2,6-Bis(pyrazolyl)pyridine functionalised with two nitronylnitroxide and iminonitroxide radicals

Article abstract of DOI:10.1002/ejoc.200400013

During the last two decades, growing interest in the field of material chemistry has been focused on the synthetic design of organic high-spin molecules for use as novel magnetic materials. Conjugated radicals based on the bis(pyrazolyl)-pyridine core are very attractive in principle, since they per se combine optical, chelating and magnetic properties on a single molecular entity. The functionalisation of this core via the bis(4′-formyl) derivatives with Ullman radicals results in novel functional biradicals, which may further be used for supramolecular architectures. In this paper we describe the synthesis, structure and properties of 2,6-bis[4′-(3-oxo-1-oxyl-4,4,5, 5-tetramethylimidazolin-2-yl)pyrazol-1′-yl]pyridine (Pz₂PyNN) and 2,6-bis[4-(1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)pyrazolyl]pyridine (Pz₂PyIN). The two biradicals can clearly be differentiated by UV/Vis, IR and EPR spectroscopy. The optical absorptions of the blue Pz ₂PyNN appear around λ_max= 610 nm (ε ≈ 1700 M^-1 cm^-1) while for Pz₂PyIN the orange-red colour arises from absorptions at λ_max= 468 nm (ε ≈ 1400 M^-1 cm^-1). A triplet ground state for the bis(nitronylnitroxide) Pz₂PyNN with ΔE_S-T5 ≈ 11.8 ± 4.8 cm^-1 can be deduced from the EPR studies because of the increase of the double integrated intensity times temperature at very low temperatures, indicating complete population of the triplet state. For the related bis(iminonitroxide) Pz₂PyIN a much smaller (at least one order of magnitude lower) singlet-triplet energy gap with an upper limit of 0.7 cm^-1 has been derived (0.7 ≥ ΔE_S-T ≥ -0.07 cm^-1). The new multifunctional biradicals represent the first example of high-spin molecules in the bis(pyrazolyl)pyridine family. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).

Full text of DOI:10.1002/ejoc.200400013

FULL PAPER
2
,6-Bis(pyrazolyl)pyridine Functionalised with Two Nitronylnitroxide and
Iminonitroxide Radicals
[a]
[a]
[a]
[a]
Giorgio Zoppellaro, Ahmed Geies, Volker Enkelmann, and Martin Baumgarten*
Keywords: Heterocycles / Radicals / UV/Vis and EPR spectroscopy / Open-shell molecules / High-spin molecules
−
1
cm−1
)
During the last two decades, growing interest in the field of
material chemistry has been focused on the synthetic design
of organic high-spin molecules for use as novel magnetic tions at λmax= 468 nm (ε ഠ 1400
Pz
2
PyNN appear around λmax= 610 nm (ε ഠ 1700
M
while for Pz
2
PyIN the orange-red colour arises from absorp-
−
1
−1
M
cm ). A triplet ground
materials. Conjugated radicals based on the bis(pyrazolyl)-
pyridine core are very attractive in principle, since they per
se combine optical, chelating and magnetic properties on a
single molecular entity. The functionalisation of this core via
the bis(4Ј-formyl) derivatives with Ullman radicals results in
novel functional biradicals, which may further be used for
supramolecular architectures. In this paper we describe
the synthesis, structure and properties of 2,6-bis[4Ј-(3-
oxo-1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)pyrazol-1Ј-
state for the bis(nitronylnitroxide) Pz
2
PyNN with ∆ES-T ഠ 11.8
−
1
± 4.8 cm can be deduced from the EPR studies because of
the increase of the double integrated intensity times temper-
ature at very low temperatures, indicating complete popula-
tion of the triplet state. For the related bis(iminonitroxide)
Pz
2
PyIN a much smaller (at least one order of magnitude
lower) singlet−triplet energy gap with an upper limit of 0.7
−
1
−1
cm has been derived (0.7 у ∆ES-T у −0.07 cm ). The new
multifunctional biradicals represent the first example of
high-spin molecules in the bis(pyrazolyl)pyridine family.
yl]pyridine (Pz
2
PyNN) and 2,6-bis[4-(1-oxyl-4,4,5,5-tetra-
PyIN). The two
methylimidazolin-2-yl)pyrazolyl]pyridine (Pz
2
biradicals can clearly be differentiated by UV/Vis, IR and
EPR spectroscopy. The optical absorptions of the blue
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
1
. Introduction
direct coupling that leads in general to antiferromagnetic
interactions, and tuning of the indirect coupling, by choos-
ing the types of bonds involved (connectivity) and molecu-
Studies of magneto-structural correlations in organic-
lar topology. Then, the nature of the radicals defines the
based materials have fascinated chemists, physicists and
theoretical chemists, because the rich variety of properties
and related physical phenomena that these materials exhibit
macroscopically provide inspiration for their potential ap-
plications in material science.
chemists,
the magnetic properties by modification of the molecular
unit, thereby gaining control of the intramolecular interac-
tions among the spin carriers. Several theoretical
guidelines
non-Kekule´
much debated,
magnitude of the spin polarisation contribution.[
23,25]
In
this regard, much of the progress achieved in this field over
the last twenty years, and the enormous knowledge gained
about the magnetism phenomenologically, has been driven
[
1Ϫ3]
For preparative organic
[
26Ϫ28]
[
4Ϫ6]
by the chemistry of nitroxides
nitronylnitroxide (NN) and
Their inherent stability
and the discovery of
the challenge in this field consists in tailoring
iminonitroxide
(IN)
[
29Ϫ31]
[31Ϫ35]
radicals.
in fact allows
experimental probing of all the effects governing the intra-
molecular interaction among spin units. Finally, the goal as
commonly recognised is to arrange each unit into a supra-
[
7Ϫ11]
for the design of the organic backbone in
hydrocarbons have been described and
[
12Ϫ16]
[
36Ϫ40]
[
17Ϫ20]
molecular network.
One approach is based on inter-
and these in turn allow prediction of
molecular interactions between pure organic carriers, and
this led to the achievement of bulk ferromagnetism where
the p-nitrophenyl nitronylnitroxide made by Kinoshita’s
the ground-state multiplicity of the single magnetic entity.
Conceptually, the intramolecular interactions among un-
paired spins can be visualised as the fine balance of three
[
41]
physical mechanisms[
1,21Ϫ24]
group and the 1,3,5,7-tetramethyl-2,6-diazaadamantane-
: (i) direct coupling, (ii) indirect
[
42]
N,NЈ-dioxyl made by Rassat et al.
among many outstanding examples.
represent just two
Another ap-
coupling and (iii) spin polarisation. The design of the mo-
lecular backbone (spacer) allows a decrease in the effect of
[
43Ϫ48]
proach relies on the combination between paramagnetic
metal ions and pure organic radicals, leading to a new class
of hybrid organic-inorganic materials described in the
[
a]
Max Planck Institute for Polymer Research,
Ackermannweg 10, 55128, Mainz, Germany
E-mail: baumgart@mpip-mainz.mpg.de
Fax: (internat.) ϩ49-6131-379370
[49Ϫ55]
extensive work carried out by several groups.
We
therefore thought to follow the last approach by building
chelating ligands as a core and functionalising them with
Eur. J. Org. Chem. 2004, 2367Ϫ2374
DOI: 10.1002/ejoc.200400013
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2367

G. Zoppellaro, A. Geies, V. Enkelmann, M. Baumgarten
FULL PAPER
nitronyl- and iminonitroxide radical moieties. In this in-
stance, a clear understanding of the intramolecular ex-
change interaction (J) between the radical fragments
through the coupler is a prerequisite to their possible use
2
. Results and Discussion
2.1. Synthesis of Pz PyNN and Pz PyIN
2 2
as novel building blocks for supramolecular architectures.
The bis(radical) derivatives Pz PyNN (2,6-bis[4Ј-(3-oxo-
2
In our previous work,[ we showed how, following design 1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)pyrazol-1Ј-yl]-
56]
of the molecular backbone, the novel nitronylnitroxide and pyridine, 7) and Pz PyIN (2,6-bis[4-(1-oxyl-4,4,5,5-tetra-
2
iminonitroxide biradical systems, attached to a terpyridine methylimidazolin-2-yl)pyrazolyl]pyridine, 9) have been syn-
spacer, exerted a strong intramolecular ferromagnetic ex- thesised following the multistep approach outlined in
change interaction between the radical units. Those rep- Scheme 1. The condensation of triformylmethane 1[ with
resented the first reported example of S ϭ 1 coupled Ull- hydrazine monohydrate in acidified alcoholic medium, af-
man radicals appended on a terpyridine system, and their forded the pyrazol-4-carboxaldehyde 2.[59] Then, the 2,6-
triplet ground-state spin multiplicity had been predicted on bis(4-formyl-pyrazolyl)pyridine 4 was synthesized in a one-
the basis of topological considerations. In order to further step reaction by condensation between the nucleophilic pot-
58]
[
60]
extend this work on similar heterosystems, we substituted assium salt of 2 with 2,6-dibromopyridine 3 in diethylene
the pyridines with pyrazolyl moieties[ which led to the glycol dimethyl ether (diglyme) as reaction solvent (yield
57]
novel symmetric bis(pyrazolyl)pyridine (Pz Py) derivative 45%). The choice of diglyme was dictated by the obser-
2
presented in this paper. This unit may be regarded as a ter- vation that while the first bromine group in 3 is replaced
pyridine-analogue, since it reproduces the tridentate nitro- relatively easily (70 °C, 48 h), substitution of the strongly
gen binding motif of the terpyridine core as schematically inactivated second bromine group requires a higher tem-
shown in Figure 1. On the other hand, it is worth men- perature and prolonged reaction time. Diglyme offers the
tioning that in Pz Py the topological guidelines for intra- advantages with respect to other ethereal solvents (e.g.
2
molecular coupling for π-conjugated radicals cannot be ap- THF, Et O) of high boiling point and stability at high tem-
2
plied because the presence of two pyrazolyl fragments (five- perature. In addition, its ability to chelate cations (the pot-
membered ring) renders this system nonalternant. However, assium cation in our case) leaves the nucleophile much more
besides the unfavourable spin-coupling pathways, the triplet active. The subsequent condensation between the diformyl
ground state was experimentally found for the bis(nitronyl- derivative 4 with 2,3-bis(hydroxylamino)-2,3-dimethylbutane
[
29Ϫ31,61]
nitroxide) radical with an appreciable singletϪtriplet energy (5)
in dioxane occurred while stirring at room tem-
Ϫ1
gap ∆ES-T of 17 K (ഠ 11.8 Ϯ 4.8 cm ) while a much perature under argon over ten days, and afforded the radical
smaller gap of ഠ 1 K (0.70 у ∆ES-T у Ϫ0.07 cm ) has been precursor 6 (2,6-bis[4-(1,3-dihydroxy-4,4,5,5-tetramethyl-
Ϫ1
estimated for the related bis(iminonitroxide) radical. One imidazolidin-2-yl)-pyrazolyl]pyridine) as a yellowish pow-
can recognise that the search for new organomagnetic units der (yield 42%). Heating the reaction mixture increased the
in turn stimulates further progress in theoretical consider- decomposition of 6 (dehydration) in solution, and led to
ations.
the formation of compound 8 (2,6-bis[4-(1-hydroxy-4,4,5,5-
tetramethylimidazolin-2-yl)pyrazolyl]pyridine). The sodium
periodate oxidation of 6 carried under phase-transfer con-
ditions (CHCl /H O) gave the crude biradical Pz PyNN in
3
2
2
the organic phase. The side products (i.e. mono-nitronylni-
troxide, mono-iminonitroxide) were easily recognised on
TLC and separated by column chromatography (silica gel,
acetone/light petroleum ether, b.p. 30Ϫ40 °C, 2:8), then
highly pure Pz PyNN was collected (R ϭ 0.34) and recrys-
2
f
tallised from CHCl (blue crystals, yield 27%). Similarly,
3
oxidation of the radical precursor 8 under phase-transfer
conditions (CHCl /H O) afforded the crude biradical
3
2
PzPyIN in the organic phase. Purification of the crude mix-
ture was carried out by column chromatography (silica gel,
acetone/light petroleum ether, b.p. 30Ϫ40 °C, 2:8, R ϭ
f
0
.46), then recrystallisation from CH Cl2 gave pure
2
Pz PyIN (orange powder, yield 51%). As expected, the bi-
2
radical Pz PyIN was obtained in larger yield than
2
Pz PyNN. Once isolated, the biradicals are stable as powder
2
over several months, while in protic solvents (e.g. CH Cl2
2
or CHCl ), which may catalyse loss of water molecules, a
3
clear decrease of their stability was observed upon pro-
longed storage. However, in aprotic solvents such as tolu-
ene, both biradicals can safely be kept for a long time, and
even heated up to 60 °C for several hours, with no hint of
Figure 1. The tridentate nitrogen core in the terpyridine (terpy)
2
and in the bis(pyrazolyl)pyridine (Pz Py); the position in which the
radical units have been appended are indicated with arrows
2368
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 2367Ϫ2374

Reactions with Two Nitronylnitroxide and Iminonitroxide Radicals
FULL PAPER
decomposition, providing that the medium is maintained
oxygen free.
2 2
Figure 2. UV/Vis spectra of Pz PyNN (solid line) and Pz PyIN
(
dashed-dotted line) recorded in toluene solutions at room tem-
perature; the inset shows magnified their visible region of the
spectra
Scheme 1
2.2. Optical Properties
In solution the biradical Pz PyNN is light blue (toluene,
2
Ϫ1
Ϫ1
λmax ϭ 610 nm and ε ഠ 1700  cm , nǞπ* transitions)
2 2
Figure 3. IR spectra of Pz PyNN (solid line) and Pz PyIN (dashed
while Pz PyIN shows the typical orange-red colour (tolu-
2
dotted line) recorded in KBr pellets at room temperature
Ϫ1
Ϫ1
ene, λmax ϭ 468 nm and ε ഠ 1400  cm , nǞπ* tran-
sitions). For both biradicals, the broad absorptions in the
visible region of the spectra are characterized by several vi-
were structurally characterized by X-ray diffraction (Fig-
ure 4). One of the structural features of the molecule are
the torsional angles ϕ and ϕ because they reflect the pos-
bronic components (Figure 2, solid line for Pz PyNN and
2
dashed-dotted line for Pz PyIN, respectively), and their mo-
2
1
2
lar extinction ε is more than three times greater than in
sibility of free rotation around the single bonds (σ, N2ϪC1,
C7ϪC5) and hence can be a measure of the effective π-
conjugation through the spacer. Since the molecules are lo-
cated on a twofold rotation axis, angles ϕ and ϕ are ident-
[56]
the analogous terpyridine system
(Figure 1), where two
nitronylnitroxide or iminonitroxide radicals have been at-
tached in position 5,5ЈЈ (toluene, λmaxϭ 605 nm, ε ϭ 480
1
2
Ϫ1
Ϫ1
Ϫ1


cm for the bisnitronyl- and λmaxϭ 459 nm, ε ϭ 380
ical for each half-molecule. ϕ (C6ϪN2ϪC1ϪN1) is 4.6°
1
Ϫ1
cm for the bisiminonitroxide). Their infrared spectra
and the torsional angle (C6ϪN2ϪN3ϪC4) is 0.29°, so the
recorded in KBr pellets are shown in Figure 3. In the case
two pyrazolyl rings and the pyridinyl central core are nearly
of Pz PyNN (solid line) a strong absorption originating
2
coplanar. ϕ , between the imidazolidinyl and the pyrazolyl
2
Ϫ1
from the NϪO stretching mode is visible around 1359 cm
moieties (N5ϪC7ϪC5ϪC6), is very similar at 4.2°. Hence,
while for Pz PyIN (Figure 3, dashed-dotted line) such a sig-
2
the overall quasi-planar structure of Pz PyNN in the crys-
2
nal, much weaker and commonly shifted to higher wave-
tals satisfies the geometrical prerequisite for effective π-con-
jugation between the two radical fragments. The imidazoli-
dinyl rings show relatively high intraring torsion in the per-
iphery, with (C8ϪC9ϪN5ϪC7) being Ϫ18.86° and
[
31,60,62]
number,
appears to be embedded in the spectrum.
2
.3. The Crystal Structure of Pz PyNN
2
Upon slow diffusion of hexane into a CHCl solution of (C9ϪC8ϪN4ϪC7) Ϫ22.58°. The intramolecular (O2ϪO*
radical Pz PyNN, blue crystals were obtained and these 2) and the shortest intermolecular through-space distances
3
2
Eur. J. Org. Chem. 2004, 2367Ϫ2374
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G. Zoppellaro, A. Geies, V. Enkelmann, M. Baumgarten
FULL PAPER
(
short magnetic contacts) between the imidazolidinyl oxy- vent. The double integrated EPR signal intensities for
˚
gen atoms are 5.642 and 4.738 A, respectively. The intra- Pz PyNN and Pz PyIN in solution accounts for 2.0 Ϯ 0.1
2
2
molecular distances between the two ONCNO groups uncorrelated spins, thus a rather small singletϪtriplet en-
C7ϪC*7), where most of the radical-spin densities are lo- ergy gap (∆EST) is present, and contributions arising from
(
cated, and that between the pyrazolyl carbon atoms thermally accessible spin states are active in the high-tem-
˚
(
C5ϪC*5) are 9.130 and 7.706 A, respectively. Finally, the perature regime. In such systems, when the fraction of bi-
NϪO bond lengths are in the range of the nitroxide radicals radicals under the strong exchange limit (|J/a | ϾϾ 1) rep-
N
˚
˚
reported in literature (1.283 A for O2ϪN5, 1.281 A for resents the unique or the dominating component in the ob-
[
31,69]
O1ϪN4).
served spectrum,
the simulation of the entire isotropic
solution EPR spectrum by numerical diagonalisation of the
spin Hamiltonian [Equation (1)].
2
Figure 4. The crystal structure of Pz PyNN; the thermal ellipsoids
are drawn at 50% probability. The hydrogen atoms are omitted
for clarity
2
.4. EPR Characterisation of Pz PyNN and Pz PyIN in
2 2
Solution and in the Frozen State
In order to disclose the magnitude and sign of the intra-
molecular exchange interaction between the radical moiet-
ies in Pz PyNN and Pz PyIN, detailed EPR analyses have
2
2
been carried out. Owing to its high sensitivity, the EPR
technique allows working with magnetically diluted
samples, thus minimising the effects of intermolecular con-
tributions among the paramagnetic centres. The EPR spec-
Ϫ4
trum of Pz PyNN (toluene, 10 , 298 K) displayed a re-
2
solved nine-line pattern [Figure 5 (A), solid line] at giso
ϭ
2.0065(1), where the observed line-spacing (aN1obs
ϭ
0.38 mT) corresponds to half of that found for the related
mono-nitronylnitroxide radical (where aN ϭ 0.76 mT).
Similarly, a 13-line pattern at giso ϭ 2.0060(1) is exhibited
Ϫ4
in Pz PyIN [Figure 5 (B), solid line, toluene, 10 , 298 K]
2
where again half of the line-spacing (aN1obs ϭ 0.21 mT, Figure 5. (A) EPR spectrum of Pz PyNN (solid line) recorded in
2
dilute and oxygen-free toluene solution at 298 K and its computer
aN2obsϭ 0.45 mT) is observed, compared with the related
mono-iminonitroxide radical (where aN1 ϭ 0.42 mT and
simulation (dashed-dotted line, Gaussian line-width: 6.12 MHz,
modulation amplitude: 0.84 MHz, |a | ϭ 21.44 MHz; experimen-
iso
aN2 ϭ 0.91 mT). Such findings indicate that in these birad- tal parameters: 9.4033 GHz, 100 kHz modulation frequency,
4
.0 mW power, 0.03 mT modulation amplitude; (B) EPR spectrum
icals, the intramolecular exchange interaction (J) between
the radical fragments through the spacer bis(pyrazolyl)pyri-
of Pz
2
PyIN (solid line) recorded in dilute and oxygen-free toluene
solution at 298 K and its computer simulation (dashed-dotted line,
dine is much larger than the hyperfine terms (|J/a | ϾϾ 1). Lorentzian line-width: 4.78 MHz, modulation amplitude:
N
0
N
.84 MHz, |aiso| ϭ 18.67 MHz where |aiso| ϭ [|aN1| ϩ |a |/2] with
The total spectral width, shape and number of observed
|aN1| ϭ 11.62 MHz and |aN2| ϭ 25.7211 MHz, weight factor: 0.73
peaks did not change upon changing solvent (CHCl3, for the averaged molecular conformations in strong exchange limit,
the other factors are kept the same as the experimentally recorded
CH Cl , acetone, THF, MeOH) either for Pz PyNN and
2
2
2
spectrum); experimental parameters: 9.4027 GHz, 100 kHz
modulation frequency, 5.0 mW power, 0.03 mT modulation
Pz PyIN, so toluene was used for all the measurements
2
since the radicals showed the highest stability in this sol- amplitude
2370  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org
Eur. J. Org. Chem. 2004, 2367Ϫ2374

Reactions with Two Nitronylnitroxide and Iminonitroxide Radicals
FULL PAPER
H ϭ gβB(S
a
ϩ S
b
) Ϫ 2JS
a
S
b
ˆ
ϩ Σ
Ni ϩ S
i
a
Ni
ˆ
ϫ
(
1)
ˆ
N N
(S
a
I
b
I
Ni) Ϫ g
N
β
BI
may provide an estimate of the lower limiting value for the
exchange interaction, but clearly not its sign (either ferro, J
Ͼ 0, or antiferromagnetic, J Ͻ 0). The simulation accounts
Ϫ1
for |∆ES-T| ϭ |2J/k | ϭ 0.07 cm (rms ϭ 0.31) in the case
b
of Pz PyNN [Figure 5 (A), dashed-dotted line] and for |2J/
2
Ϫ1
kb| ϭ 0.06 cm (rms ϭ 0.38) in the case of Pz PyIN [Fig-
2
ure 5 (B), dashedϪdotted line]. In frozen solution, the EPR
spectrum of the allowed transitions in Pz PyNN (∆M ϭ 1,
2
s
Ϫ3
toluene, 10 , 120 K, gav ϭ 2.0065) showed a poorly re-
solved anisotropic pattern. This arises from dipolar interac-
tions characteristic of randomly oriented triplet species and
the relatively large distance between the radical centres.
However, the zero-field-splitting parameter DЈ (DЈ ϭ |D|/
Ϫ3
Ϫ1
hc) has been estimated as 4.8 mT (ഠ 4.50 ϫ 10 cm ). In
the point-dipole approximation,[
71Ϫ74]
the DЈ value is con-
sistent with the average distance between the centres of the
˚
ONCNO groups of about 8.3 A, and correlates nicely with
the average distances between the radical centres observed
in the crystal structure.
Similarly, a poorly resolved anisotropic pattern was ob-
served in the case of Pz PyIN (∆M ϭ 1, toluene, 1.2 ϫ
Figure 6. (A) The evolution of the double integrated signal intensit-
ies (DI, open circles) and the product DI·T (closed circles ) versus
2
s
Ϫ3
10
, 120 K, gav ϭ 2.0060); the zero-field-splitting param- T of the ∆M ϭ2 signal for Pz PyNN in the low temperature range
s
2
(
T Ͻ 40 K); the solid lines in DI·T represent the best fitting of the
eter DЈ (DЈ ϭ |D|/hc) has been estimated as 5.1 mT (ഠ 4.78
data with parameters given in the text according to the
Ϫ3
Ϫ1
ϫ 10 cm ), which is consistent with the shorter distance
BleaneyϪBowers model for two interacting S ϭ 1/2 systems; the
˚
between the centres of the ONCN groups of about 8.2 A.
2
inset shows the half-field transition of Pz PyNN recorded in frozen
solution at T ϭ 4.1 K; experimental parameters: 9.4193 GHz,
The triplet state nature of Pz PyNN was supported by
2
1
00 kHz modulation frequency, 0.4 mT modulation amplitude,
the appearance of the forbidden transition at half-field 0.320 mW power; (B) the evolution of the double integrated signal
Ϫ3
intensities (DI, open circles) and the product DI·T (closed circles)
(
∆M ϭ 2, toluene, 10 , T ϭ 120 K, gav ഠ 4.01, ∆Bpp ഠ
s
s 2
versus T of the ∆M ϭ2 signal for Pz PyIN in the low temperature
1.05 mT) and its temperature dependence was followed
range (T Ͻ 40 K); the solid line in DI·T represents the best fitting
down to cryogenic temperature [Figure 6 (A) in the inset, of the data for the upper 2J/k limit (S ϭ 1) with parameters given
T ϭ 4.1 K, gav ഠ 4.01, ∆Bpp ϭ 1.04 mT]. The microwave
powers applied in the measurements were carefully con-
trolled in such a way that the signal intensities were pro- field transition of Pz
b
in the text according to the BleaneyϪBowers model; similarly, the
dashed-dotted line represents the correspondent fitting for DI·T
assuming the singlet ground state (S ϭ 0); the inset shows the half-
2
PyIN recorded in frozen solution at T ϭ
.2 K; experimental parameters: 9.4192 GHz, 100 kHz modulation
frequency, 0.4 mT modulation amplitude, 0.320 mW power
4
portional to the square root of the power. As shown in Fig-
ure 6 (A), the double integration (DI) of the ∆M ϭ 2 signal
s
increased upon decreasing the temperature (open circles).
Furthermore, the quantity DI·T (closed circles), which is
proportional to the population in the triplet state, also in- dence has also been followed down to cryogenic tempera-
creased with decreasing temperature. Such findings indicate ture [Figure 6 (B) in the inset, T ϭ 4.2 K, gav ഠ 4.01,
that the ground state in Pz PyNN is the triplet (S ϭ 1), and ∆Bpp ϭ 0.96 mT].
2
the singlet (S ϭ 0) should be associated with a thermally
While the ∆M ϭ 2 signal clearly increased upon decreas-
s
accessible excited state. Fitting of the data according to the ing the temperature [Figure 6 (B), open circles], the product
BleaneyϪBowers model for two interacting spin systems as DI·T appeared nearly constant (closed circles), with a very
described in Equation (2),[69] where
small rise at the lowest temperature. In such a case, either
the triplet state (S ϭ 1) is the lowest-energy state separated
from the singlet (S ϭ 0) by a substantial gap relative to
2
2
DI ϭ χEPR ϰ {(2Ng β /3k
b
T) ϫ [1/3 ϩ exp(Ϫ2J/k
b
T)]}
(2)
the thermal energy (|2J| ϾϾ k T), or the energy difference
b
between triplet (S ϭ 1) and singlet (S ϭ 0) is extremely
gave a separation ∆ES-T ϭ 2J/kb of 11.8 Ϯ 4.8 cmϪ1 small leading to degeneracy of those levels. Therefore, any
between the magnetic ground (S ϭ 1) and the excited states change in temperature does not shift the thermal equilib-
(
S ϭ 0). In the Pz PyIN system, the triplet-state nature was rium between the two states, and they remain statistically
2
also confirmed by the appearance of the forbidden tran- populated according to the Boltzmann distribution (Nu/
Ϫ3
sition at half-field (∆M ϭ 2, toluene, 1.2 ϫ 10 , T ϭ N ϭ [g /g ] ϫ expϪ[∆E/k T], e.g. 75% of the molecules
s
0
u
0
b
120 K, gav ഠ 4.01, ∆Bpp ϭ 0.99 mT). Its temperature depen- occupy the triplet state and 25% the singlet state). Since
Eur. J. Org. Chem. 2004, 2367Ϫ2374
www.eurjoc.org  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2371

G. Zoppellaro, A. Geies, V. Enkelmann, M. Baumgarten
FULL PAPER
from the solution EPR studies discussed on Pz PyIN, it was trometer) at room temperature. Mass spectra were obtained on FD-
2
MS, VG Instruments ZAB-2 mass spectrometer. Elemental analy-
clear that the energy gap between singlet and triplet cannot
ses were performed at the University of Mainz, Faculty of Chemis-
be large, such separation has to be very small, ranging far
try and Pharmacy on a Foss Heraeus Varieo EL. Melting points
below the temperature available with our experimental set-
were measured on a Büchi B-545 apparatus (uncorrected) by using
ting (4 K). Fitting the DI·T data according to the
open-ended capillaries. The 4-formylpyrazole 2[ and 2,3-bis(hyd-
59]
BleaneyϪBowers model for two interacting spin system,
roxylamino)-2,3-dimethylbutane (5) (free base) were prepared ac-
gave an upper limit for the singletϪtriplet separation ∆ES-
[61]
cording to literature procedures.
Ϫ1
ϭ 2J/k of 1 K (i.e. about 0.70 cm ) assuming a triplet
T
b
Triformylmethane (1): Compound was prepared according to a bri-
ground state. If the singlet ground state (S ϭ 0) is con-
efly reported procedure,[ which is described fully here. POCl
(13.8 mL, 0.147 mol) was added dropwise to a cooled solution of
DMF (C NO, 40 mL, 0.518 mol, at about 0 °C) over a period
58]
3
sidered, the separation ∆ES-T is further reduced by a factor
Ϫ1
of ten (2J/k of about Ϫ0.07 cm ), in close agreement with
b
3
H
7
the lower limiting value obtained from the simulation of the
solution EPR spectra. Since complete spin reversal of the
ground-state multiplicity (from nitronyl- to iminonitroxide)
seems improbable but cannot be excluded, we may suggest
of 1 h keeping the temperature below 5 °C during the addition. The
solution initially appeared greenish but turned pale orange by the
end of the addition. Then the ice-bath was removed and the dense
mixture was stirred at room temperature for 1 h. Bromoacetic acid
that in Pz PyIN also the ground state is the triplet. These (7.15 g, 0.051 mol) was added in portions, and the mixture was
2
findings are consistent with previous observations where ni- heated for 24 h at 70 °C. The brownish mixture was decomposed
tronylnitroxide-based biradicals exert a much stronger ex- with ice/water (200 mL) and solid Na CO was carefully added in
2 3
large excess until pH ഠ 8. Absolute ethanol was added (2 L), and
the inorganic salts were filtered off. The organic filtrate was evapo-
rated slowly under a stream of air, and the pale yellowish residue
change interaction than the parent bis(iminonitroxide) sys-
tem.
[
31,70]
In conclusion we have described a hitherto novel func-
tionalisation of the bis(pyrazolyl)pyridine core with neutral
radical moieties of the NN and IN type. Clear differen-
tiations between their spectroscopic behaviour (UV, IR,
EPR spectroscopy) is described, allowing also their use as
was neutralized with H
2
SO
4
(50%, 10 mL), extracted with CHCl
. After solvent removal, tri-
3
(
3 ϫ 200 mL) and dried over MgSO
4
formylmethane 1 was obtained as yellowish crystals that were
further purified by sublimation (2.3 g, yield 45% based on bromo-
acetic acid). The infrared data and elemental analysis have not been
fingerprints for contaminated samples. While the bis(nitron- reported previously.[58,59]
ylnitroxides) are relatively strong ferromagnetic exchange
M.p. 102Ϫ103 °C (ref. M.p, 101 °C). FT-IR (KBr): ν˜ ϭ 2828 (w,
coupled in intramolecular fashion the exchange coupling
of the bis(iminonitroxides) must presumably still undergo
ferromagnetic exchange but by at least an order of magni-
tude lower than for the NIT series, making them less
favourable in such functionalised building blocks.
ν
CϪH), 2731 (m, νCϪH), 1699 (s, CϭO), 1599(s), 1566 (s), 1561 (m),
Ϫ1
4 4 3
1207 (s), 1166 (m), 826 (s), 723 (m) cm . C H O (100.07): calcd.
C 48.01, H 4.03; found C 48.25, H 4.11.
2,6-Bis(4Ј-formylpyrazol-1Ј-yl)pyridine (4): Potassium metal (0.4 g,
1
1
0 mmol) was added to a solution of 4-formylpyrazole 2 (1 g,
0 mmol) dissolved in dry diglyme (30 mL, b.p. 162 °C), then the
mixture was heated under argon at 70 °C whilst stirring until all of
the potassium had reacted to form the pyrazolate derivative. Then,
3. Experimental Section
2
,6-dibromopyridine 3 (1.18 g, 5 mmol) was added with a cannula,
3
.1. General Remarks: Solvents were distilled before use and kept
and the reaction mixture was heated at 110 °C for a further 72 h
under argon. The mixture was cooled to room temperature and the
solid product formed was filtered off, washed first with cold water
and then with small portions of ethanol. Finally the residue was
air dried. Recrystallisation from toluene afforded 4 (0.6 g, yield
dry over molecular sieves; all other reagents were used as received.
ESR spectra were recorded in dilute, oxygen-free solutions in tolu-
ene, concentration 10 mol ϫ dm unless otherwise stated, using
a Bruker X-band spectrometer ESP300 E, equipped with an NMR
gaussmeter (Bruker ER035),
Ϫ4
Ϫ3
a frequency counter (Bruker
4
5%) as pale yellowish crystals. M.p. 283Ϫ284 °C (from toluene).
ER041XK) and a variable temperature control continuous flow N
2
FT-IR (KBr): ν˜ ϭ 3129 (w, νCϪH, aromatic), 3097 (w, νCϪH, aro-
matic), 2857 (w, νCϪH), 1682 (s, νCϭO), 1610 (m), 1583 (m), 1552
cryostat (Bruker B-VT 2000) or with an Oxford system (ESR, 910)
helium continuous flow cryostat. The g-factor corrections were ob-
tained using the PNT radical (g ϭ 2.0026) or DPPH (g ϭ 2.0037)
as standard. Calculations of the spin concentrations were carried
(
s), 1473 (s), 1409 (m), 1336 (w), 1290 (w), 1217 (m) (pyridinyl
Ϫ1
1
and pyrazolyl moieties) cm
6
. H NMR (250 MHz, [D ]DMSO,
298 K): δ ϭ 9.97 (s, 2 H, 2-CHO), 9.73 (s, 2 H, 2-CH), 8.34 (s, 2
Ϫ4
out on dilute solutions (10 ), where the double integrated signal
H, 2-CH), 8.25 (t, J ϭ 7.2 Hz, 1 H, ϪCH), 7.95 (d, J ϭ 7.9 Hz, 2
intensities of 2,6-bis[4Ј-(3-oxo-1-oxyl-4,4,5,5-tetramethylimidazo-
lin-2-yl)pyrazol-1Ј-yl]pyridine (7) and 2,6-bis[4-(1-oxyl-4,4,5,5-
tetramethylimidazolin-2-yl)pyrazolyl]pyridine (9) were compared
with those of the corresponding nitronylnitroxide and iminonitro-
xide monoradicals recorded under identical conditions (modulation
amplitude, receiver gain, power, sweep time, temperature) using the
13
H, 2-CH) ppm. C NMR (63 MHz, [D
85.3, 149.9, 143.0, 142.4, 133.1, 125.8, 111.3 ppm. FD-MS (8 kV,
CH Cl ): m/z (%) ϭ found 266.90 (100%), calcd. for C13
267.24). C13 (267.24): calcd. C 58.43, H 3.39, N 26.21;
found C 58.28, H 3.56, N, 26.10.
6
]DMSO, 298 K): δ ϭ
1
2
2
9 5 2
H N O
(
9 5 2
H N O
same EPR tube, and with careful control of the filling factors. UV/ 2,6-Bis[4Ј-(1,3-dihydroxy-4,4,5,5-tetramethylimidazolidin-2-yl)-
Vis spectra were recorded in toluene solutions with pyrazol-1Ј-yl]pyridine (6): A mixture of 2,6-bis(4Ј-formylpyrazol-1Ј-
PerkinϪElmer spectrometer (UV/Vis/NIR Lambda 900) using a 1- yl)pyridine (4) (300 mg, 1.12 mmol) and 2,3-bis(hydroxylamino)-
a
1
13
cm optical path quartz cell at room temperature. H and C NMR
spectra were recorded on a Bruker AMX 250 spectrometer. IR
spectra were recorded in KBr pellets (Nicolet 730 FT-IR spec-
2,3-dimethylbutane (5) (600 mg, 4 mmol) in dioxane (40 mL) was
stirred under argon for 10 days. The solvent was removed under
reduced pressure and the solid product was collected, washed first
2372
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 2367Ϫ2374

Reactions with Two Nitronylnitroxide and Iminonitroxide Radicals
FULL PAPER
with water then with ethanol and air dried to afford 265 mg of
ene): λmax (ε, Ϫ1 ϫ cmϪ1) ϭ 552 (90), 500 (850), 468 (1400), 442
crude 6 that was used for the next step without further purification
(1285), 415 (910), 315 (23850) nm. C25
31 9 2 2
H N O ϫ H O (507.59):
(
yield of the crude product 42%). M.p. 253Ϫ254 °C (from ethanol).
FT-IR (KBr): ν˜ ϭ 3215 (s, broad, νOH), 3121 (s, νCϪH, aromatic),
977 (s, νCϪH), 2932 (s, νCϪH), 1620 (s), 1472 (s), 1405 (s), 1320
calcd. C 59.16, H 6.55, N 24.84; found C 58.96, H 6.66, N 24.68.
3
.3. Crystal Structure Determination and Refinements for 2,6-Bis[4Ј-
(3-oxo-1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)-pyrazol-1Ј-yl]py-
ridine (Pz PyNN) (7): Formula C25 , M ϭ 521.57, mono-
clinic, space group C2/c (N°15), a ϭ 18.5194(7), b ϭ 9.6934(5), c ϭ
2
Ϫ1
1
(
(
m), 1207 (m) (pyridinyl and pyrazolyl moieties) cm . H NMR
250 MHz, [D ]DMSO, 298 K): δ ϭ 8.85 (s, 2 H, 2-CH), 8.17 (t,
2
H
31
N
9
O
4
w
6
J ϭ 2.8 Hz, 1 H, ϪCH), 8.0 (s, 4 H, 4-OH), 7.86 (s, 2 H, 2-CH),
7
1
˚
˚
3
1
c
4.4537(6) A, β ϭ 103.4960(10)°, V ϭ 2522.92(19) A , D ϭ 1.373 g
.82 (d, J ϭ 8.2 Hz, 2 H, 2-CH), 4.75 (s, 2 H, 2-CH imidazolidyn),
Ϫ3
) ppm. 1 C NMR (63 MHz,
3
ϫ cm , Z ϭ 4, T ϭ 120 K. Crystal color, shape and size: dark
blue, needles, 0.43 ϫ 0.26 ϫ 0.18 (mm). Dataset: 3057 total, 2889
.15 (d, J ϭ 8.4 Hz, 24 H, 8-CH
]DMSO, 298 K) δ ϭ 149.8, 143.7, 141.5, 126.7, 108.5, 83.1,
6.2, 24.0, 27.0 ppm. C25 ϫ 2H O (563.65): calcd. C
3
[D
6
unique reflections (4.1Ͻ θ Ͻ27.5°) of which 1895 were observed [I
Ͼ 2.0 σ(I )], Nref ϭ 1895, Npar ϭ 175. The structure was solved
0
6
5
H
37
N
9
O
4
2
3.27, H 7.33, N 22.37; found C 52.96, H 7.44, N 22.10.
0
by direct methods (SHELXS) and refined by a full-matrix least-
squares procedure to R1 value of 0.0385 (wR2 ϭ 0.0453, all data),
GOF ϭ 1.05. The crystallographic data were collected on a
2
1
,6-Bis[4Ј-(3-oxo-1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)pyrazol-
Ј-yl]pyridine (Pz PyNN) (7): Compound 6 (200 mg, 0.38 mmol)
2
dissolved in chloroform (30 mL) was oxidised under phase-transfer
conditions with NaIO
NoniusϪKappa CCD (Mo-K , µ ϭ 0.71073 A˚ ) diffractometer
α
4
(400 mg, 1.85 mmol), previously dissolved equipped with a graphite monochromator. Crystallographic data
in water (20 mL), during 30 min. The color of the organic phase
gradually changed to give finally a deep-blue organic solution. The
organic phase was collected and the solvents evaporated under air.
The bluish residue was dissolved in acetone (3 mL) and chromato-
(excluding structure factors) for the structure reported in this paper
have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC-217301. Copies of
the data can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, UK [Fax: (internat.) ϩ44-
graphed on a SiO
ether (2:8) as eluent to afford Pz
that was further recrystallised from CHCl
M.p. 220Ϫ221 °C (from CHCl
). FT-IR (KBr): ν˜ ϭ 3164 (w, νCϪH
aromatic), 2984 (w, νCϪH), 2936 (w, νCϪH), 1596 (s), 1470 (s), 1429 BiRad, written by Priv.-Doz. Dr. Gunnar Jeschke and Mr. Dariush
s), 1403 (s), 1359 (s, νNϪO), 1312 (m), 1190 (m) (pyridinyl and Hinderberger (Prof. Dr. Hans W. Spiess, Polymer Spectroscopy
2
column using a mixture of acetone/petroleum
PyNN as a blue solid (R
(54 mg, yield 27%).
2
f
ϭ 0.34) (1223)336-033; E-mail: deposit@ccdc.cam.ac.uk].
3
,
Calculations: The simulations were performed using the program
3
(
Ϫ1
Ϫ1
pyrazolyl moieties) cm . UV/Vis (toluene): λmax (ε, 
Group, Max Planck Institute for Polymer Research, Mainz). This
cm ) ϭ 668 (1460), 610 (1696) 563 (930), 518 (328), 375 (16960), program is available free to the public upon request (E-mail:
ϫ
Ϫ1
3
28 (24850), 323 (31220) nm. FAB-MS (NBA as matrix): m/z (%) ϭ
jeschke@mpip-mainz.mpg.de).
ϩ
found 521.30 (100%) [M ϩ H] , calculated for C25
521.57). C25 (521.57): calcd. C 57.57, H 5.99, N 24.17;
found C 57.34, H 5.72, N, 23.94.
31 9 4
H N O
(
31 9 4
H N O
Acknowledgments
2
,6-Bis[4-(1-hydroxy-4,4,5,5-tetramethylimidazolin-2-yl)- The authors thank the DFG for continuous support and also the
pyrazolyl]pyridine (8): A mixture of 2,6-bis(4Ј-formylpyrazol-1Ј-yl)-
pyridine 4 (260 mg, 1 mmol) and 2,3-bis(hydroxylamino)-2,3-di-
methylbutane 5 (300 mg, 2.02 mmol) in dioxane (40 mL) was
heated at 60 °C under argon for seven days. The solvent was re-
moved under reduced pressure, and the solid product was collected,
washed with water then with ethanol and dried under a nitrogen
stream to give the highly air-sensitive compound 8 (225 mg, crude
yield 46%), which was used in the next step without further purifi-
cation. FT-IR (KBr): ν˜ ϭ 3225 (broad, ϪOH), 3121 (s, νCϪH, aro-
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Eur. J. Org. Chem. 2004, 2367Ϫ2374
www.eurjoc.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2373

G. Zoppellaro, A. Geies, V. Enkelmann, M. Baumgarten
FULL PAPER
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