Preparation of poly(phenyl)acetylenes having diazo groups and magnetic characterization of poly(carbene)

Article abstract of DOI:10.1021/ja0455345

(2,6-Dibromo-4-ethynylphenyl)(2,6-dimethyl-4-tert-butylphenyl)diazomethane was found to be stable enough to survive under Rh-complex-catalyzed polymerization conditions and underwent polymerization to give a phenylacetylene polymer having diazo functional

Full text of DOI:10.1021/ja0455345

Published on Web 01/19/2005
Preparation of Poly(phenyl)acetylenes Having Diazo Groups and Magnetic
Characterization of Poly(carbene)
Tetsuji Itoh,† Yoshinobu Jinbo,† Katsuyuki Hirai,‡ and Hideo Tomioka*,†
Chemistry Department for Materials, Faculty of Engineering, and Instrumental Analysis Facilities,
Life Science Research Center, Mie UniVersity, Tsu, Mie 514-8507 Japan
Received July 25, 2004; E-mail: tomioka@chem.mie-u.ac.jp
Scheme 1
Many attempts have been made in order to realize organic
ferromagnetic materials.1,2 Triplet carbenes are regarded as one of
the most effective spin sources since the magnitude of the exchange
coupling between the neighboring centers is large.3 However, these
systems have not been employed extensively due to two disadvan-
tages that hinder their further extension to usable magnetic materials.
First, a triplet carbene unit is highly unstable and lacks the stability
for practical application under ambient conditions. To overcome
these difficulties, we have made great efforts toward stabilization
and succeeded in preparing fairly stable ones that survive for days.4
The second disadvantage that arises is that diazo groups are also
generally labile5 and, hence, are usually introduced at the last step
of synthesis.6
We found that a diphenyldiazomethane for a persistent triplet
carbene is also persistent for the diazo compound7 and, hence, can
be further modified, with the diazo group intact, into a more
due to the CdNdN stretch of the diazo group, whose intensity
relative to the other bands was essentially identical with that
observed for the starting monomer (2-N2). This clearly indicates
complicated diazo compound.8 Thus, the diazo compound can be
used as a building block to prepare a polydiazo compound, which
can then generate high-spin poly(carbene) with considerable thermal
that the diazo group is not decayed during the polymerization
stability. This is a report of the preparation of a poly(phenyl)-
process.
acetylene bearing diazo unit using Rh-complex-catalyzed polym-
erization9,10 and the characterization of poly(carbene) generated from
A 2-methyltetrahydrofuran solution of 3-nN2 was placed inside
the sample compartment of a superconducting quantum interference
those precursor polydiazo compounds.
device (SQUID) magnet/susceptometer and irradiated at 5-10 K
(2,6-Dibromo-4-ethynylphenyl)(2,6-dimethyl-4-tert-butylphenyl)-
with a light (λ ) 488 nm) from an argon ion laser through an optical
fiber. The development on magnetization (M (emu)) at 5 K in a
constant field of 5 kOe over a period of irradiation was measured
in situ. As the irradiation time was increased, the M values gradually
increased and reached a plateau after several hours. After the M
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, Mb, were
also measured under the same conditions. The magnetization (M)
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/
s) versus the temperature-normalized magnetic field (H/T) were
analyzed in terms of the Brillouin function as follows:1c,6,12
diazomethane (2-N2)8c was selected as a starting diazo precursor
for the following reasons. First, it is expected to survive the
chemical manipulations required to prepare the corresponding
polymer since the diazo carbon is well-shielded by four relatively
bulky groups at the ortho positions. Second, it is also expected to
generate a fairly persistent triplet carbene unit upon photoexcitation,
as it has been shown that 1-N2 generates the corresponding triplet
carbene (31), which survives for minutes in solution at room
temperature.11 Finally, alkyl groups, i.e., methyl and tert-butyl
groups, on one side of phenyl ring in 2-N2, are expected to both
protect the carbenic center and also improve solubility, which will
become a serious problem at a higher degree of polymerization.
The starting diazo compound 2-N2 was easily prepared from the
corresponding brominated diazo compound 1-N2 by Sonogashira
coupling reaction, followed by deprotection of the TMS group
(Scheme 1).8c Treatment of 2-N2 with [Rh(norbornadienedNBD)-
Cl]2 complex in the presence of triethylamine as a cocatalyst in
toluene for 1 day at room temperature followed by quenching by
MeOH and filtration gave a red solid. The solid was dissolved in
CHCl3 and purified by recycled gel permeation chromatography
to obtain a main fraction. The average molecular weight of polymers
(3-nN2) was estimated to be approximately 86 000 (190 mer) from
GPC calibration using a polystyrene standard. IR spectra of the
polymer (3-nN2) showed a strong absorption band at 2100 cm-1
M ) Ma - Mb ) FNgJµBB(ø)
(1)
where F is the photolysis factor of diazo compound, N is the number
of the molecule, J is the quantum number for the total angular
momentum, µB is the Bohr magneton, and g is Lande´ g-factor. Since
these 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 versus H/T plots are shown in
Figure 1a.
The observed data for the photoproduct from polydiazo com-
pound indicate that the magnetization data at two different
† Chemistry Department for Materials.
‡ Instrumental Analysis Facilities.
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10.1021/ja0455345 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
structures do not have a planar polyene backbone and also that the
dihedral angle between the backbone and the attached phenyl ring
is significantly twisted. A defect in the backbone structure due the
nature of phenylacetylene polymerization is also suggested.16c
A
similar explanation can be applied for the unexpectedly low spin
multiplicities observed for polycarbene 3. It is interesting to note
here that the spin multiplicity of 3 is significantly higher than that
of a component spin unit, i.e., triplet carbene (S ) 1), rather than
an almost complete lack of magnetic interaction, as observed for
the poly(phenyl)acetylene bearing stable radical units. This is partly
ascribable to the higher spin density of the present system composed
of triplet carbene units than that composed of doublet radical units
and demonstrates the usefulness of triplet carbene as the spin source
for constructing high-spin organic molecules.
Acknowledgment. The authors 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, GPC, IR, NMR,
and UV/vis charts of 3-nN2, plot of magnetization vs irradiation time
in the photolysis of 3-nN2, and field dependence of the magnetization
of the photoproduct from 3-nN2 (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 1. (a) Plot of M vs H/T of the photoproduct from polydiazo
compound (3-nN2) measured at 2.0 (O) and 5.0 K (0). (b) Plot of M/Ms vs
H/T of the photoproduct from polydiazo compound (3-nN2) measured at
5.0 K (0). The red lines represent two-component theoretical curves with
S ) 9.63 (F ) 0.15) and S ) 1.94 (F ) 0.22).
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