Geometric photoisomerization of chiral platinum(II) sulfoxide compounds

Article abstract of DOI:10.1023/A:1013096802361

Photochemical behavior of platinum(II) sulfoxide-containing complexes in methyl chloride was studied by polarimetry and ¹H NMR spectroscopy.

Full text of DOI:10.1023/A:1013096802361

Russian Journal of Applied Chemistry, Vol. 74, No. 6, 2001, pp. 921 924. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 6, 2001,
pp. 898 900.
Original Russian Text Copyright
2001 by Spevak, de Vekki, Skvortsov.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Geometric Photoisomerization of Chiral Platinum(II)
Sulfoxide Compounds
V. N. Spevak, D. A. de Vekki, and N. K. Skvortsov
St. Petersburg State Technological Institute, St. Petersburg, Russia
Received January 17, 2001
Abstract Photochemical behavior of platinum(II) sulfoxide-containing complexes in methyl chloride was
studied by polarimetry and 1H NMR spectroscopy.
Effective catalysts for hydrosilylation of unsatu-
rated compounds [1, 2] and antitumor preparations [3]
were found among platinum(II) aminosulfoxide com-
pounds [Pt(RR SOAmCl2] (Am is amine; R and R
are Alk or Ar). Their catalytic properties and biolog-
ical activity depend on the arrangement of ligands in
the inner coordination spheres of Pt(II) square-planar
complexes. For example, diamino derivatives are bio-
logicaly active if they have cis-arrangement of neutral
ligands, whereas in the case of [Pt(RR SO)AmCl2] the
trans-isomer is the most active [3].
Chiral sulfoxides in the complex compounds under
consideration are coordinated via a sulfur atom
{specific rotation of ( )trans-[Pt(Me-p-TolSO)(Py)Cl2]
measured in methyl chloride is 22 in contrast to
117 for the cis-isomer}, which makes it possible
to readily monitor any processes involving a sulfoxide
ligand in both the resulting geometrical isomers by
means of polarimetry. In this case, the isolation of
a free sulfoxide [specific rotation (+)Me-p-TolSO is
+142 ] results in a sharp change in the observed angle
of rotation for the reaction mixture, to the point of
a full inversion of the sign of rotation.
cis-Aminosulfoxide complexes appeared to be
more active in catalytic hydrosilylation, and diamino
coordination compounds were completely catalytically
inactive in this reaction.
A kinetic ORD monitoring by the change in the ob-
served rotation angle of the chiral farmulation de-
In this work, we made an attempt to study photoin-
duced isomerization of the complex [Pt(RR SOAmCl2].
The photoinduced effect is known [4 8] to change in
some cases the geometry of certain coordination com-
pounds, which may be useful for photoactivated cat-
alytic hydrosilylation.
As subjects for this study we chose geometrical
isomers of ( )[Pt(Me-p-TolSO)(Py)Cl2], investigated
1
earlier [9, 10] by X-ray diffraction analysis, H and
13C NMR and IR spectroscopy, and the method of
optical rotation dispersion (ORD), enabled by the
presence of a chiral sulfoxide. Along with NMR spec-
troscopy, the ORD method provides valuable informa-
tion, for example, in analyzing the ligand exchange
in platinum(II) coordination compounds [2, 11 13]
or for determining kinetic and thermodynamic param-
eters of changes in the inner coordination sphere of
complexes in solution [11].
Rotation angle
chloride vs. time . (1) Photolysis of ( )cis-[Pt(Me-p-
of the chiral composition in methyl
4
TolSO)PyCl ] (20 C, c = 1.7 10 M, irradiation time
2
0
166 min); (2) irradiation of ( )cis-[Pt(Me-p-TolSO) Cl ]
2
2
4
(25 C, c = 1.1 10
M, irradiation time 41 min);
0
(3) dark relaxation of ( )cis-[Pt(Me-p-TolSO) Cl ] after
2
2
4
irradiation (25 C, c = 1.1 10 M).
0
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922
SPEVAK et al.
monstrated that platinum(II) trans-pyridinesulfoxide
complexes are inert when irradiated with near UV
light, whereas cis-compounds rather easily trans-
form to trans-products (Figure, curve 1). Prolonged
photolysis of a solution of ( )cis-dichloropyridine-
(methyl-para-tolylsulfoxide)platinum(II) in methylene
chloride gives solely the chiral complex ( )trans-
[Pt(Me-p-TolSO)(Py)Cl2]. When the solvent is grad-
ually removed, 100% conversion into the desired
product is reached in 1.5 h.
processes occur along with photoisomerization (figure,
curve 2). These are fast reverse dark isomerization
(curve 3), and, on prolonged photolysis (60 min and
more), formation of colloidal platinum, which is not
characteristic of pyridine-sulfoxide compounds.
This phenomenon can be explained on the basis
of X-ray data [9, 10, 12]. For example, the length of
the Pt S bond is 2.214 in ( )cis-[Pt(Me-p-TolSO)-
(Py)Cl2] and 2.249
in ( )cis-[Pt(Me-p-TolSO)2Cl2]
(length of the Pt N bond in the pyridine-sulfoxide
complex is 2.032 ), which makes the isomerization
and destruction of the metal complex easier and, there-
fore, is responsible for the high mobility of sulfoxide
in platinum(II) bissulfoxide complexes.
Reverse slow dark isomerization of the trans-
compound occurs in a toluene solution at higher tem-
perature (boiling water bath, yield of the cis-complex
in several days 15%). Thus, the quantitative photo-
isomerization is attributable in this case to the absence
of a fast spontaneous interconversion of such com-
pounds in uncoordinated solvents, in contrast to solu-
tions in dimethyl sulfoxide [14], where isomerization
is catalyzed by sulfoxide:
The aforementioned researches show that plati-
num(II) pyridine-coordination compounds, catalyzing
hydrosilylation, can be further photoactivated with
the homogeneous metal-complex center preserved.
It is necessary to note in conclusion that the pho-
tolysis of optically active geometrical isomers of
( )[Pt(Me-p-TolSO)(Py)Cl2] does not result in loss
of chirality of both the complexes themselves and the
coordinated sulfoxide, as opposed to the data of [15],
indicating that free (+)Me-p-TolSO undergoes pho-
toracemization. However, this process requires high-
energy irradiation for several hours, whereas in our
case the reaction occurs under mild conditions in
the course of 1.5 2 h.
cis-[Pt(Me-p-TolSO)PyCl2]
h
cis-[Pt(Me-p-TolSO)PyCl2].
1
Additional monitoring by the intensity of the H
NMR signal of the methyl group in cis- (CDCl3; ,
ppm: 3.59 t, JPt H 23 Hz) and trans-complexes
(CDCl3; , ppm: 3.47 t, JPt H 20 Hz) demonstrated
that, along with both the geometrical isomers, up
to 1% uncoordinated (+)methyl-para-tolylsulfoxide
(CDCl3; , ppm: 2.70 s) is present in the reaction so-
lution; the signal from this compound disappears upon
complete conversion of the starting compound. The
photolysis of such cis-complexes seems to give rise to
dissociation of sulfoxide, leading to a coordinatively
unsaturated intermediate, stabilized via formation of
a thermodynamically unstable chiral trans-product,
which is not photoactivated:
The whole set of the obtained data allows pho-
toirradiation to be recommended as a method for
synthesizing thermodynamically unstable Pt(II) sulf-
oxide trans-compounds, which at the same time do
not isomerize spontaneously.
EXPERIMENTAL
1
The H NMR spectra were recorded in CDCl3 on
a Bruker AC-200 instrument with spectrum accumula-
tion, operating at 200 MHz.
Optical rotation dispersion was measured on a Per-
kin Elmer 241MC polarimeter in methyl chloride in
temperature-controlled quartz cells of length 10 cm in
the range 450 610 nm. The concentration of the com-
The photolysis of bissulfoxide complexes is known
4
5
[8] to result also in cis-
trans-isomerizations;
plex was varied in the range 1 10
1
10 M.
however, attempts to isolate the trans-product in pure
state were unsuccessful. A study of the photochemical
behavior of ( )cis-dichlorobis(methylpara-tolylsulfox-
ide)platinum(II) in methylene chloride {the specific
rotation of ( )cis-[Pt(Me-p-TolSO)2Cl2] measured
in acetone is 220 C [12]} demonstrated that side
The IR spectra were taken on a Perkin Elmer Spek-
1
trum-1000 spectrometer (4000 400 cm ) in chloro-
form, l = 0.135 mm, 1% solution; the FIR spectrum
was recorded on a Hitachi FIS-3 spectrometer (400
1
100 cm ) in KBr pellets.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 6 2001

GEOMETRICAL PHOTOISOMERIZATION OF CHIRAL PLATINUM(II) SULFOXIDE COMPOUNDS 923
Photolysis of metal complexes was carried out in a
polarimeter cell in methylene chloride solution under
the action of broadband UV light (DRL-400 lamp,
quartz or pyrex filter) passing through a water layer
of 40 mm thickness.
1145, (CH, Py) 1077; (Me) 969; (CH, Ph) 813.
FIR spectrum (cm ): (Pt Cl) 353.
1
CONCLUSION
Irradiation of a solution of ( )cis-[Pt(Me-p-
TolSO)PyCl2] with broadband light results in the iso-
merization of the complex into the trans-complex,
which is photochemically inert. Platinum(II) cis-bis-
sulfoxide coordination compounds are more sensitive
to irradiation, decomposing to colloidal platinum.
Coordinated chiral sulfoxides in the systems under
study are not racemated under the action of light.
Thermal activation of ( )trans-[Pt(Me-p-TolSO)PyCl2]
leads to its conversion to the cis-form.
Methylene chloride of Merk production was used
in the experiments. ( )cis-Dichlorobis(methyl-para-
tolylsulfoxide)platinum(II), potassium ( )trichloro-
(methyl-para-tolylsulfoxide)platinite(II), and (+)meth-
yl-para-tolylsulfoxide were obtained as described in,
respectively, [12], [16], and [17].
( )cis-Dichloropyridine(methyl-para-tolylsulfox-
ide)platinum(II). Potassium ( )trichloropyridineplat-
inite(II) (79.6 mg, i.e., 0.190 mmol) was dissolved in
4 ml of water in a beaker. Then 29.3 mg (0.190 mmol)
of (+)methyl-para-tolylsulfoxide was added slowly
with stirring. The reaction mixture was kept at room
temperature for 1 day, then the resulting white floc-
culent precipitate was filtered off and dried in a
thermostat at 50 C to give 85.6 mg (0.171 mmol,
yield 90.4%) of ( )cis-[Pt(Me-p-TolSO)PyCl2].
ACKNOWLEDGMENTS
This work was financially supported by the Rus-
sian Foundation for Basic Research (grants nos. 00-
03-32670a and 01-03-32598) and the Ministry of
Education (grant
E00-5.0-373).
1
[ ]589 = 117 C (5 g in 100 ml of CH2Cl2). H NMR
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