Angewandte
Chemie
TS2 follows, leading to I2, which rapidly yields the products
[
13]
by re-coordination of the displaced arsane. In good agree-
0
ment with the calculated DG value for the transmetalation
0
À1
equilibrium M1 + M2 = M3 + M4 (DG = À8.6 kcalmol ),
the experimental transmetalation reaction is completely
shifted to the right, as observed by F NMR spectroscopy
Figure 1). This DG value cannot be determined experimen-
1
9
0
(
tally because the concentration of 1 in the final equilibrium is
not detectable.
The free energy order of the theoretical transition states,
TS1 < TS2, is as required from the kinetic experimental
studies, and intermediates I1 and I2 are sufficiently high in
energy to be undetectable by NMR spectroscopy. The actual
theoretical values in solvent are 3.5 (for TS1) and 2.1 (for
Figure 6. Calculated structure found for I1, induced by Au···Pd inter-
actions, compared with a plausible structural alternative (I1a).
À1
TS2) kcalmol higher than the corresponding values calcu-
lated from the experimental data. As for the energy
separation between the two kinetically relevant transition
bonds to make room for the metallophilic AuÀPd interaction.
For example, the structure of intermediate I1 might have been
expected to be I1a (Figure 6), having Au and Pd simply
À1
states, the experimental difference TS ÀTS is 2.9 kcalmol ,
2
1
À1
and the DFT difference TS2ÀTS1 is 1.5 kcalmol in solvent
À1
and 4.6 kcalmol in gas phase, both of which are close to
connected by a bridging Cl ligand, with a PdÀCl distance of
[
14]
[16]
2
.9. These results are in line with the kinetic proposal.
The theoretical calculations provide details of the mech-
about 2.3 ꢀ,
most.
and an AuÀCl distance of about 2.5 ꢀ, at
[
17]
Far from this naive prediction, the calculated
anistic profile and other very interesting features of the
system. First of all, they show that the transition state with the
highest energy in the transmetalation multistep process is
TS2, which has a structure in which Au and Pd are exchanging
X (Cl) for R (Ph) (Figure 5). This is in common with the
intermediate (I1) shows a very large PdÀCl distance
[18]
(2.74 ꢀ),
which seems to be a bonding compromise to
make room for a metallophilic AuÀPd interaction at 2.97 ꢀ.
In fact, the whole transmetalation process pivots around these
metallophilic interactions, which come close to the range of
covalent PdÀAu bonds at the heart of the transmetalation
[
14,15]
Negishi and Stille processes studied before,
in which the
Zn/Pd or Sn/Pd TS exchanging X (Cl) for R (Ph) is also the
highest barrier in the multistep transmetalation.
(the Cl for Ph exchange step) in TS2 (Figure 5).
In conclusion, the study of the transmetalation in Au/Pd
systems shows that aryl transmetalation from [AuArL] to
[
PdArXL ] complexes is thermodynamically disfavored and
2
will require a subsequent irreversible reductive elimination
from [PdAr L ] to form ArÀAr and pull the reaction forward.
2
2
The starting and final steps of the transmetalation process
involve initial L release giving rise to a bimetallic system, and
final L re-coordination splitting the metal–metal PdÀAu
interaction. Strong AuÀPd interactions in the intermediates
and transition states seem to be crucial to their stabilization.
The Cl for R exchange step has the highest activation energy.
The features observed herein might occur in other systems
prone to produce metallophilic interactions (as observed in
the PtÀCu and PtÀAu cationic systems studied by Chen), and
are particularly expected for heavier Group 10, 11, and 12
metals and their combinations.
Received: November 15, 2011
Revised: January 9, 2012
Figure 5. Calculated structure of TS2.
Published online: && &&, &&&&
Keywords: gold · homogeneous catalysis · palladium ·
reaction mechanisms · transmetalation
The calculated structures of TS2 (Figure 5), I1 (Figure 6),
and the other intermediates and transition states (SI), are very
informative. The AuÀPd distances in all the bimetallic species,
.
shown in the reaction profile in Figure 4, are in the range
2
.97–2.80 ꢀ, which is much shorter than the sum of van der
[
b) For bimetallic synthesis and catalysis, including Au/Pd
systems, see: M. H. Pꢁrez-Temprano, J. A. Casares, P. Espinet,
Chem. Eur. J. 2012, 18, 1864 – 1884.
Waals radii of Au and Pd (3.29 ꢀ) and very close to covalent
bond distances (the sum of covalent radii is 2.75 ꢀ). This
indicates very strong metallophilic interactions, which are
formed even if this requires elongation of other alternative
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3
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