Magnetic Interactions through a Nonconjugated Framework Observed in Back-to-Back Connected Triazinyl–Nitroxyl Biradical Derivatives

Article abstract of DOI:10.1002/chem.201800163

Three hetero-biradical derivatives, with the structure of a back-to-back connected benzotriazinyl and tetramethyl or tetraethylisoindoline N-oxyl sharing a common benzo ring, 3-tert-butyl-1-phenyl-1,4,6,8-tetrahydro-6,6,8,8-tetramethyl-pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-7-oxyl (1-tBu), 1,3-diphenyl-1,4,6,8-tetrahydro-6,6,8,8-tetramethyl-pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-7-oxyl (1-Ph), and 3-tert-butyl-1-phenyl-1,4,6,8-tetrahydro-6,6,8,8-tetraethyl-pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-7-oxyl (2-tBu), were synthesized and characterized by single-crystal X-ray analyses, variable-temperature magnetic susceptibility studies, and DFT calculations. Temperature dependences of the magnetic susceptibilities of 1-tBu, 1-Ph, and 2-tBu exhibit broad maxima at 70, 71, and 43 K, respectively. Although these radical derivatives form a columnar or chained assembly in the solid state, magnetic measurements of diluted samples in the polymer matrices and computational results imply that the magnetic properties of the polycrystalline sample can be explained by a two-spin system with an intramolecular antiferromagnetic interaction. The magnetic behavior can be reproduced by using the Bleaney–Bowers model, with 2J=?80.0 cm^?1 for 1-tBu, 2J=?77.1 cm^?1 for 1-Ph, and 2J=?48.9 cm^?1 for 2-tBu. The moderately strong intramolecular antiferromagnetic interactions can be interpreted by a through-bond interaction through the nonconjugated framework and/or through-space interactions based on molecular orbital theory. The strong distance dependency between the N?O spin site and vinylic carbon atoms indicates that the orbital interaction plays an important role in the intramolecular magnetic interaction. The reduced magnetic interaction in 2-tBu relative to those of 1-tBu and 1-Ph can be attributed to restricted rotation of the tetraethyl group.

Full text of DOI:10.1002/chem.201800163

A Journal of
Accepted Article
Title: Magnetic Interaction through Non-Conjugated Framework
Observed in Back-to-Back Connected Triazinyl-Nitroxyl Biradical
Derivatives
Authors: Yusuke Takahashi, Ryo Matsuhashi, Youhei Miura, and Naoki
Yoshioka
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201800163
Link to VoR: http://dx.doi.org/10.1002/chem.201800163
Supported by

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Magnetic Interaction through Non-Conjugated Framework
Observed in Back-to-Back Connected Triazinyl-Nitroxyl Biradical
Derivatives
Yusuke Takahashi, Ryo Matsuhashi, Youhei Miura, and Naoki Yoshioka*
4
5
Abstract: Three hetero-biradical derivatives having a structure of a
back-to-back connected benzotriazinyl and tetramethyl or
tetraethylisoindoline N-oxyl sharing a common benzo ring, 3-tert-
butyl-1-phenyl-1,4,6,8-tetrahydro-6,6,8,8-tetramethyl-pyrrolo[4,5-g]-
basic skeleton, the transformation of functional groups, and the
computational study on the -electron system.6 Interesting
physicochemical properties,7 such as spin transition
7
g
accompanying a change in the molecular packing,
crystalline behavior,7 metal coordination,
liquid
m
7h,j,p
1
,2,4-benzotriazin-4-yl-7-oxyl
tetrahydro-6,6,8,8-tetramethyl-pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-
-oxyl (1-Ph), and 3-tert-butyl-1-phenyl-1,4,6,8-tetrahydro-6,6,8,8-
(1-tBu),
1,3-diphenyl-1,4,6,8-
intramolecular
5e
ferromagnetic interaction in the hetero biradical, formation of the
high-spin molecular assembly,7i and zwitterionic singlet state
accompanying a fluorescence,7a have been observed in the
benzotriazinyl radical family.
Besides the chemical stability of the benzotriazinyl radical
derivatives against ambient conditions, the distributions of the
SOMO and spin density are also interesting, i.e., the unpaired
electron delocalized on the carbon atoms at the 6- and 7-positions,
leading to the same sign of the spin density on these carbon
atoms (Figure 1(a)).
In the present paper, the synthesis and magneto-structural
correlation of novel triazinyl-nitroxyl hetero biradical derivatives,
1-tBu, 1-Ph, and 2-tBu (Figure 2) to evaluate the magnetic
interaction through the non-conjugated framework is described.
We have already reported the magneto-structural correlation of
7
tetraethyl-pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-7-oxyl (2-tBu), were
synthesized and characterized by single crystal X-ray analyses,
variable-temperature magnetic susceptibility studies, and DFT
calculations. Temperature dependences of the magnetic susceptibility
of 1-tBu, 1-Ph, and 2-tBu exhibit broad maxima at 70, 71, and 43 K,
respectively. Though these radical derivatives form a columnar or
chained assembly in the solid state, magnetic measurement of the
polymer diluted sample and computational results imply that the
magnetic property of the polycrystalline sample can be explained by
two-spin system with an intramolecular antiferromagnetic interaction.
The magnetic behavior can be reproduced using the Bleaney-Bowers
-
1
-1
model with 2J = -80.0 cm for 1-tBu and 2J = -77.1 cm for 1-Ph, and
-
1
2
J = -48.9 cm for 2-tBu. The moderately strong intramolecular
antiferromagnetic interaction can be interpreted by a through-bond
interaction via the non-conjugated framework and/or through-space
interaction based on the MO theory. The strong distance dependency
between the N-O spin site and vinylic carbons indicates that the orbital
interaction plays an important role in the intramolecular magnetic
interaction. The reduced magnetic interaction in 2-tBu compared to
that of 1-tBu and 1-Ph can be attributed to the restricted rotation of
the tetraethyl group.
the hetero biradical derivative,
3
containing
a
2,2,5,5-
tetramethylpyrrolin-1-oxyl unit and observed the intramolecular
ferromagnetic interaction through the non-conjugated framework
corresponding to 2J = + 14.9 cm-1 by fitting the Bleaney-Bowers
equation.8 The intramolecular ferromagnetic interaction can be
rationally explained by spin polarization on the vinylic moiety
without distribution of the SOMO. Based on the characteristic
distributions of the SOMO and spin density mentioned above, we
can evaluate the magnetic interaction with the same sign of the
spin density which was polarized on the carbon atoms at the 6-
and 7-positions where SOMO also distributes taking advantage of
the electronic state of benzotriazinyl skeleton.
Introduction
The isoindoline-based nitroxyl radical, a benzologue of pyrroline
N-oxyl radical, has an enhanced chemical stability and conversion
of a functional group has occurred without protection of the radical
Since the discovery of the purely-organic ferromagnetic
ordering in mono and biradical derivatives, the research field of
1
molecule-based magnetic materials has attracted attention and
the understanding of magnetic interactions in molecular systems
9
site similar to TEMPO derivatives. Various applications, such as
10
spin labeling and an oxidation catalyst in a porous coordination
2
has been rapidly developed. The chemistry of the benzotriazinyl
polymer11 and organic materials12 have been developed for the
radical, known as Blatter’s radical,3 has been developed in the
isoindoline-based nitroxy radical derivatives. Its SOMO distributes
on the nitroxyl and tetramethyl groups. The EPR and NMR studies
of 1,1,3,3-tetramethylisoindolin-2-yloxyl, denoted as TMIO,
past decade from the viewpoints of synthetic chemistry for the
revealed a smaller hyperfine coupling constant on the aromatic
carbons than that of the methyl protons and carbons.13 Regarding
Department of Applied Chemistry, Faculty of Science and Technology,
Keio University
the corresponding tetraethyl substituted isoindolin N-oxyl,
denoted as TEIO, the hyperfine coupling constant of the nitrogen
nuclei for TEIO (aiso = 13.4 G) was almost identical to that of TMIO
3
-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
E-mail: yoshioka@applc.keio.ac.jp
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
1
4
(aiso =13.8 G) .
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Figure 1. Resonance structure, SOMO (isovalue 0.02) and spin density (isovalue 0.0004) of (a) benzotriazinyl radical (CSD Refcode: TICMOH01) and (b) 1,1,3,3-
tetramethylisoindolin-2-yloxyl (CSD Refcode: DIXREH) at UB3LYP/6-31G(d) level.
The NMR study of TEIO revealed the hyperfine coupling of the
hydrogen nuclei of the ethyl groups and benzo ring,15 suggesting
distribution of the spin density. The small but non-negligible
hyperfine coupling of carbon and hydrogen nuclei of benzo ring
for TMIO and TEIO indicate the spin density distributes not only
nitroxyl and tetramethyl moieties but also the benzo ring.
and tetraethyl groups, on the magnetic interaction is also
discussed based on magneto-structural correlation and
computational study.
Molecular design16 and crystal design17 for the organic magnetic
solids are mainly applied for -conjugated molecules and -
stacked systems. Other than -conjugated molecular system,
magnetic interactions in the biradical through spiro-conjugated,18
cycloalkenyl,2d,19
and
methylene-bridged20
have
been
theoretically and experimentally investigated. Considering the
SOMO and spin density distributions, the TMIO and TEIO
derivatives can be regarded as a spin center having a localized
partially occupied molecular orbital (LPOMO) and indirect
magnetic coupling through a non-conjugated moiety can be
expected like the indirect exchange mechanism in -conjugated
polymers carrying the LPOMO proposed by Tyutyulkov et al.21
Few reports describe the magnetic interaction of biradicals
carrying localized spin centers except for the ELDOR analysis of
the rigidly-fixed TMIO biradical derivative.22 Herein, the
substituent effect of isoindolinenitroxy radical site, i.e. tetramethyl
Figure 2. The molecular structure of the biradicals.
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Results and Discussion
Synthesis of Biradical
The synthetic procedure of 1-tBu and 1-Ph is described in
Scheme 1. The 5-amino-substituted TMIO derivative 4 was
prepared according to the literature method.23 The condensation
between the hydazonyl chloride and 4 gave the corresponding
hydrazonamide derivative 5 carrying the nitroxyl group in a
moderate yield. Cyclization and aerial oxidation of the precursor
in the presence of DBU and 10% Pd/C proceeded and the desired
biradical derivatives 1-tBu and 1-Ph were obtained. The
tetraethyl-substituted biradical 2-tBu was also obtained by a
similar procedure. It was notable that the ethyl-group substituted
hydrazonamide was easily converted to the dihydro-1,2,4-
benzotriazine and was oxidized to desired biradical 2-tBu,
therefore, it was immediately used for the next reaction.
Figure 3. The ORTEP drawings of (a) 1-tBu (dN1–N2 = 1.370(2) Å, dN2–C1
=
1.324(2) Å, dC1–N3 = 1.336(3) Å, dN4–O1 = 1.273(2) Å, dN4–C5 = 1.481(3) Å, dN4–C6
= 1.471(3) Å) and (b) 2-tBu (dN1–N2 = 1.366(3) Å, dN2–C1 = 1.329(3) Å, dC1–N3
=
1.328(3) Å, dN4–O1 = 1.265(3) Å, dN4–C5 = 1.486(3) Å, dN4–C6 = 1.492(3) Å).
Thermal ellipsoids are shown at 50% probability level.
Scheme 1. Synthetic route of biradicals 1-tBu and 1-Ph.
Crystallographic Study
The single crystals of each biradical suitable for the
crystallographic study were obtained by slow evaporation from a
CH
and 1-Ph belong to the monoclinic space group P2
belongs to the monoclinic space group P2 /n (Table S1). The
2
Cl
2
/ n-hexane solution. The crystal structures of both 1-tBu
1
/c, and 2-tBu
1
bond lengths (Figures 3 and S1) of the benzotriazinyl moiety are
similar in value to the other benzotriazinyl radical derivatives
suggesting delocalization of an unpaired electron on the
heterocyclic ring. Focusing on the nitroxyl moiety, the N-O and C-
N bond lengths are also typical of the TMIO derivatives.11,12a In
the crystal packing, 1-tBu forms a slipped column with a CH-
interaction between the benzotriazinyl moiety and H15 atom of
3.22 Å (Figure 4(a)). 1-Ph also forms a 1D slipped columnar
structure along the a axis because of the - interaction between
the C1-phenyl ring and the triazinyl ring of 3.83 Å (Figure 4(b)). In
the packing of 1-tBu and 1-Ph, there is no direct contact of the
nitroxyl moiety which is shorter than 2.38 Å (Figure S2). The
biradical 2-tBu forms the 1D chain structure due to the CH-
interaction between the N1-phenyl ring and triazinyl ring and the
CH- interaction was also confirmed between one of the ethyl
groups and N1-phenyl ring (Figure 4(c)).
Figure 4. Crystal structure of (a) 1-tBu, (b) 1-Ph and (c) 2-tBu. The selected
hydrogen atoms and methyl groups are omitted for clarity.
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EPR Study
The EPR spectra in a toluene glass matrix were measured at 4
K and that of 1-tBu is shown in Figure 5(a) (the spectra of 1-Ph
and 2-tBu is shown in Figures S3(a), S4). The spectra of the Δm
±1 region show a fine structure due to zero field splitting (ZFS).
s
=
The signals at the center of the spectra arise from the
corresponding monoradical impurities. The ZFS parameters (|2D|
=
440, 443 and 448 G for 1-tBu, 1-Ph, and 2-tBu, respectively)
are estimated from the outer most signals. The point-dipole
3
approximation, |D| = |-3gμ
B
/2r | gave a dipole-dipole distance r of
5.02 Å for 1-tBu, 5.01 Å for 1-Ph, and 4.99 Å for 2-tBu, which
are shorter than the distance between the center of the nitroxyl
and nitrogen atoms of the triazinyl moiety in the crystal
coordination. The estimated distances are acceptable because
the unpaired electron on the benzotriazinyl ring was delocalized
s
onto the conjugation system. In the Δm = ±2 region, half-field
signals were observed (inset of Figures 5(a), S3(a), S4) and the
temperature dependence of the signal intensities was measured.
The relationship of the signal intensities with the reciprocal
temperature for 1 is shown in Figures 5(b), S3(b). There was non-
linear relationship, which indicate that the biradical derivatives
have a ground singlet state. For 2-tBu, the thermal behaviour of
the signal intensity cannot be measured due to the weak signal
s
intensity in the Δm = ±2 region.
Magnetic Property
The temperature dependence of the molar magnetic
susceptibility (χ ) for the polycrystalline sample of the biradicals
was measured in the temperature range of 1.8 – 300 K under the
applied field of 5000 Oe. At room temperature, the χ T value for
-tBu, 1-Ph and for 2-tBu was 0.673, 0.669 and 0.703 emu K mol-
m
Figure 5. (a) EPR spectrum of 1-tBu in toluene glass matrix at 4 K. Inset shows
m
s
the corresponding signal in Δm =±2 region. (b) The relationship of the signal
1
1
intensities of Δm =±2 to the reciprocal temperature.
s
,
respectively, which are lower values than that of the calculated
-1
S = 2 × 1/2 spin system (0.750 emu K mol ), indicating the
presence of an antiferromagnetic interaction. With the decreasing
temperature, the χ value of 1-tBu increased and reached the
m
maximum value at 73 K, then converged to zero. The behaviour
can be described using the Bleaney-Bowers two spin model24
(
equation 1) with 2J = -80.0 cm-1 including a 1.7% paramagnetic
impurity (Figure 6). The antiferromagnetic interaction was
dominant in the solid state. To diminish the intermolecular
magnetic interaction, the PSt diluted sample was prepared by the
mixing of CHCl
dried. Its magnetic susceptibility is shown in the inset of Figure 6.
The χ value behaves similar to those of the polycrystalline
sample, indicating a presence of appreciable intramolecular
magnetic interaction. The thermal behavior of χ for the biradical
-Ph was almost identical to that of 1-tBu and analyzed using the
3 3
solution of PSt and CHCl solution of 1-tBu, then
m
m
1
-1
same model with 2J = -77.1 cm . In spite of the different molecular
arrangement, a similar magnetic susceptibility behavior was
observed. Therefore, the estimated 2J value was assigned to the
m
intramolecular interaction. Interestingly, the χ for 2-tBu exhibited
a similar behavior and reaching the maximum value at 43 K, which
can be described by the same model with 2J = -48.8 cm-1
including a 0.99% paramagnetic impurity. The thermal behavior
of the PSt diluted sample exhibited a similar behavior to that of
the polycrystalline sample. It is implied that the magnitude of the
intramolecular magnetic interaction decreased by the substituent
Figure 6. Figure Caption. Temperature dependence of polycrystalline samples
of χ for 1-tBu (○), 1-Ph () and 2-tBu (△) under the applied field of 5000 Oe.
The values showed the temperature of the maximum χ . The inset represents
the thermal behavior of χ for the PSt-diluted sample. The solid line represents
the best fit to the Bleaney-Bowers two spin model including the paramagnetic
impurity.
m
m
m
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ퟑ+퐞퐱퐩
)
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effect of the ethyl group.
ퟐ
ퟐ
퐁
ퟐ푵퐀품 흁
ퟏ
ퟏ
흌퐦
=
∙
∙
(1)
ퟐ푱
풌퐁
푻−휽
풌퐁푻
DFT Calculation of Biradicals Based on Crystallographic
Coordinate
To estimate the SOMO and the spin density distributions in our
system, the DFT calculations at the UB3LYP/6-31G(d) level were
carried out for crystal coordinate using the Gaussian 09 program
package25 (Figures 7, S5). The spin density was distributed over
the entire molecule. While the SOMO distribution of the nitroxyl
site was similar to the parent radical, the SOMO of the
benzotriazinyl site was delocalized not only for the benzotriazinyl
and N1-phenyl rings, but also for the tetramethyl groups due to
the effect of hyperconjugation.26 This indicated that the SOMO
interacted through the sp3 carbons of tetramethyl groups. The
difference in the SOMO and spin density distribution was not
observed between 1-tBu and 2-tBu. The energies of the triplet
state and broken symmetry singlet state were determined and the
magnitude of the calculated exchange parameter 2J was
Figure 7. The SOMO (isovalue 0.02) of (a, c) nitroxyl site and (b, d)
benzotriazinyl site of 1-tBu and 2-tBu at UB3LYP/6-31G(d) level.
estimated using the expression J = (EBS-E
where E and S2 are the total energy and total spin angular
T T
)/(<S2> -<S2>BS),
2
7
-1
momentum, respectively. The negative 2J value of -100.1 cm
-1
-1
for 1-tBu, -116.0 cm for 1-Ph and -100.6 cm for 2-tBu indicated
the presence of an intramolecular antiferromagnetic interaction
(
Table S2). Although the calculated 2J values were overestimated,
the computational results were consistent with the experimental
results. However, the computation could not reproduce the
substituent effect of the alkyl group on the intramolecular
magnetic interaction. The computation also suggested that there
were negligible intermolecular magnetic interactions (Figure S6,
Table S3).
Mechanism of Intramolecular Magnetic Interaction
At first, the mechanism of the intramolecular magnetic
interaction was considered based on the spin density distribution.
When the spin center exhibits the intramolecular magnetic
interaction against the other spin center in the biradical, the spin
density has to distribute on the cross-linking moiety of each spin
center. Focusing on the calculated spin density of 2,2,5,5-
tetramethyl- and tetraethylpyrrolin-1-yloxyl at the UB3LYP/6-
Figure 8. Spin density distribution of (a) tetramethyl- and (b) tetraethyl pyrroline
N-oxyl, (c) 1’-tBu and (d) 2’-tBu at UB3LYP/6-31G(d) level using optimized
geometry at UB3LYP/6-31G(d) level.
31G(d) level (Figure 8(a, b)), it mainly distributes on the nitroxyl
moiety and the spin was polarized not only the adjacent C(CH
3
)
2
3
moieties, but also the vinylic group through the sp carbon, which
is consistent with the spectroscopic studies of the analogues of
TMIO13 and other cyclic nitroxyl derivatives.28 According to these
studies, there are negative spins on the C atoms of the adjacent
N atom and positive spin on the vinylic C atoms. For the ethyl-
substituted pyrroline N-oxyl, there is a low spin density on the
terminal methyl groups. Focussing on the spin density of the
vinylic moiety, the substituent effect was not observed. The
calculated spin density of the benzotriazinyl radical 1’-tBu, 1’-Ph
and 2’-tBu attached to the pyrroline skeleton is shown in Figures
3
8
(c, d), S7. There is a negative spin on the sp carbons and a
Figure 9. Schematic illustration of spin density of 1-tBu. Phenyl ring and tert-
positive spin on the N atom of the pyrroline ring. There are higher
butyl group were omitted for clarity.
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2
spin densities on the C(alkyl) moieties and N atom of the
pyrroline ring for 2’-tBu than that of 1’-tBu, suggesting a stronger
through-bond interaction of 2-tBu than that of 1-tBu. Considering
the spin density distribution, the predicted intramolecular
magnetic interaction based on the through-bond consideration will
be ferromagnetic (Figure 9), which is incompatible with the
experimental results. Because of the spin polarization through the
3

-bond, there is too low spin density on the sp carbons of
pyrroline ring to propagate an effective through-bond interaction
as the primary magnetic pathway.
The intramolecular magnetic interaction of the triazinyl-nitroxyl
biradical is also interpreted by the through-bond and through-
space interaction based on MO theory. The SOMOs of the
benzotriazinyl and nitroxyl sites can distribute on the tetramethyl
groups which may be derived from the hyperconjugation. These
Figure 10. Figure Caption. The energy diagram of biradical derivatives 1-tBu,
1-Ph, and 2-tBu. TS and TB represent the through-space interaction and
through-bond interaction, respectively.
3 2
molecular orbitals of the C(CH ) moieties were assumed as
pseudo  orbitals, producing the interaction between the SOMOs
through the pseudo  orbitals (through-bond interaction). The
through-space interaction between the radical sites also existed
due to the short distances between the N4 and C4/C7 atoms (ca.
2
.27 Å). Dougherty proposed the mechanism of the magnetic
interaction in cyclobutanediyl, 2d and suggested that there is a
large HOMO-LUMO gap in the cyclobutanediyl because of the
through-space interaction, however, the through-bond interaction
raises the energy level of the HOMO and nearly produces a
degeneration of the HOMO and LUMO, leading to the
ferromagnetic interaction. Considering our system based on the
mechanism proposed by Dougherty, it is predicted that the energy
level of the MO with the same phase of the SOMOs for the nitroxyl
and benzotriazinyl sites (MO_A in Figure 10) is lower than that of
the different phase of both spin sites (MO_B in Figure 10) due to
the through-space interaction. The pseudo  orbitals on the sp3
carbons interacted with MO_A, resulting in raising the energy
level of MO_A. Therefore, the antiferromagnetic interaction was
preferred for biradical derivatives 1-tBu, 1-Ph, and 2-tBu. In
addition, due to the small energy gap (0.42 eV for 1-tBu, 0.33 eV
for 1-Ph and 0.38 eV for 2-tBu) of the SOMO of both spin sites
(
nitroxyl site: -4.82 eV for 1-tBu and 1-Ph, and -4.76 eV for 2-tBu,
benzotriazinyl site: -4.40 eV for 1-tBu and -4.49 eV for 1-Ph, and
4.38 eV for 2-tBu), the SOMO-SOMO interaction between the
-
two spin sites enhances the magnetic exchange. Based on these
proposed mechanisms of intramolecular magnetic interaction, the
through-space interaction can affect their intramolecular magnetic
interaction in biradical derivatives carrying the 2,2,5,5-
tetramethylpyrrolin-1yloxyl unit although the through-bond
interaction exists. Due to stronger through-bond ferromagnetic
interaction, the magnitude of the intramolecular interaction of 2-
tBu decreased.
The Distance Dependency on the Intramolecular Magnetic
Interaction
As already described, the through-space interaction is dominant
in the intramolecular magnetic interaction. To estimate the
correlation of the distance between the spin centers with the
magnitude of the magnetic interaction, the 2J values of 1-Ph were
calculated based on the optimized geometry with the distance d
between the N4 and C4/C7 atoms in the range of 2.25 – 2.55 Å.
Figure 11. The correlation of the distance between spin centers with the
magnitude of the magnetic interaction for (a) 1-Ph and (b) 3. The open circles
represent the calculated values based on the optimized geometry. The filled
circle represents the calculated values estimated from the optimized geometry
at UB3LYP/6-31G(d) level.
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The absolute values of the spin density on the C4/C7 atoms were
almost constant, however, the estimated 2J value increased with
the increasing d value (Figure 11(a), Table S4). The sign of 2J
was inversed from negative to positive between 2.45 and 2.50 Å,
suggesting that the through-bond interaction was preferred to the
through-space interaction. On the other hand, the 2J value of 3
decreased in an exponential fashion with the increasing spin
density on vinylic carbons (Figure 11(b), Table S5).
result was qualitatively consistent with the result of the magnetic
susceptibility measurement and suggested rotation of the
tetramethyl group in the solid state. The computation also implied
that the direction of the C-H bond of the alkyl group might affect
the intramolecular magnetic interaction because the C(alkyl)
moieties were on the pathways of the through-bond interaction.
2
Conclusions
Substituent Effect of Alkyl Groups Based on Computational
Study
Three novel hetero biradicals carrying localized nitroxyl and
delocalized benzotriazinyl sites were synthesized and their
spectroscopic property and the magneto-structural correlation
investigated. The magnetic susceptibility measurement for the
polycrystalline and PSt-diluted samples revealed the existence of
an appreciable intramolecular antiferromagnetic interaction
following the Bleaney-Bowers two-spin model. Ferromagnetic
interaction is favored based on through-bond type interaction
taking account of the spin density distribution of the biradical
derivatives. Through-space antiferromagnetic interaction from the
close contact between localized SOMO of nitroxyl moiety and
delocalized one of benzotriazinyl moiety surpass the through-
bond interaction. The magnetic interaction is modulated by the
media, i.e. dilution in polymer matrices. However, 2-tBu carrying
a tetraethyl group at isoindoline moiety exhibited lower magnetic
interaction than those of 1-tBu and 1-Ph. The computation also
suggested that the rotation of the methyl groups contributes the
intramolecular interaction, i.e. the direction of the C-H bond plays
an important role for the modulation of intramolecular magnetic
interaction. As described, the localized-delocalized biradicals can
become promising components to construct molecule-based
magnets from the point of enhancement of the chemical stability
of radical site.
The computational study based on crystallographic geometry
implied that there were no alkyl substituents on the sp carbons
3
existed for the through-bond interaction, which is incompatible
with the experimental result. To evaluate the substituent effect,
we also focussed on the rotation of the alkyl groups. Compared
to the ethyl group, the methyl group is able to rotate in the solid
state and has been studied by a spectroscopic method.29
Therefore, considering the rotation of the methyl groups, the
single point calculations were carried out using the optimized
structure of 1-tBu with the rotation of the tetramethyl groups to
evaluate the rotation effect of the methyl group on the
intramolecular magnetic interaction. The dihedral angle  is
defined as that between one of the three CH bonds and the N-C
bond of the pyrroline ring (Figure 12(a)) and  was changed in the
range of 60 – 180°. The estimated 2J value was -89.9 cm-1 at  =
6
0°. Surprisingly, the magnitude of the J value increased with the
increasing dihedral angle and was reached the maximum value of
-1
-
142.3 cm , then decreased to the initial value (Figure 12(b),
Table S6). Without considering the weighting factor of the position
-1
of the H atoms, the average 2J value was -112.9 cm , which was
a stronger antiferromagnetic interaction than that of 2-tBu. This
Experimental Section
General information
Acetonitrile and dichloromethane were distilled from calcium hydride. N-
phenylbenzohydrazonoyl chloride30 was prepared according to the
literature method. The other chemicals were purchased from Wako, TCI
Chemicals, Junsei Chemical and Sigma-Aldrich and used as received. The
1H and 13C NMR spectra were recorded by a JEOL JNM-LA 300
3
spectrometer using tetramethylsilane as the internal reference in CDCl .
The chemical shifts and coupling constants were expressed in ppm and
Hz, respectively. The mass spectroscopy was carried out using a Bruker
Ultraflex II (MALDI-TOF, sinapic acid was used as the matrix) and JEOL
GC-mate(EI). The IR spectra were recorded using JASCO FT/IR-4100
(KBr method) and Bruker ALPHA FTIR spectrometer (ATR method). The
crystal data were collected using a Bruker D8 Venture with MoKα (0.71073
Å) radiation. The structures of 1-tBu and 1-Ph were solved by the direct
2
method using SHELXT-2013 and refined by the F full matrix least squares
using SHELXL-2013 in the Bruker APEX-II program package. The
structure of 2-tBu was solved by the by the Intrinsic method using
SHELXT-2013 and refined by the F2 full matrix least squares using
SHELXL-2014 in the Bruker APEX-III program package. The magnetic
susceptibility measurements were performed using a Quantum Design
MPMS-XL SQUID magnetometer in the temperature range of 1.8-300 K at
Figure 12. (a) The definition of the dihedral angle  for 1-tBu. (b) The correlation
between the magnitude of the 2J value and the dihedral angle
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Chemistry - A European Journal
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5
000 Oe. The diamagnetic contribution from the sample holder and
To a solution of 5-amino-1,1,3,3-tetramethylisoindolin-2-yloxyl (205 mg,
1.00 mmol) and N-phenylbenzohydrazonyl chloride (254 mg, 1.10 mmol)
in EtOH (3.0 mL) was added triethylamine (0.139 mL, 1.00 mmol) under a
nitrogen atmosphere and stirred for 16 h at room temperature. The
samples estimated using the Pacault method 1-tBu: -2.47×10 emu mol-
3
1
-4
1,
The X-band EPR spectra were recorded using
spectrometer. The signal positions were calibrated by
teslameter. The samples were degassed by the freeze-pump-thaw method.
The elemental analyses were carried out at the Central Laboratory of the
Faculty of Science and Technology, Keio University. Cyclic voltammogram
measurements were carried out using an ALS CHI 6102B electrochemical
1-Ph: -2.49×10-4 emu mol , 2-tBu: -2.96×10 emu mol ) were corrected.
Bruker E500
ER035M
-1
-4
-1
a
a
reaction mixture was extracted with CH
Na SO and evaporated under reduced pressure. The residue was purified
by column chromatography on SiO (EtOAc / n-hexane = 1 / 3 as eluent)
2 2
Cl , washed with water, dried over
2
4
2
to give a yellow solid which gradually turned brown. Yield: 282 mg, 0.706
+
mmol, 71%; mp: 154-156 °C; HRMS(EI): Calcd. for C25
H
27
N
4
O[M] : m/z =
-
1
analyzer and AgNO
3
was used as the reference electrode. The observed
399.2185, Found 399.2193; IR(KBr pellet, cm ): 3321, 3052, 2975, 2927,
1601.
redox potentials were corrected using ferrocene as the internal reference.
UV/Vis spectra were recorded using a JASCO V-650 spectrometer (cyclic
voltammograms and UV/Vis spectra were shown in supporting
information).
1,3-Diphenyl-1,4,6,8-tetrahydro-6,6,8,8-tetramethyl-pyrrolo[4,5-g]-
1,2,4-benzotriazin-4-yl-7-oxyl (1-Ph)
N-phenylpivalohydrazonoyl chloride
To a solution of N’-phenyl-N-(1,1,3,3-tetramethyl-2-oxo-isoindolin-5-
yl)benzenecarbohydrazonamide (282 mg, 0.706 mmol) and a catalytic
amount of 10% Pd/C in CH Cl (12.0 mL) was added DBU (0.105 mL, 0.706
To a solution of N’-phenyl-N-pivaloylhydrazine (231 mg, 1.20 mmol) and
2
2
triphenylphosphine (393 mg, 1.25 mmol) in distilled CH
3
CN (3.0 mL) was
mmol) and stirred for 16 h at room temperature. The reaction mixture was
concentrated and the residue was purified by column chromatography on
added carbon tetrachloride (0.096 mL, 1.25 mmol) under a nitrogen
atmosphere and stirred for 16 h at room temperature. The solvent was
evaporated under reduced pressure and the residue was purified by
SiO
2
(EtOAc / n-hexane = 1 / 3 as eluent) to give a brown solid.
Recrystallization from CH
2
Cl
2
/ n-hexane gave black crystals. Yield: 174
+
column chromatography on SiO
give a clear oil which turned brown in the air. Yield: 227 mg, 1.08 mmol,
0%; 1H NMR(CDCl
, 300 MHz) δ: 7.62(s, 1H), 7.26(dd, 2H, J = 7.8, 7.2
Hz), 7.04(d, 2H, J = 7.8 Hz), 6.87(t, 1H, 7.2 Hz) 1.31(s, 9H); 13C
NMR(CDCl , 75 MHz) δ: 144.13, 135.71, 129.24, 120.41, 113.04, 40.93,
8.33; HRMS(EI): Calcd. for C11
[M] +: m/z = 210.0924, Found
10.0914.
2
2
(CH Cl
2
/ n-hexane = 2 / 3 as eluent) to
mg, 0.439 mmol, 62%; mp: 210-211 °C; LRMS(MALDI): m/z = 396 [M] ;
-
1
IR(ATR, cm ): 3052, 2967, 2925, 1592; Elem. Anal.: Calcd. for C25
24 4
H N O:
9
3
C 75.73, H 6.10, N 14.13, Found: C 75.71, H 6.14, N 13.79.
3
N’-phenyl-N-(1,1,3,3-tetraethyl-2-oxo-isoindolin-5-
yl)pivalohydrazonamide
2
2
H15ClN
2
To a solution of 5-amino-1,1,3,3-tetraethylisoindolin-2-yloxyl (207 mg,
.897 mmol) and N-phenylpivalohydrazonoyl chloride (264 mg, 0.987
mmol) in EtOH (8.0 mL) was added triethylamine (0.20 mL, 1.12 mmol)
under a nitrogen atmosphere and stirred for 17 h at room temperature. The
N’-phenyl-N-(1,1,3,3-tetramethyl-2-oxo-isoindolin-5-
yl)pivalohydrazonamide (5-tBu)
0
To a solution of 5-amino-1,1,3,3-tetramethylisoindolin-2-yloxyl (146 mg,
.711 mmol) and N-phenylpivalohydrazonoyl chloride (171 mg, 0.812
mmol) in EtOH (3.0 mL) was added triethylamine (0.099 mL, 0.771 mmol)
under a nitrogen atmosphere and stirred for 16 h at room temperature. The
reaction mixture was extracted with CH
Na SO and evaporated under reduced pressure. The residue was purified
by column chromatography on SiO (CH Cl / n-hexane = 3 / 1 as eluent)
to give an orange oil, which is easily converted to dihydro-1,2,4-
benzotriazine and was oxidized to 2, therefore, it was used immediately
for the next reaction. Yield: 326 mg, 0.749 mmol, 84%; LRMS(MALDI): m/z
= 412[M-23] +
2 2
Cl , washed with water, dried over
0
2
4
2
2
2
reaction mixture was extracted with CH
Na SO and evaporated under reduced pressure. The residue was purified
by column chromatography on SiO (EtOAc / n-hexane = 1 / 4 as eluent)
2 2
Cl , washed with water, dried over
2
4
2
to give a yellow solid which gradually turned orange. Yield: 178 mg, 0.469
+
mmol, 66%; mp: 71-73 °C; HRMS(EI): Calcd. for C23
3
2
H
31
N
4
O[M] : m/z =
3
-tert-Butyl-1-phenyl-1,4,6,8-tetrahydro-6,6,8,8-tetraethyl-
-
1
79.2498, Found 379.2523; IR(KBr pellet, cm ): 3348, 3050, 2973, 2928,
868, 1602.
pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-7-oxyl (2-tBu)
To
yl)pivalohydrazonamide (242 mg, 0.556 mmol) and a catalytic amount of
0% Pd/C in CH Cl (6.0 mL) was added DBU (0.083 mL, 0.556 mmol)
a solution of N’-phenyl-N-(1,1,3,3-tetraethyl-2-oxo-isoindolin-5-
3
-tert-Butyl-1-phenyl-1,4,6,8-tetrahydro-6,6,8,8-tetramethyl-
pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-7-oxyl (1-tBu)
1
2
2
and stirred for 16 h at room temperature. The reaction mixture was
concentrated and the residue was purified by column chromatography on
To a solution of N’-phenyl-N-(1,1,3,3-tetramethyl-2-oxo-isoindolin-5-
yl)pivalohydrazonamide (198 mg, 0.522 mmol) and a catalytic amount of
SiO
Recrystallization from CH
Yield: 173 mg, 0.400 mmol, 72%; mp: 160-161 °C; LRMS(MALDI): m/z =
409 [M-23] ; IR(ATR, cm ): 2971, 2932, 1590; Elem. Anal.: Calcd. for
C H N O: C 74.96, H 8.39, N 12.95, Found: C 75.32, H 8.36, N 12.89.
27 36 4
2
(EtOAc / n-hexane = 1 / 10 as eluent) to give a reddish-brown solid.
1
0% Pd/C in CH
stirred for 16
concentrated and the residue was purified by column chromatography on
SiO (EtOAc / n-hexane = 1 / 5 as eluent) to give a reddish-brown solid.
Recrystallization from CH Cl / n-hexane gave reddish-black crystals.
Yield: 107 mg, 0.284 mmol, 54%; mp: 168-170 °C; LRMS(MALDI): m/z =
2
Cl
h
2
(5.0 mL) was added DBU (0.079 mL, 0.706 mmol) and
at room temperature. The reaction mixture was
2
Cl / n-hexane gave reddish-black crystals.
2
+
-1
2
2
2
Preparation of the PSt-diluted Sample
+
-1
376. [M] ; IR(ATR, cm ): 2972, 2952, 2926, 1592; Elem. Anal.: Calcd. for
23 28 4
C H N O: C 73.37, H 7.50, N 14.88, Found: C 73.49, H 7.57, N 14.49.
Biradical 1-tBu: To a solution of polystyrene (17 mg, 0163 mmol (based
on MW of monomer), average MW: 260000) in CHCl
dropwise a solution of 1-tBu (13 mg, 0.0345 mmol) in CHCl
3
(5.0 mL) was added
(5.0 mL) and
N’-phenyl-N-(1,1,3,3-tetramethyl-2-oxo-isoindolin-5-yl)benzenecarbo
3
hydrazonamide (5-Ph)
stirred for 1 day at room temperature under dark condition. The resulting
solution was vaporized on the PTFE sheet to afford the PSt-diluted sample.
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Chemistry - A European Journal
10.1002/chem.201800163
FULL PAPER
Biradical 1-Ph: PSt-diluted sample was prepared according to the same
procedure as described above using polystyrene (18.5 mg, 0.178 mmol
based on MW of monomer)) and 1-Ph (33 mg, 0.0832 mmol).
Bendikov, J. M. Rawson, P. A. Koutentis, Chem. Eur. J. 2014, 20, 5388;
g) C. P. Constantinides, A. A. Berezin, G. A. Zissimou, M. Manoli, G. M.
Leitus, M. Bendikov, M. R. Probert, J. M. Rawson, P. A. Koutentis, J. Am.
Chem. Soc. 2014, 136, 11906; h) I. S. Morgan, A. Peuronen, M. M.
Hänninen, R. W. Reed, R. Clérac, H. M. Tuononen, Inorg. Chem. 2014,
(
Biradical 2-tBu: PSt-diluted sample was prepared according to the same
procedure as described above using polystyrene (18 mg, 0173 mmol
based on MW of monomer)) and 2-tBu (15 mg, 0.0347 mmol).
5
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Chemistry - A European Journal
10.1002/chem.201800163
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The magneto-structural correlation of
novel triazinyl-nitroxyl hetero biradical
Yusuke Takahashi, Ryo Matsuhashi,
Youhei Miura, Naoki Yoshioka*
derivatives
moderately
was
strong
reported.
intramolecular
The
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antiferromagnetic interaction was
observed and can be interpreted by a
through-bond interaction via the non-
conjugated framework and through-
space interaction based on the MO
theory.
Magnetic Interaction through Non-
Conjugated Framework Observed in
Back-to-Back Connected Triazinyl-
Nitroxyl Biradical Derivatives
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