Preparation of sterically congested diphenyldiazomethanes having a pyridine ligand and magnetic characterization of photoproducts of their 2:1 copper complexes

Article abstract of DOI:10.1021/jo0504363

To confirm whether high-spin species can be generated as a result of ferromagnetic interaction between the 3d spin of metal ions and the 2p spins of triplet carbene through the pyridyl group located remote from the carbene center, [2,6-dibromo-4-(3- and 4

Full text of DOI:10.1021/jo0504363

Preparation of Sterically Congested Diphenyldiazomethanes
Having a Pyridine Ligand and Magnetic Characterization of
Photoproducts of Their 2:1 Copper Complexes
Masayoshi Matsuno,§ Tetsuji Itoh,*,§ 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
t-itoh@chem.mie-u.ac.jp
Received March 8, 2005
To confirm whether high-spin species can be generated as a result of ferromagnetic interaction
between the 3d spin of metal ions and the 2p spins of triplet carbene through the pyridyl group
located remote from the carbene center, [2,6-dibromo-4-(3- and 4-pyridyl)phenyl](4-tert-butyl-2,6-
dimethylphenyl)diazomethanes were prepared and the corresponding carbenes were generated
either in the absence or presence of Cu(hfac)2. These were characterized by ESR and UV/vis in a
matrix at low temperature, and by laser flash photolysis in solution at room temperature. These
studies indicated that although both carbenes generated a fairly stable complex with copper ions,
the 4-pyridyl isomer formed a high-spin species as a result of ferromagnetic interaction between
the 3d spin of metal ions and the 2p spins of triplet carbene. Such an interaction in the corresponding
3-isomer is likely to be antiferromagnetic. This is further confirmed by magnetic measurements
using a Superconducting Quantum Interference Device (SQUID). The results demonstrate that
extension of this method will enable stable high-spin polycarbenes to be obtained.
Introduction
amount of interest. Many attempts to prepare organic
ferromagnetic materials have been made.1,2 The spin
sources used for such studies are mostly thermodynami-
cally stable radicals such as phenoxyls,3 triphenylmeth-
yls,4 and nitroxides,5 mainly due to their ease of prepa-
ration and use. These radicals have some potential
problems. For instance, exchange coupling between neigh-
boring nitroxides is weak. Triplet carbenes are regarded
as one of the most effective spin sources, since the
magnitude of the exchange coupling between the neigh-
boring centers is large.6,7 Iwamura and co-workers have
prepared a “starburst”-type nona(diazo) compound, a
Molecular magnetism, in which the spins of unpaired
electrons in π-orbitals of light atoms such as carbon,
nitrogen, and oxygen are mainly responsible for the
magnetic properties, is attracting an ever-increasing
§ Chemistry Department for Materials, Faculty of Engineering.
‡ Instrumental Analysis Facilities, Life Science Research Center.
(1) (a) Iwamura, H. Adv. Phys. Org. Chem. 1990, 26, 179. (b)
Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88. (c) Rajca, A. Chem.
Rev. 1994, 94, 871. (d) Lahti, P. M., Ed. Molecular Magnetism in
Organic-Based Materials; Marcel Dekker: New York, 1999. (e) Itoh,
K.; Kinoshita, M., Eds. Molecular Magnetism; Kodansha-Gordon and
Breach: Tokyo, Japan, 2000.
(2) (a) Kahn, O. Molecular Magnetism; VCH Publishers: Weinheim,
Germany, 1993. (b) Gatteschi, D. Adv. Mater. 1994, 6, 635. (c) Miller,
J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 385. (d)
Miller, J. S.; Epstein, A. J. Chem. Eng. News 1995, October 2, p 30.
(e) Kahn, O., Ed. Magnetism: A Supramolecular Function; NATO ASI
Seri. C; Kluwer: Dordrecht, The Netherlands, 1996. (f) Turnbull, M.
M.; Sugimoto, T.; Thompson, L. K., Eds. Molecule-Based Magnetic
Materials; ACS Symp. Ser. No. 644; American Chemical Society:
Washington, DC, 1996. (g) Gatteschi, D. Curr. Opin. Solid State Mater
Sci. 1996, 1, 192. (h) Miller, J. S.; Epstein, A. J., Eds. MRS Bull. 2000,
25, 21. (i) Miller, J. S.; Drillon, M., Eds. Magnetism: Molecules to
Materials II; Wiley-VCH: New York, 2001.
(3) (a) Nishide, H.; Kaneko, T. In ref 1d, pp 285-303. (b) Nishide,
H.; Maeda, K.; Oyaizu, K.; Tsuchida, E. J. Org. Chem. 1999, 64, 7129.
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2002, 8, 4835. (c) Raica, A.; Wongsriatanakul, J.; Raica, S. J. Am.
Chem. Soc. 2004, 126, 6608.
(5) (a) Bushby, R.-J. In ref 2i, pp 149-187. (b) Amabilino, D. B.;
Viciana, J. In ref 2i, pp 1-60.
(6) (a) Koga, N.; Iwamura, H. In Carbene Chemistry; Bertrand, G.,
Ed.; Fontis Media: Lausanne, Switzerland, 2002; pp 271-296. (b)
Matsuda, K.; Nakamura, N.; Takahashi, K.; Inoue, K.; Koga, N.;
Iwamura, H. Molecule-based Magnetic Materials; ACS Symp. Ser. No.
644; American Chemical Society: Washington, DC, 1996; p 142.
10.1021/jo0504363 CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/05/2005
7054
J. Org. Chem. 2005, 70, 7054-7064

Sterically Congested Diphenyldiazomethanes
SCHEME 1
precursor of nonacarbene, and have demonstrated that
the nine diazo groups are photolyzed at low temperature
to give a nonadecet ground state (S ) 9).7
However, these systems have two disadvantages that
hinder their extension to usable magnetic materials.
First, a triplet carbene unit is highly unstable and lacks
stability for practical applications under ambient condi-
tions.8 To overcome this difficulty, we attempted to
stabilize triplet carbenes, and succeeded in preparing
fairly stable ones that survive for several days in solution
at room temperature.9,10 The next step is to explore a way
of connecting them, while retaining a robust π-spin
polarization.
The second disadvantage is that diazo groups, gener-
ally employed as precursors for carbenes, are also, in
general, labile,11 and hence cannot be used as building
blocks for the preparation of more complicated poly(diazo)
compounds. We found that a diphenyldiazomethane
prepared to generate a persistent triplet carbene is also
persistent for the diazo compound and, hence, can be
further modified, with the diazo group intact, into a more
complicated diazo compound. For instance, bis(2,4,6-
tribromophenyl)diazomethane is found to be stable enough
to survive under Sonogashira coupling reaction conditions
and undergoes a coupling reaction with (trimethylsilyl)-
acetylene to give bis(2,6-dibromo-4-trimethylsilylethyn-
ylphenyl)diazomethane. Three units of the diazo com-
pound are then introduced at the 1-, 3-, and 5-positions
of the benzene ring through the ethynyl group by
employing a similar coupling reaction to give a tris(diazo)
compound, which eventually generates a fairly stable
septet ground-state tris(carbene) upon irradiation.12 Thus
it is potentially possible to prepare a poly(diazo) com-
pound by extending this method.13
The strategy is based on the supramolecular chemistry
exhibited by pyridine- and polypyridine-metal ions.16 For
instance, bis(4-pyridyl)diazomethane, DPy(CdN2), formed
a polymeric chain structure [DPy(CdN2)-M-] by ligation
with coordinatively unsaturated metal ions (M). The 3d
spins on metal ions in the chain do not interact with each
other, but once triplet bis(4-pyridyl)carbene, DPy(C:), is
generated upon irradiation, they start to interact with
each other through the 2p spins on the carbene center,
thereby generating a high-spin system comprised of 2p
and 3d spins (Scheme 1).14
This suggests that if a pyridyl group is introduced on
a phenyl ring of a sterically congested diphenyldiazo-
methane, a precursor for a persistent triplet carbene, a
polymeric diazo compound chain should be attainable
after coordination with metal ions. However, it needs to
be confirmed whether the 2p spins of triplet carbene,
generated upon photolysis of the chain, can indeed
interact magnetically with the 3d spin of metal ions
through the pyridyl group located remote from the
carbene center. If so, the overall magnetic properties will
also be affected by the position of the coordinating
nitrogen in the pyridine ring. Iwamura and co-worker
have investigated the exchange coupling of the spins of
copper(II) ion coordinated with the ring nitrogen atoms
and N-tert-butylaminoxyl radical attached as a substitu-
ent on the pyridine and N-phenylimidazole rings and
shown the 2p spins of N-tert-butylaminoxyl radicals
indeed interact magnetically with the 3d spin of metal
ions but the magnitude of interaction in the complex
using N-phenylimidazole rings is much smaller than that
observed for the complex using pyridine ring. This
indicates that the interaction through a remote π-system
is possible but is considerably weakened due to the
presence of many intervening π-bonds.17 However, the
mode and magnitude of similar interactions of the 2p
spins of triplet carbene through a distant ligating site is
not known.
Another method for preparing poly(diazo) compounds
is to use heterospin systems comprised of 2p spins of
organic radicals and 3d spins of magnetic metal ions.14,15
(7) (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.
(8) For reviews of general reactions of carbenes, see: (a) Kirmse,
W. Carbene Chemistry, 2nd ed.; Academic Press: New York, 1971. (b)
Carbenes; ; Moss, R. A., Jones, M., Jr., Ed.; Wiley: New York, 1973
and 1975; Vols. 1 and 2. (c) Carbene(oide), Carbine; Regitz, M., Ed.;
Thieme; Stuttgart, Germany, 1989. (d) Wentrup, C. Reactive Interme-
diates; Wiley: New York, 1984; pp 162-264.
(9) (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 S. A.; Lansanne, Switzer-
land, 2002; pp 103-152.
(14) (a) Koga, N.; Iwamura, H. In ref 1d, pp 629-659. (b) Koga, N.;
Iwamura, H. Mol. Crsyt. Liq. Cryst. 1997, 305, 415. (c) Iwamura, H.;
Koga, N. Mol. Crsyt. Liq. Cryst. 1999, 334, 437. (d) Iwamura, H.; Koga,
N. Pure Appl. Chem. 1999, 71, 231.
(15) (a) Sano, Y.; Tanaka, M.; Koga, N.; Matsuda, K.; Iwamura, H.;
Rabu, P.; Drillon, M. J. Am. Chem. Soc. 1997, 119, 8246. (b) Karasawa,
S.; Koga, N. Polyhedron 2001, 20, 1387. (c) Karasawa, S.; Sano, Y.;
Akita, T.; Koga, N.; Itoh, T.; Iwamura, H.; Rabu, P.; Drillon, M. J.
Am. Chem. Soc. 1998, 120, 10080. (d) Karasawa, S.; Kumada, H.; Koga,
N.; Iwamura, H. J. Am. Chem. Soc. 2001, 123, 9685. (e) Morikawa,
H.; Imamura, F.; Tsurukami, Y.; Itoh, T.; Kumada, H.; Karasawa, S.;
Koga, N.; Iwamura, H. J. Mater. Chem. 2001, 11, 493.
(16) (a) Lehn, J.-M. Supramolecular Chemistry; VCH Publisher:
New York, 1995. (b) Nierengarten, J.-F.; Dietrich-Buchecker, C. O.;
Sauvage, J.-P. J. Am. Chem. Soc. 1994, 116, 375. (c) Leininger, J.-F.;
Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100, 853.
(17) (a) Ishimura, Y.; Inoue, K.; Koga, N.; Iwamura, H. Chem. Lett.
1994, 1693. (b) Ishimura, Y.; Kitano, M.; Kumada, H.; Koga, N.;
Iwamura, H. Inorg. Chem. 1998, 37, 2273.
(10) (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.
(11) Regitz, M.; Maas, G. Diazo Compounds-Properties and Synthe-
sis; Academic Press: Orland, FL, 1986.
(12) Tomioka, H.; Hattori, M.; Hirai, K.; Sato, K.; Shiomi, D.; Takui,
T.; Itoh, K. J. Am. Chem. Soc. 1998, 120, 1106.
(13) (a) Furukawa, K.; Takui, T.; Itoh, K.; Miyahara, I.; Hirotsu,
K.; Watanabe, T.; Hirai, K.; Tomioka, H. Synth. Met. 1997, 85, 1330.
(b) Tomioka, H.; Hattori, M.; Hirai, K.; Sato, K.; Shiomi, D.; Takui, T.;
Itoh, K. J. Am. Chem. Soc. 1998, 120, 1106. (c) Itoh, T.; Hirai, K.;
Tomioka, H. J. Am. Chem. Soc. 2004, 126, 1130. (d) Itoh, T.; Morisaki,
F.; Kurono, M.; Hirai, K.; Tomioka, H. J. Org. Chem. 2004, 69, 5870.
(e) Ohtsuka, Y.; Itoh, T.; Hirai, K.; Tomioka, H.; Takui, T. Org. Lett.
2004, 6, 847. (f) Itoh, T.; Maemura, T.; Ohtsuka, Y.; Ikari, Y.; Wildt,
H.; Hirai, K.; Tomioka, H. Eur. J. Org. Chem. 2004, 2991.
J. Org. Chem, Vol. 70, No. 18, 2005 7055

Matsuno et al.
SCHEME 2
SCHEME 3
To examine these issues, we have prepared diphenyl-
diazomethanes having pyridyl groups and have charac-
terized the magnetic properties of the photoproducts
formed from these diazo compounds.
Results and Discussion
(A) Design and Preparation of Precursor Diazo-
methane. A diphenyldiazomethane unit used in this
study was (2,4,6-tribromophenyl)(4-tert-butyl-2,6-dim-
ethylphenyl)diazomethane, 1-N2, which can generate a
fairly persistent triplet diphenylcarbene (31).18 Before
preparing a desired precursor, the spin densities of
organic spin sources need to be considered. The nonbond-
ing molecular orbitals (NBMOs) of 31 were studied using
the simple Hu¨ckel molecular orbital method, in which
the atoms having nonzero NBMO coefficients have been
stared. To a first approximation, the spin densities are
assumed to be generated on the atoms with such nonzero
coefficients. Thus, ligand pyridyl groups need to be
introduced at the ortho and/or para positions, which have
NBMO spin densities such that the interactions of the
spins are transmitted effectively to a ligand.19,20 We
therefore decided to introduce pyridyl groups at the para
position of 1-N2, for synthetic reasons (Scheme 2).
The spin densities at the bridging groups are also
important factors to take into account. The nonbonding
molecular orbitals (NBMOs) of the isomeric carbenes 3-
and 4-pyridylcarbene suggest that, while in the para
isomer 4-pyridylcarbene, nonzero NBMO coefficients are
expected to be on the pyridyl nitrogen, zero NBMO
coefficients are expected on the pyridyl nitrogen atom in
the meta isomer 3-pyridylcarbene, and hence the mag-
nitude of the spin interactions is expected to be small.
The sign of the spin polarization of the π-electrons on
the nitrogen atom is also different between the two
isomers.
under Suzuki coupling conditions21 were unsuccessful.
However, the coupling reaction of the precursory car-
bamate took place smoothly to give [2,6-dibromo-4-(3- and
4-pyridyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)meth-
ylcarbamate, which was converted to the corresponding
diazo compounds (3- and 4-Py-1-N2) by a routine method
(Scheme 3).
(B) Spectroscopic Studies of Metal-Free Diazo-
methanes (3- and 4-Py-1-N2). Before examining the
magnetic properties of the species generated by photoly-
sis of a complex between the desired diazo compounds
Py-1-N2 and metal ions, we characterized free carbenes
from 3- and 4-Py-1-N2.
Electron Spin Resonance (ESR). Irradiation (λ >
350 nm) of 4-Py-1-N2 (2.7 × 10-2 M) in 2-methyltetrahy-
drofuran (2-MTHF) at 77 K gave ESR signals of randomly
oriented triplet molecules (Figure 1a). The signals were
analyzed in terms of the zero-field splitting (ZFS) pa-
rameters with D ) 0.420 cm-1 and E ) 0.0290 cm-1 (E/D
) 0.069).22 The values showed unequivocally that the
triplet signals are due to triplet [2,6-dibromo-4-(4-py-
ridyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)carbene (4-
Py-31) generated by photodissociation of 4-Py-1-N2. Simi-
lar irradiation of 3-Py-1-N2 (3.9 × 10-2 M) in 2-MTHF
gave triplet signals ascribable to triplet [2,6-dibromo-4-
(3-pyridyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)car-
bene (3-Py-31) with ZFS parameters of D ) 0.410 cm-1
and E ) 0.0270 cm-1 (E/D ) 0.066) (Figure 1b).
These values can be compared with those reported for
the triplet carbene before pyridination, (4-tert-butyl-2,6-
These factors will affect the overall spin states if the
2p and 3d spins interact magnetically. To test this idea,
we prepared sterically congested diaryldiazomethanes
having a 3- and 4-pyridyl group (3- and 4-Py-1-N2,
respectively) as a ring substituent, and characterized the
magnetic properties of the complex formed as a result of
coordination with Cu(II) ions followed by irradiation.
All attempts to prepare a desired diazo compound by
reacting pyridylboronic acid with the diazomethane
(18) (a) Tomioka, H.; Watanabe, T.; Hirai, K.; Furukawa, K.; Takui,
T.; Itoh, K. J. Am. Chem. Soc. 1995, 117, 6376. (b) Tomioka, H.; Hattori,
M.; Hirai, K. J. Am. Chem. Soc. 1996, 118, 8723. (c) Tomioka, H.;
Watanabe, T.; Hattori, M.; Nomura, N.; Hirai, K. J. Am. Chem. Soc.
2002, 124, 474. (d) Hirai, K.; Iikubo, T.; Tomioka, H. Chem. Lett. 2002,
1226.
(19) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99,
4587.
(20) (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-Interscience: New York, 1982; pp 1-12.
(21) For reviews, see for example: (a) Suzuki, A. In Metal-Catalyzed
Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-
VCH: Weinheim, Germany, 1998; pp 49-97. (b) Stanforth, S. P.
Tetrahedron 1998, 54, 263. (c) Miyaura, N.; Suzuki, A. Chem. Rev.
1995, 95, 2457.
(22) ESR signals of brominated diarylcarbenes are generally very
broad18 and hence not all transitions are clearly observed. To determine
ZFS parameters, we employed computer simulation.
7056 J. Org. Chem., Vol. 70, No. 18, 2005

Sterically Congested Diphenyldiazomethanes
FIGURE 2. UV/vis spectra obtained by irradiation of diazo
compound 4-Py-1-N2: (a) spectra of 4-Py-1-N2 in 2-methyltet-
rahydrofuran at 77 K; (b) the same sample after irradiation
(λ > 350 nm); (c-e) the same sample after thawing to 100 (c),
140 (d), and 150 K (e).
FIGURE 1. ESR spectra obtained by irradiation of compound
4-Py-1-N2 (a) and 3-Py-1-N2 (b) in 2-methyltetrahydrofuran
at 77 K. The signal at 334 mT is ascribable to a doublet species
fortuitously produced during the photolysis of diazo compound.
dimethylphenyl)(2,4,6-tribromopheyl)carbene (31) (D )
0.423 cm-1 and E ) 0.0326 cm-1, E/D ) 0.0771)17 and a
phenylated analogue, (2,6-dibromo-4-phenylphenyl)(4-
tert-butyl-2,6-dimethylphenyl)carbene (Ph-31) (D ) 0.348
cm-1 and E ) 0.0326 cm-1, E/D ) 0.0937).23
enough to allow thermal stability. Instead, the thermal
stability of Py-31 was estimated more accurately by
monitoring the UV/vis spectra of 31 as a function of
temperature (vide infra).
UV/Visible Spectroscopy (UV/Vis). Optical spectro-
scopic monitoring in the frozen medium gave analogous
results. Irradiation (λ > 350 nm) of 4-Py-1-N2 (5.4 × 10-4
M) in 2-MTHF at 77 K resulted in the appearance of new
absorption bands at the expense of the original absorption
due to 4-Py-1-N2. The new spectrum consists of two
identifiable features: an intense UV band with a maxi-
mum at 333 nm and weak, broad bands extending from
410 to 550 nm (Figure 2). These features are usually
present in the spectrum of triplet diarylcarbenes.25 Since
ESR signals due to triplet carbene were observed under
identical conditions, we can safely assign the absorption
spectrum to the triplet carbene 4-Py-31. The bands can
be compared with those observed for carbene before
The structure of triplet carbenes is characterized by
ESR ZFS parameters D and E.24 The D value is related
to the separation between the unpaired electrons. The E
value, when weighted by D, is a measure of the deviation
from axial symmetry. For diarylcarbenes, this value will
thus depend on the magnitude of the central C-C-C
angle. Since the E value depends on the magnitude of
the central angle, the reduction in E indicates that the
carbene adopts a structure with an expanded C-C-C
angle upon annealing. This interpretation is supported
by the observation that a substantial reduction in E is
usually accompanied by a significant reduction in D,
indicating that the electrons are becoming more delocal-
ized.
3
pyridination, 1, which exhibited rather weak bands at
The ZFS parameters of Py-31, especially the D values,
are closer to those reported for the triplet carbene before
pyridination (31) and apparently larger than those for the
phenylated analogue Ph-31. This suggests that the extent
of delocalization of unpaired electrons into pyridyl groups
at para positions is not significant compared to that of
the phenyl ring.
The ESR signals not only were stable at this temper-
ature but disappeared irreversibly when the matrix was
thawed to room temperature. The thermal stability of the
triplet carbenes could be estimated by thawing the matrix
containing triplet carbenes gradually and then recooling
it to 77 K, to measure the signal. This procedure can
compensate for weakening of signals due to Curie’s law.23
The characteristic bands due to Py-31 started to decay
slowly at 100 K and decayed rather sharply at 120 K.
The triplet signals of brominated DPCs are generally
weak and broad. The signals of Py-31 were not strong
337 and 349 nm. The rather strong bands in the longer
wavelength region exhibited by Py-31 are similar to those
observed for the phenylated analogue, Ph-31, and show
the role of the pyridyl groups at the para position.
The glassy solution did not exhibit any changes until
the matrix temperature was raised to 100 K, but the
bands started to disappear rather sharply at around 140
K, and disappeared irreversibly when the temperature
was raised to 150 K. The thermal stability is thus
essentially similar to that observed for 31 and Ph-31.
Similar results were obtained in the photolysis of the
meta isomer, 3-Py-1-N2 (Figure 3). The absorption spec-
trum and thermal stability of 3-Py-31 are essentially
identical with those observed for the corresponding
4-isomer.
Laser Flash Photolysis (LFP). Laser flash photoly-
sis (LFP) of 4-Py-1-N2 (1.5 × 10-4 M) in degassed benzene
at room temperature produced a transient species show-
ing a strong absorption band at 380 nm (Figure 4). Due
to overlap of the absorption maxima of the diazo precur-
sor 4-Py-1-N2, the sample was not sufficiently transpar-
ent for adequate monitoring in the 300-370 nm region.
(23) Monguchi, K.; Itoh, T.; Hirai, K.; Tomioka, H. J. Am. Chem.
Soc. 2004, 126, 11900.
(24) See for reviews of the EPR and UV/vis spectra of triplet
carbenes: (a) Sander, W.; Bucher, G.; Wierlacher, S. Chem. Rev. 1993,
93, 1583. (b) Trozzolo, A. M.; Wasserman, E. In Carbenes; Jones, M.,
Jr., Moss, R. A., Eds.; Wiley: New York, 1975; Vol. 2, pp 185-206. (c)
Tomioka, H. In Advances in Strained and Interesting Organic Mol-
ecules; Halton, B., Ed.; JAI Press: Greenwich, CT, 2000; Vol. 8, pp
83-112.
(25) (a) Platz, M. S. In Diradicals; Borden, W. T., Ed; Wiley: New
York, 1982; pp 195-258. (b) Wasserman, E.; Hutton, R. S. Acc. Chem.
Res. 1977, 10, 27. (c) Breslow, R.; Chang, H. W.; Wasserman, E. J.
Am. Chem. Soc. 1967, 89, 1112.
J. Org. Chem, Vol. 70, No. 18, 2005 7057

Matsuno et al.
FIGURE 3. UV/vis spectra obtained by irradiation of diazo
compound 3-Py-1-N2: (a) spectra of 3-Py-1-N2 in 2-methyltet-
rahydrofuran at 77 K; (b) the same sample after irradiation
(λ > 350 nm); (c-e) the same sample after thawing to 100 (c),
140 (d), and 150 K (e).
FIGURE 5. Absorption of transient products obtained upon
308-nm excitation of diazo compound 3-Py-1-N2 in degassed
benzene at room temperature recorded 10 µs (red), 100 ms
(green), and 1 s (blue) after excitation. The inset shows the
time course of the absorption at 340 nm (oscillogram trace).
SCHEME 4
and show a broad absorption band centered at 390-450
nm. Thus, the observations can be interpreted as indicat-
ing that 4-Py-31 is trapped by oxygen to form the carbonyl
oxide (4-Py-1-O2), which confirms that the transient
absorption quenched by oxygen is due to 4-Py-31 (Scheme
4).
The apparent build-up rate constant, kobs, of the
carbonyl oxide (4-Py-1-O2) is essentially identical with
that of the decay of 4-Py-31, and kobs is expressed as given
in eq 1
FIGURE 4. Absorption of transient products obtained upon
308-nm excitation of diazo compound 4-Py-1-N2 in degassed
benzene at room temperature recorded 10 µs (red), 100 ms
(green), and 1 s (blue) after excitation. The inset shows the
time course of the absorption at 350 nm (oscillogram trace).
kobs ) k0 + kO [O2]
(1)
Therefore, direct comparison of the transient bands with
those observed in the matrix at low temperature was not
possible. It is likely that we are observing the tailing part
of the absorption band due to 4-Py-31. This assignment
was supported by trapping experiments (vide infra). All
absorption bands decayed in a similar manner. The inset
in Figure 4 shows the decay of 4-Py-31 in the absence of
trapping reagents, which is found to be second order (2k/
ꢀl ) 1.6 s-1). The rough lifetime of 4-Py-31 was estimated
in the form of the half-life, t1/2, to be 0.8 s.
When LFP was carried out on a nondegassed benzene
solution of 4-Py-1-N2 (1.5 × 10-4 M), the half-life of 4-Py-
31 decreased dramatically, and a broad absorption band
with a maximum at 420 nm appeared at the expense of
the absorption due to 4-Py-31 (Figure S1, Supporting
Information). The spent solution was found to contain
the corresponding ketone 4-Py-1-O. It is well docu-
mented26,27 that diarylcarbenes with triplet ground states
are readily trapped by oxygen to generate the corre-
sponding diaryl ketone oxides, which are easily observed
directly, either by matrix isolation or by flash photolysis,
2
where k0 represents the rate of decay of 4-Py-31 in the
absence of oxygen and kO is the rate constant for the
2
quenching of 4-Py-31 by oxygen. A plot of the observed
pseudo-first-order rate constant of the formation of the
oxide against [O2] is linear (Figure S1, Supporting
Information). From the slope of this plot, kO was deter-
2
mined to be 2.2 × 107 M-1 s-1
.
Similar LFP of the meta isomer 3-Py-1-N2 (1.5 × 10-4
M) gave a transient absorption band with a maximum
at 380 nm, which decayed in a second-order manner (2k/
ꢀl ) 1.3 s-1) with a half-life of 0.8 s (Figure 5). The rate
constant for the quenching by oxygen, kO , was deter-
2
mined to be 2.6 × 107 M-1 s-1 (Figure S2, Supporting
Information).
Once again, these values are comparable to that
3
observed for the triplet carbene before pyridination, 1
(t1/2 ) 0.55 s),18 and that for the phenylated analogue Ph-
31 (t1/2 ) 0.5 s),23 suggesting that the introduction of the
pyridyl group does not decrease the stability of the triplet
state.
(26) See for review: Sander, W. Angew. Chem., Int. Ed. Engl. 1990,
29, 344. (b) Bunnelle, W. Chem. Rev. 1991, 91, 336.
(27) Scaiano, J. C.; McGimpsey, W. G.; Casal, H. L. J. Org. Chem.
1989, 54, 1612.
(C) Spectroscopic Studies of Photoproducts of
Complex between Py-1-N2 and Cu(hfac)2. All of the
above observations suggest that we now have a fairly
7058 J. Org. Chem., Vol. 70, No. 18, 2005

Sterically Congested Diphenyldiazomethanes
persistent triplet carbene unit that has a pyridine ligand
bridgeable with metal ions. The next step is to character-
ize the magnetic properties of the photoproducts of the
complex between Py-1-N2 and a metal ion.
It has been demonstrated that the sign and magnitude
of the exchange coupling between metal ions and an
organic radical depend not only on the periodicity of the
ligand π-orbitals, but also on the orbitals occupied by the
unpaired d electrons of the metal ions.14a For instance,
ferromagnetic interaction is observed between the car-
benes and the copper ions in the [Cu(hfac)2{4-DPy(C:)}]
(hfac ) hexafluoroacetylacetonate) system, while anti-
ferromagnetic interaction is observed between the car-
benes and the manganese ions in the [Mn(hfac)2{4-
DPy(C:)}] system. This is explained in terms of the
difference in the overlapping mode of the magnetic
orbitals in the metal ions and the nitrogen atom on the
pyridine ligands. In the Cu(II) complexes, the magnetic
orbital is dx -y and orthogonal to the pπ-orbital at the
pyridyl nitrogen, in which the spin is polarized due to
the presence of the triplet carbene. Hund’s rule would
predict the coupling to be ferromagnetic. On the other
hand, in the Mn(II) complexes, the magnetic orbital that
interacts with carbene is dxz and hence the interaction
with the 2pπ-orbital at the pyridine nitrogen is a π-type
magnetic one. In this case, the unpaired electrons are
expected to couple antiferromagnetically.
FIGURE 6. ESR spectra obtained by irradiation of 4-Py-1-
N2 and Cu(hfac)2 in 2-MTHF mixed in a 2:1 molar ratio: (a,
b) ESR spectrum before (a) and after (b) irradiation (λ > 350
nm) at 77 K; (c-e) ESR spectra observed at 77 K in 2-meth-
yltetrahydrofuran after warming the matrix to 120 (c), 130
(d), and 140 K (e).
2
2
Thus, if Cu(II) is used as a metal ion, a ferromagnetic
interaction would be expected in the complex with 4-Py-
31. Since the sign of the spin polarization of the π-elec-
trons on the nitrogen atom in 3-Py-31 may be opposite to
that of 4-Py-31, an antiferromagnetic interaction will be
induced between the carbene center and the copper ion.
We thus chose Cu(hfac)2 as a metal ion for this work.
ESR. Solutions of 4-Py-1-N2 (13.6 × 10-3 M) in
2-MTHF and Cu(hfac)2 (6.8 × 10-3 M) in 2-MTHF were
mixed at a 2:1 molar ratio at room temperature, and the
mixture was allowed to stand overnight.
FIGURE 7. ESR spectra obtained by irradiation of 3-Py-1-
N2 and Cu(hfac)2 in 2-MTHF mixed in a 2:1 molar ratio: (a,
b) ESR spectrum before (a) and after (b) irradiation (λ > 350
nm) at 77 K; (c-e) ESR spectra observed at 77 K in 2-meth-
yltetrahydrofuran after warming the matrix to 120 (c), 130
(d), and 140 K (e).
The solution was placed in the cryostat of the low-
temperature ESR cavity. ESR signals at 260, 275, 291,
312, and 321 mT, which are characteristic of the magnetic
orbital dx -y in the Cu(II) ion of Cu(hfac)2,28 were seen
before irradiation. When the solution was irradiated (λ
> 350 nm) at 77 K, rather broad signals gradually
appeared at around 331 mT, at the expense of the signals
due to the isolated Cu(II) ion in Cu(hfac)2 (Figure 6). The
original signals due to Cu(II) were recovered when the
sample was warmed to room temperature and recooled.
Similar ESR spectral changes have been observed for the
2:1 complex of (phenyl)(4-pyridyl)carbene with the Cu-
(II) ion, which has been shown to have a ferromagnetic
interaction between the carbene and the copper ion. The
observation that no significant signals due to isolated
triplet carbene 4-Py-31 were observed suggests that the
pyridine moiety binds with Cu(hfac)2 essentially in a
nearly quantitative manner under these cryogenic condi-
tions.
2
2
130 K. The signals were replaced by those due to the Cu-
(II) ion at around 140 K.
It should be noted that the signals of di(4-pyridyl)-
carbene-Cu(hfac)2 disappeared irreversibly at temper-
atures higher than 60 K. Thus, the remarkable thermal
stability of the 4-Py-1-Cu complex is seen.
Similar irradiation of a Cu(II) complex of the meta
isomer 3-Py-1-N2 (2.6 × 10-3 M) gave a different spec-
trum. Upon irradiation at 77 K, rather sharp signals at
325-354 mT were observed, at the expense of the signals
due to the isolated Cu(II) ion. Once again, the original
signals due to Cu(II) were recovered when the sample
was warmed to room temperature and recooled (Figure
7), and no significant signals due to isolated triplet
To estimate the thermal stability of the signals, the
sample was warmed to a desired temperature, allowed
to stand at this temperature for 5 min, and recooled to
77 K to measure the ESR signals. The new signals
started to disappear at 120 K but were observable up to
3
carbene 1 were observed. The shape of the signals is
completely different from that observed for the 4-Py-31-
Cu(II) complex. Upon warming the matrix, the sharp
signals started to decrease at around 120 K and were
replaced by those due to the Cu(II) ion at around 140 K.
Although the spectral changes upon irradiation of the
(28) Sugiura, Y. Inorg. Chem. 1978, 17, 2178.
J. Org. Chem, Vol. 70, No. 18, 2005 7059

Matsuno et al.
FIGURE 9. UV/vis spectra obtained by irradiation of 3-Py-
1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 2:1 molar ratio: (a,
b) UV/vis spectra before (a) and after (b) irradiation at 77 K;
(c-e) the same sample after thawing to 100 (c), 140 (d), and
150 K (e).
FIGURE 8. UV/vis spectra obtained by irradiation of 4-Py-
1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 2:1 molar ratio: (a,
b) UV/vis spectra before (a) and after (b) irradiation at 77 K;
(c-e) the same sample after thawing to 100 (c), 140 (d), and
150 K (e).
Py-1-N2Cu complexes differed between the two isomers,
they were not clear enough to judge the difference in the
mode of magnetic interaction.
UV/Vis. Irradiation of a 2:1 mixture of 4-Py-1-N2 and
Cu(hfac)2 in 2-MTHF (2.7 × 10-4 M) at 77 K gave rise to
new absorption bands at the expense of the original
absorption due to the complex. The new spectrum again
consists of two identifiable features, an intense UV band
with an apparent maximum at 361 nm and weak, broad
bands with apparent maxima around 506 and 546 nm
(Figure 8). Since the ESR signals most likely ascribable
to the carbene-Cu complex are observed under identical
conditions, we can assign the absorption spectrum to the
complex. These bands can be compared with those of the
metal-free triplet carbene 4-Py-31 (337 nm and broad
bands at 400-550 nm). A similar slight red shift of the
characteristic absorption due to carbene after complex-
ation with Cu(hfac)2 has also been noted in di(4-pyridyl)-
carbene-Cu(hfac)2 systems. The glassy solution did not
exhibit any changes for hours at this temperature, but
all the bands disappeared irreversibly when the temper-
ature was raised to 150 K. Thus, remarkable thermal
stability of the 4-Py-1-Cu complex is once again noted.
Similar irradiation of a 2:1 mixture of 3-Py-1-N2 and
Cu(hfac)2 in 2-MTHF (1.3 × 10-4 M) at 77 K gave rise to
new absorption bands at the expense of the original
absorption due to the complex (Figure 9). In this case,
the new spectrum is very similar to that of the metal-
free triplet carbene 3-Py-31, with little shift. This may
be interpreted in terms of weak interaction between the
carbene center and the copper ion and/or a difference in
the mode of magnetic interaction. The absorption bands
disappeared irreversibly at a temperature around 150 K.
Superconducting Quantum Interference Device
(SQUID) Measurements. To obtain the spin quantum
numbers of the Py-1-Cu complex more definitively, the
magnetic properties before and after photolysis of the Py-
1-N2-copper complex in 2-MTHF solutions were studied
using a SQUID magneto/susceptometer.
FIGURE 10. (a) Plot of magnetization (M in emu) as a
function of irradiation time observed in the photolysis of 4-Py-
1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 2:1 molar ratio
measured at 5.0 K and 5 kOe. (b) Field dependence of the
magnetization of the photoproduct from a 2:1 complex of 4-Py-
1-N2 with Cu(hfac)2 in 0.3 mM 2-methyltetrahydrofuran
matrix measured at 2.0 and 5.0 K. Mb and Ma refer to the
magnetization value before and after irradiation, respectively,
and M ) Ma - Mb.
kOe with irradiation time was measured in situ for the
sample and is shown in Figure 10a. As the irradiation
time increased, the M values gradually increased and
reached a plateau after several hours. After the M values
reached a plateau, the magnetization values after ir-
radiation, 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 (Figure 10b).
The 2-MTHF solution of the 2:1 4-Py-1-N2-copper
complex (3.0 × 10-4 M) was placed inside the sample
compartment of a SQUID magneto/susceptometer and
was irradiated at 5-10 K by light (λ ) 488 nm) from an
Ar ion laser through an optical fiber. The development
of magnetization (M/emu) at 5 K in a constant field of 5
7060 J. Org. Chem., Vol. 70, No. 18, 2005

Sterically Congested Diphenyldiazomethanes
that three kinds of magnetic species, the biscarbene
5
complex (S ) /2), a one-carbene defected monocarbene
complex (S ) 3/2), and a two-carbene defected noncarbene
1
complex (S ) /2) exist in the matrix after irradiation,
the magnetization expression is written as:
Mb ) MCu + Mdia
Ma ) FbisMbis + FmonoMmono + (1 - Fbis
-
Fmono)MCu + Mdia
M ) Ma - Mb ) FbisMbis + FmonoMmono - (Fbis
Fmono)MCu ) Fbis{(5/2)NgµΒB(øbis)} +
mono {(3/2)NgµΒB(ømono)} -
(Fbis + Fmono){Ng′µΒB(ø′)/2} (5)
+
FIGURE 11. Plot of M vs H/T of the photoproduct from a 2:1
complex of 4-Py-1-N2 with Cu(hfac)2 measured at 2.0 (O) and
5.0 (0) K. The red and broken lines show fitting curves by eqs
4 and 5, respectively.
F
Magnetization of the copper complex before and after
irradiation is expressed as follows:
where øbis ) (5/2)gµΒH/(kBT), ømono ) (3/2)gµΒH/(kBT), and
Fbis and Fmono are the molar fractions of biscarbene and
monocarbene complexes, respectively. When the experi-
mental data (M vs H/T) were analyzed in terms of eq 5,
the fitted curve (broken line) with Fbis ) 0.33 ( 0.01 and
Fmono ) 0.59 ( 0.02 exactly traced the experimental data
(solid line in Figure 11).30 In other words, the resulting
4-Py-31-copper complex is comprised of a 33:59:8 mixture
of copper ion complexed with biscarbene, monobarbene,
and noncarbene.31
The overall results for the field dependence of M for
the complex of Cu(hfac)2 with 4-Py-31 suggest that the
generated carbene centers interact ferromagnetically
with copper(II) ions, as was expected.
Mb ) MCu + Mdia
(2)
(3)
Ma ) FMcomp + (1 - F)MCu + Mdia
where Mcomp and MCu are the magnetization due to the
carbene-Cu complexes and the diazo-copper complexes,
respectively, and F is a photolysis factor. The magnetiza-
tion (M) due to the species generated by photolysis was
obtained as the difference between Ma and Mb: M ) Ma
- Mb ) F(Mcomp - MCu). Thus, the effect of diamagnetic
and any paramagnetic impurities could be canceled out
by this treatment.
The data obtained for the photoproducts from similar
irradiation of the 2-MTHF solution of the 2:1 3-Py-1-N2-
copper complex (1.0 mM) were also analyzed using the
Brillouin function (Figures 12 and 13). The experimental
data (M vs H/T) for the 3-Py-1-N2-copper complex after
irradiation were analyzed in terms of eq 4 with a fitting
curve with S ) 1.00 ( 0.03 and F ) 0.60 ( 0.03 (Figure
13).30 The S value of the 3-Py-31-copper complex is
distinctly lower than that of the 4-Py-31-copper complex.
Although this value corresponds to the theoretical one
for magnetically isolated carbene 3-Py-31, it is notable
that the saturation magnetization (Ms) at 2.5 × 104 Oe/K
is 3.26 × 103 emu‚Oe/mol. This value corresponds to only
29% of that expected for magnetically isolated carbene.
This may be due to incomplete photodecomposition of
3-Py-1-N2. However, the degree of photolysis was esti-
mated to be >95% by comparing the n-π* absorption of
the diazo moiety in the UV/vis spectra before and after
the SQUID measurement. In addition, ESR results for
the 3-Py-31-copper complex indicated that the carbene
centers interacted magnetically with the copper ion.
Thus, the low S and Ms values can be explained in
terms of the antiferromagnetic interaction between the
carbene centers and the copper ion in the complex.
Coordination of a Cu(II) ion to the pyridyl ligand of 4-Py-
The experimental data were analyzed by best-fitting
the Brillouin function B(ø) as given by eq 4:29
M ) Ma - Mb ) F(Mcomp - MCu) ) F{NgµΒSB(ø) -
Ng′µΒB(ø′)/2)} (4)
where ø ) gSµΒH/(kBT), ø′ ) g′µΒH/(2kBT), and the other
symbols have their usual meaning. The g′ value for the
complexes of Cu(hfac)2 with 4-Py-1-N2 was 2.14, which
was obtained from the ESR spectrum before irradiation,
under similar conditions. The experimental data (M vs
H/T) for the 4-Py-1-N2-copper complex after irradiation
were fitted with eq 4 with S ) 2.01 ( 0.01 and F ) 0.82
( 0.01. The fitting curve is shown in Figure 11 as a red
line.30
The estimated S value was thus slightly lower than
the theoretical one for when two carbene centers (S ) 2)
interact with a copper(II) ion (S ) 1/2) ferromagnetically
(S ) 2.5). The lower S value may be due to a diminished
spin density at the pyridyl nitrogen located remote from
the carbene center. However, the magnetization data at
two different temperatures were fitted to the same
Brillouin function (Figure 11), and hence the sample
generated by irradiation of the 4-Py-1-N2-copper com-
plex is considered to be a ground state, at least at 5 K.
Then the lower S value is probably ascribed to a
carbene defect due to incomplete photolysis of the diazo
compounds or decomposition of carbenes. If one assumes
(31) This does not mean that the sample with the same composition
is formed in ESR and UV/vis measurements since the conditions under
which the complex is generated are intrinsically very different among
the three methods, SQUID, ESR and UV/vis, in terms of the sample
concentration, generation temperatures, light source, sample condi-
tions, and so on. So we think that it is unlikely that the sample with
the same compositions under these circumstances should be formed
under three different spectroscopic measurements.
(29) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, Ger-
many, 1986.
(30) Ms values obtained from the best fitted curves were used.
J. Org. Chem, Vol. 70, No. 18, 2005 7061

Matsuno et al.
Conclusions
The present results demonstrate that the 2p spins of
triplet carbene can interact magnetically with the 3d spin
of metal ions, through the pyridyl group located remote
from the carbene center. Interestingly, the regiospecificity
in the exchange coupling of magnetic metal ions observed
for bis(pyridyl)carbene systems where the spins are
“directly” transmitted to the ligand nitrogen atom is
retained in the present system. Thus a ferromagnetic
interaction is observed for the copper ion complex with
the 4-pyridyl isomer, while an antiferromagnetic interac-
tion is observed in the corresponding 3-isomer.
It is interesting to compare the present results with
that observed for the complex of Cu(hfac)2 and N-{4-(N-
oxyl-tert-butylamino)phenyl}imidazole (4-NOIm).17 Since
the magnitude of the exchange coupling between a triplet
carbene and a neighboring radical center is expected to
be larger than that between radicals, one may expect the
magnitude of the interaction in the present complex is
larger than that observed for the complex using N-{4-
(N-oxyl-tert-butylamino)phenyl}imidazole. Unfortunately
we were not able to estimate the value at present.
However, it should be noted here that the magnetic
measurements for the complex of Cu(hfac)2 and 4-NOIm
were made in the crystalline state in which the chain of
the complex is expected to be held tightly, while the
present measurements were made in a frozen solution
where the chain my be more flexible. It has been shown
that the apparent spin quantum number of the ferro-
magnetic chains consisting of a 1:1 complex between bis-
(4-pyridylcarbene)(DPy) and copper ions amounts to S
) 33.6, but this value drops down to 6.4 when it is
measured in a frozen solution of 2-MTHF. Thus we need
to do magnetic measurements on a crystalline sample of
Py-1-N2-copper complex before we draw meaningful
conclusions concerning this issue.
FIGURE 12. (a) Plot of magnetization (M in emu) as a
function of irradiation time observed in the photolysis of 3-Py-
1-N2 and Cu(hfac)2 in 2-MTHF mixed in a 2:1 molar ratio
measured at 5.0 K and 5 kOe. (b) Field dependence of the
magnetization of the photoproduct from a 2:1 complex of 3-Py-
1-N2 with Cu(hfac)2 in 0.3 mM 2-methyltetrahydrofuran
matrix measured at 2.0 and 5.0 K. Mb and Ma refer to the
magnetization value before and after irradiation, respectively,
and M ) Ma - Mb.
The carbene-copper complex showed remarkable sta-
bility, surviving temperatures up to 140 K in the 2-MTHF
matrix, while the analogous copper ion complex with an
unprotected pyridylcarbene decays at temperatures higher
than 60 K. The results suggest that it should be possible
to prepare a persistent high-spin polycarbene by extend-
ing this method. Since we have a fairly stable triplet
carbene that can survive for a week in solution at room
temperature, it should not be long before we can achieve
a fairly stable high-spin polycarbene.
Experimental Section
Preparation of [2,6-Dibromo-4-(4-pyridyl)phenyl](4-
tert-butyl-2,6-dimethylphenyl)diazomethane (4-Py-1-N2).
To a mixture of ethyl N-[2,4,6-tribromophenyl(4-tert-butyl-2,6-
dimethylphenyl)methyl]carbamate13f (100 mg, 0.18 mmol),
2-(4-pyridyl)-4,4,5,5-tetramethyl-1,3-dioxaborolane32 (108 mg,
0.53 mmol), [1,1′-bis(diphenylphosphino)ferrocene]palladium
dichloride dichloromethane complex (PdCl2(dppf)‚CH2Cl2) (14
mg, 0.018 mol) and potassium phosphate (63 mg, 0.30 mmol)
were added anhydrous 1,4-dioxane (2.5 mL) under Ar atmo-
sphere and the mixture was stirred at 100 °C for 2 days. The
mixture was then filtered and the filtrate was extracted with
diethyl ether. The organic layer was washed with water, dried
over anhydrous sodium sulfate, and concentrated under
reduced pressure to leave a crude product that was column
FIGURE 13. Plot of M vs H/T of the photoproduct from a 2:1
complex of 3-Py-1-N2 with Cu(hfac)2 measured at 2.0 (O) and
5.0 (0) K. The solid line shows the fitting curve by eq 4.
31 induces a ferromagnetic exchange interaction, due to
a spin polarization mechanism and the interaction aris-
ing from the orthogonal relationship of singly occupied
2
2
dx -y orbitals on the Cu(II) ion with the π-orbitals on the
nitrogen atom of the pyridine. On the other hand, the
sign of the polarization of the π-electrons on the nitrogen
atom for 3-Py-31 may be opposite to that of 4-Py-31.
Therefore, an antiferromagnetic interaction should be
induced between the carbene center and the copper ion
to give low S and Ms values.
(32) Coudret, C. Synth. Commun. 1996, 26, 3543.
7062 J. Org. Chem., Vol. 70, No. 18, 2005

Sterically Congested Diphenyldiazomethanes
chromatographed (silica gel, hexane:diethyl ether ) 1:1) to give
ethyl N-[[2,6-dibromophenyl-4-(4-pyridyl)phenyl](4-tert-butyl-
2,6-dimethylphenyl)methyl]carbamate (32 mg, 32%) as a yel-
(M + 4, 1.3), 574 (M + 2, 2.0) 572 (M+, 1.5), 495 (97.6), 493
(96.6), 57 (100); HRMS calcd for C27H30Br2N2O2 572.0673,
found m/z 572.0649.
1
low semisolid; H NMR (300 MHz, CDCl3, ppm) δ 8.68 (br s,
To a stirred and cooled solution of the carbamate (17 mg,
0.030 mmol) in acetic anhydride (1.2 mL) and acetic acid (0.9
mL) was added sodium nitrite (50 mg, 0.72 mmol) in portions
at 0 °C over a period of 2.5 h and the mixture was stirred for
1 day at room temperature after addition was complete. The
mixture was poured into ice and extracted with diethyl ether.
The ethereal layer was washed with aqueous sodium carbonate
water, dried over anhydrous sodium sulfate, and concentrated
under reduced pressure to give ethyl N-nitroso-N-[[2,6-dibro-
mophenyl-4-(3-pyridyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)-
2H), 7.81 (s, 2H), 7.44 (d, J ) 4.41 Hz, 2H), 6.99 (s, 2H), 6.49
(d, J ) 8.82 Hz, 1H), 5.38 (d, J ) 8.64 Hz), 4.26-4.13 (m, 2H),
2.23 (s, 6H), 1.29 (s, 9H), 1.27 (t, J ) 4.41 Hz, 3H); 13C NMR
(75.5 MHz, CDCl3, ppm) δ 155.0, 150.6, 150.5, 144.7, 139.0,
138.9, 136.9, 132.5, 132.4, 127.0, 126.9, 125.3, 61.2, 58.1, 34.1,
31.2, 21.3, 14.8; EI-MS m/z (rel intensity) 576 (M + 4, 1.3),
574 (M + 2, 2.0) 572 (M+, 1.5), 495 (97.6), 493 (96.6), 57 (100);
HRMS calcd for C27H30Br2N2O2 572.0673, found m/z 572.0647.
To a stirred and cooled solution of the carbamate (94 mg,
0.16 mmol) in acetic anhydride (6 mL) and acetic acid (4.5 mL)
was added sodium nitrite (226 mg, 3.28 mmol) in portions at
0 °C over a period of 2.5 h and the mixture was stirred for 1
day at room temperature after addition was complete. The
mixture was poured into ice and was extracted with diethyl
ether. The ethereal layer was washed with aqueous sodium
carbonate water, dried over anhydrous sodium sulfate, and
concentrated under reduced pressure to give ethyl N-nitroso-
N-[[2,6-dibromophenyl-4-(4-pyridyl)phenyl](4-tert-butyl-2,6-
dimethylphenyl)methyl]carbamate (85 mg, 86%) as a brown
semisolid; 1H NMR (300 MHz, CDCl3, ppm) δ 8.69 (br s, 2H),
δ 7.84 (s, 2H), 7.46 (d, J ) 4.41 Hz, 2H), 6.99 (s, 2H), 6.93 (s,
1H), 4.38-4.36 (m, 2H), 2.23 (s, 6H), 1.28 (s, 9H), 1.25 (t, J )
3.31 Hz, 3H).
1
methyl]carbamate (19 mg, quant.) as a brown semisolid; H
NMR (300 MHz, CDCl3, ppm) δ 8.80 (br s, 1H), 8.64 (br d, J )
4.23 Hz, 1H), 7.86 (dt, J ) 8.08, 1.84 Hz, 1H), 7.77 (s, 2H),
7.45-7.38 (m, 2H), 6.99 (s, 2H), 6.93 (d, J ) 8.64 Hz, 1H),
4.38-4.36 (m, 2H), 2.23 (s, 6H), 1.28 (s, 9H), 1.25 (t, J ) 6.98
Hz, 3H).
To a stirred solution of the nitrosocarbamete (19 mg, 0.030
mmol) in dry tetrahydrofuran (5 mL) was added potassium
tert-butoxide (8 mg, 0.07 mmol) at -20 °C under argon
atmosphere. After being stirred overnight, the reaction mixture
was poured into ice and extracted with ether, and the ethereal
layer was washed with water, dried over anhydrous sodium
sulfate, and concentrated under reduced pressure to leave a
crude product that was purified by column chromatography
(alumina, hexane at -20 °C), followed by GPC (chloroform,
monitored at 320 nm) to give [2,6-dibromo-4-(3-pyridyl)phenyl]-
(4-tert-butyl-2,6-dimethylphenyl)diazometane (3-Py-1-N2) (2
mg, 28%) as an orange semisolid: 1H NMR (300 MHz, CDCl3,
ppm) δ 8.82 (br s, 1H), 8.64 (br d, J ) 4.23 Hz, 1H), 7.84 (dt,
J ) 8.08, 1.84 Hz, 1H), 7.81 (s, 2H), 7.41-7.36 (m, 2H), 7.10
(s, 2H), 2.20 (s, 6H), 1.31 (s, 9H); 13C NMR (75.5 MHz, CDCl3,
ppm) δ 150.9, 149.5, 147.9, 140.0, 137.4, 134.2, 134.1, 131.9,
131.6, 126.9, 126.0, 125.9, 124.8, 63.4, 34.3, 31.3, 21.2; IR
To a stirred solution of the nitrosocarbamate (85 mg, 0.14
mmol) in dry tetrahydrofuran (5 mL) was added potassium
tert-butoxide (32 mg, 0.28 mmol) at -20 °C under argon
atmosphere. After being stirred overnight, the reaction mixture
was poured into ice and extracted with ether, and the ethereal
layer was washed with water, dried over anhydrous sodium
sulfate, and concentrated under reduced pressure to leave a
crude product that was purified by column chromatography
(alumina, hexane at -20 °C), followed by GPC (chloroform,
monitored at 320 nm) to give [2,6-dibromo-4-(4-pyridyl)phenyl]-
(4-tert-butyl-2,6-dimethylphenyl)diazometane (4-Py-1-N2) (15
mg, 21%) as an orange semisolid: 1H NMR (300 MHz, CDCl3,
ppm) δ 8.71 (br s, 2H), 7.86 (s, 2H), 7.47 (br s, 2H), 7.10 (s,
2H), 2.20 (s, 6H), 1.31 (s, 9H); 13C NMR (75.5 MHz, CDCl3,
ppm) δ 151.0, 150.63, 150.58, 144.7, 139.1, 137.5, 133.0, 131.6,
126.0, 125.8, 124.7, 63.5, 34.3, 31.3, 21.2; IR (NaCl, cm-1) νCd
2053.
(NaCl, cm-1) νCdN 2054.
2
Irradiation for Product Analysis. In a typical run, a
solution of the diazo compound (ca. 10 mg) in solvent was
placed in a Pyrex tube and irradiated with a high-pressure,
300-W mercury lamp until all the diazo compound was
destroyed. The irradiation mixture was concentrated on a
rotary evaporator below 20 °C. Individual components were
isolated by preparative TLC and identified by NMR and MS
(see the Supporting Information).
EPR Measurements. The diazo compound was dissolved
in 2-methyltetrahydrofuran and the solution was degassed in
a quartz cell by three freeze-degas-thaw cycles. The sample
was cooled in an optical transmission EPR cavity at 77 K and
irradiated with a 500 W Xe lamp using a Pyrex filter. EPR
spectra were measured on a spectrometer (X-band microwave
unit, 100 kHz field modulation). The signal positions were read
by the use of a gaussmeter. The temperature was controlled
by a Digital Temperature Indicator/Controller, providing the
measurements accuracy within (0.1 K and the control ability
within (0.2 K. Errors in the measurements of component
amplitudes did not exceed 5%, the accuracy of the resonance
fields determination was within (0.5 mT.
Low-Temperature UV/Vis Specta. Low-temperature spec-
tra at 77 K were obtained by using a variable-temperature
liquid-nitrogen cryostat equipped with a quartz outer window
and a sapphire inner window. The sample was dissolved in
dry 2-MTHF, placed in a long-necked quartz cuvette of 1-mm
path length, and degassed thoroughly by repeated freeze-
degas-thaw cycles at a pressure near 10-5 Torr. The cuvette
was flame-sealed, under reduced pressure, placed in the
cryostat, and cooled to 77 K. The sample was irradiated for
several minutes in the spectrometer with a 300-W high-
pressure mercury lamp using a Pyrex filter, and the spectral
changes were recorded at appropriate time intervals. The
N
2
Preparation of [2,6-Dibromo-4-(3-pyridyl)phenyl](4-
tert-butyl-2,6-dimethylphenyl)diazomethane (3-Py-1-N2).
To a mixture of ethyl N-[2,4,6-tribromophenyl(4-tert-butyl-2,6-
dimethylphenyl)methyl]carbamate13f (100 mg, 0.18 mmol),
2-(3-pyridyl)-4,4,5,5-tetramethyl-1,3-dioxaborolane33 (108 mg,
0.53 mmol), [1,1′-bis(diphenylphosphino)ferrocene]palladium
dichloride dichloromethane complex (PdCl2(dppf)‚CH2Cl2) (14
mg, 0.018 mol) and potassium phosphate (63 mg, 0.30 mmol)
was added anhydrous 1,4-dioxane (2.5 mL) under Ar atmo-
sphere and the mixture was stirred at 100 °C for 2 days. The
mixture was then filtered and the filtrate was extracted with
diethyl ether. The organic layer was washed with water, dried
over anhydrous sodium sulfate, and concentrated under
reduced pressure to leave a crude product that was column
chromatographed (silica gel, hexane:diethyl ether ) 1:1) to give
ethyl N-[[2,6-dibromophenyl-4-(3-pyridyl)phenyl](4-tert-butyl-
2,6-dimethylphenyl)methyl]carbamate (36 mg, 36%) as a yel-
1
low semisolid; H NMR (300 MHz, CDCl3, ppm) δ 8.79 (br s,
1H), 8.62 (br d, J ) 4.23 Hz, 1H), 7.81 (dt, J ) 8.08, 1.84 Hz,
1H), 7.77 (s, 2H), 7.39-7.35 (m, 2H), 6.99 (s, 2H), 6.50 (d, J )
8.64 Hz, 1H), 5.39 (d, J ) 8.64 Hz, 1H), 4.26-4.11 (m, 2H),
2.24 (s, 6H), 1.29 (s, 9H), 1.27 (t, J ) 6.98 Hz, 3H); 13C NMR
(75.5 MHz, CDCl3, ppm) δ 155.0, 150.6, 149.5, 148.0, 138.7,
138.0, 137.0, 134.2, 132.6, 132.5, 127.0, 126.9, 126.6, 125.2,
61.2, 58.0, 34.1, 31.2, 21.9, 14.8; EI-MS m/z (rel intensity) 576
(33) Li, W.; Nelson, D. P.; Jensen, M. S.; Hoerrner, R. S.; Cai, D.;
Larsen, R. D.; Reider, P. J. J. Org. Chem. 2002, 67, 5394.
J. Org. Chem, Vol. 70, No. 18, 2005 7063

Matsuno et al.
spectral changes upon thawing were also monitored by care-
fully controlling the matrix temperature with a Temperature
Controller.
SQUID Measurements. Magnetic susceptibility data were
obtained on a Superconducting Quantum Interference Device
(SQUID) magnetometer/susceptometer. Irradiation with light
from an argon ion laser (488 nm) through a flexible optical
fiber that passes through the inside of the SQUID sample
holder was performed inside the sample room of the SQUID
apparatus at 5-11 K. One end of the optical fiber was located
40 mm above the sample cell (capsule) and the other was
attached to a coupler for the laser. The bottom part of the
capsule (6 mm × 10 mm) without a cap was used as a sample
cell. A 50-60 µL sample solution (0.3-1.0 mM) in 2-MTHF
was placed in the cell, which was held by a straw. The
irradiation was carried out until there was no further change
of magnetization monitored at 5 K in a constant field of 5 kOe.
The magnetization, Mb and Ma, before and after irradiation
was measured at 5 K in a field range of 0-50 kOe. The plots
of the magnetization [M ) (Ma - Mb)] versus the magnetic
field were analyzed in terms of the Brillouin function.
Flash Photolysis. All flash measurements were made on
a flash spectrometer. Three excitation light sources were used
depending on the precursor absorption bands and lifetime of
the transient species. They were (i) a cylindrical 150-W Xe
flash lamp (100 J/flash with 10-µs pulse duration), (ii) a Nd:
YAG laser (355 nm pulses of up to 40 mJ/pulse and 5-6-ns
duration; 266 nm pulses of up to 30 mJ/pulse and 4-5-ns
duration), and (iii) an excimer laser (308 nm pulses of up to
200 mJ/pulse and 17-ns duration). The beam shape and size
were controlled by a focal length cylindrical lens.
A 150 W xenon short arc lamp was used as the probe source,
and the monitoring beam, guided using an optical fiber scope,
was arranged in an orientation perpendicular to the excitation
source. The probe beam was monitored with a photomultiplier
tube through a linear image sensor (512 photodiodes used).
Timing of the excitation pulse, the probe beam, and the
detection system was achieved through a digital synchro scope
that was interfaced to a computer. This allowed for rapid
processing and storage of the data and provided printed
graphic capabilities. Each trace was also displayed on a
monitor.
A sample was placed in a long-necked Pyrex tube that had
a sidearm connected to a quartz fluorescence cuvette and
degassed using a minimum of four freeze-degas-thaw cycles
at a pressure near 10-5 Torr immediately prior to being
flashed. The sample system was flame-sealed under reduced
pressure, and the solution was transferred to the quartz
cuvette, which was placed in the sample chamber of the flash
spectrometer. A cell holder block of the sample chamber was
equipped with a thermostat and allowed to come to thermal
equilibrium. The concentration of the sample was adjusted so
that it absorbed a significant portion of the excitation light.
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 Pro-
moted Research (No. 12002007). Supports from the
Mitsubishi Foundation and the Nagase Science and
Technology Foundation is also appreciated.
Supporting Information Available: General methods,
analytical and spectroscopic data for photoproducts of diazo
compounds, transient absorption spectra obtained in LFP of
4- and 3-Py-1-N2 in nondegassed benzene, and UV-vis spectra
measure before and after SQUID measurements. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO0504363
7064 J. Org. Chem., Vol. 70, No. 18, 2005