Study on the Heteroatom Influence in Pyridine-Based Nitronyl Nitroxide Biradicals with Phenylethynyl Spacers on the Molecular Ground State

Article abstract of DOI:10.1021/jo034597n

Novel pyridine-based nitronyl nitroxide (NIT) biradicals, 3,5-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)] pyridine (1) and 2,6-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)] pyridine (2), and monoradicals

Full text of DOI:10.1021/jo034597n

Stu d y on th e Heter oa tom In flu en ce in P yr id in e-Ba sed Nitr on yl
Nitr oxid e Bir a d ica ls w ith P h en yleth yn yl Sp a cer s on th e Molecu la r
Gr ou n d Sta te
Chandrasekar Rajadurai, Anela Ivanova, Volker Enkelmann, and Martin Baumgarten*
Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany
baumgart@mpip-mainz.mpg.de
Received May 7, 2003
Novel pyridine-based nitronyl nitroxide (NIT) biradicals, 3,5-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetrameth-
ylimidazolin-2-yl)phenylethynyl)]pyridine (1) and 2,6-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimi-
dazolin-2-yl)phenylethynyl)]pyridine (2), and monoradicals, 4-(5-bromopyridine-3-ylethynyl)-1-(1-
oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (3), 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-
4,4,5,5-tetramethylimidazolin-2-yl)benzene (4), and 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-
tetramethylimidazolin-2-yl)pyridine (5), were synthesized and investigated by ESR and UV-vis
spectroscopy. The solution EPR measurements of the biradicals gave well-resolved, nine-line spectra
with exact half line spacing as compared to monoradicals (giso ) 2.0067) with isotropic line spacing
|aN|) 7.36 G. This indicates strong, intramolecular exchange coupling (J . 7 × 10-4 cm-1; J /aN
.
1) of the biradicals with in the limit of EPR. The temperature dependence on the ∆ms ) (2 signal
intensity of biradicals follow Curie behavior down to 4 K ascertaining the triplet ground state or
its near-degeneracy with the singlet state. UV-vis studies of 1-5 show characteristic differences
in the extinctions of n-π* transitions around 600 nm. Both biradicals 1 and 2 were crystallized in
monoclinic space groups C2/ c and P21/a with the intraradical distances 1.54 and 1.47 nm,
respectively. Computational studies of the biradicals 1, 2, and 1,3-bis[4-(1-oxyl-3-oxo-4,4,5,5-
tetramethylimidazolin-2-yl)phenylethynyl)]benzene (T) were performed by the AM1/CAS(8,8)
method to calculate the singlet-triplet (∆EST) energy difference and the spin density distribution.
Results show that the position of the pyridyl nitrogen in 1 and 2 in comparison with T does not
alter the triplet ground-state spin multiplicities supporting the obtained experimental results.
In tr od u ction
ridine, etc.7 For the design of high spin molecules, control
of geometry and topology are crucial.8 The sign of mo-
lecular exchange interaction (J ) can be either positive or
Organic high spin molecules are a topic of current
interest in order to understand the magnetic interactions
in and between molecules for the design and synthesis
of molecular-based magnets.1 The key in obtaining
magnetic ordering is the formation of very high spin
domains. This has proven to be very difficult for organic
polymers, since often large number of defects2a-c from
incomplete radical site formation or their loss reduce
their effectiveness. Thus, ordering of discrete high spin
molecules through hydrogen bonding3a-d or metal coor-
dination is still a promising approach. Metal complex-
ation can be achieved via stable radical sites themselves
especially with nitroxides,4 nitronyl nitroxide5 (NIT), and
imino nitroxides6 or combined with conjugated fragments
carrying heteroatoms such as pyridine, bipyridine, terpy-
(3) (a) Romero, F. M.; Ziessel, R.; Bonnet, M.; Pontillon, Y.; Res-
souche, E.; Schweizer, J .; Delley, B.; Grand, A.; Paulsen, C. J . Am.
Chem. Soc. 2000, 122, 1298-1309. (b) Ferrer, J . R.; Lahti, P. M.;
George, C.; Oliek, P.; J ulier, M.; Palacio, F. Chem. Mater. 2001, 13,
2447-2454. (c) Maspoch. D.; Catala, L.; Gerbier, P.; Ruiz-Molina, D.;
Vidal-Gancedo, J .; Wurst, K.; Rovia, C.; Veciana, J . Chem. Eur. J . 2002,
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E.; Davies, J . E. J . Chem. Soc., Dalton Trans. 1998, 2477. (b) Stroh,
C.; Ziessel, R. Tetrahedron Lett. 1999, 40, 4543-4596. (c) Stroh, C.;
Turek, P.; Rabu, P.; Ziessel, R. Inorg. Chem. 2001, 40, 5334. (d) Romero,
F. M.; Ziessel, R. Tetrahedron Lett. 1999, 40, 1895-1898. (e) Zoppellaro,
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(1) (a) Lahti, P. M. Magnetic Properties of Organic Materials; Marcel
Dekker: New York, 1999. (b) Diradicals; Borden, W. T., Ed.; J ohn
Wiley and Sons: New York, 1982. (c) Crayston, J . A.; Devine, J . N.;
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10.1021/jo034597n CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/25/2003
J . Org. Chem. 2003, 68, 9907-9915
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Rajadurai et al.
negative depending on the coupling unit between the
radical sites and their steric demands. Most often,
m-phenylene bridging, leading to non-Kekule´ structures,
have been used for high-spin ground-state formation.9 To
obtain a ferromagnetically coupled system, apart from
topology, the geometry of the molecules,10 the nature and
position of the substituents,11 and heteroatom influ-
ence12,13a are to be considered in the design of high-spin
molecules. Understanding the effect of the heteroatom
position like nitrogen in the ground-state spin multiplic-
ity is important since the role of nitrogen is pivotal for
metal complexation in constructing higher dimensional
magnetic architectures.
In this context, Dougherty’s group12a has probed the
influence of heteroatom in non-Kekule´ systems and
reported a stable quintet ground state for a series of
neutral pyridine-based tetraradicals (3,5-, 2,6-, and 2,4-
isomers) generated by the photolysis of the corresponding
neutral bisdiazenes. It was found that the position of the
pyridyl nitrogen in all the three isomers had no influence
on the ground-state spin multiplicities. Also, Lahti and
co-workers12b described high-spin ground states for dini-
trenes attached to pyridines in the 2,6- and 2,4-positions.
Interestingly, in contrast to the above reports, Takui and
co-workers13a examined the molecular ground state of
nitronyl nitroxide biradicals substituted to 2,6- and 3,5-
pyridines and reported the S ) 1 ground state for 2,6-
pyridine nitronyl nitroxide radicals based on magnetic
susceptibility measurements and S ) 0 state for the
corresponding 3,5-pyridine biradicals based on EPR and
magnetic susceptibility measurements. For carbene spin
sources,13b reverted high spin stabilities were found as
compared to nitronyl nitroxide, the low spin ground state
was described for 2,6-pyridine bridging and high spin
quintet ground state for the 3,5-pyridine bridging, rea-
soned by opposite effects of the heteroatom locations.
Wautelet et al. have reported the singlet ground state
for the methyl and methoxy carrying bisimino nitroxide
derivative of T (1,3-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetrameth-
ylimidazolin-2-yl)phenylethynyl)]benzene) (Figure 1). The
interplay between the spin polarization and the spin
delocalization (π-conjugation) mechanisms was reasoned
for the observed singlet ground state.13c,d Accidentally,
the half field signal was not observed by EPR and no
comparison with the stronger exchange coupled bisnit-
ronyl nitroxide was made.13c-e
Phenylethynyl spacers carrying nitronyl nitroxide bi-
radicals as in 1 and 2 attached to the pyridine units
without functional groups were designed in this work,
to clearly deduce the influence of the pyridyl nitrogen
position on the ground state spin multiplicities. Intramo-
lecularly separated radicals, on the other hand (e.g., 1
and 2), need strong π-conjugation for achieving a strong,
intramolecular exchange interaction. In this aspect,
arylethynyl units are versatile building blocks for spa-
tially separated radicals in a molecule because of their
rigid and conjugated nature.13f,g Formation of alternating
spin density waves along the near planar conjugated
spacer is good for high spin formation. Besides using the
EPR spectroscopy for studying the exchange interactions,
the optical spectra in the visible range have proven
valuable for the identification of the number of radicals
in solution and will be described. The synthesis of high
spin biradicals 1 and 2, which are based on the types of
reactions involved in the preparation of the model
compounds 4 and 5, is described (Figure 2). The basic
reaction sequence involves synthesis of dialdehydes,14
followed by condensation reaction15 with 2,3-dimethyl-
2,3-bis(hydroxylamino)butane, and finally oxidation us-
ing NaIO4 under phase-transfer conditions. The detailed
EPR, UV-vis, single-crystal X-ray analysis, and compu-
tational studies will also be described.
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Resu lts a n d Discu ssion
Syn th esis of 1-5. The synthetic sequence toward
high-spin biradicals 1 and 2 is based on the building block
8,14a,b which was prepared via Sonogashira coupling of
p-bromobenzaldehyde with trimethylsilylacetylene (TMSA)
to give 6, followed by deprotection of the silyl group14c
upon stirring with K2CO3 in methanol under argon to
give 8. Compound 8 was used to build the dialdehydes 9
and 11 using 3,5- and 2,6-dibromopyridines in the pres-
ence of catalysts Pd(PPh3)2Cl2, CuI, and Et3N base. The
cross-coupling reactions for 9 and 11 involve absolutely
dry argon conditions obtained by freeze-pump-thaw
cycles. This is necessary to avert homocoupling of 8 to
form a dimer. The dimers become the predominant prod-
uct if the oxygen is not properly excluded from the re-
action vessel. Together with the dialdehydes, mono-
coupled products (10 in case of reaction f) were also
obtained.
(9) (a) Ishida, T.; Iwamura, H. J . Am. Chem. Soc. 1991, 113, 4238.
(b) Kanno, F.; Inoue, K.; Koga, N.; Iwamura, H. J . Phys. Chem. 1997,
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3173. (d) Inoue, K.; Iwamura, H. Adv. Mater. 1996, 8, 73. (e) Inoue,
K.; Iwamura, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 927. (f) Borden,
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Ed. Engl. 1992, 31, 180. (b) Kanno, F:, Inoue, K.; Koga, N:, Iwamura,
H. J . Am. Chem. Soc. 1993, 115, 847.
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Chem. Soc. 1996, 118, 1452. (b) Chapyshev, S. V:, Walton, R.; Sanborn,
J . A.; Lahti, P. M. J . Am. Chem. Soc. 2000, 122, 1580-1588. (c) Rule,
M.; Matlin, A. R.; Seeger, D. E.; Hilinski, E. F.; Dougherty, D. A.;
Berson, J . A. Tetrahedron 1982, 38, 787. (d) Liao, Y.; Xie, C.; Lahti, P.
M.; Weber, R. T.; J iang, J .; Barr, D. P. J . Org. Chem. 1999, 64 (14),
5176-5182.
(13) (a) Shiomi, D.; Ito, K.; Nishizawa, M.; Sato, K.; Takui, T.; Itoh,
K. Synth. Met. 1999, 103, 2271-2272. (b) Bae, J . Y.; Yano, M.; Sato,
K.; Shiomi, D.; Takui, T.; Kinoshita, T.; Abe, K.; Itoh, K.; Hong, D.
Synth. Met. 1999, 103, 2261. (c) Turek, P.; Wautelet, P.; Moigne, J . L.;
Stanger, J . L.; Andre, J . J .; Bieber, A.; Rey, P.; Cian, A. D.; Fischer, J .
Mol. Cryst. Liq. Cryst. 1995, 272, 99-108. (d) Wautelet, P.; Moigne,
J .L; Videva, V.; Turek, P. J . Org. Chem. 2003, 68, 8025-8036. (e)
Wautelet, P.; Catala, L.; Bieber, A.; Turek, P.; Andre, J . J . Polyhedron
2001, 20, 1571-1576. (f) Catala, L.; Turek, P.; LeMoigne, J .; De Cian,
A.; Kyritsakas, N. Tetrahedron Lett. 2000, 41, 1015. (g) Murata, S.;
Iwamura, H. J . Am. Chem. Soc. 1991, 113, 5547. (h) Catala, L.; Moigne,
J . L.; Kyritsakas, N.; Rey, P.; Novoa, J . J .; Turek, P. Chem. Eur. J .
2001, 7, 2466-2480.
The dialdehydes have low Rf values as compared to
dimers and monocoupled products in chloroform and
(14) (a) Sonogashira, K.; Tohada, Y., Hagihara, N. Tetrahedron Lett.
1975, 4467-4470. (b) Thorand, S.; Krause, N. J . Org. Chem. 1998,
63, 8551-8553. (c) Austin, W. B.; Bilow, N.; Kelleghan, W. J .; Lau, K.
S. Y. J . Org. Chem. 1981, 46, 2280-2286.
(15) Osiecki, J . H.; Ullman, E. F. J . Am. Chem. Soc. 1968, 90, 1078.
9908 J . Org. Chem., Vol. 68, No. 26, 2003

Heteroatom Influence in Pyridine-Based Nitronyl Nitroxide Biradicals
F IGURE 1. Mono- and bis(nitronyl nitroxide) radicals 1-5 T.
dichloromethane. Purification of the dialdehydes was
done by column chromatography. The condensation reac-
tions of aldehydes (6, 9-11) with 2,3-dimethyl-2,3-bis-
(hydroxylamino)butane were performed in MeOH with
cosolvents, THF for 6, dichloromethane for 9, and toluene
for both 10 and 11, respectively. Mild reflux under
constant argon bubbling for 9-11 and for 6 stirring for
1 day led to the precipitation of the condensation prod-
ucts. The oxidation reactions of the condensed products
were performed in a phase-transfer solution (water/
CHCl3) with NaIO4 by stirring for 10 min to prevent the
overoxidation of NIT to the imino nitroxides. The bi-
radicals 1 and 2 are blue in color and are stable up to 1
year even in toluene solution. The biradicals were crys-
tallized in toluene, and the obtained blue single crystals
were used for X-ray crystal and molecular structure
analysis.
Monoradical 5 was prepared in four steps. The first
step involves synthesis of 2-formyl-5-bromopyridine,16
which was carried out in two steps: (1) selective mono-
lithiation of 2,5-dibromopyridine in the 2-position to
generate 5-bromo-2-lithiopyridine using n-BuLi (1.6 M
in hexane) at -78 °C in toluene (50 mL for 1 g of 2,5-
dibromopyridine); followed by (2) addition of dry DMF
and quenching with saturated solution of NH4Cl. The
yield was up to 23% after column chromatography. The
coupling reaction of the aldehyde 15 with TMSA in order
to form 16 was performed via Sonogashira coupling
(Figure 3). The TMSA-coupled product was obtained as
sticky dark brown mass, which was difficult to purify by
regular workup and column chromatography. Alterna-
tively, direct sublimation of the dark brown mass was
carried out in order to get analytically pure, light brown
crystalline powder up to 33% yield. During condensation
reaction of 16 with 2,3-dimethyl-2,3-bis(hydroxylamino)-
butane, no precipitate was obtained after 2 days stirring
in methanol. Following solvent evaporation, the obtained
crude product was used for oxidation reaction with NaIO4
in CHCl3/water to afford 5.
Op tica l p r op er ties. The UV-vis spectra of the NIT
radicals are very useful tools to estimate approximately
the number of radical units in a single molecule based
on the extinction coefficient of the n-π* transition in the
visible range of the spectrum at ∼600 nm, provided that
the functionality and the backbones are quite similar
(Figure 4). In this scope we have attempted to compare
the extinctions (n-π* transition) of 4 with those of 1 and
2. In contrast to our expectation, the biradicals 1 and 2
have extinctions of 470 and 530 M-1 cm-1, respectively,
which are not exactly twice as compared to the extinction
of 4 which is 339 M-1 cm-1 at 611 nm. The vibronic
coupling pattern on the other hand is very similar for 1,
2, and 4. The lowered extinctions of 1 and 2 compared to
4 may be due to the presence of pyridine unit. Indeed
monoradicals 3 and 5 have extinctions of 252 M-1 cm-1
at 620 nm and 266 M-1 cm-1 at 581 nm (blue shifted
without vibronic coupling due to loss in symmetry),
respectively, which are much lower than for the benzene
based monoradical 4. The extinctions of 3 and 5 are now
reasonable as compared to the doubled extinction’s of 1
and 2, proving that the pyridine unit has predominant
role in lowering some of the extinction coefficient of the
biradicals even in the presence of phenylethynyl spacers.
(16) Wang, X.; Rabbat, P.; O’Shea, P.; Tillyer, R.; Grabowski, E. J .
J .; Reider, P. J . Tetrahedron Lett. 2000, 41, 4335-4338.
J . Org. Chem, Vol. 68, No. 26, 2003 9909

Rajadurai et al.
F IGURE 2. Synthesis of mono- (3 and 4) and biradicals (1 and 2). Reagents and conditions: (a) trimethylsilylacetylene (TMSA)/
Pd(PPh3)2Cl2/CuI/Et3N/THF, (b) 2,3-dimethyl-2,3-bis(hydroxylamino)butane/MeOH/THF, (c) NaIO4/CHCl3/H2O, (d) K2CO3/MeOH,
(e) Pd(PPh3)2Cl2/CuI/Et3N/THF, (f) Pd(PPh3)2Cl2/CuI/Et3N/THF, (g) 2,3-dimethyl-2,3-bis(hydroxylamino)butane/MeOH/toluene,
(h) 2,3-dimethyl-2,3-bis(hydroxylamino)butane/MeOH/Dichloromethane, (i) 2,3-dimethyl-2,3-bis(hydroxylamino)butane/MeOH/
toluene; (j-l) NaIO4/CHCl3/H2O.
F IGUR E 3. Synthesis of monoradical (5). Reagents and conditions: (m) (1) n-BuLi/-78 °C/toluene, (2) DMF; (n) TMSA/
Pd(PPh3)2Cl2/CuI/Et3N/THF; (o) 2,3-dimethyl-2,3-bis(hydroxylamino)butane/MeOH; (p) NaIO4/CHCl3/H2O.
EP R Stu d ies. The EPR (X-band) investigations of the
degassed toluene solutions of the mono and biradicals
1-5 were performed. The monoradicals show five lines
due to the hyperfine coupling (hfc) of electron spin with
two equivalent NIT nitrogens (14N) with giso value cen-
tered at 2.0067. The spectra of the monoradicals were
best simulated with hfc values (|aN|) 7.40, 7.40, and 7.36
G for 3-5, respectively. The biradicals 1 and 2 display
all dilute concentrations (c ) 10-3-10-5 M) further
supports that the interactions between the radicals are
purely intramolecular.
To ensure pure anisotropic intramolecular magnetic
dipole-dipole interaction, 10-4 M solutions were used to
observe the fine structures (at 120 K using 0.2 mW
microwave power) (Figure 6). The zero-field splitting
parameter or fine structure (D) gives an indication about
the intraradical distance (r) as described by Mukai17 and
co-workers
nine line patterns in liquid solution phase with giso
)
2.0067 yielding intensity ratios close to 1:4:10:16:19:16:
10:4:1. Temperature-dependent spectral behavior of the
∆ms ) (1 transition from 300 to 200 K clearly ruled out
the possibility of J -modulation effect in both 1 and 2. For
these cases of strong exchange coupling within the EPR
limits, the simulation perfectly fits by assuming four
nitrogens with half the hyperfine coupling with J . 7 ×
10-4 cm-1 (J /aN . 1). These nine line patterns seen at
D ) 3/ g2â2 (r - 3mij2)FiFj /rij
2
5
∑
4
ij
where rij is the distance between atoms of i and j, mij is
(17) (a) Mukai, K.; Tamaki, T. Bull. Chem. Soc. J pn. 1977, 50, 1239-
1244. (b) Mukai, K.; Sakamoto, J . J . Chem. Phys. 1978, 68, 1432-1438.
(c) Mukai, K.; Inagaki, N. Bull. Chem. Soc. J pn. 1980, 53, 2695-2700.
9910 J . Org. Chem., Vol. 68, No. 26, 2003

Heteroatom Influence in Pyridine-Based Nitronyl Nitroxide Biradicals
F IGURE 4. UV-vis spectra of 1-5 (in toluene), inset shows the differences of the n-π* transitions as molar extinction coefficients.
F IGURE 5. Experimental ESR spectra (∆ms ) (1) of 1 and 2 in toluene (c ) 10-3 M) at 240 K (s) and their simulated spectra
(‚‚‚).
pattern (Figure 6). This additionally proves that, there
is no monoradical contribution in the biradicals. Based
on this considerations, the derived distances for both
biradicals are r ∼ 1.02 nm.
To ascertain the intramolecular biradical nature of 1
and 2, 10-3 M concentrations were used to measure
(microwave power ) 3.9 mW, number of scans ) 10,
receiver gain ) 8 × 105, modulation frequency ) 100
kHz, modulation amplitude ) 0.4 mT) the forbidden
half field transitions (∆ms ) (2) down to liquid he-
lium temperature. The measurements were repeated at
different microwave powers in order to ensure that
the signals were not saturated, by plotting the ob-
served signal intensities with respect to the square root
F IGURE 6. ESR spectra (∆ms ) (1) of 1 and 2 in toluene
glass (c )10-4 M) at 120 K (0.2 mW microwave power) in
comparison with monoradical 5 (arrows show the enlarged
outermost zfs components).
of the microwave power used. The plot of the doubly
integrated and of the peak to peak signal intensities
of the ∆ms ) (2 signal versus inverse temperature
(Figure 7) followed a Curie pattern with strong increase
in signal intensity down to liquid helium temperature
indicating triplet ground state or its very near degeneracy
with the singlet state. Certainly the exchange coupling
J cannot be very large, but a ferromagnetic exchange of
J ∼ 10-15 K is in line with the results, while an
antiferro-magnetic exchange could only be as small as
|J | < 2 K.
the distance vector along the i and j axis, and Fi and Fj
are the spin densities on atoms i and j. In NIT the spin
densities are mainly delocalized on O-N-C-N-O bonds,
besides that, for extended π-conjugated systems like 1
and 2 the electron spins are delocalized along the
conjugated pathways.
The approximate D′ values taken from the nearly
symmetric fine structures of the biradicals 1 and 2 are
Molecu la r St r u ct u r es of 1 a n d 2 in t h e Solid
Sta te. Single crystals suitable for X-ray studies were
selected and the structures were determined and con-
-4
D′ ∼ 0.24 × 10 cm-1. The monoradical 5 exhibits very
different anisotropy with strong asymmetric spectral
J . Org. Chem, Vol. 68, No. 26, 2003 9911

Rajadurai et al.
F IGURE 7. Temperature dependence of the doubly integrated signal intensities and the peak-to-peak signal intensities of the
∆ms ) ( 2 (Curie plot) transitions of 1 and 2. The insets show the respective ∆ms ) ( 2 signal at 4.5 K.
F IGURE 8. ORTEP diagrams of 1 and 2 with thermal ellipsoid plot (50% probability) with numbering scheme.
TABLE 1. Sin glet-Tr ip let sp littin g (∆EST) Ca lcu la ted
firmed with monoclinic crystal system in C2/c and P21/a
space groups for 1 and 2 (with toluene molecule), re-
spectively.
w ith ROHF /AM1/CAS (8,8)a
molecule geometry
1
2
T
θ1 ) θ2 ) θ3 ) 0°
3.6160
3.2638
4.1582
1.4007
2.8524
2.4690
3.3262
0.9369
3.7943
3.1781
4.5220
1.2700
As shown in the ORTEP diagram (Figure 8), radical 1
is symmetric to the central pyridine while radical 2
is slightly distorted from symmetry. The intramolecu-
lar distance between the two radical carbon centers
(O-N-C-N-O) were measured for 1 and 2, which are
1.54 and 1.47 nm, respectively. Per unit cell four mol-
ecules of 1 and three molecules of 2 (with four toluene)
were found and the molecules of 1 and 2 are packed in
the c-axis. The torsional angles between the benzene ring
and radical plane are 28.41° (C11-C10-C13-N4), and
due to the location of toluene molecule close to the radical
the plane is distorted by 33.15° (C24-C25-C28-N5) for
2, and in 1 both arms have 21.03° (C10-C9-C12-N3).
The O-N-C-N-O moiety is planar, with nearly equiva-
lent O-N bond lengths of 1.299(1) Å (O1-N3); 1.29(1) Å
(O2-N4); 1.285(1) Å (O3-N5); 1.294(1) Å (O4-N6) for 2
and 1.279(3) Å (N2-O1); 1.276(4) Å (N3-O2) for 1. The
bond angles are 126.1(3)° (O2-N3-C12) and 126.2(3)°
(O1-N2-C12) for 1 and 127.2(8)° (O1-N3-C13); 124.4-
(8)° (O2-N4-C13); 124.2(9)° (O3-N5-C28); 125.8(1)°
(O4-N6-C28) for 2. The CtC lengths are 1.195(5) Å
(C5-C4) for 1 and 1.193(1) Å (C6-C5) and 1.201(1) Å
(C20-C21) for 2.
θ1 ) 0°, θ2 ) 21°, θ3 ) 3°
θ1 ) 20°, θ2 ) θ3 ) 0°
θ1 ) 20°, θ2 ) 21°, θ3 ) 3°
a
The values are in kJ /mol.
TABLE 2. Hea ts of for m a tion ca lcu la ted w ith ROHF /
AM1/CAS (8,8)a
molecule geometry
1
2
T
θ1 ) θ2 ) θ3 ) 0°
1227.611
1252.284
1232.322
1224.602
1262.359
1292.634
1267.827
1259.351
1187.645
1210.456
1191.888
1184.900
θ1)0°, θ2 ) 21°, θ3 ) 3°
θ1 ) 20°, θ2 ) θ3 ) 0°
θ1 ) 20°, θ2 ) 21°, θ3 ) 3°
a
The values are in kJ /mol.
The geometry of the biradicals were optimized with
inclusion of 6 frontier MOs occupied by 6 electrons in the
active space of the CI. Two minima of T with close energy
were detected (see Table 2): one corresponding to a
planar structure (θ1 ) θ2 ) θ3 ) 0°, see Figure 9)
separated by ∼2.7 kJ /mol from a molecule with θ1 ) 20°,
θ2 ) 21°, and θ3 ) 3°. Substitution of one carbon atom in
the central benzene ring with nitrogen led to a slightly
larger difference between the energies of the planar form
(∼3 kJ /mol) as compared to the respective twisted
conformation. To determine more accurately ∆EST, single-
point energy calculations of the optimized structures with
extended CI were made. The correlation included con-
figurations resulting from the mixing of 8 electrons in 8
MOs (or CAS(8,8)). The effect of the out-of-plane rotation
Th eor etica l Ca lcu la tion s. Semiempirical (AM1) cal-
culations with extended configuration interaction (CI)
were performed for the biradicals 1, 2, and T (the ana-
logue of 1 and 2 with three benzene rings in the spacer).
The singlet-triplet splitting (∆EST) and the spin densities
were determined. The influence of structure alternation
on the magnitude of ∆EST was estimated as well.
9912 J . Org. Chem., Vol. 68, No. 26, 2003

Heteroatom Influence in Pyridine-Based Nitronyl Nitroxide Biradicals
Con clu sion s
In this paper, we demonstrate that two NIT attached
to phenylethynyl spacer, which are linked (meta-type) at
both ends of 3,5- and 2,6-pyridines by ∼1.5 nm distance
(X-ray) are intramolecularly exchange coupled (1 and 2)
with J . hfc. Biradicals 1 and 2 have approximately the
same zero-field splitting values independent of the posi-
tion of the pyridyl nitrogen. In both molecules 1 and 2,
the position of the pyridyl nitrogen has no influence on
the molecular ground state as evidenced by cryogenic
EPR measurements, where both exhibit Curie like be-
havior. These experimental findings point toward triplet
ground state or its near-degeneracy with a singlet state
while the theoretical calculations support the triplet
entities. In addition to EPR, the extinction coefficient of
the n-π* transition of the UV-vis spectra in the visible
region can also be used to estimate roughly the number
radical units in a molecule. This approach can be used
to distinguish a monoradical from a biradical or even a
triradical. The preparation of the extended metal com-
plexes of 1 and 2 are underway.
F IGURE 9. Spin density distribution of biradicals 1 (green)
and T (black) calculated with ROHF/AM1/CIS (20,20).
on the magnitude of the singlet-triplet splitting was
estimated by consideration of two intermediate struc-
tures, one with θ1 ) 20° and θ2 ) θ3 ) 0° and the other
with θ1 ) 0°, θ2 ) 21° and θ3 ) 3°. For comparison the
X- ray results for the dihedral angles of 1 were found
with (θ1 ) 21°, θ2 ) 2.4°, and θ3 ) 5.7°). The results for
∆EST are summarized in Table 1, while the heats of
formation of all molecules are given in Table 2.
All values in Table 1 are positive, which means that
all biradicals have a triplet ground state, in accordance
with the topological predictions. It can be seen, that the
replacement of the central benzene ring from the pyridine
has a moderate effect on the exchange interaction. The
3,5-substituted pyridine is comparable to benzene as a
coupling unit. The 2,6-substituted biradical features
slightly smaller singlet-triplet gap but the triplet is still
well below the singlet. The variation of the singlet-triplet
splitting can be explained in terms of MOs involved in
the interaction. A scheme of the frontier MOs of 1
included in the active space is shown in Figure 10. The
same MO pattern was preserved in the other two radicals
as well.
Substitution of carbon with nitrogen at position 1 or 4
in the central ring of the coupler constitutes the only
structural difference between the three biradicals, which
should be reflected in the exchange interaction. From all
depicted MOs, only two have non-zero coefficients on one
of these atomic sites, namely HOMO - 1 and LUMO +
1. Moreover, the value in position 4 is much larger than
that in 1. Therefore, the 3,5-substitution of pyridine in 1
practically does not alter the singlet-triplet separation.
On the other hand, the 2,6-pattern in 2 results in MOs
with nitrogen contribution. This could be responsible for
the slight decrease of ∆EST. The largest ∆EST is obtained
when the bridging π-system is planar. This is an indica-
tion that the high-spin ground state is stabilized by the
spin-polarization through the bridge.
Exp er im en ta l Section
3,5-Bis(4-for m ylph en yleth yn yl)pyr idin e (9). Twenty mil-
liliters of triethylamine and 20 mL of THF were frozen in a
Schlenk flask using liquid nitrogen, and 0.94 g (4 mmol) of
3,5-dibromopyridine, 0.141 g (0.2 mmol) of Pd(PPh3)2Cl2, 1.3
g (10 mmol) of 8, and 0.0762 g (0.4 mmol) of CuI were added
into the flask under argon over flow and deaerated by freeze-
pump-thaw cycles five times. The brown mixture was refluxed
at 60 °C for 4 days. After evaporation of the solvents the crude
product was extracted with dichloromethane and evaporated
to brownish yellow powder. In TLC (CHCl3) the first spot from
the top was dimer (Rf ∼ 0.75) 8 [m/z 258.0 (100%)], the second
spot was monocoupled product (Rf ∼ 0.5) 10, yield 0.550 g
(54%), and the third one was the desired dialdehyde (Rf ∼ 0.2)
9. The fractions were separated by column chromatography
(CHCl3) to get yellowish mono coupled aldehyde and the
desired yellow dialdehyde. Yield: 0.335 g (27%). 1H NMR (250
MHz, CDCl3, RT) δ: 10 (s, 2H), 9.7 (s, 2H,), 7.95 (s,1H,), 7.89
(d, 4H, 3J ) 8.21 Hz), 7.69 (d, 4H, 3J ) 8.22 Hz). 13C NMR
(62.5 MHz, CDCl3, RT) δ: 191.3, 151.4, 140.9, 136.0, 132.3,
129.7, 128.3, 119.6, 92.4, 88.7. FD MS (70 eV): m/z 335.0 (100).
UV (toluene, c ) 10-4 M): 305 nm (ꢀ ) 32 613 M-1 cm-1). FTIR
(KBr disk; ν in cm-1): 3030 (s, Py-C-H), 2754 and 2856 (w,
C-H, carbonyl), 1685 (vs, CdO), 825 (vs, aromatic CH out of
plane deformation). Mp: 185-187 °C (decomposed before
melting).
4-(5-Br om op yr id in -3-yleth yn yl)ben za ld eh yd e (10). 1H
NMR (250 MHz, CDCl3, RT) δ: 10.02 (s, 1H), 8.68 (s, 1H), 8.64
3
(d,1H, 4J ) 1.9 Hz), 7.99 (s, 1H), 7.89 (d, 2H, J ) 8.21 Hz),
7.38 (d, 2H, 3J ) 7.9 Hz). 13C NMR (62.5 MHz, DMSO-d6, RT)
δ: 191.98, 149.9, 140.4, 135.5, 131.8, 129.2, 126.5, 120.0, 92.1,
87.5. FD MS (70 eV): m/z 285.2 (∼89) and 287.2 (100). FTIR
(KBr disk; ν in cm-1): 3012 (w, Py CH stretching), 1680 (vs,
CdO), 829 (s, aromatic CH out of plane deformation). Mp:
150-152 °C.
Good illustrations of this phenomenon are the calcu-
lated spin densities. The values for the structures with
largest ∆EST of 1 (green) and T (black) are shown in the
Figure 9.
It can be seen that (1) there is sign alternation
throughout the molecules, characteristic of the ferromag-
netic coupling of the unpaired electrons, and (2) introduc-
tion of nitrogen affects only the energy of the system but
does not change the spin density distribution.
Thus, nitrogen can be considered as suitable replace-
ment of carbon in the coupling unit, sustaining the triplet
ground state and furthermore offering ligation advan-
tages.
4-(5-Br om opyr idin -3-yleth yn yl)-1-(1,3-dih ydr oxy-4,4,5,5-
tetr a m eth ylim id a zolin -2-yl)ben zen e (14). A 0.200 g (0.7
mmol) portion of 10 was taken in 10 mL each of toluene and
methanol with 0.120 g (0.8 mmol) of 2,3-dimethyl-2,3-bis-
(hydroyamino)butane18 and refluxed at 90 °C for 3 days under
argon bubbling. The formed turbid yellow solution was filtered,
and the condensation product 14 was obtained as yellow
(18) (a) Ovcharenko, V. I.; Fokin, S. V:, Romanenko, G. V.; Korobkov,
I. V.; Rey, P. Russ. Chem. Bull. 1999, 48, 1519-1525. (b) Ovcharenko,
V. I.; Fokin, S. V.; Rey, P. Mol. Cryst. Liq. Cryst. 1999, 334, 109-119.
J . Org. Chem, Vol. 68, No. 26, 2003 9913

Rajadurai et al.
F IGURE 10. ROHF/AM1-calculated frontier MOs of biradical 1; pictured with contour value of 0.035 and 64 × 64 grid points.
1
powder. Yield: 0.134 g (46%). H NMR (250 MHz, DMSO-d6,
RT) δ: 8.73 (s, 1H), 8.72 (s, 1H), 8.3 (s, 1H), 7.55 (s), 7.23 (d,
2H), 7.16 (d, 2H), 4.54 (s, 1H), 1.08, 1.04. 13CNMR (62.5 MHz,
DMSO-d6, RT) δ: 150.1 and 149.7, 143.8, 140.6, 131.0, 128.9,
128.2, 120.1, 120.0, 94.0, 89.9, 84.4, 66.3, 24.4 and 17.2.
4-(5-Br om op yr id in -3-yleth yn yl)-1-(1-oxyl-3-oxo-4,4,5,5-
tetr a m eth ylim id a zolin -2-yl)ben zen e (3). A 0.130 g (0.313
mmol) portion of 14 was taken with 0.069 g (0.32 mmol) of
NaIO4 in 25 mL each of chloroform and water. The solution
was stirred for 20 min in an ice-cold bath. The formed blue
organic layer was separated and dried over MgSO4 and
evaporated to yellow powder. The radical 3 was obtained as
blue crystalline powder after column chromatography on silica
gel (1:2 acetone/petroleum ether boiling range 30-40°C; Rf ∼
0.74). Yield: 26 mg (20%). EPR (in toluene 10-3 M; RT; ν ) 9.
10 min. The blue organic layer was separated, and the aqueous
layer was extracted with chloroform repeatedly, and the
combined organic layers were concentrated into deep blue
solution at 45°C under 75 mbar and purified by preparative
chromatography (1:1 acetone/hexane) to afford 1 as blue
powder. Yield: 35 mg (35%). EPR (in toluene 10-3 M, RT, ν )
9.405596 GHz, 2.01 mW power, 2 Scans): nine lines, giso
)
2.0067, aN/2 ) 3.65 G. UV/vis (toluene c ) 10-5 M): 617 nm (ꢀ
) 470 M-1 cm-1), 387 nm (ꢀ ) 15 972 M-1 cm-1). FTIR (KBr
disk; ν in cm-1): 1390 (s, N-O). Anal. Calcd for C35H35N5O4 :
C, 71.29; H, 5.98; N, 11.88. Found: C, 70.28; H, 6.20; N, 11.49.
2,6-Bis(4-for m ylp h en yleth yn yl)p yr id in e (11). A 20 mL
portion of triethylamine and 20 mL of THF were frozen in a
dry Schlenk flask using liquid nitrogen, and 0.94 g (4.0 mmol)
of 2,6-dibromopyridine, 0.067 g (0.095 mmol) of Pd(PPh3)2Cl2,
1.3 g (10 mmol) of 8, and 0.0381 g (0.2 mmol) of CuI were added
into the flask under argon over flow and deaerated using
Schlenk line by freeze-pump-thaw cycles 5 times. The brown
mixture was refluxed with stirring at 60 °C for 3 days under
argon. The solvents from the crude reaction mixture were
evaporated and extracted with dichloromethane and evapo-
rated to a brownish yellow powder. TLC (CH2Cl2): the first
spot from the top was monocoupled [m/z 285.2 and 287.1 (100)]
and the second was the desired dialdehyde 11. The fractions
were separated by column chromatography (CH2Cl2) to get
397562 GHz; 2.01 mW power; 2 scans): Five lines, giso
)
2.0067, aN ) 7.40 G. UV/vis (toluene c ) 10-5 M): 389 nm (ꢀ
) 9474 M-1 cm-1), 620 nm (ꢀ ) 252 M-1cm-1). FTIR (KBr disk;
ν in cm-1): 1354 (s, N-O). Mp: 167-169 °C.
3,5-Bis[4-(1,3-dih yd r oxy-4,4,5,5-tetr a m eth ylim id a zolin -
2-yl)p h en yleth yn yl)]p yr id in e (13). A 0.154 g (0.456 mmol)
portion of 9 was dissolved in methanol and dichloromethane
15 mL each. After addition of 0.225 g (1.5 mmol) of 2,3-
dimethyl-2,3-bis(hydroxylamino)butane, the clear yellow solu-
tion was refluxed under argon bubbling at 60 °C for 12 h. The
light yellow precipitate of 13 was filtered and washed with
chloroform and methanol, respectively. Yield: 0.100 g (37%).
Compound 13 was used for the next step without purification
and characterization.
3,5-Bis[4-(1-oxyl-3-oxo-4,4,5,5-tetr a m eth ylim id a zolin -
2-yl)p h en yleth yn yl)]p yr id in e (1). A 0.1 g (0.168 mmol)
portion of 13 was taken with the phase-transfer solution of
10 mL each of chloroform and water. A 0.1 g (0.465 mmol)
portion of NaIO4 was added into the solution and stirred for
1
yellowish dialdehyde. Yield: 0.8 g (60%). H NMR (250 MHz,
3
CDCl3, RT) δ: 10.02 (s, 2 H), 7.88 (d, 4 H, J ) 8.53 Hz), 7.74
(m, 5 H, the pyridine C4-H and benzene protons also appears
3
13
at the same ppm), 7.54 (d, 2 H, J ) 7.58 Hz). C NMR (62.5
MHz, CDCl3, RT) δ: 191.3, 143.4, 136.8, 136.1, 132.6, 129.6,
128.1, 127, 91.4, 88.5. FD MS (70 eV): m/z 335.3 (100). UV
(toluene, c ) 10-4 M): 338 nm (ꢀ ) 28 841 M-1 cm-1). FTIR
(KBr disk; ν in cm-1): 3051 (w, Py-C-H), 2850 and 2736 (w,
carbonyl C-H), 2210 (m, CdC), 1697 (vs, CdO), 1601 (vs,
9914 J . Org. Chem., Vol. 68, No. 26, 2003

Heteroatom Influence in Pyridine-Based Nitronyl Nitroxide Biradicals
aromatic CdC), 1558 (vs, aromatic CdN). Mp: 192-194 °C
(decomposed before melting).
power, 2 scans): nine lines, giso ) 2.0067, aN/2 ) 3.65 G. UV/
Vis (in toluene c ) 10-5 M): 629 nm (ꢀ ) 499 M-1 cm-1), 387
nm (ꢀ ) 19 146 M-1 cm-1). FTIR (KBr disk; ν in cm-1): 1365
(s, N-O).
2,6-Bis[4-(1,3-dih yd r oxy-4,4,5,5-tetr a m eth ylim id a zolin -
2-yl)p h en yleth yn yl)]p yr id in e 12). As described for the
synthesis of 13, 0.2 g (0.597 mmol) of 11 were dissolved in 10
mL of toluene and 5 mL of methanol, and after addition of
0.225 g (1.5 mmol) of 2,3-dimethyl-2,3-bis(hydroxylamino)-
butane the turbid yellow solution was heated under argon
bubbling to 80 °C to get a clear solution and then refluxed for
2 days. The formed yellow precipitate of 12 was filtered and
washed with chloroform and methanol, respectively. Yield:
Ack n ow led gm en t. C.R. and M.B. gratefully ac-
knowledge the Graduate College, TU- Darmstadt for the
Ph.D. fellowship. We thank Giorgio Zoppellaro (MPI
Polymer Research, Mainz) and Dr. Sergei Fokin and
Prof. Dr. Victor Ovcharenko, International Tomography
Center, Russian Academy of Sciences, Novosibirsk,
Russia, for many helpful discussions. A.I. kindly ac-
knowledges the support given by the Marie Curie
program. M.B. also thanks the DFG for its continuous
support.
1
0.215 g (61%). H NMR (500 MHz, DMSO-d6, RT) δ: 7.89 (t, 1
3
3
3
H, J ) 8.01 Hz and J ) 7.78 Hz), 7.81 (s), 7.64 (d, 2H, J )
3
3
7.86 Hz), 7.60 (d, 4 H, J ) 7.93 Hz), 7.58 (d, 4 H, J ) 7.99
Hz), 4.55 (s, 2 H), 1.08 (s, 12 H) and 1.04 (s, 12 H).
2,6-Bis[4-(1-oxy-3-oxo-4,4,5,5-tetr a m eth ylim id a zolin -2-
yl)p h en yleth yn yl)]p yr id in e (2). A 0.15 g (0.252 mmol)
portion of 12 was taken in a phase-transfer solution of 15 mL
each of chloroform and water. A 0.15 g (0.698 mmol) por-
tion of NaIO4 was added into the solution and stirred for
10 min. The blue organic layer was extracted with chloroform
and concentrated into deep blue solution at 45 °C under 75
mbar and purified by preparative chromatography (1:1 ace-
tone/hexane) to blue powder (2). Yield: 50 mg (34%). EPR
(in toluene c ) 10-3 M, RT, ν ) 9.405379 GHz, 2.01 mW
Su p p or tin g In for m a tion Ava ila ble: General details and
synthetic procedure toward compounds 4 and 5. ESR spectra
(∆ms ) (1) of 1 and 2 at various temperatures. X-ray
crystallographic data and experimental parameters of 1 and
2. This material is available free of charge via the Internet at
http://pubs.acs.org.
J O034597N
J . Org. Chem, Vol. 68, No. 26, 2003 9915