Solvent effects on the rate of oxidative addition of methyl iodide to di(4-thiocresol)(2,2′-bipyridyl) platinum(II)

Article abstract of DOI:10.1016/S0022-328X(99)00759-7

The reaction of methyl iodide with di(4-thiocresol)(2,2′-bipyridyl) platinum(II) to give iodo (methyl) di(4-thiocresol)(2,2′-bipyridyl) platinum(IV) follows the rate law, rate=K₂[Pt-(4-thiocresol)₂ (2,2′-bipyridyl)] [MeI]. The values of K₂ increase with increasing polarity of the solvent, suggesting a polar transition state for the reaction.

Full text of DOI:10.1016/S0022-328X(99)00759-7

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 599 (2000) 166–169
Solvent effects on the rate of oxidative addition of methyl iodide
to di(4-thiocresol)(2,2%-bipyridyl) platinum(II)
Ja’afar K. Jawad a,*, Faisal N.K. Al-Obaidy a, Jasim A. Hammud a, Fathi Al-Azab b
a Department of Chemistry, Faculty of Education, Uni6ersity of Salahaddin, Erbil, Iraq
b Department of Chemistry, Faculty of Science, Sana’a Uni6ersity, Sana’a, Yemen
Received 24 November 1998; received in revised form 9 December 1999
Abstract
The reaction of methyl iodide with di(4-thiocresol)(2,2%-bipyridyl) platinum(II) to give iodo (methyl) di(4-thiocresol)(2,2%-
bipyridyl) platinum(IV) follows the rate law, rate=K2[Pt-(4-thiocresol)2 (2,2%-bipyridyl)] [MeI]. The values of K2 increase with
increasing polarity of the solvent, suggesting a polar transition state for the reaction. © 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Oxidative addition; Platinum; Solvent effects
1. Introduction
The ability of the aryl platinum(II) complexes to
undergo oxidative addition at all is remarkable since
similar complexes stabilized by tertiary phosphine lig-
ands do not give stable aryl platinum(IV) complexes [7].
This is probably due to the greater s-donor and poorer
p-acceptor power of 2,2%-bipyridyl with respect to ter-
tiary phosphines and to its lower steric requirements.
The oxidative-addition reactions of methyl iodide to
(2,2%-bipyridyl) and (1,10-phenanthroline) platinum (II)
complexes has been reported [8]. The solvent effects on
the rate of oxidative addition of methyl iodide to
[PtPh2-(bipy)] indicate a polar transition state in the
reaction such as is predicted for the SN2 mechanism
[16], provided the platinum atom acts as a nucleophilic
centre during the oxidative addition.
It has been shown [1] that the UV–vis spectra of
bipyridyl platinum(II) complexes contain two intense
metal to ligand charge-transfer (MLCT) bands, whose
energies are strongly dependent on the nature of the
other ligands bound to platinum and on the solvent.
More electronegative substituents on platinum and
more polar solvents cause the bands to move to higher
energy.
The reactivity of alkyl and aryl-(2,2%-bipyridyl) plat-
inum(II) complexes towards oxidative addition reac-
tions can be correlated with the energy of the
lowest-energy MLCT band [2], suggesting that it is the
energy of the platinum d-orbitals that primarily deter-
mines the reactivity [1,3–5], and since in such reactions
the metal centre acts as a nucleophile [6]. We are
looking for this effect by studying the oxidative addi-
tion of methyl iodide to the complexes [4X-C6H4S)2
Pt(2,2%-bipyridyl)] as a function of the substituent X.
Thus steric effects due to X should be negligible in
these compounds, and it has already been shown [1]
that the energy of the first MLCT band (and hence the
energy of the platinum d orbitals) correlates well with
Hammett s-values of X.
Bipyridine platinum (II) thiolates have been synthe-
sized and characterized [9]. Some transition-metal thio-
late complexes have been synthesized recently [10–14].
This prompted us to report the solvent effects on the
rate of oxidative addition of methyl iodide to di(4-
thiocresol) (2,2%-bipyridyl) platinum(II).
2. Results
Orange solutions of di(4-thiocresol)(2,2%-bipyridyl)
platinum(II) in common organic solvents turned yellow
* Corresponding author. Present address: Ma’een Post Office, PO
Box 13220, Sana’a, Yemen.
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00759-7

J.K. Jawad et al. / Journal of Organometallic Chemistry 599 (2000) 166–169
167
on addition of methyl iodide and yellow crystals of
[PtIMe(4-thiocresol)2(2,2%-bipyridyl)] could be isolated.
Once formed, the product was very sparingly soluble in
common organic solvents, but a satisfactory NMR
spectrum was obtained by adding one drop of methyl
iodide to
a
suspension of [Pt(4-thiocresol)2(2,2%-
bipyridyl)] in chloroform in an NMR tube and record-
ing the spectrum before the product began to crystallize
from the resulting solution. The methyl of the 4-
thiocresol appeared as a singlet at 2.34 ppm, and the
methyl platinum resonance appeared as a singlet 1.34
ppm with satellites due to coupling of 195Pt with
2j(PtH) 70.6 Hz. The magnitude of the coupling con-
stant in such complexes is usually sufficient to deter-
mine the stereochemistry [15], but because the
trans-influences of 2,2%-bipyridyl and iodide are very
similar, either stereochemistry I or II is possible.
The stereochemistry of oxidative addition of methyl
iodide to cis-[PtMe2(PMe2Ph)2] has previously been
shown to be trans by deuterium labelling studies [16].
The NMR spectrum indicates that the complex has
structure I, corresponding to trans-oxidative addition.
Thus for the 4-methyl group of the thiocresol only one
resonance was observed for the methyl protons. The
only other likely structure is II, formed by cis-oxidative
addition, but this contains non-equivalent aryl groups
and so is not consistent with the NMR spectrum.
Since the platinum(IV) product has no MLCT band
in the UV–vis spectrum, the kinetics of the reaction
could be followed conveniently by monitoring the de-
cay of the MLCT band due to [Pt(4-thiocresol)2(2,2%-
bipyridyl)] by UV–vis spectrophotometry. The changes
in the spectrum during a typical run are shown in Fig.
1. Graphs of log (At−Aꢀ) versus time gave good
straight lines, indicating a first-order dependence of the
rate on [Pt(4-thiocresol)2(2,2%-bipyridyl)] concentration,
and the pseudo-first-order rate constants obtained were
proportional to the concentration of methyl iodide for
a given solvent as shown in Fig. 2. Thus the reactions
are first order in both platinum complex and methyl
iodide. The resulting second-order rate constants for
four solvents at 30°C are given in Table 1, which also
includes the second-order rate constants for oxidative
addition of methyl iodide to [PtPh2(bipy)] [4] and [Ir-
Cl(CO)PPh3)2] [17] in various solvents together with the
polarity parameter [18] ET for each solvent. In each
case there is an increase in rate by a factor of about
10–15 between the least-polar and most-polar solvents,
suggesting a common mechanism with a polar transi-
tion state. Activation energy and entropy of activation
have been measured for the reaction, and they are 55 kJ
Fig. 1. Changes in the UV–vis spectrum during the reaction in
chloroform at 30°C with [MeI]: (a) t=0; (b) t=20 min; (c) t=40
min; (d) t=60 min; (e) t=ꢀ.
mol−1 and −140 J mol−1
K
−1, respectively. It can be
seen that the entropy of activation has a large negative
value typical of oxidative-addition reactions of methyl
iodide [19,20]. The mechanism of the reaction of diaryl
(2,2%-bipyridyl) platinum (II) complexes with methyl
iodide has already been shown [4,5] to involve nucleo-
philic attack by platinum on the carbon atom of MeI to
give [PtMePh2(2,2%-bipyridyl)]+I−, which rapidly rear-
ranges to [PtIMePh2(2,2%-bipyridyl)]. Thus by analogy,
the most likely mechanism of reaction of [Pt(4-
thiocresol)2(2,2%-bipyridyl)] with methyl iodide involves
nucleophilic attack by platinum on the carbon
Fig. 2. Graph of the psuedo-first-order rate constants (kobs.) vs.
concentration of methyl iodide of the reaction at 30°C in solvents: (a)
methyl cyanide; (b) acetone; (c) dichloromethane; (d) chloroform.

168
J.K. Jawad et al. / Journal of Organometallic Chemistry 599 (2000) 166–169
Table 1
Solvent effects on the second-order rate constants for reactions with methyl iodide
Solvent
ET
Pt(4-thiocresol)2(bipy) k2
PtPh2(bipy) k2
IrCl(CO)(PPh3)2 k2
(30°C) (l mol−1 min−l
)
(40°C) (l mol−1 min−1
)
(25°C) (l mol−1 min−1
)
Toluene
Benzene
33.9
34.5
37.4
37.5
38.1
39.1
41.1
42.2
43.8
46.0
46.3
0.15
6.5
0.112
Tetrahydrofuran
Chlorobenzene
Ethyl acetate
Chloroform
Dichloromethane
Acetone
N,N-Dimethyl-formamide
Acetonitrile
Nitromethane
0.46
11.3
0.14
0.71
0.88
1.85
1.10
1.20
21
40.5
1.70
1.40
89
atom of MeI to give the intermediate, which rapidly re-
arranges to the final product.
band at 402 cm−1 belonging to the band (PtꢀN). Also a
strong band at 1000 cm−1 appears, which is attributed
to the (CꢀS) vibration in the spectra of the ligand and the
new complex formed, whereas there is no such band in
the spectrum of dichlorobipyridyl platinum(II).
The UV–vis spectra of (2,2%-bipyridyl) platinum(II)
complexes contain two intense MLCT bands, whose en-
ergies are strongly dependent on the nature of the lig-
ands bound to platinum and on the solvent. More
electronegative substituents on platinum and more polar
solvents cause the bands to move to higher energy [1].
We have found this to be so.
Similar assignment can be made for the dithiolato
(2,2%-bipyridyl) platinum(II) complexes studied here,
though the energy of the MLCT bands was very sensitive
to the nature of the thiolato group bound to platinum.
The lowest energy MLCT band for [Pt(p-
ZC6H4)2(bipy)] complexes was as follows: Z=CH3 (522
nm), H (475 nm), Br (462 nm), Cl (457 nm). The same
band of these complexes in methylcyanide solution was
at 502 nm and in acetone the band had moved to 516 nm
while in dichloromethane it was at 522 nm and in chloro-
form the band had moved to 530 nm.
The UV–vis spectra were used mainly to characterize
the dithiolato (2,2%-bipyridyl) platinum(II) complexes by
comparison with similar samples prepared earlier [1].
The elemental analyses of the new complexes were
generally in good agreement with theoretical values.
The IR, UV–vis spectra and elemental analyses are
consistent with complexes of the form:
3. Experimental
Di(4-thiocresol)(2,2%-bipyridyl) platinum(II) was pre-
pared as shown below.
Recrystallized dichloro (2,2%-bipyridyl) platinum(II)
(1.0 mmol) in chloroform (15ml) was added to a magnet-
ically stirred solution of 4-thiocresol (2.0 mmol) in
deionized distilled water (15 ml) containing potassium
hydroxide (2.2 mmol) and benzyl triphenyl phospho-
nium chloride (PTC) (0.1 mmol) at 30°C in a 100 ml
flask, equipped with an N2 inlet, water-cooled condensor
and a thermometer. After stirring for 1 h under N2, the
organic layer was separated and the aqueous layer ex-
tracted (three times) with chloroform.
The chloroform fractions were dried over anhydrous
magnesium sulphate. The solution was filtered and the
chloroform evaporated under vacuum, to give a red
solid.
Yield (80%). M.p. 336°C (dec.) found: C, 48.06; H,
3.64; N, 4.62. Anal. Calc. for C24H22N2S2: C, 48.23; H,
3.71; N, 4.69%.
The comparison of IR spectra shows the disappear-
ance of the weak band at (2600–2620) cm−1, which was
attributed to the stretching peak of (SꢀH) of the free lig-
and the two bands at (300, 340) cm−1, which was at-
tributed to the (PtꢀCl) stretch of dichlorobipyridylꢀ
platinum (II). At the same time a new strong band at 485
cm−1 appeared, which was attributed to the (PtꢀS) bond
stretch. This may confirm the formation of a new com-
plex, which may be a square-planar cis-complex because
the bonding of the stabilizing ligand (bipy) is in the cis-
form. This suggestion is confirmed by the formation of
one band for the bond (PtꢀS) as well as one medium

J.K. Jawad et al. / Journal of Organometallic Chemistry 599 (2000) 166–169
169
[2] R. Brady, B.R. Flynn, G.L. Geoffory, H.B. Gray, J. Peone, L.
Vaska, Inorg. Chem. 15 (1976) 1485.
[3] N. Chaudhury, M.G. Kekre, R.J. Puddephatt, J. Organomet.
Chem. 73 (1979) C17.
[4] J.K. Jawad, R.J. Puddephatt, J. Organomet. Chem. 117 (1976)
297.
[5] J.K. Jawad, R.J. Puddephatt, J. Chem. Soc. Dalton Trans.
(1977) 1466.
[6] A.J. Deeming, M.T.P. International Review of Science, Inor-
ganic Chemistry, series one, Butterworth, London, 1972, Chap-
ter 4.
[7] T.G. Appleton, H.C. Clark, L.E. Manzer, J. Organomet. Chem.
65 (1974) 275.
Iodo (methyl) di(4-thiocresol)(2,2%-bipyridyl) platinum-
(IV): To suspension of [Pt(4-thiocresol)2(2,2%-
a
bipyridyl)] (1 mmol) in dichloromethane (10 ml) was
added methyl iodide (excess). After stirring the mixture
for 10 min a yellow solution was obtained. Diethyl
ether was added dropwise until the solution became
cloudy and the mixture was set aside in the refrigerator,
when yellow crystals of the product crystallized. Yield
85%, m.p. 267°C (dec.) Found: C, 40.34; H, 3.39; N,
3.64. Anal Calc. for C25H25IN2S2Pt: C, 40.59; H, 3.40;
N, 3.78%.
[8] R. Uson, J. Fronies, P. Espinet, J. Garin, Syn. React. Inorg.
Met. Org. Chem. 7 (1977) 211.
[9] J.A. Hammud, MSc. Thesis, 1989, University of Salahaddin,
Erbil, Iraq.
4. Kinetic studies
[10] D. Cruz-Garritz, P. Sosa, H. Torrens, A. Hill, D.L. Hughes,
R.L. Richards, J. Chem. Soc. Dalton Trans. 3 (1989) 419.
[11] S. Dev, K. Imagawa, Y. Mizobe, G. Cheng, Y. Wakatsuki., H.
Yamazaki, M. Hidai, Organometallics 8 (1989) 1232.
[12] A. Shaver, R.D. Lai, Inorg. Chem. 27 (1988) 4664.
[13] C. Xu, J.W. Sirice, G.K. Anderson, Inorg. Chim. Acta 206
(1993) 123.
[14] V.K. Jain, S. Kannan, R.J. Butcher, J.P. Jasinsa, J. Chem. Soc.
Dalton Trans. 10 (1993) 1509.
[15] D.E. Clegg, J.R. Hall, G.A. Swile, J. Organomet. Chem. 38
(1972) 403.
Standard
solutions
of
[Pt(4-thiocresol)2(2,2%-
bipyridyl)] and MeI in the required solvent were pre-
pared. They were then mixed in the required
proportions, and the resulting solution transferred to a
1 cm quartz UV cell, which was held in the electrically
heated thermostatted cell compartment of a Philips pu
8620 UV/VIS/NIR spectrophotometer. Spectra were
then recorded as the reaction proceeded. The concen-
tration of [Pt(4-thiocresol)2(2,2%-bipyridyl)] was about
10−4 M at the start of each reaction.
[16] M.P. Brown, R.J. Puddephatt, C.E.E. Upton, J. Chem. Soc.
Dalton Trans. (1974) 2457.
[17] H. Stieger, K. Kelm, J. Phys. Chem. 77 (1973) 29.
[18] C. Reichardt, Angew. Chem. Int. Ed. 4 (1965) 29.
[19] R. Ugo, A. Pasini, A. Fusi, S. Cenini, J. Am. Chem. Soc. 94
(1974) 7364.
References
[20] P.B. Chock, J. Halpern, J. Am. Chem. Soc. 88 (1966)
3511.
[1] N. Chaudhury, R.J. Puddephatt, J. Organomet. Chem. 84 (1975)
105.
.