Secondary kinetic deuterium isotope effects in the reaction of mel with organoplatinum(II) complexes

Article abstract of DOI:10.1021/om0500016

Secondary α-deuterium (α-D) kinetic isotope effects (KIEs) have been determined for the reactions of the (diimine)diarylplatinum(II) complexes [Pt(p-MeC₆H₄)₂(NN)] (NN = bipyridine (bpy), 1a; NN = 4,4′-dimethyl-2,2′-bipyrid

Full text of DOI:10.1021/om0500016

2528
Organometallics 2005, 24, 2528-2532
Secondary Kinetic Deuterium Isotope Effects in the
Reaction of MeI with Organoplatinum(II) Complexes
Mehdi Rashidi,* S. Masoud Nabavizadeh, Alireza Akbari, and
Sepideh Habibzadeh
Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Iran
Received January 2, 2005
Secondary R-deuterium (R-D) kinetic isotope effects (KIEs) have been determined for the
reactions of the (diimine)diarylplatinum(II) complexes [Pt(p-MeC6H4)2(NN)] (NN ) bipyridine
(bpy), 1a; NN ) 4,4′-dimethyl-2,2′-bipyridine (Me2bpy), 1b; NN ) 4,4′-di-tert-butyl-2,2′-
bipyridine (tBu2bpy), 1c) with MeI in acetone solvent at different temperatures. Consistent
with an SN2-type mechanism, very small normal secondary deuterium isotope effects
(kH/kD) values of 1.00-1.11 are obtained for the reactions studied. The trends in KIEs and
activation parameter values are discussed in terms of recent experimental and theoretical
1
investigations. The platinum(IV) products have been fully characterized using H and 13C
NMR spectroscopy.
Scheme 1
Introduction
There has been considerable effort to use equilibrium
or kinetic isotope effects to study reactions.1-5 In
particular, secondary R-deuterium (R-D) kinetic isotope
effects (KIEs) have been used to study Menschutkin-
type reactions6 and Stang et al.7 have used KIEs to
confirm an SN2 type mechanism for the oxidative
addition reaction of Vaska’s compound, (Ph3P)2Ir(CO)-
Cl, with MeI. The oxidative addition reaction of alkyl
halides with transition-metal complexes is a key step
in many industrially important catalytic processes.8
Reactions using d8 square-planar (diimine)organoplati-
num(II) complexes have been extensively studied during
the past three decades.9-11 In these reactions, the
electron-rich platinum center attacks alkyl halides
usually by a classical SN2 type mechanism. However,
sometimes the reactions are proceeded by a radical
mechanism. The concerted three-center mechanism is
also a possibility, and although it has been proposed for
some reactions, it has never been demonstrated by
experimental evidence.9 The last mechanism is usually
proposed in the oxidative addition of C-H and C-C
bonds, and also we have recently proposed it in the
oxidative addition of some peroxides, including H2O2,
to (diimine)diarylplatinum(II) complexes.12
In the present work, we have studied the kinetics of
reactions of CH3I/CD3I with some (diimine)diarylplati-
num(II) complexes9,13 (Scheme 1); the KIE results have
confirmed the operation of an SN2 type mechanism in
this important type of oxidative addition reaction. The
complexes used were reacted at rates that could easily
be monitored by UV-vis spectroscopy by conventional
methods, and thus careful measurements were possible.
The results are discussed in terms of recent theoretical
and experimental studies. The platinum(IV) products
* To whom correspondence should be addressed. E-mail: rashidi@
chem.susc.ac.ir. Fax: (+98) 711-228-6008. Phone: (+98) 711-228-4822.
(1) Janak, K. E.; Parkin, G. J. Am. Chem. Soc. 2003, 125, 6889.
(2) Janak, K. E.; Parkin, G. J. Am. Chem. Soc. 2003, 125, 13219.
(3) Janak, K. E.; Parkin, G. Organometallics 2003, 22, 4378.
(4) Hu, W. P.; Truhlar, D. G. J. Am. Chem. Soc. 1994, 116, 7797.
(5) Davico, G. E.; Bierbaum, V. M. J. Am. Chem. Soc. 2000, 122,
1740.
1
have been fully characterized using H and 13C NMR
spectroscopy.
Experimental Section
The 1H and 13C NMR spectra of the complexes were recorded
as CDCl3 solutions on a Bruker Avance DPX 250 MHz
spectrometer, and TMS (0.00) was used as an external refer-
ence. 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 spectropho-
(6) (a) Balaban, A. T.; Bota, A.; Oniciu, D. C.; Klatte, G.; Roussel,
C.; Metzger, J. J. Chem. Res. Synop. 1982, 44. (b) Leffek, K. T.;
Matheson, A. F. Can. J. Chem. 1972, 50, 986.
(7) Stang, P. J.; Schlavelll, M. D.; Chenault, H. K.; Breldegam, J.
L. Organometallics 1984, 3, 1133.
(8) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. A.
Principles and Applications of Organotransition Metal Chemistry, 2nd
ed.; University Science Books: Mill Valley, CA, 1987.
(9) Rendina, L. M.; Puddephatt, R. J. Chem. Rev. 1997, 97, 1735.
(10) Rashidi, M.; Esmaeilbeig, A. R.; Shahabadi, N.; Tangestanine-
jad, S.; Puddephatt, R. J. J. Organomet. Chem. 1998, 568, 53.
(11) Rashidi, M.; Momeni, B. Z. J. Organomet. Chem. 1999, 574,
286.
(12) Rashidi, M.; Nabavizadeh, S. M.; Hakimelahi, R.; Jamali, S. J.
Chem. Soc., Dalton Trans. 2001, 3430.
(13) Jawad, J. K.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans.
1977, 1466.
10.1021/om0500016 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/15/2005

Secondary Kinetic Deuterium Isotope Effects
Organometallics, Vol. 24, No. 10, 2005 2529
tometer with temperature control using an EYELA NCB-3100
constant-temperature bath. The diimine ligand 4,4′-di-tert-
butyl-2,2′-bipyridine (tBu2bpy)14 and the starting complex cis-
[Pt(p-MeC6H4)2(SMe2)2]15 were made by literature methods.
The complex [PtMe2(tBu2bpy)] was prepared as reported.16
The following complexes were made similarly using the
appropriate platinum(II) complex and CH3I or CD3I.
[PtIMe(p-MeC6H4)2(tBu2bpy)] (2c). Yield: 90%. Mp: 188
°C dec. Anal. Calcd for C33H41N2IPt: C, 50.3; H, 5.3; N, 3.6.
Found: C, 51.0; H, 5.5; N, 3.8. 13C{1H} NMR (trans isomer):
δ 20.2 (s, 1J(PtC) ) 670 Hz, MePt), 32.0 (s, terminal C atoms
of tBu groups), 37.1 (s, central C atoms of tBu groups); aromatic
C atoms of tBu2bpy ligand 151.0 (s, 2J(PtC) ) 16 Hz, C6), 125.9
(s, C5), 156.8 (s, C4), 121.6 (s, C3), 165.2 (s, C2); aromatic C
1
13C{1H} NMR: δ -17.5 (s, J(PtC) ) 801 Hz, MePt), 29.8 (s,
t
terminal C atoms of Bu groups), 35.4 (s, central C atoms of
t
tBu groups); aromatic C atoms of Bu2bpy ligand 146.5 (s,
3
2J(PtC) ) 34 Hz, C6), 123.8 (s, J(PtC) ) 19 Hz, C5), 156.3 (s,
1
3
4J(PtC) ) 9 Hz, C4), 118.6 (s, J(PtC) ) 11 Hz, C3), 160.7 (s,
atoms of p-tolyl ligands 134.0 (s, J(PtC) ) 716 Hz, C atoms
directly attached to platinum), 133.3 (s, Co atoms), 128.3-
131.0 (Cm and Cp atoms). 13C{1H} NMR (cis isomer): δ 5.2 (s,
C2).
[Pt(p-MeC6H4)2(tBu2bpy)] (1c). To a solution of cis-[Pt(p-
t
1J(PtC) ) 657 Hz, MePt), 32.0 (s, terminal C atoms of Bu
MeC6H4)2(SMe2)2] (500 mg, 1 mmol) in ether (100 mL) was
t
t
groups), 37.0 (s, central C atoms of Bu groups); aromatic C
added Bu2bpy (270 mg, 1 mmol). A red solution was formed.
t
2
atoms of Bu2bpy ligand 148.4 (s, J(PtC) ) 13 Hz, C6), 149.9
(s, 2J(PtC) ) 14 Hz, C6), 125.5 and 125.6 (s, C5), 156.2 (s, C4),
121.7 and 121.9 (s, C3), 165.2 (s, C2); aromatic C atoms of
This was stirred for 30 min, and then the solvent was removed.
The resulting red needles were washed with ether (2 × 2 mL)
and dried under vacuum; yield 87%, mp 226 °C dec. Anal.
Calcd for C32H38N2Pt: C, 59.5; H, 5.9; N, 4.3. Found: C, 60.0;
H, 6.2; N, 4.4. UV-vis (λmax/nm): in acetone, 434, in benzene,
475 (shoulder 450); in n-hexane, 510 (shoulder 478). 13C{1H}
NMR: δ 20.0 (s, Me groups on aryl ligands), 29.8 (s, terminal
C atoms of tBu groups), 35.2 (s, central C atoms of tBu groups);
aromatic C atoms of tBu2bpy ligand 149.5 (s, 2J(PtC) ) 40 Hz,
1
p-tolyl ligands 134.5 and 136.0 (s, J(PtC) obscured, C atoms
directly attached to platinum), 133.3 (s, Co atoms), 128.3-131.0
(Cm and Cp atoms). H NMR (trans isomer): δ 1.3 (s, J(PtH)
1
2
t
obscured, 3H, MePt), 1.38 (s, 18 H, Bu); aromatic protons of
tBu2bpy ligand 8.85 (d, 3J(PtH) ) 12.4 Hz, 3J(H5H6) ) 6.0 Hz,
2H, H6), 6.7 (H5), 8.09 (s, 2H, H3), 2.22 (s, 6H, Me groups on
p-tolyl ligands); aromatic protons of p-tolyl ligands 7.2-7.6
3
C6), 123.6 (s, J(PtC) ) 20 Hz, C5), 155.8 (s, C4), 118.1(s, C3),
(4H, Ho), 6.2-6.6 (4H, Hm). H NMR (cis isomer): δ 2.06 (s,
1
161.5 (s, C2); aromatic C atoms of p-tolyl ligands 140.9 (s,
1J(PtC) ) 1070 Hz, C atoms directly attached to platinum),
2J(PtH) ) 71.1 Hz, 3H, MePt), 1.34 (s, 9 H, tBu), 1.37 (s, 9 H,
tBu); aromatic protons of Bu2bpy ligand 8.98 (d, 3J(PtH) )
t
2
3
127.6 (s, J(PtC) ) 83 Hz, 2 Co), 137.6 (s, J(PtC) ) 35 Hz, 2
3
11.6 Hz, J(H5H6) ) 5.9 Hz, 2H, H6), 6.7 (H5), 8.02 and 8.04
Cm), 135.7(s, 4J(PtC) ) 38 Hz, Cp). 1H NMR: δ 2.26 (s, 6H,
(s, 2H, H3), 2.23 (s, 6H, Me groups on p-tolyl ligands); aromatic
protons of p-tolyl ligands 7.2-7.6 (4H, Ho), 6.2-6.6 (4H, Hm).
[PtI(CD3)(p-MeC6H4)2(tBu2bpy)]. This compound had 1H
NMR data similar to data obtained for 2c, except for MePt
peaks that were missed.
t
Me groups on p-tolyl ligands), 1.41(s, 18 H, Bu); aromatic
protons of tBu2bpy ligand 8.55 (d, 3J(PtH) ) 23.5 Hz, 3J(H5H6)
) 5.8 Hz, 2H, H6), 7.27 (d, 3J(H6H5) ) 5.0 Hz, 2H, H5), 7.97 (s,
2H, H3); aromatic protons of p-tolyl ligands 7.38 (d, 3J(PtH) )
3
3
68.8 Hz, J(HmHo) ) 7.5 Hz, 4H, Ho), 6.88 (d, J(HoHm) ) 7.0
[PtIMe(p-MeC6H4)2(Me2bpy)] (2b). Yield: 78%. Mp: 190
°C dec. Anal. Calcd for C27H29N2IPt: C, 46.1; H, 4.2; N, 4.0.
Found: C, 45.3; H, 4.3; N, 4.1. 1H NMR (trans isomer): δ 1.89
(s, 2J(PtH) ) 69.7, 3H, MePt), 2.55 (s, 6H, Me groups on
Me2bpy ligand); aromatic protons of Me2bpy ligand 8.87 (d,
3J(PtH) ) 13.0 Hz, 3J(H5H6) ) 5.8 Hz, 2H, H6), 6.87 (H5), 8.14
(s, 2H, H3), 2.32 (s, 6H, Me groups on p-tolyl ligands); aromatic
protons of p-tolyl ligands 7.28-7.5 (4H, Ho), 6.28-6.7 (4H, Hm).
1H NMR (cis isomer): δ 1.45 (s, 2J(PtH) ) 70.9 Hz, 3H, MePt),
2.21, 2.15 (s, 6H, Me groups on Me2bpy ligand); aromatic
protons of Me2bpy ligand, 9 (d, 3J(PtH) ) 11.1 Hz, 3J(H5H6) )
5.4 Hz, 2H, H6), 6.89 (H5), 8.04 and 8.06 (s, 2H, H3), 2.33 (s,
6H, Me groups on p-tolyl ligands); aromatic protons of p-tolyl
ligands 7.28-7.5 (4H, Ho), 6.28-6.7 (4H, Hm).
Hz, 4H, Hm).
[Pt(p-MeC6H4)2(Me2bpy)] (1b). This was made similarly
using Me2bpy in CH2Cl2, and the final product was washed
with n-hexane; yield 90%, mp 225 °C dec. Anal. Calcd for
C26H26N2Pt: C, 55.6; H, 4.6; N, 5.0. Found: C, 55.7; H, 4.7; N,
1
4.9. UV-vis (λmax/nm): in acetone, 435. H NMR: δ 2.23 (s,
6H, Me groups on p-tolyl ligands), 2.41(s, 6H, Me groups on
Me2bpy); aromatic protons of Me2bpy ligand 8.51 (d,
3J(PtH) ) 18.4 Hz, 3J(H5H6) ) 5.5 Hz, 2H, H6), 7.19 (d,
3J(H6H5) ) 5.5 Hz, 2H, H5), 7.86 (s, 2H, H3); aromatic protons
3
3
of p-tolyl ligands 7.37 (d, J(PtH) ) 62.5 Hz, J(HmHo) ) 6.7
Hz, 4H, Ho), 6.88 (d, J(HoHm) ) 6.7 Hz, 4H, Hm).
3
[PtIMe3(tBu2bpy)] (2d). An excess of MeI (1 mL) was
added to a solution of [PtMe2(tBu2bpy)] (100 mg) in CH2Cl2
(15 mL). The reaction mixture was stirred for 30 min, after
which time an orange-yellow solution had developed. The
solvent was removed at reduced pressure, leaving a yellow
solid. This was washed with n-hexane (4 mL) and dried under
vacuum; yield 95%, mp 236 °C dec. Anal. Calcd for C21H33N2-
IPt: C, 39.7; H, 5.2; N, 4.4. Found: C, 40.0; H, 5.4; N, 4.3.
1
[PtI(CD3)(p-MeC6H4)2(Me2bpy)]. This compound had H
NMR data similar to data obtained for 2b, except for MePt
peaks that were missed.
Kinetic Studies of the Reaction of [Pt(p-MeC6H4)2-
(Me2bpy)] with MeI. A solution of [Pt(p-MeC6H4)2(Me2bpy)]
(1b) in acetone (3 mL, 2.64 × 10-4 M) in a cuvette was
thermostated at 25 °C, and a known excess of MeI was added
using a syringe. After rapid stirring, the absorbance at λ 435
nm was collected with time (Figures 1 and 2). The absorbance-
time curves were analyzed by pseudo-first-order methods. 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). A plot of kobs versus [MeI] was
1
13C{1H} NMR: δ -5.3 (s, J(PtC) ) 666 Hz, Me groups trans
1
to N), 9.2 (s, J(PtC) ) 682 Hz, Me group trans to I), 31.7 (s,
t
terminal C atoms of Bu groups), 36.7 (s, central C atoms of
t
tBu groups); aromatic C atoms of Bu2bpy ligands 148.1 (s,
3
2J(PtC) ) 16 Hz, C6), 125.2 (s, J(PtC) ) 14 Hz, C5), 155.9 (s,
3
1
C4), 121.2 (s, J(PtC) ) 8 Hz, C3), 164.4 (s, C2). H NMR: δ
2
0.55 (s, J(PtH) ) 73.5 Hz, 3H, Me group trans to I), 1.45 (s,
2J(PtH) ) 70.3 Hz, 6H, Me groups trans to N), 1.38 (s, 18 H,
Abst ) Abs∞ + (Abs0 - Abs∞) exp(-kobst)
(1)
tBu); aromatic protons of Bu2bpy ligand 8.80 (d, 3J(PtH) )
t
13.4 Hz, J(H5H6) ) 6.0 Hz, 2H, H6), 7.52 (d, J(H6H5) ) 5.7
3
3
linear (Figure 3), and the slope gave the second-order rate
constant. The same method was used at other temperatures
(see Table 1), and activation parameters were obtained from
the Eyring equation (eq 2 and Figure 4).
Hz, 2H, H5), 8.05 (s, 2H, H3).
(14) Adams, C. J.; James, S. L.; Liu, X.; Raithby, P. R.; Yellowlees,
L. J. J. Chem. Soc., Dalton Trans. 2000, 63.
(15) Puddephatt, R. J.; Thomson, M. A. J. Organomet. Chem. 1982,
238, 231.
(16) Achar, S.; Scott, J. D.; Vittal, J. J.; Puddephatt, R. J. Organo-
metallics 1993, 12, 4592.
∆Sq ∆Hq
k2
(T)
kB
( h )
ln
) ln
+
-
(2)
R
RT

2530 Organometallics, Vol. 24, No. 10, 2005
Rashidi et al.
Similar methods were used to study the other reactions at
the corresponding λmax values, and the data are collected in
Table 1.
Results and Discussion
Synthesis and Characterization of the Com-
plexes. The (p-tolyl)platinum(II) precursors [Pt(p-
MeC6H4)2(NN)] (NN ) bipyridine (bpy), 1a; NN ) 4,4′-
dimethyl-2,2′-bipyridine (Me2bpy), 1b; NN ) 4,4′-di-tert-
butyl-2,2′-bipyridine (tBu2bpy), 1c) were made by the
reaction of cis-[Pt(p-MeC6H4)2(SMe2)2] with an equimo-
lar amount of the corresponding NN, by displacement
of SMe2 ligands. As shown in Scheme 1, the platinum-
(II) complexes 1 reacted cleanly with MeI to give the
organoplatinum(IV) complexes [PtIMeR2(NN)] (2), in
each case as an approximately 2:1 mixture of cis and
trans isomers which could not be separated. The com-
plexes were fully characterized by NMR spectroscopy.
The analogous methylplatinum(II) complex 1d and the
corresponding platinum(IV) complex 2d were made
similarly, and their more straightforward NMR results
were used to ascertain the assignments for the aryl
analogues. Full data are collected in the Experimental
Section. The platinum(II) and platinum(IV) complexes
Figure 1. Absorbance-time curves for the reaction of
[Pt(p-MeC6H4)2(Me2bpy)] with MeI (2.65-15.9 mM with
[MeI] increases reading downward) in acetone at 25 °C.
The inset shows the changes in the UV-vis spectrum
during the reaction of [Pt(p-MeC6H4)2(tBu2bpy)] (2.64 ×
10-4 M) and MeI (15.9 mM) in acetone at T ) 20 °C: (a)
initial spectrum (before adding MeI); (b) spectrum at t )
0; successive spectra recorded at intervals of 16 s.
t
with Bu2bpy as ligand show characteristic 1H NMR
signals which are based on that of free tBu2bpy and that
of the dimethyl complexes 1d and 2d. Particularly
useful are H6 protons, which are shifted downfield upon
coordination and are coupled to platinum with a typical
value of 3J(PtH) ) 23.5 Hz for the platinum(II) complex
[Pt(p-MeC6H4)2(tBu2bpy)] (1c), which as expected is
considerably reduced in the corresponding platinum(IV)
complex [PtIMe(p-MeC6H4)2(tBu2bpy)] (2c) (3J(PtH) )
11.6 Hz in the cis isomer and 3J(PtH) ) 12.4 Hz in the
trans isomer). Integration of these peaks is used to
estimate the isomer ratios. The remaining signals in the
aromatic region are assigned to Hm and Ho protons of
p-tolyl ligands. Similarly the other diimine complexes
Figure 2. First-order plots for the reaction of [Pt(p-
MeC6H4)2(NN)] with methyl iodide at different tempera-
tures in acetone: (a) CD3I (2.65 mM), T ) 10 °C, NN )
t
were characterized. The Bu2bpy complexes were more
t
soluble and gave good 13C NMR spectra. In particular,
the signal for aryl carbon directly attached to platinum
in the platinum(II) complex 1c gave a PtC coupling
Me2bpy; (b) MeI (5.3 mM), T ) 10 °C, NN ) Bu2bpy; (c)
CD3I (7.95 mM), T ) 25 °C; NN ) bpy; (d) MeI (10.6 mM),
T ) 25 °C, NN ) tBu2bpy; (e) CD3I (13.25 mM), T ) 25 °C,
t
NN ) Bu2bpy.
1
value of J(PtC) ) 1070 Hz, which appeared further
upfield with a considerably reduced coupling value of
tivation parameters were also determined from mea-
surement at different temperatures (Figure 4), and the
data are given in Table 1. These reactions followed good
second-order kinetics, first order in both platinum(II)
and halide reagent, with remarkable reproducibility
((2%), and the rates were not affected by the addition
of p-benzoquinone as radical scavenger. The rates of the
reaction with MeI in benzene at different temperatures
were slower than the rates of similar reactions in
acetone by a factor of around 7. The kinetics of other
platinum(II) complexes were studied similarly, and the
data are collected in Table 1. The reaction of MeI with
the methyl analogue [PtMe2(tBu2bpy)] (1d) was very fast
and could not be measured easily. These observations
suggest an SN2 mechanism of oxidative addition of
methyl iodide to the platinum(II) complex.17 Also, the
large negative values of ∆Sq are typical of oxidative
addition by a common SN2 mechanism which involves
1J(PtC) ) 716 Hz in the platinum(IV) complex 2c.
2
t
Similarly, the J(PtC) value for the C6 of the Bu2bpy
ligand was 2J(PtC) ) 40 Hz in the platinum(II) complex
1c, which was reduced to 2J(PtC) ) 16 Hz in the
platinum(IV) complex 2c.
Kinetic Study. The kinetics of oxidative addition of
MeI or CD3I to [Pt(p-MeC6H4)2(tBu2bpy)] (1c) in acetone
were studied by using UV-vis spectroscopy. In each
case, excess halide reagent was used and the disappear-
ance of the MLCT band at λ ) 435 nm was used to
monitor the reaction. For minimization of errors, the
rates of addition of MeI and CD3I were measured in
parallel with the identical stock solution of the plati-
num(II) complex. The reactions followed good first-order
kinetics (Figures 1 and 2). Graphs of these first-order
rate constants against the concentration of the halide
gave good straight-line plots passing through the origin,
showing a first-order dependence of the rate on the
concentration of the halide (Figure 3). Thus, the overall
second-order rate constants were determined. The ac-
(17) Monaghan, P. K.; Puddephatt, R. J. J. Chem. Soc., Dalton
Trans. 1988, 595.

Secondary Kinetic Deuterium Isotope Effects
Organometallics, Vol. 24, No. 10, 2005 2531
Table 1. Second-Order Rate Constants and Activation Parametersa for Reaction of [Pt(p-MeC6H4)2(NN)] (1)
with MeI and CD3I in Acetone (or Benzene)b
k2/L mol-1 s-1
kH/kD
(25 °C)
∆Hqc/
∆Sqc/
compd no.
NN
reagent
5 °C
10 °C
15 °C
25 °C
kJ mol-1
J K-1 mol-1
1a
1a
1b
1b
1cd
bpy
MeI
CD3I
MeI
CD3I
MeI
0.21 ( 0.01 0.28 ( 0.01
0.21 ( 0.02 0.27 ( 0.01
0.43 ( 0.01 0.57 ( 0.02
0.42 ( 0.04 0.57 ( 0.01
0.64 ( 0.03
0.37 ( 0.02
0.36 ( 0.04
0.77 ( 0.01
0.76 ( 0.01
0.84 ( 0.02
0.62 ( 0.01
0.60 ( 0.01
1.29 ( 0.01
1.24 ( 0.03
1.43 ( 0.05
1.03 ( 0.02 34.9 ( 0.4
34.1 ( 0.7
-132 ( 2
-135 ( 2
-123 ( 1
-126 ( 2
-123 ( 1
(-161 ( 3)
-126 ( 1
bpy
Me2bpy
Me2bpy
tBu2bpy
1.04 ( 0.03 35.6 ( 0.3
34.8 ( 0.3
1.11 ( 0.05 35.3 ( 0.4
(29.2 ( 0.8)
(0.10 ( 0.01) (0.12 ( 0.02) (0.19 ( 0.03)
0.44 ( 0.02 0.60 ( 0.02 0.77 ( 0.03 1.30 ( 0.04
1c
tBu2bpy
CD3I
34.7 ( 0.4
a Values given on the basis of 95% confidence limits from least-squares regression analysis. b Values in parentheses are in benzene.
c Obtained from the Eyring equation. The errors were computed from the Girolami formula.18 d The values for k2/L mol-1 s-1: at 20 °C,
1.08 ( 0.04 in acetone (0.16 ( 0.02 in benzene); at 30 °C, 1.84 ( 0.03 in acetone (0.23 ( 0.07 in benzene).
of the platinum center. The (tBu2bpy)PtII complex 1c
reacted slightly faster than did the (Me2bpy)PtII complex
1b, but the rate increase is not significant at all. Also,
note that, as expected, the complexes studied in the
present work reacted with MeI much faster than did
Vaska’s complex, [IrCl(CO)(PPh3)2], in a related reac-
tion.7 For example, for [Pt(p-MeC6H4)2(tBu2bpy)] (1c) k2
at 25 °C in acetone is 1.43 L mol-1 s-1, while for Vaska’s
complex, the rate at 25 °C in acetonitrile is 0.0229 L
mol-1 s-1, even though acetonitrile is more polar than
acetone.7
As shown in Table 1, consistent with the proposed SN2
mechanism, the small normal KIEs (kH/kD ) 1.03-1.11
at 25 °C) were observed for the three systems studied.
This is also consistent with the previous results obtained
for the Menschutkin-type reactions of methyl halides
Figure 3. Plots of first-order rate constants (kobs/s-1) for
with different nucleophiles,6 for the reduction of MeI
the reaction of [Pt(p-MeC6H4)2(tBu2bpy)] with MeI in
with hydride ion,19 and for the oxidative addition
acetone at different temperatures versus concentration of
reaction of Vaska’s compound, (Ph3P)2Ir(CO)Cl, with
MeI.
MeI;7 an SN2 mechanism has been suggested in each
case with small inverse to small normal KIEs of 0.93-
1.11, 0.98-1.056, and 0.94-1.00, respectively. Also,
small normal KIEs of 1.06-1.16 have recently been
reported for the reaction of cis-[M(CO)2I2]- (M ) Rh,
Ir) with MeI, which are in excellent agreement with the
theoretical values of 1.09-1.15 obtained for the reac-
tions having a classical SN2 type intermediate.20 Haynes
et al. in the latter reference have also reported ab initio
molecular orbital calculations for the reaction of the Rh
complex, with MeI approaching the complex from the
carbon end, but in a side-on manner as compared to the
linear approach in the SN2 type; a much larger com-
puted KIE value of 1.92 is obtained for the bent
transition state. Although they emphasize that the bent
TS does not correspond to a three-center concerted
Figure 4. Eyring plots for the reaction of (a) [Pt(p-MeC6-
addition of MeI to the metal center, it probably elimi-
nates the possibility of such a mechanism. As mentioned
before, we have recently studied the kinetics of addition
of some peroxides, R2O2, to (diimine)diarylplatinum(II)
complexes12 and suggested a concerted three-center
intermediate. We found that, probably because the
intermediate is not ionic as in a classical SN2 type one,
reactions were not sensitive to the solvent polarity and
in fact the rates were slightly faster in the nonpolar
benzene solvent than in the polar acetone solvent. In
H4)2(tBu2bpy)] + MeI in benzene, (b) [Pt(p-MeC6H4)2(Me2-
bpy)] + CD3I in acetone, and (c) [Pt(p-MeC6H4)2(tBu2bpy)]
+ MeI in acetone.
nucleophilic attack of the metallic center at the methyl
group of MeI and formation of the cationic intermediate
[(p-MeC6H4)2(tBu2bpy)Pt-Me]+ I-.
When the 4- and 4′-positions of the bpy ligand are
substituted with alkyl groups (whether Me or tBu), the
reaction rates of the corresponding complexes are
increased slightly more than twice compared to the case
where the bpy complex is used. This is consistent with
the proposed mechanism, as the alkyl substitutions
render more basic character to the diimine nitrogen
donor atoms, which in turn increase the nucleophilicity
(18) Morse, P. M.; Spencer, M. D.; Wilson, S. R.; Girolami, G. S.
Organometallics 1994, 13, 1646.
(19) Holm, T. J. Am. Chem. Soc. 1999, 121, 515.
(20) Griffin, T. R.; Cook, D. B.; Haynes, A.; Pearson, J. M.; Monti,
D.; Morris, G. E. J. Am. Chem. Soc. 1996, 118, 3029.

2532 Organometallics, Vol. 24, No. 10, 2005
Rashidi et al.
the present study, the complex 1c reacted nearly 7 times
faster in acetone than in benzene, which further sup-
ports the operation of a classical SN2 type mechanism.
Our results also eliminate the possibility of the radical
mechanism in the systems studied, as higher KIE
values are normally expected for this kind of mecha-
nism. Holm in his recent study19 addresses KIEs in
radical mechanisms for reactions between metals and
MeI, and he has reported for example a value of
kH/kD ) 1.68 for electron transfer to MeI from a
platinum cathode.
Finally, an interesting result from Table 1, which has
not been discussed in the previous studies, is that in
all reactions both ∆∆Hq ) 0.8 kJ mol-1 (∆∆Hq
)
∆Hq
∆Sq
- ∆HqCD I) and ∆∆Sq ) +3 J K mol-1 (∆∆Sq )
MeI
3
- ∆SqCD I) are very close to zero within experi-
MeI
mental error. O3n the basis of Halpern’s discussion21 it
is not possible to rule out artifactual as opposed to real
compensation effects. The small KIE could actually be
a result of the different vibrational modes having
opposite contributions.
Truhlar4 has shown that, in determining the total
KIE, the picture based on changes in zero point energies
upon deuteration (vibrations) is incomplete and transla-
tions and rotations must also be considered. Using ab
initio calculations, Bierbaum5 has investigated the
factors influencing the total KIE (η) in terms of the
Conclusions
Very small normal secondary deuterium kinetic iso-
tope effects of 1.00-1.11 were observed for the reactions
of (diimine)organoplatinum(II) complexes with MeI at
different temperatures, which presents further support
for the operation of an SN2-type mechanism in these
important oxidative addition reactions. The results are
discussed in terms of recent experimental and theoreti-
cal studies.4,5,19,20 The KIE for the reaction of the (tBu2-
bpy)PtII complex 1c (1.11) is rather larger than those
of the other nucleophiles (1a, 1.03; 1b, 1.04). This is
attributed to complex 1c being a heavier nucleophile
than the others. For the reaction of the (tBu2bpy)PtII
complex 1c, kH/kD values increase as the temperature
is raised. This is attributed to the effect of temperature
on the ηrot factor (rotational contribution to the total
KIE, η). In all the reactions studied, the ∆∆Hq and ∆∆Sq
values are very close to zero and thus the small KIE is
attributed to different vibrational modes having op-
posite contributions.
contributions (ηtrans, translational; ηrot, rotational; ηvib
,
vibrational) as η ) ηtransηrotηvib. He has shown that the
translational contribution does not change appreciably
for the reactions he studied, but ηrot clearly shows large
variations among the different reactions. He then has
related the rotational partition function to the moments
of inertia, involving the heaviness of the nucleophile.
He concludes that “the final ηrot becomes larger for
heavier nucleophiles reacting with the same neutral”.
Our results in the present study seem to be consistent
with Bierbaum’s studies. First, as Table 1 shows, for
the reaction of the (tBu2bpy)PtII complex 1c, kH/kD
)
1.11, which is rather significantly larger than the values
of kH/kD ) 1.03 and 1.04 for the (bpy)PtII complex 1a
and the (Me2bpy)PtII complex 1b, respectively. The three
systems we studied are very closely related; especially
the environments around the reacting areas (crowded-
ness, which is an important factor5) in different com-
plexes are very similar. The higher kH/kD value observed
for the reaction of a significantly heavier nucleophile,
the (tBu2bpy)PtII complex 1c, is thus understandable.
Second, as the temperature rises, the value of kH/kD also
increases in the reaction of the (tBu2bpy)PtII complex
1c; kH/kD ) 1.07, 1.09, and 1.11 at 10, 15, and 25 °C,
respectively. This might be due to the effect of temper-
ature on the ηrot factor.
Acknowledgment. We thank the Shiraz University
Research Council, Iran (Grant No. 83-GR-SC-25) and
the Iran TWAS chapter based at ISMO for financial
support. We also thank Dr. Ghatee for his key help in
purchasing the UV-vis spectrophotometer.
OM0500016
(21) Halpern, J. Bull. Chem. Soc. Jpn. 1988, 61, 13.