C-H bond activation by dicationic platinum(II) complexes

Article abstract of DOI:10.1021/om060792r

Double protonolysis of diimine platinum dimethyls [(N-N)PtMe₂] (N-N = ArN=C(Me)C(Me)=NAr) generates dicationic Pt(II) complexes that can activate a variety of C-H bonds, liberating 1 equiv of acid and forming organoplatinum species that are moderately stable to the resulting acidic conditions. Ethylbenzenes lead to η³-benzyl complexes; mechanistic experiments suggest that η³-benzyl product formation proceeds via C-H bond activation at the benzylic methylene position. In some cases π-arene complexes can be observed, but their role in the C-H activation process is not clear. Cyclohexane and 1-pentene react to give η³-allyl complexes; allylbenzene gives a chelated phenyl-)η²-olefin structure, as determined by X-ray diffraction. No stable C-H activation products are obtained from methylbenzenes, benzene itself, or alkanes.

Full text of DOI:10.1021/om060792r

294
Organometallics 2007, 26, 294-301
C-H Bond Activation by Dicationic Platinum(II) Complexes
Tom G. Driver,† Travis J. Williams, Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
ReceiVed August 31, 2006
Double protonolysis of diimine platinum dimethyls [(N-N)PtMe2] (N-N ) ArNdC(Me)C(Me)dNAr)
generates dicationic Pt(II) complexes that can activate a variety of C-H bonds, liberating 1 equiv of
acid and forming organoplatinum species that are moderately stable to the resulting acidic conditions.
Ethylbenzenes lead to η3-benzyl complexes; mechanistic experiments suggest that η3-benzyl product
formation proceeds via C-H bond activation at the benzylic methylene position. In some cases π-arene
complexes can be observed, but their role in the C-H activation process is not clear. Cyclohexane and
1-pentene react to give η3-allyl complexes; allylbenzene gives a chelated phenyl-η2-olefin structure, as
determined by X-ray diffraction. No stable C-H activation products are obtained from methylbenzenes,
benzene itself, or alkanes.
Our investigations of the C-H bond activation step have
made extensive use of reactions of diimine-ligated platinum
methyl complexes 2 (eq 1).9 In comparison to conditions involv-
ing aqueous HBF4,9g,h reactivity is enhanced when 2 is generated
under anhydrous conditions (by adding B(C6F5)3 to trifluor-
oethanol-d3 (TFE)asshownineq1),butthe[(F6C5)3BOCD2CF3]-
counterion leads to a (competing or consecutive) side reactions
transfer of a pentafluorophenyl group from B to Ptsthat in many
cases complicates the kinetics and/or product characterization.9i,j
Introduction
The area of C-H bond functionalization has generated
considerable activity over the last three decades.1 One of the
earliest discoveriessthe so-called Shilov system for selective
platinum-mediated oxidation of alkanes to alcohols2shas con-
tinued to stimulate interest, both as a potentially useful
methodology and as a topic for mechanistic investigation. The
reaction is believed to occur in three stages: (1) reversible
electrophilic C-H bond activation to afford a platinum(II) alkyl
complex with liberation of a proton,3 (2) oxidation of R-Pt(II)
to R-Pt(IV) by hexachloroplatinate,4,5 and (3) reductive elimi-
nation via SN2 displacement by water (or chloride) to give ROH
(or RCl) and regenerate the platinum(II) catalyst.6,7 Because
R-Pt(II) is highly labile with respect to protonolysis, the second
step must be at least as fast as the reverse of the first.8
(4) For recent leading mechanistic reports on the oxidation of platinum-
(II) complexes to platinum(IV) complexes, see: (a) Zhang, F. B.; Brocz-
kowski, M. E.; Jennings, M. C.; Puddephatt, R. J. Can. J. Chem. 2005, 83,
595-605. (b) Canty, A. J.; Denney, M. C.; van Koten, G.; Skelton, B. W.;
White, A. H. Organometallics 2004, 23, 5432-5439. (c) Rostovtsev, V.
V.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem. 2002, 41,
3608-3619. (d) Scollard, J. D.; Day, M.; Labinger, J. A.; Bercaw, J. E.
HelV. Chim. Acta 2001, 84, 3247-3267.
(5) Hexachloroplatinate can be replaced with cheaper oxidants; for
leading references, see: (a) Lin, M.; Shen, C.; Garcia-Zayas, E. A.; Sen,
A. J. Am. Chem. Soc. 2001, 123, 1000-1001. (b) Weinberg, D. R.; Labinger,
J. A.; Bercaw, J. E. Organometallics, in press.
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Bercaw, J. E.; Horva´th, I. T.; Eller, K. Organometallics 1993, 12, 895-
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1990, 112, 5628-5629.
(7) For recent mechanistic reports on oxidative addition/reductive
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(b) Procelewska, J.; Zahl, A.; Liehr, G.; van Eldik, R.; Smythe, N. A.;
Williams, B. S.; Goldberg, K. I. Inorg. Chem. 2005, 44, 7732-7742. (c)
Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125,
9442-9456. (d) Jensen, M. P.; Wick, D. D.; Reinartz, S.; White, P. S.;
Templeton, J. L.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 8614-8624.
(8) Wang, L.; Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Mol. Catal.
A 1997, 116, 269-275.
(9) (a) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999,
121, 1974-1975. (b) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J.
E. J. Am. Chem. Soc. 2000, 122, 10846-10855. (c) Johansson, L.; Tilset,
M. J. Am. Chem. Soc. 2001, 123, 739-740. (d) Johansson, L.; Ryan, O.
B.; Rømming, C.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 6579-6590. (e)
Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124,
1378-1399. (f) Procelewska, J.; Zahl, A.; van Eldik, R.; Labinger, J. A.;
Bercaw, J. E. Inorg. Chem. 2002, 41, 2808-2810. (g) Heyduk, A. F.;
Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2003, 125, 6366-6367.
(h) Heyduk, A. F.; Driver, T. G.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2004, 126, 15034-15035. (i) Driver, T. G.; Day, M. W.;
Labinger, J. A.; Bercaw, J. E. Organometallics 2005, 24, 3644-3654. (j)
Owen, J. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2006, 128,
2005-2016. (k) Lersch, M.; Dalhus, B.; Bercaw, J. E.; Labinger, J. A.;
Tilset, M. Organometallics 2006, 25, 1055-1058.
* To whom correspondence should be addressed. E-mail: jal@caltech.edu
(J.A.L.); bercaw@caltech.edu (J.E.B.).
† Current address: Department of Chemistry, University of Illinois at
Chicago, Chicago, IL 60607.
(1) For recent reviews on C-H activation and functionalization, see:
(a) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc.
Chem. Res. 1995, 28, 154-162. (b) Shilov, A. E.; Shul’pin, G. B. Chem.
ReV. 1997, 97, 2879-2932. (c) Erker, G. Angew. Chem., Int. Ed. 1999, 38,
1699-1712. (d) Davies, H. M. L.; Antoulinakis, E. G. J. Organomet. Chem.
2001, 617-618., 47-55. (e) Crabtree, R. H. Dalton Trans. 2001, 17, 2437-
2450. (f) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507-514. (g)
Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003, 54, 259-320. (h) Lersch,
M.; Tilset, M. Chem. ReV. 2005, 105, 2471-2526.
(2) (a) Shilov, A. E.; Shteinman, A. A. Coord. Chem. ReV. 1977, 24,
97-143.(b) Shilov, A. E.; Shul’pin, G. B. Russ. Chem. ReV. 1987, 56, 442.
(3) For recent mechanistic discussions on platinum-mediated C-H bond
activation, see: (a) Wik, B. J.; Ivanovic-Burmazovic, I.; Tilset, M.; van
Eldik, R. Inorg. Chem. 2006, 45, 3613-3621. (b) Wik, B. J.; Lersch, M.;
Krivopapic, A.; Tilset, M. J. Am. Chem. Soc. 2006, 128, 2682-2696. (c)
Labinger, J. A.; Bercaw, J. E.; Tilset, M. Organometallics 2006, 25, 805-
808. (d) Thomas, C. M.; Peters, J. C. Organometallics 2005, 24, 5858-
5867. (e) Romeo, R.; Plutino, M. R.; Romeo, A. HelV. Chim. Acta 2005,
88, 507-522. (f) Dick, A. R.; Kampf, J. W.; Sanford, M. S. Organometallics
2005, 24, 482-485. (g) Plutino, M. R.; Scolaro, L. M.; Albinati, A.; Romeo,
R. J. Am. Chem. Soc. 2004, 126, 6470-6484. (h) Thomas, J. C.; Peters, J.
C. J. Am. Chem. Soc. 2003, 125, 8870-8888. (i) Ingleson, M. J.; Mahon,
M. F.; Weller, A. S. Chem. Commun. 2004, 2398-2399. (j) Gerdes, G.;
Chen, P. Organometallics 2003, 22, 2217-2225. (k) Konze, W. V.; Scott,
B. L.; Kubas, G. J. J. Am. Chem. Soc. 2002, 124, 12550-12556. (l) Norris,
C. M.; Reinartz, S.; White, P. S.; Templeton, J. L. Organometallics 2002,
21, 5649-5656.
10.1021/om060792r CCC: $37.00 © 2007 American Chemical Society
Publication on Web 12/15/2006

C-H Bond ActiVation by Dicationic Pt(II) Complexes
Organometallics, Vol. 26, No. 2, 2007 295
Table 1. Qualitative Comparison of Platinum Methyl Cation
Stabilitya
[(N-N)Pt-
Me(TFE)]+
(mM)
t1/2(dec)
(22 °C)
entry
Ar
X-
1
2
3
4
5
3,5-(t-Bu)2C6H3 [(F5C6)3BOCD2CF3]-
2,4,6-Me3C6H2 [(F5C6)3BOCD2CF3]-
2,4,6-Me3C6H2 [OTf]-
26
26
26
13
26
8 h
6 days
9 h
27 h
60 h
2,4,6-Me3C6H2 [OTf]-
3,5-t-Bu2C6H3
[F3BOCD2CF3]-
To circumvent this complication, we have examined alternate
methods for generating anhydrous 2. In the course of this work
we discovered that it is possible to doubly protonolyze the
starting platinum dimethyl complex 1 to generate methyl-free,
dicationic Pt(II) complexes.
a Abbreviations: N-N ) ArNdC(Me)C(Me)dNAr; TFE ) 2,2,2-
trifluoroethanol-d3.
at all, presumably a consequence of strong coordination of X-
(acetate, trifluoroacetate).
Note that in the model system 2, unlike the “real” Shilov
system, the H of the activated C-H bond is lost as CH4 rather
than as a proton. As a consequence, whether or not the detailed
mechanistic information gleaned from the former is applicable
to the latter, complexes such as 2 are not directly applicable as
catalysts for practical alkane functionalization. In contrast,
formation of an organoplatinum complex via C-H activation
by a dicationic species would generate a proton; hence, whether
a particular hydrocarbon leads to an observable organoplatinum
product will depend on the issues of thermodynamic stability
toward protonolysis and the kinetic reactivity of the C-H bond
with the dicationic center. The latter may or may not resemble
reactivities toward the monocationic models 2.
We report here that a platinum dication complex can indeed
activate a variety of C-H bonds, that the thermodynamic
stability of the resulting product in the presence of liberated
proton is strongly dependent upon the specific structure, and
that the mechanism for the formation of one class of product is
substantially different from that previously established for the
corresponding reaction of platinum methyl monocations.
Several qualitative decomposition rates are shown in Table
1. With X ) [(F6C5)3B(OCD2CF3)]- 2a (Ar ) 3,5-(t-Bu)2C6H3)
decomposes by pentafluorophenyl transfer to platinum with a
half-life of about 8 h at 22 °C; increasing the steric crowding
around platinum (2b; Ar ) 2,4,6-Me3C6H2) slows the rate of
decomposition (entry 2). When X ) triflate, the decomposition
rate is comparable (entry 3), although it is significantly slower
at lower concentration (entry 4), suggesting a bimolecular
pathway. Using BF3 as the Lewis acid for protonolysis affords
a substantial increase in the stability of the platinum methyl
cation (entry 5).
Diprotonolysis of 1 by Triflic Acid or BF3. In contrast to
the behavior observed with B(C6F5)3, reaction of 1a or 1b with
2.05 equiv of triflic acid effects protonolysis of both methyl
groups to liberate 2 equiv of methane and new methyl-free
platinum complexes. In the case of 1b, the 1H NMR shows an
unsymmetrical environment for the diimine ligand, suggesting
structure 3b (eq 2). Addition of methanol to the solution results
Results and Discussion
Protonolysis of 1 by Various Acids. Generation of the
platinum methyl cation 2 by protonolysis of the dimethyl
complex 1 in trifluoroethanol-d3 (TFE) is effected by a variety
of acids (HBF4‚OEt2, DOAc-d3, DO2CCF3, DOTf, HN(Tf)2) and
acid precursors (B(C6F5)3, BF3, PF5, Tf2O) (eq 1). In general,
more than 1 equiv of acid is required for clean formation of 2.
Both the stability and the reactivity of the resulting cationic
species depend on the counteranion as well as the aryl group
on the diimine ligand (Ar). In particular, when acetic or
trifluoroacetic acid is used, no C-H activation can be observed
in the formation of a new symmetrical species, presumably 4b.
Similar protonolysis of 1a gives a product that is symmetrical,
but with broad NMR signals.
Scheme 1

296 Organometallics, Vol. 26, No. 2, 2007
DriVer et al.
Figure 1. UV/visible spectral shanges during titration of 5a (0.505 mM) with a solution of NaOCH2CF3 (30 mM).
Diprotonolysis can also be achieved with BF3; the detailed
behavior is somewhat different. Exposure of 1a to 2.1 equiv of
BF3 (in trifluoroethanol-d3) results in rapid protonolysis of the
first methyl group to generate 2a, followed by slow loss of the
second methyl group to produce a new symmetrical product,
again exhibiting broad NMR signals. Addition of excess LiOTf
to this solution gives the same (symmetrical) NMR spectrum
observed when 1a is treated with excess triflic acid. These
findings strongly suggest that the symmetrical species are the
bis(solvento) dication 5a and the bis(triflato) neutral species 6a
(Scheme 1). Addition of acetonitrile to a solution of 5a gives a
new, sharp, symmetrical spectrum, assigned to [(N-N)Pt-
(NCCH3)2]2+ (7a). We have not able to cleanly isolate platinum
dication 5a; concentration of a solution of 5a produces yet
another symmetrical species, tentatively assigned as a dimer.10
Additional support for the proposed identity of dication 5a
was obtained from a titration experiment. Addition of NaOCH2-
CF3 to the platinum dication results in a color change of the
solution (light orange to blood red) accompanied by an upfield
shift of the diimine methyl resonance in the 1H NMR. The peak
initially appears at δ 2.32; after the addition of 1 equiv of
NaOCH2CF3, it shifts to δ 1.82 and, after 3 equiv, to δ 1.72.
(The peak gradually disappears over time due to base-catalyzed
H/D isotope exchange with the solvent.9e) Addition of 4 equiv
of F3B‚DOCD2CF3 at this point regenerates the original
spectrum. Following the same titration by UV/visible spectros-
copy reveals only one well-defined transition, corresponding
to the addition of 1 equiv of base (Figure 1); the overlaid spectra
show a number of apparent isosbestic points, but several are
not perfect. These results strongly imply that the three NMR
signals above correspond to dicationic, monocationic, and
neutral complexes respectively (eq 3), with the UV/visible
spectrum differing significantly between the first and second
but much less so between the second and third.
plexes shown in Table 2 undergo diprotonolysis, but whereas
1c (like 1a) gives complete formation of the dication 5c in a
few hours at room temperature, both 1b and 1d require more
prolonged reaction at elevated temperature. This difference in
reactivity presumably reflects inhibition of protonation by steric
crowding (1b) or reduced electron density (1d). The fact that
diprotonolysis of 1a and 1b with triflic acid gives different
products (5a and 3b, respectively) is probably also a steric
effect: the larger ligand in 1b disfavors coordination of two
relatively bulky triflate groups.
Observation of Platinum π-Arene Complexes. Addition of
1,4-diethylbenzene or p-xylene to a solution of 5a produces new
NMR signals, including sharp singlets at 6.78 and 6.87 ppm,
respectively, assigned to arene complexes 8a and 8a′ (eq 4).
On the basis of literature precedent3a,b,11 the coordination is
expected to be η2; if so, the observation of a single peak for
the aryl protons requires that the molecules be fluxional.12 The
relative intensities of the NMR signals for 5a and 8a vary with
the amount of added arene, as expected for an equilibrium
situation. Attempts to determine equilibrium constants met with
complications, however. Duplicate determinations with different
preparations of 5a gave apparent values of Keq for replacement
of TFE by 1,4-diethylbenzene ranging from 45 to 115 at room
The steric and electronic environment around platinum affects
the conditions required for diprotonolysis of 1. All the com-
(11) (a) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J.
Am. Chem. Soc. 2001, 123, 12724-12725. (b) Iverson, C. A.; Lachicotte,
R. J.; Mu¨ller, C.; Jones, W. D. Organometallics 2002, 21, 5320-5333. (c)
Berenguer, J. R.; Fornie´s, J.; Mart´ın, F.; Mart´ın, A.; Menjo´n, B. Inorg.
Chem. 2005, 44, 7265-7267.
(10) For a recent report of a related hydroxy-bridged diimine-ligated
platinum dimer, see: Kannan, S.; James, A. J.; Sharp, P. R. Polyhedron
2000, 19, 155-163.

C-H Bond ActiVation by Dicationic Pt(II) Complexes
Organometallics, Vol. 26, No. 2, 2007 297
Table 2. Conditions Required for Complete Diprotonolysis
of 1 to 5
Table 3. Formation of η3-Benzyl Complexes from Platinum
Dications
complex
Ar
time (h)
T (°C)
complex
Ar
T (°C)
time (h)
yield of 10 (%)
1a
1b
1c
1d
3,5-(t-Bu)2C6H3
2,4,6-Me3C6H2
3,5-Me2C6H3
8
15
8
22
60
22
50
5a
5b
5c
5d
3,5-(t-Bu)2C6H3
2,4,6-Me3C6H2
3,5-Me2C6H3
50
50
50
60
15
20
15
15
93
30
76
97
3,5-(CF3)2C6H3
36
3,5-(CF3)2C6H3
temperature. Furthermore, the apparent Keq value generally
increases as the temperature is raised but does not return to its
original value upon recooling. A group of new NMR signals
grows in intensity during the latter experiments; similar peaks
are observed when 5a is allowed to decompose by itself at
elevated temperature. It is probable that decomposition generates
one or more NMR signals that coincide with those used to
quantify 8a. Additionally, the amount of acid used in the
protonolysis of 1swhich is difficult to control or determine with
high precisionsappears to affect both the initial apparent
equilibrium constant and the rate of decomposition. Hence, it
is not possible to obtain reliable Keq values.
displacement of coordinated solvent by hydrocarbon is a
requisite (often rate-determining) first step, and one might expect
that solvent would be more tightly bound to a dicationic center,
thus reducing reactivity. On yet another hand, though, equilib-
rium displacement of TFE by 1,4-diethylbenzene can be
observed, even at room temperature (see above). We find that
formation of η3-benzyl complexes 10 from the reaction of
platinum dications 5 with 1,4-diethylbenzene (eq 6) is slower
The formation of π-arene complexes is sensitive to both the
steric and electronic environment around platinum. When the
steric crowding is increased (5b) or the electron density around
platinum is reduced (5d), no π-arene complexes are observed.
Also, irrespective of the ligand environment around platinum,
no π-arene complex formation is observed for monosubstituted
arenes or benzene itself. On the other hand, for the bulkier arene
1,3,5-triethylbenzene a π-arene complex does form, but more
slowly: at room temperature only about 20% of 5a converts to
9a, but heating to 50 °C results in complete consumption of
the platinum dication and formation of the π-arene complex
(eq 5).
than the corresponding reactions of 2 and requires elevated
temperature, as shown in Table 3. 10b is also obtained on
starting from triflate complex 3b, but the reaction requires higher
temperatures (95 °C) and gives low yields (around 35%), in
part a consequence of product instability: independently
synthesized 10b decomposes in the presence of excess triflic
acid at 95 °C with a t1/2 value of approximately 6 h. Complexes
with more tightly binding ligands, such as platinum ditriflate
6a, methanol-ligated complex 4b, and bis(acetonitrile) complex
7a, all fail to react productively with 1,4-diethylbenzene.
As shown in eq 6, net activation of a benzylic C-H bond
would liberate a proton that ends up as HOCD2CF3 with
concomitant formation of BF3/TFE. (Indeed, the 1H NMR peak
associated with the residual “OH” present in solution grows
significantly over the course of the reaction, but this is primarily
due to acid-catalyzed arene H/D exchange, which takes place
under reaction conditions even in the absence of any Pt complex.
Demonstration that benzylic C-H activation results in growth
of the OH peak requires a specifically labeled substrate: see
below.) Hence, the η3-benzyl complexes 10 must be stable to
the presence of 1 equiv of acid. Reactions of 5b were carried
out in the presence of added base (K2CO3, CsCO3, 2,6-lutidine,
2,6-di-tert-butylpyridine, or NaOCH2CF3) to determine whether
the low yields are a consequence of a less favorable equilibrium
between C-H activation and protonolysis, but all such additives
only inhibited the formation of η3-benzyl 10b. Furthermore, no
H/D exchange was observed in the ethyl group of unreacted
1,4-diethylbenzene, arguing against any equilibrium.
Coordinated arenes are readily displaced by acetonitrile to
afford 7a, similar to the reactivity reported by related (pyra-
zolylborate)platinum π-arene complexes above -30 °C.13 The
addition of less than 2 equiv of acetonitrile to 8a does not result
in a mixed acetonitrile-arene complex but rather a mixture of
dinitrile 7a and π-arene complex 8a, indicating that the
equilibrium strongly disfavors a mononitrile complex and/or that
it undergoes a second ligand substitution much faster than the
first.
C-H Bond Activation of Alkylbenzenes at Dicationic
Platinum Centers. Methyl- and ethyl-substituted benzenes react
readily at room temperature with methylplatinum cations 2,
leading to η3-benzyl complexes as the final product.9h,i Since
these C-H activations are formulated as electrophilic reactions,
one might expect dicationic species to be even more reactive.
On the other hand, all previous mechanistic studies indicate that
Increasing the steric bulk of the substrate from 1,4-diethyl-
benzene to 1,3,5-triethylbenzene slows the reaction. After 15 h
at 50 °C, the η3-benzyl species 11a was observed in only 30%
conversion, the balance being π-arene complex 9a. After
prolonged heating at 70 °C 11a was obtained in 92% yield (eq
7).
(12) No change was observed in the NMR down to -30 °C. The rotation
barrier for a related platinum complex was estimated to be 9.4 kcal mol-1
(-90 °C): Norris, C. M.; Reinartz, S.; White, P. S.; Templeton, J. L.
Organometallics 2002, 21, 5649-5656.
(13) Norris, C. M.; Templeton, J. L. Organometallics 2004, 23, 3101-
3104.
It is notable that both 5b and 5d activate 1,4-diethylbenzene,
even though no intermediate arene complex is detectable.

298 Organometallics, Vol. 26, No. 2, 2007
DriVer et al.
Similarly, heating 5a in the presence of excess ethylbenzene
(which also forms no observable arene complex) yields η3-
benzyl 12a in 91% yield (eq 8). On the other hand, p-xylene
Figure 2. Dependence of rate of reaction of 5a to 10a on [1,4-
diethylbenzene].
(which does form an arene complex) does not react with 5a to
give the expected η3-benzyl species. This appears to be a
consequence of product instability: the complex prepared by
reaction of p-xylene with 2a is completely decomposed after
10 h at 50 °C in a solution of BF3/TFE.
Mechanism of η3-Benzyl Formation: Kinetics. The kinetics
of formation of the platinum η3-benzyl complex 10a were
determined for 1,4-diethylbenzene concentrations ranging from
about 0.2 to 0.6 M; disappearance of the dicationic platinum
complex followed good first-order behavior in all cases. Previous
studies strongly indicate that benzene C-H activation by 2
proceeds via an intermediate η2-arene complex.9b,e Although we
have been unable to measure a reliable equilibrium constant
for formation of the η2-diethylbenzene complex 8a (see above),
the qualitative observations indicate that much or most of the
dicationic platinum species in solution should be in the form
of 8a over the range of arene concentrations used; hence, we
expected the rate to exhibit approximate zero-order dependence
on diethylbenzene. In contrast, the reaction was found to be
first-order in 1,4-diethylbenzene over the entire range studied
(Figure 2), with a second-order rate constant of 5.1 × 10-4 M-1
s-1. (The large y intercept undoubtedly reflects the competing
decomposition processes discussed above.) Only small kinetic
isotope effects (determined from separate kinetics runs) were
found using 1,4-diethylbenzene-(aryl)d4 (kH/kD ) 1.38) or 1,4-
diethylbenzene-d10 (kH/kD ) 1.19).
intermediate might involve coordination of a second π bond or
a C-H bond generating a σ complex. The low KIE values could
be consistent with either; they do appear to rule out the
possibility that actual C-H activation is rate determining.
However, since the NMR observations indicate that not all of
the platinum is in the form of 8a under these conditions, that
mechanism would not be expected to give such clean first-order
dependence on arene concentration. At present we do not have
a fully satisfying explanation of this finding.
Mechanism of η3-Benzyl Formation: Isotopic Labeling.
Isotopic labeling experiments were previously used to elucidate
the mechanism of formation of η3-benzyl products from the
reactions of ethyl- and 1,4-diethylbenzene isotopologues with
platinum methyl cations 2 (eq 10).9i Information was obtained
Further indication that C-H activation is not a simple
intramolecular reaction of coordinated arene was obtained from
a competition experiment. A solution of π-arene complex 8a,
generated by adding 1,4-diethylbenzene (8.4 equiv, 0.183 M)
to platinum dication 5a, was treated with ethylbenzene addition
(53 equiv, 0.9 M). No change in platinum speciation was
observed, consistent with the failure to observe any π-arene
complex of ethylbenzene (see above). After the temperature was
raised to 50 °C, the resulting product consisted of a 44:56
mixture of η3-benzyls 10a (from 1,4-diethylbenzene) and 12a
(from ethylbenzene). The ratio of the products reflects neither
the initial ratio of substrates (1:6) nor the preferential coordina-
tion to platinum.
from both the C/H distribution of the liberated methane and
from the observation of isotope exchange between the various
positions of η3-benzyl groups derived from regiospecifically
labeled substrates. We concluded that initial attack occurs
exclusively at aryl positions (in contrast to the analogous
reactions of methylbenzenes, where initial attack at aryl and
methyl positions compete), followed by a complex pathway
One possible interpretation of these findings is shown in eq
9. The rate-determining step is substitution of trifluoroethanol
by 1,4-diethylbenzene in 8a. Formation of the bis(arene)

C-H Bond ActiVation by Dicationic Pt(II) Complexes
Organometallics, Vol. 26, No. 2, 2007 299
Scheme 2
Table 4. Isotope Exchange in η3-Benzyl Products from Two
tion under these conditions: after the reaction mixture is heated
for 24 h, addition of an excess of 1,4-diethylbenzene still gives
η3-benzyl product 10a in 98% yield. The possibility that
reversible aryl C-H bond activation takes place but the resulting
phenyl product is unstable to the liberated acid cannot be tested
by isotope exchange, as that process occurs in the acidic medium
even in the absence of platinum complex. Evidence in support
of platinum-catalyzed reversible exchange was obtained by
exposing platinum phenyl cation 13a (prepared from 2a and
benzene) to the reaction conditions (eq 11). While 13a is stable
Routesa
D in 10a
D in 10a
from 2a (%)
from 5a (%)
entry
substrate
Ha
Hb
Hc
Ha
Hb
Hc
1
2
3
4
5
1,4-(CH3CH2)2C6H4
1,4-(CD3CD2)2C6H4
1,4-(CH3CH2)2C6D4
CH3CH2C6H5
16
87
73
18
30
0
100
0
0
100
12
84
27
0
21
96
20
12
18
b
100
b
b
100
65
82
87
20
94
CH3CD2C6D5
34
a See eq 10 for position labels. b Could not resolve necessary peaks,
(Scheme 2) that eventually leads to cleavage of the benzylic
C-H bond, effecting substantial isotope exchange between the
aryl Hc and methyl Ha positions whenever these two positions
contain different labels in the starting 1,4-diethyl- or ethylben-
zene isotopologue (Table 4).
For C-H activation at a dicationic platinum center, we no
longer have liberated methane to look at, but we can examine
the liberated proton. Unfortunately, as noted above, the acidic
conditions effect facile H/D exchange between solvent OH and
aryl positions; thus, evidence for initial attack at an aryl position
cannot be obtained in this way. However, when 5a is allowed
to react with 1,4-C6D4(CH2CH3)2, the NMR signal for the
hydroxylic proton increases by almost exactly the amount
expected for liberation of one proton per Pt. A corresponding
experiment with C6D5CD2CH3 results in a negligible increase
in the OH signal. Taken together, these results imply that 2
and 5 follow different mechanisms for C-H activation of
ethylbenzenes: the former reacts at an aryl position and the
latter at a benzylic methylene.
to the acidic reaction environment at room temperature, warming
the reaction mixture to 50 °C results in complete dephenylation
to give the solvated platinum complex 5a in 76% yield. Addition
of 1,4-diethylbenzene to the resulting solution again gives η3-
benzyl 10a.
In striking contrast, 5a reacts rapidly with 1 equiv of
allylbenzene at room temperature to cleanly afford the ortho-
metalated product 14a (eq 12); the structure was confirmed
crystallographically.14 If more than 1 equiv of allylbenzene is
As a further test, the isotope scrambling patterns for the two
routes to η3-benzyls are compared in Table 4. Although the
pattern for activation by 5a is complicated by the substantial
acid-catalyzed H/D exchange between solvent and aryl C-H
bonds, comparison of entries 1 and 3 seems definitive. There
is some exchange of D into the methyl Ca position in both cases;
this was previously shown to take place after formation of the
product. However, entry 3 for reaction of 2a shows much more
extensive deuteration of Ha by transfer from the original aryl
positions; no such increase is observed for the reaction of 5a.
Hence, in this case net benzylic C-H activation does result
from direct attack at the benzylic position, unlike the previous
case of activation by 2. The activation is irreversible: analysis
of unreacted diethyl- and ethylbenzene isotopologues by GC/
MS shows no H/D exchange in the alkyl groups.
used, the excess substrate is all consumed and the products
apparently an oligomeric speciessoils out of solution. No
oligomerization of allylbenzene takes place in BF3/TFE in the
absence of platinum.
Compounds with allylic C-H bonds also react rapidly at
room temperature. Reaction of cyclohexene with the platinum
Other C-H Bond Activations at Dicationic Platinum
Centers. Addition of benzene to the platinum dication 5a results
in neither arene complex formation nor C-H activation; no new
products are formed, even after heating. There is no decomposi-
(14) See the Supporting Information for details. Full crystallographic data
have been deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ,
U.K., and copies can be obtained on request, free of charge, by quoting the
publication citation and the deposition number 614486.

300 Organometallics, Vol. 26, No. 2, 2007
DriVer et al.
Scheme 3
dication 5a at 25 °C gives the η3-allyl complex 15a in 94%
Scheme 4
yield (eq 13). In contrast to the case for allylbenzene, excess
substrate could be employed without oligomerization. The
addition of cyclohexene to a solution of 5a at -20 °C did not
1
result in the observation of a discernible π complex using H
NMR spectroscopy.
1-Pentene reacts similarly to give a mixture of isomeric allyls,
16a and 17a, in a 26:74 ratio (eq 14). The ratio of diastereomers
p-xylene (2.5 equiv) was added to 5a, giving the platinum
π-arene complex 8a′ (eq 4). The reaction mixture was then
saturated with cyclohexane (94 equiv) and heated. As before,
alkane C-H bond activation did not occur.
Conclusions
We have demonstrated that a platinum dication can activate
C-H bonds to afford organoplatinum complexes that are stable
to acid. The ultimate goal of this program is the development
of catalysts, based on the Shilov system, that can selectively
oxidize or otherwise functionalize hydrocarbons, especially
alkanes. Major challenges clearly remainsthe range of hydro-
carbons that can be activated by the complexes studied here is
rather limited and methods are needed for completing the
functionalization sequence that will not interfere with C-H bond
activation, as well as others.
These dicationic platinum complexes (5) exhibit complicated
behavior, most notably sensitivity to subtle changes in hydro-
carbon reagent, major differences of reactivity and mechanism
as compared to those of the corresponding methylplatinum
monocations (2). Much of this can be understood by recognizing
the central role of ligand replacement in C-H activation
chemistry. It is generally accepted that a C-H σ complex is
the immediate precursor to C-H activation and that a C-H σ
bond is a relatively weak ligand, so that a readily displaceable
ligand is a requisite component of any C-H activator, hence
the role of TFE as solvent/ligand in these model systems.3c
The presence of a “soft” electron-releasing methyl group
should weaken the bonding of TFE to the monocationic platinum
center in 2, relative to that in the dicationic 5. Accordingly,
activation of diethylbenzene by 5 not only is slower but also
(according to one interpretation of the kinetics) requires prior
replacement of one TFE by a π-arene before a second arene
of 17a was estimated to be approximately 18:82. These product
ratios did not change upon warming the reaction mixture to 50
°C for several days. A plausible mechanism for the formation
of η3-allyl isomers is shown in Scheme 3.
In contrast to olefin or arene substrates, no C-H bond
activation products were observed when the solvated platinum
dication 5a was exposed to methane, n-pentane, or cyclohexane.
No reaction was observed until 90 °C, at which point the starting
material slowly decomposed (in the presence or absence of
alkane) to give Pt(0) along with the same symmetrical product
1
observed by H NMR on concentration of 5a (see above). No
H/D exchange of unreacted alkane was observed in any case.
The plausible inference (from kinetics) that activation of a
benzylic C-H bond requires prior coordination of a π-arene
ligand raises the possibility that the same might be true for
alkane C-H bond activation. To test that, a small amount of

C-H Bond ActiVation by Dicationic Pt(II) Complexes
Organometallics, Vol. 26, No. 2, 2007 301
reagents were commercially obtained and, where appropriate,
purified prior to use. Tris(pentafluorophenyl)borane (B(C6F5)3) was
purified by sublimation (90 °C, 0.5 mmHg). Trifluoroborane was
purified by following the procedure of Brown and co-workers:16
the borane was condensed into benzonitrile and evacuated. The
borane-nitrile complex was decomposed by mild heating to 60
°C. The borane was passed through two -78 °C traps and
condensed at -196 °C. It was then vacuum-distilled by warming
to -78 °C. A known volume of BF3 was then condensed into
trifluoroethanol to form a 0.4776 M solution. Trifluoroethanol-d3
was dried over 3 Å molecular sieves for at least 5 days and then
was vacuum-distilled onto B(C6F5)3. After 6 h, the trifluoroethanol-
d3 was vacuum-distilled to and stored in a valved reaction vessel.
The platinum dimethyl complexes were synthesized by following
earlier reported procedures.9g Trifluoroethanol-d3, B(C6F5)3, and
platinum dimethyl complexes were stored in a Vacuum Atmo-
spheres nitrogen atmosphere drybox.
molecule undergoes C-H activation. We suggest, then, that the
(unspecified) intermediate shown in eq 9 above is in fact a σ
complex of a benzylic C-H bond, which is not capable of
displacing TFE directly from 5a but can do so from 8a. Labeling
experiments indicate that the benzylic position is the site of
initial C-H bond activation by 8a, whereas it is an aryl position
for 2; the reasons for this difference are not clear.
The facile C-H activations of allylbenzene and of allylic
positions on olefins are consistent with this picture. Initial
coordination of the olefinic π bond, a considerably better ligand
than an η2-arene, generates a species electronically similar to
8a, which furthermore has an aryl or allylic C-H bond
appropriately placed for intramolecular activation, as illustrated
for cyclohexene in Scheme 4. We cannot determine whether
benzene itself undergoes C-H activation by 5; the expected
product (13) would not be stable, and background acid-catalyzed
H/D exchange obviates that test for reversible activation. It is
also not clear why methylbenzenes do not react with 5 to give
η3-benzyls, in contrast to reactions of 2, where methyl- and
ethylbenzenes react similarly (although by different mechanisms).9i
We believe this probably reflects product stability rather than
inherent reactivity, as decomposition products are observed. A
similar situation is found in the formation of π-arene complexes,
where p-dialkylbenzenes readily displace a TFE ligand whereas
monoalkylbenzenes (and benzene itself) show no observable
complexation at all. Presumably this represents a delicate
balance between the bonding abilities of the arene vs those of
TFE, which is just tipped by the additional electron donation
conferred by two alkyl substituents.15
The failure of alkanes to react at dicationic platinum may be
a consequence of kinetics and/or thermodynamics. A simple
alkyl product, [(N-N)PtR(TFE)]+, is not be expected to be stable
to the proton generated in its formation, since the corresponding
methyl compound 2 is not. The lack of any observable alkyl
H/D exchange, however, suggests that there is a kinetic barrier
as well. Previous studies have shown that methane is consider-
ably less reactive than benzenes or alkylbenzenes toward 2;9e,h,j
also, there is no ligand available to generate an intermediate
(like 8) that may be more prone to undergo TFE replacement
by a C-H bond. An attempt to provide the latter, in the form
of p-xylene, had no effect. As shown above, a C-H bond at a
saturated carbon can be activated intramolecularly, at least in a
case where a relatively stable product such as an η3-allyl group
can be reached. It remains to be seen whether a suitable
combination of reagents and reaction conditions will make it
possible to access a much wider range of C-H bonds for
functionalization by this route. Progress toward this end is
ongoing in our laboratories.
Representative Formation of [(N-N)Pt(TFE-d3)2]2+ (5a) from
[(N-N)PtMe2] (1a) (N-N ) ArNdC(Me)C(Me)dNAr). To a
suspension of the platinum dimethyl complex 1a (0.010 g, 0.015
mmol) in 0.700 mL of trifluoroethanol-d3 was added 0.070 mL of
a 0.455 M solution of F3B in trifluoroethanol-d3. The reaction
1
progress was analyzed periodically using H NMR spectroscopy.
After 15 h, analysis revealed complete consumption of 1a and
formation of 5a (in situ characterization): 1H NMR (300 MHz,
CF3CD2OD) δ 7.84 (br s, 2H), 7.35 (br s, 4H), 2.32 (br s, 6H),
1.38 (br s, 36H); 13C NMR (125 MHz, CF3CD2OD) δ 189.8, 157.9,
144.6, 127.8, 117.9, 37.0, 31.8, 20.6; 19F NMR (282 MHz, CF3-
CD2OD) δ -77.3, -150.5 (br s).
Representative Formation of the η3-Benzyl Complex 10a from
[(N-N)Pt(TFE-d3)2]2+ (5a). To 0.700 mL of a 0.014 M solution of
the platinum dication 5a in trifluoroethanol-d3 was added 0.050
mL of 1,4-diethylbenzene (0.321 mmol). Analysis of the resulting
mixture using 1H NMR spectroscopy revealed partial formation of
the π-arene complex 8a (R ) Et): 1H NMR (300 MHz, CF3CD2-
OD) δ 7.73 (t, J ) 1.8 Hz, 2H), 7.22 (d, J ) 1.5 Hz, 4H), 6.77 (s,
4H), 2.41 (s, 6H), 1.88 (q, J ) 7.5 Hz, 4H), 1.43 (s, 36H), 1.12 (t,
J ) 7.5 Hz, 6H); 13C NMR (125 MHz, CF3CD2OD) δ 181.3, 157.1,
150.4, 143.5, 132.4, 117.0, 113.6; 36.7, 31.9, 27.2, 20.0, 13.0; 19
F
NMR (282 MHz, CF3CD2OD) δ -77.2, -81.8, -150.9. The
reaction mixture was heated to 50 °C. After 13 h, 1H NMR analysis
revealed formation of the η3-benzyl complex 10a in 98% yield:
1H NMR (600 MHz, CF3CD2OD) δ 7.61 (s, 1H), 7.59 (s, 1H),
7.24 (br s, 1H), 7.45 (s, 1H), 6.96 (br s, 2H), 6.80 (br s, 1H), 6.47
(s, 1H), 6.39 (s, 1H), 5.88 (d, J ) 6.6 Hz, 1H), 2.82 (q, J ) 6.6
Hz, 1H), 2.08 (dd, J ) 14.7, 7.5 Hz, 1H), 2.02 (s, 3H), 2.00 (dd,
J ) 14.7, 7.5 Hz, 1H), 1.93 (s, 3H), 1.44 (s, 9H), 1.39 (br s, 9H),
1.36 (br s, 9H), 1.30 (s, 9H), 0.93 (t, J ) 7.5 Hz, 3H), 0.29 (d, J
) 6.6 Hz, 3H). The 1H NMR data matched those previously
reported.9i
Acknowledgment. Funding for this work was provided by
the bp MC2 program and the NIH in the form of NRSA
fellowships to T.G.D. (Grant No. GM070272) and T.J.W. (Grant
No. GM075691). We thank Tom Dunn for assistance with NMR
spectrometry, Dr. Mona Shahgholi for mass spectrometry data,
and Mr. Larry M. Henling for crystallographic analysis.
Experimental Section
General Considerations. 1H NMR and 13C NMR spectra were
recorded at ambient temperature using Varian 600 or 300 spec-
trometers. The data are reported as follows: chemical shift in ppm
from internal tetramethylsilane on the δ scale, multiplicity (br )
broad, s ) singlet, d ) doublet, t ) triplet, q ) quartet, m )
multiplet), coupling constants (Hz), and integration. Mass spectra
were acquired on a Finnigan LCQ ion trap or Agilent 5973 Network
mass selective detector and were obtained by peak matching. All
reactions were carried out under an atmosphere of nitrogen in
glassware which had been oven-dried. Unless otherwise noted, all
Supporting Information Available: Text, tables, and figures
giving detailed experimental procedures, characterization data, and
integrated rate laws and a CIF file giving X-ray diffraction data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM060792R
(15) Displacement of TFE by ethylbenzene and (to a lesser extent)
benzene can be observed at low TFE concentrations by using the weaker
ligand hexafluoroisopropyl alcohol as solvent: Driver, T. G. Unpublished
results.
(16) Brown, H. C.; Johannesen, R. B. J. Am. Chem. Soc. 1953, 75, 16-
20.