Mechanistic information from low-temperature rapid-scan and NMR measurements on the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II) dimethyl complex

Article abstract of DOI:10.1021/ic0519537

A detailed kinetic study of the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II) dimethyl complex was undertaken in dichloromethane over the temperature range of -90 to +10°C by stopped-flow techniques. Time-resolved UV-vis monitoring of the reaction allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. The second-order rate constant for the protonation step was determined to be 15200 ± 400 M^-1 s^-1 at -78°C, and the corresponding activation parameters are ΔH^? = 15.2 ± 0.6 kJ mol^-1 and ΔS^? = -85 ± 3 J mol^-1 K^-1, which are in agreement with the addition of a proton that results in the formation of the platinum(IV) hydrido complex. The kinetics of the second, methane-releasing reaction step do not show an acid dependence, and the MeCN concentration also does not significantly affect the reaction rate. The activation parameters for the second reaction step were found to be ΔH^? = 75 ± 1 kJ mol^-1, ΔS ^? = +38 ± 5 J mol^-1 K^-1, and ΔV^? = +18 ± 1 cm³ mol^-1, strongly suggesting a dissociative character of the rate-determining step for the reductive elimination reaction. The spectroscopic and kinetic observations were correlated with NMR data and assisted the elucidation of the underlying reaction mechanism.

Full text of DOI:10.1021/ic0519537

Inorg. Chem. 2006, 45, 3613−3621
Mechanistic Information from Low-Temperature Rapid-Scan and NMR
Measurements on the Protonation and Subsequent Reductive
Elimination Reaction of a (Diimine)platinum(II) Dimethyl Complex
Bror J. Wik,† Ivana Ivanovic-Burmazovic,‡ Mats Tilset,*,† and Rudi van Eldik*,‡
Department of Chemistry, UniVersity of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway,
and Institute for Inorganic Chemistry, UniVersity of Erlangen-Nu¨rnberg, Egerlandstrasse 1,
91058 Erlangen, Germany
Received November 11, 2005
A detailed kinetic study of the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II)
dimethyl complex was undertaken in dichloromethane over the temperature range of 90 to 10 C by stopped-
flow techniques. Time-resolved UV vis monitoring of the reaction allowed the assessment of the effects of acid
concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. The second-order rate constant
−
+
°
−
1
1
for the protonation step was determined to be 15200
±
400 M- s- at
−
78
°C, and the corresponding activation
1
1
parameters are
∆
Hq
)
15.2
±
0.6 kJ mol- and
∆
Sq ) −85
±
3 J mol- K-1, which are in agreement with the
addition of a proton that results in the formation of the platinum(IV) hydrido complex. The kinetics of the second,
methane-releasing reaction step do not show an acid dependence, and the MeCN concentration also does not
significantly affect the reaction rate. The activation parameters for the second reaction step were found to be
∆
Hq
1 cm3 mol-1, strongly suggesting a
1
1
)
75
±
1 kJ mol-
,
∆
Sq ) +38
±
5 J mol- K-1, and
∆
Vq ) +18
±
dissociative character of the rate-determining step for the reductive elimination reaction. The spectroscopic and
kinetic observations were correlated with NMR data and assisted the elucidation of the underlying reaction mechanism.
Introduction
provided significant mechanistic insight, and the activation
and functionalization of hydrocarbons in the “Shilov system”
are considered to take place by the general mechanism
depicted in Scheme 1.8-10
The development of direct, selective methods for catalytic
conversion of hydrocarbons to value-added products remains
a significant challenge.1-4 Early work by Garnett and Shilov
established that aqueous PtII salts are capable of activating
C-H bonds of arenes and alkanes.5-7 Mechanistic studies
on the aqueous K2PtCl4 system and on model systems have
We and others11-24 have recently carried out a series of
mechanistic studies pertaining to the C-H activation steps
(8) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471-2526.
(9) Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003, 54, 259-320.
(10) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
118, 5961-5976.
* To whom correspondence should be addressed. E-mail:
mats.tilset@kjemi.uio.no (M.T.), vaneldik@chemie.uni-erlangen.de (R.v.E.).
† University of Oslo.
(11) Heiberg, H.; Johansson, L.; Gropen, O.; Ryan, O. B.; Swang, O.; Tilset,
M. J. Am. Chem. Soc. 2000, 122, 10831-10845.
(12) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2000, 122, 10846-10855.
‡ University of Erlangen-Nu¨rnberg.
(1) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507-514.
(2) Shilov, A. E.; Shul’pin, G. B. ActiVation and catalytic reactions of
saturated hydrocarbons in the presence of metal complexes; Kluwer
Academic: Dordrecht, The Netherlands, 2000.
(3) Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2001, 2437-2450.
(4) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc.
Chem. Res. 1995, 28, 154-162.
(5) Garnett, J. L.; Hodges, R. J. J. Am. Chem. Soc. 1967, 89, 4546-
4547.
(6) Gol’dshleger, N. F.; Shteinman, A. A.; Shilov, A. E.; Eskova, V. V.
Zh. Fiz. Khim. 1972, 46, 1353-1354.
(7) Gol’dshleger, N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman, A. A.
Zh. Fiz. Khim. 1969, 43, 2174-2175.
(13) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 739-740.
(14) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. J. Am. Chem.
Soc. 2001, 123, 6579-6590.
(15) Wik, B. J.; Lersch, M.; Tilset, M. J. Am. Chem. Soc. 2002, 124,
12116-12117.
(16) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002,
124, 1378-1399.
(17) Procelewska, J.; Zahl, A.; van Eldik, R.; Zhong, H. A.; Labinger, J.
A.; Bercaw, J. E. Inorg. Chem. 2002, 41, 2808-2810.
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10.1021/ic0519537 CCC: $33.50
Published on Web 04/04/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 9, 2006 3613

Wik et al.
Scheme 1
Scheme 3
Scheme 2
and (N-N)PtIV(Me)2(H)(NCMe)+. Increasing concentrations
of MeCN led to reduced PtII/PtIV product ratios. This is
consistent with protonation occurring mostly, if not exclu-
sively, at Pt rather than at a methyl group and also
demonstrates that, under the reaction conditions, C-H
reductive coupling and trapping by the added MeCN ligand
occur at comparable rates.15
However, no quantitative data have been available on the
kinetics of the protonation step and the accompanying MeCN
trapping starting from (N-N)PtMe2. In this paper, we report
the details of a kinetic study of these reactions. Concentra-
tion-, temperature-, and pressure-dependent kinetic measure-
ments allow us to evaluate the kinetics of the protonation
and trapping of MeCN at low temperatures and of MeCN
dissociation and C-H reductive coupling at higher temper-
atures.
using model complexes (N-N)PtR2, where N-N is a diimine
ligand. One question that has been addressed15,25 is whether
the proton to be removed from the hydrocarbon dissociates
directly from an activated metal-hydrocarbon complex (a
so-called σ complex) or from a hydrido ligand that is formed
via oxidative cleavage of the C-H bond of the σ-complex
intermediate (Scheme 2). The fact that scrambling of isotope
labels occurs11-13,16,25 between hydrido and alkyl sites
demonstrates that these species interconvert in solution and
complicates probing of the true, kinetically preferred site of
deprotonation.
The question of the site of deprotonation was recently
addressed by us by investigating the reverse reaction, i.e.,
protonation of (N-N)PtMe2 (I; Scheme 3).15 Protonation
might occur either at a coordinated methyl ligand to produce
the transient σ-methane complex (IV, bottom) or at the metal
(top), resulting in a transient pentacoordinate platinum(IV)
hydridodimethyl species (II). Either of these species is
expected to be trapped by an added ligand, in this case
acetonitrile, in an associative fashion12-14,17 to produce the
platinum(II) solvento complex V or the hexacoordinate
platinum(IV) solvento complex III, respectively. (The pro-
tonation and acetonitrile ligation at Pt may also occur in a
concerted fashion, which will be equally consistent with our
analysis.) Protonation of (N-N)PtMe2 in deuterated dichlo-
romethane (CD2Cl2) at -78 °C in the presence of MeCN
led to mixtures of (N-N)PtII(Me)(NCMe)+ (plus methane)
Experimental Section
General Considerations. Solvents for the kinetic measurements
were used without further purification. P.a.-grade and ultradry
acetonitrile and CH2Cl2 were supplied by Acros. The measurements
in p.a. grade and ultradry solvents did not show any significant
differences. The specific complex studied in this work, (N′-N′)-
PtMe2, where the diimine N′-N′ is ArNdC(Me)sC(Me)dNAr with
Ar ) 2,6-Me2C6H3, was synthesized according to reported meth-
26
ods12 from [Me2Pt(µ-SMe2)]2 and the diimine.27
Sample Preparation. In the kinetic studies, two different
protocols were used for the preparation of the solutions of the Pt
complex (N′-N′)PtMe2 and the acid HBF4‚Et2O. In one protocol,
a solution of the Pt complex in neat CH2Cl2 was mixed with the
acid solution, which was premade in MeCN/CH2Cl2 solvent
mixtures to yield the desired concentrations of acid, MeCN, and
(N′-N′)PtMe2 after mixing. Using this procedure, different experi-
ments were conducted in which the acid concentration was varied
at a constant concentration of MeCN or vice versa. In the other
protocol, CH2Cl2 solutions of the Pt complex and of the acid were
separately prepared with the appropriate concentration of MeCN
being present in both solutions. Thus, in the study of the acetonitrile
dependence of the kinetics, a new solution of the acid as well as
the complex had to be prepared for each set of measurements. In
the observation cell, the concentration of (N′-N′)PtMe2 was 0.125
mM, whereas the acid concentration was varied between 0.5 and 3
mM, and the concentration of acetonitrile was adjusted to be
between 1 and 30 vol % after mixing. A series of experiments with
(19) Heyduk, A. F.; Driver, T. G.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2004, 126, 15034-15035.
(20) Driver, T. G.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Organo-
metallics 2005, 24, 3644-3654.
(21) Gerdes, G.; Chen, P. Organometallics 2003, 22, 2217-2225.
(22) Labinger, J. A.; Bercaw, J. E.; Tilset, M. Organometallics 2006, 25,
805-808.
(23) Gerdes, G.; Chen, P. Organometallics 2006, 25, 809-811.
(24) Wik, B. J.; Lersch, M.; Krivokapic, A.; Tilset, M. J. Am. Chem. Soc.
2006, 128, 2682-2696.
(26) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643-1648.
(27) Tom Dieck, H.; Svoboda, M.; Greiser, T. Z. Naturforsch. B 1981,
36B, 823-832.
(25) Tilset, M.; Johansson, L.; Lersch, M.; Wik, B. J. ACS Symp. Ser. 2004,
885, 264-282.
3614 Inorganic Chemistry, Vol. 45, No. 9, 2006

(Diimine)platinum(II) Dimethyl Complex
1
CN was previously studied by H NMR spectroscopy.15
different concentrations of Bu4NPF6 were performed in order to
check whether the ionic strength of the solution influences the
protonation reaction. The ionic strength did not show any significant
effects on the rate of the reaction up to 3 mM Bu4NPF6.
Measurements. UV-vis spectra were recorded on Shimadzu
UV-2101 and Hewlett-Packard 8542A spectrophotometers. Low-
temperature kinetic data were obtained by recording time-resolved
UV-vis spectra using a modified Bio-Logic stopped-flow module
µSFM-20 combined with a cryo-stopped-flow accessory (Huber
CC90 cryostat) and equipped with a J & M TIDAS high-speed
diode array spectrometer with a combined deuterium and tungsten
lamp (200-1015-nm bandwidth). Isolast O rings were used for all
sealing purposes. Data were analyzed using the integrated Bio-
Kine software, version 4.23, and the software package Specfit/32
global analysis program. Measurements under high pressure were
carried out using a homemade high-pressure stopped-flow instru-
ment,28,29 for which Isolast O rings were also used for all syringe
seals.
At least 10 kinetic runs were recorded under all conditions, and
the reported rate constants represent the mean values. The UV-
vis spectrophotometers and stopped-flow instrument were thermo-
stated to the desired temperature ( 0.1 °C. Values of ∆Hq and
∆Sq were calculated from the slopes and intercepts of plots of ln-
(k/T) versus 1/T, respectively, and values of ∆Vq were calculated
from the slope of plots of ln(k) versus pressure in the usual way.30
The activation parameters and corresponding error limits were
calculated from a weighted linear least-squares fit of the data, in
which the experimental points were weighed according to the
magnitude of the error (Origin version 7).
Protonation occurs at Pt with concomitant MeCN coordina-
tion to form the platinum(IV) hydrido complex (N′-N′)Pt-
(Me)2(H)(NCMe)+, which is kinetically stabilized at low
temperatures by the apically coordinated MeCN ligand.31 At
temperatures of about -40 °C, a gradual release of methane
and the formation of (N′-N′)Pt(Me)(MeCN)+ were observed
in a second reaction step. This reaction sequence has now
been subjected to a systematic kinetic investigation in CH2-
Cl2 containing variable amounts of MeCN over the temper-
ature range of -90 to +10 °C by stopped-flow techniques
with time-resolved UV-vis monitoring of the reaction. In
general, the same two reaction steps are observed by UV-
vis monitoring.
Protonation below -40 °C. The first reaction step was
observed at -78 °C in MeCN/CH2Cl2 mixtures of different
compositions (see the Experimental Section). The resulting
time-resolved UV-vis spectra clearly indicate a one-step
reaction as shown in Figure 1. The characteristic absorption
bands of (N′-N′)PtMe2 at 540, 504, and 377 nm decay, while
the absorbance at wavelengths below 320 nm increases,
resulting in a clean isosbestic point at 320 nm. These spectral
changes and the resulting pseudo-first-order kinetics do not
depend on [MeCN] in the range of 5-30% (v/v). Lower
acetonitrile concentrations could not be used for the kinetic
measurements (but were utilized in the previous NMR
study15) because of the decrease in solubility of HBF4‚Et2O
at low temperature.32 The presence of MeCN in the solution
of (N′-N′)PtMe2 in CH2Cl2 prior to the addition of acid, or
the addition of MeCN together with the acid, does not cause
any differences in the spectra or the kinetics. To avoid
potential complications arising from mixing of two different
solvent compositions at low temperatures, it was preferable
to have the same concentration of MeCN in both the
solutions of the Pt complex and of the acid before mixing.
The acid concentration dependence (0.5-3 mM acid) of the
pseudo-first-order rate constants at -78 °C in the presence
of 30% MeCN is shown in Figure 2. At the two lowest acid
concentrations, the observed rate constants for protonation,
Check for H/D Scrambling in Unreacted Pt Methyl Groups
1
by H NMR. An NMR tube equipped with a Teflon needle valve
was loaded with (N′-N′)PtMe2 (5 mg, 9.7 × 10-3 mmol), and CD3-
CN/CD2Cl2 (1:1, v/v; ca. 0.4 mL) was added by vacuum transfer.
The tube was shaken to dissolve the content, and an ambient-
temperature NMR spectrum (200 MHz) was recorded: δ 0.63 (s,
2J(195Pt-H) ) 86.7 Hz, 6 H, Pt-Me), 1.23 (s, 6 H, NdCMe), 2.11
(s, 12 H, aryl-Me), 6.98-7.21 (m, 6 H, aryl-H). The tube was
immersed in a -45 °C cooling bath, and DOTf (9 µL, ca. 10 equiv)
was added against a flush of argon. The tube was quickly shaken
to mix the reactants and held at the cooling bath temperature for
1.5 h. The 1H NMR spectrum was then recorded at ambient
temperature. The 1H NMR spectrum (200 MHz) revealed the
presence of CH4 and CH3D (ca. 1:1) as well as (N′-N′)Pt(CH3)-
k
obs(H+), were obtained from initial rate measurements
2
(NCCD3)+: δ 0.14 (t, CH3D), 0.15 (s, CH4), 0.38 (s, J(195Pt-H)
because the kinetic traces exhibited a significant deviation
from first-order behavior as a result of the absence of pseudo-
first-order conditions. It appeared to us that the deviations
may also be caused by incomplete acid or ion-pair dissocia-
tion to furnish the “free acid”, whatever its exact identity
might be,33 under the low-temperature conditions. The
initially rapid protonation caused by “free acid” would then
be slowed at longer reaction times, either because further
dissociation is needed to furnish the “free acid” or because
protonation by “undissociated acid” does occur but at a
considerably slower rate than that by “free acid”. It is
) 75.2 Hz, 3 H, Pt-Me), 2.15 (s, 3 H, NdCMe), 2.25 (s, 3 H,
NdCMe), 2.35 (s, 12 H, aryl-Me), 7.23-7.39 (m, 6 H, aryl-H).
There was no detectable quantity of the isotopomer (N′-N′)Pt-
(CH2D)(NCCD3)+, which is readily13 discernible by 1H NMR. There
was also no evidence for D incorporation into the aryl-Me groups
of the product or unconsumed reactant. Protonation with HBF4‚
Et2O in the presence of small quantities of methanol-d4 led to
substantial amounts of CH3D, demonstrating that isotopic “leakage”
can occur with protic impurities.
Results and Discussion
The protonation of the platinum(II) dimethyl complex
(N′-N′)PtMe2 with HBF4‚Et2O in CD2Cl2 containing CD3-
(31) Puddephatt, R. J. Coord. Chem. ReV. 2001, 219-221, 157-185.
(32) Triflic acid undergoes phase separation in CH2Cl2 at low tempera-
tures: Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996,
15, 2504-2516.
(33) Protonated ether is the likely active proton donor because the
approximate pKa of protonated acetonitrile (ca. -12) is considerably
lower than that of protonated ethers (-2 to -5): Lowry, T. H.;
Richardson, K. S. Mechanism and Theory in Organic Chemistry;
Harper & Row: New York, 1987; p 297.
(28) van Eldik, R.; Palmer, D. A.; Schmidt, R.; Kelm, H. Inorg. Chim.
Acta 1981, 50, 131-135.
(29) van Eldik, R.; Gaede, W.; Wieland, S.; Kraft, J.; Spitzer, M.; Palmer,
D. A. ReV. Sci. Instrum. 1993, 64, 1355-1357.
(30) van Eldik, R.; Hubbard, C. D. In Chemistry at Extreme Conditions;
Riad Manaa, M., Ed.; Elsevier: Amsterdam, The Netherlands, 2005;
Chapter 4, pp 109-164.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3615

Wik et al.
Figure 1. Time-resolved spectra for the protonation reaction of (N′-N′)PtMe2 in CH2Cl2 at -78 °C. Experimental conditions: [PtII] ) 0.125 mM, [MeCN]
) 5.74 M (30%), [HBF4‚Et2O] ) 1 mM, and reaction time ) 0.6 s.
Figure 2. kobs(H+) vs [HBF4‚Et2O] for the protonation reaction in CH2-
Figure 3. Eyring plot for the protonation of (N′-N′)PtMe2 with
Cl2. Experimental conditions: [PtII] ) 0.125 mM, [MeCN] ) 5.74 M (30%),
and T ) -78 °C.
HBF4‚Et2O in CH2Cl2. Experimental conditions: [PtII] ) 0.125 mM,
[MeCN] ) 5.74 M (30%), and [HBF4‚Et2O] ) 1 mM.
the protonation step was found to be 15 200 ( 400 s-1 M-1
at -78 °C. The absence of a significant nonzero intercept
suggests that there is no back or parallel reaction that is not
first-order in [H+].
Temperature-dependent kinetic measurements (Table S1
in the Supporting Information) were done by recording the
time-dependent spectra of the protonation of (N′-N′)PtMe2
with a 1 mM acid solution with 30% MeCN present in the
therefore important to have a substantially greater than 10-
fold excess of acid compared to the (N′-N′)PtMe2 concen-
tration in order to ensure a good first-order behavior. Because
of the significant change in absorbance in the monitored
UV-vis range caused by protonation of (N′-N′)PtMe2, the
experiments could be performed with a lower concentration
(0.03 mM) of the Pt complex at the lowest HBF4‚Et2O
concentrations (but unchanged MeCN concentrations), and
indeed a clean first-order kinetic behavior resulted. To our
gratification, under these conditions the kobs(H+) value is in
good agreement with the value obtained from the initial rates
when operating at the higher Pt complex concentration. From
the slope of the resulting good linear plot of kobs(H+) versus
[HBF4‚Et2O] (Figure 2), the second-order rate constant for
temperature range of -90 to -40 °C. The rate constant kobs
-
(H+) could not be accurately obtained at higher temperatures
because of the intervention of the subsequent reaction step
(vide infra). The Eyring plot in Figure 3 shows an excellent
linear fit, and the resulting activation parameters are ∆Hq )
15.2 ( 0.6 kJ mol-1 and ∆Sq ) -85 ( 3 J mol-1 K-1.
3616 Inorganic Chemistry, Vol. 45, No. 9, 2006

(Diimine)platinum(II) Dimethyl Complex
These appear to be rather unique data for the kinetic and
activation parameters of protonation reactions at metal
centers. Previous studies have revealed that protonation
reactions of organotransition-metal complexes can be mecha-
nistically complex. Protonation can occur at multiple sites
in such complexes, either at the metal or at a ligand. There
is a definite possibility that rapid proton transfer may occur,
inter- or intramolecularly, between these sites, and this makes
it far from trivial to assess which is the kinetically preferred
site of protonation.34-38 It is commonly considered that proton
transfers at carbon sites or at metal centers are intrinsically
slow because of the considerable geometric and electronic
rearrangements that arise as a consequence of the protonation
event and that protonation of a hydrido ligand may be
kinetically preferred because the accompanying structural/
electronic changes are less significant.39-41 The protonation
process described herein appears to be rather fast for a well-
documented15 metal-centered process and is characterized by
a low activation enthalpy. The large negative value for the
activation entropy and the low activation enthalpy strongly
suggest that the observed process has an associative character
and is in agreement with the addition of a proton and MeCN,
formally an oxidative addition process. This results in the
formation of the platinum(IV) hydrido complex (N′-N′)Pt-
(Me)2(H)(NCMe)+, which, as already mentioned, was spec-
troscopically identified by 1H NMR in the same temperature
range and concluded to be the kinetically preferred proto-
nation product.15 We infer that the product of our first
reaction step has to be the hexacoordinate platinum(IV)
hydrido complex, which is stabilized31 by the axial MeCN
ligand.
Figure 4.
k
obs(H+) for the protonation reaction vs [MeCN] in CH2Cl2.
Experimental conditions: [PtII] ) 0.125 mM, [HBF4‚Et2O] ) 1 mM, and
T ) -40 °C.
Protonation above -40 °C. At -40 °C, a modest
influence (less than a factor of 2) of [MeCN] on the observed
rate constant for the protonation step, kobs(H+), is observed
when [MeCN] is varied from 0.95 to 5.9 M (Figure 4). Our
interpretation of this, already discussed above in the context
of low-temperature protonations at -78 °C, is that the
increased solvent polarity will enhance acid or ion-pair
dissociation to furnish the “free acid”. Hence, the effective
“free acid” concentration will increase with increasing
[MeCN] at constant [HBF4‚Et2O] but only until complete
dissociation is achieved. Only at -40 °C, when the proto-
nation has become considerably faster than that at -78 °C,
does the extent of dissociation start to affect the proton-
transfer kinetics. Alternatively, the concentration dependence
in Figure 4 might represent a “saturation effect” that is typical
of saturation kinetics. Such behavior might result for a two-
step protonation reaction in which MeCN is involved in the
second step; i.e., capture of the pentacoordinate intermediate
would be rate-limiting at low [MeCN]. Such behavior was,
however, not seen at -78 °C, and it appears that the simplest
unifying explanation for the [MeCN] dependence at all
temperatures would be the acid/ion-pair dissociation model.
At -40 °C, the hexacoordinate platinum(IV) hydrido (N′-
N′)Pt(Me)2(H)(NCMe)+ (based on the previous NMR stud-
ies15) undergoes a gradual methane release to furnish (N′-
N′)Pt(Me)(NCMe)+ sufficiently fast to be monitored together
with the protonation step on the same time scale. In Figure
5a, a rapid decay of the absorbances at 544, 509, and 369
nm, which is characteristic for the conversion of the starting
platinum(II) complex to the hexacoordinate platinum(IV)
hydrido complex, is followed by a slow increase in absor-
bance at 397 and 306 nm related to the formation of the PtII
product (N′-N′)Pt(Me)(NCMe)+. The calculated spectra of
the three Pt species involved in the two-step reaction
mechanism are presented in Figure 5b, clearly depicting the
individual spectra of starting, intermediate, and product
complexes. On an even longer time scale, the second reaction
step goes to completion (Figure 5c).
The low-temperature protonation that was investigated
by NMR15 was conducted at varying MeCN concentrations
(see the Introduction). At low [MeCN], intramolecular
reductive H-CH3 coupling with subsequent associative
methane displacement competed with MeCN trapping,
concerted or stepwise, of the putative pentacoordinate
intermediate (N′-N′)Pt(Me)2(H)+ to give a mixture of
the PtII and PtIV complexes (N′-N′)Pt(Me)(NCMe)+ and
(N′-N′)Pt(Me)2(H)(NCMe)+, respectively, as products
(Scheme 3). However, at higher [MeCN], trapping of the
pentacoordinate species by MeCN was the predominant
reaction pathway, and a rather clean formation of (N′-N′)-
Pt(Me)2(H)(NCMe)+ was observed. The stopped-flow kinetic
experiments described herein were conducted at similarly
high MeCN concentrations and, consequently, the PtIV
product is the only observable product of the protonation
step at -78 °C.
(34) Autissier, V.; Brockman, E.; Clegg, W.; Harrington, R. W.; Henderson,
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J. Am. Chem. Soc. 2000, 122, 12235-12242.
(39) Kristjansdottir, S. S.; Norton, J. R. In Transition Metal Hydrides;
Dedieu, A., Ed.; VCH: Weinheim, Germany, 1992; pp 309-359.
(40) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155-284.
(41) Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913-926.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3617

Wik et al.
Figure 6. kobs(elim) for the methane elimination vs [MeCN] for the reaction
in CH2Cl2. Experimental conditions: [PtII] ) 0.125 mM, [HBF4‚Et2O] )
1 mM, and T ) -40 to +10 °C.
Table 1. Kinetic Data for the Second Step of the Reaction between
(N′-N′)PtMe2 and HBF4‚Et2O in CH2Cl2 as a Function of Pressurea
P (MPa)
k )
obs(elim) (s-1
8
50
90
3.7 ( 0.2
2.7 ( 0.1
2.0 ( 0.1
1.43 ( 0.04
130
∆V* ) +18 ( 1 cm3 mol-1
a Experimental conditions: [PtII] ) 0.125 mM, [MeCN] ) 5.74 M,
[HBF4‚Et2O] ) 1 mM, and T ) 5 °C.
dependence, as might be anticipated. However, somewhat
surprisingly, the MeCN concentration also does not signifi-
cantly affect the reaction rate. The dependence of kobs(elim)
on [MeCN] as a function of temperature is shown in Figure
6. Only a slight MeCN dependence was observed at the
highest applied temperature (+10 °C), which probably results
from a medium effect. From the average values of measured
k
obs(elim) at each temperature (Table S2 in the Supporting
Information), the activation parameters for the second
reaction step were found to be ∆Hq ) 75 ( 1 kJ mol-1 and
∆Sq ) +38 ( 5 J mol-1 K-1. The kinetics of this step were
furthermore monitored at elevated pressures at 5 °C for a
solution containing 30% MeCN by volume. From the good
linear correlation between ln[kobs(elim)] and the pressure data
summarized in Table 1, the activation volume was found to
be ∆Vq ) +18 ( 1 cm3 mol-1. The high activation enthalpy,
large positive value of the activation entropy, and large
positive volume of activation strongly suggest a dissociative
character of the rate-limiting step for the methane-releasing
reaction.
The reaction between (N′-N′)PtMe2 and HBF4‚Et2O was
investigated at room temperature in a tandem cuvette. The
Pt complex was dissolved in neat CH2Cl2 and mixed with
the acid at a constant concentration in MeCN/CH2Cl2 solvent
mixtures of different [MeCN]. No observable changes were
seen in the spectra after mixing of the reactants when
[MeCN] was varied. At the same time, although the spectrum
of the product and the rate constants for the product
formation at different temperatures were not affected by
varying the MeCN concentration, significant differences were
Figure 5. Time-resolved spectra for the reaction of (N′-N′)PtMe2 with
HBF4‚Et2O in CH2Cl2 at -40 °C. (a) Time scale: 1 min. (b) UV-vis spectra
of (N′-N′)PtMe2 (A), intermediate (B), and (N′-N′)Pt(Me)(MeCN)+ (C).
(c) Time scale: 7 min. Experimental conditions: [PtII] ) 0.125 mM, [MeCN]
) 5.74 M (30%), and [HBF4‚Et2O] ) 1 mM.
The kinetics of the methane-releasing reaction step, kobs
-
(elim), were studied as a function of the acid and MeCN
concentrations within the temperature range of -40 to +10
°C. The time-resolved spectra were fitted to a single-
exponential function that resulted in kobs(elim) values. The
rate of this methane release shows no acid concentration
3618 Inorganic Chemistry, Vol. 45, No. 9, 2006

(Diimine)platinum(II) Dimethyl Complex
Scheme 4
pling. In principle, each of the reactions for which kinetics
are reported here may or may not proceed via the hexaco-
ordinate intermediate. This will be taken into account, and
for clarity the two reactionssthe protonation of I to produce
III and the methane elimination from III to produce Vs
will be discussed separately.
Figure 7. Time-resolved spectra for the reaction of (N′-N′)PtMe2 with
HBF4‚Et2O in CH2Cl2 at -40 °C. Experimental conditions: [PtII] ) 0.125
mM, [MeCN] ) 0.96 M (5%), and [HBF4‚Et2O] ) 1 mM. Time scale: 1
min.
seen in an intermediate spectrum, which is characteristic for
the PtIV species that is formed between the protonation step
and the subsequent reductive elimination. In the presence
of higher [MeCN] at -40 °C (Figure 5), the spectroscopic
signature of the intermediate, identical with that of the
platinum(IV) hydrido product of the protonation step at lower
temperatures (Figure 1), is detected. At the same temperature
and [HBF4‚Et2O] but at low [MeCN], the intermediate PtIV
species can no longer be clearly observed (Figure 7). The
calculated spectra of the species involved in the overall
reaction under these conditions and the time-resolved spectra
on the longer time scale are shown in Figure S1 in the
Supporting Information. After the initial rapid protonation,
the starting spectrum for the following step immediately
assumes the characteristics of the product (N′-N′)Pt(Me)-
(NCMe)+, and the overall reaction appears to proceed
without passing through an observable intermediate. This
effect is more prominent at higher temperatures (Figure S2
in the Supporting Information). There may be two important
reasons for this behavior. The first is the fact that the
protonation step has a very low activation enthalpy compared
to the reductive elimination. This results in a dramatic
acceleration of the methane elimination relative to the initial
protonation when the temperature is raised and, eventually,
the elimination starts to interfere with the protonation
kinetics. This occurs most notably at low [MeCN], where
the first step is slowed at lower [MeCN], as was discussed
above (see the discussion of Figure 4). The second reason
is that at lower [MeCN] the formation of the hexacoordinate
PtIV intermediate is less efficient because of competing
reductive H-CH3 coupling and methane loss, which is
consistent with the previously reported NMR studies.
Suggested Mechanism. The results of the present kinetic
investigation are fully consistent with the previously proposed
mechanism15 (Scheme 3) in which the putative (unobserved)
pentacoordinate platinum(IV) hydrido complex is trapped by
a solvent molecule and/or undergoes reductive C-H cou-
The protonation reaction, which proceeds with an experi-
mentally determined rate kobs(H+), can be presented by
Scheme 4. The top pathway represents a concerted proto-
nation/MeCN coordination that does not involve the penta-
coordinate intermediate. The transition state comprises the
starting PtII complex, a proton, and a MeCN molecule and
should ideally be first-order with respect to each component.
The protonation kinetics at -78 °C are zero-order in [MeCN]
over a wide concentration range, and this strongly argues
against the concerted pathway. The stepwise path is consis-
tent with the low-temperature kinetics, provided that the
addition of the proton, i.e., k1, is rate-limiting and the MeCN
addition k2 is fast. At -78 °C, the platinum(IV) hydride is
stable and k-2 (which produces II, the putative intermediate
also for the elimination of methane) is slow.42
Two alternative pathways for the methane elimination
reaction, which proceeds with the experimentally measured
rate kobs(elim), are depicted in Scheme 5. The bottom path
involves initial MeCN dissociation (k-2) to form the penta-
coordinated intermediate II, which yields the σ complex IV
after C-H reductive coupling. The top path is a concerted
process that furnishes the same σ-complex intermediate. Both
alternatives are dissociative with respect to MeCN; they only
differ in the timing of the MeCN release relative to C-H
coupling.43 Reductive X-Y eliminations from octahedral L2-
PtIV(X)(Y) compounds are usually considered to be dissocia-
tive if a readily dissociable ligand is present; whether the
overall process occurs in a concerted or stepwise fashion is
(42) A third alternative in which MeCN addition precedes protonation may
also be discounted on the basis of zero-order MeCN dependence: First-
order kinetics with respect to [MeCN] should arise whether MeCN
addition is rate-limiting or in preequilibrium with rate-limiting
protonation.
(43) A third alternative that produces V directly from III in a concerted
reductive elimination/MeCN dissociation bypasses both intermediates
II and IV. This alternative is discounted on the basis of the substantial
body of experimental evidence that implies complexes akin to IV in
C-H activation and reductive elimination chemistry at Pt.8,9
Inorganic Chemistry, Vol. 45, No. 9, 2006 3619

Wik et al.
Scheme 5
strongly dependent on the nature of X, Y, and the supporting
ligands L.8,9,44,45 (An interesting consequence, imposed by
the principle of microscopic reversibility, of a concerted
process would be that isotopic exchange between hydrido
and methyl sites at Pt, which proceeds by methane “rotation”
at II, would require transition-state stabilization by a donor
ligand, in this case MeCN.)
resonance that would signal the scrambling product suggests
that the reductive coupling k3 is essentially irreversible under
the actual reaction conditions of this study (unless coupling
exhibits a substantial isotope effect that would serve to
attenuate the extent of scrambling). The 1H NMR spectrum
showed that CH4 as well as CH3D were formed in this
experiment. The presence of CH4 is not at odds with the
lack of Pt-CH2D because CH4 would always arise from
unavoidable traces of protic contaminants (see the Experi-
mental Section). It should be pointed out that under quite
different reaction conditions (trifluoroethanol-d3 solvent,
ambient temperature) extensive H/D scrambling between
hydrido and methyl sites was seen when (N′-N′)Pt(CH3)2
was treated with DOTf.13 This suggests that a more polar
solvent could promote scrambling by assisting proton transfer
or by stabilizing the pentacoordinate PtIV intermediate and
making the reaction step II f IV reversible.
At higher temperatures, the subsequent reductive elimina-
tion of methane is observed in the stopped-flow experiments.
The measured rate constant kobs(elim) of this reaction is
independent of [MeCN], suggesting that k-2 must be the rate-
determining step, followed by the rapid reaction sequence
II f IV f V. The combined kinetic experiments have
established that the protonation, kobs(H+), proceeds with a
relatively low ∆Hq of ca. 15 kJ mol-1, whereas the reductive
elimination, kobs(elim), has a ∆Hq value of ca. 75 kJ mol-1.
When the temperature is raised, kobs(elim) will therefore be
accelerated to a much greater extent than kobs(H+). Thus,
whereas kobs(H+) . kobs(elim) at -78 °C, i.e., the platinum-
(IV) hydrido complex is relatively stable because of slow
The putative pentacoordinate species II is a common
intermediate for the protonation (Scheme 4) and elimination
(Scheme 5) reactions. There have been no direct observations
of II in any previous experiments. There is also no direct
evidence for the discrete pentacoordinate hydrido species II
in the rapid-scan experiments reported here, but we note that
a few relatively stable, neutral pentacoordinate platinum-
(IV) alkyl complexes have been recently reported.46-49 The
hexacoordinate species III is stable at -78 °C under the
rapid-scan conditions and does not convert to V on the time
scale of the experiment (30 min). Under the NMR conditions
at this temperature, V is formed, in part, during the
protonation, most notably at low [MeCN].15 At higher
[MeCN], the formation of III predominates. Protonation at
low [MeCN] will favor k3 over k2[MeCN] (see Scheme 3)
to produce a mixture of III and V. There is a possibility
that step 3, reductive coupling/oxidative cleavage, might be
reversible; i.e., k3 and k-3 are both kinetically significant.
This scenario should give rise to H/D scrambling between
hydrido and methyl sites in a reaction between (N′-N′)Pt-
(CH3)2 and deuterated acid and would be detected by the
1
generation of the H NMR-observable scrambling product
(N′-N′)Pt(CH2D)(NCMe)+ after the associative methane
displacement13 from IV by MeCN. This possibility was
investigated under relevant reaction conditions (see the
Experimental Section). The absence of the Pt-CH2D NMR
k
obs(elim), the converse is true at elevated temperatures and
therefore the platinum(IV) hydrido complex cannot be
observed at the higher temperatures. Under such conditions,
k3 must be .k2[MeCN]; if this is not the case, the observed
rate constant should show a significant [MeCN] dependence.
Figure 6 illustrates how kobs(elim) slightly increases with
increasing [MeCN], whereas, in fact, an increase in [MeCN]
should cause a decrease in kobs(elim) according to the rate
law presented in eq 1. Such behavior was recently observed
in related work on a pyridine-coordinated complex.50 If the
(44) Arthur, K. L.; Wang, Q. L.; Bregel, D. M.; Smythe, N. A.; O’Neill,
B. A.; Goldberg, K. I.; Moloy, K. G. Organometallics 2005, 24, 4624-
4628.
(45) Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003,
125, 9442-9456.
(46) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am.
Chem. Soc. 2001, 123, 6425-6426.
(47) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001,
123, 6423-6424.
(48) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804-6805.
(49) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003,
125, 15286-15287.
(50) Procelewska, J.; Zahl, A.; Liehr, G.; van Eldik, R.; Smythe, N. A.;
Williams, B. S.; Goldberg, K. I. Inorg. Chem. 2005, 44, 7732-7742.
3620 Inorganic Chemistry, Vol. 45, No. 9, 2006

(Diimine)platinum(II) Dimethyl Complex
k-2k3 + k-3k2[MeCN]
and quantitative picture of two important reactions, proto-
nation/deprotonation at the metal center and C-H bond
formation, which are relevant to C-H activation at Pt.
Kinetic and activation parameters obtained for the two well-
separated reaction steps clearly suggest the oxidative addition
and reductive elimination character of the two studied
processes. These results are quite unique because kinetic
studies on protonation reactions at metals are rather rare,
and its spectral visualization is difficult because of their rapid
nature. Although in the time-resolved UV-vis spectra there
is no direct evidence for the existence of the pentacoordinate
platinum(IV) hydrido complex, the agreement between
stopped-flow and NMR results supports the mechanism in
which it is the pentacoordinate intermediate that undergoes
reductive elimination. The formation of the crucial penta-
coordinate species proceeds by rate-limiting dissociation of
the apical ligand MeCN from the hexacoordinate PtIV
complex. In a recent report on a closely related system, it
was clearly shown that ligand dissociation induces reductive
C-C coupling,50 which is in full agreement with the findings
of this study.
kobs(elim) )
(1)
k3 + k2[MeCN]
reaction from II to IV is irreversible as supported by the
NMR experiment described above, this rate expression
simplifies to eq 2; furthermore, if k3 . k2[MeCN], it reduces
to kobs(elim) ) k-2. This is entirely consistent with the
positive activation entropy and activation volume values
found for kobs(elim) ()k-2) under these conditions.
k-2k3
kobs(elim) )
) k-2
(2)
k3 + k2[MeCN]
The rapid-scan experiments at -40 °C also show the
formation of III and V in the same time interval, in good
agreement with NMR observations. At higher [MeCN], the
rapid-scan experiments at -40 °C and higher temperatures
clearly show the formation of III (Figure 5) as observed at
-78 °C, whereas at lower [MeCN], the overall reaction
proceeds more through the direct I f II f IV reaction
sequence (Figure 7) than through the I f III f II f IV
sequence.
Acknowledgment. We gratefully acknowledge generous
financial support from the Norwegian Research Council and
the Deutsche Forschungsgemeinschaft and also DAAD for
providing travel support.
Conclusions
In this paper, the results from rapid-scan UV-vis spec-
troscopic measurements on the protonation and subsequent
methane-release reaction of a (diimine)platinum(II) dimethyl
complex within the temperature range of -90 to +10 °C
were reported. In combination with the results of previously
reported NMR studies, this study provides a more complete
Supporting Information Available: Three figures presenting
rapid-scan spectroscopic data and two tables with kinetic data as a
function of temperature. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC0519537
Inorganic Chemistry, Vol. 45, No. 9, 2006 3621