Detection of Pd(III) and Pd(IV) intermediates during the aerobic oxidative C-C bond formation from a Pd(II) dimethyl complex

Article abstract of DOI:10.1021/om300752w

The dimethyl Pd^II complex (^MeN4)Pd ^IIMe₂ (^MeN4 = N,N′-dimethyl-2,11-diaza[3, 3](2,6)pyridinophane) is readily oxidized by dioxygen in the presence of protic solvents to selectively eliminate ethane. UV-vis, EPR, ESI-MS, and NMR studies reveal the formation of several Pd^III and Pd^IV intermediates during the aerobically induced C-C bond formation reaction, including the key intermediate [(κ³-^MeN4)Pd ^IVMe₃]⁺, which leads to ethane elimination. The latter complex was also synthesized independently and structurally characterized to reveal a distorted octahedral geometry that is proposed to promote facile reductive elimination. Overall, this study represents a rare example of aerobic oxidation of an organometallic Pd^II precursor that leads to a well-defined Pd^IV species, which undergoes selective C-C bond formation under ambient conditions.

Full text of DOI:10.1021/om300752w

Article
pubs.acs.org/Organometallics
Detection of Pd(III) and Pd(IV) Intermediates during the Aerobic
Oxidative C−C Bond Formation from a Pd(II) Dimethyl Complex
Fengzhi Tang,† Ying Zhang,† Nigam P. Rath,‡ and Liviu M. Mirica*,†
†Department of Chemistry, Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899, United States
‡Department of Chemistry and Biochemistry, University of Missouri−St. Louis, One University Boulevard, St. Louis, Missouri
63121-4400, United States
S
* Supporting Information
ABSTRACT: The dimethyl PdII complex (MeN4)PdIIMe2
(MeN4 = N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane)
is readily oxidized by dioxygen in the presence of protic
solvents to selectively eliminate ethane. UV−vis, EPR, ESI-MS,
and NMR studies reveal the formation of several PdIII and PdIV
intermediates during the aerobically induced C−C bond
formation reaction, including the key intermediate [(κ3-MeN4)-
PdIVMe3]+, which leads to ethane elimination. The latter
complex was also synthesized independently and structurally
characterized to reveal a distorted octahedral geometry that is proposed to promote facile reductive elimination. Overall, this
study represents a rare example of aerobic oxidation of an organometallic PdII precursor that leads to a well-defined PdIV species,
which undergoes selective C−C bond formation under ambient conditions.
formation of ethane under ambient conditions (Scheme 1).
In addition, several PdIII and PdIV intermediates were detected
INTRODUCTION
■
The recent development of Pd-catalyzed aerobic trans-
formations provides a promising and practical use of dioxygen
(O2) as an environmentally friendly and cheap oxidant in a
variety of oxidative organic transformations.1 Most of the
reported reactions involve Pd0/II catalytic cycles in which O2
acts as the terminal oxidant to regenerate the PdII species.1 In
addition, high-valent PdIV and/or PdIII species have been
recently proposed as active intermediates in several catalytic
and stoichiometric aerobic transformations.2 However, the
detection of such PdIII or PdIV intermediates has been reported
only in a very limited number of cases.3−5 In this regard, use of
a ligand system that can stabilize both PdIII and PdIV complexes
and promote aerobic oxidation of organometallic PdII
precursors can provide key insight into the mechanisms of
these important aerobic transformations, as well as enable the
development of new Pd-catalyzed aerobic transformations.
We have recently reported that the dimethyl complex
Scheme 1. Aerobic C−C Bond Formation Reactivity of
(MeN4)PdIIMe2
in the reaction mixture by UV−vis, EPR, ESI-MS, and NMR,
including the PdIV species [(κ3-MeN4)PdIVMe3]+, which
generates ethane upon reductive elimination. The [(κ3-MeN4)-
PdIVMe3]+ complex was also independently synthesized, and its
C−C bond formation reactivity was investigated. Overall, this
system represents a rare example of aerobic oxidation of an
organometallic PdII complex to generate detectable high-valent
Pd intermediates and lead to oxidative C−C bond formation
reactivity. In addition, these studies provide strong evidence for
the recently proposed mechanism of inner-sphere O2 reduction
(
tBuN4)PdIIMe2 tBuN4 = N,N′-di-tert-butyl-2,11-diaza[3,3]-
(
(2,6)pyridinophane) is oxidized by O2 and leads to selective
elimination of ethane from the PdIII intermediate [(tBuN4)-
PdIIIMe2]+.4 Although several PdIV species were proposed as
transient intermediates in that case, they were not observed
experimentally. In addition, the formation of a PdIV species
[(Me3tacn)PdIVMe3]+ was observed upon the aerobic oxidation
of (Me3tacn)PdIIMe2 (Me3tacn = N,N′,N″-trimethyl-1,4,7-
triazacyclononane).5 However, in the latter case the PdIV
species eliminates ethane only at elevated temperatures. Herein,
we report the aerobic oxidation of the dimethyl complex
(MeN4)PdIIMe2 (1, MeN4 = N,N′-dimethyl-2,11-diaza[3,3]-
(2,6)pyridinophane), which leads to facile and selective
Received: August 6, 2012
Published: August 31, 2012
© 2012 American Chemical Society
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at a PdII center followed by C−C reductive elimination from a
EPR Studies of Oxidation of 1 with O2. A 4.5 mM solution of 1
in degassed 1:1 MeOH/PhH was prepared immediately before the
experiment. Oxygen was bubbled through the solution for 2 min, and
the reaction mixture was left to stand in the dark for 10 min at 20 °C.
The solution was then transferred to a quartz EPR tube, and the EPR
spectrum was recorded at 77 K.
PdIV center.4,5
EXPERIMENTAL DETAILS
■
General Specifications. All manipulations were carried out under
a nitrogen atmosphere using standard Schlenk and glovebox
techniques if not indicated otherwise. All reagents for which the
synthesis is not given were commercially available from Aldrich, Acros,
STREM, or Pressure Chemical and were used as received without
further purification. Solvents were purified prior to use by passing
through a column of activated alumina using an MBRAUN SPS. N,N′-
Dimethyl-2,11-diaza[3,3](2,6)pyridinophane (MeN4)6 was prepared
according to the literature procedure, and (MeN4)PdIIMe2 (1) and
[(MeN4)PdIIIMe2]ClO4 ([1+]ClO4) were synthesized in a manner
similar to the tBuN4 analogues.7,8 NMR spectra were obtained on a
Varian Mercury-300 spectrometer (300.121 MHz) or a Varian Unity
Inova-600 spectrometer (599.746 MHz). UV−vis spectra were
recorded on a Varian Cary 50 Bio spectrophotometer and are
reported as λmax, nm (ε, M−1 cm−1). EPR spectra were recorded on a
JEOL JES-FA X-band (9.2 GHz) EPR spectrometer at 77 or 298 K.
ESI-MS experiments were performed using a Bruker Maxis Q-TOF
mass spectrometer with an electrospray ionization source. Elemental
analyses were carried out by the Columbia Analytical Services Tucson
Laboratory. Cyclic voltammetry experiments were performed with a
BASi EC Epsilon electrochemical workstation. Electrochemical grade
Bu4NClO4 (Fluka) was used as the supporting electrolyte. Electro-
chemical measurements were performed under a blanket of nitrogen,
and the analyzed solutions were deaerated by purging with nitrogen. A
glassy carbon disk electrode and a Ag/0.01 M AgNO3/MeCN
electrode were used as the working and reference electrode,
respectively. The reference electrode was calibrated against Cp2Fe
(Fc).
DMPO Spin Trapping Studies of Oxidation of 1 with O2.
Under N2, 250 μL of a freshly prepared 9.0 mM solution of 1 in
degassed benzene was mixed with 250 μL of a freshly prepared 90 mM
solution of DMPO in degassed MeOH and placed in a vial equipped
with a septum ([1] = 4.5 mM, [DMPO] = 45 mM). O2 was bubbled
through the solution for 1 min, the solution was then transferred to an
EPR tube, and the EPR spectrum was recorded immediately at 293 K.
ESI-MS Studies of Oxidation of 1 with O2. A 0.1 mg amount of
1 was dissolved in 1 mL of 5% H2O/MeCN, and air or O2 was
bubbled through the solution for about 30 s. Then 50 μL of the
solution was injected into the MS instrument at various time points to
detect any transient intermediates present in the reaction mixture.
X-ray Structure Determination of 1 and [2+]I. Suitable crystals
were mounted on Mitgen cryoloops in random orientations in a
Bruker Kappa Apex-II CCD X-ray diffractometer equipped with an
Oxford Cryostream LT device and a fine focus Mo Kα radiation X-ray
source (λ = 0.71073 Å). Preliminary unit cell constants were
determined with a set of 36 narrow frame scans. Typical data sets
consist of combinations of ϕ and ϕ scan frames with a typical scan
width of 0.5° and a counting time of 15−30 s/frame at a crystal-to-
detector distance of ∼4.0 cm. The collected frames were integrated
using an orientation matrix determined from the narrow frame scans.
Apex II and SAINT software packages9 were used for data collection
and data integration. Analysis of the integrated data did not show any
decay. Final cell constants were determined by global refinement of
xyz centroids of reflections from the complete data sets. Collected data
were corrected for systematic errors using SADABS9 based on the
Laue symmetry using equivalent reflections. Structure solutions and
refinement were carried out using the SHELXTL-PLUS software
package.10 The structures were refined with full matrix least-squares
Synthesis of [(MeN4)PdIVMe3]I ([2+]I). Under N2, 1 (20.1 mg,
49.6 μmol) was dissolved in 3 mL of acetone. Upon addition of 1
equiv of MeI (31 μL, 49.6 μmol), a brownish precipitate formed
immediately. The reaction mixture was stirred at room temperature for
about 20 min and then stored at −35 °C for 3 h. The clear yellow
solution was decanted, and the brown product was washed with 2 × 1
mL of precooled acetone and dried under vacuum for 10 min. Yield:
14.6 mg, 54%. The resulting solid was dissolved in CD3CN, and the
NMR was taken immediately. X-ray quality crystals were obtained
from a solution in acetone at −35 °C. 1H NMR (CD3CN), 20 °C (δ,
ppm): 7.56 (t, J = 7.8 Hz, 2 H, py-H), 7.08 (d, J = 7.8 Hz, 4 H, py-H),
4.73 (d, J = 15.0 Hz, 4 H, −CH2−), 4.00 (d, J = 18.3 Hz, 4 H,
−CH2−), 2.65 (s, 6 H, −N−CH3), 1.80 (s, 9H, PdIV−CH3). ESI-MS:
m/z 419.1420, calcd for [(MeN4)PdIVMe3]+ 419.1427. The elemental
analysis of this complex could not be obtained due to its thermal
sensitivity.
2
refinement by minimizing ∑w(Fo − Fc2)2. All non-hydrogen atoms
were refined anisotropically to convergence, and the hydrogen atoms
were added at the calculated positions in the final refinement cycles.
Crystal data and intensity data collection parameters are listed in
Tables S11−S14.
RESULTS AND DISCUSSION
■
Characterization and Aerobic Reactivity of (MeN4)-
PdIIMe2 (1). The MeN4 ligand was synthesized using a reported
synthetic procedure,6 and the PdII precursor (MeN4)PdIIMe2, 1,
was obtained in a manner similar to the tBuN4 analogue.4,8 The
X-ray structure of 1 reveals a square-planar geometry around
the PdII center with two methyl ligands and the two pyridine N
atoms of the MeN4 ligand binding equatorially, while the two
amine donors are pointing away from the metal center (Figure
1, left). The cyclic voltammogram of 1 in 0.1 M Bu4NClO4/
MeCN exhibits an anodic wave at −520 mV vs Fc+/Fc that is
assigned to a PdII/III oxidation (Figure 2), which is significantly
lower than that of the reported analogue (tBuN4)PdIIMe2
(−360 mV vs Fc+/Fc).4 In addition, a reversible second
General Procedure for the Oxidation of 1 with O2. A 4−5 mM
solution of 1 with an equimolar amount of 1,3,5-trimethoxybenzene or
1,4-dioxane (used as internal standards) was dissolved in 2.4 mL of
O2-saturated CD3OD/C6D6 or D2O/CD3CN solution, and an NMR
tube was filled to the top (to avoid the escape of volatiles into the
headspace) and sealed with a septum. The reaction mixture was kept
1
in the dark and periodically analyzed by H NMR. The yields of Pd
intermediates and organic/Pd products were determined by
integration versus the internal standard, calculated as [moles of
product]/[moles of 1] × 100% and given as an average of two runs.
UV−Vis Studies of Oxidation of 1 with O2. A ∼5 mM solution
of 1 in 1:1 MeOH/PhH was placed into a quartz cuvette (path length
1.0 cm) equipped with a septum-sealed cap and a magnetic stir bar.
Oxygen was bubbled through the solution for 2−3 min, and the
reaction mixture was stirred under O2 at 20 °C in the dark. The
reaction progress was monitored by UV−vis. The concentration of 1+
during the reaction was calculated based on the extinction coefficient
of the 620 nm absorption band (ε = 422 M−1 cm−1 in 1:1 MeOH/
PhH). The same aerobic oxidation experiment was performed for a 5.2
mM solution of (MeN4)PdIIMe2 in 5% H2O/MeCN.
oxidation wave assigned to the PdIII/IV oxidation is found to
have an uncommonly low E1/2
value of −400 mV vs Fc+/
III/IV
Fc.8 These low oxidation potentials are likely due to the ability
of the MeN4 ligand to provide two additional N-donors to
stabilize six-coordinate PdIII and PdIV species, while the less
bulky N-Me groups can accommodate the more compact
structure of a PdIV center.11 In light of these results, it is
expected that 1 can be easily oxidized by mild oxidants to
generate detectable PdIII and/or PdIV species. Indeed, 1 was
found to readily react with O2 in the presence of protic solvents
(i.e., MeOH/PhH or H2O/MeCN solvent mixtures), and
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Figure 1. ORTEP representation (50% probability ellipsoids) of 1
(left) and the cation of [1+]ClO4 (right). Selected bond distances (Å)
and angles (deg): 1, Pd1−C1 2.035(2), Pd1−C2 2.037(2), Pd1−N1
2.1585(18), Pd1−N2 2.1718(18); 1+, Pd1−C1 2.0432(10), Pd1−C1i
2.0433(10), Pd1−N1 2.1393(7), Pd1−N1i 2.1393(7), Pd1−N2
2.4013(11), Pd1−N3 2.3506(12), N2−Pd1−N3 145.37(4). The
structure of [1+]ClO4 is taken from ref 11 and is shown only for
comparison.
Figure 3. Formation of 1+ during aerobic oxidation of 1 (5.2 mM) in
5% H2O/MeCN followed by UV−vis spectroscopy (Δt = 2 min, tmax
=
30 min). Inset: Plot of the concentration of 1+ during oxidation of 1
under the same conditions.
Figure 4. EPR spectra of a solution of [1+]ClO4 in 3:1 PrCN/MeCN
(black line), the product of oxidation of 1 with O2 in 1:1 MeOH/PhH
(red line), and a simulated EPR spectrum (blue line) using the
following parameters: gx = 2.170 (AN = 16.0 G); gy = 2.170 (AN = 16.0
G); gz = 1.990 (AN = 22.5 G). The features marked with an asterisk are
due to the hyperfine coupling to the 105Pd isotope (I = 5/2, natural
abundance 22.3%).
Figure 2. Cyclic voltammogram of 1 in 0.1 M Bu4NClO4/MeCN, 100
mV/s scan rate.
monitoring of the reaction by NMR shows an aerobically
induced ethane formation in up to 53% yield (Scheme 2),
Scheme 2. Aerobic Reactivity of (MeN4)PdIIMe2 (1)12
(calcd for 1+: 404.1192). Interestingly, the PdIII species 1+
accumulates during the reaction up to a maximum yield of 5%
in 5% H2O/MeCN and 12% in 1:1 MeOH/PhH, respectively.
Moreover, 1+ decays as ethane is formed, suggesting the
involvement of this species as a transient intermediate in the
aerobic oxidation of 1.4 To confirm the role of 1+ in ethane
formation, we investigated the reactivity of the independently
synthesized [1+]ClO4. While 1+ is stable in CD3CN in the dark,
addition of OD− leads to rapid formation of ethane in 24%
yield.8 Hydroxide is likely generated during aerobic oxidation as
a product of O2 reduction in the presence of protons.
Furthermore, this coordinating anion can promote C−C
bond formation by binding to the PdIII center of 1+ and
displacing one of the axial amine groups (vide infra).4
similar to the previously reported (tBuN4)PdIIMe2 complex.4
Moreover, the observed C−C bond formation reactivity is not
affected by the presence of the alkyl radical trap TEMPO,8
suggesting a nonradical mechanism (vide infra).
Detection of the PdIII Intermediate [(MeN4)PdIIIMe2]+
(1+). When a solution of 1 is exposed to O2 or air in the
presence of protic solvents, a green color develops and the
UV−vis spectrum reveals absorption peaks at 620 and 490 nm
that are identical to those of the independently synthesized
complex [(MeN4)PdIIIMe2]ClO4, [1+]ClO4 (Figure 3).8 More-
over, the resulting solution exhibits an axial EPR spectrum
identical to [1+]ClO4, with superhyperfine coupling to the two
amine N atoms in both parallel and perpendicular directions
(Figure 4), in line with the distorted octahedral geometry of the
PdIII center (Figure 1, right).11 The formation of 1+ was further
confirmed by ESI-MS, which reveals a peak at m/z 404.1213
Detection and Characterization of the PdIV Inter-
mediate [(κ3-MeN4)PdIVMe3]+ (2+). When the aerobic
oxidation of 1 was monitored by NMR, peaks corresponding
to a transient intermediate were observed in addition to those
of ethane and the PdII product(s).8 The NMR spectrum
suggests the presence of three methyl groups attached to the Pd
center, and by comparison with other reported analogous
species5,13 we assign this intermediate as a [(κ3-MeN4)-
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PdIVMe3]+ (2+) species. During the course of the reaction, 2+
forms in up to 34% and 10% yields in O2-saturated 1:1
CD3OD/C6D6 and 5% D2O/CD3CN, respectively, and its
decay correlates with ethane formation, suggesting the
involvement of 2+ in C−C bond formation.8 In addition, the
maximum yield of 2+ also increases with an increasing
concentration of the protic solvent: from only a 6% yield of
2+ in 1% H2O/MeCN to a 32% yield in 50% H2O/MeCN, as
observed by NMR.8 This suggests that 2+ is the product of the
aerobic oxidation of 1 in the presence of protons in solution.4
Species 2+ was independently synthesized by treating 1 with
MeI in MeCN or acetone, to yield the [(k3-MeN4)PdIVMe3]I
([2+]I) product, which has an identical NMR spectrum to that
of the PdIV intermediate observed during the aerobic oxidation
of 1.8 The X-ray structure of [2+]I reveals a distorted octahedral
PdIV center with two Me groups in the equatorial positions,
while the third methyl group replaces one of the axial N atoms
of the MeN4 ligand (Figure 5). The Pd−Meeq distances are
Scheme 3. Reaction of 1 with CD3I
preceding the formation of 2+ during the aerobic oxidation of
1. Gratifyingly, ESI-MS analysis of 1 in O2- or air-saturated 5%
H2O/MeCN revealed several isotopic patterns that were
assigned to the following cationic PdIII and PdIV species:
[(MeN4)PdIIIMe2]+ (1+, m/z 404.1213, calcd 404.1192),
[(MeN4)PdIVMe3]+ (2+, m/z 419.1440, calcd 419.1429),
[(MeN4)PdIVMe2(OOH)]+ (4+, m/z 437.1196, calcd
437.1169), and [(MeN4)PdIVMe2(OH)]+ (5+, m/z 421.1253,
calcd 421.1221, Figure 6).15 These peaks were not observed
Figure 5. ORTEP representation (50% probability ellipsoids) of the
cation of [2+]I. Selected bond distances (Å) and angles (deg): Pd1−
C1 2.035(3), Pd1−C1i 2.035(3), Pd1−C2 2.061(4), Pd1−N1
2.283(2), Pd1−N1i 2.283(2), Pd1−N2 2.231(3), N2−Pd1−C2
169.22(13), C1−Pd1−C1i 92.50(18), C1−Pd1−C2 80.62(12).
Figure 6. ESI-MS spectrum showing the isotopic pattern of cationic
PdIII and PdIV intermediates formed during the aerobic oxidation of 1
in 5% H2O/MeCN, 15 min after exposure to air (top), and simulation
of the isotopic pattern corresponding to a 1.2:1.0:0.4:0.75 ratio of
[(MeN4)PdIIIMe2]+, [(MeN4)PdIVMe3]+, [(MeN4)PdIVMe2(OH)]+, and
[(MeN4)PdIVMe2(OOH)]+ (bottom).
2.035 Å, similar to other reported PdIVMe3 complexes,5,13 while
the Pd−Meax distance is slightly longer (2.061 Å) and
substantially tilted toward the equatorial plane, likely due to
the steric clash with the adjacent methylene H atoms (Figure
S14).8 This distortion is likely responsible for ethane
elimination: while 2+ is fairly stable at low temperatures (t1/2
≈ 6.7 days at −20 °C), it easily eliminates ethane quantitatively
at RT (t1/2 ≈ 68 min, Figure S13). By comparison, ethane
elimination from the more symmetric [(Me3tacn)PdIVMe3]+
complex is much slower and occurs only at elevated
temperatures.5 The 1H NMR of [2+]I in CD3CN at RT
shows one averaged singlet at 1.80 ppm corresponding to the
Pd-Me H’s, suggesting a rapid rearrangement of the equatorial
and axial methyl groups, while two separate Pd-Me peaks are
observed at −20 °C in a 1:2 ratio (Figures S10, S11).8 The
fluxionality of 2+ was confirmed by studying the reductive
elimination reactivity of [(κ3-MeN4)PdIV(CD3)(CH3)2]+ (2+-
d3), formed by reacting 1 with CD3I in CD3CN, which leads to
formation of CH3CH3 and CH3CD3 in a 1:2 ratio,8 as expected
for a fast rearrangement of the three methyl groups and
reductive elimination of any two of the three methyl groups
(Scheme 3).4,14
during the ESI-MS analysis of 1 in degassed MeCN or 5%
H2O/MeCN, suggesting that these PdIII and PdIV intermediates
are likely formed during the aerobic oxidation of 1 and not due
to an artifact of the ESI-MS experiment. Interestingly, the
relative ratio of these intermediates changes with time during
the course of the aerobic oxidation. While all four species were
detected at the early stages of the reaction, species 2+ is the
longest-lived intermediate (Figures S21−S24), suggesting that
1+, 4+, and 5+ are precursors to 2+.8 In addition, the presence of
2+ at reaction times comparable to the time scale of ethane
formation further confirms its direct involvement in the
reductive elimination of ethane (vide infra).
Crossover Experiments. To further probe the formation
of ethane through reductive elimination from the trimethyl PdIV
intermediate 2+, crossover experiments were performed using a
1:1 mixture of 1 and (MeN4)PdII(CD3)2, 1-d6. Monitoring of
the aerobic oxidation reaction by NMR in O2-saturated 5%
D2O/CD3CN reveals formation of CH3CH3 and CH3CD3 in
14% and 15% yields, respectively. Given the typical yield of
42% ethane formed upon oxidation of 1 under similar
ESI-MS Studies of the Aerobic Oxidation of 1.
Electrospray mass spectrometry (ESI-MS) studies were
employed to obtain additional information on the steps
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conditions, the crossover result suggests a 1:1:1 ratio of
CH3CH3, CH3CD3, and CD3CD3 (Scheme 4), which is
strongly suggest that formation of mixed-isotope intermediates
occurs only through methyl group transfer to yield the
[(MeN4)PdIVMe3]+ species 2+, which then undergoes reductive
elimination to form ethane (vide infra).
Proposed Mechanism for the Aerobic C−C Bond
Formation Reactivity of 1. Based on the experimental results
described above, and by analogy with the aerobic oxidation of
PtII and PdII dimethyl complexes,16,17 we propose the following
mechanism for the aerobically induced C−C bond formation
from 1 (Scheme 5). Oxidation of 1 by O2 through an inner-
Scheme 4. Crossover Experiments for the Aerobic Oxidation
of 1 and 1-d6 and the Reaction of 1+ and 1+-d6 with
Hydroxide
Scheme 5. Proposed Mechanism for Aerobic Oxidation of 1
a
and Subsequent Ethane Elimination
consistent with the formation of 2+, which undergoes a rapid
rearrangement and elimination of any two of the three methyl
groups.4 In addition, a 1:1 mixture of the PdIII complexes
[1+]ClO4 and [(MeN4)PdIII(CD3)2]ClO4, [1+-d6]ClO4, in
CD3CN yields both CH3CH3 and CH3CD3 in 8% yield upon
addition of 1 equiv of OD−, corresponding also to a
CH3CH3:CH3CD3:CD3CD3 ratio of 1:1:1 (Scheme 4). Control
NMR experiments show that no methyl group exchange occurs
at PdII centers between 1 and 1-d6.
ESI-MS analysis of the aerobic oxidation under similar
conditions reveals the formation of a ∼1:1 ratio of 1+ and
[(MeN4)PdIII(CD3)2]+, 1+-d6, and no mixed-isotope PdIII
species was observed, suggesting that no methyl exchange
occurs between 1+ and 1+-d6.4 Interestingly, a complex isotopic
pattern corresponding to [(MeN4)PdIVMe3]+ isotopologs was
observed that can be assigned to a ∼1:1:1:1 ratio of
[(MeN4)PdIV(CH3)3]+, [(MeN4)PdIV(CH3)2(CD3)]+, [(MeN4)-
PdIV(CH3)(CD3)2]+, and [(MeN4)PdIV(CD3)3]+ (Figure 7).
These four PdIV trimethyl species can reductively eliminate any
two of the three methyl groups and generate CH3CH3,
CH3CD3, and CD3CD3 in a 1:1:1 ratio.8 Overall, these studies
a
Intermediate species were detected by the techniques listed.
sphere mechanism is expected to generate a transient PdIII-
superoxide intermediate, [(κ3-MeN4)PdIIIMe2(O2•−)] (6). This
is similar to the analogous (tBuN4)PdII(Me)2 complex4 and
supported by the detection of an EPR-active metal-bound
radical adduct when the oxidation is performed in the presence
of the spin trap DMPO (DMPO = 5,5-dimethyl-1-pyrroline-N-
oxide, Figure S29). In addition, the obtained hyperfine coupling
constants are different from those for the DMPO adducts of C-
center radical, hydroxyl, alkylperoxide, or superoxide radicals,8
suggesting that a non-metal-based radical mechanism is likely
not operative.4
Upon protonation, species 6 would yield the detected PdIV-
hydroperoxo species 4+, which can further oxidize another
molecule of 1 to give two molecules of the observed PdIV-
hydroxo species 5+. Similar PtIV species have been detected
during the aerobic oxidation of PtIIMe2 precursors,16 while
analogous PdIV intermediates were also recently reported
during the aerobic oxidation of (Me3tacn)PdIIMe2.5 The PdIV-
hydroxo species 5+ can undergo rapid comproportionation in
the presence of (MeN4)PdIIMe2 (1) to yield the PdIII species
Figure 7. ESI-MS spectrum showing the isotopic pattern of
[(MeN4)PdIV(CH3)n(CD3)3−n]+ (n = 0−3) isotopologs formed during
aerobic oxidation of a 1:1 mixture of (MeN4)PdII(CH3)2 and
(
MeN4)PdII(CD3)2 in 5% H2O/MeCN, 3 min after exposure to air
(top), and simulation of the isotopic pattern corresponding to a
0.9:1.1:1.0:0.8 ratio of [(MeN4)PdIV(CH3)3]+, [(MeN4)-
PdIV(CH3)2(CD3)]+, [(MeN4)PdIV(CH3)(CD3)2]+, and [(MeN4)-
PdIV(CD3)3]+ (bottom).
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(κ3-MeN4)PdIIIMe2(OH) (7) and [(MeN4)PdIIIMe2]+ (1+). The
two PdIII species can interconvert through displacement of one
of the axial N donors of MeN4 by a hydroxide anion.4,18 The
trimethyl PdIV species 2+ can then be formed by two different
pathways, one of which involves a Me group transfer between
the two PdIII dimethyl species 1+ and 7. Similar Me group
transfer was also reported for other PdII and PtII complexes
upon one-electron oxidation.19 Another possible mechanism
may involve a PdIV-to-PdII Me group transfer between 5+ and 1
to give 2+ and 3 (Scheme 5).4 Such methyl group transfers
from electrophilic PdIVMe3 complexes to nucleophilic PtIIMe2
or PdIIMe2 species have been observed previously.20 In
addition, a very similar Me group transfer reaction from a
[(Me3tacn)PdIVMe2(OH)]+ to (Me3tacn)PdIIMe2 center was
recently proposed by our group.5
We also thank Dr. Julia R. Khusnutdinova for experimental
assistance. L.M.M. is a Sloan Fellow.
REFERENCES
■
(1) (a) Stoltz, B. M. Chem. Lett. 2004, 33, 362. (b) Stahl, S. S. Angew.
Chem., Int. Ed. 2004, 43, 3400. (c) Stahl, S. S. Science 2005, 309, 1824.
(d) Beck, E. M.; Grimster, N. P.; Hatley, R.; Gaunt, M. J. J. Am. Chem.
Soc. 2006, 128, 2528. (e) Sigman, M. S.; Jensen, D. R. Acc. Chem. Res.
2006, 39, 221. (f) Gligorich, K. M.; Sigman, M. S. Chem. Commun.
2009, 3854. (g) Campbell, A. N.; Stahl, S. S. Acc. Chem. Res. 2012, 45,
851. (h) Shi, Z.; Zhang, C.; Tang, C.; Jiao, N. Chem. Soc. Rev. 2012, 41,
3381.
(2) (a) Zhang, J.; Khaskin, E.; Anderson, N. P.; Zavalij, P. Y.;
Vedernikov, A. N. Chem. Commun. 2008, 3625. (b) Zhang, Y.-H.; Yu,
J.-Q. J. Am. Chem. Soc. 2009, 131, 14654. (c) Wang, A.; Jiang, H.;
Chen, H. J. Am. Chem. Soc. 2009, 131, 3846. (d) Boisvert, L.; Denney,
M. C.; Kloek, H. S.; Goldberg, K. I. J. Am. Chem. Soc. 2009, 131,
15802. (e) Zhu, M.-K.; Zhao, J.-F.; Loh, T.-P. J. Am. Chem. Soc. 2010,
132, 6284. (f) Vedernikov, A. N. Acc. Chem. Res. 2012, 45, 803.
(3) Chuang, G. J.; Wang, W.; Lee, E.; Ritter, T. J. Am. Chem. Soc.
2011, 133, 1760.
CONCLUSION
■
In summary, reported herein is a rare example of aerobic
oxidation of an organometallic PdII precursor to form
detectable PdIII and PdIV species, which lead to selective C−
C bond formation under ambient conditions. The detailed
mechanisms of the aerobic oxidation of (MeN4)PdIIMe2 and
subsequent ethane elimination were investigated by various
spectroscopic methods. UV−vis and EPR studies reveal the
formation of a [(MeN4)PdIIIMe2]+ intermediate, and a PdIII-
superoxide species is proposed to form upon the initial inner-
sphere oxidation of the PdII center by O2. Interestingly, ESI-MS
and NMR studies show the formation of several PdIV
intermediates, including the PdIV-OOH and PdIV-OH species
involved in the aerobic oxidation steps, as well as the key
intermediate [(MeN4)PdIVMe3]+ responsible for ethane elimi-
nation. The latter PdIV species was also synthesized
independently, and its observed ethane elimination reactivity
provides unambiguous evidence for the proposed mechanism.
Overall, these results strongly suggest that both PdIII and PdIV
oxidation states are involved in the aerobically induced C−C
bond formation from a PdIIMe2 precursor: while the initial
formation of a PdIII species is likely necessary for an efficient
aerobic oxidation, the formation of a PdIV intermediate is
required for a selective and facile C−C bond formation step.
Current research efforts are aimed at using the MeN4 ligand
system for catalytic aerobic C−C and C−heteroatom bond
formation reactions.
(4) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc.
2012, 134, 2414.
(5) Khusnutdinova, J. R.; Qu, F.; Zhang, Y.; Rath, N. P.; Mirica, L. M.
Organometallics 2012, 31, 4627.
(6) Bottino, F.; Di Grazia, M.; Finocchiaro, P.; Fronczek, F. R.;
Mamo, A.; Pappalardo, S. J. Org. Chem. 1988, 53, 3521.
(7) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc.
2010, 132, 7303.
(8) See Supporting Information.
(9) SADABS; Bruker Analytical X-Ray: Madison, WI, 2008.
(10) Sheldrick, G. M. Acta Crystallogr. Sect. A: Found. Crystallogr.
2007, 64, 112.
(11) We have recently employed the MeN4 ligand to stabilize a series
of PdIII and PdIV complexes: Tang, F.; Qu, F.; Khusnutdinova, J. R.;
Rath, N. P.; Mirica, L. M., submitted for publication in Chem. Sci.
(12) A small amount of methane forms during aerobic oxidation,
likely due to ligand decomposition from the PdII precursor or the PdII
product (see Supporting Information). The mechanism of ligand
decomposition is currently under investigation.
(13) (a) Byers, P. K.; Canty, A. J.; Skelton, B. W.; White, A. H. J.
Chem. Soc., Chem. Commun. 1986, 1722. (b) Byers, P. K.; Canty, A. J.;
Skelton, B. W.; White, A. H. J. Chem. Soc., Chem. Commun. 1987, 1093.
(c) Byers, P. K.; Canty, A. J.; Skelton, B. W.; White, A. H.
Organometallics 1990, 9, 826. (d) Canty, A. J.; Honeyman, R. T.;
Roberts, A. S.; Traill, P. R.; Colton, R.; Skelton, B. W.; White, A. H. J.
Organomet. Chem. 1994, 471, C8. (e) Canty, A. J.; Jin, H.; Roberts, A.
S.; Skelton, B. W.; Traill, P. R.; White, A. H. Organometallics 1995, 14,
199. (f) Canty, A. J.; Dedieu, A.; Jin, H.; Milet, A.; Skelton, B. W.;
Trofimenko, S.; White, A. H. Inorg. Chim. Acta 1999, 287, 27.
(g) Bayler, A.; Canty, A. J.; Edwards, P. G.; Skelton, B. W.; White, A.
H. J. Chem. Soc., Dalton Trans. 2000, 3325.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental details, spectroscopic characterization,
aerobic oxidation studies, and X-ray crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(14) Byers, P. K.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott, J.
D. Organometallics 1988, 7, 1363.
(15) The additional peaks around the m/z value of 405 are
tentatively assigned to [(MeN4)PdO2-H]+ (calcd 405.0543), which is
likely a product of ligand oxidation and/or PdII product decom-
position. The MeN4 ligand is flexible even when bonded to the Pd
center and contains benzylic C−H bonds; thus it can undergo
oxidation and ligand decomposition, as suggested by the less-than-
quantitative yield of ligand-containing products at the end of the
aerobic reactions. However, the formation of the PdIII and PdIV species
occurs much faster than any potential ligand oxidation/decomposition
reactions, which are thus not responsible for the observed aerobic
oxidation of the PdII center and the subsequent C−C bond formation.
The ligand decomposition most likely occurs from the PdII-
monomethyl products (see page S7 in the Supporting Information).
A detailed characterization of the reaction side products is currently
under way.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mirica@wustl.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Department of Chemistry at Washington
University for startup funds, and the American Chemical
Society Petroleum Research Fund (49914-DNI3) and DOE
Catalysis Science Program (DE-FG02-11ER16254) for support.
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(16) (a) Rostovtsev, V. V.; Henling, L. M.; Labinger, J. A.; Bercaw, J.
E. Inorg. Chem. 2002, 41, 3608. (b) Prokopchuk, E. M.; Jenkins, H. A.;
Puddephatt, R. J. Organometallics 1999, 18, 2861. (c) Prokopchuk, E.
M.; Puddephatt, R. J. Can. J. Chem. 2003, 81, 476.
(17) (a) Vedernikov, A. N.; Binfield, S. A.; Zavalij, P. Y.;
Khusnutdinova, J. R. J. Am. Chem. Soc. 2006, 128, 82. (b) Vedernikov,
A. N. Chem. Commun. 2009, 4781.
(18) Wieghardt, K.; Koeppen, M.; Swiridoff, W.; Weiss, J. J. Chem.
Soc., Dalton Trans. 1983, 1869.
(19) (a) Lanci, M. P.; Remy, M. S.; Kaminsky, W.; Mayer, J. M.;
Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 15618. (b) Seligson, A. L.;
Trogler, W. C. J. Am. Chem. Soc. 1992, 114, 7085. (c) Johansson, L.;
Ryan, O. B.; Romming, C.; Tilset, M. Organometallics 1998, 17, 3957.
(20) (a) Aye, T.-K.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott,
J. D.; Watson, A. A. Organometallics 1989, 8, 1518. (b) Markies, B. A.;
Canty, A. J.; Boersma, J.; van Koten, G. Organometallics 1994, 13,
2053.
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