Mechanistic investigation of benzene C-H activation at a cationic platinum(II) center: Direct observation of a platinum(II) benzene adduct

Article abstract of DOI:10.1021/ja0017460

The platinum(II) methyl cation [(N-N)Pt(CH₃)(solv)]⁺BF₄^- (N-N = ArN=C(Me)C(Me)=NAr, Ar = 2,6-(CH₃)₂C₆H₃, solv = H₂O (2a) or TFE = CF₃CH₂OH (2b)) is prepared by treatment of (N-N)Pt(CH₃)₂ with 1 equiv of aqueous HBF₄ in TFE. Reaction of a mixture of 2a and 2b with benzene in TFE/H₂O solutions cleanly affords the platinum(II) phenyl cation [(N-N)Pt(C₆H₅)(solv)]⁺BF₄^- (3). Investigations of the kinetics and isotopic labeling experiments indicate that reaction of 2 with benzene proceeds via benzene coordination, reversible oxidative addition of benzene C-H bonds, reversible formation of a methane C,H-σ complex, and final dissociation of methane. Under conditions where [(N-N)Pt(CH₃)(H₂O)]⁺BF₄^- (2a) is the major starting complex, rate-determining benzene coordination to 2b is implicated by the observed kinetic rate law (inverse first order in [H₂O] and first order in [C₆H₆] to 3.8 M) and the small kinetic deuterium isotope effect for C₆H₆ vs C₆D₆ (k(H)/k(D) = 1.06 ± 0.05 at 25 °C). When deuterated benzenes C₆D₆ and 1,3,5C₆H₃D₃ are used, almost full statistical scrambling of deuterium from one benzene into methane is achieved, indicating that the energetic barriers for dissociating benzene and methane are considerably higher than interconversions of intermediate hydrocarbon complexes and [(N-N)Pt(C₆H₅)(CH₃)H]⁺. Protonation of (N-N)Pt(CH₃)(C₆H₅) with HBF₄ in TFE, which provides an independent route into the manifold of postulated intermediates, gives a mixture of 3 + CH₄ (82%) and 2 + C₆H₆ (18%). Protonation of (N-N)Pt(CH₃)(C₆H₅) with triflic acid in methylene chloride/diethyl ether mixtures at -69 °C allows direct low-temperature NMR observation of a fluxional π benzene complex, [(N-N)Pt(CH₃)(C,C-η²-C₆H₆)]⁺.

Full text of DOI:10.1021/ja0017460

10846
J. Am. Chem. Soc. 2000, 122, 10846-10855
Mechanistic Investigation of Benzene C-H Activation at a Cationic
Platinum(II) Center: Direct Observation of a Platinum(II) Benzene
Adduct
Lars Johansson,1a Mats Tilset,1a Jay A. Labinger,1b and John E. Bercaw*,1b
Contribution from the Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute
of Technology, California 91145, and Department of Chemistry, UniVersity of Oslo, P.O. Box 1033,
Blindern, N-0315 Oslo, Norway
ReceiVed May 19, 2000
-
Abstract: The platinum(II) methyl cation [(N-N)Pt(CH3)(solv)]+BF4 (N-N ) ArNdC(Me)C(Me)dNAr,
Ar ) 2,6-(CH3)2C6H3, solv ) H2O (2a) or TFE ) CF3CH2OH (2b)) is prepared by treatment of (N-N)Pt-
(CH3)2 with 1 equiv of aqueous HBF4 in TFE. Reaction of a mixture of 2a and 2b with benzene in TFE/H2O
solutions cleanly affords the platinum(II) phenyl cation [(N-N)Pt(C6H5)(solv)]+BF4 (3). Investigations of
-
the kinetics and isotopic labeling experiments indicate that reaction of 2 with benzene proceeds via benzene
coordination, reversible oxidative addition of benzene C-H bonds, reversible formation of a methane C,H-σ
complex, and final dissociation of methane. Under conditions where [(N-N)Pt(CH3)(H2O)]+BF4- (2a) is the
major starting complex, rate-determining benzene coordination to 2b is implicated by the observed kinetic
rate law (inverse first order in [H2O] and first order in [C6H6] to 3.8 M) and the small kinetic deuterium
isotope effect for C6H6 vs C6D6 (kH/kD ) 1.06 ( 0.05 at 25 °C). When deuterated benzenes C6D6 and 1,3,5-
C6H3D3 are used, almost full statistical scrambling of deuterium from one benzene into methane is achieved,
indicating that the energetic barriers for dissociating benzene and methane are considerably higher than
interconversions of intermediate hydrocarbon complexes and [(N-N)Pt(C6H5)(CH3)H]+. Protonation of (N-
N)Pt(CH3)(C6H5) with HBF4 in TFE, which provides an independent route into the manifold of postulated
intermediates, gives a mixture of 3 + CH4 (82%) and 2 + C6H6 (18%). Protonation of (N-N)Pt(CH3)(C6H5)
with triflic acid in methylene chloride/diethyl ether mixtures at -69 °C allows direct low-temperature NMR
observation of a fluxional π benzene complex, [(N-N)Pt(CH3)(C,C-η2-C6H6)]+.
Introduction
Scheme 1
The reactions of hydrocarbon C-H bonds with organo-
transition metal complexes have been a major focus of research
over the last two decades, motivated in large part by the
tantalizing possibility of direct utilization of alkanes as chemical
feedstocks. Many examples of C-H actiVation, some remark-
ably efficient, have been reported,2 but relatively few have been
incorporated into schemes for overall alkane functionalization.
Selective oxidation is a particularly attractive target, but the
majority of alkane-activating complexes are unstable to oxidiz-
ing conditions. Most exceptions to this limitation are found in
the class of electrophilic activations by late transition metals.3
The most extensively studied example of the latter is the so-
called Shilov system, consisting of aqueous Pt(II) and Pt(IV)
salts, which converts alkanes to alcohols at 120 °C. Earlier work
in the Caltech group4 and elsewhere5 has delineated the scope
of this reaction, and established the mechanism depicted in
Scheme 1.
Mechanistic details of the actual C-H activation, step (i) in
Scheme 1, cannot be obtained by direct examination of the
working catalytic system, so we resorted to models. Studies of
protonolysis of complexes L2PtRX, the formal microscopic
reverse of the C-H activation step, provide evidence for the
involvement of two intermediatessa Pt(II) C,H-η2-alkane
* To whom correspondence should be addressed. E-mail:
bercaw@caltech.edu.
(1) (a) University of Oslo. (b) California Institute of Technology.
(2) (a) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H.
Acc. Chem. Res. 1995, 28, 154. (b) SelectiVe Hydrocarbon ActiVation;
Davies, J. A., Watson, P. L., Liebman, J. F., Greenberg, A., Eds.; VCH:
New York, 1990. (c) ActiVation and Functionalization of Alkanes; Hill, C.
L., Ed.; John Wiley & Sons: New York, 1989.
(4) (a) Labinger, J. A.; Herring, A. M.; Lyon, D. K.; Luinstra, G. A.;
Bercaw, J. E.; Horvath, I. T.; Eller, K. Organometallics 1993, 12, 895. (b)
Luinstra, G. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1993,
115, 3004. (c) Luinstra, G. A.; Wang, L.; Stahl, S. S.; Labinger, J. A.;
Bercaw, J. E. Organometallics 1994, 13, 755. (d) Luinstra, G. A.; Wang,
L.; Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Organomet. Chem. 1995,
504, 75.
(3) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed.
1998, 37, 2180-92.
10.1021/ja0017460 CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/21/2000

Benzene C-H ActiVation at a Pt(II) Center
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10847
complex and a PtIV(R)(H) oxidative addition productswhich
interconvert rapidly.6 These model complexes differ from the
Shilov system in that they include stabilizing (nitrogen or
phosphorus) ligands, raising the question of their relevance to
the catalytic alkane oxidations. But relevance has been dem-
onstrated, perhaps most dramatically by the finding that such a
complex with L2 ) 2,2′-bipyrimidyl catalyzes the oxidation of
methane to methyl bisulfate by concentrated sulfuric acid in
over 70% yield, by far the most effective example of homo-
geneously catalyzed alkane functionalization yet reported.7
While the above-mentioned model studies concentrated on
the reverse of the reaction of interest, the deduced scheme
indicated that C-H activation might be observed in the
chemistry of [L2PtR(solv)]+, where solv is a weakly bonded
solvent molecule or other ligand. This was first achieved for
the case L2 ) tetramethylethylenediamine (tmeda), R ) methyl,
and solv ) pentafluoropyridine: this cationic Pt(II) complex
reacts with hydrocarbons such as benzene and 13CH4 at 85 °C,
generating methane and the corresponding new organoplatinum-
(II) product.8 More recently the Oslo group9 found that the
analogous diimine complex [(Nf-Nf)Pt(CH3)(OH2)]+BF4- (Nf-Nf
) ArfNdC(Me)C(Me)dNArf, Arf ) 3,5-(CF3)2C6H3) exhibits
the same reactivity under milder conditions in trifluoroethanol
(TFE), readily activating benzene and methane C-H bonds at
25 and 45 °C, respectively.
These stoichiometric C-H activations offer the possibility
of gaining information about several key mechanistic questions.
(1) Does activation proceed via oxidative addition, giving
the PtIV(R)(H) intermediate implicated by the protonolysis
studies (as well as by a related system wherein a PtIV(R)(H)
species generated by C-H activation is the stable final
product10), or by some alternative route such as σ-bond
metathesis? Calculations on a cationic Ir(III) system which had
earlier been shown to undergo similar transformations11 support
oxidative addition,12 whereas several theoretical studies on the
Shilov system and models have been interpreted as supporting
a variety of detailed C-H activation mechanisms.13
Scheme 2
Scheme 3
18-electron species and presumably reacts dissociatively, but
[L2PtR(solv)]+ is an open-shell 16-electron complex, so either
mechanism is in principle possible.
As part of an ongoing study into the effects of ligand
-
variations, we discovered that [(N-N)Pt(CH3)(solv)]+BF4
(N-N ) ArNdC(Me)C(Me)dNAr, Ar ) 2,6-(CH3)2C6H3, solv
) H2O or TFE) (2) undergoes similar C-H activation in TFE,
but the reactions are somewhat cleaner and the products more
stable than in the previous cases. We report here a detailed
kinetic and mechanistic investigation of the C-H activation of
benzene reaction by 2.
Results and Discussion
Preparation of Platinum Complexes and C-H Activation
of Benzene. Following the procedure for preparation of
(Nf-Nf)Pt(CH3)2,9 protonolysis of (N-N)Pt(CH3)2 (1) with 1
equiv of aqueous HBF4 in TFE and evaporation of solvent give
an air-stable orange-brown powder. When the product is
1
dissolved in dry TFE-d3, the H NMR data show the presence
(2) What is the rate-limiting stepscoordination of the
incoming hydrocarbon, the actual C-H activation, or something
else?
(3) Does the reacting hydrocarbon enter the coordination
sphere by an associative or dissociative mechanism? The iridium
system mentioned above, [Cp*Ir(PMe3)(Me)(CH2Cl2)]+, is an
of a mixture of two [(N-N)Pt(CH3)(L)]+ species, in an
approximately 3:1 ratio. Addition of water to the solution results
in an increase in the peaks of the major species and a
corresponding decrease of the minor species. We attribute this
observation to an equilibrium between the expected aqua
-
complex [(N-N)Pt(CH3)(OH2)]+BF4 (2a) and a minor sol-
(5) (a) Kushch, L. A.; Lavrushko, V. V.; Misharin, Y. S.; Moravsky, A.
P.; Shilov, A. E. NouV. J. Chim. 1983, 7, 729. (b) Horvath, I. T.; Cook, R.
A.; Millar, J. M.; Kiss, G. Organometallics 1993, 12, 8. (c) Hutson, A. C.;
Lin, M.; Basickes, N.; Sen, A. J. Organomet. Chem. 1995, 504, 69. (d)
Zamashchikov, V. V.; Popov, V. G.; Rudakov, E. S.; Mitchenko, S. A.
Dokl. Akad. Nauk SSSR 1993, 333, 34.
(6) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1995, 117, 9371. (b) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 1996, 118, 5961-76.
(7) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii,
H. Science 1998, 280, 560.
(8) (a) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1997, 119, 848. (b) Holtcamp, M. W.; Henling, L. M.; Day, M. W.;
Labinger, J. A., Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467.
(9) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999, 121,
1974.
(10) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235.
(11) (a) Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 10462.
(b) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970.
(12) (a) Strout, D. L.; Zaric, S.; Niu, S.; Hall, M. B. J. Am. Chem. Soc.
1998, 118, 6068. (b) Su, M. D.; Chu, S. Y. J. Am. Chem. Soc. 1997, 119,
5373.
(13) (a) Siegbahn, P. E. M.; Crabtree, R. H. J. Am. Chem. Soc. 1996,
118, 4442. (b) Mylvaganam, K.; Bacskay, G. B.; Hush, N. S. J. Am. Chem.
Soc. 1999, 121, 4633. (c) Mylvaganam, K.; Bacskay, G. B.; Hush, N. S. J.
Am. Chem. Soc. 2000, 122, 2041. (d) Heiberg, H.; Swang, O.;, Ryan, O.
B.; Gropen, O. J. Phys. Chem. A 1999, 103, 10004.
vento adduct [(N-N)Pt(CH3)(TFE)]+BF4- (2b) (Scheme 2). The
equilibrium constant Keq ) [2b][H2O]/[2a][TFE] was estimated
at (1.2 ( 0.3) × 10-3 at 25 °C by H NMR analysis of the
1
ratio between 2a and 2b at different water concentrations in
TFE. Analytical data indicate the isolated solid is primarily 2b.
Addition of ca. 30 equiv of benzene (ca. 0.4 M) to a solution
of 2 in TFE-d3 at ambient temperature results in clean formation
of the corresponding phenyl species 3 (“2” and “3” denote the
equilibrium mixture of 2a/b and 3a/b, respectively) with
1
concurrent production of methane, as observed by H NMR
spectroscopy (Scheme 3).14
Complete conversion to 3 is achieved after approximately
16 h. No intermediates can be observed during the course of
the reaction, and the ratio between 3a and 3b is constant and
approximately the same as for 2a and 2b (3:1). Addition of
water after complete reaction leads to an increased 3a:3b ratio,
confirming that the major species 3a is the aqua adduct.
Subsequent addition of acetonitrile to the product mixture
(14) An X-ray crystal structure determination for [(N-N)Pt(C6H5)(CH3-
CN)]+BF4- has been carried out. Its description will be reported separately.

10848 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Johansson et al.
Figure 1. (a) UV/vis spectral changes for the reaction of benzene with 2a in TFE. Isosbestic points appear at 439 and 484 nm. (b) Absorbance at
390 nm versus time: (9) points used for the first-order kinetics least-squares fit (s); (0) points omitted from the fit, which illustrate the “background”
increase in absorbance attributed to slow degradation of the Pt species.
quantitatively produces [(N-N)Pt(C6H5)(NCMe)]+BF4 (3c).
-
The spectroscopic data for 3c are identical to those of an
authentic sample independently synthesized from (N-N)Pt-
(C6H5)2 (4) and HBF4 in CH3CN. Benzene activation by the
mixture of 2a and 2b in TFE thus appears to proceed at a lower
rate, but otherwise in a fashion very similar to that of the
analogous reaction of [(Nf-Nf)Pt(CH3)(H2O)]+.9
Kinetics of the Benzene Activation Reaction. Since the
diimine platinum species described above are intensely colored,
the reaction can be conveniently followed by UV/vis spectros-
copy. The typical spectral change over time for the reaction
between 2a (excess H2O) and benzene in TFE is shown in Figure
1a. The appearance of isosbestic points (439 and 484 nm) is
consistent with the conclusion from 1H NMR that no intermedi-
ates accumulate to a significant level.
Figure 2. kobsd versus [C6H6]/[H2O] or [C6D6]/[H2O] for the benzene
activation at 2a in TFE at various benzene concentrations ([C6H6] and
Kinetic data are obtained by following the absorbance at 390
nm as a function of time. Benzene concentrations in these
experiments were always high enough to ensure pseudo-first-
[C6D6] ) 0.6-3.8 M) and almost constant water concentration ([H2O]
) 0.11-0.15 M). [Pt] ) 5.3-7.6 × 10-3 M; [Me4N+BF4-] ) 0.13-
order conditions ([C6H6] > 10[Pt]). Constant ionic strength in
0.19 M; T ) 25 °C. Calculated rate constants: kH ) (1.98 ( 0.03) ×
10-5 s-1 and kD ) (1.87 ( 0.04) × 10-5 s-1. Calculated kinetic isotope
the system was maintained by the addition of excess of an inert
salt (Me4N+BF4-). Control experiments showed that the pres-
effect: kH/kD ) 1.06 ( 0.05. kobsd ) k[C6H6]/[H2O].
ence of salt has no measurable effect on the reaction rate. As
1
described above, at low water concentrations substantial amounts
of the solvento species 2b and 3b are present in the reaction
mixture. To simplify analysis, the kinetic experiments were
performed at water concentrations where at least 90% of the
platinum species are present as the aqua adducts 2a and 3a
([H2O] > 0.11 M), so that the data can be treated in terms of
a simple transformation of 2a to 3a. Of course, the potential
role of the solvento species 2b/3b must be taken into account
with regard to mechanistic interpretation.
of new H NMR signals in both cases. Furthermore, a TFE
solution of 2 (no benzene present) exhibited a small increase
in absorbance at 390 nm over 15 h. The exact nature of these
decomposition pathways is not fully understood, but the absence
of incorporation of deuterium in the liberated methane (in TFE-
d3 solutions) suggests some kind of inter- or intramolecular C-H
activation of the diimine ligand. However, the rate of the
decomposition, and more importantly the rate of the corre-
sponding increase in absorbance at 390 nm, for 2a/3a is at least
1 order of magnitude lower than that of the benzene C-H
activation reaction and therefore should not affect the observed
rate constants to any significant extent.
The change in absorbance at 390 nm over time exhibited clean
first-order behavior over 3-4 half-lives (Figure 1b); control
experiments showed that the reaction rate was independent of
the initial platinum concentration. When the absorbance was
followed for more than 4 half-lives, deviation from first-order
behavior was manifested by a slow additional increase in
intensity. We believe that this deviation is due to slow
degradation of 2 and 3. In control experiments, slow decom-
position of 2 and 3 in TFE-d3 could indeed be observed by 1H
NMR over several days, accompanied by elimination of methane
and benzene, respectively, and formation of complex patterns
Two sets of experiments were performed: varying benzene
(both C6H6 and C6D6) concentration, keeping water concentra-
tion nearly constant, and the converse. The first-order rate
constants thus obtained, kobs, are plotted as a function of the
ratio [benzene]/[water] in Figures 2 and 3, respectively. The
reaction rate exhibits first-order dependence on benzene con-
centration, with no signs of saturation kinetics in the concentra-
tion range investigated (0.6-3.8 M), and inverse first-order

Benzene C-H ActiVation at a Pt(II) Center
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10849
Table 1. Methane Isotopomer Distributions Obtained in Reactions
with C6H6, 1,3,5-C6H3D3, and C6D6
C6H6
1,3,5-C6H3D3
C6D6
isotopomer found (%) found (%) calcd (%)a found (%) calcd (%)a
CH4
100
-
31
37
20
11
-
12
47
36
5
-
33
<1b
17
32
32
19
-
5
36
47
12
67
CH3D
CH2D2
CHD3
CD4
-
-
-
total % D
-
28
63
a Calculated statistical distribution. b Presumably formed due to slow
concurrent decomposition of 2a.
Scheme 4
Figure 3. kobsd versus [C6H6]/[H2O] for the benzene activation at 2a
in TFE at various water concentrations ([H2O] ) 0.13-1.08 M) and
almost constant benzene concentration ([C6H6] ) 1.42-1.50 M). [Pt]
) 6.7 × 10-3 M; [Me4N+BF4-] ) 0.15 M; T ) 25 °C. Calculated rate
constant: k ) (1.53 ( 0.03) × 10-5 s-1. kobsd ) k[C6H6]/[H2O].
The absence of deuterium in methane generated in the
reaction with C6H6 rules out any H/D exchange between
intermediates and either solvent or water.16 In contrast, the
reactions with 1,3,5-C6H3D3 and C6D6 produce substantial
1
amounts of CH3D, CH2D2, and CHD3 (easily detected by H
NMR spectroscopy) and, in the latter case, significant amounts
2
of CD4 (identified by H NMR spectroscopy). This behavior
may be explained by an extension of the same mechanism
previously proposed,8 shown in Scheme 4.
Isotope scrambling is effected by two processes: intercon-
version between the hydrocarbon complex intermediates
[(N-N)Pt(II)(CH3)(C6H6)]+ and [(N-N)Pt(II)(CH4)(C6H5)]+ by
way of the platinum(IV) methyl/phenyl/hydride, and rearrange-
ment of the C,H-η2-coordinated benzene and methane adducts
from one C-H bond to another. Three additional features should
be noted. First, no discernible incorporation of deuterium into
the Pt-CH3 group of 2a could be observed by 1H NMR during
the course of the reaction.17 Second, when the total extent of
deuterium incorporation in these experiments is calculated,
almost full statistical scrambling of deuterium into methane is
achieVed (63% in the reaction with C6D6 and 28% in the reaction
with 1,3,5-C6H3D3), assuming one molecule of benzene ex-
changes isotopes with one molecule of 2a. Third, the deproto-
nation of [(N-N)Pt(H)(CH3)(C6H5)]+ must be relatively slow,
a difference from the tetramethylenediamine system.6a These
findings imply that (a) once a molecule of benzene has
coordinated and undergone C-H activation, initiating the
exchange process, loss of benzene to regenerate 2a is consider-
ably slower than loss of methane to produce 3a, i.e., scrambling
involving more than one molecule of benzene prior to methane
loss is not significant, and (b) the energetic barriers for
elimination of benzene and methane from the respective
Figure 4. Eyring plot for the reaction of benzene with 2a in TFE.
[C6H6] ) 1.50 M; [H2O] ) 0.24 M; [Pt] ) 6.7 × 10-3 M; [Me4N+BF4
]
-
) 0.15 M; T ) 23, 35, 45, and 55 °C. Calculated activation
parameters: ∆Hq ) 19 ( 0.3 kcal mol-1, ∆Sq ) -16 ( 1 cal mol-1
K-1, and ∆Gq
) 24 ( 1 kcal mol-1
.
298
K
dependence on water concentration, giving the rate law shown
as eq 1. The kinetic isotope effect is close to unity; kH/kD
)
[C6H6]
[H2O]
d[2a]
dt
-
) k
[2a]
(1)
1.06 ( 0.05. The temperature dependence of the reaction was
studied over the range 23-55 °C, and the activation parameters
were calculated from the Eyring plot shown in Figure 4: ∆Hq
) 19 ( 0.3 kcal mol-1, ∆Sq ) -16 ( 1 cal mol-1 K-1, and
(15) The relative amounts of the different isotopomers were determined
by NMR integration with one of the diimine arene methyl signals in the
product serving as internal standard. The quantity of the 1H NMR inactive
isotopomer CD4 was calculated from the total amount of methane generated
(as determined in the reaction with C6H6).
(16) Water is mainly present as D2O in the solution due to facile H/D
exchange with TFE-d3.
∆Gq
) 24 ( 1 kcal mol-1
.
298K
Deuterium Scrambling. As in the previously reported studies
on the closely related species [(tmeda)Pt(CH3)(NC5F5)]+[B{3,5-
C6H3(CF3)2}4]- and [(Nf-Nf)Pt(CH3)(OH2)]+BF4
,
reaction
- 8,9
(17) More recent studies of the reaction of benzene-d6 with a related
platinum methyl complex reveals at partial reaction a very weak 1:1:1 triplet
slightly upfield of the 1H NMR signal for [Pt-CH3], indicative of [Pt-
CH2D]. The NMR data for reaction of 2a with deuterated benzenes is not
of sufficient quality to reveal this minor signal, but we believe that there is
a small extent of deuterium incorporation into the methyl group, as is
expected from the relative rates of loss of benzene and methane on
protonation of (N-N)Pt(II)(CH3)(C6H5) (vide infra). Zhong, A. H.; Lab-
inger, J. A.; Bercaw, J. E. Unpublished results.
with deuterated hydrocarbon substrates leads to multiple
incorporation of deuterium in the liberated methane. In three
parallel NMR tube experiments, C6H6, 1,3,5-C6H3D3, and C6D6,
respectively, were allowed to react with 2a in TFE-d3, and the
methane isotopomer distributions after complete reaction were
1
determined from H NMR analysis.15 Results are summarized
in Table 1.

10850 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Johansson et al.
Scheme 5
Scheme 6
reveals clean formation of what appears to be a single species
(i), stable at this temperature, which we formulate as the benzene
adduct [(N-N)Pt(CH3)(C,C-η2-C6H6)]+, on the basis of the
following: (1) No signals attributable to free methane or
benzene, or to any platinum hydride, can be detected in the 1H
NMR spectrum. (2) The platinum methyl group (integrating for
3 H) appears as a broad signal at δ -1.59, far upfield from
where methyl groups coordinated to neutral or cationic {(N-N)-
Pt(II)} and {(N-N)Pt(IV)} normally resonate20 (Figure 5). (3)
In the aromatic region, a singlet is observed at δ 6.96 integrating
for 6 H, in addition to signals for the diimine aryl protons. (4)
At -33 °C (by which temperature i has begun to convert to
new species) the singlet in the aromatic region exhibits 195Pt
satellites (J(195Pt-H) ) 23.5 Hz); the platinum methyl signal
also exhibits satellites, as well as a substantial shift downfield
(δ -1.35, (2J(195Pt-H) ) 65.9 Hz).
intermediates must be much higher than the barriers involved
in the interconversion between these species.
Synthesis and Protonation of (N-N)Pt(CH3)(C6H5) (5).
Generation of [(N-N)Pt(CH3)(C,C-η2-C6H6)]+ at Low Tem-
perature. Protonation of the methyl phenyl complex 5 offers
an alternate route to the manifold of postulated intermediates
involved in the H/D scrambling process (Scheme 4). Given the
relative activation barrier heights discussed above, it is not
implausible that one or more intermediates might be sufficiently
long-lived for direct observation by NMR at low temperature,
as has been accomplished for several platinum(IV) alkyl hydride
species.6 Furthermore, the relative amounts of the final products
(2, 3, benzene, and methane) resulting from the protonation of
5 in TFE should give a direct measure of the difference in energy
barriers for benzene and methane loss.
Attempted preparation of 5 by direct reaction of (N-N)Pt-
(C6H5)(Cl) with methyllithium or (N-N)Pt(CH3)(Cl) with
phenyllithium led to extensive decomposition and formation of
platinum black. This does not take place with the corresponding
tetramethylethylenediamine complexes, and may be due to low-
lying diimine-centered LUMOs facilitating reduction of the
platinum species by the lithium reagents. We therefore decided
to perform the arylation at platinum before the introduction of
the diimine ligand.
These observations suggest that i contains a C,C-η2-benzene
coordinated to the Pt(II) center. By analogy to platinum(II) olefin
complexes, the coordinated CdC bond would be expected to
be oriented perpendicular to the coordination plane. As shown
in Scheme 6, there are two possible conformers, of which the
one on the left appears likely to be more favorable on steric
grounds. In this conformer, the platinum methyl group should
lie in the shielding region created by the benzene ring current
effect, accounting for the abnormally high upfield shift of the
methyl signal.
This interpretation would also explain the large temperature
dependence of that shift, since the relative population of the
two conformers would be expected to be temperature dependent,
as well as the fact that the methyl signal is relatively broad, if
the interconversion between conformers is not extremely fast
on the NMR time scale at these low temperatures. In addition
to the conformer interconversion, of course, the platinum must
also be rapidly interchanging among the six possible C,C-η2
coordination modes for the benzene ligand to render all the
hydrogens apparently chemically equivalent.
Reaction between Pt(CH3)(Cl)(SMe2)2 and phenyllithium
gave Pt(CH3)(C6H5)(SMe2)2 as the major product. Small
18
amounts of Pt(C6H5)2(SMe2)2 were also formed in the reaction.
Attempts to isolate Pt(CH3)(C6H5)(SMe2)2 by recrystallization
at this stage were unsuccessful. Subsequent reaction of the crude
Pt(CH3)(C6H5)(SMe2)2 with the diimine ligand (N-N) gave a
mixture of about 90% 5 and 10% byproduct (N-N)Pt(C6H5)2
(4). Separation of the desired product was achieved by flash
chromatography on basic alumina. Analytically pure 5 was
obtained in 31% overall yield.
Although C,H-η2 benzene coordination is also possible in
principle, it seems to us much less likely on several grounds.
First, there is no obvious reason a conformation placing the
coordinated benzene in a position to shield the methyl group
would be favorable; alternatives wherein the benzene ring lies
outside the coordination plane seem at least as attractive. Second,
such a structure would be expected to give a substantially higher
value for J(195Pt-H). Third, there is no good precedent for such
a coordination modesseveral determined attempts to generate
a C,H-η2-methane complex from analogous platinum methyl
complexes were unsuccessful21swhereas a C,C-η2-benzene
species has been isolated in a related Rh system where the C-H
activation step is sterically blocked.22
In an NMR tube, 5 was protonated with 1 equiv of aqueous
HBF4 in TFE-d3 at 25 °C. H NMR spectroscopy revealed
1
that 18% methyl species (2a + 2b) and 82% phenyl species
(3a + 3b) were formed in the reaction (Scheme 5). Subsequent
addition of acetonitrile to the product mixture led to the more
stable 2c and 3c, in the same 18:82 ratio. This result implies
that the transition state for elimination of benzene from iii in
Scheme 7 lies about 0.9 kcal/mol higher (∆∆Gq) than that for
elimination of methane.
In a second NMR tube experiment, 10 equiv of HOTf was
added to 5 at -78 °C in dichloromethane.19 A reaction took
place immediately upon addition of the acid, resulting in a pale
1
orange-yellow solution. The H NMR spectrum at -69 °C
At temperatures above ca. -40 °C, elimination of methane
and benzene is observed. At -23 °C, all of the benzene adduct
i is cleanly converted to products within an hour. Presumably
(18) The 1H NMR spectrum of the product is highly complex. We
presume that this is due to the formation of a mixture of cis-Pt(CH3)(C6H5)-
(SMe2)2 (major species), trans-Pt(CH3)(C6H5)(SMe2)2, cis-Pt2(CH3)2(C6H5)2-
1
(µ-SMe2)2, and trans-Pt2(CH3)2(C6H5)2(µ-SMe2)2. Selected H NMR data
for cis-Pt(CH3)(C6H5)(SMe2)2: (300 MHz, dichloromethane-d2) δ 0.56 (s,
(20) See for instance compounds 2a-c and 5, or similar complexes
reported in ref 9.
(21) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem. 1998, 37,
2422-31.
(22) Jones, W. D.; Feher, F. H. J. Am. Chem. Soc. 1984, 106, 1650.
2J(195Pt-H) ) 84.5 Hz, 3 H, PtMe), 2.11 (s, 3J(195Pt-H) ) 24.3 Hz, 6 H,
3
PtSMe2), 2.39 (s, J(195Pt-H) ) 22.8 Hz, 6 H, PtS′Me2).
(19) Low-temperature experiments at -78 °C cannot be performed in
TFE due to its high freezing point (-45 °C).

Benzene C-H ActiVation at a Pt(II) Center
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10851
1
Figure 5. Selected parts of the H NMR spectra showing the species formed after protonation of 5 with HOTf in dichloromethane-d2 at -78 °C:
(a) at -69 °C; the benzene adduct [(N-N)Pt(CH3)(C6H6)]+ (i); (9) PtCH3, ([) PtC6H6, (*) ArH; (b) at -33 °C; a mixture of i and the products
(N-N)Pt(CH3)(OTf) (6), (N-N)Pt(C6H5)(OTf) (7), benzene, and methane; (×) 6 and 7, ArH; (b) 7, PtC6H5; (1) CH4; (2) C6H6; (c) at -23 °C;
after full conversion to the products.
the latter are the corresponding methyl and phenyl triflates 6
and 7, although we have not yet isolated them in pure form.23
The ratio of 6:7 is about 40:60, indicating that loss of benzene
is relatively less disfavored compared to that of methane at these
low temperatures in dichloromethane than in TFE at room
temperature. No other intermediates could be observed prior to
and, therefore, that the barriers between these intermediates are
considerably lower than those for elimination of either methane
or benzene. The relative height of the two latter barriers may
be determined from the ratio of products obtained in the
protonolysis of 5 (methane:benzene ) 82:18 at room temper-
ature). (2) The diagram implies that substitution of benzene for
water in 2a to afford i should be the rate-limiting step in benzene
activation, consistent with the rate law, which is first-order in
benzene as well as 2a. If initial coordination of benzene involves
CdC interaction rather than CsH, as argued above, this is also
consistent with the very small kinetic isotope effect found.
1
methane/benzene elimination by H NMR.
Proposed Mechanism. All of the above results may be
accounted for in terms of Scheme 7, where the manifold of
intermediates may be accessed either by protonation of 5 or by
substitution of benzene for water in 2a.
We can construct a qualitative reaction coordinate diagram,
shown in Figure 6. The details are based on the following
observations of the kinetics and isotopic exchange: (1) The
observation of almost complete statistical scrambling of deu-
terium into the produced methane in the reactions between 2a
and C6D6/1,3,5-C6H3D3 implies a rapid dynamic equilibrium
between the benzene adducts i and ii and methane adduct v
The observed inverse first-order dependence on the water
concentration precludes a direct, associative pathway for the
water-benzene exchange at 2a, but there are still (at least) two
possible scenarios that agree with the observed kinetics: either
a dissociative pathway via the coordinatively unsaturated 14
e- intermediate [(N-N)Pt(Me)]+ (Scheme 8) or a solvent-
assisted associative pathway via the TFE complex 2b (Scheme
9). Utilizing steady-state and fast preequilibrium approximations,
the corresponding rate laws for the two alternative mechanisms
can be formulated as given in eqs 2 and 3, respectively.
(23) The closely related complex (Nf-Nf)Pt(CH3)(OTf) can be generated
in dichloromethane-d2 by protonation of the corresponding platinum
dimethyl complex with 1 equiv of HOTf and characterized by 19F NMR,
but it is unstable at ambient temperature. Johansson, L.; Tilset, M. Preceding
paper in this issue.
For situations where k-1[H2O] . k2[C6H6] and [H2O] .

10852 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Johansson et al.
Figure 6. Qualitative reaction coordinate diagram for the reaction between 2a and benzene.
Scheme 7
Scheme 8
were not reached. The rather negative activation entropy
obtained from the Eyring plot (-16 cal mol-1 K-1) also argues
against a dissociative pathway; however, solvation effects might
contribute to a significant extent to this value. Thus, while the
solvent-assisted associative route seems somewhat more con-
sistent with observations, we do not want to draw any definitive
conclusion at this time.24
Once the benzene adduct i is formed, a dynamic equilibrium
between it and the postulated intermediates ii-v is established.25
From the protonation experiment of 5 at low temperature we
K3[TFE], and neglecting any contribution from a direct associa-
tive H2O-benzene substitution on 2a (k5), both rate laws are
in agreement with the observed kinetics (eq 1). The absence of
rate saturation in [C6H6] appears to support eq 3, but of course
it might just be that sufficiently high benzene concentrations
(24) After experiments described in this paper, two of us have obtained
evidence supporting associative rather than dissociative substitution of
methane for acetonitrile for 2c (Johansson, L.; Tilset, M. J. Am. Chem.
Soc., in press).

Benzene C-H ActiVation at a Pt(II) Center
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10853
Scheme 9
The reaction is completed by loss of methane from v to give
3a. By microscopic reversibility, this may proceed either
dissociatively (via the coordinatively unsaturated 14 e- inter-
mediate vi) or associatively (via the solvento species 3b).
Although Scheme 4 shows this step as reversible, we have seen
no evidence for the reverse reaction under our conditions, partly
because the concentration of methane is very low. But we expect
that the rate of methane activation by 3a is inherently very low,
as this process (measured by isotope exchange) is generally
slower than benzene activation;9 also 3a is likely to be more
stable than 2a, further increasing the reaction barrier.
In conclusion, the ArNdC(Me)C(Me)dNAr (Ar ) 2,6-
(CH3)2C6H3) ligand-stabilized methylplatinum(II) cation under-
goes relatively clean reactions with benzene. Rate-determining
benzene coordination leads to isomeric C,C-η2- and C,H-η2-
benzene adducts that rapidly interconvert via the platinum(IV)
methyl/phenyl/hydride complex with a C,H-η2-methane adduct.
Loss of methane to afford the phenylplatinum(II) cation,
although somewhat faster than loss of benzene, is surprisingly
much slower than interconversions of the isomeric benzene and
methane adducts.
1
have direct H NMR evidence for the benzene adduct i in
dichloromethane, and we assume that this species is formed in
TFE as well. On the basis of the 1H NMR features of i, we also
propose that benzene is rapidly rotating in this complex, and is
probably CdC π-coordinated in its energetically most stable
form. However, a σ-coordinated benzene intermediate ii,
coordinated through a C-H bond, also likely lies on the reaction
coordinate for the subsequent breaking of a C-H bond in
benzene. This process yields a transient 5-coordinated platinum-
(IV) hydride intermediate (iii)26 which can eliminate methane
via v.
Experimental Section
General Considerations. Trifluoroethanol (TFE) and TFE-d3 were
distilled from CaSO4 and a small amount of NaHCO3 and stored over
3 Å molecular sieves. All other solvents were dried according to
standard procedures. The UV/vis experiments were performed on an
HP 8452A spectrophotometer; the cuvette holder was electronically
thermostated with an HP 89090A instrument (Peltier element). 1H NMR
spectra were recorded on General Electric QE300, Bruker Advance
We have no direct evidence for the relative energy, or even
the existence, of the ligand-stabilized six-coordinate platinum-
(IV) hydride iv (L ) water or TFE). Analogous compounds
may be generated by low-temperature protonation or other
routes10,27 with different nitrogen ligand systems. One might
suppose that, in the absence of an η2-benzene alternative, a
species such as iv might be observable, but on protonolysis of
the dimethyl complex (N-N)Pt(CH3)2 with HBF4 at low temper-
ature, no intermediates at all could be observed prior to methane
loss. However, when an analogous experiment was performed
with (Nf-Nf)Pt(CH3)2 and HBF4 in dichloromethane/diethyl
ether at -78 °C, clean formation of [(Nf-Nf)Pt(CH3)2(H)(L)]+
1
DXP 300, and Innova 500 instruments. H NMR chemical shifts are
reported in parts per million relative to tetramethylsilane, with the
residual solvent proton resonance as internal standard. Elemental
analyses were carried out at the Caltech Elemental Analysis Facility
by Fenton Harvey or Ilse Beetz, Mikroanalytisches Laboratorium,
Kronach, Germany. Pt(CH3)4(µ-SMe2)2,29a Pt(CH3)(Cl)(SMe2)2,26a Pt-
(C6H5)2(SMe2)2,26b and the diimine ligand N-N ) ArNdC(Me)C-
(Me)dNAr, Ar ) 2,6-Me2C6H4,30 were prepared as described in the
literature. (N-N)Pt(CH3)2 (1) was prepared according to the procedure
developed by Scollard et al.31
[(N-N)Pt(CH3)(OH2)]+(BF4-) (2a(BF4-)) and [(N-N)Pt(CH3)-
(TFE)]+(BF4-) (2b(BF4-)). A 100 mL Schlenk flask loaded with 1
(111 mg, 0.214 mmol) was cooled to -15 °C, whereupon a cooled
mixture of TFE (25 mL) and a 48% solution of HBF4 in H2O (28 µL,
0.21 mmol) was added with a syringe. The solution was slowly heated
to 0 °C and stirred until all starting material had reacted to give an
orange solution. The solution was cooled to -15 °C, and the solvent
was slowly removed by vacuum transfer. The residue was dried in vacuo
at ambient temperature to give an orange-brown powder: 1H NMR
(300 MHz, TFE-d3) (2a) δ 0.63 (s, 2J(195Pt-H) ) 73.1 Hz, 3 H, PtMe),
1.64 (s, 3 H, NCMeC′MeN), 1.83 (s, 3 H, NCMeC′MeN), 2.21 (s, 6
H, ArMe), 2.31 (s, 6 H, Ar′Me), 7.19-7.28 (m, 6 H, ArH); (2b) δ
0.65 (s, 3 H, PtMe), 1.76 (s, 3 H, NCMeC′MeN), 1.91 (s, 3 H,
NCMeC′MeN), 2.21 (s, 6H, ArMe), 2.34 (s, 6H, Ar′Me) 7.19-7.28
(m, 6 H, ArH). Anal. Calcd for C21H29BF4N2OPt (2a): C, 41.53; H,
4.81; N, 4.61. Anal. Calcd for C23H30BF7N2OPt (2b): C, 40.07; H,
4.39; N, 4.06. Found: C, 39.93; H, 4.50; N, 4.11.
(stable up to ca -40 °C) was observed by H NMR.28 When
1
the temperature was gradually increased, no other intermediates
were observed prior to methane loss, suggesting that the
platinum(IV) hydride is energetically more stable than any
putative [(Nf-Nf)Pt(CH3)(σ-CH4)]+ intermediate. If we assume
that the same trend applies to the present benzene activation
system, the order of stability would be i > iv > ii ≈ v. In any
case, as long as intermediate iv is less stable than i, it will not
be detectable either by direct observation or by affecting the
interconversion between ii and v in the present system.
(25) One might also argue that there exists only one benzene adduct,
that is to say that intermediates i and ii in Scheme 7 should be collapsed
into one, the C,C-η2-benzene isomer. But C,H-η2-alkane adducts (as for v)
are commonly invoked intermediates in oxidative addition and reductive
elimination of C-H bonds. Moreover, it is difficult to envision how the
C,C-η2-coordinated benzene would undergo direct C-H oxidative addition,
or how reductive elimination of the C-H bond of the phenyl hydride
intermediate would lead directly to the C,C-η2-benzene isomer. Thus, we
prefer the interpretation shown in Scheme 7.
(26) It has been shown that reductive elimination from 6-coordinated
Pt(IV) species with a labile ligand L takes place via initial dissociation of
L to a 5-coordinated Pt(IV) intermediate. By the principle of microscopic
reversibility, we assume that iii is the intermediate formed after insertion
into a C-H bond in ii and v, respectively. See: Crumpton, D. M.; Goldberg,
K. I. J. Am. Chem. Soc. 2000, 122, 962 and references therein.
(27) (a) Fekl, U.; Zahl, A.; van Eldik, R. Organometallics 1999, 18, 4156.
(b) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics 1995,
14, 4966.
(N-N)Pt(CH3)(NCMe)]+(BF4-) (2c(BF4-)). To a solution of 1 (127
mg, 0.245 mmol) in 12 mL of acetonitrile was added a 54% solution
of HBF4 in diethyl ether (34 µL, 0.25 mmol), and the reaction mixture
was stirred for 15 min. The solvent was removed, and the residue was
washed with several portions of diethyl ether. The product was obtained
(29) (a) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643.
(b) Hadj-Bagheri, N.; Puddephatt, R. J. Polyhedron 1988, 7, 2695.
(30) tom Dieck, H.; Svoboda, M.; Grieser, T. Z. Naturforsch. 1981, 36B,
823.
(31) Scollard, J. D.; Labinger J. A.; Bercaw, J. E. To be submitted for
publication.
(28) Johansson, L.; Tilset, M. Preceding paper in this issue.

10854 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Johansson et al.
as an orange powder (152 mg, 98%) after drying in vacuo: 1H NMR
6.85-6.94 (m, 3 H, ArH), 7.12-7.25 (m, 3 H, Ar′H). Anal. Calcd for
C27H32N2Pt: C, 55.95; H, 5.56; N, 4.83. Found: C, 55.62; H, 5.38; N,
4.44.
2
(300 MHz, TFE-d3) δ 0.57 (s, J(195Pt-H) ) 75.2 Hz, 3 H, PtMe),
1.85 (s, 3 H, 4J(195Pt-H) ) 12.1 Hz, PtNCMe), 1.88 (s, 3 H,
4J(195Pt-H) ) 9.2 Hz, NCMeC′MeN), 1.94 (s, 3 H, NCMeC′MeN, 2.18
(s, 6 H, ArMe), 2.29 (s, 6 H, Ar′Me), 7.21-7.31 (m, 6 H, ArH); 13C-
{1H} NMR (75 MHz, dichloromethane-d2) δ -13.2 (1J(195Pt-C) )
671 Hz, PtMe), 2.7 (3J(195Pt-C) ) 13.8 Hz, PtNCMe), 17.4 and 17.6
(ArMe and Ar′Me), 19.2 (3J(195Pt-C) ) 17.4 Hz, NCMeC′MeN), 20.1
(3J(195Pt-C) ) 46.9 Hz, NCMeC′MeN), 118.9 (2J(195Pt-C) ) 290 Hz,
PtNCMe), 127.9 (Ar Cp), 128.5 (3J(195Pt-C) ) 10.0 Hz, Ar Co), 128.8
(Ar′ Cm), 128.9 (Ar′ Cm), 129.0 (Ar′ Cp), 129.8 (3J(195Pt-C) ) 14.9
Hz, Ar Co), 142.7 (2J(195Pt-C) ) 11.1 Hz, Ar Cipso), 142.8 (2J(195Pt-
C) ) 30.5 Hz, Ar′ Cipso), 174.6 (2J(195Pt-C) ) 60.0 Hz, NCMeC′MeN),
183.3 (NCMeC′MeN). Anal. Calcd for C23H30BF4N3Pt: C, 43.82; H,
4.80; N, 6.67. Found: C, 44.11; H, 4.50; N, 6.37.
Protonation of (N-N)Pt(CH3)(C6H5) (5). To an NMR tube loaded
with 5 (4.9 mg, 8.5 × 10-3 mmol) was added a 48% solution of aqueous
1
HBF4 (1.2 µL, 9 × 10-3 mmol) dissolved in 0.7 mL of TFE-d3. H
NMR showed a product composition consisting of 2a (14%), 2b (4%)
3a (70%), and 3b (12%). After addition of acetonitrile-d3 (1.0 µL, 0.02
mmol) to the solution, 1H NMR revealed clean formation of 2c (18%)
and 3c (82%).
Generation of [(N-N)Pt(CH3)(C6H6)]+(OTf-) (i(OTf-)) at Low
Temperature. To an NMR tube loaded with a solution of 5 (3.0 mg,
5.2 × 10-3 mmol) in 400 µL of dichloromethane-d2 was slowly added
150 µL of dichloromethane d2 to form an upper, separate layer. After
subsequent slow addition of a solution of HOTf (4.5 µL, 5.2 × 10-2
mmol) in 100 µL of dichloromethane-d2/40 µL of diethyl ether, the
tube was sealed and cooled to -78 °C. The tube was shaken to mix
the reactants (care was taken to minimize any heating of the sample),
and a pale orange-yellow solution was immediately obtained. The tube
was then transferred to a precooled NMR probe: 1H NMR (300 MHz,
-69 °C) δ -1.58 (s, 3 H, PtMe), 1.99 (s, 3 H, NCMeC′MeN), 2.04 (s,
3 H, NCMeC′MeN), 2.10 (s, 6 H, ArMe), 2.33 (s, 6 H, Ar′Me), 6.96
(s, 6 H, PtC6H6), 7.09-7.28 (m, 6 H, ArH and Ar′H); (300 MHz, -33
(N-N)Pt(C6H5)2 (4). A solution of Pt(C6H5)2(SMe2)2 (358 mg, 0.756
mmol) and the diimine ligand (221 mg, 0.756 mmol) in toluene (40
mL) was stirred at 45 °C for 18 h. Pentane (ca 100 mL) was added to
let the product precipitate, and the mixture was slowly cooled to ambient
temperature. The mother liquid was decanted, and the product was
washed with several portions of pentane and finally dried in vacuo.
The product was obtained as a dark purple powder (436 mg, 90%):
1H NMR (300 MHz, dichloromethane-d2) δ 1.64 (s, 6 H, NCMeCMeN),
2.20 (s, 12 H, ArMe), 6.40 (m, 2H, PhHp), 6.49 (m, 4H, PhHm), 6.86
2
°C) δ -1.35 (s, J(195Pt-H) ) 65.9 Hz, 3 H, PtMe), 1.98 (s, 3 H,
NCMeC′MeN), 2.02 (s, 3 H, NCMeC′MeN), 2.12 (s, 6 H, ArMe), 2.32
(s, 6 H, Ar′Me), 7.01 (s, J(195Pt-H) ) 23.5 Hz, 6 H, PtC6H6), 7.13-
7.27 (m, 6 H, ArH and Ar′H); 13C{1H} NMR (75 MHz, dichlo-
romethane-d2, -62 °C) δ not obsd (PtMe), 17.1 and 17.3 (ArMe and
Ar′Me), 19.5 (NCMeC′MeN), 19.8 (NCMeC′MeN), 120.7 (br, Pt-
(C6H6)), 127.6, 127.9, 128.2, 128.2 (?), 128.6, 129.2 (Ar and Ar′ Co,
Cm, and Cp), 139.6 and 142.0 (Ar and Ar′ Cipso), not obsd (NCMeC′MeN
and NCMeC′MeN).
3
3
(“d”, J(H-H) ) 7.2 Hz, J(195Pt-H) ) 70.3 Hz, 4H, PhHo), 6.88-
6.92 (m, 6 H, ArH). Anal. Calcd for C32H34N2Pt: C, 59.89; H, 5.34;
N, 4.37. Found: C, 59.68; H, 5.10; N, 4.07.
[(N-N)Pt(C6H5)(NCMe)]+(BF4-) (3c(BF4-)). To a solution of 4
(104 mg, 0.162 mmol) in 10 mL of dichloromethane and 1 mL of
acetonitrile was added a 54% solution of HBF4 in diethyl ether (23
µL, 0.17 mmol), and the reaction mixture was stirred for 30 min. The
solvent was removed, and the residue was washed with several portions
of diethyl ether. The product was obtained as an orange powder (108
mg, 94%) after drying in vacuo: 1H NMR (300 MHz, TFE-d3) δ 1.77
(s, 3 H, PtNCMe), 2.00 (s, 3 H, NCMeC′MeN), 2.10 (s, 3 H,
NCMeC′MeN, 2.14 (s, 6 H, ArMe), 2.38 (s, 6 H, Ar′Me), 6.68-6.71
(m, 3H, PhHm and PhHp), 6.75-6.80 (m, 2H, PhHo), 6.88-6.99 (m, 3
H, ArH).), 7.25-7.35 (m, 3 H, Ar′H). 13C{1H} NMR (75 MHz,
dichloromethane-d2) δ 2.8 (3J(195Pt-C) ) 12.5 Hz, PtNCMe), 17.8 and
17.8 (ArMe and Ar′Me), 19.5 (3J(195Pt-C) ) 16.7 Hz, NCMeC′MeN),
20.4 (3J(195Pt-C) ) 47.4 Hz, NCMeC′MeN), 118.2 (PtNCMe), 124.3
(PtPh Cp), 126.7 (2J(195Pt-C) ) 48.7 Hz, PtPh Co), 128.3 (Ar Cp),
128.4 (Ar Co), 128.6 (Ar Cm), 129.0 (Ar′ Cm), 129.1 (Ar′ Cp), 129.4
(3J(195Pt-C) ) 15.4 Hz, Ar′ Co), 132.2 (1J(195Pt-C) ) 894 Hz, PtPh
NMR Data of the Products (N-N)Pt(CH3)(OTf) (6) and (N-
N)Pt(C6H5)(OTf) (7). 1H NMR (300 MHz, -23 °C) (6) (40%) δ 0.71
2
(s, J(195Pt-H) ) 73.9 Hz, 3 H, PtMe), 1.53 (s, 3 H, NCMeC′MeN),
1.78 (s, 3 H, NCMeC′MeN), 2.18 (s, 6 H, ArMe), 2.25 (s, 6 H, Ar′Me),
7.14-7.24 (m, 6 H, ArH and Ar′H); (7) (60%) δ 1.66 (s, 3 H,
NCMeC′MeN), 1.94 (s, 3 H, NCMeC′MeN), 2.14 (s, 6 H, ArMe), 2.34
(s, 6 H, Ar′Me), 6.49-6.61 (m, 3 H, PhHm and PhHp) 6.75 (“d”, 3J(H-
H) ) 7.9 Hz, 2 H, PhHo), 6.85-6.96 (m, 3 H, ArH), 7.14-7.24 (m, 3
H, Ar′H).
UV/Vis Kinetics. General Procedure for the Preparation of Stock
Solutions. A Schlenk flask loaded with 1 (51.8 mg, 0.100 mmol) was
cooled to -15 °C. A mixture of HBF4 (48% in water, 13.1 µL, 0.10
mmol) in 10 mL of 0.2 M Me4NBF4 in TFE was added to the flask,
and the reaction mixture was slowly allowed to warm to ambient
temperature and stirred for 30 min. An appropriate amount of water
was then added to the thus generated solution of 2a. The actual water
concentration in the stock solution was determined as follows: Two
oven-dried NMR tubes were loaded with a known amount of ferrocene
dissolved in 600 µL of dichloromethane-d2. To one of the tubes was
C
ipso), 135.1 (PtPh Cm), 142.5 (2J(195Pt-C) ) 12.0 Hz, Ar Cipso), 143.2
(Ar′ Cipso), 176.8 (2J(195Pt-C) ) 59.6 Hz, NCMeC′MeN), 183.5
(NCMeC′MeN). Anal. Calcd for C28H32BF4N3Pt: C, 48.57; H, 4.66;
N, 6.07. Found: C, 48.64; H, 4.92; N, 5.88.
(N-N)Pt(CH3)(C6H5) (5). A suspension of Pt(CH3)(Cl)(SMe2)2 (228
mg, 0.617 mmol) in 40 mL of diethyl ether was cooled to -20 °C,
and a 1.8 M solution of phenyllithium in diethyl ether/cyclohexane
(360 µL, 0.65 mmol) was added dropwise with a syringe. After the
resulting solution was stirred for 45 min, 100 mL of water was added.
The layers were separated, and the aqueous phase was extracted with
diethyl ether (2 × 75 mL). A small amount of charcoal was added to
the combined organic phases, which were then dried over MgSO4. After
filtration the diethyl ether solution was added to a 45 °C solution of
the diimine ligand (200 mg, 0.68 mmol) in 50 mL of toluene, and the
reaction mixture was stirred for 26 h at this temperature. The solvent
was removed and the residue washed with several portions of pentane.
To separate 5 from the byproduct (4, ca. 10%), flash chromatography
(basic alumina, diethyl ether/hexanes, 1:1) was employed. The desired
product was collected as the first dark purple fraction. The solvent was
removed, and the product was washed with pentane and dried in vacuo,
giving a dark purple powder (111 mg, 31%): 1H NMR (300 MHz,
1
added 100 µL of the stock solution. From the H NMR integration of
the water signal in the two samples (the ferrocene signal serving as
internal standard), the actual water concentration in the stock solution
was calculated. The stock solutions were stored under argon at -20
°C.
Varying Benzene Concentration. A 500 µL portion of stock
solution ([H2O] ) 0.16 M) was transferred to a 0.1 cm UV Pyrex cell.
Benzene or benzene-d6 (27, 53, 100, and 250 µL) (caution: cancer
suspect agent) was then added to the cell with a syringe. The cell was
sealed with a Teflon plug and shaken thoroughly. The reactions were
followed by UV/vis spectroscopy at 390 nm at 25 °C over at least 3
half-lives.
Varying Water Concentration. A 400 µL portion of stock solution
([H2O] ) 0.20 M) was transferred to a 0.1 cm UV Pyrex cell. To seven
different 2 mL solutions containing 4.5 M benzene in TFE were added
0, 11, 22, 32, 43, 54, and 108 µL of water, respectively. Then, 200 µL
of the appropriate water/benzene/TFE solution was added to the UV
cell with a syringe, and the reaction was followed as described above.
Varying Temperature. A 1.50 mL portion of stock solution ([H2O]
) 0.36 M) was transferred to a 1 cm UV Pyrex cell equipped with a
2
dichloromethane-d2) δ 0.75 (s, J(195Pt-H) ) 87.8 Hz, 3 H, PtMe),
1.41 (s, 3 H, NCMeC′MeN), 1.52 (s, 3 H, NCMeC′MeN), 2.10 (s, 6
H, ArMe), 2.24 (s, 6 H, Ar′Me), 6.44-6.57 (m, 3 H, PhHm and PhHp),
3
3
6.72 (“d”, J(H-H) ) 8.1 Hz, J(195Pt-H) ) 66.4 Hz, 2 H, PhHo),

Benzene C-H ActiVation at a Pt(II) Center
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10855
data, where k ) kobsd[H2O]/[C6H6].
magnetic stirbar. A 750 µL portion of a 4.5 M solution of benzene in
TFE was then added to the cell with a syringe. The cell was sealed
with a Teflon screwcap and shaken thoroughly. The reactions were
followed by UV/vis spectroscopy at 390 nm at 23, 35, 45, and 55 °C
over at least 3 half-lives. Rate constants were obtained from a first-
order three-parameter (kobsd, A0, and A∞) least-squares fit of the
Acknowledgment. Support by the National Science Founda-
tion (Grant No. CHE-9807496) and by Akzo-Nobel are grate-
fully acknowledged. We also gratefully acknowledge generous
support from the Norwegian Research Council, NFR (stipend
to L.J.).
absorbance at 390 nm (A) versus time (t) data employing the equation
t
A ) A∞ + (A0 - A∞)e-k . The activation parameters for the reaction
obsd
were derived from a linear least-squares analysis of ln(k/T) versus T-1
JA0017460