Interacting networks of purely organic spin-1/2 dimers

Article abstract of DOI:10.1039/c4tc00399c

In the present study we report the synthesis of some novel nitronyl nitroxide biradical systems 1-4c with various π-bridges between the radical centres. UV-Vis, IR, EPR and X-ray diffraction studies, along with MS and NMR data where appropriate, are described. Magnetic measurements revealed that the biradicals 1c, 3c and 4c exhibit a moderately strong antiferromagnetic intra-molecular exchange, whereas nitroxide 2c shows a significantly higher exchange coupling, which can only be explained by the presence of strong inter-molecular interactions. From DFT calculations performed on the basis of the X-ray crystal structure of compound 4c, a theoretical value of the intra-dimer coupling constant J_intra = -8.6 K is obtained. Direct proof also for inter-molecular arrangement with J_inter ～ -2 K was provided by the low temperature AC studies of biradical 4c. According to the magnetic characterization, the nitronyl biradical 4c is a promising candidate for a purely organic-based low-dimensional quantum magnet. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00399c

Journal of
Materials Chemistry C
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PAPER
Interacting networks of purely organic spin–1/2
dimers†
Cite this: J. Mater. Chem. C, 2014, 2,
Yulia B. Borozdina,‡a Evgeny Mostovich,ab Volker Enkelmann,a Bernd Wolf,c
6618
c
c
c
a
*
Pham T. Cong, Ulrich Tutsch, Michael Lang and Martin Baumgarten
In the present study we report the synthesis of some novel nitronyl nitroxide biradical systems 1–4c with
various p-bridges between the radical centres. UV-Vis, IR, EPR and X-ray diffraction studies, along with
MS and NMR data where appropriate, are described. Magnetic measurements revealed that the biradicals
1c, 3c and 4c exhibit a moderately strong antiferromagnetic intra-molecular exchange, whereas
nitroxide 2c shows a significantly higher exchange coupling, which can only be explained by the
presence of strong inter-molecular interactions. From DFT calculations performed on the basis of the X-
Received 27th February 2014
Accepted 25th June 2014
ray crystal structure of compound 4c, a theoretical value of the intra-dimer coupling constant Jintra
¼
ꢀ8.6 K is obtained. Direct proof also for inter-molecular arrangement with Jinter ꢁ ꢀ2 K was provided by
the low temperature AC studies of biradical 4c. According to the magnetic characterization, the nitronyl
biradical 4c is a promising candidate for a purely organic-based low-dimensional quantum magnet.
DOI: 10.1039/c4tc00399c
www.rsc.org/MaterialsC
approach,5 with purely organic radical entities. Inorganic
materials have some drawbacks, as the size of the cluster, which
Introduction
can be compressed only till a certain limit, and that the struc-
ture of the complex is difficult to predict depending on seren-
dipity. On the other hand, organic compounds are usually
transparent in many regions and could be obtained in optical
active chiral forms.4 Thus, they might be used as magneto-
optical switches and for the manipulation of polarized light in
optical devices. Latterly they were extensively studied due to
their potential application as radical-based batteries,6–8 electric
conductors6,9,10 and spintronic devices.6,9,11
Recently, coupled spin–dimer systems composed of closely-
spaced pairs of spin-1/2 carrying entities, have been used as
model systems for exploring critical phenomena under well-
controlled conditions.1 As a prominent example of special
features observed here we mention the eld-induced delocal-
ization of bosonic excitations in reduced dimensions. Likewise,
for three-dimensional spin–dimer networks, these systems can
be used to study the phenomenon of Bose-Einstein condensa-
tion (BEC) of magnetic excitations.1
Importantly, chemistry techniques permits to perform
structural modications in organic materials in order to control
the intra- and inter-molecular exchange interactions between
the spin carriers.5 Thus, the former can be designed and
adjusted through synthesis of different conjugated bridges
between the spin-containing sites, although the latter requires
supramolecular approaches employing hydrogen bonding, p-
stacking, and metal complexation. Such tunability is unprece-
dented in most of the traditional inorganic materials and is the
key to obtain materials with unique magnetic properties.
As the spin carrier we have chosen nitronyl nitroxide (NN)
radicals,1 due to their thermal and environmental stability and
chelating properties. As in other biradical species, such as
imino nitroxide, verdazyl or tert-butyl nitroxide radicals, the
spin density of the unpaired electron is delocalized over two
sites of coordination, leaving open the possibility to arrange
molecular units into a supramolecular network with unprece-
dented magnetic structure.12,13
The most commonly used spin carriers for constructing
magnetic materials are metal ions and nitroxide radicals.2
Hence, various approaches toward molecular magnets can be
classied into inorganic and organic ones, where the spins are
provided by metal ions, the so-called “metal-radical” or hybrid
approach, which combines magnetically active metal ions and
persistent organic radicals as ligands,3,4 and the organic
aMax Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz,
Germany
bNovosibirsk Institute of Organic Chemistry Siberian Branch of the Russian Academy of
Science, Lavrentiev avenue 9, 630090 Novosibirsk, Russia
cPhysikalisches Institut, Johann Wolfgang Goethe- University Frankfurt, 60438
Frankfurt am Main, Germany
† Electronic supplementary information (ESI) available: Fig. S1–12 UV-Vis spectra
of the nitronyl nitroxides 1–4c, IR spectra of dialdehyde 4a, imidazolidine 4b,
biradicals 2c, 3c and 4c, EPR spectra of the biradical 3c, magnetic susceptibility
data of compounds 1–3c, TOC graphic of 4c, respectively. CCDC 822066,
816633, 816635. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4tc00399c
In our study we considered organic spin–dimer systems with
singlet ground state and moderate intra-dimer interactions in
‡ Present address: Institute of Biochemistry, Ernst-Moritz-Arndt University
Greifswald, Felix-Hausdorff-Straße 4, 17487 Greifswald, Germany.
6618 | J. Mater. Chem. C, 2014, 2, 6618–6629
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the range of ꢀ2 to ꢀ8 K (the numbers refer to the exchange
coupling dened in accordance with the Hamiltonian used in
the DFT calculations). In addition, the compounds should form
interacting networks due to small inter-dimer interactions,
giving rise to eld-induced magnetic states, depending on the
dimensionality of the interactions, at elds below the saturation
eld.2 Since it is difficult to predict the precise magnitude of the
intra- and inter-molecular exchange interactions, which can
further affect the strength of the bulk magnetic coupling, we
started from approximate computational estimations based on
the coordination geometry and the bond distances in the
molecules, and have targeted four biradicals for in-depth
characterization.
Herein we report the synthesis of biradicals 1c–4c, designed
for the ne tuning of spin exchange interactions, their charac-
terization, including X-ray structural analysis, and, nally, the
magnetic studies, which are discussed with respect to the
quantum chemical calculations based on a broken symmetry
DFT approach.
Scheme 1 Synthetic route towards dialdehyde 1a.
dimethoxyphenyl-derivatives). Class (ii) is made by rigid and
planar molecules (compounds 3 and 4, in other words, acety-
lene derivatives). Finally, group (iii), which includes ve-
membered hetero-rings as functional part of the coupling unit,
which provides non-alternate spin-polarization paths
(compound 2a–c, pyrazole derivatives).
Results and discussion
Synthesis of the rst target dialdehyde 1a (class (i)) is rep-
resented in Scheme 1. Regardless of the different coupling
conditions tested, the 4-[4-(4-formylmethyl)-2,5-dimethoxy-
phenyl]benzaldehyde (compound 1a) could not be synthesised
via direct coupling of the dibromo derivative 5 with 4-for-
mylbenzeneboronic acid. In fact, only the mono-substituted
product was isolated at best. Thus, an alternative route was
explored successfully, as shown in Scheme 1. Initially the
dimethoxyether 5 was treated with n-BuLi in dry THF at low
temperature (T ¼ ꢀ78 ꢂC), and reaction mixture was le stirring
under argon atmosphere at the same temperature for 5 hours.
At the end of this time period excess of boron triisopropoxide
(tri(propan-2-yloxy)borane) was added dropwise. The mixture
was treated with 5% HCl, which afforded diboronic acid deriv-
ative (compound 6) as white solid. Here, the resonance 1H-NMR
signal of the hydroxy groups (OH) present in the isolated
product falls at 7.82 ppm and conrmed, therefore, formation
of the desired derivative. The compound was furthermore
recrystallized from H2O–CH3CN (1 : 1) mixture, affording highly
pure derivative 6 in a fairly good yield (45%).
Synthesis of biradicals
The synthesis of the nitronyl nitroxide radicals (NN) relies, in
general, on the condensation between 2,3-bis(hydroxyamino)-
2,3-dimethylbutane (BHA) with an aldehyde functional group,
followed by the oxidation reaction of the condensation products
(Fig. 1). In order to achieve ne tuning of the magnetic exchange
interactions in biradical systems, we used conjugation control
of the bridging unit with topologically aligned radicals in the
para-position. Theoretical studies emphasized the importance
of the mutual orientation and relative distances in the crystal
packing for the unimpeded magnetic coupling propagation.13
We engineered four novel p-conjugated formyl derivatives (1a–
4a), the key precursors towards NN synthesis, via diverse multi-
steps approaches, as outlined in Schemes 1–4. These p-conju-
gated systems vary from each other, and can be divided into
three different classes. The rst group (i) contains electron-
donor substituents, which are able to increase the electron
density along the spin-coupling backbone (radical 1a–c,
Diboronic acid 6 was then used in the Suzuki coupling
reaction with 4-bromobenzaldehyde. The dialdehyde 1a was
obtained only aer prolonged reaction under reux (5 days) in
62% yield. The 1H-NMR spectrum of 1a shows the appearance of
new signals in the aromatic region, at 7.80 and 7.95 ppm,
together with 1H-resonance signal at 10 ppm, which is the clear
ngerprint for the presence of undamaged aldehyde moieties.
Scheme 2 outlines the synthetic route towards 4-formyl-1-[4-
(4-formyl-1H-pyrazol-1-yl)phenyl]-1H-pyrazole (compound 2a,
class (iii)). The reaction of 1,4-dibrombenzene 7 with excess of
pyrazole in the presence of CuI and potassium carbonate
(nitrobenzene, T ¼ 210 ꢂC) afforded 1,4-bis(1-pyrazol-1H-yl)-
benzene 8 in a high yield (76%). Subsequent iodination reaction
was carried out using a mixture of 8, iodic acid, and iodine. The
reactants were heated at 80 ꢂC in acetic acid and let to react for
Fig. 1 Model compounds 1–4 with emphasized bridging units.
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Scheme 3 Synthetic route towards dialdehydes 3a and 4a.
Scheme 2 Synthetic route towards dialdehyde 2a.
aldehyde-protected intermediate. The dioxolane derivative 11
was subjected to Glaser coupling. The reaction required excess
of copper (I) and (II) chlorides (7 eq. and 5 eq., respectively) in
pyridine media, followed by acidic cleavage of the dioxolane
three days. The process progress was monitored by TLC and FD-
mass analysis of the reaction mixture aliquots. Crude product
precipitated upon cooling as colourless plates. Although the
reaction yield appeared initially very good, the crude contained
not only the target di-iodo derivative 9, but also substantial
amount of mono-iodinated species (ꢁ25%). Extension of the
reaction time did not signicantly improve the ratio between
mono- and di-iodo derivatives. However, the mono-iodinated
compound could be easily separated from the mother solution
by ltration, leading to the di-iodinated compound 9 in fairly
good yield (ꢁ60%). Notably, the mono-iodinated compound
could be used itself as a starting material, in order to force the
completion of the iodinating reaction, in other words, to func-
tionalize the position 4 of the le unsubstituted pyrazole ring.
Here, the disappearance of the pyrazole proton signal at 4th
position (6.50 ppm), is accompanied by a signicant down-shi
of the H signal at the 5th position, from 7.75 ppm to 8.76 ppm in
1H NMR spectra, and those resonances can be used as nger-
prints to probe the progresses of conversion into compound 9.
The nal step towards dialdehyde 2a, containing pyrazole-
phenyl bridging unit, was accomplished via formylation reac-
tion of the di-iodo derivative 9 through initial formation of the
1
ring in THF–water mixture (5 : 3). Here, appearance in the H,
13C NMR spectra of strong signals typical for the aldehyde
fragments at 10.0 (1H) and 191 (13C) ppm, conrmed successful
completion of the coupling reaction and formation of the
desired 4,40-(buta-1,3-diyne-1,4-diyl)dibenzaldehyde 3a.
To explore the potential of another closely associated deriv-
ative, the tolane dialdehyde 4a was synthesized via Sonoga-
shira–Hagihara cross-coupling reaction.16 This system contains
a shorter p-conjugated linker between the radical centres,
hence the resulting through-bond exchange interaction is
expected to be stronger in comparison to 3a. The cross-coupling
reaction between 4-bromobenzaldehyde and 4-ethynyl-
benzaldehyde 10 was carried out employing catalytic mixture of
Pd(PPh3)2Cl2, CuI, PPh3 in the presence of base (Et3N) and led to
the formation of the dialdehyde 4a in high yield (70%).
The target nitronyl nitroxide molecules could be routinely
prepared as illustrated in Scheme 4. The condensation of the
2,3-diamino-N,N0-dihydroxy-2,3-dimethylbutane with carbal-
dehydes is an air-sensitive process and required, therefore,
strict anaerobic conditions and degassed solvents.
corresponding Grignard reagent. Compound
9
reacted
smoothly with ethylmagnesium bromide. The magnesium
intermediate was quenched with the stoichiometric amounts of
DMF and afforded the target dialdehyde 2a in high yield (89%).
The diacetylene fragment was introduced as the spacer into
the p-system, giving rise to the biradical 3c (class (ii)). It is
expected that 3c should feature little or no torsion within the
molecule, which is essential for the weak intra-molecular
coupling. One of the most efficient methods for the assembly of
the molecules containing acetylene fragments relies on Glaser
homo-coupling.14 Regardless of the different coupling condi-
tions tested, we could achieve only traces amounts of the dia-
ldehyde 3a employing direct Glaser coupling methodology to 4-
ethynylbenzaldehyde 10 (Scheme 3). Thus, another route was
undertaken, where the aldehyde function was at rst protected
with ethylene glycol affording the acetal precursor 11.15 The 1H
NMR spectrum of 11 showed characteristic signals of dioxolane
ring at 4.0 ppm (–CH3, methylene groups) and at 6.7 ppm (–CH).
The signal associated to the aldehyde group, on the contrary,
was absent, hence ensuring the complete conversion into the
Scheme 4 Synthesis of the imidazolidine 4b and biradical 4c.
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As a general rule, reaction with dialdehydes 1–4a was per- the radicals into diamagnetic products occurred within a day,
ꢂ
formed in deaerated toluene under argon at 110 C, and affor- due to sequential release of water molecules (dehydration), as
ded the corresponding N,N0-dihydroxyimidazolidines 1b–4b in witnessed from the decrease of the radical signal intensity in
quantitative yields. Once formed, the radical precursors (e.g. 4b both UV-Vis absorption and in the EPR spectra (UV-Vis spec-
in Scheme 4) precipitated during the condensation step, and troscopy with absorption signals around 600 nm, and EPR
they were isolated by ltration as analytically pure (from 1H- resonance signals around giso ¼ 2.0063).
NMR spectra) white or yellowish powders. The 1H-NMR spectra
of these intermediates exhibited characteristic peaks of the
hydroxy groups (ꢁ7.8–8.0 ppm, marked with the letter a in
Absorption spectra (UV-Vis and IR spectroscopic studies)
Fig. 2.) and the nodal proton located at the second position of
The relevant UV-Vis and IR data recorded for 1c–4c are given
the imidazolidine ring (ꢁ4.7 ppm, –CH group, labelled with the
together in Table 1 for a better comparison.
letter d). To illustrate this, the 1H NMR spectrum of the
The UV-Vis absorption spectra of the biradicals systems 1c–
condensation product 4b is represented in Fig. 2. Here, the
4c were recorded in toluene solutions and exhibited the char-
remaining in the sample toluene is specied with an asterisk.
acteristic deep blue colour, which is typical for this class of
Oxidation of the N,N0-dihydroxyimidazolidines 1b–4b with
radicals. Two distinctive set of absorption bands dominate the
sodium periodate in a biphasic media of dichloromethane and
optical spectra (see ESI Fig. S1†). The rst set of bands is
water afforded nitronyl biradicals 1c–4c (Scheme 4). In order to
characterized by differently enhanced vibronic components,
avoid both further oxidation of the radicals to imino nitroxides,
and takes place at a longer wavelength, around 600 nm. Those
and to diminish the loss of the radical units, the reactions were
signals correspond to the n–p* transitions of the aminoxyl
ꢂ
carried out at T ¼ 0–5 C using an ice bath. This method was
oxides moiety. The second set of absorption values includes the
preferred over the one, where solid metal oxides were applied as
p–p* transitions of the aromatic systems, and falls at much
oxidizing agents,17,18 because it allowed a better control of the
higher energy in the 300–380 nm range. Complementary to UV-
oxidation reaction by using stoichiometric amounts of period-
Vis, the appearance of a novel signal aer oxidation of the
ate in solution. The progress of the reaction was monitored by
imidazolidine derivatives at around y ꢁ1350 cmꢀ1 (Table 1) was
TLC of the reaction mixture aliquots. All the radicals were
assumed to arise from the N–O stretching vibration of the
puried using ash column chromatography on silica (SiO2).
imidazoline moiety, which was in harmony with comparable
The synthesized nitroxide systems 1c–4c were found fairly
signals observed for several other NN systems described in the
well soluble in most of the organic solvents tested (e.g. MeOH,
literature.19,20 Other strong signals appear at ꢁ1550 cmꢀ1, but
CHCl3, THF). However, in aprotic media (e.g. toluene), longer
those are less characteristic since they belong to the C]N
stability of the radical molecules over the time was observed. In
stretching vibrations. In addition, the IR spectra ensured
protic solvents (e.g. CH2Cl2 or CHCl3), partial decomposition of
absence of the traces of the dialdehyde precursors or imidazo-
lidine condensation products in the puried biradical powders
(see for comparison Fig. S2 and S3†).
EPR spectroscopy
The solution EPR spectra of the biradical systems 1c–4c were
recorded in oxygen-free toluene solutions (see ESI Fig. S4–S7†).
at room temperature (T ¼ 298 K). The nine lines splitting
pattern witnessed in the studied systems highlights efficient
through-bond coupling of the two nitronyl nitroxide residues
via spin-polarization effects of the p-spacer. Regardless of the
different distances between the radical centres, as well as in the
molecular identity of the coupling units employed, this property
hold for 1c, 2c and 4c. Thus, within the limit of X-band EPR
spectroscopy, the attainment of strong exchange interactions (J)
|J/aN| [ 1 between the spin carriers were experimentally veri-
ed in all studied biradical systems. The spin-concentrations
for 1c–4c at T ¼ 298 K were always consistent with two spin
centres, where singlet and triplet states are thermally populated
and statistically distributed according to the Boltzmann law.
The calculated values of the hyperne coupling constants (aN)
for 1c–4c are given in Table 1 and fall between 0.7 and 0.8 mT
Fig. 2 NMR spectrum of 4a, b in DMSO at 250 MHz (128 and 105
(giso z 2.0063). These values are in harmony with those typically
scans, respectively). The intensities of the solvent signals differ so
observed in the nitronyl nitroxide radical systems found in the
much due to the significantly lower solubility of the dialdehyde 4a in
DMSO. The asterisk indicates toluene.
literature.
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Table 1 Selected spectroscopic data for the nitronyl nitroxides 1c–4c
Biradical
IRa nNO/cmꢀ1
UV-Visb lmax nmꢀ1 (3b Mꢀ1 cmꢀ1
)
Massc g molꢀ1
EPR aN/mT
1c
2c
3c
4c
1358
1354
1365
1356,1364
627 (557)
599 (3808), 654 (4011)
622 (578)
599
519
511
487
0.752
0.768
0.744
0.762
611 (868)
a
Measured in solid state at room temperature (T ¼ 293 K). b Measured in toluene solution except for compound 2c that was measured in freshly
prepared CHCl3 solution. c Measured in dichloromethane by FAB. The m/z+ (100%) peak corresponds to [M]+.
Table 2 Selected structural data for the biradicals 1c, 3c and 4c
Crystal structures analysis
Single-crystals suitable for X-ray analysis were obtained by slow
diffusion of dichloromethane–hexane mixture (1 : 4) for bir-
adicals 1c, 3c and 4c at room temperature. Deep-blue needle
crystals of biradicals 1c, 3c and 4c were analysed by X-ray
diffraction and their ORTEP drawing as given in Fig. 3–5.
Pertinent crystallographic parameters and renement data are
1c
3c
4c
Formula
M
C34H40N4O6
C30H32N4O4
512
C28H32N4O4
488
600
Crystal system
Space group
Monoclinic
Pn (no. 7)
13.045(5)
8.447(4)
13.703(5)
91.593(12)
1509.3(11)
4
9.41
1.322
0.711
4156
Monoclinic
P21/c (no. 14)
8.1768(2)
19.0191(8)
8.6983(3)
103.4(2)
1315.89(8)
4
7.32
1.294
0.711
3687
Monoclinic
P21/n (no. 14)
6.0834(4)
11.2581(5)
19.1236(9)
90.832(3)
1309.59(12)
4
4.53
1.239
0.711
3784
˚
a/A
˚
listed in Table 2, while selected torsional angles for the studied b/A
biradicals are reported in Table 3.
˚
c/A
b/deg
As shown in the ORTEP diagram (Fig. 3), radical 1c is
symmetric to the central phenyl ring and has C2 symmetry axis
through C1–C4 atoms of the benzene ring. The dihedral angles
3
˚
V/A
Z
Rfactor(%)
between the phenyl ring and radical plane are 11.2ꢂ (N11/ rc/g ꢃ cmꢀ3
Mo ka/mmꢀ1
Nref
Npar
S
C17/C14/C13) and 5.75ꢂ (N12/C17/C14/C15). The O–N–
C–N–O moiety is planar, with nearly equivalent O–N bond
lengths of 1.31(O11/N11), 1.33 (O12/N12), 1.26 (O31/N31),
193
5.633
822066
173
0.985
816633
163
1.081
816635
˚
1.24 (O32/N32) A. Closer analysis of the crystal packing shows
CCDC
that the N–O entities of neighboring molecules are attached to
the benzene ring (O11/H161* 2.48, O12/H51* 2.6, O12/
˚
C36* 3.18 A) and methoxy group of phenyl moiety to a radical
Table 3 Selected torsion angles Q and bond lengths d for nitronyl
nitroxide biradicals 1c, 3c and 4c
fragment of the neighboring unit (O32/O1* 2.94, O32/C7*
˚
3.02 A); the shortest intersheet contact is found between the
a
ꢀ1
Qb/deg
˚
Biradical
d A
1c
3c
4c
1.443
1.460
1.459
27.5 ꢄ 1
25.0 ꢄ 1
24.4 ꢄ 1
a
b
˚
Distance values were averaged (error within 0.002 A). Angle values
were averaged (error within 1ꢂ).
˚
nitronyl oxygen and methyl group (O312/H203* 2.36 A).
Therefore, the intermolecular distances are short enough to
favour magnetic interactions.
The crystallographic data were collected on Nonius Kappa
˚
CCD (Mo ka, m ¼ 0.71073 A) diffractometer equipped with a
graphite monochromator.
Compound 3c is crystallized in a transoid arrangement with
a center of symmetry located on the central C1–C1* bond
(Fig. 4). The derivative is fairly planar, and there is a dihedral
angle between the mean plane of the C6 ring and the imida-
zolidine moiety of 25ꢂ. In the present structure the distances
between the N–O and C–N atoms of the radical subunits are
Fig. 3 X-ray structure and the crystal packing of 1c with ORTEP drawn
at the 50% of the probability level: (a) molecule; (b) crystal packing.
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Fig. 5 X-ray structure of 4c with ORTEP drawn at the 50% of the
probability level: (a) molecule; (b) crystal packing.
Fig. 4 X-ray structure and the crystal packing of 3c with ORTEP drawn
at the 50% of the probability level: (a) molecule; (b) crystal packing.
View along the c axe of the crystal packing of compound 3c.
intermolecular distances but the whole molecular structure of
the biradical 4c favours intermolecular magnetic interactions.
In order to rationalize the magnetic properties of molecular
compounds, analysis of the molecular packing (so, as to nd
out the possible magneto-structural correlations) was carried
out. The studied biradical NN systems shared nearly planar
arrangements of the coupling-core. Structural analyses revealed
rather small torsions between the imidazolyl and the aromatic
core (<30ꢂ), which facilitates efficient communication between
the radical moieties through the coupler. Additionally, the
similar: N(16)–O(18) 1.276; N(17)–O(19) 1.281; N(16)–C(9) 1.36;
˚
N(17)–C(9) 1.345 A. This symmetry is due to the complete
delocalization of the odd electron in the non-bonding orbitals
of the ‘ONCNO’ core. The N–O bond and the C–N bond
distances are between the values of a single and a double bond.
Closer examination of the crystal packing shows that the radi-
˚
cals are organized in a zig-zag fashion (O18/H133* 2.53 A,
Fig. 4(b)) with additional hydrogen bonds between the sheets
formed by C5 atom of the benzene ring with hydrogen atoms
˚
˚
(C5/H71* 2.8, C5/H152* 2.86 A, Fig. 4).
intersheet contacts are found to be short enough (ꢁ2.5 A) to
The molecular structure of compound 4c (Fig. 5) is similar to
the one previously reported21 which has the same space group
(P21/n). Probably, such similar cell organization can be
explained by crystallization conditions. The torsion angle
between the plane of the benzene ring and the radical moiety is
25ꢂ. Adjacent radicals are organized in a head-to-tail fashion
forming 1D network: the rst N–O fragment of each radical unit
is attached to C7, H71 atoms of the benzene ring, the second NO
binds to the hydrogen atoms of the methyl group of an imida-
zolidine fragment. Geometrical parameters are: O2/H71* 2.52,
favour magnetic interactions.
DFT calculations and magnetic measurements
In order to understand the inuence of the bridging unit on the
electronic properties and especially on the exchange coupling,
DFT calculations 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.24 For the simplest case of two spins S ¼ 1/2 this
form of the spin Hamiltonian (H ¼ ꢀ2J12S1S2, here the exchange
parameter J will be negative in the case of antiferromagnetic
interaction) gives the exchange interaction as J ¼ (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 case
˚
O1/H141* 2.59, O2/C7* 3.16, C7/C13 3.4 A; the distance
˚
between H-bonded sheets totals 2.36 A (O1/H101*).
Reasonably to assume, that the intermolecular interaction in
two radicals oriented in a head-to-tail fashion is at maximum
when the two NO groups are arranged in such a way that the O–
N/O* angle a is equal to 90ꢂ and the angle b between the p*
orbitals of the nitronyl nitroxide radicals with a vector normal to
the N2O2 plane is also 90ꢂ.22,23 Thus, not only the
This journal is © The Royal Society of Chemistry 2014
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Table 4 The intra-dimer magnetic exchange coupling calculated Jcalc
and experimental constant Jexp values. Tmax denotes the position of
the maximum in cmol(T)
Biradical
Tmax/K
Jcalc/Ka
Jcalc/Kb
Jexp/Kc
1c
2c
3c
4c
0.7
19.5
3.2
ꢀ2.7
ꢀ2.5
ꢀ2.0
—
ꢀ0.9
ꢀ16.0!
ꢀ2.4
ꢀ14.4
ꢀ16.8
ꢀ5.8
6.3
ꢀ8.6
ꢀ4.8
a
Optimized geometries were obtained from DFT calculations (B3LYP, 6-
31G*), then the broken symmetry approach was applied (BLYP, 6-31G*)
for the evaluation of J. b The exchange interactions were evaluated for X-
ray crystal structures applying the broken symmetry approach with
c
BLYP functionals and 6-31G* basis set. Expected from the isolated
dimer model, where 2J/kBTmax ¼ 1.599.
Fig. 6 Magnetic susceptibility in a representation cmolT of biradical 4c.
The broken line indicates the theoretical value of 0.75 cm3 K molꢀ1 for
a dimer system formed by two spin S ¼ 1/2 entities. Inset: molar
susceptibility cmol (left scale) and its inverse 1/cmol (right scale) as a
function of temperature together with fits performed by using a
Curie–Weiss expression (solid line) and a Bleaney–Bowers equation26
(broken line). For fitting parameter and details see text.
of weak interaction without spin contamination S2(T) ¼ 2 and
S2(BS) ¼ 1 the direct exchange yields J ¼ E(BS) ꢀ E(T). The
obtained theoretical exchange coupling constants J are listed in
Table 4 together with the experimental values for a better
comparison.
The magnetic properties of the samples 1c–4c were deter-
mined in the temperature range 2 K # T # 270 K and in
magnetic elds B # 5 T by using a Quantum Design SQUID
magnetometer. All data have been corrected for the tempera-
ture-independent diamagnetic core contribution of the
constituents25 and the magnetic contribution of the sample
holder. For temperatures below 2 K the ac-susceptibility data
shown in Fig. 7 were recorded using an ultra-high resolution ac-
susceptometer adapted to a 3He–4He top-loading dilution
refrigerator. The compensated-coil susceptometer is optimized
for measuring small single crystals in the mg range. The system
is equipped with a 12 T superconducting magnet.
Generally, all the biradicals under investigation (1c–4c)
featured a similar magnetic behaviour in the studied tempera-
ture range. The high-quality single crystalline materials 1c, 3c,
and 4c exhibit magnetic contributions of Curie type, assigned to
uncoupled spins, the concentration of which is much less than
1%. The Curie contribution for the biradical 2c was slightly
larger. Furthermore the small Curie contribution for 2c is
indicated by the broken line in the inset of Fig. S9.†
K, cf. straight line in the inset of Fig. 6. The t yields an anti-
ferromagnetic Weiss temperature qW ¼ ꢀ(8.2 ꢄ 0.5) K. For a
more precise determination of the dominant magnetic inter-
actions, the temperature dependence of the molar magnetic
susceptibility over the whole temperature range 2 K # T # 270 K
was tted using the Bleaney–Bowers equation26 for isolated
dimers, cf. inset to Fig. 6. The intra-dimer magnetic exchange
coupling constant Jintra between two S ¼ 1/2 spins used in this
expression refers to a Hamiltonian of the form H ¼ ꢀ2J12S1S2,
which was taken also for the DFT calculations. The inset of
Fig. 6 demonstrates that the isolated-dimer model provides a
reasonable description of the overall behaviour of cmol(T) for
the biradical 4c. The biradicals 1c and 3c exhibited a similar
behaviour, in accordance with the theoretical prediction, and
their cmol(T) data were analyzed by an analogous procedure.
Their intra-dimer exchange constants are listed in Table 4.
However, a closer inspection of the low-temperature (T < 4 K)
susceptibility data as a function of temperature, and at T ¼
const. as a function of magnetic eld, for the various biradicals
(see Fig. 7 below for 4c) revealed signicant deviations from the
As an example for the magnetic behaviour of the biradicals
1c–4c, we show the magnetic susceptibility, plotted as cmolT, for
the system 4c in Fig. 6. The data reveal an almost temperature-
independent behaviour in the range from about 100 K to 270 K,
with a value cmolT ¼ (0.734 ꢄ 0.005) cm3 Kmolꢀ1. This value is
very close to the theoretically expected value of 0.75 cm3 Kmolꢀ1
for two independent spin S ¼ 1/2 entities (broken line in Fig. 6).
Very similar values of cmolT ¼ (0.731 ꢄ 0.017) cm3 Kmolꢀ1 were
observed for the biradicals 1c–3c in the same temperature
range. On cooling cmolT becomes progressively reduced below
about 140 K, a clear signature of dominant antiferromagnetic
through-bond interactions between radical centers (intra-dimer
coupling). An estimate of the mean value for the magnetic
exchange coupling in the high-temperature regime can be
obtained by tting the inverse magnetic susceptibility 1/cmol by
a Curie–Weiss model in the temperature range 70K # T # 270
Fig. 7 AC-susceptibility of the biradical 4c at T ¼ 191 mK. The data
feature rounded peaks at B1 z 8.6 T and B2 z 9.2 T.
6624 | J. Mater. Chem. C, 2014, 2, 6618–6629
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Journal of Materials Chemistry C
isolated-dimer model. These deviations were attributed to the be perfectly suited for the requirement of intra-dimer exchange
small magnetic intra- and inter-molecular interactions between coupling J ꢁ 10 K. The structural analyses showed that the small
neighboring biradicals mediated via hydrogen bonds (1c, 3c, torsions between the radical moieties and the modied diac-
4c). The strength of these interactions depends on the distance etylene bridging subunit assist unimpeded through-bond
and relative orientation of the biradicals.27–29
exchange, as well as the small intersheet distances (p-stacking)
The results of AC-susceptibility measurements as a function favour efficient magnetic interactions. Magnetic measurements
of magnetic eld at a constant temperature of 191 mK are and their interpretation revealed that the biradical 2c exhibits
shown in Fig. 7 on example of biradical 4c. The rounded double- surprisingly strong antiferromagnetic interactions, much larger
peak structure in the data, with the peak positions B1 z 8.6 T than predicted for intra-molecular exchange interaction. In all
and B2 z 9.2 T, indicates a eld-induced magnetic state for B1 # other cases weak through-space antiferromagnetic interactions
B # B2. Note that B2 roughly corresponds to the saturation eld are predominant.
Bs. Similar anomalies in the susceptibility are found in other
Summarizing, we found promising candidates for the quest
spin ladder compounds, like (C5H12N)2CuBr4,30 which represent to synthesize a purely organic molecular magnet. Quantum
a class of low-dimensional materials with structural and phys- Monte Carlo simulations for analysing the low-temperature
ical properties lying between those of 1D and 2D systems. While susceptibility data are underway in order to derive the under-
a rounded double-peak structure is expected when the inter- lying effective exchange coupling topology of this compound.
dimer couplings are one- or quasi two- dimensional, sharp Efforts towards the preparation of extended networks through
peaks reect three-dimensional interactions. In fact, a quasi the formation of complexes with different metal ions coordi-
two-dimensional magnetic behaviour for the biradical 4c is also nated to the radical unit and/or p-bridge are presently on-going.
suggested by its crystal structure.
It would be of interest to extend these studies to other series of
In leading order, the elds B1 and B2 (z Bs) are related to tolane-bridged biradicals, in particular the idea to introduce
the dominant Jintra and an averaged Jinter by gmBB1/2 ¼ Jintra
(Jinter aver (see, e.g. ref. 30 and
and gmBB2/2 ¼ Jintra + 2(Jinter aver
references cited therein), resulting in (Jinter aver of the order of
ꢀ
additional chelating sites within the core is considered.
)
)
)
Experimental
General Remarks
ꢀ1 to ꢀ2 K for 4c. These are typical values for magnetic inter-
actions mediated by hydrogen bridges.27–29 Thus, according to
our magnetic characterization, the nitronyl biradical 4c is a All chemicals and reagents were used as received from
promising candidate for a purely organic-based quasi two- commercial sources (Acros Organics, Aldrich, Fluka, Lancaster,
dimensional quantum magnet.
Merck and Strem) without additional purication. Solvents for
Remarkably, the biradical 2c shows signicant deviations synthesis were used as received, unless otherwise mentioned.
from the theoretical expectation, cf. Table 4. For this system the ESR spectra were recorded in dilute, oxygen-free solutions in
maximum of cmol was located at a temperature Tmax ¼ 19.5 K, toluene, concentration ꢁ10ꢀ4 mol L, using a Bruker X-band
distinctly higher than expected from DFT calculations for an spectrometer ESP300 E, equipped with an NMR gaussmeter
isolated single biradical entity. This is different for all other (Bruker ER035), a frequency counter (Bruker ER041XK) and a
biradicals shown in Table 4 where the agreement between variable temperature control continuous ow N2 cryostat
experimental value and theoretical prediction is reasonably (Bruker B-VT 2000). The g-factor corrections were obtained
good. This might be the result of a peculiar magneto-structural using the DPPH (g ¼ 2.0037) as standard. UV-Vis spectra were
correlation inuence and/or strong inter-molecular exchange recorded in toluene solutions with a Perkin-Elmer spectrometer
interactions existing in the material 2c. For instance, attraction (UV/Vis/NIR Lambda 900) using a 1 cm optical path quartz cell
of a neighboring radical to the lone pair of the nitrogen atom of at room temperature, unless otherwise specied.1H and 13C
the pyrazolyl moiety in 2c or stronger orbital overlap between NMR spectra were recorded on a Bruker DPX 250, Bruker DMX
the radical centres might cause such a drastic change in the 300 spectrometer. Solid powders were pressed and IR spectra of
magnetic properties of this biradical. In fact, the magnetic the samples were recorded as they were (Nicolet 730 FT-IR
interactions based on structural peculiarities of this type are spectrometer). Mass spectra were obtained on FDMS, VG
difficult to predict. The strength of these interactions strongly Instruments ZAB-2 mass spectrometer. Elemental analyses were
depends on the relative geometry between the interacting performed at the University of Mainz, Faculty of Chemistry and
magnetic orbitals which is also determined by the crystal Pharmacy on a Foss Heraeus Varieo EL. The melting points were
¨
packing. Better understanding of the origin of such an unex- measured on a Buchi B-545 apparatus (uncorrected) by using
pected behaviour in 2c therefore requires a detailed analysis of open-ended capillaries.ESI.†
the crystal structure. Work on obtaining a suitable crystal for X-
ray measurement is in progress.
2,3-Dimethyl-2,3-bis(hydroxylamino)butane
(BHA).
A
mixture of 17.6 g 2,3-dimethyl-2,3-dinitrobutane and 27.0 g
of Zn (dust) in 300 mL THF and 50 mL H2O was cooled down to
ꢂ
0–5 C in an ice bath. Keeping this temperature constant solu-
Conclusions
tion of 43 g NH4Cl in 150 mL H2O was added dropwise under
In this study a series of novel nitronyl nitroxide biradicals 1c–4c vigorous (mechanical) stirring. Aer addition was completed
coupled via modied tolane linkers was synthesized and their (ca. 2.5 h) the reaction was stirred for 1.5 h at the same
magnetic behaviour was studied. The tolane bridge seemed to temperature, and an additional 0.5 h at room temperature. The
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white-gray precipitate of Zn slurry was ltered off, and was 7.82 (s, 4H, –B(OH)2). 13C-NMR ((CD3)2SO, 63 MHz, 298K, 256
washed with THF (4 ꢃ 20 mL). Filtrate was evaporated to scan), d ppm: 55.6, 116.8, 124.6, 157.4. FAB+ m/z 225.9 [M]+.
ꢂ
viscous residue, cooled to ꢀ15 C (the mixture was kept in the
4-[4-(4-Formylmethyl)-2,5-dimethoxyphenyl]benzaldehyde 1a
freezer for 1 h). Aer that the ask was lled with argon and a (ref. 32). A mixture of 2,5-dimethoxy-1,4-phenylenediboronic acid
mixture of 50 g Na2CO3 and 30 g NaCl salts was added at once. 6 (0.76 g, 3.40 mmol), 4-bromobenzaldehyde (1.24 g, 6.68 mmol),
The ask was shaken for 15 min, and aer homogenization the K2CO3 (23 g), H2O (10 mL) and THF (75 mL) was degassed using
white solid was charged into a Soxhlet apparatus, protected argꢂon bubbling for 30 min. Aer that the mixture was heated to
from air (argon balloon) and extracted with 300 mL of 80 C and solution of Pd(PPh3)4 (0.19 g, 0.167 mmol) in THF (5
dichloromethane (72 h). White precipitate was ltered off, mL) was added through the rubber septum. Light yellow mixture
washed with dichloromethane (2 ꢃ 15 mL), hexane (2 ꢃ 10 mL) was stirred under reux for 120 h. The solvent was removed,
and dried on air. Yield is 6.1 g (42%). M p 160–161 ꢂC. IR residue was mixed with 500 mL of H2O and stored in fridge for 24
(powder, n/cmꢀ1): 3257 (vs. and broad, nOH), 2987 (vs., nC–H), h. Light yellow needles were ltered off, washed with EtOH–H2O
1
1479–1374 (vs., several bands), 1261 (s), 1178 (vs.), 1145 (vs.), (1 : 1) mixture and dried (0.71 g, 62%). H-NMR ((CD3)2SO, 250
1080 (s), 1035 (vs.), 989 (m), 952 (vs.), 904 (vs.), 852 (m), 790 (m), MHz, 298 K, 64 scan), d ppm: 3.80 (s, 6H, –OCH3), 7.12 (s, Ar-H),
690 (m).
7.80, 7.95 (both dd, 4H, 3J ¼ 8.15 Hz, Ar-H), 10.04 (s, 2H, –CHO).
A. General procedure of the condensation reaction between 13C-NMR ((CD3)2SO, 176 MHz, 323 K, 51200 scan), d ppm: 56.4,
2,3-dimethyl-2,3-bis(hydroxylamino)butane with aldehydes. A 114.8, 129.2, 129.4, 130.0, 134.9, 143.8, 150.5, 192.7. FAB+ m/z
suspension of BHA (1.1 eq. for each aldehyde group) and an 346.52 [M]+.
aldehyde (1.0 eq.) in toluene (5 mL mmolꢀ1) was carefully
2-(4-{4-[4-(1,3-Dihydroxy-4,4,5,5-tetramethyl-imidazolidine-
deaerated using argon bubbling for ꢁ20 minutes. The ask was 2-yl)phenyl]-2,5-dimethoxyphenyl}phenyl)-4,4,5,5-tetramethyl-
provided with a condenser containing an argon balloon, and imidazolidine-1,3-diol 1b. Compound 1b was synthesized
moved into an oil bath. The mixture was heated under argon following the method A as slightly yellowish crystals in 97%
near reux for 2–18 h. The progress of the reaction was moni- yield (170 mg). 1H-NMR ((CD3)2SO, 250 MHz, 298 K, 64 scan), d
tored by TLC (SiO2, hexane/ethyl acetate or dichloromethane ppm: 1.09, 1.11 (both s 12H, –CH3), 3.77 (s, 6H, –OCH3), 4.57 (s,
with 5% of methanol). Aer the process was completed and the 2H, –CH–), 7.01 (s, 2H, Ar-H), 7.53 (s, 8H, Ar-H), 7.80 (s, 4H, –N–
ask was cooled down till room temperature formed precipitate OH). 13C-NMR ((CD3)2SO, 63 MHz, 298 K, 256 scan), d ppm: 17.3,
(white or yellowish) was ltered off, washed with toluene (2 ꢃ 10 24.5, 56.2, 66.1, 90.2, 114.5, 128.3, 128.6, 129.7, 137.0, 140.8,
mL), heptane (1 ꢃ 10 mL) and dried on air. The product was 150.3. FAB+ m/z 606.74 [M]+.
used for the next step without additional purication.
B. General procedure for oxidation of 4,4,5,5-tetramethyli- lin-2-yl)phenyl]-2,5-dimethoxyphenyl}phenyl)-4,4,5,5-tetramethyl-
midazolidine-1,3-diol derivatives to nitronyl nitroxides. imidazoline-1-oxyl-3-oxo 1c. Chromatography purication on
2-(4-{4-[4-(1-Oxyl-3-oxo-4,4,5,5-tetramethyl-4H,5H-imidazo-
A
suspension of 4,4,5,5-tetramethylimidazolidine-1,3-diol (1 silica gel with CH2Cl2–MeOH (30 : 1) eluent afforded dark-blue
mmol) in H2O–CH2Cl2 (1 : 2) mixture (ꢁ50 mL) was cooled prisms of biradical 1c in 54% yield (0.15 g). UV-Vis (toluene): l/nm
down to 0–5ꢂC (to avoid overoxidation and formation of imino (3/Mꢀ1 cmꢀ1) 627 (557), 356 (20425). (Calc. for C34H40N4O6: C,
nitroxide species) using an ice bath. To that mixture NaIO4 5% 67.98; H, 6.71; N, 9.33C. Found: C, 67.90; H, 6.66; N, 9.43%).
aqueous solution (0.8 eq.) was added dropwise. Aer reaction
1-[4-(1H-pyrazol-1-yl)phenyl]-1H-pyrazole
8
(ref. 33).
A
was complete (0.5–3 h), dark-blue organic layer was separated. mixture of 1,4-dibromobenzene 7 (2.37 g, 0.01 mol), pyrazole
The aqua phase was extracted with dichloromethane (3 ꢃ 25 (6.71 g, 0.10 mol), CuI (4.95 g, 0.026 mol) and K2CO3 (16.58 g,
mL). The combined organic extracts were additionally washed 0.12 mol) in PhNO2 (20 mL) was reuxing for 3 h. Solvent was
with water (2 ꢃ 30 mL), NaCl saturated aqueous solution (1 ꢃ removed in vacuo, and the residue was washed with CH2Cl2 (3 ꢃ
50 mL), dried over MgSO4 (in some cases – Na2SO4). The residue 150 mL). Filtrate was evaporated on rotary and puried on a
aer ltration was evaporated and puried using a chromato- chromatographic column (SiO2, CH2Cl2), giving aer evapora-
graphic column (SiO2, hexane/ethyl acetate or CH2Cl2/MeOH).
tion of the solvents 1.6 g of the colorless needles of compound 8
1
2,5-Dimethoxy-1,4-phenyldiboronic acid 6.31 To a solution of (76%). H-NMR (CDCl3, 250 MHz, 298 K, 64 scan), d ppm: 6.50
1,4-dibromo-1,5-dimethoxybenzene 5 (2.96 g, 10.0 mmol) in dry (dd, 2H, J ¼ 1.62, J ¼ 2.39 Hz), 7.75 (d, 2H, J ¼ 1.62 Hz), 7.80 (s,
THF (100 mL) n-BuLi (13.3 mL, 21.25 mmol, of 1.6 M in hexane) 4H, Ar-H), 7.95 (d, 2H, J ¼ 2.39 Hz). 13C-NMR (CDCl3, 63 MHz,
was added dropwise in 30 minutes at ꢀ78 ꢂC. The mixture was 298 K, 274 scan), d ppm: 107.9, 120.0, 126.7, 138.4, 141.3. FAB+
stirred during 5 h with cooling, and B(Oi-Pr)3 (10 mL, 43.4 m/z 210.10 [M]+.
mmol) was added slowly keeping the temperature constantly at
4-Iodo-1-[4-(4-iodo-1H-pyrazol-1-yl)phenyl]-1H-pyrazole 9. A
ꢀ78 ꢂC. The resulted solution was stirred overnight and allowed mixture of 1-[4-(1H-pyrazol-1-yl)phenyl]-1H-pyrazole 8 (1.47 g,
to warm slowly to room temperature. The mixture was decom- 7.0 mmol) and 30% H2SO4 (2.5 mL) in conc. AcOH (20 mL) was
posed with 5% HCl (20 mL) and THF was removed in vacuo. heated up to 80 C. To this mixture a solution of I2 (1.42 g, 5.6
ꢂ
White crystalline residue was ltered off and washed with large mmol), HIO3 (0.49 g, 2.8 mmol) in AcOH (400 mL) and H2SO4 (2
amount of water. Compound 6 was recrystallized from H2O– mL) was added dropwise within 60 min. The resulted red-brown
CH3CN (1 : 1, 100 mL) mixture, affording white needles of the solution was heated for 60 h at the same temperature. During
1
product 6 in 45% yield (1.02 g). H-NMR ((CD3)2SO, 250 MHz, reaction colourless crystals were formed. Reaction mixture was
298 K, 16 scan), d ppm: 3.78 (s, 6H, –OCH3), 7.16 (s, 2H, Ar-H), cooled to ambient temperature, and sodium thiosulfate was
6626 | J. Mater. Chem. C, 2014, 2, 6618–6629
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Journal of Materials Chemistry C
added until the solution became colourless. Crystalline solid neutralized with NaHCO3 aqueous solution and the phases were
was ltered off and washed with AcOH (3 ꢃ 5 mL), water (3 ꢃ 10 separated. The aqueous layer was extracted with small portions
mL), NaHCO3 saturated aqueous solution (3 ꢃ 5 mL), and EtOH of toluene (3 ꢃ 10 mL) and the combined organic layers were
(2 ꢃ 5 mL). Drying compound on air resulted in 1.9 g (59%) of collected, dried over MgSO4 and ltrated. Toluene solution was
the titled derivative 9 (in addition 0.62 g (26%) of monoiodo evaporated under reduced pressure. Crude orange oil was
derivative was isolated from the ltrate). 1H-NMR ((CD3)2SO, puried using column chromatography (SiO2, dichloro-
250 MHz, 298 K, 64 scan), d ppm: 7.86 (w. s, 2H), 7.95 (w. s, 4H, methane). Compound 11 was obtained as slightly yellow oil in
1
Ar-H), 8.76 (w. s, 2H). 13C-NMR ((CD3)2SO, 63 MHz, 298 K, 256 85% yield (2 g). H-NMR (CDCl3, 250 MHz, 298 K, 16 scan), d
scan), d ppm: 101.7, 119.4, 132.2, 142.2, 145.9. FAB+ m/z 462.03 ppm: 3.02 (s, 1H), 3.99 (m, 4H, -CH2), 6.73 (s, 1H), 7.91 (dd, 4H,
[M]+. (Calc. for C12H8I2N4: C, 31.19; H, 1.75; N, 12.13. Found: C, 3J ¼ 8 Hz, Ar-H). 13C-NMR (CDCl3, 63 MHz, 298K, 256 scan), d
31.08; H, 1.16; N, 12.03%).
ppm: 67.2, 82.4, 84.2, 103.9, 123.2, 127.2, 134.5, 137.8. FAB+ m/z
4-Formyl-1-[4-(4-formyl-1H-pyrazol-1-yl)phenyl]-1H-pyrazole 173.0 [M]+.
2a. A suspension containing starting compound 9 (1.4 g, 3.03
1,4-Bis(4-(1,3-dioxolan-2-yl)phenyl)buta-1,3-diyne 3. Glaser
mmol) in THF (50 mL) was subjected reaction with EtMgBr (6.3 coupling.14 A mixture of 2-(4 ethynyl phenyl)-1,3-dioxolane 11
mL, 6.36 mmol, 1 M in THF) under argon. The mixture was (3.57 g, 20.52 mmol), CuCl (14 g, 141.4 mmol), CuCl2 (14.3 g,
stirring at 60ꢂC for 1 h, and then solution of dry DMF (0.5 mL, 106.3 mmol) and dry pyridine (120 mL) was stirred at room
6.36 mmol) in dry THF (1 mL) was added dropwise. Cold light temperature for 24 hours; then it was washed with 10% HCl (1
yellow reaction mixture was decomposed with NH4Cl saturated ꢃ 40 mL) and extracted with Et2O (5 ꢃ 60 mL). The combined
aqueous solution (20 mL). Excess of THF was evaporated, water organic extracts were washed with NH4Cl saturated solution (to
residue was extracted with EtOAc (4 ꢃ 100 mL). The combined remove excess of copper) and dried over MgSO4. As the main
organic fractions were dried over Na2SO4, ltrated and solvent product 1,4-bis(4-(1,3-dioxolan-2-yl)phenyl)buta-1,3-diyne 3 was
was removed in vacuo. Yellow precipitate was washed with isolated from the reaction mixture using column chromatog-
1
hexane (3 ꢃ 15 mL) and dried on air. Obtained 0.71 g of dia- raphy (SiO2, dichloromethane) in 60% yield (4.26 g). H-NMR
ldehyde 2a (89%). 1H-NMR ((CD3)2SO, 250 MHz, 298 K, 64 scan), (CDCl3, 250 MHz, 298 K, 16 scan), d ppm: 3.95 (m, 8H, –CH2),
d ppm: 8.06 (w. s, 2H), 8.28 (w. s, 4H, Ar-H), 9.26 (w. s, 2H), 9.92 5.77 d (s, 2H), 7.38 d (d, 4H, 3J ¼ 8 Hz), 7.45 d (d, 4H, 3J ¼ 8 Hz).
(w. s, 2H, –CHO). 13C-NMR ((CD3)2SO, 63 MHz, 298K, 256 scan), 13C-NMR (CDCl3, 63 MHz, 298 K, 256 scan), d ppm: 66.5, 73.3,
d ppm: 120.3, 125.6, 132.4, 137.6, 141.5, 184.9. FAB+ m/z 266.28 82.4, 103.1, 122.6, 126.8, 133.1, 138.9. FAB+ m/z 346.2 [M]+.
[M]+. (Calc. for C14H10N4O2: C, 63.15; H, 3.79; N, 21.04. Found:
C, 63.18; H, 3.68; N, 21.13%).
4,40-(Buta-1,3-diyne-1,4-diyl)dibenzaldehyde 3a (ref. 34).
Solution of 3 (1.78 g, 5.14 mmol) in THF (50 mL) was cooled to
ꢂ
2-(1-{4-[4-(1,3-Dihydroxy-tetramethyl-imidazolidine-2-yl)-1H- 5 C using an ice bath, and 3% HCl (30 mL) was added. The
pyrazol-1-yl]phenyl}-1H-pyrazole-4-yl)-4,4,5,5-tetramethyl-imi- mixture was stirred for 45 min the ice bath and additional 1 h at
dazolidine-1,3-diol 2b. The condensation was realized using room temperature. Aer that THF was evaporated. The residue
procedure A: starting from 0.2 g (0.75 mmol) of dialdehyde 2a was diluted with Et2O (60 mL), washed with water (3 ꢃ 35 mL)
and 0.27 g of BHA (1.84 mmol) 0.34 g of the imidazolidine 2b and dried over MgSO4. Solvent was evaporated under reduced
was obtained (87%). 1H-NMR ((CD3)2SO, 250 MHz, 298 K, 64 pressure. Dialdehyde 3a was obtained as light yellow solid in
scan), d ppm: 1.06, 1.08 (both s 12H, –CH3), 4.67 (s, 2H,–CH–), 86% yield (1.15 g). 1H-NMR ((CD3)2SO, 250 MHz, 298 K, 16
3
3
7.71 (s, 2H), 7.81 (w. s, 4H, –NOH), 7.90 (s, 4H, Ar-H), 8.38 (s, scan), d ppm: 7.91 (d, 4H, J ¼ 8 Hz), 8.03 (d, 4H, J ¼ 8 Hz),
2H). 13C-NMR ((CD3)2SO, 63 MHz, 298 K, 256 scan), d ppm: 17.4, 10.12 (s, 2H). 13C-NMR ((CD3)2SO, 75 MHz, 353 K, 1801 scan), d
23.9, 65.0, 83.1, 119.0, 125.4, 126.7, 137.5, 140.9. FAB+ m/z ppm: 75.8, 82.1, 125.5, 129.1, 132.7, 136.3, 191.7. FAB+ m/z 258.0
526.63 [M]+.
2-(1-{4-[4-(1-Oxyl-3-oxo-4,4,5,5-tetramethyl-4H,5H-imidazo-
[M]+.
2-(4-{4-[4-(1,3-Dihydroxy-4,4,5,5-tetramethyl-imidazolidine-
lin-2-yl)-1H-pyrazol-1-yl]phenyl}-1H-pyrazole-4-yl)-4,4,5,5-tetra- 2-yl)phenyl]buta-1,3-diyn-1-yl}phenyl)-4,4,5,5-tetramethyl-imi-
methyl-imidazoline-1-oxyl-3-oxo 2c. Compound 2c was dazolidine-1,3-diol 3b. Following protocol A dibenzaldehyde 3a
synthesized using methodology B from bisimidazolidine 2b (0.3 g, 1.19 mmol) was converted into imidazolidine 3b aer 3 h
(0.50 g, 0.95 mmol) in good yield (0.30 g, 61%) as dark-blue ne of reuxing with BHA in toluene in 97% yield (0.58 g). 1H-NMR
needles: UV-Vis (CHCl3): l/nm (3/Mꢀ1 cmꢀ1) 599 (3804), 654 ((CD3)2SO, 250 MHz, 298 K, 128 scan), d ppm: 1.28 (d, 24H,
(4011), 321 (69198), 366 (31143). FT-IR (powder, n/cmꢀ1) 3179, CH3), 4.84 (s, 2H, –CH–), 7.92 (d, 4H, 3J ¼ 8 Hz), 8.11 (d, 4H, 3J ¼
3164 (w, nC–H, aromatic), 2985, 2930 (m, nC–H), 1680 (s), 1601 (m, 8 Hz). 13C-NMR ((CD3)2SO, 63 MHz, 298K, 256 scan), d ppm: 23,
nPh), 1528 (vs.), 1450 (m), 1425 (s), 1417 (s), 1400 (m), 1386 (vs., 69.7, 74.9, 82.3, 103.1, 123.6, 130.1, 134.0, 138.6.
nmethyl), 1354 (vs., nNO), 1332, 1314 (vs., pyrazolyl-moiety), 814
2-(4-{4-[4-(1-Oxyl-3-oxo-4,4,5,5-tetramethyl-4H,5H-imidazo-
(vs., np-sub-Ph). (Calc. for C26H32N8O4: C, 59.99; H, 6.20; N, 21.52. lin-2-yl)-phenyl]buta-1,3-diyn-1-yl}phenyl)-4,4,5,5-tetramethyl-
Found: C, 60.18; H, 6.16; N, 21.34%).
imidazoline-1-oxyl-3-oxo 3c. Biradical 3c was obtained in 24ꢀ%1
2-(4-Ethynyl phenyl)-1,3-dioxolane 11. 4-Ethynylbenzalde- yield (0.140 g). M p 220–221 ꢂC. UV-Vis (CHCl3): l/nm (3/M
hyde 10 (1.76 g, 13.53 mmol) was charged into a ask together cmꢀ1) 607 (912), 349 (33844); (toluene) l/nm (3, molꢀ1ꢃ cmꢀ1
)
with ethyleneglycol (1.4 mL, 25.15 mmol), para-toluene-sulfonic 622 (578), 352 (71875). FT-IR (powder, n/cmꢀ1) 3099, 3049
acid (0.2 g, 1 mmol) and toluene (60 mL). The solution was (w, nC–H, aromatic), 2991, 2940 (m, nC–H), 1602 (m, nPh), 1543 (w),
heated to reux with Deane-Stark for one day. Then it was 1522 (m), 1481 (w), 1447 (m), 1428 (s), 1427(s), 1418 (vs.), 1386
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(vs., nmethyl), 1365 (vs., nNO), 1302 (vs.), 1212 (s), 1164 (s), 1141 (s),
1130 (vs.), 831 (vs, np-sub-Ph). (Calc. for C30H32N4O4: C, 70.29; H,
6.29; N, 10.93. Found: C, 70.30; H, 6.25; N, 10.90%).
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4-[2-(4-Formylphenyl)ethynyl]benzaldehyde 4a (ref. 35).
Starting compounds 4-ethynylbenzaldehyde 10 (0.2 g, 1.54
mmol), 4-bromobenzaldehyde 12 (0.284 g, 1.54 mmol) were
transferred to a ame-dried ask in argon stream. Dry solvents
mixture – DMF–NEt3 (10 mL, 1 : 1) was added through the
rubber septum. Solution was degassed using argon bubbles for
20 min, and then catalytic mixture of Pd(PPh3)2Cl2 (54 mg, 0.077
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raphy (SiO2, CH2Cl2) afforded the titled compound 4a in 92%
yield (0.33 g). 1H-NMR (CDCl3, 250 MHz, 298 K, 64 scan), d ppm:
7.71, 7.90 (both dd 2H, 3J ¼ 8.2 Hz, Ar-H), 10.04 (s, 2H, –CHO).
13C-NMR (THF-d8, 75 MHz, 298 K, 1725 scan), d ppm: 92.4,
129.0, 130.1, 132.9, 137.5, 191.3. FAB+ m/z 234.063 [M]+.
´
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2-(4-{2-[4-(1,3-Dihydroxy4,4,5,5-tetramethyl-imidazolidine-
2-yl)phenyl]ethynyl}phenyl)-4,4,5,5-tetramethyl-imidazolidine-
1,3-diol 4b. Following method A from 0.45 g (1.94 mmol) of
dialdehyde 4a bisimidazolidine 4b was synthesized as 0.94 g
1
(98%) of yellow powder. H-NMR ((CD3)2SO, 250 MHz, 298 K,
105 scan), d ppm: 1.05, 1.09 (both s 12 H, –CH3), 4.54 (s, 2H,
–CH–), 7.53 (w. s, 8H, Ar-H), 7.84 (w. s, 4H, –N–OH). 13C-NMR
((CD3)2SO, 63 MHz, 298 K, 256 scan), d ppm: 17.4, 24.5, 66.4, 10 S. K. Pal, M. E. Itkis, F. S. Tham, R. W. Reed, R. T. Oakley and
88.2, 90.1, 121.6, 128.2, 131.0, 143.5. FAB+ m/z 494.293 [M]+.
2-(4-{2-[4-(1-Oxyl-3-oxo-4,4,5,5-tetramethyl-4H,5H-imidazo-
lin-2-yl)phenyl]-ethynyl}phenyl)-4,4,5,5-tetramethyl-imidazo-
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611 (868), 342 (59101). FT-IR (powder, n/cmꢀ1) 3100 (w, nC–H
aromatic), 2989, 2943 (m, nC–H), 1677 (m), 1606 (s, nPh), 1530 (w),
1484 (w), 1447 (m), 1422 (s), 1390(s, nmethyl), 1365, 1356 (vs., nNO),
1301 (vs.), 1211 (s), 1166 (s), 1131 (vs.), 833 (vs., np-sub-Ph). (Calc.
for C28H32N4O4: C, 68.83; H, 6.60; N, 11.47. Found: C, 68.67; H,
6.56; N, 11.37%).
)
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Acknowledgements
The authors are pleased to acknowledge continued support
from the Max Planck Society. This work was also supported by
SFB TR49. We warmly thank Dr Giorgio Zopellaro for lively and
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