Preparation of Multi-Spin Systems: A Case Study of Tolane-Bridged Verdazyl-Based Hetero-Diradicals

Article abstract of DOI:10.1002/ejoc.202000044

Iodine- and ethynyl-containing 'Kuhn'-verdazyls, oxoverdazyls, and nitronyl nitroxides were investigated as building blocks for the preparation of multi-spin systems via the Sonogashira reaction, and, as a result, eleven diradicals were prepared with fair yields. The reactivity of the building blocks indicates that oxoverdazyl iodides are effective starting components for the synthesis of diradicals via the Sonogashira coupling. The described one-step protocol allows combining different spin units, thereby facilitating the design of tolane-bridged diradicals and screening of their properties. The novel compounds were characterized by cyclic voltammetry, UV/Vis and electron spin resonance (ESR) spectroscopy. Although the electrochemical investigation and electronic spectra showed a negligible influence of radical moieties on each other, ESR data revealed a strong exchange interaction between two unpaired electrons. The prepared verdazyl-nitronylnitroxide diradicals have high stability at storage and hold promise for further investigation and application.

Full text of DOI:10.1002/ejoc.202000044

A Journal of
Accepted Article
Title: Preparation of Multi-spin Systems: a Case Study of Tolane-
bridged Verdazyl-based Hetero-diradicals
Authors: Darya E. Votkina, Pavel V. Petunin, Svetlana I. Zhivetyeva,
Irina Yu. Bagryanskaya, Mikhail N. Uvarov, Maxim S.
Kazantsev, Marina E. Trusova, Evgeny V. Tretyakov, and
Pavel S. Postnikov
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10.1002/ejoc.202000044
European Journal of Organic Chemistry
FULL PAPER
Preparation of Multi-spin Systems: a Case Study of Tolane-
bridged Verdazyl-based Hetero-diradicals
Darya E. Votkina,[a] Pavel V. Petunin,*[a,b] Svetlana I. Zhivetyeva,[c] Irina Yu. Bagryanskaya,[c,d] Mikhail
N. Uvarov,[d,e] Maxim S. Kazantsev,[c] Marina E. Trusova,[a] Evgeny V. Tretyakov,[c,d] Pavel S.
Postnikov*[a]
[a]
D.E. Votkina, Dr. P.V. Petunin, Prof. M.E. Trusova, Dr. P.S. Postnikov
Research School of Chemistry & Applied Biomedical Sciences,
Tomsk Polytechnic University,
30 Lenin Avenue, Tomsk 634050, Russia.
E-mail: petuninpavel@tpu.ru; postnikov@tpu.ru
Dr. P.V. Petunin
Siberian State Medical University
[b]
[c]
[d]
[e]
2 Moskovskiy trakt, Tomsk 634050, Russia.
Dr. S.I. Zhivetyeva, Prof. I.Yu. Bagryanskaya, Dr. M.S. Kazantsev, Prof. E.V. Tretyakov
N. N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences (SB RAS)
9 Ac. Lavrentiev Avenue, Novosibirsk 630090, Russia
Prof. I.Yu. Bagryanskaya, Dr. M.N. Uvarov, Prof. E.V. Tretyakov
Novosibirsk State University
2 Pirogova Str., Novosibirsk 630090, Russia
Dr. M.N. Uvarov
V.V. Voevodsky Institute of Chemical Kinetics and Combustion, SB RAS
3 Institutskaya Str., Novosibirsk 630090, Russia.
Supporting information for this article is given via a link at the end of the document.
Abstract: Iodine- and ethynyl-containing 'Kuhn'-verdazyls,
oxoverdazyls, and nitronyl nitroxides were investigated as building
blocks for the preparation of multi-spin systems via the Sonogashira
reaction, and, as a result, eleven diradicals were prepared with fair
yields. The reactivity of the building blocks indicates that oxoverdazyl
iodides are effective starting components for the synthesis of
diradicals via the Sonogashira coupling. The described one-step
protocol allows combining different spin units, thereby facilitating the
design of tolane-bridged diradicals and screening of their properties.
The novel compounds were characterized by cyclic voltammetry,
UV-Vis and electron spin resonance (ESR) spectroscopies. Although
the electrochemical investigation and electronic spectra showed a
negligible influence of radical moieties on each other, ESR data
revealed a strong exchange interaction between two unpaired
electrons. The prepared verdazyl-nitronylnitroxide diradicals have
high stability at storage and hold promise for further investigation
and application.
(Figure 1: A,[8a] B[6] and C[8b]) requires a multi-step complicated
experimental procedures. The main limitations are related to the
low stability of radical moieties under reaction conditions as well
as the influence of unpaired electron on the chemical
transformation processes.[9]
In this paper, we focus on the general approach to prepare a
wide range of organic high-spin systems based on verdazyl
radicals. The significance of this study is strongly related to a
range of issues that arose in verdazyl chemistry. First of all,
verdazyl high-spin systems have not been systematically studied
previously: there are only two examples of verdazyl-containing
hetero-diradicals; namely, compound D[10] developed by Y.
Takahashi and tolane-bridged diradical E (Figure 1).[11a] In
addition, combination of verdazyls and nitroxides, as predicted[12]
and reported,[10d,11] could be considered promising for the
synthesis of multi-spin molecules and possible materials for two-
dimensional (2D) and 3D ordering of triplon excitations of spin
dimers in a crystal lattice.
Introduction
The purely organic high-spin systems based on nitroxides,
verdazyls, trityls, and phenoxides are widely applied as labels
for structural studies of biological objects with the EPR
technique,[1] as agents for dynamic nuclear polarisation,[2] and as
potential components of a quantum computer.[3] Furthermore,
such high-spin paramagnets have been used for the tuning of
magneto-optic properties of a chiral polymer[4] and fabrication of
electronic devices, e.g., as electrochemical switchers and
memory devices,[5] hole-transporting materials[6], and spin
filters.[7]
Despite the high demand for multi-spin systems in various fields
of science and technology, the development of an efficient
synthetic pathway towards exchange-coupled hetero-spin
molecules still remains a challenge. The synthesis of high-spin
systems based on the nitronyl nitroxide building blocks
Figure 1. Examples of known hetero-diradicals.
1
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10.1002/ejoc.202000044
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For the preparation of the systematic series of high-spin
molecules, we applied the same strategy that was used for the
preparation of diradical E, where Sonogashira coupling of
iodophenyl- and ethynylphenyl-substituted radical blocks was
employed.[11a] Moreover, the same reaction was successfully
applied to obtain nitronylnitroxide derivatives. [13] Considering the
previous efforts, here we describe a convenient and reliable
one-step approach for the coupling of different spin-carriers via
the Sonogashira reaction. Eleven verdazyl-verdazyl and
verdazyl-nitronylnitroxide diradicals were prepared with
moderate yields and fully characterized by cyclic voltammetry
(CVA), UV-Vis and ESR spectroscopies. The molecular and
crystal structure of one verdazyl-nitronylnitroxide diradical was
solved by X-Ray structural analysis.
yield of desired 3ca (Table 1, Entry 2) after 24 hours of reaction.
On the contrary, the reaction of 1c and 2a gave target diradical
3ca in 50% isolated yield after 6 hours (Table 1, Entry 1). These
observations were considered in the development of a synthetic
route to ‘Kuhn’-verdazyl-oxoverdazyl diradicals 3ba (Table 1,
Entry 4) and 3bb (Table 1, Entry 5), which were prepared with
acceptable yields (56% and 21% respectively).
As has been reported previously, the position of the iodo-phenyl
substituent in ‘Kuhn’-verdazyl structure dramatically affects on
the reaction rate: ‘Kuhn’-verdazyl 1a with the 4-iodophenyl
substituent at N1 demonstrates higher reactivity than 1d in
Sonogashira coupling[10c] and oxidative addition.[9] In our study,
we observed a similar effect in the reactions of 1a with 2c or 2d,
yielding hetero-diradicals 4ac and 4ad, respectively (Table 1,
Entries 6 and 7).
It is also possible to synthesise oxoverdazyl-nitronylnitroxides 5
using two approaches similar to diradicals 3. The reaction of
iodine-containing oxoverdazyl 1c with the ethynyl-substituted
nitronyl nitroxide 2e gives the corresponding diradical 5ce with
higher yields than the vice versa cross-coupling of 1e with 2c
(Table 1, Entries 8 and 9). Lower reactivity of 1e in comparison
with 1c may be explained by the results of oxidative addition to
Pd(PPh3)4, as shown previously for verdazyl halides.[9] After the
reaction of 1e with Pd(PPh3)4 in THF, we isolated Pd-
nitronylnitroxide derivative only in 40% yield (see SI Section S1).
At the same time, radical 1c gave Pd-verdazyl derivative in a
92% yield.[9] Keeping this in mind, other hetero-spin diradicals
5be (Table 1, Entry 11) and 5bf (Table 1, Entry 12) were
prepared in the same way by the cross-coupling of 1b with 2e or
2f, respectively. Moreover, hetero-diradical 8fe was successfully
isolated after the reaction between oxoverdazyl 1f and nitroxide
2e in 51% yield (Table 1, Entry 17).
Highly reactive ‘Kuhn’-verdazyl 1a was successfully coupled with
ethynyl-substituted nitronyl nitroxides 2e, 2f to prepare hetero-
diradicals 6ae and 6af in average yields (Table 1, Entries 13 and
14). Similar to 4ac and 4ad, the full conversion of starting
compounds into 6ae and 6af required ~8 h. Therefore, there are
significant differences in reactivity between C-linked iodophenyl-
substituted ‘Kuhn’-verdazyls (like 1d) and their N-linked
analogue 1a that seems to be more reactive (Table 1, Entry 2 vs.
Entries 6, 7, 13 and 14).
Results and Discussion
Our study was focused on the Sonogashira coupling of iodine-
substituted radicals 1a-1f with ethynyl-containing building blocks
2a-2f (Figure 2). The reaction was carried out in the previously
optimized conditions[10c] for stable radical alkynylation: iodide 1
(1 eq.) and ethynyl building block 2 (1.5 eq.) with Pd(PPh3)4
(10% mol.) and CuI (10% mol.) as catalysts, Et3N as a base in
THF at room temperature under inert atmosphere. Reaction
times, yields, and structures of the products are summarised in
Table 1. General information about the preparation of starting
compounds 1 and 2 is given in the Experimental part.
Previously we have shown[9] that the reactivity of verdazyl-
halides in Sonogashira coupling depends on the radical
structure: 6-oxoverdazyls are more reactive than ‘Kuhn’-
verdazyls. It was interesting to confirm this trend further, so two
pairs of building blocks that give the same product 3ca were
involved in Sonogashira coupling: iodine-containing 6-
oxoverdazyl 1c with ethynyl-containing ‘Kuhn’-verdazyl 2a and
vice versa 1d plus 2c. The last combination gave only an 8%
It is interesting that diradicals 3bb and 5bf were isolated only in
low yields. A possible reason could be associated with the steps
of hindered trans/cis-isomerization and reductive elimination of
bulky organic moieties, which blocked the rotation and
decreased the torsional freedom.[14]
Unfortunately, hetero-diradical 7de was unstable (Table 1,
Entries 15, 16) and was not isolated for both combinations of the
starting compounds (1d plus 2e and 1e plus 2a). We observed
the formation of the diradical by thin-layer chromatography
(TLC) and its full decomposition after several minutes of elution.
Supposedly, one or both radical moieties in 7de were oxidized
by atmospheric oxygen, and only by-products were observed
after the column chromatography. Changing the substituent
position did not improve stability of this hetero-diradical type.
Figure 2. Structures of the building blocks under study.
2
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10.1002/ejoc.202000044
European Journal of Organic Chemistry
FULL PAPER
Table 1 The synthesis of verdazyl hetero-diradicals via Sonogashira coupling [a]
Entry
Iodine/ethynyl blocks
1c (1 eq.)/2a (1.5 eq.)
1d (1 eq.)/2c (1.5 eq.)
Time [b]
6 h
Yield, % [c]
Product
1
2
50
8
24 h
3
4
1g (1 eq.)/2a (1.5 eq.)
15 min
55
56
1b (1 eq.)/2a (1.5 eq.)
6 h
5
6
1b (1 eq.)/2b (1.5 eq.)
1a (1 eq.)/2c (1.5 eq.)
6 h
8 h
21
46
7
1a (1 eq.)/2d (1.5 eq.)
8 h
42
8
9
1c (1 eq.)/2e (1.5 eq.)
1e (1.5 eq.)/2c (1 eq.)
1g (1 eq.)/2e (1.5 eq.)
6 h
6 h
46
31
51
10
15 min
11
1b (1 eq.)/2e (1.5 eq.)
6 h
34
12
13
1b (1 eq.)/2f (1.5 eq.)
1a (1 eq.)/2e (1.5 eq.)
6 h
8 h
15
54
14
1a (1 eq.)/2f (1.5 eq.)
8 h
46
15
16
1d (1 eq.)/2e (1.5 eq.)
1e (1 eq.)/2a (1.5 eq.)
12 h
6 h
0
0
17
1f (1 eq.)/2e (1.5 eq.)
6 h
51
[a] Reaction conditions: iodine-containing compound 1 (0.1 mmol) and ethynyl-containing compound 2 (0.15 mmol) were stirred in a deoxygenated mixture of
THF (3 mL) and Et3N (1 mL) with Pd(PPh3)4 (10% mol) and CuI (10% mol) as catalysts; [b] reaction progress was monitored by thin-layer chromatography; [c]
preparative yields.
3
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10.1002/ejoc.202000044
European Journal of Organic Chemistry
FULL PAPER
15000
12000
9000
6000
3000
0
In total, the four series of hetero-diradicals were prepared via
Sonogashira coupling. The presented approach may be
considered universal, versatile, and widely applicable. The
reactivity of the radical iodides in Sonogashira coupling
decreases in the following order: oxoverdazyls > N-linked
‘Kuhn’-verdazyl > nitronyl nitroxides > C-linked ‘Kuhn’-verdazyls.
This experimentally confirmed pattern can be of great help in the
choice of the building blocks to obtain the exchange-coupled
verdazyl-verdazyl and verdazyl-nitronylnitroxide diradicals.
In all experiments, 1.5 eq. of ethynyl-containing building blocks
were used to achieve the full conversion of the iodo-derivatives.
We have performed control experiments with the ethynyl
derivatives 2a and 2c that confirmed their significant
decomposition (60–80% after 24 h) in the presence of catalysts
in THF in an inert atmosphere but in the absence of iodine
compound (see Supporting Information, Section S1). At the
same time, the verdazyl iodides were relatively stable under the
same conditions, as the previously reported.[9] It is not possible
to unambiguously describe the decomposition pathway of the
ethynyl compounds. However, it is worth noting that neither the
formation of any paramagnetic by-products nor the dimerisation
into buta-1,3-diyne derivatives was not observed. Several
attempts to optimize the reaction conditions in order to avoid
decomposition of ethynyls 2 in the reaction were made. However,
neither increasing the temperature up to 60 °C nor changing the
solvent to DMF or benzene or the base from Et3N to N,N-
diisopropylethylamine was not successful. Moreover, coupling
reaction with ethynyl derivatives 2a or 2e in the presence of CuI
with the specially synthesised Pd-verdazyl derivative 1f[9] gave
only a minor (5%) increase in the reaction yields, despite a
decrease in the reaction time (Table 1, Entries 3 and 10). This
experiment indicates that the yields in Sonogashira coupling are
mainly affected by the decomposition of the ethynyl building
blocks during transmetalation and elimination processes but not
during the oxidative addition of iodides to the Pd catalyst.
3bb
2b
2d
400
500
600
nm
700
800
Figure 3. UV-Vis spectra in CH2Cl2 solution of biradical 3bb as
superposition of spectra of starting building blocks: 2d as ethynyl-containing
equivalent of 1b and 2b.
superposition of electronic spectra of initial compounds 2d as
ethynyl-containing analogue of 1b and 2b, meaning that two
radical moieties influence each other only in a slight manner.
The same trend is present in the electronic absorption spectra of
other synthesised diradicals (see Supporting Information).
Electrochemical analysis of diradicals 3–6 and 8 was carried out
in the deoxygenated CH2Cl2 solution. For compounds 3, 5, and 8,
the observed CVA curves seem to be the superposition of redox
waves of the two independent radical moieties with negligible
differences (less than 0.1 V) in comparison with the starting
building blocks (Figure 4, Supporting Information Section S3). It
is noteworthy that the sufficiently different redox potentials of the
two spin moieties may allow the selective reduction or oxidation
of one of two paramagnetic groups, opening the possibilities for
the electronic devices design.
In the CVA data for diradicals 4 and 6, there is only one
reduction process, but at the same time, two reversible oxidation
waves are present, corresponding to the oxidation of two
different radical groups (SI, Section S3). It is not possible to
correctly assign the reduction process to a certain radical moiety
since the reduction potential is different from those of the
A stability test was performed on crystal powders of all the
prepared diradicals (3–6 and 8) at the refrigerator and room
temperatures in air. ‘Kuhn’-verdazyl-oxoverdazyls 3 and 4
demonstrated the shortest lifetime; a significant amount of a by-
product was observed after one month of storage in the
refrigerator and after one week of storage at room temperature.
Nitroxide-containing diradicals 5, 6, and 8 did not demonstrate
significant decomposition, and only traces of by-products were
observed on TLC after three months of storage at 3°C and after
one month at room temperature.
3bb
2d
2b
Numerous attempts to grow diradical crystals suitable for X-ray
diffraction led to the formation of fine powders. Suitable single
crystals were obtained only for diradical 8fe (crystal structure of
diradical 5ce was solved previously [10d]), and its molecular and
crystal structure was successfully solved (see SI, Section S2).
The obtained diradicals have conjugated linker, so the expected
mutual influence of the unpaired electrons was studied using
spectral and electrochemical methods. The diradical 3bb was
used as an example to demonstrate the general trends. Figure 3
shows the electronic absorption spectrum of diradical 3bb
together with spectra of building blocks 2d as ethynyl-containing
analogue of 1b and 2b in CH2Cl2 at room temperature. The
spectra of the starting monoradicals agree well with the existing
data[15]: 2b has a broad charge transfer characteristic band in
the visible region at ~700 nm, and 2d has a double peak at ~540
and ~565 nm. The resulting spectrum of diradical 3bb is a
-2,0 -1,5 -1,0 -0,5
0,0
0,5
1,0
1,5
E, V vs Fc/Fc+
Figure 4. Cyclic voltammogram of biradical 3bb as superposition of redox
waves of starting building blocks: 2d as ethynyl-containing equivalent of 1b
and 2b. CVAs of all compounds were recorded in CH2Cl2 solution
(100 mV/s with 0.1 M Bu4NPF6 electrolyte).
4
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10.1002/ejoc.202000044
European Journal of Organic Chemistry
FULL PAPER
corresponding starting monoradicals. Some conjugation of
molecular orbitals undetectable in the UV-Vis spectra may be
responsible for the absence of the second reduction peak in the
diradical systems 4 and 6.
Continuous-wave ESR spectra of the prepared monoradicals 1
and 2 and diradicals 3–8 were recorded at room temperature in
deoxygenated toluene solution (Figure 5 and SI, Section S1).
ESR spectra of monoradicals were successfully simulated, and
the hyperfine coupling constants were measured that are in
good agreement with known data.[9,15] The ESR spectra of
diradicals have rather complex line shapes because of the
were obtained, and their stability, ESR and UV-Vis spectra, and
electrochemical properties were studied. The mutual influence of
the unpaired electrons in the diradicals with a conjugated linker
was not observed in the UV-Vis spectroscopy studies but was
clearly present in the ESR spectra. In addition, the observed
changes in the CVA data for some diradicals indirectly confirm
the presence of the radical moieties’ interaction. The reactivity
and applicability of the radical-containing building blocks in
Sonogashira coupling were described, and it is shown that
iodine-containing oxoverdazyls perform as the most reactive
building blocks for the Sonogashira reaction, and the reactivity of
radical iodides decreases as follows: oxoverdazyls > N-linked
‘Kuhn’-verdazyl > nitronyl nitroxides > C-linked ‘Kuhn’-verdazyls.
As for diradicals stability, ‘Kuhn’-verdazyl-oxoverdazyls have the
shortest lifetime, and the nitronyl nitroxide-containing diradicals
are much more stable. The diradicals have sufficient differences
in the redox potentials of the spin moieties; therefore, they may
be independently reduced or oxidized, providing novel
opportunities for the switchable materials design.[17] In general,
the Sonogashira cross-coupling reaction represents a versatile
synthetic tool for the fast and straightforward screening of novel
organic magnetic materials based on multi-spin systems.
Authors would like to encourage further research into the
magnetic characteristics of the hetero-diradicals, revealing their
magneto-structural correlations.
significant
contributions
of
electron-electron
spin-spin
interactions and the hyperfine interactions. For example, the
ESR spectra of diradicals 3ba, 3bb, and 3ca have unique
spectral line shapes in each case, significantly different from that
of initial radicals. The strong exchange interaction is confirmed
by the distinctly reduced splitting between small peaks in
comparison with the characteristic value of hyperfine splitting, as
observed for some other diradicals.[16] It should be noted, that
the beautiful single narrow line of the ESR spectrum of 3ca has
peak-to-peak linewidth of 1.6 G. In the case of motional
averaging of the ESR spectrum, such a narrow line should be
caused by decreasing in the hyperfine interaction between
electron spins and nitrogen atoms because of a specific electron
spin density distribution over 3ca backbones. Strict
determination of exchange interaction strength and explanation
of the narrow ESR line of 3ca are a question of further
investigation.
Experimental Section
General information. All organic reagents and solvents were purchased
from commercial suppliers (Sigma-Aldrich, Alfa Aesar, and others) and
were used as received. THF was purified according to a previously
reported procedure.[18] Radicals 1a,[10c] 1b,[9] 1c[9] and acetylene-
substituted nitronyl nitroxide radicals 2e[15] and 2f[15] were obtained
according to previously reported procedures. The synthesis of 1,3,5-
substituted verdazyls 1d,[10c] 2a, and 2b[10c] and nitroxide 1e[19] was
carried out using procedures adapted from other papers. Synthetic
pathways for the building blocks are given in SI, Section S1. 1Н and 13С
NMR spectra were recorded on a Bruker Avance III HD (400 MHz)
instrument. Elemental analysis was performed using a Euro EA 3000
(EuroVector). Absorption spectra were registered on a Specord 250
PLUS UV-Vis spectrophotometer. Melting points were measured using
the Mettler Toledo Melting point system MP50. High-resolution mass
spectra were recorded on an Agilent 6550 Q-TOF LC-MS instrument with
electrospray ionization (ESI) and a Thermo Fisher Scientific LC-MS LTQ-
Orbitrap Velos instrument with ESI.
Conclusion
A series of verdazyl-verdazyl and verdazyl-nitronylnitroxide
hetero-diradicals was synthesized via Sonogashira coupling with
fair to moderate yields. The described synthetic pathway allowed
us to combine different spin-carriers in one conjugated molecule
by a one-step procedure. As a result, eleven hetero-diradicals
Oxoverdazyls
3ba 3bb
'Kuhn'-verdazyls
3ca
ESR spectra were acquired using a Bruker Elexsys ESP-580E X-band
instrument with the ER 4118 X-MD5 dielectric cavity and neutron-
irradiated LiF as a standard (g = 2.00229  0.00001[20]). The spectra
were obtained in a deoxygenated toluene solution (three freeze-pump-
thaw cycles) at a concentration of ~5 mM in glass tubes of a 4.8 mm
outer diameter. Microwave power was 20 mW, modulation frequency
100 kHz, and modulation amplitude 0.1 G. Simulation of ESR spectra
was conducted in the Winsim2002 freeware, and hyperfine coupling
constants were measured directly from the simulation.[21] CVA
measurements were performed in a deoxygenated CH2Cl2 solution by a
computer-controlled P-8nano potentiostat/galvanostat (Elins, Russia) in
combination with
a
three-electrode cell (Gamry), with 0.1 M
tetrabutylammonium hexafluorophosphate as a supporting electrolyte. Pt,
a Pt wire and Ag/AgCl served as a working, counter, and reference
electrode, respectively. The reference electrode was calibrated by
measurement of the redox potentials of ferrocene. Routine monitoring of
the reactions was performed using silica gel–coated aluminum plates
(Merck, Silica gel 60, F254), which were analyzed under UV light at
254 nm.
336
337
338
339
340
341
342
Magnetic field, mT
Figure 5. Experimental ESR spectra in deoxygenated toluene solution of
compound 3ba, 3bb and 3ca in comparison with starting building blocks.
5
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10.1002/ejoc.202000044
European Journal of Organic Chemistry
FULL PAPER
X-ray diffraction analysis. Single crystals for compound 8fe were grown
from a CH2Cl2–hexane solution in an open flask at –5 °C. X-ray
crystallography of a single crystal of 8ef was carried out in a Bruker
Kappa Apex II CCD diffractometer via φ,ω-scans of narrow (0.5°) frames
with MoKα radiation (λ = 0.71073 Å) and a graphite monochromator. The
structure was solved by direct methods in the SHELX-97 software[22] and
was refined by the full-matrix least-squares method against all F2 in
anisotropic approximation using the SHELXL-2014/7 software.[23] The
hydrogen atoms’ positions were calculated via the riding model.
Absorption correction was applied by the empirical multi-scan method in
the SADABS software.[24]
Free solvent-accessible volume in the crystal of 8ef derived from
PLATON routine analysis[25] was found to be 26.4% (988.0 Å3). This
volume is occupied by highly disordered solvent molecules that could not
be modeled as a set of discrete atomic sites. We employed the
PLATON/SQUEEZE procedure to calculate the contribution to the
diffraction from the solvent region and thereby obtained a set of solvent-
free diffraction intensities.
Synthesis
oxide-1-oxyl 1e. To
of
2-(4-iodophenyl)-4,4,5,5-tetramethylimidazoline-3-
a
solution of 2-(4-iodophenyl)-4,4,5,5-
tetramethylimidazolidine-1,3-diol 13 (362 mg, 1 mmol) in CH2Cl2 (25 mL),
a solution of NaIO4 (214 mg, 1 mmol) in water (15 mL) was added, and
the mixture was stirred vigorously for 30 min until the organic phase
turned deep blue. The organic layer was separated, washed with water
(2  20 mL), and dried with Na2SO4, and the solvent was evaporated in
vacuo. The radical was purified via column chromatography
(hexane:CH2Cl2 at 1:1 to 1:4) to obtain blue crystals after evaporation of
the eluent (298 mg, 83% yield). Mp = 141.7 °C (decomposition). UV-vis
(CH2Cl2): λmax (log ε) = 370 (4.12), 585 (2.60), 629 (2.60) nm. FT-IR (KBr
disk): 2992, 2928, 1584, 1467, 1415, 1361, 1301, 1165, 1008, 816 cm1
.
ESR (toluene, 9.5 GHz): quintet, aN = 7.42 G (2N), g-value 2.0062. Anal.
calcd. for C13H16IN2O2: C, 43.47; H, 4.49; N, 7.80. Found: C, 43.49; H,
4.50; N, 7.81. HRMS (ESI/Q-TOF) m/z: [M+H]+ calcd. for C13H17IN2O2:
360.0335. Found: 360.0329.
The general procedure for the preparation of 1,3,5-substituted 6-
oxoverdazyl radicals 1f, 2c and 2d. A solution of Na2CO3 (2.12 g, 20
mmol) and K3[Fe(CN)6] (2.963 g, 9 mmol) in 20 mL of water was added
to a solution (50 mL) of tetrazinan-3-ones 11 (2 mmol) and Et4NBr
(84 mg, 0.4 mmol) in CH2Cl2 (50 mL). Radicals 1f, 2c and 2d were
extracted with CH2Cl2 (2  30 mL), washed with water, dried over Na2SO4
and subjected to flash chromatography (hexane:CH2Cl2 at 2:1). Pure
radicals 1f, 2c and 2d were obtained after evaporation of the eluate in
vacuo.
Crystallographic data on the structure of 8fe were deposited in the
Cambridge Crystallographic Data Centre as supplementary publication
No. CCDC 1961924, containing supplementary crystallographic data for
this paper. A copy of the data can be obtained, free of charge, by
contacting Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB21EZ, UK (fax: +44 122 3336033 or e-mail:
deposit@ccdc.cam.ac.uk; Internet: https://www.ccdc.cam.ac.uk).
The general procedure for the preparation of verdazyls 1d, 2a, and
2b: Ba(OH)2∙8H2O (473 mg, 1.5 mmol) and 1,3,5-substituted formazan
(0.5 mmol) were dissolved in DMF (8 mL), and the resulting mixture was
heated to 65 °C. After 15 min, the mixture was cooled to room
temperature, MeI (125 µL, 2 mmol) was added, and the reaction mixture
was stirred at room temperature for 10 min, at 60 °C for 30 min and
90 °C for 10 min. Ba(OH)2 was separated by centrifugation, and the
verdazyl radical was precipitated from the solution with cracked ice (70 g).
The solid product was filtered out and washed with water (20 mL).
1,3-Diphenyl-5-(4-iodophenyl)-6-oxoverdazyl radical 1f. Oxidation of
2-(4-ethynylphenyl)-4,6-diphenyl-1,2,4,5-tetrazinan-3-one[26]
(913 mg,
2 mmol) according to the general procedure afforded 1f (725 mg, 80%
yield) as deep-violet solids, mp = 178.3–182.9 °C. UV-vis (CH2Cl2): λmax
(log ε) = 413 (3.16), 541 (3.43), 565 (3.45) nm. FT-IR (KBr disk): 3441,
3062, 3030, 1694, 1481, 1409, 1363, 1309, 1249, 1121, 1001, 824, 748,
684, 654, 606, 501 cm1. ESR (toluene, 9.5 GHz): multiplet, aN = 6.43 G
(N2, N4), aN = 4.45 G (N1, N5), g-value 2.0040. Anal. calcd. for
C20H14IN4O: C, 53.00; H, 3.11; N, 12.36. Found: C, 53.04; H, 3.10; N,
12.32. HRMS (ESI/Q-TOF) m/z: [M]+ calcd. for C20H14IN4O: 453.0212.
Found: 453.0213.
1,5-Diphenyl-3-(4-iodophenyl)verdazyl radical 1d. According to the
general
procedure,
the
reaction
of
3-(4-iodophenyl)-1,5-
diphenylformazan[10c] (213 mg, 0.5 mmol) with MeI afforded compound
1d (198 mg, 90% yield) as light-green solids, mp = 130.0–131.4 °C. UV-
vis (CH2Cl2): λmax (log ε) = 413 (3.72), 438 (3.72), 721 (3.42) nm. Fourier
transform infrared spectroscopy (FT-IR; KBr disk): 3058, 2925, 2856,
1,5-Diphenyl-3-(4-ethynylphenyl)-6-oxoverdazyl radical 2c. Oxidation
of 6-(4-ethynylphenyl)-2,4-diphenyl-1,2,4,5-tetrazinan-3-one 11c (709 mg,
2 mmol) according to the general procedure produced 2c (816 mg, 84%
yield) as violet solids, mp = 318.3–319.1 °C. UV-vis (CH2Cl2): λmax = 429
(3.29), 548 (3.41), 580 (3.32) nm. FT-IR (KBr disk): 3435, 3377, 3070,
3038, 1694, 1589, 1487, 1357, 1249, 1123, 1005, 754, 692 cm1. ESR
(toluene, 9.5 GHz): nonet, aN = 6.27 G (N2, N4), aN = 4.62 G (N1, N5), g-
value 2.0039. Anal. calcd. for C22H15N4O: C, 75.20; H, 4.30; N, 15.94.
Found: C, 75.28; H, 4.29; N, 16.01. HRMS (ESI/Q-TOF) m/z: [M]+ calcd.
for C22H15N4O: 351.1246. Found: 351.1279.
1744, 1591, 1489, 1393, 1215, 1139, 1121, 1004, 828, 752, 688 cm1
.
ESR (toluene, 9.5 GHz): nonet, aN1,5 = 5.48 G, aN2,4 = 5.49 G, g-value
2.0033. Anal. calcd. for C20H16IN4: C, 54.68; H, 3.67; N, 12.75. Found: C,
54.71; H, 3.66; N, 12.76. High-resolution mass spectrometry
(HRMS; ESI/Q-TOF) m/z: [M]+ calcd. for C20H16IN4: 439.0420. Found:
439.0417.
1,5-Diphenyl-3-(4-ethynylphenyl)verdazyl radical 2a. According to the
1,5-Diphenyl-3-(3-ethynylphenyl)-6-oxoverdazyl radical 2d. Oxidation
of 6-(3-ethynylphenyl)-2,4-diphenyl-1,2,4,5-tetrazinan-3-one 11d (709 mg,
2 mmol) according to the general procedure gave 2d (597 mg, 85% yield)
as violet solids, mp = 148.8–150.4 °C. UV-vis (CH2Cl2): λmax (log ε) = 415
(3.23), 538 (3.43), 563 (3.41) nm. FT-IR (KBr disk): 3439, 3270, 3064,
general
procedure,
the
reaction
of
3-(4-ethynylphenyl)-1,5-
diphenylformazan 12a (162 mg, 0.5 mmol) with MeI produced compound
2a (144 mg, 85% yield) as light-green solids, mp = 95.2–95.9 °C. UV-vis
(CH2Cl2): λmax (log ε) = 362 (3.86), 448 (3.65), 725 (3.34) nm. FT-IR (KBr
disk): 3281, 3062, 2924, 2852, 2104, 1598, 1565, 1495, 1448, 845, 754,
690 cm1. ESR (toluene, 9.5 GHz): nonet, aN1,5 = 5.88 G, aN2,4 = 5.88 G,
g-value 2.0033. Anal. calcd. for C22H17N4: C, 78.32; H, 5.08; N, 16.61.
Found: C, 78.29; H, 5.06; N, 16.60. HRMS (ESI/Q-TOF) m/z: [M]+ calcd.
for C22H17N4: 337.1453. Found: 337.1451.
2114, 1692, 1595, 1485, 1359, 1257, 1187, 760, 734, 682, 622, 602 cm1
.
ESR (toluene, 9.5 GHz): nonet, aN = 6.34 G (N2, N4), aN = 4.54 G (N1,
N5), g-value 2.0039. Anal. calcd. for C22H15N4O: C, 75.20; H, 4.30; N,
15.94. Found: C, 75.03; H, 4.31; N, 15.96. HRMS (ESI/Q-TOF) m/z: [M]+
calcd. for C22H15N4O: 351.1246. Found: 351.1239.
1,5-Diphenyl-3-(3-ethynylphenyl)verdazyl radical 2b. According to the
general
procedure,
the
reaction
of
3-(3-ethynylphenyl)-1,5-
The general procedure for the Sonogashira cross-coupling reaction:
Pd(PPh3)4 (11.6 mg, 0.01 mmol) was added to a deoxygenated (freeze-
pump-thaw method, 3 cycles) solution of iodine-containing building block
1a–1e (0.1 mmol), ethynyl-containing building block 2a–2g (0.15 mmol)
and CuI (1.9 mg, 0.01 mmol) in a mixture of THF (3 mL) and Et3N (1 mL).
The solution was stirred until full conversion of one of the building blocks,
1 or 2, according to thin-layer chromatography (reaction times are
reported in Table 1). The product was isolated by column
chromatography with an appropriate eluent (see below), the solvent was
evaporated, and the resultant solid product was reprecipitated from a
CH2Cl2–hexane system to obtain pure diradicals 3–6 and 8.
diphenylformazan 12b (162 mg, 0.5 mmol) with MeI generated
compound 2b (149 mg, 88% yield) as light-green solids, mp = 87.3–
88.4 °C. UV-vis (CH2Cl2): λmax (log ε) = 432 (3.65), 715 (3.35) nm. FT-IR
(KBr disk): 3411, 3285, 3060, 3032, 2924, 2852, 2106, 1597, 1563, 1495,
1377, 1297, 1197, 1076, 800, 748, 688 cm1. ESR (toluene, 9.5 GHz):
nonet, aN1,5 = 5.89 G, aN2,4 = 5.88 G, g-value 2.0033. Anal. calcd. for
C22H17N4: C, 78.32; H, 5.08; N, 16.61. Found: C, 78.25; H, 5.07; N, 16.59.
HRMS (ESI/Q-TOF) m/z: [M]+ calcd. for C22H17N4: 337.1453. Found:
337.1449.
6
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000044
European Journal of Organic Chemistry
FULL PAPER
Compound 3ca. According to the general procedure, the reaction of 1c
(45.3 mg, 0.1 mmol) with 2a (50.6 mg, 0.15 mmol) was followed by
column chromatography (hexane:CH2Cl2 at 1:1 to 0:1), which afforded
compound 3ca (33.1 mg, 50% yield) as olive solids, mp = 210.3–
210.8 °C. UV-vis (CH2Cl2): λmax (log ε) = 539 (3.29), 565 (3.30), 723
(3.48) nm. FT-IR (KBr disk): 3422, 3068, 3040, 2212, 1695, 1595, 1495,
1407, 1357, 1304, 1246, 1147, 1122, 845, 751, 690, 654 cm1. Anal.
calcd. for C42H30N8O: C, 76.12; H, 4.56; N, 16.91. Found: C, 76.19; H,
4.57; N, 16.88. HRMS (ESI/LTQ Orbitrap) m/z: [M]+ calcd. for C42H30N8O:
662.2542. Found: 662.2540.
Compound 3ba. According to the general procedure, the reaction of 1b
(45.3 mg, 0.1 mmol) with 2a (50.6 mg, 0.15 mmol) was followed by
column chromatography (hexane:CH2Cl2 at 1:1 to 0:1), which afforded
compound 3ba (37.1 mg, 56% yield) as olive solids, mp = 170.5–
171.5 °C. UV-vis (CH2Cl2): λmax (log ε) = 450 (3.79), 538 (3.21), 566
(3.21), 728 (3.44) nm. FT-IR (KBr disk): 3439, 3065, 3037, 2210, 1700,
1590, 1493, 1399, 1144, 745, 687, 607 cm1. Anal. calcd. for C42H30N8O:
C, 76.12; H, 4.56; N, 16.91. Found: C, 76.11; H, 4.58; N, 16.89.
HRMS (ESI/LTQ Orbitrap) m/z: [M]+ calcd. for C42H30N8O: 662.2542.
Found: 662.2534.
Compound 3bb. According to the general procedure, the reaction of 1b
(45.3 mg, 0.1 mmol) with 2b (50.6 mg, 0.15 mmol) followed by column
chromatography (hexane:CH2Cl2 at 1:1 to 0:1) gave compound 3bb
(13.9 mg, 21% yield) as olive solids, mp = 151.2–151.5 °C. UV-vis
(CH2Cl2): λmax (log ε) = 537 (3.32), 566 (3.33), 716 (3.49) nm. FT-IR (KBr
disk): 3400, 3059, 2924, 2855, 2150, 1700, 1595, 1493, 748, 690, 610,
515 cm1. Anal. calcd. for C42H30N8O: C, 76.12; H, 4.56; N, 16.91. Found:
C, 76.22; H, 4.59; N, 16.83. HRMS (ESI/LTQ Orbitrap) m/z: [M]+ calcd.
for C42H30N8O: 662.2542. Found: 662.2545.
Compound 4ac. According to the general procedure, the reaction of 1a
(51.5 mg, 0.1 mmol) with 2c (50.6 mg, 0.15 mmol) followed by column
chromatography (hexane:CH2Cl2 at 1:1 to 0:1) afforded compound 4ac
(34.0 mg, 46% yield) as olive solids, mp = 174.1–175.5 °C. UV-vis
(CH2Cl2): λmax (log ε) = 443 (4.16), 543 (3.40), 590 (3.23), 736 (3.46) nm.
FT-IR (KBr disk): 3314, 3057, 2923, 2204, 1700, 1595, 1493, 1252, 1136,
748, 693, 518 cm1. Anal. calcd. for C48H34N8O: C, 78.03; H, 4.64; N,
15.17. Found: C, 78.19; H, 4.63; N, 15.10. HRMS (ESI/LTQ Orbitrap)
m/z: [M]+ calcd. for C48H34N8O: 738.2856. Found: 738.2849.
Compound 4ad. According to the general procedure, the reaction of 1a
(51.5 mg, 0.1 mmol) with 2d (50.6 mg, 0.15 mmol) followed by column
chromatography (hexane:CH2Cl2 at 1:1 to 0:1) generated compound 4ad
(31.0 mg, 46% yield) as olive solids, mp = 150.9–151.8 °C. UV-vis
(CH2Cl2): λmax (log ε) = 434 (4.14), 524 (3.48), 567 (3.36), 731 (3.52) nm.
FT-IR (KBr disk): 3437, 3063, 2927, 2207, 1703, 1593, 1510, 1493, 1249,
1133, 754, 690 cm1. Anal. calcd. for C48H34N8O: C, 78.03; H, 4.64; N,
15.17. Found: C, 77.99; H, 4.66; N, 15.18. HRMS (ESI/LTQ Orbitrap)
m/z: [M]+ calcd. for C48H34N8O: 738.2856. Found: 738.2848.
Compound 5ce. According to the general procedure, the reaction of 1c
(45.3 mg, 0.1 mmol) with 2e (38.6 mg, 0.15 mmol) followed by column
chromatography (hexane:CHCl3 at 1:1 to 0:1) produced compound 5ce
(26.8 mg, 46% yield) as violet solids, mp = 209.2–210.8 °C. UV-vis
(CH2Cl2): λmax = 387 (4.22), 452 (3.35), 554 (2.41), 590 (3.33) nm. FT-IR
(KBr disk): 3447, 3068, 2987, 2924, 2212, 1701, 1487, 1363, 845, 756,
693 cm1. Anal. calcd. for C35H30N6O3: C, 72.15; H, 5.19; N, 14.42.
Found: C, 72.21; H, 5.17; N, 14.56. HRMS (ESI/LTQ Orbitrap) m/z: [M]
calcd. for C35H30N6O3: 582.2379. Found: 582.2465.
Compound 5be. According to the general procedure, the reaction of 1b
(45.3 mg, 0.1 mmol) with 2e (38.6 mg, 0.15 mmol) was followed by
column chromatography (hexane:CHCl3 at 1:1 to 0:1), which afforded
compound 5be (19.8 mg, 34% yield) as violet solids, mp = 165.1–
165.9 °C. UV-vis (CH2Cl2): λmax (log ε) = 384 (3.84), 539 (3.08), 567
(3.07) nm. FT-IR (KBr disk): 3444, 3068, 2987, 2932, 2212, 1702, 1484,
1363, 1301, 1235, 1130, 837, 759, 693, 612 cm1. Anal. calcd. for
C35H30N6O3: C, 72.15; H, 5.19; N, 14.42. Found: C, 72.20; H, 5.21; N,
14.46. HRMS (ESI/LTQ Orbitrap) m/z: [M] calcd. for C35H30N6O3:
582.2379. Found: 582.2466.
(8.8 mg, 15% yield) as violet solids, mp = 132.2–133.0 °C. UV-vis
(CH2Cl2): λmax (log ε) = 369 (4.28), 539 (3.47), 566 (3.47) nm. FT-IR (KBr
disk): 3445, 3066, 2992, 2930, 2211, 1702, 1485, 1361, 800, 761, 690,
602, 541 cm1. Anal. calcd. for C35H30N6O3: C, 72.15; H, 5.19; N, 14.42.
Found: C, 72.11; H, 5.20; N, 14.40. HRMS (ESI/LTQ Orbitrap) m/z: [M]
calcd. for C35H30N6O3: 582.2379. Found: 582.2465.
Compound 6ae. According to the general procedure, the reaction of 1a
(51.5 mg, 0.1 mmol) with 2e (38.6 mg, 0.15 mmol) was followed by
column chromatography (hexane:CH2Cl2 at 4:1 to 1:2), which afforded
compound 6ae (34.8 mg, 54% yield) as olive solids, mp = 129.1–
129.9 °C. UV-vis (CH2Cl2): λmax (log ε) = 445 (4.15), 517 (3.20), 734
(3.54) nm. FT-IR (KBr disk): 3453, 3056, 2986, 2926, 2207, 1591, 1495,
1391, 1363, 1261, 1131, 836, 746, 692, 517 cm1. Anal. calcd. for
C41H36N6O2: C, 76.38; H, 5.63; N, 13.03. Found: C, 76.43; H, 5.65; N,
12.99. HRMS (ESI/LTQ Orbitrap) m/z: [M] calcd. for for C41H36N6O2:
644.2900. Found: 644.2993.
Compound 6af. According to the general procedure, the reaction of 1a
(51.5 mg, 0.1 mmol) with 2f (38.6 mg, 0.15 mmol) followed by column
chromatography (hexane:CH2Cl2 at 4:1 to 1:2) produced compound 6af
(29.6 mg, 46% yield) as olive solids, mp = 130.0–130.7 °C. UV-vis
(CH2Cl2): λmax (log ε) = 366 (4.28), 436 (4.13), 518 (3.19), 731 (3.58) nm.
FT-IR (KBr disk): 3439, 3058, 2952, 2926, 2856, 2205, 1591, 1505, 1389,
1363, 1263, 1137, 746, 690, 515 cm1. Anal. calcd. for C41H36N6O2: C,
76.38; H, 5.63; N, 13.03. Found: C, 76.36; H, 5.64; N, 13.00.
HRMS (ESI/LTQ Orbitrap) m/z: [M] calcd. for C41H36N6O2: 644.2900.
Found: 644.2988.
Compound 8fe. According to the general procedure, the reaction of 1f
(45.3 mg, 0.1 mmol) with 2e (38.6 mg, 0.15 mmol) was followed by
column chromatography (hexane:CH2Cl2 at 1:1 to 1:3), which afforded
compound 8fe (29.7 mg, 51% yield) as violet solids, mp = 114.4–
116.3 °C. UV-vis (CH2Cl2): λmax (log ε) = 381 (4.35), 571 (3.48) nm. FT-IR
(KBr disk): 3459, 3060, 2990, 2928, 2858, 2212, 2118, 1702, 1600, 1487,
1421, 1389, 1365, 1307, 1247, 1167, 1125, 838, 756, 692, 521 cm1
.
Anal. calcd. for C35H30N6O3: C, 72.15; H, 5.19; N, 14.42. Found: C,
72.20; H, 5.18; N, 14.45. HRMS (ESI/Q-TOF) m/z: [M] calcd. for
C35H30N6O3: 582.2379. Found: 582.2398.
The general procedure for the preparation of hydrazones 9a and 9b.
A solution of phenyl hydrazine (6 mmol) and aromatic aldehyde (6 mmol)
in EtOH (20 mL) was refluxed for 1 h in an inert atmosphere. Water
(8 mL) was added, and hydrazone was isolated by filtration followed by
washing with 70% EtOH (10 mL).
1-(4-Ethynylbenzylidene)-2-phenylhydrazine 9a. According to the
general procedure, the reaction of phenyl hydrazine (594 µL, 6 mmol)
with 4-ethynylbenzaldehyde (781 mg, 6 mmol) afforded compound 9a
(1.084 g, 82% yield) as light-yellow solids, mp = 112.5–113.2 °C. 1Н
NMR (DMSO-d6, 400 MHz): δ 4.25 (s, 1H), 6.77 (t, J = 7.26 Hz, 1H), 7.09
(d, J = 7.62 Hz, 2H), 7.21–7.25 (m, 2H), 7.48 (d, J = 8.28 Hz, 2H), 7.65 (d,
J = 8.30 Hz, 2H), 7.85 (s, 1H), 10.51 (s, 1H) ppm. 13C NMR (DMSO-d6,
100 MHz): δ 81.6, 83.7, 112.2, 119.1, 120.7, 125.6, 129.2, 129.2, 132.0,
135.3, 136.4, 145.0 ppm. FT-IR (KBr disk): 3303, 3284, 3054, 3029,
2924, 2102, 1603, 1581, 1523, 1498, 1257, 1141, 751, 698, 659, 546,
510 cm1. Anal. calcd. for C15H12N2: C, 81.79; H, 5.49; N, 12.72. Found:
C, 81.71; H, 5.50; N, 12.69.
1-(3-Ethynylbenzylidene)-2-phenylhydrazine 9b. According to the
general procedure, the reaction of phenyl hydrazine (594 µL, 6 mmol)
with 3-ethynylbenzaldehyde (781 mg, 6 mmol) produced compound 9b
(1.123 g, 85% yield) as light-yellow solids, mp = 108.8–109.7 °C. 1Н
NMR (DMSO-d6, 400 MHz): δ 4.23 (s, 1H), 6.76 (t, J = 7.14 Hz, 1H), 7.08
(d, J = 7.96 Hz, 2H), 7.21–7.24 (m, 2H), 7.38–7.42 (m, 2H), 7.69 (d, J =
6.24 Hz, 1H), 7.73 (s, 1H), 7.84 (s, 1H), 10.47 (s,1H) ppm. 13C NMR
(DMSO-d6, 100 MHz): δ 80.9, 83.3, 112.1, 119.0, 122.1, 126.0, 128.6,
129.1, 129.2, 130.9, 135.1, 136.4, 145.1 ppm. FT-IR (KBr disk): 3315,
3291, 3271, 3056, 3034, 2962, 2926, 2108, 1593, 1563, 1519, 1475,
1447, 1257, 1143, 908, 800, 752, 692, 630, 509 cm1. Anal. calcd. for
C15H12N2: C, 81.79; H, 5.49; N, 12.72. Found: C, 81.76; H, 5.48; N, 12.70.
Compound 5bf. According to the general procedure, the reaction of 1b
(45.3 mg, 0.1 mmol) with 2f (38.6 mg, 0.15 mmol) followed by column
chromatography (hexane:CHCl3 at 1:1 to 0:1) gave compound 5bf
Chlorocarbamoyl hydrazones 10c and 10d. Hydrazone 9a or 9b
(1.101 g, 5 mmol) was dissolved in dry CH2Cl2 (50 mL) in a Schlenk flask
7
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000044
European Journal of Organic Chemistry
FULL PAPER
in an inert atmosphere. Anhydrous pyridine (495 µL, 6.1 mmol) and a
solution of triphosgene (1.484 g, 5 mmol) in 15 mL of dry CH2Cl2 were
added, and the resulting solution was stirred for 3 h at room temperature.
Next, 1 M HCl (20 mL) was added to the mixture, and the product was
extracted with CH2Cl2, washed with water (3  35 mL) and dried with
MgSO4. Chlorocarbamoyls 10c and 10d were purified by flash
chromatography (CH2Cl2 as an eluent) with subsequent evaporation of
the solvent.
solution was stirred for 10 min until full consumption of diazonium salts.
Then, the solvent was removed in a rotary evaporator in vacuo, cold 90%
EtOH (10 mL) was added to the residue, and the precipitated formazan
was filtered off and washed with cold 90% EtOH (2  20 mL).
3-(4-Ethynylphenyl)-1,5-diphenylformazan 12a. According to the
general procedure, the reaction of phenyldiazonium tosylates with 9a
produced compound 12a as dark violet solids (396 mg, 61% yield),
mp = 204.8–205.4 °C. UV-vis (CH2Cl2): λmax (log ε) = 489 (4.34) nm. 1Н
NMR (CDCl3, 400 MHz): δ 3.12 (s, 1H), 7.30 (t, 2H, J = 7.31 Hz), 7.45–
7.49 (m, 4H), 7.56 (d, 2H, J = 8.33 Hz), 7.68 (d, 4H, J = 7.91 Hz), 8.11 (d,
2H, J = 8.17 Hz) ppm. 13C NMR (CDCl3, 100 MHz): δ 77.9, 84.2, 119.0,
121.0, 125.6, 127.9, 129.6, 132.4, 138.1, 140.4, 147.8 ppm. FT-IR (KBr
disk): 3281, 3062, 2926, 2854, 2098, 1746, 1597, 1501, 1453, 1355,
1241, 1185, 1034, 838, 764, 746, 688, 660, 636, 485 cm1. Anal. calcd.
for C21H16N4: C, 77.76; H, 4.97; N, 17.27. Found: C, 77.65; H, 4.98; N,
17.28.
2-(4-Ethynylphenyl)-α-chloroformyl-4-phenylhydrazone
10c.
According to the general procedure, the reaction of 1-(4-
ethynylbenzylidene)-2-phenylhydrazine 9a (1.102 g, 5 mmol) with
triphosgene afforded compound 10c (1.117 g, 79% yield) as light-yellow
solids, mp = 96.0–96.9 °C. 1H NMR (DMSO-d6, 400 MHz): δ 4.26 (s, 1H),
6.75 (t, J = 7.25 Hz, 1H), 7.09 (d, J = 8.51, 1H), 7.21 (dd, J = 8.47 Hz, J =
7.30 Hz, 1H), 7.46 (d, J = 8.31 Hz, 1H), 7.63 (d, J = 8.32 Hz, 1H), 7.88 (s,
1H) ppm. 13C NMR (DMSO-d6, 100 MHz): δ 81.7, 83.7, 112.2, 119.1,
120.7, 120.8, 124.3, 125.6, 129.2, 132.1, 135.2, 136.5, 145.1 ppm. FT-IR
(KBr disk): 3267, 1706, 1389, 1321, 1291, 1233, 1185, 886, 840, 829,
710, 660, 547, 312 cm1. Anal. calcd. for C16H11ClN2O: C, 67.97; H, 3.92;
N, 9.91. Found: C, 67.92; H, 3.93; N, 9.89.
3-(3-Ethynylphenyl)-1,5-diphenylformazan 12b. According to the
general procedure, the reaction of phenyldiazonium tosylates with 9b
afforded compound 12b as dark violet solids (402 mg, 62% yield),
mp = 126.9–127.1 °C. UV-vis (CH2Cl2): λmax (log ε) = 485 (4.24) nm. 1Н
NMR (CDCl3, 400 MHz): δ 3.13 (s, 1H), 7.30 (t, 2H, J = 7.29 Hz), 7.38–
7.42 (m, 1H), 7.45–7.49 (m, 5H), 7.69 (d, 4H, J = 7.92 Hz), 8.14 (d, 1H, J
= 7.80 Hz), 8.27 (s, 1H) ppm. 13C NMR (CDCl3, 100 MHz): δ 77.1, 84.2,
119.0, 122.3, 126.3, 127.8, 128.6, 129.5, 129.6, 131.3, 137.8, 140.2,
147.8 ppm. FT-IR (KBr disk): 3303, 3062, 2926, 2854, 2106, 1744, 1597,
2-(3-Ethynylphenyl)-α-chloroformyl-4-phenylhydrazone
10d.
According to the general procedure, the reaction of 1-(3-
ethynylbenzylidene)-2-phenylhydrazine 9b (1.102 g, 5 mmol) with
triphosgene gave compound 10d (1.117 g, 79% yield) as light-yellow
solids, mp = 91.2–91.6 °C. 1Н NMR (DMSO-d6, 400 MHz): δ 4.23 (s, 1H),
6.75 (t, J = 7.21 Hz, 1H), 7.08 (d, J = 7.79 Hz, 2H), 7.19–7.23 (m, 2H),
7.35–7.41 (m, 2H), 7.67 (d, J = 6.65 Hz, 1H), 7.71 (s, 1H), 7.86 (s,
1H) ppm. 13C NMR (DMSO-d6, 100 MHz): δ 81.0, 83.4, 112.1, 119.0,
122.1, 124.3, 126.0, 128.6, 129.2, 130.9, 135.1, 136.5, 145.1 ppm. FT-IR
(KBr disk): 3281, 1732, 1377, 1293, 1195, 832, 708, 620 cm1. Anal.
calcd. for C16H11ClN2O: C, 67.97; H, 3.92; N, 9.91. Found: C, 67.92; H,
3.93; N, 9.89.
1509, 1481, 1453, 1351, 1241, 1167, 1044, 904, 764, 686, 652, 483 cm1
Anal. calcd. for C21H16N4: C, 77.76; H, 4.97; N, 17.27. Found: C, 77.68; H,
4.99; N, 17.26.
.
Synthesis of 2-(4-iodophenyl)-4,4,5,5-tetramethylimidazolidine-1,3-
diol 13. 4-Iodobenzaldehyde (464 mg, 2 mmol) and N,N-(2,3-
dimethylbutane-2,3-diyl)bis(hydroxylamine) (326 mg, 2.2 mmol) were
dissolved in MeOH (8 mL), and the mixture was deoxygenated. The
reaction mass was stirred for 12 h at gentle reflux then cooled to –15 °C,
and the precipitate was filtered off and washed with 70% MeOH (10 mL).
Yield: 616 mg, 85%, mp = 219.5 °C (decomp.). 1Н NMR (CDCl3, 400
MHz): δ 1.02 (s, 6H), 1.06 (s, 6H), 4.46 (s, 1H), 7.28 (d, J = 8.18 Hz, 2H),
7.69 (d, J = 8.17 Hz, 2H), 7.79 (s, 2H) ppm. 13C NMR (CDCl3, 100 MHz):
δ 17.3, 24.4, 66.2, 89.7, 93.2, 130.9, 136.4, 141.9 ppm. FT-IR (KBr
disk): 3238, 2976, 2926, 2910, 1589, 1485, 1461, 1373, 1147, 1086,
1006, 917, 872, 796 cm1. Anal. calcd. for C13H19IN2O2: C, 43.11; H,
5.29; N, 7.73. Found: C, 43.22; H, 5.28; N, 7.77.
2,4,6-Substituted
1,2,4,5-tetrazenane-3-ones
11c
and
11d.
Chlorocarbamoyl hydrazone 10c or 10d (565.5 mg, 2 mmol) was
dissolved in deoxygenated EtOH (20 mL), and Et3N (304 µL, 2.2 mmol)
and phenylhydrazine (236 µL, 2.2 mmol) were added. The mixture was
heated at 65 C for 8 h in an inert atmosphere and cooled to 0 C. Water
(2 mL) was added, and the precipitate was filtered off and washed with
80% EtOH (5 mL).
6-(4-Ethynylphenyl)-2,4-diphenyl-1,2,4,5-tetrazinan-3-one
11c.
According to the general procedure, the reaction of 2-(4-ethynylphenyl)-
α-chloroformyl-4-phenylhydrazone 10c with phenylhydrazine produced
compound 11c as greyish-white solids (638 mg, 90% yield) mp = 230.1–
234.7 °C (decomp.). 1Н NMR (DMSO-d6, 400 MHz): δ 4.21 (s, 1H), 5.42–
5.44 (m, 1H), 6.46 (d, J = 8.68 Hz, 1H), 7.08 (t, J = 7.32 Hz, 2H), 7.32–
7.35 (m, 4H), 7.47 (d, J = 8.16 Hz, 1H), 7.55 (d, J = 8.06 Hz, 1H), 7.60 (d,
J = 7.71 Hz, 2H) ppm. 13C NMR (DMSO-d6, 100 MHz): δ 72.5, 81.2, 83.2,
121.1, 121.5, 123.4, 127.4, 128.1, 131.7, 138.6, 142.7, 157.1 ppm. FT-IR
(KBr disk): 3286, 3245, 3065, 3037, 2927, 2103, 1706, 1620, 1598, 1493,
1376, 1307, 917, 831, 759, 695 cm1. Anal. calcd. for C22H18N4O: C,
74.56; H, 5.12; N, 15.81. Found: C, 74.52; H, 5.14; N, 15.85.
Acknowledgments
The authors would like to acknowledge the Multi-Access
Chemical Research Center SB RAS for spectral and analytical
measurements and the Laboratory of Physicochemical
Analytical Methods of Tomsk State University for NMR and
HRMS measurements. The authors thank Dr. Evgeny A.
Mostovich (Novosibirsk State University) for a fruitful discussion
and useful comments. This research was supported by Tomsk
Polytechnic University (project No. VIU-RSCABS-68/2019) and
the Russian Science Foundation (grant No. 18-13-00173).The
electrochemical analysis was supported by a state assignment
of the Ministry of Science and Higher Education of the Russian
Federation (Project No. 0238-2019-0004).
6-(3-Ethynylphenyl)-2,4-diphenyl-1,2,4,5-tetrazinan-3-one
11d.
According to the general procedure, the reaction of 2-(3-ethynylphenyl)-
α-chloroformyl-4-phenylhydrazone 10d with phenylhydrazine afforded
compound 11d as greyish-white solids (581 mg, 82% yield) mp = 215.3–
217.1 °C (decomp.). 1Н NMR (DMSO-d6, 400 MHz): δ 4.19 (s, 1H), 5.41
(t, J = 8.92 Hz, 1H), 6.46 (d, J = 8.99 Hz, 2H), 7.07 (t, J = 7.30 Hz, 2H),
7.31–7.44 (m, 6H), 7.56-7.63 (m, 6H) ppm. 13C NMR (DMSO-d6,
100 MHz): δ 72.5, 81.0, 121.1, 121.7, 123.4, 127.7, 128.1, 128.8, 130.3,
131.5, 138.4, 142.7, 157.2 ppm. FT-IR (KBr disk): 3286, 3248, 3062,
2927, 2102, 1708, 1620, 1595, 1493, 1376, 1307, 756, 697 cm1. Anal.
calcd. for C22H18N4O: C, 74.56; H, 5.12; N, 15.81. Found: C, 74.51; H,
5.14; N, 15.79.
Keywords: stable radicals • verdazyl radicals • nitroxides • multi-
The general procedure for the preparation of formazans 12a and
12b: Phenyldiazonium tosylate (2.4 mmol, 553 mg) was added in six
portions to a solution of hydrazone 9a or 9b (441 mg, 2 mmol) and Et3N
(557 µL, 4 mmol) in CHCl3 (20 mL) at room temperature. The resulting
spin systems • Sonogashira coupling
8
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European Journal of Organic Chemistry
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Entry for the Table of Contents
Sonogashira coupling was investigated as a promising strategy for the synthesis of a multi-spin system based on ‘Kuhn’-verdazyls,
oxoverdazyls, and nitronyl-nitroxides. Eleven hetero-diradicals were isolated in up to 56% yields using a one-step protocol.
Oxoverdazyl radicals manifested the highest reactivity among the studied radicals. This research opens up access to simple design
and investigation of conjugated diradicals.
Keywords: stable radicals; verdazyl radicals; nitroxides; multi-spin systems; Sonogashira coupling
Key Topic: high-spin systems
10
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