A multifunctional high-spin biradical pyrazolylbipyridine- bisnitronylnitroxide

Article abstract of DOI:10.1021/ol0479040

(Chemical Equation Presented) Synthesis and UV-vis, IR, and EPR spectroscopic characterizations, together with X-ray structural analysis, of functional nitronyl- and iminonitroxides attached to pyrazolylbipyridine are described. The exchange interactions between the nitronylnitroxides are found to be stronger than for the iminonitroxides. Although the substitution of a pyridine with the pyrazole ring leads to shorter distances and larger dipolar couplings, the exchange interaction is diminished. While compound 1 forms dimers in the solid state, the terpyridine 3 leads to supramolecular π-stacking.

Full text of DOI:10.1021/ol0479040

ORGANIC
LETTERS
2004
Vol. 6, No. 26
4929-4932
A Multifunctional High-Spin Biradical
Pyrazolylbipyridine-bisnitronylnitroxide
Giorgio Zoppellaro, Volker Enkelmann, Ahmed Geies, and Martin Baumgarten*
Max Planck Institute for Polymer Research, Ackermannweg 10,
D-55128, Mainz, Germany
baumgart@mpip-mainz.mpg.de
Received October 10, 2004
ABSTRACT
Synthesis and UV−vis, IR, and EPR spectroscopic characterizations, together with X-ray structural analysis, of functional nitronyl- and
iminonitroxides attached to pyrazolylbipyridine are described. The exchange interactions between the nitronylnitroxides are found to be stronger
than for the iminonitroxides. Although the substitution of a pyridine with the pyrazole ring leads to shorter distances and larger dipolar
couplings, the exchange interaction is diminished. While compound 1 forms dimers in the solid state, the terpyridine 3 leads to supramolecular
π
-stacking.
Various approaches toward magnetic properties or molecular
magnets with organic spin units have been dealt with
experimentally and theoretically.1 They can be classified into
pure organic ones through synthesis of extended polyradicals
or ordering of discrete radical molecules via π-stacking or
hydrogen bonding and into the organic inorganic hybrid
approach combining organic radicals with spin active metal
ions or metal complexes. It has been demonstrated that linear
alternating chains of nitronylnitroxides and M(hfac)2 can be
formed, even leading to ferro- or ferrimagnetic materials.2
Therefore, high spin molecules with two or more nitronylni-
troxides should not only help to extend the magnetic
structures toward higher dimensions but also overcome the
usual antiferromagnetic interaction between chains. We now
report the synthesis of two such models, the organic
biradicals 4′′,5′-bis[3-oxide-1-oxyl-4,4,5,5-tetramethyl-imi-
dazoli-din-2-yl]-6-(pyrazol-1′′-yl)-2,2′-bipyridine 1 and the
4′′,5′-bis[3-oxide-1-oxyl-4,4,5,5-tetramethyl-imidazolin-2-yl]-
6-(pyrazol-1′′-yl)-2,2′-bipyridine 2. Because detailed studies
of doubly disjoint3 biradicals are scarce compared with
nondisjoint systems,4 we compare their structure and ex-
change coupling behavior to the previously reported terpy-
ridines (3 and 4)5 where now also the X-ray structure and
the exchange couplings are elucidated (Scheme 1). We
experimentally found that despite being doubly disjointed,
their estimated through-bond exchange interaction (2J) is
both ferromagnetic and unexpectedly large as compared with
nondisjointed biradicals, like those based on disemiquinone
and dinitroxide systems.6 The assembly of diradicals 1 and
2 requires the synthesis of the nonsymmetric dicarbaldehyde
9. This is achieved via a multistep synthetic approach
involving Stille coupling of fragments 7 and 8 (Scheme 1)
(1) (a) Magnetic Properties of Organic Materials; Lahti, P. M., Ed.;
Marcel Dekker: New York, 1999. (b) Molecular Magnetism; Kahn, O.,
Ed.; VCH: Cambridge, U.K., 1993. (c) Magnetism: Molecules to Materials;
Miller, J. S., Drillon, M., Eds.; Wiley-VCH: Weinheim, 2001-2003; Vols.
I-IV.
(2) (a) Caneschi, A.; Gatteschi, D.; Lalioti, N.; Sangregorio, C.; Sessoli,
R. J. Chem. Soc., Dalton Trans. 2000, 21, 3907. (b) Minguet, M.; Luneau,
D.; Lhotel, E.; Viller, V.; Paulsen, C.; Amabilino, D. B.; Veciana, J. Angew.
Chem., Int. Ed. 2002, 41, 586.
(3) Matsumoto, T.; Ishida, T.; Koga, N.; Iwamura, H. J. Am. Chem. Soc.
1992, 114, 9952.
(4) Rajca, A. Chem. ReV. 1994, 94, 871.
(5) Zoppellaro, G.; Enkelmann, V.; Geies, A.; Baumgarten, M. Polyhe-
dron 2003, 22, 2099.
(6) (a) Shultz, D. A.; Boal, K. A.; Lee, H.; Farmer, G. T. J. Org. Chem.
1999, 64, 4386. (b) Shultz, D. A. Polyhedron 2001, 20, 1627. (c) Shultz,
D. A.; Fico, R. M.; Bodnar, S. H.; Kumar, R. K.; Vostrikova, K. E.; Kampf,
J. W.; Boyle, P. D. J. Am. Chem. Soc. 2003, 125, 11761.
10.1021/ol0479040 CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/23/2004

Scheme 1
followed by acid hydrolysis of the acetal. Condensation of
9 with bis(hydroxylamino)-dimethylbutane 10 and then by
periodate oxidation under phase transfer conditions affords
1 (9%) or 2 (18%) depending on the amount of oxidant used.
The blue nitronylnitroxide biradical 1 (pzpy2NN) is stable
as a solid and in toluene solutions. A typical broad band is
observed in the visible spectrum (toluene, λmax ) 610 nm, ꢀ
) 1114 M-1 cm-1, n f π* transition). Also the orange-red
iminonitroxide biradical 2 (pzpy2IN) shows similar stability,
Figure 1. X-ray structures of 1 and 3 with ORTEP drawn at the
50% of the probability level. The hydrogen atoms are omitted for
clarity.
inclusion of CHCl3 prevents formation of extended π-net-
works. The biradical 3, on the other hand, leads to a
supramolecular stacking motif in solid state, with units 180°
rotated on top to each at d ∼ 3.3 Å.
with the expected blue-shifted absorption (toluene, λmax
)
467 nm, ꢀ ) 1082 M-1 cm-1). Comparison with the earlier
reported aminoxyl-oxide transitions in 3 (toluene, λmax 605
nm, ꢀ ) 480 M-1 cm-1)5 and 4 (toluene, λmax 459 nm, ꢀ )
380 M-1 cm-1)5 show a hampered radical’s optical absorption
when the spin carriers are directly bound to the pyridine ring
with respect to those directly connected to pyrazole moieties.7
The IR of 1 features an intense νN-O stretching vibration at
1352 cm-1, which is weaker and shifted in 2 (νN-O ) 1371
cm-1). These are consistent both in frequency and intensity
with those observed in 35 and 4.5 Suitable crystals were
grown in CHCl3 for 18 and in acetone for 3.9 Their structures
are reported in Figure 1 together with structural parameters
such as dihedral angles (Φ) and intramolecular radical
distances. The two systems share nearly planar arrangements
of the coupling core and rather small twisting angles between
the imidazolyl and the pyridyl/pyrazolyl rings. Therefore,
the geometrical prerequisite to enable excellent conjugation
between the radical fragments through the coupler is ensured.
The biradical 1 forms dimers in the solid state, where
Although such stacking arrangement should give rise to
ferromagnetic chains, at 300 K the µeff value corresponds to
2.44 µB, consistent with the theoretical 2.45 µB for two
uncorrelated spins, hence suggesting the near degeneracy of
singlet and triplet states at high temperature. This value
remains almost constant down to 70 K; then, it increases
weakly, reaching a maximum at ∼18 K (2.55 µB) followed
by a sharp decrease at lower temperatures (Figure 2). The
trend in µeff is analyzed in term of singlet-triplet equilibrium
within the molecule, including an averaged intermolecular
interaction, θ, adopting the mean-field approximation model.10
Such analyses provides a fairly large ferromagnetic
through-bond interaction, with 2J/kb ) 29.4 ( 5.6 K, and
an averaged antiferromagnetic through-space interaction θ
) -2.6 ( 0.2 K (rms ) 0.71). To obtain a better estimation
of the ∆ES-T, low-temperature EPR studies are then carried
out on the dilute toluene solution of 3. In this way, the
(7) Zoppellaro, G.; Geies, A.; Enkelmann, V.; Baumgarten, M. Eur. J.
Org. Chem. 2004, 11, 2367.
(9) CCDC 234179. Formula for 3: C29H37N7O6, Mw ) 579.66, mono-
clinic, space group P21/c (no. 14), a ) 6.5690 Å, b ) 21.5610 Å, c )
20.6620 Å, R ) 90°, â ) 90.8°, γ ) 90°, V ) 2926.16 Å3, Fc ) 1.316 g
× cm-3, µ (Mo KR/mm-1) ) 0.094, Z ) 4, T ) 120 K. Crystal color,
shape, and size: blue, prism, 0.09 × 0.14 × 0.42 (mm). Dataset: 5870
total, 1924 unique reflections (4.1 < υ < 27.4°) of which 1911 were
observed [I0 > 2.0σ(I0)], Nref 1911, Npar 379, S ) 1.07. The structures were
solved by direct methods (SHELXS) and refined by a full-matrix least-
squares procedure to R1 value of 0.0645 (wR2 ) 0.0666, all data). The
crystallographic data were collected on Nonius Kappa CCD (Mo KR, µ )
0.71073 Å) diffractometer equipped with a graphite monochromator. The
full crystallographic details, excluding structure factors, were deposited with
the Cambridge Crystallographic Data Centre.
(8) CCDC 229003. Formula for 1: C28H33N8O4Cl3, Mw ) 651.98,
triclinic, space group P1 (no. 2), a ) 11.5796(4) Å, b ) 12.3160(5) Å, c
) 12.7410(5) Å, R ) 69.9597(13)°, â ) 65.7760(14)°, γ ) 88.9354(12)°,
V ) 1540.85(11) Å3, Fc ) 1.405 g cm-3, µ (Mo KR/mm-1) ) 0.346, Z )
2, T ) 120 K. Crystal color, shape, and size: blue, prism, 0.09 × 0.21 ×
0.38 (mm). Dataset: 31 506 total, 8016 unique reflections (4.0 < υ < 29.0°)
of which 3793 were observed [I0 > 2.0σ(I0)], Nref 3793, Npar 388, S )
1.07. The structure was solved by direct methods (SHELXS) and refined
by a full-matrix least-squares procedure to R1 value of 0.0512 (wR2
)
0.0571, all data). The crystallographic data were collected on Nonius Kappa
CCD (Mo KR, µ ) 0.71073 Å) diffractometer equipped with a graphite
monochromator. The full crystallographic details, excluding structure factors,
were deposited with the Cambridge Crystallographic Data Centre.
(10) Bino, A.; Johnston, D. C.; Goshorn, D. P.; Halbert, T. R.; Stiefel,
E. I. Science 1988, 241, 1479.
4930
Org. Lett., Vol. 6, No. 26, 2004

Figure 2. Plot of the observed magnetic susceptibility ø (O) and
µeff (b, units in Bohr magneton µB) vs temperature (T, in K) for
the crystalline sample of biradical 3. The inset shows a view down
the crystallographic c-axis of a section for the π-stacking of 3. The
best fit curves for µeff and ø are shown as dashed and plain lines
with parameters given in the text.
intermolecular contributions are made negligible (vide infra).
The liquid solution EPR spectra (toluene) exhibits 9 lines
for 1 (giso ) 2.0066(1)) and 13 lines for 2 (giso ) 2.0061-
(1)), confirming the strong exchange coupling (J . aN)
between the radical fragments. A lower limit of |J/aN| g 45
(i.e., |2J/kb| g 0.06 cm-1) is extracted upon simulations of
their entire EPR envelopes, without, however, gaining
knowledge of the sign of J. In frozen solution, the typical
zero-field-splitting (zfs) components of |D/hc| ) 3.8 × 10-3
cm-1 for 1 and |D/hc| ∼ 3.9 × 10-3 cm-1 for 2 were
detected, corresponding to r ∼ 8.8 and 8.7 Å from a point
dipole approach,11 together with the forbidden ∆ms ) 2
transitions at half field (gav ∼ 4.01).
At rt and at 120 K, the ratio of double-integrated intensity
of the ∆ms ) 1 transitions for 1-4 were only 2:1 compared
with related monoradicals. Such information already provides
the range of energies in which the singlet-triplet gaps are
located (0.06 cm-1 e |∆ES-T| e 83 cm-1). The intensities
of the forbidden ∆ms ) 2 signals increased in both 1 and 2
almost linearly upon lowering the temperature down to 4 K
(Curie-like). The product DI × T vs T, on the other hand,
increased upon lowering the temperature (Figure 3A) for 1,
showing its full triplet occupation. Fitting the DI × T data
according to the Bleaney-Bowers model12 for two interact-
ing S ) 1/2 systems gave 2J ) ∆ES-T ) 13.3 ( 4.9 cm-1
for 1. For 2, the singlet-triplet gap, however, is much smaller
and falls in the range of 0.06 cm-1 e ∆ES-T e 0.7 cm-1
(Figure 3B) with the lower limit being consistent with the rt
simulation of its EPR envelope. These findings confirm the
greater proclivity of nitronylnitroxides to adopt high-spin
ground states compared to iminonitroxides. For the terpy-
ridine biradicals 3 and 4, careful control of the low-
Figure 3. Evolution of the double-integrated signal intensities (DI,
2) and the product DI × T (b) vs T of the ∆ms ) 2 signal for 1
(A), 2 (B), 3 (C), and 4 (D) in the low-temperature range. The
solid lines represent the theoretical fitting according to the Bleaney-
Bower model, and the insets show the observed ∆ms ) 2 transition
at the lowest temperatures (T ) 4 K for (A), (B), and (C), T ) 7
K for (D)). The dashed line in (B) shows the fitting assuming the
singlet ground state (S ) 0) and the solid line shows the fitting for
the triplet ground state (S ) 1).
(11) Electron Paramagnetic Resonance; Weil, J. A.; Bolton, J. R.; Wertz,
J. E., Eds.; VCH-Wiley Interscience: New York, 1994; p 174.
(12) Bleaney, B.; Bowers, K. Proc. R. Soc. London 1952, A214, 451.
temperature data of the ∆ms ) 2 transitions followed by the
DI × T vs T fitting yielded a more accurate ∆ES-T ) 15.4
Org. Lett., Vol. 6, No. 26, 2004
4931

( 2.0 cm-1 for 3 (Figure 3C) and the smaller ∆ES-T ) 8.7
( 1.0 cm-1 for 4 (Figure 3D). Thus, although the shorter
distance in pyrazole-containing biradicals 1 and 2 leads to
enhanced dipolar couplings (zfs) compared to 3 and 4, the
exchange interaction is lowered. This is due to the occurrence
of two different pathways for the spin-polarization in 1 and
2, arising from the presence of a five-membered ring
(pyrazole).
In conclusion, we have described new biradicals and
compared their structure and exchange interaction, which is
sizable and ferromagnetic. In contrast with previous reports
in similar pyridine-based nitronylnitroxide biradical sys-
tems,13 where no significant intramolecular interactions are
found, the present work shows an efficient propagation of
the spin polarization through bond even when a second or a
third pyridine ring is introduced as a spacer. Their use in
extended networks through their radical units and/or size
recognition of their tridentate nitrogen binding cage is
presently underway. Such functionalized cores, even without
radical units, may further be considered as ligands for
promising spin-crossover compounds.14
Acknowledgment. Support by the DFG and the Max
Planck Society is gratefully acknowledged. The authors thank
Prof. W. Haase and Dr. K. Falk (TU Darmstadt) for the
magnetic susceptibility measurements.
Supporting Information Available: Detailed synthetic
1
procedures toward biradicals 1 and 2, H, 13C NMR, UV-
vis, and IR data, EPR in solution for 1 and 2 (with
simulations), in the frozen state and power saturation
behaviors for all the radical systems, and X-ray structures
of 1, 3, and a monoradical (which was used for spin
quantization) in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL0479040
(13) (a) Ziessel, R.; Ulrich, G.; Lawson, R. C.; Echegoyen, L. J. Mater.
Chem. 1999, 9, 1435. (b) Romero, F. M.; Ziessel, R. Tetrahedron Lett.
1999, 40, 1895. (c) Ziessel, R. Mol. Cryst. Liq. Cryst. 1995, 273, 101. (d)
Stroh, C.; Ziessel, R. Tetrahedron Lett. 1999, 40, 4543.
(14) Money, V. A.; Evans, I. R.; Halcrow, M. A.; Goeta, A. E.; Howard,
J. A. K. Chem. Commun. 2003, 158.
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