Platform for High-Spin Molecules: A Verdazyl-Nitronyl Nitroxide Triradical with Quartet Ground State

Article abstract of DOI:10.1021/jacs.1c02938

Thermally resistant air-stable organic triradicals with a quartet ground state and a large energy gap between spin states are still unique compounds. In this work, we succeeded to design and prepare the first highly stable triradical, consisting of oxoverdazyl and nitronyl nitroxide radical fragments, with a quartet ground state. The triradical and its diradical precursor were synthesized via a palladium-catalyzed cross-coupling reaction of diiodoverdazyl with nitronyl nitroxide-2-ide gold(I) complex. Both paramagnetic compounds were fully characterized by single-crystal X-ray diffraction analysis, superconducting quantum interference device magnetometry, EPR spectroscopy in various matrices, and cyclic voltammetry. In the diradical, the verdazyl and nitronyl nitroxide centers demonstrated full reversibility of redox process, while for the triradical, the electrochemical reduction and oxidation proceed at practically the same redox potentials, but become quasi-reversible. A series of high-level CASSCF/NEVPT2 calculations was performed to predict inter- and intramolecular exchange interactions in crystals of di- and triradicals and to establish their magnetic motifs. Based on the predicted magnetic motifs, the temperature dependences of the magnetic susceptibility were analyzed, and the singlet-triplet (135 ± 10 cm-1) and doublet-quartet (17 ± 2 and 152 ± 19 cm-1) splitting was found to be moderate. Unique high stability of synthesized verdazyl-nitronylnitroxide triradical opens new perspectives for further functionalization and design of high-spin systems with four or more spins.

Full text of DOI:10.1021/jacs.1c02938

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Platform for High-Spin Molecules: A Verdazyl-Nitronyl Nitroxide
Triradical with Quartet Ground State
Evgeny V. Tretyakov,* Pavel V. Petunin,* Svetlana I. Zhivetyeva, Dmitry E. Gorbunov, Nina P. Gritsan,*
Matvey V. Fedin, Dmitri V. Stass, Rimma I. Samoilova, Irina Yu. Bagryanskaya, Inna K. Shundrina,
Artem S. Bogomyakov, Maxim S. Kazantsev, Pavel S. Postnikov, Marina E. Trusova,
and Victor I. Ovcharenko*
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ABSTRACT: Thermally resistant air-stable organic triradicals with a quartet
ground state and a large energy gap between spin states are still unique
compounds. In this work, we succeeded to design and prepare the first highly
stable triradical, consisting of oxoverdazyl and nitronyl nitroxide radical
fragments, with a quartet ground state. The triradical and its diradical
precursor were synthesized via a palladium-catalyzed cross-coupling reaction
of diiodoverdazyl with nitronyl nitroxide-2-ide gold(I) complex. Both
paramagnetic compounds were fully characterized by single-crystal X-ray
diffraction analysis, superconducting quantum interference device magneto-
metry, EPR spectroscopy in various matrices, and cyclic voltammetry. In the
diradical, the verdazyl and nitronyl nitroxide centers demonstrated full
reversibility of redox process, while for the triradical, the electrochemical reduction and oxidation proceed at practically the same
redox potentials, but become quasi-reversible. A series of high-level CASSCF/NEVPT2 calculations was performed to predict inter-
and intramolecular exchange interactions in crystals of di- and triradicals and to establish their magnetic motifs. Based on the
predicted magnetic motifs, the temperature dependences of the magnetic susceptibility were analyzed, and the singlet−triplet (135
10 cm−1) and doublet-quartet (17 2 and 152 19 cm−1) splitting was found to be moderate. Unique high stability of synthesized
verdazyl-nitronylnitroxide triradical opens new perspectives for further functionalization and design of high-spin systems with four or
more spins.
representative of carbon-based triradical, where chlorine
atoms prevent dimerization and restrict rotation of aryl groups
owing to steric hindrance.27 Triradical 1 is stable in air up to
250 °C; it was isolated as two stereoisomers with C2 and D3
symmetries. Magnetic measurements have revealed that the
energy gap between the quartet ground state and the lowest
energy doublet state (ΔEDQ/k) exceeds 290 K for both forms.
Among annulated hydrazyls, 1,3,5-trisverdazyl-benzene (2) has
been prepared as black prisms decomposing at 242−243 °C.28
It has been shown that the quartet and doublet states of
triradical 2 are equally populated in the temperature range
from 230 to 340 K.
INTRODUCTION
■
π-Conjugated purely organic high-spin molecules (di- and
polyradicals) with substantial intramolecular exchange inter-
actions and intriguing magnetic properties have a significant
potential for advanced technological applications and funda-
mental science.1−5 The stable di- and polyradicals are
considered as working elements of spintronic devices,6−8
multifunctional molecule-based magnetic materials, and
quantum machines.9,10 Guidelines for the design of such
high-spin molecules with exchange-coupled stable radical
moieties have been developed using molecular orbital11−15
and valence bond16,17 theories. Based on these recommenda-
tions, a number of diradicals with m-phenylene or alkylidene
bridging units with strong ferromagnetic interaction between
radical parts and stability at room temperature have been
designed.18−26 Despite the obvious breakthrough in the
chemistry of diradicals with a large energy gap between the
high-spin ground state and low-spin excited state, air-stable tri-
and polyradicals are still exotic compounds.
The greatest success in the synthesis of triradicals with a
quartet ground state has been achieved for nitroxides, which
Received: March 18, 2021
Published: May 21, 2021
Previously reported examples of thoroughly characterized
organic triradicals with a quartet ground state are shown in
Figure 1. Polychlorinated triradical 1 is the only one
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Figure 1. Previously reported air-stable organic triradicals with a quartet ground state.
Figure 2. Step-by-step design of triradicals 7 and NN-Vrd-NN using ethylene-1,1-diyls as coupling units.
can be purified by column chromatography and applied as
well-defined high-spin units for metal complexation.29,30 In
crystals of the first nitronyl nitroxide (NN) triradical 3 (ref 31;
Figure 1), intramolecular exchange interactions have been
overlapped with intermolecular antiferromagnetic interactions.
To estimate the intramolecular magnetic coupling, a mixed
crystal of 3 with 1,3,5-trinitrobenzene (3·TNB, 1:1) was
prepared. Superconducting quantum interference device
(SQUID) magnetometry of 3·TNB has proved the intra-
molecular ferromagnetic interactions with J/kB = 23 K.32−34
The intramolecular ferromagnetic coupling is quite weak due
to the small spin population of the C(2) atom in the NN part.
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Scheme 1. Synthesis of Diradical I-Vrd-NN and Triradical NN-Vrd-NN
Figure 3. ORTEP views of diradical I-Vrd-NN (a) and triradical NN-Vrd-NN (b). Thermal ellipsoids are drawn at the 50% probability level.
Similar weak intramolecular exchange interactions with J/kB =
6.8−15.3 K have been observed in tert-butyl nitroxide
triradicals 4 and 5.35−37 The shortening of linkers between
the paramagnetic centers has led to a significant increase in the
exchange interaction parameters. Thus, in triradical 6, the
energy gap between the quartet ground and the lowest excited
doublet states is reported to be 240 K.38 Finally, the magnetic
measurements of mixed triradicals 7 and 8 have clearly proved
quartet ground states with J/kB = 231 and 127 K,
respectively.19
In the current work, we present a new type of triradical
(NN-Vrd-NN, Figure 2) as a verdazyl (Vrd)-containing analog
of compound 7 (Figure 1). Replacing the nitroxide moiety
with Vrd is justified due to higher stability and a great potential
for structure modification. In addition to the obvious
possibility of conversion of the NN group into an
iminonitroxide one, the NN-Vrd-NN triradical contains one
more phenyl substituent suitable for addition of functional
groups including paramagnetic centers. This property allows us
to regard the NN-Vrd-NN triradical as a platform for the
design of high-spin polyradicals. To create such molecules, it is
important for future research to investigate intramolecular
exchange interactions in NN-Vrd-NN.
RESULTS AND DISCUSSION
■
Our approach to the synthesis of triradical NN-Vrd-NN is
related to the previously reported pathway for the Vrd-NN
diradicals with a triplet ground state, proving the high stability
of 6-oxoverdazyls in reaction conditions.26 It was demon-
strated26 using spin alternation rule,39,40 quantum chemical
calculations and SQUID magnetometry that both the para-
phenylene bridging by N1 or N5 nitrogen atoms of Vrd and
meta-phenylene bridging by C3 carbon atom of Vrd lead to the
triplet ground state of the Vrd-NN diradicals. To get the high-
spin ground state, diradical I-Vrd-NN and triradical NN-Vrd-
NN were also designed using para-phenylene bridging
connected to nitrogen atoms. The synthesis of di- and
triradical were performed via a palladium-catalyzed cross-
coupling reaction of novel diiodoverdazyl I-Vrd-I with a
gold(I) nitronyl nitroxide-2-ide complex Ph3P-Au-NN41,42
(Scheme 1, Experimental Section, and Supporting Information
Section S1).
Mixtures of I-Vrd-NN and NN-Vrd-NN were formed in all
experiments, and the best result was achieved under the
heating of diiodoverdazyl I-Vrd-I (1 equiv) and Ph3P-Au-NN
(1.5 equiv) with 10 mol % of Pd(PPh3)4 in dimethylformamide
(DMF) in an inert atmosphere. Analytically pure samples of
the di- and triradical were isolated by column chromatography
on silica followed by recrystallization from an n-heptane-
acetone mixture. This system was also applied for crystal
growth.
Here, we report the synthesis, molecular and crystal
structure, and magnetic properties of air-stable triradical NN-
Vrd-NN and its diradical precursor I-Vrd-NN. Analysis of
temperature dependence of magnetic susceptibility and pulse
EPR-detected transient nutations confirmed the triplet and
quartet ground states of the diradical and triradical,
respectively.
Crystal structures of I-Vrd-NN and NN-Vrd-NN were
solved by X-ray crystallographic analysis. Monoradical
precursor I-Vrd-I upon crystallization from various solvents
gives tightly intertwined fine-needle crystals. The crystals were
too thin, not suitable for single crystal X-ray study. ORTEP
drawings with a numbering scheme of both paramagnets are
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Figure 4. A fragment of the crystal structure showing a ribbon of NN-Vrd-NN triradicals (a), a dimer of ribbons (b), and their packing (c). Short
intermolecular contacts, including those of nitroxide oxygens are indicated by dotted lines.
given in Figure 3a,b; the selected bond distances and angles
with their estimated standard deviations are listed in Table
S2.1.
In crystalline paramagnets I-Vrd-NN and NN-Vrd-NN,
bond lengths in the NN and Vrd parts are close to the average
values reported for these types of radicals.43,44 The Vrd and
phenyl rings are nearly planar. The imidazoline rings have a
twisted shape (C2 symmetry): in I-Vrd-NN, the C10 and C11
atoms are located on opposite sides of the plane of the NN
moiety at distances of 0.145 and 0.195 Å, and in NN-Vrd-NN,
atoms C10, C11, C29, and C30 are located at distances of
0.220−0.284 Å. The NN groups are twisted at different angles
(28.3−51.3°) relative to the phenyl rings, as the phenyl rings
are twisted in relation to the Vrd ring (6.9° and 11.5° for
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Figure 5. Experimental UV−vis spectra of the I-Vrd-NN diradical (red) and NN-Vrd-NN triradical (black). The vertical bars indicate positions
and oscillator strengths (right axis) of the electronic transitions calculated at the TD-UB3LYP/def2-TZVP level for I-Vrd-NN (red bars) and NN-
Vrd-NN (black bars) (a). The excitations (HOMO → SOMO1) that contribute mainly to the long-wavelength bands of the I-Vrd-NN diradical
and NN-Vrd-NN triradical (b).
slightly twisted C22−C27 phenyl rings and from 22.40° to
53.2° for the other rings).
showed that the long-wavelength bands of all the compounds
have similar maxima (562−576 nm) and intensities. By
contrast, the spectra revealed the sufficient differences in the
300−400 nm region (or 25,000−33,000 cm−1): If we go from
the Vrd radical I-Vrd-I to the di- and triradical (I-Vrd-NN and
NN-Vrd-NN), the broad structureless band of the monoradical
was replaced by a band characteristic of NN radicals with
vibrational structure.45 To assign the absorption bands and to
understand similarities and differences between these spectra,
we performed time-dependent DFT calculations at the TD-
UB3LYP/def2-TZVP level of theory.
The calculations were in good agreement with experimental
UV−vis spectra of I-Vrd-NN and NN-Vrd-NN (Figure 5a).
The long-wavelength transitions in di- and triradicals mainly
cause a redistribution of the electronic density within the Vrd
moiety, similar to the case of the I-Vrd-I radical (Figure 5b and
Figure S3.5).
According to the calculations, SOMO1 orbitals in all three
compounds have similar energies and are localized mainly on
Vrd moieties with minor delocalization to NN moieties.
SOMO2 in I-Vrd-NN as well as SOMO2 and SOMO3 in NN-
Vrd-NN are localized exclusively on NN moieties, and their
energies are almost identical (Figure S3.6). The arrangement
of the orbitals in the energy diagram and the corresponding
distributions of the electron density are reflected in the
electrochemical properties of paramagnets (Figure 6 and Table
1). The electrochemical properties of the synthesized di- and
triradicals are consistent with the previous data obtained for
the Vrd-NN diradicals.26 Due to the proximity of the Eox values
for Vrd and NN moieties, we used the computational results
(SI Section S3) to assign the electrochemical waves in di- and
triradicals. Based on the known relationship46 between ESOMO
and Eox and that ESOMO(NN) is higher than ESOMO(Vrd), we
concluded that Eox(NN) is lower than Eox(Vrd). Electro-
chemical reduction of the Vrd moiety in the mono-, di-, and
triradicals in CH2Cl2 appears as one-electron reversible
reduction waves with half-wave potentials of approximately
−0.90 V (versus Fc/Fc+, hereinafter). Likewise, the oxidation
processes associated with the Vrd fragment are characterized
by reversible or quasi-reversible waves with similar half-wave
The I-Vrd-NN diradical crystallizes as a 1:1 solvate with
acetone in the triclinic P-1 space group. The crystal packing of
the diradical is characterized by short intermolecular contacts
O3···H21 (2.533 Å) leading to the formation of centrosym-
metrical dimers (Figure S2.1 and Table S2.2). Each dimer is
linked with two adjacent dimers by short contacts O2···H18
(2.656 Å), thus yielding a ribbon structure. Acetone molecules
fill the space between the ribbons occupying a fixed position
due to numerous short contacts (such as OAce···HMe, OAce···
HPh, and HAce···ONO) with diradical molecules and with
neighboring acetone (Figure S2.2).
The NN-Vrd-NN triradical crystallizes in the tetragonal
P42/n space group where free volume is occupied by highly
disordered solvent molecules. In the triradical’s crystals, oxygen
atoms of one nitroxide group form short intermolecular
contacts O3···H13A (2.540 Å) and O2···H7 (2.555 Å) with
hydrogen atoms of one of the methyl groups and phenyl
moiety, respectively (Figure 4a, Table S2.2). Such contacts
give rise to ribbons in which the oxygen atom of another
nitroxide group forms the O5···H31C contact (2.575 Å).
Rotation of the crystal structure of NN-Vrd-NN clearly
demonstrated that the ribbon is connected by short O4···
H31A contacts (2.448 Å) with the neighboring ribbon to form
a centrosymmetric fragment (Figure 4b). Finally, the dimeric
units are packed due to intermolecular short contacts with the
participation of Vrd oxygen atoms O1···H13B, O1···H14B, and
O1···H15B (2.487−2.700 Å) and nitroxide oxygen atoms O3···
H33B (2.494 Å) with the formation of a parquet motif (Figure
4c). Another important feature of the crystal structure is the
formation of infinite π-stacks between the Vrd and phenyl rings
(C22−C27): The distances between the molecular planes
inside the stacks are 3.55 Å [Cg···Cg 3.573(2) Å] for I-Vrd-
NN and 3.54 Å [Cg···Cg 3.710(2) Å] for NN-Vrd-NN (Figure
S2.4)
To study the electronic structure of the mono-, di-, and
triradical, we used UV−vis spectroscopy, cyclic voltammetry,
and theoretical methods for the interpretation of results.
Analysis of the UV−vis spectra (Figure S3.1 and SI Section S3)
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for NN radicals.49 The set of parameters “Ai = 0.322, 0.317,
0.227, 0.213, 0.363, 0.361 mT (for i = 1−6) and g = 2.008”
leads to the best fit.
Similarly, the EPR spectrum of the NN-Vrd-NN triradical
can be simulated taking into account eight nitrogen atoms
(sum of Vrd and two of NN moieties) and their HFI constants
divided by 3 owing to significant exchange interactions among
three radical centers. The set of parameters “Ai = 0.215, 0.210,
0.153, 0.143, 0.243, 0.253, 0.249, 0.247 mT (for i = 1−8) and g
= 2.0085” corresponds to the best fit (Figure 7d). Analysis of
all best-fit parameters revealed perfect agreement of the
experimental scaling factors with the expected values of 2 and 3
for the di- and triradical, respectively. This is convincing
evidence that in liquid solutions synthesized derivatives I-Vrd-
NN and NN-Vrd-NN are in the form of a di- and triradical,
respectively, and intramolecular exchange interactions are
strong (on the EPR scale). Auxiliary pulse EPR experiments
in frozen solutions provided additional confirmations of these
findings (SI Section S4).
Bulk magnetic properties of the I-Vrd-I radical, I-Vrd-NN
diradical, and NN-Vrd-NN triradical were experimentally
investigated by SQUID magnetometry of polycrystalline
samples. In the case of the I-Vrd-I radical, χT vs T dependence
is in excellent agreement with the Curie−Weiss law for molar
Figure 6. Cyclic voltammograms of the mono-, di-, and triradical in
CH2Cl2 solution (100 mV/s with 0.1 M Bu4NPF6 as an electrolyte).
Table 1. Oxidation and Reduction Potentials of the Mono-,
a
Di-, and Triradicals in CH2Cl2
magnetic susceptibility with negative Weiss temperature TW
=
1/2
Red2
1/2
Red1
1/2
Ox1
1/2
compound
E
E
E
E
Ox2
−4 K, indicating the weak antiferromagnetic (AF) exchange
interactions between radicals. For the I-Vrd-NN diradical, the
high-temperature value of χT is 0.94 cm3·K·mol−1, which is
between a theoretical one of 0.75 cm3·K·mol−1 for two non-
interacting spins S = 1/2 and 1.00 cm3·K·mol−1 for one
paramagnetic center with spin S = 1. With decreasing
temperature, χT increases reaching 1.00 cm3·K·mol−1 at 70
K and then decreases to 0.51 cm3·K·mol−1 at 2 K (Figure 8a).
The shape of the χT dependence on temperature for the
sample of the NN-Vrd-NN triradical is similar to that of the
diradical, except that it starts from a high-temperature value of
1.18 cm3·K·mol−1, then reaches a maximum value of 1.31 cm3·
K·mol−1 at 50 K, and drops to 0.64 cm3·K·mol−1 at 2 K.
Therefore, at high temperatures, χT is close to the theoretical
value of 1.125 cm3·K·mol−1 for three non-interacting spins S =
1/2. Maximum χT value, 1.31 cm3·K·mol−1, turned out to be
much lower than the theoretical value 1.875 cm3·K·mol−1 for S
= 3/2 and close to 1.375 cm3·K·mol−1 for non-interacting spins
S1 = 1 and S2 = 1/2, which is the indication of one moderate
ferromagnetic (FM) exchange interaction and two much
weaker interactions. Our quantum chemical calculations
confirm the significant difference in Jij values. A decrease of
the χT with lowering T below 50 K is most likely caused by the
intermolecular AF interactions, although, in the general case,
two weak intramolecular AF interactions can also lead to a
decrease in χT to 0.375 cm3·K·mol−1. To shed light on the
origin of the observed magnetic behavior of the I-Vrd-NN
diradical and NN-Vrd-NN triradical, intra- and intermolecular
exchange interactions were analyzed theoretically using XRD
crystal structures.
I-Vrd-I
NN-R
I-Vrd-NN
NN-Vrd-NN
−
−0.89
−
−0.88
−0.88
−
0.52
−
0.62
0.61
b
−1.45
−1.48
−1.45
0.40
0.41
0.39
c
c
c
a
b
Potentials are presented in V vs Fc/Fc+. Previously found potentials
for NN-moiety in Vrd-NN diradicals.26 Quasi-reversible processes.
c
potentials: +0.52 V for I-Vrd-I and approximately +0.60 V for
I-Vrd-NN and NN-Vrd-NN. The observed potentials are
typical of radicals of the Vrd family.47 The next set of redox
waves (approximately −1.46 and +0.40 V) is associated with
reduction and oxidation of the NN groups, respectively. It
should be noted that the redox processes are reversible only in
the I-Vrd-NN diradical and become quasi-reversible in the
NN-Vrd-NN triradical.
Figure 7 shows continuous-wave (CW) EPR spectra of I-
Vrd-I, I-Vrd-NN, and NN-Vrd-NN recorded in toluene
solutions at room temperature. In this series, as the number
of radical centers per molecule increases, the EPR spectrum
becomes progressively narrower and less resolved due to a
combination of verdazyl and NN moieties (Figure 7a).
The EPR spectrum of the I-Vrd-I monoradical can be well
reproduced using four isotropic 14N hyperfine interaction
(HFI) constants A1 = 0.644, A2 = 0.633, A3 = 0.454, and A4 =
0.427 mT and g = 2.007 (Figure 7b). These parameters are in
good agreement with the literature data on 6-oxoverdazyl
radicals.48
For a simulation of the EPR spectrum of the I-Vrd-NN
diradical, it is necessary to take into account six nitrogen nuclei
with six HFI constants. Due to the significant exchange
interaction between the two radical centers, each HFI constant
must be halved. Thus, the EPR spectrum of I-Vrd-NN can be
simulated by means of the constants A1−A4 obtained above,
divided by 2, as well as the two HFI constants of the NN
moiety (A5 = 0.726 and A6 = 0.722 mT), also divided by 2
(Figure 7c). The nitrogen atoms of the NN moiety are almost
equivalent, and A5 and A6 are very close to the typical values
Prior to discussing the calculated values of Jij parameters (
11
̂
̂
̂
H = −2 ∑ J SiSj), we briefly consider the electronic structure
ij
of I-Vrd-NN and NN-Vrd-NN. Figure S6.1 displays the nine
active MOs of the CASSCF(9,10) calculations for the I-Vrd-
NN diradical; the occupation numbers of two of these MOs
are equal to 1 (1.0016 and 0.9999). Moreover, in both states,
triplet and singlet, the configuration with four doubly occupied
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Figure 7. X-band room-temperature CW EPR spectra of I-Vrd-I, I-Vrd-NN, and NN-Ver-NN in the toluene solutions. (a) Experimental spectra of
all the compounds. (b−d) Experimental (black) and simulated (red) spectra of the indicated compounds; parameters of the simulations are given
in the text.
Figure 8. SQUID magnetometry of the polycrystalline samples of the I-Vrd-NN diradical (a) and the NN-Vrd-NN triradical (b): Plots of χT vs T
at H = 5000 Oe in warming mode (open circles) and their best-fitting (solid curves) using magnetic models and parameters described in the text.
and two singly occupied MOs prevails (>83%). Thus, the I-
Vrd-NN molecule has a purely diradical character in the lowest
triplet and singlet states. The NN-Vrd-NN molecule has also a
purely triradical character, since the occupation numbers of
three of nine active MOs are equal to 1.000 (Figure S6.2).
To compute the parameters of intramolecular exchange
interactions in I-Vrd-NN and NN-Vrd-NN, both the high-level
CASSCF/NEVT2 and BS-UB3LYP calculations were per-
formed. For the I-Vrd-NN diradical, CASSCF(9,10) (Figure
S6.1) calculations predicted a moderate ferromagnetic
interaction with singlet−triplet splitting ΔEST/kB = 69.9 K
(J/kB = 35.0 K). The consideration of the dynamic electron
correlation at the NEVPT2 level led to a 2-fold increase in
ΔEST/kB to 134.6 K (Jdir/kB = 67.3 K). BS-DFT computations
for the I-Vrd-NN diradical predicted the same sign of J, but a
significantly higher absolute value of J/kB = 283 K. The value
of ΔEST/kB = 2J/kB = 566 K predicted at this level is much
higher than ambient temperature (∼300 K). It should be noted
that such a phenomenon is similar to previous calculations for
a series of Vrd-NN diradicals:26 both procedures predict the
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Figure 9. An energy diagram of the lowest-energy states of the NN-Vrd-NN triradical, as computed at the CASSCF(9,9)/CASPT2/def2-TZVP
level of theory (a). Chemical structure of NN-Vrd-NN and proposed variants of exchange interactions between radical moieties described by two
parameters (b and c).
Figure 10. Magnetic motifs of I-Vrd-NN (a), simplified model (b), and NN-Vrd-NN (c).
same sign, but BS-DFT provides much higher absolute values
of J.
contacts of 2.487−2.700 Å. The magnetic interactions resulting
from these contacts were calculated at the BS-B3LYP/def2-
TZVP level via the conventional model of the nearest
diradicals, in which H atoms are attached to remote radical
centers. Parameters J/kB of the intermolecular exchange
interactions exceeding 2 K were predicted only for intradimer
contacts in I-Vrd-NN (for details see SI Section S6).
Parameters of these intradimer exchange interactions were
also calculated at the CASSCF(10,10)/NEVPT2 level (Table
S6.1).
Therefore, a magnetic motif of I-Vrd-NN crystals consists of
isolated pairs of diradicals, the interactions between which can
be accounted in the mean-field approximation (Figure 10a).
This model is quite complicated and contains four para-
magnetic centers and five parameters (Jdir, J1, J2, J3, and zJ′).
Attempts to use this model for simulation have demonstrated
its overparametrization (for details see SI, Section S5). Thus,
we used for the simulation simplified model of isolated
diradicals (Figure 10b) in the mean field approximation
(Figure 8a), which gives the following best-fit parameters: Jdir/
kB = 97 7 K and zJ′/kB = −2.5 0.3 K. Note that the most
accurate calculations predict Jdir/kB = 67.3 K, which is in good
agreement with experiment.
As for the NN-Vrd-NN triradical, the magnetic motif
represents isolated paramagnetic species (a triradical with two
ferromagnetic interactions, Figure 10c); the intermolecular
interactions can be described in the mean field approximation
(see for details SI, Section S5). The best agreement with
experiment was achieved with the following parameters: J1/kB
= 105 16 K, J2/kB = 17 2 K, and zJ′/kB = −5.6 0.2 K
(Figure 8b). These J1 and J2 values correspond to the following
energies of the doublet states: ED1 = 17 cm−1 and ED2 = 152
cm−1. It is worth mentioning that the most accurate theoretical
predictions are 9.8 and 59.8 cm−1, respectively (Figure 9),
which are in fair agreement with the experimental estimates.
For the NN-Vrd-NN triradical, CASSCF(9,9)/NEVPT2
(Figure S6.2) calculations predicted an energy splitting
between the quartet ground state and two excited doublet
states (Figure 9a). To determine the parameters of the
exchange interactions between three radical moieties, the
following assumptions should be made. If we assume a
symmetrical geometry with two equivalent JVN parameters
(Figure 9b), then we obtain the following: JVN/kB = 28.2 K and
J
NN/kB = −7.3 K. If we ignore the interaction between the NN
radical centers, then the remaining J1/kB and J2/kB values
(Figure 9c) are estimated to be 38.8 and 10.1 K.
To select a more adequate interaction scheme (Figure 9b,c),
we also performed calculations of three J parameters by the BS-
DFT method. The exchange interaction between NN radical
centers turned out to be negligibly small (JNN/kB = −0.3 K),
whereas J1/kB and J2/kB were predicted to be 257 and 79 K,
respectively. The difference between the J1 and J2 is related to
different geometries of π-conjugated paths in the triradical:
Although the Vrd and NN groups are practically coplanar
(within ∼5°), the planes of phenylene linkers form different
angles relative to this common plane (∼29° and ∼52°).
Consequently, the model shown in Figure 9c adequately
describes the important exchange interactions in the triradical,
and the values of J1/kB = 38.8 and J2/kB = 10.1 K are their best
theoretical estimates. Furthermore, BS-DFT calculations
predict much higher (by approximately an order of magnitude)
values of J1 and J2 in comparison to more accurate CASSCF/
NEVPT2.
To assess intermolecular magnetic interactions, we first
investigated intermolecular contacts with the participation of
atoms having a relatively large spin density. For the di- and
triradical, short contacts of the N−O···HMe type and N−O···
HPh type were identified at distances of 2.448 to 2.656 Å. In
addition, NN-Vrd-NN crystals have short CO···HMe
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Article
characterization of starting diiodoverdazyl building block I-Vrd-I are
outlined in Scheme S1.
Synthesis of Diradical I-Vrd-NN and Triradical NN-Vrd-NN.
Verdazyl iodide I-Vrd-I, Ph3P-Au-NN, and Pd(PPh3)4 were dissolved
in DMF under argon (amounts of the reagents are given in Table 2).
CONCLUSION
■
In this work, we succeeded in preparing a heterospin triradical
containing oxoverdazyl and two NN moieties. The triradical
has bridging units of the ethylene-1,1-diyl type and, therefore, a
quartet ground state. The triradical is stable in air and
possesses good thermal stability in an inert atmosphere. The
Vrd and NN centers showed reversible (at −0.88 and 0.41 V vs
Fc/Fc+) and quasi-reversible (at −1.48 and ∼0.61 V vs Fc/Fc+,
in CH2Cl2) redox waves. The magnetic properties were
characterized by SQUID magnetometry of polycrystalline
powders and by EPR spectroscopy in various matrices and
analyzed with reference to XRD data and high-level quantum
chemical calculations. The results point to rather strong
intramolecular ferromagnetic exchange interactions. The
obtained heterospin triradical with quartet ground state can
be further functionalized which opens possibility for design
and synthesis of stable polyradicals with very high-spin ground
state for the development of purely organic magnetic and
electronic materials.
Table 2. Yields of I-Vrd-NN and NN-Vrd-NN Depending on
the Reagent Ratio and Reaction Time
yields (%)
I-
Vrd-
NN
NN-
Vrd-
NN
I-Vrd-I
entry (mmol)
Ph3P-Au-NN
(mmol)
Pd(PPh3)4
(% mol)
time
(h)
1
2
3
0.135
0.4211
0.0863
0.202
0.6318
0.259
10
10
20
1.5
1.5
3
34
20
27
27
3
14
The mixture was heated at 70 °C until full conversion of the starting
Au complex (monitored by TLC in dichloromethane-EtOAc at 35:1).
Then, the reaction mixture was cooled to room temperature, and the
solvent was removed in vacuo. Products I-Vrd-NN and NN-Vrd-NN
were isolated by column chromatography (SiO2, from EtOAc-hexane
at 1:3 to EtOAc-hexane at 1:1 to EtOAc alone) and next crystallized
from an acetone-heptane solution.
1-(4-(4,4,5,5-Tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imida-
zol-2-yl)phenyl)-3-(4-iodophenyl)-5-phenyl-2-oxoverdazyl diradi-
cal (I-Vrd-NN).
EXPERIMENTAL SECTION
■
General Procedures. DMF was dried and distilled over calcium
hydride. Other solvents were of reagent quality. All organic reagents
and solvents were purchased from commercial suppliers (Sigma-
Aldrich, Alfa Aesar, and others) and were used as received. Nitronyl
nitroxide-2-ide gold(I) complex (Ph3P-Au-NN) was prepared
according to a previously reported procedure.39 All reactions were
carried out under argon in oven-dried Schlenk flasks. Routine
monitoring of the reactions was performed by means of silica-coated
aluminum plates (Merck, Silica gel 60, F254), which were analyzed
under UV light at 254 nm. Chromatography was carried out on silica
(0.050−0.160 mm) for column chromatography.
IR spectra were recorded with a Bruker Tensor 27 FTIR
spectrometer in KBr pellets. UV−vis spectra were acquired in a
CH2Cl2 solution on a Bruker Vector-22 spectrometer. Melting points
were determined by means of an FP 900 Thermosystem microscope
̈
melting point apparatus (Mettler-Toledo International Inc., Zurich,
Switzerland), and elemental analysis was performed on a Carlo Erba
CHN analyzer.
Cyclic voltammetry measurements were performed at a sweep rate
of 0.1 V·s−1 in a deoxygenated CH2Cl2 solution by a computer-
controlled P-8 nano potentiostat/galvanostat (Elins, Russia) in
combination with a three-electrode cell (Gamry), with 0.1 M
Bu4NPF6 as a supporting electrolyte. Pt, a Pt wire, and Ag/AgCl
served as working, counter, and reference electrodes, respectively. The
reference electrode was calibrated by measurement of the redox
potentials of ferrocene. The redox event centered at 0 mV
corresponds to the ferrocene/ferrocenium redox couple (ferrocene
added as an internal reference).
Dark purple powder. Mp (5 °C/min): 129.7 °C (decomp.). UV
(CH2Cl2) λmax (lg ε): 574 (3.49), 368 (4.36), 350 (4.25), 242 (4.50)
nm. IR (KBr, cm−1): 3439, 3398, 3090, 3064, 3036, 2987, 2949, 2926,
2868, 2854, 1761, 1703, 1599, 1581, 1522, 1481, 1450, 1414, 1390,
1365, 1311, 1279, 1248, 1217, 1167, 1124, 1101, 1061, 1041, 1022,
1005, 970, 895, 868, 837, 825, 806, 773, 712, 694, 656, 634, 619, 542,
527, 509, 451. High-resolution mass spectrometry (HRMS; ESI-
QTOF) calcd for C27H25IN6O3 [M−]: 608.1033. Found: 608.1039.
Anal. calcd for C27H25IN6O3·acetone: C, 54.06; H, 4.69; N, 12.61.
Found: C, 54.11; H, 4.64; N, 12.57.
1,3-(4-(4,4,5,5-Tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imi-
dazol-2-yl)phenyl)-5-phenyl-2-oxoverdazyl triradical (NN-Vrd-NN).
All EPR data were collected using X/Q-band pulse/cw EPR
spectrometer Bruker Elexsys E580 equipped with an Oxford
Instruments temperature control system (T = 4−300 K). For all
spectral simulations, the EasySpin toolbox for Matlab was utilized.50
The magnetic susceptibility of the polycrystalline samples was
measured with a Quantum Design MPMSXL SQUID magnetometer
in the temperature range of 2−300 K with a magnetic field of up to 5
kOe. None of complexes exhibited any field dependence of molar
magnetic susceptibility at low temperatures. Diamagnetic corrections
were made using the Pascal constants.51 The spin Hamiltonian in
1 1
Dark purple powder. Mp (1 °C/min): 133.6 °C (decomp.). UV
(CH2Cl2) λmax, (lg ε): 583 (3.56), 369 (4.55), 263 (4.48) nm. FT-IR
(KBr, cm−1): 3437, 3091, 3057, 2987, 2937, 2872, 1705, 1599, 1520,
1483, 1468, 1450, 1435, 1421, 1412, 1389, 1365, 1311, 1248, 1217,
1190, 1167, 1134, 1101, 1074, 1043, 1018, 968, 868, 839, 773, 760,
721, 694, 656, 634, 619, 542, 527, 446. HRMS (ESI-QTOF) calcd for
C34H37N8O5 [M−]: 637.2887. Found: 637.2878. Anal. calcd for
C34H37N8O5·acetone: C, 63.87; H, 6.23; N, 16.10. Found: C, 63.71;
H, 6.25; N, 16.13.
̂
̂
general form of H = −2JS1S2was used for an analysis of the
experimental χT(T) dependencies.
Synthesis of Diiodoverdazyl Radical I-Vrd-I, Diradical I-Vrd-
NN, and Triradical NN-Vrd-NN. Standard techniques for synthesis
in an argon atmosphere were used, including Schlenk glassware and
manifold high-vacuum lines. The multistep, efficient synthesis and
X-ray Crystallography. Crystals of I-Vrd-NN and NN-Vrd-NN
for this analysis were prepared by slow evaporation from a solution in
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Article
acetone-heptane at 5 °C. Data collection was performed on a Bruker
Kappa Apex II CCD diffractometer using φ,ω-scans of narrow (0.5°)
frames with Mo Kα radiation (λ = 0.71073 Å) and a graphite
monochromator. The structures were solved by direct methods in the
SHELX-97 software52 and were refined by the full-matrix least-squares
method over all F2 in anisotropic approximation using the SHELXL-
2014/7 software.53 Absorption corrections were made by an empirical
multiscan method with the SADABS program.54 Hydrogen atom
positions were calculated with the riding model. The crystallographic
data are listed in Table 3.
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Computational Details. For the I-Vrd-I radical, all calculations
were performed for the geometry optimized at the UB3LYP/def2-
TZVP level.58−60 For the I-Vrd-NN diradical and NN-Vrd-NN
triradical, the calculations were mainly performed for XRD structures.
Electronic absorption spectra (UV−vis spectra) of the paramagnetic
compounds were computed via time-dependent DFT calculations61
with the B3LYP functional58,59 and the def2-TZVP basis set.60 In the
case of the I-Vrd-NN diradical and NN-Vrd-NN triradical, the UV−
vis spectra were calculated for their high-spin ground states (a triplet
state for I-Vrd-NN and a quartet state for NN-Vrd-NN).
The parameters of the intramolecular exchange interactions (J)
1 1
Table 3. XRD Data for Compounds I-Vrd-NN and NN-Vrd-
NN
̂
̂
were computed for pairs of paramagnetic centers (H = −2JS1S2) by
two procedures. The accurate ab initio CASSCF62 and CASSCF/
NEVPT2.63,64 procedures were mainly employed. The spin-
unrestricted broken-symmetry (BS) approach65 was utilized too.
compound
I-Vrd-NN
NN-Vrd-NN
empirical formula
formula weight
temperature (K)
wavelength (Å)
crystal system
space group
C27H25N6O3I·C3H6O
666.51
200(2)
0.71073
triclinic
C34H37N8O5·solvent
637.71
200(2)
0.71073
tetragonal
P42/n
The energies of the high-spin states (EHS) and low-spin states within
LS
BS
the BS approach (E ) were calculated at the UB3LYP level of theory,
and the J values were computed according to the following formula:66
P-1
LS
EHS − E
BS
unit cell dimensions a (Å) 10.8641(6)
35.3952(18)
35.3952(18)
6.2920(4)
90
90
90
7882.7(9)
8
1.075
0.074
2696
J =
⟨S2⟩HS − ⟨S2⟩LBSS
b (Å)
c (Å)
α (°)
β (°)
γ (°)
volume (Å3)
Z
11.4045(5)
12.9351(7)
85.947(2)
85.585(2)
66.789(2)
1467.1(1)
2
1.509
1.137
676
Parameters of the intermolecular exchange interactions (J) were
calculated mainly at the BS-UB3LYP/def2-TZVP level. These values
were calculated only for pairs of model radicals. To obtain model
radicals, hydrogen atoms were added to remote radical centers of the
di- and triradical. All the calculations were performed using the
ORCA 4.0.1 software suite.67
density (calcd) (mg·m−3
)
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c02938.
abs. coefficient (mm−1
F(000)
)
■
sı
crystal size (mm3)
0.04 × 0.20 × 0.90
2.04−30.07
0.04 × 0.08 × 0.90
1.15−26.12
Θ range for data
collection (°)
Additional synthetic details, SQUID, XRD, ESR data,
computation details and NMR and FTIR spectra (PDF)
−15 ≤ h ≤ 15
−16 ≤ k ≤ 13
−18 ≤ l ≤ 16
31551
−43 ≤ h ≤ 43
−43 ≤ k ≤ 43
−7 ≤ l ≤ 7
91377
index ranges
Accession Codes
CCDC 2068922−2068923 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
reflections collected
independent reflections
completeness to θ (%)
7843 R(int) = 0.034
99.9
7820 R(int)= 0.125
99.9
data/restraints/
parameters
goodness-of-fit on F2
7843/0/376
7820/0/432
1.005
1.03
final R indices I > 2σ(I)
R1 = 0.0375,
wR2 = 0.0951
R1 = 0.0619,
wR2 = 0.1298
AUTHOR INFORMATION
Corresponding Authors
■
final R indices (all data)
R1 = 0.0606,
wR2 = 0.1096
R1 = 0.1345,
wR2 = 0.1561
Evgeny V. Tretyakov − N.D. Zelinsky Institute of Organic
Chemistry, Russian Academy of Sciences, Moscow 119991,
Russian Federation; Email: tretyakov@ioc.ac.ru
Pavel V. Petunin − Tomsk Polytechnic University, Tomsk
634050, Russian Federation; orcid.org/0000-0001-7101-
5996; Email: petuninpavel@tpu.ru
largest diff. peak/hole (e· 1.01/−0.56
0.26/−0.21
Å−3
)
CCDC
2068922
2068923
The obtained crystal structures were analyzed for short contacts
between nonbonded atoms using PLATON55,56 and MERCURY
programs.57 The crystal structure of I-Vrd-NN is formed by one
solvating molecule of acetone per unit. The free volume accessible to
the solvent in the NN-Vrd-NN crystal derived from the PLATON54
routine analysis is 19% (1504 Å3). This volume is occupied by highly
disordered solvent molecules, which could not be modeled as a set of
discrete atomic sites. We employed the PLATON/SQUEEZE
procedure to calculate the diffraction contribution from the solvent
region and thereby produced a set of solvent-free diffraction
intensities. The most probable solvate molecule for NN-Vrd-NN is
acetone.
Victor I. Ovcharenko − International Tomography Center,
Siberian Branch of Russian Academy of Sciences, Novosibirsk
630090, Russian Federation; Email: ovchar@tomo.nsc.ru
Nina P. Gritsan − V.V. Voevodsky Institute of Chemical
Kinetics and Combustion, Siberian Branch of Russian
Academy of Sciences, Novosibirsk 630090, Russian
Federation; orcid.org/0000-0002-2263-1300;
Email: gritsan@kinetics.nsc.ru
Authors
CCDC 2068922 and 2068923 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
via https://www.ccdc.cam.ac.uk/, or from the Cambridge Crystallo-
Svetlana I. Zhivetyeva − N.N. Vorozhtsov Novosibirsk
Institute of Organic Chemistry, Siberian Branch of Russian
8173
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Article
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Academy of Sciences, Novosibirsk 630090, Russian
Federation
Dmitry E. Gorbunov − V.V. Voevodsky Institute of Chemical
Kinetics and Combustion, Siberian Branch of Russian
Academy of Sciences, Novosibirsk 630090, Russian
Federation
Matvey V. Fedin − International Tomography Center,
Siberian Branch of Russian Academy of Sciences, Novosibirsk
630090, Russian Federation; orcid.org/0000-0002-0537-
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Dmitri V. Stass − V.V. Voevodsky Institute of Chemical
Kinetics and Combustion, Siberian Branch of Russian
Academy of Sciences, Novosibirsk 630090, Russian
Federation; Novosibirsk State University, Novosibirsk
630090, Russian Federation
Rimma I. Samoilova − V.V. Voevodsky Institute of Chemical
Kinetics and Combustion, Siberian Branch of Russian
Academy of Sciences, Novosibirsk 630090, Russian
Federation
Irina Yu. Bagryanskaya − N.N. Vorozhtsov Novosibirsk
Institute of Organic Chemistry, Siberian Branch of Russian
Academy of Sciences, Novosibirsk 630090, Russian
Federation
Inna K. Shundrina − N.N. Vorozhtsov Novosibirsk Institute of
Organic Chemistry, Siberian Branch of Russian Academy of
Sciences, Novosibirsk 630090, Russian Federation
Artem S. Bogomyakov − International Tomography Center,
Siberian Branch of Russian Academy of Sciences, Novosibirsk
630090, Russian Federation; orcid.org/0000-0002-6918-
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Maxim S. Kazantsev − N.N. Vorozhtsov Novosibirsk Institute
of Organic Chemistry, Siberian Branch of Russian Academy
of Sciences, Novosibirsk 630090, Russian Federation;
orcid.org/0000-0002-2869-1777
Pavel S. Postnikov − Tomsk Polytechnic University, Tomsk
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Marina E. Trusova − Tomsk Polytechnic University, Tomsk
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Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c02938
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Russian Science
Foundation (project no. 20-73-00236). The authors would
like to acknowledge the Multi-Access Chemical Research
Center SB RAS for spectral and analytical measurements. The
authors thank Dr. Alexander Maryasov (Novosibirsk Institute
of Organic Chemistry) for a fruitful discussion and useful
comments. The authors would like to acknowledge the
Research Resource Center “Irkutsk Supercomputer Center
SB RAS” for the computational resources.
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DEDICATION
■
In memory of Professor Keiji Okada.
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