Dynamic processes in silyl palladium complexes: Evidence for intermediate Si-H and Si-Si σ-complexes

Article abstract of DOI:10.1021/ja063344j

The silyl palladium complexes (dcpe)PdH(SiH^tBu₂) and (dcpe)Pd(SiHMe₂)₂ display NMR spectra that vary with temperature. The dynamic NMR behavior is consistent with long-lived σ-complexes as intermediates. In the case of (dcpe)PdH(SiH^tBu₂), the intermediate is believed to be a symmetric complex with doubly bridged hydrogen atoms between the silicon and palladium. Dynamic interchange of the two silicon atoms in (dcpe)Pd(SiHMe₂)₂ is consistent with an intermediate Si-Si σ-complex. Copyright

Full text of DOI:10.1021/ja063344j

Published on Web 06/23/2006
Dynamic Processes in Silyl Palladium Complexes: Evidence for Intermediate
Si-H and Si-Si σ-Complexes
Robert C. Boyle, Douglas Pool, Heiko Jacobsen, and Mark J. Fink*
Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118
Received May 13, 2006; E-mail: fink@tulane.edu
The activation of Si-H and Si-Si bonds by transition metals is
a key step in catalytic hydrosilation1 and bis-silation2 reactions.
Knowledge of the detailed mechanism of these bond activations is
important for controlling these catalytic reactions as well as for
providing useful comparisons to C-H and C-C bond activations.
In the case of silicon, multicentered Si-H and Si-Si σ-complexes
likely play an important role.3,4 One characteristic of these
complexes is their ability to undergo rapid fluxional processes
involving either rotations perpendicular to the Si-H or Si-Si bond
axis or exchange of hydrides between the metal and the silicon.
We now report evidence for both processes, including evidence
for a transitory Si-Si σ-complex.
We have previously reported that the reactions of tertiary silanes
with the low-valent palladium complex, [(µ-dcpe)Pd]2 (1, dcpe )
1,2-bis(dicyclohexylphosphino)ethane), give silyl palladium hy-
drides which exhibit fluxionality due to intramolecular Si-H
coordination interchange about the palladium center.5 The temper-
ature-dependent isotope effects associated with this process were
Figure 1. Room-temperature 29Si-1H HMQC spectrum of 2. Inset: Cross-
interpreted as evidence for intermediate Si-H σ-complexes. We
now report the first reactions of 1 with secondary silanes to give
silyl palladium hydrides as well as bis(silyl) palladium complexes.
Both types of complexes display fluxional behavior that we attribute
to the intermediacy of a σ-complex.
peaks due to exchanging hydrogens.
a single resonance at δ ) 53.8. Unexpectedly, the coupled room-
temperature 31P NMR spectrum of 2 gives a broadened triplet (JP-H
) 50 Hz), not the expected doublet that has been previously
observed for tertiary silyl palladium hydrides. This implies that, at
room temperature, the phosphorus nuclei are strongly coupled to
two equivalent protons, which indicates the presence of a second
dynamic process not observed for tertiary silyl palladium hydrides.
The 1H NMR spectrum at room temperature shows no discernible
resonances due to Si-H or Pd-H. Since the expected average of
these two chemical shifts is δ ) 1.47, it is likely that the averaged
resonance would be obscured by the tert-butyl resonance or the
broad signal of the dcpe ligand. The resonance of the exchanging
The reaction between 1 and excess H2SitBu2 yields the silyl
palladium hydride, (dcpe)Pd(H)SiHtBu2 (2), which is in equilibrium
with the starting materials. The complex could not be isolated since
attempted removal of solvent in vacuo results in the reversion of 2
to the dinuclear palladium complex 1 and the volatile H2SitBu2.
The complex 2, however, does form stable solutions in the presence
of excess silane.6
1
protons, however, can be resolved in the two-dimensional H-
29Si HMBC and HMQC spectra (Figure 1). A cross-peak is found
2
which correlates the 29Si triplet (δ ) 33, JSiP ) 66 Hz) with a
proton resonance at δ ) 1.4, roughly the average of the low-
temperature Si-H and Pd-H resonances. All of these observations
are consistent with a dynamic process by which the palladium
hydride and the silicon hydride rapidly exchange at room temper-
ature.
Exchange of hydrogen (or H/D exchange) between a metal and
an R-carbon atom is a classic test for determining the presence of
a C-H σ-complex.7 The exchange is usually slow, requiring
observation of H/D exchange in isotopically labeled complexes.
Exchange rapid enough to be observed on the NMR time scale is
very rare, and to our knowledge has been reported only for the
metal-alkane complex, (C5Me5)Os(dmpm)(CH3)H+.8 Recently, the
related fast interchange of coordinated and pendant Si-H groups
in the silane complex, Mo(CO)5(η2-H2SiPh2), has been reported
on the basis of NMR line broadening.9
As previously reported for mononuclear silyl palladium hydrides,
2 displays NMR spectra that vary with temperature.5 In toluene-d8
at -80 °C, the NMR spectrum of 2 is consistent with a square
planar Pd(II) coordination environment. Two distinct resonances
(δ ) 60 and 63) are observed in the 31P{1H} NMR spectrum.
Removal of broadband 1H decoupling causes the upfield 31P
resonance to become a doublet (2JPH ) 156 Hz) due to coupling to
1
the trans Pd-H. In the H NMR spectrum, both the palladium
hydride resonance and the silicon hydride resonance appear as a
doublet of doublet of doublets. The palladium hydride resonance
appears at δ ) -1.86 (2JPH ) 156 Hz, 2JPH ) 6 Hz, 3JHH ) 6 Hz),
while the silicon hydride signal is centered at δ ) 4.80 (3JPH ) 30
3
3
Hz, JPH ) 20 Hz, JHH ) 6 Hz).
As the solution is warmed to room temperature, the resonances
in the 31P NMR spectrum broaden and eventually coalesce to give
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10.1021/ja063344j CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
deuterated analogue, (dcpe)Pd(SiDMe2)2, gives activation param-
eters of Ea ) 17.7(8) kcal/mol, ∆Hq ) 17.1(8) kcal/mol, and ∆Sq
) 11(2) cal. The activation energies are only slightly higher than
those previously obtained for tertiary silyl palladium hydrides (9-
13 kcal/mol).5
Silyl palladium hydrides with tertiary silyl groups show extraor-
dinary primary kinetic isotope effects which increase with temper-
ature. The bis(silyl) complex 3, however, shows a much less
pronounced isotope effect which decreases with temperature (at
-20 °C, kH/kD ) 1.37; at 20 °C, kH/kD ) 0.85). Although large for
a typical secondary isotope effect, it is much smaller than the
primary isotope effect that is observed for tertiary silyl palladium
hydrides. The deuterium kinetic isotope effect for this process,
however, does indicate that the silyl groups are not merely spectators
in the rotational process but these groups undergo significant
electronic changes affecting the Si-H bond. The likely cause is
the formation of an intermediate Si-Si σ-complex. Disilane
σ-complexes have recently been described in an unusual trinuclear
palladium complex in which one of the palladium atoms participates
in two agostic Si-Si interactions.13 The rigid geometry of the
complex supports these Si-Si interactions. In contrast, the fluxional
behavior of 3 provides the first example of an intermediate with
an unsupported Si-Si interaction.
Figure 2. Variable-temperature 29Si{1H} NMR of (dcpe)Pd(SiHMe2)2. Top,
80 °C; middle, 20 °C; bottom, -70 °C; experimental on left, simulation on
right.
Density functional theory calculations on the model complex
(dmpe)PdH(SiMe2H) show three local minima, A-C.10 The minima
Acknowledgment. We thank a DOE Laboratory Partnership
Award (DE-FG02-03ER46046) for financial support. We also thank
Luigi Cavallo for providing access to the MoLNaC Computing
Facilities at the Dipartimento di Chimica, Universita` di Salerno,
as well as Larry Byers and R. Morris Bullock for helpful
discussions.
A and B each correspond to rotational isomers possessing the silyl
palladium hydride structure. The trans H-H isomer, A, is the global
minimum. It is 1.1 kcal/mol more stable than its cis H-H
counterpart, B. Another intermediate, C, has a doubly hydrogen-
bridged structure with the hydrogen atoms bisecting the P2PdSi
coordination plane. The doubly bridged structure is 6.9 kcal/mol
higher in energy relative to A. This symmetric intermediate is likely
responsible for both the interchange about the palladium coordina-
tion environment and the observed Si-H/Pd-H scrambling process.
The activation energy for the rearrangement A f C is calculated
to be 8.1 kcal/mol. Reductive elimination to give (dmpe)Pd + Me2-
SiH2, in contrast, is a much more unfavorable process (∆E ) 27.5
kcal/mol; ∆G(298 K) ) 16.0 kcal/mol).
Supporting Information Available: Experimental details for
synthesis of new compounds, kinetic analysis, and spectra, as well as
computational details and Cartesian coordinates for optimized molecules
are available. This material is available free of charge via the Internet
at http://pubs.acs.org.
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