Synthesis and identification of a trimethylenemethane derivative π-extended with three pyridinyl radicals

Article abstract of DOI:10.1021/ol902937k

A trimethylenemethane (TMM) derivative 1^2., In which the parent TMM is π-extended by the symmetric Insertion of three pyridine rings Into the C-C bonds of TMM, has been synthesized by the alkali metal reduction of the Isolated corresponding dication. Although the frozen-glass X-band cw-ESR spectrum of 1^2. gave unresolved fine structures due to the small ZFS parameters, pulsed ESR two-dimensional electron spin transient nutation (2D-ESTN) spectroscopy unambiguously can afford to Identify diradical 1 ^2. as a triplet species.

Full text of DOI:10.1021/ol902937k

ORGANIC
LETTERS
2010
Vol. 12, No. 4
836-839
Synthesis and Identification of a
Trimethylenemethane Derivative
π-Extended with Three Pyridinyl
Radicals
Kouzou Matsumoto,*,† Daisuke Inokuchi,† Yasukazu Hirao,† Hiroyuki Kurata,†
Kazunobu Sato,‡ Takeji Takui,‡ and Takashi Kubo*,†
Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka,
Osaka 560-0043, Japan, and Departments of Chemistry and Materials Science,
Graduate School of Science, Osaka City UniVersity, Sugimoto,
Sumiyoshi-ku, Osaka 558-8585, Japan
kmatsu@chem.sci.osaka-u.ac.jp; kubo@chem.sci.osaka-u.ac.jp
Received December 21, 2009
ABSTRACT
A trimethylenemethane (TMM) derivative 12•, in which the parent TMM is π-extended by the symmetric insertion of three pyridine rings into the
C-C bonds of TMM, has been synthesized by the alkali metal reduction of the isolated corresponding dication. Although the frozen-glass
X-band cw-ESR spectrum of 12• gave unresolved fine structures due to the small ZFS parameters, pulsed ESR two-dimensional electron spin
transient nutation (2D-ESTN) spectroscopy unambiguously can afford to identify diradical 12• as a triplet species.
Trimethylenemethane (TMM) is the simplest non-Kekule´
hydrocarbon1 and has attracted much attention in variuos
fields of chemistry and materials science, such as theoretical
chemistry,2 reactive intermediate,3 organic synthesis,4 and
molecular magnetism.5 Since the first detection of TMM by
Dowd,6 a number of TMM derivatives have been studied.7
A drawback to the studies of TMM derivatives is that the
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† Osaka University.
‡ Osaka City University.
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10.1021/ol902937k  2010 American Chemical Society
Published on Web 01/14/2010

parent TMMs can survive only at cryogenic temperature.
Thus, a synthetic challenge relevant to TMM derivatives is
to chemically stabilize this fascinating π-electron system.
To our knowledge, there are only a few examples of stable
ground-state triplet TMM derivatives. Yang’s diradical, as
shown in Figure 1, is such a TMM derivative and has been
lecular ferromagnetic coupling.12 Here we report the syn-
thesis and direct identification of diradical 12· by pulsed ESR-
based two-dimensional electron spin transient nutation (2D-
ESTN) spectroscopy.13
The synthesis of dication 22+, which is the precursor of
the targeted diradical 12·, is illustrated in Scheme 1.
Scheme 1.
Synthesis of 22+
Figure 1. Stable TMM-based diradicals.
thoroughly investigated. Its ESR spectrum, magnetic sus-
ceptibility, and X-ray crystallographic analysis were re-
ported.8 Iwamura and Matsuda reported a TMM-based
bis(nitroxide) diradical (see Figure 1) whose MnII complex
exhibited the phase transition to a molecule-based bulk
magnet at 5 K.9 Among intriguing stable TMM-based
diradicals, more recently, Rajca and co-workers reported a
remarkably stable hydrocarbon diradical based on the TMM
framework (see Figure 1).10 The quest for new stable TMM
derivatives can afford to contribute to the further develop-
ment of TMM-based open shell chemistry. We have designed
a new TMM diradical 12· (see Figure 1), in which TMM is
π-extended symmetrically with three pyridinyl radical moi-
eties. Since the 2,4,6-triphenylpyridinyl radical is fairly
stable11 and the unpaired electron spins of pyridinyl radicals
are delocalized over the sizable pyridinyl ring, 12· is expected
to be a stable TMM-based diradical with strong intramo-
Kumada-Tamao coupling of 2,6-diphenyl-4-chloropyridine
(3)14 with methylmagnesium iodide in the presence of nickel
catalyst afforded the corresponding 4-methyl derivative 415
in excellent yield. A tris(4-pyridyl)methane derivative 6 was
obtained from 4 by iterative deprotonation-nucleophilic
substitution of 3 by way of bis(4-pyridyl)methane derivative
5.16 Tri-N-methylation of 6 was accomplished with trim-
ethyloxonium tetrafluoroborate to give dication 22+. Since
the BF4 salt of 22+ hardly crystallized, counterion exchange
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Org. Lett., Vol. 12, No. 4, 2010
837

was carried out to obtain the PF6 salt of 22+, which was
To estimate the stability of diradical 12·, the redox
-
obtained as dark red crystals with green luster.
properties of 22+·2(PF6 ) were examined by cyclic voltam-
-
A single crystal of 22+·2(PF6 ) for X-ray crystallographic
metry (Figure 3). Dication 22+ exhibited two reversible
analysis was obtained from a chloroform solution by the
technique of slow vapor diffusion with hexane (Figure 2).17
Figure 3.
Cyclic voltammogram of 22+·2(PF6-) in acetonitrile.
Figure 2
.
ORTEP drawing of 22+ (50% probability). Hydrogen
-
atoms, PF6 ions, and CHCl3 molecules are omitted for clarity.
reduction waves (redE1 ) -1.61 V, redE2 ) -2.04 V vs Fc/
Fc+) as well as one reversible oxidation wave (oxE1 ) +0.40
V).18 Peak intensities observed in the CV and NPV (normal
pulse voltammetry) measurements19 revealed that the two
reduction and oxidation waves involve two-electron and one-
electron processes, respectively. Therefore, dication 22+
exhibits a multiredox system from tricationic monoradical
to dianion with high reversibility. This suggests that diradical
12· is a fairly stable species although the overreduction yields
the corresponding anion or dianion.
A relatively large R1 value is mainly due to the disorder of
chloroform molecules, and the molecular structure of 22+ is
well determined. Although 22+ does not take C3 symmetry
in the crystal, the structures of the three N-methylpyridinium
moieties are similar. The valence bond plane of the central
carbon atom is approximately planar (the sum of the bond
angles around C1 is 359.9°). Three pyridinium rings form a
propeller structure, and the dihedral angles between the
pyridinium rings and the central C1-C2-C8-C14 plane
are 47.0°, 23.7°, and 30.8°. Judging from the bond lengths,
the contribution of the quinonoid structure is appreciable in
all the pyridinium rings, indicating that two positive charges
are delocalized over the three pyridinium rings. On the other
hand, the dihedral angles between the phenyl groups and
pyridinium rings are relatively large (in the range of
44.8°-79.1°). This may be due to the steric repulsion
between the phenyl group and the adjacent methyl group on
the nitrogen site. The repulsion causes the deformation of the
pyridinium ring into a boat form. As a result, the methyl groups
are deviated from the plane of the pyridinium ring.
-
Alkali metal reduction of 22+·2(PF6 ) was performed with
3% Na-Hg in the mixture of degassed acetonitrile-2-
methyltetrahydrofuran (2-MTHF). The reduction was moni-
tored by UV-vis-NIR spectroscopy (Figure 4a). The ab-
sorption peak at 540 nm due to 22+ slightly decreased in
intensity by reduction, and a new broadband appeared in the
range of 700-1000 nm. This broad absorption is ascribable
(12) The singlet-triplet energy gap (∆EST) of TMM was estimated to
be 13-16 kcal mol-1. (a) Wenthold, P. G.; Hu, J.; Squires, R. R.; Lineberger,
W. C. J. Am. Chem. Soc. 1996, 118, 475. (b) Wenthold, P. G.; Hu, J.;
Squires, R. R.; Lineberger, W. C. J. Am. Soc. Mass. Spectrom. 1999, 10,
800.
(13) 2D-ESTN spectroscopy was applied to frozen glasses containing
chemical species with different spin multiplicities and different contributing
weights, for the first time: see ref 20d.
(14) Petrenko-Kritschenko, P.; Scho¨ttle, S. Ber. Dtsch. Chem. Ges. 1909,
42, 2020.
(15) Dilthey, W. J. Prakt. Chem. 1916, 94, 53.
(16) Van Allan, J. A.; Reynolds, G. A. J. Heterocycl. Chem. 1976, 13,
577.
(17) Crystallographic data for 22+·2(PF6-)·1.3CHCl3: C56.3H46.3Cl3.9F12N3P2,
FW ) 1193.10, monoclinic, space group P21/a (no. 14), a ) 17.276(5), b )
17.432(5), c ) 18.466(5) Å, ꢀ ) 91.919(13)°, V ) 5558(3) Å3, Z ) 4,
Dcalcd ) 1.426 g·cm-3, T ) 200 K, of the 49 171 reflections which were
collected, 12 553 were unique (Rint ) 0.037) used in refinement. R1 ) 0.098
(8475 data, I > 2σ(I)), wR2 ) 0.342 (all data), GOF ) 1.305.
Figure 4. (a) UV-vis-NIR spectral change upon the reduction of
22+·2(PF6-) with 3% Na-Hg. (b) X-band ESR spectrum of diradical
12· in CH3CN-MTHF at 77 K.
838
Org. Lett., Vol. 12, No. 4, 2010

to the formation of diradical 12·, noting that Yang’s biradical
also exhibits the weak absorption (λmax ) 810 nm)8f in a
similar range. The spectrum of 12· remained unchanged under
the inert atmosphere at room temperature, suggesting the high
stability of 12·. This absorption band due to 12· disappeared
by further reduction, probably due to formation of the
corresponding dianion species. When the intensity of the
absorption in 700-1000 nm became maximized, we mea-
sured X-band cw-ESR spectra at 77 K in the same acetoni-
trile-2-MTHF glass (Figure 4b). As expected from the
extended nature of the π-electron network of 12· and the
geometrical symmetry of its molecular structure, only a broad
signal with shoulder wings corresponding to canonical peaks
due to the fine-structure D tensor was observed, suggesting
the existence of triplet species in addition to doublet species
such as radical cation or radical anion species. The signal
region of the ESR spectrum covers about 10 mT, and the
estimated zero-field parameter, D-value (|D| ) 0.002 cm-1),
is comparable to those of reported TMM derivative
diradicals.7b,8i,9a,10 Apparently, the signals of diradical 12·
were masked by the doublet species. The fine-structure
forbidden transitions due to ∆Ms ) (2 were too weak to be
detected, being consistent with the calculated transition
intensities.
to the canonical orientations are observed at 35.5 MHz. The
latter peak at 35.5 MHz is unequivocally recognized from
the field slice of the ESTN spectrum at 345.9 mT (Figure
5a). A ratio of the observed nutation frequencies, 35.5/25.2,
corresponds to that of ꢀ2/1 expected for a triplet state from
the equation of ωn ) [S(S + 1) - Ms(Ms - 1)]1/2ω1 in the
weak limit of microwave excitation for S ) 1, where ω1
denotes the nutation frequency for S ) 1/2.19 Therefore, the
nutation frequency peaks at 35.5 MHz are assignable to the
XY canonical peaks of triplet-state diradical 12 in a straight-
forward manner. The contour plot illustrates that the triplet
species is not a trace species but contributes a great deal to
the field-swept ESTN spectra. It seems reasonable that the
|D| value of 12· is comparable to that of Yang’s diradical, by
confirming that the observed nutation peaks at 35.5 MHz
are ascribed to the canonical transitions from the triplet state
in our ESTN experiments. Sophisticated quantum chemical
calculations of zero-field splitting tensors for TMM and
sizable TMM derivatives are underway.21
In summary, we have synthesized a novel TMM diradical
12· as a stabilized TMM derivative. Diradical 12· with three
pyridinyl moieties is stable under an inert atmosphere at room
temperature. The spin multiplicity of 12· has unambiguously
been identified as a triplet species by pulse ESR-based 2D-
ESTN spectroscopy. The experimental D value of 12· has
been derived from the contour plot of the 2D-ESTN spectra
and is comparable to that of Yang’s diradical. This finding
suggests that the spin distribution of 12· is extended over the
three pyridinyl moieties, being consistent with the molecular
orbital calculation. The experimental determination of the
ground state for diradical 12·, the isolation, and quantum
chemical calculations of the zero-field splitting tensors for
TMM and its sizable derivatives are in progress.
To confirm the existence of the targeted diradical 12· in a
straightforward manner, we measured pulse ESR-based
X-band two-dimensional electron spin transient nutation (2D-
ESTN) spectra of 12·. The 2D-ESTN spectroscopy as
transition moment spectroscopy is a powerful method for
the discrimination of spin multiplicity when the sample
contains the chemical species of different spin multiplicity.20
Acknowledgment. This work was supported by a Grant-
in Aid for Scientific Research (No. 21750045) from the Japan
Society for the Promotion of Science and the Global COE
program “Global Education and Research Center for Bio-
Environmental Chemistry” of Osaka University.
Supporting Information Available: Experimental pro-
cedures and product characterization for all new compounds
and the X-ray crystallographic data for compound 22+. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL902937K
(18) The oxidation potential of 22+ is comparable to that of N,N′,N′′-
trimethyl-4,4′,4′′-tripyridocyanine (+0.435 V vs Fc/Fc+). Salbeck, J. Anal.
Chem. 1993, 65, 2169.
Figure 5. Slices and contour plot of the 2D-ESTN spectra of
diradical 12· (T ) 50 K, νMW ) 9.667854 GHz). (a) Field slice of
the 2D-ESTN spectra at 345.9 mT. (b) Contour plot. The broken
lines at 25.2 and 35.5 MHz correspond to the doublet and triplet
species, respectively. (c) Frequency slice of the 2D-ESTN spectra
at 35.5 MHz.
(19) See Supporting Information.
(20) (a) 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. (b) Takui, T.; Sato, K.; Shiomi, D.; Itoh, K. Magnetic Properties
of Organic Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999;
pp 197-236. (c) Sawai, T.; Sato, K.; Ise, T.; Shiomi, D.; Toyota, K.; Morita,
Y.; Takui, T. Angew. Chem., Int. Ed. 2008, 47, 3988. (d) Nakazawa, S.;
Sato, K.; Shiomi, D.; Franco, M. L. T. M. B.; Lazana, M. C. R. L. R.;
Shohoji, M. C. B. L.; Itoh, K.; Takui, T. Inorg. Chim. Acta 2008, 361,
4031.
Figure 5 shows the field-swept 2D-ESTN spectra of
diradical 12· observed at 50 K. Besides an intense nutation
frequency peak at 25.2 MHz, nutation peaks corresponding
(21) Sugisaki, K.; Toyota, K.; Sato, K.; Shiomi, D.; Kitagawa, M.; Takui,
T. Chem. Phys. Lett. 2009, 477, 369. In addition to spin-spin interactions,
the calculations include the contribution from spin-orbit interactions.
Org. Lett., Vol. 12, No. 4, 2010
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