Preparation of platinum(II) cyclooctadiene complexes by photoirradiation

Article abstract of DOI:10.1007/s11176-005-0224-z

Preparation of platinum cyclooctadiene complexes under photoirradiation was studied, and the reaction mechanism was suggested. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0224-z

Russian Journal of General Chemistry, Vol. 75, No. 3, 2005, pp. 321 324. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 3, 2005,
pp. 353 356.
Original Russian Text Copyright
2005 by de Vekki, Uvarov, Skvortsov.
Preparation of Platinum(II) Cyclooctadiene Complexes
by Photoirradiation
D. A. de Vekki, V. M. Uvarov, and N. K. Skvortsov
St. Petersburg State Institute of Technology, St. Petersburg, Russia
Received June 17, 2004
Abstract Preparation of platinum cyclooctadiene complexes under photoirradiation was studied, and the
reaction mechanism was suggested.
It was shown previously [1, 2] that heating of pla-
tinum(II) disulfoxide complexes in a solution contain-
ing excess cycloocta-1,5-diene (COD) results in the
is formed (Fig. 1, curve 3). The replacement rate after
preirradiation is 1.5 times higher (after 120 min) than
without irradiation (Fig. 1, curves 2 and 4), but much
formation of dichloro(cycloocta-1,5-diene)platinum(II), lower than during the photoactivation.
[Pt(COD)Cl2], in a high yield. This complex was
formed most readily when starting from ( )-cis-
[Pt(Me-p-TolSO)2Cl2] (Me-p-TolSO is methyl p-tolyl
sulfoxide). The chirality of the complex allowed the
reaction progress to be monitored by optical rotation,
as the rotation of the reaction mixture changed sing
owing to consumption of the ( )-complex and forma-
tion of the (+)-ligand [2].
Thus, the rate of the formation of the cycloalkene
complex is higher under irradiation than during the
subsequent keeping of the reaction mixture in the dark.
This is due to the fact that, under the conditions of
, deg
0.03
1
2
( )-cis-[Pt(Me-p-TolSO)2Cl2] + COD
[Pt(COD)Cl2] + 2(+)-Me-p-TolSO.
A
0.06
0.09
0.12
0.15
On the other hand, the photoirradiation of sulfoxide-
containing platinum(II) complexes of a cis structure
causes their isomerization into a photochemically in-
active trans form [3, 4], which proceeds through
elimination of the free sulfoxide [4]. This fact allowed
us to expect the positive effect of photoirradiation on
the replacement of sulfoxide by COD.
3
4
0
50 100 150 200 250
Time, min
We found that the irradiation of a mixture of ( )-
[Pt(Me-p-TolSO)2Cl2] isomers (the complex exists in
solution as an equilibrium mixture of 14.4% trans and
85.6% cis forms [5]) in methylene dichloride in the
presence of a tenfold molar excess of COD accelerates
the variation of the observed rotation angle of the re-
action mixture (Fig. 1, curve 1). The rate of variation
of the rotation angle upon irradiation through quartz
is 14 times higher than without it (40 min after the
start of the reaction, 25 C). This results from the re-
placement of the sulfoxide by cyclooctadiene (Fig. 1,
curve 2) and from the concurrent cis trans photoiso-
merization of the complex (Fig. 1, curve 3). As the
irradiation is stopped, the reverse trans cis isomeriza-
tion starts, until an equilibrium mixture of the isomers
Fig. 1. Variation with time of the observed rotation angle
( ) of the reaction mixture (25 C, c 1.8 10 M): (1)
reaction of ( )-cis-[Pt(Me-p-TolSO) Cl ] with COD
(molar ratio of reactants 1:10, irradiation for 41 min,
quartz); (2) calculated curve for the reaction of ( )-cis-
[Pt(Me-p-TolSO) Cl ] with COD, corrected for isomer-
ization (the result of subtracting curve 3 from curve 1;
(3) irradiation of ( )-cis-[Pt(Me-p-TolSO) Cl ] in the
absence of COD (irradiation for 41 min, quartz); (4) re-
action of ( )-cis-[Pt(Me-p-TolSO) Cl ] with COD
(molar ratio of reactants 1:10, without irradiation).
(A) Instant of switching off the irradiation.
3
c
2
2
2
2
2
2
2
2
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322
DE VEKKI et al.
photolysis, sulfoxide is replaced by COD by another
mechanism than in the thermal reaction.
unsaturated platinum intermediate, which, in turn,
instantaneously reacts with COD to give complex B.
We failed to detect an intermediate of such a structure
1
The photolysis of the disulfoxide complex A re-
sults in the dissociation of the Pt S bond with the
formation of the free sulfoxide and a coordination-
by H NMR spectroscopy [2]; however, Sanger [6]
reported on the formation of a rhodium complex
containing 2-cyclooctadiene.
R2
O
X
Pt
X
S
X
R1
Pt
X
B
D
h
h
COD
R2
S
R1R2SO
R2
R1R2SO
O
O
R1
R1
X
S
X
R1
Pt
R2
Pt
X
X
S
D
O
A
C
R1R2SO
X
, COD
Pt
R1R2SO
X
R2
E
O
S
R1
X
R2
S
Pt
X
R1
O
F
The resulting complex C eliminates the second
sulfoxide molecule under the action of the next light
quantum, yielding coordination-unsaturated short-lived
complex D which is immediately stabilized by an
intramolecular conversion to give reaction product E.
temperature over 40 C under continuous irradiation
additionally causes the formation of colloidal pla-
tinum owing to decomposition of the metal complex.
When the irradiation time is decreased (20 min), with
the subsequent keeping in the dark (Fig. 2, curve 4),
the reaction occurs as an independent two-step process:
at a high rate under irradiation and at a low rate (equal
to that of the dark process), in its absence. At de-
creased energy of the luminous flux (Pyrex filter), the
metal complex does not decompose, but the reaction
rate noticeably decreases (Fig. 2, curve 3).
Thus, the reaction ofplatinum(II) sulfoxide-contain-
ing complexes with cycloalkenes under photoirradia-
tion occurs by a dissociative mechanism in contrast
to the dark process where an associative mechanism
is observed: a slow initial stage is the alkene coordina-
tion in the fifth site of platinum (complex F),
is followed by intramolecular recoordination {for
example, in the case of transformations of alkenes
catalyzed by platinum(II) complexes [7]}.
The replacement of COD by 1-methylcycloocta-
1,5-diene (MeCOD) somewhat decreases the reaction
rate under photoirradiation (Fig. 2, curves 1 and 2)
and the yield of the final reaction product, which is
probably associated with hindered coordination of
MeCOD owing to its steric features (the presence of
the methyl group at the coordinating double bond).
The optimal irradiation time at 20 50 C is 40
50 min (quartz); the subsequent prolonged photolysis
only slightly affects the observed change in the rota-
tion angle of the reaction mixture (Fig. 2, curve 1) and,
correspondingly, the reaction rate. The increase in
The data obtained allow us to recommend photo-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 3 2005

PREPARATION OF PLATINUM(II) CYCLOOCTADIENE COMPLEXES
323
irradiation as an alternative mild procedure for the
, deg
0.10
preparation of platinum(II) cycloalkene complexes
from disulfoxide complexes as opposed to the thermal
route (for example, the irradiation for 40 min at 20 C
followed by the dark reaction affords 98% yield of
[Pt(COD)Cl2] in 5 h, whereas without the preliminary
irradiation this takes 22 h at 40 C).
1
2
3
0.05
4
0
EXPERIMENTAL
0.05
The IR spectra were taken on IKS-29 (4000
1
1
400 cm ) and Hitachi FIS-3 (400 100 cm ) spec-
5
1
trometers in KBr. The H and 13C NMR spectra were
0.10
0.15
6
recorded on a Bruker AC-200 instrument with an
integrating device at an operating frequency of 200
(1H) and 50 (13C) MHz in CDCl3 without additional
references, using as reference the residual proton
signal of the deuterated solvent.
0
20
40
60
80
100
Time, min
Fig. 2. Variation with time of the observed rotation angle
of the chiral composition ( ) in methylene dichloride
Polarimetric measurements were carried out on a
Perkin Elmer-241MC instrument in temperature-
controlled quartz cells of length 10 cm. The concentra-
tion of the complexes was varied over the range
under continuous irradiation of ( )-cis-[Pt(Me-p-TolSO)
2
Cl ] in the presence of cycloalkene (molar ratio 1:10,
2
3
40 C, c 1.6 10 M). (1, 3, 4, 6) COD, (2) MeCOD, and
3
4
c
1.8 10
solvent.
10 M. Methylene chloride was used as a
(5) without cycloalkene; (1, 2, 4) irradiation for 20 min;
(5) quartz, (3) Pyrex, and (6) no irradiation.
Photolysis of complexes was carried out by irradiat-
ing their solutions in methylene chloride with light
of a DRL-400 medium-pressure mercury quartz
burner directly in a polarimetric cell; light was filtered
through Pyrex and a 30-mm layer of water.
octa-1,5-diene was added, and the solution was ir-
radiated for 1.5 h and then allowed to stand for 2 h.
The resulting solution was evaporated to dryness in
an air stream, and the solid residue was recrystallized
from an acetone cyclohexane mixture (10:1 by
volume). The resulting precipitate was dried at 70 C.
Yield of the colorless complex 20 mg (66%). IR spec-
In the experiments we used methylene dichloride
(Merck), cycloocta-1,5-diene, 1-methylcycloocta-1,5-
diene (both chemically pure grade), acetone, and
cyclohexane (both analytically pure grade).
1
trum, , cm : 3395 3375, 2950 2870 [ (CH)]; 1510,
1480 [ (C=C)]; 1430, 1405, 1370, 1335, 1315, 1238,
1190, 1172, 1100, 1062 [ (CH, Me)]; 1041, 1025,
1008 [ (CH, COD)]; 967, 903 [ (Me)]; 873, 853, 832,
795, 742, 561, 362 [ (CH, COD)]; 336, 318, 246
( )-cis-Dichlorobis(S-methyl p-tolyl sulfoxide)-
platinum(II) was prepared as described in [5], and cis-
dichlorobis(diethyl sulfoxide)platinum(II), as de-
scribed in [8].
1
[ (PtCl)]; 211, 168 [ (PtCl)]. H NMR spectrum, ,
ppm: 1.94 t (3H, CH3, JPtH 36 Hz), 2.35 m (4H, CH2,
JPtH 67 Hz), 2.70 m (4H, CH2, JPtH 55 Hz), 5.51
Dichloro[1,2:5,6- 4-(cycloocta-1,5-diene)]pla-
tinum(II). ( )-cis-[Pt(Me-p-TolSO)2Cl2] (25.0 mg)
and 4.4 ml of 0.1 M cycloocta-1,5-diene solution in
methylene chloride were mixed, and the volume was
brought to 5 ml. Then the reaction mixture was trans-
ferred to a polarimetric cell, irradiated for 41 min, and
then kept in the dark for 4 h. The reaction solution
was poured into 20 ml of cyclohexane, and the result-
ing precipitate was filtered off and dried in air. Yield
m (3H, CH, JPtH 88 Hz). 13C NMR spectrum,
,
ppm: 28.9 t (1C, CH3, JPtC 29 Hz), 29.3 s (1C, CH2C),
30.6 s (1C, CH2), 31.5 s (1C, CH2), 37.9 s (1C, CH2),
96.1 t (1C, CH, JPtC 150 Hz), 97.8 t (1C, CH, JPtC
152 Hz), 99.9 t (1C, CH, JPtC 164 Hz), 124.0 t (1C,
CH, JPtC 126 Hz).
ACKNOWLEDGMENTS
1
15.5 mg (93%). The H and 13C NMR spectra agree
with the data from [1].
This work was financially supported by President
of the Russian Federation (project no. MK-133.2003.
03), the Russian Foundation for Basic Research
(project no. 04-03-32632a), and the Russian Federa-
tion Ministry of Education (project no. PD02-1.3-130).
Dichloro[1,2:5,6- 4-(1-methylcycloocta-1,5-di-
ene)]platinum(II).
A
37.1-mg portion of cis-
[Pt(Et2SO)2Cl2] was dissolved in 7 ml of methylene
chloride in a quartz flask, 0.1 ml of 1-methylcyclo-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 3 2005

324
DE VEKKI et al.
5. Spevak, V.N., Lobadyuk, V.I., Skvortsov, A.N., Ko-
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