Kinetics and mechanism of the reductive elimination of oxiranes from "platinahydrins" (Platinum β-hydroxyethyl complexes)

Article abstract of DOI:10.1134/S0023158410010040

The β-hydroxyethyl platinum(IV) derivatives with the general formula [Cl₅Pt^IV-CH₂CH(OH)R]^2-, where R = H (1) or CH₂Cl (2), are fairly stable in aqueous solution at pH < 7. In neutral and weakly alkali

Full text of DOI:10.1134/S0023158410010040

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 1, pp. 18–24. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © S.L. Lintvinenko, I.R. Chanysheva, M.V. Beskrovnaya, N.S. Yanat’eva, V.V. Zamashchikov, 2010, published in Kinetika i Kataliz, 2010, Vol. 51, No. 1,
pp. 21–27.
Kinetics and Mechanism of the Reductive Elimination of Oxiranes
from “Platinahydrins” (Platinum ꢀHydroxyethyl Complexes)
β
a
a
b
b
c
S. L. Lintvinenko , I. R. Chanysheva , M. V. Beskrovnaya , N. S. Yanat’eva , and V. V. Zamashchikov
a
Litvinenko Institute of Physical Organic Chemistry and Coal Chemistry, National Academy of Sciences of Ukraine,
Donetsk, 83114 Ukraine
Donetsk National University, Donetsk, 83056 Ukraine
b
c
Azov–Black Sea State Agroengineering Academy, Zernograd, Rostov oblast, 347740 Russia
eꢀmail: sllit1@rambler.ru
Received August 18, 2008
Abstract—The
β
ꢀhydroxyethyl platinum(IV) derivatives with the general formula [Cl PtIV–
5
CH CH(OH)R]2–, where R = H (
1
) or CH Cl (
2), are fairly stable in aqueous solution at pH < 7. In neutral
2
2
and weakly alkaline solutions, they decompose, yielding the corresponding epoxide CH (O)CHR and the
2
2
ꢀ
PtCl anion. This decomposition takes place via reductive elimination caused by an intramolecular nucleoꢀ
4
philic attack of the oxygen atom on the αꢀcarbon atom of “platinahydrin” 1 or 2.
DOI: 10.1134/S0023158410010040
β
ꢀHydroxyalkyl derivatives with the general forꢀ fragments of the organic moiety of the platinahydrin
mula L –M–CH –CH(OH)–CH –R are key interꢀ and a platinum(II) complex. This makes them conveꢀ
n
2
2
mediates in a number of important catalytic processes nient objects for investigating the kinetics and mechaꢀ
1], including the wellꢀknown olefin oxidation reacꢀ nism of particular stages of catalytic reactions.
tion catalyzed by palladium(II) [2, 3]. Also known are
For the first time, the formation of the simplest
fairly stable representatives of this class of organomeꢀ platinahydrin Pt(IV)–CH –CH –OH in aqueous
[
2
2
tallic compounds, for example, L–Hg–CH – solutions was observed in the oxidation of Zeise’s salt
2
–
CH(OH)CH R [4].
anion Cl Pt(CH =CH ) with molecular chlorine [6].
2
3 2 2
The
βꢀhydroxyalkyl platinahydrins synthesized by It was noted that alkalifying the platinahydrin solution
us, L –Pt(IV)–CH –CH(OH)CH R [5], are interꢀ yields ethylene oxide. The mechanism of this process
5
2
2
mediate in reactivity. They are sufficiently stable to be was not discussed in that work [6], but now it can be
isolated and to study their spectral and other properꢀ hypothesized that ethylene oxide resulted from reducꢀ
ties. At the same time, they readily undergo chemical tive elimination through intramolecular attack by the
conversions, such as reductive elimination yielding alkoxide ion on the
αꢀcarbon atom.
Ka
PtIV CH2 CH2 OH
PtIV CH2 CH2
O
−
#
O
O
PtIV CH2 CH2
−
PtIV CH2 CH2
k*
PtII + CH2 CH2
O
Scheme 1.
Preliminary kinetic data [5] demonstrated that the ing the solution yields epichlorohydrin, reductive
–
regularities of the decomposition of an analogue of the
elimination is slowed down by Cl admixtures, and the
apparent rate constant of the decomposition of 2
compound Cl –Pt(IV)–CН –CH –OH (
1
)
, namely,
5
2
2
2–
the complex PtCl (CH CHOHCH Cl) (2) obtained
5
2
2
increase with increasing pH. These observations are
consistent with the intramolecular mechanism of
differ from the regularities of reductive elimination for reductive elimination (Scheme 1). A similar mechaꢀ
most platinum(IV) alkyl chloride complexes: alkalifyꢀ nism was suggested in a recent work [7] for the decomꢀ
2
4
−
by the oxidative addition of epichlorohydrin to PtCl
,
18

KINETICS AND MECHANISM OF THE REDUCTIVE ELIMINATION
19
IV
position of the platinahydrin L–Pt (OH) (CH – ([KCl] + [KNO ] = 3.0 mol/l) at 60°C undergoes
3
2
2
reductive elimination yielding a mixture of ethylene
glycol and chloroethanol. The yield of the latter
increases with increasing [Cl ]. The authors of that
study believed that the reductive elimination mechaꢀ
nism in this case is the same as in the reactions of the
other known monoalkyl derivatives of platinum(IV).
CH –OH) (Scheme 2). The halfꢀlife of the initial
2
unsymmetrical platinahydrin 3а (according to the terꢀ
minology of Khusnutdinova et al. [7]) is approxiꢀ
mately 1 h at 80°С. Oxirane formation begins only
after the isomerization of 3а into symmetrical comꢀ
plex 3b, which has a weakly bound sulfo group oxygen
atom trans to the hydrin fragment. The resulting
oxirane hydrolyzes rapidly into ethylene glycol
–
Specifically, an S 2 attack of the external nucleophile
N
on the
α
ꢀcarbon atom of the substrate takes place:
IV
II
(
Scheme 2).
Nu + R–Pt X –L
4
NuR + Pt X
2−
4
+ L
(I)
The first detailed kinetic study of the decomposiꢀ
–
–
–
–
2−
(Nu = Cl , Br , I [9], SCN , S2O3 [10], PtCl4 [11]
tion of the platinahydrin Cl –Pt(IV)–CН –CH –
5
2
2
and H O [9]; X = Cl or Br; R = CH , C H ,
+
2
3
2
5
OH (
1
)
, carried out by Luinstra et al. [8], demonꢀ
strated that compound
1
in aqueous solutions CH
2
COOH, CH COCH
2
3
, CH CH
2
2
NH
3
).
−
O
O
O
OH
N
OH
N
OH
OH
OH
N
+
O
Pt+
Pt
Pt
N
N
N
OH2
HO
OH
H O
2
3
а
3b
SO3−
HO(CH ) OH
2
2
O
N
=
N
N
N
Scheme 2.
The platinum(II) complex in these reactions is a
Kinetically, this is manifested as the fact that the
on the
leaving group. It differs from “ordinary” (nonꢀmetalꢀ dependence of the apparent rate constant
k
complex) leaving groups in that the breaking of the nucleophile concentration is described by the followꢀ
platinum–carbon bond is accompanied by the breakꢀ ing equation [9–16]:
ing of the bond between the platinum atom and the
ligand trans to the alkyl group of the initial complex.
Thus, at given Nu and R, the reactivity of platinum
k = kNu[Nu]/(1 + KL[L]),
where kNu is the rate constant of the reaction between
R–Pt and the nucleophile Nu and KL is the equilibꢀ
(1)
IV
alkyl complexes in reaction (I) is determined by the rium constant (see Scheme 3):
strength of the Pt–L bond. In this class of reductive
elimination reactions, nearly all of the reactions studꢀ
ied to date proceed at a noticeable rate only when the
trans position to the alkyl group is occupied by an aqua
IV
IV
KL = [L–Pt –R]/([L][H O–Pt –R]).
(2)
2
In the general case, Nu and L may be the same (e.g.,
–
–
Nu = L = Cl [9–11] and Br [12]) or different (e.g.,
–
Nu = Cl and L = Py [13] or Im [14]).
ligand (L = H O, Scheme 3). Replacing this ligand
2
Luinstra et al. [8] did not determine the pH of the
solution. As judged from their results, measurements
were taken at pH <7. It is likely for this reason that the
possibility of intramolecular reductive elimination was
not discussed (see below).
with a group forming a more stable complex with platꢀ
–
–
inum (Cl [9, 10], Br [11], pyridine [12], imidazole
13]) dramatically decreases the reductive elimination
[
–
rate: no reaction occurs even at L = Cl
.
Here, we report the decomposition kinetics of
KL
IV
IV
platinahydrins
pH 7–8.
1 and 2 in aqueous buffer solutions at
L + H O–Pt –R
H O + L–Pt –R
2
2
Nu
kNu
–OH2
RESULTS AND DISCUSSION
After holding a solution of (~5
10–5 mol/l) in
.5 M HCl at 25°C for 4 h, absorbance at = 262 nm
is unchanged; that is, ꢀhydroxyethylplainum(IV) is
fairly stable under these conditions, which is in agreeꢀ
δ−
δ+
NuR + PtII
1
×
Nu
C
Pt
0
λ
β
Scheme 3.
KINETICS AND CATALYSIS Vol. 51
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2010

20
LINTVINENKO et al.
Table 1. Dependence of the rate constant of the reductive
In salt solutions ([NaCl] + [NaClO4] = 3.0 mol/l),
complexes and decompose at an appreciable rate,
but the kinetics of these process does not obey a simple
firstꢀorder equation: the slope of the ln curve,
where is absorbance and is the reaction time,
decreases during the run (see, e.g., Fig. 1). The main
decomposition products of under these conditions
are 1,3ꢀdichloropropanꢀ2ꢀol and epichlorohydrin,
whose yields, as determined by GLC, are 35 and 65%,
respectively.
elimination of ethylene oxide from the complex
1
2
Cs Pt(CH CH OH)Cl (
1
)
on the acidity of the medium
2
2
2
5
D–t
3
–1
pH
k × 10 , s
D
t
6
.63
.03
.23
.40
.20
.20
.45
.58
.72
.80
.97
0.112
0.152
0.230
0.298
1.56
±
±
±
±
±
±
±
±
±
±
±
0.002
0.002
0.001
0.003
0.03
2
7
7
7
8
8
8
8
8
8
8
These kinetic deviations are not observed when the
1.68
0.01
reaction is conducted in a buffer solution: at pH
≥
7, 1
2.27
0.05
and decompose according to a first order equation
2
with respect to platinum(IV). The reaction rate under
these conditions is well above the reaction rate in the
solutions containing only NaCl and NaClO4. For this
reason, these experiments were performed at a lower
temperature of 298 K. Specialꢀpurpose experiments
demonstrated that the decrease in the absorbance of
3.04
2.86
0.05
0.09
4.29
0.03
7.20
0.09
Note: The solvent is water; T = 298 K; [NaCl] = 3.0 mol/l.
IV
the R–Pt solution is due only to reductive eliminaꢀ
tion: the UV spectra of solutions of K PtCl6 and
2
–
5
Table 2. Dependence of the rate constant of the reductive Cs PtCl (CH ) (10 mol/l) containing 3 M NaCl or
2
4
3
elimination of ethylene oxide from complex (1) on the Cl– NaClO4 at pH 8.15 do not change within the measureꢀ
concentration
ment time, suggesting that equatorial chloro ligand are
not replaced by hydroxo or aqua ligands. Note that no
3
–1
k × 10 , s
Cl–], mol/l
glycol and chlorohydrin formation from
1 and 2 is
[
observed at pH and the only products detectable by
GLC and H NMR [9, 11] are ethylene oxide and
≥
7
observed
calculated
1
0
0
0
0
0
1
2
3
.00
.07
.20
.39
.91
.50
.50
.00
2.56
2.25
1.72
1.64
1.07
0.83
0.63
0.66
±
±
±
±
±
±
±
±
0.03
0.02
0.02
0.01
1.07
0.01
0.01
0.01
2.50
2.19
1.78
1.46
1.01
0.79
0.63
0.58
epichlorohydrin, respectively.
Tables 1 and 2 list the rate constants of the reductive
elimination of ethylene oxide from complex 1 at variꢀ
ous pH values and chloride ion concentrations. Unlike
the other known reductive eliminations from R–
Pt(IV), this reaction speeds up as the acidity of the
medium decreases and is slowed down by Cl . Since
the primary decomposition product at pH > 7 is ethylꢀ
ene oxide, it is natural to assume that, under the conꢀ
–
ditions examined,
1 decomposes intramolecularly via
Note: The solvent is water; T = 298 K; pH 8.15; [NaCl + NaClO ] =
4
a nucleophilic attack of the hydroxyethyl oxygen atom
on the carbon atom bonded to platinum. According to
this mechanism, the concentration of the nucleophile
3
.0 mol/l.
(
see Scheme 1) does not appear in the kinetic equaꢀ
ment with earlier data [8]. The decomposition of
1
tion (1), which takes the following form at Nu =
L = Cl:
yielding gaseous ethylene (identifiable by GLC) is
observed only when the complex is heated in concenꢀ
trated HCl in a boiling water bath.
–
k = kCl/(1 +
K
Cl[Cl ]).
(3)
–
Cl , KCl
IV
IV
H O–Pt –CH –CH –OH
2
Cl–Pt –CH –CH –OH
2 2
2
2
H O
2
Ka
Ka
IV
–
+
IV
–
+
H O–Pt –CH –CH –O + H Cl–Pt –CH –CH –O + H
2
2
2
2
2
k1
k2
H CH + PtII
–
C
C
H
2
2
O
Scheme 4.
KINETICS AND CATALYSIS Vol. 51
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2010

KINETICS AND MECHANISM OF THE REDUCTIVE ELIMINATION
21
3
−1
lnD [arb. units]
k
× 10 , s
1
1
0
.5
1
5
.0
1
0
5
.5
1
2
3
Cl ], mol/l
20
40
60
80
100
, min
−
[
t
Fig. 1.Semilogarithmic plot of the absorbance of the soluꢀ
tion versus time for the decomposition of complex (water
as the solvent, = 323 K, [NaCl] = 0.125 mol/l,
NaClO4] = 0.375 mol/l).
Fig. 2. k–1 versus [Cl–] for the reductive elimination of
ethylene oxide from complex (water as the solvent,
298 K, pH 8.15, [NaCl] = 3.0 mol/l).
1
Т
1
Т =
[
However, the reductive elimination rate constants 298 K in buffer solutions with pH 8.15. The reductive
–
1
elimination rate constant in this case is a more compliꢀ
cated function of [Cl ], passing through a maximum
determined in this study disobey Eq. (3):
k as a funcꢀ
–
–
tion of [Cl ] is nonlinear (Fig. 2). This behavior is conꢀ
sistent with Scheme 4, which includes the intramolecꢀ
ular nucleophilic attack of the alkoxide both in the
complex with an aqua ligand trans to the alkyl group
and in the complex with a chloride ligand in the same
position. The following kinetic equation corresponds
(
Fig. 3). This dependence can be qualitatively
explained in terms of the reaction proceeding accordꢀ
ing to Scheme 5.
IV
As distinct from ethyleneplatinahydrin, (Pt –
CH CH OH)
(
,
β
ꢀplatinumchloropropanol, and
2
IV
2
α
IV
–
to this mechanism at [Pt –CH –CH –O ]
Ӷ
β
γ
2
2
Pt –C H –C H(OH)–C H Cl) each have two
2 2
IV
[
Pt
⎯CH –CH –OH] (which is typically the case for
2 2
reactive sites accessible to intramolecular attack by the
alkoxide oxygen atom. Attack on the ꢀcarbon atom
1
α
aqueous solutions of alcohols):
leads to the formation of reductive elimination prodꢀ
ucts, namely, epichlorohydrin and a platinum(II)
–
–
k =
(k1
+
k2KCl[Cl ]/(1 + KCl[Cl ]),
(4)
where k1 and k2 are the rate constants of intramolecuꢀ
lar reductive elimination via the mechanism presented
in Scheme 4. Nonlinear regression calculations give,
complex. Attack on the ꢀcarbon atom yields epiplatiꢀ
γ
nahydrins (2c and 2d) in equilibrium with the initial
complex. The presently available experimental data
are insufficient to calculate all of the rate and equilibꢀ
rium constants appearing in Scheme 5. However, if the
contribution from innerꢀsphere reductive elimination
from the complex containing a chloride ligand trans to
for pH 8.15, the following values of k1
, k2, and the
equilibrium constant KCl (with a multiple correlation
–1
coefficient of
k2 = 0.0003
R
= 0.990): k1 = 0.0025
±
±
0.0001 s ,
0.5 mol/l.
–1
±
0.0001 s , and KCl = 2.3
The goodness of fit between the experimental data the alkyl group is neglected (the inequality k2
Ӷ
k1 is
and Eq. (4) with the above values of the rate and equiꢀ
librium constants is illustrated by the data presented in
Table 2, in which the experimental rate constants of
assumed to be true, as in the case of
platinum), it can be demonstrated that, in the first
approximation, the apparent rate constant of Pt –
β
ꢀhydroxyetylꢀ
IV
the reductive elimination of ethylene oxide from
1 are
CH –CH(OH)CH Cl decomposition is given by
2
2
compared with the corresponding calculated data.
–
–
–
k = k1[Cl ]/{K1 + (1 +K K2)[Cl ] + KCl[Cl ]} (5)
Table 3 lists the decomposition rate constants meaꢀ
Cl
sured for the complex Cs PtCl (CH CHOHCH Cl) at
2
5
2
2
with k1 = 0.0034 s–1, KCl = 2.40 l/mol, K1 = 0.37 mol/l,
and K2 = 0.18 mol/l. It is clear from Fig. 3 that Eq. (5)
with the above rate and equilibrium constants
1
Here, the measured rate constant
k
is an effective quantity:
k
=
IV
+
k'Ka[Pt –CH –CH –OH]/[H ].
2 2
KINETICS AND CATALYSIS Vol. 51
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2010

22
LINTVINENKO et al.
II
Pt + CH –CH–CH Cl
2
2
O
k1
k2
H+
–H+
–
–
Cl , KCl
IV
Cl PtIV CH2 CH CH Cl
H O Pt
CH2 CH CH Cl
2
2
2
OH
OH
2
а
2b
–
–
–
–H
Cl
–Cl
K1
+
K2
–H+
–
Cl ,
K
'
Cl
IV
Cl PtIV CH2 CH CH2
H O Pt
2
CH2 CH CH2
O
O
2
d
2c
Scheme 5.
as a function of [Cl–] passes tions of
from the reaction slow down reductive elimination,
and, accordingly, the slope of the ln curve
[
1
]
≈
5
×
10–5 mol/l. The Н+ ions resulting
accounts for the fact that
k
0
2
through an extremum.
D–t
Thus, the data of this study demonstrate that, decreases with time. For the reaction conducted in
unlike the other alkyl complexes, the platinum(IV)
3 M HCl at 318 K, the apparent decomposition rate
hydroxyethyl complex decomposes at pH >7 mainly constant
via reductive elimination that includes an innerꢀ value of 8.8
sphere nucleophilic attack of the alkoxide oxygen
atom on the ꢀcarbon atom.
Now it is possible to explain the difference between hydroxyalkyl complexes. For example, the decompoꢀ
βꢀ
–5 –1
1
k
for
10 s , reported by Luinstra et al. [8].
It is likely that the innerꢀsphere reductive eliminaꢀ
tion mechanism suggested here is not unique to the
1
is 6.9
×
10 s , which is close to the
–
5 –1
×
α
βꢀ
IV
the complex
1
decomposition kinetics reported by sition of trans
ꢀ
L–Pt –CH
2
–CH
), where L = H O
2
–NH (3, initial
2 2
Luinstra et al. [8] and that observed in this study. In complex—KPtCl
(CH CH NH
)
3
5
2
2
–
both cases, the decomposition kinetics was studied or Cl , yields aziridine [17]. The authors of that study
spectrophotometrically as the decline of the absorꢀ believe that innerꢀsphere reductive elimination is posꢀ
IV
II
sible only when L is the water molecule [17]. In this
case, the dependence of the reaction rate on the chloꢀ
ride ion concentration is given by Eq. (3). However,
the experimental data on the effect of Сl concentraꢀ
tion on the rate of the reductive elimination of aziriꢀ
bance of solutions of
1
due to the Pt
Pt converꢀ
sion as a result of reductive elimination. However,
Luinstra et al. [8] recorded spectra at a wavelength of
–
λ
= 350 nm, at which the extinction coefficient is
–
1
–1
ε
3
≈
500 l mol cm . Therefore, in order to ensure
an absorbance value of 0.5, they most likely had to
use initial concentrations of
initial concentrations, as the reaction proceeds to an
50
dine from complex 3 are also consistent with Eq. (4) with
± ×
D
≈
–4
–1
–5 –1
k1 = (5.4
KCl = 0.90
Thus, the innerꢀsphere reductive elimination
mechanism suggested here for complexes and is
±
0.3)
×
10 s , k2 = (2.7 1.2) 10 s , and
–3
[
1
]0 10 mol/l. At these
≈
±
0.06 mol/l.
+
extent of 5%, it raises the
H
concentration by
5
×
1
2
2
⎯
5
10
mol/l, as follows from Scheme 1. This brings the
IV
apparently common to transꢀL–Pt –CH –CHX–R
solution to pH ~ 4.5, and the innerꢀsphere reductive
elimination route yielding oxirane is thus completely
retarded. The only process observed is the slow
complexes, where X is a substituent possessing nucleoꢀ
philic properties and L is a ligand strongly bonded to
platinum.
decomposition of
1
under the action of a nucleophile
In conclusion, note that
num(IV) derivatives and are representatives of the
large family of metallahydrins, M–CH –CH –
βꢀhydroxyalkyl platiꢀ
–
(
Cl or H O) via the “ordinary,” outerꢀsphere mechaꢀ
2
1
2
nism.
2
2
In this study, the decrease in the concentration of CH(OH)–CH –R, which vary widely in reactivity.
2
II
complexes was monitored at
λ
= 262 nm. At this waveꢀ With M = Hg , they decompose only under the action
is
of strong acids, yielding olefins [4]. As was mentioned
5000 l mol cm [15], so we used initial concentraꢀ above, platinahydrin
behaves in a similar way when
length, the extinction coefficient for complex
1
ε
~
–
1
–1
2
1
heated in concentrated HCl. The decomposition of
palladahydrins (M = Pd ), which are intermediates in
the familiar olefin oxidation reaction [2, 3], yields carꢀ
bonyl derivatives and, according to Moiseev’s hypothꢀ
esis [2], can be viewed as reductive elimination involvꢀ
2
The initial parameters of Eq. (5) were determined from the maxꢀ
II
–
–
imum condition for
k as a function of [Cl ]: dk/d[Cl ] = 0.
Next, fixing three parameters by turns, we determined the fourth
parameter by least squares. This procedure was repeated until
the sought values stopped changing.
KINETICS AND CATALYSIS Vol. 51
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2010

KINETICS AND MECHANISM OF THE REDUCTIVE ELIMINATION
23
Table 3. Dependence of the rate constant of the reductive involving outerꢀsphere attack by chloride ions on
elimination of epichlorohydrin from complex 2 on the Cl–
concentration
Pd(II) hydrin–oxidizer intermediate complexes or on
IV
Pd(IV) hydrins, Pd –CH –CH(OH)–CH –R
.
2
2
Cl–], mol/l
k
×
10 , s
3
–1
[
It is likely that intramolecular reductive eliminaꢀ
tion yielding olefin oxides will be observed for pallaꢀ
dium and mercury derivatives if М–RO species are
0.00
0.20
0.40
0.72
1.00
1.50
2.00
2.50
3.00
0.61
0.82
1.15
1.06
0.68
0.63
0.32
0.40
0.25
±
±
±
±
±
±
±
±
±
0.01
0.01
0.01
0.03
0.01
0.02
0.01
0.01
0.01
–
generated under conditions such that the leaving
0
0
group Pd or Hg is additionally stabilized, e.g., by
conducting the reaction in the presence of palladium
black or mercury metal.
EXPERIMENTAL
The decomposition kinetics of platinum(IV) comꢀ
plexes was studied spectrophotometrically (SFꢀ16
spectrophotometer) by monitoring the decreasing
Note: The solvent is water; T = 298 K; pH 8.15; [NaCl + NaClO ] =
4
absorbance of their solution at a wavelength of
λ
=
3
.0 mol/l.
262 nm, which corresponds absorption maximum for
the platinum(IV) complexes [12]. When the platinum
complex concentration is far below the concentration
of the Сl ligand and when the equilibrium between
different platinum(IV) species is established rapidly,
absorbance at the given wavelength is directly proporꢀ
tional to the total concentration of the platinum(IV)
complexes. Decomposition rate constants were
ing intramolecular nucleophilic attack by the hydride
ion in the transition state:
–
#
H
.
Pd CH2 CR OH
derived from the slopes of ln(
D
–
D
)–
t
straight lines.
∞
When this reaction is conducted in excess oxidizer
Ethyleneplatinahydrin complexes
synthesized via earlier reported procedures (see [9]
and [5], respectively) by reacting the corresponding
1
and were
2
(
CuCl2), the formation of carbonyl compounds is
accompanied by the formation of chlorohydrins [18,
9]. The relevant data available from the literature
1
α
ꢀoxide with K PtCl4 solutions in aqueous HClO4.
2
[
20–22] are quite consistent with the mechanisms
The products were isolated as comparatively lowꢀsoluꢀ
ble cesium derivatives, Cs Pt(R)Cl5, by adding a satuꢀ
2
rated CsCl solution to the redaction mixture.
k
, s−
1
Buffer solutions were prepared by dissolving
Na PO4 and H PO4 in bidistilled water. The chloride
ion concentration was varied using NaClO4 (reagent
grade). Ionic strength was maintained with NaClO4
0
.0012
.0010
.0008
.0006
.0004
.0002
3
3
0
0
0
0
0
(
reagent grade). The pH of the resulting solutions was
measured with an Iꢀ02 pH meter.
ACKNOWLEDGMENTS
The authors are grateful to Prof. O.N. Temkin for
reading the manuscript and making helpful remarks
and suggestions.
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1.0
1.5
2.0
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[
3.0
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No. 1
2010