A Stable Radical-Substituted Radical Cation with Strongly Ferromagnetic Interaction: Nitronyl Nitroxide-Substituted 5,10-Diphenyl-5,10-dihydrophenazine Radical Cation

Article abstract of DOI:10.1021/ja0367748

A stable radical-substituted radical ion with strongly ferromagnetic intramolecular interaction (J) between the radical and radical ion sites is an attractive spin building block of organic magnets. We prepared 2-nitronyl nitroxide-substituted 5,10-diphen

Full text of DOI:10.1021/ja0367748

Published on Web 12/11/2003
A Stable Radical-Substituted Radical Cation with Strongly Ferromagnetic
Interaction: Nitronyl Nitroxide-Substituted
5,10-Diphenyl-5,10-dihydrophenazine Radical Cation
Shinsuke Hiraoka, Toshihiro Okamoto, Masatoshi Kozaki, Daisuke Shiomi, Kazunobu Sato,
Takeji Takui, and Keiji Okada*
Departments of Chemistry and Materials Science, Graduate School of Science, Osaka City UniVersity,
Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
Received June 19, 2003; E-mail: okadak@sci.osaka-cu.ac.jp
Scheme 1 a
Recently, the magnetic properties of organic compounds have
attracted a great deal of attention. Various approaches toward
organic magnetic materials have been taken, both theoretically and
experimentally.1 Of these, an approach with a radical-substituted
radical ion as a spin building block is particularly interesting.
Yamaguchi and co-workers theoretically explored organic ferro-
and ferrimagnets with various patterns of radical-substituted donor-
acceptor charge transfer (CT) complexes.2 Although the related
experimental studies have been examined using aniline-,3 ferro-
cene-,4 and TTF-frameworks5,6 as basic structures of CT-complexes,
paramagnetic behavior has been observed in most cases and the
a Reaction conditions: (a) 2,3-bis(dihydroxylamino)-2,3-dimethylbutane
expected ferro- or ferrimagnetic properties have not been observed.
In general, spins on donors and acceptors in usual CT-complexes
have large intermolecular antiferromagnetic interactions ranging
between -100 and -1000 K.7 To realize the expected properties,
a comparably large intramolecular J (half the singlet-triplet energy
gap ∆EST) between the radical and radical ion moieties3a should
therefore be essential. In this context, the thianthrene-based radical
cations recently reported by Sugawara and co-workers seem to be
attractive, although the J values of these spectroscopically detected
species have not been determined because of their instability.8 The
development of a molecular system with isolable stability and a
large J is extremely important in the construction of molecular
magnets. Dihydrophenazines are superior electron donors,9 giving
highly stable radical cations that have large spin densities on the
nitrogen and the C2 carbon atoms.10 We report the synthesis,
structure, and magnetic properties of a nitronyl nitroxide-substituted
5,10-diphenyl-5,10-dihydrophenazine radical cation (1+).
The synthesis of 1+ is outlined in Scheme 1. Cyclic bis-
(hydroxylamine) 2 was synthesized by the condensation of dimethyl
acetal 3 with 2,3-bis(hydroxylamino)-2,3-dimethylbutane in the
presence of a catalytic amount of pyridinium tosylate in dry
methanol. The treatment of 2 with silver(I) oxide in methylene
chloride gave the neutral radical 1. The oxidation of 1 to 1+ was
achieved using tris(4-bromophenyl)aminium perchlorate in dry
(1.6 equiv) in the presence of pyridinium tosylate (0.3 equiv) in dry
methanol; (b) silver(I) oxide (5 equiv) in methylene chloride; (c) tri(4-
bromophenyl)aminium perchlorate (1 equiv) in dry acetonitrile.
Figure 1. Molecular structure (I), a dimer structure in the crystal packing
(IIa), and a schematic diagram of magnetic interaction (IIb) for 1+. (I) Drawn
at 50% ellipsoids level; hydrogen atoms, the counteranion, and the solvent
are eliminated for clarity. Selected bond lengths (Å) and dihedral angles
(deg) for 1+‚ClO4-‚CH2Cl2: a, a′: 1.386(2), 1.380(2), b, b′: 1.414(3),
1.411(3), c, c′: 1.374(2), 1.383(2), d: 1.450(3), e, e′: 1.280(2), 1.279(2),
A/C, C/E: 88.70, 102.26, D/F (F:-NdC-N- plane): 20.83. Selected bond
lengths (Å) and dihedral angles (deg) for the neutral 1: a, a′: 1.407(4),
1.409 (4), b, b′: 1.408(5), 1.403(5), c, c′: 1.395(4), 1.410(4), d: 1.456(4),
e, e′: 1.286(3), 1.279(3), A/C, C/E: 84.65, 85.52, D/F: 25.34. (IIa) The
distance of C7-C8′ contact: 3.566(3) Å. (IIb) The circles represent S )
1/2 spins, NN: nitronyl nitroxide; DPhz: dihydrophenazine.
shape of the HOMO of the neutral dihydrophenazine structure; the
elongated (shortened) bonds in 1+ have a bonding (antibonding)
relation in the HOMO of 5,10-dihydrophenazine. The N-O bond
length (e, e′) of 1+ is similar to that of neutral 1 and longer than
that of a nitrosonium nitroxide (1.23 Å).12
A short intermolecular contact was observed between the C7
atom and the C8′ atom in adjacent 1+, both of which had large
spin densities, forming a dimer structure (Figure 1-IIa).13 The
distance of the contact (3.57 Å) is close to the van der Waals contact
(3.54 Å).14 A schematic model of the magnetic interaction is
expressed in Figure 1-IIb.
acetonitrile in a glovebox. Concentration of the solvent and the
-
addition of dry ether gave fine precipitates of the salt of 1+‚ClO4
.
Recrystallization of the crude salt from the methylene chloride-
toluene mixture gave dark-red crystals of 1+‚ClO4-‚CH2Cl2. The
salt was stable under aerated conditions at room temperature.
X-ray crystal structure analyses were achieved for the neutral
111 and the 1+ salt.11 Figure 1-I shows the molecular structure of
1+. The phenyl groups at 5 and 10 positions had large dihedral
angles to the dihydrophenazine plane in both 1 and 1+ (A/C and
C/E in Figure 1-I). The dihedral angle D/F was a little smaller in
cation 1+. The bond lengths, a, a′, c, c′ were shortened for 1+,
whereas those of b, b′ were elongated. These changes in bond
lengths are in qualitative accord with the expectations from the
Figure 2-Ia,Ib shows the EPR spectrum for 1+ in butyronitrile
at 123 K and its simulation spectrum. The weak forbidden |∆ms|
) 2 signal was observed at low temperatures. The intensity of the
|∆ms| ) 2 signal linearly increased to the reciprocal temperature
9
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J. AM. CHEM. SOC. 2004, 126, 58-59
10.1021/ja0367748 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
B.; Veciana, J. In Magnetism: Molecules to Materials II; Miller, J. S.,
Drillon, M., Eds.; Wiley-VCH: New York, 2001; pp 1-60. For
ferrimagnetic approaches: (c) Hosokoshi, Y.; Katoh, K.; Nakazawa, Y.;
Nakano, H.; Inoue, K. J. Am. Chem. Soc. 2001, 123, 7921-7922. (d)
Shiomi, D.; Kanaya, T.; Sato, K.; Mito, M.; Takeda, K.; Takui, T. J. Am.
Chem. Soc. 2001, 123, 11823-11824. For CT approaches: (e) Breslow,
R. In Magnetic Properties of Organic Materials; Lahti, P. M., Ed.; Marcel
Dekker: New York, 1999; pp 27-60. (f) Sugimoto, T. In Molecular
Magnetism; Itoh, K., Kinoshita, M., Eds.; Kodansha or Gordon and
Breach: Tokyo, 2000; pp 223-239.
(2) (a) Yamaguchi, K.; Namimoto, H.; Fueno, T. Chem. Phys. Lett. 1990,
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3281-3284. (b) Sakurai H.; Izuoka A.; Sugawara T. J. Am. Chem. Soc.
2000, 122, 9723-9734.
Figure 2. EPR spectra (Ia,b) and temperature dependence of øpT of
1+‚ClO4-‚CH2Cl2 (II). (Ia) Measured at 123 K in a frozen butyronitrile
matrix with ν0 ) 9.251147 GHz. (Ib) The simulation spectrum with
parameters |D/hc| ) 0.00725 cm-1, |E/hc| ) 0.0011 cm-1, gxx ) 2.0065,
(4) Nakamura, Y.; Koga, N.; Iwamura, H. Chem. Lett. 1991, 69-72.
(5) (a) Nakazaki, J.; Ishikawa, Y.; Izuoka, A.; Sugawara, T.; Kawada, Y.
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M.; Izuoka, A.; Sugawara, T. Tetrahedron Lett. 1999, 40, 5027-5030.
(c) Kumai, R.; Matsushita, M. M.; Izuoka, A.; Sugawara, T. J. Am. Chem.
Soc. 1994, 116, 4523-4524. (d) Kumai, R.; Izuoka, A.; Sugawara, T.
Mol. Cryst. Liq. Cryst. 1993, 232, 151-154.
gyy ) 2.0020, gzz ) 2.0040, gav ) [(gxx + gyy +gzz2)/3]1/2 ) 2.004, (II)
Measured for a powder sample under B ) 0.1 T. The solid line represents
the simulation curve with J1/kB ) 700 K and J2/kB ) -18 K.
2
2
(6) (a) Sugimoto, T.; Yamaga, S.; Nakai, M.; Ohmori, K.; Tsuji, M.; Nakatsuji,
H.; Fujita, H.; Yamauchi, J. Chem. Lett. 1993, 1361-1364. (b) Sugimoto,
T.; Yamaga, S.; Nakai, M.; Tsuji, M.; Nakatsuji, H.; Hosoito, N. Chem.
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(6-50 K). The linearity indicates either the quasi-degeneracy of
singlet and triplet states, ∆EST/kB , 6 K, or a triplet ground state
with a large S-T gap, ∆EST/kB . 50 K.
A much clearer insight was obtained by measuring the magnetic
susceptibility of 1+ (Figure 2-II). The øpT value at room temperature
was 0.98 emu K mol-1, which is close to that expected for S ) 1
state (gav ) 2.004). The value decreased with lowering temperature,
which is ascribable to intermolecular magnetic interactions. The
temperature dependence of øpT clearly demonstrates a large S-T
(7) Nordio, P. L.; Soos, Z. G.; McConnell, H. M. Annu. ReV. Phys. Chem.
1966, 17, 237-260.
(8) Izuoka, A.; Hiraishi, M.; Abe, T.; Sugawara, T.; Sato, K.; Takui, T. J.
Am. Chem. Soc. 2000, 122, 3234-3235.
(9) Okamoto, T.; Terada, E.; Kozaki, M.; Uchida, M.; Kikukawa, S.; Okada,
K. Org. Lett. 2003, 5, 373-376.
(10) Cauquis, G.; Delhomme, H.; Serve, D. Tetrahedron Lett. 1971, 4649-
4652.
(11) Crystallographic data for 1: monoclinic, space group C2/c, a ) 37.16(1)
Å, b ) 6.127(2) Å, c ) 24.091(9) Å, â ) 111.392(7)°, V ) 5106.8(3)
Å3, Z ) 8, Fcalcd ) 1.273 g/cm3, T ) 113 K, R ) 0.070, Rw ) 0.088,
GOF ) 1.142, Crystallographic data for 1+‚ClO4-‚CH2Cl2, Triclinic, space
group P-1, a ) 10.282(1) Å, b ) 11.232(1) Å, c ) 15.539(2) Å, R )
80.70(1), â ) 77.67(1)°, γ ) 64.483(7), V ) 1577.2(3) Å3, Z ) 2, Fcalcd
) 1.419 g/cm3, T ) 113 K, R ) 0.045, Rw ) 0.065, GOF ) 0.988.
gap of ∆EST/kB . 300 K. The spin Hamiltonian, H ) -2J1(SNN1
‚
SDPhz1 + SNN2‚SDPhz2) - 2J2SDPhz1‚SDPhz2, was used to analyze the
øpT value on the basis of the molecular packing in the crystal (Figure
1-IIb). The øpT values were reproduced using parameters of J1/kB
g +700 K and J2/kB ) -18 ( 0.6 K (solid line in Figure 2-II).15
The large J1 is qualitatively understandable with the spin polariza-
tion mechanism via a large positive spin density of the C2-carbon
of dihydrophenazine radical cation.10,16 The intramolecular ferro-
magnetic interaction J1 falls within the same range as those of
general CT-type intermolecular magnetic interactions.7
(12) Caneschi, A.; Laugier, J.; Rey, P. J. Chem. Soc., Perkin Trans. 1 1987,
1077-1079.
(13) Two additional short contacts were observed between atoms bearing
smaller spin densities: One was between the neighboring dimers through
the O2 and the C3-attached hydrogen (2.85 Å), resulting in a dimer chain
in the [011] direction. The other was between the O2 atom and the C13-
attached hydrogen (2.41 Å) of another neighboring 1+, giving a 1+ chain
in the [010] direction.
To obtain theoretical insights into the exchange interaction in
this system, DFT calculations were performed using the Gaussian
98 program (UB3LYP/6-31G*).17 The geometry of the radical
cation was taken from X-ray structure analysis. For the calculation
of low-spin singlet state, the trial UHF wave function was generated
by the broken symmetry (BS) approach.18-20 The triplet state had
a lower energy than the BS-singlet state. The energy difference
between the triplet and the BS-singlet state was 1.98 kcal/mol,
which corresponds to J/kB ) +498 K or J/kB ) +987 K, depending
on the estimation methodologies.19,20
In sum, we have found that 1+ is a stable triplet species with
strong intramolecular ferromagnetic coupling. The following ad-
ditional findings are noteworthy: (1) the corresponding 5,10-
dimethyl derived radical cation was unstable and slowly decom-
posed even under inert atmosphere. (2) The radical cation of the
2,7-bis[(nitronyl nitroxide)-2-yl]-substituted 1+-analogue had a clean
EPR spectrum with a quartet pattern but was too unstable to isolate
in pure form. The details will be published elsewhere. Preparation
of charge-transfer complexes and related magnetic materials based
on 1+ is in progress.
(14) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
(15) The øpT values in the high-temperature region (>100 K) were reproducible
with J1/kB g +700 K. The unsatisfactory fitting of the low-temperature
region (<50 K) may be due to the other contacts described in ref 13 being
neglected. Trials with some accessible conventional models taking into
account weaker contacts, i.e. a two-spin singlet-triplet model with a mean
field (θ) or an S ) 1 chain model did not improve the fitting.
(16) Dihydrophenazine radical cation has nonalternant spin distribution, where
both the nitrogen and the C2-carbon have large positive spin densities.
See also Supporting Information.
(17) The total energy E (hartree) and S2 before spin projection were E )
-1567.36685314 with S2 ) 2.0685 for triplet state, E ) -1567.36369601
with S2 ) 1.0594 for the BS-singlet state using Gaussian 98, revision
A.9; Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M.
W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A.
Gaussian, Inc., Pittsburgh, PA, 1998.
(18) (a) Yamaguchi, K. Chem. Phys. Lett. 1975, 33, 330-335. (b) Noodleman,
L. J. Chem. Phys. 1981, 74, 5737-5743. (c) Adao, C.; Barone, V.; Bencini,
A.; Totti, F.; Ciofini, I. Inorg. Chem. 1999, 38, 1996-2004. (d) Okada,
K.; Nagao, O.; Mori, H.; Kozaki, M.; Shiomi, D.; Sato, K.; Takui, T.;
Kitagawa, Y.; Yamaguchi, K. Inorg. Chem. 2003, 42, 3221-3228.
Supporting Information Available: X-ray crystallographic files
(CIF) and a list of spin-densities (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
(19) Ruiz, E.; Cano, J.; Alvarez, S.; Alemany, P. J. Comput. Chem. 1999, 20,
1391-1400.
(20) (a) Yamanaka, S.; Kawakami, T.; Nagao, H.; Yamaguchi, K. Chem. Phys.
Lett. 1994, 231, 25-33. (b) Yamaguchi, K. Int. J. Quantum Chem. 2002,
90, 370-385.
References
(1) For general approaches: (a) Lahti, P. M.; Ed. Magnetic Properties of
Organic Materials; Marcel Dekker: New York, 1999. (b) Amabilino, D.
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