Secondary kinetic isotope effects in oxidative addition of benzyl bromide to dimethylplatinum(II) complexes

Article abstract of DOI:10.1021/om400084b

The secondary α-deuterium kinetic isotope effects (KIEs), (k _H/k_D)_α, have been determined, at different temperatures and in solvents having different polarities, for reaction of PhCH₂Br/PhCD₂Br with t

Full text of DOI:10.1021/om400084b

Article
pubs.acs.org/Organometallics
Secondary Kinetic Isotope Effects in Oxidative Addition of Benzyl
Bromide to Dimethylplatinum(II) Complexes
Marzieh Dadkhah Aseman,† Mehdi Rashidi,*,† S. Masoud Nabavizadeh,† and Richard J. Puddephatt*,‡
†Department of Chemistry, Faculty of Sciences, Shiraz University, Shiraz 71454, Iran
‡Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7
ABSTRACT: The secondary α-deuterium kinetic isotope
effects (KIEs), (kH/kD)α, have been determined, at different
temperatures and in solvents having different polarities, for
reaction of PhCH2Br/PhCD2Br with the dimethylplatinum(II)
complexes [PtMe2(NN)], in which the bidentate NN ligand is
bpy (=2,2′-bipyridine) or bu2bpy (=4,4′-di-tert-butyl-2,2′-
bipyridine). The values obtained for the secondary α-
deuterium KIEs in acetone solution are close to 1 and may
be normal or inverse, but much larger values are found for the
reactions in benzene. An explanation is presented on the basis
of solvent dependence of the degree of looseness of the
transition state in the SN2 mechanism.
Scheme 1. Proposed Mechanism of Oxidative Addition (S =
Solvent)
INTRODUCTION
■
The oxidative addition of alkyl halides with d8 square-planar
complexes is often a key step in catalytic reactions that are of
importance to the petrochemical industry,1 and studies of
oxidative addition to platinum(II) complexes have provided
much of the information needed to understand the reactivity
and mechanism in these reactions.2 The secondary deuterium
kinetic isotope effect (KIE) can give information about the
reaction mechanism and the structure of the transition state,3
and it has previously been applied to studies of oxidative
addition of methyl or ethyl iodide to square-planar complexes.4
Benzyl halides are known to be very reactive in oxidative
addition reactions.2j,5 These reactions can find significant
applications in cross-coupling reactions of benzyl halides5m
and in the synthesis of functional organoplatinum(IV)
complexes, ranging from dendrimers to building blocks for
single resonances in the 1H NMR spectrum for the
2
methylplatinum groups at δ 1.52 (s, 6H, J(PtH) = 70 Hz)
and for the methylene protons of the benzylplatinum group at δ
2.84 (s, 2H, 2J(PtH) = 95 Hz). None of the alternative product
of cis oxidative addition, which would give two MePt
resonances and two resonances for diastereotopic PtCHAHBPh
protons, was observed.
The kinetics of the oxidative addition reactions were studied
in benzene and acetone solution. The reactions were rapid and
were most conveniently followed by using equimolar
concentrations of 1a or 1b and PhCH2Br, in a way similar to
that used previously in reactions with methyl iodide.4c A typical
set of UV−visible spectra taken during a kinetic run is shown in
Figure 1, using the decay of the MLCT band of complex 1a to
monitor the reaction.
2a,l,6
self-assembly of polymers, polyrotaxanes, and nanotubes.1d,
Nevertheless, there have been no studies of kinetic isotope
effects in these reactions, and this paper provides the first
information on this topic, using oxidative addition of PhCH2Br
or PhCD2Br to the complexes [PtMe2(NN)], where NN is bpy
(=2,2′-bipyridine) or its derivative bu2bpy (=4,4′-di-tert-butyl-
2,2′-bipyridine).
RESULTS AND DISCUSSION
■
Under these conditions, the reactions followed good second-
order kinetics, and the resulting rate constants are given in
Table 1. It should also be noted that the rates of the reactions
were not affected by the addition of the radical scavenger p-
benzoquinone.
The oxidative addition reactions of benzyl bromide to
dimethylplatinum(II) complexes of general formula
[PtMe2(NN)] (1) are thought to occur according to the
mechanism shown in Scheme 1, to give the products of trans
oxidative addition, 2, by the polar SN2 mechanism, by way of
the ionic intermediate A.1d,2i For the reactions with NN = bpy,
bu2bpy, the products were 2a,b (Scheme 1). For example, the
stereochemistry of 2b was readily shown by the presence of
Received: January 30, 2013
© XXXX American Chemical Society
A
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Figure 2. Eyring plots for the reactions with PhCH2Br in acetone: (a)
for complex 1a; (b) for complex 1b.
Figure 1. Changes in the UV−visible spectrum during the reaction of
[PtMe2(bpy)] (1a; 3 mL of 3 × 10−4 M solution), with PhCH2Br,
under second-order 1:1 stoichiometric conditions, in acetone at 25 °C:
(a) initial spectrum (before adding PhCH2Br); (b) spectrum at t = 0;
successive spectra recorded at intervals of 30 s.
A comparison of the secondary α-deuterium kinetic isotope
effects for some related oxidative addition reactions is given in
Table 2, using the more accurate data from kinetic studies. The
Table 2. Secondary α-Deuterium Kinetic Isotope Effects for
the Reaction of Complex [PtMe2(bpy)] (1a) in Acetone or
Benzene
Eyring plots for the reactions in acetone solution are given in
Figure 2, from which the activation parameters were calculated
(Table 1). The large negative entropies of activation are typical
values for oxidative addition by the SN2 mechanism. For
example, in reactions of 1a in acetone, benzyl bromide gives
ΔH⧧ = 34.7 3.0 kJ mol−1 and ΔS⧧ = −105 9 J K−1 mol−1
while methyl iodide gives ΔH⧧ = 22.0 1.2 (1) kJ mol−1 and
ΔS⧧ = −138 4 J K−1 mol−1.4d
solvent
acetone
reagent
T/ °C
(kH/kD)α
PhCH2Br
25
30
25
30
25
25
25
25
0.97 0.01
0.98 0.01
1.25 0.01
1.33 0.01
1.02 0.02
0.93 0.04
1.24 0.01
1.28 0.06
benzene
PhCH2Br
1
We also measured the KIE using H NMR spectroscopy,
a
acetone
benzene
acetone
benzene
MeI
typically for the reaction of PhCH2Br/PhCD2Br (50%/50%
mixture) with complex 1a in either acetone or benzene solvent.
Thus, after the reaction is carried out at 20 °C, the final solid
products [PtBr(CH2Ph)Me2(bpy)]/[PtBr(CD2Ph)Me2(bpy)]
(2a/2a*) were obtained and the KIEs were determined from
the 1H NMR spectrum in CDCl3 by proper integration (see the
description in the Experimental Section), and the data are
collected in Table 1. The results are in agreement with those
obtained from UV−visible spectroscopy.
a
MeI
b
EtI
b
EtI
a
b
From ref 4d, using CH3I/CD3I. From ref 4e, using CH3CH2I/
CH3CD2I.
reactions in acetone give only a small secondary α-deuterium
KIE, which is inverse at lower temperatures and normal at
Table 1. Rate Constants, Kinetic α-Deuterium Isotope Effects, and Activation Parameters for the Reaction of Complexes
[PtMe2(bpy)] (1a) and [PtMe2(bu2bpy)] (1b) with PhCH2Br/PhCD2Br in Acetone or Benzene
a
b
c
complex
solvent
acetone
T (°C)
k2(H) (L mol−1s−1
)
k2(D) (L mol−1s−1
)
(kH/kD)α
(kH/kD)α NMR
1a
15
20
25
30
35
15
20
25
30
35
15
20
25
30
30
9.30 0.01
11.61 0.01
15.60 0.01
19.55 0.01
25.30 0.14
0.81 0.01
1.05 0.01
1.38 0.01
1.82 0.01
2.22 0.01
20.06 0.02
25.70 0.02
31.82 0.01
37.16 0.01
3.85 0.01
10.07 0.01
12.30 0.03
15.98 0.04
19.85 0.05
24.02 0.10
0.66 0.01
0.85 0.01
1.10 0.01
1.36 0.01
1.73 0.01
23.51 0.01
27.88 0.01
31.77 0.01
35.89 0.02
2.78 0.01
0.92 0.01
0.94 0.01
0.97 0.01
0.98 0.01
1.05 0.01
1.22 0.01
1.23 0.01
1.25 0.01
1.33 0.01
1.28 0.01
0.85 0.01
0.92 0.01
1.00 0.01
1.05 0.01
1.38 0.01
0.94
1a
benzene
1.26
1b
1b
acetone
benzene
a
Values for PhCH2Br. Activation parameters in acetone: for 1a, ΔH⧧ = 34.7 3.0 kJ mol−1, ΔS⧧ = −105 9 J K−1 mol−1; for 1b, ΔH⧧ = 27.5 3.0
b
kJ mol−1, ΔS⧧ = −124 9 J K−1 mol−1. Activation parameters in benzene: for 1a, ΔH⧧ = 35.3 3.0 kJ mol−1, ΔS⧧ = −123 9 J K−1 mol−1. Values
for PhCD2Br. Activation parameters in acetone: for 1a, ΔH⧧ = 30.2 2.0 kJ mol−1, ΔS⧧ = −120 8 J K−1 mol−1; 1b, ΔH⧧ = 17.9 2.0 kJ mol−1,
ΔS⧧ = −156 6 J K−1 mol−1. Activation parameters in benzene: for 1a, ΔH⧧ = 33.0 2.0 kJ mol−1, ΔS⧧ = −133 8 J K−1 mol−1. Error bars were
estimated by Girolami’s method.3c KIE from NMR product analysis at 20 °C.
c
B
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higher temperatures (Table 1). These are typical values for an
SN2 reaction.3a,7 However, in benzene solution the value of
(kH/kD)α is 1.33, which is the highest value yet observed for an
oxidative addition reaction.4e
Scheme 2. Proposed Mechanism of the Reaction (NN = bpy)
Table 3 gives values of the secondary α-deuterium KIE for
nucleophilic substitution reactions of benzyl derivatives. Many
Table 3. Secondary α-Deuterium Kinetic Isotope Effects for
Nucleophilic Substitution Reactions of Benzyl Derivatives
reagent
PhCH2Cl
nucleophile
solvent
T/°C
kH/kD
ref
Cl−
Cl−
dmf
40
35
25
25
40
40
30
30
0
1.05
1.08
0.99
1.01
1.02
1.26
1.25
1.19
1.48
7a
7b
7c
7c
7d
7d
7e
7e
7f
PhCH2Cl
dmso
PhCH2Br
Et3N
a
−
PhCH2Br
N3
a
PhCH2Cl
CN−
CN−
Br−
b
4-MeC6H4CH2Cl
PhCH2NMe2Ph+
PhCH2NMe2Ph+
PhCH2NMe2Ph+
b
CHCl3
acetone
dmf
Br−
PhS−
a
b
Solvent 80/20 dioxane/water. Solvent 55/45 water/2-methoxyetha-
nol.
of these reactions give values of (kH/kD)α in the range 0.99−
1.08, which are typical values for the SN2 mechanism.7a−d The
magnitude of these KIEs depends on the changes in zero point
energy on going to the transition state, and this in turn is
determined by changes in the Cα−H(D) vibrations on going
from reactants to the transition state. As described by
Westaway,3a changes in C−H(D) stretching vibrations would
typically give a small inverse KIE, but this may be balanced by
changes in the C−H(D) out-of-plane bending vibration, which
may contribute to a normal KIE. The net effect is that there is
usually a small KIE, which may be inverse or normal. However,
some much larger values of (kH/kD)α in the range 1.19−1.48
have been observed.3a,7d−f In some cases these have been
interpreted in terms of an SN1 mechanism, sometimes within
tight ion pairs, but it has also been argued that these large
values are consistent with an SN2 mechanism with an
unsymmetrical or very loose transition state.3a,7f The easier
out-of-plane bending vibration is the main contributor to these
larger KIE values.
The oxidative addition of benzyl bromide in benzene clearly
does not occur by an SN1 mechanism; therefore, the large value
of (kH/kD)α must be attributed to one of the other effects. A
radical mechanism, with intermediate formation of free benzyl
radicals, might give a high value of (kH/kD)α, but no evidence
for radical intermediates was found.2j,4e
In order to understand the observed differences in kinetic
isotope effects when S = acetone, benzene, DFT calculations
were carried out on the complexes and potential intermediates.
Because polar intermediates are invoked and the solvent effect
is critically important in this context, it was necessary to
incorporate both general solvation effects and, for intermedi-
ates, individual solvent molecules. The conductor-like screening
model (COSMO) was used to model general solvation effects
by acetone or benzene.8 The likely mechanism of reaction of
Figure 3. Proposed initiation of the SN2 oxidative addition.
transition state B1. The displacement of acetone from A1 by
the bromide ion, formed in the SN2 oxidative addition step,
must occur by a dissociative mechanism by way of a five-
coordinate intermediate or transition state C,9 to finally give the
product 2a.
Figure 4 shows the calculated structures and energies for the
compounds shown in Scheme 2 in acetone solution. The
calculation indicates an easy reaction to give the cationic
acetone complex A1, followed by a slower reaction to give 2a,
consistent with the experimental observations.2i Note that, in
the kinetics experiments, the loss of complex 1a is measured so
that the rates refer to formation of A1. The calculation of the
energy of the transition state B1 (+24 kJ mol−1) is in
benzyl bromide with the complex [PtMe2(bpy)] (1a) is shown
2d,i
in Scheme 2.1d,
The reaction is initiated by nucleophilic
reasonable agreement with the observed value of ΔH⧧
=
attack by the 5dz HOMO of [PtMe2(bpy)] on the σ* LUMO
of benzyl bromide (Figure 3). For the reaction in acetone
solution, the cationic intermediate A1 has been detected by
NMR at low temperature2i and is probably formed via the
34.7(1) kJ mol−1. The calculated activation energy is higher in
benzene solution, partially due to the low polarity, which does
not stabilize the polar intermediates, and partially because it is a
poor ligand and so cannot stabilize the platinum(IV) oxidation
2
C
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CONCLUSIONS
■
In the present study, we have observed that although (kH/kD)α
for the reaction of PhCH2Br/PhCD2Br with the
dimethylplatinum(II) complexes [PtMe2(bpy)] (1) in the
polar solvent acetone are close to unity, much larger values
of around 1.3 are found for the reactions in the nonpolar
solvent benzene. Our calculations (vide supra) suggest that the
transition state of the reaction in benzene solvent is looser than
that of the related reaction in acetone solvent. This looser
transition state for the reaction in benzene solvent will give
more room for the Cα−H(D) out-of-plane bending vibration,
which is believed to play a major role in determining the
magnitude of (kH/kD)α for SN2 reactions in which there is a
combination of bulkier alkyl halides and heavier nucleophiles.3a
In the present case, the Cα−H(D) bond in the transition state
involving the nonpolar solvent benzene (Scheme 3; R = Ph,
Figure 4. Calculated structures and relative energies (kJ mol−1) for
product 2a, intermediate A1, and transition states B1 and C1, arising
from the reaction of 1a + PhCH2Br + acetone (E = 0) in acetone
solution.
state by coordination. The ionic intermediate A2 will be formed
only transiently.
A summary of some calculated properties of the compounds
of Scheme 2 is given in Table 4. The charge on platinum is
Scheme 3. Proposed Transition State Structures
Table 4. Calculated Hirshfeld Atomic Charges and Selected
Distances (Å) for the Compounds of Scheme 2
c
c
c
d
Q(Pt)
Q(Br)
Q(C)
Pt−Br Pt−CH2 C−Br Pt−S
a
b
BnBr
BnBr
−0.13
−0.11
−0.03
−0.04
2.06
2.04
a
1a
0.11
0.12
0.39
0.39
0.44
0.44
0.44
0.33
0.46
0.45
b
1a
a
2a
b
−0.48
−0.42
−1
−0.12
−0.13
−0.11
−0.07
−0.09
−0.05
−0.07
−0.06
2.71
2.67
2.12
2.13
2.09
2.12
2.13
2.47
2.11
2.12
structure ii) has an almost planar carbon center, with long Pt−
C and Br−C distances, giving room for the C−H (or C−D)
bending vibration. This can be compared to the transition state
in acetone (Scheme 3; R = Ph, structure i), in which the polar
solvent molecule is coordinated more tightly to the platinum
center, bringing the geometry close to octahedral at platinum
and in which there is a shorter Pt−C bond and more
tetrahedral carbon center, with less room for the C−H bending
vibration. In the analogous reaction of CH3CH2I/CH3CD2I
with the dimethylplatinum(II) complex [PtMe2(bpy)] at 25 °C,
the kinetic isotope effect (kH/kD)α was found to be high for
both acetone and benzene solvents (being 1.24 in acetone and
slightly higher, 1.28, in benzene).4e Here, the donor ability of R
= Me (as compared to the withdrawing ability of R = phenyl) is
expected to make the nucleophilic attack by platinum(II) more
difficult, so that the transition state in both acetone and
benzene is closer to the structure type ii (Scheme 3), making
(kH/kD)α values similar and large in both solvents.
2a
A1
A2
B1
B2
C1
C2
2.32
−1
3.67
2.83
3.97
−0.84
−0.54
−1
5.27
5.06
3.35
2.74
−1
a
b
c
d
Solvent acetone. Solvent benzene. Hirshfeld charge. O or C atom
when S = acetone, benzene.
significantly lower in the platinum(II) complex 1a than in the
other compounds, supporting the view that 1a acts as the
nucleophile in the reaction with benzyl bromide (Figure 3).
The SN2 reactions can occur with a single or double energy
maximum, but only one transition state could be found for each
reaction. The major difference is that the transition state in
acetone solution was calculated to be productlike, with Pt−C =
2.13 Å and C−Br = 3.35 Å, whereas in benzene solution it was
more symmetrical, with Pt−C = 2.47 Å and C−Br = 2.74 Å.
The charge on the bromine atom was calculated to be −0.84e
and −0.54e in the transition states B1 and B2, respectively. The
Pt−C−Br unit was nonlinear in both cases, the angles being
150 and 152° in acetone and benzene, respectively. In benzene
solution, the benzyl group in B2 is calculated to be essentially
flat, and the long distances to both the platinum and bromine
atoms may allow it to vibrate in a way similar to that for a
benzyl radical. This calculated transition state can be considered
to be very loose and corresponds to those in some of the rare
cases in which SN2 reactions are found to give large normal
isotope effects.3a,7f The difference in the kinetic isotope effects
for the reaction of complex 1a with benzyl bromide in acetone
or benzene can therefore be attributed to the lower ability of
benzene to stabilize the polar transition state.
Finally, it is noted that the observed higher isotope effect in
the less polar solvent contrasts with data from the solvolysis of
benzyl derivatives, in which the isotope effect increases in more
polar solvents as the transition state moves closer to the SN1
limiting case.7h,j
EXPERIMENTAL SECTION
■
The benzyl halides PhCH2Br (99%) and PhCD2Br (98 atom % D)
were purchased from Aldrich Chemical. The 1H and 13C NMR spectra
of the complexes were recorded as CDCl3 solutions on a Bruker
Avance DPX 250 (or 300 MHz) spectrometer, and TMS (0.00) was
used as an external reference. All the chemical shifts and coupling
constants are given in units of ppm and Hz, respectively. Kinetic
studies were carried out by using a Perkin-Elmer Lambda 25
spectrophotometer. Temperature was carefully controlled by a
EYELA NCB-3100 low-temperature thermoregulation bath. The
starting organoplatinum(II) complexes [PtMe2(bpy)] (1a)10 and
D
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Article
[PtMe2(tBu2bpy)] (1b)11 were synthesized and characterized as
reported elsewhere. The H NMR labeling is depicted in Scheme 4.
containing 15 μL of each of the reagents PhCH2Br and PhCD2Br
(overall 10 times excess). The solution was stirred at 20 °C for 1 h,
and then the solvent was removed and the residue was dried under
1
1
vacuum. The kinetic isotope effects (KIEs) was obtained from the H
1
Scheme 4. H NMR Labeling of Complexes 2
NMR spectrum of the product mixture [PtBr(CH2Ph)Me2(bpy)]/
[PtBr(CD2Ph)Me2(bpy)] (2a/2a*) in CDCl3. A sufficiently long
delay time was used to ensure that minor differences in relaxation
2
times T1, arising from the H isotope effect in 2a*, did not affect the
integration. Thus, integration of the left satellite of Pt−CH2 protons
was used to determine the relative concentration of 2a. The relative
integration of the left satellite of Pt−CH3 protons due to Me ligands of
2a + 2a* was used to determine the relative concentration for the total
of 2a + 2a*; note that these two satellites, used to determine the
relative concentrations, were the most clear peaks in the spectrum.
Subtraction of concentration of 2a from this total gave the relative
concentration of 2a*. Dividing the relative concentration of 2a by the
relative concentration of 2a* gives the KIE. Similarly, the KIE for the
same reaction in benzene solvent was determined.
trans-[PtBr(CH2Ph)Me2(bpy)] (2a). [PtMe2(bpy)] (1a; 100 mg,
0.26 mmol), was dissolved in acetone (15 mL), and benzyl bromide
(31 μL, 0.26 mmol) was added to it. The mixture was stirred at room
temperature over a period of 3 h. The solvent was evaporated from the
resulting white solution, and the residue was washed with ether. The
product as a white solid was dried under vacuum. Yield: 91%. Mp: 220
°C dec. Anal. Calcd for C19H21BrN2Pt: C, 41.31; H, 3.96; N, 5.07.
AUTHOR INFORMATION
Corresponding Author
*E-mail: rashidi@chem.susc.ac.ir (M.R.); pudd@uwo.ca
(R.J.P.).
■
1
Found: C, 41.66; H, 3.75; N, 5.11. H NMR data (in CDCl3): δ(1H)
Notes
2
2
The authors declare no competing financial interest.
1.55 (s, 6H, J(PtH) = 69.9 Hz, PtMe), 2.83 (s, 2H, J(PtH) = 93.3
Hz, PtCH2Ph), 6.28 (d, 2H, J(HαHβ) = 7.0 Hz, J(PtHα) = 12.5 Hz,
Hα of Ph), 6.56 (t, 2H, 3J(HαHβ) = 3J(HβHγ) = 7.0 Hz, Hβ of Ph), 6.67
(m, 1H, Hγ of Ph), 7.45 (m, 2H, H5 of bpy), 7.87 (dt, 2H, 3J(H4H3) =
3
4
ACKNOWLEDGMENTS
■
4
We thank the NSERC (Canada) and Shiraz University for
financial support.
3J(H4H5) ≈ 7.8 Hz, J(H4H6) = 1.2 Hz, H4 of bpy), 7.96 (d, 2H,
3J(H3H4) = 8.1 Hz, H3 of bpy), 8.70 (d, 2H, J(H6H5) = 6.3 Hz,
3
3J(PtH6) = 12.0 Hz, H6 of bpy); 13C NMR data: δ(13C) −3.0 (s, 2C,
1J(PtC) = 678 Hz, PtMe), 23.4 (s, 1C, J(PtC) = 641 Hz, PtCH2Ph);
1
REFERENCES
■
aromatic carbons 123.0 (s, J(PtC) = 9 Hz), 124.0 (s, J(PtC) = 17 Hz),
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1538.
t
2
1.42 (s, 18H, Me groups of 2 Bu), 1.53 (s, 6H, J(PtH) = 69.6 Hz,
PtMe), 2.84 (s, 2H, 2J(PtH) = 94.8 Hz, PtCH2Ph), 6.29 (d, 2H,
3J(HαHβ) = 7.2 Hz, 4J(PtHα) =12.8 Hz, Hα of Ph), 6.56 (t, 2H,
3
3J(HαHβ) = J(HβHγ) = 7.2 Hz, Hβ of Ph), 6.70 (m, 1H, Hγ of Ph),
3
t
7.44 (d, 2H, J(H6H5) = 5.7 Hz, H5 of Bu2bpy), 7.88 (s, 2H, H3 of
tBu2bpy), 8.60 (d, 2H, J(H5H6) = 6.0 Hz, J(PtH6) = 12.5 Hz, H6 of
3
3
tBu2bpy). 13C NMR data: δ(13C) −3.3 (s, 2C, J(PtC) = 675 Hz,
1
1
PtMe), 22.8 (s, 1C, J(PtC) = 645 Hz, PtCH2Ph), 30.4 (s, 6C, Me
carbon atoms of tBu groups), 35.4 (s, 2C, central carbon atoms of tBu
groups); aromatic carbons 119.4 (s, J(PtC) = 9 Hz), 123.6 (s, J(PtC)
= 14 Hz), 123.7 (s, J(PtC) = 17 Hz), 127.4 (s, J(PtC) = 16 Hz), 127.5
(s, J(PtC) = 24 Hz), 145.1 (s, J(PtC) = 56 Hz), 146.4 (s, J(PtC) = 14
Hz), 154.8 (s), 162.8 (s, J(PtC) = 59 Hz).
Kinetic Study. A solution of complex 1a or 1b in acetone or
benzene (3 mL, 3.0 × 10−4 M) in a cuvette was thermostated at 25 °C,
and benzyl bromide with a known concentration was added using a
microsyringe. After rapid stirring, the absorbance at λ 475 nm for
complex 1a in acetone and at λ 508 nm in benzene and for complex 1b
at λ 465 nm in acetone and at λ 550 nm in benzene decreased with
time. The absorbance−time curves were analyzed by 1:1 stoichio-
metric techniques. The rate of the reaction was not affected by
addition of the radical scavenger p-benzoquinone. The data at other
temperatures were obtained similarly.
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dx.doi.org/10.1021/om400084b | Organometallics XXXX, XXX, XXX−XXX