Cleavage of carbon-carbon bonds of diphenylacetylene and its derivatives via photolysis of Pt complexes: Tuning the C-C bond formation energy toward selective C-C bond activation

Article abstract of DOI:10.1021/ja071698k

Carbon-carbon bond activation of diphenylacetylene and several substituted derivatives has been achieved via photolysis and studied. Pt⁰- acetylene complexes with η²-coordination of the alkyne, along with the corresponding Pt^II C-C activated photolysis products, have been synthesized and characterized, including X-ray crystal structural analysis. While the C-C cleavage reaction occurs readily under photochemical conditions, thermal activation of the C-C bonds or formation of Pt^II complexes was not observed. However, the reverse reaction, C-C reductive coupling (Pt ^II→Pt⁰), did occur under thermal conditions, allowing the determination of the energy barriers for C-C bond formation from the different Pt^II complexes. For the reaction (dtbpe)Pt(-Ph)(- C≡CPh) (2) → (dtbpe)Pt(η²-PhC≡CPh) (1), ΔG^? was 32.03-(3) kcal/mol. In comparison, the energy barrier for the C-C bond formation in an electron-deficient system, that is, (dtbpe)Pt(C₆F₅)(C≡CC₆F₅) (6) → (dtbpe)Pt(η²-bis(pentafluorophenyl)acetylene) (5), was found to be 47.30 kcal/mol. The energy barrier for C-C bond formation was able to be tuned by electronically modifying the substrate with electron-withdrawing or electron-donating groups. Upon cleavage of the C-C bond in (dtbpe)Pt(η²-(p-fluorophenyl-p-tolylacetylene) (9), both (dtbpe)Pt(p-fluorophenyl)(p-tolylacetylide) (10) and (dtbpe)Pt(p-tolyl)(p- fluorophenylacetylide) (11) were obtained. Kinetic studies of the reverse reaction confirmed that 10 was more stable toward the reductive coupling [the term "reductive coupling" is defined as the formation of (dtbpe)Pt(η²-acetylene) complex from the Pt^II complex] than 11 by 1.22 kcal/mol, under the assumption that the transition-state energies are the same for the two pathways. The product ratio for 10 and 11 was 55:45, showing that the electron-deficient C-C bond is only slightly preferentially cleaved.

Full text of DOI:10.1021/ja071698k

Published on Web 06/20/2007
Cleavage of Carbon-Carbon Bonds of Diphenylacetylene and
Its Derivatives via Photolysis of Pt Complexes: Tuning the
C-C Bond Formation Energy toward Selective C-C Bond
Activation
Ahmet Gunay and William D. Jones*
Contribution from the Department of Chemistry, UniVersity of Rochester,
Rochester, New York 14627
Received March 10, 2007; E-mail: jones@chem.rochester.edu
Abstract: Carbon-carbon bond activation of diphenylacetylene and several substituted derivatives has
been achieved via photolysis and studied. Pt0-acetylene complexes with η2-coordination of the alkyne,
along with the corresponding PtII C-C activated photolysis products, have been synthesized and
characterized, including X-ray crystal structural analysis. While the C-C cleavage reaction occurs readily
under photochemical conditions, thermal activation of the C-C bonds or formation of PtII complexes was
not observed. However, the reverse reaction, C-C reductive coupling (PtII f Pt0), did occur under thermal
conditions, allowing the determination of the energy barriers for C-C bond formation from the different PtII
complexes. For the reaction (dtbpe)Pt(-Ph)(-CtCPh) (2) f (dtbpe)Pt(η2-PhCtCPh) (1), ∆Gq was 32.03-
(3) kcal/mol. In comparison, the energy barrier for the C-C bond formation in an electron-deficient system,
that is, (dtbpe)Pt(C6F5)(CtCC6F5) (6) f (dtbpe)Pt(η2-bis(pentafluorophenyl)acetylene) (5), was found to
be 47.30 kcal/mol. The energy barrier for C-C bond formation was able to be tuned by electronically
modifying the substrate with electron-withdrawing or electron-donating groups. Upon cleavage of the C-C
bond in (dtbpe)Pt(η2-(p-fluorophenyl-p-tolylacetylene) (9), both (dtbpe)Pt(p-fluorophenyl)(p-tolylacetylide)
(10) and (dtbpe)Pt(p-tolyl)(p-fluorophenylacetylide) (11) were obtained. Kinetic studies of the reverse reaction
confirmed that 10 was more stable toward the reductive coupling [the term “reductive coupling” is defined
as the formation of (dtbpe)Pt(η2-acetylene) complex from the PtII complex] than 11 by 1.22 kcal/mol, under
the assumption that the transition-state energies are the same for the two pathways. The product ratio for
10 and 11 was 55:45, showing that the electron-deficient C-C bond is only slightly preferentially cleaved.
Introduction
C-C bond into close proximity to the transition metal cen-
ter.1,2,14,15 A preliminary report from our group using bis-
Activation of C-C bonds with the help of the transition metal
complexes remains one of the most challenging fields in
organometallic chemistry.1-4 These fascinating reactions have
attracted the attention of many chemists because of their
potential applications in organic synthesis and petroleum and
pharmaceutical research.5 While C-H bond activation has found
many successful applications in organic synthesis and the
chemical industry, C-C bond activation is far from practical
use.5
(diisopropylphosphino)ethane, bis(di-t-butylphosphino)methane,
and hemilabile diisopropylphosphinodimethylaminoethane chel-
ates on platinum-acetylene complexes showed evidence for
photochemical C-C cleavage.16 Here, this new method that does
not take advantage of the aforementioned driving forces for
C-C bond cleavage is further examined. A series of novel Pt0-
η2-acetylene complexes has been synthesized and fully char-
acterized along with their C-C bond-activated PtII counterparts.
Understanding the factors influencing C-C bond activation
and figuring out the mechanism of metal insertion into the C-C
bond will be key to the future development of this important
field of organometallic chemistry.2 In order to provide a driving
force for C-C bond cleavage, relief of ring strain and attainment
of aromaticity have been employed,6-13 as has forcing the target
(6) Perthuisot, C.; Jones, W. D. J. Am. Chem. Soc. 1994, 116, 3647-3648.
(7) Perthuisot, C.; Edelbach, B. L.; Zubris, D. L.; Jones, W. D. Organometallics
1997, 16, 2016-2023.
(8) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 1998,
120, 2843-2853.
(9) Wick, D. D.; Northcutt, T. O.; Lachicotte, R. J.; Jones, W. D. Organome-
tallics 1998, 17, 4484-4492.
(10) Bessmertnykh, A. G.; Blinov, K. A.; Grishin, Y. K.; Donskaya, N. A.;
Beletskaya, I. P. Tetrahedron Lett. 1995, 36, 7901-7904.
(11) Nishimura, T.; Ohe, K.; Uemura, S. J. Am. Chem. Soc. 1999, 121, 2645-
2646.
(1) Murakami, M.; Ito, Y. Cleavage of Carbon-Carbon Single Bonds by
Transition Metals. In Topics in Organometallic Chemistry; Murai, S., Ed.;
Springer: Berlin, 1999; Vol. 3, pp. 97-129.
(2) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870-883.
(3) Jennings, P. W.; Johnson, L. L. Chem. ReV. 1994, 94, 2241-2290.
(4) Bishop, K. C., III. Chem. ReV. 1976, 76, 461-486.
(5) Jun, C. H.; Moon, C. W.; Lee, H.; Lee, D. Y. J. Mol. Catal. A 2002, 189,
145-156.
(12) Nishimura, T.; Uemura, S. J. Am. Chem. Soc. 1999, 121, 11010-11011.
(13) Jones, W. D. Nature 1993, 364, 676-677.
(14) Rosenthal, U.; Pellny, P. M.; Kirchbauer, F. G.; Burlakov, V. V. Acc. Chem.
Res. 2000, 33, 119-129.
(15) Garcia, J. J.; Jones, W. D. Organometallics 2000, 19, 5544-5545.
(16) Muller, C.; Iverson, C. N.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem.
Soc. 2001, 123, 9718-9719.
9
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J. AM. CHEM. SOC. 2007, 129, 8729-8735
8729

A R T I C L E S
Gunay and Jones
Figure 3. Energy profile of activation reaction.
complex is depicted in Figure 1, and selected metrical data are
given in Table 1.
Figure 1. Molecular structure of (dtbpe)Pt(η2-PhCtCPh) (1) (ORTEP,
30% probability ellipsoids).
While d10 Pt0 complexes might be expected to have a
tetrahedral geometry, the structural analysis revealed a distorted
square-planar geometry at the metal center with the alkyne
ligand in the P-Pt-P plane. This distorted square-planar
geometry can be explained in terms of π back-donation,17-19
which is suggested by an elongated C-C triple bond and
bending of CtC-C angles compared to free diphenylacetyl-
ene.20 In addition, any π back-donation to the chelating
phosphine (dtbpe) would also promote the formation of the
observed geometry at the metal center.
To look for C-C bond activation, 1 was heated at 160 °C
for 3 days. Under these conditions, no C-C bond cleavage or
formation of PtII complex was observed. Under UV irradiation
(λ > 300 nm), however, oxidative addition of the sp2-sp C-C
bond was observed and (dtbpe)Pt(Ph)(phenylacetylide) (2) was
produced in high yield (eq 2). Compound 1 has an absorption
that tails into the near UV to ∼360 nm (see Supporting
Information). Product 2 displays two singlets in the 31P NMR
spectrum at δ 71.36 and 77.45. Characteristic P-Pt coupling
1
constants are seen for each phosphorus, with JP-Pt ) 1978.3
Figure 2. Molecular structure of (dtbpe)PtII(Ph)(phenylacetylide) (2)
(ORTEP, 30% probability ellipsoids).
Hz (trans to acetylide)21 and 1JP-Pt ) 1250.6 Hz (trans to Ph).22
In addition, 2 was characterized by single-crystal X-ray dif-
fraction as shown in Figure 2. The molecule has the expected
square planar geometry for a PtII d8 complex, with the platinum-
bound phenyl group lying at an angle of 80.9° to the square
plane. Selected metrical data are given in Table 2.
By modification of the aryl groups of the acetylene derivatives,
the C-C bond formation energy could be tuned. A kinetic
preference for C-C bond formation of the electron-rich Pt-
aryl bond over the electron-poor Pt-aryl bond was investigated
by using both electron-donating groups (EDG) and electron-
withdrawing groups (EWG) as substituents on the aryl rings of
the substrate.
Results and Discussion
In order to study C-C bond cleavage in aryl acetylenes, the
η2-coordinated Pt0 complex (dtbpe)Pt(η2-PhCtCPh) (1) was
synthesized stoichiometrically by 1:1:1 ratio reaction of bis(di-
t-butylphosphino)ethane (dtbpe), diphenylacetylene, and Pt-
(COD)2 as shown in eq 1. Heating was required to produce only
1 as product. The 31P NMR spectrum of the product clearly
showed the formation of 1 as a singlet at δ 94.23 with platinum
satellites (1JPt-P ) 2516.4 Hz), which is typical for Pt0
complexes. The single-crystal structure of this Pt-acetylene
Upon heating the C-C activated complex 2, reversion to the
Pt0 complex 1 occurred. Therefore, even though the forward
reaction does not proceed thermally, the reverse reductive
coupling reaction does. Similar chemistry was observed earlier
with the dippe analogue but was not quantified.16 These
9
8730 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

C−C Bond Activation of Diphenylacetylenes
A R T I C L E S
Table 1. Selected Bond Lengths and Angles for 1, 3, 5, 7, and 9
1
3
5
7a
9
Bond Lengths (Å)
Pt(1)-P(1)
Pt(1)-P(2)
Pt(1)-C(1)
Pt(1)-C(2)
C(1)-C(2)
C(1)-C(3)
C(2)-C(9)
2.2878(13)
2.2823(14)
2.056(5)
2.037(5)
1.299(6)
1.455(6)
1.466(6)
2.2951(9)
2.2766(9)
2.055(3)
2.042(3)
1.303(5)
1.456(5)
1.460(5)
2.2807(5)
2.2860(5)
2.0369(18)
2.0553(18)
1.309(2)
2.2676(8)
2.2676(8)
2.044(3)
2.044(3)
1.295(6)
1.456(4)
1.456(4)
2.2744(13)
2.2913(13)
2.038(5)
2.044(5)
1.304(7)
1.454(7)
1.463(7)
1.458(3)
1.465(2)
Bond Angles (deg)
C(1)-Pt(1)-C(2)
C(2)-Pt(1)-P(2)
P(2)-Pt(1)-P(1)
P(1)-Pt(1)-C(1)
Pt(1)-P(1)-C(31)
Pt(1)-P(2)-C(32)
C(3)-C(1)-C(2)
C(9)-C(2)-C(1)
36.99(17)
113.32(14)
88.32(5)
121.43(14)
107.03(16)
108.41(16)
136.0(5)
37.10(13)
114.50(10)
87.91(3)
120.09(10)
107.36(12)
108.38(12)
140.0(3)
37.31(7)
121.50(5)
88.316(17)
112.92(5)
107.66(6)
107.39(6)
139.98(17)
135.55(17)
36.92(18)
117.19(9)
88.70(4)
117.19(9)
107.80(10)
107.80(10)
142.22(18)
142.22(18)
37.2(2)
121.02(16)
88.61(5)
112.36(15)
107.34(17)
107.33(17)
142.4(5)
143.9(5)
142.5(3)
138.8(5)
a Complex 7 sits on a crystallographic mirror plane bisecting the PtP2 fragment. Therefore C1A, C2A, P1A, and C31A in complex 7 are listed here as
C2, C9, P2, and C32, respectively.
observations indicate that the overall C-C cleavage process is
thermodynamically uphill at [Pt(dtbpe)]. The free energy model
shown in Figure 3 describes the situation best. The PtII complex
must lie at a higher energy than the Pt0 complex because the
reverse reaction proceeds spontaneously. Compound 1 does not
observably convert to 2 thermally, indicating that 2 must be
several kilocalories per mole higher in energy than 1. To
quantify the stability of the C-C cleavage product, 2 was heated
at 80 °C and the reductive coupling reaction to form 1 was
monitored by 31P NMR spectroscopy. The first-order rate
constant (k) for the reverse reaction was determined by the slope
of a plot of ln (concn) versus time, giving a barrier ∆Gq for the
formation of the C-C bond of 32.03(3) kcal/mol. This energy
barrier has a correlation with both the C-C bond strength of
the target C-C bond and the metal-carbon (M-C) bond
strengths in the PtII complex.
It has been pointed out that C-C bond activation can be
facilitated by lowering the energy of the C-C bond-cleaved
oxidative addition products.23 Metal-aryl complexes with
electron-withdrawing substituents on the aryl group are generally
more stable than those with electron-donating substituents.1
Therefore, alkynes with electron-deficient aryl groups should
produce more stable PtII complexes following C-C bond
activation than alkynes with electron-rich aryl groups. Electron-
withdrawing groups might also be expected to stabilize the Pt0-
(alkyne) complex but probably to a lesser extent.24 By this
approach, it might be possible to tune the C-C bond activation
energy such that it would be thermodynamically favorable. The
electron-rich alkyne bis(3,5-dimethylphenyl)acetylene was syn-
thesized by a modified Sonogashira coupling method. The η2-
coordinated Pt0 complex 3 was synthesized via eq 1 in high
yield. Compound 3 displays a single 31P resonance at δ 94.14
(1JP-Pt ) 2513.1 Hz) and has been characterized by X-ray
analysis (Table 1).
Complex 3 was heated at 200 °C for 3 days, during which
the reaction was monitored by 31P NMR spectroscopy. Similar
to 1, C-C bond activation was not observed under thermal
conditions. Compound 3 was then irradiated in C6D6 for 6 h (λ
> 300 nm), under which conditions C-C bond cleavage
occurred cleanly. A high yield (90%) of the oxidative addition
complex (dtbpe)Pt(3,5-dimethylphenyl)(3,5-dimethylphenyl-
acetylide) 4 was obtained. The 31P NMR spectrum shows two
singlets with platinum satellites, and the single-crystal X-ray
structure shows the anticipated square-planar structure with the
platinum-bound aryl group rotated 79.8° to the Pt square plane.
Quantitative reversion from 4 to 3 was observed by 31P NMR
spectroscopy upon heating 4 at 80 °C. Therefore, this more
electron-rich system also shows thermodynamically uphill
behavior toward C-C cleavage. The first-order kinetics of the
reverse reaction showed a barrier for the reverse reaction of
31.33(21) kcal/mol. Compound 4 is therefore slightly more labile
toward reductive coupling than 2.
(17) Otsuka, S.; Nakamura, A. AdV. Organomet. Chem. 1976, 14, 245-283 and
references therein.
(18) Boag, N. M.; Green, M.; Grove, D. M.; Howard, J. A. K.; Spencer, J. L.;
Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1980, 2170-2181.
(19) (a) Bartik, T.; Happ, B.; Iglewsky, M.; Bandmann, H.; Boese, R.; Heimbach,
P.; Hoffmann, T.; Wenschuh, E. Organometallics 1992, 11, 1235-1241.
(b) Rosenthal, U.; Schulz, W. J. Organomet. Chem. 1987, 321, 103-117.
(c) Tatsumi, K.; Hoffmann, R.; Templeton, J. L. Inorg. Chem. 1982, 21,
466-468. (d) Hey, E.; Weller, F.; Dehnicke, K. Z. Anorg. Allg. Chem.
1984, 514, 18-24. (e) Dewar, M. J. S.; Ford, G. P. J. Am. Chem. Soc.
1979, 101, 783-791. (f) Hartley, F. R. Angew. Chem. 1972, 84, 657-667.
(g) Nelson, J. H.; Wheelock, K. S.; Cusachs, L. C.; Jonassen, H. B. J. Am.
Chem. Soc. 1969, 91, 7005-7008. (h) Greaves, E. O.; Lock, C. J. L.;
Maitlis, P. M. Can. J. Chem. 1968, 46, 3879-3891. (i) Dewar, M. J. S.
Bull. Soc. Chim. Fr. 1951, 18, C79. (j) Chatt, J.; Duncanson, L. A. J. Chem.
Soc. 1953, 2939-2947.
Since the electron-rich substituents destabilized the oxidative
addition complex, electron-withdrawing substituents should
stabilize the C-C cleavage product. For this reason, the electron-
deficient substrate bis(p-fluorophenyl)acetylene was attached to
Pt to give (dtbpe)Pt(η2-bis(p-fluorophenyl)acetylene) 5 (31P
1
NMR: δ 94.14, s, JP-Pt ) 2510.7 Hz).
Analogous to the previous sets of Pt0 and PtII complexes, the
C-C bond-activated complex was not observed upon heating
5, yet photolysis produced the corresponding PtII complex
(dtbpe)Pt(C6H4F)(CtCC6H4F) 6. The 31P NMR spectrum shows
(20) Mavriris, A.; Moustakali-Mavridis, I. Acta Crystallogr. 1977, B33, 3612.
The CtC bond length in free diphenylacetylene is 1.198(4) Å.
(21) Long, N. J.; Wong, C. K.; White, A. J. P. Organometallics 2006, 25, 2525-
2532.
(22) Iverson, C. N.; Lachicotte, R. J.; Muller, C.; Jones, W. D. Organometallics
2002, 21, 5320-5333.
(23) Jun, C. H. Chem. Soc. ReV. 2004, 33, 610-618.
(24) (a) Oshiki, T.; Yamada, A.; Kawai, K.; Arimitsu, H.; Takai, K. Organo-
metallics 2007, 26, 173-182. (b) Yu, Y.; Smith, J. M.; Flaschenriem, C.
J.; Holland, P. L. Inorg. Chem. 2006, 45, 5742-5751.
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8731

A R T I C L E S
Gunay and Jones
Table 2. Selected Bond Lengths and Angles for 2, 4, 6, 8, and 11
2
4
6
8
11
Bond Lengths (Å)
Pt(1)-P(1)
Pt(1)-P(2)
Pt(1)-C(9)
Pt(1)-C(1)
C(1)-C(2)
C(2)-C(3)
2.3168(5)
2.3253(5)
2.0943(18)
2.0001(18)
1.209(3)
2.3110(18)
2.3326(18)
2.098(7)
2.017(7)
1.190(10)
1.447(10)
2.322(2)
2.324(2)
2.088(8)
1.982(9)
1.219(11)
1.425(11)
2.3220(4)
2.3070(4)
2.0745(14)
1.9946(15)
1.207(2)
2.3161(14)
2.3222(15)
2.080(6)
1.986(6)
1.204(8)
1.429(8)
1.435(3)
1.426(2)
Bond Angles (deg)
C(9)-Pt(1)-C(1)
C(1)-Pt(1)-P(2)
P(2)-Pt(1)-P(1)
P(1)-Pt(1)-C(9)
Pt(1)-C(1)-C(2)
C(1)-C(2)-C(3)
80.32(7)
92.62(5)
87.731(16)
99.20(5)
171.16(16)
175.4(2)
84.9(3)
90.50(19)
87.90(7)
97.0(2)
173.5(7)
174.0(7)
81.4(3)
92.8(2)
87.98(7)
97.8(2)
170.0(7)
177.3(9)
82.37(6)
92.82(4)
88.413(13)
98.28(4)
168.76(13)
175.19(16)
82.5(2)
93.58(18)
87.95(5)
96.44(15)
170.2(6)
177.5(7)
Table 3. Effect of Modification of Substrate on the Reductive
Coupling Reaction
substrate
reaction
∆
Gq (kcal/mol)
T (°C)
bis(3,5-dimethylphenyl)acetylene
diphenylacetylene
bis(p-fluorophenyl)acetylene
bis(perfluorophenyl)acetylene
p-tolyl-p-fluorophenylacetylene
4 f 3
2 f 1
6 f 5
8 f 7
10 f 9
11 f 9
31.33 (3)
32.03 (21)
32.78 (1)
47.30
33.32 (17)
32.10 (2)
80
80
120
300
100
100
two singlets at δ 71.35 (1JP-Pt ) 1963.4 Hz, P trans to acetylide)
and 77.74 (1JP-Pt ) 1296.6 Hz, P trans to p-fluorophenyl).
Complex 6 showed similar behavior to the previous two PtII
complexes, quantitatively reverting to 5 upon heating at 120
°C. This electron-deficient system also shows thermodynam-
ically uphill behavior toward C-C cleavage. The first-order
kinetics of the reverse reaction showed a barrier for the reverse
reaction of 32.78(1) kcal/mol, indicating that 6 is slightly more
stable toward reductive coupling than 2 (∼3 times slower).
As an electron-deficient substrate stabilizes the corresponding
PtII complex, a more electron-deficient substrate might permit
thermal C-C cleavage. The η2-coordinated Pt0 complex (dtbpe)-
Pt(η2-bis(pentafluorophenyl)acetylene) (7) was synthesized and
fully characterized (31P NMR: δ 94.26, s, 1JP-Pt ) 2636.9 Hz).
A light yellow solution of 7 in C6D6 was heated at 300 °C for
3 days in a sealed tube. The reaction was monitored by 31P
NMR spectroscopy periodically but no C-C activated product
was observed. During this high-temperature reaction of 7, no
decomposition was seen and the color of the solution remained
light yellow. Upon overnight photolysis of 7, the C-C bond
cleavage product (dtbpe)Pt(C6F5)(CtCC6F5) (8) was obtained.
Figure 4. Molecular structure of 9 (ORTEP diagram, 30% probability
ellipsoids). Final R indices [I > 2 σ(I)], R1 ) 0.0472. Monoclinic, P21/c.
withdrawing group substitution has substantially stabilized the
PtII complex, this stabilization was not enough to render the
forward C-C cleavage reaction thermodynamically downhill.
Variations in the stability of the complexes upon modification
of the substrate by adding electron-donating and electron-
withdrawing groups are summarized in Table 3. Electron-
withdrawing groups have a profound effect upon stabilizing the
PtII C-C cleavage product. Electron-donating groups, in
comparison, have only a small effect. It is noteworthy that earlier
methyl hydride reductive elimination studies on platinum
showed that electron-withdrawing groups on the phosphine led
to more rapid elimination whereas electron-donating groups
slowed elimination,26 as in this case stabilization of the platinum
product by the phosphine dominates the rate of elimination.
The data in Table 3 indicate that electron-deficient substit-
uents on the acetylene provide the more stable C-C bond-
activated complexes while electron-rich substituents destabilize
the PtII complexes. Therefore, use of a heterosubstituted substrate
like p-tolyl-p-fluorophenylacetylene might be expected to show
selective C-C bond activation. The heterosubstituted alkyne
p-tolyl-p-fluorophenylacetylene was synthesized and reacted
with Pt(COD)2 as shown in eq 1. The η2-coordinated Pt0
complex of (dtbpe)Pt(η2-p-tolyl-p-fluorophenylacetylene) was
The 31P NMR spectrum shows two singlets at δ 77.0 (1JP-Pt
)
2442.77 Hz) and 79.6 (1JP-Pt ) 2273.1 Hz).
In order to measure the effect of the electron-withdrawing
groups on the C-C and M-C bond strengths, complex 8 was
heated at 300 °C and reversion to 7 was monitored by 31P NMR
spectroscopy. From the observed rate, the energy barrier for
the reversion of 8 to 7 was estimated to be 47.30 kcal/mol. The
tremendous thermal stability of 8 can be attributed to the high
strength of the Pt-aryl bond due to the presence of two ortho-
fluorines. The enhancement of metal-aryl bond strength with
the number of ortho-fluorines has been impressively demon-
strated in the literature.25 This demonstrates that while electron-
(25) Clot, E.; Besora, M.; Me´gret, C.; Eisenstein, O.; Oelckers, B.; Perutz, R.
N. Chem. Commun. 2003, 490-491.
(26) Abis, L.; Sen, A.; Halpern, J. J. Am. Chem. Soc. 1978, 100, 2915-2916.
9
8732 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

C−C Bond Activation of Diphenylacetylenes
A R T I C L E S
fully characterized (Figure 4), and the 31P NMR spectrum of
this Pt0 derivative shows a singlet at δ 94.25 (1JP-Pt ) 2506.9
Hz), despite the fact that the two phosphorus nuclei are opposite
different aryl groups.
Since the Pt-aryl bond with the electron-deficient aryl is
expected to be stronger than the Pt-aryl bond with the electron-
rich aryl, C-C bond cleavage was expected to form a Pt-aryl
bond with the electron-withdrawing group-substituted aryl.
Before irradiation with UV light, complex 9 was heated at 200
°C for a day and the corresponding C-C activated PtII complex
was not observed. Irradiation of complex 9 with UV light
overnight led to 96% conversion to a mixture of the PtII
complexes (dtbpe)Pt(p-fluorophenyl)(p-tolylacetylide) 10 and
(dtbpe)Pt(p-tolyl)(p-fluorophenylacetylide) 11, observed in a 55:
45 ratio, respectively, by 31P NMR spectroscopy (eq 3). There
were four phosphorus resonances observed at δ 70.9 (1JPt-P
)
2007.3 Hz, major), 71.2 (1JPt-P ) 1980.0 Hz, minor), 77.7
(1JPt-P ) 1298.3 Hz, major), and 77.8 (1JPt-P ) 1259.1 Hz,
minor), two for each complex. The spectral assignments were
made based upon analogies to symmetric activation products 6
and 2. The phosphorus resonance in complex 6 for the P trans
to p-fluorophenyl showed a coupling constant of 1JPt-P ) 1296.6
Hz, which is very similar to the value observed for the
phosphorus resonance at δ 77.7. Therefore, the major product
resonance at δ 77.7 can be assigned to the P trans to
p-fluorophenyl in 10. The other major resonance at δ 70.9 is
assigned to the P trans to p-tolylacetylide in 10. Hence the
phosphorus resonance at δ 77.8 (1JPt-P ) 1259.1 Hz) can be
assigned to the P trans to p-tolyl in 11, and the one at δ 71.2
(1JPt-P ) 1980.0 Hz) to the P trans to acetylide in 11.
Figure 5. Molecular structure of (dtbpe)Pt(p-tolyl)(CtCC6H4F) 11 (ORTEP
diagram, 30% probability ellipsoids).
Figure 6. Reverse reactions 10 f 9 ([) and 11 f 9 (9) at 100 °C in
C6H6.
thermal pathways, then the energies of the transition states
leading to the two products 10 and 11 should be similar (∆∆Gq
≈ 0.1 kcal/mol). On the other hand, reductive coupling barriers
differ by 1.22 kcal/mol, suggesting a thermodynamic preference
for the more electron-deficient aryl group being attached to Pt
(Hammond postulate). Therefore, the enhanced stability ob-
served for C-C cleavage products containing electron-deficient
aryl groups can be associated with a stabilization of the PtII
product rather than changing the energy of the transition state
for C-C cleavage.
With regard to kinetic selectivity, since the C-C bond-
activated product 10 was 55% while the C-C bond-activated
product 11 was 45%, greater selectivity in C-C bond activation
might be achieved by increasing the difference between electron
deficiency and richness on the two target aryl groups.
The mixture of the complexes 10 and 11 was recrystallized
and a single-crystal structure determination showed one of the
two possible products, 11 (Figure 5). There was no evidence
for disorder of the methyl and fluoro groups, as the bond
distances were noticeably different (C-F ) 1.3 Å and C-Me
) 1.5 Å).
Similar to the previous PtII oxidative addition complexes, the
mixture of 10 and 11 showed reversion to Pt0 complex 9 upon
heating at 100 °C. However, the rates of reversion from 10 and
11 to 9 were slightly different for the two compounds. The
kinetics of the reversion reactions of 10 f 9 and 11 f 9 were
monitored and the rate constants were determined from the
slopes of the first-order plots (Figure 6), giving ∆Gq ) 33.32-
(17) kcal/mol for 10 and 32.10(2) kcal/mol for 11.
Conclusions
An analysis of the kinetic selectivity in the photolysis of 9
allows a comparison of the energies of the C-C cleavage
products to be made. Since photolysis of 9 gave almost a 1:1
ratio of products 10 and 11, this implies similar barriers to C-C
cleavage. This reaction is photochemical, and the transition states
need not lie at the same energies as in the thermal pathways,
since excited states are involved. However, if we assume that
the energetics of the excited-state pathways follow those of the
Pt0 complexes of η2-coordinated diarylacetylenes have been
synthesized and fully characterized along with the corresponding
C-C bond-activated PtII complexes. C-C bond activation
occurred only under photochemical conditions. Under thermal
conditions, up to 300 °C, C-C bond activation was not
observed. However, reductive coupling, that is, PtII f Pt0, did
occur under thermal conditions and allowed determination of
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8733

A R T I C L E S
Gunay and Jones
crystallized at room temperature via solvent evaporation. Yield: 27.6
the barrier to C-C bond formation. According to the activation
energy barriers for the reverse reaction, electron-deficient
substituents stabilize the PtII complex while electron-rich ones
destabilize it. The transition state is not affected as much, leading
to the notion that the energy barrier for C-C bond formation
can be tuned by modifying the substrate by adding the EDGs
and EWGs. In no case was sufficient stabilization obtained to
change the overall energy profile and make the activation
reaction thermodynamically downhill.
1
mg (82%) of colorless air-sensitive crystals of 1. H NMR (C6D6) δ
3
7.752 (d, JH-H ) 7.5 Hz, 4H, o-C6H5), 7.244 (t, 4H, m-C6H5), 6.984
(t, 3JH-H ) 7.5 Hz, 2H, p-C6H5), 1.351 (d, 2JH-P ) 6.5 Hz, 4H, P-CH2),
3
1.144 [virtual t, JH-P ) 12.5 Hz, 36H, C-(CH3)3]. 13C{1H} NMR
1
(CDCl3) δ 26.61 (d, P-CH2), 30.95 (m, C-CH3), 35.46 (t, JC-P
)
31.91 Hz, P-C-), 125.27 (s, ipso-C), 128.74 (s, p-C), 129.18 (s, m-C),
132.45 (s, o-C), 141.57 (m, CtC). 31P NMR (C6D6) δ 94.23 (s, with
1
platinum satellites, JPt-P ) 2516.4 Hz). Anal. Calcd for C32H50P2Pt:
C, 55.54; H, 7.29. Found: C, 55.39; H, 7.25.
The C-C cleavage reaction in a substrate containing both
electron-donating and electron-withdrawing substituents has
produced 55% activation product through the C-C bond
adjacent to the EWG-substituted aryl ring and 45% activation
product through the one adjacent to the EDG-substituted aryl,
implying that modest kinetic selectivity can be obtained in C-C
bond cleavage. In this report, all the activated C-C bonds were
sp-sp2-type hybridized carbons. Further studies will be required
with similar Pt0-η2-acetylene complexes to extend these
cleavage results to sp-sp3-hybridized C-C bonds.
Preparation of (dtbpe)Pt(-Ph)(-CtCPh) (2). Ten milligrams of 1
was dissolved in 1 mL of C6D6 and placed into an NMR tube with a
Teflon stopcock. The sample was irradiated with UV light (λ > 300
nm) for 4 h. The sample was crystallized by solvent evaporation at
room temperature. Colorless and air-stable crystals of 2 (8.9 mg, 89%)
3
were obtained. 1H NMR (C6D6) δ 1.03 [d, JH-P ) 10.4 Hz, 18H,
3
C-(CH3)3], 1.34 [d, JH-P ) 10.4 Hz, 18H, C-(CH3)3], 1.12-1.22 (m,
4H, -CH2), 6.97 (t, 3H, m-H and p-H), 7.29 (distorted t, 3H, m-H and
p-H), 7.42 (d, 2H, o-H), 7.97 (t, 2H, o-H). 13C{1H} NMR (C6D6) δ
24.1 (m, P-CH2), 26.58 (m, P-CH2), 30.71 [m, -(CH3)3], 36.2 (d, 1JC-P
) 56.4 Hz, P-CH2), 36.78 (d, 1JC-P ) 65.2 Hz, P-CH2), 111. 33 (d,
3
3JC-P ) 120 Hz, CtC-Ph), 112.9 (d, JC-P ) 680 Hz, CtC-Ph),
Experimental Section
122.53, 125.14, 127.62, 131.72, 140.51 (aromatic carbon resonances).
31P NMR (C6D6) δ 71.36 (s, with Pt satellites, 1JPt-P ) 1978.3 Hz, 1P,
General Considerations. All experiments were carried out under a
nitrogen atmosphere, either on a high-vacuum line using modified
Schlenk techniques or in a Vacuum Atmosphere Corp. glove box unless
otherwise stated. The solvents were available commercially and were
distilled from dark purple solutions of benzophenone ketyl.
1
P trans to acetylide), 77.45 (s, with Pt satellites, JPt-P ) 1250.6 Hz,
1P, P trans to -Ph). Anal. Calcd for C32H50P2Pt: C, 55.54; H, 7.29.
Found: C, 55.18; H, 7.00.
Preparation of (dtbpe)Pt(η2-RCtCR), R ) 3,5-xylyl (3), p-C6H4F
(5), or C6F5 (7). These compounds were prepared by procedures
analogous to that described above for 1. Full details including 1H, 31P,
and 13C NMR data, analyses, and X-ray structures are given in the
Supporting Information.
1H, 13C{H}, and 31P{H} NMR spectra were recorded on Bruker
Avance-400, Bruker AMX-400, and Bruker Avance-500 spectrometers.
1
All H chemical shifts were referenced to residual proton resonances
or to tetramethylsilane (TMS) in the deuterated solvents. An external
standard of 85% H3PO4 was used to reference the 31P{H} NMR data.
All crystal structures were determined by use of a Siemens-SMART
3-Circle CCD diffractometer. All photolysis experiments were per-
formed with an Oriel arc source using a 200 W Hg(Xe) lamp in sealed
NMR tubes. Elemental analyses were obtained from Desert Analytics.
Diphenylacetylene was obtained from commercial sources and Pt-
(COD)2,27,28 bis(pentafluorophenyl)acetylene,2 and bis(di-t-butylphos-
phino)ethane30 were synthesized according to reported procedures.
Kinetic analyses were carried out by use of Microsoft Excel with the
Solver statistics module added.31 Errors are reported as standard
deviations. Due to the slowness of the reductive coupling reactions,
data from <3 half-lives was used in the kinetic determinations of 2, 4,
and 10. Reductive coupling in 8 was so slow that the rate was estimated
from a single point (10% change after 3.5 h at 300 °C).
Preparation of (dtbpe)Pt(η2-PhCtCPh) (1). Diphenylacetylene
(8.66 mg, 48.6 mmol) was dissolved in 0.7 mL of C6D6 and added to
white-beige crystals of Pt(COD)2 (20 mg, 48.6 mmol). As the color of
the solution changed from colorless to light yellow, a solution of dtbpe
(15.47 mg, 48.65 mmol) in 0.3 mL of C6D6 was added. The above
order and time of adding the dtbpe is crucial to obtaining good yields
of 1. The resultant solution was transferred into a sealed NMR tube
and then heated at 80 °C for 40 min, allowing all of the dtbpe to
coordinate to Pt. The reaction was monitored by 31P{H} NMR
spectroscopy, and once all of the free dtbpe was consumed and 1 was
produced, solvent and free COD [released from the initial Pt(COD)2
precursor] were removed under high vacuum. The light yellow powder
1 was taken back into the glove box and redissolved in C6H6. It was
Preparation of (dtbpe)Pt(R)(CtCR), R ) 3,5-xylyl (4), p-C6H4F
(6), or C6F5 (8). These compounds were prepared by photochemical
procedures analogous to that described above for 2. Full details
1
including H, 31P, and 13C NMR data, analyses, and X-ray structures
are given in the Supporting Information.
Synthesis of p-Tolyl-p-fluorophenylacetylene. Cuprous iodide (0.25
g, 1.25 mmol) was added to a triethylamine solution (50 mL) of t-Cl2-
Pd(PPh3)2 (0.90 g, 1.25 mmol), iodo-p-fluorobenzene (4 g, 18 mmol),
and p-tolylacetylene (20.8 g, 18 mmol) in a round-bottom flask equipped
with a stir bar and a condenser under a nitrogen atmosphere. The
mixture was refluxed for 6 h and then water was added. The organic
layer was extracted with ether and the extract was dried over anhydrous
Na2SO4. The solution was filtered and the ether was removed under
reduced pressure until product started to precipitate. The residue was
left for crystallization in ether at -10 °C. White crystals of p-tolyl-p-
fluorophenylacetylene (2.61 g) were obtained in 69.0% yield. 1H NMR
(CDCl3) δ 2.39 (s, CH3), 7.05 (t, J ) 17.4 Hz), 7.18 (d, J ) 7.9 Hz),
7.44 (d, J ) 8.0 Hz), 7.52 (m). 13C{1H} NMR (CDCl3) δ 21.5 (s, CH3),
87.6 (s, CtC), 89.0 (s, CtC), 115.0 (d, 2JC-F ) 22.9 Hz, m-C), 119.0
3
(s), 119.9 (s), 129.0 (s), 131.3 (s), 133.3 (d, JC-F ) 7.0 Hz, o-C on
fluorophenyl), 138.2 (s), 162.6 (d, 1JC-F ) 251.5 Hz, C-F). 19F NMR
(CDCl3) δ 2.9 (s, p-F) (referenced to fluorobenzene).
Preparation of (dtbpe)Pt(η2-p-Fluorophenyl-p-tolylacetylene) (9).
White crystals of p-fluorophenyl-p-tolylacetylene (30.65 mg, 0.146
mmol) were dissolved in 4 mL of C6D6. In another vial, white crystals
of dtbpe (46.41 mg, 0.146 mmol) were dissolved in 2 mL of C6D6.
White-beige powder of Pt(COD)2 (60 mg, 0.146 mmol) was placed in
a separate vial. First, the colorless solution of p-fluorophenyl-p-
tolylacetylene was transferred onto the Pt(COD)2 and stirred for 10 s.
While the color of the solution was turning from colorless to orange,
the solution of dtbpe was transferred in. The resultant light yellow
solution was stirred for 3 min. The solution was transferred into a sealed
tube and heated at 80 °C for 1.5 h. One milliliter of the solution was
taken for an NMR experiment. According to the 31P NMR spectrum,
(27) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem. Soc. 1976,
98, 6521-6528.
(28) Osborn, T. A.; Wilkinson, G.; Mrowca, J. J. Inorg. Synth. 1990, 28, 77.
(29) Gastinger, R. G.; Tokas, E. F.; Rausch, M. D. J. Org. Chem. 1978, 43,
159-161.
(30) Porschke, K. R.; Pluta, C.; Proft, B.; Lutz, F.; Kruger, C. K. Z. Naturforsch.
B: Chem. Sci. 1993, 48, 608-626.
(31) Billo, E. J. Excel for Chemists, A ComprehensiVe Guide; Wiley-VCH:
New York, 1997; pp 297-299.
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8734 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

C−C Bond Activation of Diphenylacetylenes
A R T I C L E S
∼4% of the Pt(COD)2 was not reacted with p-fluorophenyl-p-
tolylacetylene. To obtain only 9, the solution was heated at 100 °C for
1 day. The sealed tube was attached to the vacuum line, and both solvent
and COD were removed. The light yellow powder 9 was taken back
into the glovebox and redissolved in C6H6 for crystallization. Colorless
and air-sensitive crystals of 9 (82.3 mg, 78%) were obtained after 3
days upon crystallization at room temperature via solvent evaporation:
2H, o-H in p-fluorophenyl). 13C{1H} NMR (CDCl3) δ 21-24 (4 m,
P-CH2 cis and trans to Pt-Ph in both complexes), 26.6 (s, p-CH3
both in p-tolyl and p-tolylacetylide), 30.2 [m, C-(CH3)3 cis and trans
to Pt-Ph in both complexes], 36.2 (d of m, P-C cis and trans to Pt-
Ph in both complexes), 113.2 and 115.8 (dd, Pt-CtC in both
complexes), 114.4 (d, Pt-CtC in both complexes), 127-134 (six
resonances, aromatic carbons in both complexes), 139 (m, o-C in
p-tolylacetylide and p-fluorophenylacetylide in both complexes), 160.5
(d, p-C-F in both p-fluorophenyl and p-fluorophenylacetylide). 31P
NMR (CDCl3) δ 70.9 (s, with platinum satellites, 1JPt-P ) 2007.3 Hz,
1P, P trans to p-tolylacetylide), 71.2 (s, with platinum satellites, 1JPt-P
) 1980.0 Hz, 1P, P trans to p-fluorophenylacetylide), 77.7 (s, with
platinum satellites, 1JPt-P ) 1298.3 Hz, P trans to p-fluorophenyl), 77.8
3
1H NMR (C6D6) δ 1.14 (t, JH-P ) 20 Hz, 36H, C-CH3), 1.35 (d,
2JH-P ) 7 Hz, 4H, -CH2), 2.16 (s, 3H, p-CH3), 6.90 (t, 2H, m-H on
Ph-CH3), 7.05 (d, 2H, m-H on Ph-F), 7.60 (t, 4H, o-H). 13C{1H}
NMR (CDCl3) δ 21.20 (s, 1 C, p-CH3), 25.78 (m, 2 C, P-CH2), 30.16
(s, 12 C, C-CH3), 34.59 (m, 4 C, P-C), 114.35 (d, 2 C, ipso-C), 128.38
(s, p-C on Ph-CH3), 129.21 (s, 4 C, m-C), 133.97 (s, 4 C, o-C), 162
(s, 2 C, CtC), 170 (d, JC-F ) 1800 Hz, p-C on Ph-F). 31P NMR
(C6D6) δ 94.25 (s, with platinum satellites, 1JPt-P ) 2506.9 Hz). Anal.
Calcd for C33H51FP2Pt: C, 54.74; H, 7.11. Found: C, 54.93; H, 6.84.
Preparation of both (dtbpe)Pt(p-Fluorophenyl)(p-tolylacetylide)
(10) and (dtbpe)Pt(p-tolyl)(p-Fluorophenylacetylide) (11). Twenty
milligrams of 9 was dissolved in 1 mL of C6D6, and the light yellow
solution was placed in a sealed NMR tube. The sample was irradiated
overnight, and 96% conversion to 10 and 11 was observed via 31P NMR
spectroscopy. No further conversion or formation of PtII complex was
observed after irradiation for one more day. The NMR tube was taken
back into the glove box and the solution was transferred to a vial. It
was left for crystallization at room temperature by solvent evaporation.
After 4 days, colorless and air-stable crystals of 10 and 11 (14.2 mg,
71%) were obtained. 1H NMR (CDCl3) δ 1.13 (d of m, 36H, C-(CH3)3
trans to p-fluorophenyl and trans to p-tolyl), 1.37 (d of m, 36H, C-(CH3)3
trans to p-fluorophenylacetylide and trans to p-tolylacetylide), 2.07 (s,
6H, p-methyl in p-tolyl and p-tolylacetylide), 6.7-6.94 (m, 12H, m-H
in p-fluorophenyl and p-tolyl and o-H and m-H in p-fluorophenyl-
acetylide and p-tolylacetylide), 7.33 (m, 2H, o-H in p-tolyl), 7.41 (m,
1
(s, with platinum satellites, JPt-P ) 1259.1 Hz, P trans to p-tolyl).
Anal. Calcd for C33H51FP2Pt‚C6H6: C, 58.39; H, 7.17. Found: C, 59.14;
H, 7.42.
Acknowledgment. Acknowledgement is made to the U.S.
Department of Energy for support (Grant FG02-86ER13569),
to William Brennessel for his help in reorganizing the single-
crystal structure analysis reports, and to Professor Richard
Eisenberg for his suggestions about Pt complexes and photolysis
reactions.
Supporting Information Available: Details of preparations
of 3, 5, 7, 4, 6, 8, and alkynes; data collection parameters; crystal
data for complexes 1-9 and 11; UV-vis spectra for complexes
1, 3, and 7; and kinetics for reactions 2 f 1, 4 f 3, and 6 f
5. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA071698K
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8735