Formation of the palladium(IV) complex [(Me3tacn)Pd IVMe3]+ through aerobic oxidation of (Me 3tacn)PdIIMe2 (Me3tacn = N, N ′, N ″-trimethyl-1,4,7-triazacyclononane)

Article abstract of DOI:10.1021/om300426r

The dimethyl Pd^II complex (Me₃tacn)Pd ^IIMe₂ (Me₃tacn = N,N′,N″-trimethyl- 1,4,7-triazacyclononane) undergoes facile aerobic oxidation to yield the stable species [(Me₃tacn)Pd^IVMe₃]⁺. EPR, UV-vis, and ESI-MS studies suggest an inner-sphere mechanism for the oxidation of (Me₃tacn)PdMe₂ by O₂ and formation of Pd^IV species. In addition, the structurally characterized complex [(Me₃tacn)Pd^IVMe₃]I undergoes selective elimination of ethane at elevated temperatures. Overall, this system represents one of the first examples of aerobic oxidation of a Pd^II organometallic precursor to yield a well-defined Pd^IV product, supporting the role of Pd^IV species as viable intermediates in Pd-mediated catalytic or stoichiometric aerobic oxidative transformations.

Full text of DOI:10.1021/om300426r

Communication
pubs.acs.org/Organometallics
Formation of the Palladium(IV) Complex [(Me3tacn)PdIVMe3]+
through Aerobic Oxidation of (Me3tacn)PdIIMe2 (Me3tacn = N,N′,N″-
Trimethyl-1,4,7-triazacyclononane)
Julia R. Khusnutdinova,† Fengrui Qu,† 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 (Me3tacn)PdIIMe2
(Me3tacn = N,N′,N″-trimethyl-1,4,7-triazacyclononane) under-
goes facile aerobic oxidation to yield the stable species
[(Me3tacn)PdIVMe3]+. EPR, UV−vis, and ESI-MS studies
suggest an inner-sphere mechanism for the oxidation of
(Me3tacn)PdMe2 by O2 and formation of PdIV species. In
addition, the structurally characterized complex [(Me3tacn)-
PdIVMe3]I undergoes selective elimination of ethane at elevated temperatures. Overall, this system represents one of the first
examples of aerobic oxidation of a PdII organometallic precursor to yield a well-defined PdIV product, supporting the role of PdIV
species as viable intermediates in Pd-mediated catalytic or stoichiometric aerobic oxidative transformations.
4
erobic transformations catalyzed by palladium complexes
are among the most important synthetic tools in organic
Scheme 1. Reported Aerobic Reactivity of (tBuN4)PdMe2
A
synthesis and provide a practical means of dioxygen utilization
as an environmentally benign and inexpensive oxidant.1 While
the majority of Pd-catalyzed aerobic transformations have been
proposed to involve the formation of Pd0 complexes and their
subsequent reoxidation by dioxygen,1 the intermediacy of high-
valent PdIV or PdIII species has recently been proposed in some
catalytic and stoichiometric aerobic transformations.2−4 In this
context, there are only two reports on the aerobic oxidation of
PdII complexes to generate spectroscopically detected PdIII
species.3b,4 However, the formation of PdIV complexes by
aerobic oxidation of PdII precursors has not been documented,
and their relevance to aerobic oxidation catalysis is often based
on indirect evidence.2 Importantly, the direct aerobic oxidation
of PdII organometallic complexes to PdIV may not only provide
alternate reactivity patterns to the commonly employed PdII/
Pd0-mediated transformations but also avoid common prob-
lems associated with Pd black formation.1g
Scheme 2. Aerobic Reactivity of (Me3tacn)PdIIMe2
We have previously shown that a PdIIMe2 complex supported
by the tetradentante ligand tBuN4 (tBuN4 = N,N′-di-tert-butyl-
2,11-diaza[3.3](2,6)pyridinophane) undergoes facile inner-
sphere oxidation by O2 or peroxides to yield the PdIII complex
[(tBuN4)PdIIIMe2]+, which then undergoes elimination of
ethane under ambient conditions (Scheme 1).4 Herein we
report the aerobic oxidation of (Me3tacn)PdIIMe2 (Me3tacn =
N,N′,N″-trimethyl-1,4,7-triazacyclononane) to yield the stable
complex [(Me3tacn)PdIVMe3]+ (Scheme 2), one of the first
examples of formation of a well-defined PdIV species upon
aerobic oxidation of a PdII precursor.5
Me3tacn.6 X-ray analysis of 1 reveals a square-planar geometry
of the PdII center, which is bound to two methyl ligands and
two N atoms, while the third N atom of Me3tacn points away
from the metal center (Figure 1a), similar to the reported
structure of (Me3tacn)PdIICl2.7 The Pd−Me distances are
2.032 and 2.034 Å, in line with those for other PdIIMe2
complexes supported by diamine ligands.8 The two methyl
1
ligands of 1 give rise to a singlet at 0.37 ppm in the H NMR
The PdII complex (Me3tacn)PdIIMe2 (1) was obtained in
82% yield through the reaction of (COD)PdMe2 with
Received: May 17, 2012
Published: June 11, 2012
© 2012 American Chemical Society
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Communication
∼−1.3 V,15 significantly more negative than the oxidation
potential of 1. Moreover, the addition of 1 to O2-saturated
EtCN at −70 °C leads to the immediate formation of an
orange-yellow species (λmax = 407 nm, ε ≥ 11 000 M−1 cm−1),
which then slowly decays over the course of several hours.
Although this intermediate was not isolated, we tentatively
assign the observed intense absorption band to a charge
transfer transition reminiscent of a Pd−superoxo or −(hydro)-
peroxo species.16,17 For example, intense LMCT bands can be
observed in superoxo or peroxo complexes of various transition
metals.18 Moreover, the aerobic oxidation of 1 in presence of
the spin trap DMPO (DMPO = 5,5-dimethyl-1-pyrroline N-
oxide) yields an EPR-active DMPO radical adduct (or adducts)
that was not formed in the absence of 1 or O2,6 supporting an
inner-sphere oxidation of 1 by O2.4 On the basis of previous
mechanistic studies of the aerobic oxidation of PtII and PdII
dimethyl complexes,4,16,19 such an inner-sphere oxidation may
involve the initial formation of a PdIII−superoxo intermediate,
which upon protonation yields a PdIV−hydroperoxo species
(Scheme 3). The PdIV−OOH intermediate can then oxidize
Figure 1. ORTEP representation of 1 (a) and the cation of [2]I (b).
Selected bond lengths (Å) and angles (deg) are as follows. 1: Pd1−C1,
2.032(2); Pd1−C2, 2.0339(19); Pd1−N1, 2.220(6); Pd1−N2,
2.249(4). 2+: Pd1−C1, 2.0375(14); Pd1−C2, 2.0380(14); Pd1−C3,
2.0384(14); Pd1−N1, 2.2180(11); Pd1−N2, 2.2192(11); Pd1−N3,
2.2193(11); C1−Pd1−C2, 87.18(6); C1−Pd1−C3, 86.79(6); C2−
Pd1−C3, 86.06(7).
spectrum, while Me3tacn shows a fluxional behavior even at low
temperatures with only one average singlet at 2.18 ppm for the
three N-Me groups, likely due to the facile exchange between
the free and coordinated donor atoms.9 The cyclic voltammo-
gram (CV) of 1 in 0.1 M Bu4NPF6/MeCN exhibits an
irreversible oxidation wave at Epa ≈ −0.71 V vs Fc+/Fc, which is
significantly lower than those for analogous PdIIMe2 complexes
with bidentate N-donor ligands.4 Such a low oxidation peak
potential is likely due to the ability of the tridentate tacn-type
ligands to effectively stabilize the octahedral or distorted-
octahedral geometry of high-valent Pd and Pt centers.7,10
Notably, the oxidation peak potential of 1 is even lower than
that for the previously reported (tBuN4)PdIIMe2 (Epa = −0.36 vs
Fc+/Fc),4,11 presumably due to the presence of amine donors in
the equatorial plane and the less rigid structure of Me3tacn.
Prompted by the previously observed aerobic oxidation of
Scheme 3. Proposed Mechanism for Formation of
[(Me3tacn)PdIVMe3]+ (2+) by Aerobic Oxidation of
(Me3tacn)PdIIMe2 (1)
(
tBuN4)PdIIMe2,4 we set out to investigate the reactivity of 1
toward O2. When a solution of 1 in MeCN was exposed to O2,
the NMR revealed the rapid formation of a new diamagnetic Pd
complex identified as [(Me3tacn)PdIVMe3]+ (2+) (Scheme 2).12
The yield of 2+ was 44% and 34% after 10 min in acetone and
MeCN, respectively (the theoretical yield of 2+ is 50%),13 and
did not change in the presence of O2 for up to 12 h.6
Interestingly, the aerobic oxidation of 1 in the presence of water
generates 2+ in a similar yield but leads to a lower amount of
side products, suggesting that the presence of protons is needed
to facilitate PdII oxidation and O2 reduction (vide infra). The
electrospray mass spectrometry analysis (ESI-MS) of 1 in
MeCN in the presence of O2 reveals a strong signal at m/z
322.1482 (calcd for 2+ 322.1474). The identity of 2 was further
confirmed by the independent synthesis of [(Me3tacn)-
PdIVMe3]I ([2]I) through the reaction of 1 with 1 equiv of
MeI in acetone or MeCN.6 The NMR spectrum of [2]I is
identical with that of 2+ obtained by the aerobic oxidation of 1.
The X-ray analysis of [2]I reveals an octahedral Pd center
surrounded by three Me groups and a κ3-Me3tacn ligand
coordinated to the PdIV center by three N atoms (Figure 1b).
The Pd−Me bond distances are 2.037−2.038 Å, similar to
those of other reported PdIVMe3 complexes,14 while the three
Me−Pd−Me angles are all close to 90°, confirming a
symmetric, pseudo-C3v geometry.
another molecule of 1, also assisted by protons,16 to give two
molecules of the PdIV−OH species. Interestingly, ESI-MS
analysis of 1 in O2-saturated MeCN reveals the formation in
addition to 2+ of two new peaks at m/z 340.1208 and 324.1262,
which can be assigned to [(Me3tacn)PdIVMe2(OOH)]+ (calcd
m/z 340.1216) and [(Me3tacn)PdIVMe2(OH)]+ (calcd m/z
By analogy with (tBuN4)PdMe2,4 the aerobic reactivity of 1
likely involves an inner-sphere oxidation of 1 by O2, as the
•−
formal reduction potential of the O2/O2 couple in MeCN is
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Organometallics
Communication
324.1266), respectively. The two peaks decay rapidly and
disappear within 1 h, supporting their role as intermediates in
the formation of 2+.6 However, when the oxidation of 1 was
performed in 5% H2O/95% MeCN, the two intermediates
exhibited increased stability and were present in the reaction
mixture even after 1 day, supporting the need for protons in the
proposed PdII oxidation and O2 reduction mechanism.20
Analogous PtIVMe2(OOH)16b and PtIVMe2(OH)16,19,21 com-
plexes resulting from aerobic oxidation of PtIIMe2 precursors
have been isolated and structurally characterized previously,
while the aerobic oxidation of the [(tacn)PtII(dmso)Cl]+
complex has been reported to form a peroxo-bridged PtIV
complex.22 In contrast, in our case the electrophilic [(Me3tacn)-
PdIVMe2X]+ intermediates (X = OOH, OH) likely undergo
methyl group transfer to a nucleophilic (Me3tacn)PdIIMe2 (1)
species to afford the trimethyl PdIV complex 2+ (Scheme 3). A
similar methyl group transfer from electrophilic PdIVMe3
complexes to nucleophilic PtIIMe2 and PdIIMe2 complexes
has been previously reported.23
similar to that for the analogous complex [(tacn)PdIVMe3]+
(tacn = 1,4,7-triazacyclononane), leading to ethane elimination
at 140 °C.27 Notably, the reductive elimination of ethane from
the PdIVMe3 complexes supported by tridentate Me3tacn and
tacn ligands is significantly slower compared to that for
analogous PdIVMe3 complexes with bidentate N-donor ligands,
as the latter systems are expected to more easily access a five-
coordinate intermediate.13,14c,28 In addition, we have recently
reported ethane elimination in high yield at room temperature
from the PdIVMe3 complex [(κ3-MeN4)PdIVMe3]+ (MeN4 =
N,N′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane), and the
faster C−C bond formation reactivity for the latter complex
vs that for 2+ is likely due to the distorted-octahedral geometry
of [(κ3-MeN4)PdIVMe3]+.5
In summary, we have shown that (Me3tacn)PdIIMe2
undergoes facile aerobic oxidation to yield a stable [(Me3tacn)-
PdIVMe3]+ complex. Moreover, the latter species can selectively
eliminate ethane at elevated temperatures. UV−vis and EPR
spectroscopic studies indicate that the aerobic oxidation likely
involves an inner-sphere oxidation of (Me3tacn)PdIIMe2 with
O2, while ESI-MS studies suggest the formation of PdIV−OOH
and PdIV−OH intermediates, similarly to analogous Pt
complexes. The observed facile aerobic reactivity is likely due
to the ability of the Me3tacn ligand to support the octahedral
geometry of a PdIV center. Moreover, subsequent methyl group
transfer leads to further stabilization by formation of a
symmetric trimethyl PdIV complex. Overall, reported herein is
one of the first examples of aerobic oxidation of a PdII
organometallic precursor that generates a well-defined PdIV
product.5 These results suggest that PdIV complexes can act as
viable intermediates in the Pd-catalyzed aerobic oxidation of
organic substrates in general and in oxidative C−C bond
coupling in particular. Our current research efforts are aimed at
probing the mechanism of this aerobic oxidation and
characterization of the observed transient intermediates.
The aerobic oxidation of 1 resembles the recently reported
oxidatively induced reactivity of (tBu2bpy)PdIIMe2 leading to
ethane elimination. In that case, chemical oxidation of
(tBu2bpy)PdIIMe2 with Fc+, Ag+, or thianthrenyl followed by
methyl group transfer generates an unstable (tBu2bpy)PdIVMe3
+
intermediate that reductively eliminates ethane at room
temperature and forms cleanly the PdII product [(tBu2bipy)-
PdMe(solv)+].13 In contrast, the aerobic oxidation of 1 did not
yield a stable PdII monomethyl product, likely due to the
instability of the [(Me3tacn)PdIIMeX]+ species (X = OOH,
OH) under the employed oxidizing conditions.24
To probe whether the methyl group transfer is required for
an efficient aerobic oxidation, preliminary aerobic oxidation
studies have employed the palladacyclic PdII precursor
(Me3tacn)PdII(CH2CMe2C6H4-κ2C,C′) (3), which lacks methyl
ligands.6,25 While 3 is stable under O2 in MeCN for up to 1 day,
addition of 5% H2O to the reaction mixture leads to formation
of two peaks in the ESI-MS at m/z 426.1732 and 442.1685,
which are tentatively assigned to (Me3 tacn)-
PdIV(CH2CMe2C6H4-κ2C,C′)(OH)]+ (calcd m/z 426.1739)
and [(Me3tacn)PdIV(CH2CMe2C6H4-κ2C,C′)(OOH)]+ (calcd
m/z 442.1688), respectively.6 Moreover, NMR studies show
the formation in up to 48% yield of a species tentatively
assigned as [(Me3tacn)PdIV(CH2CMe2C6H4-κ2C,C′)X]+ (X =
OH, OOH, or other ligand).25b,c,26 While this reaction is not
quantitative, suggesting that the aerobic oxidation may be
hampered by steric factors,16b these preliminary results imply
that the oxidation of (Me3tacn)PdII precursors to yield PdIV
species does not require an alkyl group transfer.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF files giving experimental details,
spectroscopic characterization, and X-ray crystallographic data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mirica@wustl.edu.
■
Notes
The authors declare no competing financial interest.
We next examined the reactivity of 2+ toward the reductive
elimination of ethane. While [2]I is stable in solution at room
temperature, its thermolysis at 110 °C in DMSO-d6 for 60 h
leads to the formation of ethane, CH4, and CH3D in 93%, 7%,
and 3% yields, respectively (Scheme 4). Such reactivity is
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.
L.M.M. is a Sloan Fellow.
Scheme 4. Ethane Elimination from [(Me3tacn)PdIVMe3]I
REFERENCES
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