Mechanism of ketone and alcohol formations from alkenes and alkynes on the head-to-head 2-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex

Article abstract of DOI:10.1021/ja020953s

Reactions of the head-to-head 2-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex having nonequivalent two platinum atoms, Pt(N₂O₂) and Pt(N₄), with p-styrenesulfonate, 2-methyl-2-propene-1-sulfonate, 4-penten-1-ol, and 4-pentyn-ol were studied kinetically. Under the pseudo first-order reaction conditions that the concentration of the Pt^III dinuclear complex is much smaller than that of olefin, a consecutive basically four-step reaction was observed: the olefin π-coordinates preferentially to the Pt(N₂O₂) in the first step (step 1), followed by the second π-coordination of another olefin molecule to the Pt(N₄) (step 2). In the next step (step 3), the nucleophilic attack of water to the coordinated olefin triggers the π-σ bond conversion on the Pt(N₂O₂), and the second π-bonding olefin molecule on the Pt(N₄) is released. Finally, reductive elimination occurs to the alkyl group on the Pt(N₂O₂) to produce the alkyl compound (step 4). The first water substitution with olefin (step 1) occurs to the diaqua and aquahydroxo forms of the complex, whereas the second substitution (step 2) proceeds either on the coordinated OH-on the Pt(N₄) (path a) or on the coordinatively unsaturated five-coordinate intermediate of the Pt(N₄) (path b), in addition to the common substitution of H₂O (path c). The reactions of p-styrenesulfonate and 2-methyl-2-propene-1-sulfonate proceed through paths b and c, whereas the reactions of 4-penten-1-ol and 4-pentyn-1-ol proceed through paths a and c. This difference reflects the difference of the trans effect and/or trans influence of the π-coordinated olefins on the Pt(N₂O₂). The pentacoordinate state in path b is employed only by the sulfo-olefins, because these exert stronger trans effect. The steps 3 and 4 reflect the effect of the axial alkyl ligand (R) on the charge localization R-Pt^IV(N₂O₂)-Pt^II (N₄))and delocalization (R-Pt^III(N₂O₂)-Pt^III (N₄)(N₄)-OH₂); when R is p-styrenesulfonate having an electron withdrawing group, the charge localization in the dimer is less pronounced and the water molecule on the Pt(N₄) atom is retained (RPt^III(N₂O₂)-Pt^III (N₄)-OH₂) in the intermediate state. In both routes, the alkyl group undergoes nucleophilic attack of water, and the oxidized products are released via reductive elimination.

Full text of DOI:10.1021/ja020953s

Published on Web 03/04/2003
Mechanism of Ketone and Alcohol Formations from Alkenes
and Alkynes on the Head-to-head 2-Pyridonato-Bridged
cis-Diammineplatinum(III) Dinuclear Complex
†
†
†
†
Nami Saeki, Noriko Nakamura, Takayuki Ishibashi, Moritatsu Arime,
†
,†,‡
,†,§
Hideo Sekiya, Koji Ishihara,* and Kazuko Matsumoto*
Contribution from the Department of Chemistry, School of Science and Engineering, Waseda
UniVersity, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan, Materials Research Laboratory for
Bioscience and Photonics, Graduate School of Science and Engineering, Waseda UniVersity,
Okubo, Shinjuku-ku, Tokyo 169-8555, Japan, and AdVanced Research Institute for Science and
Engineering, Waseda UniVersity, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
Received July 11, 2002; E-mail: ishi3719@waseda.jp
Abstract: Reactions of the head-to-head 2-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex
2 2 4
having nonequivalent two platinum atoms, Pt(N O ) and Pt(N ), with p-styrenesulfonate, 2-methyl-2-propene-
1
-sulfonate, 4-penten-1-ol, and 4-pentyn-1-ol were studied kinetically. Under the pseudo first-order reaction
III
conditions that the concentration of the Pt dinuclear complex is much smaller than that of olefin, a
consecutive basically four-step reaction was observed: the olefin π-coordinates preferentially to the Pt-
(N O
2
2
) in the first step (step 1), followed by the second π-coordination of another olefin molecule to the
) (step 2). In the next step (step 3), the nucleophilic attack of water to the coordinated olefin triggers
the π-σ bond conversion on the Pt(N ), and the second π-bonding olefin molecule on the Pt(N ) is
released. Finally, reductive elimination occurs to the alkyl group on the Pt(N ) to produce the alkyl
Pt(N
4
2
O
2
4
2 2
O
compound (step 4). The first water substitution with olefin (step 1) occurs to the diaqua and aquahydroxo
-
forms of the complex, whereas the second substitution (step 2) proceeds either on the coordinated OH
on the Pt(N
4
) (path a) or on the coordinatively unsaturated five-coordinate intermediate of the Pt(N
4
) (path
2
b), in addition to the common substitution of H O (path c). The reactions of p-styrenesulfonate and 2-methyl-
2
1
-propene-1-sulfonate proceed through paths b and c, whereas the reactions of 4-penten-1-ol and 4-pentyn-
-ol proceed through paths a and c. This difference reflects the difference of the trans effect and/or trans
2 2
influence of the π-coordinated olefins on the Pt(N O ). The pentacoordinate state in path b is employed
only by the sulfo-olefins, because these exert stronger trans effect. The steps 3 and 4 reflect the effect of
IV
)-Pt (N
)) and delocalization (R-PtIII-
II
the axial alkyl ligand (R) on the charge localization (R-Pt (N
2
O
2
4
III
(
N
2
O
2
)-Pt (N
4
)-OH
2
); when R is p-styrenesulfonate having an electron withdrawing group, the charge
) atom is retained (R-
) in the intermediate state. In both routes, the alkyl group undergoes nucleophilic
localization in the dimer is less pronounced and the water molecule on the Pt(N
4
III
III
Pt (N
2
O
2
)-Pt (N
4
)-OH
2
attack of water, and the oxidized products are released via reductive elimination.
Introduction
Scheme 1
III
The head-to-head (HH) Pt dinuclear complexes [Pt2(NH3)4-
n+
-
-
-
(
NkO)2L2] (NkO ) amidate, L ) NO3 , NO2 , H2O, or Cl )
derived from “platinum blues” have been found to act as
catalysts for the oxidation of olefins to aldehydes, ketones,
epoxides, and R,â-diols (Scheme 1).1 In these reactions, it
was suggested that olefins, which initially π-coordinates to the
-4
III
axial site of the Pt dinuclear complexes, are converted to the
corresponding alkyl σ-complexes by nucleophilic attack of water
†
Department of Chemistry.
Materials Research Laboratory.
Advanced Research Institute for Science and Engineering.
‡
§
(
1) Matsumoto, K.; Mizuno, K.; Abe, T.; Kinoshita, J.; Shimura, H. Chem.
Lett. 1994, 1325.
(
2) Matsumoto, K.; Nagai, Y.; Matsunami, J.; Mizuno, K.; Abe, T.; Somazawa,
R.; Kinoshita, J.; Shimura, H. J. Am. Chem. Soc. 1998, 120, 2900.
3) Lin, Y.-S.; Takeda, S.; Matsumoto, K. Organometallics 1999, 18, 4897.
4) Matsumoto, K.; Sakai, K. AdV. Inorg. Chem. 1999, 49, 375.
to the coordinated olefins. Finally the oxidized products are
released by reductive elimination. The R-carbon atom bound
to the Pt atom receives the second nucleophilic attack of water
(
(
10.1021/ja020953s CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 3605-3616
9
3605

A R T I C L E S
Saeki et al.
to release diols.3,4 Several alkyl PtIII dinuclear complexes have
been synthesized by the reactions of [Pt2(NH3)4(pival-
amidato)2L2] with olefins such as 4-penten-1-ol and ethylene
our recent study revealed that PtIII dinuclear complexes catalyze
olefin oxidation to a certain extent, and to our surprise,
n+
III
â-hydroxyalkyl-Pt dinuclear complexes were isolated and the
2
,3
glycol vinyl ether, and their crystal structures have been
crystal structures were solved. These complexes correspond
to the intermediates of the olefin oxidation on Pt(III), and are
very significant considering the fact that analogous hydroxy-
2
determined by X-ray crystallography. The two platinum atoms
in the HH form are not equivalent: one is coordinated by two
amidate oxygen and two ammine nitrogen atoms (Pt(N2O2)),
and the other is coordinated by two ammine nitrogen and two
amidate nitrogen atoms (Pt(N4)). The alkyl groups in the alkyl
II
alkyl-Pt complexes cannot be isolated.
In this paper, we report the first kinetic and mechanistic study
III
on the reactions of the Pt dinuclear complex [Pt (NH ) (R-
2
3 4
III
4+
Pt complexes mentioned above and the halide ions in the
2 2 2
pyridonato) (H O) ] with olefins and alkyne. The study is
focused on how the dinuclear Pt complex utilizes its Pt(II)
and Pt(IV) character at each step of the reaction to stabilize the
III
monohalo complexes always bind to the Pt(N2O2), which means
that the Pt(N2O2) atom would have higher reactivity to olefins
and halides. Thus, it seems probable that the Pt(N2O2) plays an
III
alkyl Pt complexes. The present study clearly shows the
III
III
III
important role in the catalytic reactions of the dinuclear Pt
conversion of the π-coordinated olefin-Pt to σ-alkyl-Pt on
nucleophilic attack of water. The kinetic data are significant
considering the relevance of the reaction to Wacker process.
complexes. This is an important feature, especially if we think
III
III
of the novel dinuclear structure having a Pt -Pt bond, through
which the actual electron density of the two platinum atoms is
adjusted, depending on the axial ligand.2
Experimental Section
Reagents. cis-[Pt(NH
Kikinzoku Kogyo K. K., Tokyo) according to the Dhara’s method.
3 2 2 2 4
) Cl ] was prepared from K [PtCl ] (Tanaka
2
1
Reagent grade 2-pyridone (Kanto Chemical Co. Inc., Tokyo) was
recrystallized once from benzene. Sodium p-styrenesulfonate (Tokyo
Chemical Industry Co. Ltd.) and 5-Norbornene-2,2-dimethanol (Aldrich)
were used without further purification. 4-Penten-1-ol (Tokyo Chemical
Industry Co. Ltd.) and 4-pentyn-1-ol (Aldrich) were purified once by
vacuum distillation. Sodium 2-methyl-2-propene-1-sulfonate (Aldrich)
was recrystallized once from water. Perchloric acid (60% UGR for trace
analysis, Kanto) was used as received. Sodium perchlorate was prepared
and purified according to the literature.22 Head-to-head [(H
Pt(µ-C NO) Pt(NH (NO )](NO ‚2H O (HH complex) was pre-
pared according to the literature.
2 3 2
O)(NH ) -
5
H
4
2
)
3 2
3
3
)
3
2
23
Measurements. Ionic strength was maintained at 2.00 M with
perchloric acid and sodium perchlorate. All the sample solutions were
prepared by using twice distilled water just before measurement.
Spectrophotometric measurements were performed with a rapid-scan/
stopped-flow spectrophotometer USP-500 (Unisoku Scientific Instru-
ments, Osaka) and spectrophotometers UV-160A (Shimadzu, Kyoto),
UV-2200 (Shimadzu), and UV-2400 (Shimadzu). Rate constants were
measured by monitoring the absorbance change at 350 nm for the
reaction of the HH complex with sodium p-styrenesulfonate, 265 nm
for the reactions with 4-penten-1-ol and sodium 2-methyl-2-propene-
sulfonate, and 268 nm for the reaction with 4-pentyn-1-ol, as a function
of time after mixing the solutions of the complex and the ligands. The
solution of 4-penten-1-ol was prepared with 10% (v/v) ethanol because
of the insolubility of the ligand in water, whereas a 100% aqueous
solution was prepared for the HH complex, since the complex is slowly
reduced by alcohols. Therefore, the final reaction solution contained
5% ethanol after mixing. Under the conditions that the concentrations
It is well-known that Pt(II) complexes react with olefins to
5
form π-complexes such as Zeise’s salt K[PtCl3(C2H4)]‚H2O,
and their reactivities are also known.6-11 In contrast, Pt(IV)
usually does not react with olefins, and only an exceptional Pt-
(
IV) complex is known to react with olefin. The reactivity of
12,13
II
IV
the complex is not known.
The role of Pt -olefin and Pt -
alkyl intermediates are emphasized as the key compounds to
understand the mechanistic aspects of the oxidative function-
alization of ethane and ethanol by Pt(II) salts in aqueous
medium.1
4-16
However, no definite conclusion has been drawn
about the mechanism, and the kinetics data are reported only
to the ethene exchange kinetics of Pt(II) chloro ethene π-com-
plexes.7
,17
As for Pt(III) complexes, the knowledge of the reactivity is
much more limited compared to Pt(II) and Pt(IV).18-20 However,
L
of the ligands (C ) are in large excess over that of the complex (CHH),
(
(
(
(
(
5) Zeise, W. C. Magn. Pharm. 1830, 35, 105.
the reaction was found to consist of several consecutive first-order steps.
Kinetic measurements for the p-styrenesulfonate system were performed
also under the conditions, CHH . C .
L
The absorbance change for the 2-methyl-2-propene-1-sulfonate
system was similar to that for the p-styrenesulfonate, i.e., a four-step
reaction was observed, but the absorbance changes for the steps 1 and
6) Cramer, R. Inorg. Chem. 1965, 4, 445.
7) Olsson, A.; Kofod, P. Inorg. Chem. 1992, 31, 183.
8) Leden, I.; Chatt, J. J. Chem. Soc. 1955, 2936.
9) Lokken, S. J.; Martin, D. S. Inorg. Chem. 1963, 2, 562.
(
(
(
(
10) McMane, D. G.; Martin, D. S., Jr. Inorg. Chem. 1968, 7, 1169.
11) Chatt, J.; Williams, A. A. J. Chem. Soc. 1951, 3061.
12) Howard, W. A.; Bergman, R. G. Polyhedron 1998, 17, 803.
13) Persons, E. J.; Larsen, R. D.; Jennings, P. W. J. Am. Chem. Soc. 1985,
3
were too small to calculate the reliable rate constants. For the
1
07, 1793.
(
(
(
(
14) Hutson, A. C.; Lin, M.; Basickes, N.; Sen, A. J. Organomet. Chem. 1995,
4-penten-1-ol and 4-pentyn-1-ol systems, the initial two steps were
succeeded by a very slow reaction which could not be analyzed
5
04, 69.
15) Fanizzi, F. P.; Intini, F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.;
Tiripicchio, A. J. Chem. Soc., Dalton Trans. 1991, 1007.
16) Fanizzi, F. P.; Maresca, L.; Pacifico, C.; Natile, G.; Lanfranchi, M.;
Tiripicchio, A. Eur. J. Inorg. Chem. 1999, 1351.
(19) Saeki, N.; Hirano, Y.; Sasamoto, Y.; Sato, I.; Toshida, T.; Ito, S.; Nakamura,
N.; Ishihara, K.; Matsumoto, K. Eur. J. Inorg. Chem. 2001, 2001, 2081.
(20) El-Mehdawi, R.; Bryan, S. A.; Roundhill, D. M. J. Am. Chem. Soc. 1985,
107, 6282.
(21) Dhara, S. C. Indian J. Chem. 1970, 8, 193.
(22) Funahashi, S.; Haraguchi, K.; Tanaka, M. Inorg. Chem. 1977, 16, 1349.
(23) Hollis, L. S.; Lippard, S. J. Inorg. Chem. 1983, 22, 2605.
17) (a) Plutino, M. R.; Otto, S.; Roodt, A.; Elding, L. I. Inorg. Chem. 1999,
3
2
8, 1233. (b) Otto, S.; Elding, L. I. J. Chem. Soc., Dalton Trans. 2002,
354.
(
18) Saeki, N.; Hirano, Y.; Sasamoto, Y.; Sato, I.; Toshida, T.; Ito, S.; Nakamura,
N.; Ishihara, K.; Matsumoto, K. Bull. Chem. Soc. Jpn. 2001, 74, 861.
3606 J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003

Ketone and Alcohol Formations
A R T I C L E S
Figure 2. (a) Change of the UV-vis spectrum for steps 3 and 4 in Scheme
-
5
3
2
, taken every 9 min at I ) 2.0 M and 25 °C. CHH ) 2 × 10 M, CL )
-3 +
.50 × 10 M, and [H ] ) 0.0318 M. (b) The absorbance change with
time at 350 nm in Figure 1b. The inset shows the semilog plot of the data.
Figure 1. (a) Change of the UV-vis spectrum for steps 1 and 2 in Scheme
-
5
3
2
, taken every 100 ms at I ) 2.0 M and 25 °C. CHH ) 2 × 10 M, CL )
-3 +
.42 × 10 M, and [H ] ) 0.0318 M. (b) The absorbance change with
four steps; the fastest step (step 1), two intermediate steps (steps
time at 350 nm in Figure 1a. The inset shows the semilog plot of the data.
2
and 3), and the slowest step (step 4). The insets in Figures 1b
and 2b show that the reaction consists of four consecutive first-
order reactions. The pseudo first-order rate constants, kobs1, kobs2,
and kobs3 for steps 1-3, respectively, were dependent on the
excess ligand concentration, which suggests that steps 1-3 are
intermolecular processes, and kobs1 and kobs3 were also dependent
kinetically. The reaction with 5-norbornene-2,2-dimethanol was also
studied, but the H NMR spectra were too complicated to identify the
product, and so no further analysis was performed.
The 1H NMR spectra were recorded on a JEOL Lambda 270
spectrometer. The chemical-shift was referenced to TMA (tetramethy-
lammonium perchlorate, 3.190 ppm to TMS).
1
+
on [H ], but k was not, as shown in Figure 3. It is noteworthy
obs2
that kobs3 decreases drastically with increasing CL. On the other
hand, the kobs4 was not dependent either on CL or [H ] (Figure
Results and Discussion
+
Reaction with Sodium p-Styrenesulfonate. The reaction
with sodium p-styrenesulfonate gives p-sulfonatoacetophenone
3
).
We reported previously the axial ligand substitutions with
(p-acetylbenzenesulfonate) and the Pt(II) dimer complex as
halides on the same HH complex as used in this study, and
proposed the mechanisms depicted in Scheme 2 for the observed
shown in eq 1. Changes in UV-vis absorption spectra with
time under the pseudo first-order condition, CHH , CL, are
shown in Figures 1a and 2a. Figures 1b and 2b show the time
course change in absorbance at 350 nm in Figures 1a and 2a,
18
consecutive two-step reactions. In this scheme, the first ligand
substitution reaction with halide ion occurs to the Pt(N2O2) of
the HH complex (the left and upper step in Scheme 2), and the
first deprotonation of the aqua ligand occurs to the H2O-Pt-
(N4) exclusively (the left equilibrium in Scheme 2), and the
second substitution takes place on the Pt(N4) (the right and upper
steps in Scheme 2). The reactive species having an unsaturated
coordination site is postulated for the second ligand substitution
reaction (Scheme 2). The rate constants of steps 1 and 2 for the
present p-styrenesulfonate system (Figure 3) varied with CL and
+
[
H ] in the same way as those for the first (kobs1) and second
kobs2) halide substitutions on the HH complex, respectively. The
(
close similarity between the present system and the halide
system led us to consider similar reaction steps in the present
system (see steps 1 and 2 in Scheme 3). In Scheme 3, the first
π-coordination of the olefin occurs to the Pt(N2O2) (step 1) and
respectively. Both indicate that the present reaction consists of
J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003 3607

A R T I C L E S
Saeki et al.
Figure 3. Dependence of the observed rate constants on CL for the reaction
of the HH dimer with p-styrenesulfonate at I ) 2.0 M. (a) For step 1, at 25
+
°
(
(
0
C and [H ]/M ) 0.0101 (O), 0.0201 (4). 0.121 (0), 0.201 (]), and 0.403
+
b). (b) For step 2, at 25 °C and [H ]/M ) 0.201 (O), 0.403 (4). 0.805
+
0), and 1.21 (]). (c) For step 3, at 25 °C and [H ]/M ) 0.03181 (O),
+
.153 (4). 0.403 (0), and 0.805 (]). (d) For step 4, at 35 °C and [H ]/M
Figure 4. Dependence of kf1 (a) and kd1 (b) (for step 1 in Figure 3a) on
)
0.206 (O), 0.411 (4). 0.822 (0), and 1.64 (]).
+
[H ] for the reaction with p-styrenesulfonate.
Scheme 2
constants of the forward and the reverse reactions of step 1 in
Scheme 3, respectively
#
1
k K
h1
k1 +
+
[
H ]
k )
(4)
(5)
f1
Kh1
1
+
+
[
H ]
#
k K
-
1
h2
k-1
+
+
+
#
-1
[H ]
k
K
+
h2
k )
Z k-1 +
d1
Kh2
[H ]
1
+
[H ]
The pH dependencies of kf1 and kd1 are shown in Figure 4. In
+
the denominator of eq 5, Kh2/[H ] is negligible compared to
the second π-coordination of the olefin occurs to the Pt(N4)
unity under the present conditions, since the plot in Figure 4b
is linear within the experimental errors.
(step 2).
The experimental data in Figure 3a give eq 2 for kobs1, and
eq 3 is derived from the reaction paths in Scheme 3
On the other hand, kobs2 for step 2 in Scheme 3 is expressed
with eq 6, when the steady-state approximation is applied to
the coordinatively unsaturated complex (step 2 in Scheme 3)
1
8
as was the halide case
kobs1 ) k C + k
d1
(2)
(3)
f1
L
#
1
#
k3[L]
k-3
k K
k K
h1
-1 h2
kobs2 ) k [L] +
(6)
k1 +
k-1
+
2
+
+
[
H ]
[H ]
Kh2
[
L] +
kobs1
)
[L] +
k4
Kh1
1
+
1 +
+
+
[
H ]
[H ]
The rate constants for steps 1 and 2 in this system are
comparable to those in the halide system as shown in Table 1,
which supports the validity of the proposed mechanism in
Scheme 3.
Equations 4 and 5 are obtained from eqs 2 and 3, under the
present condition CHH , CL. The values kf1 and kd1 are the rate
3608 J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003

Ketone and Alcohol Formations
A R T I C L E S
Scheme 3. Reaction Mechanism for the Reaction of the HH Dimer with Sodium p-Styrenesulfonate
Table 1. Rate and Equilibrium Constants for the Reactions of the HH 2-Pyridonato-Bridged Diammineplatinum(III) Dinuclear Complex with
Sodium p-Styrenesulfonate, Sodium 2-Methyl-2-propene-1-sulfonate, 4-Penten-1-ol, 4-Pentyn-1-ol, and Sodium Halides at 25 °C and I ) 2.0
M
p-styrenesulfonatea
2-methyl-2-propene-1-sulfonate
4-penten-1-ol
4-pentyn-1-ol
Cl- a
Br- a
3
-1
) 3.1 × 10 M s-1
3
-1
) 7.2 × 10 M s-1
3
-1
step 1
k
k
k
k
1
) 1.3 × 10 M s-1
k
k
1
1
k
k
1
1
#
3
-1 -1
#
5
-1 -1
#
5
-1 -1
1
) 4.4 × 10 M
s
) 4.6 × 10 M
s
) 2.4 × 10 M
s
-1 ) 0.90 s-
1
#
h2 ) 8.9 × 10 M s-1
-2
-1
K
-4
K
h2 ) 3.8 × 10
M
) 31 ( 3 M-
1
s
-1
k
k
k
) 26 ( 9 M-1
s
-1
k
k
K
k
k
k
) 29 M-1 s-1
k
k
K
k
k
k
) 29 M-1 s-1
k
k
k
) 550 ( 10 M s-1
) 3.1 ( 0.3 s
-1
k
k
k
) 450 ( 36 M s-1
) 4.4 ( 0.8 s
-1
step 2
step 3
k
k
k
2
2
3
2
2
2
2
2
2
-
1
-1
#
2
-1
#
2
-1
-1
-1
3
) 0.64 ( 0.06 s
) 0.23 ( 0.12 s
K
h2 ) 3.9 × 10 s
K
h2 ) 1.2 × 10 s
3
3
-3
-3
-3
-2
-3
-3
-3/k
4
) 4.0 × 10
M
-3/k
4
) (2.2 ( 1.4) × 10
M
h2 ) 6.3 × 10
M
h2 ) 1.0 × 10
M
-3/k
4
) 3.7 × 10
M
-3/k
4
) 3.7 × 10
M
#
) 6.2 × 10 M s-1
4
-1
#
) 1.2 × 10 M s-1
4
-1
2
2
h3 ) 7.1 × 10 M s-1
-4
-3 ) 44 M
K
-1 -1
s
-3 ) 0.070 M
-1 -1
s
k
5
K
h3 ) 8.4 × 10 Ms-1
-3
KKh3 ) 7.3 × 10 Ms
-3
-1
-
-
-
-
K
h3 ) 0.21 M
-
3
3
k
-6/k
6
) 0.39 M- s-1
1
K
h3 ) 7.2 × 10
-3
M
KKh3+Kh3 ) 2.4 × 10
-3
M
k
k
k
k
k
-5
2
-1
-5/k
6
) 7.5 × 10 M
-
3
-1
5
) 3.7 × 10
s
-
4
-1
-6 ) 5.4 × 10
s
-
3
-1 b
) 1.0 × 10 s-1
-4
step 4
7
) 3.4 × 10
s
k
6
-
-
a
Kh1 ) 0.0195 M (25 °C, I ) 2.0 M) was used.18 At 35 °C.
b
1
Figure 5 shows the change of the H NMR spectrum after
complex and oxidation of the alcoholyl group in the σ-complex
the reaction was started under the condition, CHH < CL. At t )
to free ketone takes place at step 4.
The observed rate constant for step 3, kobs3, varied with [H+]
40 min, only the peaks corresponding to the σ-complex (1) were
observed, whereas at t ) 46 h, the peaks of the free ketone and
and decreased drastically with increasing CL (Figure 3c). The
π-bond a σ-bond conversion must be involved in step 3,
because no other reaction is expected to release proton. The
π-σ conversion seems to take place on the Pt(N2O2), because
all the alkyl groups in the alkyl-Pt(III)2 complexes reported so
II
2+
HH [Pt 2(NH3)4(R-pyridonato)2] were observed. The latter is
III
4+
the reduction product of the HH [Pt 2(NH3)4(R-pyridonato)2] .
1
The formation of the σ-complex was confirmed by the H NMR
spectrum because the isolation of the σ-complex was unsuc-
cessful. The coupling constants among Ha, Hb, and Hc, and the
2,24,25
195
far are ligated to the Pt(N2O2).
Our previous Pt NMR
2
chemical shifts depicted in Figure SI-1 are J(Ha - Hb) ) 9
studies of the alkyl-Pt(III)2 complexes show that the electrons
3
3
Hz, J(Hb - Hc) ) 9 Hz, and J(Ha - Hc) < 2 Hz, and δ(Ha)
4.67 ppm, δ(Hb) ) 4.24 ppm, and δ(Hc) ) 5.04 ppm, which
in the Pt-Pt bonds are localized and the real electronic state is
)
II
IV
24
close to Pt (N4)-Pt (N2O2). Therefore, the olefin on the Pt-
confirm the formation of the σ-complex. The peak height change
with time in Figure 5 reasonably corresponds to steps 3 and 4
in Figure 2, i.e., the σ-complex 1 is formed at step 3, and
(24) Matsumoto, K.; Matsunami, J.; Mizuno, K.; Uemura, H. J. Am. Chem. Soc.
1
996, 118, 8959.
(
25) Sakai, K.; Tanaka, Y.; Tsuchiya, Y.; Hirata, K.; Tsubomura, T.; Iijima, S.;
III
II
reduction of the Pt dinuclear complex to the Pt dinuclear
Bhattacharjee, A. J. Am. Chem. Soc. 1998, 120, 8366.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003 3609

A R T I C L E S
Saeki et al.
kfK
kobs3
)
+ k [L]
(8)
d
+
[
H ] + K
The reactive intermediate having the unsaturated coordination
site at step 3 in Scheme 3 is necessary to explain the drastic
decrease of kobs3 with increasing CL. Equation 9 is
k5Kh3
k k [L]
-5
-6
k6
+
+
[
H ] + K
h3
kobs3
)
(9)
k [L]
-
5
+
1
k6
derived when the steady-state approximation is applied to the
intermediate at step 3 in Scheme 3. This equation agrees
excellently with the experimental data as shown in Figure 3c.
+
The kobs4 is independent of both CL and [H ] as shown in
Figure 3d. This fact coincides with step 4 in Scheme 3, and
therefore eq 10 holds. At step 4, the intramolecular reaction
III
II
proceeds and the Pt complex is reduced to the HH Pt
dinuclear complex to produce the ketone
1
Figure 5. Change of the H NMR spectrum with time at 5 °C, after mixing
-
3
the HH dimer and the p-styrenesulfonate solutions. CHH ) 4.3 × 10 M,
kobs4 ) k7
(10)
-
2
CL ) 2.2 × 10 M, and [DClO4] ) 0.5 M. The chemical shift was
referenced to TMA (tetramethylammonium perchlorate).
The best-fit curves in Figure 3 that were drawn by applying
a nonlinear least-squares fitting of eqs 3, 6, 9, and 10 to the
experimental data, agree well with the experimental results. All
the rate constants in these equations were calculated by using
(
N2O2) is more easily subjected to nucleophilic attack by H2O.
If step 3 involves a simple substitution reaction preceded by
the preequilibrium as shown in eq 7, which is actually the case
for the 4-penten-1-ol system mentioned later, kobs3 is given by
eq 8. However, eq 8 is inconsistent with the experimental result.
-
2
Kh1 ) 1.95 × 10
M (25 °C, I ) 2.0 M) determined
1
4
previously, and are tabulated in Table 1.
Scheme 4. Reaction Mechanism for the Reaction of the HH Dimer with Sodium 2-Methyl-2-propene-1-sulfonate
3610 J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003

Ketone and Alcohol Formations
A R T I C L E S
Figure 7. Dependence of the observed rate constants on CL for the reaction
of the HH dimer with 2-methyl-2-propene-1-sulfonate at I ) 2.0 M and 25
+
+
°
C. (a) For step 2, [H ] ) 0.6-1.4 M. (b) For step 3, [H ]/M ) 0.617 (0)
+
and 0.411 (O). (c) For step 4, [H ]/M ) 0.0514 (O), 0.103 (0), 0.206 (4),
1
Figure 6. Change of the H NMR spectrum with time at room temperature
and 0.411 (b).
after mixing the HH dimer and the 2-methyl-2-propene-1-sulfonate solutions.
CL/CHH ) 5 and [DClO4] ) 0.03 M. (a) At t ) 0.5 h, (b) at t ) 12 h, and
reaction, but the absorbance change for steps 1 and 3 was too
small to calculate the reliable rate constants, as shown in Figure
SI-2. Figure 6 shows the time dependent H NMR spectra of
(
c) at t ) 30 h. The spectrum at lower field is 10-fold magnified.
1
There seems to be another reaction path (k2* path in Scheme
3
) which is not through the di-π-coordinated complex, under
the system. The spectrum at t ) 0.5 h mainly consists of the
peaks of the free ligand and the σ-complex 2. The peaks of the
σ-complex diminished with time, while the new peaks of the
the condition CL , CPt2, since the formation of the σ-complex
and the ketone was also observed in the H NMR spectrum
1
II
under such condition, and the UV-vis spectral change was very
slow, showing the decrease of the mono-π-complex.
Reaction with Sodium 2-Methyl-2-propene-1-sulfonate.
The reaction with 2-methyl-2-propene-1-sulfonate gave 2-meth-
yl-2,3-propanediol-1-sulfonate as the final product (eq 11). The
reason for not having the axial ligand in the intermediate
complex (2) in eq 11 is given later in the section of “Effect of
the Axial Ligands on the Kinetics”.
1,2-diol and the HH Pt dinuclear complex increased at the same
+
time. As Figure 7a shows, kobs2 was invariable with [H ] within
the experimental error, but increased along a curved line with
increasing C , which is very similar to step 2 in the p-
L
styrenesulfonate system. The kobs3 clearly increased with CL,
whereas the pH dependence of kobs3 was not obvious (Figure
7b), since the values are less reliable due to the small spectral
change mentioned above. This fact is a striking contrast to step
3
in the p-styrenesulfonate system (Figure 3c). On the other
+
hand, kobs4 for step 4 did not depend on either [H ] or CL (Figure
7
c), which is the same as for the p-styrenesulfonate system.
The four-step reaction mechanism of 2-methyl-2-propene-1-
sulfonate can be constructed as in Scheme 4, on the basis of
Scheme 3. Equations 6 and 12 can be derived for steps 2 and
4
in Scheme 4
kobs4 ) k6
(12)
Reaction with 4-Penten-1-ol. The reaction with 4-penten-
-ol gave the tetrahydrofurfuryl complex as shown in eq 13.
A two-step reaction was observed under the pseudo first-
1
The absorbance change for this system was similar to that of
order conditions (CHH , CL) as shown in Figure SI-3. The faster
the p-styrenesulfonate system, showing a similar four-step
step and the slower step were exactly first order, and no further
J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003 3611

A R T I C L E S
Saeki et al.
ol, the bridging pyridonato ligands, and the tetrahydrofurfuryl
protons of the σ-complex, and some unknown compound(s)
were observed. The structure of the σ-complex would be
analogous to that of the previously reported pivalamidato-
bridged analogue whose structure was elucidated by X-ray
2
crystallography. As the reaction proceeded, the σ-complex
precipitated as fine crystals, whose elemental analysis agreed
with the expected structure (Anal. Calcd for [Pt2(NH3)4(C5H4-
NO)2(C5H9O)](NO3)2(ClO4): C, 18.86; H, 3.06; N, 11.73.
1
Found: C, 18.63; H, 2.59; N, 11.40.). The H NMR spectra of
reaction was observed for at least a couple of hours, but
extremely slow and slight spectral change was observed in
several hours, which corresponds to the decomposition of the
the σ-complex precipitate dissolved in 1 M DClO4 and measured
at t ) 45 min and 5 h after dissolution are depicted in Figure
8b and 8c, respectively, and show that the σ-complex changes
1
II
σ-complex 3. Figure 8a shows the H NMR spectrum measured
very slowly to unknown product(s), and that no Pt dinuclear
at t ) 30 min after the reaction was started. Free 4-penten-1-
complex is produced [the bridging R-pyridonato peaks of the
1
-2
-2
Figure 8. Change of the H NMR spectrum with time at room temperature. CHH ) 1.2 × 10 M, CL ) 2 × 10 M, and [DClO4] ) 1 M. (a) At t ) 30
min after mixing the HH dimer and the 4-penten-1-ol solutions. The reaction solution gave precipitate of the σ-complex after 30 min, and the following
measurements [(b) and (c)] were done with the DClO4 solution of once isolated precipitate of the σ-complex. (b) at t ) 45 min and (c) at t ) 5 h after the
σ-complex precipitate was dissolved into 1 M DClO4 (see text).
3612 J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003

Ketone and Alcohol Formations
A R T I C L E S
#
+
Figure 9. Dependence of the observed rate constants on CL for the reaction
with 4-penten-1-ol at I ) 2.0 M and 25 °C. (a) For step 2, (b) for step 3.
Figure 10. Dependence of kf2 () k2 + k2 Kh2/[H ], slopes in Figure 9a)
+
+
and kf3 () k3Kh3/[H ], intercepts in Figure 9b) on [H ] for the reaction
+
with 4-penten-1-ol.
[
H ]/M ) 0.0510 (O), 0.102 (4), 0.204 (0), and 0.408 (]).
PtII complex are easily discernible from those of the PtIII
the π-coordinated ligand on the Pt(N4) is liberated. There seems
no water ligand on the Pt(N4) of the tetrahydrofurfuryl complex
even in aqueous solution. Similar structure is found in the
corresponding di-pivalamidato-bridged complex, which has no
ligand on the Pt(N4) in solid state as shown by X-ray
crystallography.
Reaction with 4-Pentyn-1-ol. The reaction gave 1,5-dihy-
droxy-2-pentanone as the final product (eq 16). Much the same
complex (see Figure 6)]. From these facts, the unknown product-
III
(
s) seems to be still Pt alkyl complex(es) and is not released
via reductive elimination.
The substitution reaction on the PtIII dinuclear complexes
usually proceeds consecutively, i.e., the first substitution occurs
to the Pt(N2O2) and the secondary to the Pt(N4) of the head-
to-head complexes.18 Even for the head-to-tail complex having
2
1
9
two equivalent Pt(III) atoms, the reaction is consecutive. If
the faster step for the 4-penten-1-ol system corresponds to step
+
1
in the p-styrenesulfonate system, the plot of (1 + Kh1/[H ])-
+
kf1 vs 1/[H ] must be linear according to eq 4. The result showed
that this is not the case. The kinetic measurement was also
+
performed at lower temperature (15 °C) and higher [H ]. Under
these conditions, step 1 in the p-styrenesulfonate system
becomes slower, but the faster step in the 4-penten-1-ol system
was not resolved into two steps. It seems that the step
corresponding to step 1 in the p-styrenesulfonate system is still
too fast to detect. Consequently, the observed faster step and
the slower step were assigned to steps 2 and 3 in the
p-styrenesulfonate system, respectively. The observed rate
constants kobs2 and kobs3 for the faster (step 2) and slower (step
spectral change was observed for the reaction with 4-pentyn-
1-ol (Figure SI-4). The dependence of kobs2 and kobs3 on CL was
also the same as for the 4-penten-1-ol system as shown in Figure
3
) steps in the 4-penten-1-ol system, increased linearly with
increasing CL, and the slopes of the plot of kobs2 vs CL decreased
+
1
with increasing [H ] (Figure 9). On the other hand, the slope
11. The H NMR spectrum shows that the structure of the
of the plot of kobs3 vs CL was constant, whereas the intercept
decreased with increasing [H ] as shown in Figure 9. Thus,
σ-complex 4 would be similar to that of the pivalamidato-
bridged analogue [(NH3)2Pt(µ-((CH3)3CCONH)2Pt(NH3)2(CH2-
+
3
+
the reaction mechanism of the 4-penten-1-ol system is con-
structed as in Scheme 5 on the basis of Schemes 3 and 4.
According to the mechanism in Scheme 5, kobs2 and kobs3 are
expressed as eqs 14 and 15, respectively, under the conditions,
CO(CH2)3OH)] , whose structure was solved by X-ray crys-
2
6
tallography. Therefore, the reaction mechanism seems to be
analogous to Scheme 5 and is shown in Scheme 6. The rate
constants kobs2 and kobs3 are given by eqs 14 and 17, respectively.
The analysis was reasonably performed as in Figure 12, and
the obtained values are listed in Table 1
+
CHH , CL and [H ] ) 0.05-1.6 M.
The data in Figure 9 were successfully analyzed according
to eqs 14 and 15 as
k3KK
h3
kobs3 ) k + k [L] )
+ k [L]
(17)
+
#
2
#
f3
d3
-3
+
k [H ] + k K
k K
2 h2
2
h2
[H ] + K + KK
h3
h3
kobs2 ) k [L] )
[L] Z k +
[L] (14)
f2
2
+
(
+
)
[
H ] + K
[H ]
h2
Effect of the Axial Ligands on the Kinetics. The axial bond
distances of some HH amidato-bridged dinuclear platinum(III)
complexes are tabulated in Table 2. The Pt(N2O2)-carbon (X1)
distance does not change significantly among complexes 5-7,
whereas the Pt-Pt and the Pt(N4)-X2 distances become longer
k3K
h3
k K
3
h3
kobs3 ) k + k [L] )
+ k [L] Z
+
f3
d3
-3
+
+
[
H ] + K
[H ]
k [L] (15)
h3
-
3
in the order: -CH2COCH3 < -CH2CHO < -CH2CH(CH2)3O,
as the electron density on the carbon atom (in X1) ligated to
the Pt(N2O2) is higher, which is in accordance with the trans
shown in Figure 10. The rate constants in eqs 14 and 15 are
given in Table 1.
At step 2 in Scheme 5, the π-coordinated ligand on the Pt-
(
N2O2) is converted to the σ-coordinated ligand via intramo-
(
26) Lin, Y.-S.; Yamada, J.; Misawa, H.; Matsumoto, K. to be published
lecular cyclization with proton release. After the conversion,
elesewhere.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003 3613

A R T I C L E S
Saeki et al.
Scheme 5. Reaction Mechanism for the Reaction of the HH Dimer with 4-Penten-1-ol
Figure 11. Dependence of the observed rate constants on CL for the reaction
#
+
Figure 12. Dependence of kf2 () k2 + k2 Kh2/[H ], slopes in Figure 11a)
+
with 4-pentyn-1-ol at I ) 2.0 M and 25 °C. (a) For step 2, [H ]/M ) 0.206
+
+
and kf3 () k3KKh3/([H ] + Kh3 + KKh3), intercepts in Figure 11b) on [H ]
+
(
(
O), 0.411 (4), 0.805 (0), and 1.618 (]); (b) for step 3. [H ]/M ) 0.0411
3), 0.103 (O), 0.206 (]), 0.411 (0), and 1.23 (4).
for the reaction with 4-pentyn-1-ol.
influence order of these ligands. In other words, as the trans
influence of X1 is stronger, the electron localization in the
dinuclear complex X1-Pt (N2O2)-Pt (N4)-X2 becomes more
end of the Pt-Pt bond, and the Pt(N4)-X2 becomes longer.
The order of trans influence in complexes 1-4 in Schemes 3-6,
IV
II
-
would be -CH2CH(OH)-p-C6H4SO3 < -CH2C(OH)(CH3)-
-
prominent, i.e., the trans influence of X1 appears on the other
CHSO3 , -CH2CO(CH2)2OH, -CH2CH(CH2)3O. The depen-
3614 J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003

Ketone and Alcohol Formations
A R T I C L E S
Scheme 6. Reaction Mechanism for the Reaction of the HH Dimer with 4-Pentn-1-ol
Table 2. Axial Bond Distances (Å) of the HH Amidato-Bridged cis-Diammineplatinum(III) Dinuclear Complexes
complex
5a
6b
7b
8c
9c
10d
11d
bridging ligand
pivalamidate
pivalamidate
pivalamidate
pivalamidate
pivalamidate
2-pyrrolidonate
2-pyrrolidonate
-CH2CH(CH2)3O
X1
-CH2COCH3
2.095(9)
-CH2CHO
2.121(9)
-CH2COC6H5
2.15(2)
2.676(1)
none
-CH2COCH2COCH
2.114(8)
2.7206(6)
none
Cl-
2.360(4)
2.624(1)
2.27(1)
Cl-
Pt(N2O2)-X1
2.11(1)
2.455(4)
2.6373(7)
2.392(4)
Pt-Pt
2.6892(6)
2.667(7)
2.7106(7)
2.7498(8)
2.7486(8)
2.92(1)
Pt(N4)-X2
X2
-
-
-
-
-
NO3
NO3
NO3
none
none
NO3
Cl
a
ref 24. b ref 2. c ref 23. d ref 25.
III
dence of k3 on CL for step 3 in Scheme 3 (Figure 3c) indicating
that H2O is ligated on the Pt(N4) in complex 1 in aqueous
solution is very much different from those of other systems in
Schemes 4-6 (Figures 7b, 9b, and 11b). This fact suggests that
there is no water molecule on the Pt(N4) in complexes 2-4 in
aqueous solution. In fact, complex 7 having the ligand with the
highest trans influence on the Pt(N2O2), similar to complex 3,
complex of the HH R-pyridonato-bridged Pt dinuclear complex
is much more stable than the complexes 1-4.
Step 2 in Schemes 4 and 5 has two paths, k2 and k3; in the k2
path, direct substitution of the H2O on the Pt(N4) with an olefin
takes place, whereas the k3 path involves a coordinatively
unsaturated intermediate formed by the dissociation of H2O on
the Pt(N4). In contrast, step 2 in Schemes 5 and 6 has two paths,
2
4
has no substantial axial bonding at the Pt(N4) in solid. The
fact that only complex 1 has a water molecule on the Pt(N4) in
aqueous solution can be explained as follows. Introduction of
the electron-withdrawing group such as p-benzenesulfonate into
the σ-coordinated alkyl group on the Pt(N2O2) atom reduces
#
-
k2 and k2 , in which OH on the Pt(N4) is replaced by olefin.
These difference among Schemes 3 - 6 can be explained by
the properties of the π-coordinated olefins on the Pt(N2O2). The
double bond character of p-styrenesulfonate and 2-methyl-2-
propene-1-sulfonate would be reduced, compared with other
olefins, by the electron-withdrawing benzenesulfonate and
sulfonate groups, respectively. As a result, the π-bonds between
the Pt(N2O2) and the olefins for π-styrenesulfonate and 2-meth-
yl-2-propene-1-sulfonate systems would be more effectively
strengthened by accepting more electrons from the filled orbitals
of the Pt(N2O2) atom, which, contrary to the σ-complex
mentioned above, enhances the electron-localization from the
IV
the electron-localization in the dinuclear complex (X1-Pt -
II
(
N2O2)-Pt (N4)-X2) to the less localized dimer having the
III
III
electronic structure X1-Pt (N2O2)-Pt (N4)-X2, and to fa-
cilitate water coordination to the Pt(N4) atom, whereas the
coordination of the substituted alkyl group with more electron-
donating property to the Pt(N2O2) atom effects stronger trans
influence, and invokes higher electron localization in the Pt-
Pt bond, giving the structure X1-Pt (N2O2)-Pt (N4)-X2, and
stabilizes the σ-coordinated alkyl complex. However, stabiliza-
tion of the σ-coordinated alkyl complex is much influenced by
the stability of the Pt-C bond to the nucleophilic attack. In
IV
II
III
III
dinuclear complex (X1-Pt (N2O2)-Pt (N4)-H2O) to the
IV
II
electronic structure X1-Pt (N2O2)-Pt (N4)-H2O, and reduces
the acidity of the water molecule on the Pt(N4) atom. The water
molecule on the Pt(N4) is therefore easily released. In fact, the
1
fact, it was confirmed by H NMR spectrometry that the acetonyl
J. AM. CHEM. SOC.
9
VOL. 125, NO. 12, 2003 3615

A R T I C L E S
Saeki et al.
-
2
Kh2 values in Table 1 decreases from 1.0 × 10 M for 4-pentyn-
the nature of the ligand on the opposite axial site, and vice versa.
The stability of the σ-complex depends on the nature of the
ligand (R) on the Pt(N2O2); the more the ligand is electron
-
4
1
-ol to 3.8 × 10 M for p-styrenesulfonate.
-
The axially coordinated OH on the Pt(N4) atom trans to the
III
π-bonding olefin on the Pt(N2O2) atom can be replaced by the
olefin at step 2 for the reactions with 4-penten-1-ol and
-pentyn-1-ol as shown in Schemes 5 and 6. This is unusual
considering the standard substitution reactions on mononuclear
octahedral metal complexes, however, such a substitution was
also observed in the second step of the substitution reaction
between the HT 2-pyridonato-bridged Pt dimer and Cl . The
comparison of the complexes 10 and 11 in Table 2 shows that
the Pt(N2O2)-X1 as well as the Pt-Pt distances depend on X2
withdrawing, the more the electron in the Pt dimer is localized
IV
II
to R-Pt (N2O2)-Pt (N4), to form more stable σ-complex.
However, the stability is much more influenced by the suscep-
tibility of the σ-complex to nucleophilic attack as exemplified
by the acetonyl complex.
4
Under the conditions CHH > C , where no olefin can
L
III
- 19
coordinate to the Pt(N ) atom, the σ-complex would be formed
4
directly from the mono-π complex.
III
The amidato-bridged Pt dimer complex presents a rare
-
even for the same X1 on the Pt(N2O2). When NO3 on the Pt-
N4) (10) is replaced by Cl (11) having stronger trans influence,
opportunity to study the mechanism of the olefin coordination
and the successive nucleophilic attack. These reaction steps are
closely related to Wacker reaction, and in the latter reaction,
-
(
the Pt(N2O2)-Cl distance becomes longer, and probably the
Pt(N4)-X2 distance also becomes slightly longer, which suggests
that the Pt(N4)-OH distance is longer than the Pt(N4)-OH2
distance with the same X1, in other words, the electron
localization in the HH dimer is less in the hydroxo complex
27
the π to σ conversion is slow. In contrast, the conversion is a
rapid equilibrium process in the present reactions. With regard
to the axial olefin π-coordination, Natile et al. reports novel
olefin coordination to PtCl2(dmphen) (dmphen ) 2,9-dimethyl-
IV
II
than the aqua complex (X1-Pt (N2O2)-Pt (N4)-OH2, X1-
1
,10-phenanthroline) at the axial site to give the five coordinate
III
III
Pt (N2O2)-Pt (N4)-OH). It can also be expressed as the inert
2
15
Pt(η -olefin)Cl2(dmphen). The formation constants for the
reactions with several olefins are in the order of 10 M at 25
IV
III
Pt (N4) in the aqua complex becomes more labile Pt (N4) in
the hydroxo complex. More generally, the Pt(N2O2)-X1, the
Pt(N4)-X2, and the Pt-Pt distances are very dependent on both
X1 and X2. These would be the reason OH on the Pt(N4) atom
is subjected to substitution for the 4-penten-1-ol and 4-pentyn-
2
-1
III
°
C. The formation constant for the reaction of the present Pt
dimer with p-styrenesulfonate in step 1 is calculated to be k1/
-
3
-1
k-1 ) 10 M , which is 1 order of magnitude larger than that
of the above Pt(II) monomer reaction. Although no correspond-
ing rate constant is reported for PtCl2(dmphen), the rate
1
-ol systems.
In contrast to the p-styrenesulfonate system, step 1 was not
III
constants of the Pt dimer complex for the π to σ conversion
observed for the 4-penten-1-ol and 4-pentyn-1-ol systems,
probably because step 1 is too fast to detect, whereas only the
tail of the reaction curve was observed for the 2-methyl-2-
propene-1-sulfonate system. These observations are explained
by the degree of nucleophilicity of the olefins or by the steric
hindrance of the olefins.
by water attack must be considerably higher than for the Pt(II)
complex. The rate constants of the amidato-bridged Pt
III
dinuclear complex with olefins are comparable to those with
3
-1 -1 18,19
halide ions (in the order of 10 M s ),
and considerably
higher than those of the ligand substitution on Pt(II) complexes
0
-3
-1 -1 28
(
in the range of 10 - 10
M
s ).
Conclusions
Acknowledgment. We thank 21COE “Practical Nano-
Chemistry” from MEXT, Japan for the financial support.
In the reactions of the HH 2-pyridonato-bridged cis-diam-
mineplatinum(III) dinuclear complex with olefins in acidic
aqueous solution, the first substitution with an olefin molecule
occurs to the Pt(N2O2) atom to produce the mono-π complex,
and the second substitution occurs to the Pt(N4) atom to form
di-π complex. These steps are followed by dissociation of the
second olefin on the Pt(N4) atom and the conversion of the first
π-bonding olefin to the σ-bonding alkyl ligand on the Pt(N2O2)
atom under the conditions CHH , CL. When the resultant
σ-complex is less stable, the σ-complex is reduced to produce
Supporting Information Available: Numerical values of the
observed kobs under various conditions, and related figures. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA020953S
(
27) Henry, P. M. J. Am. Chem. Soc. 1964, 86, 3246, and references therein.
28) (a) Basolo, F.; Grey, H. B.; Pearson, R. G. J. Am. Chem. Soc. 1960, 82,
4200. (b) Belluco, U.; Ettore, U.; Basolo, F.; Pearson, R. G.; Turco, A.
Inorg. Chem. 1966, 5, 591. (c) Paxson, J. R.; Martin, D. S., Jr. Inorg. Chem.
(
II
the corresponding Pt dinuclear complex and ketone. The
reactivity of the axial site of the Pt(N4) is much influenced by
1
971, 10, 1551. (d) Palmer, D. A.; Kelm, H. Inorg. Chim. Acta 1976, 19,
117.
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