Photo- and proton-coupled isomerization of novel azo-conjugated platinadithiolene complex

Article abstract of DOI:10.1246/cl.2001.852

A platinum tolylazophenylenedithiolato complex was synthesized. This new azo-conjugated complex shows reversible trans-to-cis photoisomerization, protochromism, and novel proton-coupled cis-to-trans isomerization.

Full text of DOI:10.1246/cl.2001.852

852
Chemistry Letters 2001
Photo- and Proton-Coupled Isomerization of Novel Azo-Conjugated
Platinadithiolene Complex
Masayuki Nihei, Masato Kurihara, Jun Mizutani, and Hiroshi Nishihara*
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
(Received June 8, 2001; CL-010537)
A platinum tolylazophenylenedithiolato complex was syn-
thesized. This new azo-conjugated complex shows reversible
trans-to-cis photoisomerization, protochromism, and novel pro-
ton-coupled cis-to-trans isomerization.
Azobenzene derivatives undergo reversible trans-to-cis pho-
toisomerization and cis-to-trans thermal isomerization.1
Combining the azo moiety with functional metal complexes is a
tactic for creating intelligent molecular systems, but only a limited
number of reports on the photoisomerizable azo-conjugated com-
plexes are available.2 Metalladithiolenes3 with aromatic nature
exhibit a number of interesting properties including reversible
redox activity4 and various chemical reactivities.5 Thus, the com-
bination of dithiolene complexes with the azo group can afford
photo-responsive multi-functional molecules. In the present
study, we synthesized a precursor for a new azo-conjugated ben-
zenedithiolato ligand 3, as well as its Pt complex 4, and found
reversible photoisomerization and protonation behavior and the
protonation catalyzed cis-to-trans isomerization of 4.
torsion angles in the footnote.8 The configuration around the Pt
center is a typical tetracoordinated square-planar structure. The
bond lengths of Pt–S1 and Pt–S2 are 2.310 Å and 2.316 Å,
respectively, both of which lie within the usual range of Pt–S
distances for Pt dithiolene complexes.9 The configuration of
the azo moiety is trans, but the planarity of the azobenzene moi-
ety10 is significantly distorted, with the torsion angle of
C12–C7–C5–C4 being 55.3°. The bond length of N1–N2 is
1.32(2) Å, which is significantly longer than the usual
1.23–1.24 Å values for azobenzene compounds.10 These novel
features indicate the existence of strong conjugation between
the azo and the metalladithiolene moieties.
UV–vis spectra of 4 before and after photoirradiation with
a UV light as shown in Figure 2. A π–π* transition band
ascribable to the azo moiety can be observed at 405 nm (ε =
25600 mol–1dm3cm–1), which is longer wavelength (lower ener-
gies) compared with regular azobenzene derivatives. This sup-
ports the strong azo-metalladithiolene electronic interaction as
indicated in the X-ray structure. The π–π* transition band
decreased in intensity after photoirradiation (Figure 2), indicat-
ing the occurrence of trans-to-cis photoisomerization.
The cis-to-trans back reaction occurred, allowing for a per-
fect recovery of the spectra of the trans forms, in response to
The precursor of a tolylazobenzodithiolato ligand 3 was
synthesized from 5-nitro-2-thioxo-1,3-benzodithiole 16 by a
three-step reaction (Scheme 1). In the first step, 1 was reduced
to afford a nitroso compound 2, 7 by zinc and iron(III) chloride
via a hydroxyamino compound as an intermediate. A reaction
of 2 and p-toluidine afforded 37 in a moderate yield.
Tolylazobenzenedithiolato complex 47 was prepared by a reac-
tion of 3 with phenyllithium, which led to a cleavage of the pro-
tecting group, followed by an addition of dichloro[1,2-
bis(diphenylphosphiono)ethane]platinum(II). The compound 4
was characterized by elemental analysis, 1H NMR, and UV–vis
spectroscopy. The low yield of 4, 8.2% from 3, appears to have
been due to nucleophilic side reactions accompanying the
cleavage of the protecting group by phenyllithium.
An ORTEP diagram of the molecular structure of 4 is dis-
played in Figure 1 with the selected bond lengths, angles and
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
853
thermal reaction or photoirradiation at 310 nm. Although the
transition corresponding to these UV light energies have not
been specified at present, it is intriguing that the cis-to-trans
isomerization is promoted by UV light with an energy higher
than the π–π* transition, since the regular azobenzenes undergo
isomerization by irradiating the n–π* band in the visible
region.1 The rate constant of the thermal cis-to-trans isomeriza-
tion of 4 at 23.3 °C is estimated to be 3.5 × 10–5 s–1.
The color of trans-4 in acetonitrile was changed drastically
from yellow to deep blue by an addition of 7.5 equivalents of
CF3SO3H (Figure 2), showing that the π–π* transition band of
the azo moiety at 400 nm decreases in intensity, and that a new
strong band appears at 590 nm. The reverse spectral changes
were achieved with the addition of potassium tert-butoxide.
This spectral behavior is similar to that of N-heterocycle-substi-
tuted metalladithiolenes,11 indicating that there is protonation of
the azo moiety (Scheme 2). The single protonation is supported
by a result of ESI-mass spectroscopic measurement, which
shows a peak at m/z 852.3 and its isotope pattern of 4·H+. The
protonation to the nitrogen atom bound to the tolyl moiety is
reasonable in affording the conjugated structure that delocalizes
the charge beyond the metalladithiolene moiety. The 590 nm
band can be assigned to the metalladithiolene π→azo group π*
charge-transfer transition. The metalladithiolene moiety, which
is strongly conjugated with the azo group, increases the basicity
of the azo group with a resonance stabilization effect to cause
the facile protonation behavior.
10149102, 11167217, and 11309003) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
References and Notes
1
2
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4
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Selected Data for 2: Yield 74.2 %, Anal. Calcd for
C7H3N1O1S3·0.6H2O: C,37.52; H, 1.89; N, 6.25%. Found: C,
37.43; H, 1.95; N, 5.99%. 1H NMR (270 MHz, CDCl3): δ
8.03 (dd, J = 11.1, 2.2 Hz 1H, Ph), 7.87 (d, J = 2.2 Hz 1H,
Ph), 7.73 (d, J = 11.1 Hz 1H, Ph). Selected Data for 3: 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): δ 8.01 (d, J = 2.2 Hz 1H, Ph), 7.97
(dd, J = 11.5, 2.2 Hz 1H, Ph), 7.84 (d, J = 11.1 Hz 2H, Tol),
7.61 (d, J = 11.5 Hz 1H, Ph), 7.34 (d, J = 11.1 Hz 2H, Tol),
2.46(s, 3H, Me). Selected Data for 4: Yield 8.2 %, Anal.
Calcd for C39H34N2P2S2Pt·2H2O : C, 54.99 ; H, 4.02 ; N, 3.29
%. Found: C, 54.90; H, 4.08; N, 3.30%. 1H NMR (270 MHz,
CDCl3): δ 8.12 (d, J = 3.0 Hz 1H, Ph), 7.92–7.78 (m, 8H,
dppe), 7.74 (d, J = 11.5 Hz 2H, Tol), 7.50–7.47 (m, 12H,
dppe), 7.62 (d, J = 11.1 Hz 1H, Ph), 7.43 (dd, J = 3.0 Hz, 11.1
Hz 1H, Ph), 7.24 (d, J = 11.5 Hz 2H, Tol), 2.51 (d, J = 24.8
Hz 4H, dppe), 2.39(s, 3H, Me).
When a slight amount of acid is added to a solution contain-
ing cis-4 prepared by photoirradiation, cis-4 transforms into
trans-4 immediately. The rate constant of this isomerization on
addition of 0.01 equivalents of CF3SO3H is 4.6 × 10–3 s–1, which
is larger than that of the thermal isomerization by two orders of
the magnitude. This phenomenon indicates that a protonated cis
form, cis-4·H+ instantly produces the trans form trans-4·H+
(Scheme 2). It can be deduced that the N=N bonding is weak-
ened by the protonation to the azo group, and the rupture of the
N=N π-bond forms a –NH–N= bonding as a limiting structure.
The rotation around the N–N bond is facile, and consequently,
thermodynamically favorable trans-4 is generated (Scheme 2).
The results described above indicate that the azo-conjugat-
ed metalladithiolene system shows trans-cis isomerization
behavior responsive to both photon and proton, which would be
useful to develop a multi-mode switching and information stor-
age system in the molecular level.
8
Crystal data for 4: C40H36N2P2S2PtCl2, Mr = 936.80, mono-
clinic, space group P21/a, a = 15.4919(11), b = 16.100(3), c =
15.215(2) Å, β = 94.7(7)°, V = 3782.2(9) Å3, Z = 4, ρχalc
=
1.645 g cm–3, of 40337 reflections (2θmax = 55.0°), 8867 were
unique. R = 0.063, Rw = 0.089. Crystallographic data
(excluding structural factors) for the structure(s) reported in
this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC-155723. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (+44) 1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk).
9
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The authors are grateful to Professor J. Nakayama of
Saitama University for his helpful discussion. This work was
supported by Grants-in-Aid for Scientific Research (Nos
11 S. P. Kaiwar, J. K. Hsu, L. M. Liable-Sands, A. L. Rheingold,
and R. S. Pilato, Inorg. Chem., 36, 4234 (1997).