Distinct electronic effects on reductive eliminations of symmetrical and unsymmetrical bis-aryl platinum complexes

Article abstract of DOI:10.1021/ja0480365

Symmetrical bis-aryl platinum complexes (DPPF)Pt(C₆H ₄-4-R)₂ (R = NMe₂, OMe, CH₃, H, Cl, CF₃) and electronically unsymmetrical bis-aryl platinum complexes (DPPF)Pt(C₆H₄-R)(C₆H₄-4-X) (R = CH₃, X = NMe₂, OMe, H, Cl, F, CF₃; R = OMe, X = NMe₂, H, Cl, F, CF₃; R = CF₃, X = H, Cl, NMe₂; and R = NMe₂, X = H, Cl) were prepared, and the rates of reductive elimination of these complexes in the presence of excess PPh₃ are reported. The platinum complexes reductively eliminated biaryl compounds in quantitative yields with first-order rate constants that were independent of the concentration of PPh₃. Plots of Log(k _obs/k_obs(H)) vs Hammett substituent constants (σ) of the para substituents R and X showed that the rates of reductive elimination reactions depended on two different electronic properties. The reductive elimination from symmetrical bis-aryl platinum complexes occurred faster from complexes with more electron-donating para substituents R. However, reductive elimination from a series of electronically unsymmetrical bis-aryl complexes was not faster from complexes with the more electron-donating substituents. Instead, reductive elimination was faster from complexes with a larger difference in the electronic properties of the substituents on the two platinum-bound aryl groups. The two electronic effects can complement or cancel each other. Thus, this combination of electronic effects gives rise to complex, but now more interpretable, free energy relationships for reductive elimination.

Full text of DOI:10.1021/ja0480365

Published on Web 09/18/2004
Distinct Electronic Effects on Reductive Eliminations of
Symmetrical and Unsymmetrical Bis-Aryl Platinum Complexes
Shashank Shekhar and John F. Hartwig*
Contribution from the Department of Chemistry, Yale UniVersity, P.O. Box 208107,
New HaVen, Connecticut 06520-8107
Received May 5, 2004; E-mail: John.Hartwig@yale.edu
Abstract: Symmetrical bis-aryl platinum complexes (DPPF)Pt(C6H4-4-R)2 (R ) NMe2, OMe, CH3, H, Cl,
CF3) and electronically unsymmetrical bis-aryl platinum complexes (DPPF)Pt(C6H4-4-R)(C6H4-4-X) (R )
CH3, X ) NMe2, OMe, H, Cl, F, CF3; R ) OMe, X ) NMe2, H, Cl, F, CF3; R ) CF3, X ) H, Cl, NMe2; and
R ) NMe2, X ) H, Cl) were prepared, and the rates of reductive elimination of these complexes in the
presence of excess PPh3 are reported. The platinum complexes reductively eliminated biaryl compounds
in quantitative yields with first-order rate constants that were independent of the concentration of PPh3.
Plots of Log(kobs/kobs(H)) vs Hammett substituent constants (σ) of the para substituents R and X showed
that the rates of reductive elimination reactions depended on two different electronic properties. The reductive
elimination from symmetrical bis-aryl platinum complexes occurred faster from complexes with more electron-
donating para substituents R. However, reductive elimination from a series of electronically unsymmetrical
bis-aryl complexes was not faster from complexes with the more electron-donating substituents. Instead,
reductive elimination was faster from complexes with a larger difference in the electronic properties of the
substituents on the two platinum-bound aryl groups. The two electronic effects can complement or cancel
each other. Thus, this combination of electronic effects gives rise to complex, but now more interpretable,
free energy relationships for reductive elimination.
or electrophilicity,16-21 or capacity to stabilize an accumulation
of charge.13,15,22 The potential for synergistic or cooperative
Introduction
Electronic effects lie at the heart of all reactivity. In transition
metal chemistry, the electronic properties of both the ligands
ancillary to the reaction and ligands participating directly in
the reaction affect rates and selectivity.1,2 The electronic
properties of ancillary ligands generally control the overall
electron density of the transition metal center and can influence
reactivity at a position cis or trans to one of the ligands.3-8 The
effect of the electronic properties of a ligand that participates
directly in the reaction can be complex and is sometimes
interpreted in terms of ground-state energy,9-15 nucleophilicity
effects from changes to the electronic properties of two ligands
simultaneously participating in a reaction may increase the
complexity of the effect of electronic perturbations. The
properties of one ligand participating in a reaction could
influence the magnitude or even direction of the effect of an
electronic perturbation of the second ligand.16,17,22-24 Such
synergistic effects could be important in reductive elimination,
migratory insertion, and nucleophilic attack on coordinated
ligands. The work here focuses on assessing potential synergistic
electronic effects on reductive elimination.
Theoretical studies have suggested that reductive elimination
is faster from complexes with more electron-donating ligands
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
3rd ed.; John Wiley & Sons: New York, 2001.
(2) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and
Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987.
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2448.
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9
13016
J. AM. CHEM. SOC. 2004, 126, 13016-13027
10.1021/ja0480365 CCC: $27.50 © 2004 American Chemical Society

Reductive Eliminations of Bis-Aryl Platinum Complexes
A R T I C L E S
participating in the bond-forming process.10,11 The origin of this
prediction can be traced to stronger bonds between a metal and
a ligand with an electron-withdrawing substituent because of
increased ionic character.25,26 The theoretical studies, therefore,
imply that the barriers for reductive elimination are affected
more by changes in the energies of the reactants than by changes
in the energies of the transition states.10 From this information,
one might predict that the rate of reductive elimination of
unsymmetrical compounds would relate closely to the sum of
the strengths of the two metal-ligand bonds broken in the
reductive elimination process.
Some results are consistent with faster reductive elimination
from complexes in which more electron-donating groups
undergo reductive elimination.9,27-29 For example, the reductive
elimination of biaryl from (PPh3)2Pt(p-tol)2 was faster than
elimination from (PPh3)2PtPh2, which contains a less electron-
donating aryl group.30 Similarly, reductive elimination of amine
from (DPPF)Pd(p-tol)(NAr2)17 was much faster than reductive
elimination of ether from (DPPF)Pd(p-tol)(OAr).27,31 However,
changes in the rates for reductive elimination from electronic
perturbations cannot be traced to the electron-donating or
-withdrawing property of the ligand in other cases. For example,
reductive eliminations of amines,17 ethers,22,31 and sulfides16
from arylpalladium amide, alkoxide, and thiolate complexes are
slower when the complexes contain more electron-donating
groups on the palladium-bound aryl group, and cis-(PPh3)2Pt-
(4-CH3C6H4)(4-NO2C6H4) was stated to be too unstable to
isolate, while cis-(PPh3)2Pt(4-CH3C6H4)2 requires elevated tem-
perature to undergo reductive elimination.23
The reductive elimination of biaryls from arylmetal
complexes23,28,30,32-34 could provide a context to reveal rigor-
ously whether increased or decreased electron-donating proper-
ties of a covalent, reactive ligand accelerate the rate of reductive
elimination and the extent to which the electronic properties of
the two groups undergoing reductive elimination are synergistic.
The electronic properties of an aryl group can be varied
systematically within bis-aryl metal complexes without varying
significantly the steric properties, and both ligands would be
bound through similar types of metal-ligand interactions.35
Moreover, reductive elimination of biaryls is the step in catalytic
cross-coupling between main-group organometallic compounds
and organic halides or triflates that forms the new carbon-
carbon bond in the product.36-39 Thus, an understanding of the
factors that influence the reductive elimination of biaryls will
not only improve our understanding of electronic effects on
organometallic reactivity, but could improve our ability to
predict the scope of cross-coupling and to design improved
systems.
We have used the reductive elimination from symmetrical
and unsymmetrical platinum bis-aryl complexes as a case study
to reveal the electronic effects in an organometallic reaction
that is common and that involves simultaneous participation of
two covalent ligands. Palladium and nickel complexes are often
used as catalysts for coupling reactions that form biaryls, but
the bis-aryl complexes of these metals have not been isolated.
Platinum complexes are now known that catalyze such coupling
reactions,40-42 and platinum bis-aryl complexes43,44 can be
isolated that undergo reductive elimination.23,28,30,32,33
We show that reductive elimination from unsymmetrical bis-
aryl platinum complexes is not simply slower from complexes
with more electron-withdrawing substituents and stronger
metal-ligand bonds. Instead, reductive elimination is faster from
complexes with a larger difference between the electron-
donating properties of the two aryl groups. The electronic
properties of one group affect the magnitude and direction of
the result of electronic perturbations to a second group. The
electronic effects on reductive elimination are synergistic.
Results
1. Selection of Model Complexes for Analysis. To find
platinum bis-aryl complexes with a broad spectrum of electronic
properties that are stable enough to isolate but reactive enough
to undergo reductive elimination, we prepared platinum bis-
aryl complexes with a series of ancillary ligands. Bis-aryl
platinum complexes with 1,5-cyclooctadiene (COD), triphen-
ylphosphine, 1,2-bis(diphenylphosphino)ethane (DPPE), and
1,1′-bis(diphenylphosphino)ferrocene (DPPF) as ancillary ligands
were prepared. Complexes containing COD as ancillary ligand
required a temperature of 150 °C to reductively eliminate biaryls
at a rate convenient to be monitored by NMR spectroscopy.
Moreover, several side products were observed, in addition to
the biaryls. Some of the complexes containing PPh3 as ligands
were unstable at room temperature and were difficult to isolate.
For example, complexes of the type (PPh3)2Pt(C6H4-4-NMe2)-
(C6H4-4-X), in which X ) NMe2, OMe, CH3, and Cl,
decomposed at room temperature. Elimination of biaryls from
bis-aryl complexes ligated by DPPE required 2-3 days at 150
°C. However, bis-aryl complexes ligated by DPPF were simple
to prepare and formed biaryl products in quantitative yields at
95 °C over 1-20 h. Thus, our detailed studies focused on the
reductive elimination of biaryls from DPPF-ligated platinum
complexes.
(25) Holland, P. L.; Anderson, R. A.; Bergman, R. G.; Huang, J.; Nolan, S. P.
J. Am. Chem. Soc. 1997, 119, 12800-12814.
(26) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. J.
Am. Chem. Soc. 1987, 109, 1444-1456.
(27) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852-860.
(28) Braterman, P. J.; Cross, R. J.; Young, G. B. J. Chem. Soc., Dalton Trans.
1976, 1306-1310.
(29) Ozawa, F.; Mori, T. Organometallics 2003, 22, 3593-3599.
(30) Braterman, P. S.; Cross, R. J.; Young, G. B. J. Chem. Soc., Dalton Trans.
1976, 1892-1897.
2. Synthesis and Characterization of Symmetrical Bis-Aryl
Platinum Complexes (1a-f). The synthesis of symmetrical bis-
aryl platinum complexes (DPPF)Pt(C6H4-4-R)2 is summarized
in Scheme 1. Reaction of aryl Grignard reagents with (COD)-
PtCl2 in ether formed bis-aryl platinum complexes (COD)Pt-
(31) Mann, G.; Shelby, Q.; Roy, A. H. Organometallics. 2003, 22, 2775-2789.
(32) Merwin, R. K.; Schnabel, R. C.; Koola, J. D.; Roddick, D. M. Organo-
metallics 1992, 11, 2972-2978.
(33) Braterman, P. S.; Cross, R. J. J. Chem. Soc., Perkin Trans. 1973, 2, 271-
294.
(34) Brune, H.-A.; Hess, R.; Schmidtberg, G. Z. Naturforsch 1984, 39b, 1772-
1780.
(40) Mateo, C.; FernandeziRivas, C.; Cardenas, D. J.; Echavarren, A. M.
Organometallics 1998, 17, 3661.
(35) For mechanistic and theoretical studies on reductive elimination from metal
alkyl complexes, see refs 10, 11, 36-48, and 51-52.
(36) Suzuki, A. J. Organomet. Chem. 1999, 576, 147-168.
(37) Hassan, J.; Sevingo, M.; Gozzi, C.; Emmanuelle, L.; Lemaire, M. Chem.
ReV. 2002, 102, 1359-1469.
(41) Clarke, M. L. Polyhedron 2001, 20, 151-164.
(42) Bedford, R. B.; Hazelwood, S. L.; Abisson, D. A. Organometallics 2002,
21, 2599-2600.
(43) Brown, J. M.; Perez-Torrente, J. J.; Alcock, N. W. Organometallics 1995,
14, 1195-1203.
(38) Stanforth, S. P. Tetrahedron 1998, 54, 263-303.
(39) Negishi, E. Handbook of Organopalladium Chemistry for Organic Synthesis;
John Wiley & Sons: New York, 2002; Vol. 1.
(44) Alvadro-Rodriguez, J. G.; Toscano, R. A.; Boyko, W. J. J. Organomet.
Chem. 1998, 557, 169-179.
9
J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13017

A R T I C L E S
Scheme 1
Shekhar and Hartwig
Scheme 3
Scheme 2
95% yield. Complexes 2b-f, 3b-e, and 4a,b were characterized
1
by H and 31P NMR spectroscopy and elemental analysis, and
complexes 3b and 3c, were sufficiently soluble to be character-
ized by 13C NMR spectroscopy.
Unsymmetrical bis-aryl platinum complexes 2a, 3a, and 5a,b
were inaccessible by Route A because the COD-ligated biaryl
species (COD)Pt(C6H4-4-NMe2)(C6H4-4-X) (X ) OMe, CH3,
H, Cl) decomposed at room temperature, and efforts to generate
(COD)Pt(C6H4-4-NMe2)Cl by protonolysis of the p-dimethyl-
aminophenyl group in 1a led to the decomposition of 1a without
generation of the aryl platinum chloride. Thus, complexes 2a,
3a, and 5a,b were synthesized by Route B in Scheme 2.
Reaction of DPPF with (COD)Pt(C6H4-4-R)Cl formed (DPPF)-
Pt(C6H4-4-R)Cl, and this complex was converted to the unsym-
metrical bis-aryl complex by reaction with the Grignard reagent
BrMgC6H4-4-X. Complexes 2a, 3a, and 5a,b were unstable to
(C6H4-4-R)2 in 55-75% yield. Reaction of DPPF with (COD)-
Pt(C6H4-4-R)2 in benzene formed DPPF-ligated bis-aryl platinum
complexes (DPPF)Pt(C6H4-4-R)2 in 80-95% yield. The com-
1
moisture and were characterized by H and 31P NMR spectro-
scopy.
1
plexes were characterized by H and 31P NMR spectroscopy
The unsymmetrical bis-aryl complex (DPPF)Pt(C6H4-4-
NMe2)(C6H4-4-CF3) (4c) was prepared by a circuitous route
shown in Scheme 3 because of the inaccessibility of either
(DPPF)Pt(C6H4-4-NMe2)Cl or (DPPF)Pt(C6H4-4-CF3)Cl by
protonolysis of (DPPF)Pt(C6H4-4-NMe2)2 or (DPPF)Pt(C6H4-
4-CF3)2. Protonolysis of L2Pt(C6H4-4-NMe2)2 (L2 ) COD or
DPPF) was fast, but not clean, while protonolysis of L2Pt(C6H4-
4-CF3)2 (L2 ) COD or DPPF), even with HCl in refluxing
benzene, was slow.
and elemental analysis, and complexes 1b and 1e, were
sufficiently soluble to be characterized by 13C NMR spectros-
copy.
3. Synthesis and Characterization of DPPF Ligated
Unsymmetrical Bis-Aryl Platinum Complexes. The synthesis
of DPPF-ligated unsymmetrical bis-aryl platinum complexes
2a-f, 3a-e, 4a,b and 5a,b is summarized in Scheme 2.
Complexes 2b-f, 3b-e, and 4a,b were prepared by Route A.
Reaction of CH3COCl with (COD)Pt(C6H4-4-R)2 (b-e) in a
mixture of CH2Cl2 and CH3OH formed (COD)Pt(C6H4-4-R)Cl
in 73-89% yield.45 Protonolysis of the aryl group of 1b-d
proceeded in 5-30 min at room temperature, but protonolysis
of the p-chlorophenyl group in 1e required refluxing in CH2Cl2
and MeOH. Reaction of the resulting aryl platinum chloride
complexes with BrMgC6H4-4-X (X ) OMe, CH3, H, Cl, F, CF3)
formed the unsymmetrical COD-ligated, bis-aryl complexes
(COD)Pt(C6H4-4-R)(C6H4-4-X) in 55-75% yield. Reaction of
DPPF with (COD)Pt(C6H4-4-R)(C6H4-4-X) in ether formed the
DPPF-ligated, unsymmetrical, bis-aryl platinum complexes
(DPPF)Pt(C6H4-4-R)(C6H4-4-X) (2b-f, 3b-e, 4a,b) in 70-
Reaction of CH3COCl with (COD)Pt(C6H4-4-OMe)2 in a
mixture of CH2Cl2 and MeOH formed (COD)Pt(C6H4-4-OMe)-
Cl in 92% yield. This complex was converted to (COD)Pt(C6H4-
4-OMe)(C6H4-4-CF3) in 72% yield by reaction with BrMgC6H4-
4-CF3. Treatment of (COD)Pt(C6H4-4-OMe)(C6H4-4-CF3) with
CH3COCl in a mixture of CH2Cl2 and MeOH then formed
(COD)Pt(C6H4-4-CF3)Cl in 94% yield. Reaction of DPPF with
(COD)Pt(C6H4-4-CF3)Cl in refluxing benzene formed (DPPF)-
Pt(C6H4-4-CF3)Cl in 87% yield, and conversion of this complex
to (DPPF)Pt(C6H4-4-CF3)(C6H4-4-NMe2) occurred upon addi-
tion of BrMgC6H4-4-NMe2 in 35% yield after recrystallization.
1
This final complex was characterized by H and 31P NMR
(45) Peters, B. T.; Zheng, Q.; Stahl, J.; Bohling, J. C.; Arif, A. M.; Hampel, F.;
Gladygz, J. A. J. Organomet. Chem. 2002, 641, 53-61.
spectroscopy.
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13018 J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004

Reductive Eliminations of Bis-Aryl Platinum Complexes
A R T I C L E S
Scheme 4
Table 1. First-Order Rate Constants for Thermal Decomposition
of Symmetrical Bis-Aryl Platinum Complexes
((DPPF)Pt(C6H4-4-R)2)a
1
5
R
complex
k ) ×
obs (s- 10-
NMe2
OMe
CH3
H
Cl
CF3
1a
1b
1c
1d
1e
1f
32.3
5.35
15.5
9.22
3.77
1.39
Figure 1. Hammett plot of rates of reductive elimination of(DPPF)Pt(C6H4-
4-R)2 (1a-f).
para substituent constants (σ-), and electrophilic substituent
constants (σ+). The correlation coefficient of the plot of Log-
(kobs/kobs(H)) vs σ shown in Figure 1 was higher than that vs σ-
or σ+. This plot had a slope of -0.9 and a correlation coefficient
of 0.88.
a Measured at 95 °C. [Pt] ) 0.011 M, [PPh3] ) 0.110 M.
4. Reductive Elimination from Bis-Aryl Platinum Com-
plexes. Heating of bis-aryl platinum complexes in toluene at
95 °C in the presence of excess of PPh3 formed the correspond-
ing biaryl products in quantitative yields (Scheme 4). Pt(PPh3)3,
which rapidly exchanges with free PPh3,46 was the predominant
metal product, as observed by 31P NMR spectroscopy, along
with small quantities of a material with a 31P NMR resonance
at δ 27.2 (JPt-P ) 4103 Hz) that we attributed to the previously
reported (DPPF)2Pt.47 Warming of the bis-aryl platinum com-
plexes in the absence of a reagent that would trap the Pt(0)
fragment formed the biaryl products in lower yields. In the
absence of added phosphine, biaryl products derived from the
phenyl groups of the phosphine were generated.16,48
Log(kobs/kobs(H)) was also plotted using dual substituent
parameters based on resonance and inductive effects.49,50 The
synthetic Hammett parameter (σj) was generated from a linear
combination of the inductive and resonance substituent constants
σI and σR with the formula σj ) (σI + λσR)/(1 + λ). In accord
with established procedures, the value of λ was generated from
the best fit to the equation Log(kobs/kobs(H)) ) Fjσj.49,50 The
correlation coefficient was somewhat higher for this plot of Log-
(kobs/kobs(H)) vs σj.
6. Rates of Reductive Elimination from Unsymmetrical,
Bis-Aryl Platinum Complexes (DPPF)Pt(C6H4-4-R)(C6H4-
4-X). To study potential synergistic effects of electron-donating
and electron-withdrawing substituents on the platinum-bound
aryl groups, we compared the rates of reductive elimination from
several series of electronically unsymmetrical bis-aryl platinum
complexes of the type (DPPF)Pt(C6H4-4-R)(C6H4-4-X). In each
series, the para substituent R was maintained constant, and the
para substituent X was varied. Reductive eliminations from all
the unsymmetrical bis-aryl platinum complexes (2a-f, 3a-e,
4a-c, 5a,b) followed first-order behavior over more than three
half-lives. The values of kobs for the reactions of complexes
(DPPF)Pt(C6H4-4-CF3)(C6H4-4-X) (1f, 2f, 3e, 4a-c), (DPPF)-
Pt(C6H4-4-NMe2)(C6H4-4-X) (1a, 2a, 3a, 4c, 5a,b), (DPPF)Pt-
(C6H4-4-CH3)(C6H4-4-X) (1c, 2a-f), and (DPPF)Pt(C6H4-4-
OMe)(C6H4-4-X) (1b, 3a-e) are listed in Tables 2-5.
5. Rates of Reductive Elimination from Symmetrical Bis-
Aryl Platinum Complexes (DPPF)Pt(C6H4-R)2. Rate constants
for reductive elimination of biaryls from bis-aryl platinum
complexes were determined from plots of the decay of the
platinum complexes vs time, generated from 1H and 19F NMR
data obtained over greater than three half-lives. The reactions
of bis-aryl complexes 1a-e, 2a-f, 3a-e, and 5a-c were
1
monitored by H NMR spectroscopy, and the reactions of 1f
and 4a,b were monitored by 19F NMR spectroscopy. Reactions
of the bis-aryl platinum complexes were first order for greater
than three half-lives. Values of kobs for reductive elimination
of complexes 1a-f are listed in Table 1. The rates of reductive
elimination of 1b and 1c were independent of the concentration
of PPh3, and we presumed that the rate for reaction of the other
derivatives was also independent of the concentration of added
phosphine.
The values of Log(kobs/kobs(H)) were plotted vs the Hammett
substituent constants σ, σ-, and σ+ for the groups X in (DPPF)-
Pt(C6H4-4-R)(C6H4-4-X). Again, the correlation coefficients
were higher for plots of Log(kobs/kobs(H)) vs σ than vs σ- or σ+.
Plots of Log(kobs/kobs(H)) vs synthetic Hammett parameters σj
generated different values of λ for the different series of
compounds with different groups R. Hence, we focused our
interpretation of the rates of the reaction of the unsymmetrical
compounds on the plots obtained by fitting Log(kobs/kobs(H)) vs
σ.
The rate constant for reductive elimination from bis-di-
methylaminophenyl complex 1a was approximately 20 times
larger than that for elimination from bis-trifluoromethylphenyl
complex 1f. The overall order of reactivity was R ) NMe2 >
CH3 > H > OMe > Cl > CF3.
Correlations between the rate constants and the electronic
parameters of the para substituents were explored by plotting
Log(kobs/kobs(H)) (kobs(H) ) kobs for 1d) vs various Hammett
parameters based on ionization of benzoic acids (σ), enhanced
(46) Sen, A.; Halpern, J. Inorg. Chem. 1980, 19, 1073-1075.
(47) Chaloner, P. A.; Broadwood-Strong, G. T. L. J. Chem. Soc., Dalton Trans.
1996, 1049-1043.
(49) Wells, P. R.; Ehernson, S.; Taft, R. W. Prog. Phys. Org. Chem. 1968, 6,
147-189.
(50) Mann, G.; Baranano, D.; Hartwig, J. F.; Rheingold, A. L.; Guzel, I. A. J.
Am. Chem. Soc. 1998, 120, 9205-9219.
(48) Kong, K. C.; Cheng, C. H. J. Am. Chem. Soc. 1991, 113, 6313-6315.
9
J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13019

A R T I C L E S
Shekhar and Hartwig
Table 2. First-Order Rate Constants for Thermal Decomposition
of Platinum Complexes of the Type
(DPPF)Pt(C6H4-4-CF3)(C6H4-4-X)a
1
5
X
complex
k ) ×
obs (s- 10-
CF3
CH3
OMe
H
Cl
NMe2
1f
2f
3f
4a
4b
4c
1.39
17.6
23.3
6.63
3.89
159
a Measured at 95 °C. [Pt] ) 0.011 M, [PPh3] ) 0.110 M.
Table 3. First-Order Rate Constants for Thermal Decomposition
of Platinum Complexes of the Type
(DPPF)Pt(C6H4-4-NMe2)(C6H4-4-X)a
1
5
X
complex
k
obs (s-
)
×
10-
Figure 2. Hammett plot of rates of reductive elimination from (DPPF)-
Pt(C6H4-4-CF3)(C6H4-4-X) (1f, 2f, 3f, 4a-c).
NMe2
CH3
OMe
H
1a
2a
3a
5a
5b
4c
32.3
33.2
32.3
30.8
52.3
Cl
CF3
159
a Measured at 95 °C. [Pt] ) 0.011 M, [PPh3] ) 0.110 M.
Table 4. First-Order Rate Constants for Thermal Decomposition
of Platinum Complexes of the Type
(DPPF)Pt(C6H4-4-CH3)(C6H4-4-X)a
1
5
X
complex
k ) ×
obs (s- 10-
CH3
NMe2
OMe
H
Cl
F
1c
2a
2b
2c
2d
2e
2f
15.5
33.3
27.1
11.9
7.19
2.29
17.6
Figure 3. Hammett plot for the rates of reductive elimination from (DPPF)-
Pt(C6H4-4-NMe2)(C6H4-4-X) (1a, 2a, 3a, 4c, 5a,b).
CF3
a Measured at 95 °C. [Pt] ) 0.011 M, [PPh3] ) 0.110 M.
A plot of Log(kobs/kobs(H)) (kobs(H) ) kobs for 5a) vs the
Hammett substituent constant (σ) of X in (DPPF)Pt(C6H4-4-
NMe2)(C6H4-4-X) is presented in Figure 3. These data are
distinct from those in Figure 2 and show that the rates of
reductive elimination from complexes with X ) NMe2, CH3,
OMe, and H are similar and that complexes with electron-
withdrawing groups X react faster than those with electron-
donating groups. For example, the rate constant for reductive
elimination from p-chlorophenyl complex 5b was 1.5 times
greater than that for reductive elimination from 1a, 2a, 3a, and
5a. The rate constant for reductive elimination from p-
trifluoromethylphenyl complex 4c was about 5 times greater
than that for elimination from dimethylaminophenyl complex
1a.
Table 5. First-Order Rate Constants for Thermal Decomposition
of Platinum Complexes of the Type
(DPPF)Pt(C6H4-4-OMe)(C6H4-4-X)a
1
5
X
complex
k ) ×
obs (s- 10-
OMe
NMe2
CH3
H
Cl
F
1b
3a
3b
3c
3d
3e
3f
5.35
32.6
27.1
16.0
7.16
3.25
23.3
CF3
a Measured at 95 °C. [Pt] ) 0.011 M, [PPh3] ) 0.110 M.
Plots of Log(kobs/kobs(H)) vs the Hammett substituent constant
(σ) of the group X in the series of compounds (DPPF)Pt(C6H4-
4-CH3)(C6H4-4-X) and (DPPF)Pt(C6H4-4-OMe)(C6H4-4-X) with
one constant substituent in the middle of the range of electronic
parameters (Figure S1 and S2) lacked a simple linear free energy
relationship for reductive elimination. However, several com-
parisons with this series of compounds will be shown in the
Discussion section to follow from the proposed origin of the
electronic effects on these reductive eliminations. Complex 2a,
which contains one p-dimethylaminophenyl group, and complex
2f, which contains one p-trifluoromethylphenyl group, were the
compounds with one p-tolyl group that underwent the fastest
reductive elimination. These two compounds reacted with nearly
the same rate constant, even though one aryl group possesses a
A plot of Log(kobs/kobs(H)) (kobs(H) ) kobs for 4a) vs the
Hammett substituent constant (σ) of X in (DPPF)Pt(C6H4-4-
CF3)(C6H4-4-X) is presented in Figure 2. These data demonstrate
a linear free energy relationship (R2 ) 0.99), with a F-value of
-1.5. The absolute value of this F-value is larger than that for
reductive elimination from the symmetrical bis-aryl platinum
complexes, even though a single aryl group is varied in this
series of compounds, instead of two aryl groups as in the series
of compounds of Figure 1. The rate constant for reductive
elimination from trifluoromethylphenyl complex 1f was 115
times smaller than that for elimination from dimethylaminophen-
yl complex 4c. The overall order of reactivity was X ) NMe2
> OMe > CH3 > H > Cl > CF3.
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Reductive Eliminations of Bis-Aryl Platinum Complexes
A R T I C L E S
strongly electron-donating substituent and the other possesses
a strongly electron-withdrawing substituent. Further, complex
3a with a p-dimethylamino group and complex 3e with a
p-trifluoromethyl group were the compounds with one p-
methoxyphenyl group that underwent the fastest reductive
elimination. In contrast to the prediction that reductive elimina-
tion will be slower from complexes with less electron-donating
groups undergoing reductive elimination, complex 3e, with one
trifluoromethyl group and one methoxy group, eliminated
roughly 5 times faster than the compound 1b with two methoxy
groups.
Discussion
Reductive Elimination of Biaryls from Bis-Aryl Platinum
Complexes. Both symmetrical and unsymmetrical bis-aryl
platinum complexes produced biaryls by C-C bond-forming
reductive elimination. Two trends were found to affect the rates
of the reductive elimination reactions. Complexes with aryl
groups containing more electron-donating substituents under-
went reductive elimination faster than complexes with aryl
groups containing more electron-withdrawing substituents, and
complexes with a greater difference in electron-donating capaci-
ties of the substituents on the two aryl groups underwent
reductive elimination faster than those with a smaller difference
in electron-donating capacities of the substituents on the two
aryl groups. As a result of these two effects, the influence of
the electronic properties of one aryl group on the rate of
reductive elimination depended on the electronic properties the
second aryl group.
Reductive Elimination from Symmetrical Bis-Aryl Plati-
num Complexes (DPPF)Pt(C6H4-4-X)2. The negative F-value
for the Hammett plot of the reductive elimination from
symmetrical complexes (Figure 1), which indicates faster
reaction of complexes with more electron-rich aryl groups, is
consistent with theoretical calculations10,11 and the previous
experimental observation that (PPh3)2Pt(C6H4-4-CH3)2 under-
went reductive elimination of biaryl faster than (PPh3)2Pt-
(C6H5)2.53 A rationalization for the faster reaction of complexes
with more electron-donating covalent ligands can be made by
comparing the metal-carbon bond strengths of the symmetric
bis-aryl platinum complexes. The strengths of M-X covalent
bonds tend to be greater to groups that possess stronger H-X
bonds.9,25,26,54-58 Although data on C-H bond strengths of
substituted arenes are limited, the C-H bond strengths in
substituted toluenes increase with an increase in the electron-
withdrawing capacity of the para substituent on toluene.59
Further, the strengths of the M-X bonds tend to be more
sensitive to inductive effects than the strengths of the H-X
bond.25 Thus, the strengths of the platinum-aryl bonds in
Figure 4. Qualitative energy diagram for the reductive elimination from
symmetrical bis-aryl platinum complexes.
complexes with electron-withdrawing groups will be greater than
those in complexes with electron-donating groups on the aryl
ring.
A qualitative free energy diagram for reductive eliminations
from complexes 1a and 1f that results from this analysis is
shown in Figure 4. If the difference in Pt-C bond strengths in
complexes 1f and 1a is larger than the difference in energies of
the transition states as shown, then reductive elimination from
complex 1f will be slower than reductive elimination from
complex 1a. This argument has been used previously to
rationalize the faster rate of reductive elimination of arene from
(C5Me5)Rh(PMe3)(3,5-C6H3(t-Bu)2)(H) than from (C5Me5)Rh-
(PMe3)(3,5-C6H3(CF3)2(H).9
Electronic Effects on the Reductive Elimination from
(DPPF)Pt(C6H4-4-CF3)(C6H4-4-X) Complexes. The slope of
the Hammett plot for reductive elimination from a series of bis-
aryl platinum complexes (DPPF)Pt(C6H4-4-CF3)(C6H4-4-X)
containing one p-trifluoromethylphenyl group and one variable
aryl group was also negative (Figure 2), but the absolute value
(-1.5) was larger than that of the F-value of the Hammett plot
derived from reactions of symmetric bis-aryl complexes. As
noted in the Results section, the F-value of the plot in Figure 2
is larger than that in Figure 1, even though the differences in
rate constants result from changes to a single aryl group, rather
than to changes to both aryl groups. This larger F-value from
the plot in Figure 2 provided the first evidence that the rates of
reductive elimination result from factors other than the sum of
the electronic parameters of the two aryl groups on platinum.
A qualitative energy diagram that depicts graphically our
proposed origin of the difference in rates for reductive elimina-
tion from (DPPF)Pt(C6H4-4-CF3)2 (1f) and (DPPF)(C6H4-4-
CF3)(C6H4-4-NMe2) (4c) is shown in Figure 5. We propose that
the large difference in rate between reductive elimination from
the mixed trifluoromethylphenyl dimethylaminophenyl complex
4c and from the symmetrical bis-trifluoromethylphenyl complex
1f results from a combination of significant differences in
energies of both the ground states and transition states. Just as
the ground state of bis-dimethylaminophenyl complex 1a surely
lies at higher energy than the ground state of bis-trifluorom-
ethylphenyl complex 1f, the ground state of 4c would lie at
higher energy than the ground state of 1f because of the presence
of a more electron-donating substituent on one of the aryl
groups. However, it does not seem reasonable to attribute the
(51) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195.
(52) Connors, K. A. Chemical Kinetics. The Study of Reaction Rates in Solution;
John Wiley & Sons: New York, 1990.
(53) Komiya, S.; Abe, Y.; Yamamoto, A.; Yamamoto, T. Organometallics 1983,
2, 1466-1468.
(54) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
3rd ed.; John Wiley & Sons: New York, 2001.
(55) Bulls, A. R.; Bercaw, J. E.; Manriquez, J. M.; Thompson, M. E. Polyhedron
1988, 7, 1409-1428.
(56) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701-7715.
(57) Clot, E.; Oelckers, B.; Klahn, A. H.; Eisenstein, O.; Perutz, R. N. Dalton
Trans. 2003, 4065-4074.
(58) Clot, E.; Besora, M.; Maseras, F.; Megret, C.; Eisenstein, O.; Oelckers,
B.; Perutz, R. N. Chem. Commun. 2003, 490-491.
(59) Luo, Y. Handbook Of Bond Dissociation Energies In Organic Compounds;
CRC Press: Boca Raton, FL, 2003.
9
J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13021

A R T I C L E S
Shekhar and Hartwig
Figure 5. Qualitative energy diagram for the comparison of the rates of reductive elimination from (DPPF)Pt(C6H4-4-CF3)2 and (DPPF)Pt(C6H4-4-CF3)-
(C6H4-4-NMe2).
Figure 6. Qualitative energy diagram for the comparison of the rates of reductive elimination from (DPPF)Pt(C6H4-4-NMe2) and (DPPF)Pt(C6H4-4-CF3)-
(C6H4-4-NMe2).
large difference between the rate constants for reaction of 1f
and 4c to ground-state effects alone. The difference between
the rates of reaction of mixed trifluoromethylphenyl dimethyl-
aminophenyl complex 4c and the symmetric bis-trifluorometh-
ylphenyl complex 1f is larger than the difference between the
rates of reaction of the symmetric dimethylamino- and trifluoro-
methyl-substituted 1a and 1f, even though 4c and 1f differ by
a single aryl group instead of two.
Thus, we propose that the difference in the substituents in 1f
and 4c leads not only to a higher ground-state energy of 4c but
also to a substantially lower energy transition state for reaction
of 4c. In this case, the F-value for reaction of the unsymmetrical
complexes with one trifluoromethylphenyl group would be
larger than the F-value for reaction of the symmetrical com-
plexes because of two reinforcing effects: the electron-donating
group in 4c would destabilize the ground state by making the
metal-ligand bond weaker and would stabilize the transition
state by creating a larger difference in electronic properties
between the trifluoromethylphenyl group and the second aryl
group.
complexes with one dimethylaminophenyl group, increased
electron-donating properties of an aryl group would give rise
to two counteracting effects, depicted graphically in Figure 6.
Increased electron-donating properties of the second aryl group
would lead to higher ground-state energies of the reactants for
reasons noted above, and this effect would accelerate the
reaction. However, increased electron-donating properties of the
second aryl group would also lead to higher energies of the
transition states in this case because of decreased differences
between the electronic properties of the dimethylaminophenyl
group and the variable group. This effect would retard the
reaction.
Therefore, the differences in rates that would result from
changes in the electronic properties of the aryl group among
this series of compounds would be small, and the Hammett plot
for reductive elimination of a series of complexes with one
dimethylaminophenyl group and one variable group would have
a slope that is smaller in magnitude than that of the plots in
Figures 1 and 2. The slope of the plot would be positive if the
accelerating effect of two electronically distinct aryl groups is
larger than the accelerating effect of an electron-donating group
and would be negative if the accelerating effect of two
electronically distinct aryl groups is smaller than the accelerating
effect of an electron-donating group.
Reductive Elimination from (DPPF)Pt(C6H4-4-NMe2)-
(C6H4-4-X) Complexes. Our proposed combination of electronic
effects suggests that the influence of substituents in complexes
with one constant dimethylaminophenyl group will be different
from the influence of substituents in complexes with one
constant trifluoromethylphenyl group. Among a series of
The Hammett plot generated from the rate constants for
reductive elimination from the series of compounds with one
9
13022 J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004

Reductive Eliminations of Bis-Aryl Platinum Complexes
A R T I C L E S
dimethylaminophenyl group and one variable group did display
a small positive slope (Figure 3). Most striking, the compound
with two electron-donating dimethylaminophenyl groups reacted
slower than the compound with one electron-donating dimethyl-
aminophenyl group and one electron-withdrawing trifluorom-
ethylphenyl group. This decelerating effect of the increasing
electron-donating property of substituents in this series of
compounds is the opposite of the effect of electron-donating
substituents in the series of compounds with a constant
trifluoromethylphenyl group or in the series of symmetrical
compounds.
Conclusions. We have demonstrated two distinct electronic
effects on reductive elimination of biaryls from bis-aryl platinum
compounds. One effect was predicted by theoretical methods
and causes reductive elimination to be faster from compounds
with more electron-donating groups. A second effect had not
been identified clearly and causes reductive elimination to be
faster from compounds with two aryl groups possessing more
dissimilar electronic properties. This second effect appears to
be roughly equal in magnitude to the more commonly cited
effect of faster elimination from compounds with more donating
groups. These two effects lead to complex, though interpretable,
linear free energy relationships.
Thus, the electronic effects on reductive elimination are
synergistic; the properties of one group dictate the effect of
electronic perturbations of a second group. A large increase in
the rate of reductive elimination from (DPPF)Pt(C6H4-4-CF3)-
(C6H4-4-X) was observed when X was more electron-donating,
but small differences in the rates of reductive elimination were
observed from (DPPF)Pt(C6H4-4-NMe2)(C6H4-4-X) when X was
more electron-donating. Further, the versions of (DPPF)Pt(C6H4-
4-NMe2)(C6H4-4-X) with the most electron-donating groups X
eliminated slower than those with the most electron-withdrawing
groups X. In the first set of compounds, the more electron-
donating X groups destabilized the ground state and stabilized
the transition state, and this distinct effect on the ground- and
transition-state energies led to large differences in the rate of
reaction. In the second set of compounds, the electron-donating
groups destabilized the ground state and the transition state,
and this similar effect on both ground and transition states led
to small differences in reaction rates.
The slower reductive elimination from compounds with more
electron-withdrawing groups is most likely due to ground-state
effects, such as stronger metal-ligand bonds in the reactants.
Metal-ligand covalent bonds tend to be stronger to groups that
are less electron-donating than to similar groups that are more
electron-donating. In contrast, the increased rates for reductive
elimination from compounds with larger differences in the
properties of the two aryl groups most likely result from
transition-state effects. To explain the difference in reaction rates
with ground-state arguments, one must invoke unfavorable
thermodynamics for a compound with one donating and one
withdrawing group, and such a proposal seems unlikely. Instead,
the favorable pairing of an electron-rich and electron-poor group
lowers the energy of the transition state for reductive elimination,
and this favorable pairing of two groups with discrete electronic
properties may be considered to parallel the favorable pairing
of a nucleophile and electrophile in many organic reactions or
the favorable pairing of an electron-poor olefin with an electron-
rich diene in a Diels-Alder reaction. Because many organo-
metallic reactions involve bond formation between two ligands,
these synergistic electronic effects may contribute to the trends
in reactivity among many sets of organometallic compounds.
The small differences in rate constants combined with subtle
differences in the relative magnitudes of the two compensating
effects on the rate of reductive elimination of individual
compounds makes the relationship between the rate constants
and the σ-values less linear for the series of compounds with a
dimethylaminophenyl group than for the series of compounds
with larger differences in rate constants for reductive elimination.
Nevertheless, the difference in magnitude and direction of the
electronic effects on the rate of reductive elimination from
compounds with one constant dimethylaminophenyl group and
on the rate of reductive elimination from compounds with one
constant trifluoromethyl group or with two equivalent aryl
groups is striking.
Reductive Elimination from (DPPF)Pt(C6H4-4-CH3)(C6H4-
4-X) and (DPPF)Pt(C6H4-4-OMe)(C6H4-4-X) Complexes. The
proposal that both the overall electron-donating ability of an
aryl group and the difference in electron-donating ability of two
aryl groups affect the rate of reductive elimination also predicts
that the electronic effects of substituents in a series of
compounds with one aryl group containing a constant substitu-
ent, such as methyl or methoxy, that lies in the middle of the
spectrum of donating and withdrawing groups should lead to a
complex Hammett plot. The number of compounds that could
be prepared with substituents that are more donating and less
donating than a methyl or methoxy group were too small to
examine in detail the rates of reductive elimination from such
series of compounds.
Nevertheless, it was clear that the compounds in these series
that eliminated the fastest did not possess the most negative
σ-values. Instead, compounds 2a with one trifluoromethylphenyl
group and 2f with one dimethylaminophenyl group reacted with
nearly the same rate constants, and 3a and 3f that similarly
contained one trifluoromethylphenyl or one dimethylamino-
phenyl group reacted with nearly the same rate constants.
Moreover, the rate constants for reaction of compounds 2a and
2f and for reaction of 3a and 3f were among the largest of the
series of compounds that contain one p-tolyl or one p-
methoxyphenyl aryl group, and compounds 2a and 2f and 3a
and 3f contained the largest difference in electronic properties
between the variable group and the common p-tolyl or p-
methoxyphenyl group.
Compounds 2a and 2f reacted with nearly the same rate
constant, despite the disparate electronic properties of the two
variable substituents, and compound 3f with one trifluoro-
methylphenyl group reacted faster than compound 1b with two
p-methoxyphenyl groups. These results are, again, consistent
with a large influence of the difference in electronic properties
of the two aryl groups on the energy of the transition state for
reductive elimination.
Experimental Section
General Methods. Unless otherwise noted, all reactions and
manipulations were performed in an inert atmosphere glovebox. All
31P{1H} NMR chemical shifts are reported in parts per million relative
to an 85% H3PO4 external standard. Shifts downfield of the standard
are reported as positive. Toluene, benzene, ether, and pentane were
distilled from sodium/benzophenone prior to use. Dichloromethane was
distilled from calcium hydride. Absolute methanol was used as obtained.
9
J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13023

A R T I C L E S
Shekhar and Hartwig
Phenylmagnesium bromide, 4-methylphenylmagnesium bromide, 4-meth-
oxyphenylmagnesium bromide, 4-chlorophenylmagnesium bromide,
4-fluorophenylmagnesium bromide, N,N-dimethylaminophenylmagne-
sium bromide, and 1,5-cyclooctadiene were all purchased from Aldrich
Chemicals. Potassium tetrachloroplatinate and DPPF were obtained
from Strem Chemicals. 4-Trifluorophenylmagnesium bromide60 and 1,5-
cyclooctadieneplatinumdichloride61 were synthesized by literature
procedures.
Me-H). 31P{1H} NMR (202 MHz, benzene): δ 15.51 (s, JPtP ) 1772
Hz). Anal. Calcd for C48H42FeP2Pt: C, 61.87; H, 4.54. Found: C, 62.12;
H, 4.56.
Synthesis of (DPPF)Pt(C6H5)2 (1d).62 (COD)PtCl2 (0.205 g, 0.549
mmol) and C6H5MgBr (1.2 mL, 1.2 mmol) in 30 mL of ether gave
0.181 g (72%) of (COD)Pt(C6H5)2. DPPF (0.092 g, 0.166 mmol) and
(COD)Pt(C6H5)2 (0.076 g, 0.150 mmol) in 10 mL of benzene gave 0.131
1
g (87%) of (DPPF)Pt(C6H5)2. H NMR (400 MHz, CDCl3): δ 7.63-
General Procedure for the Preparation of Symmetrical Bis-Aryl
Platinum Complexes of the Type (DPPF)Pt(C6H4-4-R)2. In a drybox,
(COD)PtCl2 was weighed into a 100 mL round-bottom flask. A stir
bar was added, followed by 30 mL of ether. The Grignard reagent (2.2
equiv) was added, and the reaction mixture was allowed to stir at room
temperature for 12 h. The reaction mixture was brought outside the
glovebox, and saturated aqueous NH4Cl (20 mL) was added. The ether
layer was separated, and the water layer was extracted with ether. The
combined ether extracts were dried over anhydrous MgSO4 and filtered
through a 1 cm pad of Celite and a 2 cm pad of decolorizing carbon.
Ether was evaporated from the filtrate under reduced pressure. The
white solid obtained was recrystallized from a mixture of dichlo-
romethane and pentane.
(COD)Pt(C6H4-4-R)2, obtained in the above step, was weighed into
a screw-capped vial. A stir bar was added, followed by 10 mL of
benzene. DPPF (1 equiv) was added, and the mixture was allowed to
stir for 1 h at room temperature. The solvent was evaporated under
reduced pressure. The yellow solid obtained was recrystallized from a
mixture of dichloromethane and pentane.
Synthesis of (DPPF)Pt(C6H4-4-NMe2)2 (1a). (COD)PtCl2 (0.418
g, 1.12 mmol) and 4-NMe2-C6H4MgBr (4.50 mL, 2.25 mmol) in 30
mL of ether gave 0.347 g (59%) of (COD)Pt(C6H4-4-NMe2)2. DPPF
(0.097 g, 0.175 mmol) and (COD)Pt(C6H4-4-NMe2)2 (0.089 g, 0.159
mmol) in 10 mL of benzene gave 0.165 g (95%) of (DPPF)Pt(C6H4-
4-NMe2)2. 1H NMR (400 MHz, CDCl3): δ 7.49-7.44 (m, 10H, DPPF),
7.29-7.25 (m, 4H, DPPF), 7.17-7.15 (m, 6H, DPPF), 6.74-6.55 (m,
4H, Pd-Ar), 6.03 (d, 4H, J ) 7.6 Hz, Pd-Ar), 4.23-4.21 (b, 4H, Cp-
H), 4.14-4.12 (b, 4H, Cp-H), 2.52 (s, 12H, NMe2-H). 31P{1H} NMR
(202 MHz, benzene): δ 15.27 (s, JPtP ) 1772 Hz). Anal. Calcd for
C50H48N2FeP2Pt: C, 60.67; H, 4.89; N, 2.83. Found: C, 60.39; H, 4.81;
N, 2.62.
7.59 (m, 2H, DPPF), 7.54-7.45 (m, 8H, DPPF), 7.37-7.35 (m, 4H,
DPPF), 7.29-7.25 (m, 8H, DPPF), 6.96-6.82 (m, 4H, Pd-Ar), 6.39-
6.36 (m, 4H, Pd-Ar), 4.33 (b, 4H), 4.24 (b, 4H). 31P{1H} NMR (121
MHz, CDCl3): δ 15.56 (s, JPtP ) 1784 Hz). Anal. Calcd for C46H38-
FeP2Pt: C, 61.14; H, 4.24. Found: C, 61.01; H, 4.34.
Synthesis of (DPPF)Pt(C6H4-4-Cl)2 (1e). (COD)PtCl2 (0.441 g, 1.18
mmol) and 4-Cl-C6H4MgBr (2.5 mL, 2.5 mmol) in 30 mL of ether
gave 0.455 g (73%) of (COD)Pt(C6H4-4-Cl)2. DPPF (0.136 g, 0.245
mmol) and (COD)Pt(C6H4-4-Cl)2 (0.114 g, 0.222 mmol) in 10 mL of
benzene gave 0.209 g (88%) of (DPPF)Pt(C6H4-4-Cl)2. 1H NMR (400
MHz, CDCl3): δ 7.43-7.40 (m, 8H, DPPF), 7.31-7.28 (m, 4H,
DPPF), 7.22-7.17 (m, 8 H, DPPF), 6.73-6.62 (m, 4H, Pd-Ar), 6.36
(d, 4H, J ) 7.9 Hz, Pd-Ar), 4.23 (b, 4H, Cp-H), 4.15 (b, 4H, Cp-H).
31P{1H} NMR (121 MHz, benzene): δ 15.17 (s, JPtP ) 2566 Hz). 13C-
{1H} NMR (126 MHz, CDCl3): δ 73.06 (app t, 2.89 Hz), 74.95 (app
t, 4.90 Hz), 75.95 (m), 126.89 (s), 127.13 (app t, 3.02 Hz), 128.07
(app t, 5.03 Hz, JPtC ) 33.58 Hz), 130.24 (s), 133.66 (dd, 54.7 Hz,
3.77 Hz), 134.765 (app t, 5.28 Hz), 137.10 (b, JPtC ) 33.96 Hz), 158.49
(dd, 115.95 Hz, 13.21 Hz). Anal. Calcd for C46H36Cl2FeP2Pt: C, 56.81;
H, 3.73. Found: C, 56.90; H, 3.69.
Synthesis of (DPPF)Pt(C6H4-4-CF3)2 (1f). (COD)PtCl2 (0.107 g,
0.286 mmol) and 4-CF3-C6H4MgBr (3.5 mL, 0.64 mmol) in 30 mL of
ether gave 0.118 g (69%) of (COD)Pt(C6H4-4-CF3)2. DPPF (0.098 g,
0.177 mmol) and (COD)Pt(C6H4-4-CF3)2 (0.099 g, 0.161 mmol) in 10
mL of benzene gave 0.153 g (83%) of (DPPF)Pt(C6H4-4-CF3)2. 1H
NMR (400 MHz, CDCl3): δ 7.45-7.38 (m, 8H, DPPF), 7.32-7.29
(m, 4H, DPPF), 7.23-7.19 (m, 8H, DPPF), 6.97-6.83 (m, 4H, Pd-
Ar), 6.58 (d, 4H, J ) 7.6 Hz, Pd-Ar), 4.26 (b, 4H, Cp-H), 4.17 (b, 4H,
Cp-H). 31P{1H} NMR (202 MHz, benzene): δ 15.44 (s, JPtP ) 1856
Hz). Anal. Calcd for C48H36F6FeP2Pt: C, 55.44; H, 3.49. Found: C,
55.43; H, 3.26.
Synthesis of (DPPF)Pt(C6H4-4-OMe)2 (1b). (COD)PtCl2 (0.593 g,
1.59 mmol) and 4-OMe-C6H4MgBr (7.0 mL, 3.5 mmol) in 30 mL of
ether gave 0.597 g (70%) of (COD)Pt(C6H4-4-OMe)2. DPPF (0.110 g,
0.198 mmol) and (COD)Pt(C6H4-4-OMe)2 (0.098 g, 0.181 mmol) in
10 mL of benzene gave 0.171 g (94%) of (DPPF)Pt(C6H4-4-OMe)2.
1H NMR (400 MHz, C6D6): δ 7.44-7.38 (m, 8H, DPPF), 7.23-7.16
(m, 4H, DPPF), 6.72-6.65 (m, 12H, DPPF and Pd-Ar), 6.05 (d, 4H,
J ) 7.2 Hz, Pd-Ar), 4.32 (b, 4H, Cp-H), 4.24 (b, 4H, Cp-H), 3.51 (s,
General Procedure for the Synthesis of Unsymmetrical Bis-Aryl
Platinum Complexes of the Type (DPPF)Pt(C6H4-4-R)(C6H4-4-X)
(2b-f, 3b-e, 4a-b) via Route A (Scheme 2). (COD)Pt(C6H4-4-R)2
was taken in a screw-capped vial. A stir bar was added, followed by a
mixture of 5 mL of CH2Cl2 and 5 mL of anhydrous MeOH. CH3COCl
(1 equiv) was added, and the reaction mixture was stirred for 10-30
min at room temperature or at 70 °C, depending upon the nature of R.
The solvent was evaporated under reduced pressure. The white solid
obtained was recrystallized from a mixture of dichloromethane and
pentane. In a drybox, (COD)Pt(C6H4-4-R)Cl obtained above was taken
in a 100 mL round-bottom flask. A stir bar was added, followed by 30
mL of ether. The Grignard reagent (2.2 equiv) was added, and the
reaction mixture was allowed to stir for 12 h at room temperature. The
reaction mixture was brought outside the glovebox, and saturated
aqueous NH4Cl (20 mL) was added. The ether layer was separated,
and the water layer was extracted with ether. The combined ether
extracts were dried over anhydrous MgSO4 and filtered through a 1
cm pad of Celite and a 2 cm pad of decolorizing carbon. Ether was
removed from the filtrate under reduced pressure. The white solid
obtained was recrystallized from a mixture of dichloromethane and
pentane. (COD)Pt(C6H4-4-R)(C6H4-4-X) obtained above was weighed
into a screw-capped vial. A stir bar was added, followed by 10 mL of
benzene. DPPF (1 equiv) was added to the above reaction mixture and
allowed to stir for 1 h at room temperature. The solvent was evaporated
6H, OMe-H). 31P{1H} NMR (202 MHz, benzene): δ 15.27 (s, JPtP
)
1796 Hz). 13C{1H} NMR (126 MHz, CDCl3): δ 55.38 (s), 72.77 (app
t, J ) 2.76 Hz), 74.89 (app t, J ) 4.78 Hz), 76.80 (m), 113.58 (b),
127.82 (app t, J ) 4.78 Hz), 129.85 (s), 134.22 (m), 134.92 (app t, J
) 4.78 Hz), 136.08 (b, JPtC ) 36.35 Hz), 150.60 (dd, J ) 102.63 Hz,
13.33 Hz), 154.74 (s). Anal. Calcd for C48H42O2FeP2Pt: C, 59.82; H,
4.39. Found: C, 59.96; H, 4.26.
Synthesis of (DPPF)Pt(C6H4-4-CH3)2 (1c). (COD)PtCl2 (0.505 g,
1.35 mmol) and 4-CH3-C6H4MgBr (3.0 mL, 3.0 mmol) in 30 mL of
ether gave 0.372 g (57%) of (COD)(C6H4-4-CH3)2. DPPF (0.059 g,
0.106 mmol) and (COD)Pt(C6H4-4-CH3)2 (0.045 g, 0.096 mmol) in 10
1
mL of benzene gave 0.102 g (95%) of (DPPF)Pt(C6H4-4-CH3)2. H
NMR (400 MHz, C6D6): δ 7.73-7.72 (m, 8H, DPPF), 7.33-7.31 (m,
4H, DPPF), 7.02-7.01 (m, 12H, DPPF and Pd-Ar), 6.62 (d, 4H, J )
7.3 Hz, Pd-Ar), 4.40 (b, 4H, Cp-H), 3.87 (b, 4H, Cp-H), 2.04 (s, 6H,
(60) Sawamura, M.; Iikura, H.; Hirai, A.; Nakamura, E. J. Am. Chem. Soc. 1998,
120, 8285.
(61) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem. Soc. 1976,
98, 6521-6528.
(62) Colacot, T. J.; Teichman, R. A.; Cea-Olivares, R.; Alvaradao-Rodriguez,
J. G.; Toscano, R. A.; Boyko, W. J. J. Orgnaomet. Chem. 1998, 557, 169.
9
13024 J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004

Reductive Eliminations of Bis-Aryl Platinum Complexes
A R T I C L E S
under reduced pressure. The yellow solid obtained was recrystallized
from a mixture of dichloromethane and pentane.
1H NMR (400 MHz, C7D8): δ 7.70-7.64 (m, 8H, DPPF), 7.55-7.52
(m, 4H, DPPF), 7.47-7.42 (m, 8H, DPPF), 7.19-7.16 (m, 2H, Pd-
Ar), 6.93-6.90 (m, 2H, Pd-Ar), 6.81 (d, 2H, J ) 6.8 Hz, Pd-Ar), 6.49
(d, 2H, J ) 6.8 Hz, Pd-Ar), 4.55-4.53 (m, 2H, Cp-H), 4.45-4.32 (m,
6H, Cp-H), 2.09 (s, 3H, Me-H). 31P{1H} NMR (202 MHz, toluene): δ
Synthesis of (COD)Pt(C6H4-4-CH3)Cl.45 The above general pro-
cedure was followed using (COD)Pt(C6H4-4-CH3)2 (0.204 g, 0.421
mmol) and CH3COCl (30 µL, 0.42 mmol) in a mixture of 5 mL of
dichloromethane and 5 mL of methanol to give 0.132 g (73%) of
(COD)Pt(C6H4-4-CH3)Cl.
16.19 (d, JPP ) 14 Hz, JPtP ) 1896 Hz), 14.65 (d, JPP ) 14 Hz, JPtP
)
1930 Hz).
Synthesis of (COD)Pt(C6H4-4-OMe)(Cl). The above general pro-
cedure was followed with (COD)Pt(C6H4-4-OMe)2 (0.294 g, 0.568
mmol) and CH3COCl (40 µL, 0.57 mmol) in a mixture of 5 mL of
dichloromethane and 5 mL of methanol to give 0.202 g (80%) of
(COD)Pt(C6H4-4-OMe)Cl.
Synthesis of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-OMe) (2b). 4-OMe-
C6H4MgBr (0.50 mL, 0.25 mmol) and (COD)Pt(C6H4-4-CH3)Cl (0.098
g, 0.228 mmol) in 30 mL of ether gave 0.062 g (54%) of (COD)Pt-
(C6H4-4-CH3)(C6H4-4-OMe). DPPF (0.068 g, 0.123 mmol) and (COD)-
Pt(C6H4-4-CH3)(C6H4-4-OMe) (0.056 g, 0.118 mmol) in 10 mL of
benzene gave 0.099 g (87%) of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-OMe).
1H NMR (400 MHz, CDCl3): δ 7.45-7.39 (m, 8H, DPPF), 7.29-
7.25 (m, 4H, DPPF), 7.15-7.20 (m, 8H, DPPF), 6.73-6.56 (b, 4H,
Pd-Ar), 6.22 (d, 2H, J ) 7.2 Hz, Pd-Ar), 6.06 (d, 2H, J ) 7.6 Hz,
Pd-Ar), 4.22-4.21 (m, 4H, Cp-H), 4.14-4.13 (m, 4H, Cp-H), 3.44 (s,
3H, OMe-H), 1.87 (s, 3H, Me-H). 31P{1H} NMR (202 MHz, ben-
zene): δ 15.19 (s, JPtP ) 1778 Hz). Anal. Calcd for C48H42OFeP2Pt:
C, 60.83; H, 4.47. Found: C, 60.72; H, 4.26.
Synthesis of (DPPF)Pt(C6H4-4-CH3)(C6H5) (2c). C6H5MgBr (0.55
mL, 0.55 mmol) and (COD)Pt(C6H4-4-CH3)Cl (0.209 g, 0.486 mmol)
in 30 mL of ether gave 0.175 g (74%) of (COD)Pt(C6H4-4-CH3)(C6H5).
DPPF (0.218 g, 0.393 mmol) and (COD)Pt(C6H4-4-CH3)(C6H5) (0.175
g, 0.371 mmol) in 10 mL of benzene gave 0.307 g (91%) of (DPPF)-
Pt(C6H4-4-CH3)(C6H5). 1H NMR (400 MHz, CDCl3): δ 7.51-7.44 (m,
8H, DPPF), 7.35-7.16 (m, 12H, DPPF), 6.93-6.79 (m, 2H, Pd-Ar),
6.79-6.66 (m, 2H, Pd-Ar), 6.45-6.42 (m, 2H, Pd-Ar), 6.37-6.33 (m,
1H, Pd-Ar), 6.29-6.27 (m, 2H, Pd-Ar), 4.30-4.29 (m, 4H, Cp-H), 4.21
(s, 4H, Cp-H), 1.93 (s, 3H, Me-H). 31P {1H} NMR (202 MHz,
Synthesis of (DPPF)Pt(C6H4-4-OMe)(C6H5) (3b). C6H5MgBr (0.60
mL, 0.60 mmol) and (COD)Pt(C6H4-4-OMe)Cl (0.228 g, 0.511 mmol)
in 30 mL of ether gave 0.216 g (87%) of (COD)Pt(C6H4-4-OMe)(C6H5).
DPPF (0.271 g, 0.489 mmol) and (COD)Pt(C6H4-4-OMe)(C6H5) (0.216
g, 0.443 mmol) in 10 mL of benzene gave 0.373 g (90%) of (DPPF)-
Pt(C6H4-4-OMe)(C6H5). 1H NMR (400 MHz, C7D8): δ 7.55-7.54 (m,
10H, DPPF), 7.44-7.29 (m, 10H, DPPF), 6.96-6.73 (m, 4H, Pd-Ar),
6.49-6.41 (m, 3H, Pd-Ar), 6.18-6.17 (m, 2H, Pd-Ar), 4.35 (b, 4H,
Cp-H), 4.25 (b, 4H, Cp-H), 3.55 (s, 3H, OMe-H). 31P{1H} NMR (121
MHz, toluene): δ 15.37 (d, JPP ) 15 Hz, JPtP ) 1820 Hz), 15.06 (d,
JPP ) 15 Hz, JPtP ) 1752 Hz). 13C{1H} NMR (126 MHz, CDCl3): δ
55.41 (s), 72.82 (d, J ) 5.9 Hz), 74.14 (d, J ) 15.72 Hz), 74.86 (s),
74.94 (s), 76.71 (dd, J ) 50.56 Hz, 9.56 Hz), 76.86 (dd, J ) 50.56
Hz, 7.42 Hz), 127.20 (dd, J ) 5.9 Hz, 2.01 Hz), 127.86 (dd, J ) 9.56
Hz, 2.65 Hz, JPtC ) 35.59 Hz), 128.71 (dd, J ) 47.54 Hz, 5.03 Hz),
128.87 (s), 128.94 (s), 129.87 (s), 130.64 (b), 133.76 (s), 133.91 (s),
134.29 (dd, J ) 48.67 Hz, 7.54 Hz) 134.92 (m), 136.40 (dd, J ) 34.84
Hz, 1.88 Hz), 139.30 (s), 139.38 (s), 150.26 (dd, J ) 113.19 Hz, 11.44
Hz), 154.79 (s). Anal. Calcd for C47H40OFeP2Pt: C, 60.46; H, 4.32.
Found: C, 60.20; H, 4.07.
benzene): δ 15.11 (d, JPtP ) 1772 Hz, JPP ) 14 Hz). 16.76 (d, JPtP
)
1772 Hz, JPP ) 14 Hz). Anal. Calcd for C48H42OFeP2Pt: C, 60.83; H,
4.47. Found: C, 60.72; H, 4.26.
Synthesis of (DPPF)Pt(C6H4-4-OMe)(C6H4-Cl) (3c). 4-Cl-C6H4-
MgBr (0.42 mL, 0.42 mmol) and (COD)Pt(C6H4-4-OMe)Cl (0.180 g,
0.404 mmol) in 30 mL of ether gave 0.113 g (54%) of (COD)Pt(C6H4-
4-OMe)(C6H4-4-Cl). DPPF (0.083 g, 0.150 mmol) and (COD)Pt(C6H4-
4-OMe)(C6H4-4-Cl) (0.068 g, 0.130 mmol) in 10 mL of benzene gave
Synthesis of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-Cl) (2d). 4-Cl-C6H4-
MgBr (0.25 mL, 0.25 mmol) and (COD)Pt(C6H4-4-CH3)Cl (0.082 g,
0.191 mmol) in 30 mL of ether gave 0.045 g (46%) of (COD)Pt(C6H4-
4-CH3)(C6H4-4-Cl). DPPF (0.053 g, 0.096 mmol) and (COD)Pt(C6H4-
4-CH3)(C6H4-4-Cl) (0.037 g, 0.087 mmol) in 10 mL of benzene gave
1
0.112 g (89%) of (DPPF)Pt(C6H4-4-OMe)(C6H4-4-Cl). H NMR (400
1
0.073 g (87%) of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-Cl). H NMR (400
MHz, C7D8): δ 7.52-7.47 (m, 8H, DPPF), 7.39-7.34 (m, 5H, DPPF),
7.29-7.26 (m, 7H, DPPF), 6.79-6.76 (t, 2H, Pd-Ar), 6.71-6.67 (m,
2H, Pd-Ar), 6.43 (d, 2H, J ) 7.8 Hz, Pd-Ar), 6.13 (d, 2H, J ) 7.8 Hz,
Pd-Ar), 4.33-4.30 (m, 2H, Cp-H), 4.29-4.27 (m, 2H, Cp-H), 4.24-
4.22 (m, 4H, Cp-H), 3.52 (s, 3H, OMe-H). 31P{1H} NMR (121 MHz,
MHz, C7D8): δ 7.45-7.39 (m, 10H, DPPF), 7.31-7.26 (m, 5H, DPPF),
7.24-7.16 (m, 5H, DPPF), 6.77-6.68 (m, 2H, Pd-Ar), 6.68-6.57 (m,
2H, Pd-Ar), 6.34 (d, 2H, J ) 7.5 Hz, Pd-Ar), 6.23 (d, 2H, J ) 6.5 Hz,
Pd-Ar), 4.24 (d, 2H, J ) 1.5 Hz, Cp-H), 4.21 (d, 2H, J ) 1.5 Hz,
Cp-H), 4.14 (s, 4H, Cp-H), 1.88 (s, 3H, Me-H). 31P{1H} NMR (121
MHz, toluene): δ 15.41 (d, JPP ) 14 Hz, JPtP ) 1778 Hz), 15.02 (d,
JPP ) 14 Hz, JPtP ) 1884 Hz). Anal. Calcd for C47H39ClFeP2Pt: C,
59.29; H, 4.13. Found: C, 59.09; H, 3.87.
toluene): δ 15.94 (d, JPP ) 14 Hz, JPtP ) 1774 Hz), 15.18 (d, JPP
)
14 Hz, JPtP ) 1806 Hz). 13C{1H} NMR (126 MHz, CDCl3): δ 54.92
(s), 72.45 (d, J ) 5.9 Hz), 72.50 (d, J ) 5.9 Hz), 74.41 (s), 74.52 (s),
75.74 (dd, J ) 45.28 Hz, 5.16 Hz), 76.10 (dd, J ) 46.53 Hz, 11.44
Hz), 113.20 (d, J ) 4.78 Hz, JPtC ) 100.62 Hz), 126.55 (d, J ) 6.54
Hz, JPtC ) 88.04 Hz), 127.51 (app t, J ) 10.56 Hz), 129.59 (d, J )
13.71 Hz), 133.36 (s), 133.73 (s). 134.39 (m), 135.64 (s, JPtC ) 50.30
Hz), 136.68 (s, JPtC ) 50.30 Hz), 148.66 (s), 149.46 (s), 149.53 (s),
154.47 (s), 158.55 (s), 159.44 (s), 159.51 (s). Anal. Calcd for C47H39-
ClOFeP2Pt: C, 58.31; H, 4.06. Found: C, 58.28; H, 4.08.
Synthesis of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-F) (2e). 4-F-C6H4-
MgBr (0.30 mL, 0.60 mmol) and (COD)Pt(C6H4-4-CH3)Cl (0.231 g,
0.537 mmol) in 30 mL of ether gave 0.175 g (67%) of (COD)Pt(C6H4-
4-CH3)(C6H4-4-F). DPPF (0.218 g, 0.393 mmol) and (COD)Pt(C6H4-
4-CH3)(C6H4-4-F) (0.175 g, 0.357 mmol) in 10 mL of benzene gave
1
0.299 g (89%) of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-F). H NMR (400
MHz, C7D8): δ 7.53-7.46 (m, 8H, DPPF), 7.37-7.33 (m, 4H, DPPF),
7.29-7.16 (m, 8H, DPPF), 6.84-6.77 (m, 2H, Pd-Ar), 6.70-6.61 (m,
2H, Pd-Ar), 6.30 (d, 2H, J ) 6.8 Hz, Pd-Ar), 6.24-6.19 (m, 2H, Pd-
Ar), 4.31-4.28 (m, 4H, Cp-H), 4.19 (s, 4H, Cp-H), 1.92 (s, 3H, Me-
H). 31P{1H} NMR (121 MHz, toluene): δ 15.37 (d, JPP ) 15 Hz, JPtP
) 1778 Hz), 14.86 (d, JPP ) 15 Hz, JPtP ) 1906 Hz). Anal. Calcd for
C47H39FFeP2Pt: C, 60.33; H, 4.20. Found: C, 60.08; H, 4.05.
Synthesis of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-CF3) (2f). 4-CF3-
C6H4MgBr (0.40 mL, 0.20 mmol) and (COD)Pt(C6H4-4-CH3)Cl (0.083
g, 0.193 mmol) in 30 mL of ether gave 0.056 g (54%) of (COD)Pt-
(C6H4-4-CH3)(C6H4-4-CF3). DPPF (0.084 g, 0.151 mmol) and (COD)-
Pt(C6H4-4-CH3)(C6H4-4-CF3) (0.074 g, 0.137 mmol) in 10 mL of
benzene gave 0.130 g (87%) of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-CF3).
Synthesis of (DPPF)Pt(C6H4-4-OMe)(C6H4-F) (3d). 4-F-C6H4MgBr
(0.30 mL, 0.60 mmol) and (COD)Pt(C6H4-4-OMe)Cl (0.237 g, 0.531
mmol) in 30 mL of ether gave 0.179 g (67%) of (COD)Pt(C6H4-4-
OMe)(C6H4-4-F). DPPF (0.202 g, 0.364 mmol) and (COD)Pt(C6H4-4-
OMe)(C6H4-4-F) (0.141 g, 0.279 mmol) in 10 mL of benzene gave
1
0.264 g (99%) of (DPPF)Pt(C6H4-4-OMe)(C6H4-4-F). H NMR (400
MHz, C7D8): δ 7.01-6.92 (m, 8H, DPPF), 6.78-6.76 (m, 4H, DPPF),
6.70-6.68 (m, 8H, DPPF), 6.24-6.07 (m, 4H, Pd-Ar), 5.67-5.63 (m,
2H, Pd-Ar), 5.56 (d, 2H, J ) 8.0 Hz, Pd-Ar), 3.74 (s, 2H, Cp-H), 3.71
(s, 2H, Cp-H), 3.67-3.61 (b, 4H, Cp-H), 2.94 (s, 3H, OMe-H). 31P
{1H} NMR (121 MHz, toluene): δ 15.31 (d, JPP ) 48 Hz, JPtP ) 1950
Hz), 15.02 (d, JPP ) 48 Hz, JPtP ) 1950 Hz). Anal. Calcd for C47H39-
FOFeP2Pt: C, 59.32; H, 4.13. Found: C, 59.10; H, 3.95.
9
J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13025

A R T I C L E S
Shekhar and Hartwig
Synthesis of (DPPF)Pt(C6H4-4-OMe)(C6H4-CF3) (3f). 4-CF3-C6H4-
MgBr (0.40 mL, 0.24 mmol) and (COD)Pt(C6H4-4-OMe)Cl (0.102 g,
0.229 mmol) in 30 mL of ether gave 0.101 g (76%) of (COD)Pt(C6H4-
4-OMe)(C6H4-4-CF3). DPPF (0.111 g, 0.200 mmol) and (COD)Pt(C6H4-
4-OMe)(C6H4-4-CF3) (0.101 g, 0.182 mmol) in 10 mL of benzene gave
0.166 g (91%) of (DPPF)Pt(C6H4-4-OMe)(C6H4-4-CF3). 1H NMR (400
MHz, CDCl3): δ 7.45-7.38 (m, 8H, DPPF), 7.32-7.24 (m, 4H,
DPPF), 7.21-7.17 (m, 8H, DPPF), 6.96 (m, 2H, Pd-Ar), 6.68 (m, 2H,
Pd-Ar), 6.57 (d, 2H, J ) 7.2 Hz, Pd-Ar), 6.08 (d, 2H, J ) 7.6 Hz,
Pd-Ar), 4.29-4.28 (m, 2H, Cp-H), 4.23-4.14 (m, 6H, Cp-H), 3.45 (s,
(DPPF)Pt(C6H4-4-CH3)Cl (0.168 g, 0.195 mmol) in 20 mL of toluene
gave 0.112 g (62%) of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-NMe2). 1H NMR
(400 MHz, C7D8): δ 7.41-7.35 (m, 8H, DPPF), 7.29-7.25 (m, 4H,
DPPF), 7.20-7.15 (m, 8H, DPPF), 6.73-6.56 (b, 4H, Pd-Ar), 6.23-
6.21 (m, 2H, Pd-Ar), 6.07-6.05 (m, 2H, Pd-Ar), 4.22-4.21 (m, 4H,
Cp-H), 4.14-4.13 (m, 4H, Cp-H), 3.44 (s, 3H, OMe-H), 1.87 (s, 3H,
Me-H). 31P{1H} NMR (121 MHz, toluene): δ 16.15 (d, JPP ) 14 Hz,
JPtP ) 1782 Hz), 15.21 (d, JPP ) 14 Hz, JPtP ) 1750 Hz)
Synthesis of (DPPF)Pt(C6H4-4-OMe)(C6H4-4-NMe2) (3a). DPPF
(0.299 g, 0.539 mmol) and (COD)Pt(C6H4-4-OMe)Cl (0.219 g, 0.491
mmol) in 5 mL of refluxing benzene gave 0.376 g (90%) of (DPPF)-
Pt(C6H4-4-OMe)Cl. 4-NMe2C6H4MgBr (0.66 mL, 0.33 mmol) and
(DPPF)Pt(C6H4-4-OMe)Cl (0.255 g, 0.290 mmol) in 20 mL of toluene
gave 0.152 g (55%) of (DPPF)Pt(C6H4-OMe)(C6H4-4-NMe2). 1H NMR
(400 MHz, C7D8): δ 7.45-7.41 (m, 8H, DPPF), 7.27-7.24 (m, 5H,
DPPF), 7.20-7.17 (m, 7H, DPPF), 6.75-6.52 (m, 4H, Pd-Ar), 6.07-
6.00 (m, 4H, Pd-Ar), 4.22-4.21 (m, 4H, Cp-H), 4.13 (b, 4H, Cp-H),
3.45 (s, 3H, OMe-H), 2.52 (s, 6H, NMe2-H). 31P{1H} NMR (121 MHz,
toluene): δ 15.26 (s, JPtP ) 1786 Hz).
3H, OMe-H). 31P{1H} NMR (121 MHz, benzene): δ 16.19 (d, JPP
)
15 Hz, JPtP ) 1928 Hz), 14.56 (d, JPP ) 15 Hz, JPtP ) 1912 Hz).
Anal. Calcd for C48H39F3OFeP2Pt: C, 60.95; H, 4.16. Found: C, 60.83;
H, 4.21.
Synthesis of (DPPF)Pt(C6H4-4-CF3)(C6H5) (4a). (COD)Pt(C6H5)2
(0.311 g, 0.679 mmol) and CH3COCl (48 µL, 0.68 mmol) in a mixture
of 5 mL of dichloromethane and 5 mL of methanol gave 0.228 g (81%)
of (COD)Pt(C6H5)Cl. 4-CF3-C6H4MgBr (0.40 mL, 0.32 mmol) and
(COD)Pt(C6H5)Cl (0.115 g, 0.276 mmol) in 30 mL of ether gave 0.105
g (72%) of (1,5-(COD)Pt(C6H5)(C6H4-4-CF3). DPPF (0.122 g, 0.220
mmol) and (COD)Pt(C6H5)(C6H4-4-CF3) (0.105 g, 0.199 mmol) in 10
mL of benzene gave 0.1754 g (89%) of (DPPF)Pt(C6H5)(C6H4-4-CF3).
1H NMR (500 MHz, C7D8): δ 7.42-7.33 (m, 9H, DPPF), 7.22-7.18
(b, 1H, DPPF), 7.15-7.11 (m, 10H, DPPF), 6.93-6.79 (m, 2H, Pd-
Ar), 6.75-6.65 (m, 2H, Pd-Ar), 6.50 (d, 2H, J ) 7.5 Hz, Pd-Ar), 6.34-
6.25 (m, 3H, Pd-Ar), 4.23-4.22 (b, 2H, Cp-H), 4.13-4.07 (m, 6H,
Cp-H). 31P{1H} NMR (121 MHz, toluene): δ 15.98 (d, JPP ) 15 Hz,
JPtP ) 1770 Hz), 15.07 (d, JPP ) 17 Hz, JPtP ) 1830 Hz). Anal. Calcd
for C47H37F3FeP2Pt: C, 58.10; H, 3.84. Found: C, 57.99; H, 3.85.
Synthesis of (DPPF)Pt(C6H4-4-CF3)(C6H4-4-Cl) (4b). (COD)Pt-
(C6H4-4-Cl)2 (0.107 g, 0.203 mmol) and CH3COCl (15 µL, 0.20 mmol)
in a mixture of 5 mL of dichloromethane and 5 mL of methanol at 70
°C gave 0.082 g (89%) of (COD)Pt(C6H4-4-Cl)Cl. 4-CF3-C6H4MgBr
(0.25 mL, 0.20 mmol) and (COD)Pt(C6H4-4-Cl)Cl (0.074 g, 0.164
mmol) in 30 mL of ether gave 0.059 g (64%) of (COD)Pt(C6H4-4-
Cl)(C6H4-4-CF3). DPPF (0.064 g, 0.115 mmol) and (COD)Pt(C6H4-4-
Cl)(C6H4-4-CF3) (0.059 g, 0.105 mmol) in 10 mL of benzene gave
Synthesis of (DPPF)Pt(C6H5)(C6H4-4-NMe2) (5a). (COD)(C6H5)-
Cl (0.186 g, 0.447 mmol) and DPPF (0.248 g, 0.45 mmol) in 5 mL of
refluxing benzene gave 0.359 g (93%) of (DPPF)Pt(C6H5)Cl. 4-NMe2-
C6H4MgBr (0.50 mL, 0.25 mmol) and (DPPF)Pt(C6H5)Cl (0.196 g,
0.231 mmol) in 20 mL of toluene gave 0.146 g (68%) of (DPPF)Pt-
1
(C6H5)(C6H4-4-NMe2). H NMR (400 MHz, C7D8): δ 7.59-7.58 (m,
9H, DPPF), 7.32-7.29 (m, 1H, DPPF), 7.14-7.10 (m, 2H, DPPF),
6.94-6.88 (m, 13H, DPPF and Pd-Ar), 6.67-6.64 (m, 2H, Pd-Ar),
6.50-6.47 (m, 1H, Pd-Ar), 6.25 (d, 2H, J ) 6.8 Hz, Pd-Ar), 4.25 (b,
4H, Cp-H), 3.71 (b, 4H, Cp-H), 2.24 (b, 6H, NMe2-H). 31P{1H} NMR
(121 MHz, toluene): δ 16.15 (d, JPP ) 14 Hz, JPtP ) 1782 Hz), 15.21
(d, JPP ) 14 Hz, JPtP ) 1782 Hz).
Synthesis of (DPPF)Pt(C6H4-4-Cl)(C6H4-4-NMe2) (5b). (COD)-
Pt(C6H4-4-Cl)Cl (0.232 g, 0.515 mmol) and DPPF (0.314 g, 0.566
mmol) in 5 mL of refluxing toluene gave 0.462 g (87%) of (DPPF)-
Pt(C6H4-4-Cl)Cl. 4-NMe2-C6H4MgBr (0.30 mL, 0.15 mmol) and
(DPPF)Pt(C6H4-4-Cl)Cl (0.108 g, 0.122 mmol) in 20 mL of toluene
gave 0.079 g (71%) of (DPPF)Pt(C6H4-4-Cl)(C6H4-4-NMe2). 1H NMR
(400 MHz, C7D8): δ 7.62-7.59 (m, 9H, DPPF), 7.17-6.98 (m, 15H,
DPPF and Pd-Ar), 6.66-6.64 (m, 2H, Pd-Ar), 6.32 (b, 2H, Pd-Ar),
4.32-4.30 (m, 4H, Cp-H), 3.85 (b, 4H, Cp-H), 2.31 (b, 6H, NMe2-
H). 31P{1H} NMR (121 MHz, toluene): δ 15.98 (d, JPP ) 14 Hz, JPtP
) 1764 Hz), 15.03 (d, JPP ) 14 Hz, JPtP ) 1820 Hz).
Synthesis of (DPPF)Pt(C6H4-4-CF3)(C6H4-4-NMe2) (4c) (Scheme
3). In a drybox, (COD)Pt(C6H4-4-OMe)Cl (0.447 g, 0.100 mmol) was
taken in a 100 mL round-bottom flask. A stir bar was added, followed
by addition of 30 mL of ether and 4-CF3-C6H4MgBr (0.40 mL, 0.12
mmol), and the mixture was stirred at room temperature for 12 h. The
reaction mixture was brought outside the glovebox, and saturated
aqueous NH4Cl (20 mL) was added. The ether layer was separated,
and the water layer was extracted with ether. The combined ether
extracts were dried over anhydrous MgSO4 and filtered through a 1
cm pad of Celite and a 2 cm pad of decolorizing carbon. Ether was
removed form the filtrate under reduced pressure. The white solid
obtained was recrystallized from a mixture of dichloromethane and
pentane to give 0.399 g (72%) of (COD)Pt(C6H4-4-OMe)(C6H4-4-CF3).
(COD)Pt(C6H4-4-OMe)(C6H4-4-CF3) (0.205 g, 0.369 mmol) was taken
in a screw-capped vial, and a stir bar was added, followed by addition
of a mixture of 5 mL of dichloromethane and 5 mL of methanol. CH3-
COCl (26 µL, 0.37 mmol) was added, and the reaction mixture was
stirred for 5 min. The solvent was evaporated under reduced pressure,
and the white solid obtained was recrystallized from a mixture of
dichloromethane and pentane to give 0.167 g (94%) of (COD)Pt(C6H4-
4-CF3)Cl. DPPF (0.150 g, 0.271 mmol) and (COD)Pt(C6H4-4-CF3)Cl
(0.119 g, 0.246 mmol) were taken in a screw-capped vial. A stir bar
was added, followed by the addition of 10 mL of benzene. The reaction
mixture was stirred for 30 min in refluxing benzene. The solvent was
1
0.090 g (84%) of (DPPF)Pt(C6H4-4-Cl)(C6H4-4-CF3). H NMR (500
MHz, C7D8): δ 7.45-7.37 (m, 9H, DPPF), 7.32-7.27 (m, 8H, DPPF),
7.27-7.17 (m, 3H, DPPF), 6.94-6.82 (m, 2H, Pd-Ar), 6.76-6.65 (m,
2H, Pd-Ar), 6.57 (d, 2H, J ) 7.0 Hz, Pd-Ar), 6.37 (d, 2H, J ) 7.5 Hz,
Pd-Ar), 4.27 (b, 2H, Cp-H), 4.17-4.15 (m, 6H, Cp-H). 31P{1H} NMR
(121 MHz, toluene): δ 15.98 (d, JPP ) 15 Hz, JPtP ) 1770 Hz), 15.07
(d, JPP ) 17 Hz, JPtP ) 1830 Hz). Anal. Calcd for C47H36ClF3FeP2Pt:
C, 56.11; H, 3.61. Found: C, 56.04; H, 3.35.
General Procedure for the Synthesis of Unsymmetrical Bis-Aryl
Platinum Complexes of the Type (DPPF)Pt(C6H4-4-R)(C6H4-4-
NMe2) (2a, 3a, 5a,b) via Route B (Scheme 2). (COD)Pt(C6H4-4-R)-
Cl was taken in a screw-capped vial. A stir bar was added, followed
by 5 mL of benzene. DPPF (1.1 equiv) was added, and the reaction
mixture was refluxed for 30 min to afford (DPPF)Pt(C6H4-4-R)Cl.
Yellow powder was washed with ether. In a drybox, (DPPF)Pt(C6H4-
4-R)Cl was taken in a 100 mL round-bottom flask. A stir bar was added,
followed by 20 mL of toluene. Grignard reagent (4-NMe2C6H4MgBr)
(1.1 equiv) was added, and the reaction mixture was allowed to stir
for 2 h at room temperature. The reaction mixture was concentrated to
one-third of the initial volume and then filtered through a 1 cm pad of
Celite and a 2 cm pad of decolorizing charcoal. The filtrate obtained
was layered with pentane and stored overnight in a freezer (-35 °C).
Yellow precipitate of (DPPF)Pt(C6H4-4-X)(C6H4-4-NMe2) was obtained.
The solvent was decanted, and the product was dried under reduced
pressure.
Synthesis of (DPPF)Pt(C6H4-4-CH3)(C6H4-4-NMe2) (2a). DPPF
(0.201 g, 0.362 mmol) and (COD)Pt(C6H4-4-CH3)Cl (0.140 g, 0.326
mmol) in 5 mL of refluxing benzene gave 0.249 g (92%) of (DPPF)-
Pt(C6H4-4-CH3)Cl. 4-NMe2C6H4MgBr (0.44 mL, 0.22 mmol) and
9
13026 J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004

Reductive Eliminations of Bis-Aryl Platinum Complexes
A R T I C L E S
removed under reduced pressure, and the yellow solid obtained was
washed with ether to give 0.199 g (87%) of (DPPF)Pt(C6H4-4-CF3)Cl.
In a drybox, (DPPF)Pt(C6H4-4-CF3)Cl (0.101 g, 0.109 mmol) was taken
in a screw-capped vial. A stir bar was added, followed by addition of
15 mL of toluene. The reaction mixture was stirred at room temperature
for 2 h. The solvent was reduced to one-third of the initial volume
under reduced pressure and layered with 5 mL of pentane. The reaction
mixture was stored overnight at -35 °C to give yellow crystals. The
solvent was decanted, and the crystals were dried under reduced pressure
a screw-capped NMR tube. The NMR tube was added to a preheated
probe at 95 °C. The temperature in the probe cavity was measured by
inserting a copper thermocouple into an NMR tube containing toluene
in the probe.63 Decay of the biaryl platinum complexes was monitored
1
by H NMR and spectroscopy and 19F NMR spectroscopy (1f, 4a,b)
over at least three half-lives. Kinetic data were fit to the expression y
) m1 + m2 exp-kt, in which k is the first-order rate constant kobs
.
Acknowledgment. We thank Dr. Timothy Boller for useful
discussions on the interpretation of the rates of reductive
elimination.
1
to give 0.038 g (35%) of (DPPF)Pt(C6H4-4-CF3)(C6H4-4-NMe2). H
NMR (400 MHz, C7D8): δ 7.64-7.55 (m, 9H, DPPF), 7.32-7.29 (m,
15H, DPPF and Pd-Ar), 7.13-6.97 (m, 2H, J ) 8.0 Hz, Pd-Ar), 6.26
(b, 2H, Pd-Ar), 4.37 (s, 2H, Cp-H), 4.27-4.26 (m, 2H, Cp-H), 3.86
(m, 2H, Cp-H), 3.83 (m, 2H, Cp-H), 2.39 (s, 6H, NMe2-H). 31P{1H}
NMR (121 MHz, toluene): δ 16.71 (d, JPP ) 12 Hz, JPtP ) 1798 Hz),
14.48 (d, JPP ) 12 Hz, JPtP ) 1826 Hz).
Determination of the Rate Constants (kobs) for the Reductive
Elimination of Biaryls from Bis-Aryl Platinum Complexes. Bis-
aryl platinum complexes (1a-f, 2a-f, 3a-e, 4a-c, 5a,b) (9.23 × 10-6
mol), PPh3 (9.23 × 10-5 mol), and 1,3,5-trimethoxybenzene (6.4 ×
10-5 mol, internal standard) were dissolved in toluene-d8 (0.8 mL) in
Supporting Information Available: 1H and 31P NMR spectra
of complexes of the type (DPPF)Pt(C6H4-4-NMe2)(C6H4-4-X),
and Hammett plots for the rates of reductive elimination from
(DPPF)Pt(C6H4-4-OMe)(C6H4-4-X) and (DPPF)Pt(C6H4-4-CH3)-
(C6H4-4-X). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA0480365
(63) Bornais, J.; Brownstein, S. J. Magn. Reson. 1978, 29, 207-211.
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J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 13027