1106
J. Am. Chem. Soc. 1998, 120, 1106-1107
Scheme 1a
Persistent High-Spin Polycarbene. Generation of
Polybrominated 1,3,5-Tris-[2-[4-(Phenylcarbeno)-
phenyl]ethynyl]benzene (S ) 3) and Spin
Identification by Two-Dimensional Electron Spin
Transient Nutation Spectroscopy
Hideo Tomioka,*,† Makoto Hattori,† Katsuyuki Hirai,†
Kazunobu Sato,‡ Daisuke Shiomi,§ Takeji Takui,*,‡ and
Koichi Itoh*,§
Chemistry Department for Materials, Faculty of Engineering
Mie UniVersity, Tsu, Mie 514, Japan
Departments of Chemistry and Material Science
Faculty of Science, Osaka City UniVersity
Sumiyoshi-ku, Osaka 558, Japan
ReceiVed June 2, 1997
Since bis(m-phenylenecarbene) was established to have a
quintet ground state in 1967,1 triplet diphenylcarbene units have
served as the source of electron spins in constructing high-spin
molecules as models for purely organic ferromagnetics.2 Thus,
poly(m-phenylenecarbenes) have been most systematically studied
inter alia and are accepted as the highest spin organic molecules.3,4
The highly transient nature of the species,5 however, is an inherent
drawback for further extension to usable magnetic materials. In
this light, it is highly desirable to synthesize persistent triplet
carbenes and to connect them in a ferromagnetic fashion with an
appropriate topological coupler. During the course of our efforts
to stabilize a triplet carbene, we were able to generate fairly stable
triplet diarylcarbenes; polybrominated diphenylcarbenes not only
survived for minutes in solution and but also existed for years in
the crystalline state at room temperature.6 So, the next step should
be to explore a way to connect them with robust π-spin
polarization retained. We prepared polybrominated tris(diazo-
diarylmethane) 5 coupled by a topological linker, 1,3,5-triethyn-
ylbenzene, and demonstrated that the tris(carbene) 6 generated
therefrom is persistent and spin-septet in its electronic ground
state.
Diazo groups are generally labile, and hence, these groups are
usually introduced at the last step of the synthesis when preparing
not only mono(diazo)7 but also poly(diazo) compounds.4 Ap-
plication of this strategy to these highly sterically congested diazo
compounds, prepared mostly by base treatment of N-nitrosocar-
bamates,7,8 is almost impossibly difficult. Fortunately, however,
in this work, the polybromodiphenyldiazomethane 3 was found
to be stable enough to survive in the presence of Pd(0) and CuI
under Sonogashira coupling reaction conditions.9 This is obvi-
ously due to the fact that the four bromine groups at the ortho
a (i) HNO3/H2SO4; (ii) H2/Pd-C; (iii) NaBH4; (iv) Br2; (v) NaNO2/
H+/H3PO2; (vi) SOCl2; (vii) H2NCO2Et/AgBF4; (viii) NaNO2; (ix) tBuOK;
(x) Me3SiCtCH/(Ph3P)2PdCl2/CuI; (xi) KOH/MeOH; (xii) 1,3,5-I3C6H3/
(Ph3P)2PdCl2/CuI; (xiii) hν (λ > 300 nm).
positions which effectively protect the carbene center from the
external reagents also are able to protect equally effectively the
diazo carbon. Moreover, each of the four ortho carbons is also
protected by these bromine groups from the approach of external
reagents. This enabled us to introduce connecting groups at the
para position of the monomer diazo compound (e.g., 3) to give
the functionalized monomer 4 and then to connect it to an
appropriate linker satisfying the topological requirement for
intramolecular ferromagnetic spin alignment. Thus, we were able
to introduce an ethynyl functional group very selectively at the
para position in a reasonable yield. Deprotection of the trimeth-
ylsilyl group proceeded almost quantitatively to give 4. Subse-
quent coupling with 1,3,5-triiodobenzene took place smoothly to
produce tris(diazo) compound 5,10 where three carbene precursor
units are properly introduced so as to generate tris(carbene)
connected in a ferromagnetic fashion.2,3,11
Photolysis (λ > 300 nm) of 5 in a 2-methyltetrahydrofuran
(2-MTHF) glass at 5 K gave a fine-structure ESR spectrum12
which was different from that observed by the photolysis of
hexabromodiphenyldiazomethane.6 Salient features13,14 of organic
high-spin species apparently failed to show up in the observed
spectrum. Canonical peaks dominated in the g ∼ 2 region, and
(9) Sonogashira, K. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon Press: Oxford, U.K., 1991; Vol. 3, pp 521-
549.
(10) The tris(diazo) compound (5) was obtained as rather stable orange
solid after purification by gel permeation chromatography on a Shodex GPC
H-2001 column with CHCl3 eluent and characterized by NMR spectroscopy.
See Supporting Information for details.
(11) (a) Iwamura, H. Pure Appl. Chem. 1993, 65, 57. (b) Ichimura, A. S.;
Koga, N.; Iwamura, H. J. Phys. Org. Chem. 1994, 7, 207. (c) Borden, W. T.;
Iwamura, H.; Berson, J. A. Acc. Chem. Res. 1994, 27, 109.
(12) ESR/ESTN measurements were made on Brucker ESR 300/380 FT-
pulsed spectrometers. Sample temperatures were controlled by a Oxford ESR
910 helium gas flow system equipped on the ESP 300 spectrometer.
† Mie University.
‡ Department of Chemistry, Osaka City University.
§ Department of Material Science, Osaka City University.
(1) (a) Itoh, K. Chem. Phys. Lett. 1967, 1, 235. (b) Wasserman, E.; Murray,
R. W.; Yager, W. A.; Trozzolo, A. M.; Smolinski, G. J. Am. Chem. Soc.
1967, 89, 5076.
(2) (a) Itoh, K. Solid-State Phys. (Bussei) 1971, 12, 635. (b) Takui, T.;
Itoh, K. Chem. Phys. Lett. 1973, 19, 120. (c) Takui, T. Ph.D. Thesis, Osaka
University, 1973. (d) Itoh, K. Pure Appl. Chem. 1978, 50, 1251. (e) Iwamura,
H. AdV. Phys. Org. Chem. 1990, 26, 179.
(3) (a) Morimoto, S.; Tanaka, F.; Itoh, K. Mataga, N. The Preprints of
Molecular Structure; Chemical Society of Japan: Tokyo, 1968; pp 67-68.
(b) Mataga, N. Theor. Chim. Acta 1968, 10, 372.
(4) Matsuda, K.; Nakamura, N.; Inoue, K.; Koga, N.; Iwamura, H. Bull.
Chem. Soc. Jpn. 1996, 69, 1483 and references therein.
(5) Lifetime of triplet diphenylcarbene is estimated to be only 2 µs in
solution at room temperature: Hadel, L. M.; Maloney, V. M.; Platz, M. S.;
McGimpsey, W. G.; Scaiano, J. C. J. Phys. Chem. 1986, 90, 2488.
(6) Tomioka, H. Acc. Chem. Res. 1997, 30, 315.
(7) Regitz, M.; Maas, G. Diazo Compounds. Properties and Synthesis;
Academic Press: London, 1986.
(8) Zimmerman, H. E.; Paskovich, D. H. J. Am. Chem. Soc. 1964, 86, 2149.
S0002-7863(97)01792-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/27/1998