2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1- H-imidazole-3-oxide-1-oxyl, a hydrogen-bonded organic quasi-1D ferromagnet

Article abstract of DOI:10.1021/ja074211g

The title radical (F4BImNN) is a stable nitronylnitroxide that forms hydrogen-bonded NH?ON chains in the solid state. The chains assemble the F4BImNN molecules to form stacked contacts between the radical groups, in a geometry that is expected to exhibit

Full text of DOI:10.1021/ja074211g

Published on Web 12/11/2007
2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1-
H-imidazole-3-oxide-1-oxyl, A Hydrogen-Bonded Organic
Quasi-1D Ferromagnet
Hidenori Murata,†,| Yuji Miyazaki,‡ Akira Inaba,‡ Armando Paduan-Filho,§
Valdir Bindilatti,§ Nei Fernandes Oliveira Jr.,§ Zeynep Delen,† and Paul M. Lahti*,†
Department of Chemistry, UniVersity of Massachusetts, Amherst, Massachusetts 01003, Research
Center for Molecular Thermodynamics, Graduate School of Science, Osaka UniVersity,
Toyonaka, Osaka 560-0043, Japan, and Instituto de F´ısica, UniVersidade de Sa˜o Paulo, Caixa
Postal 66.318, 05315-970, Sa˜o Paulo, SP, Brazil
Received June 9, 2007; E-mail: lahti@chem.umass.edu
Abstract: The title radical (F4BImNN) is a stable nitronylnitroxide that forms hydrogen-bonded NH‚‚‚ON
chains in the solid state. The chains assemble the F4BImNN molecules to form stacked contacts between
the radical groups, in a geometry that is expected to exhibit ferromagnetic (FM) exchange based on spin
polarization (SP) models. The experimental magnetic susceptibility of F4BImNN confirms the expectation,
showing 1-D Heisenberg chain FM exchange behavior over 1.8-300 K with an intrachain exchange constant
of Jchain/k ) +22 K. At lower temperatures, ac magnetic susceptibility and variable field heat capacity
measurements show that F4BImNN acts as a quasi-1-D ferromagnet. The dominant ferromagnetic exchange
interaction is attributable to overlap between spin orbitals of molecules within the hydrogen-bonded chains,
consistent with the SP model expectations. The chains appear to be antiferromagnetically exchange coupled,
giving cusps in the ac susceptibility and zero field heat capacity at lower temperatures. The results indicate
that the sample orders magnetically at about 0.7 K. The magnetic heat capacity ordering cusp shifts to
lower temperatures as external magnetic field increases, consistent with forming a bulk antiferromagnetic
phase below a Ne´el temperature of TN(0) ) 0.72 K, with a critical field of Hc ≈ 1800 Oe. The interchain
exchange is estimated to be zJ/k = (-)0.1 K.
Introduction
for attempting to control the crystallography of stable organic
open-shell molecules and thereby their magnetic behaviors.3
Purely organic molecular magnetic materials have been targets
of continually increasing interest since they were proposed in
the 1960s and realized in 1990s by discoveries of nitronylni-
troxides and nitroxides that exhibit metal-free ferromagnetism
at low temperatures, such as pNPNN.1 There has been much
effort to discern crystallographic structure-property relation-
ships between intermolecular packing and magnetic behavior.2
Hydrogen bonding has been a particularly promising strategy
We4 and others5-7 have had a strong interest in using
benzimidazoles to organize organic radicals in magnetic systems.
In particular, benzimidazoles readily form crystallographic
(2) Some general references include: (a) Gatteschi, D., Kahn, O., Miller, J.
S., Palacio, F., Eds. Magnetic Molecular Materials; Kluwer: Dordrecht,
The Netherlands (1991). (b) Kahn, O. Molecular Magnetism; VCH: New
York, 1993. (c) Iwamura, H.; Inoue, K.; Hayamizu, T. Pure Appl. Chem.
1996, 68, 243. (d) Veciana, J.; Cirujeda, J.; Rovira, C.; Molins, E.; Novoa,
J. J. J. Phys. I 1996, 6, 1967. (e) Turnbull, M. M., Sugimoto, T., Thompson,
L. K., Eds. Molecule-Based Magnetic Materials. Theory, Techniques, and
Applications; American Chemical Society, Washington, DC, 1996. (f)
Nakatsuji, S.; Anzai, H. J. Mater. Chem. 1997, 7, 2161. (g) Pilawa, B.
Ann. Phys. 1999, 8, 191. (h) Lahti, P. M., Ed. Magnetic Properties of
Organic Materials; Marcel Dekker: New York, 1999. (i) Itoh, K.;
Kinoshita, M. Molecular Magnetism: New Magnetic Materials; Gordon
and Breach: Newark, NJ, 2000. (j) Miller, J. S.; Drillon, M., Eds.:
Magnetism: Molecules to Materials, Volumes I-III: Wiley-VCH: Wein-
heim, Germany, 2001. (k) Blundell, S. J.; Pratt, F. L. J. Phys.: Condens.
Matter 2004, 16, R711. (l) Palacio F., Markova, T., Eds. Carbon-based
magnetism: An oVerView of the magnetism of metal-free carbon-based
compounds and materials; Elsevier: Amsterdam, The Netherlands, 2006.
(3) (a) Minguet, M.; Amabilino, D. B.; Vidal-Gancedo, J.; Wurst, K.; Veciana,
J. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1999, 334, 347. (b) Cirujeda,
J.; Mas, M.; Molins, E.; Lanfranc de Panthou, F.; Laugier, J.; Park, J. G.;
Paulsen, C.; Rey, P.; Rovira, C.; Veciana, J. Chem. Commun. 1995, 709.
(c) Cirujeda, J.; Hernandez-Gasio, E.; Rovira, C.; Stanger, J. L.; Turek, P.;
Veciana, J. J. Mater. Chem. 1995, 5, 243. (d) Roques, N.; Maspoch, D.;
Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Tejada, J.; Rovira, C.; Veciana,
J. Chem. Commun. 2005, 4801. (e) Lahti, P. M. In Carbon-based
magnetism: An oVerView of the magnetism of metal-free carbon-based
compounds and materials; Makarova, T., Palacio, F., Eds.; Elsevier:
Amsterdam, 2006; p 23ff.
† University of Massachusetts.
‡ Osaka University.
§ Universidade de Sa˜o Paulo.
| Present address: Department of Pure and Applied Chemistry, Faculty
of Science and Technology, Tokyo University of Science, Noda, Chiba 278-
8510, Japan.
(1) (a) Kinoshita, M.; Turek, P.; Tamura, M.; Nozawa, K.; Shiomi, D.;
Nakazawa, Y.; Ishikawa, M.; Takahashi, M.; Awaga, K.; Inabe, T.;
Maruyama, Y. Chem. Lett. 1991, 1225. (b) Takahashi, M.; Turek, P.;
Nakazawa, Y.; Tamura, M.; Nozawa, K.; Shiomi, D.; Ishikawa, M.;
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Y.; Shiomi, D.; Nozawa, K.; Hosokoshi, Y.; Ishikawa, M.; Takahashi, M.;
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K.; Shiomi, D.; Awaga, K.; Inabe, T.; Maruyama, Y.; Kinoshita, M. Chem.
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186
J. AM. CHEM. SOC. 2008, 130, 186-194
10.1021/ja074211g CCC: $40.75 © 2008 American Chemical Society

A Hydrogen-Bonded Organic Quasi-1D Ferromagnet
A R T I C L E S
chains due to their 1-D NH donor to azole N acceptor NH‚‚‚N
interactions and offer a convenient crystallographic scaffolding
to try to arrange attached radicals. Ferrer et al.4a and Miyazaki
et al.4e reported magnetostructural and calorimetric studies
showing 2-tert-butylnitroxylbenzimidazole (BABI) to be an
antiferromagnet with a Ne´el temperature of 1.7 K. Yoshioka
and co-workers have reported magnetostructural studies of
2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imi-
dazole-3-oxide-1-oxyl (BImNN) showing it to exhibit strong
1-D chain ferromagnetic (FM) exchange interactions.5a-c Ad-
ditional studies on BImNN by Blundell and co-workers con-
firmed the dominance of 1-D FM chain interactions and
suggested the presence of additional interchain exchange
interactions.7
similarities between BImNN and F4BImNN (or their organic
alloys), given that fluorination can induce large changes in
molecular properties. Indeed, Yoshioka’s group has found that
a number of other BImNN structural analogues are quite
different from BImNN in magnetic and crystallographic
behavior.5d-h
The preliminary report4g about F4BImNN noted that, despite
the similarity of crystallography and magnetism to those of
BImNN, the crystallographic packing was not identical. Because
subtle changes in molecular packing can give significant changes
in magnetic behavior, further study of F4BImNN seemed
appropriate. This article gives full details of the synthesis and
magnetostructural analysis of F4BImNN, including new mag-
netic and calorimetric studies that show it to act as a quasi 1-D
organic ferromagnet whose strongly FM coupled, hydrogen-
bonded chains are antiferromagnetically coupled to give mag-
netic ordering below a critical temperature of about 0.7 K.
Experimental Section
General Methods. Ethanol (anhydrous, water <0.02%) and metha-
nol (anhydrous, water <0.01%) were dried over 3 Å molecular sieves
before use. Triethylamine (99%) and chloroform (HPLC grade) were
obtained from Fisher Scientific and used as received. Other reagents
were used as received.
Some of us recently reported that 2-(4,5,6,7-tetrafluoroben-
zimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-
3-oxide-1-oxyl (F4BImNN) shows strong 1-D chain ferromag-
netic (FM) exchange that is quite similar to that of BImNN.4g
This showed that two molecules (BImNN and F4BImNN)
having similar shapes, similar placement of similar hydrogen
bonding donor and acceptor groups, but strong electronic
perturbation of the polarity of substituent groups (F for H
substitutions on the benzenoid ring) can still yield similar crystal
packing with commensurately similar magnetic behavior. Also,
an organic alloy of BImNN and F4BImNN gives similar 1-D
crystallographic chain formation and 1-D FM exchange interac-
tions to the behaviors noted for the pure constituent radicals.4h
It was not obvious that there would be strong crystallographic
Instrumental Analyses. All melting points are reported uncorrected.
Mass spectrometry was carried out at the University of Massachusetts
Amherst Mass Spectrometry Facility, which is supported in part by
the National Science Foundation. Solution-phase electronic spin
resonance experiments were carried out at room temperature, using a
Bruker Biospin Elexsys E-500 spectrometer system. Samples were
dissolved in spectro-grade toluene, placed in 4 mm o.d. quartz tubes,
subjected to freeze-pump-thaw degassing and measured at 9.6 GHz,
modulation frequency 100 MHz, and modulation amplitude 0.05 G.
DC magnetization and AC susceptibility measurements between 1.8
and 300 K were carried out on crushed polycrystalline samples of
F4BImNN placed in gelatin capsules and held in place with a small
amount of cotton. The samples were placed in a plastic straw and
inserted into a Quantum Design MPMS-5 SQUID magnetometer. The
magnetic behavior was measured at various applied fields (1000-
50 000 Oe).
(4) (a) Ferrer, J. R.; Lahti, P. M.; George, C.; Antorrena, G.; Palacio, F. Chem.
Mater. 1999, 11, 2205. (b) Xie, C.; Lahti, P. M. Tetrahedron Lett. 1999,
40, 4305. (c) Ferrer, J. R.; Lahti, P. M.; George, C.; Oliete, P.; Julier, M.;
Palacio, F. Chem. Mater. 2001, 13, 2447. (d) Lahti, P. M.; Ferrer, J. R.;
George, C.; Oliete, P.; Julier, M.; Palacio, F. Polyhedron 2001, 20, 1465.
(e) Miyazaki, Y.; Sakakibara, T.; Ferrer, J. R.; Lahti, P. M.; Antorrena,
G.; Palacio, F.; Sorai, M. J. Phys. Chem. B 2002, 106, 8615. (f) Delen, Z.;
Xie, C.; Hill, P. J.; Choi, J.; Lahti, P. M. Cryst. Growth Des. 2005, 5,
1867. (g) Murata, H.; Delen, Z.; Lahti, P. M. Chem. Mater. 2006, 18, 2625.
(h) Murata, H.; Mague, J. T.; Aboaku, S.; Yoshioka, N.; Lahti, P. M. Chem.
Mater. 2007, 19, 4111.
AC susceptibility measurements between 0.5 and 4.2 K were carried
out by the mutual inductance method, using polycrystalline samples in
polyvinylchloride sample holders. The experimental setup is described
elsewhere.8
(5) (a) Yoshioka, N.; Irisawa, M.; Mochizuki, Y.; Aoki, T.; Inoue, H. Mol.
Cryst. Liq. Cryst. Sci. Technol., Sect. A 1997, 306, 403. (b) Yoshioka, N.;
Irisawa, M.; Mochizuki, Y.; Kato, T.; Inoue, H.; Ohba, S. Chem. Lett. 1997,
251. (c) Yoshioka, N.; Inoue, H. Crystal Control in Organic Radical Solids.
In Magnetic Properties of Organic Materials; Lahti, P. M., Ed.; Marcel
Dekker: New York, 1999; p 553. (d) Yoshioka, N.; Matsuoka, N.; Irisawa,
M.; Ohba, S.; Inoue, H. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1999,
334, 239. (e) Nagashima, H.; Irisawa, M.; Yoshioka, N.; Inoue, H. Mol.
Cryst. Liq. Cryst. Sci. Technol., Sect. A 2002, 376, 371. (f) Nagashima,
H.; Hashimoto, N.; Inoue, H.; Yoshioka, N. New J. Chem. 2003, 27, 805.
(g) Nagashima, H.; Inoue, H.; Yoshioka, N. Polyhedron 2003, 22, 1823.
(h) Nagashima, H.; Fujita, S.; Inoue, H.; Yoshioka, N. Cryst. Growth Des.
2004, 4, 19.
(6) (a) Fegy, K.; Sanz, N.; Luneau, D.; Belorizky, E.; Rey, P. Inorg. Chem.
1998, 37, 4518. (b) Fegy, K.; Luneau, D.; Ohm, T.; Paulsen, C.; Rey, P.
Angew. Chem., Int. Ed. 1998, 37, 1270. (c) Lescop, C.; Luneau, D.;
Belorizky, E.; Fries, P.; Guillot, M.; Rey, P. Inorg. Chem. 1999, 38, 5472.
(d) Lescop, C.; Luneau, D.; Bussiere, G.; Triest, M.; Reber, C. Inorg. Chem.
2000, 39, 3740. (e) Liu, Z.-L.; Zhao, B.; Jiang, Z.-H.; Yan, S.-P. Gaodeng
Xuexiao Huaxue Xuebao 2001, 22, 5. (f) Beaulac, R.; Bussiere, G.; Reber,
C.; Lescop, C.; Luneau, D. New J. Chem. 2003, 27, 1200.
Heat capacity measurements under a magnetic field were carried
out with a commercial calorimeter based on a relaxation method
(Quantum Design, PPMS 6000). To verify the absolute heat capacity
values, adiabatic calorimetry measurements were also done using a
laboratory-made apparatus that is described elsewhere.9
Crystallographic analyses were carried out at the University of
Massachusetts Chemistry Department X-ray Structural Characterization
Facility (NSF CHE-9974648) on a Bruker Nonius CCD instrument.
Data workup was carried out using SHELXTL10 and PLATON.11
ORTEP diagrams were generated using ORTEP-3 for Windows.12
(8) Oliveira, N. F., Jr.; Paduan-Filho, A.; Salinas, S. R.; Becerra, C. C. Phys.
ReV. B 1978, 18, 6165.
(9) Kume, Y.; Miyazaki, Y.; Matsuo, T.; Suga, H. J. Phys. Chem. Solids 1992,
53, 1297.
(7) (a) Sugano, T.; Blundell, S. J.; Hayes, W.; Day, P. Polyhedron 2003, 22,
2343. (b) Sugano, T.; Blundell, S. J.; Hayes, W.; Day, P. J. Phys. IV:
Proceedings 2004, 114, 651. (c) Sharmin, S.; Blundell, S. J.; Sugano, T.;
Ardavan, A. Polyhedron 2005, 24, 2360.
(10) Sheldrick, G. M. SHELXTL97 Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Germany, 1997.
(11) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7-13.
(12) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
9
J. AM. CHEM. SOC. VOL. 130, NO. 1, 2008 187

A R T I C L E S
Murata et al.
Figure 1. Synthesis of F4BImNN.
2,3-Bis(hydroxylamino)-2,3-dimethylbutane hydrogen sulfate was
prepared from 2-nitropropane according to the procedure of Ovcharenko
et al.13
3,4,5,6-Tetrafluoro-1,2-phenylenediamine (F4DiAm) was prepared
from commercially available nitropentachlorobenzene (Aldrich) ac-
cording to the procedure of Heaton et al.14
(0.772 g, 79%) as a white solid. Mp: 176-177 °C (decomp). 1H NMR
(400 MHz, acetone-d6, ppm): δ 12.04 (br s, 1H, NH), 5.10 (s, 1H,
CH), 1.19 (s, 6H, CH3), 1.17 (s, 6H, CH3). IR (KBr pellet, cm-1): 3285
(OH str), 2943 (NH str). MS (EI): calcd for C14H16N4F4O2 m/z ) 348.3.
Found m/z ) 348. Elemental analysis calcd for C14H16N4F4O2: C, 48.30;
H, 4.63; N, 16.10. Found: C, 49.42; H, 5.16; N, 14.08.
4,5,6,7-Tetrafluorobenzimidazole-2-carbaldehyde Diethyl Acetal
(F4BAc). Sodium (0.483 g, 21.0 mmol) was carefully added to dry
ethanol (12 mL) followed by 3,4,5,6-tetrafluoro-1,2-phenylenediamine
(1.60 g, 8.88 mmol) and ethyl diethoxyacetate (2.11 g, 12.0 mmol).
The mixture was heated at reflux for 24 h and cooled to room
temperature, and then the solvent was removed under vacuum. The
residue was dissolved in water and extracted with dichloromethane.
The organic layer was separated to dryness. Chromatography on silica
gel with ethyl acetate/hexane (3/7) as the eluent yielded the acetal (1.48
2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethy-4,5-
dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN). To a suspension
of 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-1,3-dihydroxy-4,4,5,5-tet-
ramethylimidazolidine (0.624 g, 1.79 mmol) in 75 mL of chloroform
were added 76 mL of 0.25 M aqueous sodium periodate (1.90 mmol).
A deep blue color formed as soon as the oxidizing solution was added.
The mixture was stirred at room temperature under argon for 10 min.
The chloroform layer was separated, and the aqueous layer was
extracted with chloroform. The combined organic layers were dried
over anhydrous calcium chloride and evaporated to dryness. Chroma-
tography on silica gel with ethyl acetate/hexane (1/1) yielded the product
(0.305 g, 49%), which was recrystallized from chloroform/methanol
to give crystallographic quality, shiny blue-black prisms. Mp: 170-
172 °C (decomposition). ESR (toluene, 9.63086 GHz): 7.22 G (2 N).
IR (KBr pellet, cm-1): 3396 (NH, str). MS (EI): calcd for C14H13N4F4O2
m/z ) 345.1, found m/z ) 345. Elemental analysis calcd for
C14H13F4N4O2: C, 48.70; H, 3.80; N, 16.22; F, 22.01. Found: C, 49.27;
H, 3.99; N, 15.71; F, 22.00. Single-crystal X-ray diffraction analysis
details were deposited with the Cambridge Crystallographic Database,
CCDC No. 295499.
1
g, 51%) as a white powder. Mp: 208-209 °C. H NMR (400 MHz,
CDCl3, ppm): δ 11.19 (br s, 1H, NH), 5.74 (s, 1H, CH), 3.68 (m, 4H,
CH2), 1.17 (t, 6H, J ) 7.0 Hz, CH3). IR (KBr pellet, cm-1): 2930
(NH str). MS (EI): calcd for C12H12N2F4O2 m/z ) 292.2. Found m/z
) 292. Elemental analysis calcd for C12H12N2F4O2: C, 49.30; H, 4.14;
N, 9.59. Found: C, 49.53; H, 4.16; N, 9.51.
4,5,6,7-Tetrafluorobenzimidazole-2-carbaldehyde (F4BAld). A
solution of 4,5,6,7-tetrafluorobenzimidazole-2-carboxaldehyde diethyl
acetal (1.34 g, 4.59 mmol) in 1 M aqueous sulfuric acid (300 mL) was
stirred for 1 h at room temperature and then boiled for 10 min. The
solution was cooled to room temperature, and the pH was adjusted to
ca. 9 with aqueous sodium carbonate. The resulting precipitate was
filtered, washed with water, and dried under vacuum to yield the product
Results and Discussion
1
(0.766 g, 77%) as a white powder. Mp: 206-207 °C. H NMR (400
MHz, acetone-d6, ppm): δ 9.99 (s, 1H, CHO). IR (KBr pellet, cm-1):
3076 (NH str), 1700 (CdO str). MS (EI): calcd for C8H2N2F4O m/z )
218.1. Found m/z ) 218. Elemental analysis calcd for C8H2N2F4O: C,
44.10; H, 0.92; N, 12.80. Found: C, 42.81; H, 1.00; N, 12.56.
2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-1,3-dihydroxy-4,4,5,5-
tetramethylimidazolidine (F4BImNNH3). 2,3-Bis(hydroxylamino)-
2,3-dimethylbutane hydrogen sulfate (0.692 g, 2.81 mmol) and 4,5,6,7-
tetrafluorobenzimidazole-2-carbaldehyde (0.613 g, 2.81 mmol) were
dissolved in 21 mL of HPLC-grade methanol. To this mixture was
added triethylamine (0.284 g, 2.81 mmol). The mixture was stirred for
48 h under argon at room temperature and filtered. The resulting white
solid was washed with ice-cold methanol and dried to give the product
Synthesis and Characterization. The synthesis of F4BImNN
is summarized in Figure 1. The procedure of Heaton et al.14
was used to make diamine F4DiAm in good yield. The diamine
was then condensed with ethyl diethoxyacetate to give acetal
F4BAc and deprotected to the corresponding aldehyde F4BAld.
The aldehyde was condensed with 2,3-bis(hydroxylamino)-2,3-
dimethylbutane hydrogen sulfate13 in the presence of triethy-
lamine to make the bis-hydroxylamine precursor of the desired
radical, F4BImNNH3. Oxidation with aqueous sodium periodate
followed by column chromatography gave crude F4BImNN,
which can be recrystallized to give large blue-black prisms that
are highly stable to ambient conditions. This sequence works
sufficiently well that synthesis of up to a 0.5 g of radical is
readily accomplished.
(13) Ovcharenko, V.; Fokin, S.; Rey, P. Mol. Cryst. Liq. Cryst. Sci. Technol.,
Sect. A 1999, 334, 109.
(14) Heaton, A.; Hill, M.; Draksmith, F. J. Fluorine Chem. 1997, 81, 133.
9
188 J. AM. CHEM. SOC. VOL. 130, NO. 1, 2008

A Hydrogen-Bonded Organic Quasi-1D Ferromagnet
A R T I C L E S
Figure 2. Spin populations (Fspin) of F4BImNN computed at UB3LYP/
LANL2DZ level of theory using crystallographic geometry.
Electron spin resonance (ESR) spectroscopy of F4BImNN
in toluene solution at room temperature shows a typical pentet
pattern hyperfine coupling (hfc) of 7.2 G from two equivalent
nitrogens. Computational estimates of the spin density popula-
tions at the UB3LYP/LANL2DZ level of theory15,16 are given
in Figure 2, based on the F4BImNN crystal structure described
below. The major computed spin density is on the nitronylni-
troxide N-O groups, consistent with experiment. The qualitative
spin distribution follows π-spin polarization (SP) expectations,
except for the nonalternant imidazole unit, and the same-sign
polarization of fluorines as the benzenoid π-carbons to which
they are attached. The computations show significant spin
density on the imidazole unit, but this was not resolved in the
experimental ESR spectra.
Figure 3. Selected polycrystalline solid ESR spectra and temperature
dependence of gavg of F4BImNN. The spectral ordinate scales are the same.
Table 1. Crystallographic Summary for F4BImNN, CCDC No.
295499
Crystal Data
formula
C14H13F4N4O2
formula weight
crystal system
space group
a, b, c [Å]
345.2
monoclinic
P21/c (no. 14)
8.6540(4), 20.9710(9), 8.8500(4)
R, â, γ [deg]
90, 110.397(2), 90
1505.42(12)
V [Å3]
Z
4
Figure 3 shows solid polycrystalline ESR spectra of F4BImNN
as temperature decreases. The spectral gavg determined from the
absorption mode maxima decreases sharply at lower tempera-
tures, consistent with changes in the g-tensor in low dimensional
magnetic systems having dominant exchange along one crystal-
lographic axis.17 The spectra split to exhibit multiple features
with cooling, and the spectral extent becomes broad, especially
below about 25 K. These temperature variations are all
reversible. Qualitatively similar changes were observed by
Sharmin et al.7c in ESR studies of BImNN single crystals. They
also noted the appearance of additional transitions in the ESR
spectrum at lower temperatures, which they tentatively ascribed
either to a structural phase transition or an onset of hyperfine
structure features in their spectra.
D(calc) [g/cm3]
Mu(Mo KR) [mm-1
F(000)
1.523
0.138
708
0.01 × 0.03 × 0.09
]
crystal size [mm3]
Data Collection
temperature (K)
293
radiation [Å, Mo KR]
θ (min, max) [deg]
0.710 73
1.9, 25.0
dataset (-h, h; -k,k; -l,l)
tot., uniq. data, R(int)
observed data [I > 2σ(I)]
-10, 10; -24, 24; -10, 10
5128, 2653, 0.031
1783
Refinement
N
ref, Npar
2653, 219
R(I > 2σ), wR(I > 2σ)
0.0517, 0.1613
0.0863, 0.1932
1.16
R(all), wR2(all)
S(all)
min, max resd density [e/Å3]
-0.24, 0.27
Table 1 summarizes the X-ray diffraction analysis of a single
crystal of F4BImNN. Figure 4 shows an ORTEP style repre-
sentation of the structure. The most notable features of the
crystal packing are the hydrogen-bonded chains where the N-H
units are donors and both radical O-N and azole N act to some
degree as acceptors, as shown in Figure 5. This chain motif is
analogous to that in BImNN.5a-c Table 2 shows important
intermolecular, intrachain contacts generated by the symmetry
operation (x, 1/2 - y, 1/2 + z). The double dashed line in Figure
5 shows the (N)H‚‚‚O hydrogen bond contact, with a fairly short
(N1)H1‚‚‚O2_4 distance of 2.01 Å (r(N1‚‚‚O2) ) 2.814(4) Å).
The (N1)H1‚‚‚N2_4 contact of about 2.7 Å ((r(N1‚‚‚N2) )
3.152(4) Å)) shown by the dashed single line in the figure is
significantly longer; its length indicates that it is weaker than
the (N1)H1‚‚‚O2_4 interaction. Both contacts are also signifi-
(15) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b) Becke,
A. D. J. Chem. Phys. 1993, 98, 5648. (c) Stephens, P. J.; Devlin, F. J.;
Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623.
(16) Computations were carried out using Gaussian 03, rev B.03: Frisch, M.
J., et al. Gaussian Inc.: Pittsburgh, PA, 2003.
(17) Nagata, K.; Tazuke, Y. J. Phys. Soc. Jpn. 1972, 32, 337.
9
J. AM. CHEM. SOC. VOL. 130, NO. 1, 2008 189

A R T I C L E S
Murata et al.
the data. Figure 6 shows the data and the fitted curve, where
both are shown after correction for the TIC: the fitted Jchain/k
) (+)22.1 ( 0.6 K, where the uncertainty is the statistical 95%
confidence limits. Sugano et al.7a reported Jchain/k ) (+)22 K
for BImNN, measuring magnetic susceptibility at 200 Oe to
limit saturation effects. The intrachain exchange strengths for
BImNN and F4BImNN are thus very similar, consistent with
the similarities of the crystallographic chains in these systems.
The molar magnetization of F4BImNN easily saturates at 1.8
K, as shown in Figure 7. The experimental saturation magne-
tization Msat ≈ 5400 emu/mol from the high field region is
1
compatible with expectations for S ) /2 spin per radical. But,
Figure 4. ORTEP diagram for F4BImNN showing 50% probability
ellipsoids.
the experimental M(H) curve is most consistent with an effective
spin quantum number of Seff ≈ 9 using eq 3, where B(y) is a
Brillouin function of magnetic field H and spin quantum number
S ) Seff. A comparison of the theoretical Seff ) 9 magnetization
curve at 1.8 K to the data is given in Figure 7, normalized to
the experimental Msat. Thus, Seff is much larger than the value
cantly bent by comparison to the ideal of linearity for hydrogen
bonds (155° for N1-H1‚‚‚O2_4, and 110° for N1-H1‚‚‚N2_4),
but both probably contribute to forming the chains in Figure 5.
Magnetic Measurements. Figure 6 shows the results of static
field, dc magnetic susceptibility measurements (ø ) M/H) at
1000 Oe for polycrystalline F4BImNN, plotted as øT over 1.8-
300 K. The increase in øT to about 3.0 emu‚K/Oe‚mol as
temperature decreases to 1.8 K shows the presence of ferro-
magnetic (FM) exchange interactions. F4BImNN is prone to
susceptibility saturation,5g like BImNN,7a but magnetization vs
field measurements at 1.8 K (Figure 7) show a nearly linear
response up to 1000 Oe. The inset of Figure 6 shows the lower
temperature portion of a Curie-Weiss plot of the data as 1/ø
versus T. A linear fit of the higher temperature portion of the
data gives a Curie constant of 0.389 emu‚K/Oe‚mol and a Weiss
intercept of θ ) (+)11 K. The Curie constant is in good accord
with the value expected for S ) 1/2 spin carriers, and the Weiss
constant shows that there are quite strong FM exchange
interactions for an organic solid.
1
expected for nearly isolated S ) /2 F4BImNN spin carriers.
This is due to the strong exchange between radicals.
M ) N µB g ‚ S ‚ B(y)
2S + 1
2S
2S + 1
2S
1
2S
y
2S
B(y) )
coth
y - coth
(
)
(
)
gµB ‚ S ‚ H
y )
(3)
kT
The ac susceptibility behavior of F4BImNN was also
measured in a number of experiments. Figure 8 shows øac data
for F4BImNN obtained at several external fields from 0 to 7000
Oe, all at 50 Hz modulation frequency and 1 Oe modulation
field, between 2.1 and 10.0 K. No significant imaginary
component to the øac was found under any of these conditions.
As the figure shows, øac exhibits field dependent maxima. These
results are virtually the same at modulation frequencies of 17
and 1500 Hz, so the øac peak maximum positions are static rather
than dynamic in behavior (independent of ac modulation
frequency.) This behavior arises from short-range fluctuations
in the magnetization20 and indicates a crossover between low
temperature behavior dominated by the temperature proximity
of an ordered phase and the high temperature behavior of the
paramagnetic phase.
Idealized, completely isolated 1-D FM chains should not show
ordering above absolute zero.21 But, experimentally realistic
systems with even small interchain interactions can give a
magnetically ordered state. Therefore the magnetic behavior of
F4BImNN was investigated below 2 K using a 3He refrigerator
system.8 Figure 9 shows the øac data obtained at zero applied
field, 155 Hz modulation frequency, 10 Oe modulation field,
over 0.5-4.1 K. The data exhibit a characteristic λ-shaped cusp
at 0.72 K, consistent with bulk magnetic ordering.
The øT data can be fit to a 1-D Heisenberg FM chain model
using the Hamiltonian in eq 1, where the exchange constant J
is the intrachain exchange constant between neighboring
molecules i and j
H ) -2J
S ‚ S
(1)
∑
i
j
From eq 1, the susceptibility dependence on temperature for
isolated 1-D ferromagnetic chains is given by eq 2:18-19
2/3
1 + A ‚ x + B ‚ x2 +
2
Ng2µB
4kT
C ‚ x3 + D ‚ x4 + E ‚ x5
ø )
‚
+ TIC
[
]
1 + F ‚ x + G ‚ x2 + H ‚ x3 + I ‚ x4
x ) J/(2kT)
A ) 5.797 991 6, B ) 16.902 653,
C ) 29.376 885, D ) 29.832 959, E ) 14.036 918
F ) 2.797 991 6, G ) 7.008 678 0,
H ) 8.653 864 4, I ) 4.574 311 4 (2)
Heat Capacity Measurements. Thermochemical measure-
ments were carried out on polycrystalline samples of F4BImNN
to investigate the nature of its ordering transition. Figure 10
shows heat capacity measurements Cp as a function of temper-
ature at zero field and at multiple external applied magnetic
fields up to 20 000 Oe. The zero-field results show an ordering
where the various physical constants have their usual meanings.
The Lande´ constant for F4BImNN was fixed at g ) 2.0066
from the room-temperature polycrystalline ESR measurements;
the exchange constant Jchain/k ) J/k and the TIC (temperature
independent correction) were optimized by a nonlinear fit to
(18) Baker, G. A., Jr.; Rushbrooke, G. S.; Gilbert, H. E. Phys. ReV. 1964, 135,
A1272.
(19) Swank, D. D.; Landee, C. P.; Willet, R. D. Phys. ReV. B 1979, 20, 2154.
(20) Becerra, C. C.; Paduan-Filho, A. Solid State Commun. 2003, 125, 99.
(21) Ising, E. Z. Phys. 1925, 31, 253.
9
190 J. AM. CHEM. SOC. VOL. 130, NO. 1, 2008

A Hydrogen-Bonded Organic Quasi-1D Ferromagnet
A R T I C L E S
Figure 5. Intermolecular chain contacts for F4BImNN. Methyl groups are omitted in the ORTEP view for clarity. See Table 2 for contact distances.
Figure 6. øT versus temperature data for F4BImNN at 1000 Oe, corrected
for temperature-independent magnetism. Solid line in main chart shows
1-D Heisenberg chain fit to data using eq 2. Solid line in inset shows Curie-
Weiss line extrapolated from higher temperature 1/ø(T) data.
Figure 8. Field-cooled ac magnetic susceptibility for F4BImNN at 50 Hz
modulation frequency, 1 Oe modulation field, and constant applied fields
as follows: 0 Oe (4, upper), 1000 Oe (0, upper), 1500 Oe (O, upper),
2000 Oe (+), 2250 Oe (2), 2500 Oe (9), 3000 Oe (4), 4000 Oe (0), 5000
Oe (O), 6000 Oe (]), 7000 Oe ([).
Figure 7. Magnetization versus field data at 1.8 K for F4BImNN. The
solid line shows eq 3 for T ) 1.8 K, S ) 9; the data and the curve have
been normalized to the experimental Msat
.
Table 2. Selected Intrachain Intermolecular Distances (Å) for
F4BImNN; See Figure 5
O1‚‚‚C8_4
O1‚‚‚O2_4
N1‚‚‚O2_4
N1‚‚‚N2_4
3.139(4)
3.165(4)
2.814(4)
3.152(4)
O1‚‚‚N4_4
O1‚‚‚N2_4
H1‚‚‚O2_4
H1‚‚‚N2_4
3.024(4)
3.198(4)
2.01
2.75
Figure 9. Ac magnetic susceptibility for F4BImNN at zero field, 155 Hz
modulation frequency, 10 Oe modulation field.
1
cusp anomaly at 0.72 K, in good accord with the results from
the ac magnetic measurements.
served Tc ) 0.7 K yields |zJinter/k| ) 0.09 K for S ) /2 spin
carriers.
Equation 4 from mean field theory yields a rough estimate
of the magnitude of interchain exchange |zJinter| between the
strongly FM exchange coupled chains, where z is the
(here unspecified) number of neighboring sites interacting
with the chain. Using Jchain/k ) +22 K from above, the ob-
1/2
Tc ≈ 2S2(|zJinter| ‚ Jchain
)
(4)
The changes in the critical temperature cusp in the magnetic
Cp data with increasing external field are similar to the behavior
9
J. AM. CHEM. SOC. VOL. 130, NO. 1, 2008 191

A R T I C L E S
Murata et al.
Figure 11. Tc-H phase diagram from data in Figure 10. Solid line is a fit
to eq 6; see the text for details.
Previous work has suggested some reasons for T 3/2 dependence
of the low temperature heat capacity when magnetic studies
indicate antiferromagnetic ordering.26 At present, it is clear that
F4BImNN forms an ordered state below Tc, and the variation
in Tc with the external field is consistent with a low dimensional
magnet having strongly anisotropic 1-D FM chain type exchange
and weak AFM exchange between the chains. Further details
of the nature of the ordered state are under investigation.
MagnetostructuralConsiderations.Yoshiokahasdescribed5a-c
the most likely exchange path governing FM exchange in
BImNN, which presumably also applies to the crystallographi-
cally similar F4BImNN. The F4BImNN hydrogen bond scaf-
folding assembles molecules into 1-D stacks in which radical
N-O groups have excellent intermolecular overlap with the
central carbon in neighboring nitronylnitroxide groups. Figure
12 shows spin density isosurface maps16 of high spin FM and
low spin AFM exchange coupled states of a pair of molecules
from the stack, with z-clipping at a plane that shows the N-O‚
‚‚C contact shown in the simplified structural inset. The
computations were done using spin-unrestricted B3LYP hybrid
density functionals with a 6-31G* basis set (Gaussian16 keyword
Guess ) Mix for the S ) 0 state); the methyl groups were
replaced with hydrogen atoms on the nitronylnitroxide unit. The
favorable contact with the opposite sign of spin density is readily
apparent in the high spin FM state (green‚‚‚blue N-O‚‚‚C
contact), while unfavorable contact between sites with the same
sign (green‚‚‚green) occurs in the AFM state. The observed FM
1-D chain type magnetic exchange is thus completely consistent
with the expectations of a SP model following N-O‚‚‚C
contacts as shown in Figure 13.
If exchange followed the hydrogen bond contacts by an SP
pathway instead, AFM chain exchange would be expected
(Figure 13), contrary to observation. Although the hydrogen
bonds in the chains of F4BImNN bring the donor N-H groups
into close proximity with the large spin populations on the
nitronylnitroxide N-O units, the spin densities on the N-H
unit are quite small (Figure 2) and apparently do not provide
effective exchange linkage between the radicals. A similar
situation occurs in some benzimidazole tert-butylnitroxides,4d
where very close N-H‚‚‚O-N contacts hold pairs of radicals
together, but the dominant exchange path occurs by direct
dipolar interaction between nitroxide units, instead of through
the small spin densities of N-H.
Figure 10. Experimental heat capacity for F4BImNN at several external
magnetic fields versus temperature.
of other quasi-1D ferromagnetic systems having very weak
interchain exchange interactions, such as 3(4-chlorophenyl)-1,5-
diphenyl-6-oxoverdazyl, CDVO.22 In particular, the behavior
is consistent with formation of a ground state bulk antiferro-
magnetic phase below Tc. This is also consistent with the fact
that øac for polycrystalline F4BImNN at the lowest measured
temperature approaches two-thirds of its maximum value at the
Tc cusp. For an antiferromagnet, eq 5 gives the relationship
expected23 between the Ne´el temperature Tc ) TN and the
applied external magnetic field, where all terms have the typical
meanings. The Tc versus field data from Figure 10 give a good
fit to eq 5 as shown in Figure 11 for Tc(0) ) TN(0) ) 0.717 (
0.001 K, critical field Hc ) 1.81 ( 0.07 kOe, and coefficients
R ) 1.7 ( 0.1, ê ) 0.31 ( 0.05; the uncertainties are statistical
95% confidence limits. The interchain exchange interaction
estimated from Hc using eq 6 with g ) 2.0066 (from the ESR
studies above) is zJinter/k ) (-)0.12 K. This is in good accord
with the mean field estimated magnitude obtained from Tc and
Jchain/k using eq 4.
TN(H) ) TN(0)[1 - (H/Hc)R]ê
(5)
(6)
Hc ≈ (2|zJinter| ‚ S2/gµB)1/2
Interestingly, the Cp data below the ordering cusp (where the
lattice contribution is very small) are proportional to T 3/2
.
According to spin wave theory,24 a Cp ≈ T 3/2 dependence is
more consistent25-26 with ferromagnetic ordering below Tc,
while Cp ≈ T 3 dependence is expected26 for antiferromagnets.
(22) Takeda, K.; Hamano, T.; Kawae, T.; Hidaka, M.; Takahashi, M.; Kawasaki,
S.; Mukai, K. J. Phys. Soc. Jpn. 1995, 64, 2343.
(23) (a) Mouritsen, O. G.; Kjaersgaard Hansen, E.; Knak Jensen, S. J. Phys.
ReV. B 1980, 22, 3256. (b) Becerra, C. C.; Oliveira, N. F., Jr.; Paduan-
Filho, A.; Figueiredo, W.; Souza, M. V. P. Phys. ReV. B 1988, 38, 6887.
(c) Kobayashi, T.; Takiguchi, M.; Amaya, K.; Sugimoto, H.; Kajiwara,
A.; Harada, A.; Kamachi, M. J. Phys. Soc. Jpn. 1993, 62, 3239. (d) Takeda,
K.; Hamano, T.; Kawae, T.; Hidaka, M.; Takahashi, M.; Kawasaki, S.;
Mukai, K. J. Phys. Soc. Jpn. 1995, 64, 2343. (e) Takeda, K.; Mita, M.;
Kawae, T.; Takumi, M.; Nagata, K.; Tamura, M.; Kinoshita, M. J. Phys.
Chem. B 1998, 102, 671. (f) Takeda, K.; Yoshida, Y.; Inanaga, Y.; Kawae,
T.; Shiomi, D.; Ise, T.; Kozaki, M.; Okada, K.; Sato, K.; Takui, T. Phys.
ReV. B 2005, 72, 024435/1. (g) Peres, N. M. R. J. Phys.: Condens. Matter
2003, 15, 7271.
(24) de Jongh, L. J.; Miedema, A. R. AdV. Phys. 1974, 23, 1.
(25) Miyazaki, Y.; Matsumoto, T.; Ishida, T.; Nogami, T.; Sorai, M. Bull. Chem.
Soc. Jpn. 2000, 73, 67.
(26) Ohmae, N.; Kajiwara, A.; Miyazaki, Y.; Kamachi, M.; Sorai, M. Thermo-
chim. Acta 1995, 267, 435.
The FM SP model of Figure 13 gives a good account of the
dominant Jchain 1-D chain exchange in F4BImNN. The weaker
9
192 J. AM. CHEM. SOC. VOL. 130, NO. 1, 2008

A Hydrogen-Bonded Organic Quasi-1D Ferromagnet
A R T I C L E S
Figure 12. Spin density surfaces for F4BImNN radicals in a stack for high spin FM (left) and low spin AFM coupled (right) states, z-clipped at a plane
including the N-O‚‚‚C contact. Green and blue surfaces denote opposite spin density sign. Picture from GaussView 3.07 using an surface isovalue of
0.0004.
Figure 13. Possible spin polarization (SP) exchange pathways in F4BImNN. N-H‚‚‚O-N contacts shown as dashed lines; N-O‚‚‚C contacts shown as
arrows. Qualitative spin polarizations taken from Figure 2.
interchain exchange Jinter that leads to bulk ordering is harder
to assign. There are numerous intermolecular contacts of e3.7
Å involving the nitroxide oxygen atoms in F4BImNN, but
almost all of these are within the 1-D chains shown in Figures
5 and 13. For example, Figure 14 shows interchain (N4)O2‚‚
‚(H)C5 nitroxide NO to nitroxide methyl contacts (r((N4)O2‚
‚‚H5B(C5)) ) 3.68 Å, r(O2‚‚‚C5) ) 4.294(6) Å) that link the
1-D chains to form 2-D sheets in the crystallographic ac-plane.
These are shown in Figure 14: the hydrogen-bonded chains
run into the plane of the figure along the c-axis, and the (N4)-
O2‚‚‚(H)C5 contacts are in the middle, roughly along the a-axis.
The nitroxide methyl groups have only small spin densities from
Figure 2 but could provide pathways for weak exchange.
A third dimension of intermolecular contacts occurs between
nitroxide NO groups and peripheral benzenoid CF and nitroxide
methyl groups on a neighboring 2-D sheet. Each NO group is
4.4-4.5 Å from two CF groups and one methyl CH. These
Figure 14. Nitroxide to F-C and nitroxide to methyl contacts between
hydrogen-bonded chains in F4BImNN; chains run along the c-axis into the
figure.
contacts are labeled in Figure 14 as NO‚‚‚FC contacts, as red
dashed lines roughly along the b-axis, on the upper and lower
9
J. AM. CHEM. SOC. VOL. 130, NO. 1, 2008 193

A R T I C L E S
Murata et al.
Figure 15. Contacts between hydrogen-bonded chains in BImNN (left, CSD deposition code REFXUV) and F4BImNN, showing unit cells; chains run into
the figure.
peripheries of the figure. The exchange role of the CF groups
is not obvious, but the computational data from Figure 2 indicate
up to 0.1% of a spin resides on the fluorines to provide
additional exchange possibilities. We must emphasize that the
suggested electronic exchange roles for the NO‚‚‚HC and NO‚
‚‚FC contacts shown in Figure 14 are speculative at present,
but there appear to be no closer intermolecular exchange contacts
between sites of a larger spin density.
Assuming a priori that neither BImNN nor F4BImNN
undergoes significant changes in crystallographic packing upon
cooling to below a few kelvins, the difference between the two
is the packing between the 1-D stacked chains. The CF units in
F4BImNN form favorable CF/methyl contacts with nitronylni-
troxide methyl groups, bringing the benzenoid rings into closer
proximity to the nitronylnitroxide units than occurs in BImNN,
for which methyl/methyl and aryl/aryl contacts dominate inter-
chain contacts. Figure 15 shows comparison views of the regions
of interchain contact. The chains in the two molecules are
geometrically “offset” in different ways, despite their very
similar intrachain crystallography.
bonded chains. Variable temperature solid-phase ESR spectros-
copy shows g-value shifts and line shape changes consistent
with dominant 1-D axial exchange interactions in the solid. Heat
capacity measurements are also consistent with the quasi 1-D
nature of the exchange interactions. Both magnetic susceptibility
and heat capacity measurements at zero field indicate a critical
temperature of Tc ) 0.7 K. The variation of magnetic heat
capacity of F4BImNN with applied magnetic field is consistent
with an ordering transition to a bulk antiferromagnetic ground
state below Tc. The quasi-1D ferromagnetic chain exchange is
fairly strong for an organic system, Jchain ≈ +22 K; the low
temperature heat capacity variation as a function of temperature
allows a rough estimate of a ratio of intrachain to interchain
exchange, |Jchain/zJinter| ≈ 220-250.
The use of benzimidazole-based hydrogen bonding to as-
semble F4BImNN into a piled stack motif was a crucial strategic
choice in finding this addition to the family of pure organic
systems that exhibit bulk magnetic ordering. Further use of
hydrogen-bonding strategies seems promising to find additional
organic materials with magnetic ordering.
This difference raises the question of whether or how the
radicals’ magnetisms fundamentally differ. Above 2 K there is
little difference in magnetic behavior, consistent with the
dominant 1-D ferromagnetic SP exchange pathway created by
the intrachain similarity. But, the present study clearly shows
that F4BImNN orders below 0.7 K. Analogous behavior has
not been reported for BImNN, although Sugano et al.7a estimated
that zJinter ) 0.24 K based on susceptibility measurements over
1.8-5 K. Using eq 4 with zJinter ) 0.24 K and Jchain ) (+)22
K from the work of Sugano et al. would predict an ordering
temperature slightly above 1 K for BImNN. Barring clearer
evidence of bulk ordering in BImNN, it is not possible to tell
whether the interchain geometry differences between it and
F4BImNN correlate with significant differences in their low-
temperature magnetic behavior.
Acknowledgment. This work was supported in part by the
U.S. National Science Foundation by grants CHE 0415716
(H.M., Z.D., P.M.L.), CHE-9974648 (UMass-Amherst X-ray
Structural Characterization facility), CTS-0116498 (UMass-
Amherst Nanomagnetic Characterization Facility), and CHE-
0443180 (UMass-Amherst Electron Paramagnetic Resonance
Facility). Support was also provided by the Fundac¸a˜o de Amparo
a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) in Brazil (A.P.F.,
V.B., N.F.O.). We thank Dr. Gregory Dabkowski of the
University of Massachusetts Amherst Microanalytical Labora-
tory for elemental microanalyses.
SupportingInformationAvailable: Completeref16,F4BImNN
crystallographic summary and HPLC trace, F4BImNN computed
spin density, computed F4BImNN dyad triplet-single energy
gap in crystallographic chain. This information is available free
of charge on the Internet at http://pubs.acs.org.
Conclusions
F4BImNN is a quasi 1-D organic ferromagnet that exhibits
strong 1-D FM chain exchange attributable to favorable spin
polarization induced inter-radical interactions along hydrogen-
JA074211G
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194 J. AM. CHEM. SOC. VOL. 130, NO. 1, 2008