Novel aromatic N-oxyl radical based on the benzo[g]quinoline skeleton: Synthesis and intermolecular ferromagnetic interaction

Article abstract of DOI:10.1016/j.cplett.2004.11.132

A novel stable organic radical, 2,2-diphenyl-1,2-dihydrobenzo[g]quinoline- N-oxyl (2), has been synthesized and structurally characterized in order to examine the ring extension effect on the known derivative, 2,2-diphenyl-1,2- dihydroquinoline-N-oxyl (1). Based on SQUID measurement, 2 exhibited a ferromagnetic interaction obeying the Bleaney-Bowers equation with 2J = +4.6 cm^-1. The X-ray analysis revealed an intermolecular edge-edge association of the aryl ring and N-oxyl moiety in the crystal of 2. The bifurcated edge-edge contacts between the α-spin site (nitroxyl O atom) and the β-spin site (aryl-H atoms) are responsible for the ferromagnetic interaction of 2.

Full text of DOI:10.1016/j.cplett.2004.11.132

Chemical Physics Letters 402 (2005) 11–16
www.elsevier.com/locate/cplett
Novel aromatic N-oxyl radical based on the benzo[g]quinoline
skeleton: synthesis and intermolecular ferromagnetic interaction
*
Masaru Yao, Hidenari Inoue, Naoki Yoshioka
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Yokohama 223-8522, Japan
Received 13 August 2004; in final form 17 November 2004
Available online 18 December 2004
Abstract
A novel stable organic radical, 2,2-diphenyl-1,2-dihydrobenzo[g]quinoline-N-oxyl (2), has been synthesized and structurally char-
acterized in order to examine the ring extension effect on the known derivative, 2,2-diphenyl-1,2-dihydroquinoline-N-oxyl (1). Based
on SQUID measurement, 2 exhibited a ferromagnetic interaction obeying the Bleaney–Bowers equation with 2J = +4.6 cmꢀ1. The
X-ray analysis revealed an intermolecular edge–edge association of the aryl ring and N-oxyl moiety in the crystal of 2. The bifur-
cated edge–edge contacts between the a-spin site (nitroxyl O atom) and the b-spin site (aryl-H atoms) are responsible for the fer-
romagnetic interaction of 2.
ꢀ 2004 Elsevier B.V. All rights reserved.
1. Introduction
a node on the central carbon atom; therefore, the spin
densities found in the adjacent group are usually low.
To enhance the magnetic interaction in molecular
systems, it is favorable to design multipoint close con-
tacts between the SOMOs, because the intermolecular
magnetic interaction is described by the sum of the
spin-density product of nearby atoms belonging to
neighboring molecules, as suggested by McConnell [6].
We have focused on 2,2-diphenyl-1,2-dihydroquino-
line-N-oxyl (1) originally synthesized by Colonna and
co-workers [7] based on its chemical stability and spin
delocalization over the quinoline p-system. Several
N-alkylarylnitroxyl radicals, such as N-tert-butyl-phe-
nylnitroxyl, have been reported to be unstable due to a
disproportion reaction; however, 1 is fairly stable due
to the steric effect of the two phenyl rings at the 2-posi-
tion. The diphenyl substituents and the fused coplanar
structure could synergistically contribute to the preven-
tion of the direct close contact between the N–O moie-
ties, which usually cause a strong antiferromagnetic
interaction, and to the increase in the spin density over
the benzo-ring by delocalization.
The research on molecule-based magnetic materials
has greatly progressed in recent years [1,2]. In order to
realize the tailored magnets based on molecular materi-
als, we have to explore the supramolecular synthons
suitable for intermolecular ferromagnetic interactions
[3,4]. We have already discovered the formation of mag-
netic self-assemblies in nitronyl nitroxyl derivatives car-
rying heterocycles with the NH site, such as the
imidazol-2-yl, benzimidazol-2-yl, and naphth[2,3-d]imi-
dazol-2-yl groups [3,5]. Among these three compounds,
the benzimidazole derivative formed a piled columnar
assembly induced by bifurcated intra- and intermolecu-
lar hydrogen bondings between an NH site and two ni-
troxyl O atoms [3]. The assembly exhibited a 1-D
ferromagnetic chain behavior that could be rationalized
by the close contact between the singly occupied molec-
ular orbitals (SOMOs). The nitronyl nitroxyl radical has
*
Corresponding author. Fax: +81 45 566 1551.
E-mail address: yoshioka@applc.keio.ac.jp (N. Yoshioka).
0009-2614/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.11.132

12
M. Yao et al. / Chemical Physics Letters 402 (2005) 11–16
Though the synthesis and crystal structure of 1 have
adjacent aromatic rings are very small due to the charac-
teristic fused structure (1: 2.25ꢁ, 2: 2.38ꢁ). Although
N-alkylarylnitroxyl radicals have been utilized for spin
centers, the large angle between the N–O site and the
adjacent aromatic rings often reduce the spin density
over the aromatic rings [10,11]. The observed coplanar
structures in 1 and 2 could increase the spin density over
the aromatic rings. In the crystal, 1 has no appreciable
close contact; however, 2 formed a centrosymmetric
edge–edge dimer structure in its crystal. A more detailed
discussion will be given in Section 2.5.
already been reported [7,8], its magnetic property in
the solid state has not yet been reported. In the present
study, a novel derivative, 2,2-diphenyl-1,2-dihydrobenzo-
[g]quinoline-N-oxyl (2), has been synthesized as one
of the benzologues of 1 and its crystal structure and
electronic and magnetic properties were studied. Its
magneto-structural correlation was also discussed in
connection with that of 1.
Ph
Ph
Ph
Ph
2.3. ESR spectroscopy
N
O
N
O
The ESR spectra of 1 and 2 in benzene solutions
exhibited very complex patterns due to the presence of
many magnetically non-equivalent aromatic nuclei.
The spectrum of 1 was mainly composed of three lines,
which are attributable to the nitroxyl nitrogen, and was
essentially identical to the reported one [7]. In contrast,
the spectrum of 2 was basically composed of six lines,
which indicates the presence of a hydrogen atom having
high spin density (Fig. 2). The spin distributions can be
evaluated by comparing the hyperfine coupling con-
stants (hfccs), since the values are proportional to the
spin densities. Generally, the dialkyl N-oxyl radicals ex-
hibit hfccs of ca. 1.4–1.5 mT for the N-oxyl nitrogen.
These values indicate that the spin density is localized
on the N–O group. A computer simulation of the spec-
tra of 1 and 2 gave aN values of 0.982 and 0.986 mT,
respectively, which confirmed the delocalization of the
unpaired electron in the p-conjugated systems. In addi-
tion, we performed molecular orbital calculations based
on the density functional theory (DFT) to determine the
sign of the hfccs, using the coordinates obtained by
X-ray analysis. The calculated hfccs of 2 are compared
with the experimental ones in Table 1. Although the cal-
culated hfcc for the N–O is slightly smaller than the
experimental one, as commonly found in the DFT calcu-
lations of the N-oxyl radicals [12], good agreements be-
tween the observations and the calculations were
obtained for the other hfccs. Interestingly, the spin den-
1
2
2. Results and discussion
2.1. Synthesis
1 was synthesized according to Colonnaꢀs method [7].
The benzologue 2 was obtained via a 9-step synthesis.
First, 3-amino-2-naphthaldehyde was derived from
commercially available 3-amino-2-naphthoic acid in five
steps using Thummelꢀs method [9]. Next, the Friedla¨n-
der condensation with acetophenone gave 2-phen-
ylbenzo[g]quinoline. After the oxidation of the
quinolinic nitrogen atom by m-CPBA, another phenyl
group was introduced at the 2-position by the reaction
with phenylmagnesiumbromide. The aerial oxidation
of the resulting N-hydroxyl derivative in chloroform
gave a brown colored 2.
2.2. X-ray crystallographic analysis
The ORTEP drawings of 1 and 2 are shown in Fig. 1.
The dihedral angles between the N–O group and the
H(4)
H(5)
H(4)
H(6)
H(3)
H(5)
H(3)
H(7)
H(6)
H(7)
H(8)
N
N
O
O
H(8)
H(10)
H(9)
1
2
Fig. 1. ORTEP drawings of 1 (30% ellipsoids) and 2 (50% ellipsoids).

M. Yao et al. / Chemical Physics Letters 402 (2005) 11–16
13
sity of H(10) of 2 is greater than that of H(8) of 1, which
reflects the difference in the electronic structures of the
naphtho-ring and benzene ring. Fig. 3 shows the calcu-
lated SOMOs and spin density distributions for 1 and
2. In contrast to the SOMOs of the dialkyl N-oxyls,
those of 1 and 2 are delocalized over the aromatic rings.
(a)
2.4. Solid state magnetic measurements
1.0 mT
The temperature dependence of vmT and vꢀm1 for 1
and 2 are shown in Fig. 4. The vmT value of 1 was vir-
tually constant, suggesting that 1 has a nearly isolated
doublet state in the crystal. The X-ray analysis also sup-
ports this result; 1 has no effective contact for intermo-
lecular magnetic interactions in the crystal. On the
other hand, the vmT value of 2 increased as the temper-
ature decreased and reached a maximum at 2.2 K. The
observed intermolecular interaction was analyzed using
the Bleaney–Bowers equation for a dimer model [13],
(b)
Fig. 2. The ESR spectra of 2 in a benzene solution at room
temperature. g = 2.0055, aN = 0.986 mT: (a) Experimental. (b)
Simulation.
NAg2b2
vm ¼ f
:
ð1Þ
kBT ½3 þ expðꢀ2J=kBT Þꢁ
The best-fit parameters were a correction factor of f =
0.95 and a singlet–triplet energy gap of 2J = +4.6 cmꢀ1
Table 1
Experimental and calculated hfccs of 2
.
Site
Exptl. hfccsa/mT
Calcd. hfccsb/mT
2.5. Magneto-structural correlation
N
0.99
0.08
0.04
0.09
0.06
0.16
0.06
0.12
0.47
0.80
ꢀ0.11
0.05
H(3)
H(4)
H(5)
H(6)
H(7)
H(8)
H(9)
H(10)
a
We initially expected a p-stacked structure for 2 due
to its extended p-conjugation system; however, 2 formed
an edge–edge dimer structure without p-stacking in the
crystal (Fig. 5). Recently, Nguyen et al. [14] reported a
similar edge–edge association for the aromatic N-oxide
derivatives . In the dimer of 2, the nearest Oꢂ ꢂ ꢂO dis-
0.12
0.09
ꢀ0.16
0.08
ꢀ0.14
ꢀ0.49
˚
tance is 4.94 A. There seems to be almost no direct inter-
In dilute benzene solution.
b
UB3LYP/6-31G*.
action between the N–O sites [15]. However, the short
Fig. 3. Singly occupied molecular orbital (SOMO; a,c) and spin density distribution (b,d) for 1 and 2, respectively. The positive spin densities are
shown in blue and the negative ones are in green.

14
M. Yao et al. / Chemical Physics Letters 402 (2005) 11–16
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
300
250
200
150
100
50
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
350
300
250
200
150
100
50
χ
χ
χ
χ
χ
χ
χ
χ
0
100
0
100
0
20
40
60
80
0
20
40
60
80
(a)
(b)
T / K
T / K
Fig. 4. Temperature dependence of vꢀm1 and vmT for (a) 1 and (b) 2. Solid lines correspond to calculated ones (see text).
crystal structure should not be used for the detailed dis-
cussion about the magnetic interaction between the
hydrogen atoms and the radical center. We performed
the optimization for the positions of the hydrogen atoms
by AM1-RHF semi-empirical computations [18] before
the DFT calculation. The calculation for the dimer
found a triplet ground state with 2J = +3.9 cmꢀ1, which
was in good agreement with the experimental result
(2J = +4.6 cmꢀ1). Therefore, this dimeric structure leads
to the ferromagnetic interaction, in which the negatively
spin-polarized hydrogen atoms located near the N–O
site play an important role.
3. Conclusion
Fig. 5. Dimeric structure of 2. The atomic coordinates for the
hydrogen atoms were optimized with the AM1-RHF semi-empirical
method.
A novel aromatic N-oxyl radical based on the
benzo[g]quinoline skeleton 2 was prepared. The ESR
measurements and DFT calculations for 1 and 2 re-
vealed the delocalization of the unpaired electron at
the p-conjugated system. The X-ray analysis revealed
that 2 formed an edge–edge dimer in the crystal. In
the SQUID measurement, 1 exhibited a paramagnetic
behavior; however, 2 showed a ferromagnetic behavior
which is best fitted by the Bleaney–Bowers equation
for a dimer model with 2J = +4.6 cmꢀ1. This value
was also reproduced by the DFT calculation. In the di-
mer, the negatively spin-polarized hydrogen atoms lo-
cated near an N–O site play an important role in the
ferromagnetic interaction. The magneto-structural cor-
relation reveals that the new packing mode has a poten-
tial significance as a supramolecular synthon for
constructing ferromagnetic molecular self-assemblies.
distances between the N–O site and the aromatic hydro-
gen atoms at the 9- and 10-positions of an adjacent mol-
ecule are observed. The DFT calculation for 2 gave
relatively large negative spin densities for these hydro-
gen atoms. The observed ferromagnetic coupling is qual-
itatively interpreted by the McConnellꢀs theorem [6].
Nogami et al. [16] have also proposed the importance
of the hydrogen atoms carrying negative spin densities
for the ferromagnetic ordering in the crystals of a series
of the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)
derivatives . Furthermore, we performed the DFT calcu-
lation for the dimer of 2 to obtain a theoretical insight
into the exchange interaction in this system. The geo-
metric structure of the dimer extracted from the X-ray
analysis was used for the computation except for that
of the hydrogen atoms. Generally, X-ray analysis gives
˚
apparent C–H bonds that are about 0.1 A shorter than
4. Experimental
those from the neutron diffraction because the involve-
ment of the single electron of hydrogen atom in the
C–H bond shifts the center of gravity of the electron
density toward the carbon atom [17]. Therefore, the
coordinates of the hydrogen atoms extracted from the
4.1. Measurements
The ESR spectra were recorded using a JEOL JES-
RE3X spectrometer. The simulation was performed

M. Yao et al. / Chemical Physics Letters 402 (2005) 11–16
15
using PEST WinSIM [19]. The magnetic properties of
the microcrystalline samples were measured by a Quan-
tum Design MPMS-5 SQUID susceptometer at the field
strength of 0.5 T. In addition to the correction of the
diamagnetic contribution from the sample holder, the
diamagnetic contribution from the sample was esti-
mated using the Pacault method (1: 1.76 · 10ꢀ4, 2:
2.15 · 10ꢀ4 emu/mol) [20].
solvent, the residue was purified by column chromatog-
raphy on silica eluting with toluene. The yellow band
was collected and then recrystallized from the MeOH
to produce a pale yellow solid. Yield: 250 mg, 64%;
1
mp: 190–192 ꢁC (lit. 192.5–193 ꢁC [28]), H NMR (300
MHz, DMSO-d6) d 8.77 (s, 1H), 8.66 (s, 1H), 8.64 (d,
J = 9 Hz, 1H), 8.26–8.22 (m, 2H), 8.23–8.14 (m, 2H),
8.18 (d, J = 9 Hz, 1H), 7.63–7.55 (m, 5H). FAB-MS:
m/z, calcd. for C19H13N, 255.1. Found: 256 [M + H]+.
Anal. Calcd. for C19H13N: C, 89.38; H, 5.13; N, 5.49.
Found: C, 89.09; H, 5.18; N, 5.44%.
4.2. X-ray diffraction analysis
The single crystals were mounted on a glass fiber and
measured using a Rigaku four-circle AFC-7R dffrac-
tometer with graphite monochromated Mo Ka radia-
4.4.2. 2-Phenylbenzo[g]quinoline-N-oxide (4)
A solution of 200 mg of m-CPBA acid in 3 ml of chlo-
roform was gradually added to an ice-cooled, stirred
solution of 3 (160 mg, 0.63 mmol) in chloroform (20
ml). The stirring was continued for 22 h, and then basi-
fied with an NaHCO3 aqueous solution. The organic
layer was washed several times with water and dried
over Na2SO4. After evaporation of the solvent, the res-
idue was purified by column chromatography on silica
gel eluting with EtOAc/n-hexane = 1/1 and recrystallized
from the mixed solvent of acetone and hexane which
gave a white solid. Yield: 98 mg, 57%; mp: 180–183
˚
tion (k = 0.71073 A). The structure was solved by a
direct method [21] and refined by SHELXL-97 [22] with
the program system teXsan [23]. The positions of the
hydrogen atoms were introduced by calculation. The
crystallographic parameters are as follows. For 1:
monoclinic, P21/n, a = 11.673(4), b = 17.415(5), c =
˚
8.254(3) A, b = 107.61(3)ꢁ, Z = 4, R (I > 2r(I)) = 0.066,
Rw (all data) = 0.136, S = 1.29. For 2: monoclinic, P21/
˚
c, a = 12.718(3), b = 12.646(3), c = 12.437(3) A, b =
117.49(2)ꢁ, Z = 4, R (I > 2r(I)) = 0.069, Rw (all data) =
0.164, S = 0.86. The crystallographic data will be avail-
able at the Cambridge Crystallographic Data Centre.
1
ꢁC. H NMR (300 MHz, DMSO-d6), d 9.31 (s, 1H),
8.79 (s, 1H), 8.34–8.31 (m, 1H), 8.22–8.20 (m, 1H),
8.15 (d, J = 9 Hz, 1H), 8.14–8.10 (m, 2H), 7.73 (d,
J = 9 Hz, 1H), 7.70–7.67 (m, 2H), 7.61–7.53 (m, 3H).
FAB-MS (m/z), Calcd. for C19H13NO: 271.1. Found:
272 [M + H]+. Anal. Calcd. for C19H13NOÆ1/8H2O: C,
83.42; H, 4.88; N, 5.12. Found: C, 83.23; H, 4.89; N,
5.07%.
4.3. Computational details
All DFT calculations were performed using the
AUSSIAN 03 program package [24]. The popular
G
UB3LYP hybrid functional [25,26] with the standard
split valence 6-31G(d) basis set was used for the compu-
tation. The J-value was estimated using the equation
developed by Yamaguchi et al. [27].
4.4.3. 2,2-Diphenyl-1,2-dihydrobenzo[g]quinoline-N-oxyl
(2)
To a solution of 4 (200 mg, 0.74 mmol) in 15 ml of dry
tetrahydrofuran was added a phenylmagnesium bromide
THF solution (6.0 ml, 6.0 mmol) under stirring in a
nitrogen atmosphere. After 3 h, the mixture was treated
with aqueous NH4Cl and extracted with CHCl3. The or-
ganic layer was washed with water and dried on Na2SO4.
After removal of the solvent, the residue was purified by
column chromatography on silica gel eluting with tolu-
ene followed by recrystallization from CH2Cl2 with lay-
ered EtOH to give dark brown micro-crystals of 2.
Yield: 16 mg, 6%; mp: 196–199 ꢁC. FAB-MS (m/z),
Calcd. for C25H18NO: 348.1. Found: 348 [M]+. Anal.
Calcd. for C25H18NOÆ1/4H2O: C, 85.08; H, 5.28; N,
3.97. Found: C, 85.26; H, 5.44; N, 3.91%.
4.4. Synthesis
1
The H NMR spectra were measured using a JNM-
LA 300 spectrometer (JEOL Co., Japan). The elemental
analyses were performed at the Central Laboratory of
the Faculty of Science and Technology, Keio University.
Tetrahydrofuran was refluxed over and distilled from
LiAlH4. All other commercially available reagents were
used without further purification. The 3-amino-2-naph-
thaldehyde was synthesized according to the literature
[9].
4.4.1. 2-Phenylbenzo[g]quinoline (3)
A mixture of 3-amino-2-naphthaldehyde (260 mg, 1.5
mmol), acetophenone (175 ll, 1.5 mmol) and saturated
ethanolic KOH (0.7 ml) in EtOH (30 ml) was refluxed
for 11 h. The mixture was diluted with water and the
EtOH was removed in vacuo. The resulting aqueous sus-
pension was extracted with CH2Cl2, and the organic
layer was dried over Na2SO4. After evaporation of the
Acknowledgments
This work was supported in part by a Grant-in-Aid
for Scientific Research (B) 15310094, and Exploratory
Research 116651070. Financial support from the Keio

16
M. Yao et al. / Chemical Physics Letters 402 (2005) 11–16
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451.
Gijyuku Academic Development Funds is also acknowl-
edged. M.Y. gratefully acknowledges the grant-in-aid
for the 21st Century COE program ꢁKEIO Life Conju-
gate Chemistryꢀ from the Ministry of Education, Cul-
ture, Sports, Science, and Technology, Japan.
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