The first stable mononuclear silyl palladium hydrides

Article abstract of DOI:10.1021/ja029853c

The reaction of tertiary silanes with the low valent palladium complex [(μ-dcpe)Pd]₂ affords equilibrium mixtures with mononuclear silyl palladium hydrides. These complexes have been characterized by NMR spectroscopy and, in one case, by X-ray

Full text of DOI:10.1021/ja029853c

Published on Web 02/21/2003
The First Stable Mononuclear Silyl Palladium Hydrides
Robert C. Boyle, Joel T. Mague, and Mark J. Fink*
Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118
Received December 20, 2002 ; E-mail: fink@tulane.edu
Palladium-catalyzed reactions of hydrosilanes are involved in a
palladium complex 1 result in the fast establishment of an
equilibrium represented by eq 2. Measurement of equilibrium
constants by 31P{1H} NMR results in a stability order of 3a > 3b
> 3c > 3d with Keq relative to 3d (R ) Et) of 2400:310:24:1,
respectively. A similar dependence of complex stability with
increasing substitution by electronegative groups has been observed
for the ∆Hq for the reductive elimination of silanes from Cp-
(CO)2MnHSiR3 complexes.9
number of important synthetic methodologies.1 Silyl palladium
complexes are certainly key intermediates in these reactions; yet
as compared to the better known platinum congeners, the properties
of these complexes and their mechanistic roles are much less
understood. Stable silyl palladium complexes are rare and are
typically stabilized by either very electronegative substituents (i.e.,
SiCl3),2 inclusion of the silyl substituent in a chelate ring,3 or via
interactions between two or more metal centers.4 Previously, we
have reported a class of simple bis(silyl) palladium complexes,
(dcpe)Pd(SiR3)2, bearing the chelating bisphosphine ligand dcpe,5
from the reaction of either [(dcpe)Pd(µ-H)]2 or [(µ-dcpe)Pd]2 with
primary or secondary hydrosilanes.6,7 Presumably, the bis(silyl)
palladium complexes are formed by sequential oxidative additions
via a silyl palladium hydride intermediate, although these complexes
were not detected in our early experiments. We now report the
first known examples of mononuclear silyl palladium hydrides, the
first solid state structure of such a complex, and highly unusual
kinetic isotope effects associated with their dynamic NMR behavior
in solution.
(dcpe)PdH(SiR3) + HSiR′ h (dcpe)PdH(SiR′) + HSiR3
3
3
(2)
An X-ray structure determination was performed on a crystal of
3a grown in hexane solution.10 The two independent molecules of
3a in the unit cell are essentially identical; one of the molecules is
shown in Figure 1. Although the position of the metal bound
hydrogen could not be determined,11 the effects of its coordination
are apparent in the structure. The coordination geometry about the
palladium is reasonably described as distorted square planar with
the sum of the bond angles involving the non-hydrogen atoms being
357° (ideal planar, 360°).
The dinuclear complex [(µ-dcpe)Pd]2 (1) was reacted with the
tertiary hydrosilanes (2a-2d) at room temperature in toluene
solution (eq 1). Stoichiometric reaction of Ph3SiH or Ph2MeSiH
with 1 results in the instantaneous dissipation of the red color of 1
and the quantitative formation (by 31P{1H} NMR) of the silyl
palladium hydrides 3a and 3b, respectively. The reaction of 1 with
stoichiometric amounts of PhMe2SiH, however, is much slower (3
days) and gives only 30% yield of 3c along with a 25% yield of
the palladium hydride, [(dcpe)Pd(µ-H)]2 (4).8
The silyl palladium hydrides 3a-3d show highly temperature-
dependent NMR spectra. In d8-toluene solution at room temperature,
each complex shows a single resonance in the 31P{1H} NMR
spectrum (δ ) 55-58 ppm), which upon cooling to -80 °C
broadens and recoalesces to two distinct resonances. One of the
31P resonances (trans to hydride) is shown to couple strongly with
one proton. In the 1H NMR spectrum at room temperature, a 1:2:1
triplet is observed for the metal hydride resonance (δ ) -1 to -2
ppm), which at -80 °C evolves into a partially resolved doublet
of doublets from coupling to two unique phosphorus atoms. Both
1
the low temperature H and 31P{1H} NMR spectra (-80 °C) are
fully consistent with the classical square planar structure that is
found for 3a in the solid state. The temperature dependence of these
spectra is explainable by rapid intramolecular interchange of
coordination environments between the silyl and hydride substit-
uents.12 Activation parameters for this process were obtained for
several complexes and their metal deuteride analogues as shown
in Table 1.13
Stereochemical nonrigidity has been previously observed for
related Pd(II) and Pt(II) systems which include P2PdSn2,14 P2PtSi2,14
P2PtH2,15 and P2PtHSi16 (P ) phosphine) complexes. In the case
of P2PdSn2 and P2PtSi2 complexes, a twist-rotational mechanism
via a pseudo-tetrahedral transition state has been proposed for the
interconversion of phosphine sites. An alternative mechanism
involving the intermediacy of σ-complexes has been proposed for
interchange of phosphorus environments in certain P2PtH2 and P2-
PtHSi systems. The two mechanisms are potentially distinguishable
on the basis of kinetic isotope effects involving isotopic substitution
at the metal hydride. In the case of the silyl palladium hydrides
3a-c, there exists a remarkably strong temperature-dependent
deuterium kinetic isotope effect (KIE) when the metal hydride is
replaced by a deuterium. The metal deuterides, (3a-c)-d1, have
When PhMe2SiH is present in 20-fold excess, the reaction
proceeds more quickly (1 day), and the yields of 3c improve
considerably (80%), with only 11% of 4 being formed. The reaction
of 1 with Et3SiH is the most sluggish of the silanes (7 days), giving
only 41% yield of 3d even with a 20-fold excess of silane. The
stability of the silyl palladium hydrides in solution is limited due
to the reversibility of the reaction of the silanes with 1. For instance,
attempted removal of solvent in vacuo from a solution containing
complex 3c results in reversion to the dipalladium precursor 1 as
the volatile silane is also removed. Complexes 3c and 3d are only
stable in the presence of excess silane, a condition which suppresses
dissociation.
These results additionally suggest that the stability of the silyl
palladium hydrides is strongly dependent on the degree of phenyl
substitution on the silyl substituent. Competition experiments in
which an equimolar excess of two different silanes is added to the
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J. AM. CHEM. SOC. 2003, 125, 3228-3229
10.1021/ja029853c CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. We thank Frank Froncek at the LSU
Department of Chemistry for obtaining the crystallographic data
set used for the X-ray structure determination, and Professor Larry
Byers for helpful discussions. The Dow Corning Corp. and the NSF
(Grant No. 0092001) are acknowledged for financial support, and
the NSF is acknowledged (CHE-0078054) for the purchase of a
400 MHz NMR.
Supporting Information Available: Crystallographic data for 3a
(CIF), experimental preparations for 3a-d, and kinetic data (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
Figure 1. ORTEP drawing of one of the two independent molecules of
(dcpe)Pd(H)SiPh3 (3a) in the unit cell. Thermal ellipsoids are at the 50%
probability level; cyclohexyl carbons are represented by spheres of arbitrary
radius. Important bond lengths (Å) and angles (deg) for molecule 1 followed
by equivalent values for molecule 2: Pd1-Si1, 2.335(2), 2.330(2); Pd1-
P1, 2.350(2), 2.350(2); Pd1-P2, 2.323(2), 2.323(2); Si1-Pd1-P1, 166.09-
(8), 166.58(8); Si1-Pd1-P2, 104.28(8), 104.99(8); P1-Pd1-P2, 86.27(8),
87.12(8).
(1) (a) ComprehensiVe Handbook of Hydrosilylation; Marcineac, B., Ed.;
Pergamon Press: New York, 1992. (b) Suganome, M.; Ito, Y. Chem. ReV.
2000, 100, 3221. (c) Metal-Catalyzed Cross-Coupling Reactions; Dieder-
ich, F., Stang, P. J., Eds.; Wiley-VCH: New York, 1998. (d) Cross-
Coupling Reactions; A Practical Guide; Miyuura, N., Ed.; Springer: New
York, 2002.
(2) (a) Eaborn, C.; Griffiths, R. W.; Pidcock, A. J. Organomet. Chem. 1982,
225, 331. (b) Schubert, U.; Muller, C. J. Organomet. Chem. 1989, 373,
165.
(3) (a) Curtis, M. D.; Greene, J. J. Am. Chem. Soc. 1978, 100, 6362. (b)
Seyferth, D.; Goldman, E. W.; Escudie, J. J. Organomet. Chem. 1984,
271, 337.
Table 1. Activation Parameters for Intramolecular Si-H
Interchange in Selected Silyl Palladium Hydrides and Deuterides
q
q
(4) (a) Kim, Y.-J.; Lee, S.-C.; Park, J. I. Organometallics 1998, 17, 4929.
(b) Kim, Y.-J.; Lee, S.-C.; Park, J. I.; Kohtano, O.; Choi, J.-C.; Yanamato,
T. J. Chem. Soc., Dalton Trans. 2000, 417. (c) Choi, S.-H.; Lin, Z. J.
Organomet. Chem. 2000, 608, 42.
Ea
∆H
(kcal mol-1
∆S
(cal mol-1 K-1
)
complex
(kcal mol-1
)
)
(dcpe)Pd(H)SiPh3
(dcpe)Pd(D)SiPh3
(dcpe)Pd(H)SiPh2Me
(dcpe)Pd(D)SiPh2Me
(dcpe)Pd(H)SiMe2Ph
(dcpe)Pd(H)SiEt3
12.9(4)
8.0(6)
10.7(6)
6.8(7)
10.7(6)
9.6(6)
11.8(4)
8.7(4)
12.4(4)
6.5(3)
10.2(6)
6.4(7)
10.3(6)
9.2(6)
11.6(5)
7.6(4)
8.3(8)
-15.6(6)
-1(1)
-18.3(2)
-1(1)
-3(1)
6.8(1)
(5) dcpe ) 1,2-bis(dicyclohexylphosphino)ethane; dippe ) 1,2-bis(diisopro-
pylphosphino)ethane.
(6) Pan, Y.; Mague, J. T.; Fink, M. J. Organometallics 1992, 11, 3495.
(7) Pan, Y.; Mague, J. T.; Fink, M. J. J. Am. Chem. Soc. 1993, 115, 3842.
(8) The remaining phosphorus-containing product (45%) is the phosphine
oxide of dcpe from the reaction of 1 with adventitious oxygen.
(dippe)Pd(H)SiPh3
(dippe)Pd(D)SiPh3
(9) (a) Schubert, U. AdV. Organomet. Chem. 1990, 30, 151. (b) Corey, J. Y.;
-10.0(3)
Braddock-Wilking, J. Chem. ReV. 1999, 99, 175.
(10) Complete characterization (room temperature) for 3a, 1H NMR (δ, ppm):
-1.81 (1H, triplet; JP-H ) 77), 0.8-2 (48H, broad), 7.1 (3H, mult), 7.2
(6H, mult), 8.4 (6H, mult). 13C{1H}NMR (δ, ppm): 26.3, 26.9, 27.0, 27.2,
activation enthalpies 4-6 kcal mol-1 less than their corresponding
hydrides, whereas the activation entropies are almost 17-25 cal
mol-1 K-1 more negative. The resultant effect is a crossover in the
KIE (kH/kD) over the measured temperature range. The KIE is
inverse at low temperatures (3a: -60 °C; kH/kD ) 0.13) but normal
at higher temperatures (3a: -10 °C; kH/kD ) 2.0).17 Temperature-
dependent changes in KIE are typically indicative of a change of
the rate-limiting step, implying that there exist two or more
consecutive steps (and one or more intermediates) in the fluxional
process. A reasonable intermediate would be a discrete well-defined
Si-H σ-complex.18
The fluxionality in silyl palladium hydrides may parallel the steps
involved in C-H reductive elimination. Both normal and inverse
isotope effects have been observed for the reductive elimination
of C-H bonds in alkylmetal hydrides, a process proposed to
proceed via intermediate C-H σ-complexes.19 The differences in
the observed isotope effects (normal vs inverse) for C-H reductive
elimination have been attributed to differences in the rate-
determining steps for the formation (via reductive coupling) and
dissociation of the alkane σ-complexes.20 The fluxionality of the
silyl palladium hydrides may reflect a similar situation, whereupon
the relative free energies of the transition states associated with
the formation of the Si-H σ-complex (via reductive coupling) and
its isomerization (via rotational interchange) are highly temperature
dependent, resulting in both inverse and normal KIEs for the same
complex. We are currently pursuing computational and mechanistic
studies to help elucidate the details of the hydride exchange and to
determine the origin of this remarkable isotope effect.
1
2
27.3, 29.1, 30.1, 30.2, 34.6 (d of d: JPC ) 13.0, JPC ) 2.3), 126.9,
127.2, 128.2, 136.1, 137.4, 148.6 (t: JPC ) 5.4). 31P{1H}NMR (δ, ppm):
2
58.23 (singlet). 29Si NMR: δ -5.08. J(SiH) ) 29 Hz. IR (KBr, cm-1):
1895 (PdH), 1427 (SiPh). Anal. Calcd for C44H64SiP2Pd: C, 66.98; H,
8.18. Found: C, 66.10; H, 8.44.
(11) The presence of a palladium hydride in the solid is clearly evident from
the stretching frequency ν(Pd-H, KBr) ) 1895 cm-1 in the IR spectrum.
(12) The intramolecular nature of the interchange is evidenced by the retention
of P-H (hydride) and P-Si coupling at both low- and high-temperature
exchange limits as well as by the insensitivity of the exchange rate to the
concentration of the complex and to extraneous silane.
(13) Rate constants were obtained at various temperatures employing full line
shape analysis with the gNMR 4.1 software package (Budzelaar, P. H.
M. Cherwell Scientific Limited, 1999). Erying plots of the rate constants
over the measured temperature range afforded the experimental activation
parameters.
(14) (a) Tsuji, Y.; Obura, Y. J. Organomet. Chem. 2000, 611, 343. (b) Tsuji,
Y.; Nishiama, Y.; Hori, S.; Ehihara, M.; Kawamura, T. Organometallics
1998, 17, 507.
(15) (a) Clark, H. C.; Hampden-Smith, M. J. J. Am. Chem. Soc. 1986, 108,
3829. (b) Clark, H. C.; Hampden-Smith, M. J. Coord. Chem. ReV. 1987,
79, 229.
(16) Azizian, H.; Dixon, K. R.; Eaborn, C.; Pidcock, A.; Shuaib, N. M.; Vinaixa,
J. J. Chem. Soc., Chem. Commun. 1982, 1020.
(17) If the KIE for 3a was extrapolated to ambient temperatures (20 °C), then
kH/kD ) 6.3. The relatively extreme values of the KIEs at both low and
high temperatures imply that quantum mechanical tunneling may be an
important contributor for the hydride interchange.
(18) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure Theory
and ReactiVity; Kluwer Academic/Plenum Publishers: New York, 2001;
Chapter 11 and references therein.
(19) Parkin, G.; Bercaw, J. E. Organometallics 1989, 8, 1172.
(20) For recent discussions of kinetic isotope effects and C-H reductive
elimination, see the following. (a) Churchill, D. C.; Janak, K. E.;
Wittenburg, J. S.; Parkin, G. J. Am. Chem. Soc. 2003, 125, 1403. (b) Jones,
W. D. Acc. Chem Res. 2003, 36, in press.
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