as has been found also in
other materials where the magnetic units are coupled via hydro-
gen bonds.11,22 From the deviations between the experimental
data and the fits, based on the isolated-dimer model, the upper
limit of J′/kB is estimated to be of order 1 K, consistent with
the magnetic exchange couplings via H-bonds reported in the
literature.11 To determine J′ accurately, the experiments have to
be performed at temperatures significantly below 2 K and mag-
netic fields around the saturation field Bs estimated to be far in
excess of 20 T, which is out of the scope of the present work.
The main panel of Figure 7 exhibits the magnetization of the
biradical 4 at 10 K and fields up to 5 T, the highest field
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic information file (CIF) is available for
compound 4. The data have been deposited with Cambridge
Crystallographic Data Centre allocation number CCDC 816629.
This material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: martin.baumgarten@mpip-mainz.mpg.de; phone: +49
(0) 6131 379 142; fax: +49 (0) 6131 379 514.
■
Figure 7. Magnetization of 4 as a function of magnetic field at a
temperature of 10 K. The open squares are the experimental data.
From a fit to the experimental data using a singlet−triplet model, we
extract the magnetic coupling constant J = −14.7 1.2 K. The inset
exhibits the calculated field dependence of the magnetization at 10 K
up to the saturation field Bs.
Present Address
#N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry
SB RAS, Lavrent’eva 9, 630090, Novosibirsk, Russia.
accessible. As for the χmol(T) data, the data were corrected for the
diamagnetic core and contributions of the empty sample holder.
The solid line in Figure 7 is a fit using a singlet−triplet model for
isolated dimers, from which a magnetic coupling constant J/kB =
−14.7 1.2 K can be determined. The error bar of the intradimer
coupling constant J gives an estimate for the interdimer coupl-
ing constant J′. Within the error bars, this result matches the
intradimer coupling constant obtained from fitting the χmol(T)
data. The inset in Figure 7 shows the calculated magnetization
curve using the J/kB = −14.7 K and g = 2. The saturation field Bs
is reached above 60 T at 10 K, only accessible in pulse-field
experiments, where SQUID magnetometers cannot be operated.
ACKNOWLEDGMENTS
■
We gratefully acknowledge funding through the SFB-TR49.
E.M. expresses his thanks for a year of support made possible
by the International Max Planck Research School (IMPRS).
REFERENCES
■
(1) Diradicals (a) Borden, W. T. Diradicals; John Wiley and Sons:
New York, 1982. (b) Baumgarten, M. High spin molecules directed
towards molecular magnets, In EPR of Free Radicals in Solids: Trends in
Methods and Application; Lund, A. Shiotani, M., Eds.; Kluwer:
Dordecht, 2004; Chapter 12, pp 491−528. (c) Kahn, O. Magnetism:
A Supramolecular Function; Kluwer: Dordrecht, 1996. (d) Miller, J. S.,
58
dx.doi.org/10.1021/cg201224g | Cryst. Growth Des. 2012, 12, 54−59

Crystal Growth & Design
Article
Drillon, M., Eds.; Magnetism: Molecules to Materials; Wiley-VCH:
Weinheim, 2003; Vols. I−IV.
(c) Yamaguchi, K.; Jensen, F.; Dorigo, A.; Houk, K. N. Chem. Phys. Lett.
1988, 149, 537−542.
(20) Kahn, O. Molecular Magnetism; VCH: Weinheim-New York,
(2) (a) Tamura, M.; Nakazawa, Y.; Shiomi, D.; Nozawa, K.;
Hosokoshi, Y.; Ishikawa, M.; Takahashi, M.; Kinoshita, M. Chem. Phys.
Lett. 1991, 186, 401−404. (b) Kinoshita, M.; Turek, P.; Tamura, M.;
Nozawa, K.; Shiomi, D.; Nakazawa, Y.; Ishikawa, M.; Takahashi, M.;
Awaga, K.; Inabe, T.; Maruyama, Y. Chem. Lett. 1991, 20, 1225−1228.
1993.
(21) Johnston, D. C.; Kremer, R. K.; Troyer, M.; Wang, X.; Klumper,
̈
A.; Bud’ko, S. L.; Panchula, A. F.; Canfield, P. C. Phys. Rev. B 2000, 61,
9558.
(22) Removic-Langer, K.; Haussuhl, E.; Wiehl, L.; Wolf, B.; Sauli, F.;
́
̈
̀ ́
(c) Cirujeda, J.; Hernandez-Gasio, E.; Rovira, C.; Stanger, J.-L.; Turek,
Hasselmann, N.; Kopietz, P.; Lang, M. J. Phys.: Condens. Matter 2009,
21, 185013.
P.; Veciana, J. J. Mater. Chem. 1995, 5, 243. (d) Cirujeda, J.; Mas, M.;
Molins, E.; Panthou, F. L.; de; Laugier, J.; Park, J. G.; Paulsen, C.; Rey,
P.; Rovira, C.; Veciana, J. J. Chem. Soc. Chem. Commun. 1995, 709−
710. (e) Matsushita, M. M.; Izuoka, A.; Sugawara, T.; Kobayashi, T.;
Wada, N.; Takeda, N.; Ishikawa, M. J. Am. Chem. Soc. 1997, 119,
4369−4379. (f) Hirel, C.; Luneau, D.; Pec
́
aut, J.; Ohrstrom, L.;
̈
Bussier
̀
e, G.; Reber, C. Chem.Eur. J. 2002, 8, 3157−61.
(3) (a) Miller, J. S.; Epstein, A. Angew. Chem., Int. Ed. 1994, 33, 385−
415. (b) Nakatsuji, S.; Anzai, H. J. Mater. Chem. 1997, 7, 2161−2174.
(4) (a) Wautelet, P.; Le Moigne, J.; Videva, V.; Turek, P. J. Org.
Chem. 2003, 68, 8025−8036. (b) Nau, W. M. Angew. Chem., Int. Ed.
Engl. 1997, 36, 2445. (c) Crayston, J. A.; Devine, J. N.; Walton, J. C.
Tetrahedron 2000, 56, 7829.
(5) (a) Zoppellaro, G.; Ivanova, A.; Enkelmann, V.; Geies, A.;
Baumgarten, M. Polyhedron 2003, 22, 2099−2110. (b) Zoppellaro, G.;
Enkelmann, V.; Geies, A.; Baumgarten, M. Org. Lett. 2004, 6, 4929−4932.
(c) Kanzakia, Y.; Shiomia, D.; Isea, T.; Satoa, K.; Takui, T. Polyhedron
2007, 26, 1890−1894. (d) Zoppellaro, G.; Geies, A.; Andersson, K. K.;
Enkelmann, V.; Baumgarten, M. Eur. J. Org. Chem. 2008, 1431−1440.
(6) Trein, C.; Norel, L.; Baumgarten, M. Coord. Chem. Rev. 2009,
253, 2342−2351.
(7) (a) Hicks, R. G., Ed.; Stable Radicals: Wiley: New York, 2010.
(b) Lahti, P. M. (ed.), In Magnetic Properties of Organic Materials,
Marcel Dekker, New York, 1999.
(8) Itoh, K.; Kinoshita, M., Eds. Molecular Magnetism; Kodansha
(Gordon and Breach): Tokyo, 2000.
(9) Giamarchi, T.; Ruegg, Ch.; Tchernyshyov, O. Nat. Phys. 2008, 4,
̈
198−204.
(10) Ruegg, Ch.; Kiefer, K.; Thielemann, B.; McMorrow, D. F.; Zapf,
V.; Normand, B.; Zvonarev, M. B.; Bouillot, P.; Kollath, C.; Giamarchi,
̈
T.; Capponi, S.; Poilblanc, D.; Biner, D.; Kramer, K. W. Phys. Rev. Lett.
̈
2008, 101, 247202.
(11) (a) Romero, F. M.; Ziessel, R.; Bonnet, M.; Pontillon, Y.;
Ressouche, E.; Schweizer, J.; Delley, B.; Grand, A.; Paulsen, C. J. Am.
Chem. Soc. 2000, 122, 1298−1309. (b) Rancurel, C.; Daro, N.; Benedi
Borobia, O.; Herdtweck, E.; Sutter, J.-P. Eur. J. Org. Chem. 2003,
167−171.
(12) Wolf, B.; Cong, P. T.; Removic-Langer, K.; Borozdina, Y. D.;
́
Mostovich, E.; Baumgarten, M.; Lang, M. J. Phys. Conf. Ser. 2010, 200,
012225.
(13) Tutsch, U. et al., unpublished results.
́
(14) Hirel, C.; Vostrikova, K. E.; Pecaut, J.; Ovcharenko, V. I.; Rey, P.
Chem.Eur. J. 2001, 7, 2007−2014.
(15) Hoffmann, A. K.; Thomas, W. M. J. Am. Chem. Soc. 1959, 81,
580−582.
(16) Ullman, E. F.; Osiecki, J. H.; Boocock, D. G. B.; Darcy, R. J. Am.
Chem. Soc. 1972, 94, 7049−7059.
(17) Kumada, M.; Tamao, K.; Sumitani, K. Org. Synth. 1978, 58,
127−133.
(18) Crystal data for 4: C26H32N4O4, Mw = 464.56 g/mol, dark-blue
prisms (0.09 × 0.24 × 0.38 mm), monoclinic, space group P21/n, a =
10.26101(5) Å, b = 11.1001(5) Å, c = 11.4935(5) Å, α = 90°, β =
112.8641(13)°, γ = 90°, V = 1206.17(10) Å3, Z = 4, Dcalcd = 1.279 g
cm−3, μ = 0.087 mm−1, T = 120 K, total reflections: 3336, unique
reflections: 2707 (Rint = 0.0618), R1 = 0.0610 (I > 2σI), wR2 = 0.0571
(all data), GOF = 0.857. CCDC-816629 contains supplementary
crystallographic data for this paper.
(19) (a) Shoji, M.; Koizumi, K.; Kitagawa, Y.; Kawakami, T.; Yamanaka,
S.; Okumura, M.; Yamaguchi, K. Chem. Phys. Lett. 2006, 432, 343−347.
(b) Soda, T.; Kitagawa, Y.; Onishi, T.; Takano, Y.; Shigeta, Y.; Nagao, H.;
Yoshioka, Y.; Yamaguchi, K. Chem. Phys. Lett. 2000, 319, 223−230.
59
dx.doi.org/10.1021/cg201224g | Cryst. Growth Des. 2012, 12, 54−59