A dendrimer approach to high-spin polycarbenes. Conversion of connectivity from disjoint to non-disjoint by perturbation of nonbonding molecular orbital coefficients

Article abstract of DOI:10.1021/ol060378n

Pentakis(diazo) compound was prepared by coupling 3,5-bis[4-[diazo(4-tert- butyl-2,6-dimethylphenyl)methyl]-3,5-dibromophenylethynyl]phenylacetylene with bis(4-iodo-2,6-dimethylphenyl)diazomethane under Sonogashira reaction conditions. Pentakis(carbene) g

Full text of DOI:10.1021/ol060378n

ORGANIC
LETTERS
2006
Vol. 8, No. 9
1847-1850
A Dendrimer Approach to High-Spin
Polycarbenes. Conversion of
Connectivity from Disjoint to
Non-Disjoint by Perturbation of
Nonbonding Molecular Orbital
Coefficients
Katsuyuki Hirai,*,† Eiko Kamiya,‡ Tetsuji Itoh,‡ and Hideo Tomioka*,‡,§
Instrumental Analysis Facilities, Life Science Research Center, and Chemistry
Department for Materials, Faculty of Engineering, Mie UniVersity,
Tsu, Mie 514-8507, Japan, and Department of Applied Chemistry,
Aichi Institute of Technology, Toyota, Aichi 470-0392, Japan
hirai@chem.mie-u.ac.jp
Received February 13, 2006
ABSTRACT
Pentakis(diazo) compound was prepared by coupling 3,5-bis[4-[diazo(4-tert-butyl-2,6-dimethylphenyl)methyl]-3,5-dibromophenylethynyl]-
phenylacetylene with bis(4-iodo-2,6-dimethylphenyl)diazomethane under Sonogashira reaction conditions. Pentakis(carbene) generated by
irradiation of the pentakis(diazo) compound was shown to have a high-spin state with S
) 4.4 at 2.0 K.
Triplet carbenes are regarded as more effective spin sources
than radicals since the spin states (S )1) are double that of
radicals (S ) 1/2) and the magnitude of the exchange
coupling between the neighboring centers is larger in triplet
carbenes than in radicals.1,2 Moreover, the photolytic produc-
tion of polycarbenes is possible even in solid solution at
cryogenic temperatures if poly(diazo) precursors are avail-
able. Actually, Iwamura and co-workers have prepared a
starburst-type nonakis(diazo) compound, which gives a
nonakis(carbene) with a nonadecet ground state (S ) 9) upon
irradiation at low temperature.2
However, those systems have two disadvantages that
hinder their further extension to usable magnetic materials.
First, a triplet carbene unit is highly unstable and lacks
stability for practical application under ambient conditions.
† Instrumental Analysis Facilities, Mie University.
‡ Chemistry Department for Materials, Faculty of Engineering, Mie
University.
(2) (a) Nakamura, N.; Inoue, K.; Iwamura, H.; Fujioka, T.; Sawaki, Y.J.
Am. Chem. Soc. 1992, 114, 1484. (b) Nakamura, N.; Inoue, K.; Iwamura,
H. Angew. Chem., Int. Ed. Engl. 1993, 32, 872. (c) Matsuda, K.; Nakamura,
N.; Takahashi, K.; Inoue, K.; Koga, N.; Iwamura, H. J. Am Chem. Soc.
1995, 117, 5550. (d) Matsuda, K.; Nakamura, N.; Inoue, K.; Koga, N.;
Iwamura, H. Chem. Eur. J. 1996, 2, 259. (e) Matsuda, K.; Nakamura, N.;
Inoue, K.; Koga, N.; Iwamura, H. Bull. Chem. Soc. Jpn. 1996, 69, 1483.
§ Aichi Institute of Technology.
(1) (a) Koga, N.; Iwamura, H. In Carbene Chemistry; Bertrand, G., Ed,;
Fontis Media: Lausanne, 2002; pp 271-296. (b) Matsuda, K.; Nakamura,
N.; Takahashi, K.; Inoue, K.; Koga, N.; Iwamura, H. Molecule-Based
Magnetic Materials; ACS Symposium Series 644; American Chemical
Society: Washington, 1996; p 142.
10.1021/ol060378n CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/07/2006

Second, diazo groups are also generally labile,3 and, hence,
the diazo compound cannot be used as a building block to
prepare a more complicated poly(diazo) compound.
peripheral 6 diazo units, which was expected to generate
hexakis(carbene) with an S ) 6 ground state.9 However,
carbenes generated from the hexakis(diazo) compound were
found to have a low spin state of less than 2. The results
can be reasonably interpreted in terms of the disjoint-
nondisjoint concept based on the MO theory.10,11 The
hexakis(carbene) consists of a dendritic structure, which has
three bis(carbene) units on the terminal of each branches.
The connectivity of two carbene units within this bis(carbene)
unit is nondisjoint, and hence, they interact in a ferromagnetic
fashion with the spin quantum number S ) 2. However, the
connectivity between three bis(carbene) units through the
central benzene ring is disjoint. Thus, there are no magnetic
interactions between three bis(carbene) units through the core
part in the hexakis(carbene) (Scheme 1).
However, the connectivity can be improved from disjoint
to nondisjoint by introducing a new spin system so as to
disturb NBMO coefficients in the original spin systems. In
the hexakis system, for instance, this will be achieved by
incorporating a new carbene unit between the bis(carbene)
units and core benzene. To prove this idea, we prepared a
model pentakis(diazo) compound where two bis(diazo) units
are connected by an interior diazo group so as to make all
five carbene units interact ferromagnetically and character-
ized pentakis(carbene) generated from the pentakis(diazo)
compound.
We have made great efforts to stabilize a triplet carbene
and succeeded in preparing fairly stable ones surviving for
days in solution at room temperature.4,5 We also found that
a diphenyldiazomethane prepared to generate a persistent
triplet carbene is also persistent for the diphenyldiazo
compound and, hence, can be further modified, with the diazo
group intact, into a more complicated diazo compound.6-9
For instance, (2,4,6-tribromophenyl)(2,6-dibromo-4-tert-
butylphenyl)diazomethane is stable enough to survive under
mild Sonogashira coupling reaction conditions and undergoes
a coupling reaction with trimethylsilylacetylene to give (2,6-
dibromo-4-trimethylsilylethynylphenyl)(2,6-dibromo-4-tert-
butylphenyl)diazomethane. Three units of the diazo com-
pound are introduced at the 1-, 3-, and 5-positions of a
benzene ring through the ethynyl group by employing a
similar coupling reaction to give a tris(diazo) compound,
which eventually generates fairly persistent tris(carbene) with
a septet ground state upon irradiation (Scheme 1).6
Scheme 1
The interior diazo compound used in this study was bis-
(4-iodo-2,6-dimethylphenyl)diazomethane (1), which was
prepared from the corresponding dibromocarbamate12 ac-
cording to the procedure outlined in Scheme 2. Coupling of
Scheme 2
1 with 2 equiv of a bis(diazo) compound (2),9 carrying two
persistent triplet carbene precursor, resulted in the formation
(4) (a) Tomioka, H. Acc. Chem. Res. 1997, 30, 315. (b) Tomioka, H. In
AdVances in Carbene Chemistry; Brinker, U., Ed.; JAI Press: Greenwich,
CT, 1998; Vol. 2, pp 175-214. (c) Tomioka, H. In Carbene Chemistry;
Bertrand G., Ed.; Fontis Media: Lansanne, 2002; pp 103-152.
(5) (a) Tomioka, H.; Iwamoto, E.; Itakura, H.; Hirai, K. Nature 2001,
412, 626. (b) Iwamoto, E.; Hirai, K.; Tomioka, H. J. Am. Chem. Soc. 2003,
125, 14664.
(6) Tomioka, H.; Hattori, M.; Hirai, K.; Sato, K.; Shiomi, D.; Takui, T.;
Itoh, K.; J. Am. Chem. Soc. 1998, 120, 1106.
(7) Itoh, T.; Hirai, K.; Tomioka, H. J. Am. Chem. Soc. 2004, 126, 1130.
(8) Ohtsuka, Y.; Itoh, T.; Hirai, K.; Tomioka, H.; Takui, T. Org. Lett.
2004, 6, 847.
By extending this method, we were able to prepare
phenylacetylene dendritic hexakis(diazo) compound having
(3) Regitz, M.; Maas, G. Diazo Compounds-Properties and Synthesis;
Academic Press: Orlando, 1986.
(9) Itoh, T.; Maemura, T.; Ohtsuka, Y.; Ikari, Y.; Wildt, H.; Hirai, K.;
Tomioka, H. Eur. J. Org. Chem. 2004, 2991.
1848
Org. Lett., Vol. 8, No. 9, 2006

of a desired pentakis(diazo) compound (3a) as red solids in
54% yield under mild Sonogashira coupling reaction condi-
tions (Scheme 3).13
Thawing the matrix containing pentakis(carbene) and
recooling again to 77 K to measure the signal could estimate
the thermal stability of the pentakis(carbene). This procedure
can compensate for the weakening of signals due to Curie’s
law. When the matrix containing 3b was warmed gradually
in 10 K increments, the signal became sharp around 90 K
(Figure 1b). This change was not reversible; when the sample
was recooled at 77 K, no change took place, apart from an
increase in the signal intensity according to Curie’s law. This
is interpreted in terms of geometrical change of carbenes
often observed for sterically congested carbenes.14a,15 The
signals did not decay appreciably up to 100 K, started to
decompose at around 110 K, and disappeared irreversibly
above 150 K (Figure 1c,d). The ESR signals of bis(2,6-
dimethylphenyl)carebene,14a i.e., the interior carbene unit and
(2,6-dibromo-4-trimethylsilylethynylphenyl)(4-tert-butyl-2,6-
dimethylphenyl)carebene,16 i.e., the exterior carbene unit,
were found to disappear in 2-MTHF matrix at 140 and 160
K, respectively. Thus, the thermal stability of pentakis-
(carbene) (3b) is essentially identical with that of these
monocarbenes.
Scheme 3
When similar irradiation of 3a (7.7 × 10-4 M) in a
2-MTHF matrix at 77 K was monitored by UV/vis spec-
troscopy, a sharp and strong absorption band at 386 nm was
observed at the expense of the original absorption due to
3a. Since ESR signals ascribable to pentakis(carbene) are
observed under identical conditions, the absorption spectrum
can be assigned to 3b. Upon thawing the matrix, the band
became sharper and shifted to 389 nm at around 110 K and
was observable up to 140 K (Figure S2, Supporting Informa-
tion). This change can be attributed to the geometrical change
of the carbenes, as has been revealed in ESR experiments.
To obtain evidence concerning the spin states of the
photoproducts from 3a, the magnetic susceptibility of the
photoproducts was measured. A 2-MTHF solution of pen-
takis(diazo) compound (3a, 0.3 mM) was placed inside the
sample compartment of a superconducting quantum interfer-
ence device (SQUID) magnet/susceptometer and was irradi-
ated at 5-10 K with a light (λ ) 488 nm) from an argon
ion laser through an optical fiber. The development on
magnetization at 5 K in a constant field of 5 kOe with the
irradiation time for the pentakis(diazo) compound was
measured in situ. As the irradiation time was increased, the
magnetization values gradually increased and reached a
plateau after 20 min. After the magnetization values reached
a plateau, the magnetization values after irradiation, Ma, were
measured at 2.0 and 5.0 K in a field range of 0-50 kOe.
The magnetization values of the sample before irradiation,
Irradiation (λ > 300 nm) of 3a (4.0 × 10-3 M) in
2-methyltetrahydrofuran (2-MTHF) at 77 K gave ESR
spectra that were completely different from those observed
for the corresponding triplet diphenylcarbenes.14-16 The
spectra showed rather broad signal centered around ca. 330
mT (Figure 1a). The broad signal is consistent with the
Figure 1. (a) ESR spectra obtained by irradiation of pentakis-
(diazo) compound 3a in 2-methyltetrahydrofuran at 77 K and
(b-d) the same sample observed at 77 K after warming to (b) 90
K, (c) 110 K, and (d) 150 K.
(12) Itoh, T.; Matsuno, M.; Ozaki, S.; Hirai, K.; Tomioka, H. J. Phys.
Chem. B 2005, 109, 20763.
(13) Sonogashira, K. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I. Eds.; Pergamon Press: Oxford, UK, 1991; Vol. 3, pp 521-
549.
(14) (a) Hu, Y.; Hirai, K.; Tomioka, H.J. Phys. Chem. A 1999, 103, 9280.
(b) Tomioka, H.; Hu, Y.; Ishikawa, Y.; Hirai, K. Bull. Chem. Soc. Jpn.
2001, 74, 2207.
(15) Tomioka, H.; Watanabe, T.; Hattori, M.; Nomura, N.; Hirai, K. J.
Am. Chem. Soc. 2002, 124, 474.
tendency that, as the spin multiplicity became higher, the D
value became smaller.17
(10) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587.
(11) (a) Borden, W. T.; Iwamura, H.; Berson, J. A. Acc. Chem. Res.
1994, 27, 109. (b) Borden, W. T. In Diradicals; Borden, W. T., Ed.;
Wiley: New York, 1982; pp 1-12.
(16) Itoh, T.; Jinbo, Y.; Hirai, K.; Tomioka, H. J. Org. Chem. 2004, 69,
4238.
(17) Teki, Y.; Takui, K.; Yagi, H.; Itoh, K.; Iwamura, H. J. Chem. Phys.
1985, 83, 539.
Org. Lett., Vol. 8, No. 9, 2006
1849

Mb, were also measured under the same conditions. The
magnetization due to the species generated by photolysis was
then obtained by subtracting the corresponding values
obtained before and after irradiation. Thus, the effect of any
paramagnetic impurities could be canceled by this treatment.
The plots of the magnetization (M) versus the temperature-
normalized magnetic field (H/T) were analyzed in terms of
the Brillouin function as follows2,18,19
The observed data were best fitted with eq 1 with S ) 4.4
(F ) 0.91) at 2.0 K and S ) 4.2 (F ) 0.96) at 5.0 K.
Although the S values are somewhat smaller than the
theoretical value of 5, the results clearly indicate that the
five carbene centers interact ferromagnetically. In other
words, the connectivity is switched from disjoint to nondis-
joint by the new carbene unit between the bis(carbene) units
and core benzene.
Since the magnetization data at two different temperatures
were fitted to the same Brillouin function, the sample is
considered to be free from ferro- and antiferromagnetic
intermolecular interactions. Somewhat smaller values in S
and F are probably ascribed to a carbene defect due to
photodecomposition of carbenes and/or due to purity of
precursor 3a.
The present study clearly demonstrates that the potential
problem in the dendrimer approach to generate high-spin
polycarbenes is relatively easily solved by changing the
“wrong” connectivity through a new carbene center in the
appropriate position. The results also indicate that this is
realized by the persistent nature of our diazo compounds,
which allows us to introduce a diazo unit at a desired
position. Thus, a step toward a persistent triplet carbene will
eventually lead us to a persistent high-spin polycarbene.
M ) Ma - Mb ) FNgJµBB(ø)
(1)
where F is the generation factor for carbene obtained from
saturated magnetization (Ms), N is the number of the
molecule, J is the quantum number for the total angular
momentum, µB is Bohr magneton, and g is the Lande´
g-factor. Since the carbenes are composed of light elements,
the orbital angular momentum should be negligible, and J
can be replaced with the spin quantum number S. The M/Ms
versus H/T plot is shown in Figure 2 with theoretical curves
with S ) 3, 4, and 5.
Acknowledgment. We are grateful to the Ministry of
Education, Culture, Sports, Science and Technology of Japan
for support of this work through a Grant-in-Aid for Scientific
Research for Specially Promoted Research (No. 12002007).
Supporting Information Available: Preparation of 1 and
1
2a, H and 13C NMR spectra of 3a (Figure S1), UV-vis
spectra of 3b (Figure S2), plot of magnetization vs irradiation
time in the photolysis of 3a (Figure S3), and field dependence
of the magnetization of the photoproduct from 3a (Figure
S4). This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 2. Plot of M/Ms vs H/T of the photoproduct from 3a
measured at 2.0 and 5.0 K. The solid lines represent theoretical
curves with S ) 3, 4, and 5.
OL060378N
(18) Rajca, A. Chem. ReV. 1994, 94, 871.
(19) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, 1986.
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