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 [19]

Full text of DOI:10.1021/ja9717929

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

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 5, 1998 1107
key peaks appearing in the wings away from the g ∼ 2 region
were only vague. Nevertheless, considering a highly symmetric
(C3) molecular structure of the tris(carbene) 6 and the projection
factor of spin quantum number S on fine structure constants, we
expected a vanishing E value as well as a smaller negative D
value and high spectral density in the g ∼ 2 region, but robust
π-spin polarization was anticipated for a symmetric septet (S )
3) tris(carbene) such as 6.2c It is obvious, however, that well-
established conventional continuous-wave (CW) ESR spectros-
copy for randomly oriented organic high-spin species fails to give
evidence of the unequivocal spin identification of the tris(carbene)
under study.
To identify the spin multiplicity of the tris(carbene) 6 un-
equivocally, field-swept two-dimensional electron spin transient
resonance (2D-ESTN) spectroscopy for nonoriented high-spin
systems15,16 was invoked. This technique is based a on pulsed
FT ESR spectroscopic method15-18 and is capable of elaborating
straightforward information on electronic and environmental
structures of high-spin species even in amorphous materials, which
conventional CW ESR cannot provide.15,16 According to a
quantum mechanical treatment for the nutational motion of the
spin magnetization in the presence of both the static magnetic
and microwave fields, the nutation frequency ωn observed for
the allowed |S,MS T |S,MS(1 ESR transition in the extreme
weak limit (H1 , HD) for the microwave irradiation, H1, is simply
expressed as
Figure 1. Contour plot of field-swept 2D nutation spectra of 6 observed
at 5 K in a 2-MTHF glass. On the right, an electron spin-echo detected
fine-structure ESR spectrum is given.
ωn ) [S(S + 1) - MS(MS ( 1)]1/2ω1, (S g 1)
(1)
where MS denotes the electron spin sublevel involved in the
transition13-16 and HD stands for fine-structure terms. Thus, the
nutation frequency spectrum depends on S, MS, and ω1. ω1 is in
proportion to the strength of the microwave field B1 (ω1 ) -γeB1)
and corresponds to the reference frequency for S ) 1/2. γe stands
for the gyromagnetic ratio of electron. The magnetic field swept
2D-ESTN spectroscopy has been developed and can be appreci-
ated for nonoriented high-spin mixtures15,16 as well as fine-
structure spectra in which high spectral density around the g ∼ 2
region dominates, as shown in this work.
Figure 1 shows the corresponding contour plot (bottom) of
field-swept 2D-ESTN spectra obtained in the photolysis of 5 under
the same conditions as for the CW ESR spectrum. On the right
of the contour plot, an electron spin-echo detected ESR fine-
structure spectrum is given which corresponds to the conventional
fine-structure ESR spectrum in an integrated mode. Figure 2
shows six typical slices of the nutation spectra in the range 200-
350 mT. Three dominant peaks are discriminated at 22.0, 27.5,
and 29.5 MHz which are denoted by a, b, and c, respectively.
According to eq 1, the observed ratio of 22.0:27.5:29.5 uniquely
agrees well with the theoretical value x6ω1:x10ω1:2x3ω1. These
theoretical frequency values straightforwardly correspond to the
Figure 2. Six typical slices of the 2D nutation spectra in the range 200-
350 mT of Figure 1.
allowed ESR transitions for septet states, unequivocally identifying
that the observed fine-structure spectrum is due to a spin-septet
state. Up to 90 K from 2.6 K, no other nutation peaks attributable
to lower spin states (S g 1) were observed, showing the septet
state to be the ground state with the excited low-spin states located
above 300 cm-1. We notice that these experiments alone cannot
completely rule out the possible location of spin-singlet states
nearby the septet state within a few cm-1 although it is unlikely.
A nutation peak arising from any doublet species of byproducts
was not detected, showing a remarkable chemical stability of the
tris(carbene) 6. Also, a marked thermal stability of 6 manifested
itself during an annealing process. A significant change in the
ESR signal shape was observed when the matrix temperature was
raised above 140 K and kept for 15 min. New fine-structure
signals were irreversible with temperature, and they were
characteristic of the small fine-structure constants. They were
also identified to originate in a septet ground state by 2D-ESTN
spectroscopy. This is in a sharp contrast with that observed for
septet-state 1,3,5-tris(phenylmethylene)benzene, which has been
shown to be persistent only up to 852c and 50 K19 in rigid glasses.
(13) Takui, T.; Sato, K.; Shiomi, D.; Itoh, K. In Molecular Based Magnetic
Materials; Theory, Techniques, and Applications; Turnbull, M. M., Sugimoto,
T., Thompson, L. K., Eds.; American Chemical Society: Washington, DC,
1996; pp 81-98.
(14) Teki, Y.; Takui, T.; Itoh, K.; Iwamura, H.; Kobayashi, K. J. Am. Chem.
Soc. 1986, 108, 2147.
Acknowledgment. This work was supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science and Culture,
Japan.
(15) (a) Sato, K.; Shiomi, D.; Takui, K.; Itoh, K.; Kaneko, T.; Tsuchida,
E.; Nishide, H. J. Spectrosc. Soc. Jpn. 1994, 43, 280. (b) Takui, T.; Sato, K.;
Shiomi, D.; Itoh, K.; Kaneko, T.; Tsuchida, E.; Nishide, H. In Magnetism: A
Supramolecular Function; Kahn, O., Ed.; Kluwer Academic Publishers:
Dordrecht, The Netherlands, 1996; pp 249-280.
Supporting Information Available: Experimental details and spec-
troscopic data for 5 and more detailed analysis of molecular and electronic
spin structures for tris(carbene) 6 (8 pages). See any current masthead
page for ordering and Web access instructions.
(16) (a) Yano, M.; Sato, K.; Shiomi, D.; Ichimura, A.; Abe, K.; Takui, T.;
Itoh, K. Tetrahedron Lett. 1996, 37, 9207. (b) Sato, K.; Yano, M.; Furuichi,
M.; Shiomi, D.; Takui, T.; Abe, K.; Itoh, K.; Higuchi, A.; Katsuma, K.; Shirota,
Y. J. Am. Chem. Soc. 1997, 119, 6607.
JA9717929
(17) Isoya, J.; Kanda, H.; Norris, J. R.; Tang,.; Bowman, M. K. Phys. ReV.
1990, B41, 3905.
(19) Matsuda, K.; Takahashi, K.; Inoue, K.; Koga, N.; Iwamura, H. J. Am.
Chem. Soc. 1995, 117, 5550.
(18) Astashkin, A. V.; Schweiger, A. Chem. Phys. Lett. 1990, 174, 595.