C-H activation and C-C coupling of arenes by cationic Pt(II) complexes

Article abstract of DOI:10.1021/ja020798h

The synthesis and characterization of cationic platinum complexes of the type [(R₂PC₂H₄PR₂)- PtMe(OEt₂)]BAr_F (R = Cy, Et) are reported. These electrophilic platinum cations are found to react quantitatively with arenes (benzene, toluene) at room temperature by undergoing intermolecular C-H activation with concomitant C-C coupling to generate complexes of the type [{Pt(R₂ PC₂ H₄PR₂)}₂(μ-η³: η3-biaryl)][BAr_F]₂. The dianionic biaryl ligands in these compounds exhibit a rare μ-η³:η³-bis-allyl bonding mode and can be removed from the complex with stoichiometric oxidants to generate the free biaryl and [(R₂PC₂ H₄PR₂) Pt(μ-X)]₂[BAr_F]₂ (R = Cy, Et; X = Cl, I). The cationic platinum complexes [(R₂PC₂H₄PR₂)- PtMe(OEt₂)]BAr_F (R = Cy, Et) are also quite reactive with water, forming the bridging hydroxide complexes [(R₂ PC₂H₄PR₂) Pt(μ-OH)]₂[BAr_F]₂ (R = Cy, Et). A possible mechanism is proposed for the C-C coupling reaction based upon the structures of these bridging biphenyl complexes, which provides a new perspective for the related palladium-catalyzed oxidative coupling of arenes to form biaryls.

Full text of DOI:10.1021/ja020798h

Published on Web 10/01/2002
C-H Activation and C-C Coupling of Arenes by Cationic
Pt(II) Complexes
Wayde V. Konze, Brian L. Scott, and Gregory J. Kubas*
Contribution from the Chemical Science and Technology DiVision, MS J514, Los Alamos
National Laboratory, Los Alamos, New Mexico 87545
Received June 5, 2002
Abstract: The synthesis and characterization of cationic platinum complexes of the type [(R2PC2H4PR2)-
PtMe(OEt2)]BArF (R ) Cy, Et) are reported. These electrophilic platinum cations are found to react
quantitatively with arenes (benzene, toluene) at room temperature by undergoing intermolecular C-H
activation with concomitant C-C coupling to generate complexes of the type [{Pt(R2PC2H4PR2)}2(µ-η3:
η3-biaryl)][BArF]2. The dianionic biaryl ligands in these compounds exhibit a rare µ-η3:η3-bis-allyl bonding
mode and can be removed from the complex with stoichiometric oxidants to generate the free biaryl and
[(R2PC2H4PR2)Pt(µ-X)]2[BArF]2 (R ) Cy, Et; X ) Cl, I). The cationic platinum complexes [(R2PC2H4PR2)-
PtMe(OEt2)]BArF (R ) Cy, Et) are also quite reactive with water, forming the bridging hydroxide complexes
[(R2PC2H4PR2)Pt(µ-OH)]2[BArF]2 (R ) Cy, Et). A possible mechanism is proposed for the C-C coupling
reaction based upon the structures of these bridging biphenyl complexes, which provides a new perspective
for the related palladium-catalyzed oxidative coupling of arenes to form biaryls.
Introduction
has involved intramolecular C-H activation reactions on
substrates that are already coordinated to the metal center; there
Intense efforts have recently been devoted to the study of
the C-H activation step of alkane functionalization processes.1
The majority of this research has centered around electrophilic
platinum complexes containing bidentate nitrogen ligands, with
the emphasis on the catalytic transformation of methane to
methanol.1c,2 Another field of great interest is the transition-
metal-catalyzed coupling of various reagents to generate new
carbon-carbon bonds, which has now evolved into a widespread
synthetic technique.3 A great deal of work has also recently
been devoted to the combination of these two fields, and the
applications of C-H activation reactions that lead to C-C
coupling are enormous.4 However, the majority of this work
are relatively few examples of intermolecular C-H activation
reactions that lead to C-C coupling.4 One of the more studied
and important examples of this type of reaction is the Pd(II)-
catalyzed oxidative coupling of arenes to form biaryls.5 Although
this reaction has been studied a great deal, the details of the
mechanism have been mainly deduced indirectly.
We have been involved in the study of highly electrophilic
systems for the coordination and activation of small molecules6
and reported a series of coordinated alkyl and aryl halides of
the type [(P-i-Pr3)2PtH(η1-XR)]BArF (BArF ) B[3,5-C6H3-
(CF3)2]4-), along with analogues containing coordinated Et2O,
THF, and H2 ligands.6a Since these platinum phosphine cations
are potentially more electrophilic than their nitrogen analogues,
we chose to further explore these platinum phosphine cations
with respect to their ability to activate C-H bonds. Our initial
attempts involving complexes of the type [(PR3)2PtMe(solv)]-
BArF (solv ) solvento ligand) resulted in electrophilic attack
on the BArF anion by the Pt cation as opposed to C-H
activation.7 In this paper, we report novel C-H bond cleavage
reactions of aromatic substrates with complexes of the type
* Address correspondence to this author. E-mail: kubas@lanl.gov.
(1) (a) Periana, R. A.; Taube, D. J.; Evitt, E. R.; Loeffler, D. G.; Wentrcek, P.
R.; Voss, G.; Matsuda, T. Science 1993, 259, 340. (b) Sen, A.; Benvenuto,
M. A.; Lin, M.; Basickes, N. J. Am. Chem. Soc. 1994, 116, 998. (c) Periana,
R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science
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E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879. (f) Catalysis by Metal
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Hydrocarbons in the Presence of Metal Complexes; Shilov, A. E., Shul’pin,
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(2) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
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Acta 1999 294, 240. (c) Fang, X.-G.; Vincent, J. H.; Scott, B. L.; Kubas,
G. J. J. Organomet. Chem. 2000, 609, 95. (d) Fang, X.-G.; Scott, B. L.;
John, K. D.; Kubas, G. J. Organometallics 2000, 19, 4141. (e) Fang, X.-
G.; Scott, B. L.;Watkin, J. G.; Kubas, G. J. Organometallics 2001, 20,
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(3) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b) Negishi, E.;
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9
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J. AM. CHEM. SOC. 2002, 124, 12550-12556
10.1021/ja020798h CCC: $22.00 © 2002 American Chemical Society

Reaction of Cationic Pt(II) Complexes with Arenes
A R T I C L E S
[(diphosphine)PtMe(solv)]BArF that lead to an unexpected C-C
coupling reaction to form dianionic biaryl ligands in unusual
coordination modes. The Pt-biaryl complexes can then be
oxidatively cleaved to generate the free biaryls. The conclusive
identification of these unique biaryl ligand structures offers a
new perspective for elucidating the mechanism of the Pd(II)-
catalyzed oxidative coupling of arenes, and the intermolecular
C-H activation and C-C coupling reactions described in this
paper are an important entry into this developing area of
chemistry.
Results and Discussion
Synthesis and Characterization of Cationic Methyl Plati-
num Complexes. Protonation of the platinum dimethyl com-
plexes (R2PC2H4PR2)PtMe2 (R ) Cy (cyclohexyl), Et) with 1
equiv of [H(OEt2)2][BArF]8 in Et2O results in the elimination
of methane to form the cationic solvento complexes [(R2PC2H4-
PR2)PtMe(OEt2)]BArF (R ) Cy, 92% yield (1a); Et, 89% yield
(1b)), respectively (eq 1). These complexes can be isolated as
Figure 1. Molecular structure of 2a (50% probability ellipsoids). Selected
bond distances (Å) and angles (deg) for 2a: Pt(1)-P(1), 2.2138(17); Pt(1)-
O(1), 2.147(5); O(1)-Pt(1)-O(1A), 80.5(2); O(1)-Pt(1)-P(2), 96.40(15);
O(1A)-Pt(1)-P(1), 95.81(14); P(1)-Pt(1)-P(2), 87.19(7).
1
1a, both 2a and free CH4 were observed by H NMR. This
reaction likely involves coordination of H2O, followed by
cleavage of an O-H bond and loss of CH4 to generate a
monomeric hydroxide complex, which then dimerizes to form
2a. It is interesting to note the difference in hydrolytic reactivity
between this bidentate phosphine platinum complex and its
diimine and diamine analogues,2e-h,9 which in some cases were
found to form relatively stable aqua complexes that decomposed
only with certain ligand variations. The reason for this difference
is not completely clear, but it is assumed that the more
electrophilic phosphine system causes the aqua intermediate to
be very acidic and more reactive toward protonation of the
methyl ligand.
1
solids from hexanes at -30 °C and were characterized by H
NMR, 31P NMR, and elemental analyses. Particularly diagnostic
are the 31P NMR spectra, which exhibit two resonances with
vastly different JPPt satellites (e.g., δ 70.4 (s, JPPt ) 1870 Hz),
50.9 (s, JPPt ) 4520 Hz) for 1a) that are indicative of the much
stronger trans effect of the methyl ligand relative to that of the
coordinated Et2O. These complexes are stable in Et2O solutions
at room temperature for several weeks under an inert atmosphere
but decompose even in the solid state when exposed to air and
moisture.
Hydrolysis of Cationic Methyl Platinum Complexes and
X-ray Structure of [(Cy2PC2H4PCy2)Pt(µ-OH)]2[BArF]2 (2a).
The cationic solvento complex 1a was found to react with even
adventitious amounts of water to form the bridging hydroxide
complex [(Cy2PC2H4PCy2)Pt(µ-OH)]2[BArF]2 (2a) (eq 2). This
Crystals of quality suitable for X-ray diffraction analysis were
obtained by cooling a concentrated solution of 2a in pentafluoro-
pyridine to -30 °C for several days. The structure of the cation
of 2a‚4NC5F5 is shown in Figure 1 along with selected bond
lengths and bond angles. Although the OH protons were not
1
located crystallographically, they were clearly seen in the H
NMR. The structure exhibits a nearly planar P2Pt(µ-O)2PtP2 unit
which is quite similar to analogous [L2Pt(µ-OH)]2+ complexes
that have been reported previously.10
C-H Activation and C-C Coupling of Benzene. When
either 1a or 1b is heated in neat benzene at 80 °C, the mostly
insoluble solids transform almost immediately into deep red
insoluble oils. These reactions also occur at room temperature
in 4 h. The oils form red crystalline products that are isolable
in nearly quantitative yields (98%), which is remarkable
considering the complexity of the transformation. An NMR
investigation verified that the conversion of 1a and 1b to
[{Pt(R2PC2H4PR2)}2(µ-biphenyl)][BArF]2 (R ) Cy (3a), Et (3b))
is quantitative, with concomitant formation of methane (eq 3).
Both the evolution of methane during these reactions and the
1H NMR spectra of the products led us to believe that C-H
activation had occurred. However, in addition to Pt-Ph
resonances, the 1H NMR exhibited phenyl resonances that were
shifted upfield (e.g., for 3a: 5.67 (2H), 5.48 (2H), and 4.74
complex was characterized by 1H NMR, 31P NMR, and
elemental analysis, and the molecular structure was determined
by X-ray diffraction. To prevent the formation of the bridging
hydroxide complex 2a, it was necessary to use diethyl ether
which was freshly dried over an activated alumina column
throughout the synthetic preparation of 1a. When a stoichio-
metric amount of H2O was added to a diethyl ether solution of
(9) Heiberg, H.; Johansson, L.; Gropen, O.; Ryan, O. B.; Swang, O.; Tilset,
M. J. Am. Chem. Soc. 2000, 122, 10831.
(10) Li, J. J.; Li, W.; Sharp, P. R. Inorg. Chem. 1996, 35, 604.
(7) Konze, W. V.; Scott, B. L.; Kubas, G. J. Chem. Commun. 1999, 1807.
(8) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11, 3920.
9
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A R T I C L E S
Konze et al.
represented as a phenyl-substituted benzene dianion that is
bonded to two Pt(II) fragments. The bond lengths in the
coordinated ring are also consistent with a bis-allyl type of
bonding; the average C-C distance in the allyl units is 1.412-
(15) Å, which is a typical allylic C-C distance,12 while the
C-C distances which connect the allyl moieties are closer to
C-C single bond lengths (1.480(13) and 1.500(13) Å). Also
indicative12 of an allylic type of distortion is the short Pt-C
distances to the central allylic carbon atoms (2.083(13) and
2.143(11) Å) compared to the Pt-C distances to the outer allylic
carbon atoms (average distance of 2.249(10) Å).
This type of bis-allyl bonding in arenes is quite unusual, and
only a few examples have been reported in the literature, most
of which contain boat-shaped benzene rings that are coordinated
to a dinuclear center containing a metal-metal bond.13 Recently,
Fryzuk et al. reported two dinuclear yttrium(III) complexes
which contain bridging biphenyl dianions.14 In one structure,
each yttrium fragment is bound in an η6-fashion to a different
ring of the biphenyl unit; the entire biphenyl moiety is nearly
planar in this complex, with a partial double bond connecting
the two rings. In the other structure, both of the yttrium
fragments are bonded to opposite sides of one ring of the
biphenyl unit, and this ring exhibits a bis-allyl type of distortion.
Although the bond lengths in this coordinated ring are similar
to those in 3b, the ring remains planar, with both yttrium
fragments bonded in η6-fashion instead of η3-fashion. Evidence
was also obtained for fluxionality between these two structures.
However, no fluxional behavior was found in 3a or 3b, which
retained their η3,η3-bis-allyl bonding from 25 to -75 °C.
Figure 2. Molecular structure of 3b (50% probability ellipsoids).
Table 1. Selected Bond Distances (Å) and Angles (deg) for
[{Pt(Et2PC2H4PEt2)}2(µ-Biphenyl)][BArF]2 (3b)
Distances (Å)
Pt(2)-C(23)
Pt(2)-C(24)
Pt(2)-C(25)
C(21)-C(22)
C(21)-C(26)
2.249(10)
2.143(11)
2.247(8)
1.374(15)
1.422(12)
C(22)-C(23)
C(23)-C(24)
C(24)-C(25)
C(25)-C(26)
1.500(13)
1.410(13)
1.441(15)
1.480(13)
Bond Angles (deg)
P(1)-Pt(1)-P(2)
P(3)-Pt(2)-P(4)
87.68(10) C(23)-C(24)-C(25) 115.7(10)
87.22(10) C(24)-C(25)-C(26) 118.8(8)
C(21)-C(22)-C(23) 120.6(8)
C(22)-C(23)-C(24) 118.6(8)
C(25)-C(26)-C(21) 119.6(8)
C(26)-C(21)-C(22) 117.2(10)
ppm (1H)), indicative of π-type coordination of a phenyl ring.
To characterize these products further, an X-ray diffraction study
(discussed below) was carried out on 3b (crystals of 3a exhibited
disorder, which allowed only a partial structure). Reactions
similar to that in eq 3 (e.g., with complexes containing Ph2-
C-H Activation and C-C Coupling of Other Arenes.
Complex 1a reacts in neat toluene at 80 °C almost immediately
to form a red oil, which was then crystallized from a CH2Cl2/
hexanes mixture to give red crystals. The 1H NMR, 31P NMR,
and elemental analysis are consistent with the assignment of
this complex as [{Pt(Cy2PC2H4PCy2)}2(µ-bitolyl)][BArF]2 (4a).
This species was present as a mixture of coordinated bitolyl
isomers, which made it difficult to assign NMR signals.
However, the characteristic upfield-shifted phenyl resonances
1
were present in the H NMR, and there were two singlets in
the 31P NMR (δ69.9, 70.7) with identical JPPt coupling constants
(3620 Hz), indicating that there are at least two isomers of this
compound, depending upon coincidental resonances. In addition,
a preliminary X-ray analysis indicated a structure similar to that
of 3b, but the final refinement could not be accomplished due
to disorder. Complex 1a was also dissolved in fluorobenzene
and heated to 80 °C, but no fluorobenzene coupling occurred,
as evidenced by 1H and 31P NMR. Prolonged heating for several
hours yielded methane, but no tractable solid compounds could
be isolated. When 1a was heated for several hours at 80 °C in
PCH2CH2CH2PPh2 (dppp) instead of Et2PCH2CH2PEt2) as well
as the X-ray structure of a product analogous to 3b have recently
also been found by Thomas and Peters.11
X-ray Structure Determination of 3b. The molecular
structure of 3b is shown in Figure 2, and Table 1 gives metrical
data. The structure indicates that 3b is a dinuclear complex
containing a bridging, dianionic biphenyl ligand in which two
(Et2PC2H4PEt2)Pt+ units are bonded in η3-fashion to opposite
sides of one of the rings in the biphenyl ligand. The coordinated
ring is highly distorted from planarity into a pseudo-chair
conformation (bend angle of 23.3°) and is best viewed as being
a dianionic, bis-allyl moiety, while the other ring in the biphenyl
unit is unaffected by the coordination. This ligand can also be
1
cyclohexane, methane was again evolved, but the H and 31P
NMR spectra were identical to those in the fluorobenzene
(12) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.;
Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.
(13) (a) Jonas, K.; Koepe, G.; Schieferstein, L.; Mynott, R.; Kruger, C.; Tsay,
Y. H. Angew. Chem., Int. Ed. Engl. 1983, 22, 620. (b) Muller, J.; Gaede,
P. E.; Qiao, K. Angew. Chem., Int. Ed. Engl. 1993, 32, 1697. (c) Muller,
J.; Gaede, P. E.; Qiao, K. J. Organomet. Chem. 1994, 480, 213. (d)
Schneider, J. J.; Specht, U.; Goddard, R.; Kruger, C.; Ensling, J.; Gutlich,
P. Chem. Ber. 1995, 128, 941.
(14) (a) Fryzuk, M. D.; Love, J. B.; Rettig, S. J. J. Am. Chem. Soc. 1997, 119,
9071. (b) Fryzuk, M. D.; Jafarpour, L.; Kerton, F. M.; Love, J. B.; Patrick,
B. O.; Rettig, S. J. Organometallics 2001, 20, 1387.
(11) Thomas, C.; Peters, J. C., personal communication.
9
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Reaction of Cationic Pt(II) Complexes with Arenes
A R T I C L E S
Scheme 1
reaction. It is thought that at these temperatures, in the absence
of a reactive substrate, intramolecular C-H activation is
occurring on the phosphine ligands. However, this was not
verified experimentally, as no identifiable complexes could be
isolated.
Reactions of Bridging Biaryl Complexes. Since 3a and 3b
formally contain a dianionic phenyl-substituted benzene ligand,
they could be susceptible to hydrolysis similar to the reduced
benzene intermediate in a Birch reduction. To test this, a large
excess of water was added to these complexes in THF, but no
reaction was evident after several days. However, when 8 equiv
of HCl was added to solutions of 3a or 3b in CH2Cl2, the red
color immediately disappeared. Instead of the expected phenyl-
cyclohexadiene hydrolysis product, free biphenyl was obtained
along with [(R2PC2H4PR2)Pt(µ-Cl)]2[BArF]2 (eq 4).15 Evidently,
examples of intermolecular aryl group transfer in group 10
metals,16 and there are a few examples of transmetalation in
neutral group 10 metal-aryl complexes to generate biaryl.17 In
addition, there are several examples of catalytic cross-coupling
reactions to generate biaryls which have proposed this trans-
metalation/reductive elimination mechanism.3d More recently,
palladium complexes of the type [PdX(Ar)(N-N)] (N-N )
N,N,N′,N′-tetramethylethylenediamine, 2,2′-bipyridine, 4,4′-di-
methyl-2,2′-bipyridine) were reported to react with AgBF4 to
form the cationic complexes [Pd(Ar)(bpy)(solv)]+.18 Depending
on the ligand set, these complexes either were isolable or were
found to spontaneously form biaryls. The formation of biaryl
was proposed to go through a transmetalation/reductive elimina-
tion mechanism similar to that mentioned above, where the
transmetalation was assumed to occur through a dinuclear,
bridging phenyl intermediate, although no intermediates of this
type were observed.
Possible Mechanism for Arene Coupling Reaction. In
regard to the above discussion, complexes 3a, 3b, and 4a can
be viewed as representing actual isolated C-C coupled arene
intermediates, the structure of which suggests a different type
of coupling mechanism than those postulated previously for
group 10 metal complexes. The main difference implied by the
structures of our biaryl complexes is that the coupling occurs
without a change in the formal oxidation state of the metal.
This contrasts with previously postulated mechanisms for arene
or aryl coupling, in which the formal oxidation state of the metal
is reduced by 2 during a reductive elimination of the two aryl
groups from a single metal center. The current results are more
similar to the recently reported results of Fryzuk et al., in which
complexes of the type {[PhP(CH2SiMe2NSiMe2CH2)2PPh]M}2-
(µ-Cl)2 (M ) Y and Ho) were shown to react with ArLi reagents
to generate aryl-coupled products containing dianionic, µ-η6:
η6-bridged biaryl ligands.14 Although the exact nature of this
coupling reaction was not explored, it was assumed that the
coupled products allow for a more efficient interaction of charge
between the aryl groups and the metals in these predominantly
ionically bonded complexes.
H+ is reduced to H2, while the (biphenyl)2- ligand is oxidized
to free biphenyl. This oxidation reaction was also accomplished
with elemental iodine, which resulted in the formation of free
biphenyl and [(R2PC2H4PR2)Pt(µ-I)]2[BArF]2 (eq 5). The ability
to release the biaryl ligands by oxidation prompted us to
investigate the nature of the bitolyl isomers in 4a. When a
CH2Cl2 solution of 4a was treated with iodine, the free bitolyl
isomers were present along with [(R2PC2H4PR2)Pt(µ-I)]2[BArF]2
(R ) Cy, Et). After the bitolyl isomers were isolated from this
reaction mixture, a GC-MS analysis showed that no ortho-
bitolyl isomers were formed, and that a nearly statistical mixture
of m,m′-bitolyl, m,p′-bitolyl and p,p′-bitolyl were present. The
fact that no ortho-bitolyl isomers formed is most likely due to
steric effects. More importantly, the fact that a statistical mixture
of mixed meta- and para-bitolyl isomers is obtained argues
against an electrophilic aromatic substitution mechanism for the
initial C-H activation step, which would result in an enrichment
of para-bitolyl isomers.
Aryl Coupling Background. The catalytic oxidative coupling
of arenes by palladium acetate with oxygen is a well-known
reaction. Although the mechanism of this reaction has been
debated, the most often cited version (Scheme 1) involves
electrophilic aromatic substitution of the arene by palladium to
form a σ-arylpalladium(II) moiety with loss of a proton (step
a). This σ-arylpalladium(II) species is then proposed to undergo
transmetalation to give a diarylpalladium(II) species (step b),
which can then undergo reductive elimination to give the biaryl
product and a Pd(0) species (step c), which is reoxidized by
oxygen. Although no intermediates have been directly observed
to verify this mechanism of C-C coupling, there are several
The novel structures of complexes 3a, 3b, and 4a prompted
us to postulate a possible mechanism for their formation
(Scheme 2). Although we were not able to observe any
intermediates in this reaction due to the low solubility of the
(16) (a) Ozawa, F.; Fujimori, M.; Yamamoto, T.; Yamamoto, A. Organometallics
1986, 5, 2144. (b) Ozawa, F.; Hidaka, T.; Yamamoto, T.; Yamamoto, A.
J. Organomet. Chem. 1987, 330, 253. (c) Ozawa, F.; Kurihara, K.; Fujimori,
M.; Hidaka, T.; Toyoshima, T.; Yamamoto, A. Organometallics 1989, 8,
180. (d) Casado, A. L.; Casares, J. A.; Espinet, P. Organometallics 1997,
16, 5730. (e) Casado, A. L.; Espinet, P. Organometallics 1998, 17, 3677.
(17) (a) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 7547. (b)
Amatore, C.; Jutand, A. Organometallics 1988, 7, 2203. (c) Amatore, C.;
Jutand, A. J. Am. Chem. Soc. 1991, 113, 2819. (d) Osakada, K.; Sato, R.;
Yamamoto, T. Organometallics 1994, 13, 4645. (e) Osakada, K.; Yama-
moto, T. Coord. Chem. ReV. 2000, 198, 379.
(15) The compounds [(R2PC2H4PR2)Pt(µ-Cl)]2[BArF]2 (R ) Cy, Et) were
characterized by comparing their 1H and 31P NMR spectra to those of
authentic samples (prepared by reacting (R2PC2H4PR2)Pt(Me)(Cl) with
[H(OEt2)2][BArF] in Et2O).
(18) Yagyu, T.; Hamada, M.; Osakada, K.; Yamamoto, T. Organometallics 2001,
20, 1087.
9
J. AM. CHEM. SOC. VOL. 124, NO. 42, 2002 12553

A R T I C L E S
Konze et al.
Scheme 2
incorporation. This strongly suggests that a dimeric intermediate
such as ii is involved in the coupling reaction. We feel that the
η1,η2-bridged nature of this intermediate is reasonable, especially
in light of the recent spectroscopic and structural characterization
of cationic η2-benzene complexes of platinum(II)21 and a
dinuclear palladium complex with an η2-phenyl interaction.22
Since η2-coordination of a benzene ring is expected to dearo-
matize and consequently activate the ring to some degree, we
propose that, after dissociation of one of the η2-benzene ligands
to generate compound iii, a migratory insertion of the η2-benzene
ligand into the Pt-Ph bond occurs to form complex iW, which
can be viewed as containing two η1-allyl interactions. This
complex then undergoes η1-η3-type allyl migration reactions
on the ring to which the Pt fragments are attached to generate
the final product, 3a,b.23 The fact that 3 does not show any
fluxionality in solution from 25 to -75 °C suggests that the
solid-state structure, which contains a phenyl group substituent
on the central position of one of the allyl units, also represents
the most stable conformation in solution. Although, to the best
of our knowledge, no one has previously suggested the
migratory insertion of an η2-benzene ligand into a Pt-Ph bond,
it seems reasonable that this could occur based upon the
preponderance of small-molecule insertion reactions into group
10 metal-aryl bonds. In addition, migratory insertion reactions
of palladium aryls with coordinated dienes to give coordinated
allyls are known.25 It has been claimed that these reactions are
more facile than olefin insertions because of the stability of the
metal allyl complex that forms.25a This may be a driving force
in the formation of 3a,b. In further support for this type of
insertion reaction, the migration of a methyl group to the arene
ring has been reported in the complex (η6-benzene)Mn(Me)-
(CO)2 to give the endo-methylcyclohexadienyl product.26 It is
also possible that these types of insertion mechanisms are
actually operating in the reported examples of aryl-coupling
reactions mentioned above. The fact that the biaryl ligands in
3a, 3b, and 4a are removed by mild oxidants also supports this
hypothesis.
starting materials and products in the benzene solvent, we
propose that the first step of this reaction occurs through C-H
activation of benzene, with a subsequent liberation of methane
to generate a cationic platinum phenyl complex (i). Recent
kinetic studies on benzene C-H activation reactions at cationic
platinum centers containing diimine ligands suggest that these
reactions occur by a mechanism involving coordination of an
η2-benzene, followed by reversible oxidative addition of benzene
C-H bonds, reversible formation of a methane C,H-σ-complex,
and finally methane loss.2h A similar mechanism is most likely
occurring in our diphosphine system, and methane formation
1
was detected by H NMR in sealed NMR tube reactions. As
further support, a complex analogous to i has been spectroscopi-
cally observed as an intermediate in a similar system where
the phosphine is dppp.11 The C-C coupling reaction is then
proposed to occur via an η1,η2-bridged phenyl dimer (ii), which
has a putative structure very similar to that of an η1,η2-bridged
2-furyl dimer synthesized by reacting 1a with furan. However,
Summary
The reaction of the cationic platinum complexes [(R2PC2H4-
PR2)PtMe(OEt2)]BArF (R ) Cy (1a), Et (1b)) with aromatic
substrates was expected to give C-H activation to form σ-aryl
complexes of the type [(R2PC2H4PR2)Pt(Ar)(Solv)]BArF with
elimination of methane. However, even at room temperature
these reactions proceed quantitatively to form the C-C coupled
products 3a, 3b, and 4a, which contain bridging, dianionic biaryl
ligands. It is likely that σ-aryl complexes are first formed in
these reactions by C-H activation on the arene substrates,
no C-C coupling occurred between the bridging 2-furyl groups
in this complex.19 There are also several transition metal
complexes containing bridging aryl groups that have been
structurally characterized.20
To verify that two initially formed platinum phenyl complexes
are reacting to generate a dimeric complex (ii), as opposed to
a platinum phenyl complex reacting with free benzene solvent,
we carried out an experiment in which (Cy2PC2H4PCy2)Pt(Ph)-
(Me) was reacted with 1 equiv of [H(OEt2)2][BArF] in C6D6
solvent. After the reaction was heated to 80 °C for 5 min, a
dark red oil was obtained which was isolated, dissolved in CD2-
Cl2, and shown by NMR to consist of 3a with no deuterium
(21) (a) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2000, 122, 10846. (b) Reinartz, S.; White, P. S.; Brookhart, M.;
Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 12724.
(22) Lu, C. C.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 5272.
(23) As suggested by a reviewer, we had considered a mechanism involving
reductive elimination/slippage to a bis-allyl species, but this seemed less
reasonable. The primary drawback is that, if the reductive elimination is
to occur from a bis-bridging phenyl intermediate, this would require a
dinuclear reductive elimination, which is extremely rare and has been shown
by Trinquier and Hoffmann24 to be symmetry forbidden. Although our
proposed mechanism is necessarily speculative, we feel that it is the most
reasonable pathway to account for the observed products.
(19) The complexes [(Cy2P(CH2)2PCy2)Pt(µ-η1:η2-OC4H3)]2[BArF]2 and also
[(Cy2P(CH2)2PCy2)Pt(µ-S,C-SC4H3)]2[BArF]2 were prepared and structurally
characterized by X-ray diffraction. Full experimental details, characteriza-
tion, and properties of these complexes will be disclosed in a separate
manuscript.
(20) (a) Park, J. W.; Henling, L. M.; Schaefer, W. P.; Grubbs, R. H.
Organometallics 1991, 10, 171. (b) Cabeza, J. A.; Franco, R. J.; Llamazares,
A.; Riera, V. Organometallics 1994, 13, 55. (c) Dickson, R. S.; Fallon, C.
D.; Zhang, Q.-Q. J. Chem. Soc., Dalton Trans. 2000, 1973.
(24) Trinquier, G.; Hoffmann, R. Organometallics 1984, 3, 370.
(25) (a) Lobach, M. I.; Kormer, V. A. Russ. Chem. ReV. 1979, 48, 758. (b)
Albeniz, A. C.; Espinet, P. J. Organomet. Chem. 1993, 452, 229. (c)
Albeniz, A. C.; Espinet, P.; Foces-Foces, C.; Cano, F. H. Organometallics
1990, 9, 1079.
(26) Brookhart, M.; Pinhas, A. R.; Lukacs, A. Organometallics 1982, 1, 1730.
9
12554 J. AM. CHEM. SOC. VOL. 124, NO. 42, 2002

Reaction of Cationic Pt(II) Complexes with Arenes
A R T I C L E S
Preparation of [(Et2PC2H4PEt2)Pt(Me)(OEt2)]BArF (1b). To a
stirred Et2O solution (10 mL) of [H(OEt2)2]BArF (0.469 g, 0.464 mmol)
at room temperature was added (Et2PC2H4PEt2)Pt(Me)2 (0.200 g, 0.464
mmol) as a solid. Evolution of methane began immediately, and after
5 min the solution was clear and colorless. The volume was reduced
to 3 mL, and 10 mL of hexanes was then added to obtain a light yellow,
oily solid. The flask was placed in a freezer at -30 °C for 1 day to
obtain a white, crystalline solid. The filtrate was decanted off, and the
solids were washed with 3 × 5 mL of hexanes and dried under vacuum
followed by formation of dinuclear phenyl-bridged complexes
which undergo C-C coupling to form the final products. It is
proposed that this C-C coupling occurs via migratory insertion
of an η2-coordinated benzene ligand attached to platinum into
a Pt-Ph bond. The dianionic biaryl ligands in these complexes
were removed easily by mild oxidants to generate the corre-
sponding free biaryls, while the formal oxidation state of the
platinum metal does not change throughout this transformation.
This is in contrast to the previously proposed formation of
biaryls from metal-aryl complexes involving transmetalation
reactions to generate bis-aryl metal complexes, which undergo
reductive elimination to give the biaryl and a reduced metal
species. However, it is possible that the mechanism proposed
in this paper may be operating in some of these other systems
as well and may provide insight into catalyst development and
mechanistic understanding in these systems. Complex 1 is also
quite reactive with water, forming the bridging hydroxide
complexes [(R2PC2H4PR2)Pt(µ-OH)]2[BArF]2 (R ) Cy, Et) with
even adventitious amounts of water. It is intriguing that these
diphosphine platinum cations and related species can be so
highly tuned as to exhibit dramatically different reactivities
depending on the nature of the bidentate ligands; e.g., the dppp
analogue of 1 is stable to adventitious water.11 The reason for
this is not completely clear, but it would appear that factors in
addition to electronics control the protonation of the methyl
ligand in the presumed aquo intermediate. Also, the diimine
and diamine analogues2e-h,9 have been shown to be only
moderately reactive with water and, most significantly, undergo
C-H activation with aromatic substrates to generate relatively
stable Pt-aryl complexes that do not undergo C-C coupling.
1
to give 1b (0.577 g, 92%). H NMR (C6H5F, -30 °C): δ 0.525 (d,
3H, JHP ) 6.0 Hz, JHPt ) 39.6 Hz); 1.13 (t, 6H); 1.43 (m); 1.61 (m);
2.10-2.55 (m); 3.58 (q, 4H); 7.67 (s, 4H); 8.31 (s, 8H). 31P NMR
(C6H5F, -30 °C): δ 61.1 (s, JPPt ) 1814 Hz); 50.7 (s, JPPt ) 4330
Hz).
Preparation of [(Cy2PC2H4PCy2)Pt(µ-OH)]2[BArF]2 (2a). Com-
pound 1a (0.200 g, 0.127 mmol) was dissolved in C6H5F (5 mL), and
2 equiv of H2O (0.00459 g, 0.254 mmol) was then added with stirring.
A white precipitate began to form immediately. After the mixture was
stirred for 2 h at room temperature, the white solids were collected on
a coarse frit, washed with 3 × 5 mL benzene, and dried under vacuum
to give pure 2a (0.175 g, 92%). Anal. Calcd for C116H122B2F48O2P4Pt2:
1
C, 46.51; H, 4.10. Found: C, 46.53; H, 4.08. H NMR (CD2Cl2, 25
°C): δ -0.382 (s, 2H); 1.30-1.37 (m); 1.76-1.89 (m); 2.07 (m, 8H);
7.57 (s, 8H), 7.73 (s, 16H). 31P NMR (CD2Cl2, 25°C): δ 60.4 (s, JPPt
) 3500 Hz).
Preparation of [{Pt(Cy2PC2H4PCy2)}2(µ-biphenyl)][BArF]2 (3a).
Compound 1a (0.300 g, 0.191 mmol) was suspended in benzene (5
mL) and heated to 80 °C for 5 min (alternatively, this reaction was
done at 25 °C for 4 h with the same results). A dark red oil formed
with gas evolution (a sealed-tube reaction showed this gas to be CH4).
The solution was cooled to room temperature, the solvent was decanted
off of the oil, and the oil was washed with 3 × 5 mL of benzene and
dried in a vacuum to give an oily solid. The solids were dissolved in
CH2Cl2 (3 mL) and layered with hexanes (10 mL) to obtain red crystals
of pure 3a (0.291 g, 98%). Anal. Calcd for C128H130B2F48P4Pt2: C,
Experimental Section
1
49.34; H, 4.21. Found: C, 49.13; H, 4.34. H NMR (CD2Cl2, 25 °C):
General Procedures. All manipulations were performed in dry
glassware under a helium atmosphere in a Vacuum Atmospheres drybox
or by using standard Schlenk techniques. All NMR spectra were
recorded on a Unity Series 300 MHz spectrometer. Toluene, hexanes,
and Et2O were dried by passing through activated alumina and copper
oxide. CH2Cl2 was dried over P2O5, C6H6 was dried over NaK, and
C6H5F was dried over CaH2. The complexes (R2PC2H4PR2)PtMe2, (R2-
PC2H4PR2)Pt(Me)(Ph), and (R2PC2H4PR2)Pt(Me)(Cl) (R ) Cy (cyclo-
hexyl), Et) were prepared according to the literature.27 The compounds
[H(OEt2)2]BArF and NaBArF were prepared according to the literature,28
but NaBArF was dried by azeotropic distillation in benzene for better
exclusion of water.
Preparation of [(Cy2PC2H4PCy2)Pt(Me)(OEt2)]BArF (1a). To a
stirred Et2O solution (10 mL) of [H(OEt2)2]BArF (0.438 g, 0.433 mmol)
at room temperature was added (Cy2PC2H4PCy2)Pt(Me)2 (0.280 g, 0.433
mmol) as a solid. Evolution of methane began immediately, and after
5 min the solution was clear and colorless. The volume was reduced
to 2 mL, and 15 mL of hexanes was then added to obtain an oily solid.
The flask was placed in a freezer at -30 °C for 1 day to obtain a
white, crystalline solid. The filtrate was decanted off, and the solids
were washed with 3 × 5 mL of hexanes and dried under vacuum to
give pure 1a (0.605 g, 89%). Anal. Calcd for C63H73BF24OP2Pt: C,
48.19; H, 4.69. Found: C, 48.08; H, 4.96. 1H NMR (C6H5F, -30 °C):
δ 0.497 (d, 3H, JHP ) 5.7 Hz, JHPt ) 36 Hz); 1.12 (t, 6H); 1.55-1.62
(m); 1.70-1.79 (m); 1.90 (m, 4H); 3.63 (q, 4H); 7.65 (s, 4H); 8.29 (s,
8H). 31P NMR (C6H5F, -30 °C): δ 70.4 (s, JPPt ) 1870 Hz); 50.9 (s,
JPPt ) 4520 Hz).
δ 1.21-1.31 (m); 1.69-1.83 (m); 2.33 (m, 8H); 4.74 (t, 1H, JHPt ) 29
Hz); 5.48 (m, 2H); 5.67 (m, 2H); 7.12 (m, 2H); 7.30 (t, 2H); 7.42 (t,
1H); 7.57 (s, 8H), 7.73 (s, 16H). 31P NMR (CD2Cl2, 25 °C): δ 71.8 (s,
JPPt ) 3640 Hz).
Preparation of [{Pt(Et2PC2H4PEt2)}2(µ-biphenyl)][BArF]2 (3b).
Compound 1b (0.250 g, 0.185 mmol) was suspended in benzene (5
mL) and reacted at either 80 or 25 °C as above. The crude oily solid
was dissolved in CH2Cl2 (2 mL) and layered with hexanes (15 mL) to
obtain red crystals of pure 3b (0.243 g, 98%). X-ray quality crystals
were obtained by layering a THF solution of 3b with hexanes and
storing at -30 °C for several days. Anal. Calcd for C96H82B2F48P4Pt2:
1
C, 42.97; H, 3.08. Found: C, 42.77; H, 3.26. H NMR (CD2Cl2, 25
°C): δ 0.835 (m); 1.19 (m); 2.12 (m, 8H); 4.82 (t, 1H, JHP ) 6.60 Hz,
JHPt ) 24.6 Hz); 5.32 (m, 2H); 5.57 (m, 2H); 7.07 (m); 7.16 (m); 7.32
(m); 7.57 (s, 8H), 7.73 (s, 16H). 31P NMR (CD2Cl2, 25 °C): δ 57.0 (s,
JPPt ) 3670 Hz).
Preparation of [{Pt(Cy2PC2H4PCy2)}2(µ-bitolyl)][BArF]2 (4a).
Compound 1a (0.300 g, 0.191 mmol) was suspended in toluene (5 mL)
and reacted at either 80 or 25 °C as above. The crude oily solid was
dissolved in CH2Cl2 (3 mL) and layered with hexanes (15 mL) to obtain
red crystals of pure 4a (0.294 g, 98%). Anal. Calcd for C130H134B2F48P4-
1
Pt2: C, 49.66; H, 4.30. Found: C, 49.54; H, 4.38. H NMR (CD2Cl2,
25 °C): (many signals overlapping because of isomeric mixture) δ
1.21-1.35 (m); 1.64-1.75 (m); 2.25 (m); 4.95-5.08 (m); 5.57 (m);
5.71 (m); 5.83 (m); 6.94 (br); 7.24 (br); 7.28 (br); 7.31 (br); 7.34 (br);
7.37 (br); 7.57 (s, 8H), 7.73 (s, 16H). 31P NMR (CD2Cl2, 25°C): δ
70.7 (s, JPPt ) 3620 Hz), δ 69.9 (s, JPPt ) 3620 Hz).
(27) (a) Smith, D. C. Jr.; Haar, C. M.; Stevens, E. D.; Nolan, S. P.; Marshall,
W. J.; Moloy, K. G.Organometallics 2000, 19, 1427. (b) Glockling, F.;
McBride, T.; Pollock, R. J. I. Inorg. Chim. Acta 1974, 8, 77.
Reactions of 3a and 3b with HCl. A solution containing 0.0500
mmol of either 3a or 3b in 5 mL of CH2Cl2 was treated with 0.40 mL
of a 1.0 M solution of HCl in Et2O. The red color of the solution
(28) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11, 3920.
9
J. AM. CHEM. SOC. VOL. 124, NO. 42, 2002 12555

A R T I C L E S
Konze et al.
Table 2. Crystal and Data Collection Parameters for
[(Cy2PC2H4PCy2)Pt(µ-OH)]2[BArF]2‚4NC5F5 (2a) and
[{Pt(Et2PC2H4PEt2)}2(µ-Biphenyl)][BArF]2 (3b)
disappeared immediately to give a light yellow solution. The solvents
were removed under vacuum, and the remaining light yellow solids
were extracted with 3 × 10 mL of hexanes. The hexane extracts were
combined, and the solvent was removed to give a white solid, which
was found by 1H NMR and GC-MS to be pure biphenyl. The
remaining yellow solids were dissolved in CD2Cl2, and the spectra were
identical to those of samples of [(R2PC2H4PR2)Pt(µ-Cl)]2[BArF]2 (R )
Cy, Et) which were prepared by reacting (R2PC2H4PR2)Pt(Me)(Cl) with
[H(OEt2)2][BArF] in Et2O.
Reactions of 3a and 3b with I2. To a stirred solution containing
0.0500 mmol of either 3a or 3b in acetone was added a 2-fold excess
of iodine (0.0254 g, 0.100 mmol). The color changed from dark red to
light red immediately. The solvent was removed under vacuum, and
the solids were extracted with 3 × 10 mL of hexanes. The hexane
extracts were combined, and the solvent was removed to give pure
biphenyl as above. The remaining light yellow solids were found to
contain [(R2PC2H4PR2)Pt(µ-I)]2[BArF]2 (R ) Cy, Et) based upon the
similarity of the spectra to those of [(R2PC2H4PR2)Pt(µ-Cl)]2[BArF]2
prepared above.
Reaction of 4a with I2. A stirred solution containing 0.0500 mmol
of 4a in acetone was treated with I2 as above, with similar results and
workup. The hexane extracts were combined, and the solvent was
removed to give a white solid, which was found by 1H NMR to contain
a mixture of bitolyl isomers. The isomeric mixture was found by GC-
MS to consist of 3,3′-dimethylbiphenyl (43.7%), 3,4′-dimethylbiphenyl
(44.1%), and 4,4′-dimethylbiphenyl (12.2%), which is very close to a
statistical mixture (4:4:1) of these isomers.
X-ray Crystal Structure Determinations. Crystals of 2a and 3b
were mounted from a matrix of mineral oil. The crystals were
immediately placed on a Bruker P4/CCD/PC diffractometer and cooled
to 203 K using a Bruker LT-2 temperature device. The data were
collected using a sealed, graphite-monochromatized MoKR X-ray
source. A hemisphere of data was collected using a combination of φ
and ω scans, with 30 s frame exposures and 0.3° frame widths. Data
collection and initial indexing and cell refinement were handled using
SMART software.29 Frame integration and final cell parameter calcula-
tion were carried out using SAINT software.30 The data were corrected
for absorption using the SADABS program.31 Decay of reflection
intensity was not observed. A two-fold disorder in 2a was modeled in
the carbon atom positions C9, C11, and C12 (cyclohexyl carbon atoms);
each of these positions was split over two, one-half occupancy sites.
Hydrogen atom positions were fixed (C-H ) 0.96 Å for methane,
0.97 Å for methylene, and 0.93 Å for aromatic). The hydrogen atoms
were refined using the riding model, with isotropic temperature factors
fixed to 1.2 (methylene and aromatic) or 1.5 (methyl) times the
equivalent isotropic U of the carbon atom to which they were bound.
2a
3b
formula
formula weight
color
crystal system
space group
a, Å
C
136H120B2F68N4O2P4Pt2
C96H82B2F48P4Pt2
2683.30
red
triclinic
P1h
13.0443(6)
18.1546(9)
24.857(1)
110.108(1)
98.902(1)
98.738(1)
5326.0(4)
2
3670.04
colorless
triclinic
P1h
14.1156(8)
15.2352(9)
19.112(1)
74.306(1)
70.329(1)
82.550(1)
3722.5(4)
1
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
temp, K
203(2)
203(2)
R(int)
0.0202
0.0212
no. of params refined
final R indices
(I > 2σ(I))
R indices (all data)
856
1369
R1 ) 0.0603,
wR2 ) 0.1687
R1 ) 0.0709,
wR2 ) 0.1792
1.178
R1 ) 0.0657,
wR2 ) 0.1714
R1 ) 0.0876,
wR2 ) 0.1856
1.277
GOF on F2
The final model did not include hydrogen atom positions on the
disordered cyclohexyl group in 2a, and hydrogen atoms were not placed
on the bridging portion of the biphenyl ligand in 3b. The final
refinement for 2a and 3b included anisotropic temperature factors on
all non-hydrogen atoms except for solvent atom positions.32 Structure
solution, refinement, graphics, and preparation of publication materials
were performed using SHELXTL.33 Additional details of data collection
and structure refinement are listed in Table 2.
Acknowledgment. This work is supported by the Department
of Energy, Office of Basic Energy Sciences. We are grateful to
Drs. John Watkin, Jean Huhmann-Vincent, and Richard Broene
for valuable discussions. We also thank Chris Thomas and Dr.
Jonas Peters for disclosure of their recent results on analogous
systems that confirm our findings and support the proposed
mechanism for formation of the biaryl complexes.
Supporting Information Available: X-ray data for 2a and
3b (PDF, CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA020798H
(29) SMART, Version 4.210; Bruker Analytical X-ray Systems, 6300 Enterprise
Lane, Madison, WI 53719, 1996.
(32) R1 ) σ||Fo| - |Fc||/σ|Fo| and R2w ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]1/2. The
parameter w ) 1/[σ2(Fo2) + (a*P)2], where a ) 0.1170 for 2a, and 0.095
for 3b.
(33) SHELXTL, Version 5.1; Bruker Analytical X-ray Systems, 6300 Enterprise
Lane, Madison, WI 53719, 1997.
(30) SAINT, Version 4.05; Bruker Analytical X-ray Systems, 6300 Enterprise
Lane, Madison, WI 53719, 1996.
(31) Sheldrick, G. SADABS; University of Go¨ttingen, Germany, 1996.
9
12556 J. AM. CHEM. SOC. VOL. 124, NO. 42, 2002