Characterization of peroxo and hydroperoxo intermediates in the aerobic oxidation of N-heterocyclic-carbene-coordinated palladium(0)

Article abstract of DOI:10.1021/ja046884u

An η²-peroxopalladium(II) complex, derived from dioxygen addition to Pd(IMes)₂ (IMes = bis-1,3-di(2,4,6-trimethylphenyl)imidazoline-2-ylidene), has been isolated and characterized. Subsequent addition of HOAc to (IMes)₂Pd(O₂) yields the first example of a hydroperoxopalladium species derived from molecular oxygen. The characterization and reactivity studies of these complexes provide the most detailed insights to date into the proposed mechanism for palladium(0) oxidation by molecular oxygen. Copyright

Full text of DOI:10.1021/ja046884u

Published on Web 07/28/2004
Characterization of Peroxo and Hydroperoxo Intermediates in the Aerobic
Oxidation of N-Heterocyclic-Carbene-Coordinated Palladium(0)
Michael M. Konnick, Ilia A. Guzei, and Shannon S. Stahl*
Department of Chemistry, UniVersity of Wisconsin-Madison, 1101 UniVersity AVenue, Madison, Wisconsin 53706
Received May 26, 2004; E-mail: stahl@chem.wisc.edu
Scheme 1. Simplified Mechanism for Palladium-Catalyzed
Aerobic Oxidation Reactions
Selective aerobic oxidation of organic substrates represents a key
challenge in modern chemistry, and palladium-catalyzed methods
(Scheme 1) are emerging as a versatile strategy to achieve this goal.1
Despite significant recent advances, only a small fraction of the
known palladium(II)-mediated oxidation reactions operate ef-
fectively with molecular oxygen as the stoichiometric oxidant.2 We
have been investigating the fundamental reaction between pal-
ladium(0) and molecular oxygen to facilitate further advances in
this area. Both chelating and monodentate ligands have been used
in palladium-catalyzed oxidation reactions,1 and we recently
reported the oxygenation of 1, a palladium(0) complex coordinated
by the chelating ligand, bathocuproine (bc), and dibenzylidene
acetone (dba).3 In the present study, we describe the aerobic oxi-
dation of 2, a palladium(0) complex bearing two N-heterocyclic
carbene (NHC) ligands, 1,3-di(2,4,6-trimethylphenyl)imidazoline-
2-ylidene (IMes).4 The IMes ligands are stable under the oxidizing
reaction conditions and lead to important differences in the reactivity
between 1 and 2. The ability of the monodentate IMes ligands to
undergo cis-trans isomerization has enabled isolation and char-
acterization of the first dioxygen-derived palladium(II)-hydro-
peroxide complex, which is frequently postulated as an intermediate
in palladium-catalyzed oxidation reactions.1a
spectroscopy reveals a medium-intensity band at 868 cm-1 that
shifts to 821 cm-1 when 3 is prepared with 18O-labeled dioxygen.
The 47 cm-1 shift is close to the 52 cm-1 shift predicted by a simple
diatomic oscillator model for an O-O stretch. MALDI-TOF mass
spectrometric data for 3 reveal an ion peak that corresponds to the
palladium(0) complex, 2, at m/z ) 714.3 (M)+. A softer ionization
technique (electrospray) was required to obtain the corresponding
peak for 3 at m/z ) 747.1, (M + H)+. The mass fragment for 2
also appears in the electrospray experiment (Figure S4) and suggests
dissociation of O2 from 3 may be possible; however, no evidence
for reversible oxygenation was obtained under standard preparative
conditions.7 The structure of 3 was definitively established by X-ray
crystallography. The η2-O2 fragment exhibits an O-O bond length
of 1.443(2) Å, which supports its formulation as a peroxo complex.
The bulky NHC ligands are forced into a cis arrangement to
accommodate O2 binding, and their steric crowding is evident from
low-temperature 1H NMR spectroscopic studies of 3, which reveal
restricted rotation around the N-Ar bonds. For example, decoa-
lescence of the IMes aryl resonances was detected below -80 °C.
The extremely rapid oxygenation of 2 at -78 °C contrasts the
reactivity of the (bc)Pd(0) complex, 1, which requires 20-30 min
to form the peroxopalladium(II) complex at room temperature. This
rate difference probably reflects the difference in mechanism
between the two complexes: associative olefin substitution for 1
versus simple O2 addition for 2. The reaction between O2 and
palladium(0) is formally spin-forbidden, but the rapid rate observed
for eq 1 suggests that spin-state changes do not contribute to a
large kinetic barrier.8 Relatively large spin-orbit coupling of
palladium should facilitate triplet-singlet surface crossing along
the reaction coordinate.9
Pd(IMes)2, 2, was prepared according to a published procedure;5
however, our spectroscopic data (1H, 13C NMR) differ from those
in the original report. The mass spectrum (MALDI-TOF) and X-ray
crystal structure of 2 (see above) clearly establish its identity and
support our spectral assignments.6 The discrepancy with the
literature data appears to arise from the extreme air sensitivity of
2, which reacts immediately even in the solid state upon exposure
to air. A solution of Pd(IMes)2 in toluene at -78 °C changes color
from yellow to brown upon introduction of an atmosphere of oxygen
Addition of acetic acid to a toluene solution of 3 at room
temperature leads to rapid formation of the hydroperoxopalladium-
(II) complex, 4 (eq 2). The presence of only one carbene ligand
1
into the reaction vessel. The H and 13C NMR spectroscopic data
of the new product match the characterization data originally
reported for 2.5 Further analysis establishes the identity of this
product as an η2-peroxo complex, (IMes)2Pd(O2), 3 (eq 1). Infrared
resonance in the 1H and 13C NMR spectra of 4 indicates that
protonolysis of a Pd-O bond in 3 occurs with concomitant cis-
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C O M M U N I C A T I O N S
comparison with phosphine-coordinated palladium complexes,
which undergo rapid ligand oxidation in the presence of molecular
oxygen.13 In fact, Pd(OAc)2 is among the most active homogeneous
catalysts for phosphine oxidation by molecular oxygen.14 This
instability limits the use of phosphines in both fundamental studies
and catalytic reactions of this kind.1,4 Finally, in certain palladium-
catalyzed oxidation reactions, activated oxygen species, especially
hydroperoxides, have been proposed to react directly with organic
substrates.15 The isolation of both 3 and 4 provides us with a unique
opportunity to probe these proposals directly.
Figure 1. Experimental (A) and simulated (B) electrospray ionization mass
spectra of 4 and 4-18O2. Both major peaks correspond to the ion derived
from protonation of the parent molecule, (M + H)+. The 4-18O2 sample
also contains small amounts of 16O-labeled compound. The full mass
spectrum also reveals ion fragments corresponding to loss of OAc and/or
OOH from these compounds (see Supporting Information).
Acknowledgment. We appreciate assistance with mass spec-
trometry from M. Vestling, and we thank A. Clauss for use of his
infrared spectrometer. Financial support from the following agencies
is gratefully acknowledged: NSF (CAREER, CHE-0094344), the
Dreyfus Foundation (Teacher-Scholar Award), and the Sloan
Foundation (Research Fellowship)
trans isomerization of the carbene ligands. The hydroperoxide
1
proton is evident in the H NMR spectrum at 3.87 ppm, (C6D6,
Figure S6), and the O-H stretch is detected by infrared spectros-
copy with a weak band at 3504 cm-1. Upon preparation of the
oxygen-18- and deuterium-labeled compounds, 4-18O2 and 4-d1, this
infrared band shifts to 3492 and 2591 cm-1, respectively. These
observed shifts of 12 and 913 cm-1 compare favorably with the
predicted shifts of 12 and 955 cm-1 (Figure S7). The O-O stretch
was not detected by infrared spectroscopy, and strong background
fluorescence interfered with the acquisition of resonance Raman
data. The proposed identity of 4 was further supported by
electrospray ionization mass spectrometry. The spectrum reveals
the predicted pattern for the (M + H)+ ion, which shifts as expected
for the corresponding 18O-labeled complex (Figure 1).
The isolation and characterization of a dioxygen-derived hydro-
peroxopalladium(II) complex is unprecedented,10 although it is
frequently proposed as a catalytic intermediate.1a No hydroperoxide
species was detected in protonation studies of (bc)Pd(O2), 1.3
Addition of <2 equiv of HOAc to 1 yields only unreacted starting
material, (bc)Pd(OAc)2, and hydrogen peroxide. In the present
system, rapid cis-trans isomerization of the NHC ligands appears
to provide steric protection of the remaining Pd-O bond. The
substantially slower second protonolysis step enables the isolation
of 4.
Supporting Information Available: Experimental procedures and
spectroscopic data. X-ray crystallographic data for complexes 2, 3, and
5 in CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org
References
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(4) For examples of NHC-coordinated palladium complexes used in aerobic
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(6) Full characterization data are provided in the Supporting Information.
(7) Attempts to promote O2 dissociation included sparging a toluene solution
of 3 with nitrogen gas, attempted sublimation of 3 (which led to
decomposition), and stirring a solution of 3 in mesitylene under full
vacuum at 40 °C.
(8) The role of spin-state effects on the kinetics of transition-metal reactions
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J.-L.; Harvey, J. N. J. Am. Chem. Soc. 2004, 126, 5789-5797. (b) Detrich,
J. L.; Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem.
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Unpublished results.
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Werner, W. U.S. Patent 4,908,323, 1990.
Further protonolysis of 4 to yield hydrogen peroxide (eq 3)
proceeds quite slowly, reaching 80% completion after 3 days at
room temperature. 1H NMR spectroscopic analysis of this reaction
reveals the formation of (IMes)2Pd(OAc)2, 5, which was prepared
independently by the addition of the free NHC ligand to Pd(OAc)2.
Protonolysis of 3 was also performed with H2SO4 as the acid, and
under these conditions, a colorimetric assay11 revealed a 70% yield
of H2O2.12
(12) For details, see Supporting Information. Partial disproportionation of
hydrogen peroxide may occur under the reaction conditions: Steinhoff,
B. A.; Fix, S. R.; Stahl, S. S. J. Am. Chem. Soc. 2002, 124, 766-767.
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The reactions outlined above (eqs 1-3) provide the most detailed
insights to date into the proposed mechanism for palladium(0)
oxidation by molecular oxygen. The advantageous features of
N-heterocyclic carbene ligands are immediately apparent based on
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