Crystallographic Insights into the Synthesis and Magnetic Properties of Oxoverdazyl Radicals Functionalized by Benzoic Acid

Article abstract of DOI:10.1002/ejic.201700950

The synthesis and crystallization of two verdazyl radicals, 1,5-dimethyl-3-(4′-carboxyphenyl)-6-oxoverdazyl HI_Me and 1,5-diisopropyl-3-(4′-carboxyphenyl)-6-oxoverdazyl HI_iPr, are described. The electrochemical studies reveal that the oxidation of the two radicals is reversible, whereas their reduction is irreversible. The EPR spectrum of both radicals essentially exhibits a nine-line pattern related to the mean hyperfine interaction of the unpaired electron with the nitrogen atoms of the verdazyl cycle. The single-crystal X-ray diffraction of the intermediates towards HI_iPr allows a fine description of the cyanoborane adduct, which is the key intermediate of this synthesis. The verdazyl radicals themselves are obtained as single crystals. In the case of HI_iPr, depending on the solvent, two polymorphs are crystallized. The structure resolution reveals that, in the solid state, the organization of the verdazyl radicals is governed by both H-bonding and π–π interactions and is reminiscent of the H-bonded structures that can be present in solution. Within the 1D π stacks observed in the three compounds, the verdazyl–verdazyl distance varies from 4.88 ? in HI_Me to 7.90 ? in H^aI_iPr. This modulation of the distance strongly influences the antiferromagnetic intermolecular exchange interaction between π-stacked radicals, which goes from J = –90 cm^–1 (H = –JΣS_iS_i+1) for HI_Me to –12.96(3) cm^–1 for H^bI_iPr and –0.92 cm^–1 for H^aI_iPr.

Full text of DOI:10.1002/ejic.201700950

A Journal of
Accepted Article
Title: Crystallographic insights into the synthesis and magnetic
properties of oxoverdazyl radicals functionalized by benzoic acid.
Authors: Varun Kumar, Sergiu Shova, Ghénadie Novitchi, Cyrille Train,
and Vincent Maurel
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700950
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
Crystallographic insights into the synthesis and magnetic
properties of oxoverdazyl radicals functionalized by
benzoic acid.
Varun Kumar,a Sergiu Shova,b Vincent Maurel,c Ghenadie Novitchi,a Cyrille Traina,*
a CNRS, Univ. Grenoble Alpes, LNCMI, F-38000 Grenoble, France
b “Petru Poni” Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, 700487
Iasi, Romania
c Univ. Grenoble Alpes, CEA, CNRS, INAC, SyMMES, F-38000 Grenoble, France
Keywords: Verdazyl radicals, single crystal X-ray diffraction, borohydride.
Abstract
The synthesis and crystallization of two verdazyl radicals, 1,5-dimethyl-3-(4’-carboxyphenyl)-6-
oxoverdazyl HIMe and 1,5-diisopropyl-3-(4’-carboxyphenyl)-6-oxoverdazyl HIi-Pr, are described. The
electrochemical studies reveal that the oxidation of the two radicals is reversible whereas their
reduction is irreversible. The EPR spectrum of both radicals essentially exhibits a nine-line pattern
related to the mean hyperfine interaction of the unpaired electron with the nitrogen atoms of the
verdazyl cycle. The single-crystal X-ray diffraction of the intermediates towards HIi-Pr allows a fine
description of the cyanoborane adduct which is the key intermediate of this synthesis. The verdazyl
radicals themselves were obtained as single crystals. In the case of HIi-Pr, depending on the solvent,
two polymorphs were crystallized. The structure resolution reveals that, in the solid-state, the
organization of the verdazyl radicals is governed by both H-bonding and π-π interaction and is
reminiscent of the H-bonded structures that can be present in solution. Within the 1D π-stacks
observed in the three compounds, the verdazyl-verdazyl distance varies from 4.88 Å in HIMe to 7.90 Å
in HaIi-Pr. This modulation of the distance strongly influences the antiferromagnetic intermolecular
exchange interaction between π-stacked radicals which goes from J=-90 cm-1 (H=-JSiSi+1) for HIMe
to -12.96(3) cm-1 for HbIi-Pr and -0.92 cm-1 for HaIi-Pr.
*To whom correspondence will be addressed: e-mail: cyrille.train@lncmi.cnrs.fr
1
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European Journal of Inorganic Chemistry
Introduction
1-5
Verdazyls radicals are known for long as stable organic radicals.
Their current revival
follows two lines: the synthesis of purely organic materials6 with photoswitchable7 or
conductive,8 properties and their use as proligands for original coordination compounds.2, 3, 5
It turns out that the coordination chemistry of these radicals is rather tricky and tedious and
sometimes perturbed by the limited stability of the radicals. The stability of the radical is
essentially insured by the delocalization of the electronic density in the singly-occupied
molecular orbital (SOMO) over the four nitrogen atoms of the verdazyl cycle9 but it is
tentatively reinforced by the steric protection of the groups borne by N1 and N5 (Scheme 1),
though this point is still controversial.10 To overcome the stability and solubility problems
encountered during the coordination processes, Brook and coll. have proposed to use
isopropyl groups instead of the mostly used methyl groups.11 This strategy was later used for
introducing verdazyl radicals into polymers.6
Scheme 1. The two verdazyl radicals under studies
Herein, following this line, we compare the synthesis as well as structural and magnetic
properties of 1,5-dimethyl-3-(4’-carboxyphenyl)-6-oxoverdazyl and 1,5-diisopropyl-3-(4’-
carboxyphenyl)-6-oxoverdazyl, that were obtained by two different synthetic routes.
Results and Discussion
Synthesis. Various approaches have been used to synthesize verdazyl radicals.3, 4 Scheme 2
shows the two main routes to synthesize functionalized verdazyl radicals. The classical
approach (steps (i-iii) in Scheme 2) was used to synthesize HIMe: in the first step the bis-
hydrazide compound (EMe) was obtained by reacting methyl hydrazine and triphosgene. Then
the condensation reaction between bis-hydrazide compound (EMe) and 4-formyl benzoic acid
forms the tetrazane derivate HFMe. The verdazyl free radical HIMe was obtained using the
classical oxidation of tetrazane by sodium periodate in water at room temperature. HIMe was
further purified and obtained as single-crystals by recrystallization in ethylacetate. HIMe is
soluble in hot ethyl acetate and methanol but poorly soluble in less polar organic solvents.
2
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
O
O
R
R
N
N
N
N
N
N
HN
NH
O
NH2
i)
ii)
iii)
NH
N
N
NH2
NH2
EMe
O
OH
O
OH
HFMe
O
N
N
O
HN
NH
O
N
N
viii)
ix)
x)
N
N
O
NH HN
O
+
+
Cl-
Cl-
NH3
NH3
O
O
Ei-Pr
D
HIR
R= Me, i-Pr
O
ONa
NaFi-Pr
vii)
N
O
O
O
-
O
BH2
H
N
v)
vi)
iv)
N
NH2
NH+
O
N
H
O
N
O
N
O
N
H
H
H
C
B
A
Scheme 2. Synthesis of carboxylate functionalized verdazyl radicals. In boxed molecules
characterized by single crystal X-ray diffraction (see below for details). i) triphosgene in CH2Cl2 at -
42°C, ii) 4-formyl benzoic acid in EtOH, iii) NaIO4 at r.t. in H2O, iv) acetone at r.t. in toluene v)
NaBH3CN, TSOH at r.t. in THF, vi)1M NaOH at 90°C, vii) triphosgene, Et3N at r.t. in toluene, viii)
35% HCl in ethanol at 80°C, ix) 4-formyl benzoic acid, Na2CO3 in 96% EtOH, x)NaIO4 at r.t. in H2O,
HCl.
This limited solubility causes some difficulties for further reaction and/or crystallization in
particular towards metal-radical architectures. In order to increase the solubility of free
radicals and its coordination compounds in organic solvents, Brook and coll.11 have proposed
to replace the methyl groups by isopropyl ones using an intermediate described by Gallina
and coll.12 The published procedure was slightly modified to reach HIiPr. As shown in scheme
2, the synthetic process toward HIi-Pr is then much more complicated. The main reason for the
increased complexity of the synthetic process is the decrease of reactivity of the nitrogen
bearing the bulky isopropyl substituent. This latter phenomenon calls for the protection of the
primary amine function to insure the proper regioselectivity. We have insisted on the ultimate
3
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
purification and on the crystallization of the intermediates of the synthesis in order to
characterize them by standard NMR techniques and by single crystal X-ray Diffraction
(XRD). It should be noted here that the isolation and structural characterization of reactant
(A), intermediate (B) and final product (C) of hydrogenation reaction have never been
reported for hydrazine derivatives and, more generally, are rarely reported for other
substrates.13 X-ray diffraction is of particular importance for B since the different nuclear
spins of the boron stable isotopes tend to blur the NMR spectrum of the boron-containing
derivatives.14 In our case, at the expected position for BH2CN at 1.8 ppm13, a large band can
be observed (Figure SI-7). The structural description of A, B, C, Ei-Pr will be discussed in
crystallographic part.
Compound Ei-Pr is obtained as the chloride salt of the protonated form of the molecule of
interest. This protonation acts as a hurdle for the condensation step with 4-formyl benzoic
acid. The reaction mixture is thus neutralized with sodium carbonate to allow the reaction to
occur. As a consequence, we obtained the sodium salt of the tetrazane (NaFi-Pr). Upon
oxidation in acidic conditions, we retrieve the protonated form of the targeted verdazyl HIi-Pr.
HIi-Pr exhibits a good solubility in almost all organic solvents. The detailed synthetic
procedure and purification for each intermediate product are given in SI.
Single crystals of HIi-Pr were grown in two different solvents leading to two polymorphs
named HaIi-Pr (obtained from methanol) and HbIi-Pr (obtained from dichloromethane). The IR
spectra of HIMe and HaIi-Pr are very similar and show the presence of same functional groups
and bonds redistribution (Figure SI-1). Nonetheless, in HaIi-Pr, the stretching band of the CH
bonds (νCH) is much more intense and has different shape in compared to that of HIMe,
reflecting the substitution of the two methyl groups by two isopropyl groups (Figure SI-1). A
noticeable difference between the two polymorphs is found in IR spectroscopy for the OH
(2500-3600 cm-1) and carbonyl region (1600-1800 cm-1) (Figure SI-1). In the latter case, the
valence vibration of the carbonyl group is shifted by 25 cm-1 (1720 cm-1 for HaIi-Pr and 1695
cm-1 for HbIi-Pr) while the asymmetric oscillation of the carboxylic group is shifted by 20 cm-1.
In absence of chemical modification, this suggests important reorganisations in hydrogen
bonds involving the acidic moieties and/or carbonyl groups of the radicals between these two
polymorphs. The structural description of HaIi-Pr and HbIi-Pr shall further assess this statement.
4
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
Cyclovoltammetric studies
8π anion
7π radical
6π cation
E ox = +0.21 V
HIi-Pr
Ecred = -1.47 V
HIMe
Ecred = -1.34 V
-1
E ox = +0.37 V
-2
0
1
E(V) vs. Fc/Fc+
Figure 1. Cyclovoltammograms of HIMe (blue) and HIi-Pr (green) in acetonitrile at room temperature.
The cyclovoltammograms of HIMe and HIi-Pr dissolved in acetonitrile exhibit two redox
processes for both radicals (Figure 1). The presence of these two processes is typical of
neutral oxoverdazyl radicals. The values of the oxidation (0.37 V and 0.21 V for HIMe and
HIi-Pr respectively) and reduction (-1.34 V and -1.47 V for HIMe and HIi-Pr respectively)
potentials are consistent with those previously published.6, 15-18 The assignment of the
processes is then straightforward. The first process is related to the oxidation of the
oxoverdazyl radical into a cationic oxoverdazylium 6π-aromatic system. The second process
corresponds to the reduction of the radical into and anionic 8π-antiaromatic system. For both
radicals, the reduction process appears are quasi-irreversible. This contrasts with what has
been observed in most cases.15-18 Nonetheless, such an irreversibility has been observed when
various acids were willingly added to the solution19 as well as in an imidazole-substituted
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
oxoverdazyl.16 When the acidic proton borne by the substituent of the verdazyl radical is
replaced by non-acidic group, the reversibility is recovered.6,16 Following these observations,
the irreversibility observed for HIMe and HIi-Pr is attributed to the presence of the protonated
carboxylic acid moiety of these radicals.
EPR studies in solution
HIMe
HIi-Pr
Fit
Fit
a(N2,4) = 6.4 G
a(N1,5) = 5.4 G
a(HCH3) = 5.4 G
g = 2.0036
a(N2,4) = 6.5 G
a(N1,5) = 5.3 G
a(HCH) = 1.4 G
g = 2.0036
Exp
Fit
Exp
Fit
LW = 1.05
LW = 1.05
340
342
344
346
348
338
340
342
344
346
348
Field (mT)
Field (mT)
Figure 2. Experimental (upper panel) and fitted (lower panel) X-band EPR spectra of HIMe (left) and
HIi-Pr (right) in solution at room temperature.
The EPR spectra of HIMe and HIi-Pr dissolved in acetonitrile and toluene respectively is shown
in Figure 2. They exhibit a dominant nine-line pattern with additional coupling attributed to
the hyperfine interactions with the nitrogen (IN=1) and hydrogen (IH=1/2) nuclear spins.
Accordingly, the EPR spectra of HIi-Pr and HIMe are very well fitted taking into account these
interactions (Figure 2 lower panel). Following the organic nature of the compounds, the
Landé factor perfectly matches that of DPPH. In accordance with the previous observations,6,
8, 11, 15, 17, 18, 20-25 two different hyperfine coupling constants with the nitrogen atoms are needed
to account for the observed spectra. Following the calculated electron density in the SOMO of
the verdazyl radicals,9,11 the highest coupling constant (6.4 G for HIMe and 6.5 G for HIi-Pr)
corresponds to the hyperfine interaction with N2 and N4 atoms whereas the lowest coupling
constant (5.4 G for HIMe and 5.3 G for HIi-Pr) corresponds to the hyperfine interaction with N1
and N5 atoms. Moreover, a dramatic drop of the hyperfine constant with the hydrogen atoms
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
is observed when going from HIMe (5.4 G ) to HIi-Pr (1.4 G). This evolution was already
observed and interpreted by Brook and coll.11 They attributed this phenomenon by the
conformational in-plane preference of the hydrogen borne by the isopropyl moieties.
X-ray diffraction.
The tert-butyl 2-isopropylhydrazonecarboxylate A crystalizes in I41/a space group. C6-N2,
and C5-O2 distances are worth 1.277(2)Å and 1.209(2)Å respectively, revealing the existence
of double bonds. N2-N1, C5-O1 and C4-O1 are worth 1.378(2)Å, 1.336(2)Å and 1.474(2)Å
respectively. They are typical of single bonds with a lengthening for C4-O1 corresponding to
the bulkiness of the tertiobutyl moiety. These bond lengths assess the presence of an imine, an
ester and a hydrazine functional groups within molecule A (Figure 3).
The interactions of A with NaBH3CN in THF led to the formation of the zwitterionic
cyanoborane adduct B. It crystalizes in P-1 space group. The hydrogen atoms bound to N1,
N2, C6 and B1 were resolved. This gives a direct proof of the zwitterionic structure of B.
Moreover, the metrics of the left part of B (C4-O1=1.484(3)Å; C5-O1=1.325(2)Å; C5-
O2=1.216(3)Å; N1-C5=1.369(3)Å) is preserved compared to A during the reaction. In
contrast, upon addition of the cyanoborane moiety on the imine function, N2 changes from
sp2 to sp3 hybridization with a tetrahedral environment around N2. As a consequence, one
observes an increase of the nitrogen atoms spacing within the hydrazine group (N1-
N2=1.425(2)Å) compared to A. Adduct B represents the first example of structural
characterization of zwitterionic cyanoborane adducts of hydrazine (Figure 3). This is of
utmost importance since its structure resolution allows to finely characterize this key
intermediate of the synthesis whereas, in contrast with the other intermediates, NMR does not
provide a clear view of the structure of this molecule.13
Hydrolysis with concentrated sodium hydroxide at 90°C leads to the formation of C (Figure
3). It crystalizes in Cc space group. The main feature of the structure is the observation of a
1.492(6)—1.460(5)Å bong length for N2-C6. Together with the presence of the two hydrogen
atoms bonded to N2 and C6, this 0.200Å lengthening compared to A definitely demonstrate
the reduction of the imine function by the sodium cyanuroborohydride without any effect on
the protected ester function.
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European Journal of Inorganic Chemistry
A
B
C
Figure 3. Molecular structures of A, B and C.
Upon reaction of C with triphosgene and deprotection in acidic conditions, Ei-Pr was obtained
(Figure SI-2). From structure resolution, it clearly appears that, in contrast with EMe, the two
amine groups are protonated and that the charge is balanced by the presence of two chloride
ions.
The molecular structures of the two radicals were resolved in the form of two different
polymorphs HaIi-Pr and HbIi-Pr for the isopropyl derivatives. They all crystallize in the same
space group (P-1) but it appears that the structure of HIMe (Figure 4) and HaIi-Pr (Figure 5) are
closely related whereas the structure of HbIi-Pr is markedly different. The first two compounds
will thus be described together.
8
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
a)
b)
Figure 4. 1D hydrogen-bonding (a) and π-stacks (b) of HIMe radicals.
A close observation of the molecular structure of of HIMe and HaIi-Pr reveals that the
carbon atom C8 in the verdazyl ring is sp2 hybridized, thus making the molecule planar.
Moreover, hydrogen atoms are absent on C8, N3 and N4 atoms. These observations can be
compared to the tetrazane structure and have been previously identified as a marker of the
formation of the radical upon oxidation.26 The bond lengths in the verdazyl ring are given in
Table 2 and matches well with those reported in literature.27
Because the packing plays a crucial role in the magnetic properties of these
compounds,27, 28 it is described in details thereafter. In HaIi-Pr, oxygen atom (O3) is involved in
hydrogen bonding with proton (H1) and forms one dimensional (1D) hydrogen-bonded chains
where the molecules are arranged in a head-to-tail manner (Figure 4a). Moreover, due to the
presence of both phenyl and verdazyl rings and to the overall flatness of the molecules, there
are possibilities of π-π interactions. They lead to the formation of 1D π-stacks perpendicular
to the H-bonded chains (Figure 4b). The π-stacked molecules also adopt a head-to-tail
arrangement. Surprisingly the mean distance between these molecules is 3.48Å in HIMe and
3.40Å in HaIi-Pr (Figure SI-3). In turn, the bulkiness of the isopropyl groups compared to the
methyl ones provokes a slipping of the molecules within the π-stacks (Figure 4b and Figure
5b): in HaIi-Pr, the phenyl groups are almost one above the other and the verdazyl-verdazyl
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
distance has dramatically increased from 4.88Å in HIMe to 7.90Å HaIi-Pr (Figure SI-3). These
arrangements result from an optimization between the stabilization provided by the π-π
interactions and the steric repulsion between the alkyl groups borne by the verdazyl moieties.
a)
b)
Figure 5: 1D hydrogen-bonding (a) and π-stacks (b) of HaIi-Pr radicals.
The structure of 1,5-diisopropyl-3-(4’-carboxyphenyl)-6-oxoverdazyl crystallized
from dichloromethane (HbIi-Pr) is shown in Figure 6. The molecular structure of HbIi-Pr is
much similar to that of HaIi-Pr (Table 1) but the supramolecular arrangement of the radicals is
markedly different: the proton (H1) from the carboxylic acid group is hydrogen bonded to
oxygen atom (O2) of the neighboring carboxylic acid group. The molecules are thus
arrangement in a head-to-head manner and form H-bonded dimers rather that 1D chains as
encountered in HIMe and HaIi-Pr. In turn, the hydrogen-bonded dimers present in HbIi-Pr are
often found in solution when carboxylic acids are dissolved in aprotic solvents.30 They are
characterized by very robust hydrogen-bonds that reduce the strength of the covalent O-H
bonds leading to the observation of low energy O-H elongation in IR spectroscopy. With
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
characteristic bands observed at 2676 and 2551 cm-1, this feature was indeed present in the IR
spectrum of HbIi-Pr. In thus appears that the H-bonded dimers are present in dichloromethane
and are preserved during the crystallization process. Accordingly, the absence or presence of
H-bonded dimers in methanol or dichloromethane respectively is the key to explain the
formation of the two polymorphs HaIi-Pr and HbIi-Pr.
During the crystallization process, the hydrogen-bonded dimers developed π-π
interactions leading to the formation of 1D π-stacks (Figure 6b). Due to the steric hindrance of
the isopropyl substituents, the mean distance between the stacked molecules is 3.62Å.
Despite, this rather long distance, due to the different stacking, the verdazyl-verdazyl distance
is only 5.04Å (Figure SI-3) markedly smaller than in HaIi-Pr.
a)
b)
Figure 6: hydrogen-bonded dimers (a) and π-stacks (b) of HbIi-Pr radicals.
Table 1. Selected bond lengths for the verdazyl radicals (Å)°.
Distance
C1-O1
C1-O2
C1-C2
C2-C3
HIMe
HaIi-Pr
HbIi-Pr
1.314(3)
1.193(3)
1.490(3)
1.380(3)
1.326(2)
1.209(2)
1.491(2)
1.388(2)
1.315(6)
1.208(6)
1.505(7)
1.382(7)
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C3-C4
C4-C5
C5-C6
C5-C8
C6-C7
C7-C2
C8-N3
N3-N2
N2-C9
N2-C10
C9-O3
C9-N1
N4-C8
N1-N4
N1-C11
C10-C12
C10-C13
C11-C14
C11-C15
1.382(3)
1.381(3)
1.382(3)
1.485(3)
1.376(3)
1.382(3)
1.333(3)
1.370(2)
1.357(3)
1.457(3)
1.225(2)
1.363(3)
1.325(3)
1.362(2)
1.454(3)
1.378(2)
1.393(2)
1.390(2)
1.487(2)
1.381(2)
1.393(2)
1.332(2)
1.367(2)
1.369(2)
1.484(2)
1.231(2)
1.369(2)
1.326(2)
1.367(2)
1.481(2)
1.509(2)
1.509(2)
1.520(3)
1.505(3)
1.383(7)
1.371(7)
1.381(7)
1.508(7)
1.398(7)
1.375(7)
1.328(7)
1.377(6)
1.357(7)
1.472(7)
1.223(7)
1.389(7)
1.319(7)
1.375(6)
1.456(7)
1.503(7)
1.492(8)
1.519(7)
1.511(7)
Symmetry code i)0.5-x, 0.5+y, -z
Magnetic properties. The magnetic behaviour of a polycrystalline samples of HIMe, HaIi-Pr
and HbIi-Pr in the temperature range of 2300 K (at 0.1 T) is shown in Figure 7. The value of
χMT for HIMe, HaIi-Pr and HbIi-Pr is 0.340, 0.368 and 0.352 cm3 K mol−1 at 300 K. This values
are slightly lower than the expected χMT value (0.375 cm3 K mol−1) for non-interacting free
radicals (g = 2.0, S = ½). The χMT product for HIMe and HbIi-Pr continuously decreases with
decreasing of temperature to reach 0.02 cm3 K·mol−1 at 2 K. This behaviour indicates that
exchange interaction in HIMe and HbIi-Pr is dominated by antiferromagnetic interactions. In the
case of HaIi-Pr the χMT product remains constant until approximately 35 K and then decreases
to reach 0.231 cm3 K mol−1 at 2 K.
According to X-ray diffraction study, the exchange interaction in the three compounds
originate from the 1D π-stacking (Figure 4b-6b).27 The magnetic behaviour can thus be
analysed using the classical one dimensional Heisenberg spin S=1/2 chain model of Bonner
and Fisher31, 32 based on the following spin Hamiltonian:
H = -J (SiSi+1)
where J is the exchange interaction coupling constant along the Si = ½ chain.
The fitting procedure leads to a good agreement between the experimental data and the
theoretical calculations (Figure 7; Table SI-8).
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
0.4
0.3
0.2
0.1
0
HaIi-Pr
χT
HbI
χTi-Pr
HI
χT
Me
theor
fit-(χT)
0
100
200
300
T(K)
Figure 7. Plot of χMT vs temperature of HaIi-Pr (diamond) HbIi-Pr (triangle) and HIMe (cercle).
(Hdc=0.1T). Solid lines represent the best fit using the Bonner and Fisher model.31, 32
The main exchange interaction parameters J extracted from the fit are = -90(5) cm1; -0.92(1),
and -12.96(3) for HIMe, HaIi-Pr and HbIi-Pr respectively. The efficient π-stacking observed in
HIMe leads to the highest J value within this series. It is comparable to what has been
observed and/or calculated in comparable structural situation.27, 33, 34 Then the magnitude of
the exchange interaction parameter decreases markedly. This evolution is closely to the
increase of the inter-verdazyl distances revealed by the structural study (Figure SI-3). This
qualitative observation is in line with the increase of J observed when the verdazyl-verdazyl
distance is reduced29 and with the quantitative calculation of the exchange interaction that
takes into account the detailed overlap between the -orbitals of the neighbouring radicals.34
Conclusion.
The synthesis and crystallization of 1,5-dimethyl-3-(4’-carboxyphenyl)-6-oxoverdazyl 1,5-
diisopropyl-3-(4’-carboxyphenyl)-6-oxoverdazyl was described. Varying the alkyl
substituents calls for the use of very different synthetic pathways: the methyl derivatives is
obtained through the classical route whereas, the isopropyl derivatives is synthesized through
a much more sophisticated method11 to obtain the correct regioisomer. In the latter case,
utmost care has been taken to crystallize the intermediates, leading to a precise structural
description of the elusive cyanoborane intermediate.
13
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
The supramolecular arrangement of the verdazyl radicals in the solid state is governed by both
H-bonding and π-π interaction. The crystallization of two polymorphs of ,5-diisopropyl-3-(4’-
carboxyphenyl)-6-oxoverdazyl starting from different solvents indicates that the
supramolecular arrangement in the solid state is reminiscent of the H-bonded structures that
can be present in solution. The variation of the verdazyl-verdazyl distances from less than 5Å
to almost 8Å leads to a decrease of the exchange interaction parameter J by two orders of
magnitude.
Experimental Section
Materials. All chemicals were purchased from Sigma-Aldrich and were used as received.
The precursors (A-E) for radical was prepared according the ref. 11 with small improvements.
The condition of synthesis for precursors A-E are presented in scheme 2 and detailed in the
supporting information. The crystallographic determinations of precursors along of way of
preparation of verdazyl radicals have been performed and summarised in scheme 1 (see also
the cif files in SI).
sodium 4-[6-oxo-1,5-di(isopropyl)-1,2,4,5-tetrazinan-3-yl]benzoate (NaFi-Pr):
0.5g (2mmol) of 2,4-diisopropylcarbonhydrazide bis-hydrochloride was dissolved in 100mL
96% ethanol. To this solution was added 0.45g (4.2mmol) of well grinded Na2CO3. The
reaction mixture was stirred for few minutes. Thereafter, 0.3g (2mmol) of 4-formylbenzoic
acid was added to the reaction flask. The resulting reaction mixture was refluxed for 5 hours,
cooled and filtered. Then all the solvent was evaporated under reduced pressure and a mixture
of NaCl and sodium salt of tetrazane (NaFi-Pr) was obtained. Then all the tetrazane salt was
dissolved in absolute ethanol leaving NaCl as residue. The solvent was filtered and then again
evaporated to give sodium salt of tetrazane (NaFi-Pr). Compound was characterized by IR
spectroscopy and mass spectroscopy. Yield (%) 97, Elemental analyses. Anal. Calcd for
C15H21N4NaO3 (Mr = 328.34): C, 54.87; H, 6.45; N, 17.06. Found: C, 55.43; H, 6.85; N,
17.46. IR (cm-1): 3432, 3244, 2973, 2933, 2874, 1605, 1556, 1398, 1307, 1193, 1128, 1091,
1064, 1019, 842, 783, 729, 709. 1H NMR (300 MHz, DMSO) δ 7.88 (d, 2H J = 8.04Hz ArH),
7.45(d, 2H J = 8.04Hz ArH), 4.98(d, 2H J = 11.57 Hz NH), 4.49(m, 2H J = 6.62 Hz CH),
4.33(t, 1H J = 11.57 Hz CH), 1.04 (d, 12H J = 6.62 Hz CH3).
4-[6-oxo-1,5-di(isopropyl)-1,2,4,5-verdaz-3-yl]benzoic acid (HIi-Pr):
0.164g (0.5mmol) of NaFi-Pr was dissolved in 20mL water. To this solution was added a
solution of 0.16g (0.75mmol) of NaIO4 in 5mL water. After a continuous stirring of two hours
at room temperature the color of the solution changed from yellow to red. To this solution
was added diluted hydrochloric acid and stirred for few minutes. This separated out the small
red flakes of verdazyl radical which were extracted using dichloromethane. The extracted
organic phase was dried over Na2SO4 and allowed to stand overnight. Next day, we obtained
nice red crystals of verdazyl radical (HIi-Pr). Yield (%) 72. The two polymorphs have been
14
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
crystalized from methanol and dichloromethane respectively. Suitable crystals for X-ray
crystals were obtained within two days by slow evaporation of methanol (HaIi-Pr) and
dichloromethane (HbIi-Pr). Formation of correct compounds was confirmed by IR
spectroscopy, mass spectroscopy and single crystal X-ray diffraction. HaIi-Pr IR(cm-1): 2985,
2940, 2875, 2678, 2555, 1780, 1681, 1612, 1576, 1540, 1471, 1433, 1387, 1367, 1310, 1291,
1235, 1220, 1171, 1128, 1103, 1018, 927, 868, 778, 702, 653, 545. HbIi-Pr
Elemental analyses. Anal. Calcd for C15H19N4O3 (Mr = 303.4): C, 59.39; H, 6.31; N, 18.47.
Found: C, 58.79; H, 6.10; N, 18.46. IR(cm-1): 2985, 2940, 2875, 2678, 2555, 1780, 1681,
1612, 1576, 1540, 1471, 1433, 1387, 1367, 1310, 1291, 1235, 1220, 1171, 1128, 1103, 1018,
927, 868, 778, 702, 653, 545
4-(6-oxo-1,5-dimethyl-1,2,4,5-tetrazinan-3-yl)benzoic acid (HFMe):
1.18g (10mmol) of compound EMe was dissolved in 100mL ethanol. 1.5g (10mmol) of 4-
formyl benzoic acid was added. The resulting mixture was refluxed for 4-5 hours. All the
solvent was then evaporated to give a white powder of compound HFMe. The compound was
characterized by IR spectroscopy. Yield (%) 96. Elemental analyses: Anal. Calcd for
C11H14N4O3 (Mr = 250.1): C, 52.79; H, 5.64; N, 22.39. Found: C, 52.19; H, 5.10; N, 22.42. IR
(cm-1) : 3409, 3278, 3239, 2929, 1720, 1612, 1570, 1527, 1506, 1488, 1431, 1392, 1299,
1
1224, 1164, 1104, 1086, 1014, 976, 901, 873, 837, 762, 727, 702, 630. H NMR (300 MHz,
DMSO) δ 12.84 (s, 1H CO2H) 7.95 (d, 2H J = 8.26Hz ArH), 7.66(d, 2H J = 8.26Hz ArH),
5.80(d, 2H J = 7.05Hz NH), 5.01(t, 1H J = 7.05Hz CH), 2.94 (s, 6H CH3).
4-[6-oxo-1,5-di(methyl)-1,2,4,5-verdaz-3-yl]benzoic acid (HIMe):
0.5g (2mmol) of compound HFMe was dissolved in 50mL water. To this solution was added a
solution of 0.64g (3mmol) of sodium periodate in 20mL water. The resulting reaction mixture
was stirred for 2 hours. Then, radical (compound (HIMe) was extracted using 2×25mL ethyl
acetate and allowed to crystallize by slow evaporation of the solvent. The red crystals were
obtained within two days and characterized by IR spectroscopy and single crystal X-ray
diffraction. Yield (%) 65.2. Elemental analyses: Anal. Calcd for C11H11N4O3 (Mr = 247.08):
C, 53.44; H, 4.48; N, 22.66. Found: C, 53.01; H, 4.13; N, 22.73. IR (cm-1) : 3235, 3065,
2940, 1734, 1673, 1609, 1413, 1371, 1300, 1250, 1211, 1175, 1100, 1015, 876, 784, 766, 698,
663, 545, 513, 453.
1
13
Physical Measurements and Instrumentation. H, C NMR spectra were recorded on a
Bruker Avance III spectrometer (Ultrashield Magnet) in DMSO-d6 at 25 °C using standard
1
13
pulse programs at 300.10 (1H) and 75.16 (13C) MHz. H and C chemical shifts are quoted
relative to the residual solvent signals. Elemental analyses were carried out at the
Microanalytical Service of the Faculty of Chemistry of the Grenoble University. IR spectra
were measured with a Nicolet iS50FTIR.
Cyclic voltammetry
15
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
For electrochemical experiments, the radicals were dissolved (4 mmol.L-1) in acetonitrile with
Bu4NPF6 as an electrolyte (0.1 mol.L-1). The cyclic voltammograms (scan rates 100 mV/s)
were recorded using a Bio-Logic SP-300 potentiostat with a carbon working electrode, a
platinum counter electrode and a calomel reference electrode. The data was refereed to
ferrocene/ferrocenium redox couple according to ref.36.
EPR measurements
For EPR experiments, the HIMe free radical was dissolved (0.1 mmol.L-1) in acetonitrile and
15 µL of the solution was inserted in a capillary tube (internal diameter 1mm) under argon.
The HIi-Pr free radical was dissolved (0.1 mmol.L-1) in toluene and 100 µL of the solution
inserted in a standard EPR tube (internal diameter 3mm) under argon. EPR spectra were
recorded at room temperature with a Bruker EMX spectrometer operating at X-band
frequency with an ER-4116 dual mode cavity. The following settings were used: microwave
frequency: 9.651 GHz for HIMe) and 9.659 GHz for HIi-Pr; microwave power: 2 mW;
modulation amplitude: 0.2 G. Measurement of g value is obtained by comparison with the
EPR spectrum of DPPH (g = 2.0036) recorded just after. The simulation of EPR spectra was
performed using EasySpin software.37
Crystallographic Structure Determination. Crystallographic measurements for compounds
HIMe HaIi-Pr HbIi-Pr were carried out with an Oxford-Diffraction XCALIBUR E CCD
diffractometer equipped with graphite-monochromated Mo-K radiation. The crystals were
placed 40 mm from the CCD detector. The unit cell determination and data integration were
carried out using the CrysAlis package of Oxford Diffraction.38 The structure was solved by
direct methods using Olex239 software with the SHELXS40 structure solution program and
²
refined by full-matrix least-squares on F0 with SHELXL-97.40 Atomic displacements for non-
hydrogen, non-disordered atoms were refined using an anisotropic model. The hydrogen
atoms have been placed by Fourier Difference accounting for the hybridisation of the
supporting atoms and the possible presence of hydrogen bonds in the case of donor atoms.
The molecular plots were obtained using the Olex2 program.39 The main crystallographic data
together with refinement details are summarized in Table 2. CCDC 1566557-1566559;
1566570-1566572 and 1566597 contain the supplementary crystallographic data for this
paper;
these
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html
16
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
Table 2. Crystallographic data, details of data collection and structure refinement parameters for the investigated compounds
HIMe
C11H11N4O3
247.24
HaIi-Pr
C15H19N4O3
303.34
HbIi-Pr
C15H19N4O3
303.34
A
C8H16N2O2
172.23
B
C9H20BN3O2
213.09
C
C8H18N2O2
174.24
EiPr
C28H86Cl8N16O7
1042.72
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
298
triclinic
P-1
200
triclinic
P-1
200
triclinic
P-1
200
tetragonal
I41/a
200.00(10)
triclinic
P-1
200
triclinic
P-1
200
triclinic
P-1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
V/Å3
Z
6.9401(4)
7.6113(4)
11.4209(6)
82.746(4)
77.027(4)
76.003(4)
568.78(5)
2
7.3491(6)
9.2243(6)
12.7536(10)
69.359(7)
84.961(7)
78.343(6)
792.25(11)
2
5.0412(9)
11.030(2)
14.365(3)
101.388(17)
93.829(15)
97.697(16)
772.4(3)
2
1.304
0.093
0.40×0.05×0.05
3.81 to 46.51
4399
16.3586(5)
16.3586(5)
15.7965(7)
90
90
90
4227.2(3)
16
1.082
8.8130(8)
8.8740(6)
9.0260(5)
80.065(5)
70.657(7)
80.388(7)
651.47(9)
2
14.3711(7)
15.7034(7)
25.3297(12)
71.908(4)
87.583(4)
82.184(4)
5383.2(4)
20
1.075
0.077
0.50×0.10×0.10
3.324 to 49.426
36775
6.8196(4)
13.9376(12)
14.3388(15)
96.800(8)
90.967(7)
91.015(6)
1352.8(2)
1
1.280
0.469
0.90×0.01×0.10
2.944 to 48.81
9019
Dcalc/mg/mm3
/mm-1
1.444
0.109
1.272
0.091
1.086
0.076
0.40×0.20×0.10
0.078
Crystal size/mm3
0.20×0.15×0.10 0.35×0.10×0.02
5.532 to 50.046 3.414 to 50.052
6224
0.40×0.35×0.15
3.584 to 50.054
5058
θmin, θ max()
Reflections
collected
6278
9335
Independent
reflections
2002
[Rint=0.0315]
2792
[Rint=0.0234]
2204
[Rint=0.0558]
1873
[Rint=0.0295]
2253
[Rint=0.0358]
18134
[Rint=0.0450]
4313
[Rint=0.0447]
Data/restraints/
parameters
2002/0/166
2792/0/199
2204/0/199
1873/0/114
2253/1/154
18134/0/1131
4313/1/274
a
0.0559
0.1570
1.039
0.0446
0.1096
1.035
0.0987
0.2050
1.092
0.0471
0.1182
1.083
0.0588
0.1541
1.077
0.0887
0.1993
1.034
0.0942
0.2570
1.070
R1 (I2(I)
b
wR2 (all data)
GOFc
0.20/-0.15
0.15/-0.16
0.26/-0.23
0.18/-0.14
0.47/-0.42
0.26/-0.37
1.03/-0.57
max
and
min/e/Å3
2
2 2
aR1 = Fo - Fc/Fo, bwR2 = {[w (Fo - Fc2)2] /[w(Fo ) ]}1/2.
2
cGOF = {[w(Fo - Fc2)2] /(n – p)}1/2, where n is the number of reflections and p is the total number of parameters refined
17
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10.1002/ejic.201700950
European Journal of Inorganic Chemistry
Magnetism Studies. Magnetic measurements were carried out on microcrystalline samples
with a Quantum Design SQUID magnetometer (MPMS-XL). Variable-temperature (2300 K)
direct current (dc) magnetic susceptibility was measured under an applied magnetic field of
0.1 T. All data were corrected for the contribution of the sample holder and diamagnetism of
the samples estimated from Pascal’s constants.41, 42 The analysis of the magnetic data was
carried out by fitting the χT and χ thermal variations including temperature independent
paramagnetism (TIP), impurity contribution (ρ), and intermolecular interaction (zJ’).42-44
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