Five-coordinate intermediates in carbon-carbon reductive elimination reactions from Pt(IV) [7]

Full text of DOI:10.1021/ja9912123

962
J. Am. Chem. Soc. 2000, 122, 962-963
Scheme 1
Five-Coordinate Intermediates in Carbon-Carbon
Reductive Elimination Reactions from Pt(IV)
Dawn M. Crumpton and Karen I. Goldberg*
Department of Chemistry, Box 351700
UniVersity of Washington, Seattle, Washington 98195-1700
ReceiVed April 16, 1999
Reductive elimination is a fundamental organometallic reaction
and often serves as the critical product release step in homoge-
neously catalyzed organic reactions and metal-mediated stoichio-
metric transformations.1 Although numerous studies of the
mechanisms of C-C and C-H reductive elimination reactions
have been carried out, it remains a challenge to predict whether
such reactions will occur directly or require ancillary ligand loss
) 4.2 ( 0.1 × 10-6 s-1 in benzene-d6; kobs ) 3.2 ( 0.1 × 10-6
s-1 in THF-d8). Activation parameters of ∆Hq ) 43 ( 2 kcal/
mol and ∆Sq) 15 ( 4 eu. were calculated using rate constants
measured in benzene-d6 over a temperature range of 165-205
°C.10 To examine the possibility of an intermolecular mechanism
for reductive elimination, a crossover study was performed in
which an equimolar mixture of 1 and (dppe)Pt(CD3)4 (1-d12) in
benzene-d6 was heated at 165 °C for slightly more than one half-
life (50 h). Analysis by mass spectrometry revealed that the
gaseous products consisted of CH3CH3 and CD3CD3 (<4% CH3-
CD3). No scrambling of deuterium within the unreacted Pt(IV)
species was observed. The Pt(II) products, however, were shown
to be a mixture of 4, 4-d3, and 4-d6 by 31P{1H} NMR spectros-
copy.11 That this scrambling occurs after the reductive elimination
was confirmed by a separate experiment in which the thermolysis
of a mixture of 4 and 4-d6 in benzene-d6 at 165 °C for 12 h
generated a significant amount of 4-d3. Exchange of alkyl groups
among d8 M(II) (M ) Pt, Pd) complexes has been reported.12
The results of the crossover experiment are inconsistent with
C-C reductive elimination proceeding by an intermolecular
mechanism (pathway A in Scheme 1). It is considerably more
difficult to distinguish between the possible intramolecular
mechanisms of direct reductive elimination (pathway B) and
predissociation of one end of the phosphine chelate13 followed
by reductive elimination from a five-coordinate species (pathway
C). These are kinetically indistinguishable and the measured
activation parameters are compatible with either. However, strong
support for phosphine dissociation (pathway C) is provided by
thermolysis of (dppbz)PtMe4 (2).8 With a benzene backbone,
dppbz is a more rigid diphosphine chelate than dppe and is
expected to have a smaller propensity for chelate opening.14,15
Complex 2 was heated at 165 °C in THF-d8 and after 300 h had
or even ligand association prior to the reductive coupling.1-7
A
better understanding of the preferences for these different
pathways is vital to current efforts toward rational catalyst design.
In virtually all studies of reductive elimination reactions which
form alkyl C-C and C-H bonds from octahedral Pt(IV) com-
plexes, mechanistic evidence has supported preliminary ligand
loss and the generation of a five-coordinate intermediate prior to
the elimination.3-7 However, since all the compounds investigated
have contained at least one ligand capable of facile dissociation
(e.g. PR3, halide), it is not clear how strong the preference is for
coupling to occur from a five-coordinate intermediate. To explore
the possibility of direct reductive elimination from a six-coordinate
species, we have examined the thermal reactivity of the Pt(IV)
complexes (dppe)PtMe4 (1),5 (dppbz)PtMe4 (2),8 and (dppe)PtMe3-
Et (3)8 (dppe ) Ph2PCH2CH2PPh2, dppbz ) o-Ph2PC6H4PPh2).
The chelate effect inhibits the formation of five-coordinate
intermediates via phosphine dissociation. Complexes 1-3 are
unusually robust for Pt(IV) phosphine alkyl compounds.3,4 1 has
been reported to be indefinitely stable in solution at ambient
temperature5 and we have observed similar behavior for 2 and 3.
Temperatures in excess of 150 °C are required to induce the
reductive elimination of ethane from 1. Reported herein is an
investigation of the mechanism of C-C reductive elimination
from 1, a Pt(IV) complex in which ligand dissociation should be
substantially suppressed.
The thermolysis of 1 in solution required a period of days at
165 °C to obtain quantitative conversion to (dppe)PtMe2 (4) and
ethane.9 The reaction exhibited first-order kinetic behavior (kobs
(1) 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.
(2) For example: (a) Marcone, J. E.; Moloy, K. G. J. Am. Chem. Soc.
1998, 120, 8527. (b) Yamamoto, T.; Abla, M. J. Organomet. Chem. 1997,
535, 209. (c) Hartwig, J. F.; Andersen, R. A.; Bergman, R. G. J. Am. Chem.
Soc. 1991, 113, 6492. (d) Milstein, D. Acc. Chem. Res. 1984, 17, 221.
(3) (a) Williams, B. S.; Holland, A. W.; Goldberg, K. I. J. Am. Chem. Soc.
1999, 121, 252. (b) Goldberg, K. I.; Yan, J. Y.; Breitung, E. M. J. Am. Chem.
Soc. 1995, 117, 6889 and references therein.
(4) (a) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. E. J. Chem. Soc.,
Dalton Trans. 1974, 2457. (b) Roy, S.; Puddephatt, R. J.; Scott, J. D. J. Chem.
Soc., Dalton Trans. 1989, 2121. (In the absence of added L, 1-2% of C-C
reductive elimination from L2PtMe4 (L ) MeNC, 2,6-Me2-C6H3NC) may
proceed without ligand dissociation. Significant decomposition of L under
the reaction conditions hinders complete analysis of the data.)
(5) (a) Hill, G. S.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1999,
18, 1408. (b) Hill, G. S.; Puddephatt, R. J. Organometallics 1997, 16, 4522.
(6) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996,
118, 5961 (b) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics
1995, 14, 4966. (c) Jenkins, H. A.; Yap, G. P. A.; Puddephatt, R. J.
Organometallics 1997, 16, 1946. (d) Fekl, U.; Zahl, A.; van Eldik, R.
Organometallics 1999, 18, 4156.
(10) Eyring and kinetic plots provided in Supporting Information.
(11) 31P{1H} NMR (C6D6): 4 (δ 47.35, JPt-P ) 1779 Hz); 4-d6 (δ 47.49,
JPt-P ) 1769 Hz); 4-d3 (δ 47.42, JPt-P ) 1774 Hz). The signals were not
baseline separated and integration was not feasible.
(12) Exchange of methyl, aryl, and halide ligands between d8 M(II)
centers: Casado, A. L.; Casares, J. A.; Espinet, P. Organometallics 1997, 16,
5730 and references therein.
(13) An initial opening of a chelate ring has been proposed in the
mechanisms of some substitution and oxidative addition reactions. (a)
Basallote, M. G.; Dura´n, J.; Ferna´ndez-Trujillo, M. J.; Gonza´lez, G.; Ma´n˜ez,
M. A.; Mart´ınez, M. Inorg. Chem. 1998, 37, 1623 and references therein. (b)
Bromberg, S. E.; Yang, H.; Asplund, M. C.; Lian, T.; McNamara, B. K.;
Kotz, K. T.; Yetson, J. S.; Wilkens, M.; Frei, H.; Bergman, R. G.; Harris, C.
B. Science 1997, 278, 260. (c) Rauscher, D. J.; Thaler, E. G.; Huffman, J. C.;
Caulton, K. G. Organometallics 1991, 10, 2209 and references therein. (d)
Thaler, E. G.; Folting, K.; Caulton, K. G. J. Am. Chem. Soc. 1990, 112, 2664.
(e) Landgrafe, C.; Sheldrick, W. S.; Su¨dfeld, M. Eur. J. Inorg. Chem. 1998,
407.
(7) There is support for direct reductive elimination in the coupling of aryl
groups from Pt(IV): Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am.
Chem. Soc. 1998, 120, 2843.
(14) (a) Mann, G.; Baranano, D.; Hartwig, J. F.; Rheingold, A. L.; Guzei,
I. A. J. Am. Chem. Soc. 1998, 120, 9205. (b) Sweigart, D. Inorg. Chim. Acta
1974, 317.
(15) The bite angles for dppe and dppbz are similar (85° and 83°, respec-
tively). Dierkes, P.; van Leeuwen, P. W. N. M. J. Chem. Soc., Dalton Trans.
1999, 1519.
(8) See Supporting Information for the synthesis, purification, and char-
acterization of 2, fac-3, and mer-3.
(9) See Supporting Information for safety precautions.
10.1021/ja9912123 CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/25/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 5, 2000 963
Scheme 2
ates the Pt(II) ethylmethyl complex (5). Such a Pt(II) ethyl
complex may also undergo â-hydride elimination to generate
ethylene.19 However, as 95% of the Pt was accounted for, the
vast majority of the ethylene results from â-hydride elimination
from the Pt(IV) species 3 rather than the Pt(II) complex 5.
Additional support for phosphine dissociation from (dppe)Pt
tetraalkyl species at high temperatures is the observation of
isomerization to an equilibrium mixture of fac-3 and mer-3 during
the reaction at 165 °C. At 100 °C, isomerization is complete within
4 h and occurs without further reaction to generate Pt(II)
complexes. The mer isomer is favored (Keq ) 6.3 at 100 °C).
Isomerization of Pt(IV) octahedral complexes is most commonly
proposed to occur via fluxional five-coordinate intermediates.6c,20,21
In summary, despite the strong chelate effect of dppe, complex
1 appears to undergo reductive elimination by a mechanism that
involves ligand dissociation and C-C coupling from a five-
coordinate intermediate (Scheme 1, pathway C). The observation
of isomerization and â-hydride elimination for the (dppe)Pt(IV)
ethyl complex 3 establishes the kinetic viability of dppe chelate
arm opening at Pt(IV). That the C-C reductive elimination from
1 is occurring via the five-coordinate chelate opened species is
supported by the dramatic rate reduction observed when dppe is
replaced by the more rigid chelating ligand dppbz.
Thus, there are as yet no definitive examples of C-C reductive
elimination of alkyl groups from Pt(IV) that undergo direct
coupling from the six-coordinate species. This is a critical aspect
to consider in catalyst development, as an open site should be
accessible from a six-coordinate Pt(IV) complex to promote C-C
reductive elimination. In addition, due to the presence of an open
coordination site, â-hydride elimination is likely to be a significant
competing pathway for alkyl groups undergoing C-C coupling
in such systems. Finally, based on the principle of microscopic
reversibility, C-C oxidative addition to square planar Pt(II) should
require preliminary ligand dissociation and addition of the C-C
bond to a “formally” three-coordinate Pt(II) species.22 Such
reactivity has been documented for C-H activation reactions at
Pt(II) centers.23-25
proceeded to only 4% conversion.16 For comparison, 4% conver-
sion of 1 (a first-order process) would require only ca. 3.5 h under
the same conditions. Thus, replacement of dppe with the more
rigid chelate dppbz significantly inhibits the rate of ethane
elimination.
Predissociation of the dppe phosphine linkage from Pt(IV) was
shown to be kinetically competent in the Pt(IV) C-C reductive
elimination reaction by examination of the thermal behavior of
the trimethylethyl analogue, (dppe)PtMe3Et (3). This compound
provides the potential for â-hydride elimination to compete with
or even eclipse C-C reductive elimination under thermolysis
conditions.17 As â-hydride elimination is known to require an open
coordination site at the metal center,1 the observation of such
reactivity for 3 would imply that phosphine dissociation is
occurring on the time scale of the reaction or faster.
Complex 3 (as a nonequilibrium mixture of fac and mer
isomers) was prepared by the reaction of EtMgBr and (dppe)-
PtMe3I in toluene and isolated in low yield after extensive
purification procedures.8 Pt-C bond cleavage processes were not
observed until 3 was heated above 100 °C in benzene-d6. Such
thermal stability is unusual for a Pt(IV) ethyl complex, and is
undoubtedly related to the absence of an easily accessible open
coordination site at the metal.17 When heated to 165 °C in
benzene-d6, 3 undergoes reaction to form the organic products
ethylene, methane, propane and ethane, and the Pt(II) products
(dppe)PtMe2 (4) and (dppe)PtMeEt (5). The disappearance of 3
is a first-order process with kobs ) (1.2 ( 0.1) × 10-4 s-1 at 165
°C. The yields of 4 and 5 are 91% and 4%, respectively.18 The
Studies to further investigate the importance of five-coordinate
intermediates in C-C and C-H reductive elimination from d6
octahedral complexes are underway in our laboratory. The use
of both different metals and ligand sets may change the energy
barriers associated with the strong five-coordinate preference.
1
organic products were identified by H NMR and GC/MS and
the ratio of ethylene to propane to ethane was 75:18:7 (GC
analysis).
The mechanism that most reasonably accounts for the observed
organic and Pt(II) products from thermolysis of 3 is depicted in
Scheme 2. Preliminary dissociation of a phosphine chelate arm
is followed by competitive â-hydride elimination and C-C reduc-
tive elimination from the five-coordinate intermediate. The â-hy-
dride elimination path leads to ethylene and (dppe)PtMe3H. This
Pt(IV) alkyl hydride would be expected to undergo rapid reductive
elimination of methane at 165 °C and so should not be observed.
The C-C reductive elimination pathways proceed to ethane and
propane. The majority of 3 proceeds via â-hydride elimination
and propane reductive elimination as evidenced by the high yield
of (dppe)PtMe2 (4) and the significant production of ethylene and
propane. The minor ethane reductive elimination pathway gener-
Acknowledgment is given for support from the National Science
Foundation, the Union Carbide Innovation Recognition Program, the
DuPont Educational Aid Program, and the University of Washington.
K.I.G. is an Alfred P. Sloan Research Fellow. We thank Dr. K. Dobbs
(DuPont) for helpful discussions and the referees for their suggestions.
Supporting Information Available: Synthesis and characterization
of 2 and 3; kinetic plots for the thermolysis of 1 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA9912123
(19) Bryndza, H. E.; Calabrese, J. C.; Marsi, M.; Roe, C. D.; Tam, W.;
Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 4805 and references therein.
(20) (a) Clark, H. C.; Manzer, L. E. Inorg. Chem. 1973, 12, 362. (b) Crespo,
M.; Puddephatt, R. J. Organometallics 1987, 6, 2548.
(16) Thermolyses of 2 were carried out in the presence of 2% cross-linked
polyvinylpyridine (PVP, an acid scavenger).3a In the absence of PVP, greater
initial reactivity was observed with the amount dependent on sample (after
100 h, 9-17% conversion). With continued heating, thermolyses without PVP
distinctly slowed (from 100 to 500 h, only 5-13% conversion). A trace Lewis
acid impurity in 2 (consumed over time or scavenged by PVP) is suspected.
The addition of PVP had no effect on the thermolysis rate of 1.
(17) (PMe2Ph)2Pt(Me)(Et)2I undergoes competitive â-hydride elimination
and C-C reductive elimination at 33 °C. Brown, M. P.; Puddephatt, R. J.;
Upton, C. E. E.; Lavington, S. W. J. Chem. Soc., Dalton Trans. 1974, 1613.
(18) (a) Yields were measured by integration against an internal standard
in the 1H NMR spectrum. The remaining 5% may be accounted for by some
decomposition of 5 to intractable Pt(0).19 (b) Small amounts of 1 (2-5%)
were present in the samples of 3. However, only 15% of 1 had reacted after
the thermolysis of 3 was complete (12 h), so the contribution was negligible.
(21) Five-coordinate intermediates have also been proposed in the isomer-
ization of octahedral Ir(III) complexes. Wehman-Ooyevaar, I. C. M.; Drenth,
W.; Grove, D. M.; van Koten, G. Inorg. Chem. 1993, 32, 3347.
(22) van der Boom, M. E.; Kraatz, H.-B.; Hassner, L.; Ben-David, Y.;
Milstein, D. Organometallics 1999, 18, 3873.
(23) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235.
(24) Direct oxidative addition of C-H bonds to square planar (dmpe)Pt-
(Me)X has been proposed. However, a mechanism involving preliminary
opening of the phosphine chelate could not be eliminated. Peters, R. G.; White,
S.; Roddick, D. M. Organometallics 1998, 17, 4493.
(25) (a) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999,
121, 1974. (b) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1997, 119, 848. (c) Brainard, R. L.; Nutt, W. R.; Lee, T. R.; Whitesides,
G. M. Organometallics 1988, 7, 2379. (d) Foley, P.; DiCosimo, R.; Whitesides,
G. M. J. Am. Chem. Soc. 1980, 102, 6713.