Synthesis and magnetic properties of stable radical derivatives carrying a phenylacetylene unit

Article abstract of DOI:10.3390/molecules23020371

A nitronyl nitroxide derivative, 2-phenylethynyl-4,4,5,5-tetramethyl-4,5-dihydro-1Himidazol- 1-oxyl-3-oxide (1), and two verdazyl derivatives carrying a phenylacetylene unit, 1,5-diphenyl-3-phenylethynyl-6-oxo-1,2,4,5-tetrazin-2-yl (2) and 1,5-diisopropyl-3-phenylethynyl- 6-oxo-1,2,4,5-tetrazin-2-yl (3), were synthesized and their packing structures were studied by X-ray crystallographic analysis and magnetically characterized in the solid state. While 1 and 3 had an isolated doublet spin state, 2 formed an antiferromagnetically coupled pair (2J/kB = -118 K). Density functional theory (DFT) calculations reveal that the spin density polarized in the phenyl group decreases as the dihedral angle between the phenyl ring and radical plane increases.

Full text of DOI:10.3390/molecules23020371

molecules
Article
Synthesis and Magnetic Properties of Stable Radical
Derivatives Carrying a Phenylacetylene Unit
ID
Shogo Miyashiro, Tomoaki Ishii, Youhei Miura and Naoki Yoshioka *
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Yokohama 223-8522,
Japan; jiro38460506@gmail.com (S.M.); tomoaki3272@gmail.com (T.I.); y-miura@applc.keio.ac.jp (Y.M.)
* Correspondence: yoshioka@applc.keio.ac.jp; Tel.: +81-45-566-1585
Received: 6 January 2018; Accepted: 7 February 2018; Published: 9 February 2018
Abstract: A nitronyl nitroxide derivative, 2-phenylethynyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-
imidazol-1-oxyl-3-oxide (
1,5-diphenyl-3-phenylethynyl-6-oxo-1,2,4,5-tetrazin-2-yl (
6-oxo-1,2,4,5-tetrazin-2-yl ( ), were synthesized and their packing structures were studied by X-ray
crystallographic analysis and magnetically characterized in the solid state. While and had an
isolated doublet spin state, formed an antiferromagnetically coupled pair (2J/kB = 118 K). Density
1), and two verdazyl derivatives carrying a phenylacetylene unit,
2) and 1,5-diisopropyl-3-phenylethynyl-
3
1
3
2
−
functional theory (DFT) calculations reveal that the spin density polarized in the phenyl group
decreases as the dihedral angle between the phenyl ring and radical plane increases.
Keywords: stable radical; phenylacetylene; nitronyl nitroxide; verdazyl; magnetic property;
computational analysis
1. Introduction
The study of stable organic radicals has attracted a great deal of attention because of their
wide applicability for spin probes, molecule-based magnets, and molecular conductors [1–8].
To assess the magnetic properties of organic radical solids, it is important to investigate the
correlation between molecular arrangement and magnetic properties based on crystal engineering
strategies [
van der Waals interactions [
stackings [11 15 24 27] have been introduced to various stable radical derivatives such as phenoxyl,
nitroxyl, etc. and their effects have been examined. As for verdazyl radical derivatives, strong
intermolecular magnetic interactions induced by aromatic stackings have been reported [28 35].
9
–
27]. For the purpose of inducing the formation of magnetic molecular self-assemblies,
9,
10 12 14 18], hydrogen bonds [16 17 19 23 25 26], and aromatic
,
–
,
, , – , ,
,
, ,
–
In addition to the control of intermolecular interactions, introduction of specific substituents is
also a useful tool for design of molecular packing. The ethynyl group has a rigid linker, and it
can also propagate magnetic interaction through the conjugate system and freely rotate around
the bond. In the field of crystal engineering, the free rotation of ethynyl group is widely used for
the construction of a supramolecular architecture [36
and computational study [43 45] of ethynyl-substituted derivatives were reported, a limited
number of papers have discussed their magneto-structural correlation [33 34 42]. We synthesized
three derivatives, 2-phenylethynyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide ( ),
1,5-diphenyl-3-phenylethynyl-6-oxo-1,2,4,5-tetrazin-2-yl ( ), and 1,5-diisopropyl-3-phenylethynyl-
6-oxo-1,2,4,5-tetrazin-2-yl ( ) (Chart 1). While the synthesis of has already been reported [38],
structural analysis and solid-state magnetic characterization were not conducted. In this paper,
,37]. Although the synthetic study [38–42]
–
,
,
1
2
3
1
we described the crystal structures and magnetic properties of
1
,
2
and
3. To elucidate the effect
of a directly substituted ethynyl group, the spin densities of
computational method.
1
,
2
and
3 were estimated using a
Molecules 2018, 23, 371; doi:10.3390/molecules23020371
www.mdpi.com/journal/molecules

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Chart 1. The chemical structures of 1, 2 and 3.
2. Results
2.1. Synthesis and Electron Paramagnetic Resonance Spectra
The syntheses of compounds are summarized in Scheme 1. Typically, 2,3-bis(hydroxyamino)-
2,3-dimethylbutane sulfate reacts with an aldehyde and the precursor of nitronyl nitroxide (NN) is
1–3
obtained, but this method did not lead to
group [40]. Therefore, the synthesis route of
1
1
due to the reaction between ethynyl and the hydroxy
was basically followed by Ullman’s route through alkene
and dehydrobromide compounds [38]. To obtain the verdazyl (VZ) derivatives
derivatives protected by Boc, and 10 46 48] were synthesized from 49] and
were synthesized by the
2
and
3, the hydrazine
8
[
–
6
[
9 [50], respectively.
They were combined with the aldehyde derivative [51,52] and 2 and 3
oxidation of 11 and 12, respectively.
Scheme 1. Synthesis of 1, 2, and 3.
The solution electron paramagnetic resonance (EPR) spectra of these radicals exhibited typical
patterns for NN and VZ derivatives (Figure S5). The hyperfine coupling constants (hfcc) and g value
of compounds 1–3 are summarized in Table 1.
Table 1. EPR parameters of 1, 2 and 3 in toluene.
αN(G)
g
1
2
3
7.32, 7.35
4.62, 4.64, 6.39, 6.40
5.29, 5.61, 6.52, 6.56
2.0062
2.0033
2.0031

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2.2. X-ray Structural Analysis
The single crystal of
= 5:3) and it was a blue plate-like crystal. The X-ray structure analysis of
system was monoclinic and the space group was Cc (Table S1) [53]. Figure 1 shows an ORTEP drawing
of
and the dihedral angle of the phenyl ring to the NN group was determined to be 77.81(12)◦,
1
was obtained by slow evaporation of the solvent (dichloromethane:n-hexane
1
revealed that the crystal
1
which is a very big torsion. Three types of intermolecular close arrangements were confirmed, labeled
Pair I, Pair II, and Pair III (Figure 2). In Pair I, the O atom of NN and two H atoms of the methyl group
were the closest sites. On the other hand, the O atom of NN and H atom of the phenyl group were the
closest to Pair II and Pair III, respectively.
Figure 1. ORTEP drawing of 1. Thermal ellipsoids are shown as 50% probability.
Figure 2. Crystal structure of 1.
The slow evaporation of a mixed solvent (ethyl acetate:n-hexane = 1:1) formed the single crystal of
2
as a dark red plate-like crystal. The crystal system and space group were triclinic and P-1, respectively
(Table S1) [53]. Figure 3 shows the ORTEP drawing of . The dihedral angle between the VZ ring
and phenyl group at positions 1 and 5 were 41.68(9)◦ and 50.08(6)◦, respectively, but for the phenyl
group at position 3, it was 8.26(11)◦.
had an antiparallel stacking and two intermolecular contacts
(Figure 4). The intermolecular distances of Pair IV and Pair V were 3.343(2) Å for dN1-N4’ and 3.710(3)
Å for dC15-C16’
2
2
.
Figure 3. ORTEP drawing of 2. Thermal ellipsoids are shown as 50% probability.

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Figure 4. Crystal structure of 2.
The single crystal of 3 was obtained by slow evaporation of a mixed solvent (dichloromethane:
n-hexane = 1:2) as orange needles. The crystal system and space group were monoclinic and P21/n,
respectively [53]. Details of the crystallographic data are listed in Table S1. The crystal packing
of
3 was composed of three different molecules labeled A, B, and C in Figure 5. Dihedral angles
◦
◦
◦
between the phenyl group and VZ unit were 7.42(25) (A), 3.88(25) (B), and 46.02(13) (C). There was
linear stacking between the same structural molecules; A, B, and C made Pair VI, Pair VII, and Pair
VIII (Figure 6), respectively. The intermolecular distances were 3.701(5) Å for dC2-C11’, 3.681(6) Å for
d
C18-C27’, and 4.171(6) Å for dC34-C43’ in Pair VI, Pair VII, and Pair VIII, respectively.
Figure 5. ORTEP drawing of 3. Thermal ellipsoids are shown as 50% probability.
Figure 6. Crystal structure of 3.

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2.3. Magnetic Properties
The solid-state magnetic susceptibility measurements of
1,
2, and
3
were carried out by a SQUID
magnetometer in the temperature region of 1.8–300 K. The χmT-T and χm-T plots for
1 and 3
(Figure 7) exhibit a similar pattern. Cooling did not cause any remarkable changes in the χmT values,
−1
which indicated that they were in an isolated doublet-state. The χm
their Weiss temperatures were 0.13 K and
-T plots of
1
and
3
showed that
had a
−
−
1.0 K, respectively. However, the χm value of
2
maximum at 80 K and the minimum at 15 K, and the χmT value decreased with cooling, meaning that the
antiferromagnetic interaction was dominant. Fitting with the antiferromagnetic dimer model revealed
that the magnetic interaction, 2J/kB, of 2 was −118 K, containing 0.5% of the isolated component.
(a)
(b)
(c)
Figure 7. Temperature dependence of χmT (
field of 10000 Oe (1 and 2) or 5000 Oe (3) (1.8–300 K).
#
) and χm
(ꢀ
) for
1(a),
2(b
) and 3(c) under the applied
Antiferromagnetic dimer model equation


2
NAg2µB
1
1
4


ꢀ
ꢁ
χm
=
(1 − ρ) +
ρ
2J
B
kBT
3 + exp −k T
χm: magnetic susceptibility; NA: Avogadro constant; g: g factor; µB: Bohr magneton;
kB: Boltzmann constant; ρ: isolated component.
3. Discussion
The magnetic interactions of the abovementioned pairs were calculated by DFT at the
UB3LYP/6-31G* level, and the calculation results are substituted into Yamaguchi’s equation [54
(Table 2). In and , the value of J/kB for all the pairs was nearly zero; these results agreed with
]
1
3
the SQUID measurement. In these pairs, the C, N, and O atoms of the radical ring that have a large
spin density were not close to the C, N, and O atoms of the neighboring radical ring. However,
Pair V had a low value of J/kB, and Pair IV had a large negative value that was thought to cause the

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antiferromagnetic behavior of the SQUID measurement. It was suggested that the antiferromagnetic
interaction was caused by the intermolecular interaction between the N1 and N4 atoms (x, y, z)
that have large spin density and the N1’ and N4’ atoms of neighboring molecules (1
The values of the spin density of the atoms that form the six-membered ring and phenylacetylene
−
x, 1
−
y, 1 z).
−
group of 1–3 are shown in Tables S2–S6, respectively.
Table 2. Estimation of intermolecular magnetic interactions of 1, 2, and 3.
1
2
3
Pair I
Pair II
0.190
Pair III
0.0632
Pair IV
Pair V
Pair VI
Pair VII
−2.91
Pair VIII
2J/kB (K)
0.190
−
−79.4
−1.58
−1.26
−0.948
To estimate the influence of the rotation of the ethynyl group on the extent of the spin density,
the molecules that were based on the crystal structures and rotated between the phenyl group and
radical ring at 15-degree intervals (0–90◦) were calculated by DFT. The sum of the spin density that
was taken as an absolute value was used in the plots of Figure 8. The spin density of the radical group
and the ethynyl group remained almost constant; however, that of the phenyl group decreased with
rotation. This result indicated that the spin density was difficult to expand at the higher dihedral angle
of ethynyl like other conjugated systems.
(a)
(b)
(c)
Figure 8. Correlation between dihedral angle and spin density of
the sum of the ethynyl and phenyl(#), ethynyl(4), phenyl(3) and experimental data(•, • and •).
1
(
a
),
2
(
b
) and
3
(
c
); Radical group( ),
×

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4. Experimental
Compounds
1,
4, and
5
[38],
6
[49],
9
and 10
[50], phenylpropargylaldehyde [48] were synthesized
with reference to the respective papers. The details of the methods for synthesizing the other
compounds are summarized in the Supplementary Materials.
The NMR spectra were measured by JEOL JNM-LA300 (Tokyo, Japan) (tetramethylsilane was used
as the reference). Mass spectroscopy was carried out using a Bruker Ultraflex II (MALDI-TOF, with sinapic
acid used as the matrix) (Billerica, MA, USA). The IR spectra were measured using a JASCO FT/IR-4100
(Tokyo, Japan). The crystal data were collected using a Bruker D8 Venture with Mo–Kα radiation (0.71073 Å).
The structures were solved by the direct method using SHELXT-2013 (Göttingen, Germany) [55] and refined
by F2 full matrix least squares using SHELXL-2014 (Göttingen, Germany) [56] in the Bruker APEX-II
program package. The magnetic susceptibility measurement was carried out using a Quantum Design
MPMS-XL SQUID (San Diego, CA, USA) magnetometer in the temperature range of 1.8–300 K under the
applied field of 10000 Oe (1 and 2) or 5000 Oe (3). The EPR spectra were recorded using a Bruker E500
spectrometer (Billerica, MA, USA) at room temperature. The EPR data were simulated by the Winsim
ver. 0.96 program (Bethesda, MD, USA) [57]. The DFT calculation was carried out using the Gaussian 09
program (Wallingford, CT, USA) [58].
5. Conclusions
As stable organic radicals carrying an ethynyl group, the nitronyl nitroxide derivative
verdazyl derivatives and were synthesized. X-ray crystal analysis revealed their packing structures,
and had several types of close intermolecular contacts. The magnetic susceptibility measurement
revealed that and had an isolated doublet-state and was an antiferromagnetic compound that
fitted with 2J/kB = 118 K containing 0.5% of the isolated component. DFT calculations of , and
1 and
2
3
1–3
1
3
2
−
1
,
2
3
agreed with the results of the magnetic susceptibility measurements, and indicated that the spin
density of the phenyl group decreased due to the free rotation of the acetylene unit.
Supplementary Materials: The Supplementary Materials are available online: supplementary materials contain
synthetic procedure, EPR spectra, crystallographic parameter, and distributions of SOMO and spin density.
Acknowledgments: This work was partly supported by the MEXT Supported Program for the Strategic Research
Foundation at Private Universities, 2012–2016.
Author Contributions: Shogo Miyashiro wrote the draft. He and Tomoaki Ishii contributed to the synthesis
of the radical derivatives and X-ray diffraction analyses and physicochemical characterizations. Youhei Miura
contributed support to syntheses and characterizations. Naoki Yoshioka contributed to the conception and
revision of the submitted manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
Lahti, P.M. (Ed.) Magnetic Properties of Organic Materials; Marcel Dekker: New York, NY, USA, 1999;
ISBN 9780824719760.
Miller, J.S.; Drillon, M. (Eds.) Magnetism: Molecules to Materials; Wiley–VCH: Weinheim, Germany, 2005;
Volume I–V. [CrossRef]
Hicks, R.G. (Ed.) Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds; Wiley:
Chichester, UK, 2010; ISBN 978-0-470-77083-2.
Chatgilialoglu, C.; Studer, A. (Eds.) Encyclopedia of Radicals in Chemistry, Biology and Materials; Wiley:
Hoboken, NJ, USA, 2012.
Koivisto, B.D.; Hicks, R.G. The magnetochemistry of verdazyl radical-based materials. Coord. Chem. Rev.
2005, 249, 2612–2630. [CrossRef]
Vostrikova, K.E. High-spin molecules based on metal complexes of organic free radicals. Coord. Chem. Rev.
2008, 252, 1409–1419. [CrossRef]
Retera, I.; Veciana, J. Playing with organic radicals as building blocks for functional molecular materials.
Chem. Soc. Rev. 2012, 41, 303–349. [CrossRef] [PubMed]

Molecules 2018, 23, 371
8 of 10
8.
9.
Lipunova, G.N.; Fedorchenko, T.G.; Chupakhin, O.N. Verdazyls: Synthesis, properties, application.
Russ. Chem. Rev. 2013, 82, 701–734. [CrossRef]
Mukai, K. Anomalous magnetic properties of stable crystalline phenoxyl radicals. Bull. Chem. Soc. Jpn. 1969
42, 40–46. [CrossRef]
,
10. Chouteau, G.; Veyret-Jeandey, C. Metamagnetism in tanol suberate. J. Phys. Paris 1981, 42, 1441–1444.
[CrossRef]
11. Sugawara, T.; Murata, S.; Kimura, K.; Iwamura, H.; Sugawara, Y.; Iwasaki, H. Design of molecular assembly
of diphenylcarbenes having ferromagnetic intermolecular interactions. J. Am. Chem. Soc. 1985, 107,
5293–5294. [CrossRef]
12. Awaga, K.; Sugano, T.; Kinoshita, M. Ferromagnetic intermolecular interactions in a series of organic mixed
crystals of galvinoxyl radical and its precursory closed shell compound. J. Chem. Phys. 1986, 85, 2211–2218.
[CrossRef]
13. Tamura, M.; Nakazawa, Y.; Shiomi, D.; Nozawa, K.; Hosokoshi, Y.; Ishikawa, M.; Takahashi, M.; Kinoshita, M.
Bulk ferromagnetism in the
β-phase crystal of the p-nitrophenyl nitronyl nitroxide radical. Chem. Phys. Lett.
1991, 186, 401–404. [CrossRef]
14. Chiarelli, R.; Novak, M.A.; Rassat, A.; Tholence, J.L. A ferromagnetic transition at 1.48 K in an organic
nitroxide. Nature 1993, 363, 147–149. [CrossRef]
15. Lanfranc de Panthou, F.; Luneau, D.; Laugier, J.; Rey, P. Crystal structures and magnetic properties of a
nitronyl nitroxide and of its imino analog. Crystal packing and spin distribution dependence of ferromagnetic
intermolecular interactions. J. Am. Chem. Soc. 1993, 115, 9095–9100. [CrossRef]
16. Pei, Y.; Kahn, O.; Aebersold, M.A.; Ouahab, L.; Le Berre, F.; Pardi, L.; Jean Louis, T. Synthesis, Crystal
Structure, Magnetic Properties, and Spin Densities of a Triazole-Nitronyl Nitroxide Radical. Adv. Mater.
1994, 6, 681–683. [CrossRef]
17. Cirujeda, J.; Mas, M.; Molins, E.; Lanfranc de Panthou, F.; Laugier, J.; Park, J.G.; Paulsen, C.; Rey, P.;
Rovira, C.; Veciana, J. Control of the structural dimensionality in hydrogen-bonded self-assemblies of
open-shell molecules. Extension of intermolecular ferromagnetic interactions in α-phenyl nitronyl nitroxide
radicals into three dimensions. J. Chem. Soc. Chem. Commun. 1995, 709–710. [CrossRef]
18. Lang, A.; Pei, Y.; Ouahab, L.; Kahn, O. Synthesis, crystal structure, and magnetic properties of
5-methyl-1,2,4-triazole-nitronyl nitroxide: A one-dimensional compound with unusually large ferromagnetic
intermolecular interactions. Adv. Mater. 1996, 8, 60–62. [CrossRef]
19. Matsushita, M.M.; Izuoka, A.; Sugawara, T.; Kobayashi, T.; Wada, N.; Takeda, N.; Ishikawa, M.
Hydrogen-Bonded Organic Ferromagnet. J. Am. Chem. Soc. 1997, 119, 4369–4379. [CrossRef]
20. Yoshioka, N.; Irisawa, M.; Mochizuki, Y.; Kato, T.; Inoue, H.; Ohba, S. Unusually Large Magnetic Interactions
Observed in Hydrogen-Bonded Nitronyl Nitroxides. Chem. Lett. 1997, 26, 251–257. [CrossRef]
21. Nagashima, H.; Hashimoto, N.; Inoue, H.; Yoshioka, N. Coexistence of an antiferromagnetically coupled
dimer and isolated paramagnetic spin in 4-azaindol-2-yl nitronyl nitroxide crystal. New J. Chem. 2003, 27,
805–810. [CrossRef]
22. Nagashima, H.; Inoue, H.; Yoshioka, N. An ideal one-dimensional antiferromagnetic spin system observed
in hydrogen-bonded naphth[2,3-d]imidazol-2-yl nitronyl nitroxide crystal: The role of the hydrogen bond.
J. Phys. Chem. B 2004, 108, 6144–6151. [CrossRef] [PubMed]
23. Nagashima, H.; Fujita, S.Y.; Inoue, H.; Yoshioka, N. Metamagnetic Behavior Observed in Purely Organic
5-Azaindol-2-yl Nitronyl Nitroxide Brick-Wall Architecture. Cryst. Growth Des. 2004, 4, 19–21. [CrossRef]
24. Yao, M.; Asakura, S.; Abe, M.; Inoue, H.; Yoshioka, N. Formation of a One-Dimensional Stacking Structure of
π
-Conjugated Nitroxyl Radical Bearing a 1,2,5-Thiadiazole Ring and Its Magnetic Property. Cryst. Growth Des.
2005, 5, 413–417. [CrossRef]
25. Murata, H.; Mague, J.T.; Aboaku, S.; Yoshioka, N.; Lahti, P.M. An Organic Radical Solid Solution with Strong
Ferromagnetic Exchange. Chem. Mater. 2007, 19, 4111–4113. [CrossRef]
26. Murata, H.; Miyazaki, Y.; Inaba, A.; Paduan-Filho, A.; Bindilatti, V.; Oliveira, N.F., Jr.; Delen, Z.; Lahti, P.M.
2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1-H-imidazole-3-oxide-1-oxyl,
A Hydrogen-Bonded Organic Quasi-1D Ferromagnet. J. Am. Chem. Soc. 2008, 130, 186–194. [CrossRef]
[PubMed]
27. Takahashi, Y.; Miura, Y.; Yoshioka, N. Synthesis and properties of the 3-tert-butyl-7-trifluoromethyl-
1,4-dihydro-1-phenyl-1,2,4-benzotriazin-4-yl radical. New J. Chem. 2015, 39, 4783–4789. [CrossRef]

Molecules 2018, 23, 371
9 of 10
28. Azuma, N.; Yamauchi, J.; Mukai, K.; Ohya, H.N.; Deguchi, Y. The Magnetic Properties of Verdazyl Free
Radicals. III. The Anomalous Magnetic Behavior of Symmetrical Triphenylverdazyl. Bull. Chem. Soc. Jpn.
1973, 46, 2728–2734. [CrossRef]
29. Allemand, P.M.; Srdanov, G.; Fred, F. Molecular engineering in the design of short-range ferromagnetic
exchange in organic solids: The 1,3,5-triphenylverdazyl system. J. Am. Chem. Soc. 1990, 112, 9391–9392.
[CrossRef]
30. Mito, M.; Takeda, K.; Mukai, K.; Azuma, N.; Gleiter, M.R.; Krieger, C.; Neugebauer, F.A. Magnetic Properties
and Crystal Structures of 1,5-Diphenylverdazyls with Electron Acceptor Groups in the 3-Position. J. Phys.
Chem. B 1997, 101, 9517–9524. [CrossRef]
31. Hicks, R.G.; Lemaire, M.T.; Oehrstroem, L.; Richardson, J.F.; Thompson, L.K.; Xu, Z. Strong Supramolecular-Based
Magnetic Exchange in π-Stacked Radicals. Structure and Magnetism of a Hydrogen-Bonded Verdazyl Radical:
Hydroquinone Molecular Solid. J. Am. Chem. Soc. 2001, 123, 7154–7159. [CrossRef] [PubMed]
32. Mukai, K.; Matsubara, M.; Hisatou, H.; Hosokoshi, Y.; Inoue, K.; Azuma, N. Anomalous Magnetic Behavior
in Three Kinds of 3-(Aryl-substituted)-1,5-diphenylverdazyl Radical Crystals (p-FPDV, p-PyDV and m-PyDV)
Induced by Frustrated Spin Interaction. J. Phys. Chem. B 2002, 106, 8632–8638. [CrossRef]
33. Merhi, A.; Roisnel, T.; Rigaut, S.; Train, C.; Norel, L. Ferromagnetic intermolecular exchange interaction in
ethynyl-verdazyl radical crystals. Cryst. Eng. Comm. 2014, 16, 9783–9787. [CrossRef]
34. Eusterwiemann, S.; Dresselhaus, T.; Doerenkamp, C.; Janka, O.; Niehaus, O.; Massolle, A.; Daniliuc, C.G.;
Eckert, H.; Poettgen, R.; Neugebauer, J.; et al. Cooperative Magnetism in Crystalline N-Aryl-Substituted
Verdazyl Radicals: First-Principles Predictions and Experimental Results. Chem. Eur. J. 2017, 23, 6069–6082.
[CrossRef] [PubMed]
35. Eusterwiemann, S.; Doerenkamp, C.; Dresselhaus, T.; Janka, O.; de Oliveira, M., Jr.; Daniliuc, C.G.; Eckert, H.;
Neugebauer, J.; Poettgen, R.; Studer, A. Strong intermolecular antiferromagnetic verdazyl-verdazyl coupling
in the solid state. Phys. Chem. Chem. Phys. 2017, 19, 15681–15685. [CrossRef] [PubMed]
36. Ducharme, Y.; Wuest, J.D. Use of Hydrogen Bonds to Control Molecular Aggregation. Extensive,
Self-complementary Arrays of Donors and Acceptors. J. Org. Chem. 1988, 53, 5787–5789. [CrossRef]
37. Simard, M.; Su, D.; Wuest, J.D. Use of Hydrogen Bonds to Control Molecular Aggregation. Self-Assembly of
Three-Dimensional Networks with Large Chambers. J. Am. Chem. Soc. 1991, 113, 4696–4698. [CrossRef]
38. Ullman, E.F.; Osiecki, J.H.; Boocock, D.G.B.; Darcy, R. Stable free radicals. X. Nitronyl nitroxide monoradicals
and biradicals as possible small molecule spin labels. J. Am. Chem. Soc. 1972, 94, 7049–7059. [CrossRef]
39. Dulog, L.; Kim, J.S. Stable free radical paramagnetic monomers containing aminoxylamine oxide moieties.
An intermediate step toward organic ferromagnetic polyradicals. Makromol. Chem. 1989, 190, 2604–2614.
[CrossRef]
40. Tretyakov, E.V.; Romanenko, G.V.; Podoplelov, A.; Ovcharenko, V. Synthesis of alkynyl-substituted nitronyl
nitroxides through an organosilicon derivative. Eur. J. Org. Chem. 2006, 2695–2702. [CrossRef]
41. Tretyakov, E.V.; Romanenko, G.V.; Ovcharenko, V.I. Alkynyl-substituted nitronyl nitroxide. Russ. Chem. Bull.
2006, 55, 591–592. [CrossRef]
42. Tretyakov, E.V.; Tolstikov, S.E.; Romanenko, G.V.; Bogomyakov, A.S.; Stass, D.V.; Kadirov, M.K.;
Holin, K.V.; Sinyashin, O.G.; Ovcharenko, V.I. Synthesis, structure, and magnetic properties of
2,20-(buta-1,3-diyne-1,4-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide 1-oxyl). Polyhedron
2011, 30, 3232–3237. [CrossRef]
43. Ko, K.C.; Cho, D.; Lee, J.Y. Systematic Approach to Design Organic Magnetic Molecules: Strongly Coupled
Diradicals with Ethylene Coupler. J. Phys. Chem. A 2012, 116, 6837–6844. [CrossRef] [PubMed]
44. Sarbadhikary, P.; Shil, S.; Panda, A.; Misra, A. A Perspective on Designing Chiral Organic Magnetic Molecules
with Unusual Behavior in Magnetic Exchange Coupling. J. Org. Chem. 2016, 81, 5623–5630. [CrossRef]
[PubMed]
45. Steenbock, T.; Shultz, D.A.; Kirk, M.L.; Herrmann, C. Influence of Radical Bridges on Electron Spin Coupling.
J. Phys. Chem. A 2017, 121, 216–225. [CrossRef] [PubMed]
46. Neugebauer, F.A.; Fischer, H.; Krieger, C. Verdazyls. Part 33. EPR and ENDOR studies of 6-oxo-
and 6-thioxoverdazyls. X-Ray molecular structure of 1,3,5-triphenyl-6-oxoverdazyl and 3-tert-butyl-
1,5-diphenyl-6-thioxoverdazyl. J. Chem. Soc. Perkin Trans. 1993, 2, 535–544. [CrossRef]

Molecules 2018, 23, 371
10 of 10
47. Milcent, R.; Barbier, G.; Capelle, S.; Catteau, J.P. New general synthesis of tetrahydro-1,2,4,5-
tetrazin-3(2H)-one derivatives and stable 3,4-dihydro-3-oxo-1,2,4,5-tetrazin-1(2H)-yl radical derivatives.
J. Heterocycl. Chem. 1994, 31, 319–324. [CrossRef]
48. Paré, E.C.; Brook, D.J.R.; Brieger, A.; Badik, M.; Schinke, M. Synthesis of 1,5-diisopropyl substituted
6-oxoverdazyls. Org. Biomol. Chem. 2005, 3, 4258–4261. [CrossRef] [PubMed]
49. Tayama, E.; Kobayashi, Y.; Toma, Y. Diaza [1,4] Wittig-type rearrangement of N-allylicN-Boc-hydrazines into
γ-amino-N-Boc-enamines. Chem. Commun. 2016, 52, 10570–10573. [CrossRef] [PubMed]
50. Melendez, R.E.; Lubell, W.D. Aza-Amino Acid Scan for Rapid Identification of Secondary Structure Based
on the Application of N-Boc-Aza1-Dipeptides in Peptide Synthesis. J. Am. Chem. Soc. 2004, 126, 6759–6764.
[CrossRef] [PubMed]
51. Kuroda, H.; Hanaki, E.; Izawa, H.; Kano, M.; Itahashi, H. A convenient method for the preparation of
α
-vinylfurans by phosphine-initiated reactions of various substituted enynes bearing a carbonyl group with
aldehydes. Tetrahedron 2004, 60, 1913–1920. [CrossRef]
52. Masuda, Y.; Kuratsu, M.; Suzuki, S.; Kozaki, M.; Shiomi, D.; Sato, K.; Takui, T.; Okada, K. Preparation
and magnetic properties of verdazyl-substituted dihydrophenazine radical cation tetrachloroferrate salts.
Polyhedron 2009, 28, 1950–1954. [CrossRef]
53. CCDC 1587567, 1587568 and 1587569 for
1, 2, and 3, Respectively, Contain the Supplementary
Crystallographic Data for This Paper. Available online: http://www.ccdc.cam.ac.uk/conts/retrieving.html
(11 November 2017).
54. Yamaguchi, K.; Fukui, H.; Fueno, T. Molecular orbital (MO) theory for magnetically interacting organic
compounds ab-initio MO calculations of the effective exchange integrals for cyclophane-type carbene dimers.
Chem. Lett. 1986, 625–628. [CrossRef]
55. Sheldrick, G.M. SHELXT-Integrated space-group and crystal-structure determination. Acta Crystallogr.
Sect. A 2015, 71, 3–8. [CrossRef] [PubMed]
56. Sheldrick, G.M. Crystal structure refinement with SHELXL. Acta Crystallogr. Sect. C 2015, 71, 3–8. [CrossRef]
[PubMed]
57. O’Brien, D.A.; Duling, D.R.; Fann, Y.C. Winsim v.0.96, EPR Spectral Simulation for MS-Windows; Public EPR
Software Tools; NIEHS: Bethesda, MD, USA.
58. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.;
Mennucci, B.; Petersson, G.A.; et al. Gaussian 09 (Revision D.01); Gaussian, Inc.: Wallingford, CT, USA, 2013.
Sample Availability: Samples of the compounds 1, 2 and 3 are available from the authors.
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