Ferromagnetic behavior of formyl-group-carrying stable thioaminyl radicals

Article abstract of DOI:10.1021/jo0300789

Four formyl-group-carrying thioaminyl radicals were generated, and one radical could be isolated as radical crystals. Magnetic susceptibility measurements of the isolated radical showed a ferromagnetic regular linear-chain interaction of 2J/k_B = 3.2 K, which was explained in terms of the X-ray crystallographic results.

Full text of DOI:10.1021/jo0300789

SCHEME 1
F er r om a gn etic Beh a vior of
F or m yl-Gr ou p -Ca r r yin g Sta ble Th ioa m in yl
Ra d ica ls1
Yozo Miura,*,† Shogo Nakamura,† and Yoshio Teki*,‡
Department of Applied Chemistry, Graduate School of
Engineering and Department of Material Science and
Chemistry, Graduate School of Science, Osaka City
University, Sumiyoshi-ku, Osaka 558-8585, J apan
miura@a-chem.eng.osaka-cu.ac.jp
Received March 3, 2003
Abstr a ct: Four formyl-group-carrying thioaminyl radicals
were generated, and one radical could be isolated as radical
crystals. Magnetic susceptibility measurements of the iso-
lated radical showed a ferromagnetic regular linear-chain
interaction of 2J /kB ) 3.2 K, which was explained in terms
of the X-ray crystallographic results.
X-ray crystallographic analyses were unsuccessful for a
long time because they did not give high-quality single
crystals. Fortunately, however, we quite recently suc-
ceeded in the X-ray analysis for one ferromagnetic
radical,1 N-(2-pyridythio)-2,6-diphenyl-4-cyanophenyl-
aminyl, and its magnetic behavior could be analyzed on
the basis of the X-ray results. The present paper reports
the second successful results.
N-(Arylthio)-2,6-diphenylanilines (1), the correspond-
ing precursors of N-(arylthio)-2,6-diphenyl-4-formylphen-
ylaminyls (2), were prepared according to Scheme 1.
Treatment of 4-cyano-2,6-diphenylaniline with diisobut-
ylaluminum hydride (DIBAL-H) in toluene gave 4-formyl-
2,6-diphenylaniline in 86% yield. The precursors 1 were
obtained in 22-31% yields by the reaction of 4-formyl-
2,6-diphenylaniline with 3-nitro-, 4-nitro-, 2,4-dichloro-,
and 3,5-dichlorobenzenesulfenyl chlorides in dry ether in
the presence of Et3N.
Oxidation of 1 was performed with PbO2 using benzene
as the solvent. When PbO2 was added to the stirred
colorless (a , c, d ) or light yellow (b) mixtures of 1 and
K2CO3, they immediately turned to dark green, and an
intense ESR signal was observed from the resultant
colored solutions. TLC analysis showed formation of the
two new colored compounds, 2 (green) and 3 (orange). In
the case of 1c, both the compounds (2c and 3c) could be
isolated by chromatography. However, in the cases of 1a ,
1b, and 1d , pure radical crystals could not been obtained
despite much effort. This is in part due to a partial
decomposition of the radicals during chromatography.
The structure of 2c was confirmed by the IR spectrum
showing no NH absorption peak and by satisfactory
elemental analyses. Later, the radical structure was
unequivocally determined by X-ray crystallography, as
described below.
Thioaminyl radicals have attracted much attention as
a family of isolable and oxygen-inert radicals having a
delocalized π-spin system.2 Isolable and oxygen-inert
radicals are important as a spin source or building blocks
in the study of molecule-based magnetism.3,4 Although
much effort has been paid to finding new stable free
radicals, isolable and oxygen-inert free radicals are still
rare. In the present work we prepared formyl-group-
carrying thioaminyl radicals because we planned to
prepare thioaminyl-verdazyl or thioaminyl-nitronyl ni-
troxide diradicals. Among four kinds of formyl-substi-
tuted thioaminyls prepared, one radical could be isolated
as radical crystals, and its X-ray crystallographic analysis
was successfully performed. The magnetic susceptibility
measurements with a superconducting quantum inter-
ference device (SQUID) magnetometer showed that the
isolated radical was ferromagnetic, and the ferromagnetic
behavior could be satisfactorily explained on the basis
of the X-ray crystallographic results. Although we have
so far found nine ferromagnetic thioaminyls,1,5-9 their
* To whom correspondence should be addressed (Y. M.). Phone +81-
6-6605-2798. Fax +81-6-6605-2769.
† Department of Applied Chemistry, Graduate School of Engineer-
ing.
‡ Department of Material Science and Chemistry, Graduate School
of Science.
(1) ESR Studies of Nitrogen-Centered Free Radicals. 57. Part 56.
Miura, Y.; Oyama, Y.; Teki, Y. J . Org. Chem., 2003, 68, 1225.
(2) Miura, Y. Trends Org. Chem. 1997, 6, 197. Miura, Y. Recent Res.
Dev. Org. Chem. 1998, 2, 251.
(3) Magnetic Properties of Organic Materials; Lahti, P. M., Ed.;
Marcel Dekker: New York, Basel, 1999.
The structure of 3c was determined by its 1H NMR
(400 MHz) and mass spectra, elemental analysis, and
later by X-ray crystallography, indicating that the 4-
formylanilino group is converted into the p-benzoquinone
monoimine structure, with a loss of the formyl group. To
clarify whether 3c is formed via 2c or not, 2c was treated
with PbO2 in benzene under atmospheric conditions, and
chromatography of the resultant mixture gave 3c in 58%
yields. This reaction was also carried out under a
(4) Proceedings of the 7th International Conference on Molecule-
Based Magnets; Polyhedron 2001, 20, 1115-1784.
(5) Teki, Y.; Itoh, K.; Okada, A.; Yamakage, H.; Kobayashi, T.;
Amaya, K.; Kurokawa, S.; Ueno, S.; Miura, Y. Chem. Phys. Lett. 1997,
270, 573.
(6) Miura, Y.; Momoki, M.; Nakatsuji, M.; Teki, Y. J . Org. Chem.
1998, 63, 1555.
(7) Miura, Y.; Kurokawa, S.; Nakatsuji, M.; Ando, K.; Teki, Y. J .
Org. Chem. 1998, 63, 8295.
(8) Miura, Y.; Tomimura, T.; Teki, Y. J . Org. Chem. 2000, 65, 7889.
(9) Nakatsuji, M.; Miura, Y.; Teki, Y. J . Chem. Soc., Perkin Trans.
2 2001, 738.
10.1021/jo0300789 CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/12/2003
8244
J . Org. Chem. 2003, 68, 8244-8247

SCHEME 2
TABLE 1. ESR P a r a m eter for 2 in Ben zen e Mea su r ed a t
20 °Ca ,b
c
radical
aN
aH
g
2a d
2bd
2ce
2d e
0.864
0.861
0.850
0.860
2.0061
2.0058
2.0059
2.0059
0.130 (2H),f 0.090 (1H)g
0.130 (2H),f 0.095 (3H)h
a
b
The hfc constants are given in mT. The g values were
determined by comparison with Fremy’s salt (g ) 2.0057) in an
aqueous dilute K2CO3 solution. c The numbers in parentheses refer
d
to the number of equivalent protons. The hyperfine splittings due
to aromatic protons were not observed. e The proton hfc constants
are determined by computer simulation. f Due to the anilino meta
g
h
protons. Due to the phenylthiyl ortho proton. Due to the
phenylthiyl ortho and para protons.
F IGURE 1. ESR spectrum of 2c in benzene at 20 °C: (a)
observed spectrum; (b) computer simulation.
nitrogen atmosphere, and the same compound was
obtained in a similar yield. On the basis of the above
results we conclude that 3 is formed via 2 and the
p-benzoquinone monoimine oxygen comes from the PbO2
used as the oxidant. Although we cannot provide a clear
mechanism for formation of 3 at the present time, a
plausible mechanism is drawn in Scheme 2.
In the 1H NMR spectrum of 3c (see Supporting
Information), the singlet due to the meta protons (2H)
of the benzoquinone monoimine at 6.63 ppm and the
multiplets due to the 2- and 6-phenyl protons (10H) at
7.43-7.53 ppm were significantly broadened. We previ-
ously observed a similar phenomenon for 4.10 A plausible
explanation for the broadened 1H NMR spectrum may
be formation of diradical 3c′, though the diradical was
not detected by ESR, probably due to the very low
concentration.
F IGURE 2. The total atomic spin densities in 2c. The spin
density calculations based on the density functional theory
were performed by the UBecke 3LYP hybrid method using the
6-31G basis set.
The spin density distribution for 2c was calculated by
the DFT UBecke 3LYP hybrid method using the 6-31G
basis set. The X-ray crystallographic structure was used
without further optimization of structure. All the MO
calculations were carried out on Gaussian 98.11 The total
atomic spin densities are depicted in Figure 2, and the
calculated hyperfine coupling (hfc) constants are com-
pared with the observed values in Table 2. As found in
Table 2, the calculated hfc constants are in satisfactory
The ESR spectra of 2 were measured at room temper-
ature using benzene as the solvent. The ESR parameters
are summarized in Table 1. Although the ESR spectra
of 2a and 2b were a broad 1:1:1 triplet, the other two
radicals, 2c and 2d , showed additional hyperfine split-
tings due to aromatic protons. A typical ESR spectrum
of 2c is shown in Figure 1. The spectrum is split into a
1:1:1 triplet, and each component of the 1:1:1 triplet is
further split into a broad 1:3:3:1 quartet. Computer
simulation of the spectrum gave aN ) 0.850, aH (2H) )
0.130, and aH (1H) ) 0.090 mT (g ) 2.0059).
(11) All calculations were carried out on Gaussian 98, Version 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., J r.;
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.; Baboul, A. G.; 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.;
Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.; Wong, M. W.;
Andres, J . L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople,
J . A.; Gaussian, Inc.: Pittsburgh, PA, 1998.
(10) Miura, Y.; Momoki, M. J . Chem. Soc., Perkin Trans. 2 1998,
1185.
J . Org. Chem, Vol. 68, No. 21, 2003 8245

TABLE 2. Ca lcu la ted h fc Con sta n ts for 2c a n d
Com p a r ison w ith Obser ved Va lu es
position
calcd hfc constanta,b
obsd hfc constanta,c
0.850
N1
S1
H3
H5
H8
0.7708
0.2547
0.2334
0.1977
-0.0733
0.130
0.130
0.082
a
b
The hfc constants are given in mT. The hfc constants are
calculated by the UBecke 3LYP hybrid method using the STO
6-31G basis set. c The hfc constants are shown in absolute values.
F IGURE 4. Stacking mode of 2c along the a axis. The
contacts between S1‚‚‚ C5′ (3.76 Å), N1‚‚‚ C5′ (4.26 Å), and
S1‚‚‚ C19′ (4.29 Å) are shown to be ferromagnetic, whereas the
contacts between S1‚‚‚ C4′ (4.06 Å) and S1‚‚‚ C6′ (4.34 Å) are
shown to be antiferromagnetic (see text).
Å), N1‚‚‚ C5′ (4.26 Å), S1‚‚‚ C19′ (4.29 Å), and S1‚‚‚ C6′
(4.34 Å). The spin densities on those atoms predicted by
the above MO calculations are 0.156 (S1), 0.570 (N1),
0.167 (C4), -0.093 (C5), and 0.175 (C6), respectively.
According to the McConnell rule,13 the contacts through
S1‚‚‚ C5′, S1‚‚‚ C19′, and N1‚‚‚ C5′ are ferromagnetic, and
the contracts through S1‚‚‚ C4′ and S1‚‚‚ C6′ are anti-
ferromagnetic. Since the above analysis shows that the
intrachain interaction is ferromagnetic, the contacts
through S1‚‚‚ C5′, S1‚‚‚ C19′, and N1‚‚‚ C5′are dominant.
In conclusion, we have successfully isolated a formyl-
group-carrying thioaminyl radical, and the regular linear-
chain ferromagnetic interaction could be satisfactorily
explained by the X-ray crystal structure. This is the
second ferromagnetic thioaminyl whose magnetic behav-
ior could be explained by the X-ray crystallographic
results. The magnetic data analyzed on the basis of the
X-ray crystallographic structures are particularly impor-
tant for robust developments of the magnetic study.
F IGURE 3.
theoretical values calculated using eq 1.
ømolT vs T plots of 2c. The solid curve is the
agreement with the observed ones, strongly suggesting
that the spin density distribution shown in Figure 2 is
reliable.
Magnetic susceptibility (ø) measurements for 2c were
carried out using polycrystalline samples with SQUID.
The diamagnetic components were estimated by the
usual Pascal sum rule. Figure 3 shows ømolT vs T plots.
The ømolT value at the high-temperature region (∼300
K) is 0.350 emu K mol-1, which corresponds to 93% of
the Curie constant (0.376 emu K mol-1 for the S ) 1/2
spin system), indicating the purity of the radical is 93%.
The ømolT is constant in the temperature range from
300 to 50 K and below 50 K ømolT increases with
decreasing temperature and gives a maximum in ømolT
at 10 K. Below 10 K, the ømolT decreases with decreasing
temperature. This magnetic behavior suggests that the
intrachain interaction is ferromagnetic and the inter-
chain interaction is antiferromagnetic. As found in Figure
4, the radical molecules are stacked along the a axis in
a zigzag fashion. Taking this X-ray result into account
we analyzed the ferromagnetic behavior with the regular
Heisenberg model12 (eq 1), and the best-fit value of the
regular linear-chain interaction, 2J 1/kB, was determined
to be +3.2 K (27 cal mol-1).
Exp er im en ta l Section
Gen er a l P r oced u r e for P r ep a r a tion of N-(Ar ylth io)-4-
for m yl-2,6-d ip h en yla n ilin es (1). A solution of arenesulfenyl
chloride (10 mmol) in CH2Cl2 (5 mL) was added dropwise to a
stirred solution of 4-formyl-2,6-diphenylaniline (2.00 g, 7.3 mmol)
and Et3N (4.0 mL) in dry ether (300 mL) at 0 °C. After being
stirred for 2 h at the same temperature, the mixture was filtered
and evaporated. The residue was then chromatographed on
alumina with 4:5 (1a ), 3:1 (1b), 1:3 (1c), or 1:1 CH2Cl2/hexane
(1d ) at room temperature (1a and 1b) or 0 °C (1c and 1d ).
Compounds 1a and 1b were further purified with a recycle
preparative HPLC instrument.
Isola tion of N-[(2,4-Dich lor op h en yl)th io]-4-for m yl-2,6-
d ip h en ylp h en yla m in yl (2c). A mixture of 1 (100 mg) and K2-
CO3 (300 mg) in benzene (10 mL) was vigorously stirred. After
PbO2 (2.5 g) was added during 0.5 min in some portions, the
mixture was further stirred for 1 min and filtered. After the
benzene was removed by freeze-drying, the residue was sub-
jected to chromatography on alumina. Elution of the green zone
with benzene gave 1c in 56% yield, and subsequent elution of
the orange zone with ethyl acetate gave 3c in 18% yield. Data
for 2c: dark green needles (from hexane/ethyl acetate); mp
H ) -2J 1ΣSi‚Sj
(1)
The nearest distances between the neighboring radical
molecules are found for S1‚‚‚ C5′ (3.76 Å), S1‚‚‚ C4′ (4.06
(12) Bonner, J . C.; Fisher, M. E. Phys. Rev. 1964, A135, 640.
(13) McConnell, H. M. J . Chem. Phys. 1963, 39, 1910.
8246 J . Org. Chem., Vol. 68, No. 21, 2003

128-130 °C; IR (KBr) 1680 cm-1 (CHO); UV-vis (benzene) 394
(15200), 445 (12800), 572 (2980, s), 656 nm (5590 L mol-1 cm-1).
Anal. Calcd for C25H16Cl2NOS: C, 66.82; H, 3.59; N, 3.12.
Found: C, 66.64; H, 3.70; N, 3.06.
NOS: C, 66.06; H, 3.46: N, 3.21. Found: C, 65.87; H, 3.68; N,
3.21.
N-[(2,4-Dich lor oph en yl)th io]-2,6-diph en yl-p-ben zoqu in o-
n e Mon oim in e (3c): orange prisms (from hexane/ethyl
acetate); mp 185-186 °C; IR (KBr) 1625 cm-1 (CdO); FABMS
Su p p or t in g In for m a t ion Ava ila b le: Preparation of
4-formyl-2,6-diphenylaniline and 1a -d , 1H NMR spectrum of
3c, and X-ray crystallographic data of 2c and 3c. This material
is available free of charge via the Internet at http://pubs.acs.org.
1
m/z 436 (M+ + 1, 26); H NMR (CDCl3) δ 6.63 (br s, 2H), 6.70
(d, J ) 8.8 Hz, 1H), 6.85 (dd, J ) 8.8 and 2.0 Hz, 1H), 7.16 (d,
J ) 2.0, 1H), 7.43-7.53 (br m, 10H). Anal. Calcd for C24H15Cl2-
J O0300789
J . Org. Chem, Vol. 68, No. 21, 2003 8247