Steric and solvent effects on the secondary kinetic α-deuterium isotope effects in the reaction of methyl iodide with organoplatinum(ii) complexes: Application of a second-order technique in measuring the rates of rapid processes

Article abstract of DOI:10.1021/om900778u

The kinetic isotope effects (KIEs), k_H/k_D, have been determined for reaction of CH₃I/CD₃I with several organoplatinum(II) complexes [PtR₂(NN)], in which the bidentate NN ligand is bpy = 2,2′-bipyridine, ^tBu₂bpy = 4,4′-bis(test-butyl)-2,2′-bipyridine, phen = 1,10-phenanthroline, or Me₂phen = 2,9-dimethyl1,10-phenanthroline, at different temperatures and in solvents having different polarities. The values obtained for the secondary α-deuterium KIEs are close to 1 and are dependent on the solvent; values of up to 7-10% larger are obtained for the reactions in the polar solvent acetone as compared to those obtained in the nonpolar solvent benzene. The data also indicate that the steric crowding around the squareplanar coordination sphere of the platinum(II) complexes with the ligand Me2phen results in higher KIEs. The reactions involving dimethylplatinum(II) complexes, [PtMe₂(NN)], were fast, and a 1:1 molar ratio (of complex and reagent) technique was successfully used to measure the rate constants accurately by conventional UV-visible spectroscopy. It is shown that there are significant advantages to measuring the reaction rates under second-order condition, as compared to the usual pseudo-first-order method.

Full text of DOI:10.1021/om900778u

82 Organometallics 2010, 29, 82–88
DOI: 10.1021/om900778u
Steric and Solvent Effects on the Secondary Kinetic r-Deuterium Isotope
Effects in the Reaction of Methyl Iodide with Organoplatinum(II)
Complexes: Application of a Second-Order Technique in Measuring the
Rates of Rapid Processes
Sepideh Habibzadeh,† Mehdi Rashidi,*,† S. Masoud Nabavizadeh,*,† Leila Mahmoodi,†
Fatemeh Niroomand Hosseini,‡ and Richard J. Puddephatt§
†Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran, ‡Department of
Chemistry, Islamic Azad University, Shiraz Branch, Shiraz 71993-37635, Iran, and Department of
Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7
§
Received September 7, 2009
The kinetic isotope effects (KIEs), kH/kD, have been determined for reaction of CH3I/CD3I with several
organoplatinum(II) complexes [PtR2(NN)], in which the bidentate NN ligand is bpy=2,20-bipyridine,
tBu2bpy=4,40-bis(tert-butyl)-2,20-bipyridine, phen=1,10-phenanthroline, or Me2phen=2,9-dimethyl-
1,10-phenanthroline, at different temperatures and in solvents having different polarities. The values
obtained for the secondary R-deuterium KIEs are close to 1 and are dependent on the solvent; values of
up to 7-10% larger are obtained for the reactions in the polar solvent acetone as compared to those
obtained in the nonpolar solvent benzene. The data also indicate that the steric crowding around the square-
planar coordination sphere of the platinum(II) complexes with the ligand Me2phen results in higher KIEs.
The reactions involving dimethylplatinum(II) complexes, [PtMe2(NN)], were fast, and a 1:1 molar ratio
(of complex and reagent) technique was successfully used to measure the rate constants accurately by
conventional UV-visible spectroscopy. It is shown that there are significant advantages to measuring the
reaction rates under second-order condition, as compared to the usual pseudo-first-order method.
Introduction
Bimolecular oxidative addition reactions of SN2 type
involving addition of a wide variety of alkyl halides to
dimethyl or diaryl organoplatinum(II) complexes have been
investigated extensively,5 including the diimine platinacyclo-
pentane complex [Pt(CH2CH2CH2CH2)(NN)], in which the
bidentate NN ligand is either 2,20-bipyridine or 1,10-phenan-
throline.6 Although the reactions are well documented and
established, we have recently presented some new insights by
a closer look at the kinetic behavior.7 The kinetics of reaction
of MeI with some binuclear organoplatinum(II) complexes,
held together by a robust bridging biphosphine ligand, have
recently been studied, and a stepwise oxidative addition of
MeI to the platinum(II) centers of each complex was de-
scribed.8 In all these reactions involving the oxidative addi-
tion of alkyl halides to organoplatinum(II) complexes, it is
The kinetic isotope effect (KIE) is considered an important
tool in obtaining information about reaction mechanism and
structure of the transition state.1 In this respect, investigation
of the secondary kinetic R-deuterium isotope effects in the
oxidative addition reaction of alkyl halides with d8 square-
planar complexes2 can be very useful, as these reactions are
of fundamental importance in both chemistry and applied
chemistry, with particular relevance to the petrochemical
industry.3,4
*Corresponding authors. E-mail: rashidi@chem.susc.ac.ir (M.R.);
nabavi@chem.susc.ac.ir (S.M.N.).
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& Sons: New York, 2001. (c) Atwood, J. D. Inorganic and Organometallic
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Organometallics 1997, 16, 1897. (b) Fraser, C. S. A.; Eisler, D. J.; Jennings,
M. C.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 2002, 1224.
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r
pubs.acs.org/Organometallics
Published on Web 12/08/2009
2009 American Chemical Society

Article
Organometallics, Vol. 29, No. 1, 2010 83
well established that the reactions normally proceed by an
SN2-type mechanism involving a pentacoordinated or a
solvent six-coordinated cationic intermediate.5
A secondary R-deuterium KIE study involving the reac-
tion of MeI with some organoplatinum(II) complexes has
been reported in which, as is consistent with an SN2-type
mechanism, very small inverse to very small positive second-
ary kinetic R-deuterium isotope effect, kH/kD, values of
0.94-1.14 were observed.9 The factors influencing KIE, in
reactions involving both organic and inorganic nucleophiles,
have been thoroughly discussed,2,10 but further theoretical
and experimental investigations are needed to give a more
complete understanding of the subject. In continuation of
our interest in the oxidative addition reactions of alkyl
halides with organoplatinum(II) complexes and the related
KIE, in the present work, two interesting findings are
reported. First we have investigated the effects of solvent
and of steric hindrance (implemented by ligands on the axial
directions of the square-planar geometry) on the secondary
R-deuterium KIEs in reaction of MeI with some organo-
platinum(II) complexes and confirmed that the effects are
significant. Second, many oxidative addition reactions, espe-
cially those involving diimine-dimethylplatinum(II) com-
plexes, are very rapid and so either stopped-flow or fast-
scanning techniques have usually been necessary in order to
follow the kinetics of the reactions with UV-visible spectro-
scopy.11 In the present study, we have measured the rates of
such rapid reactions under second-order conditions, and we
have shown that the reactions can be followed easily by
conventional UV-visible spectroscopy under these condi-
tions. In particular, there are significant savings of time and
material, with no loss of accuracy in the results, when
compared to kinetic data obtained under pseudo-first-order
conditions. We anticipate that the technique could easily be
used for investigating other fast oxidative addition reactions
involving platinum or other transition metal complexes.
Figure 1. Changes in the UV-visible spectrum during the
reaction of [Pt(p-MeC6H4)2(Me2phen)], 1b (3 ꢀ 10-4 M), with
MeI (133.3 mM) in acetone at T=20 ꢀC: (a) initial spectrum
(before adding MeI); (b) spectrum at t = 0; successive spectra
recorded at intervals of 35 s.
Scheme 1
Results
The reactions involved in the present study are described in
Scheme 1. The reddish diorganoplatinum(II) complexes 1
reacted with excess MeI in either acetone or benzene to form
pale yellow solutions, from which solid products 2 were
obtained with the geometry shown in Scheme 1. The starting
organoplatinum(II) complexes 1c,12 1d,13 1e,12 and 1f14 and
the product organoplatinum(IV) complexes 2c,15 2d,9 2e,16
and 2f17 have been synthesized and characterized as reported
elsewhere. The other complexes were characterized by ele-
mental analysis and 1H NMR spectroscopy, and the details
are collected in the Experimental Section.
The Kinetic Investigations.
Reactions Involving Ditolyl Complexes. The oxidative ad-
dition reactions of CH3I/CD3I with the reddish ditolyl
complexes 1a and 1b in both benzene and acetone were
studied using UV-visible spectroscopy. In each case, excess
CH3I or CD3I was used and the disappearance of the MLCT
band for the complex 1a (λmax=445 nm in acetone or λmax
480 nm in benzene) or 1b (λmax=435 nm in acetone or λmax
=
=
(9) Rashidi, M.; Nabavizadeh, S. M.; Akbari, A.; Habibzadeh, S.
Organometallics 2005, 24, 2528.
(10) Holm, T. J. Am. Chem. Soc. 1999, 121, 515.
(11) Monaghan, P. K.; Puddephatt, R. J. J. Chem. Soc., Dalton
Trans. 1988, 595.
(12) Monaghan, P. K.; Puddephatt, R. J. Organometallics 1984, 3,
444.
(13) Achar, S.; Scott, J. D.; Vittal, J. J.; Puddephatt, R. J. Organo-
470 nm in benzene) was used to monitor the reaction. For
minimization of errors, the addition reaction rates of
both CH3I and CD3I were measured in parallel with the
identical stock solution of the related platinum(II) complex.
The reactions followed good first-order kinetics (see Figures
1 and 2 for typical cases). The pseudo-first-order rate
constants (kobs) were evaluated by nonlinear least-squares
fitting of the absorbance-time profiles to a first-order
equation (eq 1):
metallics 1993, 12, 4592.
(14) Klein, A.; McInnes, E. J. L.; Kaim, W. J. Chem. Soc., Dalton
Trans. 2002, 2371.
(15) Jawad, J. K.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans.
1977, 1466.
Abst ¼ Abs¥ þ ðAbs0 -Abs¥Þ expð -kobstÞ
Graphs of these first-order rate constants against the
concentration of the halide gave good straight-line plots
ð1Þ
(16) Aye, K. T.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott,
J. D.; Watson, A. A. Organometallics 1989, 8, 1518.
(17) De Felice, V.; Giovannitti, B.; De Renzi, A.; Tesauro, D.;
Panunzi, A. J. Organomet. Chem. 2000, 593, 445.

84 Organometallics, Vol. 29, No. 1, 2010
Habibzadeh et al.
Figure 4. Eyring plots for the reactions (a) [Pt(p-MeC6H4)2-
(phen)], 1a, with CD3I in benzene; (b) [Pt(p-MeC6H4)2(Me2-
phen)], 1b, with CD3I in benzene; and (c) [Pt(p-MeC6H4)2-
(phen)], 1a, with MeI in acetone.
Figure 2. Absorbance-time curves for the reaction of [Pt(p-
MeC6H4)2(Me2phen)], 1b, with CD3I (26.7-133.3 mM with
[CD3I] increases reading downward) in acetone at 25 ꢀC.
under the pseudo-first-order conditions ([MeI]0 > 10 [diaryl-
Pt complex]) and uses eq 1 to fit the data, would not be
appropriate, and either stopped-flow or fast-scanning tech-
niques are usually needed to measure the rates.11 For this
type of fast reaction we have found that the method that uses
1:1 stoichiometry, in reactions that follow second-order
kinetics and consume one mole of each of the two reactants,
could be used to measure the reaction rates by conventional
UV-visible spectroscopy. Thus, the reactions of MeI with
the methyl-diimine complexes 1c-1f were monitored by the
same procedure described above for the corresponding
reaction involving the diaryl complexes 1a and 1b, except
that the data were obtained under second-order 1:1 stoichio-
metric conditions ([MeI]0 = [Pt complex]) and the data were
fit to eq 3 with the programs that are based on the least-
squares method.18
Abst ¼ Abs¥ þ ðAbs0 -Abs¥Þ=ð1 þ ½Pt complexꢁ0
ꢀ k2 ꢀ tÞ
ð3Þ
Figure 3. Plots of first-order rate constants (kobs/s-1) for the
reaction of [Pt(p-MeC6H4)2(phen)], 1a, with MeI in benzene at
different temperatures vs concentration of MeI.
The changes in the UV-visible spectrum during a typical
reaction are shown in Figure 5, with the absorbance/time
curves shown in Figure S1. The k2 values were evaluated by
nonlinear least-squares fitting of the absorbance-time pro-
files to eq 3. This method was used at different temperatures,
and activation parameters were obtained from the Eyring
equation (eq 2 and Figure S2). The results are collected in
Tables S3-S6.
passing through the origin, showing a first-order dependence
of the rate on the concentration of the halide (Figure 3), and
the slope gave the overall second-order rate constant. The
same method was used at other temperatures, and activation
parameters were obtained from the Eyring equation (eq 2
and Figure 4). The resulting data are collected in Tables S1
and S2.
Discussion
As has been reported previously, the reaction of
dimethylplatinum(II) complexes containing diimine ligands
with MeI is very fast and so cannot be measured accurately
by the more conventional UV-visible spectrophotometric
techniques.11 In this study we have succeeded in measuring
the rate of these fast reactions by using a 1:1 molar ratio of
ꢀ
ꢁ
ꢀ
ꢁ
k2
T
kB
h
ΔSq ΔSq
ln
¼ ln
þ
-
ð2Þ
R
RT
Reactions Involving Dimethyl Complexes. The reaction of
MeI with the dimethylplatinum(II) complexes containing
chelating diimine ligands, such as complexes 1c-1e, is
known to be very fast, and therefore measurement of the
rate of the reaction by the above technique, which works
(18) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms,
2nd ed.; McGraw-Hill: New York, 1995.

Article
Organometallics, Vol. 29, No. 1, 2010 85
Figure 6. CR-H(D) out-of-plane bending vibrations in the
substrate and in the SN2 transition state.
in agreement with the rate constant values obtained in the
present study for the same reaction, using 1:1 stoichiometric
techniques, in acetone at 20 and 30 ꢀC, which are 38.2 ( 0.4
and 55.5 ( 0.9 dm3 mol-1 s-1, respectively, with the ΔSq
value being -138 ( 4 J K-1 mol-1. It is interesting to note
that comparatively higher temperatures were used in the
latter method to obtain the ΔSq value (see Table S3).
Note also that in a separate report for the same reaction in
acetone at 30 ꢀC,15 the value of k2 has been found to be
46.8 dm3 mol-1 s-1, a value that is some 20% lower than
those obtained by the two independent procedures just
mentioned. This may support the conclusion that the 1:1
stoichiometric technique has probably provided more reli-
able data as compared to the pseudo-first-order technique,
especially at mild or elevated temperatures, where the rates
are increased and yet the measurements are much easier to
perform.
Figure 5. Changes in the UV-visible spectrum during the
reaction of [PtMe2(bpy)], 1c (3 mL of 3 ꢀ 10-4 M solution),
with MeI, under second-order 1:1 stoichiometric conditions, in
benzene at T=30 ꢀC: (a) initial spectrum (before adding MeI);
(b) spectrum at t = 0; successive spectra recorded at intervals of
30 s; (c) final spectrum.
Investigation of the data obtained from the reaction of
diorganoplatinum(II) complexes 1 with CH3I/CD3I revealed
some interesting secondary R-deuterium KIE trends. It is
believed that the vibration contribution has a major role in
the magnitude of total R-deuterium KIEs in SN2 reactions.
This has been factored into stretching and bending compo-
nents, and it has been shown that both are important in
determining KIEs. Westaway has described that, in an SN2
reaction, the tetrahedral substrate is converted into a penta-
valent transition state (Figure 6), with corresponding change
in the sp3 character of the carbon atom, and this increases the
energy of the CR-H(D) out-of-plane bending vibrations and
leads to the values of the normal or small inverse KIEs that
are observed for SN2 reactions.1a Thus, the trends observed
in the present work have been explained as follows:
complex and reagent (second-order conditions). This makes
the reaction slow enough for measurement by the conven-
tional UV-visible spectroscopy.
For reactions operating by the SN2 mechanism, both time
measurements and concentrations of all the reagents are
important, and so the accuracy of measurements under
second-order conditions is probably expected to be less than
that of those under pseudo-first-order conditions. However,
the measurements by the second-order technique used in the
present work have confirmed that this is not always true,
especially for fast reactions, and there are several more
advantages:
(i) The time needed to measure the rate constants was very
significantly shortened.
(ii) The amount of the materials needed was much less, and
this is in particular important when the precious metal
complexes, and especially expensive deuterated compounds,
such as CD3I reagent used in the present study, are consumed
during the process.
(iii) The measurements at comparatively higher tempera-
tures were easily feasible.
(iv) The slower reaction allows a greater fraction of the
reaction to be monitored, under a wider temperature range,
and gives improved accuracy of the results and a more
complete kinetic study.
(a) Solvent effect. The results in Tables S1-S6 (summari-
zed and typified in Table 1) clearly reveal that the values of
kH/kD for each reaction at the same temperatures are some
5-10% lower in the nonpolar solvent benzene than in the
polar solvent acetone. As mentioned above, the bending
contribution, (kH/kD)bend, to the total KIE, kH/kD, is sig-
nificant, and therefore a looser (the looseness is defined as
the nucleophile-leaving group distance) SN2 transition state
results in a larger kH/kD value, while a tighter one results in a
smaller or inverse kH/kD value. In a polar solvent such as
acetone, the polar [Pt] CH3 I system in the transition
3 3 3
3 3 3
The above advantages are clear in the kinetic results
obtained in the present study. The second-order rate cons-
tants, k2, reported in the literature for the reaction
of [PtMe2(bpy)], 1c, with MeI in acetone at 20, 3.6, and
state is more loosened than that in a nonpolar solvent such
as benzene, and this makes the bending contribution,
(kH/kD)bend, to the total KIE higher in acetone than in
benzene. This “looseness” suggestion can also be modified
by the “crowdedness” concept suggested by Davico and
Bierbaum.20 As such, the polar acetone solvent molecules
surround the positively charged CH3 section of the
-7.5 ꢀC are 40 ( 1, 22 ( 1, and 14 ( 1 dm3 mol-1 s-1
,
respectively;19 by extrapolation, the value of k2 at 30 ꢀC is
calculated to be 56 ( 1 dm3 mol-1 s-1. The data have been
obtained under pseudo-first-order conditions, and the ΔSq
value is found to be -129 ( 1 J K-1 mol-1. These values are
(20) Davico and Bierbaum believe that crowdedness rather than
looseness is a better way of understanding the secondary R-deuterium
KIE magnitude in polyatomic nucleophiles; see: Davico, G. E.;
Bierbaum, V. M. J. Am. Chem. Soc. 2000, 122, 1740.
(19) Byers, P. K.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott,
J. D. Organometallics 1989, 8, 1363.

86 Organometallics, Vol. 29, No. 1, 2010
Habibzadeh et al.
Table 1. Rate Constants (10 ꢀ kH or 10 ꢀ kD / dm3 mol-1 s-1 a,b
)
, Activation Parameters,b and Kinetic r-Deuterium Isotope Effectsa for the
Reaction of Complexes [PtR2(NN)], 1, with CH3I/CD3I in Acetone or Benzene at 25 ꢀC
complex
solvent
10 ꢀ kHb (dm3 mol-1 s-1
)
kH/kD
ΔH # b,c (kJ mol-1
)
ΔS# b,c (J K-1 mol-1
)
1a
acetone
benzene
acetone
benzene
acetone
benzene
acetone
benzene
acetone
benzene
benzene
8.7 ( 0.3
1.05 ( 0.04
28.9 ( 2.0
-149 ( 7
(-152 ( 7)
-156 ( 7
(-163 ( 6)
-138 ( 6
(-143 ( 6)
-162 ( 5
(-164 ( 5)
-138 ( 4
(-139 ( 4)
-153 ( 5
(-157 ( 6)
-105 ( 7
(-111 ( 7)
-104 ( 8
(-106 ( 7)
-112 ( 5
(-113 ( 5)
-108 ( 7
(-112 ( 7)
-117 ( 7
(-119 ( 6)
(8.3 ( 0.1)
1.02 ( 0.02
(1.04 ( 0.02)
22.5 ( 0.2
(28.0 ( 2.0)
32.2 ( 2.2
(30.0 ( 2.0)
29.7 ( 1.8
(28.5 ( 1.7)
26.4 ( 1.6
(26.0 ( 1.5)
22.3 ( 1.2
(22.0 ( 1.2)
21.6 ( 1.4
(20.3 ( 1.7)
30.1 ( 2.1
(28.3 ( 2.0)
34.4 ( 2.2
(33.7 ( 2.2)
29.1 ( 1.5
(28.7 ( 1.5)
34.2 ( 2.2
(32.9 ( 2.1)
30.2 ( 1.9
(29.7 ( 1.8)
0.98 ( 0.03
1.11 ( 0.01
1.07 ( 0.01
1.02 ( 0.02
0.93 ( 0.04
1.05 ( 0.01
1.01 ( 0.03
1.03 ( 0.01
0.91 ( 0.03
1.00 ( 0.01
1b
1c
(20.3 ( 0.1)
4.89 ( 0.01
(4.57 ( 0.01)
450.5 ( 3.5
(440.4 ( 6.7)
109.3 ( 2.9
(117.6 ( 3.7)
1120.1 ( 9.4
(1061.4 ( 7.1)
221.6 ( 9.0
(219.7 ( 6.0)
724.0 ( 3.9
(700.3 ( 8.1)
140.3 ( 2.9
(154.4 ( 4.1)
251.1 ( 3.2
(250.1 ( 1.1)
1d
1e
1fd
a Estimated errors were calculated from least-squares regression analysis. b Values in parentheses are for reactions with CD3I. c Obtained from the
Eyring equation. The errors were computed using the published formula.22 d The complex was not stable enough in acetone for any kinetic
measurements.
[Pt] CH3 I transition state, and this leads to an increase
3 3 3
3 3 3
in energy of the CR-H(D) out-of-plane bending vibrations
compared to the case when the solvent is benzene. Note also
that when comparing the values of kH/kD for reactions
involving different Pt nucleophilic complexes, the difference
is more pronounced in benzene than in acetone.
(b) Steric effect. As mentioned above, the magnitude of
R-deuterium KIEs in an SN2 reaction is influenced by the
amount of steric crowding at the CR-H(D) out-of-plane
bending vibrations,1 and a smaller KIE would be found for a
reaction with a tighter transition state. In the reaction of
diimineplatinum(II) complexes 1 with MeI studied in the
present work, the Me group attacks the platinum center in a
direction perpendicular to the square-planar coordination
plane of the complex, interacting with the filled Pt dz2 orbital.
The rate of the reaction is, among other factors, dependent
on the steric bulk of the ligands, and for example, the ortho-
substituted phenyl ligands inhibit the reaction.15
Figure 7. Natural populations analysis (NPA) charge of nitro-
gen atoms in bidentate N-donor ligands at B3LYP/6-311þG*.
The X-ray structural analyses of a number of Pd(II) and
Pt(II) complexes containing 2,9-dimethyl-1,10-phenanthro-
line, Me2phen, e.g., [PtPh2(Me2phen)],14 [PdMeCl(Me2-
phen)],21a and [PtCl2(Me2phen)],21b have revealed that the
plane of the Me2phen ligand in each complex has a remark-
able rotation around the N-N direction with respect to the
coordination plane, and this creates crowding around the
coordination sphere. We believe that this crowding is prob-
ably responsible for the observation that the magnitude of
R-deuterium KIEs for reactions involving the complex
[PtMe2(Me2phen)], 1f, in benzene and at different tempera-
tures is some 7-9% greater than that of [PtMe2(phen)], 1e, at
the same conditions. A similar trend was also observed for
the ditolyl analogous complexes [Pt(p-MeC6H4)2(Me2-
phen)], 1b, and [Pt(p-MeC6H4)2(phen)], 1a (see Table 1).
Theoretical studies using density functional calculations
were performed to calculate the charge on nitrogen atoms of
the NN ligands.23 The data were used in connection with the
kinetic data to confirm the steric effect of the Me2phen ligand
on the kinetic behaviors. Thus the charges were measured
and the results are indicated in Figure 7. The graph of log(k2)
for the reaction of complexes [PtMe2(NN)], 1c-1f, with
MeI at 25 ꢀC in benzene versus charges on the NN ligands,
Figure 8a and Table 2, is linear for the series of NN ligands
t
bpy, phen, and Bu2bpy, and this confirms that the rate of
reactions correlates linearly with the charges on the NN
(21) (a) De Felice, V.; Albano, V. G.; Castellari, C.; Cucciolito, M. E.;
De Renzi, A. J. Organomet. Chem. 1991, 403, 269. (b) Fanizzi, F. P.; Intini,
F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.; Tiripicchio, A. J. Chem. Soc.,
Dalton Trans. 1991, 1007.
(22) Morse, P. M.; Spencer, M. D.; Wilson, S. R.; Girolami, G. S.
Organometallics 1994, 13, 1646.
(23) Density functional calculations were performed with the pro-
gram suite Gaussian03 using the B3LYP method.24 The 6-311þG* basis
set was used with B3LYP. All geometries were optimized. To evaluate
and ensure the optimized structures of the molecules, frequency calcula-
tions were carried out using analytical second derivatives. In all cases
only real frequencies were obtained for the optimized structures.

Article
Organometallics, Vol. 29, No. 1, 2010 87
Figure 8. Plot showing a correlation between the rate constants on a logarithmic scale {log(k2)} for (a) the reaction of complexes
[PtMe2(NN)], 1c-1f, with MeI at 25 ꢀC in benzene and (b) the reaction of complexes [Pt(p-MeC6H4)2(NN)] with MeI at 25 ꢀC in
acetone and the charge of the nitrogen atom on the NN ligands. The values for the Me2phen ligand were omitted from the linear fit.
Table 2. Charges of Nitrogen Atoms on the NN Ligandsa and the k2 Values (experimental and calculated)b for the Reaction of Complexes
[PtR2(NN)] (R = Me or p-MeC6H4), 1, with MeI at 25 ꢀC, in Benzene or Acetone
NN ligand
bpy
phen
tBu2bpy
Me2phen
N chargea
-0.418
10.9 (10.5)
-0.429
14.0 (15.3)
-0.438
22.2 (21.1)
-0.478
25.1 (83.9)
experimental (or calculated)b k2 values for reaction
of [PtMe2(NN)] þ MeI in benzene at 25 ꢀC
experimental (or calculated)b k2 values for reaction
of [Pt(p-MeC6H4)2(NN)] þ MeI in acetone at 25 ꢀC
0.62c (0.60)
0.84 (0.94)
1.43c (1.37)
2.03 (7.06)
a N charge applies to the natural populations analysis (NPA) charge of nitrogen atoms in bidentate NN ligands at B3LYP/6-311þG*; see Figure 7.
b Values of k2 in parentheses were calculated according to log(k2) = m(N charge) þ b equations shown in Figure 8. c From ref 9.
ligands. However, when NN is Me2phen, the related point is
quite off the linear correlation, and if the line is extrapolated,
then the rate of reaction of complex [PtMe2(Me2phen)], 1f, at
the corresponding conditions of temperature 25 ꢀC in ben-
zene would be estimated to be nearly 84 dm3 mol-1 s-1. This
theoretical rate obtained for the reaction based on the
electronic effect of the NN ligand is considerably higher
than that obtained experimentally at the same condi-
tions, i.e., nearly 25 dm3 mol-1 s-1. A similar trend was
observed for the reaction of the analogous tolyl complexes
[Pt(p-MeC6H4)2(phen)], 1a, [Pt(p-MeC6H4)2(Me2phen)], 1b,
[Pt(p-MeC6H4)2(bpy)], and [Pt(p-MeC6H4)2(tBu2bpy)] with
MeI at 25 ꢀC (see Table 2 and Figure 8b). We attribute this
significant rate reduction to the steric effect imposed by Me
groups of the Me2phen ligand on the coordination sphere of
the square-planar complexes [PtR2(NN)].
Conclusions
The reaction of dimethyl-diimine platinum(II) complexes
with methyl iodide is fast, and therefore either stopped-flow
or fast-scanning techniques have usually been needed in
order to follow the kinetics of the reactions under pseudo-
first-order conditions with UV-visible spectroscopy.11
However, in the present work we have shown that, by using
a 1:1 stoichiometric method (under second-order con-
ditions), the rate of the reaction of MeI with several
dimethylplatinum(II) complexes [PtMe2(NN)], in which the
bidentate NN ligand is 2,20-bipyridine, 1,10-phenanthroline,
or their alkyl-substituted derivatives, can be determined
accurately, even at higher temperatures, by using more
conventional UV-visible spectroscopy. The other advan-
tages of this 1:1 stoichiometric method are significant time
and material savings. We anticipate that the technique can
successfully be used to determine the rate of other fast
reactions involving other organometallic complexes.
(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.;. Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;.
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,. K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;.
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.;. Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.;
Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Revision B03;
Gaussian, Inc.: Pittsburgh, PA, 2003.
We have determined the rate of reactions of a series of
organoplatinum(II) complexes with CH3I/CD3I, in acetone
or benzene at different temperatures, and indicated that in
each case a secondary R-deuterium KIE close to 1 (either
1.00, small inverse up to 0.91, or small positive up to 1.11)
is obtained. The results show that in cases where steric

88 Organometallics, Vol. 29, No. 1, 2010
Habibzadeh et al.
crowding is created around the coordination sphere the
bending contribution to the total KIE is affected and leads
to a considerable increase in the magnitude of the secondary
R-deuterium KIE. We also have clearly indicated that when
the nonpolar solvent benzene is used, the values of secondary
R-deuterium KIE are 7-10% lower than when the more
polar solvent acetone is used. This trend is proposed to arise
because the transition state in benzene is tighter than that in
acetone. It is also clearly shown that, in the same reactions
involving different nucleophilic platinum complexes, the
difference is more pronounced in benzene than in acetone.
We therefore suggest that, in this kind of study, using the
reaction rates obtained in a less polar solvent can provide a
more reliable comparison.
C28H26N2Pt requires: C, 57.43; H, 4.48; N, 4.78. 1H NMR: δ 2.19
(s, 6H, Me of Me2phen), 2.23 (s, 6H, ArCH3), 6.75 (d, 4H,
3J(HoHm)=7.7 Hz, Hm of Ar), 7.32 (d, 4H, 3J(HmHo)=7.7 Hz,
3J(PtHo)=73.5 Hz, Ho of Ar), 7.49 (d, 2H, 3J(H3H4) = 8.3 Hz, H3
and H8 of phen), 7.83 (s, 2H, H5 and H6 of phen), 8.31 (d, 2H,
3J(H3H4) = 8.3 Hz, H4 and H7 of phen).
[PtIMe(p-MeC6H4)2(phen)], 2a. Excess MeI (1 mL) was
added to a solution of [Pt(p-MeC6H4)2(phen)] (100 mg) in
acetone (20 mL). The mixture was stirred at room temperature
for 2 h. The solvent was evaporated from the resulting solution,
and the residue was washed with ether and dried under vacuum.
Yield: 91%; mp 234 ꢀC (dec). Anal. Found: C, 45.94; H, 3.61; N,
3.86. C27H25IN2Pt requires: C, 46.36; H, 3.60; N, 4.00. 1H
2
NMR: δ 2.32 (s, 3H, J(PtH)=72.0 Hz, Me-Pt), 2.36 (s, 6H,
ArCH3), 6.65 (d, 4H, 3J(HoHm)=7.6 Hz, Hm of Ar), 7.43 (d, 4H,
3J(HmHo)=7.6 Hz, 3J(PtHo)=34.9 Hz, Ho of Ar), 9.12 (d, 2H,
3
3J(H2H3) = 5.0 Hz, J(PtH2) =11.6 Hz, H2 and H9 of phen),
8.56 (d, 2H, 3J(H3H4) = 8.2 Hz, H4 and H7 of phen), 8.07
(s, 2H, H5 and H6 of phen), 7.82 (dd, 2H, 3J(H4H3) = 8.2 Hz,
3J(H2H3) = 5.0 Hz, H3 and H8 of phen).
Experimental Section
The 1H NMR spectra of the complexes were recorded as
CDCl3 solutions on a Bruker Avance DPX 250 MHz spectro-
meter, 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 1c,12 1d,13 1e,12 and 1f14 and the product organo-
platinum(IV) complexes 2c,15 2d,9 2e,16 and 2f17 have been
synthesized and characterized as reported elsewhere. The start-
ing complexes 1, used for kinetic measurements, nicely precipi-
tated in ether solutions, and the separated crystals were carefully
dried under vacuum. The 1H NMR data and the microanalysis
results of the complexes indicate that the complexes are of very
high purity.
The following complexes were made similarly using the
appropriate platinum(II) complex and CH3I or CD3I.
[PtIMe(p-MeC6H4)2(Me2phen)], 2b. Yield: 82%; mp 236 ꢀC
(dec). Anal. Found: C, 47.40; H, 3.60; N, 3.51. C29H29IN2Pt
1
requires: C, 47.88; H, 4.02; N, 3.85. H NMR: δ 1.21 (s, 3H,
2J(PtH)=70.3 Hz, Me-Pt), 2.22 (s, 6H, methyl of Me2phen), 2.43
3
(s, 6H, ArCH3), 6.77 (4H, d, J(HoHm) = 8.2 Hz, Hm of Ar),
7.52 (d, 4H, 3J(HmHo)=8.2 Hz, Ho of Ar), 8.55 (d, 2H, 3J(H3H4)
=8.5 Hz, H4 and H7 of phen), 7.86 (s, 2H, H5 and H6 of phen),
7.71 (d, 2H, 3J(H4H3) = 8.5 Hz, H3 and H8 of phen).
[PtI(CD3)(p-MeC6H4)2(phen)]. This compound had 1H NMR
data similar to data obtained for 2a, but without the MePt peak.
[PtI(CD3)(p-MeC6H4)2(Me2phen)]. This compound had 1H
NMR data similar to data obtained for 2b, but without the MePt
peak.
[Pt(p-MeC6H4)2(phen)], 1a. [Pt(p-MeC6H4)2(SMe2)2] (504 mg,
1 mmol) was dissolved in diethyl ether (20 mL), and 1,10-phenan-
throline (180 mg, 1 mmol) was added to it. The mixture was stirred
at room temperature over a period of 2 h, after which the solution
turned yellow. The solvent was evaporated and the yellow residue
was washed with n-hexane and dried under vacuum. Yield:
95%; mp 288 ꢀC (dec). Anal. Found: C, 55.57; H, 3.91; N, 4.85.
C26H22N2Pt requires: C, 56.01; H, 3.98; N, 5.02. 1H NMR: δ 2.30
(6H, s, ArCH3), 6.95 (4H, d, 3J(HoHm)=7.8 Hz, Hm of Ar), 7.48
(d, 4H, 3J(HmHo)=7.8 Hz, 3J(PtHo)=68.7 Hz, Ho of Ar), 9.02
Kinetic Study. In a typical experiment, a solution of complex 1
in acetone or benzene (3 mL, 3.0 ꢀ 10-4 M) in a cuvette was
thermostated at 25 ꢀC, and MeI with a known concentration was
added using a microsyringe. After rapid stirring, the absorbance
was monitored with time.
Acknowledgment. We thank the Iran National Science
Foundation and the Shiraz University Research Councils
(Grant Nos. 88-GR-SC-12 and 88-GR-SC-18) for finan-
cial support.
3
3
(d, 2H, J(H2H3) =5.0 Hz, J(PtH2) = 18.6 Hz, H2 and H9 of
phen), 8.55 (d, 2H, 3J(H3H4)=8.2 Hz, H4 and H7 of phen), 7.97
3
(s, 2H, H5 and H6 of phen), 7.77 (dd, 2H, J(H4H3) =8.2 Hz,
3J(H2H3) = 5.0 Hz, H3 and H8 of phen). [Pt(p-MeC6H4)2-
(Me2phen)], 1b, was prepared by the same method using [Pt(p-
MeC6H4)2(SMe2)2) and 2,9-dimethyl-1,10-phenanthroline. Yield:
97%; mp 276 ꢀC (dec). Anal. Found: C, 57.71; H, 4.57; N, 4.38.
Supporting Information Available: Figures S1-S2 (kinetic
data and Eyring plots), Tables S1-S6 (primary kinetic data).
These materials are available free of charge via the Internet at
http://pubs.acs.org.