Polymorphs of a pyrazole nitronyl nitroxide and its complexes with metal(ii) hexafluoroacetylacetonates

Article abstract of DOI:10.1039/b604598g

The synthesis of pyrazol-4-yl nitronyl nitroxide is reported, along with the crystal structures of its polymorphs and coordination compounds with nickel(ii) and cobalt(ii) hexafluoroacetylacetonate. The polymorphic forms of the pure radical show very different magnetic properties: the alpha form shows only antiferromagnetic interactions while those of the beta form reveal competing ferro- and anti-ferromagnetic interactions. This phenomenon can be traced to the relative orientations of the spin carriers in the crystal. These, in turn, are determined in large measure by the hydrogen bonding networks, where a competition between [N-H...O] and [N-H...N] hydrogen bonds is observed. This delicate balance is also seen in the coordination chemistry of the radical acting as a ligand for the hexafluoroacetylacetonate complexes of nickel(ii) and cobalt(ii). In the former, a 2 1 radical metal complex is formed because of coordination of the pyrazolyl nitrogen atom only. In the latter, a cyclic dimer is formed which involves both the nitrogen and oxygen as coordinating atoms. In both cases, the NH groups of the pyrazolyl moiety lead to the formation of hydrogen bonded networks in the crystals. The magnetic properties of the complexes reveal antiferromagnetic interactions. However, the pyrazolyl nitronyl nitroxide has been proven an interesting component for the formation of coordination compounds with hydrogen bonds in between the magnetic components. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b604598g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Polymorphs of a pyrazole nitronyl nitroxide and its complexes with
metal(II) hexafluoroacetylacetonates{
Laure Catala,{a Klaus Wurst,b David B. Amabilino*a and Jaume Veciana*a
Received 29th March 2006, Accepted 10th May 2006
First published as an Advance Article on the web 19th May 2006
DOI: 10.1039/b604598g
The synthesis of pyrazol-4-yl nitronyl nitroxide is reported, along with the crystal structures
of its polymorphs and coordination compounds with nickel(II) and cobalt(II)
hexafluoroacetylacetonate. The polymorphic forms of the pure radical show very different
magnetic properties: the alpha form shows only antiferromagnetic interactions while those of the
beta form reveal competing ferro- and anti-ferromagnetic interactions. This phenomenon can be
traced to the relative orientations of the spin carriers in the crystal. These, in turn, are determined
…
in large measure by the hydrogen bonding networks, where a competition between [N–H O] and
…
[N–H N] hydrogen bonds is observed. This delicate balance is also seen in the coordination
chemistry of the radical acting as a ligand for the hexafluoroacetylacetonate complexes of
nickel(II) and cobalt(II). In the former, a 2 : 1 radical : metal complex is formed because of
coordination of the pyrazolyl nitrogen atom only. In the latter, a cyclic dimer is formed which
involves both the nitrogen and oxygen as coordinating atoms. In both cases, the NH groups of
the pyrazolyl moiety lead to the formation of hydrogen bonded networks in the crystals. The
magnetic properties of the complexes reveal antiferromagnetic interactions. However, the
pyrazolyl nitronyl nitroxide has been proven an interesting component for the formation of
coordination compounds with hydrogen bonds in between the magnetic components.
nitroxide has been studied.42 In addition a bis(pyrazol-4-
Introduction
yl)pyridine bearing nitronyl and imino nitroxides has been
prepared.43 Several complexes of the 1-methyl-1H-pyrazol-4-yl
The preparation of open shell nitrogen containing compounds
which are able to form hydrogen bonds between each other
nitronyl nitroxide derivative with copper(II) hexafluoroacetyl-
acetonate have been prepared,44 and an elegant structure of
and which can also act as ligands for paramagnetic metal ions
is useful for the area of molecular magnetism. For the purely
the pyrazol-3-yl nitronyl nitroxide with silver has been
reported.42 Here, we describe the detailed synthesis of
organic species, the effect of the hydrogen bonds on the
relative orientation of the spin carriers and their participation
4PzNN, its polymorphic structures as a pure ligand, and some
in the magnetic exchange interactions provides useful informa-
tion.1–11 Specifically, the hydrogen bonds can modulate
properties by avoiding proximity of NO groups, for example,
by involving them in intra- or inter-molecular interactions,12–16
e.g. in crystals of nitronyl nitroxides derived from triazole17
and imidazole.18–20 Metal complexes of nitronyl nitroxides
with nitrogen-contaning rings have proven very fruitful.21–28
We were attracted to pyrazole derivatives because of their
interesting supramolecular29–34 and coordination35–39 chemis-
tries, and particularly the pyrazol-4-yl variety. Before we
described preliminary results of a nitronyl nitroxide with this
ligand (4PzNN, Scheme 1),40 the N-methylated compound had
been reported,41 and subsequently the pyrazol-3-yl nitronyl
aInstitut de Cie`ncia de Materials de Barcelona (CSIC), Campus
Universitari, 08193 Bellaterra, Catalonia, Spain.
E-mail: amabilino@icmab.es; vecianaj@icmab.es; Fax: +34 93 5805729;
Tel: +34 93 580 1853
bInstitut fu¨r Allgemeine Anorganische und Theoretische Chemie,
Universita¨t Innsbruck, A-6020, Innrain 52a, Austria
{ This article is part of a themed issue on Molecular Magnetic
Materials.
{ Permanent address: Laboratoire de Chimie Inorganique, Institut de
Chimie Mole´culaire et des Mate´riaux d’Orsay, Bat. 420, Universite´
Paris-Sud, 91405 Orsay, France.
Scheme 1
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of its complexes with metal hexafluoroacetylacetonates, as well
as their magnetic properties.
performed. Decomposition of the radical occurred when kept
for some time in the reaction mixture, whereas nitronyl
nitroxides are usually very stable. To prevent this decomposi-
tion, a reaction time of 12 minutes was used and the reaction
mixture was quickly worked up. In such a way, a yield of
50% could be obtained after condensation and oxidation.
Condensation with the bishydroxylamine reported by Rey and
coworkers did not prevent this side reaction.50 The pathway
using the condensation with bisamine also described by Rey
Results
Synthesis of the organic radical
The route pioneered by Ullman and coworkers45,46 was
applied to the synthesis of 4PzNN, using the condensation of
the 4-formyl pyrazole with a bishydroxylamine or its sulfate
salt (Scheme 1) to generate the N,N-bishydroxylamine
precursor. The latter was further oxidized by sodium periodate
in a biphasic mixture of water and chloroform to give the
nitronyl nitroxide derivative.
Two different routes were employed to synthesize the
4-formyl pyrazole. One route was inspired from previous
work on heteroaryl 4-pyrazolyl ketones.47 It consisted of using
direct lithiation of the 4-bromo pyrazole (commercially
available or synthesized in one step by monobromination of
pyrazole) with 3 equivalents of n-BuLi, which gave the
intermediate 1,4-dilithiopyrazole, which could in turn be
converted into the formylated compound by adding dry
dimethylformamide to the reaction mixture. In the course of
optimizing the reaction, we found that some parameters were
particularly crucial to enhance the yield. The monolithiate
formed immediately when adding the first equivalent of BuLi,
precipitating as a white powder in the reaction mixture. This
intermediate could not further react because of its insolubility,
in such a way that when maintaining the reaction at low
temperature (rise to a temperature between 220 to 0 uC), no
halogen exchange could proceed, and the bromo compound
was recovered. When warmed to 20 uC, the dilithiated
compound dissolved in THF and could react further. Indeed,
it was found that the time at room temperature influenced the
yield of formation of the dilithiated derivative, and that pass-
ing from one to three hours at room temperature decreased the
amount of starting bromopyrazole (as monitored by 1H NMR
after quenching the reaction with MeOH). Despite efforts to
improve the yield, we obtained only 41% of 4-formyl pyrazole
after the workup and chromatography.
The other convenient route was to prepare the 4-formyl
pyrazole from the condensation of triformyl methane with
hydrazine in MeOH.43,48 We tried in our laboratory the
various methods employed to synthesize the triformyl
methane, and concluded that the perbromide salt was the
most efficient intermediate to generate the target compound,
mainly because the hydrolysis procedure did proceed as
described and because the amount of DMF employed in the
synthesis was considerably reduced, thus preventing distilla-
tion during the workup. When performing the condensation
with one equivalent of hydrazine, the azine formed as a
secondary product (orange precipitate) but still a high yield of
87% of 4-formyl pyrazole was obtained.
The 4-formyl pyrazole was condensed with the 2,3-
bis(hydroxylamine)-2,3-dimethylbutane sulfate salt, synthe-
sised by the recently reported procedure of Ovcharenko and
coworkers,49 in a methanolic solution of potassium carbonate
over three days. Water was added to dissolve the salts and
partially precipitate the adduct. However, chloroform could be
added and direct oxidation by sodium periodate could be
Fig. 1 The alpha polymorph of 4PzNN: views along the a axis (top)
and along the c axis (bottom) of the crystal structure.
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and coworkers was not convenient, as oxidation by m-perben-
zoic acid led to the N-hydroxyl pyrazole derivative. During the
chromatography, acidic solvents were excluded because slight
reduction was observed when concentrating large volumes.
From the magnetic point of view, the shortest NO–NO
˚
˚
contacts were found at 3.64 A and 4.74 A inside the stacks,
between the molecules forming the lateral parts of the stacks,
as shown in Fig. 1, with hydrogen bonds between the nitroxide
oxygen atoms and the hydrogen atoms of the methyl groups
attached to the imidazolyl ring, which can be important
magnetically.
Structural characterization
In order to get modification of the magnetic properties of the
pure organic radical by varying the structures, we used the
polarity of the solvent used for crystallisation to influence
the hydrogen bonds that are formed between the molecules.
Either oxygen or nitrogen can act as hydrogen bond donors in
4PzNN. A priori, it is very difficult to predict which pattern
should be favoured, as a simple argument based on electro-
negativity or partial charges fails as far as the acceptor nature
is considered.
The beta phase of 4PzNN was obtained by evaporation of
an aqueous solution of the compound. It crystallises in the
P21/c space group with the cell parameters reported in Table 1.
Two independent molecules are present, one with a torsion
angle of 10.1u (N6–C7–C10–C12) and the other with 10.2u
(N1–C1–C4–C5), favoured by the intramolecular weak
…
…
˚
hydrogen bonds [d(CH O) = 2.36 A, A(CH O) = 114u
…
…
˚
and d(CH O) = 2.43 A, A(CH O) = 113u]. The molecular
structure is not as coplanar as in the alpha polymorph,
probably because of the absence of p–p interactions in this
beta polymorph. Directional and strong hydrogen bonds are
only formed between the pyrazolyl NH donor group and the
oxygen atom of the nitroxide group. This strong hydrogen
bond is supplemented in the solid state structure by a variety of
close contacts between the hydrogen atoms attached to the
methyl groups attached to the imidazolyl ring and electro-
negative atoms (Fig. 2). The nitrogen atom of the pyrazolyl
ring is located in a kind of pocket generated by four of these
methyl groups.
The X-ray crystal structure of 4PzNN in the alpha phase
was briefly described in a previous report.40 Further details
are reported here. The polymorph obtained from a dichloro-
¯
methane–hexane mixture packs in the triclinic space group P1
with two non-equivalent molecules in the cell. They present a
dihedral angle between the imidazolyl and the pyrazolyl rings
of 1u and 9u justified by the presence of weak intramolecular
…
…
˚
hydrogen bonds [d(CH O) = 2.37 A, A(CH O) = 114u],
…
…
˚
[d(CH O) = 2.39 A, A(CH O) = 113u]. In principle, this
conformation should lead to optimum delocalisation of the
spin density from the radical unit to the pyrazolyl ring, and
thus to the NH group. The structure could be described as
being stacks of tetramers piled up along the a axis. Within
these tetramers, two molecules related by a centre of inversion
From the structural viewpoint, hydrogen bonded chains of
molecules propagate down the c axis and along the b axis
(Fig. 3), giving rise to sheets comprised of these two
perpendicular lines of molecules. These chains of molecules
˚
are stacked by p–p interactions at 3.55 A. Furthermore, two
…
…
within the sheets are joined by a series of [CH O] hydrogen
bonds51–53 (see Table 2), a typical kind of non-covalent
interaction in these radicals.54 The undulating sheets come
together through largely non-directional interactions except
types of hydrogen bonds are present, one set of [N–H O]
…
and one other set of [N–H N] hydrogen bonds, with the
…
…
˚
parameters [d(N–H O) = 1.89 A, A(N–H O) = 163u],
…
…
…
for a very long [CH N] approach.
˚
[d(NH N) = 2.08 A, A(N–H N) = 157u], respectively.
Table 1 Crystallographic data for b-4PzNN and the metal complexes of the radical
b-4PzNN
[(4PzNN)2Ni(hfac)2]2CHCl3
[(4PzNN)Co(hfac)2]
Formula
M
Colour, habit
Dimensions/mm
T/K
C10H15N4O2
223.26
Dark blue block
0.6 6 0.35 6 0.2
223
C30H32F12N8NiO8?0.5CHCl3
979.03
Blue needle
0.22 6 0.04 6 0.015
223
C40H34Co2F24N8O12
1392.61
Red prism
0.3 6 0.18 6 0.1
218
˚
Radiation (l/A)
System
MoKa (0.71073)
Monoclinic
P21/c
13.8867(4)
13.1057(3)
13.4309(4)
90
105.917(1)
90
2.35064(11)
8
1.262
MoKa (0.71073)
Triclinic
¯
P1
MoKa (0.71073)
Monoclinic
C2/c
32.361(1)
8.8752(4)
21.1416(5)
90
118.260(2)
90
5.3483(3)
4
1.729
Space group
˚
a/A
˚
13.111(3)
13.153(4)
13.664(4)
95.901(8)
110.85(1)
103.85(1)
2.0916(10)
2
b/A
˚
c/A
a/u
b/u
c/u
V/nm3
Z
Dc/g cm23
F(000)
1.555
994
952
2784
h range/u
1.52–23.0
3267
2775
1.63–19.0
3347
1500
545
0
1.97–22.94
3665
3020
Measured reflections
Observed reflections (I . 2s(I))
Parameters
306
450
Restrictions
R (I . 2s(I))
wR2 (I . 2s(I))
0
0.0343
0.0866
0
0.0368
0.0874
0.0922
0.1718
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The nickel(II) hexafluoroacetylacetonate (Ni(hfac)2) com-
¯
plex of 4PzNN which crystallises in the triclinic P1 space
group, from a 1 : 1 mixture of the components, is comprised of
a 2 : 1 radical : metal complex, with only the nitrogen atoms of
the pyrazolyl ligands coordinated to the metal centre, in the cis
positions (Fig. 4). These complexes come together through
…
short [NH O] hydrogen bonds between the pyrazole amine
group and the nitroxide oxygen atom. Each neighbouring
complex enters into two of these interactions in a head-to-tail
manner giving rise to a corrugated ladder-type structure
(Fig. 5). These 1D structures come together through weaker
essentially non-specific interactions, the only notable short
contacts being those between the nitroxide oxygen atoms and
the chlorine atoms of the disordered chloroform molecule
˚
(3.07 A).
The complex formed between 4PzNN and Co(hfac)2 leads to
a cyclic dimer which crystallises in the monoclinic C2/c space
group, with the molecule located about an inversion centre.
Both the nitrogen atoms of the pyrazolyl ligands and the
oxygen atoms of the nitroxide groups are coordinated to the
metal centre, in the cis positions (Fig. 6). In this sense, it is
reminiscent of the complex formed between the 4-pyridyl
nitronyl nitroxide and manganese(II) hexafluoroacetyl-
acetonate,55 and copper(II), cobalt(II) and manganese(II)
hexafluoroacetylacetonate complexes of a pyrimidine-derived
tert-butylnitroxide.56 In the present case however, and in
common with the nickel(II) complex we described above, the
Fig. 2 A view of the two non-equivalent molecules of 4PzNN in the
beta polymorph (top), and a view of the unit cell showing the close
contacts between molecules (bottom).
Fig. 3 View down the c axis (top) and the b axis (bottom) showing the
perpendicular hydrogen-bonded chains.
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Table 2 Hydrogen bond and short contact distances and geometries found in crystals of b-4PzNN, and its complexes with Ni(hfac)2 and
Co(hfac)2
…
…
…
D–H A
angle/deg
D–H
distance/s
H
A
distance/A
D
A
distance/A
Symmetry operation to
generate A
˚
˚
Compound
H-bond
…
N4–H4 O1
b-4PzNN
0.89(2)
0.91(2)
0.97
0.97
0.97
0.97
0.94
0.97
0.87
0.87
0.97
0.97
0.94
0.94
0.82(3)
0.82(3)
1.83(2)
1.92(2)
2.42
2.53
2.55
2.69
2.37
2.72
1.92
1.96
2.66
2.70
2.86
2.96
2.32(3)
2.85(3)
2.704(2)
2.812(2)
3.384
3.489
3.461
3.608
3.274
3.665
2.67(2)
2.62(2)
3.575
3.538
3.086
3.162
2.926(3)
2.853(3)
165(2)
163(2)
170
170
157
158
160
163
144
139
158
144
111
94
131(3)
123(3)
1 2 X, 0.5 + Y, 1.5 2 Z
X, 0.5 2 Y, 0.5 + Z
X, 0.5 2 Y, 20.5 + Z
1 2 X, 2Y, 2 2 Z
X, Y, Z
X, 0.5 2 Y, 20.5 + Z
X, Y, Z
1 2 X, 2Y, 1 2 Z
1 2 X, 2Y, 1 2 Z
2X, 2Y, 1 2 Z
X, 21 + Y, Z
…
N8–H8 O3
…
C91–H91A O2
…
C31–H31C O1
…
C22–H22C O4
…
C81–H81C O2
…
C12–H12 O2
…
C81–H81B N3
…
…
[(4PzNN)2Ni(hfac)2]2
[(4PzNN)Co(hfac)]2
N4–H4 O3
N8–H8 O2
…
C21–H21A O4
…
C22–H22B O4
X, 21 + Y, Z
…
C12–H12 O2
2X, 2Y, 1 2 Z
1 2 X, 2Y, 1 2 Z
1.5 2 X, 20.5 + Y, 1.5 2 Z
X, Y, Z
…
C6–H6 O3
…
…
N4–H4 O1
N4–H4 O4
…
complexes come together through [NH O] hydrogen bonds
between the pyrazole amine group and the nitroxide oxygen
atom. Each neighbouring complex enters into four of these
interactions so as to generate a brick wall-like sheet which runs
down the b axis (Fig. 7). There are no other hydrogen bonding
interactions between the molecules, but a series of contacts
between the fluorine atoms of the diamagnetic ligand and
the hydrogen atoms of the methyl groups in the radical
component. These sheets come together with close contacts
between fluorine atoms so as to form a fluorine-rich layer in
between the complexes.
Magnetochemical characterization
The temperature dependence of the paramagnetic suscepti-
bility of the samples was measured with a SQUID suscept-
ometer in the temperature range from 2 to 300 K. The alpha
phase of 4PzNN behaves as paramagnet until about 30 K,
when a decrease occurs in xT when lowering the temperature.40
Fitting of the data to the Curie–Weiss law gave h = 22.1 K. In
the case of the beta phase of 4PzNN, the magnetic interactions
are expected to be much stronger than in the polymorph
Fig. 5 A view of the crystal structure of [(4PzNN)2Ni(hfac)2]2-
CHCl3 showing the corrugated ladder type structure resulting
from the hydrogen bonding interactions between the complex
molecules.
Fig. 4 The crystal structure of the complex molecule
[(4PzNN)2Ni(hfac)2].
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Fig. 8 Temperature dependence of xmT vs. log T for a ground
crystalline sample of the beta phase of 4PzNN.
˚
˚
at 3.23 A and a very short contact at 2.94 A between the NO
group and the central carbon (contact 2). This type of contact
between regions bearing spin densities of the opposite signs
(NO bearing positive and the carbon bearing negative spin
density) has been said to be responsible for the propagation of
ferromagnetic interactions.57,58 More than just taking into
account the distance, we compared the whole relative spatial
disposition of the fragments to some reference biradicals which
exhibited clearly ferromagnetic interactions. In the case of
4PzNN, we did not find the exact geometry present in the
latter, but the orthogonal disposition of the spin-bearing frag-
ments was even more favourable for i) a strictly orthogonal
Fig. 6 The cyclic dimer formed in the crystals of 4PzNN?Co(hfac)2
(disordered fluorine atoms are omitted for clarity).
because of the greater proximity of the NO groups between
the primary patterns. This hypothesis was confirmed in the
temperature dependent magnetic susceptibility measurement
(Fig. 8). The xT product experiences a sharp drop from
around 100 K indicative of antiferromagnetic interactions,
followed by a plateau between 10 and 4 K, implying competing
interactions.
˚
SOMO orbital at 3.23 A, and ii) short contacts with spin
˚
densities of opposite signs at 2.94 A. These features led us to
From the magnetic point of view, the ‘‘magnetic patterns’’ in
the solid state structure of the beta phase of 4PzNN are found
to be tetramers linked by two groups of short NO–NO
contacts. One of them (contact 1) is present between the
expect a ferromagnetic interaction to proceed through this
pattern. The interactions promoted by the hydrogen bonds are
expected to be ferromagnetic, but weak, as in the aggregates in
solution,40 since they also involve [NH O] hydrogen bonds.
…
˚
nitroxides at a distance of 3.87 A (Fig. 9), an arrangement that
As the magnitude of the magnetic exchange is much
weaker than those promoted by through-space contacts, the
ferromagnetic interaction is compensated. As we discussed, the
is likely to give rise to an antiferromagnetic interaction. The
other group of contacts (contact 2) is due to the orientation of
the nitroxide of one molecule towards the imidazolidine cycle
of the neighbouring one, giving rise to a short NO–NO contact
Fig. 7 The packing of the cyclic dimers of 4PzNN?Co(hfac)2 with the
…
[NH O] hydrogen bonds indicated (disordered fluorine atoms are
Fig. 9 A view of the solid state structure of the beta phase of 4PzNN
omitted for clarity).
showing the close contacts between spin carriers.
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Fig. 10 Temperature dependence of xmT vs. T for a ground crystal-
line sample of the compound [(4PzNN)2Ni(hfac)2]2CHCl3.
two groups of short contacts should lead to i) antiferromag-
netic interactions (J1) due to the contact 1 between coplanar
nitroxides, and ii) ferromagnetic interactions (J2) associated
with the group of contacts 2. To check this prediction based on
a strict comparison of the patterns with some reference ones,
we fitted the behaviour with a symmetric linear 4-spin model,
which gave the following parameters: J1 = 233.3 ¡ 0.2 K,
J2 = + 14.4 ¡ 0.4 K, h = 1.1 ¡ 0.1 K. This fitting corresponds
to the prediction based on the examination of the structure,
and shows that reliable predictions can be extracted from the
study of the geometry of groups of short contacts which
involve the spin-bearing regions in this case.59,60
The magnetic susceptibility measurements of the complex
[(4PzNN)2Ni(hfac)2]2CHCl3 shows a gentle decrease from 40 K
in the xT versus T plot (Fig. 10) from a value that corresponds
to the uncorrelated spin half and spin 1 nuclei. The weak
antiferromagnetic interactions which are responsible for this
decrease may occur through the pyrazolyl or between two
complexes. The latter hypothesis is reasonable since the nearest
NO–NO distance in the crystals has a distance between the
Fig. 11 Temperature dependence of xmT vs. T for a ground crystal-
line sample of [(4PzNN)Co(hfac)2] and a representation of the spin
correlation in the system.
ferromagnetic interaction may be present.64 It could be due
either to a weak ferromagnetic coupling within the dimer
through the pyrazolyl ring (J2 in Fig. 11) and/or to inter-dimer
…
[NH ON] hydrogen bonds between neighbouring molecules.
˚
oxygen atoms of 3.80 A, despite the poor overlap (the groups
are oriented end-on and displaced). Furthermore, at low tem-
perature, the value of xT obtained is close to 1.2 emu K mol21
corresponding to the remaining S = 1.
Conclusions
The pyrazol-4-yl nitronyl nitroxide radical shows competing
hydrogen bonding situations, with both pyrazole nitrogen
atoms and nitroxide oxygen atoms acting as acceptors of the
hydrogen bonds from the NH group of the pyrazolyl unit. This
competing situation is also revealed in the coordination to
metal ions in hfac complexes, providing an interesting addition
to this family of complexes.44,55,56,65–68 However, in these
The complex formed between 4PzNN and Co(hfac)2 has a
xT versus T curve which shows a steady decrease from around
50 K (Fig. 11). At higher temperature in the curve, the value
of the plateau (3.8 emu K mol21) is consistent with a strong
antiferromagnetic (J1 in Fig. 11) coupling between the spin 3/2
cobalt ion and the spin half nitronyl nitroxide, as expected in
this type of complex. This strong interaction already takes
place at room temperature, leading to a decoupled dimer
of two S = 1 entities.55 The higher than expected value of
xT can be explained in terms of spin–orbit coupling of the
cobalt ion.61,62 and corresponds to the contribution of
3.2 emu K mol21 usually reported for the Co(II) ion.63 The
temperature dependence analysis is complicated by the fact
that depopulation of the levels of the Co(II) ion occurs when
lowering the temperature until the Kramers doublet is singly
occupied. The strong antiferromagnetic coupling of the Co(II)
Kramers doublet to the radical should give rise to two S = 0
states weakly coupled through the pyrazole ring. As the xT
value obtained is higher than zero at 4 K, an additional
…
complexes the [N–H O] interactions prevail. The hydrogen
bonding capability of the pyrazole unit is therefore an
interesting one for the construction of magnetic metal organic
frameworks. In the cases presented, these interactions are
relatively weak and antiferromagnetic on the whole, except for
the beta phase of the pure radical in which ferro- and
antiferromagnetic interactions compete.
Experimental
General methods and materials
Solvents were distilled before use. In particular, THF was
dried over sodium/benzophenone, and distilled under argon.
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All the reagents were used as received from Aldrich Chemical
Company. Thin-layer chromatography (TLC) was performed
on aluminium plates coated with Merck Silica gel 60 F254.
Microanalyses were performed by the Servei d’Analisi of the
Universitat de Barcelona. 1H and 13C NMR spectra were
recorded on a Bruker ARX 300 spectrometer. FT-IR spectra
were recorded on a Perkin-Elmer Spectrum One spectrometer.
UV-visible spectra were recorded on a VARIAN Cary 5
instrument. MS spectra were recorded on a Jeol JMS-DX 300
instrument. 4-Bromopyrazole was prepared as described in
ref. 69. The perbromide salt was found to be the most
accessible way to get triformyl methane in a good yield.70
The EPR spectra were recorded on an X-band Bruker
spectrometer (ESP-300E). Temperature was measured by a
thermocouple introduced inside the tube, at 1.5 cm from the
bottom. Magnetic susceptibility measurements were obtained
with a Quantum Design SQUID magnetometer using an
applied field of 100 Oe.
column (SiO2, ethyl acetate), giving a beige powder with a yield
1
of 87%. H NMR (CDCl3) d 8.02 (d, 2H), 7.37 (d, 2H), 1.12
(s, 9H), 0.92 (s, 9H), 20.09 (sb, 6H). 13C NMR (CDCl3). IR
(KBr, cm21): 1680 (nCLO). Anal calcd for C17H29NO3Si: C,
63.12; H, 9.04; N, 4.33; Si, 8.68. Found: C, 63.16; H, 9.10; N,
4.29, Si, 8.81%.
4-(1-Oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)pyrazole
(4PzNN)
To a solution of 4-formyl pyrazole (1.050 g, 10.93 mmol)
in MeOH (70 ml) was added N,N9-dihydroxy-2,3-diamino-
2,3-dimethyl butane monosulfate monohydrate (3.175 g,
12.03 mmol) and K2CO3 (5 g). The mixture was stirred during
1 day at room temperature and then concentrated by a slow
bubbling of argon during 3 days. The mixture was con-
centrated to reduce the volume of MeOH, then water (50 mL)
was added to precipitate the adduct 4-(1,3-hydroxy-4,4,5,5-
tetramethylimidazolin-2-yl)pyrazole as a white solid (505 mg,
Crystallographic data for the structures reported in this
paper have been deposited at the Cambridge Crystallographic
Data Base as CCDC reference numbers 603117–603119.
For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b604598g. The alpha phase of 4PzNN has
REFCODE NENNUP.
1
20% yield). H NMR (DMSO-D6) d (ppm) 7.80 (s, 4H, -OH);
7.78–7.47 (m, 4H, Ph); 4.50 (s, 2H, -H imid.); 1.07 (s, 12H,
-CH3); 1.03 (s, 12H, -CH3). 13C NMR (DMSO-D6) d (ppm)
148.3, 141.1, 127.2, 115.1 (Ph), 91.0 (CH imid.), 66.0
(Cquat imid.), 24.1 (-CH3), 17.0 (-CH3). Calcd (%) for
C20H31N4O4: C 60.89, H 8.69, N 14.20; found: C 60.45, H
8.78, N 14.56%. IR (KBr, cm21): 3189 (nOH). 4PzNN could be
obtained either from the adduct or directly from the reaction
mixture described above, adding water and dichloromethane.
To the solution cooled to 0 uC was added NaIO4 (1720 mg,
1.57 mmol). The mixture was stirred at room temperature
during 12 minutes. After quick extraction and drying of the
organic dark blue phase with Na2SO4, the residue was
purified by chromatography (SiO2, diethyl ether–THF 80 :
20 to avoid any acidic chlorinated solvent) to afford the
compound as a blue oil which was dried with heptane (1.230 g,
49% yield). Mp 214 uC; IR (KBr, cm21): 1367 (nN–O); UV-vis
4-Formyl pyrazole
Method A. 4-Bromopyrazole (16.79 g, 115 mmol) was
dissolved in 100 mL of THF and cooled at 278 uC. A solution
of tert-butyllithium was then added dropwise (150.62 mL of
a 1.6 M hexane solution, 241 mmol). A white precipitate
appeared in the mixture when 100 mL of BuLi were added.
The resulting suspension was stirred at this temperature for
half an hour, and then allowed to warm slowly to room
temperature (the suspension disappeared) and stirred for three
hours at room temperature. The mixture was then cooled to
278 uC (the precipitate reappeared) and dimethylformamide
(10.0 mL, 12.7 mmol) freshly distilled under argon was
introduced dropwise by a syringe. The mixture was further
stirred at room temperature during one night and then
acidified with a saturated solution of ammonium chloride
solution. After extraction of the pale yellow solution with
4 portions of 200 mL of diethyl ether, drying with MgSO4 and
removal of the solvents, a yellow oil was obtained and left on
the vacuum pump during 24 h. This crude product was
purified by silica column chromatography (ethyl acetate–
dichloromethane 1 : 9 gradually to 3 : 7) to give a 4.5 g of a soft
creamy solid in 41% yield .
l
max/nm (e/M21 cm21): 585 (835); calcd (%) for C20H28N4O4
(388.5): C 61.84, H 7.26, N 14.42; found: C 61.80, H 7.29,
N 14.41.
Complex [(4PzNN)2Ni(hfac)2]2CHCl3
4PzNN in a mixture of CHCl3 and diethyl ether was added
to a warm heptane solution of one equivalent of nickel(II)
bis(hexafluoroacetylacetonate) and the mixture was allowed to
cool to room temperature. Upon standing blue needles of the
complex crystallized. Yield: 32%. IR (KBr, cm21): 3148, 3009,
2949, 1652, 1562, 1537, 1486, 1257, 1230, 1143, 802, 677.
Details of the crystal structure of the compound are given
below and in Table 1.
Method B. A methanolic solution (50 mL) of trisformyl-
methane (1000 mg, 10 mmol) was added dropwise (1 drop s21
)
Complex [(4PzNN)Co(hfac)2]
to a solution of hydrazine hydrate in distilled MeOH (50 mL).
Usually, the mixture turned to yellow, often accompanied by
the formation of an insoluble orange precipitate. The mixture
was left at room temperature for 5 hours, then filtered, MeOH
was evaporated off, 25 mL of water were added and a
saturated solution of NaHCO3 was added until pH = 8 was
reached. Extraction by ethyl acetate (900 mL in three portions)
was followed by concentration and purification on a short
4PzNN in CHCl3 was added to a warm heptane solution of
one equivalent of cobalt(II) bis(hexafluoroacetylacetonate) and
the mixture was allowed to cool to room temperature. Upon
standing red blocks of the complex crystallized. Yield: 52%. IR
(KBr, cm21): 3369, 2992, 2920, 1645, 1534, 1485, 1257, 1212,
1150, 800, 670. Details of the crystal structure of the
compound are given below and in Table 1.
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13 F. M. Romero, R. Ziessel, M. Drillon, J.-L. Tholence, C. Paulsen,
X-Ray measurement and structure determination
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Data collection was performed on a Nonius Kappa CCD
equipped with graphite-monochromatized Mo-Ka radiation
˚
(l = 0.71073 A). Data were integrated using DENZO and
16 L. Catala, J. Le Moigne, N. Gruber, J. J. Novoa, P. Rabu,
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5078.
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Chem., Int. Ed., 1998, 37, 1270.
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2003, 42, 1584.
27 M. T. Lemaire, Pure Appl. Chem., 2004, 76, 277.
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scaled with SCALEPACK. Several scans in w and v directions
were made to increase the number of redundant reflections,
which were averaged in the refinement cycles. This procedure
replaces in a good approximation an empirical absorption
correction. The structures were solved with direct methods
(SHELXS86) and refined against F2 (SHELX97). All non-
hydrogen atoms were refined with anisotropic displacement
parameters, except three carbon atoms of (4PzNN)2Ni(hfac)2.
Hydrogen atoms attached to carbon atoms were calculated
and refined with isotropic displacement parameters 1.2 or 1.5
times higher than the value of their carbon atoms. Hydrogen
atoms attached to nitrogen were found and refined isotropi-
cally, except for (4PzNN)2Ni(hfac)2, where all hydrogen atoms
were calculated. The solvent CHCl3 of (4PzNN)2Ni(hfac)2
lies near a symmetry centre and is disordered with partial
overlying positions. The fluorine atoms of two CF3 groups in
4PzNN?Co(hfac)2 have a 2 : 1 positional disorder.
The quality of the data set of (4PzNN)2Ni(hfac)2 is reduced
by the small crystal size with low diffraction and visible
intensities on the frames below 2H 34u.
31 I. Alkorta, J. Elguero, B. Donnadieu, M. Etienne, J. Jaffart,
D. Schagan and H.-H. Limbach, New J. Chem., 1999, 23, 1231.
32 C. N. Foces-Foces, I. Alkorta and J. Elguero, Acta Crystallogr.,
Sect. B: Struct. Sci., 2000, 56, 1018.
Acknowledgements
33 I. Boldog, E. B. Rusanov, J. Sieler, S. Blaurock and
K. V. Domasevich, Chem. Commun., 2003, 740.
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38 M. D. Ward, J. A. McCleverty and J. C. Jeffery, Coord. Chem.
Rev., 2001, 222, 251.
We warmly thank Carlos Go´mez (Universitat de Vale`ncia) for
the magnetic susceptibility data and Daniel Ruiz-Molina
(ICMAB-CSIC) for help with its interpretation. This work
was supported by grants from the Programa Nacional de
Materiales of the DGI (Spain, MAT2003-04699), the EU
Marie Curie RTN QuEMolNa (MRTN-CT-2003-504880),
and the NoE MAGMANet (515767-2). We also thank referees
for constructive and helpful comments.
39 R. Q. Zou, C. S. Liu, X. S. Shi, X. H. Bu and J. Ribas,
CrystEngComm, 2005, 7, 722.
40 L. Catala, R. Feher, D. B. Amabilino, K. Wurst and J. Veciana,
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