CH-π and CF-π interactions lead to structural changes of n-heterocyclic carbene palladium complexes

Article abstract of DOI:10.1002/anie.201309371

The role of CH-π and CF-π interactions in determining the structure of N-heterocyclic carbene (NHC) palladium complexes were studied using 1H NMR spectroscopy, X-ray crystallography, and DFT calculations. The CH-π interactions led to the formation of the

Full text of DOI:10.1002/anie.201309371

Angewandte
Chemie
DOI: 10.1002/anie.201309371
Conformation Analysis
CH–p and CF–p Interactions Lead to Structural Changes of
N-Heterocyclic Carbene Palladium Complexes**
Xiangya Xu, Benjamin Pooi, Hajime Hirao,* and Soon Hyeok Hong*
Abstract: The role of CH–p and CF–p interactions in
of the metal center, are particularly important in regulating
the structures of organometallic complexes.[4] Additionally,
determining the structure of N-heterocyclic carbene (NHC)
1
palladium complexes were studied using H NMR spectros-
weak interactions could alter the conformation and geometry
of a catalyst and would thereby exert significant effects on the
catalytic activity and selectivity, as is the case with enzymes in
nature. For the rational improvement of the catalytic perfor-
mance of complexes, such structural control should be fully
understood.
copy, X-ray crystallography, and DFT calculations. The CH–p
interactions led to the formation of the cis-anti isomers in
1-aryl-3-isopropylimidazol-2-ylidene-based
[(NHC)2PdX2]
complexes, while CF–p interactions led to the exclusive
formation of the cis-syn isomer of diiodobis(3-isopropyl-1-
pentafluorophenylimidazol-2-ylidene) palladium(II).
N-heterocyclic-carbene (NHC)-based palladium com-
plexes have proven to be effective catalysts in many
important reactions, especially in cross-coupling reactions.[5]
The enhanced catalytic activity has been largely attributed to
the electronic and steric properties of NHC ligands. Most
well-defined [(NHC)2PdX2]-type complexes reportedly have
the trans configuration, although cis complexes have been
reported to show faster catalyst initiation because of their
stronger trans effect.[6] Some cis complexes have been
reported, including those with chelating NHC ligands.[7] In
spite of the extensive research on catalyst development using
NHCs, to the best of our knowledge, there has been no study
designed to evaluate the structural effects of these weak
interactions in NHC-based transition-metal complexes.
Herein, we report the roles of CH–p and CF–p interactions
in controlling the structure of [(NHC)2PdX2]-type complexes.
The synthesis of [(NHC)2PdX2]-type complexes, by heat-
ing PdCl2, the NHC precursor 1a, and Cs2CO3 in 1,4-dioxane
at 808C, yielded only the trans isomer 2a after 6 hours
(Scheme 1). However, the trans isomer was converted into the
cis-anti isomer 3a slowly in CDCl3 even at room temperature.
The lower solubility of 3a in CDCl3 caused its crystallization
from solution. The structure was readily confirmed by X-ray
crystallography (Figure 1).[8] The methyl doublet peaks of the
isopropyl group of 2a, peaks which initially appeared at d =
1.40 ppm in the 1H NMR spectra, split into two separate
methyl doublet peaks at d = 0.54 ppm and 1.40 ppm upon
T
he CH–p interaction, a type of weak hydrogen bonding,
plays a significant role in determining or stabilizing structures
of a vast variety of chemical and biological molecules.[1] For
example, CH–p interactions serve as a driving force for
crystal packing or as a stabilizing effect for a particular
conformation of a molecule, even in solution. Intramolecular
CH–p interactions can also induce optical activity of com-
pounds.[2] In biology, CH–p hydrogen bonds play important
roles in the folding mechanisms of proteins and DNA and also
in the specific substrate recognition of proteins and en-
zymes.[3]
Understanding structure and bonding of organometallic
complexes is of fundamental importance. Electronic effects
such as the trans effect, steric effects of ligands, and properties
[*] X. Xu,[+] B. Pooi,[+] Prof. Dr. S. H. Hong
Center for Nanoparticle Research, Institute for Basic Science (IBS)
Seoul 151-742 (Republic of Korea)
and
Department of Chemistry, College of Natural Sciences
Seoul National University
Seoul 151-747 (Republic of Korea)
and
Korea Carbon Capture & Sequestration R&D Center
Daejeon 305-303 (Republic of Korea)
E-mail: soonhong@snu.ac.kr
Prof. Dr. H. Hirao
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University
Singapore 637371 (Singapore)
E-mail: hirao@ntu.edu.sg
[+] These authors contributed equally to this work.
[**] This work was supported by the Korea CCS R&D Center (NRF-
2013M1A8A1035844), the Basic Science Research Program through
the National Research Foundation of Korea (NRF-
2012R1A1A1004077), and Institute for Basic Science (IBS), funded
by the Korea government (Ministry of Science, ICT & Future
Planning). H.H. thanks Nanyang Technological University for
a startup grant and for computer resources at the High Performance
Computing Centre.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309371.
Scheme 1. Synthesis of [(NHC)2PdI2] complexes.
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with a yield of 90% upon isolation. This result
suggested that 3a is thermodynamically more
stable than 2a. The compound 3a was stable
enough to be re-isolated without any sign of
decomposition after being heated in the temper-
ature range of 23–908C in various solvents. In
contrast, 3b, bearing only isopropyl groups, did not
enjoy this increased stability when converted from
2b. Unlike 3a, the isolated 3b was slowly converted
back into 2b when heated at 308C in [D8]1,4-
dioxane or CDCl3. The system eventually reached
an equilibrium with a trans/cis ratio of 3:7. There-
fore, it is likely that CH–p interactions play a role in
stabilizing the cis isomer 3a.
Figure 1. Molecular structure of 3a.
conversion from 2a into 3a. The data suggested that the
methyl groups in 3a are no longer magnetically equivalent
because one methyl group experiences a strong shielding
effect.[9] With the help of X-ray crystallographic data, we
assumed that the close proximity of one of the methyl groups
to the isopropyl unit and the phenyl ring in 3a led to this
splitting. The electron density of the phenyl group shields the
protons on this methyl group when the methyl group rests
above the plane of the phenyl ring.
Nishio and co-workers have done database studies of CH–
p interactions since 1970s.[9a,10] They devised a set of methods
to prove the existence of CH–p interactions, as shown in
Figure S2 in the Supporting Information. Following the
Nishio method,[11] we measured the dihedral angles and
distances da, dp, d1, b, and a from the crystal structure of 3a.
Both the dihedral angles and distances of 3a (Table 1) were
The convincing proof of the existence of CH–p interac-
tions within NHC-based palladium complexes prompted us to
investigate the correlation between the electron density on
the phenyl ring and the extent of stabilization caused by CH–
p interaction. The compounds 2c, 2d, and 2e were isomerized
to form 3c, 3d and 3e, respectively, at room temperature in
CDCl3.[8] From the dihedral angle and distance data (Table 1),
it can be shown that CH–p interactions occurred in 3c and 3d.
Notably, 3d had dp distances of 2.664 and 2.626 ꢀ, whereas 3c
had dp distances of 2.750 and 2.687 ꢀ, thus suggesting that the
electronic property of the p system influences the strength of
the CH–p interaction.[10f,12] The electron-donating methoxy
group increases the electron density on the phenyl ring in 3d,
thereby resulting in a stronger CH–p interaction than those
brought about by the electron-withdrawing fluoro group in
3c. We also replaced X ligands with
bromides to see whether the CH–p
interaction still exists. The initially
Table 1: Dihedral angles and distances for cis-[(NHC)2PdX2] complexes.[a]
Complex
CH/p
da [ꢀ]
dp [ꢀ] dl [ꢀ]
b [8]
a [8]
synthesized trans isomer 2h was
successfully transformed into the
cis-anti isomer 3h, the CH–p inter-
action of which was observed in the
X-ray crystal structure (Table 1).
An interesting result was
achieved in the conversion of 2e
into 3e. The strong electronegativ-
3a: R1 =iPr,
R2 =Ph
CH15c/PlC8,9
CH4c/PlC19,20
CH18a/PlC8,9
CH5a/PlC20,21
2.812 2.757 2.787 30.36 81.29
2.861 2.817 2.845 33.60 81.67
2.920 2.750 2.845 26.97 75.61
2.827 2.687 2.757 23.54 76.99
3c: R1 =iPr,
R2 =4-FC6H4
3d: R1 =iPr,
R2 =4-MeOC6H4
CH13a/PlC20,21 2.849 2.664 2.799 31.05 72.10
cis-[(NHC)2PdI2]
CH26a/PlC7,6
2.911 2.626 2.829 42.78 68.19
2.784 2.691 2.756 4.06 77.90
2.714 2.613 2.651 22.52 80.34
2.683 2.527 2.626 16.01 73.97
2.945 2.767 2.883 32.11 73.74
2.472 2.939 2.694 29.70 71.77
3e: R1 =iPr, R2 =C6F5 CF10/PlC8,9
3 f: R1 =Ph,
R2 =Ph
CH15/PlC25,26
CH18/PlC2,3
CH13b/PlC8,9
CH2b/PlC19,20
ity of fluorine gives rise to five Cd+
F
À
3g: R1 =Ph,
R2 =Et
dÀ bond dipoles in the phenyl ring
of 3e. This feature renders the
center of the phenyl ring electro-
positive, as opposed to the electro-
negative phenyl ring in 3a. The
À
3h: R1 =iPr,
R2 =Ph
CH24a/PlC7,8
2.957 2.729 2.899 31.83 70.32
cis-[(NHC)2PdBr2]
CH12a/PlC17,18 3.059 2.831 2.994 30.59 71.05
[a] a: dihedral angle between plane C1OC2 and plane HC1C2; b: ]HCxC1; dp: distance of H/p plane
(H/I); dl: distance of H/line C1-C2 (H/J); da: distance of HC1 (C1 is the nearest carbon atom to H).
CdÀ
H
d+ bond dipoles thus disfavor
the interaction with the electron-
deficient C6F5 p system in 3e. In
dÀ
contrast, the Cd+
bond dipoles
À
F
consistent with our conjecture. The data showed that C4H and
C15H, belonging to the CH3 of the iPr group, were in Ra 1 of
the C8,C9 and C19,C20 p-system planes (Figure 1). Thus, we
concluded that there exist CH–p interactions between the
C4H, C15H, and C19,20, C8,C9 phenyl planes in 3a.
favorably interact with the C6F5 center (see Figure S3).[13] This
explains why 3e adopts the cis-syn configuration rather than
the cis-anti configuration (Figure 2). The compound 3e had
a dp value of 2.691 ꢀ, which is shorter than the values of 2.757/
2.817 ꢀ for 3a. This CF–p interaction is further supported by
19F NMR spectroscopic data. Ortho fluorine substituents,
which participate in the CF–p interactions, experience
a deshielding anisotropic effect because of the close proximity
with an electropositive pentafluorophenyl ring, while meta
To further check the stability of the cis isomer, compound
2a was heated in CDCl3 at 558C for 12 days (see Figure S1).
1
By monitoring the reaction with H NMR spectroscopy, we
observed that 2a was fully converted into the cis isomer 3a
2
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which are involved in CH–p interaction, have much
smaller chemical-shift values than the other methyl
group. The two distinct chemical-shift values
agreed reasonably well with the experimental
values. Furthermore, our atoms-in-molecules
(AIM) calculations using AIMALL software[22]
yielded bond critical points corresponding to CH–
p and CF–p interactions (see Figures S6 and S7).
Having appreciated how significantly CH–p
and CF–p interactions could affect the configura-
tion preference of the NHC complexes, we next
Figure 2. Molecular structure of 3e.
Table 2: Comparison of theoretical and experimental chemical-shift
values for 3a.
and para fluorine substituents were more shielded in the cis-
syn isomer 3e than in the trans-anti isomer 2e.
d Ha [ppm][a]
d Hb [ppm][b]
To make theoretical comparisons of CH–p and CF–p
interactions, a series of density functional theory (DFT)
calculations were performed on cis-anti (Conf-1) and cis-syn
(Conf-2) geometries of 3a and 3e using Gaussian 09.[14]
Geometry optimizations were performed with the B3LYP,
BP86, M06-2X, and MPW1B95 functionals in conjunction
with Lanl2TZ(f) for Pd, Lanl2DZdp for I, and 6-311 + G-
(2d,p) for all other atoms.[15–19] The solvent effect of chloro-
form was taken into account by the IEFPCM method.[20] DFT
calculations predicted that 3a and 3e prefer Conf-1 and Conf-
2, respectively, in accordance with the X-ray structures (see
Table S1). This result supports our hypothesis that the
configurations of 3a and 3e are determined by CH–p and
CF–p interactions, respectively. Of all the DFT functionals
examined, the MPW1B95 data showed particularly good
agreement with experimental geometry and stability
(Figure 3 and Tables S1 and S2). In the B3LYP and BP86
B3LYP
BP86
M06-2X
MPW1B95
experimental
0.62
0.58
0.21
0.18
0.54
1.40
1.48
1.44
1.48
1.40
[a] Average chemical-shift value for all Ha atoms in the methyl group that
is involved in CH–p interaction. [b] Average chemical-shift value for all
Hb atoms in the methyl group that is not involved in CH–p interaction.
turned our attention to bulkier and more rigid systems. The
compound 3 f, which bears two phenyl groups on each NHC,
was found to adopt a twisted shape (Figure 4). Notably, a CH–
p interaction occurs between the Csp2 H of the phenyl ring on
the NHC and the p system from the phenyl group on the
other NHC. The compound 3 f has dp values of 2.613 and
À
Figure 4. Molecular structure of 3 f.
Figure 3. Superposition of the experimental (blue) and MPW1B95-
optimized (yellow) geometries: (a) 3a; (b) 3e.
2.527 ꢀ, which are shorter than those of 3a, 2.757 and
2.817 ꢀ. It has been reported that the CH–p interaction is
stronger when the acidity of the hydrogen atom is greater.[23]
optimized geometries, the hydrogen atom of the isopropyl
group was hardly within the Ra 1 region (see Figure S5),
which should be attributable to the inadequate description of
dispersion force with these functionals. It should be noted that
all these functionals tended to overestimate the stability of
the trans isomers (Tables S1–S4). This problem of DFT was
previously observed for other analogous complexes.[21] Never-
theless, as far as evaluation of the energy gap between the two
cis conformers is concerned, we believe that the DFT
functionals perform reasonably well.
À
The C H bond that is involved in the CH–p interaction in 3 f
is more acidic, because the carbon center is sp2 hybridized, as
À
compared to the Csp3 H bond in 3a. Interestingly, the binding
modes of both aryl–aryl interactions in 3 f were in between
edge-to-face and face-to-face aryl–aryl interactions with the
dihedral angles of 51.98 and 54.68. In the case of the CF–p-
interaction-driven aryl–aryl interactions in 3e, these inter-
actions were closer to an edge-to-face mode with the dihedral
angle of 70.38.
The existence of CH–p and CF–p interactions was further
verified by theoretical NMR calculations. The NMR data in
Table 2 clearly show that the methyl groups of isopropyl,
Another interesting result was obtained when 3g was
synthesized from 2g. It was expected that the terminal methyl
group in the ethyl substituent would point towards the
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aromatic ring, similar to the case of 3a, because of the CH–p
the formation of the cis-anti or cis-syn isomer of
[(NHC)2PdX2] depending on the type of weak interaction
provided by substituent groups of the NHC. Further inves-
tigations on the effect of the weak interactions on catalysis are
currently ongoing to fully comprehend the roles of weak
forces in catalysis with the ultimate goal of developing highly
efficient and selective enzymelike transition-metal catalysts
which exploit such weak interactions.
À
interaction between the C H group and the phenyl ring.
However, the methyl group pointed away in the opposite
direction. Analysis of our 1H NMR and X-ray crystallo-
graphic data showed that a CH–p interaction occurred
between C2H and C19,C20 of the aromatic ring and C13H
and C8,C9 of the aromatic ring, instead of C3H and C14H
(CH3) (Figure 5). The H atoms on the CH2 group are more
acidic as they are bonded to a carbon atom which is directly
connected to the nitrogen atom of NHC and thus will interact
more favorably with the phenyl ring.
Received: October 28, 2013
Published online: && &&, &&&&
Keywords: conformation analysis · hydrogen bonds ·
.
noncovalent interactions · palladium · p interactions
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4
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Angewandte
Communications
Communications
Conformation Analysis
X. Xu, B. Pooi, H. Hirao,*
S. H. Hong*
&&&& — &&&&
CH–p and CF–p Interactions Lead to
Structural Changes ofN-Heterocyclic
Carbene Palladium Complexes
Doing the twist! The roles of CH–p and
CF–p interactions in determining the
structure of the title complexes were
studied. The CH–p interactions led to the
cis-anti isomers in 1-aryl-3-isopropylimi-
dazol-2-ylidene-based complexes, while
CF–p interactions led to the exclusive
formation of the cis-syn isomer of diiodo-
bis(3-isopropyl-1-pentafluorophenylimi-
dazol-2-ylidene)palladium(II). C gray,
N violet, Pd turquoise, F yellow, I purple.
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!