A Weakly Antiferromagnetically Coupled Biradical Combining Verdazyl with Nitronylnitroxide Units

Article abstract of DOI:10.1002/cplu.201900709

An antiferromagnetically (AFM) coupled biradical based on oxoverdazyl and nitronylnitroxide was synthesized in 46 % yield using Sonogashira coupling. The obtained heterobiradical evidenced distinct properties of both radical entities in solution. Dependin

Full text of DOI:10.1002/cplu.201900709

Accepted Article
Title: A Weakly Antiferromagnetically Coupled Biradical Combining
Verdazyl with Nitronylnitroxide Units
Authors: Pavel V. Petunin, Tatyana V. Rybalova, Marina E. Trusova,
Mikhail N. Uvarov, Maxim S. Kazantsev, Evgeny A.
Mostovich, Lars Postulka, Paul Eibisch, Bernd Wolf, Michael
Lang, Pavel S. Postnikov, and Martin Baumgarten
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COMMUNICATION
A Weakly Antiferromagnetically Coupled Biradical Combining
Verdazyl with Nitronylnitroxide Units
Pavel V. Petunin,[a,b] Tatyana V. Rybalova,[c,d] Marina E. Trusova, Mikhail N. Uvarov, Maxim S.
[a]
[d,e]
[c]
[d]
[f]
[f]
[f]
[f]
Kazantsev, Evgeny A. Mostovich, Lars Postulka, Paul Eibisch, Bernd Wolf, Michael Lang, Pavel
[a]
[g]
S. Postnikov,* and Martin Baumgarten*
Abstract: The antiferromagnetically (AFM) coupled biradical based
on oxoverdazyl and nitronylnitroxide was synthesised in 46% yield,
using Sonogashira coupling. The obtained heterobiradical evidenced
the distinct properties of both radical entities in solution. The prepared
biradical crystallised in two different crystal forms depending on the
solvent. Form II was used for SQUID magnetisation measurements
Einstein condensation (BEC) of triplon excitations.[3] Therefore a
control of Jintra as well as Jinter is essential and we have shown
earlier that this is possible in organic biradicals, where the
intramolecular interactions can be fine-tuned through the applied
spacer between the radical units.[4] Besides changing the bridging
part between the two radical units, two different stable radicals
can also be applied for varying the exchange interaction with a
given spacer as we found for a tetramethoxypyrene with mixed
nitronylnitroxide and iminonitroxide.[5] For a tolane bridge (1,2-
B B
demonstrating coupling constant JintraII/k = -2.1 K and zJinterII/k = -11.5
K. Consequently, total intermolecular exchange interactions are in five
times larger than the intramolecular ones. Further DFT calculations
well explained this phenomenon and point out the advantage of Form
I for further depth investigations.
bis(phenyl)acetylene)
nitronylnitroxides (NN) as depicted in Figure 1 as A
iminonitroxides (IN) shown as B[ achieving 2D and 3D ordering
of triplon excitations of these spin dimers in the crystal lattice. Due
to this reason investigation of new perspective multi-spin systems
we
have
earlier
tested
two
or two
[
6,7]
8]
Weakly antiferromagnetically coupled crystalline spin-½ dimers
can serve to investigate critical phenomena such as, e.g., the
formation of triplon excitations and their interactions in a magnetic
field strong enough to close the singlet-triplet gap of the biradicals.
Depending on the interdimer exchange interaction Jinter relative to
is actual challenge. In the present work we consider
combination of nitronylnitroxide (NN) and oxoverdazyl radicals
VZ) as this represent a case falling in between the described
a
(
homobiradicals A and B (Figure 1). While it is difficult to achieve
such a hetero-biradical upon two different kinds of condensation
of a dialdehyde precursor, Sonogashira coupling between the two
different radical fragments is considered as perspective pathway.
the
intradimer
exchange
interaction
J
intra
,
different
dimensionalities of the spin systems in the crystalline solid can be
observed such as one-dimensional (1D) spin-ladder systems
showing Luttinger-liquid behavior,[1] two-dimensional Berezinskii-
Kosterlitz-Thouless networks,[2] or three-dimensional Bose-
The preparation of hetero-biradical
1 was based on the
convergent approach and included the reaction between ethynyl-
and iodo-substituted radicals (Scheme 1). Both building blocks
were prepared via well-established procedures: the verdazyl 2
was prepared from 4-iodobenzaldehyde via published method[
and the NNR 3 was synthesised according to Klyatskaya et al.[10]
The most intriguing issue in the synthesis of desired biradical was
the reaction between the two building blocks. We previously
reported the evaluation of Pd-catalysed coupling reactions, where
the iodine-substituted 6-oxoverdazyl 2 exhibited higher activity in
9]
[
a]
Dr. P.V. Petunin, Dr. M.E. Trusova, Dr. P.S. Postnikov
Research School of Chemistry & Applied Biomedical Sciences
National Research Tomsk Polytechnic University
30, Lenin ave., Tomsk 634050, Russia
E-mail: postnikov@tpu.ru
Dr. P.V. Petunin
Department of Chemistry
Siberian State Medical University
[
b]
c]
[9,11]
Sonogashira reaction than appropriate Kuhn-verdazyls.
2, Moskovskiy trakt, Tomsk 634050, Russia
[
T.V. Rybalova, Dr. M.S. Kazantsev
N.N. Vorozhtzov Novosibirsk Institute of Organic Chemistry of
Siberian Branch of Russian Academy of Sciences
9, Lavrent’ev ave., Novosibirsk, 630090 Russia
[d]
T. V. Rybalova, Dr. M. N. Uvarov, Dr. M. S. Kazantsev, Dr. E. A.
Mostovich
Novosibirsk State University
2, Pirogova str., Novosibirsk 630090, Russia
[
[
[
e]
Dr. M. N. Uvarov
V. V. Voevodsky Institute of Chemical Kinetics and Combustion,
Siberian Branch, Russian Academy of Sciences
3, Institutskaya str., Novosibirsk 630090, Russia
f]
Dr. L. Postulka, P. Eibisch, Dr. B. Wolf, Prof. Dr. M. Lang
Physikalisches Institut
Goethe Universität Frankfurt
1, Max von Laue Str., 60438 Frankfurt am Main, Germany
g]
Prof. Dr. M. Baumgarten
Max Planck Institute for Polymer Research
10, Ackermannweg, 55128 Mainz, Germany
E-mail: baumgart@mpip-mainz.mpg.de
Figure 1. Homobiradicals A and B and the newly designed Heterobiradical
Supporting information for this article is given via a link at the end of
the document.
1
with tolane bridge.
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Scheme 1 Synthetic pathway for the preparation of hetero-biradical 1.
Therefore we chose this substrate as a iodine-containing moiety
for the further coupling. The Sonogashira cross-coupling reaction
between radicals 2 and 3 was realized according to the previously
published conditions.[11] The reaction proceeded with the
formation of a target product in the mixture of NNR by-products
and, due to this reason, the good yield of the desired biradical 1
was achieved only in case of addition 1.5 eq. of NNR 3. Finally,
hetero-bi-radical 1 was isolated by column chromatography.
Detailed information about the synthesis of the starting materials
and the heterobiradical 1 is presented in SI Section S1.
formed via slow diffusion of hexane into chloroform – toluene
mixture (20:1, v:v).
The main difference between the crystal structures is governed
by the different interplane angles between the phenylene moieties
in the tolane linker and the resulting angles between the two
radical planes (see SI Section 5). Form I is closer to a planar
structure (21⁰ between planes of VZ and NN) as compared with
Form II (78⁰ between planes of VZ and NN). The structural
parameters (bond length and angles) in radical moieties were
similar with typical values in verdazyl and nitronyl-nitroxide
monoradicals. Thus, crystallisation from different solvents could
be applied to fine-tune the intramolecular interactions within the
biradical 1 skeleton. This effect potentially could be used for
control of magnetic properties of the studied materials.
The electronic properties of the prepared radical 1 were evaluated
using UV-Vis spectroscopy and cyclic voltammetry (CV)
measurements. The electronic spectrum shows absorption bands
with maxima at around 555, 590 nm and 380, 650 nm, which are
in accordance with the absorbance of verdazyl and NNR moieties,
respectively (SI Section S2). The 500-750 nm region in the UV-
-1
-
Vis spectrum of 1 demonstrated weak n-π* band (ε ≈ 450 cm M
1)
of NN radical (SI, Section S2). The heterobiradical 1 was also
studied by CV measurements (SI, Section S3). We observed two
reversible reduction waves and one intensive reversible oxidation
wave with a broad shoulder. The appearance of a broad peak is
due to close energies of the SOMO in verdazyl and NN moieties.
1
2
The resulting differences between Eox and Eox potentials are less
than 0.1 V. As a result, two redox processes overlapped around
0.45 V. At the same time the reduction processes for verdazyl and
NN were well-resolved and had a good agreement with reduction
potentials of 2 and 3 as starting materials (Ered1(NN) = -1.38 V and
2
E
red (VZ) = -1.01 V, see SI Section S3).
The EPR spectrum of 1 in toluene solution exhibited the complex
pattern (SI Section S4), which is typical for the hetero-radical
systems.[12-14] Heterobiradical demonstrated g-value of 2.0050
averaged between the VZ radical with g = 2.0038 and NN
possessing g = 2.0067.
The crystallisation of biradical 1 was successful under different
conditions enabling the isolation of two different crystal solvates
Figure 2. X-Ray structure of 1 (Form I – top, Form II – bottom) and the values
of the dihedral angles between the planes of phenyl, VZ and NN moieties.
Thermal ellipsoids are drawn at 50% probability.
(
Figure 2, SI Section 5). The first one (Form I) was obtained by
slow evaporation at room temperature from 1:1 v/v DCM : hexane
mixture (Figure 2 top). The second (Form II, Fig. 2 bottom) was
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The magnetic properties were measured for single crystals of
Form II with a SQUID magnetometer in a temperature range of
set to avoid differed strongly between the two crystal solvates:
-1
Form I and Form II providing JintraI/k
B
= ‒2.94 cm = ‒4.23 K and
2
K ≤ T ≤ 270 K at a field of B = 1 T (Figure 3). The χmol T vs. T
JintraII/k
B
= ‒1.54 cm-1 = ‒2.2 K, respectively (Figure S10). The
plot is shown in the inset indicating an overall antiferromagnetic
exchange, while the main plot χmol vs. T clearly shows a maximum
at 2.5 K.
To analyse the SQUID data a model of coupled dimers was used.
Combined with a singlet-triplet model the experimental data can
be reproduced by taking an inter–dimer coupling into account
within a mean–field approximation. As shown in the SI Fig. S11
decrease of JintraII compared to JintraI is easily assigned to the large
interplane angle between the radical units (78⁰) in Form II (Figure
2) reducing the spin-spin interaction. For calculation of the
interdimer interaction the X-ray dimer structure geometries of both
forms were selected, and the one more distant radical site on
each biradical was deleted (see SI Section S6). For the Form I
two closely overlapping VZ radicals were found in short distance
-1
the very weak intradimer coupling of JintraII/k
extracted from the field dependence at different temperatures. At
the same time fits to the total susceptibility curve yield zJinterII/k
11.5 K with z the number of nearest neighbours. Thus, the total
B
= -2.1 K could be
B
providing JinterI/k = -2.13 cm = -3.06 K, which is very promising
since JintraI ~ 1.5 JinterI which should allow for a decent increase of
the dimensionality. For form II, on the other hand, two NN radical
fragments are closest packed, with a relative short NO-ON
contact of 4.38 Å dominating this exchange interaction and
B
=
-
intermolecular interaction is five times larger than the
intramolecular exchange, which is not well-suited to reach higher
dimensionalities of the spin ordered triplons in a magnetic field.
Recently DFT methods were successfully applied for calculations
leading to a larger JinterII/k
B
= -4.39 cm-1 = -6.3 K.
From the calculations, it thus appears that Form I would be much
more promising than Form II, and larger crystalline amounts of
Form I would be needed.
[
8, 14,
of spin-spin interactions and explanation of experimental data
1
5]
we performed DFT calculations to estimate the spin-density
In conclusion, we have developed approach to the preparation of
the AFM coupled heterobiradical based on oxoverdazyl and
nitronylnitroxide via Sonogashira coupling. This pathway shows
high potential for further design of tolane-bridged hetero-spin
systems. Both radical moieties have distinctly different optical and
redox properties and verdazyls are slightly less spin delocalised
to the bridging unit than nitronylnitroxides, reducing the exchange
interaction. The obtained biradical crystallised in two crystal
solvates which have principally different magnetic properties:
Form II leads to larger intermolecular than intramolecular
exchange interactions, which is reversed for Form I.
distribution within this biradical and the intra- and intermolecular
exchange interactions. The geometries of the X-ray structures
were used for define of atomic coordination in calculation model.
The Heisenberg-Dirac-Van Vleck (HDVV) Hamiltonian as first
given by Noodleman was applied.[17,18] The broken-symmetry (BS)
approach from Yamaguchi[19,20] was then used for elucidation of
B
the exchange interaction J, which becomes, J/k = (E(BS) –
2
2
E(T))/(S (T) – S (BS)), with E(BS) the energy of the broken
2
symmetry and E(T) the triplet energy. S are the eigenvalues of
the spin operator. With S (BS) close to 1 and S (T) close to 2 the
direct exchange becomes J/k = E(BS) – E(T).
2
2
B
Consequently, synthesised heterobiradical in Form
I
is
Furthermore, the singlet and triplet energy levels calculations
were carried out by UBLYP hybrid function with 6–31G(d) basis
set to avoid Hartree-Fock contamination, usually leading to larger
perspective material for further investigation of interacting triplons
in a magnetic field.
Acknowledgements
This work was supported by the Russian Ministry of Education
and Science (Scientific Program no. 4.5924.2017) and the DFG-
TR49 project. P.V.P. and P.S.P. would like to acknowledge the
Multi-Access Chemical Research Center SB RAS for spectral and
analytical measurements.
Conflicts of interest
There are no conflicts to declare.
spin alternations. The intramolecular exchange interactions (J/kB)
Furthermore, the singlet and triplet energy levels calculations
were carried out by UBLYP hybrid function with 6–31G(d) basis
Keywords: antiferromagnetism • multispin systems • magnetic
properties • stable radicals • Sonogashira coupling
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Figure 3. Molar susceptibility vs. Temperature in range 2-220 K and fields of
0.1 T (light blue full circles) and 1 T (dark blue circles) together with a fit
according the model discussed in the text. The inset shows a χmol T vs. T plot
3
(
full orange circles) with a high temperature value of 0.75 cm ∙K/mol (black
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Table of Contents
COMMUNICATION
Antiferromagnetically coupled
verdazyl-nitronylnitroxide
P.V. Petunin, T.V. Rybalova, M.E.
Trusova, M.N. Uvarov, M.S. Kazantsev,
E.A. Mostovich, L. Postulka, P. Eibisch,
B. Wolf, M. Lang, P.S. Postnikov,*
M. Baumgarten*
heterobiradical was synthesised via
Sonogashira reaction. Magnetic
properties were measured and
obtained biradical was found as
perspective material for investigation
of interacting triplons in a magnetic
field.
Page No. – Page No.
A Weakly Antiferromagnetically
Coupled Biradical Combining
Verdazyl with Nitronylnitroxide Units
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