A Palladium(II) Peroxido Complex Supported by the Smallest Steric N-Heterocyclic Carbene, IMe = 1,3-Dimethylimidazole-2-ylidene, and Its Reactivity by Oxygen-Atom Transfer

Article abstract of DOI:10.1002/ejic.201601019

Stabilized with an N-heterocyclic carbene ligand of the smallest steric profile, Pd^II(η²-O₂)(IMe)₂(IMe = 1,3-dimethylimidazole-2-ylidene) was synthesized by the direct addition of dioxygen to Pd⁰(IMe)₂. The peroxidopalladium complex with IMe, which was previously available only by computation, was characterized by IR spectroscopy and X-ray crystallography. Moreover, its oxygen-atom-transfer reactivity was elucidated by studying its thermolysis in pyridine at 60 °C to yield 1,3-dimethylimidazol-2-one (IMeO) and its reaction with oxygen-atom-transfer acceptors such as PPh₃and (Me₃Si)₂.

Full text of DOI:10.1002/ejic.201601019

DOI: 10.1002/ejic.201601019
Communication
Oxygen-Atom Transfer
A Palladium(II) Peroxido Complex Supported by the Smallest
Steric N-Heterocyclic Carbene, IMe = 1,3-Dimethylimidazole-2-
ylidene, and Its Reactivity by Oxygen-Atom Transfer
Eunsung Lee,*[a,b,c] Dae Young Bae,[b,c] Sungho Park,[b] Allen G. Oliver,[d] Yonghwi Kim,[b]
and Dmitry V. Yandulov[a][‡]
Abstract: Stabilized with an N-heterocyclic carbene ligand of characterized by IR spectroscopy and X-ray crystallography.
the smallest steric profile, PdII(η2-O2)(IMe)2 (IMe = 1,3-dimethyl- Moreover, its oxygen-atom-transfer reactivity was elucidated by
imidazole-2-ylidene) was synthesized by the direct addition of studying its thermolysis in pyridine at 60 °C to yield 1,3-dimeth-
dioxygen to Pd0(IMe)2. The peroxidopalladium complex with ylimidazol-2-one (IMeO) and its reaction with oxygen-atom-
IMe, which was previously available only by computation, was transfer acceptors such as PPh3 and (Me3Si)2.
Introduction
oxygenation, the actual synthesis and characterization of this
complex has not been reported to date. Herein, we report the
synthesis and characterization of PdII(η2-O2)(IMe)2 including its
oxygen-atom-transfer reactivity, which involves thermolysis of
the complex leading to the formation of the urea 1,3-dimethyl-
imidazol-2-one (IMeO) and reaction of the complex with PPh3
and (Me3Si)2 to give Ph3PO and (Me3Si)2O, respectively.
Dioxygen is an ideal terminal oxidant for oxidation reactions of
organic substrates with clear economic benefits and environ-
mentally benign characteristics. For example, O2 serves as a
stoichiometric oxidant for the oxidation of alcohols and the
oxidative heterocyclization of alkenes in the presence of the
Pd(OAc)2/DMSO system in addition to the recent discovery of
oxygen-promoted C–H bond activation at Pd.[1] The reaction
scope was extended by introducing N-donor auxiliary ligands
such as phenanthroline derivatives and N-heterocyclic carbenes
(NHCs) that are resistant to oxidative conditions.[2] To gain fun-
damental understanding of palladium-mediated oxygenation
processes, several theoretical and experimental mechanistic
studies have been conducted by the groups of Landis and Stahl,
who studied the reaction between Pd0(IMe)2 (IMe = 1,3-dimeth-
ylimidazole-2-ylidene) and O2 with spin-unrestricted DFT meth-
ods to corroborate the solvent dependency in the thermody-
namics of the process and to elucidate the stepwise mechanism
that involves spin crossover.[3]
Results and Discussion
The synthesis of the PdII(η2-O2) complex by treating the Pd0
precursor with O2 in the solid state was previously demon-
strated.[2a,4] Storage of crystalline 1[5] under an atmosphere of
O2 for 1 week produced surface coloration that was visible only
under a microscope. However, exposure of a yellow homogene-
ous solution of 1 in THF to an atmosphere of O2 resulted in
quick discoloration and the formation of a bisque precipitate of
peroxido complex 1O2, which slowly decomposed at 130 °C
(Figure 1, a; see also Figure S1 in the Supporting Information).
Featuring limited solubility, 1O2 was reasonably stable only in
pyridine (py) and decomposed on the timescale of hours (DMF,
CH3CN and DMSO). However, the complex decomposed in-
stantly in CH2Cl2 and acetone and was not soluble in C6H6, THF,
or Et2O (Figures S2–S4). These solubility and stability properties
are unusual, because the most-known palladium peroxido com-
plex, PdII(η2-O2)(IMes)2, is soluble in aprotic nonpolar solvents
such as C6D6 and PdII(η2-O2)bc (bc = bathocuproine) is stable
in CD2Cl2.[2a,3a] After extensive efforts, crystals of 1O2 suitable
for X-ray diffraction were obtained and analyzed, before losing
crystallinity, in less than 12 h. The crystal structure of 1O2 (Fig-
ure 1, b,c) confirmed its identity as the peroxide PdII(η2-O2)-
(IMe)2, with structural parameters within the range of values
observed for previously reported analogues.[2a,3a,4,6] Each perox-
ide oxygen atom of 1O2 accepts four to five O···HC hydrogen
bonds with distances of 2.14–2.71 Å. The O–O distances ob-
Although the PdII(η2-O2)(IMe)2 complex has been utilized ex-
tensively in computational studies as a standard product of Pd0
[a] Department of Chemistry, Stanford University,
Stanford, CA 94305-5080, USA
[b] Center for Self-assembly and Complexity (CSC), Institute for Basic Science
(IBS),
Pohang 790-784, Republic of Korea
E-mail: eslee@postech.ac.kr
http://iom.postech.ac.kr/
[c] Department of Chemistry and Division of Advanced Materials Science,
Pohang University of Science and Technology,
Pohang 790-784, Republic of Korea
[d] Department of Chemistry and Biochemistry, University of Notre Dame,
Notre Dame, IN 46556, USA
[‡] Current address: Solano Community College,
4000 Suisun Valley Road, Fairfield, CA 94534-3197, USA
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201601019.
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Communication
served in the two independent molecules, 1.475(3) and
In a previous computational study, the formation of 1O2 ac-
1.476(5) Å, are the longest for various PdII(η2-O2)(NHC)n com- cording to Figure 1 in the gas phase was characterized by a
plexes[2a,3a,4,6] among the palladium complexes containing IR positive Gibbs free energy (ΔG°) value, however, solvation in
spectroscopy data for ν(O–O), possibly as a consequence of the common organic solvents was suggested to increase the stabil-
extensive hydrogen bonding present and the strong donor ity of 1O2 to dissociation of O2 considerably.[3b] Heating solid
character of the two IMe ligands. On the other hand, the weaker 1O2 under 1 atm of N2 for 8 h did not produce 1, as observed
σ-donor character of the phosphine and nitrogen-based bc li- by 1H NMR spectroscopy in solution, whereas thermolysis of the
gands in PdII(η2-O2)[PPh(tBu)2]2 and PdII(η2-O2)bc causes the solid material at +130 °C for 8 h resulted in its decomposition
palladium(II) center to bind dioxygen weakly and to maintain to an insoluble brown tar still featuring a trace amount of resid-
shorter O–O bonds.[2a] The observed bands for ν(O–O) at ual 1O2. Thus, 1O2 appears to be relatively stable to the loss of
823 cm–1 (18O2: 782 cm–1) in the solid state and at 837 cm–1 in O2 in the solid state (Figure S1).
pyridine solution are consistent with the O–O distance/stretch-
ing frequency correlation established previously (Figures 2 and
S15).[7]
Computational studies adequately reproduce the key struc-
tural characteristics of 1 and 1O2 and suggest that CH···O
hydrogen bonding indeed serves to elongate and weaken
[ν(O–O)] both the O–O and Pd–O bonds in the solid-state struc-
ture of 1O2 (Figures 3 and S10). The modest thermodynamic
3
stability of 1O2 relative to that of 1 and O2 computed in sol-
vents (DMSO, py) in which 1O2 shows no signs of O2 loss at
room temperature, together with the low activation energy
computed for the addition of 3O2 to 1,[3b] suggests that
hydrogen-bonding interactions with solvent C–H bonds, such
as those involving a DMSO molecule in the structure of 1O2,
provides additional stabilization. We expected that the unique
combination of low steric bulk and strong donor character of
auxiliary ligands in 1 and 1O2 would enable their combination
into PdII μ-oxo dimer 12(μ-O)2, a rare type of compound that
has not been structurally characterized without electrophilic
stabilization of at least one μ-oxo ligand (Figure S10).[8] Al-
though the DFT calculations suggest that 12(μ-O)2, featuring
two very short CH···O (1.91 Å) hydrogen bonds to each μ-O,
exhibits considerable stability with respect to constituents 1
and 1O2 (Figure S10), treatment of 1 with substoichiometric 3O2
or 1O2 at room temperature in [D6]DMSO did not produce a
new compound, which suggests a kinetic barrier to such oxid-
ative dimerization.
Figure 1. (a) Synthesis of PdII(η2-O2)(IMe)2 (1O2) and its reactivity. (b,c) ORTEP
drawing of the crystal structure of 1O2·py·DMSO (50 % probability ellipsoids)
showing the pattern of CH···O hydrogen bonds (orange dotted lines) to per-
oxide oxygen atoms (red).
Figure 3. Free energies of 1, 1O2, and PdIMe complexes that can be derived
from 1 and O2 in pyridine and DMSO computed at the B3PW91/BS I//BS II,
SMD level.
Thermolysis of 1O2 in pyridine at +60 °C resulted in the for-
mation of the cyclic urea IMeO[9] and water, in addition to a
brown solid not soluble in DMSO (Figures 4, S5, and S6). Com-
plex 1O2 decomposed in under 24 h as well to form 1 (trace),
IMeO (22 %), and H2O (40 %) in thermolysis in [H5]pyridine at
+60 °C. Although 1 was not observed in the thermolysis of 1O2
Figure 2. (a) IR spectra of 1O2 and 1O2-18O2 in Nujol showing the shift of
ν(O–O). (b) Correlation between O–O distance and stretching frequency in
Pd(η2-O2)L2 compounds. Pd(η2-O2)[PPh(tBu)2]2,[2k] Pd(η2-O2)(bc),[2a] Pd(η2-O2)-
(PPh3)2,[6b] Pd(η2-O2)(IMes)2,[3a] O2 from Na2O2.[2l]
2–
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Figure 4. 1H NMR spectra of the thermolysis of 1O2 in [H5]- and [D5]pyridine and the 1H NMR spectrum of IMeO isolated from these reactions.
in anhydrous [D5]pyridine, the presence of IMeO (22 %) and dium catalysis with NHC proceeds by the formation of a palla-
H2O points to the IMe ligands as the source of reducing equiva- dium peroxido complex, and the previous examples demon-
lents through the electrophilic or hydrogen-atom-abstraction strate the formation of hydrogen peroxide with acid and per-
reactivity of 1O2. Whereas mechanistic details of these reactions oxocarbonate palladium complex with CO2.[3a,4]
remain to be investigated, the DFT calculations in Figure 3 show
the immediate products of IMe oxygenation, Pd0IMe and IMeO,
to be strongly favored thermodynamically. Formation of IMeO
from 1O2 is formally analogous to the well-known oxygenation
of phosphines in related phosphine complexes[10] and under-
scores the limits of the “oxidative stability” of NHC ligands, pre-
viously documented in the ready formation of saturated cyclic
ureas from CuI(NHC)2+ complexes and O2.[11]
The typical oxygen-atom transfer (OAT) acceptors[12] PPh3
and (Me3Si)2 were converted into Ph3PO (>90 %) and (Me3Si)2O
(>90 %) with substoichiometric 1O2 in DMSO over 26 h; how-
ever, Pd0(IMe)2 (1), which was expected as another product, was
not observed among the complex mixture of Pd products
Figure 5. (a) OAT reactivity of 1O2 to afford hexamethyldisilane and 2. (b) In-
formed concomitantly in this solvent, possibly as a conse-
quence of the reactivity of 1 with DMSO.[5] In [D5]pyridine as
the solvent, OAT from 1O2 to (Me3Si)2 resulted in 93 % of
(Me3Si)2O, accompanied by 1, which underwent slower conver-
sion with the excess amount of (Me3Si)2 to cis-Pd(IMe)2(SiMe3)2
(2) (Figures 5 and S7–S9).[13] Observation of IMeO among the
dependent synthesis of 2 and (c) its X-ray crystal structure.
Conclusion
initial products of the 1O2/(Me3Si)2 reaction in [D5]pyridine at Described herein was the synthesis and characterization of the
room temperature is notable and speaks for the reactive charac- novel complex Pd(η2-O2)(IMe)2 (1O2) supported by IMe as an
ter of partly deoxygenated 1O2. This OAT reactivity clearly sup- NHC ligand with one of the smallest steric profiles. Lack of auxil-
ports the hypothesis that the current aerobic oxidation in palla- iary steric hindrance preserved the salient features of the struc-
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Experimental Section
Pd(η2-O2)(IMe)2 (1O2): Compound 1 (400 mg, 1.34 mmol) was dis-
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stirred vigorously at room temperature under O2 pressure (1 atm)
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37.9 ppm. IR (Nujol): ν = 1237 (s), 1127 (w), 1089 (w), 823 cm–1 (s,
˜
η2-O2). C10H16N4O2Pd (330.66): calcd. C 36.32, H 4.88, N 16.94; found
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Acknowledgments
This work was supported by the Stanford University, USA and
the Institute for Basic Science (IBS), grant number IBS-R007-D1].
Keywords: Palladium · Carbene ligands · Ligand
oxygenation · Atom transfer · Oxygen
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Received: August 17, 2016
Published Online: ■
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Communication
Oxygen-Atom Transfer
E. Lee,* D. Y. Bae, S. Park,
A. G. Oliver, Y. Kim,
D. V. Yandulov ..................................... 1–5
A Palladium(II) Peroxido Complex
Supported by the Smallest Steric N-
Heterocyclic Carbene, IMe = 1,3-
Dimethylimidazole-2-ylidene, and
Its Reactivity by Oxygen-Atom
Transfer
PdII(η2-O2)(IMe)2 (IMe = 1,3-dimethyl- of the complex leads to the formation
imidazole-2-ylidene), which is used ex- of 1,3-dimethylimidazol-2-one (IMeO),
tensively in computational studies as a and its reaction with PPh3 and (Me3Si)2
standard product of Pd0 oxygenation, results in Ph3PO and (Me3Si)2O, respec-
is synthesized and its oxygen-atom tively.
transfer reactivity studied. Thermolysis
DOI: 10.1002/ejic.201601019
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