Bimetallic reductive elimination from dinuclear Pd(III) complexes

Article abstract of DOI:10.1021/ja1036644

In 2009, we reported C-halogen reductive elimination reactions from dinuclear Pd(III) complexes and implicated dinuclear intermediates in Pd(OAc)₂-catalyzed C-H oxidation chemistry. Herein, we report results of a thorough experimental and theor

Full text of DOI:10.1021/ja1036644

Published on Web 09/21/2010
Bimetallic Reductive Elimination from Dinuclear Pd(III)
Complexes
David C. Powers,† Diego Benitez,‡ Ekaterina Tkatchouk,‡ William A. Goddard III,‡
and Tobias Ritter*,†
Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street,
Cambridge, Massachusetts 02138, and Materials and Process Simulation Center, California
Institute of Technology, Pasadena, California 91125
Received April 29, 2010; E-mail: ritter@chemistry.harvard.edu
Abstract: In 2009, we reported C-halogen reductive elimination reactions from dinuclear Pd(III) complexes
and implicated dinuclear intermediates in Pd(OAc)2-catalyzed C-H oxidation chemistry. Herein, we report
results of a thorough experimental and theoretical investigation of the mechanism of reductive elimination
from such dinuclear Pd(III) complexes, which establish the role of each metal during reductive elimination.
Our results implicate reductive elimination from a complex in which the dinuclear core is intact and suggest
that redox synergy between the two metals is responsible for the facile reductive elimination reactions
observed.
Introduction
and, more importantly, gives insight into evaluating and under-
standing metal-metal redox synergy in general.
Metal-metal redox cooperation during catalysis can potentially
lower activation barriers of chemical transformations and thus
allow access to reaction pathways that are difficult to access
with mononuclear catalysts.1 Redox chemistry of multinuclear
complexes has been intensely studied, in part due to potential
advantages in catalysis; however, metal-metal cooperation
during redox chemistry has been difficult to establish. Mono-
nuclear Pd(IV) complexes have been suggested as intermediates
in palladium-catalyzed C-H oxidation reactions since 1971.2
In 2009, we proposed dinuclear Pd(III) intermediates in catalysis
as an alternative to Pd(II)/Pd(IV) redox cycles.3 In a preliminary
study of C-Cl reductive elimination from the dinuclear Pd(III)
complex 1 (eq 1), we were unable to establish whether reductive
elimination proceeded with simultaneous, co-operative redox
participation of both metals or via one-centered redox chemistry.
Herein, we report a detailed investigation of the mechanism of
C-Cl reductive elimination from 1 and present a tool for
evaluating metal-metal synergy during redox transformations
of multinuclear complexes. Our results implicate synergistic
redox chemistry of both metals during reductive elimination,
which leads to an energetic advantage as compared to related
monometallic reductive elimination reactions. This analysis is
the first detailed evaluation of reductive elimination from Pd(III)
Redox catalysis in synthesis is often accomplished by
homogeneous, mononuclear catalysts.4 Oxidative addition and
reductive elimination, two of the fundamental redox transforma-
tions of organometallic chemistry, have been well studied for
mononuclear complexes. Oxidative additions are reactions in
which the oxidation state and coordination number of a transition
metal center increase by two upon addition of a small molecule.5
Oxidative addition can proceed via concerted,6 SN2-like,7 and
radical8 mechanisms. Reductive eliminationsformally the re-
verse of oxidative additionsdescribes a transformation in which
the oxidation state and coordination number of a metal are
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Tarr, D. A. Inorganic Chemistry, 3rd ed.; Pearson Education, Inc.:
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Dougherty, D. A. Modern Physical Organic Chemistry; University
Science Books: Sausalito, CA, 2006; pp 724-726.
† Harvard University.
‡ California Institute of Technology.
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14092 J. AM. CHEM. SOC. 2010, 132, 14092–14103
10.1021/ja1036644  2010 American Chemical Society

Reductive Elimination from Dinuclear Pd(III) Complexes
A R T I C L E S
reduced by 2, with concurrent formation of a small molecule.
Like oxidative addition, reductive elimination can proceed via
various mechanisms.9 In a catalytic cycle, the mechanisms of
oxidative addition and reductive elimination need not be the
microscopic reverse of one another.
as intermediates in catalysis.3,15 The potential of utilizing metal-
metal redox synergy to accomplish challenging transformations
has long been recognized, and while proposals of mechanisms
involving metal-metal cooperation have been posited,16 iden-
tification of the intimate role of each metal center during redox
transformations at dinuclear complexes is difficult to establish
experimentally.14i-k,17 Understanding how metal-metal redox
synergy can be used in catalysis requires insight into the role
of each metal during redox chemistry.
Redox chemistry in biology10 and heterogeneous catalysis11
often occurs at multinuclear sites. For example, biologically
relevant redox catalysis, such as the reduction of dinitrogen by
nitrogenase10a,b and the oxidation of methane by methane
monooxygenase,10c-e is frequently accomplished by multi-
nuclear active sites. While redox catalysis involving more than
one metal is frequently encountered, it is difficult to ascertain
the specific role of each individual metal center during a redox
transformation.
During our efforts to utilize metal-metal redox synergy in
catalysis, we found it useful to employ a two-tiered nomencla-
ture scheme in which organometallic redox transformations are
classified by both the nuclearity of the complex undergoing the
redox transformation and the metallicity of the transformation.
Nuclearity is a descriptor of structure and refers to the number
of metal centers present in the complex undergoing the redox
transformation. Metallicity is a descriptor of the mechanism of
a redox transformation and refers to the number of metal centers
that undergo redox chemistry coupled to substrate oxidation or
reduction. The presence of multiple metal centers (nuclearity)
in a redox transformation does not necessitate redox participation
(metallicity) of all metal centers.
The mechanisms of redox transformations at multinuclear
complexes are less understood than the corresponding reactions
of mononuclear complexes.12 Stoichiometric oxidative addition13
to and reductive elimination14 from dinuclear complexes have
been observed, and dinuclear intermediates have been proposed
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• Nuclearity s descriptor of structure; the number of metal
centers in a complex that undergoes a redox transformation.
• Metallicity s descriptor of mechanism; the number of metal
centers that participate in redox chemistry during a redox
transformation.
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One can, in principle, determine the nuclearity of a redox
transformation using the tools of reaction kinetics. For example,
fragmentation of a dinuclear complex into two identical mono-
nuclear complexes prior to reductive elimination can afford a
half-order rate law with respect to the dinuclear transition metal
complex.18 While redox transformations that take place at
mononuclear complexes are by definition monometallic,19 redox
transformations at dinuclear complexes can be either mono- or
bimetallic, depending on whether one or both metals change
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(19) In this conversation, we are excluding transition metal complexes in
which significant redox participation of the ligand is involved. For
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Berry, T. E. J. Am. Chem. Soc. 1967, 89, 2866–2874. (b) Pierpont,
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A R T I C L E S
Powers et al.
their oxidation state in the redox transformation. Differentiating
between mono- and bimetallic mechanisms is experimentally
challenging.
Mono- and bimetallic transformations are kinetically indis-
tinguishable, because the transition states do not differ in
chemical composition but only in specific roles of the metal
center in the dinuclear core. Further, the identity of the transition
metal fragment produced by a redox transformation is not
sufficient to assign the metallicity of a given reaction. For
example, the oxidative addition of MeI to dinuclear Au(I)
complex 4 to afford dinuclear Au(II) complex 6 illustrates the
difficulty in assigning the metallicity of a redox transforma-
tion.13d,f,h If oxidative addition proceeds initially at a single Au
site to afford the Au(I)/Au(III) mixed-valence species 5, before
comproportionative isomerization to complex 6, the oxidative
addition to 5 is monometallic (eq 2). Alternatively, if oxidative
addition proceeds with Au-Au bond formation concurrent with
Au-C bond formation to form 6 via 7 (eq 3), the redox
transformation is bimetallic.
Figure 1. Synthesis of 1. ORTEP drawing of 9 and 1 with ellipsoids drawn
at 50% probability (hydrogen atoms omitted for clarity). Selected bond
lengths in 1 [Å]: Pd(1A)-Pd(1B), 2.5672(5); Pd(1A)-Cl(A), 2.4167(10);
Pd(1A)-C(A), 2.000(4); Pd(1A)-O(1A), 2.133(3); Pd(1A)-O(2A), 2.042(3);
Pd(1A)-N(A), 2.016(3). For comparison, the Pd-Pd distance in 9 is
2.8419(8) Å.
in the redox chemistry of reductive elimination? The redox
activity of each metal is difficult to evaluate experimentally;
no experimental observable can be directly correlated with
reaction metallicity. We have thus pursued a computational
investigation of reaction metallicity. The computational model
used to assay metallicity was benchmarked by comparing the
computed mechanism with three experimental observables: (1)
the ground-state structure of 1, (2) the activation parameters
for C-Cl reductive elimination from 1, and (3) the ability of
computed charge distributions to correlate with experimentally
obtained F-values from Hammett analysis.
Development of catalysis concepts that rely on metal-metal
synergy during redox transformations requires fundamental
understanding of the role of each metal involved in the redox
transformation. Potentially, metal-metal cooperation during
catalysis could allow development of reactions that are difficult
to accomplish with monometallic systems. We have sought to
evaluate the potential advantage of utilizing two metals during
redox chemistry in desirablesbut currently challengings
transformations. Transition-metal-mediated halogenation reac-
tions are difficult, and only a few methods are available.21
Reductive elimination from dinuclear Pd(III) complexes is facile
and provides a venue to better understand and characterize
reductive elimination from dinuclear complexes.3 Herein, we
provide evidence for synergistic redox participation of both
palladium centers during reductive elimination and show that
bimetallic reductive elimination is more facile than related
monometallic processes. The observed facility of redox trans-
formations at bimetallic Pd(III) complexes challenges the
previously proposed generality of Pd(II)/Pd(IV) redox cycles.22
Synthesis and Structure of Pd(III) Dichloride 1. Dinuclear
Pd(III) complex 1 was chosen as a suitable substrate to
investigate reductive elimination from dinuclear Pd complexes
for two reasons. First, Pd(OAc)2 is frequently used as a catalyst
in directed C-H oxidations. Second, the bridging acetate ligands
hold the two palladium centers in proximity, and therefore allow
for metal-metal interaction.23 Treatment of complex 9, derived
from cyclometalation of benzo[h]quinoline (8) with Pd(OAc)2,
with PhICl2 resulted in the formation of dinuclear Pd(III)
dichloride 1 in 92% yield (Figure 1).
By 1H NMR spectroscopy, complex 1 is diamagnetic,
consistent with a Pd-Pd single bond. At -50 °C, the 1H NMR
spectrum displays eight nonequivalent aromatic resonances and
a single resonance for the bridging acetate ligand at 2.69 ppm.
In the solid state, the Pd-Pd distance in 1 is 2.5672(5) Å, 0.27
Å shorter than the corresponding distance in 9, consistent with
the formation of a Pd-Pd bond upon oxidation.24 The Pd-O
bonds are nonequivalent; the Pd-O bond trans to the carbon
ligand is 2.133(3) Å, and the Pd-O trans to the nitrogen ligand
Results
Two fundamental questions are addressed in this manuscript.
First, does C-Cl reductive elimination from 1 proceed from a
mono- or dinuclear complex? Second, do both metals participate
(20) Tolman, C. A.; Meakin, P. Z.; Lindner, D. L.; Jesson, J. P. J. Am.
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Soc. 1998, 120, 13280–13281. (b) Cotton, F. A.; Koshevoy, I. O.;
Lahuerta, P.; Murillo, C. A.; Sanau´, M.; Ubeda, M. A.; Zhao, Q. L.
J. Am. Chem. Soc. 2006, 128, 13674–13675. (c) Berry, J. F.; Bill, E.;
Bothe, E.; Cotton, F. A.; Dalal, N. S.; Ibragimov, S. A.; Kaur, N.;
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2006, 62, 11483–11498. (b) Mun˜iz, K. Angew. Chem., Int. Ed. 2009,
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Reductive Elimination from Dinuclear Pd(III) Complexes
A R T I C L E S
Scheme 1. Rapid Exchange between Bridging and Apical Acetate
Ligands Leads to a 1:1 Mixture of 12 and 12-d3 upon Thermolysis
of 10-d6
Scheme 2. Treatment of the Reaction Mixture after Reductive
Elimination with Excess Pyridine Affords a Mixture of 2, 13, 14, 15,
16, and 17 that Establishes the +II Oxidation State for Palladium
is 2.042(3) Å, consistent with the stronger structural trans effect
of the aryl ligand as compared to the substituted pyridine
ligand.25
Figure 2. Hammett plot based on 7-benzo[h]quinolinyl substitution.
containing species, identified as Pd(II) complexes 13, 14, 15,
16, and 17 on the basis of comparison of the 1H NMR spectrum
Potential ligand fluxionality, in which the apical and bridging
ligands might exchange with one another, was probed by
variable-temperature NMR spectroscopy of dinuclear Pd(III)
tetraacetate 10-d6, in which the bridging acetate ligands were
1
of the mixture with the H NMR spectra of authentic samples
1
(Scheme 2). By H NMR, the combined yield of complexes
13-17 was determined to be 99%. The observed products of
reaction with pyridine establish that the oxidation state of Pd
in 3 is +II. The exact structure of the Pd(II) complex
immediately after reductive elimination is unknown. (See the
Supporting Information for further discussion of the identity of
the Pd-containing byproducts of reductive elimination.)
1
perdeuterated. At -60 °C, the H NMR spectrum of 10-d6
1
displayed a single H NMR resonance at 1.48 ppm (apical
acetate). Upon warming of the sample to -30 °C, the integration
of the signal at 1.48 ppm (apical acetate) decreased with
concurrent development of a new resonance at 2.71 ppm
(bridging acetate). Acetate scrambling was determined to be
fast relative to reductive elimination; warming a sample of 10-
d6 to 23 °C afforded a 1:1 mixture of 12 and 12-d3 (Scheme
1).26
Effect of 7-Substitution of the Benzo[h]quinolinyl Ligand
on the Rate of C-Cl Reductive Elimination. Dinuclear Pd(III)
dichloride complexes 18a-e with 7-substituted benzo[h]quino-
linyl ligands were prepared by cyclometalation of 7-substituted
benzo[h]quinolines with Pd(OAc)2 followed by oxidation with
PhICl2. Dinuclear Pd(III) complexes with NO2 (18f) and CN
(18g) substituents were not sufficiently soluble to be used in
kinetics experiments. Thermolysis of 18a-e at 29 °C afforded
7-substituted-10-chlorobenzo[h]quinolines 19a-e in 84-95%
yield. Reaction yields did not correlate with the σ-values of the
substituents. In all cases, C-Cl bond formation was observed
exclusively; C-O reductive elimination to afford 7-substituted
10-acetoxybenzo[h]quinolines was not detected. The rate of
C-Cl bond formation was measured by monitoring the forma-
C-Cl Reductive Elimination from 1. Upon warming of
complex 1 to 23 °C, 10-chlorobenzo[h]quinoline (2) was isolated
in 94% yield based on 1 (eq 1). Monitoring the thermolysis of
1 by 1H NMR spectroscopy established that both decomposition
of 1 and formation of 2 obey first-order rate laws. The activation
parameters of C-Cl reductive elimination were determined to
be ∆Hq ) 17.2 ( 2.7 kcal ·mol-1, ∆Sq ) -11.2 ( 9.4 cal ·K-1,
and ∆Gq
) 20.5 ( 0.1 kcal·mol-1 by monitoring the
298
evolution of 2 as a function of temperature (between 5 and 35
°C). The rate of reductive elimination was unaffected by
exogenous chloride (up to 17.1 mM in n-Bu4NCl; 1.17 equiv
with respect to 1) and acetate (up to 11.8 mM in n-Bu4NOAc;
0.81 equiv with respect to 1).
27
1
tion of 19a-e by H NMR spectroscopy. The Hammett plot
generated from these data (Figure 2) shows that C-Cl reductive
elimination from dinuclear Pd(III) complexes is accelerated by
electron-withdrawing 7-substituents on the benzo[h]quinolinyl
ligand (F ) 1.46).
Reductive elimination of 2 from 1 is accompanied by the
formation of a mixture of Pd-containing products (3). Treatment
of the crude reaction mixture after reductive elimination with
excess pyridine provided a mixture of 2 and five palladium-
Effect of Bridging Carboxylate Ligand on the Rate of
C-Cl Reductive Elimination. Benzoate-bridged dinuclear
Pd(III) complexes 20a-e were prepared by treatment of 9 with
4-substituted benzoic acids at reduced pressure followed by
oxidation with PhICl2. Decomposition of 20a-e at 29 °C
afforded compound 2 in 91-96% yield, with no C-O reductive
(25) Coe, B. J.; Glenwright, S. J. Coord. Chem. ReV. 2000, 203, 5–80.
(26) Exchange of carboxylate ligands can proceed intermolecularly (see
Supporting Information). For simplicity, here we depict only those
complexes arising from intramolecular exchange; intermolecular
exchange does not change our conclusions based on these experiments
because exchange, whether intra- or intermolecular, is fast relative to
reductive elimination.
(27) Hammett, L. P. Chem. ReV. 1935, 17, 125–136.
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A R T I C L E S
Powers et al.
Scheme 4. Carboxylate Scrambling on Pd(III) Complexes Prevents
Hammett Analysis of Electronic Demand of Apical Ligands
because we have previously observed that, in the absence of
pyridine, the products of C-O reductive elimination can react
with the palladium-containing byproducts of reductive elimi-
nation.3b
Oxidation of Pd(II) benzoate 21 with p-nitrobenzoyl peroxide
(24) afforded Pd(III) complex 22, which, after subsequent
warming to 23 °C in the presence of 20.0 equiv of pyridine,
afforded a 4:1 mixture of 26 and 27, as determined by both 1H
NMR spectroscopy and isolated yields of 26 and 27 (Scheme
3a). Oxidation of p-nitrobenzoate-bridged complex 23 with
benzoyl peroxide (25) also afforded a 4:1 mixture of 26 and 27
(Scheme 3b), which confirmed that carboxylate exchange is fast
relative to C-O reductive elimination.
Figure 3. Hammett plot based on bridging benzoate substitution.
Scheme 3. Carboxylate Scrambling on Pd(III) Complexes is Fast
Relative to C-O Reductive Elimination
Intramolecular carboxylate exchange in tetrabenzoate dipal-
ladium(III) complex 22 could generate an equilibrium mixture
of three isomers (Scheme 4).26 Reductive elimination from
isomer 28 can afford either 26 or 27, while reductive elimination
from 22 or 29 can afford only 26 or 27, respectively. Without
knowledge of all four product-forming rate constants, the ratio
of 26 and 27 cannot be deconvoluted to provide kb/kb′, the
relative rate of benzoate over 4-nitrobenzoate reductive elimina-
tion.28
Oxidation of 23 with unsymmetrical peroxide 30 at -50 °C,
followed by treatment with pyridine at 23 °C, afforded a 2:1
mixture of 26 and 27 (Scheme 5).26 Intramolecular carboxylate
exchange in dinuclear Pd(III) complex 31, which features three
4-nitrobenzoate ligands and one benzoate ligand, could generate
either one of two isomers (31 or 32). Reductive elimination
from 32 can afford only 27 (k3), while reductive elimination
from 31 can afford 26 (k1) and 27 (k2). The observed 2:1 ratio
of 26 and 27 shows that k1 > k2 + k3 ·Keq and that benzoate
undergoes reductive elimination faster than 4-nitrobenzoate.
elimination observed in any case. The Hammett plot27 (Figure
3) generated from these data shows that reductive elimination
is accelerated by electron-withdrawing substituents on the
bridging carboxylate ligands (F ) +0.71). For complexes 20f
(R ) Me) and 20g (R ) OMe), bearing electron-donating
substituents, log(kx/kH) was not linearly correlated with the
substituent σ-values.
Competitive Reductive Elimination of Substituted Benzoate
Ligands. Because the electronic properties of chloride cannot
be altered, the electronic demand of the apical ligand during
reductive elimination from 1 cannot be investigated. Dinuclear
Pd(III) tetrabenzoates were selected as substrates to probe the
electronic demand of the apical ligand during reductive elimina-
tion because the electronic properties of benzoate ligands can
be modified (Scheme 3).26 In the following experiments, the
2-phenylpyridyl ligand was used in lieu of the benzo[h]quino-
linyl ligand because 2-(pyridin-2-yl)phenyl benzoate is more
stable toward hydrolysis than benzo[h]quinolin-10-yl benzoate,
which simplified product isolation and analysis. Further, pyridine
is added to the dinuclear Pd(III) complexes prior to thermolysis
(28) Attempts to accomplish Hammett analysis using esp-bridged complex
37, anticipated to prevent carboxylate exchange, were unsuccessful
because, unlike C-Cl reductive elimination from 35, C-O reductive
elimination does not proceed from esp-bridged complex 37.
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Reductive Elimination from Dinuclear Pd(III) Complexes
A R T I C L E S
Scheme 5. Oxidation of 23 with Unsymmetrical Peroxide 30
Affords a 2:1 Mixture of 26 and 27, which Establishes the Kinetic
Preference for Reductive Elimination of More Electron-Rich
Benzoate Ligands
Figure 4. First-order rate constant of C-Cl reductive elimination as a
Reductive Elimination in the Presence of Exogenous AcOH. We
have proposed that reductive elimination from dinuclear Pd(III)
complexes similar in structure to 1 is the product-forming step
of a variety of Pd-catalyzed C-H functionalizations.3 During
catalysis, substrate is present in large excess relative to Pd, and
thus we investigated C-Cl reductive elimination in the presence
of exogenous benzo[h]quinoline (8). By 1H NMR and UV-vis
spectroscopy, no interaction between complex 1 and 8 was
observed between -50 and 23 °C. We have observed a
reproducible acceleration of C-Cl bond formation in the
presence of exogenous benzo[h]quinoline (8) at 23 °C.3a We
now report a re-examination of the observed rate enhancement
for the formation of 2 from 1 upon addition of 8, which indicates
that the observed acceleration is due to acid generated by
palladation of added 8, not N-coordination of 8 to 1, as we
previously proposed.3a
Examination of the thermolysis of 1 in the presence of
exogenous benzo[h]quinoline (8) revealed that the concentration
of 8 decreased during the evolution of 2. (For quantitative kinetic
data regarding the rate of reductive elimination from 1 as a
function of initial benzo[h]quinoline concentration, see the
Supporting Information.) Cyclometalation of 8 (for example,
8f9 shown in Figure 1) by the Pd(II)-containing byproducts
of reductive elimination (3) generates an equivalent of acid, as
a result of C-H bond cleavage during metalation. We therefore
examined the potential effect of acid on the rate of reductive
elimination from 1 by monitoring the formation of 2 as a
function of AcOH concentration ([AcOH]). AcOH was selected
because potential exchange of the conjugate base, acetate, with
the acetate ligands of 1 is a degenerate process. The rate of
formation of 2 was monitored at AcOH concentrations up to
1.16 M in CD2Cl2 by 1H NMR spectroscopy. The rate of
formation of 2 is accelerated in the presence of AcOH (Figure
4). The plot of first-order rate constant vs [AcOH] has a nonzero
intercept, reflective of the background rate of C-Cl reductive
elimination in the absence of AcOH, and shows that at least
two pathways are available for C-Cl reductive elimination: one
pathway independent of and one pathway linearly dependent
on the concentration of AcOH.
function of [AcOH].
pated to be computationally challenging using density functional
theory (DFT). The combination of high-oxidation-state metal
centers, metal-metal bonding, and the coupling of the
metal-metal bond to the electronegative chloride ligands is
demanding for exchange-correlation hybrid density functionals.29
In addition, the lack of accurate consideration of attractive
medium-range interactions such as π-π stacking in orthodox
hybrid DFT functionals30 (e.g., B3LYP, PBE0) was expected
to overestimate Pd-Pd bond distances. The M06 functional31
has been shown to accurately describe both transition-metal-
based complexes containing Au, Ru, and Pd32 and non-covalent
interactions between proximal aromatic groups.33 The M06
functional, with a triple-ꢀ basis set for Pd and Cl, predicted a
Pd-Pd distance of 2.62 Å (compared to 2.57 Å determined
experimentally) and accurately described the relative spacing
of the benzo[h]quinolinyl ligands. (See Supporting Information
for evaluation of other functionals.) In addition, the computed
structure of 1, which we will denote A, reflects the experimen-
tally observed nonequivalence of the two oxygen-based ligands
on each palladium atom (Figure 1). The M06 functional also
accurately described the Pd-Pd distance in 9, which does not
contain a formal Pd-Pd bond23 (experimental, 2.84 Å; com-
puted, 2.87 Å). In the ensuing discussion, computed structures
will be designated by compound letters, not numbers.
b. Activation Parameters for C-Cl Reductive Elimination
from 1. Transition state B has been located at ∆Gq ) 21.2
298
kcal·mol-1 (∆Hq ) 18.9 kcal ·mol-1), higher in energy than
(29) (a) Petrie, S.; Stranger, R. Inorg. Chem. 2004, 43, 2597–2610. (b)
Cramer, C. J.; Truhlar, D. G. Phys. Chem. Chem. Phys. 2009, 11,
10757–10816.
(30) Zhao, Y.; Truhlar, D. G. Org. Lett. 2007, 9, 1967–1970.
(31) (a) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241.
(b) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157–167.
(32) (a) Benitez, D.; Tkatchouk, E.; Goddard, W. A., III Organometallics
2009, 28, 2643–2645. (b) Benitez, D.; Tkatchouk, E.; Gonzalez, A. Z.;
Goddard, W. A., III; Toste, F. D. Org. Lett. 2009, 11, 4798–4801. (c)
Benitez, D.; Shapiro, N. D.; Tkatchouk, E.; Wang, Y. M.; Goddard,
W. A., III; Toste, F. D. Nat. Chem. 2009, 1, 482–486.
Computational Studies. a. Structure of 1. Description of the
halogen-metal-metal-halogen tetrad present in 1 was antici-
(33) Benitez, D.; Tkatchouk, E.; Yoon, I.; Stoddart, J. F.; Goddard, W. A.,
III J. Am. Chem. Soc. 2008, 130, 14928–14929.
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A R T I C L E S
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Figure 5. Calculated gas-phase stationary points in the mechanism of C-Cl reductive elimination from A. Energies are solvent-corrected electronic energies
at 0 K (E0 K); ∆Hq and ∆Gq (transition state B) for the reductive elimination are at 298 K.
Table 1. Comparison of Experimental and Computational
experiment, reductive elimination from a dinuclear Pd(III)
complex to form a Pd(II) complex, such as E, is energetically
favorable.
Activation Parameters for Reductive Elimination from 1
∆Hq (kcal·mol-1
)
∆Sq (cal·K-1
)
∆Gq (kcal·mol-1
)
298
c. Comparison of Computed Transition State with Hammett
Analyses. We have computed natural charge distributions of
structure A and transition state B (Figure 6) from natural
population analyses.34 The difference in computed charge
distribution in the transition state B and structure A was
compared with the experimentally determined F-values from
the Hammett analyses. The relative charge distributions in A
and B are in qualitative agreement with the experimentally
determined F-values. For example, the increased anionic charge
in the transition state of the Ca atom bound to Pda (0.05 for A;
-0.04 for B) is consistent with the positive F-value (+1.46)
that was experimentally determined for benzo[h]quinolinyl
ligand substitution.
experiment
computation
17.2 ( 2.7
-11.2 ( 9.4
-7.5
20.5 ( 0.1
18.9
21.2
structure A (Figure 5). This agrees well with the experimentally
determined activation parameters (∆Gq
) 20.5 ( 0.1
298
kcal·mol-1 and ∆Hq) 17.2 ( 2.7 kcal ·mol-1, Table 1).
Unlike in structure A, the Pd nuclei in B are non-equivalent
because C-Cl bond formation proceeds at Pda while ionization
of Cl- proceeds at Pdb. The Pd-Pd distance is longer in B (2.65
Å) than in A (2.62 Å) by 0.03 Å. The bond lengths between
Pda and the carbon (Ca) and chlorine (Cla) ligands that participate
in reductive elimination are longer by 0.20 and 0.12 Å in the
transition state, respectively. The Pd-O bond trans to Ca is 0.16
Å longer in B than it is in A.
Discussion
Structure C has been located as a local energy minimum in
the gas phase; C is not an energy minimum when a CH2Cl2
solvent model is applied but is depicted in Figure 5 to better
illustrate the path of reductive elimination. In C, the C-Cl bond
is 1.76 Å long, indicating that bond formation is nearly complete
(for comparison, the C-Cl bond in E is 1.75 Å). Ionization of
Clb from C affords ion pair D, which is 6.8 kcal·mol-1 higher
in energy than A. Experimentally, the structure of the Pd(II)
complex immediately following reductive elimination has not
been determined. Computationally, the ion pair D evolves to
Pd(II) structure E. Several other Pd(II) complexes could form
by ligand rearrangement, and structure E was only chosen as a
stationary point in Figure 5 to illustrate that, as observed by
In the following discussion we will first analyze experimental
and computational evidence consistent with reductive elimina-
tion proceeding from a dinuclear palladium complex. Subse-
quently, we will evaluate the redox participation of both metals
during reductive elimination (metallicity). Because metallicity
cannot be probed experimentally, we have pursued a compu-
tational description of metallicity.
Nuclearity of Pd Complex during Redox Transformation.
C-Cl bond formation from complex 1 could proceed via
reductive elimination from a mono- (after dissociation of 1) or
a dinuclear complex (Figure 7).
Figure 7. C-Cl reductive elimination could, in principle, proceed from
mononuclear or dinuclear complexes.
Figure 6. Partial charge distribution in structure A and transition state B
(numbers in parentheses). Hammett analyses of benzo[h]quinolinyl, bridging
carboxylate, and apical ligands are consistent with the computed charge
distributions.
Reductive elimination from a mononuclear complex following
initial Pd-Pd bond cleavage could, in principle, afford observed
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Reductive Elimination from Dinuclear Pd(III) Complexes
A R T I C L E S
Scheme 6. Exchange between 1 and 20a is Not Observed on the
Time Scale of Formation of 2, Precluding Pre-equilibrium
Dissociation of 1 during Reductive Elimination
nuclear Pd(IV) complex (Path 2, Figure 8). Such dispropor-
tionative metal-metal bond cleavage has been reported for
dinuclear Pt(III) intermediates during the oxidation of Pt(II) to
Pt(IV).36 As with homolytic Pd-Pd bond cleavage, either
heterolytic cleavage or subsequent reductive elimination could
be rate limiting.
Figure 8. Potential pathways via mononuclear Pd complexes. Path 1:
Dissociation into two Pd(III) monomers followed by reductive elimination
from mononuclear Pd(III). Path 2: Disproportionation to Pd(II) and Pd(IV)
followed by reductive elimination from Pd(IV).
product 2.35 We have considered two processes by which the
dinuclear core could fragment into mononuclear palladium
complexes prior to reductive elimination: (1) homolytic Pd-Pd
bond cleavage to afford two mononuclear Pd(III) complexes
and (2) heterolytic Pd-Pd bond cleavage to afford one Pd(IV)
complex and one Pd(II) complex (Figure 8). For each pathway,
either Pd-Pd bond cleavage or subsequent reductive elimination
from mononuclear Pd complexes could be rate determining.
Fast homolytic Pd-Pd bond cleavage to afford two identical
Pd(III) monomers, followed by rate-determining reductive
elimination, would proceed with a reaction order of 0.5 with
respect to 1 and was excluded by the observed first-order rate
law for decomposition of 1 and formation of 2.18
Possible pre-equilibrium dimer cleavage, followed by rate-
determining reductive elimination, was evaluated by a crossover
experiment between acetate-bridged complex 1 and benzoate-
bridged complex 20a (Scheme 6). Complex 33, the product of
crossover between 1 and 20a, was not observed during C-Cl
reductive elimination, precluding pre-equilibrium dimer cleav-
age. Addition of 1.0 equiv of benzo[h]quinoline (8), a potentially
coordinating ligand, did not lead to observed exchange either.
To evaluate the possibility of rate-determining heterolytic
Pd-Pd cleavage prior to C-Cl reductive elimination, the
relative rates of reductive elimination from acetate-bridged
complex 1 and esp-bridged complex 35 (eq 5) were measured
at 35 °C (R,R,R′,R′-tetramethyl-1,3-dibenzenedipropionate)
(esp). Reductive elimination from esp-bridged Pd(III) complex
35 proceeds 16 times more slowly than that from acetate-bridged
Pd(III) complex 1. However, the esp-bridged Pd(II) complex
34 dissociates at least 1500 times more slowly than acetate-
bridged Pd(II) complex 9 (see Supporting Information). The
relative rates of reductive elimination from 1 and 35 are
therefore most consistent with reductive elimination proceeding
from a dinuclear complex. In addition to the experimental data,
we computed the barrier for heterolytic cleavage of the dinuclear
core to generate a Pd(II) and Pd(IV) complex to be at least 29.5
kcal·mol-1 (see Supporting Information); the experimentally
determined activation enthalpy for reductive elimination from
Rate-determining homolytic Pd-Pd bond cleavage, followed
by fast reductive elimination, was excluded by comparison of
the activation parameters for C-Cl reductive elimination (∆Hq
) 17.2 ( 2.7 kcal ·mol-1, ∆Sq ) -11.2 ( 9.4 cal K-1, and
∆Gq ) 20.5 ( 0.1 kcal ·mol-1) with the calculated Pd-Pd
298
bond strength. The Pd-Pd bond in A was calculated to be 35.3
kcal·mol-1 by calculating the energy required to separate the
palladium nuclei to 3.65 Å. Stretching of the Pd-Pd bond led
to geometrical distortions of the bridging acetate ligands.
Therefore, the Pd-Pd bond energy was also calculated for
structure F, in which the bridging acetate ligands have been
replaced by chelating propanedialato ligands (33.2 kcal ·mol-1)
(eq 4). The computed value of 35.3 kcal · mol-1 is likely an
underestimate of the energy required to cleave the dinuclear
core because cleavage requires breaking of the Pd-Pd bond in
addition to two Pd-O bonds. The effect of solvent polarization
(Poisson-Boltzmann model) in CH2Cl2 was included while
calculating the Pd-Pd bond strength in F to account for solvent
stabilization of the complexes generated after bond cleavage.
1 is Hq ) 17.2 ( 2.7 kcal ·mol-1
.
The combination of the observed first-order disappearance
of 1 and formation of 2, the measured activation parameters
which show that reductive elimination from 1 is more facile
than homolytic Pd-Pd bond cleavage, and the lack of crossover
between 1 and 20a are all consistent with reductive elimination
from a dinuclear complex. Further, the computed reaction
pathway agrees with experimental observations of ground-state
structure, Arrhenius parameters for C-Cl reductive elimination,
Reductive elimination could potentially proceed by dispro-
portionation of 1 into a Pd(II) and a Pd(IV) complex, with
subsequent monometallic reductive elimination from a mono-
(34) (a) Landis, C. R.; Weinhold, F. J. Comput. Chem. 2007, 28, 198–
203. (b) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys.
1985, 83, 735–746.
(36) (a) Bandoli, G.; Caputo, P. A.; Intini, F. P.; Sivo, M. F.; Natile, G.
J. Am. Chem. Soc. 1997, 119, 10370–10376. (b) Canty, A. J.; Gardiner,
M. G.; Jones, R. C.; Rodemann, T.; Sharma, M. J. Am. Chem. Soc.
2009, 131, 7236–7237.
(35) Schore, N. E.; Ilenda, C.; Bergman, R. G. J. Am. Chem. Soc. 1976,
98, 7436–7438.
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A R T I C L E S
Powers et al.
charge. Conversely, the electron binding energy increases during
oxidation because the electron is less effectively shielded from
the nuclear charge. When comparing the electron binding
energies in metal centers with similar ligand sets, higher electron
binding energies correlate with higher oxidation states. We
suggest that the electron binding energies, which correlate with
oxidation state, are suitable to ascertain the redox participation
of the metals.44
We selected two hypothetical but well-defined limiting cases
for monometallic reductive elimination (paths 3 and 5, Figure
9) and computed the pathways of reductive elimination for both.
Subsequently, the electron binding energies of electrons in the
4s orbitals of both palladium centers, Pda and Pdb, have been
calculated for each of the two pathways.45
Figure 9. Potential pathways from dinuclear Pd complexes. Path 3:
Monometallic reductive elimination to form a Pd(I)/Pd(III) mixed-valence
complex and 2, followed by comproportionation to afford Pd(II). Path 4:
Computed low-energy pathway for reductive elimination (Figure 5). Path
5: Disproportionation to a Pd(II)/Pd(IV) mixed-valence complex, followed
by monometallic reductive elimination.
Hypothetical reductive elimination from Pda with Pdb as a
non-redox-active spectator ligand on Pda would proceed through
a dinuclear Pd(I)/Pd(III) mixed-valence complex (Figure 10a,
path 3). We investigated path 3 by computationally cleaving A
into two mononuclear Pd(III) complexes (G). Computed
intermediate G serves as model for dinuclear compound A, in
which potential metal-metal cooperation is eliminated. The
electron binding energies were determined computationally to
describe the redox chemistry of each Pd center during hypo-
thetical path 3 (Figure 10a). The cleavage from A to G is redox-
neutral and does not significantly affect the computed electron
binding energy of either Pda or Pdb. Monometallic reductive
elimination from Pda proceeds through transition state H to
afford structure I. Redox chemistry is occurring only at Pda
during the transformation of G to I, and thus the electron binding
energy of Pdb does not change. At the same time, the electron
binding energy of Pda decreases as G is transformed to I,
consistent with redox contribution at Pda. We selected the Pd(II)
complex J as the end point, because complex J has well-defined
oxidation states on palladium. Further, the ligand sphere of
palladium in J is closely related to the ligand sphere of palladium
in A, allowing for meaningful comparison of electron binding
energies. Formal comproportionation of the mixed-valence
structure I to J results in the electron binding energies of Pda
and Pdb converging. Pda in structure I is most accurately
described as a Pd(II) with a ligand-centered radical, and thus
the electron binding energy of Pda during comproportionation
does not increase, as would be expected for oxidation of Pd(I)
to Pd(II). The diagram in Figure 10a shows that only Pda
and Hammett F-values and also implicates dinuclear complexes
in every step of reductive elimination.
Metallicity of Reductive Elimination. Knowledge of the
nuclearity of the complex from which reductive elimination
proceeds provides no information about the role of each metal
center during reductive elimination. Depending on the extent
of metal-metal cooperation during reductive elimination from
a dinuclear complex, either monometallic or bimetallic redox
pathways are possible (Figure 9).
To probe the electronic contribution of each metal center
during reductive elimination, the electron binding energies37 of
the 4s core electrons of both Pda and Pdb were calculated as a
function of reaction progress for pathways 3-5 shown in Figure
9. The electron binding energy38 of a 4s electron is the energy
required to remove an electron from the 4s orbital42 to infinite
separation, without allowing any relaxation of the remaining
electrons. For metal centers with similar ligand environments,
electron binding energy is well correlated with formal oxidation
state.43 The electron binding energy decreases during reduction
of a metal center because the metal becomes more electron rich
and the electron is more effectively shielded from the nuclear
(37) The core and valence (available in the LACV3P++**(2f) basis set)
natural atomic orbital energies of both Pda and Pdb were obtained and
plotted as a function of reaction progress. According to Janak’s
theorem, the relative orbital energies are estimates of the relative
vertical electron binding energies. The relative natural atomic orbital
energies can be regarded as approximations of the vertical ionization
potential or the negative of the electron affinity.
(43) (a) Riggs, W. M. Anal. Chem. 1972, 44, 830–832. (b) Leigh, G. J.
Inorg. Chim. Acta 1975, 14, L35–L36. (c) Chatt, J.; Elson, C. M.;
Hooper, N. E.; Leigh, G. J. J. Chem. Soc., Dalton Trans. 1975, 2392–
2401. (d) Cisar, A.; Corbett, J. D.; Daake, R. L. Inorg. Chem. 1979,
18, 836–843. (e) Corbett, J. D. Inorg. Chem. 1983, 22, 2669–2672.
(44) Similarly analysis of the carbon 1s orbitals of CH3 bound to transition
metals has been shown to correlate well with the electrophilicity or
nucleophilicity of the methyl groups. Nielsen, R. J.; Goddard, W. A.,
III. J. Phys. Chem., manuscript in preparation.
(45) In addition to calculating the electron binding energy from A, we have
also computed the electron binding energies of both Pd centers during
C-Cl reductive elimination from cationic structure M, in which one
of the apical chloride ligands is replaced by a pyridyl ligand. The
results of these calculations are in the Supporting Information, and
the conclusions drawn from this structure mirror those from complex
A.
(38) Energies were obtained from Natural Bond Orbital (NBO)39analyses
of the M06 wavefunction using the NBO 5.0 module40 in Jaguar. The
NBO analysis method is a popular mathematical treatment of wave-
functions to study bonding effects such as hybridization and
covalency.32c,41
(39) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899–926. (b) Weinhold, F.; Landis, C. R. Valency and Bonding: A
Natural Bond Orbital Donor-Acceptor PerspectiVe; Cambridge Uni-
versity Press: Cambridge, UK, 2005.
(40) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Bohmann, J. A.; Morales, C. M.; Weinhold, F. Jaguar; Theoretical
Chemistry Institute, University of Wisconsin: Madison, 2001.
(41) (a) Kim, C. K.; Lee, K. A.; Kim, C. K.; Lee, B.-S.; Lee, H. W. Chem.
Phys. Lett. 2004, 391, 321–324. (b) Nori-Shargh, D.; Roohi, F.;
Deyhimi, F.; Naeem-Abyaneh, R. J. Mol. Struct. 2006, 763, 21–28.
(c) Leyssens, T.; Peeters, D.; Orpen, A. G.; Harvey, J. N. Organo-
metallics 2007, 26, 2637–2645.
(42) The 4s orbital was selected because it is spherically symmetrical and
more sensitive to perturbations in the electron density of the metal
center than energetically lower lying s orbitals.
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Reductive Elimination from Dinuclear Pd(III) Complexes
A R T I C L E S
Figure 10. (a) Plot of the 4s core electron binding energy of Pda (solid) and Pdb (dashed) as a function of reaction progress for monometallic reductive
elimination via a Pd(I)/Pd(III) mixed-valence species (Figure 9, path 3). (b) Plot of electron binding energy of Pda and Pdb as a function of reaction progress
for monometallic reductive elimination from a Pd(II)/Pd(IV) mixed-valence species (Figure 9, path 5). (c) Plot of electron binding energy of Pda and Pdb as
a function of reaction progress for calculated bimetallic reductive elimination (Figure 5).
participates in redox chemistry during reductive elimination;
hypothetical path 3 is therefore monometallic.
constant because Pdb is not redox-active according to path 5.
Hypothetical path 5 is therefore monometallic.
Both paths 3 and 5 were selected to illustrate and benchmark
the electron binding energies in two limiting cases of mono-
metallic reductive elimination. Path 4 is derived from the
computed low-energy pathway shown in Figure 5. Both electron
Alternatively, C-Cl bond formation could proceed by initial
disproportionation of complex 1 to a mixed-valence Pd(II)/
Pd(IV) complex prior to reductive elimination from Pd(IV), as
shown in Figure 9, path 5. Previously, it was speculated that
reductive elimination from dinuclear Pd(III) complexes may
proceed via Pd(II)/Pd(IV) mixed-valence species.46,47 As shown
in Figure 10b, the electron binding energy diagram computed
for disproportionation from A to K shows increased electron
binding energy of Pda because Pda is more oxidized in K than
it is in A. Reductive elimination proceeds through transition
state L, in which redox chemistry is occurring at Pda. The
electron binding energies of Pda and Pdb converge as reductive
elimination affords two Pd(II) centers (J). During reductive
elimination from Pda, the electron binding energy of Pdb is
(46) Deprez, N. R.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 11234–
11241.
(47) The relative energy of acetate-bridged Pd(II) dimers in which either
the Pd centers are held in proximity (as in 9) or the palladium centers
are in a planar arrangement (as in K) was investigated: Jack, T. R.;
Powell, J. Can. J. Chem. 1975, 53, 2558–2574. It was concluded that
a planar complex is greater than 12 kcal ·mol-1 higher in energy than
a structure in which the palladium centers are held in proximity. We
have been unable to locate a local minimum for a planar complex
with two Pd(II) centers; all attempts to calculate such a species result
in the structure folding to bridged structure 9. Structure K is calculated
to be 12.4 kcal ·mol-1 higher in energy than A.
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A R T I C L E S
Powers et al.
binding energies of Pda and Pdb monotonically decrease while
A is converted to C via transition state B. The electron binding
energies continue to monotonically decrease until Pd(II) struc-
ture J. Neither metal center becomes oxidized beyond Pd(III)
during the course of reductive elimination.
The calculated electron binding energies implicate simulta-
neous redox participation of both metals during reductive
elimination. Monotonic reduction in electron binding energy at
both metals is inconsistent with initial disproportionation to a
Pd(II)/Pd(IV) complex prior to monometallic reductive elimina-
tion. A simultaneous change in the electron binding energies
of both Pda and Pdb is also inconsistent with initial monometallic
reductive elimination to a Pd(I)/Pd(III) mixed-valence complex.
Therefore, we conclude that reductive elimination from 1 is
bimetallic.
Figure 11. Reductive elimination from 1 in the presence of AcOH proceeds
via at least two pathways, one linearly dependent on and one independent
of [AcOH].
Reductive Elimination in the Presence of AcOH. Previously,
on the basis of the observation of C-C,48 C-O,2d,e C-Cl,49
and C-F50 reductive elimination from isolated Pd(IV) com-
plexes, Pd(II)/Pd(IV) redox cycles have been invoked as
mechanistic rationales for Pd-catalyzed C-H oxidation reac-
tions.51 We have proposed that reductive elimination from
dinuclear Pd(III) complexes, similar to 1, is the product-forming
step in a variety of Pd-catalyzed C-H oxidation reactions on
the basis of the observation of dinuclear complexes during
chlorination3a and acetoxylation3b of 2-phenylpyridine deriva-
tives.52
Kinetic Advantage of Bimetallic Reductive Elimination.
Based on the HOMO and LUMO calculated for structure A
and transition state B (see Supporting Information), metal-metal
bonding during reductive elimination is accomplished primarily
2
by overlap of the palladium dz orbitals. We have calculated
the activation barrier for reductive elimination as a function of
Pd-Pd distance. Computationally forced elongation of the
2
Pd-Pd bond attenuates the overlap of the dz orbitals and thus
During catalysis, C-H metalation by Pd(II) generates an
equivalent of acid. We have found that the rate of reductive
elimination is linearly correlated with [AcOH]. We propose that
the observed C-Cl reductive elimination rate enhancement is
due to protonation of one of the bridging acetate ligands of 1
to afford 36 (Figure 11). In the presence of acid, protonation of
the acetate ligand would result in a pentacoordinate Pd center
of a dinuclear complex, in which one Pd-O bond is cleaved.
We suggest that reductive elimination from 36, in which one
bridging acetate ligand is protonated, is faster than that from 1.
A dinuclear transition state for reductive elimination from
cationic complex 36, similar in structure to the transition state
for reductive elimination from 1 (B), has been located compu-
tationally. The barrier to reductive elimination from 1 was
computed to be 21.8 kcal ·mol-1, whereas the barrier to reductive
decreases electronic communication between the metals as
compared to low-energy structure A (eq 6). By preventing
electronic communication between the metals, bimetallic reduc-
tive elimination is forced to become increasingly monometallic.
Table 2. Computed Reference Ground-State and Activation
Energies of Reductive Elimination as a Function of Restricted
Pd-Pd Distancea
ground-state
energy (kcal·mol-1
activation energy
(kcal·mol-1) relative to A
elimination from 36 was computed to be 20.1 kcal ·mol-1
,
Pd-Pd distance (Å)
)
consistent with the observed rate enhancement in the presence
of AcOH.45 Computed electron binding energies for both
palladium atoms in A + H+ and in B + H+ indicate that, similar
to reductive elimination from A, reductive elimination from a
protonated dinuclear core is bimetallic as well. During Pd-
catalyzed C-H functionalization, C-H metalation generates
acid, and thus acid-catalyzed reductive elimination may be
2.62
2.95
3.30
3.65
0.0 (A)
6.2
20.5
17.1
21.8
33.3
46.0
35.3
a Energies are gas-phase electronic energies at 0 K (E0 K).
The activation barrier for reductive elimination when the
Pd-Pd distance is fixed at 2.62 Å (the Pd-Pd distance in A) is
17.1 kcal ·mol-1. Computational elongation of the Pd-Pd
distance to 2.95 Å (Table 2, entry 2) increased the activation
barrier to 21.8 kcal ·mol-1. Further elongation to 3.30 and 3.65
(48) Byers, P. K.; Canty, A. J.; Skelton, B. W.; White, A. H. J. Chem.
Soc., Chem. Commun. 1986, 1722–1724.
(49) Whitfield, S. R.; Sanford, M. S. J. Am. Chem. Soc. 2007, 129, 15142–
15143.
(50) (a) Furuya, T.; Ritter, T. J. Am. Chem. Soc. 2008, 130, 10060–10061.
(b) Furuya, T.; Benitez, D.; Tkatchouk, E.; Strom, A. E.; Tang, P.;
Goddard, W. A., III; Ritter, T. J. Am. Chem. Soc. 2010, 132, 3793–
3807.
Å led to activation barriers of 33.3 and 46.0 kcal ·mol-1
,
respectively (entries 3 and 4). Pd-Pd distances beyond 3.65
Å, which is greater than the sum of the two van der Waals radii
of the palladium atoms, could not be accommodated while
maintaining the bridging geometry of the acetate ligands.
Comparison of the activation barriers for bimetallic reductive
elimination (entry 1) and monometallic reductive elimination
(entry 4) indicates that metal-metal cooperation during reductive
elimination lowers the energy barrier by ∼30 kcal·mol-1 as
compared to monometallic reductive elimination, which ap-
proximately corresponds to the Pd-Pd bond dissociation energy
in 1.
(51) (a) Giri, R.; Chen, X.; Yu, J.-Q. Angew. Chem., Int. Ed. 2005, 44,
2112–2115. (b) Jordan-Hore, J. A.; Johansson, C. C. C.; Gulias, M.;
Beck, E. M.; Gaunt, M. J. J. Am. Chem. Soc. 2008, 130, 16184–16186.
(c) Qin, C.; Lu, W. J. Org. Chem. 2008, 73, 7424–7427. (d) Neumann,
J. J.; Rakshit, S.; Dro¨ge, T.; Glorius, F. Angew. Chem., Int. Ed. 2009,
48, 6892–6895. (e) Wang, X.; Mei, T.-S.; Yu, J.-Q. J. Am. Chem.
Soc. 2009, 131, 7520–7521. (f) Zhang, Y.-H.; Shi, B.-F.; Yu, J.-Q.
Angew. Chem., Int. Ed. 2009, 48, 6097–6100. (g) Wang, G.-W.; Yuan,
T.-T. J. Org. Chem. 2010, 75, 476–479.
(52) A detailed investigation of the relevance of dinuclear Pd(III) complexes
in catalysis will be published separately: Powers, D. C.; Xiao, D. Y.;
Geibel, M. A. L.; Ritter, T. J. Am. Chem. Soc. manuscript accepted.
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Reductive Elimination from Dinuclear Pd(III) Complexes
A R T I C L E S
relevant during catalysis. On the basis of the presented data,
we cannot distinguish between pre-equilibrium and rate-
determining protonation in the acid-catalyzed pathway for
reductive elimination from 1 (k′, Figure 11).
C-heteroatom bonds can be efficiently formed by bimetallic
reductive elimination from dinuclear Pd(III) complexes. In this
article we have provided a theoretical framework for bimetallic
redox catalysis. Redox synergy between metal centers, as is seen
for C-Cl reductive elimination here, also has the potential to
lower the activation barriers for other redox processes. We
anticipate that our findings may serve as a foundation for future
reaction development.
Conclusions
In this article we sought to address the following questions:
Does the core of dinuclear Pd(III) complex 1 stay intact during
reductive elimination? Do both metal centers participate in the
redox transformation, and is there an energetic benefit to having
two metals as opposed to a single metal? How closely related
are previously proposed mononuclear Pd(IV) complexes to the
dinuclear Pd(III) complexes reported here with respect to the
mechanism of reductive elimination?
Acknowledgment. We thank Douglas M. Ho, Jessica Y. Wu,
and Takeru Furuya for X-ray crystallographic analysis, Robert
Nielsen and David Ford for insightful discussions, Sanofi Aventis
for a graduate fellowship for D.C.P., the reviewers of this
manuscript for comments, which improved the quality of the
manuscript, and the NSF (CHE-0952753) and NIH-NIGMS
(GM088237) for funding. Computational facilities were funded by
grants from ARO-DURIP and ONR-DURIP.
Experimental and theoretical analysis of C-Cl reductive
elimination from complex 1 has implicated reductive elimination
from a dinuclear core with synergistic, bimetallic redox
participation of both metals during reductive elimination. The
presence of the second metal in dinuclear complex 1 lowers
the activation barrier of reductive elimination. If reductive
elimination is artificially forced to proceed via a monometallic
pathway by eliminating metal-metal communication, the barrier
Supporting Information Available: Detailed experimental
procedures and spectroscopic data for all new compounds;
detailed computational methods and XYZ coordinates (PDF,
CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
to reductive elimination is increased by ∼30 kcal ·mol-1
.
Transition-metal-mediated C-heteroatom bond construction
remains a synthetically challenging goal. We have shown that
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