Mechanistic investigation of catalytic Carbon-Carbon bond activation and formation by platinum and palladium phosphine complexes

Article abstract of DOI:10.1021/ja973368d

The complexes Pt(PEt₃)₃ and Pd(PEt₃)₃ cleave the C-C bond of biphenylene to give (PEt₃)₂Pt(2,2'-biphenyl), 1, and (PEt₃)₂Pd(2,2'- biphenyl), respectively. Heating (PEt

Full text of DOI:10.1021/ja973368d

J. Am. Chem. Soc. 1998, 120, 2843-2853
2843
Mechanistic Investigation of Catalytic Carbon-Carbon Bond
Activation and Formation by Platinum and Palladium Phosphine
Complexes
Brian L. Edelbach, Rene J. Lachicotte, and William D. Jones*
Contribution from the Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
ReceiVed September 25, 1997
Abstract: The complexes Pt(PEt3)3 and Pd(PEt3)3 cleave the C-C bond of biphenylene to give (PEt3)2Pt-
(2,2′-biphenyl), 1, and (PEt3)2Pd(2,2′-biphenyl), respectively. Heating (PEt3)2Pt(2,2′-biphenyl) in the presence
of biphenylene leads to C-C cleavage of a second biphenylene to give (PEt3)2Pt(2,2′-tetraphenyl), 2, via a
Pt(IV) intermediate. 2 reductively eliminates tetraphenylene at 115 °C. At 120 °C the reaction is catalytic;
Pt(PEt3)3 or 1 converts biphenylene to tetraphenylene. The intermediates in the catalytic cycle have been
identified, and 1 and 2 have been characterized by X-ray analysis. Under catalytic conditions 1 and 2 approach
steady-state concentrations. Kinetic analysis reveals that the steady-state concentration ratio, resting state
species, and overall rate of catalysis, kobs, depend on the ratio of biphenylene to PEt3. This observation is
consistent with loss of PEt3 from 1, resulting in the 14-electron species (PEt3)Pt(2,2′-biphenyl), I. At 130 °C,
I coordinates to PEt3 approximately 130 times faster than it activates the C-C bond of biphenylene. The
complex (depe)Pt(2,2′-biphenyl), 7 (depe ) bis(diethylphosphino)ethane), does not cleave the C-C bond of
biphenylene. Compound 2 is also capable of activating the C-H bonds of benzene and biphenylene to give
trans-(PEt3)2Pt(R-biphenyl)(phenyl), 5, and trans-(PEt3)2Pt(R-biphenyl)(R-biphenylenyl), 6, respectively.
Compounds 5 and 6 have been characterized by X-ray analysis. Substitution of Pd for Pt results in more
rapid catalysis; (PEt3)2Pd(2,2′-biphenyl) is a very efficient catalyst (20 turnovers/h at 120 °C).
Introduction
Third, the energy necessary to overcome the large activation
barrier required for C-C bond activation often results in thermal
decomposition of the metal complex prior to C-C cleavage.
To date, most examples of C-C bond cleavage by transition
metals have been stoichiometric, often relying on ring strain,3
prearomaticity,4 or intramolecular addition in which the C-C
bond is “forced” into close proximity to the metal.5 Of this
latter variety, Milstein and co-workers have discovered a system
that is capable of removing a methyl group from an aromatic
ring attached to a phosphine ligand. â-Alkyl migration has been
observed with electrophilic early transition metal complexes.6
Catalytic C-C bond cleavage is much less common.5,7 Voll-
hardt et al. achieved a catalytic reaction in which tetraphenylene
The cleavage of C-C bonds is vitally important to the
petroleum industry since it is the central step occurring in the
breakdown of hydrocarbons into small organic molecules (i.e.,
cracking). The breaking of C-C bonds is also crucial in coal
liquefaction processes such as the Exxon Donor Solvent process
or the Microcat Coal Liquefaction process. Presently cracking
is achieved with heterogeneous catalysts at high temperatures.
Consequently, there has been renewed interest in the develop-
ment of homogeneous catalysts capable of the same chemistry
under mild conditions, and homogeneous systems offer an
excellent opportunity to study fundamental C-C cleavage
reactions.
(3) (a) Adams, D. M.; Chatt, J.; Guy, R. G.; Sheppard, N. J. Chem. Soc.
1961, 738-742. (b) Bailey, N. A.; Gillard, R. D.; Keeton, M.; Mason, R.;
Russell, D. R. J. Chem. Soc., Chem. Commun. 1966, 396-398. (c)
McQuillin, F. J.; Powell, K. C. J. Chem. Soc., Dalton Trans. 1972, 2123-
2129. (d) Barretta, A.; Cloke, F. G. N.; Feigenbaum, A.; Green, M. L. H.;
Gourdon, A.; Prout, K. J. Chem. Soc., Chem. Commun. 1981, 156-158.
(e) Baretta, A.; Chong, K. S.; Cloke, F. G. N.; Feigenbaum, A.; Green, M.
L. H. J. Chem. Soc., Dalton Trans. 1983, 861-864. (f) Flood, T. C.; Statler,
J. A. Organometallics 1984, 3, 1795-1803. (g) Eisch, J. J.; Piotrowski, A.
M.; Han, K. I.; Kru¨ger, C.; Tsay, Y. H. Organometallics 1985, 224-231.
(h) Schwager, H.; Spyroudis, S.; Vollhardt, K. P. C. J. Organomet. Chem.
1990, 382, 191-200. (i) Lu, Z.; Jun, C.-H.; de Gala, S. R.; Sigalas, M.;
Eisenstein, O.; Crabtree, R. H. J. Chem. Soc., Chem. Commun. 1993, 1877-
1880. (j) Lu, Z.; Jun, C.-H.; de Gala, S. R.; Eisenstein, O.; Crabtree, R. H.
Organometallics 1995, 14, 1168-1175. (k) Perthuisot, C.; Edelbach, B.
E.; Zubris, D. L.; Jones, W. D. Organometallics 1997, 16, 2016-2023.
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91, 5970-5977. (b) Benfield, F. W. C.; Green, M. L. H. J. Chem. Soc.,
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For coal liquefaction, homogeneous systems have attracted
attention due to their ability to penetrate the substrate. However,
several factors impede their development. First, the metal-
carbon bond resulting from C-C insertion has been calculated
to be relatively weak.1 This is certainly true for first-row
transition metals, but probably not so important for third row
metals such as iridium. Second, on the basis of simple steric
and statistical factors, the C-H bonds surrounding the target
C-C bond are kinetically more accessible. As an example,
cyclopropanes have been observed to undergo C-H activation
prior to rearrangement to the more stable metallacyclobutanes.2
(1) (a) Crabtree, R. H. In The Chemistry of Alkanes and Cycloalkanes;
Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: New York, 1992; p
653. (b) Crabtree, R. H. Chem ReV. 1985, 85, 245. (c) Halpern, J. Acc.
Chem. Res. 1982, 15, 238-244. (d) Pilchner, G.; Skinner, H. A. In The
Chemistry of the Metal-Carbon Bond; Patai, S., Ed.; John Wiley & Sons:
New York, 1982.
(2) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 7346-
7355 and references therein.
S0002-7863(97)03368-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/12/1998

2844 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Edelbach et al.
and derivatives of tetraphenylene were catalytically formed from
biphenylene derivatives using Ni(COD)(PMe3)2.3 On the basis
spectroscopic data and earlier work by Eisch et al.,3g the
mechanism shown in eq 1 was proposed. However, no detailed
The major Pt bound phosphine species was found at δ 8.44 (s,
JPt-P ) 1922 Hz); the minor species was found at δ -0.44 (s,
JPt-P ) 3173 Hz).
Identification of Intermediates in the Catalytic Reaction.
The major species at δ 8.44 was formed stoichiometrically in
high yield by heating Pt(PEt3)3 in the presence of biphenylene
1
at 80 °C. Free PEt3 was also formed in this reaction. The H
NMR spectrum showed no hydride resonances, suggesting that
the first intermediate formed in the catalytic reaction was (PEt3)2-
Pt(2,2′-biphenyl), 1, resulting from C-C bond activation of
biphenylene (eq 3). Compound 1 was independently synthe-
mechanistic studies were performed.
Here we report that several Pt and Pd complexes are capable
of catalyzing the formation of tetraphenylene from biphenylene.
In an effort to increase our understanding of the conditions
necessary to cleave strong aryl-aryl bonds, a detailed mecha-
nistic study was performed to determine the operating catalytic
cycle. We anticipated that aryl-aryl bonds should be among
the easiest to cleave for two reasons. First, while the aryl-
aryl C-C bond is one of the strongest C-C bonds, the oxidative
addition product would contain two strong M-aryl bonds, which
should make the cleavage of this type of bond one of the most
favorable. (A similar line of reasoning has been used to explain
why the strong aryl-H bond is one of the easiest to undergo
C-H activation.8) Second, the π-system of a biaryl substrate
offers the opportunity for coordination prior to C-C insertion,
thereby decreasing the kinetic barrier to C-C oxidative addition.
Biphenylene offers several advantages as a substrate for studying
C-C cleavage. In addition to having two aryl-aryl C-C
bonds, it is also a strained ring molecule, and insertion into the
C-C bond will be even more favorable than into a simple biaryl
C-C bond.
sized by reacting 2,2′-dilithiobiphenyl with cis-(PEt3)2PtCl2. The
1H and 31P NMR spectra of 1 were identical to the major species
formed in the reaction shown in eq 2. The aromatic region of
1
the H NMR spectrum of 1 exhibited an ABCD pattern, with
the two deshielded aromatic protons ortho to the Pt-C bond
found at δ 7.501 (dd, JPt-H ) 48.6, JH-H ) JP-H ) 7.2 Hz, 2
H). Three other resonances corresponding to the six remaining
aromatic protons were observed at δ 7.405 (d, JH-H ) 7.6 Hz),
6.999 (t, JH-H ) 7.2 Hz), and 6.880 (t, JH-H ) 7.2 Hz).
The X-ray structure of 1 was determined (Figure 1). The
solid-state structure shows a distorted square planar complex
with a pseudo-2-fold axis bisecting the two phosphines and the
biphenyl. The distortion is best described by a twist angle of
26.5° between the planes defined by Pt, C(1), C(12) and Pt,
P(1), P(2). Alternatively, the four ligating atoms lie an average
of 0.33 Å above and below the best plane of the PtL4 moiety.
The bond angles about the platinum deviate from ideal square
planar geometry, ranging from 79.9(2)° to 97.08(4)°.
Results
Thermal Reaction of Pt(PEt3)3 with Biphenylene. The
thermolysis of Pt(PEt3)3 in benzene-d6 with 10 equiv of
biphenylene (BP) at 120 °C resulted in the catalytic formation
1
of tetraphenylene (0.135 turnovers/day, eq 2). The H NMR
spectrum was complicated throughout the reaction. Early in
the catalytic cycle, the 31P{1H} NMR spectrum revealed the
presence of two Pt bound phosphine species as well as free PEt3.
Heating complex 1 at 80 °C in the presence of biphenylene
(with no added phosphine) led to the quantitative formation of
(PEt3)2Pt(2,2′-tetraphenyl) (2) (eq 4). The H and 31P NMR
(5) (a) Suggs, J. W.; Jun, C.-H. J. Chem. Soc., Chem. Commun. 1985,
92-93. (b) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature
1993, 364, 699-701. (c) Gozin, M.; Aizenberg, M.; Liou, S.-Y.; Weisman,
A.; Ben-David, Y.; Milstein, D. Nature 1994, 370, 42-44. (d) Liou, S.-
Y.; Gozin, M.; Milstein, D. J. Chem. Soc., Chem. Commun. 1995, 1165-
1166. (e) Liou, S.-Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc. 1995,
117, 9774-9775. (f) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein,
D. J. Am. Chem. Soc. 1996, 118, 12406-12415. (g) van der Boom, M. E.;
Kraatz, H.-B.; Ben-David, Y.; Milstein, D. J. Chem. Soc., Chem. Commun.
1996, 2167-2168. (h) Gandelman, M.; Vigalok, A.; Shimon, L. J. W.;
Milstein, D. Organometallics 1997, 16, 3981-3986.
1
(6) Watson, P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471-
6473. Bunel, E.; Burger, B. J.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110,
976-978.
spectra of 2 matched that of the minor species formed in the
1
reaction shown in eq 2. The aromatic region of the H NMR
(7) (a) Noyori, R.; Kumagai, Y.; Umeda, I.; Takaya, H. J. Am. Chem.
Soc. 1972, 94, 4018. (b) Kaneda, K.; Azuma, H.; Wayaku, M.; Teranishi,
S. Chem. Lett. 1974, 215. (c) Fujimura, T.; Aoki, S.; Nakamura, E. J. Org.
Chem. 1991, 56, 2809 and references therein. (d) Huffman, M. A.;
Liebeskind, L. S. J. Am. Chem. Soc. 1991, 113, 2771. (e) Perthuisot, C.;
Jones, W. D. J. Am. Chem. Soc. 1994, 116, 3647-3648. (f) Murakami,
M.; Amii, H.; Shigeto, K.; Ito, Y. J. Am. Chem. Soc. 1996, 118, 8285-
8290.
spectrum of 2 showed the two downfield ortho protons at δ
7.441 (d, JPt-H ) 34.2, JH-H ) 7.2 Hz). The rest of the aromatic
region integrated to 14 protons; however, individual proton
assignments were not possible due to the overlap of proton
resonances. The large platinum-phosphorus coupling of 2 as
compared to 1 suggested that the phosphines were trans to one
another in 2. Large platinum-phosphorus coupling of trans
(8) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91-100.

Catalytic Carbon-Carbon Bond ActiVation and Formation
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2845
Table 1. kobs for Reductive Elimination of Tetraphenylene from 2
(0.023 M) in Toluene-d8 at 115.5 °C
[PEt3]
(M)
kobs
(s-1
[PEt3]
(M)
kobs
(s-1
[PEt3]
(M)
kobs
)
)
(s-1
)
0
2.50 × 10-6 0.044 2.25 × 10-6
0.29
0.49
2.27 × 10-6
0.022 2.22 × 10-6 0.15
2.38 × 10-6
2.39 × 10-6
Scheme 1
Figure 1. ORTEP drawing of 1. Ellipsoids are shown at the 30% level.
Hydrogen atoms have been omitted for clarity.
was formed catalytically with the same turnover rate given by
complex 1. The resting state species was also compound 2.
Reductive Elimination Kinetics from 2. Added phosphine
has two pronounced effects on the catalytic formation of
tetraphenylene. First, free phosphine inhibits the rate of
formation of tetraphenylene. Second, with additional phosphine
present, the resting state changes from 2 to 1. The first step in
understanding these observations was to monitor the rate of
reductive elimination of tetraphenylene from complex 2. Sealed
NMR tubes containing toluene-d8 solutions of complex 2 (0.023
M) were prepared with various phosphine concentrations (0-
0.48 M) and heated at 115.5 °C. The disappearance of 2 was
followed by 1H NMR spectroscopy to 3 half-lives. The
reactions all showed the concomitant formation of tetraphe-
nylene. Pt(PEt3)3 was also produced in those samples with
added phosphine; with no added phosphine collodial platinum
was formed.10 The rate of disappearance of 2 followed first-
order kinetics and was not affected by the phosphine concentra-
tion (Table 1). The average rate constant for all reactions was
Figure 2. ORTEP drawing of 2. Ellipsoids are shown at the 30% level.
Hydrogen atoms have been omitted for clarity.
phosphines vs phosphines trans to an alkyl or aryl group has
been established empirically, and is owed in part to the larger
trans effect of an alkyl or aryl group relative to a phosphine
group.9
calculated to be 2.34(11) × 10-6 s-1
.
Kinetics. The next objective was to probe the effect of added
phosphine on the rate of formation of 2 from 1 in the presence
of biphenylene at 80 °C, a temperature at which reductive
elimination of tetraphenylene from 2 does not occur. To our
surprise even the slightest amount of added phosphine severely
inhibited the rate of formation of 2. At 80 °C and 0.000625 M
PEt3 (0.025 equiv based on 1), no detectable reaction could be
observed even after 24 h. Therefore, the concentrations of 1
and 2 were monitored under catalytic conditions at 130 °C in
p-xylene-d10 with various ratios of biphenylene to PEt3. The
concentration of PEt3 was constant throughout the reaction since
it is not consumed, and pseudo-first-order concentrations of
biphenylene were used to ensure constant concentrations early
on in the cycle. Figure 3 shows the results of this study. In all
cases the concentrations of 1 and 2 approach steady-state values
as the catalytic cycle progresses.
The structure of 2 was confirmed by X-ray crystallography
(Figure 2). The solid-state structure was very distorted from a
square planar geometry (0.46 Å average deviation from a square
plane). The P(1)-Pt(1)-P(2) bond angle was 154.27(3)° and
the C(1)-Pt(1)-C(24) angle was 157.06(12)°. A twist angle
of 33.7° was found between the planes defined by the atoms
Pt(1), P(1), C(1) and atoms Pt(2), P(2), C(24).
Complex 1 catalyzes the formation of tetraphenylene from
biphenylene. When complex 1 was heated at 120 °C in the
presence of excess biphenylene, the catalytic formation of
tetraphenylene was 5× more rapid than when Pt(PEt3)3 was
employed as the catalyst (0.6 turnovers/day), suggesting that
free phosphine inhibits the catalysis. Furthermore, when 1 was
used as the catalyst, the resting state species in the catalytic
cycle was complex 2 (no 1 was seen by 1H or 31P NMR
spectroscopy).
Given the inhibition of added phosphine on the formation of
2 and the intermediates identified, the following general kinetic
scheme for catalysis is proposed (Scheme 1). The first step
involves reversible dissociation of phosphine to give a three-
coordinate intermediate, I, which can either recoordinate phos-
Compound 2 was also shown to be an intermediate in the
catalytic formation of tetraphenylene. When 2 was heated at
120 °C in the presence of excess biphenylene, tetraphenylene
(9) Pregosin, P. S.; Kunz, R. W. In NMR 16 Basic Principles and
Progress: 31P and 13C NMR of Transition Metal Phosphine Complexes;
Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1979; pp
42-43.
(10) DiCosimo, R.; Whitesides, G. M.; J. Am. Chem. Soc. 1982, 104,
3601-3607.

2846 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Table 2. Results of Kinetic Analysis of Figure 3a
Edelbach et al.
[BP]
[PEt3]
[BP]/[PEt3]
kobs, s-1
[2]ss/[1]ss
kr, s-1
kf, s-1
0.1
0.2
0.3
0.1
0.3
0.01
0.02
0.03
0.02
10
10
10
5
1.46 × 10-5
1.46 × 10-5
1.46 × 10-5
1.04 × 10-5
1.83 × 10-5
0.28
0.22
0.24
0.13
0.53
1.14 × 10-5
1.19 × 10-5
1.17 × 10-5
9.22 × 10-6
1.20 × 10-5
3.24 × 10-6
2.66 × 10-6
2.82 × 10-6
1.22 × 10-6
6.33 × 10-6
0.0135
22
aThe concentration of 1 for each run was 0.023 M.
phine or react with biphenylene to give 2. The forward rate
constant, kf, represents a composite of these steps. The reverse
rate constant, kr, is the rate constant for reductive elimination
of tetraphenylene from 2 to generate “(PEt3)2Pt” which rapidly
C-C activates biphenylene to regenerate 1. Since only 1 and
2 are observed in the catalytic reaction, a steady-state ap-
proximation can be applied to I, giving the rate law shown in
eq 5 and the expression for kf (eq 6). Equation 6 reveals the
d[1]
dt
-
) kf[1] - kr[2]
(5)
(6)
k1
kf )
1 + k-1[PEt3]/k2[BP]
source of the inverse dependence on phosphine concentration
of the formation of 2 (at a constant BP concentration) observed
experimentally at 80 °C.
Since the concentrations of 1 and 2 change rapidly at first,
but then change only slowly over the course of the catalysis,
the overall kinetics can be treated as an approach to steady state.
Once a steady-state concentration is reached d[1]/dt ≈ 0;
therefore, eq 5 can be rewritten as eq 7, where [1]ss and [2]ss
are the concentrations of 1 and 2 at steady state. The application
of mass balance and substitution into eq 5 leads to eqs 8 and 9
(see the Experimental Section). Integration of eq 8 gives eq
10 from which kobs can be obtained. This treatment indicates
kf[1]ss ) kr[2]ss
(7)
d[1]
dt
-
) (kobs)([1] - [1]ss) where kobs ) kf + kr (8)
k1
kobs
)
+ kr
(9)
(
)
1 + k-1[PEt3]/k2[BP]
[1] ) ([1]o - [1]ss)e-k t + [1]ss
(10)
obs
that kobs should be affected by the ratio of biphenylene to PEt3
and not by the individual concentrations of biphenylene and
PEt3 (eq 9). The data for each experiment in Figure 3 were fit
to eq 10 using a least-squares approach in which kobs, [1]o, and
[1]ss were allowed to vary. The values of kobs obtained are
shown in Table 2. Parts a-c of Figure 3 display reactions at a
constant biphenylene to phosphine ratio (10:1), with each ratio
being generated from different concentrations of biphenylene
and PEt3. Table 2 indicates that the values of kobs are nearly
equal for all three runs. Furthermore, the steady-state ratios,
[2]ss/[1]ss, are approximately the same for all three runs.
Decreasing the biphenylene to PEt3 ratio to 5:1 (Figure 3d) leads
to a reduction in kobs, as predicted by eq 9, as well as a reduction
in the steady-state ratio. Increasing the biphenylene to PEt3
ratio to 22:1 (Figure 3e) leads to an increase in kobs, and an
increase in the steady-state ratio, [2]ss/[1]ss.
Figure 3. Concentration vs time plots of 1 (9) and 2 (2) at varying
biphenylene (BP) and PEt3 (L) concentrations during the catalytic
formation of tetraphenylene at 130 °C in p-xylene-d10.
The rate constant kr was calculated by using eq 11 (see the
Experimental Section), and kf was determined by rearranging

Catalytic Carbon-Carbon Bond ActiVation and Formation
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2847
Figure 4. 1/kf vs [PEt3]/[BP] for the conversion of 1 to 2 at 130 °C.
eq 7 to give eq 12. These results are also shown in Table 2.
kr ) kobs/(1 + [2]ss/[1]ss)
kf ) ([2]ss/[1]ss)kr
(11)
(12)
Figure 5. ORTEP drawing of 4. Ellipsoids are shown at the 30% level.
Hydrogen atoms have been omitted for clarity.
The reverse rate constant, kr, was also determined independently
by monitoring the rate of reductive elimination of tetraphenylene
from 2 at 130 °C in p-xylene-d10. The value of kr was 1.08(3)
× 10-5 s-1, in very close agreement to the values determined
from fits of the data displayed in Figure 3. Equation 6 predicts
that kf is also dependent on the ratio of biphenylene to PEt3,
and that decreasing this ratio leads to a decrease in kf and vice
versa. .Table 2 shows that this was indeed the case.
anaylsis. The 31P{1H} NMR spectrum of 4 appears as a singlet
at δ -11.44 with a JPt-P of 1749 Hz. The lower JPt-P coupling
of Pt(IV) vs Pt(II) species is well established.9 The trans
orientation of the phosphine ligands was confirmed by X-ray
crystallography (Figure 5).
A diethyl ether solution of 2,2′-dilithiobiphenyl was added
to a benzene solution of 4 in an attempt to form complex 3 (eq
15). All attempts to isolate 3 failed. Therefore, the mixture of
Taking the reciprocal of eq 6 leads to eq 13. A plot of 1/kf
vs [PEt3]/[BP] at 130 °C is shown in Figure 4. The values kf
k-1 [PEt3]
1
1
k1
)
+
(13)
kf k1k2
[BP]
were determined by using eq 12 (with the steady-state ratios
from Table 1, and kr ) 1.08(3) × 10-5 s-1). The intercept
allowed for the determination of k1 (3.84 × 10-5 s-1), although
the error is fairly large relative to the magnitude of k1. The
slope and k1 provide an approximate value for the branching
ratio (k-1/k2) of 131. That is, the coordinatively unsaturated
intermediate I recoordinates to phosphine 131 times faster than
it reacts with biphenylene, thereby explaining the observed
phosphine inhibition.
Evidence for a Pt(IV) Intermediate. Although complexes
1 and 2 have been identified as intermediates during the
catalysis, the question of how 1 goes on to 2 remains open.
One possibility is that 1 forms 2 via the Pt(IV) species 3. Rapid
2,2′-dilithiobiphenyl and 4 was stirred for 15 min, the resulting
suspension was quickly filtered, and the solvent was removed
under high vacuum. The solid was dissolved in benzene, and
a 31P{1H} NMR spectrum was acquired. The spectrum revealed
the presence of two Pt bound phosphine species in ap-
proximately equal ratios. One species was identified as complex
2 on the basis of the 31P{1H} NMR spectrum. The other species
appeared as a singlet at δ -31.64 (JPt-P ) 1242 Hz). The lower
Pt-P coupling constant of this species relative to 4 suggests
that the phosphines are trans to aryl groups. At room temper-
ature, this species disappeared with the concomitant formation
of complex 2, providing strong evidence that 3 is an intermediate
in the catalytic cycle. The addition of several equivalents of
PEt3 (4 mM) did not inhibit this interconversion.
Side Reactions with C6D6 and Biphenylene. During the
course of catalysis by heating 1 with excess biphenylene in C6D6
solvent, two new minor platinum phosphine complexes slowly
appeared. The first complex formed relatively early on, and
its 31P{1H} NMR spectrum displayed a singlet at δ 4.40 (s, JPt-P
) 2871 Hz). The second complex grew in very slowly, and its
reductive elimination of adjacent aryl groups from 3 would
generate 2. To test this hypothesis, the Pt(IV) species trans-
(PEt3)2Pt(2,2′-biphenyl)Br2 (4) was formed by adding Br2 to
1
complex 1 (eq 14). Complex 4 was characterized by H and
31P{1H} NMR spectrum showed a singlet at δ 5.02 (s, JPt-P
)
2805 Hz). The first complex was formed independently by
heating 1 in C6D6 at 130 °C. The complex was identified as
trans-(PEt3)2Pt(R-biphenyl)(phenyl), 5, a square planar platinum-
(II) species with trans phosphines and trans biphenyl and phenyl
groups. The solid-state structure was determined by X-ray
crystallography (Figure 6). This structure is consistent with the
larger Pt-P coupling constant of complex 5 relative to 1, given
31P NMR spectroscopy, X-ray crystallography, and elemental

2848 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Edelbach et al.
Figure 8. ORTEP drawing of 7. Ellipsoids are shown at the 30% level.
Hydrogen atoms have been omitted for clarity.
Figure 6. ORTEP drawing of 5. Ellipsoids are shown at the 30% level.
Hydrogen atoms have been omitted for clarity.
Figure 9. ORTEP drawing of 8. Ellipsoids are shown at the 30% level.
Hydrogen atoms have been omitted for clarity except the hydride ligand.
Catalysis with Pd Complexes. The effect of changing the
metal was also probed by heating Pd(PEt3)3 in the presence of
excess biphenylene in C6D6. As with Pt(PEt3)3, tetraphenylene
was formed catalytically. With Pd(PEt3)3 the turnover rate was
considerably faster (16 turnovers/day at 120 °C in C6D6).
Furthermore, the 31P{1H} NMR spectrum consisted of only two
singlets throughout the course of the catalysis. One singlet
corresponded to free PEt3; the other resonance was assigned as
(PEt3)2Pd(2,2′-biphenyl) (δ 4.94). The assignment of the Pd
Figure 7. ORTEP drawing of 6. Ellipsoids are shown at the 30% level.
Hydrogen atoms have been omitted for clarity.
that alkylphosphines are not as efficient trans directors as aryl
groups. The 1H NMR spectrum of 5-d6 isolated from the
catalytic reaction in C6D6 integrated to eight protons in the
aromatic region. This is consistent with a fully deuterated
phenyl group and one deuterium on the biphenyl group. The
second species, 6, was isolated by combining several catalytic
reaction mixtures which had gone to completion and separating
the components by column chromotagraphy. The solid-state
structure of 6 was determined by X-ray crystallography (Figure
7). Complex 6 was identified as trans-(PEt3)2Pt(R-biphenyl)-
(R-biphenylenyl), also a square planar platinum (II) species with
trans phosphines and trans biphenyl and biphenylenyl groups.
Both 5 and 6 were heated at 120 °C in the presence of excess
biphenylene, but no reaction occurred in either case. Further-
more, the formation of both complexes was inhibited by the
presence of excess phosphine. The formation of complex 5 can
be avoided by running the catalytic reaction in THF-d8, toluene-
d8, or p-xylene-d10.
Reaction of (depe)Pt(2,2′-biphenyl). (depe)Pt(2,2′-biphe-
nyl), 7 (depe ) bis(diethylphosphino)ethane), was synthesized
from (depe)PtCl2 and 2,2′-dilithiobiphenyl in a manner similar
to that of complex 1. A structure of the complex shows the
expected square planar geometry with only a slight deviation
from planarity ((0.15 Å, average, Figure 8). A THF-d8 solution
of 7 and excess biphenylene was heated at 130 °C for one week
during which time no reaction was observed.
1
species was based on its H NMR spectrum. The aromatic
region displayed a very similar ABCD pattern to that observed
for (PEt3)2Pt(2,2′-biphenyl). The splitting patterns of the methyl
and methylene groups on the phosphine were identical to those
of the Pt analogue. (PEt3)2Pd(2,2′-biphenyl) was formed
stoichiometrically by heating Pd(PEt3)3 at 60 °C in the presence
of approximately 1 equiv of biphenylene. (PEt3)2Pd(2,2′-
biphenyl) proved to be the most efficient catalyst for the
formation of tetraphenylene from biphenylene (20 turnovers/h
at 120 °C in C6D6). No side products were observed during
the course of catalysis.
Discussion
Tetraphenylene is formed catalytically from biphenylene using
Pt(PEt3)3. The catalytic cycle outlined in Scheme 2 is proposed
on the basis of kinetic analysis and the identification of
complexes 1, 2, and 3 as intermediates in the cycle. The first
step of the catalytic reaction is loss of phoshine from Pt(PEt3)3
to give the putative species Pt(PEt3)2 which rapidly inserts into
the C-C bond of biphenylene to give intermediate 1. Revers-
ible loss of phosphine from 1 followed by C-C bond activation
of a second biphenylene molecule and rapid phosphine reco-
ordination gives the Pt(IV) species 3. Reductive C-C coupling

Catalytic Carbon-Carbon Bond ActiVation and Formation
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2849
Scheme 2
of adjacent aryl groups gives complex 2.11 Reductive elimina-
tion of tetraphenylene from 2 and subsequent rapid insertion of
(PEt3)2Pt into the C-C bond of biphenylene continues the cycle.
Clearly phosphine inhibits the catalytic formation of tetraphe-
nylene. This inhibition is consistent with the reversible loss of
phosphine from intermediate 1 to give a three-coordinate Pt(II)
intermediate, I. I can then recoordinate to phosphine or cleave
the C-C bond of biphenylene. The calculated branching ratio
k-1/k2 of 131 indicates the preference of I for free phosphine
vs biphenylene.
Kinetic analysis revealed that the ratio of free phosphine to
biphenylene determined the overall rate and the resting state
species of the catalytic cycle. Starting with 1, a large excess
of biphenylene, and no added phosphine, the resting state was
complex 2. That is, the rate of reductive elimination (kr) of
tetraphenylene from 2 determined the overall rate of catalysis.
As free phosphine is added to the system, k2 becomes the slow
step in the cycle and complex 1 becomes the major resting state
complex. Figure 3 reveals that as the PEt3/biphenylene ratio
increases the steady-state concentration of 1 increases relative
to that of 2.
The bulk of experimental and theoretical evidence suggests
that phosphine loss occurs prior to reductive elimination in cis-
bis(phosphine)MIIR2 systems (M ) Pd, Pt) where R is an alkyl
group.10,12,13 However, when at least one organic group is
unsaturated, reductive elimination often occurs directly from
the four-coordinate species without prior phosphine loss.14 This
appears to be the case in the present system, as addition of free
phosphine had no effect on the rate of reductive elimination of
tetraphenylene from complex 2.15 The ability to eliminate
without phosphine dissociation has been explained, in part, by
the decreased directionality of sp and sp2 hybrid orbitals vs an
sp3 hybrid orbital. Decreased directionality of an orbital leads
to increased multicenter bonding in the transition state and lower
activation energies for reductive elimination.16 For unsaturated
organic fragments, experimental and theoretical evidence also
indicates the involvement of a transient η2 complex prior to
loss of the organic species.14 Consequently, unsaturated systems
display enhanced thermal lability relative to that of the saturated
systems. The structure of 2 shows a significant distortion toward
a tetrahedral geometry, and it is possible that reductive elimina-
tion occurs by a continuation of this distortion without isomer-
ization to a cis complex. Alternatively, the relatively high
thermal stablity of complex 2 can be attributed to a large barrier
for the rearrangement of the trans phenyl groups to a cis
geometry, from which reductive elimination can occur.
C-C vs C-H Activation. There are only a few reported
cases of intermolecular C-H bond activation of arenes by a
phosphine-stabilized Pt(0) complex. In 1977, Stone et al. found
that Pt(P(cyclohexyl)3)2 reacts with fluorobenzenes to give C-H
oxidative addition products.17 Whitesides and co-workers were
able to activate the C-H bonds of a number of hydrocarbons
(including benzene) using the reactive intermediate [bis(dicy-
clohexylphosphino)ethane]platinum(0).18 The ability of aro-
matic hydrocarbons to coordinate in the η2 mode prior to C-H
activation has been demonstrated. For the case of [bis-
(dicyclohexylphosphino)ethane]platinum(0) there was evidence
for an η2-benzene complex prior to the C-H activation of
benzene.18a Experimental evidence also suggests that C-H
activation and η2 coordination occur prior to the activation of
(14) (a) Loar, M. K.; Stille, J. K. J. Am. Chem. Soc. 1981, 103, 4174-
4181. (b) Brown, J. M.; Cooley, N. A. Chem. ReV. 1988, 88, 1031-1046
and references therin. (c) Ozawa, F.; Kurihara, K.; Fujimori, M.; Hidaka,
T.; Toyoshima, T.; Yamamoto, A. Organometallics 1989, 8, 180-188. (d)
Stang, P. J.; Kowalski, M. H. J. Am. Chem. Soc. 1989, 111, 3356-3362.
(e) Calhorda, M. J.; Brown, J. M.; Cooley, N. A. Organometallics 1991,
10, 1431-1438. (f) Kohara, T.; Yamamoto, T.; Yamamoto, A. J. Orga-
nomet. Chem. 1980, 192, 265-274. (g) Abis, L.; Sen, A.; Halpern, J. J.
Am. Chem. Soc. 1978, 100, 2915-2916.
(15) It is possible that phosphine dissociation from 2 is rate determining
(i.e., recoordination of phosphine to form 2 is slow), and that trans-to-cis
isomerization, phosphine recoordination, and reductive elimination are all
rapid and irreversible. This would also lead to a lack of dependence of the
rate of tetraphenylene formation on the phosphine concentration. We feel
it is unlikely, however, that reaction of phosphine with the three-coordinate
14-electron intermediate would be slow.
(11) A reviewer suggested that phosphine loss precedes reductive
coupling in the Pt(IV) intermediate. The addition of 4 mM PEt3, however,
did not inhibit the rate of conversion of the Pt(IV) intermediate to 2, which
argues against prior phosphine dissociation. The low concentration of
phosphine employed, however, does not completely discount this possibility.
Kinetic studies using higher concentrations of PEt3 could not be examined
due to limitations in dynamic range in the NMR determinations.
(12) (a) Ozawa, F.; Ito, T.; Nakamura, Y.; Yamamoto, A. Bull. Chem.
Soc. Jpn. 1981, 54, 1868-1880. (b) Gille, A.; Stille, J. K. J. Am. Chem.
Soc. 1980, 102, 4933-4941. (c) Moravskiy, A.; Stille, J. K. J. Am. Chem.
Soc. 1981, 103, 4182-4186. (d) Foley, P.; DiCosimo, R.; Whitesides, G.
M. J. Am. Chem. Soc. 1980, 102, 6713-6725. (e) McCarthy, T. J.; Nuzzo,
R. G. Whitesides, G. M. J. Am. Chem. Soc. 1981, 103, 3396-3403.
(13) Tatsumi, K.; Hoffmann, R.; Yamamoto, A.; Stille, J. K. Bull. Chem.
Soc. Jpn. 1981, 54, 1857-1867.
(16) Low, J. J.; Goddard, W. A. J. Am. Chem. Soc. 1986, 108, 6115-
6128.
(17) Fornies, J.; Green, M.; Spencer, J. L.; Stone, F. G. A. J. Chem.
Soc., Dalton Trans. 1977, 1006-1009.
(18) (a) Hackett, M.; Ibers, J. A.; Whitesides, G. M. J. Am. Chem. Soc.
1988, 110, 1436-1448. (b) Hackett, M.; Whitesides, G. M. J. Am. Chem.
Soc. 1988, 110, 1449-1462.

2850 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Edelbach et al.
Scheme 3
accounted for by the less frequent occurrence of C-H activation.
The preference for η2 coordination over C-H activation has
been demonstrated in the reaction of the coordinatively unsatur-
ated fragment [(C5Me5)Rh(PMe3)] with fused polycyclic are-
nes.22 Complex 5 could form in a similar manner through
reaction of I with benzene. The increased rate of formation of
5 relative to 6 is simply a result of the larger benzene
concentration (vs biphenylene substrate).
Pd vs Pt Complexes. Both Pd(PEt3)3 and (PEt3)2Pd(2,2′-
biphenyl) were capable of catalyzing the formation of tetraphe-
nylene from biphenylene more efficiently than their Pt coun-
terparts. In both cases, (PEt3)2Pd(2,2′-biphenyl) was the resting
state species with no sign of (PEt3)2Pd(2,2′-tetraphenyl), the Pd
analogue of complex 2. Both of these observations are
consistent with the catalytic cycle shown in Scheme 2. With
the Pt system there was a near balance between kf and kr.
Increasing the PEt3:biphenylene ratio had the effect of decreasing
kf relative to kr, resulting in complex 1 becoming the dominant
resting state species. However, it is well-known that reductive
elimination from Pd complexes is more facile than from Pt;
therefore, reductive elimination from (PEt3)2Pd(2,2′-tetraphenyl)
would be rapid. Consequently, the near balance that exists in
the Pt system is eliminated (as kr is now larger than kf) and
(PEt3)2Pd(2,2′-biphenyl) is the resting state species regardless
of the phosphine concentration. The increased turnover rate
results from kf being more rapid in the Pd systems relative to
the Pt systems. This arises from the weaker phosphine-Pd bond
compared to the phosphine-Pt bond. Therefore, the three-
coordinate Pd analogue of intermediate I is formed more rapidly
than the Pt system, resulting in a larger kf. Finally, no side
products were observed when the reaction was run in C6D6.
This is consistent with the decreased ability for Pd to C-H
activate relative to Pt.
C-C bonds.5b,19 In our case, there was no indication of η2
coordination or C-H activation of benzene or biphenylene (prior
to C-C cleavage) with the reactive species [(PEt3)2Pt0].
However, it is very possible that η2 coordination and C-H
activation are Very fast and reVersible and therefore not observed
on the NMR time scale. Halpern et al. demonstrated that cis-
hydridoalkyl and -hydridoaryl bisphosphine complexes of
platinum(II) undergo rapid reductive elimination of RH even
at -25 °C.14g Furthermore, trans-(PEt3)2PtH(C6H5) undergoes
reductive elimination of benzene at 65 °C in C6D12.20
The formation of compounds 5 and 6 is consistent with C-H
activation of benzene and biphenylene, respectively, by the Pt-
(II) complex 1. Examples of nonelectrophilic C-H activation
by a Pt(II) complex are rare, and often occur intramolecularly.21
The formation of both of these complexes is inhibited by added
phosphine, which is consistent with the loss of PEt3 from 1 prior
to C-H activation. A mechanism showing the formation of
complex 6 is outlined in Scheme 3. The C-H activation of
biphenylene is shown to occur at the R-carbon via intermediate
I. This is consistent with the site of C-H activation of
biphenylene by (C5Me5)Rh(PMe3)(Ph)(H).19 The final step
involves reductive elimination to give 6.
If reversible C-H bond activation precedes C-C cleavage,
the five-coordinate intermediate should exhibit a similar pro-
pensity to reductively eliminate the biphenyl group (giving 6)
as free biphenylene (dashed arrow, regenerating I). If this was
the case, 6 would form at least as rapidly as tetraphenylene;
however, this did not occur. One explanation may be that the
C-C bond cleavage from I, which ultimately leads to the
formation of tetraphenylene, does not require prior C-H
activation of biphenylene. Rather, η2 coordination may be
kinetically and thermodynamically preferred over C-H activa-
tion. In this way, C-C activation is preceded by a series of η2
coordination modes. The slow formation of 6 would be
Conclusion
The electron rich species Pt0(PEt3)2 and Pd0(PEt3)2 are able
to insert into the C-C bond of biphenylene. The resulting Pt-
(II) and Pd(II) complexes (PEt3)2M(2,2′-biphenyl) can also
cleave the C-C bond of biphenylene with prior phosphine
dissociation, resulting in a novel Pt(IV) species. The C-H bond
of benzene and biphenylene can also be cleaved by (PEt3)2Pt-
(2,2′-biphenyl). In this study, carbon-carbon bond activation
and formation were coupled in a catalytic reaction, resulting in
the generation of tetraphenylene. The resting state species was
found to vary with the ratio of biphenylene to free phosphine,
rather than the absolute concentration of either. The results
indicate that (PEt3)M(2,2′-biphenyl) (M ) Pt, Pd) has a much
greater tendency to coordinate PEt3 than cleave the C-C bond
of biphenylene. The rate of catalytic formation of tetraphe-
nylene is increased with Pd complexes relative to Pt complexes
due to the more facile reductive elimination from Pd compounds.
On the basis of the insights provided by this study, efforts are
currently underway to maximize the catalytic turnover rate by
varying the ligand system. The cleavage of C-C bonds in other
substrates is also being examined.
Experimental Section
(19) Perthuisot, C.; Jones, W. D. J. Am. Chem. Soc. 1994, 116, 3647-
3648.
General Considerations. All manipulations were performed under
an N2 atmosphere, either on a high-vacuum line using modified Schlenk
techniques or in a Vacuum Atmospheres Corp. glovebox. Tetrahy-
(20) Arnold, D. P.; Bennett, M. A. Inorg. Chem. 1984, 23, 2110-2116.
(21) For intermolecular examples, see: Yamazaki, S. Polyhedron 1992,
11, 1983-1985. Uchimaru, Y.; El Sayed, A. M. M.; Tanaka, M. Organo-
metallics 1993, 12, 2065-2069. For intramolecular examples, see ref g,
5h, and: Hietkamp, S.; Stufkens, D. J.; Vrieze, K. J. Organomet. Chem.
1979, 168, 351-361. Cavell, K. J.; Jin, H. J. Chem. Soc., Dalton Trans.
1995, 4081-4089. Williams, N. A.; Uchimaru, Y.; Tanaka, M. J. Chem.
Soc., Chem. Commun. 1995, 1129-1130. Lope`z, O.; Crespo, M.; Font-
Bard´ıa, M.; Solans, X. Organometallics 1997, 16, 1233-1239.
(22) (a) Chin, M. R.; Dong, L.; Duckett, S. B.; Partridge, M. G.; Jones,
W. D.; Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 7685-7695. (b) Chin,
M. R.; Dong, L.; Duckett, S. B.; Jones, W. D. Organometallics 1992, 11,
871-876. (c) Jones, W. D.; Dong, L. J. Am. Chem. Soc. 1989, 111, 8722-
8723.

Catalytic Carbon-Carbon Bond ActiVation and Formation
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2851
drofuran, benzene, and toluene were distilled from dark purple solutions
of benzophenone ketyl. Alkane solvents were made olefin-free by
stirring over H2SO4, washing with aqueous KMnO4 and water, and
distilling from dark purple solutions of tetraglyme/benzophenone ketyl.
Benzene-d6, toluene-d8, and xylene-d10 were purchased from Cambridge
Isotope Laboratories, and distilled under vacuum from dark purple
solutions of benzophenone ketyl, and stored in ampules with Teflon-
sealed vacuum line adaptors. The preparations of Pt(PEt3)3,23 Pd-
added dropwise at room temperature to the stirred solution. A white
precipitate formed immediately. The solution was stirred for 5 min,
layered with diethyl ether, and put in a -20 °C freezer. The white
crystals that formed were filtered and rinsed with diethyl ether (99.3
mg, 78% yield). 1H NMR (C6D6): δ 8.837 (d, JH-H ) 8.0 Hz, 2 H),
7.337 (d, JH-H ) 6.0 Hz, 2 H), 7.039 (t, JH-H ) 7.6 Hz, 2 H), 6.947
(t, JH-H ) 6.4 Hz, 2 H), 1.808-1.638 (m, 12 H), 0.522 (1:4:6:4:1
quintet, JH-H ) 7.6, JP-H ) 7.6 Hz, 18 H). 31P{1H} NMR (C6D6): δ
-11.44 (s, JPt-P ) 1749 Hz). Anal. Calcd for C24H38Br2P2Pt: C, 38.78;
H, 5.15. Found: C, 38.98; H, 5.03.
(PEt3)3,24 2,2′-dilithiobiphenyl,25 cis-PtCl2(PEt3)2,26 and (depe)PtCl2
27
have been previously reported. Biphenylene was purchased from
Aldrich Chemical Co., 2,2′-dibromobiphenyl was purchased from Alfa
Aesar (Avocado), and triethylphosphine was purchased from Strem
Chemical Co. The liquids were stirred over sieves, freeze-pump-
thaw-degassed three times, and vacuum distilled prior to use.
Preparation of Complex 3. A solution of 2,2′-dilithiobiphenyl (0.33
mmol) in ether (5 mL) was added dropwise to a stirred suspension of
(PEt3)2Pt(2,2′-biphenyl)Br2 (19.3 mg, 0.026 mmol) in 5 mL of benzene
at room temperature. The reaction mixture was stirred for ap-
proximately 15 min. The suspension was quickly filtered through a
frit to remove as much LiBr as possible. The solvent was removed
under vacuum, and the resulting solid was left under high vacuum for
1.5 days. The solid was then dissolved in benzene, and a 31P{1H} NMR
spectrum was acquired. Two distinct resonances were observed. The
resonance at δ -0.44 (s, JPt-P ) 3173 Hz) was assigned as complex 2.
The other resonance at δ -31.64 (JPt-P ) 1242 Hz) was assigned as
complex 3. At room temperature, 3 disappeared with the concomitant
formation of complex 2.
All 1H NMR and 31P NMR spectra were recorded on a Bruker
1
AMX400 spectrometer. All H chemical shifts are reported in parts
per million (δ) relative to tetramethylsilane and referenced using
chemical shifts of residual solvent resonances (C6D6, δ 7.15; CDCl3, δ
7.24; CD2Cl2, δ 5.32). 31P NMR spectra were referenced to external
30% H3PO4 (δ 0.0). GC-MS was conducted on a 5890 Series II gas
chromatograph fitted with an HP 5970 Series mass selective detector.
Analyses were obtained from Desert Analytics. A Siemens SMART
system with a CCD area detector was used for X-ray structure
determination.
Preparation of trans-(PEt3)2Pt(r-biphenyl)(Ph)-d6, 5-d6. (PEt3)2-
Pt(2,2′-biphenyl) (75 mg, 0.146 mmol) was dissolved in dry C6D6 (∼1
mL) in a sealed NMR tube under nitrogen. The tube was heated to
130 °C for 7 days. The solvent was removed and the solid dissolved
in a minimum of hot acetone. Clear, air stable crystals were obtained
by slow cooling to room temperature and then to -20 °C (45 mg, 46%
yield). 1H NMR (THF-d8): δ 7.712 (dd, JPt-H ) 39.6, JH-H ) 6.8,
1.5 Hz, 1 H), 7.355-7.290 (m, 3 H), 7.237-7.174 (m, 1 H), 6.941-
6.840 (m, 3 H), 1.431-1.305 (m, 6 H), 1.282-1.157 (m, 6 H), 0.845
(1:4:6:4:1 quintet, JH-H ) 7.6, JP-H ) 7.6 Hz, 18 H). 31P{1H} NMR
(C6D6): δ 4.40 (s, JPt-P ) 2871 Hz). Anal. Calcd for C30H38D6P2Pt:
C, 53.97; H, 5.69. Found: C, 54.86; H, 6.62.
Preparation of (PEt3)2Pt(2,2′-biphenyl), 1. All glassware was
thoroughly dried prior to use. A solution of 2,2′-dilithiobiphenyl (from
496 mg (1.59 mmol) of 2,2′-dibromobiphenyl) in ether (5 mL) was
added dropwise to a stirred suspension of cis-PtCl2(PEt3)2 (725 mg,
1.44 mmol) in 50 mL of benzene at room temperature. The mixture
became green-yellow over the course of the addition. The reaction
mixture was stirred for 3 h, during which time the color turned bright
yellow. The reaction was quenched with 10 mL of H2O. The organic
layer was separated, dried with MgSO4, and recrystallized (benzene/
methanol) to give air stable yellow crystals (650 mg, 77% yield). The
compound slowly decomposed in CDCl3 over several days. 1H NMR
(CDCl3): δ 7.501 (dd, JPt-H ) 48.6, JH-H ) 7.2, JP-H ) 7.2 Hz, 2 H),
7.405 (d, JH-H ) 7.6 Hz, 2 H), 6.999 (t, JH-H ) 7.2 Hz, 2 H), 6.880
Isolation of trans-(PEt3)2Pt(r-biphenyl)(r-biphenylenyl), 6. Sev-
eral depleted catalytic reaction mixtures were combined, and the solvent
was removed. The resulting solid was dissolved in CH2Cl2, and 6 was
isolated by thick-layer silica chromatography with a 96:4 (v/v) solution
of hexanes-acetone as eluent (Rf ) 0.5). 1H NMR (THF-d8): δ 8.177
(d, JH-H ) 6.7 Hz, 1 H), 7.760 (d, JPt-H ) 39.4, JH-H ) 7.4, 1 H)
7.420-7.160 (m, 5 H), 7.080-6.840 (m, 3 H), 6.700 (d, JH-H ) 6.3
Hz, 1 H), 6.630-6.500 (m, 2 H), 6.490-6.410 (m, 1 H), 6.390-6.290
(m, 1 H), 6.170 (d, JH-H ) 7.0 Hz, 1 H), 1.547-1.314 (overlapping
multiplets, 12 H), 0.855 (1:4:6:4:1 quintet, JH-H ) 7.4, JP-H ) 7.4 Hz,
18 H). 31P{1H} NMR (C6D6): δ 5.02 (s, JPt-P ) 2805 Hz). Anal.
Calcd for C36H46P2Pt: C, 58.77; H, 6.30. Found: C, 58.70; H, 6.21.
(t, JH-H ) 7.2 Hz, 2 H), 1.995(1:4:6:4:1 quintet, JH-H ) 7.5, JP-H
)
7.5 Hz, 12 H), 1.142 (1:2:2:2:1 quintet, JP-H ) 15.0, JH-H ) 7.5 Hz,
18 H). 31P{1H} NMR (C6D6): δ 8.44 (s, JPt-P ) 1922 Hz). Anal.
Calcd for C24H38P2Pt: C, 49.39; H, 6.56. Found: C, 49.38; H, 6.38.
If water is present during the reaction to form (PEt3)2Pt(2,2′-
biphenyl), white plates of (PEt3)2Pt(2,2′-biphenyl)HBr, 8, are formed
as a minor side product. The isolation and characterization of this
compound by NMR spectroscopy, elemental analysis, and X-ray
diffraction is given in the Supporting Information (see Figure 9).
Preparation of (PEt3)2Pt(2,2′-tetraphenyl), 2. (PEt3)2Pt(2,2′-
biphenyl) (100 mg, 0.17 mmol) and biphenylene (36.5 mg, 0.24 mmol)
were dissolved in dry benzene (∼5 mL) in an ampule under nitrogen,
and the sample was heated to 80 °C for 4 h. The solvent was removed
under vacuum and the solid dissolved in methylene chloride and layered
with acetone. Clear air stable crystals were obtained at -20 °C (114
mg, 90% yield). 1H NMR (CD2Cl2): δ 7.441 (d, JPt-H ) 34.2, JH-H
) 7.2 Hz, 2 H), 7.372 (dd, JH-H ) 7.8, 1.8 Hz, 2 H), 7.077 (dd, JH-H
) 4.8 Hz, 1.2 Hz, 4H), 6.997 (t, JH-H ) 7.2 Hz, 2 H), 6.958-6.887
(m, 4 H), 6.826 (d, JH-H ) 7.6 Hz, 2 H), 1.087-0.915 (m, 12 H),
0.638 (1:4:6:4:1 quintet, JH-H ) 7.5, JP-H ) 7.5 Hz, 18 H). 31P{1H}
NMR (C6D6): δ -0.44 (s, JPt-P ) 3173 Hz). Anal. Calcd for C36H46P2-
Pt: C, 58.77; H, 6.30. Found: C, 58.81; H, 6.20.
Preparation of (depe)Pt(2,2′-biphenyl), 7. All glassware was
thoroughly dried prior to use. A solution of 2,2′-dilithiobiphenyl (from
156 mg (0.50 mmol) of 2,2′-dibromobiphenyl) in ether (5 mL) was
added dropwise to a stirred suspension of (depe)PtCl2 (215 mg, 0.455
mmol) in 30 mL of benzene at room temperature. The reaction mixture
was stirred for 1 h, during which time the color turned bright yellow.
The reaction was quenched with 10 mL of H2O. The organic layer
was separated and dried with MgSO4 and the solvent removed under
vacuum. The resulting solid was dissolved in CH2Cl2, and 7 was
isolated by thick-layer silica chromatography with a 1:1 (v/v) solution
of CH2Cl2/benzene as eluent as air stable yellow crystals (176 mg, 70%
yield). 1H NMR (THF-d8): δ 7.562 (dd, JPt-H ) 51.7, JH-H ) 7.6,
JP-H ) 7.6 Hz, 2 H), 7.288 (d, JH-H ) 7.5 Hz, 2 H), 6.845 (t, JH-H
)
Preparation of (PEt3)2Pt(2,2′-biphenyl)Br2, 4. (PEt3)2Pt(2,2′-
biphenyl) (100 mg, 0.17 mmol) was dissolved in a minimum amount
of benzene (∼5 mL). Degassed bromine (8.8 µL, 0.17 mmol) was
7.5 Hz, 2 H), 6.739 (t, JH-H ) 6.5 Hz, 2 H), 2.113-1.888 (m, 8 H),
1.806 (d, JH-H ) 12.2 Hz, 4 H), 1.142 (1:2:2:2:1 quintet, JP-H ) 16.2,
JH-H ) 7.5 Hz, 12 H). 31P{1H} NMR (THF-d8): δ 54.71 (s, JPt-P
)
1830 Hz). Anal. Calcd for C22H28P2Pt: C, 47.74; H, 5.83. Found:
C, 47.68; H, 5.89.
(23) Yoshida, T.; Matsuda, T.; Otsuka, S. Inorg. Synth. 1990, 28, 120-
121.
(24) Kuran, W.; Musco, A. Inorg. Chim. Acta 1975, 12, 187-193.
(25) Gardner, S. A.; Gordon, H. B.; Rausch, M. D. J. Organomet. Chem.
1973, 60, 179.
(26) Parshall, G. W. Inorg. Synth. 1970, 12, 27-28.
(27) Booth, G.; Chatt, J. J. C. S. A Inorg. Phys. Theor. 1966, 6, 634-
638.
Preparation of (PEt3)2Pd(2,2′-biphenyl). (PEt3)3Pd (203 mg, 0.35
mmol) and biphenylene (80.1 mg, 0.53 mmol) were dissolved in dry
benzene (∼5 mL) in an ampule under nitrogen. The ampule was heated
to 68 °C for 14 days. The 31P{1H} spectrum showed only product and
free PEt3. The volatiles were removed under vacuum, and the solid

2852 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Edelbach et al.
Table 3. Summary of Crystallographic Data for 1, 2, 4, 5, 6, 7, and 8
crystal parameters
chemical formula
formula weight
cryst syst
space group, Z
a, Å
1
2
4
5
6
7
8
C24H38P2Pt
583.57
C36H46P2Pt
735.76
C24H38Br2P2Pt C30H44P2Pt
743.39
C39H49P2Pt
774.81
triclinic
C22H32P2Pt
553.51
monoclinic
C2/c, 24
C24H39BrP2Pt
664.49
orthorhombic
Pnma, 4
661.68
monoclinic
P21/n, 8
9.02730(10)
20.2102(4)
31.84380(10) 14.1814(4)
orthorhombic triclinic
Pna21, 4 P1h, 2
14.08140(10) 10.404(2)
16.4069(2) 10.453(2)
monoclinic
P21, 2
9.5953(4)
13.5058(6)
10.7168(5)
90
P1h, 2
11.3813(3)
11.8826(3)
24.30280(10) 18.1731(2)
b, Å
c, Å
13.4616(2)
38.9163(2)
90
13.33980(10)
10.45720(10)
90
10.34030(10) 16.837(2)
R, deg
90
81.965(9)
83.164(9)
60.777(13)
1579.6(4)
-80
90
81.2980(10)
72.5350(10)
78.4240(10)
1783.73(8)
-90
â, deg
90
110.4420(10) 92.59
98.47
90
γ, deg
90
90
90
90
90
vol, Å3
2388.94(4)
-50
1.623
14609
5558
250
1301.35(10)
-90
5803.74(13)
-90
12592.7(2)
-80
2535.09(4)
-50
temp, °C
F
calc, g cm-3
no. of data collected
no. of unique data
1.547
1.897
5335
1781
154
1.515
22380
7480
1.443
1.752
1.741
10121
10844
39390
14858
676
15021
3175
6606
353
7563
no. of params varied
607
379
154
2
R1(Fo), wR2(Fo ) (I > 2σ(I)) 0.0217, 0.0439 0.0239, 0.0572 0.0284, 0.0702 0.0475, 0.0836 0.0543, 0.1278 0.0397, 0.0743 0.0224, 0.0480
2
R1(Fo), wR2(Fo ) (all data)
0.0278, 0.0455 0.0265, 0.0582 0.0331, 0.0727 0.0707, 0.0902 0.0625, 0.1323 0.0436, 0.0753 0.0289, 0.0498
0.950 1.023 1.079 1.082 1.038 1.367 1.071
goodness of fit
was kept under vacuum to remove excess biphenylene. A brown-red
solid was obtained (170 mg, 98% yield). A trace of tetraphenylene
was also present and was not separated. 1H NMR (C6D6): 7.671 (d,
JH-H ) 7.3 Hz, 2 H), 7.537 (d, JH-H ) 7.2, JP-H ) 4.5, Hz, 2 H),
7.204 (t, JH-H ) 7.2 Hz, 2 H), 7.120 (t, JH-H ) 7.3 Hz, 2 H), 1.995-
(1:4:6:4:1 quintet, JH-H ) 7.5, JP-H ) 7.5 Hz, 12 H), 1.142 (1:2:2:2:1
quintet, JP-H ) 15.1, JH-H ) 7.2 Hz, 18 H). 31P{1H} NMR (C6D6):
δ 4.94 (s).
Once steady-state conditions are reached, the rate of disappearance
of 1 is equal to its rate of formation (eq 7). From mass balance, the
total quantity of 1 + 2 must remain constant, either at time t or once
steady state has been achieved (eq 16). Solving eq 16 for [2] and eq
7 for [2]ss and substitution into eq 5 gives eq 8, where kobs ) kf + kr.
Integration of eq 8 gives eq 10, which describes the approach of [1] to
steady state.
General Procedure for Catalysis. A solution containing the metal
complex (0.027 M) and biphenylene (10 equiv, 0.27 M) was prepared
in C6D6 and added to a resealable NMR tube. The NMR tube was
heated at 120 °C in a constant-temperature oil bath thermostatically
controlled ((0.5 °C). 1H NMR spectra were obtained at various
intervals to obtain the catalytic turnover number.
kf[1]ss ) kr[2]ss
(7)
[1]t + [2]t ) [1]ss + [2]ss
(16)
d[1]
-
) (kobs)([1] - [1]ss)
(8)
Kinetics of Reductive Elimination of Tetraphenylene from 2. Six
sealed NMR tubes were prepared with varying concentrations of PEt3
(0.49, 0.29, 0.15, 0.044, 0.022, and 0 M) in toluene-d8. The concentra-
tion of 2 in each tube was maintained at 0.023 M. The NMR tubes
were heated at 115.5 °C in a constant-temperature oil bath thermostati-
cally controlled ((0.5 °C). Samples were removed from the baths at
intervals and cooled to room temperature. 1H NMR spectra were
obtained, and thermolysis was followed for a minimum of 3 half-lives.
The methyl peak of toluene-d8 was used as an internal standard. The
observed rate constant (kobs) was independent of phosphine concentra-
tion. The average kobs ) 2.34(11) × 10-6 s-1. The same reaction was
performed three times at 130 °C in xylene-d10 with a phosphine
concentration of 0.023 M. The average rate constant was 1.08(3) ×
dt
[1] ) ([1]o - [1]ss)e-k t + [1]ss
(10)
obs
Equation 7 can be solved for kf and substituted into the expression
for kobs (eq 17), which upon rearrangement gives eq 11. kf is obtained
by rearrangement of eq 7 to give eq 12.
kobs ) kf + kr ) kr([2]ss/[1]ss) + kr
kr ) kobs/(1 + [2]ss/[1]ss)
kf ) ([2]ss/[1]ss)kr
(17)
(11)
(12)
10-5 s-1
.
Kinetics of Catalysis. Five sealed NMR tubes were prepared with
varying concentrations of PEt3 and biphenylene (see Table 2) in
p-xylene-d10. The concentration of 1 in each tube was 0.023 M. The
NMR tubes were heated at 130 °C in a constant-temperature oil bath
thermostatically controlled ((0.5 °C). Samples were removed from
the baths at intervals and cooled to room temperature. 1H NMR spectra
were obtained, and thermolysis was followed until a steady-state
concentration of 1 and 2 was attained. The methyl peak of p-xylene-
X-ray Structural Determination of 1, 2, 4, 5, 6, 7, and 8. Single
crystals of 1 and 8 were mounted with epoxy on glass fibers, and
immediately placed in a cold nitrogen stream at -50 °C on the X-ray
diffractometer. Single crystals of 2, 4, 5, 6, and 7 were mounted under
Paratone-8277 on glass fibers, and immediately placed in a cold nitrogen
stream at -80 or -90 °C on the X-ray diffractometer. The X-ray
intensity data were collected on a standard Siemens SMART CCD area
detector system equipped with a normal focus molybdenum-target X-ray
tube operated at 1.5 kW (50 kV, 30 mA) for 1 and 8 and 2.0 kW (50
kV, 40 mA) for the remaining crystals. A total of 1321 frames of data
(1.3 hemispheres) were collected using a narrow frame method with
scan widths of 0.3° in ω and exposure times of 10 s/frame using a
detector-to-crystal distance of 5.09 cm (maximum 2θ angle of 56.54°)
for all of the crystals except 5 and 7, which were collected using
exposure times of 30 s/frame. The total data collection time was
approximately 6 h for 10 s/frame exposures, and 13 h for 30 s/frame
exposures. Frames were integrated with the Siemens SAINT program
to 0.75 Å for all of the data sets; however, weak data > 45° were
omitted from the refinement of 4 and 5. The unit cell parameters for
all of the crystals were based upon the least-squares refinement of three-
d10 was used as an internal standard. The data for each experiment in
Figure 3 were fit to eq 10 using a least-squares approach (using the
solver tool in Microsoft Excel) in which kobs, [1]o, and [1]ss were allowed
to vary. These values were then used to calculate [2]ss/[1]ss () ([1]o -
[1]ss)/[1]ss ), and kr and kf using eqs 11 and 12, respectively.
Derivation of Rate Expressions. In Scheme 1, the time-dependent
concentration for 1 is governed by eq 5. Application of the steady-
state approximation to [I] allows kf to be written as in eq 6.
d[1]
dt
-
) kf[1] - kr[2]
(5)
(6)
k1
kf )
1 + k-1[PEt3]/k2[BP]

Catalytic Carbon-Carbon Bond ActiVation and Formation
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2853
dimensional centroids of >5000 reflections.28 Data were corrected for
absorption using the program SADABS.29 Space group assignments
were made on the basis of systematic absences and intensity statistics
by using the XPREP program (Siemens, SHELXTL 5.04). The
structures were solved by using direct methods and refined by full-
matrix least squares on F2. For all of the structures, the non-hydrogen
atoms were refined with anisotropic thermal parameters and hydrogens
were included in idealized positions, giving data:parameter ratios greater
than 10:1. The Pt bound hydride in 8 was located from the difference
Fourier map, and the positional and isotropic thermal parameters were
refined. There was nothing unusual about the solution or refinement
of any of the structures, with the exceptions of 5 and 7. Both complexes
crystallized in the monoclinic crystal system, and the space group
assignments were P21/n and C2/c for 5 and 7, respectively. For a Z
value of 8, 5 crystallized with two independent molecules in the
asymmetric unit. For a Z value of 24, 7 crystallized with three
independent molecules in the asymmetric unit. Further experimental
details of the X-ray diffraction studies are provided in Table 3.
Positional parameters for all atoms, anisotropic thermal parameters,
all bond lengths and angles, and fixed hydrogen positional parameters
are given in the Supporting Information for all of the structures.
Acknowledgment is made to the U.S. Department of Energy,
Grant FG02-86ER13569 for their support of this work.
Supporting Information Available: Text describing the
preparation of (PEt3)2Pt(2,2′-biphenyl)HBr and tables listing the
crystallographic data, positional parameters, anisotropic thermal
parameters, and bond lengths and angles (46 pages). See any
current masthead page for ordering information and Web access
information.
(28) It has been noted that the integration program SAINT produces cell
constant errors that are unreasonably small, since systematic error is not
included. More reasonable errors might be estimated at 10× the listed value.
(29) The SADABS program is based on the method of Blessing; see:
Blessing, R. H. Acta Crystallogr., Sect A 1995, 51, 33-38.
JA973368D