Formation and decomposition of the methylplatinum(IV) complex in the mechanically activated K2PtCl4 powder-MeI vapor system

Article abstract of DOI:10.1023/A:1019818800279

Mechanical treatment of the K₂PtCl₄ solid salt in a vibrating mill results in Pt-Cl bond heterolysis to form coordinatively unsaturated Pt(II) complexes. At room temperature, the freshly treated K₂PtCl₄ salt absorbs methyl bromide and evolves methyl chloride to the gas phase. The reaction mechanism involves the following sequence of steps: the oxidative addition of methyl iodide to Pt(II) with the intermediate formation of Pt(IV) methyl complexes and the decomposition of the latter due to intramolecular reductive elimination with methyl chloride formation. The first step of the reaction of MeI with the preactivated surface of the K₂PtCl₄ salt is assisted by active sites, which are regenerated in each act of the chemical transformation of MeI into MeCl involving in the chain substitution of halogen in methyl iodide. The coordinatively unsaturated surface platinum complexes can act as such active sites. Due to their effective positive charge, they can provide electrophilic assistance to nucleophilic substitution. Chain termination is probably due to the coordination of the complex with a coordination vacancy and an interstitial chloride ion to the inactive K₂PtCl₄ complex.

Full text of DOI:10.1023/A:1019818800279

Kinetics and Catalysis, Vol. 43, No. 4, 2002, pp. 475–483. Translated from Kinetika i Kataliz, Vol. 43, No. 4, 2002, pp. 516–525.
Original Russian Text Copyright © 2002 by Mitchenko, Khomutov, Zhikharev, Beletskaya.
Formation and Decomposition of the Methylplatinum(IV)
Complex in the Mechanically Activated
K2PtCl4 Powder–MeI Vapor System
S. A. Mitchenko1, 2, E. V. Khomutov2, I. V. Zhikharev3, and I. P. Beletskaya3
1 Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, 117813 Russia
2 Litvinenko Institute of Physicoorganic and Coal Chemistry, National Academy of Sciences of Ukraine,
Donetsk, 340114 Ukraine
3 Department of Chemistry, Moscow State University, Moscow, 117234 Russia
Received May 22, 2001
Abstract—Mechanical treatment of the K2PtCl4 solid salt in a vibrating mill results in Pt–Cl bond heterolysis
to form coordinatively unsaturated Pt(II) complexes. At room temperature, the freshly treated K2PtCl4 salt
absorbs methyl bromide and evolves methyl chloride to the gas phase. The reaction mechanism involves the
following sequence of steps: the oxidative addition of methyl iodide to Pt(II) with the intermediate formation
of Pt(IV) methyl complexes and the decomposition of the latter due to intramolecular reductive elimination
with methyl chloride formation. The first step of the reaction of MeI with the preactivated surface of the K2PtCl4
salt is assisted by active sites, which are regenerated in each act of the chemical transformation of MeI into
MeCl involving in the chain substitution of halogen in methyl iodide. The coordinatively unsaturated surface
platinum complexes can act as such active sites. Due to their effective positive charge, they can provide elec-
trophilic assistance to nucleophilic substitution. Chain termination is probably due to the coordination of the
complex with a coordination vacancy and an interstitial chloride ion to the inactive K2PtCl4 complex.
INTRODUCTION
tion state model A) [2, 3] or the mechanism of con-
certed three-center addition in which the coordinatively
unsaturated Pt(II) complex simultaneously acts as a
nucleophile attacking the carbon atom and an electro-
phile binding the leaving group [4] (reaction (II), tran-
In aqueous solutions, the [PtCl4]2– anion, similar to
chloride ions, is a metal-centered nucleophile, which
reacts with methyl iodide to form the Pt(IV) methyl
complex (MePt(IV)) [1–3]. This oxidative addition can
occur via either the SN2 mechanism (reaction (I), transi- sition state model B)
–
2–
≠
2–
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
+ I–,
Pt
CH3
Pt
+ ICH3
C
I
Pt
*
*
(I)
H H
A
–
≠
CH3
Pt
H3C
–
–
I
Cl
I
Cl
*
Cl
+ ICH3
Pt
*
Pt
.
Cl
Cl
Cl
Cl
Cl
Cl
(II)
*
B
The decomposition of MePt(II) due to reductive transformation via a three-center mechanism. Decom-
elimination with chloride ions yields methyl chloride. position via the SN2 mechanism was established [3, 5–7]
Evidently, reductive elimination, which is the reverse of for reactions in aqueous solutions. The three-center
oxidative addition, can either occur as SN2 nucleophilic mechanism of reductive elimination probably occurs
substitution (Cl– is the attacking nucleophile and during the thermolysis of the Cs2PtMeCl5 solid salt at
[PtCl4]2– is the leaving group) or as intramolecular 270°C in the absence of a solvent [8].
0023-1584/02/4304-0475$27.00 © 2002 MAIK “Nauka /Interperiodica”

476
MITCHENKO et al.
It is known that the grinding of powders under
The K2PtX4 salts (X = Cl, Br) were prepared using a
impact loading destroys the crystal structure of the standard procedure [19] and preliminarily dried for
near-surface layer, due to which the physicochemical 3 days in a desiccator at 12–140°C. An increase in the
properties of powders change and their reactivity sub- duration of drying had no noticeable effect on the kinet-
stantially increases [9–12]. After mechanical activa- ics of the reactions. All procedures were carried out in
tion, the solid KCl salt demonstrates reactivity in the a dry box.
heterogeneous nucleophilic substitution of halogen in
2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO) was
RI vapor (R = Me, Et, Pr) [13–17]. The driving force of
used as a paramagnetic probe. An aliquot (0.6 ml) of a
the reaction is the stabilization of the leaving group
solution of the nitroxyl radical (2 × 10–3 mol/l) in hex-
(iodide ion) by structural defects of the KCl lattice,
ane was injected with a syringe through a rubber gasket
which are anionic vacancies. These structural defects
into the vibrating reactor filled with the mechanically
are generated during the preliminary mechanical treat-
activated platinum salt, after which the solvent was
ment of the salt and bear an effective positive charge
evaporated in an argon flow.
facilitating the binding of a leaving group. The stoichi-
ometric consequence of the elementary act of the chem-
ical transformation of RI into RCl is the generation of a
new anionic vacancy instead of the former one and the
appearance of the foreign iodide ion in the anionic KCl
sublattice. The decay of the active site, that is, anionic
vacancy, occurs because of electron transfer to the latter
from the foreign iodide ion to form the F-site.
We can assume that the mechanical treatment of the
K2PtCl4 powder as well as K2PtCl6 [18] also results in
the appearance of the coordinatively unsaturated plati-
num complexes on the salt surface
The carbonyl complexes and Zeise salt
(K[PtCl3(C2H4)]) were prepared as follows. An atmo-
sphere of CO or ethylene was created in a closed reactor
containing the mechanically activated K2PtCl4 solid
salt. After 30 min, the unreacted gas was removed by
evacuation and/or in a dry air flow.
IR spectra were recorded using a SPECORD-IR75
in KBr pellets.
A liquid substrate (1–10 µl) was injected into the
reactor through the rubber gasket with a microsyringe
after the mechanical treatment of the salt. The con-
sumption of methyl iodide and the accumulation of
methyl chloride in the gas phase of the reactor were
monitored by GLC using an LKhM-8MD chromato-
graph with a flame-ionization detector and a 3-m col-
umn packed with the 5% SE-30 stationary phase on
Chromaton NAW. Samples were taken from the gas
phase of the reactor through the rubber gasket using a
dosing syringe with a fixed volume. The reaction was
carried out at room temperature.
mechanical treatment
–
–
[PtCl4]2–
*
[PtCl3 ] + Cl .
(III)
The point defect of the lattice in the form of the single-
–
*
charge [PtCl3 ] anion bears an effective positive
charge and, similar to the anionic vacancies on the sur-
face of the mechanically treated KCl salt, can stabilize
the leaving group (iodide ion) in the reaction with
methyl iodide facilitating the nucleophilic attack of the
carbon atom by Pt(II) to form the Pt(IV) methyl com-
plex. In addition to these processes, the methylation of
such coordinatively unsaturated complexes according
to the three-center mechanism (II) can be expected.
The number of moles of MeX in the gas phase of the
reactor (mg(MeX)) and the partial pressure of MeX
(P(MeX)) were determined as described in [14]. The
number of moles of MeX adsorbed at room temperature
on the K2PtCl4 surface (mads(MeX), µmol/g) was deter-
mined from the difference between the amount of the
substance in the gas phase of the reactor after the com-
plete desorption of MeX when the K2PtCl4 surface was
poisoned with water additives and mg(MeX). Water (its
amount was sufficient for the poisoning of the salt sur-
face, cessation of the reaction, and entire desorption of
The purpose of this work is to study the influence of
mechanical activation on the reaction of the K2PtCl4
solid salt with methyl iodide vapor.
EXPERIMENTAL
Weighted salt samples (0.3 g) were subjected to
mechanical treatment for 60 min in an MMVE-0.005
2
1
methyl halide to the gas phase of the reactor (20 µl)
vibrating micromill in air. The working frequency and
was injected with a syringe through the rubber gas-
ket while preserving the containment of the reactor.
The specific surface area of the salt surface deter-
mined by the BET method from argon desorption
was equal to 3.1 m2/g. The surface concentration of
MeX, Cads(MeX), (mol/m2) was determined from the
ratio of the mads(MeX) value to the surface area of the
salt surface.
amplitude of vibrations of the vibrating reactor were
f = 50 Hz and h0 = 5.5 mm, respectively, which corre-
sponded to a specific rating of ≈15 W/kg. A glass
17.37-ml vibrating reactor containing glass grinding
balls was hermetically closed with a rubber gasket.
After mechanical treatment, chromatographically
pure ethylene (70 ml) was bubbled through the reactor
if necessary.
1
2
Independent experiments showed that the reaction also occurred
in argon.
An increase in the amount of water added to the reactor did not
substantially change the m (MeX) value.
ads
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

FORMATION AND DECOMPOSITION OF THE METHYLPLATINUM(IV) COMPLEX
477
1H NMR spectra were recorded in methanol-d4 mental facts confirm the formation of Lewis acid sites
using a Varian GEMINI instrument with a working fre- (LAS), viz., species of coordination vacancies
–
quency of 200 MHz. The extraction of Zeise’s salt or
methyl complexes of Pt(IV) from the sample surface
was preceded by evacuation and/or dry air bubbling
through the reactor until methyl iodide and methyl
chloride completely disappeared from the gas phase
above the solid salt surface (GLC monitoring). For the
quantitative determination of the Pt(IV) methyl com-
plex and Zeise’s salt on the sample surface, extraction
was performed by a fixed volume of CD3OD containing
a known amount of sodium acetate, whose signal inten-
sity was used as an internal standard.
*
([PtCl3 ] ).
1. Adsorption of TEMPO on the mechanically acti-
vated Pt(II) salt surface indicates the presence of LAS.
When TEMPO (1.7 µmol/g) is supported, the ESR
spectrum contains the only signal corresponding to the
radicals with hindered rotational mobility. In this case,
'
the Azz parameter is 32.2 Oe.An increase in the amount
of the introduced radical broadens the ESR lines, prob-
ably due to the exchange interaction. Unlike the ESR
spectra of TEMPO on the mechanically treated K2PtCl6
salt, the signal belonging to the radicals with a higher
rotational mobility is not observed in the case of
K2PtCl4. This fact does not allow us to use this method
for the estimation of the limiting amount of LAS
formed by the mechanical treatment of K2PtCl4.
To perform chemical tests for the presence of the
Pt(IV) methyl complex, we used the known reaction of
decomposition of the latter in acidic aqueous solutions
containing a chloride ion with methyl chloride forma-
tion [3–5] in ~100% yield. With this purpose, MePt(IV)
was extracted with CH3OH as described above. The
extract was dried in an air flow without heating. The dry
residue was dissolved in distilled water (3 ml). An ali-
quot (0.2 ml) of the resulting solution was added to the
closed reactor, which was shaken and whose tempera-
ture was maintained constant, containing an aqueous
solution (2 ml) of a mixture of HClO4 (0.5 M), NaCl
(2.5 M), and NaClO4 (2.0 M) at 41°C. Under these con-
ditions, K2PtMeCl5 undergoes reductive elimination to
evolve MeCl to the gas phase [3–5]. The kinetics of
decomposition of the Pt(IV) methyl complex was mon-
itored by GLC by methyl chloride accumulation. The
concentration of the complex in the extract was deter-
mined using the preliminarily obtained calibrating plot
of the amount of the added methyl complex synthesized
according to [8] vs. the surface area of the chromato-
graphic peak of MeCl formed after the complete
decomposition of Cs2PtMeCl5. Note that the pseudo-
first-order rate constants of the decomposition of
the complex obtained in the heterogeneous reaction,
(8.8 ± 0.5) × 10–4 s–1, coincide with those of the decom-
position of Cs2PtMeCl5 obtained by the method
described in [8], (8.8 ± 0.7) × 10–4 s–1.
2. The freshly treated salt absorbs carbon oxide from
the gas phase to form the platinum carbonyl complex
[PtCl3(CO)]–. The IR spectrum of the latter in a KBr
pellet contains a band of the stretching vibration of the
3
coordinated carbonyl group, ν(CO) 2115 cm–1.
3. The consumption of ethylene from the gas phase
by the freshly treated salt leads to the formation of
Zeise’s salt, whose anion is the ethylene π complex
[PtCl3(C2H4)]–. The formation of this product was
4
detected by 1H NMR. According to the estimate from
the 1H NMR data, ~5 µmol of Zeise’s salt per gram of
treated K2[PtCl4] are formed. When the mechanically
treated K2[PtCl4] salt is kept for six months at room
temperature, the coordinatively unsaturated complexes
almost completely relax: the salt surface remains virtu-
ally unchanged, but ethylene is not noticeably absorbed
on this sample with Zeise’s salt formation.
Adsorption isotherms of MeI and MeCl on the
mechanically treated K2PtCl4 surface. The data on
MeI and MeCl (MeX) adsorption on the mechanically
activated K2PtCl4 surface are presented in Table 1. As
can be seen, the surface concentration of the adsorbed
substrates Cads(MeX) monotonically increases with an
increase in the partial pressure of methyl halide vapor
P(MeX), which can be described by curves with satura-
tion. The plot of Cads(MeX) vs. P(MeX) with saturation
corresponds to the Langmuir equation
ESR spectra were recorded in an SE/X-2544
radiospectrometer with a working frequency of
9.4 GHz using the Mn(II)/MgO system as a standard.
X-ray structure studies were carried out with a
DRON-UM1 diffractometer using CuKα irradiation and
the Ni filter.
RESULTS
Cmax(MeX)Kads(MeX)P(MeX)
----------------------------------------------------------------------------
Cads(MeX) =
, (1)
1 + Kads(MeX)P(MeX)
Mechanically active transformations in the coor-
dination sphere of the K2PtCl4 complexes. The mech-
anochemical treatment of the solid K2PtCl4 salt, unlike
K2PtCl6 [18], did not result in the homolytic cleavage of
the Pt–Cl bond: we failed to detect the formation of any
paramagnetic species. In this case, the Pt–Cl bond is
heterolyzed to form the coordinatively unsaturated
Pt(II) complexes (reaction (III)). The following experi-
3
4
For terminal carbonyls ν(CO) usually lies between 2125 and
1850 cm , that is, differs from ν(CO) for free CO, which is
–1
–1
2143 cm [20].
After unreacted ethylene was thoroughly removed from the reac-
tor, the product was extracted by deuteriomethanol. The spectrum
of the extract corresponds to that of Zeise’s salt (δ = 4.20 ppm,
195
1
J( Pt– H) = 56.0 Hz).
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

478
MITCHENKO et al.
Table 1. Surface concentration of the substrates adsorbed
on the mechanically activated K2PtCl4 surface (Cads(MeX))
as a function of the partial pressure of methyl halide vapor
(P(MeX))
face region in the vicinity of the active site resulting in
the transformation of MeI into MeCl.
Kinetics of methyl chloride accumulation. At
room temperature, the freshly treated K2PtCl4 salt
absorbs methyl iodide with methyl chloride evolution
Cads(MeCl),
µmol/m2
Cads(MeI),
µmol/m2
P(MeCl), Pa
P(MeI), Pa
5
to the gas phase.
0.10
0.71
1.04
1.75
2.22
2.46
4.43
–
0.35
2.30
1.18
3.21
2.44
3.00
4.60
–
0.34
0.89
1.28
1.90
4.14
5.34
5.70
13.03
16.46
4.50
10.10
7.06
CH3I + K2PtCl4
CH3Cl + K2PtCl3I.
(IV)
The addition of water vapor to the reactor substantially
retards the reaction, and the presence of 1 µl of water
inhibits it completely. After the treated salt was allowed
to stay for six months at room temperature, its active
sites resulting in reaction (IV) were almost completely
deactivated: the specific surface area of the salt
remained virtually unchanged, but methyl iodide con-
sumption and methyl chloride evolution on the this sur-
face did not occur at a noticeable rate.
11.33
15.70
18.70
16.09
20.19
20.77
–
–
In the case of the maximal coverage of the surface
with the substrate and the large initial amount of methyl
iodide, the curve of methyl chloride formation (taking
into account the salt adsorbed on the surface) consists
of three regions (Fig. 2). In the beginning of the reac-
tion, MeCl evolution accelerates and then the product is
accumulated for 4 to 5 h at a constant rate. The yield of
methyl chloride calculated per unreacted substrate in
the second region is close to 100%. If the reaction is
carried out for a longer time, the evolution of methyl
chloride slows down and ceases after approximately a
day reaching the limit mmax(MeCl) ≈ 83 µmol/g.
Table 2. Parameters of the Langmuir equation for MeX ad-
sorption on the activated K2PtCl4 and KCl surfaces [14]
Kads(MeX) ×103, Pa–1
K2PtCl4 KCl
MeI 0.06 ± 0.02 0.9 ± 0.2
MeCl 0.08 ± 0.03 9 ± 3
Cmax(MeX), µmol/m2
MeX
K2PtCl4
KCl
21 ± 1
21 ± 3
5.0 ± 0.8
1.6 ± 0.1
Let us compare the maximal amount of methyl chlo-
ride formed in the reaction with 1 g of the activated salt,
Nmax(MeCl) = NAmmax(MeCl) ≈ 5.0 × 1019 molecule/g (NA
is Avogadro’s number), and the amount of the complex
anions on the surface of 1 g of the salt, Nsp(Pt) = N(Pt)Ssp ≈
where Cmax is the maximum surface concentration of
MeX in the adsorption layer, and Kads(MeX) is the
adsorption equilibrium constant for MeX. The corre-
sponding linearized plots are presented in Fig. 1.
1.5 × 1019 ion/g, where N(Pt) ≈ 5 × 1018 ion/m2 is the num-
The parameters of MeX adsorption on the activated
K2PtCl4 surface determined from the linear plots of
Eq. (1) and, for comparison, those for the activated KCl
surface are presented in Table 2. The Cmax parameters
(monolayer capacity) for MeI on the K2PtCl4 and KCl
surfaces coincide, which evidently corresponds to the
almost complete coverage of the salt surface with the
adsorbed substrate (cf. [14]).
ber of the Pt complex ions per 1 m2 of the K2PtCl4 sur-
6
face. Taking into account that the [PtCl4]2– complex ion
contains four chloride ligands, we obtain that the
Nmax(MeCl) value is comparable with the amount of
chlorine atoms on the surface of platinum complexes.
Taking into account that the system does not contain
free chloride ions in an amount comparable with that of
7
methyl chloride formed and the profile of the evolution
As in the case of adsorption on the mechanically
treated KCl salt, the Cmax parameter for MeCl is lower
than that for MeI, and the adsorption equilibrium con-
stants on the K2PtCl4 surface for MeCl are somewhat
higher than the corresponding value for MeI. By anal-
ogy to [14], we can assume that MeCl formed in the
reaction is adsorbed on the surface region in the vicinity
of the active site, which induced this reaction. In this
case, the maximum surface concentration of MeCl,
Cmax(MeCl), should be determined to a great extent by
the concentration of active sites on the surface. In the
framework of this assumption, the higher Kads value for
methyl chloride compared to that for methyl iodide can
be a consequence of a higher adsorption heat due to the
specific interaction of the reaction product with the sur-
of the product, we can conclude that MeCl is formed
due to the intramolecular decomposition of the inter-
mediate Pt(IV) methyl complex.
k1
–
–
A + CH3I + [PtCl4]2–
*
A · I + [Pt(CH3)Cl4 ] , (V)
5
A similar reaction with methyl bromide formation occurs on the
freshly treated platinum bromide salt, K PtBr .
2
4
6
18
2
The value of N(Pt) ≈ 5 × 10 particle/m was estimated from
–10
the lattice parameters of K PtCl , a = b ≈ 7 × 10
m, c =
2
4
–10
4.13 × 10 m [21].
7
According to Eq. (III), the amount of free interstitial chloride ions
in the system is equal to [A] , where [A] = 5 µmol/g is the initial
0
0
concentration of the coordinatively unsaturated particles gener-
ated by the mechanical treatment.
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

FORMATION AND DECOMPOSITION OF THE METHYLPLATINUM(IV) COMPLEX
479
180
4
3
2
1
80
60
40
20
0.5
0.4
0.3
0.2
0.1
2
1
160
140
2
1
120
100
0
1
2
3
4
5
6
7
8
9
10
0
50 100 150 200 250 300 350
1/P(MeX), Pa–1
t, min
Fig. 1. Linear plots of the Langmuir isotherm for the
adsorption of (1) methyl iodide and (2) methyl chloride
formed on the mechanically activated K PtCl surface.
Fig. 2. Kinetics of (1) MeI consumption and (2) MeCl accu-
mulation in the reaction of MeI with the activated K PtCl
2
4
salt in excess MeI.
2
4
k2
–
–
The signal in the higher field is assigned to the
[Pt(CH3)Cl5]2– complex and/or trans-[Pt(CH3)Cl4S]–
(S=CD3OD or D2O), and that in the lower field is
assigned to the complex containing the iodide ligand in
the equatorial plane (see [3]). The kinetics of changes
in the amount of MePt(IV) agrees with the assumption
*
*
[Pt(CH3)Cl4 ]
[PtCl3 ] + CH3Cl,
(VI)
where A is the surface active site inducing the reaction.
In fact, in the framework of reactions (V) and (VI), pro-
vided that a change in the concentration of the active
sites and adsorbed substrate on the K2PtCl4 surface
during the first 3–5 h of the reaction can be neglected, that this complex is intermediately involved in reac-
we obtain the following equation for a change in the tion (IV) (Fig. 3). With the above assumptions, reac-
8
total amount of methyl chloride m(MeCl) in time:
m(MeCl) = k1Cmax(MeI)[A]Sspt
k1Cmax(MeI)[A]
tions (V) and (VI) lead to the following equation for the
change in the amount of the intermediate product with
time:
(2)
m(MePt(IV))
----------------------------------------
+
Ssp(exp(–k2t) – 1).
k2
(3)
k1Cmax(MeI)[A]
----------------------------------------
k2
=
Ssp(1 – exp(–k2t)) ,
In this approximation, 1/k2 is the characteristic time for
attaining the stationary concentration of the intermedi-
ate Pt(IV) methyl complex. After this stationary con-
centration was achieved, methyl chloride is linearly
accumulated, which agrees with the experimental plot
which agrees with the experimental plot at k2 = (8 ± 4) ×
k1Cmax(MeI)[A]
–4 –1
----------------------------------------
10 s and
Ssp = (7 ± 1) µmol/g. The
from these data,
k2
of m(MeCl) vs. time (Fig. 2) at k1Cmax(MeI)[A]Ssp
=
(3.9 ± 0.1) × 10–3 µmol g–1 s–1.
parameter
determined
k1Cmax(MeI)[A]Ssp = (5.6 ± 3.6) × 10–3 µmol g–1 s–1, coin-
cides within the experimental error with the value
obtained independently using Eq. (2). Then, taking into
account Cmax(MeI) = 21 ± 1 µmol/m2 and the specific
Formation of the intermediate Pt(IV) methyl
complex. In fact, the formation of the Pt(IV) methyl
complex under these conditions was confirmed by H
1
NMR and chemical tests [1, 7]. The MePt(IV) complex
was extracted with methanol after unreacted methyl
iodide was removed from the reactor by evacuation
and/or purging with dry air. The 1H NMR spectrum of
the extract in CD3OD coincides with that of the methyl
complexes obtained in [1, 3] under homogeneous con-
ditions: two signals are observed, namely, a singlet at
surface area Ssp = 3.2 m2/g, we obtain the estimate
k1[A] = 8 × 10–5 s–1.
Reactions in an ethylene atmosphere. The coordi-
natively unsaturated Pt(II) complexes generated on the
surface due to the mechanical treatment of the K2PtCl4
powder efficiently absorb ethylene from the gas phase
and are transformed into Zeise’s salt anion
2
δ = 3.09 ppm accompanied with satellites J(195Pt–1H) =
78.6 Hz and a singlet at δ = 3.20 ppm with satellites
2J(195Pt–1H) = 79.7 Hz. The ratio of the intensities of
these satellites (≈2 : 1) remains unchanged with time.
–
*
[PtCl3 ] + C2H4
[η2-(C2H4)PtCl3]–.
(VII)
It is known that the oxidative addition of methyl iodide
to the Pt(II) π-ethylene complex [Pt(η-C2H4)Cl3]– in an
aqueous solution is accompanied by ethylene elimina-
tion from the coordination sphere of platinum [22]. It
8
At the limiting surface coverage with the substrate the surface
concentration of the latter remains unchanged and equal to
C
(MeI) = C (MeI) at a considerable change in the methyl
ads
max
iodide pressure in the gas phase.
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

480
MITCHENKO et al.
m([η2-(C2H4)PtCl3]–), µmol /g
m(MePt(IV)),µmol /g
8
18
12
6
6
4
2
0
4
8
12
16
20
24
0
50
100
150
200
250
t, min
m(MeCl), µmol /g
Fig. 4. Amount of Zeise’s salt as a function of the amount of
evolved MeCl in the reaction of MeI with the activated
K PtCl salt in ethylene.
Fig. 3. Kinetics of the accumulation of the Pt(IV) methyl
complex in the reaction of MeI with the activated K PtCl
2
4
salt.
2
4
sample 2 exhibits shifts of diffraction maxima corre-
sponding to the parameter a = b (the initial structure has
a tetragonal system, space group P4/mmm) toward a
decrease in the angle, that is, an increase in the lattice
parameter (Fig. 5). It is seen from Fig. 5 that the dou-
blets [111] and [210], [101] and [200], and [211] and
[220] discernible in sample 1 almost look like singlets
for sample 2. In all presented doublets, the line with the
greater Miller index for the parameter a is shifted. For
the doublet [200] this shift estimated from the distance
between the centers of diffraction maxima reaches
0.2 Å. This shift indicates that the parameter a
increases in the near-surface layers, which can be
explained by the inclusion of iodine atoms in the coor-
dination sphere of the metal complex. At the same time,
the position of lines related to the parameter c remained
unchanged, which is probably due to the inclusion of an
iodide ion in the coordination sphere of platinum com-
plexes arranged in the crystallographic planes (h, k, 0).
After the samples were allowed to stay at room temper-
ature for three months, the structure of sample 2 relaxed
to the initial state 1. This is evidently related to the ther-
modynamic instability of state 2.
was of interest to elucidate the influence of ethylene
–
*
coordination to the [PtCl3 ] complexes on the hetero-
geneous reaction with methyl iodide.
In an ethylene atmosphere, reaction (IV) is approx-
imately twice as slow, but the patterns of substrate con-
sumption and product formation are similar to those
described above. In this case, methyl chloride accumu-
lation is accompanied by ethylene consumption with
the formation of Zeise’s salt: the amount of Zeise’s salt
formed linearly increases with an increase in the
amount of the evolved methyl chloride (Fig. 4). The
intercept on the ordinate axis on the plot of
m([η2-(C2H4)PtCl3]–) vs. m(MeCl) coincides within
experimental errors with the amount of Zeise’s salt
formed when ethylene is absorbed by the freshly treated
K2PtCl4. The slope of this straight line is 0.51 ± 0.05.
In this case, we did not detect the Pt(IV) methyl
complex containing the iodide ligand in the equatorial
1
plane in the H NMR spectrum of the methanol
(CD3OD) extract; only [Pt(CH3)Cl5]2– and/or trans-
[Pt(CH3)Cl4S]– (S=CD3OD or D2O) complexes were
observed.
X-ray structure study. The iodide ions formed in
reaction (IV) can enter the coordination sphere of plat-
inum or form the KI phase. This should be manifested
in the diffraction pattern as either a change in the lattice
Sample 3 does not exhibit these effects, and its dif-
fraction pattern is close to that for sample 1, but the
integral intensities of several X-ray maxima are redis-
tributed. Unfortunately, despite this fact, it is impossi-
ble to detect the appearance of the KI phase, because all
X-ray reflections of KI coincide with those of the basic
K2PtCl4 structure. To resolve this situation, we carried
out an experiment using the platinum bromide salt,
K2PtBr4. In this case, the reflections [100], [220], [222],
and [331] corresponding to the KI phase [23] were
detected and did not coincide with the reflections of
K2PtBr4.
9
parameters of K2PtCl4 or the appearance of new sig-
nals assigned to the KI phase. For X-ray structure stud-
ies we used polycrystalline samples of the mechani-
cally treated K2PtCl4 salt before the reaction with
methyl iodide (sample 1) and the same samples after
the reaction with MeI in air (sample 2) and ethylene
(sample 3). The diffraction pattern of sample 1 com-
pletely corresponds to the published data [23] for the
single-phase K2PtCl4 salt. The diffraction pattern of
9
The ion radius of the iodide ion is much greater than that of the
chloride ion.
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

FORMATION AND DECOMPOSITION OF THE METHYLPLATINUM(IV) COMPLEX
481
[111]
[101]
[211]
[220]
[210]
[200]
1
2
36
31
26
21
2θ, deg
Fig. 5. X-ray patterns of the mechanically treated K PtCl salt (1) before and (2) after the reaction with methyl iodide in air.
2
4
DISCUSSION
reductive elimination of methyl chloride (reactions (V)
–
*
and (VI), where A = [PtCl3 ] ).
Let us consider possible mechanisms of the reac-
tion. Provided that the coordinatively unsaturated
[PtCl3 ] complexes are active sites, we assume two
routes of the reaction. The first route is the three-center
oxidative addition of methyl iodide followed by the
reductive elimination of methyl chloride (reactions (II)
and (VIII)).
A similar electrophilic assistance by the Pt(II) aque-
ous–chloride complexes [24] and mercury(II) salts [25]
to the oxidative addition of methyl iodide to the Pt(II)
tetrachloride complexes was found for reactions in
aqueous solutions. The stoichiometric consequence of
reaction (V) is the formation of the Pt(IV) methyl com-
–
–
*
*
plex with the [Pt(CH3)Cl4 ] coordination vacancy and
–
CH3
–
–
–
*
I
Cl
I
the [PtCl3 ] · I ion pair and/or the mixed chloride–
Cl
(VIII)
+ CH3Cl.
Pt
Pt
iodide [PtCl3I]2– complex. The latter were detected by
XRD. Intramolecular reductive elimination (reaction (VI))
yields methyl chloride and results in the regeneration of
the complex with the coordination vacancy. In this case,
the active sites on salt surface are mobile. Chain termi-
nation can also occur due to the recombination of the
coordinatively unsaturated complex and interstitial
chloride ion formed in reaction (III)
Cl
Cl
Cl
*
*
The stoichiometric consequence of reaction (II) is the
formation of the Pt(IV) methyl complex with a coordi-
nation vacancy. The decomposition of the latter with
the formation of methyl chloride (reaction (VIII))
should yield a mixed chloride–iodide coordinatively
unsaturated complex. In this case, no migration over the
reaction site surface occurs, and the amount of methyl
chloride formed should not exceed 3[A]0, where [A]0 =
5 µmol/g is the initial concentration of coordinatively
unsaturated species generated by the mechanical treat-
ment. In our case, a much greater amount of methyl
chloride is evolved and, hence, the considered route
cannot be the main one.
–
–
*
[PtCl3 ] + Cl
[PtCl4]2–.
(IX)
Taking into account that the point defects, including the
interstitial chloride ions, are localized predominantly
on the salt surface [26], we arrive at a reasonable expla-
nation for the fact that the reaction is restricted by
approximately one monolayer of the platinum salt. This
chain mechanism suggests the surface concentrations
The second route is oxidative addition via the SN2
mechanism with the electrophilic assistance by the of the active states A are constant in time at the initial
coordinatively unsaturated complexes followed by the moments of the reaction (see above) when quadratic
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

482
MITCHENKO et al.
–
–
chain termination can be neglected and that the amount
of evolved methyl chloride is relatively large. Note that,
according to mechanism involving reactions (V) and
(VI), methyl chloride is formed in the vicinity of the
electrophilic site, a complex with the coordination
vacancy. This fact can be the reason for the higher
adsorption equilibrium constant of MeCl: due to the
electrostatic interaction, the adsorption heat of methyl
–
*
[PtCl3 ] · I
[PtCl3I]2–.
(X)
The formation of the KI phase in the reaction with
methyl iodide in ethylene is likely explained by the
relaxation of the [η2-(C2H4)PtCl3]– · I– defect in the
reaction
K2[Pt(η2-C2H4)Cl3] · I
KPt(η2-C2H4)Cl3 + KI.
(XI)
*
chloride on the [PtCl3 ] complex bearing a positive
According to numerous published data [27–30],
intramolecular reductive elimination from Pt(IV) com-
plexes occurs through a pentacoordinated intermediate.
Such a pentacoordinated Pt(IV) methyl complex with a
vacancy in the trans position to the methyl group
should form in the reactions under consideration. This
fact most likely explains the instability of the pentaco-
–
charge can evidently be higher than the adsorption heat
on the electroneutral surface regions.
The NMR signals from the methyl complexes with
and without the iodide ligand in the equatorial plane are
detected for methanol extracts of the methyl complex.
Assuming that, at the moment of extraction, the iodide
ligand does not enter the coordination sphere of the
*
ordinated [Pt(CH3)Cl4 ] derivative, unlike the stable
10
[Pt(CH3)Cl4]– complex and excluding a similar sec-
hexacoordinated [Pt(CH3)Cl5]2– complex in the form of
ondary reaction in the solid phase due to the hindered
mobility, we have to assume that the methyl complex
containing the iodide ligand in the equatorial plane is
formed by the three-center mechanism (II) and/or due
to the methylation of the mixed platinum(II) chloride–
iodide complex. According to reactions (V) and (VI),
the solid K2Pt(CH3)Cl5 salt. Additional stabilization of
–
*
[Pt(CH3)Cl4 ] by the crystalline field of the basic struc-
ture can be another reason for the difference between
–
*
the reactivities of [Pt(CH3)Cl4 ] in the K2PtCl4 matrix
and [Pt(CH3)Cl5]2– in the K2Pt(CH3)Cl5 matrix.
–
2–
*
the [PtCl3 ] and [PtCl3I] complexes formed in the
chemical transformation of MeI into MeCl can be
localized only near each other. This fact makes the
methylation of the last particle sterically possible in the
next act of chain propagation. It is likely that the pres-
ence of the iodide ligand in the coordination sphere
enhances the nucleophilicity of the Pt(II) complex,
which additionally favors this reaction.
ACKNOWLEDGMENTS
This work was supported by INTAS (grant no. 97-
1874).
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