The aerobic oxidation of a Pd(II) dimethyl complex leads to selective ethane elimination from a Pd(III) intermediate

Article abstract of DOI:10.1021/ja210841f

Oxidation of the Pd^II complex (N4)Pd^IIMe₂ (N4 = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane) with O₂ or ROOH (R = H, tert-butyl, cumyl) produces the Pd^III species [(N4)Pd^IIIMe₂]⁺, followed by selective formation of ethane and the monomethyl complex (N4)Pd^IIMe(OH). Cyclic voltammetry studies and use of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap suggest an inner-sphere mechanism for (N4)Pd^IIMe ₂ oxidation by O₂ to generate a Pd^III- superoxide intermediate. In addition, reaction of (N4)Pd^IIMe ₂ with cumene hydroperoxide involves a heterolytic O-O bond cleavage, implying a two-electron oxidation of the Pd^II precursor and formation of a transient Pd^IV intermediate. Mechanistic studies of the C-C bond formation steps and crossover experiments are consistent with a nonradical mechanism that involves methyl group transfer and transient formation of a Pd^IV species. Moreover, the (N4)Pd^IIMe(OH) complex formed upon ethane elimination reacts with weakly acidic C-H bonds of acetone and terminal alkynes, leading to formation of a new Pd^II-C bond. Overall, this study represents the first example of C-C bond formation upon aerobic oxidation of a Pd^II dimethyl complex, with implications in the development of Pd catalysts for aerobic oxidative coupling of C-H bonds.

Full text of DOI:10.1021/ja210841f

Article
pubs.acs.org/JACS
The Aerobic Oxidation of a Pd(II) Dimethyl Complex Leads to
Selective Ethane Elimination from a Pd(III) Intermediate
Julia R. Khusnutdinova,† 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, One University Boulevard, University of MissouriSt. Louis, Missouri 63121-4400,
United States
S
* Supporting Information
ABSTRACT: Oxidation of the PdII complex (N4)PdIIMe2
(N4 = N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane)
with O2 or ROOH (R = H, tert-butyl, cumyl) produces the
PdIII species [(N4)PdIIIMe2]+, followed by selective formation
of ethane and the monomethyl complex (N4)PdIIMe(OH).
Cyclic voltammetry studies and use of 5,5-dimethyl-1-pyrro-
line-N-oxide (DMPO) as a spin trap suggest an inner-sphere
mechanism for (N4)PdIIMe2 oxidation by O2 to generate a
PdIII-superoxide intermediate. In addition, reaction of (N4)-
PdIIMe2 with cumene hydroperoxide involves a heterolytic O−O bond cleavage, implying a two-electron oxidation of the PdII
precursor and formation of a transient PdIV intermediate. Mechanistic studies of the C−C bond formation steps and crossover
experiments are consistent with a nonradical mechanism that involves methyl group transfer and transient formation of a PdIV
species. Moreover, the (N4)PdIIMe(OH) complex formed upon ethane elimination reacts with weakly acidic C−H bonds of
acetone and terminal alkynes, leading to formation of a new PdII−C bond. Overall, this study represents the first example of C−C
bond formation upon aerobic oxidation of a PdII dimethyl complex, with implications in the development of Pd catalysts for
aerobic oxidative coupling of C−H bonds.
been recently proposed in some stoichiometric and catalytic
aerobic oxidation reactions mediated by PdII complexes, no
such species have been detected experimentally.11,12
INTRODUCTION
■
Palladium-catalyzed C−C coupling reactions are among the
most important transformations in synthetic organic chem-
istry.1 The vast majority of these reactions involve a C−C bond
formation step at a PdII center to generate a Pd0 species which
We have recently reported the one-electron oxidation of PdII
precursors to yield stable PdIII dimethyl and monomethyl
species [(N4)PdIIIMe(X)]+ (X = Me, Cl; N4 = N,N′-di-tert-
butyl-2,11-diaza[3.3](2,6)pyridinophane) that undergo light-
induced elimination of ethane.13 Herein we report the facile
oxidation of (N4)PdIIMe2 with O2 or peroxides to form the
PdIII species [(N4)PdIIIMe2]+, followed by selective formation
of ethane and the monomethyl complex (N4)PdIIMe(OH)
(Scheme 1). Moreover, the latter product can further react and
activate the weakly acidic C−H bonds of organic substrates
such as acetone or terminal alkynes, leading us to propose a
possible catalytic cycle that can be employed for the aerobic
oxidative coupling of C−H bonds (vide infra). Cyclic
voltammetry studies and spin trapping experiments with 5,5-
dimethyl-1-pyrroline-N-oxide (DMPO) suggest an inner-
sphere oxidation of (N4)PdIIMe2 by O2 to generate a PdIII-
superoxide intermediate. In addition, oxidation of (N4)PdIIMe2
by cumene hydroperoxide leads to heterolytic O−O bond
cleavage, implying a two-electron oxidation and formation of a
transient PdIV intermediate. Mechanistic studies and crossover
experiments of the C−C bond formation reaction are
is subsequently reoxidized by a functionalized substrate,2
a
sacrificial oxidant,3 or even O2.4 In addition, oxidative C−H
coupling reactions have recently been reported in which the
oxidation of an organometallic PdII species generates high-
valent PdIII or PdIV intermediates that undergo facile reductive
elimination and C−C bond formation.5,6 However, the use of
expensive and hazardous strong oxidants required to produce
such high-valent Pd species limits the practical applications of
these transformations.7 In this context, the development of
oxidative C−H coupling reactions using molecular oxygen as an
environmentally friendly and inexpensive oxidant is highly
desirable.4
While various oxidants have been employed in the C−C
coupling reactions catalyzed by high-valent Pd species,5,7 no
C−C bond formation reactions induced by aerobic oxidation of
PdII precursors have been reported to date. For example, C−C
bond formation and ethane elimination from PdII dimethyl
complexes have recently been shown to be induced by chemical
oxidation through the intermediacy of high-valent Pd species.8
However, examples of aerobic oxidation PdII complexes leading
to formation of detectable PdIII or PdIV species are very rare.9,10
Moreover, while the intermediacy of PdIII or PdIV species has
Received: November 17, 2011
Published: December 22, 2011
© 2011 American Chemical Society
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UV−Vis Study of Formation of 2 during Oxidation of 1 with
O2 or Peroxides. A 1 mL solution of 1 (4.7 mM) in C6H6 was placed
into a quartz cuvette (10 mm path length) equipped with a septum-
sealed cap and a magnetic stir bar, and 1 mL of MeOH was added. O2
was then bubbled through the solution for 10−15 min, and the
reaction mixture was stirred under O2 in the dark and the reaction
progress was monitored by UV−vis. For reaction with peroxides, the
reaction solution was prepared in degassed solvents under N2 and a
stock solution of peroxide in MeOH was added through the septum.
The concentration of 2 during the reaction was calculated based on
the extinction coefficient of the 750 nm absorption band (ε = 639 M−1
cm−1 in PhH-MeOH).
DMPO Spin Trap Experiments. A 8.9 mM stock solution of 1 in
C6H6 and a 90 mM DMPO stock solution in MeOH were prepared in
degassed solvents under N2 and used immediately after preparation.
Then 250 μL of the stock solution of 1 in C6H6 and 250 μL of the
stock solution of DMPO in MeOH were combined in a vial equipped
with a septum ([1] = 4.5 mM, [DMPO] = 45 mM). O2 was bubbled
through the solution for 1 min, and the solution was transferred to an
EPR tube. The EPR spectrum was recorded immediately after
preparation at 293 K.
General Procedure for NMR Studies of Reactivity of 2 with
NaOD and Other Additives. A 2.37 mM solution of [2]ClO4 in
CD3OD was prepared under a N2 atmosphere, placed into an NMR
tube, and 1,3,5-trimethoxybenzene was added as an internal standard.
The NMR tube was filled to the top with 2.4 mL of the resulting
solution (to avoid the escape of volatiles into the headspace), sealed
with a septum, and then 1 equiv of a CD3OD stock solution of NaOD
or other additives was added. The reaction mixture was kept in the
dark and periodically analyzed by 1H NMR. The yield of the products
were determined by NMR integration using 1,3,5-trimethoxybenzene
as an internal standard, calculated as [moles of product]/[moles of 1]
*100%, and reported as the average of two runs. For crossover
experiments, a 1:1 mixture of [2]ClO4 and [(N4)Pd(CD3)2]+, [2-
d6]ClO4 was used.
General Procedure for Electrochemical Studies of 1 and 2. A
solution of 1 in 0.1 M Bu4NBF4 1:3 PhH-MeOH was deaerated by
bubbling N2 for 10−15 min and the cyclic voltammogram (CV) was
recorded under a N2 blanket. The 1:3 PhH-MeOH solvent mixture
was used due its higher conductivity; however, similar results were
obtained in 1:1 PhH-MeOH. O2 was bubbled through the solution of
1 for 10 min, and cyclic votammograms were then recorded under a
blanket of O2. Similarly, the CV of [2]ClO4 was recorded in 0.1 M
Bu4NBF4/PhH-MeOH under a blanket of O2. As a control
experiment, the CV of the O2-saturated 0.1 M Bu4NBF4 1:3
PhH:MeOH solution was recorded.
X-ray Structure Determination of 1, 2[ClO4], 4[OTf], 5, and 6.
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 3.5 cm. The collected frames were
integrated using an orientation matrix determined from the narrow
frame scans. Apex II and SAINT software packages (Bruker Analytical
X-Ray, Madison, WI, 2008) 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 SADABS (Bruker Analytical
X-Ray, Madison, WI, 2008) based on the Laue symmetry using
equivalent reflections. Crystal data and intensity data collection
parameters are listed in Tables S8−15. Structure solutions and
refinement were carried out using the SHELXTL-PLUS software
package (see the Supporting Information). The structures were solved
by direct methods and refined successfully in the C2/c, C2/c, P-1, P-1,
and Cc space groups, respectively. Full matrix least-squares refinements
were carried out by minimizing Σw(Fo2−Fc2)2. The non-hydrogen
Scheme 1. Oxidative C−C Bond Formation Reactivity of
(N4)PdIIMe2 (1) in the Presence of O2 or Peroxides
consistent with a nonradical mechanism that involves formation
of a PdIV transient species. Overall, this study represents the
first example of C−C bond formation upon aerobic oxidation
of a PdII dimethyl complex, with direct implications in the
development of Pd catalysts for the oxidative coupling of C−H
bonds. These results suggest that generation of a PdIII
intermediate promotes the aerobic oxidation of the PdII
precursor through an inner-sphere reduction of O2, while
transient formation of PdIV species is needed for a facile C−C
bond formation.
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′-
Di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (N4),14 (COD)-
PdIICl2,15 (N4)PdIIMe2 (1),13 [(N4)PdIIIMe2]ClO4 (2[ClO4]),13 and
(N4)PdIIMeCl16 were prepared according to literature procedures. 1H
(300.121 MHz) NMR spectra were recorded on a Varian Mercury-300
spectrometer. 13C (151 MHz) NMR spectra were recorded on a
Varian Unity Inova-600 spectrometer. 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 Thermo FT or 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 or a
CHI 660D Electrochemical Analyzer. Electrochemical-grade Bu4NBF4
(Fluka) was used as the supporting electrolyte. Electrochemical
measurements were performed under a blanket of nitrogen, and the
analyzed solutions were deaerated by purging with nitrogen. A glassy
carbon disk electrode (d = 1.6 mm) and a Ag/0.01 M AgNO3/MeCN
electrode were used as the working and reference electrode,
respectively. The nonaqueous reference electrode was calibrated
against Cp2Fe (Fc).
General Procedure for Oxidation of 1 with O2 (or Peroxides).
A 6.8 mM solution of 1 in O2-saturated C6D6 was placed into an NMR
tube and 1,3,5-trimethoxybenzene was added as an internal standard.
The NMR tube was filled to the top (to avoid the escape of volatiles
into the headspace) by addition of 1.2 mL of O2-saturated CD3OD
and sealed with a septum. The concentration of 1 in the resulting
solution is 3.4 mM. The reaction mixture was kept in the dark and
1
periodically analyzed by H NMR. The yield of the products were
determined by NMR integration using 1,3,5-trimethoxybenzene as an
internal standard, calculated as [moles of product]/[moles of 1] ×
100%, and reported as the average of two runs. For oxidation with
peroxides, all solutions were prepared in degassed solvents under N2
atmosphere, and CD3OD stock solutions of H2O2, tBuOOH, or cumyl
peroxide were added to the reaction mixtures. For crossover
experiments, a 1:1 mixture of 1 (13 μmol) and (N4)Pd(CD3)2, 1-d6
(13 μmol) was used.
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atoms were refined anisotropically to convergence. The hydrogen
atoms were treated using appropriate riding model (AFIX m3).
RESULTS AND DISCUSSION
■
Characterization and Oxidation Studies of (N4)-
PdIIMe2 (1). The dimethyl complex (N4)PdIIMe2, 1, was
synthesized according to a previously published method13 and
was characterized by X-ray diffraction (Figure 1a). The X-ray
Figure 2. Formation of 2 during aerobic oxidation of 1 (2.3 mM) in
1:1 PhH-MeOH followed by UV−vis spectroscopy (Δt = 4.5 min, tmax
= 70 min). Inset: plot of the concentration of 2 during oxidation of 1
under the same conditions.
Figure 1. ORTEP representation of 1 (a) and the cation of 2[ClO4]
(b). Selected bond lengths (Å), 1: Pd1−C1 2.099; Pd1−C2 2.099;
Pd1−N1 2.173; Pd1−N2 2.173. 2: Pd1−C1 2.048; Pd1−C1i 2.048;
Pd1−N1 2.112; Pd1−N1i 2.112; Pd1−N2 2.469; Pd1−N2i 2.469.
structure of 1 reveals a square planar geometry around the PdII
center with the N4 ligand bound through the two pyridyl N
atoms, whereas the two amine donors are pointing away from
the metal center. The cyclic voltammogram (CV) of 1 in 0.1 M
Bu4NBF4/THF exhibits a PdII/III oxidation peak potential at
−0.36 V vs Fc+/Fc13 that is significantly lower than the
oxidation potentials of PdIIMe2 complexes supported by
bidentate N-donor ligands.8,17 For example, complexes
(tmeda)PdIIMe2 and (tBu2bipy)PdIIMe2 exhibit irreversible
oxidation waves at +0.30 V and +0.45 V vs Fc+/Fc, respectively
(Figures S30 and S31).17 The uncommonly low oxidation
potential of 1 is due to the ability of the tetradentate ligand N4
to form (κ3-N4)PdMe2 or (κ4-N4)PdMe2 species18 that have
lower oxidation potentials,19−21 as well as stabilize the distorted
octahedral geometry of the PdIII center (Figure 1b).13 Such a
low oxidation potential of 1 has prompted us to study its
reactivity toward O2. While solutions of 1 in aprotic solvents
such as benzene, toluene, fluorobenzene, or THF are stable
under 1 atm O2 for several days, the addition of methanol to a
solution of 1 in any of these solvents leads to the formation of a
green paramagnetic species identified as [(N4)PdIIIMe2]+, 2,
which exhibits an UV−vis spectrum identical to that of the
independently synthesized 2[ClO4] complex (Figure 2).13
Interestingly, the EPR spectrum of 2 in frozen MeOH (or in a
glassing solvent mixture)17 reveals superhyperfine coupling to
two N atoms in both parallel and perpendicular directions
(Figure 3),9,22a which is different from the broad EPR spectrum
observed in MeCN.13 This difference is likely due to a more
symmetric structure of 2 in a frozen protic solvent that results
in sharper EPR features and a larger separation between the gz
and gx/gy values.9,22b In addition, the presence of a dynamic
Jahn−Teller distortion in frozen MeCN at 77 K can also lead to
a broader EPR spectrum.23 Indirect support for this behavior in
frozen solution is provided by the solid-state C2 symmetric
Figure 3. EPR spectra of 0.7 mM solution of [2]ClO4 in MeOH
(black line), the product of oxidation of 1 (4.5 mM) with O2 in 1:1
PhH-MeOH, and a simulated EPR spectrum (red line) using the
following parameters: gx = 2.221 (AN = 12.0 G); gy = 2.191 (AN = 13.8
G); gz = 1.986 (AN = 18.0 G). The features marked with an asterisk are
due to the hyperfine coupling to the 105Pd isotope (abundance 22.3%,
I = 5/2).
structure of 2[ClO4] crystallized from MeOH (Figure 1,b), as
opposed to the C1 symmetric structure obtained from MeCN.13
Formation of the [(N4)PdIIIMe2]+ species was also confirmed
by the ESI-MS of an O2-saturated MeOH solution of 1 that
reveals a peak at m/z 488.2118 amu (calcd for 2: 488.2131
amu). Moreover, the maximum yield of 2 observed during
aerobic oxidation increases with increasing concentrations of
MeOH in solution. Stirring a solution of 1 in 1:3 PhH-MeOH
under 1 atm O2 for 60 min generates 2 in 26% yield, whereas
the maximum yield of 2 is only 16% in 1:1 PhH-MeOH.
Formation of 2 was also observed in solution mixtures
containing other protic solvents such as acetone−water and
THF−water (Table S8).17 Overall, we propose that the
presence of protons is needed to facilitate the irreversible
reduction of O2 along with the oxidation of 1 to 2, in a
mechanism similar to that proposed for the aerobic oxidation of
PtIIMe2 complexes in protic solvents (vide infra).21,24,25 As 1 is
poorly soluble in neat MeOH or water-containing solvent
mixtures, a 1:1 PhH-MeOH (v:v) solvent mixture was used for
further studies as it provides an optimal solubility for both 1
and 2.
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The UV−vis spectra obtained during the reaction of 1 with
O2 in 1:1 PhH-MeOH show that the concentration of 2
increases rapidly during the initial period of time (∼70 min)
and then slowly decays over the course of several hours (Figure
2, inset). When the oxidation of 1 in O2-saturated 1:1 C6D6-
CD3OD is monitored by 1H NMR spectroscopy, the formation
of ethane and a new diamagnetic PdII complex is detected in 52
complex (Figure 4).17,31 The current of the O2 reduction wave
is significantly higher in the presence of 1 or 2 compared to the
1% and 84
2% yields, respectively, after 8 h at RT (a
theoretical yield of 50% ethane is expected based on the
reaction shown in Scheme 1).8,13 The PdII complex formed
upon ethane elimination was identified as the PdII methyl
hydroxo complex (N4)PdIIMe(OH), 3 (vide infra).26
A similar oxidation reactivity was observed when 1 was
t
reacted with oxidants such as H2O2, BuOOH, or cumene
hydroperoxide (CumOOH). For example, the oxidation of 1
with 1 equiv of H2O2 in 1:1 PhH-MeOH generates 2 in 41%
yield after 10 min, as detected by UV−vis and EPR
1
spectroscopy, followed by its slow decay. H NMR analysis of
Figure 4. CVs in 1:3 PhH-MeOH, (scan rate 100 mV/s): O2
reduction wave in absence of Pd complexes (black line); in presence
of 2.2 mM 1 (red line); in presence of 2.2 mM [2]ClO4 (blue line);
CV of 2.2 mM 1 in absence of O2 (green line, the I axis multiplied by
10×).
the reaction of 1 with 1 equiv H2O2 in 1:1 C6D6-CD3OD
reveals the formation of ethane and 3 in 46 1% and 60 3%
yield, respectively. Similarly, the oxidation of 1 with 1 equiv
tBuOOH in 1:1 C6D6-CD3OD produces ethane and 3 in 43
1% and 78 1% yield, respectively. The oxidation of 1 with 0.5
t
equiv of H2O2 or BuOOH leads to formation of ethane in
peak current of O2 reduction in absence of Pd complexes
(Figure 4), suggestive of an electrocatalytic O2 reduction. The
appearance of an O2 reduction catalytic-like wave in the
presence of 1 (or 2) can be explained by an inner-sphere
oxidation of 1 with O2 to produce 2, which is subsequently
electrochemically reduced to 1 and thus completes an
electrocatalytic cycle (Scheme 2). Similar electrocatalytic O2
similar yields to the use of 1 equiv oxidant, suggesting a
1:peroxide stoichiometry of 2:1. For example, oxidation of 1
t
with 0.5 equiv of H2O2 or BuOOH leads to the formation of
ethane in 42 1% and 40 2% yields, respectively (Tables S3
and S4).Overall, these results suggest that both O2 and
peroxides are capable of oxidizing 1 to 2, which is likely
intimately involved in the observed C−C bond formation (vide
infra).13
Scheme 2. Catalytic O2 Electroreduction in the Presence of 1
or 2
Mechanistic Studies of the Oxidation of 1. Formation
of 2 from 1 and O2 is intriguing since, while there are several
reports on the oxidation of Pd0 complexes by O2,27 only a few
PdI and PdII complexes11,28 have been shown to react with O2,
and only two systems lead to formation of detectable PdIII
species.9,10 However, the mechanism of aerobic oxidation in
these latter systems has not been investigated. To obtain further
insight into the oxidation of (N4)PdIIMe2, we first considered
the in situ formation of H2O2 from MeOH and O2 in the
presence of 1, given the previous reports of PdII-catalyzed H2O2
formation through aerobic oxidation of primary and secondary
alcohols.29 However, no H2O2 was detected at levels above 2
μM (i.e., <0.1%) in a 2.2 mM solution of 1 in 1:1 THF-MeOH
using an Amplex red/horseradish peroxidase (HRP) assay.17
Moreover, formation of Pd black was not observed during the
reaction of 1 with O2.30 The formation of PdIII species was also
observed by UV−vis and EPR in benzene in the presence of
proton donors such as phenol or acetic acid.17 These results
indicate that MeOH likely acts as a mildly acidic source of
protons that promotes the aerobic oxidation of 1 and not as a
sacrificial reductant.30
Cyclic Voltammetry Studies of O2 Reduction in the
Presence of 1. The possibility of an outer-sphere electron
transfer from 1 to O2 to form 2 and superoxide (O2•−) also
needs to be considered. Cyclic voltammetry (CV) studies reveal
that the PdII/PdIII oxidation wave potential is ∼0.8 V more
positive than the O2 reduction peak potential, arguing against
an outer-sphere electron transfer mechanism.17 Moreover, the
CV of 1 (or 2) in O2-saturated 1:3 PhH-MeOH reveals an O2
reduction wave at a potential that is 0.5−0.6 V more positive
than the O2 reduction peak potential in the absence of the Pd
reduction has been observed in the presence of other transition
metal complexes capable of an inner-sphere O2 reduction.31,32
DMPO Spin Trap Studies. Alternatively, O2 could be
involved in an inner-sphere oxidation of 1 to a PdIII−O2•−
species. To detect any oxygen-containing intermediate, the
oxidation of 1 with O2 was carried out in the presence of 5,5-
dimethyl-1-pyrroline-N-oxide (DMPO), a commonly used spin
trap for O-centered radicals.33 When a solution of 1 (4.5 mM)
and DMPO (45 mM) in 1:1 PhH-MeOH was reacted with O2,
the EPR spectrum of the reaction mixture showed the
formation of an unstable DMPO spin adduct that exhibits
four broad lines (AN = 13.8 G, AH = 9.9 G, g = 2.004, Figure 5).
These hyperfine coupling constants are typical for DMPO
adducts of O-centered radicals, yet they are distinctly different
from the hyperfine coupling constants characteristic for DMPO
adducts of C-centered radicals, hydroxyl radical, or superoxide
radical.33 Similar broad features have been reported for the
DMPO adducts of CoIII- and CuII-superoxide complexes
formed during the oxidation of the corresponding precursors
with O2 in the presence of DMPO.34,35 Moreover, a similar
DMPO adduct was observed when O2 was reacted with a
solution containing equimolar amounts of Co(ClO4)2·6H2O
and the ligand N4 in MeOH (AN = 13.6 G, AH = 7.6 G, g =
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Scheme 4. Proposed Mechanisms of Formation of 2 during
Oxidation of 1 by O2 or Peroxides
Figure 5. EPR spectrum of 1 (4.5 mM) in 1:1 PhH-MeOH in the
presence of DMPO (45 mM), 2 min after exposure to O2.
2.005).17 Overall, these observations are suggestive of an inner-
sphere oxidation of 1 by O2 to form a proposed PdIII-
superoxide that is trapped by DMPO to form a PdIII-peroxide-
DMPO adduct (Scheme 3), similar to the one proposed for a
Co complex.34
Scheme 3. Proposed Mechanism of Oxidation of 1 by O2 and
Formation of a DMPO Radical Adduct
complexes via either heterolytic or homolytic O−O bond
cleavage leading to the formation of α-cumyl alcohol or a
cumyloxy radical, respectively; the latter subsequently produces
acetophenone as a major product via a methyl radical loss.37
Reaction of 1 with 1 equiv CumOOH produces ethane and 3 in
40
1% and 64
1% yields, respectively. The reaction is
accompanied by consumption of ∼0.63 equiv CumOOH and
formation of an equivalent amount of α-cumyl alcohol. No
acetophenone was detected in the reaction mixture, suggestive
of a heterolytic O−O bond cleavage and thus a two-electron
oxidation of 1 by CumOOH.37 Moreover, no DMPO adducts
of hydroxyl radical or solvent-derived radicals were detected
during the oxidation of 1 with 1 equiv of H2O2, ruling out a
Fenton-type radical mechanism.38 This two-electron oxidation
of the PdII center by the metal-bound peroxide would generate
a PdIV-hydroxo39 (or PdIV-oxo40) intermediate, which can react
with 1 to yield 2 equiv of the PdIII species, consistent with the
observed 1:cumene hydroperoxide stoichiometry of ∼2:1
(Scheme 4).41 Overall, these mechanistic studies suggest the
involvement of transient PdIV intermediates during the
oxidation of 1, although the steric properties of the N4 ligand
preferentially stabilize the observed PdIII complex 2.
C−C Bond Formation Reactivity Studies of 2. Based on
the reported light-induced elimination of ethane and a PdII
monomethyl complex from the isolated PdIII complex
2[ClO4],13 we propose that the detected species 2 during
oxidation of 1 is likely a reactive intermediate leading to C−C
bond formation. When a CD3OD solution of [2]ClO4 was
exposed to visible light in the absence of O2, ethane was formed
in 44 1% yield, along with 3 in 72 1% yield (after addition
of 1 equiv NaOH).17 However, the oxidation of 1 with O2 or
peroxide gives identical yields of ethane in the presence or
absence of ambient light. To further study the reactivity of 2 in
the dark, we investigated the effect of various additives on the
stability of 2. While 2 is stable in CD3OD in the dark in the
presence or absence of O2, the addition of 1 equiv NaOD leads
to elimination of ethane and 3.17 The reaction performed in
Proposed Mechanisms for Oxidation of 1 with O2. In
contrast to the dearth of PdII complexes that undergo aerobic
oxidation, there are several reports of aerobic oxidation of
analogous Pt organometallic complexes. For example, a number
of PtII dimethyl complexes supported by bidentate or
tridentante N-donor ligands have been reported to react with
O2 to generate PtIV complexes.21,24 The first steps of the
reaction of 1 with O2 can be viewed as similar to those
proposed for the PtII complexes, in which the presence of a
protic solvent may be required for the proton transfer to a
transient PtIII-superoxide complex to form high-valent PtIV-
hydroperoxo21 and/or -hydroxo species (Scheme 4).21,24
Although no PdIV species has been detected by NMR in our
reaction mixtures, (N4)PdIV-hydroperoxo and/or -hydroxo
intermediates can form transiently and undergo rapid
comproportionation in the presence of (N4)PdII complexes
to yield a PdIII species.9 Such comproportionation reaction was
observed for an analogous system in which the electrochemi-
cally generated [(N4)PdIVMeCl]2+ species reacts rapidly with 1
equiv (N4)PdIIMeCl to form 2 equiv of [(N4)PdIIIMeCl]+
quantitatively.36 Similarly, comproportionation of [(κ3-N4)-
PdIVMe2(OH)]+ and 1 would generate 2 and a [(κ3-
N4)PdIIIMe2(OH)] species, which can undergo an entropically
favored displacement of the hydroxide ligand by the amine
group20 to stabilize the distorted octahedral PdIII center and
yield another molecule of 2 (Scheme 4).13
Proposed Mechanisms of Oxidation of 1 with Peroxides.
The reaction of 1 with peroxides provides further insight into
its oxidation mechanism. For example, cumene hydroperoxide
(CumOOH) has been reported to react with transition metal
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absence of O2 gave ethane and 3 in 22% and 39% yield,
respectively after 4 h, while the reaction performed under 1 atm
O2 yielded 41% of ethane after 4 h. The yield of ethane
obtained in the latter case is similar to that observed under the
typical conditions for aerobic oxidation of 1, suggesting that 2 is
a viable reaction intermediate leading to ethane elimination.
The yield of ethane remained unaffected when 2[ClO4] reacted
with 1 equiv NaOD in the presence of the radical traps such as
TEMPO, suggesting a nonradical pathway for C−C bond
formation. The absence of a radical mechanism leads us to
propose a Me group transfer between two PdIII species to give a
transient PdIVMe3 intermediate (vide infra).8 This is supported
by the reaction of 1 with a two-electron oxidant MeI that leads
to elimination of ethane and (N4)PdIIMeI.13
The generation of hydroxide ions in solution likely results
from the reduction of O2 (or peroxides) in the presence of a
protic solvent. Moreover, hydroxide is likely the counteranion
for the cationic species 2 formed during oxidation of 1 with O2
or peroxides, and also leads to the formation of the PdII methyl
hydroxo complex 3 as a final product (vide infra). Overall, we
propose that the reversible HO− binding to the PdIII center
through displacement of one axial amine group promotes the
formation of a transient PdIV intermediate and the subsequent
C−C bond elimination (vide infra). Similarly, the addition of
other coordinating anions such as CN− or MeO− induces a
similar ethane elimination reactivity of 2[ClO4] in the dark.17
Crossover Experiments. To gain insight into the possible
mechanism of ethane formation, crossover experiments were
performed using a 1:1 mixture of (N4)Pd(CH3)2, 1, and
(N4)Pd(CD3)2, 1-d6 in C6D6-CD3OD. The control experiment
in absence of O2 showed no scrambling between 1 and 1-d6 in
C6D6 or C6D6-CD3OD solutions for at least 8 h, both in the
presence or absence of ambient light. Oxidation of a 1:1
mixture of 1 and 1-d6 with O2 in 1:1 C6D6-CD3OD leads to
formation of CH3CH3 and CH3CD3 in 16.5% and 15% yields,
respectively, after 8 h. Given the typical yield of ∼50% ethane
upon oxidation of 1, the yield of CD3CD3 can be estimated as
CH3CH3:CH3CD3:CD3CD3.42 The observed 1:1:1 ratio is
consistent with a mechanism involving a Me group transfer
between two PdIII dimethyl species to form a [(N4)PdIVMe3]+
intermediate and a PdII monomethyl species. The formed
[(N4)PdIVMe3]+ species is expected to undergo fast rearrange-
ment of the methyl groups and lead to reductive elimination of
any two of the three Me groups.43,44 This proposed mechanism
yields a 1:1:1 ratio of CH3CH3:CH3CD3:CD3CD3 for the
crossover experiment (Scheme S1), in line with the
experimental result.17
The rapid intramolecular rearrangement of the methyl
groups in the proposed [(N4)PdIVMe3]+ intermediate was
confirmed by the reaction of (N4)PdIIMe2 with CD3I, which
yields CH3CH3 and CH3CD3 in a 1:2 ratio (Scheme 5,
bottom).17 Similar results have been reported for the oxidative
addition of CD3I to the analogous (bpy)PdII(CH3)2 com-
plex43,44 to form a (bpy)PdIV(CH3)2(CD3)+ species that
undergoes rapid rearrangement of the equatorial CH3 and
axial CD3 groups to give a statistical mixture of isotopomeric
PdIV complexes.43,44 Subsequent reductive elimination would
lead to formation of CH3CH3 and CH3CD3 in a 1:2 ratio,
similar to that observed for 1.
Our mechanistic studies suggest that ethane elimination is
likely promoted by a coordinating anion such as HO−, which
displaces one of the amine groups and leads to a [(κ3-
N4)PdIIIMe2(OH)] complex. Since the increased stability of
the (κ4-N4)PdIII species is likely due to the ability of the N4
ligand to accommodate a distorted octahedral geometry of the
PdIII center,13 disruption of the κ4-coordiation mode by HO−
likely induces Me group transfer between two PdIII species to
generate [(κ3-N4)PdIVMe3]+ and [(N4)PdIIMe(OH)], 3
(Scheme 6, path A). Such reactivity has been observed
Scheme 6. Proposed Mechanisms for C−C Bond Formation
∼ 1 8 % , s u g g e s t i n g
a n e a r l y 1 : 1 : 1 r a t i o o f
CH3CH3:CH3CD3:CD3CD3 (Scheme 5, top). Similarly, the
Scheme 5. Crossover Experiment between 1 and 1-d6 (top)
and the Reaction of 1 with CD3I (Bottom)
elimination of ethane from a 1:1 mixture of [(N4)PdIII(CH3)2]-
ClO4, [2]ClO4, and [(N4)PdIII(CD3)2]ClO4, [2-d6]ClO4, in
the presence of 1 equiv NaOD produces CH3CH3 and
CH3CD3 in a 1:1 ratio after 24 h in the dark. The ESI-MS of
the 1:1 mixture of [2]ClO4 and [2-d6]ClO4 shows no methyl
group scrambling after 8 h in the dark, suggesting that methyl
group exchange does not occur at PdIII centers on this time
scale. Moreover, no scrambling was observed for the PdIII
complexes generated by aerobic oxidation of a 1:1 mixture of 1
and 1-d6.17
Proposed Mechanisms of Ethane Formation. The
observed 1:1:1 ratio of ethane isotopomers rules out a radical
mechanism of ethane formation involving Me radicals that
would lead to the statistical 1:2:1 ratio of
previously for PdII and PtII complexes supported by bidentate
(or tridentate) ligands,44 for which facile Me group transfer
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reactions occur upon one-electron oxidation.8,45 Another
possible mechanism may involve an initial disproportionation
of [(κ3-N4)PdIIIMe2(OH)] to generate [(κ3-N4)-
PdIVMe2(OH)]+ and 1 (Scheme 6, path B). Subsequent Me
group transfer from [(κ3-N4)PdIVMe2(OH)]+ to 1 could
generate 3 and [(κ3-N4)PdIVMe3]+, the latter species being
responsible for ethane formation. A similar Me group transfer
reaction from an electrophilic PdIVMe3 center to a nucleophilic
PtII center been reported previously.46
reacting independently prepared 3 with 3:1 acetone-CH3OH to
give 5 (Scheme 8), which was isolated in 46% yield and
Scheme 8. Reactivity of 3 with Acetone and 4-
Ethynyltoluene
Characterization and C−H Activation Reactivity of 3.
The identity of a PdII methyl hydroxo complex 3 formed upon
ethane elimination from 1 (Scheme 1) was confirmed by its
independent synthesis from (N4)PdIIMe(MeCN)+ and 1 equiv
OH− (Scheme 7, eq (i)). Attempts to isolate 3 in the solid state
characterized by NMR spectroscopy and X-ray diffraction
(Figure 7a).17 Similarly, 3 reacts with 4-ethynyltoluene to
Scheme 7. Independent Syntheses of 3
led to the formation of a dinuclear hydroxo-bridged complex
[(N4)PdIIMe(μ−OH)PdIIMe(N4)](OTf), 4[OTf], which was
characterized by X-ray diffraction (Figure 6). Both metal
Figure 7. ORTEP representation of 5 (a) and 6 (b). Selected bond
lengths (Å), 5: Pd1−C1 2.020; Pd1−C2 2.046; Pd1−N1 2.125; Pd1−
N2 2.196; 6: Pd1−C1 2.032; Pd1−C2 1.959; Pd1−N1 2.107; Pd1−
N2 2.144.
generate the corresponding acetylide complex (N4)PdIIMe-
(CCC6H4Me), 6, which was isolated and structurally
characterized (Figure 7b).17 The observed reactivity demon-
strates the ability of 3 to activate substrates containing weakly
acidic C−H bonds and form a new Pd−C bond.49 Interestingly,
although complex 5 does not react with O2 or peroxides, its
oxidation can be accomplished using Fc+ to produce a PdIII
species that was characterized by UV−vis and EPR (Figure
S27),17 suggesting that a range of organometallic (N4)PdII
complexes could be oxidized to PdIII intermediates and possibly
be involved in a catalytic oxidative coupling of C−H bonds
(vide infra).
Proposed Catalytic Cycle for the Aerobic Oxidative
Coupling of C−H Bonds. Based on the observed reactivity of
(N4)Pd-methyl complexes in aerobic oxidation, C−C bond
formation, and C−H bond activation reactions, we propose a
catalytic cycle for the oxidative coupling of C−H bonds using
O2 as the oxidant (Scheme 9).50 In this catalytic cycle, a
monohydrocarbyl hydroxo complex (N4)PdIIR(OH) activates
the C−H bond of an organic substrate to give a bis-
(hydrocarbyl) species (N4)PdIIR2 (Scheme 9, step (i)). The
ability of hydroxo complexes of late transition metals to activate
C−H bonds has been previously reported.47,49,51 The resulting
(N4)PdIIR2 species is expected to have a low oxidation
potential and react with O2 to form a [(N4)PdIIIR2]+
intermediate (Scheme 9, step (ii)). Subsequent C−C bond
formation through the intermediacy of a PdIVR3 species would
lead to the coupled product and regeneration of a
Figure 6. ORTEP representation of the cation of 4[OTf]. Selected
bond lengths (Å): Pd3−C3 2.018; Pd3−N10 2.029; Pd3−O34 2.034;
Pd3−N9 2.180; Pd4−C4 2.010; Pd4−O34 2.033; Pd4−N14 2.056;
Pd4−N13 2.183; Pd3···Pd4 3.832. Only one of the two conformers
present in the crystal structure is shown.
centers in 4 exhibit a square planar geometry with each PdII
atom surrounded by two pyridyl N atoms, one methyl group,
and a bridging hydroxide ligand. Notably, there are only three
other examples of structurally characterized dinuclear Pd
complexes with a single bridging hydroxo ligand.47,48 Addition
of 1 equiv OH− to 4 in MeOH quantitatively generates 3
1
(Scheme 7, eq (ii)), whose H NMR spectrum is identical to
that of 3 obtained from (N4)PdIIMe(MeCN)+.17
Interestingly, when the aerobic oxidation of 1 was performed
in 15:1 d6-acetone-D2O, ethane formed in 40% yield after 8
days along with a new PdII product identified as the acetonyl
complex (N4)PdMe(CD2COCD3), 5, obtained in 42% yield.17
The latter complex could be obtained upon deprotonation of
acetone by the hydroxo ligand of 3.49 This was confirmed by
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weakly acidic C−H bonds of acetone and terminal alkynes and
lead to formation of a new PdII−C bond.
Scheme 9. Proposed Catalytic Cycle for the Aerobic
Oxidative Coupling of C−H Bonds That Involves (i) C−H
Bond Activation, (ii) Aerobic PdII Oxidation, and (iii) C−C
Bond Formation
Overall, these results provide evidence that (N4)Pd systems
can undergo aerobic oxidation, C−C bond formation, and C−
H bond activation reactions, and thus could be involved in a
catalytic cycle for the oxidative coupling of C−H bonds using
green oxidants such as O2. Our studies suggest the use of
tetradentate ligands can promote the aerobic oxidation of
organometallic PdII complexes by lowering their oxidation
potential and stabilizing the generated PdIII species, while also
allowing the formation of transient PdIV intermediates that
undergo facile reductive elimination and C−C bond formation.
Our current research efforts aim to provide further insights into
catalyst design for the oxidative coupling of C−H bonds using
molecular oxygen as the oxidant, as well as survey the scope of
this unique oxidative reactivity.
ASSOCIATED CONTENT
■
monohydrocarbyl complex (N4)PdIIR(OH) (Scheme 9, step
(iii)).
S
* Supporting Information
Detailed experimental details, spectroscopic characterization,
and X-ray crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
Such a direct oxidative coupling of C−H bonds is currently
of great practical interest in the chemical industry. For example,
the oxidative coupling of methane into higher hydrocarbons
can provide a means of converting abundant natural gas
resources into useful chemicals.52 However, the utilization of
molecular oxygen as the oxidant in these C−H coupling
reactions remains a challenge.4 We have demonstrated herein
the viability of all three steps of the proposed catalytic cycle for
Pd complexes supported by the tetratenate ligand N4. Further
studies will be directed toward the development of metal
complexes capable of catalytic aerobic transformations. Our
results strongly suggest that stabilization of a PdIII species
promotes the aerobic oxidation of PdII precursors, while
formation of PdIV intermediates is needed for an efficient C−C
bond formation. As such, the ability of N4 or other tetradentate
ligands to access both high-valent Pd species may be necessary
to promote efficient aerobic C−H coupling reactions.
AUTHOR INFORMATION
■
Corresponding Author
mirica@wustl.edu
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.
We also thank Stephanie Tucker for help with the Amplex red/
HRP assay and Ying Zhang for ESI-MS analyses.
REFERENCES
■
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CONCLUSION
■
In summary, reported herein is the first example of an aerobic
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that after initial absorption of 0.3−0.5 equiv of O2, a slower
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