A highly substitutable palladium(II) complex stabilized by the smallest steric N-heterocyclic carbene, IMe (IMe = 1,3- dimethylimidazole-2-ylidene)

Full text of DOI:10.1002/bkcs.10988

Communication
DOI: 10.1002/bkcs.10988
BULLETIN OF THE
E. Lee et al.
KOREAN CHEMICAL SOCIETY
A Highly Substitutable Palladium(II) Complex Stabilized by the
Smallest Steric N-Heterocyclic Carbene, IMe (IMe = 1,3-
Dimethylimidazole-2-ylidene)
Eunsung Lee,†,‡,§, Dae Young Bae,‡,§ and Dmitry V. Yandulov†
*
†Department of Chemistry, Stanford University, Stanford, CA, 94305-5080, USA. *E-mail: eslee@postech.ac.kr
Center for Self-assembly and Complexity (CSC), Institute for Basic Science (IBS), Pohang 790-784, Republic of Korea
‡
§
Department of Chemistry and Division of Advanced Materials Science, Pohang University of Science
and Technology, Pohang 790-784, Republic of Korea
Received September 1, 2016, Accepted September 21, 2016, Published online October 26, 2016
Keywords: Palladium, N-heterocyclic carbene, Monodentate, Labile ligand, Isomerization
Palladium complexes supported by N-heterocyclic carbenes
followed by halide abstraction of 1 with silver triflate in THF.
Both complexes were obtained in 94% yield (Scheme 1). In
the process of formation of 2, iodide abstraction should be
carried out within 1 min, followed by a quick filtration of the
reaction mixture through a pad of Celite. Otherwise cationic
polymerization of THF starts immediately, indicating strong
interaction between THF and a highly naked palladium(II)
center due to labile ligands. Addition of pentane to the filtrate
affords immediately crystalline yellow 2, which is further col-
lected on a frit. Unlike most known stable palladium(II) NHC
complexes in a solid phase, 2 decomposes to a black tar solid
(
NHCs) have been of great interests in catalysis because of
1,2
their capability as efficient catalysts in organic synthesis.
Since the strong palladium–NHC bonds retain the stability
of palladium complexes, and steric modification of the NHC
ligands can be readily available, NHCs become more popu-
lar and important alternatives to phosphines as supporting
3,4
ligands. Thus, the synthetic direction of NHCs has been
aimed to optimize both electronic and steric properties of
NHCs, which allows one to maximize the reactivity of inor-
4
ganic catalysts for desired results. As the results, numerous
ꢀ
Pd(II)–NHC complexes have been synthesized and applied
in cross-coupling reactions, telomerization reactions, oxida-
tion reactions including a catalytic conversion of methane to
even at 50 C under dynamic vacuum, as the stability of 2 is
not secured because of three labile ligands. However, the
same approach to access the naked palladium complexes was
not successful with sterically bulky NHCs such as 1,3-bis-
5–9
methanol. Although significant efforts have been made to
1
0
20
increase steric bulkiness of NHC in particular, another
simple approach to access a reactive palladium complex is
to introduce labile ligands to a metal center. In addition,
since mono-ligated palladium complexes have been also
known as active catalysts in most palladium catalyzed
organic reactions, efficient methods to generate mono-
ligated NHC palladium complexes have been successfully
(2,6-diisopropylphenyl)imidazolylidene (IPr) and 1,3-di-
t
19
tert-butylimidazole-2-ylidene (I Bu) because metalation of
proximate C–H bonds in NHCs via the palladium metal ion
results in decomposition of the complexes, as confirmed by
1
14
H NMR spectra. Such an undesired metalation was not
observed with IMe because C–H bonds in the methyl group
stay away from the naked palladium of 2.
3
,11
developed and utilized for the organic synthesis.
Single crystal X-ray analysis of a yellow crystal of
2 reveals that one THF molecule is coordinated to a Pd cen-
ter at a trans position of IMe and two triflate anions are
coordinated to the Pd center in a trans position, allowing a
square planar conformation (Figure 1). The Pd − OTHF and
Pd − OOTf bond distances in the solid state are 2.119(6) and
2.032(2) Å and Pd − CIMe bond distance is 1.938(4) Å. In a
previously reported palladium(II) complex having both THF
and NHC, the bulky di-tert-butyl substituents of the NHC
However, most of mono-ligated NHC palladium complexes
contain bulky NHCs with relatively strongly bound anions
such as acetate anions, or halide- or carboxylate-bridged palla-
12,13
dium dimers are obtained.
With weakly bound anions, the
metalation of bulky NHC ligands has been observed in many
cases and resulted in the deterioration of the palladium–NHC
14
complexes. Therefore, the access to a mono-ligated NHC
complex with weakly coordinating ligands is very lim-
1
5,16
3
ited.
To overcome the problematic metalation of NHC,
and trans effect of η -allyl ligand in the complex appears to
16
we have utilized the smallest steric NHC and successfully
prepared mono-ligated palladium(II) complex with a weakly
elongate Pd − CNHC bond distance to 2.064 Å. The theo-
retically computed structure of 2 is also in good agreement
with above distances except the configuration of auxiliary
ligands. The discrepancy of the ligand configuration may
cause the weak coordination bonds.
1
7,18
coordinating anion and tetrahydrofuran (THF).
Here,
we report the preparation and full characterization of a
mono-ligated palladium(II) complex supported by IMe
(
IMe = 1,3-dimethylimidazole-2-ylidene), triflate, and THF.
As the palladium(II) complex 2 shows a low thermal stabil-
ity, we investigated its electronic properties at the B3PW91/
BS I level utilizing the Gaussian program (see Appendix S1,
Synthesis of Pd(IMe)(OTf ) (THF) (2) complex involves
2
19
the formation of iodo-bridged palladium(II) dimer (1),
Bull. Korean Chem. Soc. 2016, Vol. 37, 1745–1746
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BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
N
1.0 mol% 2
1
.0 IMeCO2,
DMSO, 12 h
I
I
I
PdCl2
N
Pd
Pd
I
N
C D , rt
6
6
4
KI, 100°C (20 min)
CH3CN/H2O
N
< 1 min
99% ( 1H NMR)
trans-β-methylstyrene
>
1
9
4%
allylbenzene
4
.05 AgOTf,
THF (1min), stir
4%
9
Scheme 2. Isomerization of allylbenzene to trans-β-methylstyrene
catalyzed by 2.
THF
N
N
N
TfO Pd OTf
IMe=
IMeCO2
=
CO2
N
N
N
2
ligand because there is no adjacent C–H bond nearby palla-
dium center, which is vulnerable to metalation. Thus, a highly
substitutable and naked palladium(II) complex with one NHC
could be isolated. We believe that the present palladium(II)
complex can offer a potential application as an efficient catalyst
for various organic reactions, which are in progress in our lab.
2
Scheme 1. Synthesis of Pd(IMe)(OTf) (THF) (2) complex.
Acknowledgments. This work was supported by Stanford
University and Institute for Basic Science (IBS) (IBS-R007-
D1). We thank Dr Victor G. Young, Jr. for X-ray analysis.
Supporting Information. Details for experimental, X-ray
and DFT data are available in the online version of this article.
Figure 1. X-ray crystal structure of 2. Selected experimental [cal-
culated at the B3PW91/BS I level for the optimized structure]
bond lengths [Å]. Pd1−C1 1.938(4) [1.943], Pd1−O1(O1A) 2.032
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Bull. Korean Chem. Soc. 2016, Vol. 37, 1745–1746
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