Tris[p-(N-oxyl-N-tert-butylamino)phenyl]amine, -methyl, and borane have doublet, triplet, and doublet ground states, respectively

Article abstract of DOI:10.1021/ja9920819

N,N,N-Tris[p-(N-oxyl-tert-butylamino)phenyl]amine (N) was obtained as dark violet plates by lithiation of tris(4-bromophenyl)amine, followed by reaction with 2-methyl-2-nitrosopropane and oxidation of the resulting tris(hydroxyamine) with Ag₂O.

Full text of DOI:10.1021/ja9920819

J. Am. Chem. Soc. 2000, 122, 2567-2576
2567
Tris[p-(N-oxyl-N-tert-butylamino)phenyl]amine, -methyl, and -borane
Have Doublet, Triplet, and Doublet Ground States, Respectively
Tetsuji Itoh,† Kenji Matsuda,† Hiizu Iwamura,*,†,§ and Kenzi Hori‡
Contribution from the Institute for Fundamental Research in Organic Chemistry, Kyushu UniVersity,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan and Department of Applied Chemistry and
Chemical Engineering, Faculty of Engineering, Yamaguchi UniVersity, Tokiwadai, Ube 755-8611, Japan
ReceiVed June 18, 1999
Abstract: N,N,N-Tris[p-(N-oxyl-tert-butylamino)phenyl]amine (N) was obtained as dark violet plates by
lithiation of tris(4-bromophenyl)amine, followed by reaction with 2-methyl-2-nitrosopropane and oxidation of
the resulting tris(hydroxyamine) with Ag2O. An X-ray crystal and molecular structure analysis of the monoclinic
single crystal with space group Cc showed that it has neither C3 symmetry nor zwitterionic quinonoid structure.
An EPR spectrum in MTHF solution at room temperature consisted of seven lines (aN ) 4.06 G (3N) at g )
2.0058). The temperature dependence of the magnetic susceptibility data on N in the range 2-300 K was
analyzed in terms of a triangular coupling model for three S ) 1/2 spins to give a set of best-fit parameters
which placed a doublet state as the ground state with a quartet state lying 559 K () 1.11 kcal mol-1) above
it. B3LYP/6-31G*//B3LYP/6-31G computations were performed on both spin states of a model molecule (N′)
in which all three N-tert-butyl groups in N were replaced with N-methyl groups. The doublet state was found
to be more stable than the quartet state by 0.84 kcal mol-1. Tris[p-(N-oxyl-tert-butylamino)phenyl]methyl (C)
gave a monoclinic single crystal with space group Cc (no. 9) and has been shown to have a benzoquinoneimine
N-oxide type diradical structure. Temperature dependence of its magnetic susceptibility showed that C has a
triplet ground state with a singlet state lying 410 K above the triplet. Reactions of the corresponding
organolithium compounds with BF3‚OEt2 were not applicable to give tris[4-(N-oxyl-tert-butylamino)phenyl]-
borane (B) but its 2,2′,2′′,6,6′,6′′-hexamethyl derivative (B′). In the trigonal crystal with space group R3h (no.
148), the three benzene rings are tilted by 49.2° with respect to the valence plane of the boron atom. The
magnetic susceptibility data on B′ were consistent with a doublet ground state with an energy gap to a quartet
state by 9.9 K () 0.0197 kcal mol-1). All the results together with ab initio MO studies are consistent with
a picture that the ground spin states of title compounds N, C, and B are doublet, triplet, and doublet, respectively.
The electronic structures are explained qualitatively by p-(N-oxyl-tert-butylamino)phenyl homologues of
(•H2C)3X in which X ) N, C‚, and B atoms, and two, one, and no π-electrons are contained, respectively.
Introduction
bis- and quartet tris-aminoxyl radicals formed complexes in
which spins ordered in the range 3.4-46 K.3 New polyradicals
having aminoxyl radicals arranged trigonally in cross-conjuga-
tion are of importance in constructing high TC 2p-3d hybrid
spin magnets.4 To increase basicity of the aminoxyl oxygen
Studies of N,N,N-tris[p-(N-oxyl-N-tert-butylamino)phenyl]-
amine, -methyl, and -borane (N, C, and B, respectively) have
2-fold significance. First, organic free radicals having two or
more basic coordinating centers serve as bridging ligands for
magnetic metal ions to make hybrid-spin chains or networks
consisting of organic 2p and metal 3d spins by self-assembly.1-3
Thus bis(hexafluoroacetylacetonato)manganese(II) with triplet
(2) (a) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Acc. Chem. Res.
1989, 22, 392. (b) Caneschi, A.; Gatteschi, D.; Rey, P. Prog. Inorg. Chem.
1991, 39, 3936. (c) Burdukov, A. B.; Ovcharenko, V. I.; Ikorski, V. N.;
Pervukhina, N. V.; Podberezskaya, N. V.; Grigor’ev, I. A.; Larionov, S.
V.; Volodarsky, L. B. Inorg. Chem. 1991, 30, 972. (d) Caneschi, A.; Chiesi,
P.; David, L.; Ferraro, F.; Gatteschi, D.; Sessoli, R. Inorg. Chem. 1993,
32, 1445. (e) Stumpf, H. O.; Ouahab, L.; Pei, Y.; Grandjean, D.; Kahn, O.
Science 1993, 261, 447. (f) Volodarsky, L. B.; Reznikov, V. A.; Ovcharenko,
V. I. Synthetic Chemistry of Stable Nitroxides; CRC Press: Boca Raton,
1994; Chapter 4.
(3) (a) Inoue, K.; Iwamura, H. J. Am. Chem. Soc. 1994, 116, 3173. (b)
Inoue, K.; Iwamura. H. J. Chem. Soc., Chem. Commun. 1994, 2273. (c)
Mitsumori, T.; Inoue, K.; Koga, N.; Iwamura. H. J. Am. Chem. Soc. 1995,
117, 2467. (d) Inoue, K.; Hayamizu, T.; Iwamura, H. Chem. Lett. 1995,
745. (e) Inoue, K.; Hayamizu, T.; Iwamura, H.; Hashizume, D.; Ohashi, Y.
J. Am. Chem. Soc. 1996, 118, 1803. (f) Iwamura, H.; Inoue, K.; Hayamizu,
T. Pure Appl. Chem. 1996, 68, 243. (g) Inoue, K.; Iwamura. H. AdV. Mater.
1996, 8, 73. (h) Markosyan, A. S.; Hayamizu, T.; Iwamura, H.; Inoue, K.
J. Phys.: Condens. Mater. 1998, 10, 2323. (i) Iwamura, H.; Inoue, K.;
Koga, N. New J. Chem. 1998, 201. (j) Inoue, K.; Iwahori, F.; Iwamura, H.
Chem. Lett. 1998, 737. (k) Uchiyama, R. Iwamura, H. To be published
elsewhere.
† IFOC, Kyushu University.
‡ Yamaguchi University.
§ Present address: National Institution for Academic Degrees, 4259
Nagatsuta-cho, Midori-ku, Yokohama, 226-0026, Japan.
(1) (a) Miller, J. S.; Epstein, A. J.; Reiff, W. M. Chem. ReV. 1988, 88,
201. (b) Miller, J. S.; Dougherty, D. A., Eds. Ferromagnetic and High Spin
Molecular Based Materials. Mol. Cryst. Liq. Cryst. 1989, 176. (c) Gatteschi,
D.; Kahn, O.; Miller, J. S.; Palacio, F., Eds. Magnetic Molecular Materials,
NATO ARI Series E.; Kluwer Academic Publishers: Dordtrecht, 1991;
E198. (d) Iwamura, H.; Miller, J. S. Eds. Chemistry and Physics of
Molecular Based Magnetic Materials. Mol. Cryst. Liq. Cryst. 1993, 232
and 233. (e) Kahn, O. Molecular Magnetism; VCH Publishers: Weinheim,
1993. (f) Gatteschi, D. AdV. Mater. 1994, 6, 635. (g) Miller, J. S.; Epstein,
A. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 385. (h) Miller, J. S.; Epstein,
A. J. Chem. Eng. News, October, 1995, 2, 30. (i) Kahn, O. Magnetism: A
Supramolecular Function, NATO ASI Series C; Kluwer: Dordrecht, 1996.
(j) Turnbull, M. M.; Sugimoto, T.; Thompson, L. K. Eds. Molecule-based
magnetic materials; ACS Symposium Series 644; American Chemical
Society: Washington, DC, 1996. (k) Gatteschi, D. Curr. Opin. Solid State
Mater. Sci. 1996, 1, 192.
(4) Oniciu, D. C.; Matsuda, K.; Iwamura, H. J. Chem. Soc., Perkin Trans.
2 1996, 907.
10.1021/ja9920819 CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/04/2000

2568 J. Am. Chem. Soc., Vol. 122, No. 11, 2000
Itoh et al.
Scheme 1a
-
Figure 1. Ground-state electron configuration in the basic structural
units of N, C, and B.
atoms serving as ligating sites, it was deemed worthwhile to
synthesize tris[p-(N-oxyl-tert-butylamino)phenyl]amine (N).5
a Reagents and conditions: (a) NaNO2, concd H2SO4, CuBr, aq HBr,
53%; (b) Mg, 1,1-dimethylnitrosoethane, Et2O, 31%; (c) tert-butyldi-
methylchlorosilane, imidazole, DMF, 97%; (d) n-BuLi, Et2O, BF3‚Et2O,
13%; (e) AcOH, THF, 54% (f) Ag2O, CH2Cl2, 98%.
Results and Analyses
Synthesis. 1. Triradical N. Tris(4-bromophenyl)amine was
lithiated with excess tert-butyllithium and treated with 2-methyl-
2-nitrosopropane to give the corresponding tris(hydroxyamine).
Triradical N was obtained by oxidation of this hydroxyamine
derivative with Ag2O in CH2Cl2.5
2. Diradical C. Diradical C was prepared as described
previously.4
3. Tris[2,6-dimethyl-4-(N-oxyl-tert-butylamino)phenyl]bo-
rane B′. O-Silylation of N-(p-bromophenyl)-N-tert-butylhy-
droxyamine was followed by lithiation with tert-butyllithium.
The organolithium compound thus obtained was allowed to react
with boron trifluoride etherate to give a complex mixture in
which the expected triarylborane derivative was not found.
Therefore, our recourse was to prepare its 2,2′,2′′,6,6′,6′′-
hexamethyl derivative B′. The synthesis outlined in Scheme 1
started from commercially available 4-bromo-2,6-dimethyl-
aniline which was converted to 1. The Grignard reagent prepared
from 1 and 1.0 equiv of magnesium was coupled with 2-methyl-
2-nitrosopropane to give 2. After protection of the hydroxyamino
group with a tert-butyldimethylsilyl group in quantitative yield,
3 was lithiated with tert-BuLi and coupled with 1/3 equiv of
BF3‚Et2O to give triarylborane 4. B′ was obtained by means of
the desilylation of 4 with acetic acid in THF followed by the
oxidation with Ag2O in dichloromethane.
Second, simplified models of B, C, and N by truncation of
three phenylene units and replacement of the N-tert-butylami-
noxyls with •H2C radicals may be regarded as trimethylene
derivatives (•H2C)3X (X ) B, C•, and N) (Figure 1).6 The
carbon analogue is triplet trimethylenemethane (TMM). Since
the corresponding amine has one more π-electron than the
methane, the former is expected to become isoelectronic with
the anion radical of the latter and therefore a doublet mono-
radical. The π-electron is one less in borane analogue B which
corresponds to the cation radical of TMM. It is of great interest
to see if such a simplified picture is applicable to the
understanding of the ground-state electron configuration of N,
C, and B.
X-ray Crystal and Molecular Structure. Crystallographic
data and experimental parameters for radicals N, C, and B′ are
summarized in Table 1.
1. Triradical N.5,7 A single crystal of N for an X-ray
diffraction measurement was obtained by slow evaporation of
a n-heptane-dichloromethane solution of N. The molecular
structure of N is reproduced in Figure 2. The molecule has no
(5) A preliminary account of this paper appeared in Itoh, T.; Matsuda,
K.; Iwamura, H. Angew. Chem., Int. Ed. Engl. 1999, 38, 1791.
(6) (a) Longuet-Higgins, H. C. J. Chem. Phys. 1950, 18, 265. (b)
Greenwood, H. H. Trans. Faraday Soc. 1952 48, 677. (c) Crawford, R. J.;
Cameron, D. M. J. Am. Chem. Soc. 1966, 88, 2589. (d) Yarkony, D. R.;
Schaefer, H. F., III. J. Am. Chem. Soc. 1974, 96, 3754. (e) Feller, D.; Borden,
W. T.; Davidson, E. J. Chem. Phys. 1981, 74, 2256.
(7) The crystallographic data (excluding structure factors) for the structure
have been deposited with the Cambridge Crystallographic Data Center as
supplementary publication no. CCDC-101263. Copies of the data can be
obtained free of charge on application to The Director, CCDC, 12, Union
Rode, GB- Cambridge CB2 1EZ, UK (fax: int. code +(44)1233-336-033;
e-mail: deposit@ccdc.cam.ac.uk).

Tris[p-(N-oxyl-N-tert-butylamino)phenyl]amine
J. Am. Chem. Soc., Vol. 122, No. 11, 2000 2569
Table 1. Crystallographic Parameters for N, C, and B′
Table 2. Comparison of the Bond Lengths (Å) and Angles (deg)
in Na)
N
C
B′
ring
a
A
B
C
formula
fw
crystal color
crystal description plate
crystal system
space group
a/Å
b/Å
C30H39N4O3
503.66
dark violet
C31H39N3O3
501.68
reddish purple reddish brown
C36H51B1N3O3
584.63
1.412
(1.425)
[1.427]
1.400
(1.410)
[1.409]
1.380
(1.391)
[1.393]
1.405
(1.411)
[1.409]
1.417
1.421
(1.426)
1.428
(1.427)
prismatic
monoclinic
Cc (no. 9)
13.35(1)
23.542(4)
10.134(2)
117.72(1)
2820(1)
4
prismatic
trigonal
R3h (no. 148)
19.777(2)
bb
cb
db
e
1.393
(1.409)
1.400
(1.409)
monoclinic
Cc (no. 9)
13.11(1)
23.081(5)
10.410(2)
116.80(5)
2811(2)
4
1.190
2874
2461
1.383
(1.391)
1.388
(1.391)
c/Å
16.393(3)
â/deg
V/Å3
Z
1.403
(1.410)
1.400
(1.410)
5552.9(7)
8
1.190
2630
1519
D
calcd/(g cm-3
)
1.153
1923
1597
1.414
(1.407)
1.413
(1.407)
no. of reflections
obsd data
(1.405)
[1.409]
1.291
(1.323)
[1.322]
(I > 3.00σ(I)) (I > 1.50σ(I)) (I > 3.00σ(I))
0.039
0.054
R
Rw
0.058
0.055
0.050
0.072
f
1.285
(1.322)
1.294
(1.322)
φ
29.9
34.3
57.3
(23.9)
[33.6]
(26.8)
[36.6]
(46.7)
[45.7]
a The bond lengths and angles for ab initio calculated doublet and
quartet states are in () and [ ], respectively. In the last case near C3
symmetry does not reveal meaningful difference in bond lengths among
the rings. b Average of two nearly equivalent bond lengths on the same
ring.
2. Diradical C. A single crystal of C for an X-ray diffraction
measurement was obtained by recrystallization from Et2O at
-30 °C. The crystal structure of C is reproduced in Figure 3.
The molecule has no 3-fold axis passing through the central
carbon atom. Bond alternation was clearly found on two
p-phenylene rings B and C: 1.417 Å vs 1.364 Å on an average,
while ring A does not have such a tendency (1.396 Å vs 1.388
Å on an average) (see Table 3). The bond connecting the
aminoxyl nitrogen atom with the benzene ring is normal 1.429
Å for ring A, but shorter (1.415 and 1.382 Å) for rings B and
C.9 Rings B and C are more coplanar with the plane defined by
the C2C12C22 plane (27° and 24°, respectively) and the dihedral
angles of aminoxyl moieties with the phenylene rings to which
Figure 2. ORTEP drawing of N with thermal ellipsoid plot at 50%
probability level. The hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): N4-C1 1.412(4), N4-C11 1.421-
(4), N4-C21 1.428(4), N1-O1 1.291(3), N2-O2 1.285(4) N3-O3
1.294(3), N1-C4 1.417(4), N2-C14 1.414(4), N3-C24 1.413(4), C1-
N4-C11 122.1(2), C1-N4-C21 118.9(2), C11-N4-C21 118.7(2),
O1-N1-C4 116.9(2), O2-N2-C14 116.9(3), O3-N3-C24 116.6-
(2).
3-fold axis passing through the central nitrogen atom. Intramo-
lecular distances between the oxygen atoms of the aminoxyl
groups are 9.73 Å (O1-O2), 11.30 Å (O2-O3), and 11.41 Å
(O3-O1), whereas the shortest intermolecular distance between
O1 and O2′ of neighboring molecules is 4.84 Å.8 N4 is located
only 0.04 Å out of the C1C11C21 plane from which the three
p-phenylene rings labeled A, B, and C are tilted by 29.9°, 34.3°,
and 57.3°, respectively. The C-C bond lengths of the p-
phenylene rings are not very much indicative of a potential
contribution of quinonoid structures. The bonds connecting the
aminoxyl nitrogen atom with the benzene ring are in the range
1.41-1.42 Å and only slightly shorter than 1.43-1.45 Å in
typical phenylaminoxyls9 (Table 2).
(8) Symmetry operation: x, -y, 1/2 + z.
(9) (a) Kanno, F.; Inoue, K.; Koga, N.; Iwamura, H. J. Am. Chem. Soc.
1993, 115, 847. (b) Kanno, F.; Inoue, K.; Koga, N.; Iwamura, J. Phys.
Chem. 1993, 97, 13267. (c) Hanson, A. W. Acta Crystallogr. 1953, 6, 32.
Figure 3. ORTEP drawing of C with a thermal ellipsoid plot at a
50% probability level. The hydrogen atoms are omitted for clarity.

2570 J. Am. Chem. Soc., Vol. 122, No. 11, 2000
Itoh et al.
Table 3. Comparison of Bond Length (Å) in C
ring
A
B
C
a
1.492
1.402
1.388
1.392
1.283
1.429
1.427
1.427
1.366
1.418
1.296
1.415
1.425
1.419
1.363
1.406
1.293
1.382
ba
ca
da
f
e
a
Bond lengths are averaged over assumed symmetry.
Scheme 2. Quinonoidal Resonance Structures Extending
over Two Rings B and C in C at Least in the Solid State
Figure 4. ORTEP drawing of B′ with thermal ellipsoid plot at 50%
probability level. The hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): B1-C1 1.573(2), C1-C2 1.417-
(3), C2-C3 1.383(3), C3-C4 1.387(3), C4-C5 1.388(3), C5-C6
1.377(3), C6-C1 1.417(3), C2-C7 1.508(3), C6-C8 1.516(3), C4-
N1 1.416(3), N1-O1 1.266(2), B1-C1-C2 121.0(2), B1-C1-C6
121.9(2), C1-C2-C3 120.6(2), C2-C3-C4 121.5(2), C3-C4-C5
118.3(2), C4-C5-C6 121.5(2), C5-C6-C1 120.8(2), C6-C1-C2
117.1(2), O1-N1-C4 117.3(2), N1-C4-C3 118.0(2), N1-C4-C5
123.6(2).
they are attached are 26.9° (ring A), 2.8° (ring B) and 9.3° (ring
C). It is suggested that, whereas ring A is tilted by 50° and
there is no bond alternation detected on this ring, the bis-
(aminoxyl)methylenecyclohexa-2,5-dienone N-tert-butylimine
N-oxide structure is in resonance between rings B and C in
crystals (Scheme 2).
3. Triradical B′. A single crystal of B′ for an X-ray
diffraction measurement was obtained by recrystallization from
n-hexane/CH2Cl2 (2:1) at -30 °C. The molecular structure of
B′ is reproduced in Figure 4. In contrast with the above two
radicals, triradical B′ has a crystallographic 3-fold symmetry
passing through the central boron atom, resulting in the three
aminoxyl moieties becoming equivalent in the crystalline state.
The aryl groups are tilted by 49.2° with respect to the borane
valence plane. The shortest intermolecular distance between the
aminoxyl oxygen atoms of neighboring molecules is 3.99 Å
(Figure 5).
determine rigorously whether the ground state is quartet, doublet,
or degenerate quartet/doublet states.
2. Diradical C. The EPR spectrum of biradical C in degassed
MTHF solution at room temperature exhibits a well-resolved
complex pattern, dominated by the presence of different
conformers and their hyperfine splitting.4
3. Triradical B′. The EPR spectra of triradical B′ centering
at g ) 2.0058 consisted of a septet (|aN| ) 4.11 G) due to
hyperfine coupling with the three equivalent nitrogen nuclei.
In a frozen solution, B′ showed a large main peak with a very
weak outer shoulder and no signal arising from the ∆ms ) 2 or
3 transitions was observed.
EPR Spectra in Fluid and Solid Solutions. 1. Triradical
N. X-band EPR spectra of N in degassed 2-methyltetrahydro-
furan (MTHF) solution were obtained in the temperature range
8.5-300 K. At 300 K the EPR spectrum centering at g ) 2.006
consisted of a septet in a ratio of 1:3:6:7:6:3:1 due to hyperfine
coupling with the three equivalent nitrogen nuclei (Figure 6),
suggesting that the exchange interaction is much larger than
the hyperfine interaction (|J| . |aN|). The spectrum was
simulated by an isotropic hyperfine couplings with three
equivalent nitrogen atoms (aN ) 4.06 G), six equivalent meta-
hydrogens (aH ) 0.8 G), six equivalent ortho-hydrogens (aH )
0.4 G), and a nitrogen atom (aN ) 0.5 G).
In a frozen solution, N showed a strong main peak with a
pair of weak outer shoulders separated by 20.3 G in the ∆ms )
1 region. Most of the former is due to a doublet species. The
latter was simulated by assuming a dipole-dipole interaction
with |D/hc| ) 0.0019 cm-1 and |E/hc| ) 0. The D value
corresponds to a mean distance of 11.2 Å for the unpaired
electrons.10 No signal arising from the ∆ms ) 2 or 3 transitions
were detected. The intensity of the main peak decreased as the
temperature was increased in the range 8.5-100 K in good
agreement with a Curie law. This result is not decisive to
Molar Paramagnetic Susceptibility. 1. Triradical N. The
magnetic susceptibility data of N were acquired on a SQUID
susceptometer/magnetometer. The temperature dependence of
the molar paramagnetic susceptibility (ømol) at 5000 G is given
as a ømolT vs T plot in Figure 7. At 300 K, the observed ømolT
value amounts to 0.695 emu K mol-1, a value considerably
smaller than the theoretical one of 1.125 emu K mol-1 for a
triradical having three independent spins, implying that the
coupling favoring the antiparallel alignment of the spins is
operative even at 300 K. The ømolT value decreased linearly
(theoretically speaking it should be a part of a sigmoidal curve)
as the temperature was lowered until it remained almost constant
at ca. 0.375 emu K mol-1 from 100 down to 20 K. This constant
value is typical of a paramagnetic species having single isolated
spin; N behaves as if it had only one spin per molecule under
these conditions. As the temperature was decreased below 20
K, the ømolT values increased to reach a value of 0.58 emu K
mol-1 at 2 K.
(10) (a) Lakcharst, G. R.; Rosantsev, E. G. IzV. Akad. Nauk SSSR Ser.
Khim. 1968, 8, 1708. (b) Veciana, J.; Rovira, C.; Ventosa, N.; Crespo, M.
I.; Palacio, F. J. Am. Chem. Soc. 1993, 115, 57. (c) Rajca, A.; Utamapanya,
S. J. Am. Chem. Soc. 1993, 115, 2396.

Tris[p-(N-oxyl-N-tert-butylamino)phenyl]amine
J. Am. Chem. Soc., Vol. 122, No. 11, 2000 2571
Figure 5. (a) Projection along the c-axis of B′. The methyl groups are omitted for clarity. (b) Crystal structures of B′ in which the nearest molecules
are shown. Broken lines indicate the short contacts between the neighboring molecules: O1-H1* 2.66 Å, O1-O1* 3.99 Å, O1-H5* 2.45 Å.
Figure 7. Temperature dependence of ømolT of triradical N at a constant
field of 5000 Oe. The solid curve is the theoretical one described in
text.
the interaction between the adjacent molecules, that there is no
intramolecular magnetic interaction populating the quartet state
at the cryogenic temperature, and that the ground state must be
a doublet state.6 The doublet-quartet energy gap is estimated to
be greater than 300 K (>0.6 kcal mol-1). The origin of the
ferromagnetic interaction between the neighboring molecules
in crystals may be located at contacts O1-C12′ (3.272(4) Å)
and O3-C25′ (3.131(3) Å) along the chain made by the
molecules aligned in the direction of the c-axis.
The magnetic interaction in a nonsymmetric triangular
triradical system can be written by spin Hamiltonian of eq 1,
Figure 6. X-band EPR spectra of N in 10 mM MTHF solution (a) at
room temperature (9.327 GHz) and (b) 8.5 K (9.330 GHz).
where Jij is the exchange coupling parameter between Si and
Sj.11 The molar susceptibility is given by eq 2, where ∆1 and
A further proof of the ground doublet state in N was furnished
by similar magnetic susceptibility measurements on a dilute
(11) In principle the spin Hamiltonian should contain terms due to the
sample (5%) of N in poly(vinyl chloride) in which possible
intermolecular coupling would be considerably diminished.
Under these conditions, the ømolT values were flat below 20 K,
and the continuous increase observed for the crystalline sample
was not detected. It was confirmed that the increase is due to
Zeeman, dipole-dipole, and hyperfine interactions in addition to the
exchange term. They are estimated to be 0.7 K (1 K ) 0.695 cm-1) at a
field of 5000 G, 1.0 × 10-2 K for two spins at a distance of 5 Å, and 1.3
× 10-3 K for a ) 100 G, respectively. Therefore these terms may be
neglected in the present discussion of the exchange interaction as strong as
10-500 K (|J| . |aN|).

2572 J. Am. Chem. Soc., Vol. 122, No. 11, 2000
Itoh et al.
Figure 8. Energy level diagram for the two spin doublets and the spin
quartet originating from the interaction of S1 ) S2 ) S3 ) 1/2.
Figure 9. Temperature dependence of ømolT of biradical C at a constant
field of 5000 Oe. The solid curve is the theoretical one described in
the text.
∆2 are given by eqs 3 and 4 which are energy differences defined
as in Figure 8.12
H ) -2(J12S1S2 + J23S2S3 + J31S3S1)
(1)
ø
molT )
Ng2µB 1 + exp(-∆1/kBT) + 10 exp(-∆2/kBT)
2
T
f
‚
‚
4kB
1 + exp(-∆1/kBT) + 2 exp(-∆2/kBT) (T - θ)
(2)
∆1 ) 2(J122 + J232 + J312 - J12J23 - J23J31 - J31J12)1/2 (3)
∆2 ) ∆1/2 - (J12 + J23 + J31) (4)
Figure 10. Temperature dependence of ømolT of triradical B′ in neat
crystals (∆) and 6% PVC film (O) at a constant field of 5000 Oe. The
solid curve is the theoretical one described in text.
A purity factor f was introduced to account for all the
experimental errors of weighing a sample, calibration of a
SQUID susceptometer, etc., in addition to the paramagnetic
purity of a radical sample. Any interradical interaction is taken
into account by a mean-field theory.
as large as those of rings A and B, the aminoxyl radical on ring
C may be regarded as isolated from the rest: J23 ) J31 ) 0.
Then J ) J12 ) -270 ( 3 K from eqs 3 and 4. If J12 ) 0 for
some reasons, then J23 ) J31 ) -171 ( 2 K.
When eq 2 is used for the fitting of the temperature
dependence of the magnetic susceptibility on N in the range
2-300 K, three Jij parameters are required. It is apparent that
the three Jij parameters must be highly correlated, and there
are too many parameters to determine three kinds of Js uniquely
from the fitting of eq 2 to the experimental temperature
dependence of ømolT data by a least-squares fitting procedure.
Therefore, we decided to fit the expression of the magnetic
susceptibility as a function of ∆1 and ∆2 to the experimental
data. The best-fit parameters by means of a least-squares method
were ∆1 ) 1015 ( 444 K, ∆2 ) 559 ( 9 K, θ ) 0.75 ( 0.02
K and f ) 0.958 ( 0.004.
2. Diradical C. Our new measurements in the present study
on crystalline samples of C gave ømolT vs T plots as shown in
Figure 9. The ømolT value is already 0.80 emu K mol-1 at 300
K, suggesting that the ferromagnetic coupling making the two
spins to align in parallel between the two unpaired electrons is
in the order ∼300 K. As the temperature is lowered, the ømolT
value increased steadily to reach a maximum of 0.83 emu K
mol-1 at around 150 K and then decreased continuously.
Previously, a limited quality of C restricted us from detecting
the steady increase of ømolT values with decreasing temperature.4
Application of Bleaney-Bowers-type equation (eq 7)13 for an
equilibrated singlet-triplet model, with Weiss constant θ for
an antiferromagnetic mean molecular field, gave the best fit
parameters: 2J/kB ) 410 ( 10 K, f ) 0.893 ( 0.002, θ )
7.375 ( 0.06 K.
Since the eigenvalues for eq 1 is given by:
E(3/2) ) -1/2(J12 + J23 + J31)
E(1/2) ) 1/2(J12 + J23 + J31) (
(5)
(J122 + J232 + J312 - J12J23 - J23J31 - J31J12) (6)
2Ng2µB
2
1
T
ø
molT ) f
‚
‚
(7)
k
3 + exp(-2J/kBT) (T - θ)
the ground doublet state is concluded to lie below an excited
quartet state by ∆2 ) 559 ( 9 K () 1.11 kcal mol-1) (see
Figure 8). A large standard deviation in ∆1 makes it unrealistic
to determine the exchange coupling parameters Js. It was only
by making the following assumptions when numerical values
for J were obtained. (1) For a regular triangle which is applicable
to a triradical having a 3-fold symmetry axis,9b J ) J12 ) J23
) J31 in eqs 3 and 4, and J/kB ) - 135 ( 3 K. Since ∆1 ) 0,
the ground doublet state is doubly degenerate.
(2) For isosceles triangles, J12 ) J31 * J23 in eqs 3 and 4,
there are several possibilities and no unique set of J parameters
was obtained.9b Since the torsion angle of ring C with respect
to the central nitrogen-aromatic carbon bond is almost twice
3. Triradical B′. The ømolT value of a microcrystalline sample
of B′ was 1.07 emu K mol-1 at 300 K, a value close to a
theoretical 1.125 emu K mol-1 for three isolated spins. As the
temperature is lowered, the ømolT value started to decrease
steadily at ca. 200 K; the magnetic susceptibility is dominated
by antiferromagnetic interactions. Since it is not clear whether
the antiferromagnetic interaction is intra- or interradical mol-
ecules, another measurement was carried out for a film sample
made by casting a 6% solution of B′ in poly(vinyl chloride).
Under these conditions, the continuous decrease in ømolT in the
range 300-50 K was absent (Figure 10). To understand the
magnetic interactions more quantitatively, a regular triangle
(12) (a) Bencini, A.; Benelli, C.; Dei, A.; Gatteschi, D. Inorg. Chem.
1985, 24, 695. (b) Reference 1a, pp 211-228.
(13) Bleany, B.; Bowers, K. D. Proc. R. Soc. London 1952, A214, 451.

Tris[p-(N-oxyl-N-tert-butylamino)phenyl]amine
J. Am. Chem. Soc., Vol. 122, No. 11, 2000 2573
three-spin model suggested by X-ray crystal structure analysis
of B′ was applied to ømolT plot. By assuming that the remaining
decrease in ømolT values at temperatures below ca. 50 K is due
to intraradical coupling, the ømolT vs T plots were analyzed on
the basis of a regular triangle coupling: the spin Hamiltonian
given by eq 1 where J ) J12 ) J23 ) J31. The temperature
dependence of the molar susceptibility is given by eq 6, where
all symbols have their usual meaning. The best fit to the
observed points was reached by 3J/kB ) -9.9 K ( 0.3 and f )
0.956 ( 0.004 and θ ) 0.
with the carbon analogue C where two p-phenylene rings exhibit
quinonoidal bond alternation by 0.05 Å, any bond alternation
is less than 0.02 Å in the rings of N. The dihedral angles between
the benzene rings and the central trigonal nitrogen plane are
somewhat larger for a quinonoid structure. Triradical N exhibits
a broad absorption band at 450-800 nm (λmax 534 nm, ꢀ )
5500). Although its absorption coefficient is not very strong
compared with that of quinonoid system C, the long-wavelength
absorptions may be due to weak CT transitions from the amine
nitrogen to the aminoxyl groups.
2. Diradical C. The X-ray crystal structure shows that two
p-phenylene rings exhibit bond alternation by 0.05 Å, suggesting
a contribution of the quinonoid resonance structure as shown
in Scheme 2. This resonance structure corresponds to a
p-phenylene-extended nitronyl nitroxide structure. A strong
absorption characteristic of a quinonoid structure is observed
at 554 nm, ꢀ ) 20400.
3. Triradical B′. The X-ray analysis demonstrates that the
molecule lies on a crystallographic 3-fold axis, and therefore
the boron and the three carbon atoms bonded to it are coplanar.
The aryl groups are tilted by 49.2° with respect to the borane
valence plane. The torsion angle is almost the same as that found
in the trimesitylborane (51.1° and 49.6°).15 In B, which does
not have the six aryl methyl groups, the propeller shape must
have been a bit flatter as in triphenylborane (35° and 28°).16
Magnitude of the Intramolecular exchange couplings in
N, C and B′. 1. Antiferromagnetic Coupling in Triradical
N. In stark contrast to the ferromagnetic coupling in C, the
corresponding amine N has now been found to have antiferro-
magnetic coupling. As a result, the ground state is a doublet
with a quartet state lying 559 K above the ground state. There
are too many parameters (Js) to determine for applying a general
triangular coupling model to the experimental temperature
dependence of paramagnetic susceptibility. If a regular triangle
model is applied, J/kB ) - 135 K. Note that this antiferro-
magntic coupling is almost comparable in magnitude to the
ferromagnetic coupling of J/kB ) 205 K in C. A superexchange
mechanism has been evoked in organic di- and tricarbenes,17
but there are no example of organic free radicals in which such
strong interaction as in N has been determined quantitatively.
2Ng2µB 1 + 5 exp(-3J/kBT)
2
T
ø
molT ) f
‚
‚
(8)
4kB
1 + exp(-3J/kBT) (T - θ)
Ground doublet states which are doubly degenerate are thus
concluded to be more stable than a quartet state by 3J/kB ) 9.9
K.
Ab Initio Molecular Orbital Studies on N. To delineate
the electronic structures and obtain an estimate on the energy
gap between doublet and quartet states of N, ab initio MO
studies based on the density functional theory (DFT) were
carried out. First, the molecule of N was simplified to N′ by
replacing all the N-tert-butyl with N-methyl groups. All the
geometric parameters were optimized for N′ by taking electron
correlation into account at the B3LYP/6-31G level of theory
on a Gaussian94 program.14 The optimized structures for which
the pertinent bond lengths and angles are given in Table 2 and
the Supporting Information have no 3-fold axis. The torsion
angles of the three rings with respect to the plane defined by
the three quaternary ring carbons attached to the central nitrogen
atom are 23.9°, 26.8°, and 46.7° for the doublet and 33.6°, 36.6°,
and 45.7° for the quartet. The latter is more closer to a C3
structure. The computed bond lengths and angles agree with
those observed for the doublet state of N. For these optimized
structures, B3LYP/6-31G* computations were performed to
obtain the DFT energy values of -1257.34292593 and
-1257.34162401 Hartree for the doublet and quartet states of
N′, respectively. The former is more stable than the latter by
0.84 kcal mol-1 () 423 K).
Whereas the central amine nitrogen atom does not carry high
net spin density in both states (the Mulliken spin density of
0.0093 and 0.0314 for the doublet and quartet, respectively), it
has high R and â spin densities in the 100th magnetic orbital
and the 99th doubly occupied MO, respectively, in the ground
doublet state, demonstrating the operation of a superexchange
mechanism through the lone pair of electrons on the central
nitrogen atom, with two spins on the aminoxyl radical centers
coupled antiferromagnetically, with one spin remaining intact.
The superexchange interaction is a concept introduced in
coordination chemistry to explain the interaction between two
magnetic metal ions through a bridging diamagnetic atom, e.g.,
in M-O-M. One of the most important factors contributing
to the interaction is a charge transfer from the diamagnetic
bridging atom to the metal ions. Depending on the symmetry
of the magnetic orbitals at the metal ions, the interaction
becomes antiferro- or ferromagnetic. It is very often the case
indeed to detect the LMCT band for such systems. Spin
polarization does not accompany a charge transfer as the
difference in energy of the electron repulsion between R vs R
and R vs â is concerned.
Discussion
Molecular Structures of N, C, and B′. 1. Triradical N. It
was difficult to show from the X-ray crystal structure that the
p-phenylene rings have a tendency to assume the quinonoid
structure, and there are signs of shortened bond lengths for the
bonds connecting the aminoxyl nitrogen atoms with the benzene
rings (1.43-1.45 Å is typical of phenylaminoxyls).9 Compared
Triradical N may be regarded as an organic counterpart of
bridged trinuclear metal complexes exhibiting the superexchange
(15) Blount, J. F.; Finocchiaro, P.; Gust, D.; Mislow, K. J. Am. Chem.
Soc. 1973, 95, 7019.
(16) Zettler, F.; Hausen, H. D.; Hess, H. J. Organomet. Chem. 1974,
72, 157.
(14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1992.
(17) (a) Nakamura, T.; Momose, T.; Shida, T.; Sato, K.; Nakazawa, S.;
Kinoshita, T.; Takui, T.; Itoh, K.; Okuno, T.; Izuoka, A.; Sugawara, T. J.
Am. Chem. Soc. 1996, 118, 8684. (b) Matsuda, K.; Yamagata, T.; Seta, T.;
Iwamura, H.; Hori, K. J. Am. Chem. Soc. 1997, 119, 8058.
(18) (a) Beckett, R.; Colton, R.; Hoskins, F. F.; Martin, R. L.; Vince, D.
G. Aust. J. Chem. 1969, 22, 2527. (b) Reference 1e, pp 211-228. (c)
McCusker, J. K.; Schmitt, E. A.; Hendrickson, D. N. In ref 1c, pp 297-
319.

2574 J. Am. Chem. Soc., Vol. 122, No. 11, 2000
Itoh et al.
interaction,18 e.g., [Cu3(pao)3OH]18a and µ3-oxo-bridged tri-
nuclear Fe(III) acetate complexes.18c The interaction between
any two radical centers through the nitrogen atom carrying a
lone pair of electrons is antiferromagnetic leaving the remaining
one electron spin intact. The spin states are typically frustrated
in that the ground state is degenerate. In crystals, however, the
molecular structure is frozen to one of the tautomeric structure
due probably to the crystal field.
2. Strongly Ferromagnetic Coupling in Diradical C.
Diradical C has a ground triplet state with a singlet state lying
by 2J/kB ) 410 K above the ground state. Matsumoto et al.
synthesized 1,1-bis[p-(N-tert-butyl-N-hydroxyamino)phenyl]-2-
methylpropene 6 similar to C and showed that 6 had a small
exchange coupling of 2J/kB ) 15.3 K.19 Hosokoshi et al.
determined much larger exchange coupling of 319 K between
nitronyl nitroxide and tert-butylaminoxyl radicals in 7, whereas
the exchange coupling between imino nitroxide and tert-
butylaminoxyl radical in 8 is less than 200 K.20 A contribution
of the quinonoidal resonance forms seems to be important in
strengthening the magnitude of the exchange coupling in C and
7. C may also be regarded as the aminoxyl counterpart of Yang’s
diradical in which the molecule has approximate D3 symmetry
with propeller blades of quinonoidally distorted six-membered
rings in the same direction.21
Conclusions
It is well established that the positive spin density (R spin) is
polarized at the para carbon atom of the phenyl ring in N-tert-
butyl-N-phenylaminoxyls. The three R spins thus polarized are
to interact through the central nitrogen, carbon, and boron atoms
as in (•H2C)3X where X ) N, C•, and B, respectively. In other
words, the results obtained in this work should also be
interpreted in terms of a series of three-time phenylogs of
trimethylenemethane TMM. The numbers of the π-electrons in
the simplified CH2dX(CH2•)2 are four for X ) C in the carbon
analogue, leaving the two electrons for two degenerate orbitals
making the triplet electronic configuration most stable (see
Figure 1). In the amino analogue, the number of the π-electrons
is five, making the two orbitals doubly occupied and leaving
only one SOMO. Similarly, in the borane analogue, the
π-electron is not supplied by the central atom making the total
number of the relevant π-electrons three. Here again only one
SOMO is generated. Taking the effects of dynamic electron
correlation into account by CASPT2 single-point calculations
at the geometries optimized via the CASSCF methodology,
2
2
Borden and Brown showed that A2 and B1 states are nearly
degenerate ground states and excited A2′′ states are 38.0 and
4
23.4 kcal mol-1 above the ground states for trimethyleneamine
and trimethyleneborane, respectively.22 The ground spin states
of N and B′ observed in this study are justified by this theoretical
study. The much smaller magnitude of the observed gaps are
due to the partial R spins polarized at the ring carbon atoms
attached to the central nitrogen and boron atoms.
The antiferromagnetic coupling in molecular systems is
nothing but a partial bonding. The triangular antiferromagnetic
coupling among three 1/2 spins leading to a doublet ground
state in N corresponds therefore to the π-bonding in cyclopro-
pane-1,2,3-triradical in which any two spins interact antiferro-
magnetically owing to the strong overlap of the spin-containing
orbitals and a π bond is formed to give a cyclopropenyl radical.
The π-bond energy of J ≈ -60 kcal mol-1 may be regarded as
a measure of the antiferromagnetic interaction in this case.
Finally let us add some messages pertinent to the design of
new molecular based magnets. First, the doublet ground state
of the triphenylamine N carrying three N-tert-butylaminoxyl
groups on the para positions indicates that not only the topology
but also the number of the π-electrons is important in dictating
ferromagnetic couplers for molecular-based magnets.1 Second,
if coordinatively doubly unsaturated Lewis-acidic magnetic
metal ions having spin S form 3:2 polymer complexes with N
functioning as a bridging ligand, what merit will N have over
the previously used quartet tris-aminoxyl radicals? Statistically,
N will behave as a monoradical having three ligating sites. The
total number of spins for the n units will be (S - 1/3)n in stead
of (3S - 2/2)n for the equivalent complexes from quartet tris-
aminoxyls. For a third, the idea of aligning in parallel the spins
of the cation radicals developed by acidic doping of polyary-
lamines may be in trouble by antiferromagnetic coupling through
the unreacted base centers.
3. Very Weakly Antiferromagnetic Coupling in Triradical
B′. Triradical B′ has a doubly degenerate ground doublet state
which is more stable than a quartet state by 3J/kB ) 9.9 K.
This result is in accord with the expectation based on the Hu¨ckel
MO calculations. The weakened absolute J value between tert-
butylaminoxyl radicals in B′ is considered to be caused by the
large torsion angles of the benzene rings and/or the lack of π
electrons on the boron atom. Even though perturbed by six ortho
methyl groups, the interaction is through-bonds, and it is likely
that the antiferromagnetic interaction would be much stronger
in the sterically less-biased B.
Experimental Section
1
1. Materials. H and 13C NMR spectra were recorded on a JEOL
EX-270 instrument. IR spectra were obtained on a Hitachi I-5040
spectrometer. Mass spectra were obtained on a JEOL JMS-HX110A
instrument. All solvents used in the reactions were purified by the
reported methods. THF was purified by distillation from sodium-
benzophenone ketyl under a dry nitrogen atmosphere, just before use.
(19) Matsumoto, T.; Ishida, T.; Koga, N.; Iwamura, H. J. Am. Chem.
Soc. 1992, 114, 9952.
(20) (a) Inoue, K.; Iwamura, H. Angew. Chem., Int. Ed. Engl. 1995, 34,
917. (b) Hosokoshi, Y.; Takizawa, K.; Nakano, H.; Goto, T.; Takahashi,
M.; Inoue, K. J. Magn. Magn. Mater. 1998, 177-181, 634.
(21) Yang, N. C.; Castro, A. J. J. Am. Chem. Soc. 1960, 82, 6208. Bock,
H.; John, A.; Havlas, Z.; Bats, J. W. Angew. Chem., Int. Ed. Engl. 1993,
32, 416.
(22) We thank W. T. Borden and E. Brown for disclosing their
illuminating results before publication.

Tris[p-(N-oxyl-N-tert-butylamino)phenyl]amine
J. Am. Chem. Soc., Vol. 122, No. 11, 2000 2575
All reactions were performed under an atmosphere of dry nitrogen
unless otherwise specified. All reactions were monitored by thin-layer
chromatography carried out on 0.2-mm E. Merck silica gel plates (60F-
254) using UV light as a detector. Column chromatography was
performed using silica gel (Wakogel C-200, 200 mesh) or neutral
alumina (ICN, activity grade IV or Nacalai, Alumina Activated 200
mesh, inactivated with 6% H2O). Powder poly(vinyl chloride) (n ≈
1100) used for the preparation of polymer film samples was purchased
from Wako Pure Chemical Industries Ltd.
Tris[2,6-dimethyl-4-(O-tert-butyldimethylsilyl-N-tert-butyl-N-hy-
droxyamino)phenyl]borane (4). To a solution of 3 (2.85 g, 7.35 mmol)
in ether (15 mL) was added n-butyllithium (1.6 M n-hexane solution,
4.6 mL) at -78 °C. The mixture was warmed to room temperature
over 30 min and then cooled again to -78 °C. A solution of
trifluoroboron diethyl ether complex (0.35 g, 2.46 mmol) in ether (10
mL) was added to the mixture, which was then allowed to warm to
room temperature and stirred for 1 h. The reaction mixture was
quenched by addition of saturated aqueous ammonium chloride, the
mixture was extracted with ether, washed with water, and dried over
MgSO4, and the solvent was removed under reduced pressure. The
residue was chromatographed on silica gel using n-hexane as an eluent
to give 4 as a colorless oil (305 mg, 13%). 1H NMR (270 MHz, CDCl3)
-0.11 (s, 18H), 0.89 (s, 27H), 1.10 (s, 27H), 1.93 (s, 18H), 6.78 (s,
6H). FAB MASS: m/z 930 [M+ + 1].
Tris[p-(N-tert-butyl-N-hydroxyamino)phenyl]amine. To a solution
of tris(p-bromophenyl)amine (2.0 g, 4.15 mmol) in THF (40 mL) was
added tert-butyllithium (1.6 M pentane solution, 18.2 mL) at -78 °C.
The mixture was warmed to 0 °C over 10 min and then cooled again
to -78 °C. 2,2-Dimethylnitrosoethane (1.45 g, 16.6 mmol) was added,
and the mixture was warmed to room temperature and stirred for 1 h.
Saturated aqueous ammonium chloride and ether were added, the
organic layer was separated, washed with water, and dried over MgSO4,
and the solvent was removed under reduced pressure. The residue was
washed with dichloromethane to afford 0.77 g (37%) of tris[p-(N-tert-
butyl-N-hydroxyamino)phenyl]amine as white powders: mp 153 °C
(dec); 1H NMR (270 MHz, DMSO-d6) δ ) 8.20 (s, 3H), 7.92 (d, J )
8.57 Hz, 6H), 6.84 (d, J ) 8.57 Hz, 6H), 1.06 (s, 27H); 13C NMR
(67.8 MHz, DMSO-d6) δ ) 145.34, 143.74, 125.29, 122.23, 59.19,
25.99; FAB HRMS: m/z calcd for C30H42N4O3 506.3257, found
506.3256.
Tris[2,6-dimethyl-4-(N-tert-butyl-N-hydroxyamino)phenyl]bo-
rane (5). A mixture of 4 (300 mg, 0.32 mol), glacial acetic acid (2
mL), and THF (10 mL) was stirred at room temperature for 30 min
under argon. Dilution with water gave a white precipitate which was
collected and washed with water to afford 103 mg (54%) of 5 as a
1
white solid: mp 131-132 °C (dec). H NMR (270 MHz, DMSO-d6)
1.16 (s, 27H), 2.12 (s, 18H), 6.97 (s, 6H), 8.40 (s, 3H). FAB MASS:
m/z 588 [M+ + 1].
Tris[2,6-dimethyl-4-(N-tert-butyl-N-oxyamino)phenyl]borane (B′).
To a solution of 5 (103 mg, 0.085 mmol) in dichloromethane (10 mL)
was added an excess of freshly prepared Ag2O (300 mg), and the
mixture was stirred for 30 min. After filtration, the solvent was removed
under reduced pressure at ambient temperature. The residue was
chromatographed on aluminum oxide using dichloromethane as an
eluent to give B′ as a reddish brown powder (100 mg, 98%): mp 180-
181 °C (dec). Anal. Calcd for C36H51B1N3O3: C, 73.96; H, 8.79; N,
7.19. Found: C, 74.21; H, 9.03; N, 6.87.
Samples of N, C, and B′ Diluted in Polymer Films. PCV powder
(50 mg) and a polycrystalline sample of radical N, C, or B′ (5-6 wt
%) were dissolved in CH2Cl2 (ca. 5 mL) on a watch glass, and the
solvent was slowly evaporated. The obtained PVC films were dried
under reduced pressure overnight at room temperature.
Tris[p-(N-tert-butyl-N-oxylamino)phenyl]amine (N). To a solution
of tris[p-(N-tert-butyl-N-hydroxyamino)phenyl]amine (100 mg, 0.20
mmol) in dichloromethane (30 mL) was added an excess of freshly
prepared Ag2O (ca. 300 mg), and the mixture was stirred for 2 h. After
filtration, the solvent was removed under reduced pressure at ambient
temperature. The residue was chromatographed on aluminum oxide
using dichloromethane as an eluent and recrystallized from n-heptane-
dichloromethane (2:1) to give 2 as dark violet plates (86 mg, 87%):
mp 190-192 °C; FAB MASS: m/z 503 [M+]. Anal. Calcd for
C30H39N4O3: C, 71.54; H, 7.80; N,11.12. Found: C, 7.79; H, 71.44;
N, 11.08.
2,5-Dibromo-m-xylene (1). To sodium nitrite (9.0 g, 0.130 mole)
in concentrated sulfuric acid (100 mL), maintained below 10 °C, was
added portionwise a solution of 4-bromo-2,6-dimethylaniline (25.0 g,
0.125 mole) in glacial acetic acid (100 mL). After 1 h, the mixture
was added in portions to a vigorously stirred solution of copper(I)
bromide (18 g, 0.125 mole) in 48% hydrobromic acid (125 mL) and
water (100 mL) at 50 °C. After the addition was complete, the solution
was kept at 70 °C for 0.5 h and then diluted with water (1 L). The
mixture was extracted with n-hexane, washed with water, and dried
over MgSO4, and the solvent was removed under reduced pressure.
The residue was chromatographed on silica gel using n-hexane as an
eluent to give 1 as a colorless oil (17.46 g, 53%). 1H NMR (270 MHz,
CDCl3) 2.38 (s, 6H), 7.21 (s, 2H).
2. Crystallography. The intensity data for N and C were collected
on a Rigaku RAXIS-IV imaging plate area detector with graphite-
monochromated Mo KR radiation (λ ) 0.71070 Å). Indexing was
performed from three oscillations which were exposed for 4.0 min.
The crystal-to-detector distance was 110.00 mm with the detector at a
zero swing position. Readout was performed in the 0.100 mm pixel
mode. The data were collected at a temperature of -100 °C to a
maximum 2θ value of 55°. A total of 205.0° (for N) and 185.0° (for
C) oscillation images were collected, each being exposed for 60 min
(for N) and 100 min (for C). The intensity data for B′ were collected
on a Rigaku AFC7R diffractometer with graphite monochromated Mo
KR radiation (λ ) 0.71070 Å). The data were collected at a
temperature of 23 °C using the ω-2θ scan technique to a maximum
2θ value of 55°. The structures of N and C were solved by a direct
method with MULTAN 88 and refined by the full matrix least-squares
method. The structure of B′ was solved by a direct method with
SHELXS-86 and refined by the full matrix least-squares method. All
of the non-hydrogen atoms were refined anisotropically. The final cycle
of the least-squares refinement was based on 2461 (for N), 1597 (for
C) and 1519 (for B′) observed reflections (I > 3.0σ(I) for N and B′, I
> 1.5σ(I) for C) and 335 (for N and C) and 199 (for B′) variable
parameters with R (Rw)) 0.039 (0.054) (for N), 0.058 (0.055) (for C)
and 0.050 (0.072) (for B′). The crystal data for these molecules are
summarized in Table 2. All calculations were performed using the
teXsan crystallographic software package from Molecular Structure
Corp.
N-(4-Bromo-3,5-dimethylphenyl)-N-tert-butylhydroxyamine (2).
To the Grignard compound prepared from 1 (17 g, 0.06 mole) and Mg
(1.57 g, 0.07 mole) in dry ether (100 mL) was added a solution of
1,1-dimethylnitrosoethane (6.72 g, 0.08 mol) in ether (100 mL)
dropwise. After the addition was complete, the mixture was stirred for
0.5 h and then quenched by addition of saturated aqueous ammonium
chloride. The mixture was extracted with ether, washed with water,
dried over MgSO4 and the solvent was removed under reduced pressure.
The residue was washed with n-hexane to afford 5.6 g (31%) of 2 as
a white powder: mp 117-118 °C (dec).1H NMR (270 MHz, CDCl3)
1.13 (s, 9H), 2.38 (s, 6H), 5.32 (s, 1H), 6.95 (s, 2H).
N-(4-Bromo-3,5-dimethylphenyl)-O-tert-butyldimethylsilyl-N-tert-
butylhydroxyamine (3). A solution of 2 (5.5 g, 0.02 mol), imidazole
(3.2 g, 0.05 mol), and tert-butylchlorodimethylsilane (3.0 g, 0.02 mol)
in N,N-dimethylformamide was stirred at 50 °C for 12 h. After dilution
with n-hexane and water, the organic layer was separated, washed with
water, and dried over MgSO4, and the solvent was removed under
reduced pressure. The residue was chromatographed on silica gel using
n-hexane as an eluent to give 3 as a white solid (7.58 g, 97%): mp
53.5-55 °C. 1H NMR (270 MHz, CDCl3) -0.13 (s, 6H), 0.90 (s, 9H),
1.07 (s, 9H), 2.37 (s, 6H), 6.95 (s, 2H).
3. Magnetic Measurement. Fine crystalline samples or polymer
film samples were mounted in a capsule (Japan Pharmacopoeia NO.
5, 4.5 o.d. × 11 mm) and measured on a Quantum Design MPMS-5S
SQUID susceptometer at 5000 G. The data were corrected for the
diamagnetic contribution in the range -3.7 × 10-5 to -6.7 × 10-5
emu G of sample capsules and holding straws used. Contribution of
the diamagnetic susceptibility (-1.7 × 10-4 to -2.2 × 10-4 emu G)
of the polymer matrix was determined by measuring the susceptibility

2576 J. Am. Chem. Soc., Vol. 122, No. 11, 2000
Itoh et al.
of the same amount of similar PVC films made in the absence of
dissolved free radicals. Corrections for the diamagnetic contribution
from the free radical molecules were made by using Pascal’s constants.
4. Computational Studies. The computational work was performed
on a NEC HSP computer and the library program GAUSSIAN94 at
the Computer Center, Institute for Molecular Science in Okazaki.
for a Postdoctoral Fellowship. Thanks are also due to Prof.
Noboru Koga of Faculty of Pharmaceutical Sciences, Kyushu
University, for his helpful discussion on the analysis of the
magnetic data.
Supporting Information Available: X-ray crystal structural
data on C and B′.7 Computational results: atomic coordinates
after structural optimization by B3LYP/6-31G and calculated
total energies obtained by B3LYP/6-31G*. This material is
available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by a Grant-in
Aid for COE Research “Design and Control of Advanced
Molecular Assembly Systems” (no. 08CE2005) from the
Ministry of Education, Science, Sports, and Culture, Japan. T.
I. is thankful to the Japan Society for the Promotion of Science
JA9920819