Kinetic study of reductive elimination from the complexes (diphosphine)Pd(R)(CN) [6]

Full text of DOI:10.1021/ja980762i

J. Am. Chem. Soc. 1998, 120, 8527-8528
8527
transformations: nickel-catalyzed olefin hydrocyanation10 and
palladium-catalyzed coupling reactions.8
The complexes (diphosphine)Pd(R)(CN) are prepared via the
route shown in eq 2. The synthesis of the dialkyl precursors
follows procedures described for similar complexes.11 Conversion
Kinetic Study of Reductive Elimination from the
Complexes (Diphosphine)Pd(R)(CN)
John E. Marcone and Kenneth G. Moloy*
Central Research and DeVelopment
E. I. du Pont de Nemours & Co., Inc.
Experimental Station, P.O. Box 80328
Wilmington, Delaware 19880-0328
-
ReceiVed March 9, 1998
The reductive elimination reaction (eq 1) is a key transforma-
tion in organometallic chemistry, often representing both the
product-forming and rate-determining steps in a number of
important transformations, both stoichiometric and catalytic.1
Examples include chemistries as varied as hydrogenation, hy-
droformylation, carbonylation, hydrocyanation, and coupling
reactions.2 While the importance of this transformation is clear,
of the dialkyls to the alkyl cyanide complexes is accomplished
by one of two methods: treatment with CF3CO2H, followed by
Bu4NCN, or by direct reaction with excess HCN.12 Complexes
1-5 present a series wherein the chelate ring size/bite angle is
methodically changed while maintaining constant donor properties
at phosphorus, as shown by IR.13
a quantitative understanding is not as well developed as other
fundamental reactions in organometallic chemistry,1a,3 although
C-H, Si-H, and C-C elimination have received significant
attention.1,4 The effect of ancillary ligands on the energetics of
reductive elimination is lacking to a significantly greater degree.
This latter point is particularly important, as chelating diphos-
phines with large bite angles have recently been shown to be
especially beneficial in a number of important catalytic reactions
and these ligands are now receiving a great deal of attention.5-7
In several cases it is speculated that large bite angle diphosphines
accelerate reaction rates by enhancing the rates of product-forming
reductive elimination,5a-d,8 and two prior studies offer evidence
for this conclusion.5c,9 We have found that reductive elimination
of RCN from the complexes (diphosphine)Pd(R)(CN) (R ) CH2-
TMS, CH2CMe3) provides a unique opportunity to study the
kinetics of this important transformation and a quantitative
measure of how it is influenced by the chelating diphosphine.
These complexes also serve as models for two important
Heating a colorless solution of complex 1 and g1.5 equiv of
diphos in THF-d8 leads to quantitative formation of TMSCH2-
1
CN and bright yellow Pd(diphos)2, as determined by H and 31P
NMR monitoring. A kinetic analysis shows the reaction is first
order in 1 and is independent of the excess diphos concentration,
for >5 half-lives. Events leading to the transition state are thus
intramolecular, and the excess ligand serves only to trap Pd(0).
The formation rate of TMSCH2CN is equal to the decay rate of
1. Kinetic analyses of reductive elimination from complexes 2-5
were also conducted. As with 1, all experiments were conducted
in the presence of a small excess of the appropriate diphosphine.
The entire series exhibits first-order decay in [Pd(alkyl)(cyanide)]
and zero-order dependencies on the excess diphosphine. All
kinetic runs showed clean formation of TMSCH2CN and Pd-
(diphosphine)2, the latter confirmed by independent generation
from Pd2dba3 and 4 equiv of diphosphine.14
The reductive elimination rate increases significantly with
increasing diphosphine bite angle (Table 1). Thus, progressing
from the small bite angle (∼85° 7b) ligand diphos in 1 to the larger
bite angle of DIOP (∼100° 7b) in 3 results in a nearly 104-fold
rate increase. Complex 2, with an intermediate bite angle (dppp,
∼90° 7b), lies between these two extremes. Because the substit-
uents at phosphorus are essentially identical throughout this series
the kinetic ordering is attributable to changes in the chelate ring
size and is not electronic in origin.15 Increasing chelate ring size
results in increased bite angles, chelate flexibility, and steric size,
and all would be expected to enhance reductive elimination in
the present case. These factors favor both a mechanism involving
an intact chelate ring (where PPdP is ideally larger in the
transition state than in the square planar starting material) or one
involving preequilibrium chelate ring opening. Increasing the
(1) (a) Goldberg, K. I.; Yan, J.; Breitung, E. M. J. Am. Chem. Soc. 1995,
117, 6889, and references therein. (b) Brown, J. M.; Cooley, N. A. Chem.
ReV. 1988, 88, 1031, and references therein. (c) Byers, P. K.; Canty, A. J.;
Crespo, M.; Puddephatt, R. J.; Scott, J. D. Organometallics 1988, 7, 1363,
and references therein. (d) Gillie, A.; Stille, J. K. J. Am. Chem. Soc. 1980,
102, 4933.
(2) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; Wiley: New York, 1988. (b) Collman, J. P.; Hegedus, L. S.; Norton,
J. R.; Finke, R. G. Principles and Applications of Organometallic Chemistry;
University Science Books: Mill Valley, CA, 1987. (c) Atwood, J. D. Inorganic
and Organometallic Reaction Mechanisms; Brooks/Cole: Monterey, CA, 1985.
(3) Reference 2c, p 177.
(4) Cleary, B. P.; Mehta, R.; Eisenberg, R. Organometallics 1995, 14, 2297,
and references therein.
(5) Coupling reactions: (a) Driver, M. S.; Hartwig, J. F. J. Am. Chem.
Soc. 1996, 118, 7217. (b) Kranenburg, M.; Kamer, P. C. J.; van Leeuwen, P.
W. N. M. Eur. J. Inorg. Chem. 1998, 155. (c) Brown, J. M.; Guiry, P. J.
Inorg. Chim. Acta 1994, 220, 249. (d) Hayashi, T.; Konishi, M.; Kobori, Y.;
Kumada, M.; Higuchi, T.; Hirotsu, K. J. Am. Chem. Soc. 1984, 106, 158. (e)
Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1997, 119, 8232. (f) Mann,
G.; Hartwig, Driver, J. F.; M. S.; Ferna´ndez-Rivas, C. J. Am. Chem. Soc.
1998, 120, 827. (g) Widenhofer, R. A.; Zhong, H. A.; Buchwald, S. L. J. Am.
Chem. Soc. 1997, 119, 6787.
(10) (a) Tolman, C. A.; McKinney, R. J.; Seidel, W. C.; Druliner, J. D.;
Stevens, W. R. AdV. Catal. 1985, 33, 1. (b) Ba¨ckvall, J. E.; Andell, O. S.
Organometallics 1986, 5, 2350.
(11) (a) Diversi, P.; Fasce, D.; Santini, R. J. Organomet. Chem. 1984, 269,
285. (b) Tooze, R.; Chiu, K. W.; Wilkinson, G. Polyhedron 1984, 3, 1025.
(12) The complex DIOPPd(CN)(Et) has been prepared from DIOPPdEt2
and excess HCN: Hodgson, M.; Parker, D.; Taylor, R. J.; Ferguson, G. J.
Chem. Soc., Chem. Commun. 1987, 1309.
(6) Hydrocyanation: Goertz, W.; Kamer, P. C. J.; van Leeuwen, P. W. N.
M.; Vogt, D. J. Chem. Soc., Chem. Commun. 1997, 1521.
(7) Hydroformylation: (a) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer,
P. C. J.; van Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J. Organometallics
1995, 14, 3081. (b) Casey, C. P.; Whiteker, G. T.; Melville, M. G.; Petrovich,
L. M.; Gavney, J. A., Jr.; Powell, D. R. J. Am. Chem. Soc. 1992, 114, 5535.
(8) Calhorda, M. J.; Brown, J. M.; Cooley, N. A. Organometallics 1991,
10, 1431.
(13) IR data (νCN in CH2Cl2, cm-1): 1, 2127; 2, 2123; 3, 2126; 4, 2123; 5,
2126; diphosPd(CH2CMe3)(CN), 2119.
(14) Amatore, C.; Broecker, G.; Jutand, A.; Khalil, F. J. Am. Chem. Soc.
1997, 119, 5176.
(15) Changing PPdP will almost certainly have some effect on the
electronics at palladium; it is only suggested that the donor/acceptor properties
of the phosphorus ligands are held nearly constant.
(9) Kohara, T.; Yamamoto, T.; Yamamoto, A. J. Organomet. Chem. 1980,
192, 265.
S0002-7863(98)00762-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/05/1998

8528 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Communications to the Editor
Table 1. Rate Data and Activation Parameters for Reductive
Elimination of RCN from the Complexes (diphosphine)Pd(R)(CN)
Scheme 1
temp
k at 80 °C
∆Hq
∆Sq
range
a,b
c
Complex
(s-1
)
(kcal mol-1
)
(e.u.)c
(°C)
1
2
3
4
5
2.1(0.2)e-6
5.0(0.7)e-5
1.0(0.2)e-2
2.1(0.1)e-5
7.4(0.7)e-5
30.8(3)
2.3(8) 85-119
-1(3) 60-98
12(1) 36-60
13(2) 65-94
14(3) 60-94
1(2) 66-87
27.3(1.2)
28.2(0.4)
32.9(0.6)
32.5(0.9)
28.7(0.5)
angle enlargement (Thorpe-Ingold effect). Thus, reductive
elimination by a mechanism involving chelate ring opening and
a three-coordinate intermediate should exhibit the kinetic ordering
4,5 < 2. Instead, the observed kinetic ordering tracks the bite
angles (4 < 2 < 5). While this ordering result supports a
mechanism involving an intact ring, the effect of gem-dialkyl
substitution has not been widely exploited or quantified in metal-
phosphine systems.21 Therefore, these results are offered as
evidence for an intact chelate ring but do not conclusively rule
out a phosphine dissociation preequilibrium.22
Activation parameters are provided in Table 1. These data
show that the reductive elimination is dominated by the ∆Hq term,
with a small but favorable entropic contribution of 0-14 eu.
Trends in ∆Hq and ∆Sq are not obvious, with the exception that
substituents on the chelate ring tend to increase both parameters.
An interpretation of the change in ∆Hq is difficult23 but the
increase in ∆Sq may reflect increased flexibility in the transition
state of the ring-substituted systems.
diphosPd (CH2CMe3)(CN) 2.6(0.1)e-5
a Rate constants at the common temperature of 80 °C are calculated
by interpolation or extrapolation of linear regressions of the Eyring
data (see the Supporting Information). b Values in parantheses are 95%
confidence limits. c Values in parantheses are the standard error.
diphosphine bite angle and sterics compresses the CPdC, forcing
the two carbon atoms closer together, and this would also be
expected to accelerate C-C bond formation and subsequent
elimination.16 Enhanced reductive elimination rates with increas-
ing bite angle has been previously suggested5c but quantified only
in the case of (diphosphine)Ni(CH3)2, where elimination of ethane
is 50 times faster for dppp than for diphos, similar to that observed
here.9 Computational studies indicate that reductive elimination
proceeds with reduced barriers when PPdP in hypothetical cis-
(PH3)2Pd(CH3)(CHdCH2) is allowed to increase along the reac-
tion coordinate.8
The small ∆Sq suggests that elimination of the coupled product
occurs after the transition state, and the mechanism presented in
Scheme 1 is proposed to account for these results. This
mechanism resembles the familiar migratory insertion of CO into
metal-carbon bonds to produce acyl complexes. Considering
the isoelectronic relationship between CO and CN-, this is a
reasonable conclusion. Further note that reductive elimination
of (CH3)3CCH2CN is faster than TMSCH2CN. Elimination of
CH3CN from (diphosphine)Pd(CH3)(CN) is, in turn, much slower
than TMSCH2CN,24 and thus the kinetic ordering with respect to
the alkyl group is (CH3)3CCH2 > TMSCH2 . CH3. This order,
as well as its magnitude, is the same as that observed for migratory
CO insertion,25 adding further support to the suggestion that the
mechanisms are related.26 These results have bearing on other
examples of reductive elimination reactions in that they also
appear to involve a migration mechanism involving three-center
transition states and σ or π bound intermediates, as opposed to
concerted mechanisms.1b,4,8
The rate law and activation parameters (∆Sq) rule out a
mechanism involving coordination of excess phosphine to give
an 18 e intermediate which then reductively eliminates product.17
However, the kinetic data do not distinguish between mechanisms
involving four-coordinate (16 e) or three-coordinate (14 e)
intermediates, the latter involving dissociation of one arm of the
diphosphine chelate. The ligand dissociation preequilibrium
mechanism is generally favored with related monophosphine d8
complexes such as (R3P)2M(X)(Y) (M ) Ni, Pd, Pt), and (R3P)-
Au(R′)3.1d,2,5e,18 Both mechanisms are consistent with the observed
kinetic ordering with respect to chelate ring size. Exchange of
the inequivalent P nuclei in complexes 1-5 is not observed by
31P NMR during the course of the reductive elimination, but the
time scale of this experiment is of no use for discriminating the
two mechanisms.19
An attempt to distinguish these mechanisms has been made
through the use of gem-dialkyl and Thorpe-Ingold effects.21 gem-
Dialkyl substitution is known to greatly enhance and stabilize
ring formation in organic ring systems. The new ligands Et2-
dppp and (CH2)3dppp were prepared to investigate the effect of
gem-dialkyl substitution on reductive elimination. Both disfavor
a phosphine dissociation preequilibrium due to unfavorable gauche
interactions in their ring-open forms (gem-dialkyl effect), interac-
tions which are absent in dppp. At the same time, these ligands
differ in their bite angles; of this series, Et2dppp should have the
smaller bite angle due to angle compression at the central carbon
atom and (CH2)3dppp should have the larger bite angle due to
In conclusion, relatively minor changes in the chelate bite angle
have been shown to induce a significant effect (ca. 104-fold) on
the rate of reductive elimination. Further studies of the effect of
ancillary ligands on reductive elimination reactions are in progress.
Acknowledgment. We thank Dr. E. E. Bunel, Ms. L. A. Howe, and
Ms. C. Kelly for expert assistance with the kinetics measurements.
Supporting Information Available: Text describing synthetic,
spectroscopic, and analytical data for the complexes (diphosphine)Pd-
(R)(CN), details regarding the kinetic measurements, and Eyring plots
(10 pages, print/PDF). See any current masthead page for ordering
information and Web access instructions.
(16) This is clearly seen in the structures of diphosPdCl2,16a dpppPdCl2,16a
and DIOPPdCl2,16b where the Cl‚‚‚Cl distance decreases from 3.397 Å to 3.341
Å to 3.280 Å, respectively. (a) Steffen, W. L.; Palenik, G. J. Inorg. Chem.
1976, 15, 2432. (b) Gramlich, V.; Consiglio, G. HelV. Chim. Acta 1979, 62,
1016.
(17) Evidence for an associative process in the reductive elimination of
RCN from P2Ni(R)(CN) has been presented: McKinney, R. J.; Roe, C. J.
Am. Chem. Soc. 1986, 108, 5167.
JA980762I
(21) Shaw has shown that bulky substituents on phosphorus drive ring-
forming reactions in metal-diphosphine systems: Shaw, B. L. AdV. Chem.
Ser. 1982, 196, 101.
(22) An unverified assumption built into our argument is that the energetics
of the gauche interactions manifested as the gem-dialkyl effect are at least
comparable to those driving ring opening (i.e., enlarged bite angle).
(23) Reference 2c, p 16.
(18) (a) Komiya, S.; Albright, T. A.; Hoffman, R.; Kochi, J. A. J. Am.
Chem. Soc. 1976, 98, 7255. (b) Kuch, P. L.; Tobias, R. S. J. Organomet.
Chem. 1976, 122, 429.
(24) M. J. Burn (Du Pont), unpublished observations.
(19) This assumes that isomerization of the resulting T-shaped intermediate
is rapid relative to reductive elimination. Evidence for this has been presented
(25) Cotton, J. D.; Crisp, G. T.; Latif, L. Inorg. Chim. Acta 1981, 47, 171.
(26) CO insertion into the Pd-CH3 bonds of (diphosphine)Pd(CH3)Cl and
(diphosphine)Pd(CH3)L+ is also accelerated by diphosphines with large bite
angles: Dekker, G. P. C. M.; Elsevier, C. J.; Vrieze, K.; van Leeuwen, P. W.
N. M. Organometallics 1992, 11, 1598.
18a
in the case of reductive elimination from R3PAuR′3 and (R3P)2PdR′2.1d
(20) For a good discussion of these effects, see: Jung, M.; Gervay, J. J.
Am. Chem. Soc. 1991, 113, 224.