Synthesis of neutral spin-delocalized electron acceptors for multifunctional materials

Article abstract of DOI:10.1021/ol701725r

(Chemical Equation Presented) A new synthetic route to stable spin-delocalized radicals, annelated nitronyl nitroxides, has been developed on the basis of the condensation of benzofuroxan with aryl nitrones. The electronic structure of the resulting radic

Full text of DOI:10.1021/ol701725r

ORGANIC
LETTERS
2007
Vol. 9, No. 23
4781-4783
Synthesis of Neutral Spin-Delocalized
Electron Acceptors for Multifunctional
Materials
Brynn M. Dooley,† Steven E. Bowles,‡ Tim Storr,‡ and Natia L. Frank*,†,‡
Department of Chemistry, Box 3065, UniVersity of Victoria, Victoria, British
Columbia, V8W 3V6 Canada, and Department of Chemistry, Box 351700, UniVersity
of Washington, Seattle, Washington 98195-1700
nlfrank@uVic.ca
Received August 24, 2007
ABSTRACT
A new synthetic route to stable spin-delocalized radicals, annelated nitronyl nitroxides, has been developed on the basis of the condensation
of benzofuroxan with aryl nitrones. The electronic structure of the resulting radicals was investigated through absorption spectroscopy, EPR,
electrochemistry, and computation (DFT-UB3LYP). The annelated radicals exhibit electronic transitions in the near IR (850
−900 nm) and are
excellent electron acceptors (Ered 0.0 vs SCE) ideal for the development of multifunctional magnetic materials.
∼
The development of magneto-electronic materials (OMEMs)
is central to the emerging field of correlated electronic
materials for high-density data storage and quantum comput-
ing applications.1 The dominant strategy involves the incor-
poration of inorganic spin carriers into semiconducting
materials or fabrication of magnetic-semiconducting hetero-
structures.2 Recent interest in the development of single-
phase processable magneto-electronic materials has led to
new developments in organic-based systems that may exhibit
slower spin dephasing rates.3 The development of stable
organic radical-based materials with critical conducting or
optical properties is a novel strategy toward the discovery
of organic magneto-electronic materials.
Stable radicals such as nitronyl nitroxides,4 thiazyls,5 and
verdazyls6 play a central role in development of spin-
containing materials due to their high stability and ease of
synthesis. Annelation of imidazolidinyl-based nitronyl ni-
troxides (1) leads to stable planar spin-delocalized radicals,7
the benzonitronyl nitronyl nitroxides (BNN 2), which exhibit
strong magnetic exchange through efficient π-π stack-
ing interactions in the solid state.8 BNN radicals are of
interest in that they possess (i) stability toward dimerization
in the solid state, (ii) a planar topology for strong intermo-
lecular interactions in the solid state, and (iii) delocalized
† University of Victoria.
‡ University of Washington.
(1) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.;
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2001, 294, 1488-1495. Prinz, G. A. Science 1998, 282, 1660-1663.
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D. Nature 2001, 411, 770-772.
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821-824. Ouyang, M.; Awschalom, D. D. Phys. World 2004, 17, 21-22;
Dediu, V.; Murgiaa, M.; Matacottaa, F. C.; C. T.; Barbanerab, S. Solid
State Commun. 2002, 122, 181-184. Wang, F. J.; Xiong, Z. H.; Wu, D.;
Shi, J.; Vardeny, Z. V. Synth. Met. 2005, 155, 172-175. Prigodin, V. N.;
Raju, N. P.; Pokhodnya, K. I.; Miller, J. S.; Epstein, A. J. AdV. Mater.
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10.1021/ol701725r CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/18/2007

spin density that provides multiple indirect and direct
magnetic exchange pathways critical for OMEM develop-
ment. Relative to other radical classes, investigations into
materials incorporating BNN radicals and their electronic
structure are sorely missing8 due to poor yields and low
tolerance to diverse functionality associated with existing
synthetic routes.
Early synthetic approaches involve the preparation of the
radical precursor 1-hydroxy-2-arylbenzimidazole-3-N-oxide
(4) by condensation of nitrosobenzene with benzonitrile
oxide,9 acid-catalyzed condensation of benzo1,2-dioxime
with arylaldehydes,10 and base-catalyzed condensation of
primary nitroalkanes.11 These synthetic routes require either
strongly acidic conditions, limiting the nature of the func-
tionality, or suffer from poor reproducibility and low yields.
Herein we present a generalized synthetic methodology for
annelated nitronyl nitroxides with varying functionality based
on the condensation of benzofuroxan with nitrones12 in
nonpolar solvents. The electronic structure of a subset of
the resulting radicals was investigated through computation
and spectroscopy and found to be give rise to extremely
strong electron-acceptor ability with near-IR absorption due
to lowering of the SOMO by conjugative effects.
Condensation of commercially available benzofuroxan with
the nitrones of interest in refluxing hydrocarbon solvents
cleanly leads to precipitation of the insoluble radical precur-
sors. Thus, the radical precursors can be generated under
neutral conditions in high yield. Higher yields of 4 were
generally obtained with nitrones containing π-electron-
donating groups, perhaps due to increased stability of the
nitrone. Oxidation of precursors can be accomplished with
lead(IV) oxide or silver(I) triflate in nonpolar solvents to
yield yellow-green solutions with EPR spectra characteristic
of radicals. Analytically pure samples for investigation can
be obtained by flash chromatography to yield green-brown
microcrystalline powders (2a-e, 38-41%). In general the
series of radicals are stable in solution and indefinitely in
the solid state with the exception of 2b, 2f, and 2g, which
were found to decompose in solution within 24 h.
The absorption spectra of radicals 2a-e possess absorp-
tions in the UV region (Figure 1) and a broad weak
A series of radical precursors were synthesized in which
heteroaromatic, aromatic, and alkyl substituents could be
incorporated into the C-2 position of the radical (Scheme
1). The functionality of interest can be introduced via an
Scheme 1. Synthetic Pathway to BNN Radicals
Figure 1. Absorption spectroscopy of 10-5 M ACN solutions of
nitronyl nitroxide 1a (- - -) and benzimidazolyl nitronyl nitroxide
(BNN) 2a (s).
absorption band in the near-IR (λmax 825-950 nm, ꢀ ≈ 900).
The energy of the low-energy transition is dependent on the
nature of the aryl substituent, lying at higher energy for
electron acceptors (825 nm for pyridyl) and lower energy
for electron donors (900-925 nm for thienyl) assigned to a
symmetry forbidden HOMO-SOMO transition (by TD-DFT
computation; see Supporting Information). In contrast, the
localized parent imidazolidinyl nitroxide 1 exhibits a diag-
nostic absorption band in the visible region at 600 nm
assigned to a SOMO-LUMO (n-π*) transition.16
The EPR spectra of annelated nitronyl nitroxides 2a-e
exhibit g-values of 2.0070 ( 0.003 and a 5-line pattern with
relative intensities of 1:2:3:2:1 consistent with hyperfine
coupling to two equivalent nitrogens. Hyperfine coupling
constants for the nitrogens in the benzimidazole moiety (N1
aldehyde, which upon condensation with hydroxyl-ami-
nobenzene13 generates functionalized nitrones14 in good yield.
(6) Hicks, R. G.; Lemaire, M. T.; Thompson, L. K.; Barclay, T. M. J.
Am. Chem. Soc. 2000, 122, 8077-8078. Neugebauer, F. A.; Fischer, H.;
Krieger, C. J. Chem. Soc., Perkin Trans. 1993, 2, 535-544.
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Kato, R.; Kobayashi, H. J. Mat. Chem. 1997, 7, 1377-1382. Zakrassov,
A.; Shteiman, V.; Sheynin, Y.; Tumanskii, B.; Botoshansky, M.; Kapon,
M.; Keren, A.; Kaftory, M.; Vos, T. E.; Miller, J. S. J. Mater. Chem. 2004,
14, 1827-1837.
(9) Minici, F.; Galli, R.; Quilico, A. Tetrahedron Lett. 1963, 12, 785-
790.
(10) Niclas, H.; Grundemann, E.; Patzold, F. J. Prakt. Chem. 1990, 332,
345-350.
(14) Heaney, F.; Rooney, O.; Cunningham, D.; McArdle, P. J. Chem.
Soc., Perkin Trans. 2 2001, 1, 373-378.
(11) Abu El-Haj, M. J. J. Org. Chem. 1972, 37, 2519-2520.
(12) Borah, H. N.; Boruah, R. C.; Sandhu, J. S. Heterocycles 1985, 23,
1625-1628.
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Chem. A 2003, 107, 4940-4951.
(16) Ullman, E. F.; Siecki, J. H.; Boocock, D. G. B.; Darcy, R. J. Am.
Chem. Soc. 1972, 94, 7049-7059.
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4782
Org. Lett., Vol. 9, No. 23, 2007

and N3) in 2a-e were found to be a(N) ) 4.328 ( 0.062,
roughly 60% of that found for the parent radical 1 (7.4 G),
consistent with significant spin delocalization into the
annelated aromatic moiety. Superhyperfine coupling con-
stants can be resolved for coupling to two equivalent sets of
protons on the annelated ring (HR and Hâ) and the aryl
substituent protons, consistent with ∼10% of the total spin
density residing in the annelated ring.
Figure 3. Computational (DFT UB3LYP/6-31G(d,p)) SOMO (left)
and SCF spin electron density (right) of 2a generated with
Gaussview03 (isovalue 0.04).
expected with substitution at C-2, consistent with EPR and
electronic absorption spectroscopy. In accord, spin densities
in the C-2 aryl group are dictated by spin polarization through
the nodal C-2 position, consistent with the larger class of
nitronyl nitroxides.19
Figure 2. Cyclic voltammetry of the reductive process for a TBAH/
ACN solution (50 mV/s) of radical 2a (s).
Table 1. Spectroscopic Properties of Annelated Radicals
a (N1,N3)b a (HR,Hâ)b E1/2(red) E(SOMO)
a
c
d
λmax
1
600
7.43
4.37
4.26
4.26
4.37
4.38
-0.741
0.099
The electrochemical behavior of the series of BNN radicals
2a-e was investigated by cyclic voltammetry. The parent
radical 2a exhibits a reversible one-electron reduction at
0.001 V vs SCE (∆Ep ) 80 mV) and an irreversible one-
electron oxidation at 1.60 V vs SCE in acetonitrile. Although
both the oxidation and reduction processes of pyridyl radicals
were found to be irreversible, the reduction potential is
decreased relative to the parent 2a by thienyl subtitution
(∼0.02 V vs SCE). By comparison, the cyclic voltammogram
of radical 1 was found to have an irreversible one-electron
reduction at -0.741 V versus SCE and a reversible one-
electron oxidation at 1.00 V versus SCE (∆Ep ) 90 mV).
The very low reduction potential of BNN radicals indicates
that the radical moiety is almost as easily reduced as the
standard acceptor compounds TCNE and TCNQ.17 The
strong accepting ability is most likely due to the aromatic
10 π-electron anion formed upon reduction.
Quantum mechanical calculations were carried out at the
UB3LYP/6-31G(d,p) level of theory18 for the series of
radicals 2a-e. The optimized geometry for radicals 2a-e
indicates geometries in accordance with the expected bond
lengths and angles associated with the nitronyl nitroxide and
aromatic moieties. Analysis of the SOMO suggests that this
orbital is delocalized primarily on the benzimidazole moiety
(Figure 3), leading to spin delocalization on the benzimida-
zole. Little perturbation of the energy of the SOMO is
2a 825
2b 762, 820 (sh)
2c 831
2d 975
2e 930
0.93, 0.65
0.92, 0.69
0.91, 0.73
0.99, 0.74
0.96, 0.69
0.001 -0.238
∼0.2e
-0.212
-0.225
∼0.2e
0.031 -0.216
0.020 -0.216
a Acetonitrile solution. b EPR hyperfine coupling constants in benzene
at 300 K. c Values reported vs SCE (solutions measured as 1-5 mM in 0.1
M TBAH ACN vs Ag/AgNO3, Fc/Fc+ at 50 mV/s). d Computational
UB3LYP/6-31G(d,p). e Irreversible processes, Ep(anodic) values given.
In summary, a robust and versatile synthetic methodology
for annelated nitronyl nitroxides has been developed in which
a series of heteroaromatic BNN radicals were successfully
synthesized. The effect of annelation on the nitronyl nitroxide
moiety has been examined spectroscopically and found to
be consistent with a lowering of the SOMO energy. The
resulting class of radicals possess extremely good electron
acceptor ability, long-wavelength absorption spectra, and a
spin delocalized electronic structure. This class of planar
delocalized radicals provides excellent redox and magnetic
properties for the development of multifunctional magnetic
materials and is currently under investigation for incorpora-
tion into nanoscopic systems.
Acknowledgment. This work was supported by the NSF
(MDITR), University of Washington, University of Victoria,
NSERC, CRC, CFI, and BCKDF.
(17) Gross-Lannert, R.; Kaim, W.; Olbrich-Deussner, B. Inorg. Chem.
1990, 29, 5046-5053.
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W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(19) Zheludev, A.; Barone, V.; Bonnet, M.; Delley, B.; Grand, A.;
Ressouche, E.; Rey, P.; Subra, R.; Schweizer, J. J. Am. Chem. Soc. 1994,
116, 2019-2027.
Supporting Information Available: Experimental details
including syntheses and characterization. This material is
available free of charge via the Internet at http://pubs.acs.org.
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4783