Synthesis of azo-conjugated metalladithiolenes and their photo- and proton-responsive isomerization reactions

Article abstract of DOI:10.1021/ja028080p

A versatile synthetic method of azo-conjugated metalladithiolenes was developed, and new complexes with various central metals and substituent groups were synthesized. Molecular structures of the azo-conjugated metalladithiolenes of Ni(II), Pd(II), and Pt(II) with diphenylphosphinoethane as a co-ligand were determined by X-ray crystallography. While the energy of the reversible trans-to-cis photoisomerization is considerably lower than that of azobenzene, the thermal stability of the cis form is much higher than that of the organic azobenzene derivatives showing similar low-energy trans-to-cis photoisomerization. A novel proton response of the azo group occurs, and the combination of photoisomerization and protonation leads to a novel proton-catalyzed cis-to-trans isomerization, the rate of which correlates with the redox potential of the metalladithiolene moiety. The study including other azo-conjugated metalladithiolenes has indicated that the protonation is a common feature for the azo-conjugated metalladithiolenes, but trans-to-cis photoisomerization is strongly dependent on the electronic structure of the trans form or a steric effect in the cis form.

Full text of DOI:10.1021/ja028080p

Published on Web 02/07/2003
Synthesis of Azo-Conjugated Metalladithiolenes and Their
Photo- and Proton-Responsive Isomerization Reactions
Masayuki Nihei,† Masato Kurihara, Jun Mizutani, and Hiroshi Nishihara*
Contribution from the Department of Chemistry, School of Science, The UniVersity of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received August 9, 2002 ; E-mail: nisihara@chem.s.u-tokyo.ac.jp
Abstract: A versatile synthetic method of azo-conjugated metalladithiolenes was developed, and new
complexes with various central metals and substituent groups were synthesized. Molecular structures of
the azo-conjugated metalladithiolenes of Ni(II), Pd(II), and Pt(II) with diphenylphosphinoethane as a co-
ligand were determined by X-ray crystallography. While the energy of the reversible trans-to-cis
photoisomerization is considerably lower than that of azobenzene, the thermal stability of the cis form is
much higher than that of the organic azobenzene derivatives showing similar low-energy trans-to-cis
photoisomerization. A novel proton response of the azo group occurs, and the combination of photoi-
somerization and protonation leads to a novel proton-catalyzed cis-to-trans isomerization, the rate of which
correlates with the redox potential of the metalladithiolene moiety. The study including other azo-conjugated
metalladithiolenes has indicated that the protonation is a common feature for the azo-conjugated
metalladithiolenes, but trans-to-cis photoisomerization is strongly dependent on the electronic structure of
the trans form or a steric effect in the cis form.
Azobenzene1 and its derivatives undergo reversible trans-
cis isomerization in response to photo and thermal input, a
mechanism to which much effort has been devoted.2 Such
isomerization of azobenzenes is the subject of current research
interest in the area of photon-mode high-density information
storage and photoswitching devices.3 Azo-conjugated transition
metal complexes can provide new advanced molecular functions,
based on combinations of photoisomerization of the azo group
and changes in the intrinsic properties, that is, in the optical,
redox, and magnetic properties, originating from the d-electrons.
Some metal complexes including azobenzene and related
compounds as a building block have already been synthesized,4
and a number of studies concerned with the photoisomerization
of azo-conjugated metal complexes have appeared in recent
years.5 Some of them have shown novel behaviors which are
not observed for organic azobenzenes, such as redox-combined
single light reversible isomerization,6d,h MLCT photoisomeri-
zation,6f,h and isomerization-promoted photoluminescence
switching,5a,c indicating that the type of attached metal com-
plexes controls such functionalities.
Metalladithiolene,7 which is known to exhibit a noninnocent
character originating from its quasi-aromaticity and a strong
π-electron donor ability due to the involvement of sulfur atoms,
is one of the representative transition metal complexes of present
interest. These complexes have attracted attention as components
in magnetic8 and conducting materials,9 as models for the
molybdenum cofactor (Moco),10 and as solution lumiphore.11
(5) (a) Yam, V. W. W.; Lau, V. C. Y.; Wu, L. X. J. Chem. Soc., Dalton Trans.
1998, 1461. (b) Hayami, S.; Inoue, K.; Osaki, S.; Maeda, Y. Chem. Lett.
1998, 987. (c) Tsuchiya, S. J. Am. Chem. Soc. 1999, 121, 48. (d) Otsuki,
J.; Harada, K.; Araki, K. Chem. Lett. 1999, 269. (e) Aiello, I.; Ghedini,
M.; Deda, M. L.; Pucci, D.; Francescangeli, O. Eur. J. Inorg. Chem. 1999,
1367. (f) Barigelletti, F.; Ghedini, M.; Pucci, D.; Deda, M. L. Chem. Lett.
1999, 297. (g) Miyaki, Y.; Onishi, T.; Kurosawa, H. Chem. Lett. 2000,
1334.
(6) (a) Yutaka, T.; Kurihara, M.; Nishihara, H. Mol. Cryst. Liq. Cryst. 2000,
343, 193. (b) Yutaka, T.; Kurihara, M.; Kubo, K.; Nishihara, H. Inorg.
Chem. 2000, 39, 3438. (c) Yutaka, T.; Mori, I.; Kurihara, M.; Mizutani, J.;
Kubo, K.; Furusho, S.; Matsumura, K.; Tamai, N.; Nishihara, H. Inorg.
Chem. 2001, 40, 4986. (d) Kume, S.; Kurihara, M.; Nishihara, H. Chem.
Commun. 2001, 1656. (e) Kurosawa, M.; Nankawa, T.; Matsuda, T.; Kubo,
K.; Kurihara, M.; Nishihara, H. Inorg. Chem. 1999, 38, 5113. (f) Kurihara,
M.; Matsuda, T.; Hirooka, A.; Yutaka, T.; Nishihara, H. J. Am. Chem.
Soc. 2000, 122, 12373. (g) Kurihara, M.; Nishihara, H. Coord. Chem. ReV.
2002, 226, 125. (h) Kurihara, M.; Hirooka, A.; Kume, S.; Sugimoto, M.;
Nishihara, H. J. Am. Chem. Soc. 2002, 124, 8800.
† Present address: Department of Chemistry, University of Tsukuba,
Tennodai 1-1-1, Tsukuba, Ibaraki 305-8571, Japan. E-mail: nihei@staff.chem.
tsukuba.ac.jp.
* To whom correspondence should be addressed. Fax: +81-3-5841-
8063. Tel: +81-3-5841-4346.
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9
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J. AM. CHEM. SOC. 2003, 125, 2964-2973
10.1021/ja028080p CCC: $25.00 © 2003 American Chemical Society

Azo-Conjugated Metalladithiolenes
A R T I C L E S
Electron Spray Ionization (ESI) Mass Spectrometry Measure-
ments. ESI mass spectra (ESI MS) were obtained with a Micromass
LCT time-of-flight mass spectrometer. Each sample was infused as an
acetonitrile solution. Cone voltage was varied in the range from 30 to
60 V to obtain clear peaks.
Metalladithiolenes possess a highly delocalized bonding descrip-
tion.7 Moreover, strong electronic interactions between metal-
ladithiolene and the azo group can be anticipated by the
conjugation of the metalladithiolenes and the azo group, which
introduces the potential for novel functionality of the azobenzene
moiety.
Electrochemical Measurements. A glassy carbon rod (diameter 5
mm; Tokai Carbon GC-20) was embedded in Pyrex glass, and the cross
section was used as a working electrode. Cyclic voltammetry measure-
ments were carried out in a standard one-compartment cell under Ar
equipped with a platinum-wire counter electrode and an Ag/Ag+
reference electrode (10 mM AgClO4 in 0.1 M Bu4NClO4-MeCN, E0′-
(ferrocenium/ferrocene (Fc+/Fc) in 0.1 M Bu4NClO4/CH2Cl2) ) 0.077
V vs Ag/Ag+) with a BAS CV-50W voltammetric analyzer.
Determination of Cis-to-Trans Thermal Isomerization Rate
Constants. A 1-cm light path-length quartz cell was used for the
photoisomerization measurements. The sample solution was degassed
by N2 bubbling before photoirradiation. The time course change in
absorbance was measured by a Hewlett-Packard 8453 UV-vis
spectrometer, and the temperature was controlled by a Hewlett-Packard
89090A Peltier Temperature Control Accessory. The cis-to-trans thermal
isomerization obeyed first-order kinetics, with the isomerization rate
constant, k, being evaluated from the change in absorbance by the
following equation:
In a previous communication, we reported preliminary results
concerning the synthesis of azo-conjugated platinadithiolene and
its photo and proton response and novel proton-coupled cis-to-
trans isomerization originating from the strong electronic
interactions between the metalladithiolene and the azo moi-
eties.12 Although the protonated product of azobenzene deriva-
tives has been recognized for a long time,13 reports on proton-
catalyzed isomerization of azobenzene derivatives have not been
available to our knowledge. In this paper, we report the
development of a novel versatile method of synthesizing azo-
conjugated metalladithiolene systems, and also the curious
proton response and proton-catalyzed cis-to-trans isomerization
for dppe-M (M ) Ni, Pd, and Pt) complexes. This study
provides a systematic investigation of the dependence of the
types of central metal and substituents on isomerization in the
azo moiety.
ln{(A0 - A∞)/(At - A∞)} ) kt
(1)
Experimental Section
Materials. 5-Nitro-2-thioxo-1,3-benzenedithiol, [1,2-bis(diphenyl-
phosphino)ethane]dichloroplatinum(II), [1,2-bis(diphenylphosphino)-
ethane]dichloropalladium(II), and CpCo(III)I2(CO) (Cp ) η5-cyclo-
pentadienyl) were prepared according to the literature.14 Zinc powder,
iron(III) trichloride, and nickel(II) trichloride were purchased from
Wako Chemicals, and all other materials and solvents were purchased
from Kanto Chemicals. Anhydrous solvents were used after distillation
from CaH2. Tetrahydrofuran was purchased from Kanto Chemicals and
was used after drying by passage through a molecular sieve column
and degassing. Spectroscopic-grade solvents (Kanto Chemicals) were
used for UV-vis absorption spectroscopy. The solvents for electro-
chemical measurement were of HPLC grade (Kanto Chemicals). For
the electrochemical measurements, tetrabutylammonium perchlorate was
purchased from Tomiyama Chemicals and was used after recrystalli-
zation from methanol.
Apparatus. UV-vis, IR, and 1H NMR spectra were recorded with
Jasco V-570 and Hewlett-Packard 8453 UV-vis spectrometers, a Jasco
FT/IR-620v spectrometer, and a JEOL EX270 spectrometer, respec-
tively. Photoisomerization measurements were carried out under a
nitrogen atmosphere using a USHIO 500W super high-pressure USH-
500D mercury lamp as an irradiation source. The monochromic light
was obtained with a Jasco CT-10T monochromator.
where A0, A∞, and At denote the absorbance at time t ) 0, t, and ∞,
respectively. Both k and A∞ were evaluated by a nonlinear least-squares
calculation.
X-ray Crystallography. Crystals of 12-14 obtained by recrystal-
lization from dichloromethane/hexane were mounted on a glass loop,
and data were collected at 173 K (Rigaku Mercury diffractometer
coupled with a CCD area detector with graphite monochromated Mo
KR radiation (0.7107 Å)). An empirical absorption correction was
applied, that resulted in transmission factors ranging from 0.74 to 1.03
for 13 and from 0.44 to 1.04 for 14, respectively. The structure was
solved by direct methods and expanded using Fourier techniques.
Hydrogen atoms were not included in the calculations. The final cycle
of full-matrix least-squares refinement on F was based on 6563 observed
reflections (I > -10.0σ(I)) for 12, on 8870 observed reflections (I >
1.50σ(I)) for 13, and on 3990 observed reflections (I > 3.00σ(I)) for
14. All calculations were performed using the CrystalStructure crystal-
lographic software package of the Rigaku Corp. and the Molecular
Structure Corp. (the crystallographic data are given in Table S1,
Supporting Information).
Synthesis. 5-Nitroso-1,3-benzodithiole-2-thione, 2. Zinc powder
was slowly added to a mixture of 1 (1.00 g, 4.37 mmol) in
2-methoxyethanol (120 mL) and ammonium chloride (350 mg, 6.56
mmol) in water (10 mL) over a 30 min period. After the mixture was
stirred for an additional 1 h, the residue was filtered. The filtrate was
added dropwise over a period of 1 h to a stirred solution of 2.84 g
(10.5 mmol) of FeCl3‚6H2O in ethanol (24 mL) and water (96 mL) at
-15 °C, and then it was filtered and washed with water. The residue
was dried to yield 691 mg (3.24 mmol, 74.2%) of 2. Anal. Calcd for
C7H3N1O1S3‚0.6H2O: C, 37.52; H, 1.89; N, 6.25. Found: C, 37.43;
(8) (a) Manoharan, P. T.; Noordik, J. H.; de Boer, E.; Keijzers, C. P. J. Chem.
Phys. 1981, 74, 1980. (b) Kuppusamy, P.; Manoharan, P. T. Chem. Phys.
Lett. 1985, 118, 159.
(9) (a) Cassoux, P.; Valade, L.; Kobayashi, H.; Kobayashi, A.; Clark, R. A.;
Underhill, A. E. Coord. Chem. ReV. 1991, 110, 115. (b) Olk, R. M.; Olk,
B.; Dietzch, W.; Kirmse, R.; Hoyer, E. Coord. Chem. ReV. 1992, 117, 99.
(c) Fourmigue, M.; Lenoir, C.; Guyon, F.; Amaudrut, J. Inorg. Chem. 1995,
34, 4979.
(10) (a) Hille, R. Chem. ReV. 1996, 96, 2757. (b) Hsu, J. K.; Bonangelino, C.
J.; Kaiwer, S. P.; Boggs, C. M.; Fettinger, J. C.; Pilato, R. S. Inorg. Chem.
1996, 35, 4743.
(11) (a) Zuleta, J. A.; Burberry, M. S.; Eisenberg, R. Coord. Chem. ReV. 1990,
97, 47. (b) Cummings, D. S.; Eisenberg, R. Inorg. Chem. 1995, 34, 2007.
(c) Bevilacqua, M. J.; Eisenberg, R. Inorg. Chem. 1994, 33, 2913. (d)
Cummings, D. S.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949. (e)
Kaiwer, S. P.; Vodacek, A.; Bloufh, N. V.; Pilato, R. S. J. Am. Chem. Soc.
1997, 119, 3311.
(12) Nihei, M.; Kurihara, M.; Mizutani, J.; Nishihara, H. Chem. Lett. 2001, 852.
(13) Hewitt, J. T.; Pope, F. G. Chem. Ber. 1897, 30, 1624.
(14) (a) Nakayama, J.; Sugiura, H.; Hoshino, M. Tetrahedron Lett. 1983, 24,
2585. (b) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem.
Soc. 1976, 98, 6521. (c) Ozawa, F. In Synthesis of Organometallic
Compounds; Komiya, S., Ed.; Wiley: New York, 1997; p 285. (d) King,
R. B. Inorg. Chem. 1966, 5, 82.
1
H, 1.95; N, 5.99. H NMR (270 MHz, CDCl3, 25 °C): δ 8.03 (dd, J
) 8.1, 1.6 Hz, 1H, Ph), 7.87 (d, J ) 1.6 Hz, 1H, Ph), 7.73 (d, J ) 8.1
Hz, 1H, Ph).
5-Amino-1,3-benzodithiole-2-thione, 3. An ethanol solution of
SnCl2‚6H2O (5.91 g, 26.19 mmol) was added to a stirred solution of 1
(2.00 g, 8.73 mmol) in hydrochloric acid (90 mL) and ethanol (45 mL),
and then refluxed for 13 h. The slurry was filtered, and the solid was
dissolved with ethyl acetate and washed three times with aqueous
NaHCO3. The organic solution was then evaporated and purified by
silica gel column chromatography with chloroform as an eluent. The
product was recrystallized from dichloromethane/methanol to yield
9
J. AM. CHEM. SOC. VOL. 125, NO. 10, 2003 2965

A R T I C L E S
Nihei et al.
0.794 g (3.99 mmol, 45.7%) of 3. Anal. Calcd for C7H5N1S3: C, 42.18;
H, 2.53; N, 7.03. Found: C, 42.41; H, 2.63; N, 7.08. H NMR (270
ClNi: C, 62.02; H, 4.25; N, 3.81. Found: C, 61.99; H, 4.30; N, 3.84.
1H NMR (270 MHz, CDCl3, 25 °C): δ 7.99 (s, 1H, Ph), 7.86-7.80
(m, 8H, dppe), 7.76 (d, J ) 8.6 Hz, 2H, Ph), 7.52-7.45 (m, 14H, dppe,
Ph), 7.39 (d, J ) 8.6 Hz, 2H, Ph), 2.40 (d, JPH ) 17.8 Hz, 4H, dppe).
[1,2-Bis(diphenylphosphino-KP)ethane]{4-[(4-methoxyphenyl)-
azo]-1,2-benzenedithiolato-KS,KS′}nickel(II), 11. 11 was synthesized
in the same manner as 9. Yield: 19.0%. Anal. Calcd for C39H34ON2P2S2-
Ni‚0.5H2O: C, 63.26; H, 4.76; N, 3.78. Found: C, 63.56; H, 4.53; N,
1
MHz, CDCl3, 25 °C): δ 7.22 (d, J ) 8.6 Hz, 1H, Ph), 6.77 (d, J ) 1.9
Hz, 1H, Ph), 6.72 (dd, J ) 8.6, 1.9 Hz, 1H, Ph), 3.88 (s, 2H, NH2).
5-(Phenylazo)-1,3-benzodithiole-2-thione, 4. A mixture of 2 (450
mg, 2.11 mmol) and aniline (192 µL, 2.11 mmol) in methanol (125
mL) and acetic acid (25 mL) was refluxed for 21 h. The solution was
concentrated to 40 mL and cooled to 4 °C, and the precipitate was
then filtered and washed with methanol. The resulting crude product
was purified by silica gel column chromatography with chloroform/
hexane (1/1, v/v) as an eluent. Recrystallization from methanol afforded
111 mg (0.386 mmol, 19.5%) of 4. Anal. Calcd for C13H7N2S3Cl: C,
54.14; H, 2.80; N, 9.71. Found: C, 53.96; H, 2.89; N, 9.69. 1H NMR
(270 MHz, CDCl3, 25 °C): δ 8.03 (d, J ) 2.2 Hz, 1H, Ph), 8.00 (dd,
J ) 8.4, 2.2 Hz, 1H, Ph), 7.96-7.91 (m, 2H, Ph), 7.62 (d, J ) 8.4 Hz,
1H, Ph), 7.59-7.49 (m, 3H, Ph).
1
3.84. H NMR (270 MHz, CDCl3, 25 °C): δ 7.97 (s, 1H, Ph), 7.87-
7.79 (m, 10H, dppe, Ph), 7.53-7.44 (m, 14H, dppe, Ph), 6.95 (d, J )
9.2 Hz, 2H, Ph), 3.85 (s, 3H, OMe), 2.40 (d, JPH ) 17.8 Hz, 4H, dppe).
[1,2-Bis(diphenylphosphino-KP)ethane]{4-[(4-methylphenyl)-azo]-
1,2-benzenedithiolato-KS,KS′}nickel(II), 12. 12 was synthesized in
the same manner as 9. Yield: 13.1%. Anal. Calcd for C39H34N2P2S2-
Ni‚2H2O: C, 62.33; H, 5.10; N, 3.73. Found: C, 62.30; H, 5.09; N,
1
3.81. H NMR (270 MHz, CDCl3, 25 °C): δ 7.98 (s, 1H, Ph), 7.86-
5-[(4-Methylphenyl)azo]-1,3-benzodithiole-2-thione, 5. 5 was syn-
thesized in the same manner as 4. Yield: 43.0%. Anal. Calcd for
C14H10N2S3‚0.25H2O: C, 54.78; H, 3.45; N, 9.13. Found: C, 54.86;
H, 3.32; N, 9.19. 1H NMR (270 MHz, CDCl3, 25 °C): δ 8.01 (d, J )
1.6 Hz, 1H, Ph), 7.97 (dd, J ) 8.4, 1.6 Hz, 1H, Ph), 7.84 (d, J ) 8.1
Hz, 2H, Tol), 7.61 (d, J ) 8.4 Hz, 1H, Ph), 7.34 (d, J ) 8.1 Hz, 2H,
Tol), 2.46 (s, 3H, Me).
7.80 (m, 8H, dppe), 7.53-7.45 (m, 14H, dppe, Ph), 7.71 (d, J ) 8.4
Hz, 2H, Tol), 7.23 (d, J ) 8.4 Hz, 2H, Tol), 2.39 (d, JPH ) 17.6 Hz,
4H, dppe), 2.38 (s, 3H, Me).
[1,2-Bis(diphenylphosphino-KP)ethane]{4-[(4-methylphenyl)-azo]-
1,2-benzenedithiolato-KS,KS′}palladium(II), 13. Phenyllithium (0.38
mL, 0.40 mmol) was added dropwise to a stirred solution of 5 (60 mg,
0.20 mmol) in THF (25 mL) at -15 °C and stirred for 10 h, allowing
the temperature to increase to room temperature. dppePdCl2 powder
(115 mg, 0.20 mmol) was then added to the solution, which was stirred
further for 2 h. The resulting mixture was separated by silica gel column
chromatography using dichloromethane/hexane (1/1, v/v) as an eluent.
The yellow-band component was collected, and the solvent was
evaporated. Recrystallization from dichloromethane/hexane yielded 7.9
mg (0.010 mmol, 5.2%) of 13 as fine yellow crystals. Anal. Calcd for
C39H34N2P2S2Pd: C, 61.38; H, 4.49; N, 3.67. Found: C, 61.25; H, 4.66;
5-[(4-Chlorophenyl)azo]-1,3-benzodithiole-2-thione, 6. 6 was syn-
thesized in the same manner as 4. Yield: 19.5%. Anal. Calcd for
C13H7N2S3Cl: C, 48.36; H, 2.19; N, 8.68. Found: C, 48.14; H, 2.28;
1
N, 8.71. H NMR (270 MHz, CDCl3, 25 °C): δ 8.02 (d, J ) 1.4 Hz,
1H, Ph), 7.98 (dd, J ) 8.4, 1.4 Hz, 1H, Ph), 7.89 (d, J ) 8.4 Hz, 2H,
Ph), 7.62 (d, J ) 8.4 Hz, 1H, Ph), 7.51 (d, J ) 8.4 Hz, 2H, Ph).
5-[(4-Methoxylphenyl)azo]-1,3-benzodithiole-2-thione, 7. 7 was
synthesized in the same manner as 4. Yield: 35.0%. Anal. Calcd for
C14H10ON2S3: C, 52.80; H, 3.17; N, 8.80. Found: C, 52.55; H, 3.19;
1
N, 3.72. H NMR (270 MHz, CDCl3, 25 °C): δ 7.98 (d, J ) 2.2 Hz,
1
1H, Ph), 7.89-7.79 (m, 8H, dppe), 7.72 (d, J ) 8.1 Hz, 2H, Tol),
7.54-7.26 (m, 14H, dppe), 7.24 (d, J ) 8.1 Hz, 2H, Tol), 2.54 (d, JPH
) 20.1 Hz, 4H, dppe), 2.38 (s, 3H, Me).
N, 8.80. H NMR (270 MHz, CDCl3, 25 °C): δ 7.97 (d, J ) 1.9 Hz,
1H, Ph), 7.95 (dd, J ) 8.1, 1.9 Hz, 1H, Ph), 7.93 (d, J ) 9.5 Hz, 2H,
Ph), 7.59 (d, J ) 8.1 Hz, 1H, Ph), 7.03 (d, J ) 9.5 Hz, 2H, Ph), 3.91
(s, 3H, OMe).
[1,2-Bis(diphenylphosphino-KP)ethane]{4-[(4-methylphenyl)-azo]-
1,2-benzenedithiolato-KS,KS′}platinum(II), 14. Phenyllithium (0.38
mL, 0.40 mmol) was added dropwise to a stirred solution of 5 (60 mg,
0.20 mmol) in THF (25 mL) at -15 °C and was stirred for 10 h,
allowing the temperature to increase to room temperature. dppePtCl2
powder (133 mg, 0.20 mmol) was then added to the solution, which
was stirred further for 2 h. The resulting mixture was separated by
silica gel column chromatography using dichloromethane/hexane (1/
1, v/v) as an eluent. The yellow-band component was collected, and
the solvent was evaporated. Recrystallization from dichloromethane/
hexane yielded 14.0 mg (0.016 mmol, 8.2%) of 14 as fine yellow
crystals. Anal. Calcd for C39H34N2P2S2Pt: C, 54.99; H, 4.02; N, 3.29.
5,5′-Azobis(1,3-benzodithiole-2-thione), 8. A mixture of 2 (326 mg,
1.53 mmol) and 3 (350 mg, 1.53 mmol) in chloroform (200 mL),
methanol (70 mL), and acetic acid (70 mL) was refluxed for 3 days.
The clear solution was then turned to slurry, and the orange precipitate
was collected and washed with methanol to yield 296 mg of 8 (0.75
mmol, 49.0%). Anal. Calcd for C14H6N2S6: C, 42.61; H, 1.53; N, 7.10.
1
Found: C, 42.36; H, 1.67; N, 6.94. H NMR (270 MHz, CDCl3, 25
°C): δ 8.04 (d, J ) 1.9 Hz, 1H, Ph), 8.00 (dd, J ) 8.7, 1.9 Hz, 1H,
Ph), 7.64 (d, J ) 8.7 Hz, 2H, Ph).
All manipulations described below were carried out under a nitrogen
atmosphere.
1
Found: C, 54.90; H, 4.08; N, 3.30. H NMR (270 MHz, CDCl3, 25
1,2-Bis(diphenylphosphino-KP)ethane][4-(phenylazo)-1-,2-ben-
zenedithiolato-KS,KS′]nickel(II), 9. Phenyllithium (0.38 mL, 0.40
mmol) was added dropwise to a stirred solution of 4 (58 mg, 0.20 mmol)
in THF (25 mL) maintained at -15 °C, and the resulting solution was
then stirred for 10 h, while the temperature was allowed to increase to
room temperature. dppeNiCl2 powder (105 mg, 0.20 mmol) was then
added to the solution, which was stirred further for 2 h. The resulting
mixture was separated by silica gel column chromatography using
dichloromethane/hexane (1/1, v/v) as an eluent. The yellow-band
component was collected, and the solvent was evaporated. Recrystal-
lization from dichloromethane/hexane yielded 25.6 mg (0.026 mmol,
18.2%) of 9 as a red powder. Anal. Calcd for C38H32N2P2S2Ni: C,
65.07; H, 4.60; N, 3.99. Found: C, 64.85; H, 4.53; N, 4.01. 1H NMR
(270 MHz, CDCl3, 25 °C): δ 8.00 (s, 1H, Ph), 7.86-7.79 (m, 10H,
dppe, Ph), 7.54-7.27 (m, 16H, dppe, Ph), 7.31 (d, J ) 8.4 Hz, 1H,
Ph), 2.40 (d, JPH ) 17.6 Hz, 4H, dppe).
°C): δ 8.12 (d, J ) 2.2 Hz, 1H, Ph), 7.92-7.78 (m, 8H, dppe), 7.74
(d, J ) 8.4 Hz, 2H, Tol), 7.50-7.47 (m, 12H, dppe), 7.62 (d, J ) 8.1
Hz, 1H, Ph), 7.43 (dd, J ) 2.2, 8.1 Hz, 1H, Ph), 7.24 (d, J ) 8.4 Hz,
2H, Tol), 2.51 (d, JPH ) 18.1 Hz, 4H, dppe), 2.39 (s, 3H, Me).
Results and Discussion
Synthesis of Azo-Conjugated Metalladithiolenes. Several
synthetic methods of metalladithiolenes have been previously
reported. The methods used for the synthesis of metallo-1,2-
ethylenedithiolates have been well established,7,15 but the
(15) (a) Wang, K.; McConnachie, J. M.; Stiefel, E. I. Inorg. Chem. 1999, 38,
4334. (b) Kaiwer, S. P.; Hsu, J. K.; Vodacek, A.; Yap, G.; Liable-Sands,
L. M.; Rheingold, A. L.; Pilato, R. S. Inorg. Chem. 1997, 36, 2406. (c)
Davies, E. S.; Beddoes, R. L.; Collison, D.; Dinsmore, A.; Docrat, A.;
Joule, J. A.; Wilson, C. R.; Garner, C. D. J. Chem. Soc., Dalton Trans.
1997, 3985. (d) Hsu, J. K.; Bonangelino, C. J.; Kaiwar, S. P.; Boggs, C.
M.; Fettinger, J. C.; Pilato, R. S. Inorg. Chem. 1996, 35, 4743. (e)
Nakayama, J.; Choi, K. S.; Akiyama, I.; Hoshino, M. Tetrahedron Lett.
1993, 34, 115.
[1,2-Bis(diphenylphosphino-KP)ethane]{4-[(4-chlorophenyl)-azo]-
1,2-benzenedithiolato-KS,KS′}nickel(II), 10. 10 was synthesized in
the same manner as 9. Yield: 23.3%. Anal. Calcd for C38H31N2P2S2-
9
2966 J. AM. CHEM. SOC. VOL. 125, NO. 10, 2003

Azo-Conjugated Metalladithiolenes
A R T I C L E S
Scheme 1. Synthesis of Azo-Conjugated Metalladithiolenesa
a Reagents: (i) Zn, NH4Cl, MeOCH2CH2OH; (ii) FeCl3‚6H2O; (iii) SnCl2‚6H2O, HCl; (iv) CH3COOH, MeOH; (v) MCl2(dppe); (vi) CpCoI2(CO); (vii)
NiCl2‚6H2O, Bu4NBr; (viii) CH3COOH, MeOH, CHCl3; (ix) NiCl2.
methods for synthesizing metallo-1,2-benzenedithiolates remain
few in number.16 The most versatile and general method of
preparing metallo-benzenedithiolenes is a reaction involving a
benzenedithiolate dianion and a metal halide.16a Benzenedithi-
olate is highly sensitive to oxidation17 and is unstable in air.
Thus, the key elements in the synthetic strategy of the azo-
conjugated metalladithiolene system are (1) how to protect the
dithiolate group from reduction and oxidation while constructing
the azo group, and (2) how to create the metalladithiolene
component after a cleavage of the protecting group. To address
these issues, we developed a versatile method of synthesizing
azo-conjugated metalladithiolenes, as described in Scheme 1.
Nitroso compound 2 derived from nitro compound 1 is a
novel dithiolate-protected compound, which can react with a
number of organic and other functional molecules containing
amino groups to afford various asymmetric and symmetric azo-
conjugated metalladithiolenes. This protecting group can be
cleaved by either basic media or nucleophilic reagents, whereas
it is stable in strong acidic media and is resistant to strong
reduction and mild oxidation. Thus, this group is one of the
most suitable protecting groups for the construction of the azo
group by reduction and oxidation in acidic media. The versatility
of this synthetic method was demonstrated by the preparation
of the precursors, 4-8, from compound 2 and the corresponding
anilines. Reactions of 2 and aniline derivatives in mild acidic
media afforded precursors of metalladithiolene ligands with
various substituent groups, 4-7, in moderate yields (Scheme
1C). The precursor of azo-bridged dinuclear metalladithiolene
ligand 8 was prepared by the reaction of 2 and amino compound
3, obtained by the reduction of nitro compound 1 with tin(II)
chloride in hydrochloric acid in a moderate yield (Scheme 1B).
This condensation reaction can be carried out under gentler
conditions than can other preparations of the azo group; thus
this method is applicable for the generation of various types of
azo-conjugated metalladithiolenes that contain functional groups.
These precursors could derive corresponding metalladithi-
olenes by cleavage of the protecting group with phenyllithium
or potassium hydroxide (Scheme 2), followed by the addition
of metal halides.
The one-pot preparation used here provided two advantages:
the first was selectivity of the reaction conditions due to the
choice between two different types of cleavage, and the second
was a preparation of the metalladithiolene without isolation of
air-sensitive dithiolate or dithiol compounds. The cleavage with
phenyllithium yielded dithiolate in dry THF accompanying the
loss of diphenylthioketone (Scheme 2, cleavage 1). Because
some metalladithiolenes and metal halides are unstable in protic
solvent, reaction in aprotic solvent is often required for
metalladithiolene synthesis. In contrast, many metal halides and
some dithiolate dianions are water-soluble or methanol-soluble,
(16) (a) Heck, R. F. Inorg. Chem. 1968, 7, 1513. (b) Cocker, T. M.; Bachman,
R. E. Inorg. Chem. 2001, 40, 1550. (c) Xi, R.; Abe, M.; Suzuki, T.;
Nishioka, T.; Isobe, K. J. Organomet. Chem. 1997, 549, 117.
(17) Kuster, W.; Mayo, P. J. Am. Chem. Soc. 1974, 96, 3502.
9
J. AM. CHEM. SOC. VOL. 125, NO. 10, 2003 2967

A R T I C L E S
Nihei et al.
Scheme 2. Cleavages of the Protecting Group
Figure 1. ORTEP plot of 12 with 50% probability of ellipsoids.
whereas they are insoluble in most organic solvents. Cleavage
with potassium hydroxide in alcohol proved suitable for using
such compounds as reaction reagents (Scheme 2, cleavage 2).
By employing both methods, we successfully prepared various
types of dithiolenes as 9-18.
Reaction of dppeNiCl2 and dithiolate derived from 5 by
cleavage 1 gave a mixture containing air-stable Ni dithiolene
12 (Scheme 1C). 12 could be purified by silica gel column
chromatography and recrystallization from CH2Cl2/hexane to
yield orange crystals. Substituting dppePdCl2 or dppePtCl2 for
the Ni complex in this reaction provided the corresponding
metalladithiolenes, 13 and 14, in the same manner. Low yields
of these complexes, 5-13% from 5, appear to have been the
result of nucleophilic side reactions accompanying the cleavage
of the protecting group by phenyllithium. Replacing the methyl
group with other substituent groups had no effect on the yield
of 9-11.
Figure 2. ORTEP plot of 13 with 50% probability of ellipsoids.
Because CpCoI2(CO) is air- and water-sensitive,14d cleavage
1 was used to prepare 15 (Scheme 1C). The yield from 5 was
similar to that of the dppe-metalladithiolene complexes 12-
14. On the other hand, because NiCl2‚6H2O is soluble in water
or methanol and insoluble in most organic solvents, the cleavage
was carried out with potassium hydroxide (cleavage 2) to
prepare 16 (Scheme 1C). The yield from 5 was similar to that
of the other dithiolenes.
Reaction of 8 and phenyllithium in THF (cleavage 1) gave a
slurry; the subsequent addition of dppeNiCl2 to this slurry
afforded the mononuclear complex 17 as a red solution (Scheme
1C). On the other hand, treatment of 8 with potassium hydroxide
in alcohol (cleavage 2) gave a clear red solution, and the addition
of dppeNiCl2 led to a precipitation of the dinuclear complex
18 (Scheme 1C). This result can be attributed to the solubility
of the dithiolate anion. In cleavage 1, the cleavage of the two
protecting groups of 8 most likely proceeded stepwise, and the
initially produced dianion was immediately precipitated because
the charged dianion compound was insoluble in THF. Addition
of dppeNiCl2 for the complexation reaction gave a clear solution
of THF-soluble mononuclear complex 17. In the case of
cleavage 2, the dianion and the tetraanion derived from 8 were
soluble in alcohol; accordingly, the complete cleavage of the
two protecting groups was achieved. The following complex-
ation reaction afforded the alcohol-insoluble dinuclear complex
18 as a red precipitate. It is of note that utilizing two different
cleavage conditions (i.e., cleavages 1 and 2) provided for quite
different structures of the complex.
Figure 3. ORTEP plot of 14 with 50% probability of ellipsoids.
the Ni, Pd, and Pt complexes 12-14 by recrystallization from
dichloromethane/hexane. ORTEP diagrams for 12-14 are
displayed in Figures 1-3 (the selected bond lengths, angles,
and torsion angles are summarized in Table S2, Supporting
Information). In the Pd complex, 13, there were two types of
molecules having slightly different structures. The configuration
around the metal center was a typical tetracoordinated square-
planar structure, and a dichloromethane molecule was included
as the crystal solvent. The metalladithiolene rings were planar
five-membered rings, which indicated the existence of the quasi-
aromaticity in these metallacycles.7 The bond lengths of M-S1
and M-S2 were within the usual range of M-S distances for
other square-planar metalladithiolene complexes.18 In the Ni
complex, 12, the Ni-S and the S-C bonds can be best described
as single bonds with lengths of 2.158(3) and 2.159(3) Å, and
X-ray Crystallographic Analysis of 12-14. Single crystals
suitable for X-ray crystallographic analysis were obtained for
9
2968 J. AM. CHEM. SOC. VOL. 125, NO. 10, 2003

Azo-Conjugated Metalladithiolenes
A R T I C L E S
Table 1. Reduction and Oxidation Potentialsa of 12-14
0′
E or Ep,c or Ep,a/V vs Fc+/Fc
b
complex
metalladithiolene
azo
metalladithiolene
azonium
12
13
14
-1.930 (rev)
-2.25 (2e-)
-2.60 (2e-)
-1.85
-1.96
-1.95
-1.98
-2.08
-2.22
0.31
0.38
0.45
-0.29
-0.34
-0.35
a E0′ ()(Ep,c + Ep,a)/2) values denote reversible reactions (rev. is given
in parentheses), and Ep,c and Ep,a values are given for irreversible reduction
and oxidation. The reduction is a 1e- process unless 2e- is specified in the
parentheses. b Potentials of a new reduction wave upon the addition of acid.
1.775(9) and 1.750(9) Å, respectively, and the C1-C2 bond at
1.368(12) Å is best described as a double bond. The Pd and Pt
complexes 13 and 14 had structural features in the metalla-
dithiolene moiety similar to that of the Ni complex, 12. Regular
organic azobenzenes have a planar structure in the trans form,
and the azo double bond length lies in the range of 1.23-1.27
Å.19 The configurations of the azobenzene moiety in 12-14
were all trans, but the planarity and the N1-N2 bond length of
the azobenzene moiety were significantly different from each
other in a comparison of complexes. In the Ni complex, 12,
the five-membered metalladithiolene ring was coplanar with a
fused phenyl ring and an azo double bond; however, the
planarity between the tolyl and the azo moieties was lost with
a N1-N2-C7-C8 torsion angle of -158.5(11)°. On the other
hand, the Pd complex 13 had an almost planar azobenzene
structure that was coplanar with the metalladithiolene ring. The
Pt complex, 14, contained a significantly distorted azobenzene
moiety, as can be seen in the C12-C7-C5-C4 torsion angle
of 55.3°. The N1-N2 bond length in 12 and that in 13 were
1.252(9) and 1.26(2) Å, respectively, values which lie in the
normal range of 1.23-1.27 Å for organic azobenzenes. The Pt
complex, 14, includes a significantly long N1-N2 double bond
with a length of 1.32(2) Å. This value is closer to the single
bond length of charge-distributed triazene (-N--N+dN-),20
rather than to that of a typical double bond. The unusual bond
length indicated the existence of strong conjugation between
the metalladithiolene and the azobenzene moieties.
Electrochemistry of 12-14. Cyclic voltammetry of 12-14
was carried out at a glassy carbon electrode in Bu4NClO4-
acetonitrile (Figure S1, Supporting Information), and the results
are summarized in Table 1. The complexes 12-14 show a
reduction wave ascribable to the metalladithiolene moiety, in
addition to irreversible two-step 1e- reduction waves of the azo
moiety from -1.8 to -2.0 V versus Fc+/Fc, respectively. The
Ni complex, 12, shows a reversible wave at -1.9 V, which can
be attributed to a Ni(II)/Ni(I) 1e- reduction.20a Compounds 13
and 14 underwent an irreversible one-step 2e- reduction on the
metal center at -2.2 and -2.6 V, respectively. The potential
and reversibility of the reduction of the metalladithiolene
moieties were similar to those of the related complexes,21 and
the reduction potential was found to be more negative, that is,
in the order of 14 > 13 > 12. The reduction potentials of the
Figure 4. UV-vis spectral changes of 12 (a), 13 (b), and 14 (c) upon
addition of CF3SO2H in MeCN. The numbers in the figure refer to the
mole equivalents of the acid to the complex.
azo moiety in 12-14 were more negative than that of azotoluene
(-1.70 V vs Fc+/Fc), and this potential shift can be attributed
to the strong electron donation from the metalladithiolene
moiety. The donation effect was more efficient, that is, in the
order of 14 > 13 > 12. In the oxidation of the metalladithiolene
moiety, all complexes revealed a one-step irreversible wave at
0.3-0.5 V versus Fc+/Fc, and the order of these potentials was
identical to that of the reduction potential. The results of these
electrochemical measurements indicated that the π electron
delocalization and the π donation ability of the metalladithiolene
moiety were both the most significant in the Pt complex, 14.
Proton Response of 9-14. Compounds 12-14 exhibited an
unusual proton response together with a drastic change in color,
for example, from yellow to deep green in the Ni complex, 12,
and to deep blue in the Pd and Pt complexes, 13 and 14. Figure
4 shows the spectral changes in 12-14 upon addition of
trifluoromethanesulfonic acid in MeCN. With increases in the
amount of acid, the π-π* transition band of the azo moiety
decreased in intensity, and a new strong band appeared in the
visible region. Reverse spectral changes were achieved with the
addition of potassium tert-butoxide. This spectral behavior was
similar to that of the N-heterocycle-substituted metalladithiol-
enes,11e,15b,22 thus indicating that protonation of the azo moiety
had taken place (Scheme 3). To examine the proton response
(18) (a) Darkwa, J. Inorg. Chim. Acta 1997, 257, 137. (b) Landis, K. G.; Hunter,
A. D.; Wagner, T. R.; Curtin, L. S.; Filler, F. L.; Jansen-Varunum, S. A.
Inorg. Chim. Acta 1998, 282, 155.
(19) (a) Is¸ik, S¸.; O¨ ztu¨rk, S.; Fun, H. K.; Agar, E.; S¸as¸maz, S. Acta Crystallogr.,
Sect. C 2000, 95. (b) Handrosch, C.; Dinnebier, R.; Bondarenko, G.; Bothe,
E.; Heinemann, F.; Kisch, H. Eur. J. Inorg. Chem. 1999, 1259.
(20) (a) Boyd, G. V.; Norris, T.; Lindley, P. F.; Mahmoud, M. M. J. Chem.
Soc., Perkin Trans. 1 1977, 1612. (b) Boyd, G. V.; Norris, T.; Lindley, P.
F. J. Chem. Soc., Perkin Trans. 1 1977, 965.
(21) (a) Bowmaker, G. A.; Boyd, P. D. W.; Campbell, G. K. Inorg. Chem. 1982,
21, 2403. (b) Bond, A. M.; Tedesco, V. Inorg. Chem. 1994, 33, 5761.
9
J. AM. CHEM. SOC. VOL. 125, NO. 10, 2003 2969

A R T I C L E S
Nihei et al.
Scheme 3. Resonance Structures of Protonated
Metalladithiolenes
Table 2. λmax of Trans and Protonated Forms of 9-12 and Rate
Constantsa of Cis-to-Trans Thermal Isomerization of 9-12 in
MeCN
trans-M
+
M−H
λmax/nm
complex
(10-4 ꢀ/M-1 cm-1
)
λ
max/nm
104 k/s-1
9
422 (1.92)
430 (2.02)
421 (2.22)
418 (1.82)
641
653
656
642
0.63
1.2
2.8
1.2
10
11
12
a Rate constants were estimated from the first-order plots of absorption
change at 35 °C.
further,electrochemical measurements were carried out upon
addition of acid to 12-14. The results are given in Table 1
(cyclic voltammograms are displayed in Figure S2, Supporting
Information). Upon the addition of acid, the reduction wave of
the azo moiety disappeared, and a new reduction wave was
observed at a more positive potential (-0.22 to -0.28 V vs
Fc+/Fc). This finding clearly demonstrates that the protonation
of the azo moiety induces a significantly positive shift of the
azo reduction potential. The reduction potentials of the proto-
nated azo moiety were in the following order: 14 > 13 > 12,
indicating the order of stability of the protonated azo moiety. It
can be deduced from these results that the protonated azo moiety
is stabilized by the π electron donation from the dithiolene ring,
which is the most efficient in the Pt complex, 14. The single
protonation is supported by the results of the ESI-mass
spectroscopic measurement, which show a peak at m/z 715.1
in 12, m/z 763.1 in 13, and 852.3 in 14, and an isotope pattern
of M-H+ (M ) 12-14), respectively. The protonation to the
nitrogen atom bound to the tolyl moiety reasonably afforded
the conjugated structure that delocalized the charge beyond the
metalladithiolene moiety (Scheme 3). The metalladithiolene
moiety, which was strongly conjugated with the azo group,
increased the basicity of the azo group with a resonance
stabilization effect, which in turn led to the observed facilitated
protonation behavior. A strong visible band was assigned to
the metalladithiolene π f azo group π* charge-transfer transi-
tion. Appearance of this band was caused by the protonation to
the azo group, because the protonation, accompanying the strong
conjugation with an alternative bond structure, decreased the
energy gap between the metalladithiolene moiety and the azo
moiety. The wavelengths of this charge-transfer band in azo-
conjugated Ni dithiolenes with various substituent groups, R
(R ) H, 9; R ) Cl, 10; R ) OMe, 11; R ) CH3, 12), are
summarized in Table 2, which shows that no significant
difference can be seen among the various substituted dithiolenes.
This result indicates that the wavelength of this band was
dominated by the electronic structure of the metalladithiolene
moiety and was furthermore independent of the substituent group
on the phenyl ring.
Figure 5. UV-vis spectra of 12 (a), 13 (b), and 14 (c) in MeCN before
(solid line) and after (dotted line) photoirradiation by UV light at 405 nm.
Photo Response of 12-14. Figure 5 shows UV-vis absorp-
tion spectra of 12-14 in MeCN before and after photoirradiation
with a UV light at 405 nm. A π-π* transition band ascribable
(22) Kaiwer, S. P.; Vodacek, A.; Blough, N. V.; Pilato, R. S. J. Am. Chem.
Soc. 1997, 119, 9211.
9
2970 J. AM. CHEM. SOC. VOL. 125, NO. 10, 2003

Azo-Conjugated Metalladithiolenes
A R T I C L E S
to the azo moiety in the trans form can be observed at 405 nm
(18 200 M-1 cm-1), 405 nm (22 300 M-1 cm-1), and 405 nm
(23 700 M-1 cm-1) for 12-14, respectively, because the
M(S2C6H4)(dppe) (M ) Ni, Pd, Pt) does not give a strong band
in this region. The bands are significantly shifted to a lower
energy for 12-14, as compared with those of regular azoben-
zene derivatives,1 an effect which was due to strong conjugation
and high delocalization between the metalladithiolene and the
azo moiety. In regular organic azobenzenes, a nearly linear
correlation exists between the lower energy shift of the π-π*
transition band and the stability of the cis form,23 and thus some
of these compounds with a lower shifted π-π* transition band
may have shown no efficient photo response.1a Despite the large
low-energy shift of the π-π* transition, 12-14 showed a
distinguishable reversible photo response. The π-π* transition
band decreased in intensity after photoirradiation, indicating the
occurrence of trans-to-cis photoisomerization. The cis-to-trans
reverse reaction occurred, allowing perfect recovery of the
spectra of the trans forms, in response to photoirradiation at
360 nm in 12, and at 310 nm in 13 and 14, respectively. The
Figure 6. Correlation between ∑σc-t and log(k/k0).
these complexes are listed in Table 2. The wavelengths of
absorptions in these complexes were almost identical to each
other and were independent of the substituent group. In contrast,
the rate constants of the cis-to-trans isomerization were strongly
dependent on the substituent group. Nishimura et al.23 reported
that the introduction of substituents almost invariably accelerated
the rate, irrespective of the nature of the substituents. The
substituent effect was of an additive nature, and the following
Hammett-type equation holds approximately, as
1
reversible isomerization reaction was also observed by a H
NMR spectroscopic measurement. The integration ratio of the
signals for trans and cis forms indicated that ca. 40% of the
trans form was converted to the cis form by photoirradiation at
405 nm in a photostationary state. Although the transition
corresponding to these UV light energies has not been specified,
it is intriguing that the cis-to-trans isomerization is promoted
by UV light with an energy higher than the π-π* transition,
because the regular azobenzenes undergo isomerization by
irradiating the n-π* band in the visible region.1b
log k/k )
σ
(2)
∑
0
c-t
where k0 and k refer to the rate constant of an azobenzene24
and of substituted azobenzene derivatives, and σc-t denotes a
substituent constant characteristic of the type and the position
of the substituent in question. Using this equation and the rate
constant for the nonsubstituted Ni dithiolene complex, 9, we
Thermal Isomerization of 9-12. Organic azobenzenes have
been known to show cis-to-trans thermal isomerization, the rate
of which is strongly affected by para-substituent groups.23
Referring to the ability of the electron donor and acceptor
substituted on the two phenyl groups, Rau1a classified the
azobenzenes into three groups: (i) the azobenzene type, (ii) the
aminoazobenzene type, and (iii) the pseudo-stilbene type. In
the case of (i), that is, azobenzene without donor or acceptor
substituents, an absorption peak corresponding to the π-π*
transition appeared around 350 nm, and thermal isomerization
was very slow (more than 1 day at ambient temperature). For
type (ii) with the donor group on the para-position of the phenyl
ring, an absorption peak assigned to the π-π* transition
appeared at ca. 400 nm, which was longer than the wavelength
of type (i). The thermal isomerization was also faster in the
case of the type (ii) compounds (several hours). The last type
(iii), with donor and acceptor substituents on the para-position
of each phenyl group (the so-called push-pull type), exhibited
a maximum absorption peak attributed to the π-π* transition
in the visible region (longer than 400 nm). The thermal
isomerization of type (iii) was much faster (several minutes or
seconds) than that of type (ii) and strongly depended on the
polarity of the environment. In this study, cis-to-trans isomer-
ization was investigated for azo-conjugated Ni dithiolenes with
various substituent groups, R (R ) H, 9; R ) Cl, 10; R ) OMe,
11; and R ) CH3, 12). The estimated first-order rate constants
for cis-to-trans isomerization for 9-12 at 35 °C and the λmax of
estimated the substituent constant for Ni dithiolene as σc-t
)
1.02. Using this value and the known substituent constant (CH3;
σc-t ) 0.22, Cl; σc-t ) 0.22, OMe; σc-t ) 0.46), we plotted
log k/k0 values against σc-t for 9-12 according to eq 2 (Figure
6). A positive correlation was found to exist between log k/k0
and σc-t, which indicated that metalladithiolenes also obey the
additive rule for the cis-to-trans isomerization rate constant of
organic azobenzenes and that the stability of the cis form for
substituted azo-conjugated metalladithiolenes can thus be
predictable. Furthermore, although metalladithiolenes exhibit the
π-π* transition band shifted to a lower energy, the stability of
the cis form was higher than that of such organic azobenzenes.
It can be considered that tactics to induce a lower-shifted π-π*
transition band in organic azobenzenes are most likely limited,
and the donor-acceptor substituent groups are introduced for
most of the compounds25 (i.e., type (iii)), for which the rate of
rotational isomerization is much accelerated. Consequently, azo-
conjugated Ni dithiolenes 9-12 can be classified as type (ii)
or type (iii) in the absorption peak of the π-π* transition, as
well as type (i) in regards to the stability of the cis form. The
results described above indicate that the use of azo-conjugated
metalladithiolenes offers an efficient method of lowering the
transient energy needed for isomerization, while maintaining
the stability of the cis form.
(24) Talaty, E. R.; Fargo, J. C. J. Chem. Soc., Chem. Commun. 1967, 65.
(25) (a) King, N. R.; Whale, E. A.; Davis, F. J.; Gilbert, A.; Mitchell, G. R. J.
Mater. Chem. 1997, 7, 625. (b) Charlton, M. H.; Docherty, R.; Mcgeein,
D. J.; Morley, J. O. J. Chem. Soc., Faraday Trans. 1993, 89, 1671. (c)
Wildes, P. D.; Pacifici, J. G.; Irick, G., Jr.; Whitten, D. G. J. Am. Chem.
Soc. 1971, 93, 2004.
(23) (a) Nishimura, N.; Kosako, S.; Sueishi, Y. Bull. Chem. Soc. Jpn. 1984, 57,
1617. (b) Nishimura, N.; Sueyoshi, T.; Yamanaka, H.; Imai, E.; Yamamoto,
S.; Hasegawa, S. Bull. Chem. Soc. Jpn. 1976, 49, 1381. (c) Sueyoshi, T.;
Nishimura, N.; Yamamoto, S.; Hasegawa, S. Chem. Lett. 1974, 1131.
9
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A R T I C L E S
Nihei et al.
Scheme 4. Photo and Proton Responses of Azo-Conjugated
Metalladithiolenes
Figure 7. Correlation between redox potentials and rate constant k (white)
and kacid (black) for 12-14.
Proton-Catalyzed Cis-to-Trans Isomerization of 12-14.
The proton response of the cis form was then investigated. When
a slight amount of acid was added to a solution containing cis-
12-14 prepared by photoirradiation, immediate cis-to-trans
transformations were observed. The rate constants of cis-to-
trans thermal and proton-catalyzed isomerization upon addition
of 0.01 equiv of CF3SO3H for 12-14 at 23.3 °C are displayed
in correlation with the reduction potentials of nonprotonated
forms in Figure 7. The rate constant of thermal isomerization,
k (s-1), was independent of the differences among the central
metals, whereas the rate constant of proton-catalyzed isomer-
ization, kacid (s-1), strongly depended on the metal center. The
kacid reaches 5.1 × 10-3 s-1, which is 150 times larger than k,
in the Pt complex 14, whereas the significant acceleration by
addition of the acid in the same condition was not observed for
azobenzene (k ) 2 × 10-6 s-1, kacid ) 3 × 10-6 s-1). A positive
correlation existed between the rate constant, kacid, and the
reduction potential of the metalladithiolenes. This result indi-
cated that the rate constants of the proton-catalyzed isomeriza-
tion were exponentially proportional to the redox potentials of
the metalladithiolenes and that the acceleration was efficient
by shifting the reduction potential in the negative direction. It
can be deduced that the reduction potential of the metal center
is closely related to the electron-withdrawing ability of the metal
center, which accompanies the electron-donation ability of the
dithiolene ring. In the case of the Pt complex 14, the reduction
potential of which was more negative than that of 12 and 13,
strong donation to the azo group was exhibited; such donation
stabilized the transient state with the resonance effects. Ac-
cordingly, the acceleration of isomerization was closely related
to the reduction potential of the metal center.
The proton-assisted phenomenon indicated that a protonated
cis form, cis-H+, instantly produced the trans form, trans-H+,
followed by the formation of the trans form accompanying
deprotonation (Scheme 4). In this isomerization, it can be
presumed that the transient structure was the charge-distributed
structure (Scheme 4), in which the protonated nitrogen atom
changed its hybridization from sp2 to sp3. This activated species,
with substantial single-bond character in the azo group, was
stabilized by the strong conjugation between metalladithiolene
and the azo moiety, and thus the rotational barrier for the cis-
to-trans isomerization was expected to be much smaller than
that expected for the thermal isomerization. The reduced
activated energy caused acceleration in the rate of the cis-to-
trans isomerization. In conclusion, azo-conjugated metalladithi-
olenes exhibited a novel isomerization behavior, that is, “proton-
catalyzed cis-to-trans isomerization”. The mechanism of the
proton-catalyzed cis-to-trans isomerization can be used to predict
that the π donation ability of the dithiolene ring might be
considerably influential in regards to the isomerization behavior.
The type of central metal was dominant to the electronic state
of the dithiolene ring, and thus isomerization could be controlled
in such instances by use of different metal centers.
Conclusion
A versatile synthetic method of the azo-conjugated metalla-
dithiolene system using a dithiolate-protected nitroso compound
2 was developed. In this system, a novel proton response of
the azo group occurred due to the strong electron-donating effect
of the metalladithiolene moiety. This led to a novel proton-
catalyzed cis-to-trans isomerization in dppe-M complexes 12-
14, and this isomerization rate correlated with the redox potential
of the metalladithiolene moiety. The trans-to-cis photoisomer-
ization of 12-14 occurred at a considerably lower energy as
compared to that of azobenzene, and the thermal stability of
the cis form was much higher than that of the organic
azobenzene derivatives showing similar low-energy trans-to-
cis photoisomerization. The results of this systematic study of
various azo-conjugated metalladithiolenes suggested that a wide
range of isomerization behaviors of azo-conjugated metalla-
dithiolenes can be controlled by adjusting the chemical structure,
the electronic structure, the substituent groups, and the redox
potential of metalladithiolenes.
Acknowledgment. This work was supported by Grants-in-
Aid for scientific research (Nos. 13022212, 14050032, and
14204066) from the Ministry of Education, Science, Sports and
Culture, Japan. The authors thank Y. Namiki for his experi-
mental assistance.
Supporting Information Available: Experimental section for
synthesis and text for the photo and proton response of 15-18,
crystal parameters and X-ray diffraction data (Table S1) and
selected bond lengths, angles, and torsion angles for 12, 13,
and 14 (Table S2), cyclic voltammograms of 12-14 in the
absence of protons (Figure S1) and in the presence of protons
(Figure S2), cyclic voltammograms of 15 in the absence of
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2972 J. AM. CHEM. SOC. VOL. 125, NO. 10, 2003

Azo-Conjugated Metalladithiolenes
A R T I C L E S
protons (Figure S3) and in the presence of protons (Figure S4),
UV-vis spectral change by photoirradiation of 15 (Figure S5),
cyclic voltammogram of 16 (Figure S6), UV-vis spectra of 16
and its reduced form (Figure S7), UV-vis spectral change in
16 by protonation (Figure S8), and UV-vis spectral change by
photoirradiation and by protonation of 17 and 18 (Figures S9
and S10, respectively) (PDF and TXT). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA028080P
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J. AM. CHEM. SOC. VOL. 125, NO. 10, 2003 2973