Cooperative Magnetism in Crystalline N-Aryl-Substituted Verdazyl Radicals: First-Principles Predictions and Experimental Results

Article abstract of DOI:10.1002/chem.201700988

We report on a series of eight diaryl-6-oxo-verdazyl radicals containing a tert-butyl group at the C(3) position with regard to their crystal structure and magnetic properties by means of magnetic susceptibility measurements in combination with quantum chemical calculations using a first-principles bottom-up approach. The latter method allows for a qualitative prediction and detailed analysis of the correlation between the solid-state architecture and magnetic properties. Although the perturbation in the molecular structure by varying the substituent on the N-aryl ring may appear small, the effects upon the structural parameters controlling intermolecular magnetic coupling interactions are strong, resulting in a wide spectrum of cooperative magnetic behavior. The non-substituted 1,5-diphenyl-tert-butyl-6-oxo-verdazyl radical features a ferromagnetic one-dimensional spin ladder type magnetic network—an extremely rarely observed phenomenon for verdazyl radicals. By varying substituents at the phenyl group, different non-isostructural compounds were obtained with widely different magnetic motifs ranging from linear and zigzag one-dimensional chains to potentially two-dimensional networks, from which we predict magnetic susceptibility data that are in qualitative agreement with experiments and reveal a large sensitivity to packing effects of the molecules. The present study advances the fundamental understanding between solid-state structure and magnetism in organically based radical systems.

Full text of DOI:10.1002/chem.201700988

A Journal of
Accepted Article
Title: Cooperative Magnetism in Crystalline N-Aryl-substituted Verdazyl
Radicals: First-Principles Predictions and Experimental Results
Authors: Steffen Eusterwiemann, Thomas Dresselhaus, Carsten
Doerenkamp, Oliver Janka, Oliver Niehaus, Anja Massolle,
Constantin Daniliuc, Hellmut Eckert, Rainer Pöttgen,
Johannes Neugebauer, and Armido Studer
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To be cited as: Chem. Eur. J. 10.1002/chem.201700988
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Supported by

Chemistry - A European Journal
10.1002/chem.201700988
Cooperative Magnetism in Crystalline N-Aryl-substituted Verdazyl
Radicals: First-Principles Predictions and Experimental Results
[a]
[b]
[d]
[c]
Steffen Eusterwiemann , Thomas Dresselhaus , Carsten Doerenkamp , Oliver Janka , Oliver
Niehaus , Anja Massolle , Constantin G. Daniliuc , Hellmut Eckert*[d][e], Rainer Pöttgen*[c],
[
c]
[b]
[a]
[b]
[a]
Johannes Neugebauer* , Armido Studer*
[
a]
S. Eusterwiemann, C. G. Daniliuc, Prof. A. Studer
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster (Germany)
E-mail: studer@uni-muenster.de
[
b]
T. Dresselhaus, A. Massolle, Prof. J. Neugebauer
Organisch-Chemisches Institut and Center for Multiscale Theory and Computation
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster (Germany)
Email: j.neugebauer@uni-muenster.de
[
c]
O. Janka, O. Niehaus, Prof. R. Pöttgen
Institut für Anorganische und Analytische Chemie
Westfälische Wilhelms-Universität Münster
Corrensstraße 28/30, 48149 Münster (Germany)
Email: pottgen@uni-muenster.de
[
d]
C. Doerenkamp, Prof. H. Eckert
Institut für Physikalische Chemie
Westfälische Wilhelms-Universität Münster
Corrensstraße 28/30, 48149 Münster (Germany)
Email: eckerth@uni-muenster.de
[
e]
present address: Prof. H. Eckert
Instituto da Física em Sao Carlos
Universidade de Sao Paulo
Avenida Trabalhador Saocarlense 400, Sao Carlos, SP 13590, (Brazil)
Status:
For:
resubmission - 17.03.2017
Chemistry – A European Journal (SFB858 Special Issue)
1
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Chemistry - A European Journal
10.1002/chem.201700988
Abstract
We report on a series of eight diaryl-6-oxo-verdazyl radicals containing a tert-butyl group at the
C(3) position regarding their crystal structure and magnetic properties by means of magnetic
susceptibility measurements in combination with quantum chemical calculations using the first-
principles bottom-up approach. The latter method allows for a qualitative prediction and detailed
analysis of the correlation between solid state architecture and magnetic properties. Although the
perturbation in the molecular structure by varying the substituent of the N-aryl-ring may appear
small, the effects upon the structural parameters controlling intermolecular magnetic coupling
interactions are strong, resulting in a wide spectrum of cooperative magnetic behavior. The non-
substituted 1,5-diphenyl-t.butyl-6-oxo-verdazyl radical features a ferromagnetic one-dimensional
spin ladder type magnetic network, an extremely rarely observed phenomenon for verdazyl
radicals. By varying substituents at the phenyl group, different non-isostructural compounds were
obtained with widely different magnetic motifs ranging from linear and zig-zag one-dimensional
chains to potentially two-dimensional networks, from which we predict magnetic susceptibility data
that are in qualitative agreement with experiments and reveal a large sensitivity to packing effects
of the molecules. The present study advances the fundamental understanding between solid state
structure and magnetism in organically based radical systems.
Introduction
Spin alignment in the solid state of organic radicals is an important structural feature affecting
molecular charge transfer[1] and magnetic exchange coupling instrumental for the design of
[2]
organic magnetic materials. One key element is an organic spin-carrying building block, which
is stable enough to be purified under ambient atmospheric conditions and to be stored in a
[3]
crystalline form over several months to years without the loss of integrity. With this objective and
prospect, carbon centered radicals such as polychlorinated triphenylmethyl radicals and
nitrogen/oxygen centered radicals, especially nitronyl and imino nitroxides, have been studied
most commonly, since they do not show any disproportionation or hydrogen abstraction
[4]
reactions. To obtain cooperative behavior (i.e. a ferromagnetic or antiferromagnetic interaction
between two spin centers), the geometry of the mutual orientations of the spin sites is of critical
importance. There are different approaches for controlling these interactions within organic
magnetic materials. Whereas the “through bond” approach uses di- or polyradicals to achieve
[5]
intramolecular spin communication through a π-conjugated network , metal ions have also been
2
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Chemistry - A European Journal
10.1002/chem.201700988
[6]
used to coordinate to the radical moiety to give rise to metal-organic radical frameworks. Overall,
the control of the intermolecular “through space” interactions in organic radical crystals is of
fundamental interest for understanding, manipulating, and designing the magnetic properties of
these materials in the solid state. Following this approach, in the early 1990’s the first fully organic
[
7]
ferromagnetic material – the para-nitrophenyl nitronyl nitroxide – was characterized with a Curie
[8]
temperature of T = 0.6 K and further materials have been discovered since then. However, in
C
nitroxide radicals, the presence of bulky alkyl groups in the α-position tends to attenuate such
intermolecular magnetic interactions, owing to steric hindrance. Alternatively, 6-oxo-verdazyl
[9]
radicals, well known for their uses as mediators in polymerizations , as substrates in organic
synthesis[10] and as profluorescent spin probes[11], may be considered as suitable magnetic
building blocks. As they show an almost planar radical heterocycle and no bulky substituents near
the radical center, they appear to be rather promising building blocks for the design of cooperative
magnetic materials.[12] Following their discovery by Kuhn[13] (saturated carbon at C(6)) and
Neugebauer (carbonyl or thiocarbonyl group at C(6))[14], there have been only few studies
focusing on the magnetic properties of structurally characterized verdazyl radicals (Figure 1).[15]
R
X
R
R
N
N
N
N
N
N
N
N
'
'
eu e auer s ver az s
g b
yl
u n s ver az s
K h
d
yl
N
d
=
,
X
M
O S
,
-
,
r r
=
e
i P A
R
Figure 1. Chemical structures of the two verdazyl radical classes.
At the same time, there are many reports about different magnetic phenomena of not fully
structurally characterized radicals.[12] However, for revealing structure-property relationships and
for designing new systems based on cooperative magnetic interactions crystal structure
information is indispensable.[12] Ferromagnetic interactions – the parallel alignment of spins in the
solid state – are of special interest and to the best of our knowledge only two examples based on
structurally characterized verdazyl radicals are known in the literature.[16] In this contribution we
present the results of a collaborative effort, combining synthesis, physical property analysis and
quantum chemical modelling of magnetic topologies, to uncover new structure-property
relationships for a series of crystalline tert-butyl substituted 1,5-diaryl-6-oxo verdazyl radicals. The
3
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Chemistry - A European Journal
10.1002/chem.201700988
results will be important for designing promising candidates with unique solid state architectures
and enhanced intermolecular magnetic interactions.
Results and Discussion
Structure and Ferromagnetic Interactions in a 1,5-diphenyl-6-oxo-verdazyl radical
Although verdazyls are generally considered to be hard to obtain in the form of single crystals[12]
we were able to solve and refine the structure of a 1,5-diphenyl-6-oxo-verdazyl radical bearing a
tert-butyl group at the C(3) position. Verdazyl radical 3a[11b] (for the synthesis, see Scheme 1)
crystallizes in the monoclinic crystal system with space group P2 /c and unit cell parameters a =
1
15.8770(9), b = 5.9373(3), c = 17.0945(5) Å and four molecules within the unit cell (for more
detailed crystallographic data, see Table S1, Supporting Information (SI)). As can be seen from
the CNNC dihedral angle of 2.8° the verdazyl heterocycle adopts an almost planar 7π-system
(
Figure 2). As also found for other verdazyl radicals[11a, 17], the N-phenyl rings appear twisted with
respect to the central heterocycle with CCNN torsion angles of around 28.6° (-28.9°). A
characteristic of the solid state structure of this system is an arrangement of the molecules to
chains along the c-axis, which are interconnected in a head-to-tail structure by C=O∙∙∙H-C (tert-
butyl group) interactions. Neighboring chains are oriented antiparallel to each other, as can be
seen by the view on the ac-plane (Figure 3, right). Along the b-axis, linear chains are formed with
the same orientation of the verdazyl radical through π-π interactions involving the N-phenyl rings.
The shortest distance between the verdazyl ring and the neighboring phenyl ring C1∙∙∙C15 is 3.367
Å in this direction (Figure 3, left). We also note that the orientation of two neighboring verdazyl
rings is almost parallel with an angle between the two planes of 4.5°. However, the bulky tert-
butyl C(3) substituent prevents an efficient intermolecular π-π stacking of the verdazyl
heterocycles, as indicated by the translation vector b of 5.937 Å
4
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Chemistry - A European Journal
10.1002/chem.201700988
Figure 2. Molecular entity in the crystal structure of verdazyl radical 3a (displacement ellipsoids are shown
with 30% probability).
Figure 3. Solid state structure of verdazyl radical 3a. Left: π−π interaction between two verdazyl radicals
responsible for J
1
(the shortest distance between the verdazyl ring and the neighboring phenyl ring, C1∙∙
C15, is 3.367 Å); right: formation of linear chains along the b-axis.
∙
Next, we investigated the magnetic properties of a polycrystalline sample of 3a. The temperature
dependent magnetic susceptibilities χ, measured in an external magnetic field of 10 kOe (Figure
–1
S29, SI), increase with decreasing temperature. The χ data can be fitted using the Curie-Weiss
law, with a Curie constant of C = 0.376(1) K (μeff = 1.73(1) μ ), which corresponds to the theoretical
value for a one electron system. In addition, a positive Weiss constant θ = 0.7(1) K indicates
B
p
weak ferromagnetic interactions in the paramagnetic region. These are also revealed by an
increase of χT with decreasing temperature (see Figure 4).
5
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10.1002/chem.201700988
Figure 4. Experimental (black crosses) and calculated temperature-dependent magnetic susceptibility of
3a, plotted as molar susceptibility times temperature (left) and molar susceptibility (right) versus
temperature, shown for magnetic models of various size (solid lines).
The experimental magnetization isotherms, measured at 2 and 5 K using an external magnetic
field ranging from 0 to 50 kOe are shown in S29, SI (bottom). Whereas the magnetization M(H)
shows a linear dependence at 5 K, the corresponding magnetization isotherm at 2 K is curved,
both typical behavior of paramagnetic compounds. As the external magnetic field is increased at
2
K, an increase of the magnetization is observed, in agreement with a preferential parallel
alignment of the spins. A saturation magnetization of μsat = 0.24(1) µ was observed at 2 K and
0 kOe.
B
5
In order to explain the ferromagnetic interactions, we applied a systematic computational
procedure,[18] which implements the first-principles bottom-up approach by Deumal et al..[19]
Analyzing the solid state structure of 3a, we calculated three relevant coupling constants of J =
1
−
1
−1
−1
+
0.54 cm , J
neighboring sites in all cases. The strongest magnetic interaction J
structure and parallel orientation of the verdazyl radicals along the b-axis (see Figure 3, left). The
less pronounced ferromagnetic coupling described by J is perpetuated in a zig-zag manner
2
= +0.45 cm and J
3
= +0.09 cm , representing ferromagnetic interactions between
1
is caused by the linear chain
2
between two antiparallel oriented staples along the b-axis (Figure 5). In this case, the shortest
distance between two neighboring phenyl rings is found to be 3.497 Å (between C21 and C25’).
The combination of J
1
and J
2
results in a one-dimensional ladder type structure along the b-axis
(
illustrated by the blue and red lines in Figure 5, bottom). A much weaker interaction to the other
-1
neighboring staples along the a-axis is described by a third coupling constant J = +0.09 cm ,
3
which induces a two dimensional magnetic model at lower temperatures (purple lines in Figure 5,
bottom). In total, the system can be described as a ferromagnetically coupled 2D system of weakly
interacting 1D spin ladders with zig-zag rungs.
6
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The magnetic models for which the magnetic susceptibility has been simulated implement this
motif. The results are shown in Figure 4. Note, that the crystallographic axes do not necessarily
represent the best choice for the directions, into which a minimal magnetic model is enlarged.[18-
19]
Thus, in our procedure a better suited set of axes is automatically chosen. These axes are at
the same time ordered by their expected relevance for the total system, so that the first of the new
axes represents the direction, in which the strongest interactions occur. For 3a, the new axes are
shown in the bottom panel of Figure 5. The labels for the model size given in Figure 4 and
analogous plots for the other compounds (see further below) are to be understood in terms of
axes chosen in this way.
Figure 5. View along the c-axis onto the solid state structure (top) and the magnetic model (bottom) of
verdazyl radical 3a. Top: The shortest contacts responsible for J
.723 Å) are indicated; bottom: Interactions within the unit cell (solid lines), entering or leaving the unit cell
dashed lines) and outside the unit cell (dotted lines), are shown with a line thickness qualitatively
2 3
(C21∙∙∙C25’, 3.497 Å) and J (C13∙∙∙C15,
3
(
reflecting the strength of the coupling constant indicated with the corresponding color. The directions into
which the model is extended are indicated as the x- and y-axes. The origin of the unit cell has been
shifted to enable a more convenient representation.
7
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Chemistry - A European Journal
10.1002/chem.201700988
All couplings calculated for 3a have a rather small strength, which can be explained by the large
distance and by unfavorable slips and tilt angles between the interacting aromatic rings.
Interestingly, no relevant magnetic interactions were found for the head-to-tail structural motif
along the c-axis involving the discussed hydrogen bonds. In summary, for these specific 1,5-
diaryl-6-oxo verdazyl radicals such ferromagnetic interactions are revealed for the first time.
Synthesis and Characterization of substituted 1,5-diphenyl-6-oxo-verdazyl radicals
Encouraged by these initial results we decided to systematically investigate the effect of various
substituents at the N-bonded phenyl ring of the verdazyl radical on the relation between crystal
structure and magnetic properties. We aimed at similar ferromagnetic networks such as in 3a by
replacing the H atom in the para-position by other substituents (compounds 3b–3g). The
preparation of these verdazyl radicals (Scheme 1) starts with a diaryl substituted
carbonohydrazide 1a–1h, readily prepared by Ullmann coupling (for more experimental details,
see the SI). A treatment with pivalaldehyde afforded the corresponding tetrazinan-3-ones 2a–2h
in moderate to good yields by simple filtration of the alcoholic reaction mixture. Improved yields
were obtained by recrystallization of the filtrate in MeOH. We also noticed that the electron rich
compound 2f is easily oxidized to the verdazyl radical upon simple exposure to air, as indicated
by a color change from colorless to red in the solid state. All the other derivatives did not show
this behavior. In order to quantitatively oxidize these precursors, they were reacted with
1,4-benzoquinone to afford the verdazyl radicals 3b–3h in good yields as crystalline red solids.
O
O
O
a
b
)
)
R
N
N
R
R
N
N
N
N
R
R
N
N
R
HN
NH
NH2 NH2
yield
:
y
iel :
d
=
=
86%a
76%
48%
52%
72%
56%
67%
=
=
=
=
=
=
=
=
a
1
1
1
1
1
1
1
1
a
R
R
R
R
R
R
R
R
H
2a
2b
2c
2d
2e
2f
R
R
R
R
R
R
R
R
H
3a
3b
3c
3d
3e
3f
R
R
R
R
R
R
R
R
H
55%
50%
49%
37%
57%
64%
49%
-
-
-
=
=
=
=
=
=
=
=
=
=
=
=
=
=
b
c
p F
p F
p F
-
-
-
p Cl
p Cl
p Cl
-
-
-
d
e
p Br
p Br
p Br
-
-
-
p Me
p Me
p Me
-
-
-
f
g
h
p OMe
p OMe
p OMe
-
-
-
m Br
m m Me
2g
2h
m Br
3g
3h
m Br
,
-
,
-
,
-
m m Me 51%
m m Me 68%
2
2
2
Scheme 1. General reaction scheme for the preparation of tert-butyl substituted verdazyl radicals 3a–3h.
a) Pivalaldehyde (1.0 eq.), MeOH, 40–60 °C, 2–3 h; b) 1,4-benzoquinone (1.7 eq.), CH Cl , sealed tube,
2
2
a
6
0 °C, 3–4 h. See ref 11b.
8
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All the verdazyl radicals of the present study were characterized by electron spin resonance
ESR) in CH Cl (1 mM) and the optical and redox properties were determined through UV/Vis
(
2
2
absorption spectroscopy and cyclic voltammetry measurements. Figure 6 shows the experimental
ESR spectra, together with best-fit simulations (dashed lines). The spectra can be explained in
14
terms of isotropic hyperfine coupling with two sets of two N nuclei, corresponding to N
1 5
,N , and
1
N
2
,N
4
, respectively. In addition, the spectra are influenced by weak hyperfine coupling with the H
nuclei through σ-π spin polarization, which were considered with their appropriate multiplicities.
In compound 3b, a moderately strong hyperfine coupling to the two 19F nuclei was detected,
whereas hyperfine coupling with the chlorine and bromine nuclear magnetic moments in
compounds 3c, 3d, and 3g was found to be negligible. The complete set of g-factors and hyperfine
coupling parameters is summarized in Table 1. These were obtained, using theoretically
calculated values from quantum mechanical (QM) calculations as starting values in the fitting
procedure (see SI for a brief description of the procedure used). The complicated spectra caused
by the large number of couplings lead to difficulties in their interpretation. Already the information
which nuclei have a significant coupling is hard to obtain directly from the experimental data.
Furthermore, a fitting procedure can quickly lead to ambiguous results, if the starting values are
not chosen very carefully. While a straightforward computation of ESR parameters for an
optimized structure[20] leads to unsatisfactory spectra, inclusion of vibrational effects as in our
current approach[21] (see the SI for details) yields much more accurate results. A rough agreement
for the shape of the spectra can already be obtained without recourse to fitted hyperfine coupling
constants and g-factors (see red curves in Figure 6). This fact and the small deviations to the
values optimized after fitting to the experimental data give a lot of confidence in the quality of the
final results.
9
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Chemistry - A European Journal
10.1002/chem.201700988
O
N
N
N
N
F
F
O
N
N
N
N
Cl
Cl
O
O
O
N
N
N
N
Br
Br
N
N
N
N
N
N
N
N
O
O
O
O
O
N
N
N
N
Br
N
N
N
N
Br
N
N
N
N
Figure 6. Experimentally measured, QM calculated and fitted ESR spectra of the verdazyl radicals 3a-3h
in degassed CH Cl
2
2
.
10
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Table 1. QM calculated and fitted ESR-parameters for the verdazyl radicals 3a–3h. Coupling constants
1
with | A | < 1 MHz are indicated as small. The coupling constants for the methyl hydrogens are on
2
average 1.6230 MHz (QM) and 1.8350 MHz (fitted). The coupling constants of the methyl hydrogen
3
atoms were calculated to be small for these compounds. The coupling constant of the bromine atom in
meta-position was calculated to be small.
A(N1,5
)
A(N2,4
)
A(Hortho
)
A(Hmeta
)
A(Hpara
)
A(Xpara)
R
g-factor
MHz
MHz
MHz
MHz
MHz
MHz
-
QM
2.0041
2.0038
2.0041
2.0038
2.0042
2.0041
2.0046
2.0042
11.924
12.387
13.025
12.386
12.123
12.113
11.412
11.964
17.477
18.155
18.084
18.466
17.559
18.501
17.032
18.447
-
1.904
|-1.833|
2.055
-2.050|
2.299
-2.130|
2.458
-2.275|
1.446
1.356
-1.667
|-1.572|
3
3
3
3
3
3
3
3
a
b
c
d
e
f
H
-
Fitted
QM
3
.933
.935
-
1.448
1.474
1.647
1.719
1.765
1.700
1.420
1.428
1.335
-
-
-
-
-
-
-
-
-
p-F
3
Fitted
QM
|
small
-
p-Cl
p-Br
1p-Me
2p-OMe
-
Fitted
QM
|
small
-
-
-
-
-
-
-
-
-
-
Fitted
QM
|
2.0041
2.0038
2.0041
2.0038
12.898
12.558
12.901
17.574
18.285
17.293
-1.822
|-1.839|
-1.918
Fitted
QM
Fitted
QM
12.816
12.381
18.151
17.911
|-1.705|
-1.989
1.192
small
-
2.0039
.0037
2.0041
.0038
-1.702
g
h
3m-Br
Fitted
QM
2
11.909
12.444
12.621
18.602 |-2.1400|
small
|-1.676|
-1.675
2m,m-
Me
17.458
18.450
-1.883
-
-
2
Fitted
2
|-1.689|
|-1.575|
For a further analysis of the optical properties of these radicals, we looked at the absorption
maxima in dichloromethane solution at room temperature. In the VIS spectral region we note a
characteristic band appearing from 490 nm to 516 nm, which is due to the nature of the substituent
and explains the color of the verdazyl radical (Figure 7, for full spectra, see Figure S26, SI). The
red shift of these derivatives can be understood in terms of more effective π conjugation of the N-
aryl substituent to the verdazyl heterocycle. Therefore, the electron rich methyl- (3e: λmax = 501
nm) and methoxy-substituted radicals (3f: λmax = 516 nm) show a more bathochromic shift of the
absorption maximum compared to radicals featuring electron withdrawing groups. For example,
in the series of the para-halogen substituted derivatives the wavelength of the absorption
maximum increases from fluorine (3b: λmax = 495 nm) to chlorine (3c: λmax = 504 nm) to bromine
(
3d: λmax = 505 nm) in accordance with the decrease of electronegativity of the substituent.
Interestingly, the non-substituted compound 3a (λmax = 490 nm) shows a clear hypsochromic shift,
which is also found for the meta-substituted derivatives 3g and 3h. For the latter two compounds
11
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10.1002/chem.201700988
this may be due to steric effects, thereby prohibiting an efficient delocalization of the N-aryl
substituent to the heterocycle. For instance, the 3,5-dimethyl verdazyl radical 3h shows a larger
CCNN torsion angle (42.2°/42.1°) than the para-methyl substituted one (3e: −31.9°/25.3°).
Assuming that the geometries in solution and the solid state are comparable this may explain the
slight shifts in the absorption maxima for these different compounds.
Figure 7. Normalized absorption spectra of the verdazyl radicals 3a–3h in degassed CH
2
2
Cl .
The different electronic properties of the derivatives are also evident in cyclic voltammetry
measurements. In general, for all derivatives reversible oxidation and reduction waves are found,
when measured against ferrocene[22] as an internal standard (Figure 8; for more information, see
Table or Figures S27 and S28, SI). Electron donating groups like methyl in 3e (Eox = +0.31 V),
methoxy in 3f (Eox = +0.23 V) or the 3,5-dimethyl group in 3h (Eox = +0.32 V) effect lower oxidation
potentials than derivatives with electron withdrawing groups like fluorine (3b: Eox = +0.41 V),
chlorine (3c: Eox = +0.44 V), bromine (3d: Eox = +0.45 V) or meta-bromine (3g: Eox = +0.49 V).
Not surprisingly, the reduction potentials show the same trend vice versa, since the ∆Ecell
potentials (1.40–1.44 V) are comparable.[23]
12
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Figure 8. Cyclic voltammetry curves for selected verdazyl radicals 3a, 3f and 3g (0.1 M TBATFB in MeCN
+
vs. Fc /Fc); the red dashed line belongs to 3a measured with ferrocene as an internal standard.
Table 2. Summary of analytical data for the verdazyl radicals 3a–3h. Optical properties (λmax) and
+
oxidation and reduction potentials Eox and Ered, measured against Fc /Fc.
R
λ
max (nm)
E
Ox (V)
E
Red (V)
∆Ecell (V)
1.44
3
3
3
3
3
3
3
3
a
b
c
d
e
f
H
490
495
504
505
501
516
496
494
0.36
0.41
0.44
0.45
0.31
0.23
0.49
0.32
-1.08
-1.02
-0.97
-0.95
-1.12
-1.17
-0.92
-1.11
p-F
1.43
p-Cl
1.41
p-Br
1.40
p-Me
p-OMe
m-Br
m,m-Me
1.43
1.40
g
h
1.41
2
1.43
13
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Crystal Structures and Magnetic Interactions in substituted 1,5-diphenyl-6-oxo-verdazyl
radicals
For all the derivatives, the solid state structures could be solved and refined by single crystal X-
Ray diffractometry. Suitable crystals of 3b–3h were obtained by slow evaporation of a CHCl or
3
EtOAc solution of the sample at room temperature. In this section the magnetic properties of these
derivatives are described and discussed in the context of their crystal structures. As in the case
of 3a, the magnetic couplings are most conveniently discussed within a different coordinate
system than that given by the crystallographic axes. Table 1 summarizes the choice of axes in
the magnetic models in terms of the crystallographic axes for all magnetic models constructed in
this work.
Table 3. Information about the magnetic models used in simulations for the magnetic susceptibility. The
sizes of the magnetic models in the corresponding figures are in terms of the axes x, y and z as denoted
here. Axis definitions given in round brackets do not feature periodic magnetic interactions, i.e. magnetic
models do not need to be extended into that direction.
Compound Couplings Axes of the magnetic cells Molecules per
Model
Considered
x
y
z
magnetic cell
3
3
3
a
b
c
J
1
, J
2
, J
3
b
a
(c)
2
Net (weakly coupled
spin ladders)
Chain
J
1
2
a
a
(b)
(c)
2
4
J
1
, J
, J
3
b
a + b − c
Network (weakly
interacting chains)
Dimer
J
J
1
2
(a)
(a)
a − b
a
(b)
(b)
b
(c)
(c)
(c)
(b)
(c)
(c)
(c)
(c)
(c)
(c)
2
2
8
2
2
2
8
2
2
4
Dimer
J
1
-J
4
Net
3
d-α
d-β
J
1
, J
2
c
Net
J
1
2
2
1
2
2
(a)
(a)
a + b
b
(b)
(b)
(a)
(a)
(a)
a
Dimer
3
J
Dimer
J
J
1
, J
J
, J
, J
3
3
Chain
Chain
J
b
Chain
1
, J
b
Net (weakly coupled
chains)
3g
J
1
-J
4
b
a
b + c
8
Network (weakly
interacting chains)
14
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Interestingly, the verdazyl radicals 3b–3h do not crystallize isostructurally to 3a. Despite the
comparable sizes of fluorine and hydrogen,[24] the crystal structures of 3a and 3b are completely
different, probably because of the unique electronic effects of the fluorine atom. 3b (Figure S2,
SI) crystallizes in the orthorhombic crystal system with space group Pnma and unit cell
parameters of a = 6.8786(2), b = 19.0272(5), and c = 12.6718(4) Å with four molecules in a unit
cell (see Table S1, SI). In detail, two neighboring molecules are oriented almost perpendicular to
each other with an angle between the two planes of 77°. In that way the carbonyl group is pointing
towards the heterocycle of a neighboring molecule with a distance of 3.042 Å between O1 and
C1’ (Figure 9, left and Figure S3, SI). Through this C=O∙∙∙π interaction, linear chains are formed
along the a-axis (Figure 10). Consequently, the two neighboring N-phenyl substituents are
arranged in an eclipsed manner with distances of around 3.388 Å, which leads to a staple-like
structure along the a-axis, built through π-π interactions of the N-phenyl rings.
Figure 9. Left: Solid state structure of verdazyl radical 3b. The C=O∙∙∙π interaction between two verdazyl
−
1
1
radicals corresponding to J =−4.34 cm is a result of linear chains along the a-axis with verdazyl radicals
oriented almost perpendicular to each other (shortest distance between two verdazyl radicals: O1∙∙∙C1 =
.042 Å); right: the magnetic model of 3b implementing this interaction. Interactions within the unit cell
3
(
solid line), entering or leaving the unit cell (dashed lines) and outside the unit cell (dotted lines), are
shown. The direction into which the model is extended is labeled as the x-axis.
15
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Figure 10. View along the a-axis of verdazyl radical 3b emphasizing the formation of linear chains
involving verdazyl radicals orientated almost perpendicular to each other.
Because of the different solid state architecture of 3a and 3b significantly different magnetic
interactions are expected. Indeed, the experimental results show antiferromagnetic interactions
with a Neél temperature of T
a Curie constant of C = 0.352(1) (μeff = 1.68(1) μ
.73 μ ) and a Weiss constant of θ = −2.6(1) K could be extracted (Figure S30, SI). In good
agreement with the experiment (Figure 11, comparison of theoretical and experimental values)
N
= 7.2(1) K for 3b. From the paramagnetic region of compound 3b
B
), which is close to the theoretical value (μcalc
=
1
B
p
−
1
the calculations delivered also one antiferromagnetic coupling constant J
corresponds to the chain arrangement along the a-axis. Further calculated coupling constants of
= −0.09 cm−1 and J = −0.07 cm−1 were found to be negligible in this case and are thus not
1
= −4.34 cm , which
J
2
3
depicted. Figure 11 also shows an approximation of the magnetic susceptibility data by the
Bleaney-Bowers equation[25] (blue curve), which is based on a simple singlet-triplet transition
model for an isolated dimeric system. This model predicts a magnetic susceptibility maximum at
-1
7
.2 K based on an assumed magnetic coupling constant J’ = −3.95 cm , which agrees reasonably
well with J1. However, the shape of the curve is incompatible with the experimental data. A chain
model according to the Bonner-Fisher equation[26], which corresponds to the magnetic topology
suggested by our implementation of the first-principles bottom-up approach, yields an excellent
description of the measured susceptibility with an assumed magnetic coupling constant J’ = −3.25
-
1
cm .
16
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Figure 11. Experimental (black crosses) and calculated temperature-dependent molar magnetic
susceptibility of 3b for magnetic models of various size (solid red and green lines). Enclosed is also a
[
25]
comparison with susceptibility curves calculated from the Bleaney-Bowers model, assuming J’ = −3.95
-1
[26]
-1
cm (blue curve), and from the Bonner-Fisher model , assuming J’ = −3.25 cm (purple curve).
Antiferromagnetic behavior is also observed in the para-chlorine substituted verdazyl radical 3c
(
Figure S4, SI), which crystallizes in a different crystal system (monoclinic with space group C2/c)
than verdazyl radical 3b. A Neél temperature of T = 5.9(1) K was observed with elevated values
for the Curie constant (C = 0.466), the effective magnetic moment μeff = 1.93(1) μ and a Weiss
= −27.9(2) K (see Figure S31, SI). The solid state structure is depicted in Figures 12
N
B
constant θ
p
and 13.
Figure 12. Formation of tetrameric units within the solid state structure of verdazyl radical 3c involving
C=O∙∙∙π interactions (O1A∙∙∙C1B = 2.889 Å; O1B∙∙∙C15A = 2.941 Å).
17
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Tetrameric units are formed through the existence of C=O∙∙∙π-π interactions between O1A and
C1B (2.889 Å) and O1B and C15A (2.941 Å) (see Figure 12 and Figure S5, SI). The angle
between the planes of the verdazyl rings is 58°. These tetramers are connected to each other via
π-π interactions with shortest distances between two neighboring phenyl rings of C26∙∙∙C26 =
3
.299 Å building linear chains along the b-axis. Quantum chemical calculations revealed a
negative magnetic coupling constant of J
= −7.26 cm−1 for this interaction. The tetramers are
further connected by π-π interactions with shortest distances between neighboring phenyl and
verdazyl rings of C1∙∙∙C15 = 3.280 Å and a coupling constant of J
= −4.56 cm−1 leading to a net
structure in the ab-plane (Figure 13). The coupling constants inside the tetramer are J = −1.54
= −0.37 cm . Further couplings were found to be negligible. The relative strength of
1
2
3
−
1
−1
cm and J
4
the couplings can be easily explained by the tilt angle between the aromatic systems. The
predicted magnetic susceptibility for this compound is depicted along with the experimental curve
in Figure 14. Neglecting the interactions inside the tetramers, J
interactions only (red curve) and the result becomes independent of the number of cells used in
the simulation. This changes if further interactions are included (J -J : green curve, J -J : blue
curve). In that case, however, only the minimal magnetic model can be simulated with the
procedure used by us. The effect of including J and in particular J in the simulation is
comparatively small and does not change the good qualitative agreement with experiment. This
good agreement supports the assumption that considering multiple cells with all couplings J -J
1
and J
2
lead to isolated
1
3
1
4
3
4
1
4
will not significantly change this picture. The susceptibility maximum is predicted at higher
temperatures than measured, suggesting that the calculation tends to over-estimate the dominant
J-values in this case.
18
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Figure 13. View along the c-axis onto the solid state structure (top) and the magnetic model (bottom) of
verdazyl radical 3c. Top: Tetramers are connected to each other through classical π-π interactions along
the b-axis (shortest distance between the phenyl rings: C26∙∙∙C26 = 3.299 Å, J
diagonal (C1∙∙∙C15 = 3.279 Å, J ) (H atoms were omitted for clarity); bottom: Interactions within the unit
1
) and along the ab-
2
cell (solid lines), entering or leaving the unit cell (dashed lines) and outside the unit cell (dotted lines), are
shown with a line thickness qualitatively reflecting the strength of the coupling constant indicated with the
corresponding color. The directions into which the model shows periodic interactions are indicated as the
x- and y-axes. Molecules outside the unit cell are shown in grey.
19
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Figure 14. Experimental (black crosses) and calculated temperature-dependent molar magnetic
susceptibility of 3c for the magnetic models of isolated dimers (red line) and the minimal magnetic models
including J -J (green line) and J -J (blue line). Enclosed is also a comparison with a susceptibility curve
calculated from the Bleaney-Bowers model, assuming J’ = −3.22 cm (purple curve), and from the
1
3
1
4
[
25]
-1
[
26]
-1
Bonner-Fisher model , assuming J’ = −3.22 cm (turquoise curve).
For comparison, Figure 14 includes an approximation by the Bleaney-Bowers equation and one
according to Bonner and Fisher, which reproduce the experimental susceptibility maximum for an
-1
assumed J’- value of −3.22 cm On the other hand, the curvature of the susceptibility data is
much better predicted by the QM-based approach than by the model equations, indicating the
necessity to consider more than a single coupling for the description of 3c.
With these two antiferromagnetic networks in hand, we expected interesting magnetic properties
also for the para-bromine system 3d. In contrast to all other discussed solid state structures, we
observed dimorphism under the same crystallization conditions. Whereas the orthorhombic phase
(
3d-α) (Figure S6, SI) crystallizes with the same space group type Pnma (crystal parameters: a =
.4879(4), b = 23.0270(11), c = 10.4930(5) Å and with four molecules in a unit cell) as 3b, also a
second polymorph, a monoclinic (3d-β) (Figure S9, SI) phase is found. The latter crystallizes like
7
3c in the monoclinic group C2/c with cell parameters a = 22.1850(14), b = 15.5200(10) and c =
21.7749(14) Å and with 16 molecules in a unit cell (for a comparison of crystallographic data, see
Table S1, SI). Interestingly, the 3d-β phase was calculated to show mainly antiferromagnetic
–1
–1
–1
interactions with J
1
= −6.66 cm , J
2
= +1,34 cm and J = −1.18 cm . The solid state of 3d-β is
3
built from C=O∙∙∙π interactions between two verdazyl radicals with a shortest distance between
O1A and C1B of 2.830 Å, resulting in an angle between the two verdazyl heterocycles’ planes of
62° (Figure S10, SI). Furthermore, a linear chain of radical pairs (AB) in the order BA-AB-BA-AB
is formed through the presence of π-π interactions of neighboring N-phenyl substituents with
alternating radical pair distances of up to 3.415 A (between BA-AB) and 3.370 Å (between AB-
20
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BA) along the ab-diagonal (Figure 15, left). The corresponding one-dimensional magnetic model
is shown in the right panel of Figure 15.
Figure 15. Solid state structure (left) and magnetic model (right) of verdazyl radical 3d-β (A representing
the molecule “A” and B representing the molecule “B”). Left: Formation of linear chains involving π-π
interactions between the N-phenyl rings of two neighboring verdazyl radicals in the order BA-AB-BA-AB
(
H atoms were omitted for clarity). Several intermolecular distances are displayed. [C26B∙∙∙C23A 3.394 Å
); C21B∙∙∙C22A 3.376 Å (J ); C23A∙∙∙C22A 3.415 Å (J ); C26B∙∙∙C21B 3.370 Å (J )]; right: Interactions
(
J
3
3
1
2
within the unit cell (solid lines) and entering or leaving the unit cell (dashed lines), are shown with a line
thickness qualitatively reflecting the strength of the coupling constant indicated with the corresponding
color. The direction into which the model shows periodic interactions is indicated as the x-axis. Molecules
outside the unit cell are shown in grey. The origin of the unit cell has been shifted to enable a more
convenient representation.
In the experimental data (see Figure S32, SI), however, we could not observe any clear
cooperative effects. One might explain the paramagnetic character of 3d in terms of a dominant
contribution from the 3d-α phase, which has rather weak interactions (calculated coupling
= +0.6 cm–1 and J
= –0.17 cm ) compared to the ones of 3a, 3b and 3c. In the
–1
constants: J
1
2
solid state of 3d-α zig-zag chains along the a-axis are formed through the presence of C=O∙∙∙π
interactions (J ) between the verdazyl radicals (Figure S7, SI). We note shortest distances
1
between the two verdazyl rings of 3.175 Å (O1A∙∙∙C1) or 3.143 Å (O1A∙∙∙C2), while the angle of
the two heterocycle planes is 71°. Although the solid-state structure is similar to that of the para-
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fluorine system 3b, no antiferromagnetic behavior is found in the experiments or calculations. A
careful analysis of the crystallographic data revealed that the shortest distances, resulting from
the C=O∙∙∙π interaction (i. e. C1∙∙∙O1) of two neighboring verdazyl rings are a little bit longer for
the para-bromine system (3d-α: 3.143 Å vs. 3b: 3.042 Å; for a comparison, see also Table 4).
Along these lines, further effects should be considered as well: a) different angles of two
neighboring verdazyl heterocycle planes (3d-α: 71° vs. 3b: 77°), b) different angles between the
planes of two neighboring phenyl rings (3d-α: 47° vs. 3b: 27°). All of this indicates that the
intermolecular magnetic interactions for 3d-α are weaker than for the antiferromagnetically
coupled para-fluorine system 3b, thus explaining the Curie-type paramagnetism over the whole
studied temperature range. Hence, these data demonstrate that the observed magnetic behavior
is complex and shows a strong dependence on packing effects. The magnetic model and a
packing diagram of the compound are shown in Figure 16.
Figure 16. Solid state structure (left) and magnetic model (right) of verdazyl radical 3d-α (view along the
b-axis. Left: The shortest intermolecular distances responsible for J
1
(O1A∙∙∙C2 3.143 Å) and J (C5∙∙∙C5
2
3
.802 Å) are indicated (H atoms were omitted for clarity); right: Interactions within the unit cell (solid lines),
entering or leaving the unit cell (dashed lines) and outside the unit cell (dotted lines), are shown with a
line thickness qualitatively reflecting the strength of the coupling constant indicated with the
corresponding color. The directions into which the model is extended are indicated as the x- and y-axis.
Molecules outside the unit cell are shown in grey. The origin of the unit cell has been shifted to enable a
more convenient representation.
22
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In Figure 17, the magnetic susceptibility of the investigated sample 3d is depicted. As seen from
comparison with the calculations the expected superposition of the magnetic behavior of 3d-α
and 3d-β is observed. Additional information about the magnetic models used is given in the SI.
Figure 17. The temperature-dependent magnetic susceptibility of 3d, calculated for the structures of the α
(
red) and the β (green) phases. The experimental results (black crosses) remain almost constant in this
plot (indicating paramagnetism) and are in-between the results calculated for the different phases.
Next, we studied the effect of the position of the bromine on the structure-property relation, starting
with the solid state structure of verdazyl radical 3g bearing the bromine in the meta positions of
the N-phenyl rings. The solid-state structure shows π-π interactions between two verdazyl radicals.
The verdazyl rings present a parallel shifted orientation into opposite directions, so that the
shortest distances are formed between the verdazyl ring and the phenyl ring of 3.256 Å (between
C1A and C16B) and 3.267 Å (between C1B and C15A; Figure S18, SI). Along the b-axis additional
π-π interactions are present between the verdazyl ring and the phenyl ring of a neighboring
molecule (N1B∙∙∙C25B = 3.311 Å), forming an antiferromagnetically coupled zig-zag Heisenberg
-1
chain (Figure 18) with a coupling constant of J = –3.27 cm . A secondary structure comprises a
1
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-1
chain parallel to the first one, characterized by a smaller coupling constant of J = −1.18 cm . The
2
resulting antiferromagnetic behavior is barely visible in the experimental temperature range
-1
(
Figure 19). A third, ferromagnetic coupling (J
3
= +0.39 cm , Figure S18, SI), which connects the
different chains and thus would promote the magnetic topology to a net, and an even weaker
-1
antiferromagnetic coupling (J = –0.15 cm , Figure S19, SI) are present in the system. However,
4
a comparison of the green and blue curves in Figure 19 as well as the plots shown in Figure S25
in the SI clearly demonstrate that these couplings have a negligible influence on the macroscopic
magnetic behavior. Considering only J
1
and J , the total system can be described by an equal
2
number of parallel zig-zag chains for the two different coupling strengths of the same order of
magnitude. Additional information about the simulations is given in the SI.
Figure 18. Solid state structure (left) and magnetic model (right) of verdazyl radical 3g. Left: Formation of
chains along the b-axis involving additionally π-π interactions between the verdazyl ring and phenyl ring of
the molecule “B” (N1B∙∙∙C25B 3.311 Å, J
.426 Å, J ). Another contact visible in this view (C1A∙∙∙C16B 3.256 Å, J
atoms were omitted for clarity; right: Interactions within the unit cell (solid lines), entering or leaving the
1
) and between the phenyl rings of molecule “A” (C12A∙∙∙C21A
3
2
3
) leads to a negligible coupling. H
unit cell (dashed lines) and outside the unit cell (dotted lines), are shown with a line thickness qualitatively
reflecting the strength of the coupling constant indicated with the corresponding color. The direction into
which the model is extended is indicated as the x-axis. Molecules outside the unit cell are shown in grey.
24
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Figure 19. Experimental (black crosses) and calculated temperature-dependent molar magnetic
susceptibility of 3g for magnetic models of various size (solid lines). J is neglected in all curves.
4
Similar to 3g also in the solid state structures of 3e (R = p-Me) and 3f (R = p-OMe) the verdazyl
radicals are parallel shifted, i.e. at an angle between the verdazyl planes of 0°. For 3e and 3f π-π
interactions between the verdazyl heterocycle and the phenyl rings are dominant, which leads to
the formation of linear chains (found for 3f, see Figure S16, SI) or linear chains of dimeric units
(found for 3e, see Figure S13 and S14, SI). However, for both verdazyl radicals we calculated
rather small magnetic coupling constants, which are also summarized in Table 4. Furthermore,
magnetic susceptibility measurements indicate simple paramagnetic behavior of these species
over the whole studied temperature range from 3–300 K. (see Figure S33 and S34, SI). For a
more detailed discussion of the structure property relation of these species, see the SI. Also in
the case of the dimethylated verdazyl radical 3h linear chains along the a-axis are formed through
CH∙∙∙π interactions, in which the verdazyl rings are oriented antiparallel but stacked on top of each
other (Figure 20 and S21, SI). However, no relevant π-π interactions between the verdazyl rings
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Chemistry - A European Journal
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or/and the N-phenyl rings are present, thus leading to large intermolecular distances around 4.4 Å
between the radical heterocycles. Not surprisingly, the large radical-radical distances manifest
–1
–1
themself in comparatively small coupling constants of J
1
= −0.66 cm , J
2
= −0.11 cm and J =
3
–1
−
0.10 cm , which explains the experimental paramagnetic behavior (Figure S36, SI).
Figure 20. Solid state structure of verdazyl radical 3h. Formation of linear chains along the a-axis of
alternating verdazyl radicals built through CH∙∙∙π interactions. No relevant π-π interactions between the
verdazyl rings or/and phenyl rings were observed. The angle between the two planes corresponding to
the phenyl rings is 80°. The verdazyl rings present an antiparallel orientation with large distances about
4.4 Å.
Table 4. Summary of the solid state characteristics along with the most relevant calculated magnetic
-1
n
exchange coupling constants J (in cm ) and the experimentally observed macroscopic behavior. VV
angle, i.e. the angle between the planes of two neighboring verdazyl heterocycles; FM = ferromagnetic
a
interactions, AFM = antiferromagnetic interactions; PM = paramagnetism; the paramagnetic behavior
b
was observed for 3d, being a mixture of the two phases 3d-α and 3d-β; same orientation of the two
c
d
verdazyl rings with d ~ 4.50 Å; opposite orientation of the two verdazyl rings; opposite orientation of the
two verdazyl rings with d ~ 4.40 Å; n.a. = not assigned.
compound
solid state characteristics
computational
parameters
macroscopic
behavior /
magnetic
interaction
type
packing
type
linear
VV
angle / °
0°
J
cm
+0.54
+0.45
+0.09
n
/
shortest
distances/ Å
3.367 (C1-C15)
-
1
topology
b
3a
π-π
FM
R = H
verdazyl-
phenyl
chains
3.497 (C21-C25) 2D: weakly
3.723 (C13-C15) interacting 1D-spin
ladders with zig-
zag rungs
3
b
C=O∙∙∙π
verdazyl-
verdazyl
zig-zag
chains
77°
−4.34
3.042 (C1-O1)
AFM
= 7.2(1) K
D-zig-zag chains
R = p-F
T
N
1
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3
c
C=O∙∙∙π
verdazyl-
verdazyl
linear
58°
−7.26
3.299 (C26-C26) AFM
R = p-Cl
chains of
tetrameric
units
3.280 (C1-C15)
2.889 (C1-O1)
2.941 (C15-O1)
N
T = 5.9(1) K
2D: isolated
dimers weakly
coupled to a net
−
−
−
4.56
1.54
0.37
3
d-α
C=O∙∙∙π
verdazyl-
verdazyl
zig-zag
chains
71°
62°
+0.60
−0.17
3.143 (O1-C2)
3.802 (C5-C5)
PMa
2D: weakly
interacting net
R = p-Br
linear
chains
with
ABBA-
pattern
−6.66
+1.34
−1.18
3.415 (C22-C23) PMa
3.370 (C21-C26) 1D: linear chain
2.830 (C1-O1)
3d-β
C=O∙∙∙π
verdazyl-
verdazyl
R = p-Br
with ABBA-pattern
c
3
e
π-π
verdazyl-
phenyl
linear
0°
−1.50
+0.44
3.604
3.266 (C1-C26)
PM
R = p-Me
chains of
dimeric
units
c
3
f
π-π
verdazyl-
phenyl
linear
chains
0°
+0.28
−0.16
3.389
3.754
PM
R = p-OMe
c
3
g
π-π
verdazyl-
phenyl
linear
0°
−3.27
3.311 (N1-C25)
3.426 (C12-C21)
AFM
T ≈ 3.0 K
N
parallel zig-zag
chains
R = m-Br
chains of
dimeric
units
−
1.18
d
3
h
CH∙∙∙π
linear
chains
0°
−0.66
3.572
3.809
PM
R = m,m-Me
2
−
−
0.11
0.10
3.810
Conclusion
Ferromagnetic interactions between spin centers have been observed in a phenyl substituted
verdazyl radical, a behavior rarely observed for this kind of radical compounds. The magnetic
coupling network has been analyzed using our previously developed first-principles bottom up
approach based on crystallographic input. The magnetic network consists of weakly interacting
one-dimensional spin ladders with zig-zag rungs and calculated magnetic exchange coupling
−
1
−1
constants of comparable magnitude (J
1
= +0.54 cm , J
2
= +0.45 cm ) as well as a third weaker
−
1
interaction characterized J = +0.09 cm . In particular, the linear chain of parallel oriented
3
verdazyl radicals caused by the π-π interaction of the verdazyl ring and the phenyl ring may be
held responsible for the stronger ferromagnetic interaction.
By varying substituents at the phenyl group, a series of seven additional verdazyl radicals were
prepared, characterized, and their bulk magnetic properties measured and analyzed in terms of
their magnetic coupling networks.Table 4 gives a summary of the crystal structural characteristics,
27
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Chemistry - A European Journal
10.1002/chem.201700988
the most relevant calculated magnetic exchange coupling constants J
n
and the experimentally
observed macroscopic magnetic behavior. The cooperative magnetic properties found in the
present series of molecular verdazyl radicals are the result of complex interaction networks whose
topology is strongly dependent on the solid state structure. Along these lines, the angles,
distances and translational displacements describing the relative orientation of neighboring
molecules with respect to each other are crucial in controlling the relevant magnetic coupling
interactions. Our recent black box implementation of the first-principles bottom up approach
allows for accurate qualitative prediction and detailed analysis of the correlation between solid
state architecture and magnetic properties. Although the perturbation in the molecular structure
by varying the substituent of the N-aryl-ring may appear small, the effects upon the structural
parameters controlling intermolecular magnetic coupling interactions are strong, resulting in a
wide spectrum of cooperative magnetic behavior. They include dimorphism (3d), and can range
from simple paramagnetic behavior (3d, 3e, 3f and 3h) to pronounced antiferromagnetic (3b and
3
c) and even ferromagnetic interactions (3a). For 3b, featuring a para-fluorine substituent, C=O∙∙
π interactions between neighboring radicals lead to a formation of one-dimensional zig-zag
chains, in which the molecules are coupled antiferromagnetically to each other, observable in the
χ(T)-plot with a maximum of the magnetic susceptibility at around T
= 7 K. The substitution of
∙
N
fluorine with chlorine in 3c resulted in a two-dimensional network, in which dimeric units are
connected to a weakly coupling net. Besides a weaker C=O∙∙∙π interaction, an additional π-π
interaction between the aryl substituents leads to a macroscopic antiferromagnetic state with T
N
=
6 K. In 3g, the dominant structural motif is a π-π interaction between a verdazyl heterocycle and
an N-Aryl-ring, which is responsible for the dominant antiferromagnetic coupling extending
through a parallel zig-zag chain.
The important role played by the intermolecular π−π interactions between the verdazyl ring and
the phenyl substituent groups of adjacent molecules in establishing the magnetic coupling
topology is a quite general result of the present study. Based on this finding, the incorporation of
additional planar substituents into the radical’s molecular architecture appears to be a promising
approach for exercising further structural control of magnetic interactions in these materials.
Experimental Section
28
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Chemistry - A European Journal
10.1002/chem.201700988
All reactions involving air or moisture sensitive reagents or intermediates were carried out under
argon atmosphere. All glassware was dried by the use of a heat gun under high vacuum prior to
use. Concentration of the reaction mixture was performed under reduced pressure at 40 °C at the
appropriate pressure. Purified compounds were further dried under high vacuum.
General procedure for the synthesis of diaryl substituted carbonohydrazides 1a–1h (GP1):
According to literature procedures[11a, 27] in a Schlenk tube CuI (5 mol%), 1,10-phenanthroline
(
10 mol%), K
3
PO (3.05 equiv.) and carbohydrazide (1.0 equiv.) were added, evacuated and
4
backfilled with argon two consecutive times. Aryl iodide (2.2 - 2.3 equiv.) and DMF (2 mL/mmol
carbohydrazide) were added and the mixture was heated to 90 °C in the dark and stirred for 48 h
at this temperature. After the reaction mixture was allowed to cool to room temperature it was
filtered through a short pad of silica and eluted with EtOAc. Water was added and the two layers
were separated. The aqueous layer was extracted with EtOAc twice and the combined organic
layers were washed with brine and dried over MgSO
. Filtration, evaporation of the solvent in
4
vacuo and FC afforded the diaryl-substituted carbohydrazides.
General procedure for the synthesis of tetrazinan-3-ones 2a–2h (GP2): In a two-necked flask
with condenser carbonohydrazide 1 (1.0 eq.) was dissolved in MeOH and stirred at 45 °C for
5
min. A methanolic solution of pivalaldehyde (1.0 eq.) was added dropwise using a syringe pump
within 30 min. After addition the mixture was refluxed for 3–4 h. Stirring was stopped and the
mixture was allowed to cool to rt. The precipitate was filtered off and carefully washed with little
amounts of cold MeOH. The filtrate was cooled to –14 °C overnight and the achieved precipitate
filtered off again and washed with cold MeOH. The combined solids were dried in vacuo to afford
the tetrazinan-3-one without the need of a further purification step.
General procedure for the synthesis of 6-oxo-verdazyl radicals 3a–3h (GP3): In a Schlenk
tube tetrazinan-3-one 2 (1.0 eq.) and 1,4-benzoquinone (1.7 eq.) were added and dissolved in
CH
2
Cl . The mixture was heated to 60 °C for an appropriate time. After full conversion of the
2
starting material (monitored by TLC) the mixture was cooled to rt and filtered. The solvent was
removed in vacuo and the residue subjected to FC to afford the 6-oxo-verdazyl radical as a red
crystalline solid.
For detailed analytical data and yields of the precursor compounds 1a–1h, 2a–2h and the radicals
3a–3h (including crystal structures, CCDC 1508710-1508718), see SI.
29
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