A PdII complex bearing a benzimidazole-derived ligand with potentially "mesoionic and remote" character and its catalytic activity

Article abstract of DOI:10.1002/ejic.201300334

The oxidative addition of palladium(0) to the C2-protected 5-bromo-2-methylbenzimidazolium tetrafluoroborate 3 affords the complex trans-[PdBr(bimy)(PPh₃)₂]BF₄ (4), which is the first complex of an unprecedented benzimidaz

Full text of DOI:10.1002/ejic.201300334

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Mesoionic Compounds
H. V. Huynh,* H. L. Ong,
J. C. Bernhammer, G. Frison ............. 1–9
A PdII Complex Bearing a Benzimidazole-
Derived Ligand with Potentially “Me-
soionic and Remote” Character and Its
Catalytic Activity
The oxidative addition of Pd0 to a C2-pro-
tected 5-bromobenzimidazolium salt af-
fords a complex of an unprecedented and
formally neutral benzimidazole-derived li-
gand (bimy). DFT calculations indicate
that the bimy ligand bridges the gap be-
tween neutral mesoionic and anionic aryl
ligands. The catalytic activity of the com-
plex in the Sonogashira reaction has been
studied.
Keywords: Carbene ligands / Mesoionic
compounds / Palladium / Cross-coupling /
Density functional calculations
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DOI:10.1002/ejic.201300334
A PdII Complex Bearing a Benzimidazole-Derived Ligand
with Potentially “Mesoionic and Remote” Character and
Its Catalytic Activity
Han Vinh Huynh,*[a] Hong Lee Ong,[a] Jan C. Bernhammer,[a] and
Gilles Frison[b]
Keywords: Carbene ligands / Mesoionic compounds / Palladium / Cross-coupling / Density functional calculations
The oxidative addition of palladium(0) to the C2-protected 5-
bromo-2-methylbenzimidazolium tetrafluoroborate 3 affords
the complex trans-[PdBr(bimy)(PPh3)2]BF4 (4), which is the
first complex of an unprecedented benzimidazole-derived li-
gand (bimy) with a potentially “mesoionic and remote” char-
acter. DFT calculations revealed that the bimy ligand bridges
the gap between the mesoionic pyrazolin-4-ylidene and the
anionic phenyl ligand. A preliminary catalytic study revealed
the promising activity of 4 in the Sonogashira coupling of aryl
bromides with phenylacetylene.
nature of the benzimidazolium-derived ligand obtained by
such an approach. DFT calculations to probe the poten-
tially remote and mesoionic character of the ligand and a
preliminary catalytic study of the complex in the Sonoga-
shira coupling reaction are also reported.
Introduction
The chemistry of classical N-heterocyclic carbenes
(NHCs),[1–3] in which the carbene donor atom is adjacent
to two nitrogen atoms, has recently been extended to so-
called remote (rNHC) and mesoionic (or abnormal) carb-
enes (MICs).[4–9] rNHCs are formally neutral ligands con-
taining remote heteroatom(s) with respect to the carbene
donor atom. The term mesoionic, on the other hand, de-
scribes dipolar heterocyclic compounds as a subclass of be-
taines, in which both the negative and the positive charge
are delocalized, that cannot be represented by any neutral
resonance form.[10] Notably, imidazolin-4/5-ylidenes, as di-
rect isomers of the now ubiquitous imidazolin-2-ylidenes,
that coordinate through the C4 or C5 carbon atom are the
best investigated ligands among various types of
MICs.[11–13] On the other hand, ligands that are formally
derived from the deprotonation of benzimidazolium salts,
are so far exclusively limited to the well-established and
“normal” benzimidazolin-2-ylidenes. This is not surprising
as the aromatic protons at the backbone of such salts are
very weakly acidic compared to the C2 proton of the NCN
fragment. To circumvent the difficult and nonselective de-
protonation at the aromatic ring and to enforce metalation
at the benzimidazole backbone, we attempted the oxidative
addition of Pd0 to a 5-bromobenzimidazolium salt. This cu-
riosity-driven research attempts to provide insights into the
Results and Discussion
Syntheses and Characterizations
Inspired by advances made in the imidazole system, for
which blocking of the usual coordination site at C2 and
subsequent metalation at C4/5 yields mesoionic NHC com-
plexes,[6,12] we attempted a similar approach on the benz-
imidazole system. In this approach, C2 is protected by a
methyl group, and metalation at the aromatic ring is antici-
pated by oxidative addition to a C5–Br bond of the respec-
tive benzimidazolium ligand precursor. Although the re-
sulting ligand is formally neutral, no neutral canonical reso-
nance structure can be obtained, as highlighted in
Scheme 1. This renders the ligand potentially mesoionic in
nature.
Thus, C2-protected 5-bromo-2-methylbenzimidazole
(1)[14] was subjected to a two-step dialkylation process to
generate 1,3-dibenzyl-5-bromo-2-methylbenzimidazolium
bromide (2) in 69% yield (Scheme 2).[15,16] The positive
mode ESI mass spectrum of 2 shows a base peak at m/z =
391 for the [M – Br]+ cation, and the C5–Br carbon reso-
nates at δ = 120.4 ppm in the 13C NMR spectrum, a value
typical for aromatic C–Br groups. Subsequently, the Br–
counteranion in salt 2 was exchanged by using NaBF4 in
methanol to afford salt 3 in 61% yield. The presence of the
BF4– anion in 3 is supported by ESI mass spectrometry and
19F NMR spectroscopy. As expected, the 13C and 1H NMR
[a] Department of Chemistry, National University of Singapore,
3 Science Drive 3, 117543 Singapore
E-mail: chmhhv@nus.edu.sg
Homepage: www.carbene.de
[b] Laboratoire des Mécanismes Réactionnels, Department of
Chemistry, Ecole Polytechnique and CNRS,
91128 Palaiseau Cedex, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300334.
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Scheme 1. Canonical resonance structures for a benzimidazole-de-
rived mesoionic metal complex.
spectra of 3 remain largely unchanged compared to those
of 2. Single crystals of 3 subjected to X-ray crystallographic
analysis for the purpose of comparison confirmed its ident-
ity, and the molecular structure is depicted in Figure 1.
Figure 1. Molecular structure of 3 showing 50% probability ellip-
–
soids; disordered atoms, hydrogen atoms and the BF4
counteranion are omitted for clarity. Selected bond lengths [Å] and
angles [°]: C1–C8 1.486(11), C1–N1 1.334(10), C1–N2 1.341(9),
N1–C2 1.388(9), N2–C7 1.383(9), C2–C7 1.364(10), C2–C3
1.428(11), C3–C4 1.306(10), C4–C5 1.391(11), C5–C6 1.410(10),
C6–C7 1.365(10), C5–Br1 1.878(7); N1–C1–N2 109.0(7), C1–N1–
C2 108.1(6), C1–N2–C7 108.7(6).
to the formation of a cationic Pd complex. Moreover, the
positive mode ESI mass spectrum shows a base peak at m/z
= 1023 corresponding to the [M – BF4]+ molecular cation.
Further confirmation of its identity was obtained by single-
crystal X-ray diffraction. The molecular structure of 4 is
shown in Figure 2, and selected crystallographic data are
Scheme 2. Synthesis of a benzimidazole-derived PdII complex 4.
The synthesis of the first complex containing a benzimid- summarized in Table 1.
azole-derived potentially mesoionic ligand (bimy) was
As expected, the geometry around the metal centre is dis-
achieved by heating 3 and [Pd(PPh3)4] in dichloromethane torted square planar. The carbocycle containing the carbon
under reflux (Scheme 1). Concentration of the reaction mix- donor atom is perpendicular to the coordination plane and
ture and subsequent precipitation with diethyl ether led to intersects it at a dihedral angle of 90°. The Pd–C1 distance
the formation of a white powder, which contained crude of 1.986(10) Å is in the range generally observed for Pd–
trans-[PdBr(bimy)(PPh3)2]BF4 (4). Owing to its low solubil- NHC complexes, but offers little insight into whether 4 is a
ity in tetrahydrofuran (THF), analytically pure 4 can be ob- carbene complex, as the Pd–C or Pd–X bond lengths of
tained as a white solid by precipitation from a saturated trans-[PdX(aryl)(PPh3)2] and trans-[PdX(NHC)(PPh3)2]
THF solution at ambient temperature. Comparison of the (X = Br, Cl) complexes are generally undistinguishable.[17]
integrals of the benzylic and aromatic proton signals in the On the other hand, the alternating C–C and C=C bond
1H NMR spectrum of 4 reveals a ratio of 4:43, which indi- lengths (average 1.388 Å) in the carbocycle are significantly
cates that the complex contains two PPh3 ligands, one car- different (within 3σ). This may imply a mesoionic ground
bon donor ligand and a bromido ligand. In the 13C NMR state, in which the carbon donor ligand is formally
spectrum, the carbon donor atom resonates at δ = monoanionic and charge-separated from the adjacent cat-
159.5 ppm as a triplet (2JP,C ≈ 4 Hz), which is shifted down- ionic NCN azolium moiety, similar to the situation for
field by 38.8 ppm relative to the respective signal in the pre- carbodicarbenes and related ligands.[18] Notably, signifi-
cursor salt 3 and may indicate carbene formation. The 31P cantly different carbocyclic distances (average 1.377 Å) are
NMR spectrum of 4 contains a single resonance at δ = also observed in the precursor 3, which undoubtedly has a
25.1 ppm, which suggests that the two phosphane donors delocalized aryl moiety (Figure 3).
are arranged trans to each other. Signals at δ = –76.64 and
Despite this observation, it has been noted by Raub-
–76.68 ppm in the 19F NMR spectrum of 4 corroborate the enheimer et al. that molecular structural data have not been
presence of 10BF4– and 11BF4– counter anions, which points a useful probe for the confirmation or dismissal of carbene
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Figure 3. Comparison of the ground-state resonance structures for
the ligand precursor 3 and the potentially mesoionic ligand in 4.
DFT Calculations
To gauge the true nature of the ligand in complex 4, DFT
calculations were performed on the formally neutral C li-
gands 5–9 and their respective trans-[PdBrL(PPh3)2]+ com-
plexes. The anionic phenyl ligand 10 has also been com-
puted for comparison. The results summarized in Table 2
indicate that “benzimimdazolin-5-ylidenes” (bimy) 5 and 6
have larger first and second proton affinities (PA) and
metal–ligand interaction energies (ΔEint) than their classical
NHC isomer 8 (benzimidazolin-2-ylidene) and MIC 9. Res-
onance structures A–C in Scheme 1 illustrate that the six-
membered ring of the bimy ligand could be seen as a nor-
mal monoanionic aryl system. However, comparison of 5
and 6 with 10 shows that the anionic phenyl ligand 10 has
Figure 2. Molecular structure of 4 showing 50% probability ellip-
–
soids; hydrogen atoms, BF4 and the aromatic rings of the benzyl
substituents are omitted for clarity. Selected bond lengths [Å] and
angles [°]: Pd1–C1 1.986(10), Pd1–P1 2.325(3), Pd1–P2 2.321(3),
Pd1–Br1 2.5180(13), C1–C2 1.336(15), C2–C3 1.419(14), C3–C4
1.342(15), C4–C5 1.413(15), C5–C6 1.369(14), C6–C1 1.451(15),
C3–N2 1.423(13), N2–C7 1.316(14), C7–N1 1.379(15), N1–C4
1.386(13), C7–C22 1.473(14); C1–Pd1–Br1 172.5(4), P1–Pd1–P2 much larger first and second PAs. On the other hand, simi-
174.45(11), C1–Pd1–P1 86.6(3), C1–Pd1–P2 89.8(3), P1–Pd1–Br1
91.57(8), P2–Pd1–Br1 92.57(8).
larities are observed between 5 and 6 and the true zwitter-
ionic structure 7, which includes both localized and remote
positive and negative charges. A larger deviation is observed
between the second PAs of 5 and 6 and that of 7, which
Table 1. Selected crystallographic data for 3 and 4.
reveals significant differences in the π systems of 5 and 6
3
4
Formula
Formula weight
Crystal size [mm]
Temperature [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C
479.12
0.22ϫ0.17ϫ0.06
223(2)
triclinic
22H20BBrF4N2
C58H50BBrF4N2P2Pd
1110.06
0.50ϫ0.34ϫ0.30
100(2)
monoclinic
P21/c
12.4554(10)
13.9046(11)
29.570(2)
90
100.219(2)
90
5040.0(7)
4
1.463
1.280
1.40–25.00
8875
5280
0.7000, 0.5670
R1 = 0.0992
wR2 = 0.1947
R1 = 0.1664
wR2 = 0.2226
1.036
Table 2. Computational results obtained for ligands 5–9 and their
trans-[PdBrL(PPh3)2]+ complexes.
¯
P1
9.834(2)
10.713(3)
11.724(3)
81.420(4)
67.166(3)
70.022(3)
1069.7(4)
2
1.487
1.966
1.88–25.00
3759
2272
γ [°]
5
6
7
8
9
10
V [Å3]
1st PA[a] 1365[b]
1368,
1359[b]
475,
1339
1132
1203
1713
Z
Dcalcd. [gcm–3
]
μ [mm–1
θ range [°]
]
2nd PA[a] 526[b]
364
432
254
788
475[b]
–422
925
[c]
Reflections collected
Independent reflections
Max., min. transmission 0.8911, 0.6716
Final R indices [IϾ2σ(I)] R1 = 0.0719
wR2 = 0.173
ΔEint
–427
901
–911
–399
853
–821
–279
827
–777
–323
811
–791
–740
1080
–1324
[c]
[c]
ΔEPauli
ΔEelstat
–946
(68.6%) (70.2%) (65.6%) (70.2%) (69.7%) (72.7%)
–417 –401 –430 –329 –343 –496
(31.4%) (29.8%) (34.4%) (29.8%) (30.3%) (27.3%)
[c]
ΔEorb
R indices (all data)
R1 = 0.1235
wR2 = 0.2005
1.032
d(Pd-C)[d] 2.021
q(C)[e] –0.32[b]
2.019
2.012
–0.25
2.008
+0.13
2.014
–0.39
2.025
–0.39
Goodness-of-fit on F2
–0.30,
Peak/hole [eÅ–3
]
0.526/–0.432
3.463/–1.577
–0.32[b]
[a] Proton affinities [kJ/mol] computed at the B3LYP/aug-cc-pVTZ
level. [b] Computed at the B3LYP/aug-cc-pVDZ level. [c] EDA re-
sults [kJ/mol] for the Pd–C bonds of trans-[PdBr(5–9)(PPh3)2]+
computed at the BP86/TZ2P//BP86/SVP level. [d] Pd–C bond
length in Å. [e] Atomic charge at the divalent carbon atom.
formation based on their findings in a quinoline-based sys-
tem.[17] Similarly, 13C NMR spectroscopy cannot be used
to dismiss carbene formation.
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compared to that in 7. Indeed, the ammonium group in 7 low yield (Table 3, Entry 6), which accentuates the limita-
has only minor π-donor capability, whereas the NCN moie- tions of 4 in the coupling of aryl chlorides. In light of recent
ties in 5 and 6 allow significant π delocalization, which is a advances in copper- and amine-free Sonogashira coupling
requirement for a mesoionic system. These differences and reactions,[20,21] we attempted the coupling of 4-bromobenz-
similarities are also confirmed by a comparison of the aldehyde and phenylacetylene under similar conditions.
atomic charges at the divalent donor atoms, which reveals However, satisfactory results could not be obtained as the
that the bimy ligands 5 and 6 can be placed intermediately reactions attempted gave incomplete conversions, side prod-
between the true zwitterion 7 and the MIC 9 and phenyl ucts and undesired homocoupling products.
ligand 10. The classical NHC is uniquely different, as it is
the only system calculated with a positive atomic charge of NHC character of the bimy ligand in 4, we compared its
+0.13 at the divalent carbon atom. catalytic activity with those of the two structurally related
To pinpoint the effects of the mesoionic and remote
Overall, both the structural data of 4 and the calculated complexes shown in Figure 4 in the coupling of deactivated
electronic data for various C ligands 5–10 suggest that the substrate 4-bromotoluene with phenylacetylene.
new ligands 5 and 6 bridge the gap between mesoionic and
simple anionic aryl ligands. Ligands 5 and 6 show signifi-
cant π delocalization and unique neutral structures cannot
be written for them, which reveals their mesoionic nature.
On the other hand, their bonding properties and PA values
approach those of the monoanionic aryl ligand. This under-
pins the fact that there is no clear-cut electronic differentia-
Figure 4. Structurally related complexes 11 and 12.
tion between different carbon donors. Instead, there is con-
tinuous spectrum of electronic structures ranging from clas-
sical neutral NHCs to anionic aryl ligands through me-
soionic and zwitterionic species.[19]
Complex 11 bears an anionic phenyl ligand instead of
the bimy ligand and, thus, retains the steric properties of
the original complex with significantly modified electronic
properties. In 12, the bimy ligand is replaced by its isomeric
and normal NHC counterpart. In this case, together with
Catalytic Sonogashira Coupling Reactions
Complex 4 was also subjected to a preliminary test for the modification in electronic structure, changes in the
its catalytic activity in the Sonogashira coupling of aryl steric bulk of the ligand cannot be avoided.
bromides and phenylacetylene in the presence of CuI as a
As shown in Table 4, complex 4 performs distinctly bet-
cocatalyst. By employing standard conditions and a catalyst ter than the other two complexes. The poor performance of
loading of 1 mol-%, complex 4 proved to be a suitable cata- 11 can be attributed to the rapid loss of the anionic phenyl
lyst precursor and led to quantitative yields for activated ligand by reductive elimination during the catalysis to leave
aryl bromides (Table 3, Entries 1 and 2). The coupling of a phosphane-stabilized complex as the true catalytically
electron-rich aryl bromides resulted in good-to-moderate active species. A similar ligand loss is unlikely for complex
yields (Table 3, Entries 3–5). The coupling of 2-(4-bro- 4. NHC complex 12 performs better than aryl complex 11,
mophenyl)-1-ethylbenzimidazole with phenylacetylene led but is a less efficient catalyst precursor than bimy complex
to the formation of a new benzimidazole substituted tolane 4. The superiority of 4 can be explained by the increased
in a yield of 63% (Table 3, Entry 5). The coupling of phen- donor strength of the bimy ligand compared to that of its
ylacetylene with 4-chlorobenzaldehyde resulted in a very normal NHC counterpart.
Table 3. Sonogashira coupling reactions[a] catalyzed by 4.
Entry
Aryl halide
Time [h]
Temperature [°C]
Yield [%][b]
1
2
3
4
5
6
4-bromobenzaldehyde
4-bromoacetophenone
4-bromotoluene
4-bromoanisole
2-(4-bromophenyl)-1-ethylbenzimidazole
4-chlorobenzaldehyde
1
3
24
24
24
24
80
80
80
80
80
80
Ͼ99
Ͼ99
76
40
63[c]
8
[a] Reaction conditions: 1 mmol of aryl halide, 2 mmol of phenylacetylene, 1.2 mmol of NEt3, 5 mol-% of CuI, 1 mol-% of 4, 1 mL of
1
degassed DMF. [b] Yields were determined by H NMR spectroscopy for an average of two runs. [c] Isolated yield.
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Table 4. Catalytic comparison of 4, 11 and 12.[a]
The bimy ligand bridges the gap between MIC 9 and an-
ionic phenyl ligand 10. More importantly, the electronic
structures of various divalent carbon donor ligands cannot
be clearly differentiated. A continuous range of electronic
structures are found from classical NHCs to MICs to an-
ionic aryl ligands.[19] A preliminary catalytic study revealed
the promising activity of 4 in the Sonogashira coupling of
aryl bromides with phenylacetylene, and it is superior to
that of its structurally related analogues bearing an aryl (11)
or a normal NHC ligand (12). This warrants future studies
on the coordination chemistry of these new species.
Entry Catalyst
Time [h]
Temperature [°C] Yield [%][b]
1
2
3
4
11
12
24
24
24
80
80
80
76
38
45
[a] Reaction conditions: 1 mmol of 4-bromotoluene, 2 mmol of
phenylacetylene, 1.2 mmol of NEt3, 5 mol-% CuI, 1 mol-% palla-
dium catalyst, 1 mL of degassed DMF. [b] Yields were determined
1
by H NMR spectroscopy for an average of two runs.
In a separate study, the influence of the catalyst loading
of 4 was investigated. At 1 mol-%, the coupling of 4-bromo-
benzaldehyde with phenylacetylene reached completion
within an hour (Table 5, Entry 1). At 0.02 mol-%, a maxi-
mum turnover number (TON) of 3800 was achieved by in-
creasing the reaction time to 72 h (Table 5, Entry 5). On the
other hand, increased reaction temperatures are unfavour-
able as indicated by the decreased yield at 120 °C (Table 5,
Experimental Section
General Considerations: If not noted otherwise, all manipulations
were performed without precautions to exclude air and moisture.
All solvents and chemicals were used as received without further
purification. 1H, 13C, 31P, and 19F NMR spectra were recorded with
Entry 6). Further reduction of catalyst loading resulted in a Bruker ACF 300 spectrometer, and the chemical shifts (δ) were
internally referenced to the residual solvent signals relative to tet-
ramethylsilane (1H, 13C) or externally to 85% H3PO4 (31P) and
CF3CO2H (19F). Mass spectra were measured with a Finnigan
MAT LCQ (ESI) spectrometer. Elemental analyses were performed
with an Elementar Vario Micro Cube elemental analyzer at the
Department of Chemistry, National University of Singapore. 1,3-
Dibenzylbenzimidazolium tetrafluoroborate[25] and [PdBr(Ph)-
(PPh3)2] (11)[26] were synthesized by modification of the reported
procedures.
the formation of only trace amounts of products (Table 5,
Entries 7–8).
Although the performance of 4 in the Sonogashira reac-
tion pales in comparison to that of the highly efficient phos-
phane catalyst systems with Na2[PdCl4]/PtBu3,[22] ferrocen-
ylphosphane[23] or tetraphosphane[24] ligands, the achieved
TON of 3800 is comparatively high for NHC- and related
carbon-donor-based catalysts.[20] Further modifications of
such benzimidazole-derived ligands and their complexes,
which are important and interesting subjects for future in-
vestigations, could possibly improve the performance and
robustness of the catalyst.
6-Bromo-2-methylbenzimidazole (1): Glacial acetic acid (3 mL) was
added to a Schlenk tube fitted with a reflux condenser containing
4-bromo-1,2-diaminobenzene (0.560 g, 3.0 mmol) under an atmo-
sphere of nitrogen. The mixture was heated under reflux overnight
under an inert atmosphere of nitrogen. The excess acetic acid was
then removed in vacuo, and the residue was neutralized with a
small amount of saturated aqueous K2CO3 solution. The residue
was collected by filtration and washed with water to yield 1
(0.510 g, 2.41 mmol, 81%) as a brown powder. The analytical data
of this compound was identical to the literature values.[14]
Conclusions
The oxidative addition of [Pd(PPh3)4] to 1,3-dibenzyl-5-
bromobenzimidazolium tetrafluoroborate 3 affords trans-
[PdBr(bimy)(PPh3)2]BF4 (4), which is the first complex of
an unprecedented benzimidazole-derived ligand (bimy)
with a potential “mesoionic and remote” character. The so-
lid-state molecular structure of the complex has been
studied by single-crystal X-ray diffraction, and the nature
5-Bromo-1,3-dibenzyl-2-methylbenzimidazolium Bromide (2): To a
suspension of 1 (0.506 g, 2.4 mmol) in CH3CN (30 mL), NaOH
(0.423 mL, 2.64 mmol, 6.25 m) was added. The resulting mixture
was stirred for 30 min before benzyl bromide (0.285 mL, 2.4 mmol)
was added. The mixture was left to stir overnight at ambient tem-
of the bimy ligand has been probed by DFT calculations. perature. After the removal of all volatiles in vacuo, the residue was
Table 5. Effect of loading of 4 in the Sonogashira coupling reactions.[a]
Entry
[Pd] [mol-%]
Time [h]
Temperature [°C]
Yield [%][b]
TON
1
2
3
4
5
6
7
8
1
0.5
0.2
0.2
0.02
0.02
0.002
0.002
1
80
80
80
80
80
120
80
120
Ͼ99
Ͼ99
91
Ͼ99
76
51
trace
trace
100
200
455
500
3800
2850
0
24
24
30
72
72
72
72
0
[a] Reaction conditions: 1 mmol of 4-bromobenzaldehyde, 2 mmol of phenylacetylene, 1.2 mmol of NEt3, 5 mol-% CuI, 1 mL of degassed
1
DMF. [b] Yields were determined by H NMR spectroscopy for an average of two runs.
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FULL PAPER
trans-[PdBr(NHC)(PPh3)2]+BF4 (12): A suspension of 1,3-dibenz-
ylbenzimidazolium tetrafluoroborate (154 mg, 0.40 mmol) and sil-
ver(I) oxide (46 mg, 0.20 mmol) in dichloromethane (30 mL) was
stirred at ambient temperature for 15 h, shielded from light. Palla-
dium(II) bromide (106 mg, 0.40 mmol) was dissolved in acetonitrile
(10 mL), and dichloromethane (20 mL) and triphenylphosphane
(210 mg, 0.80 mmol) were added. After 5 min, the two solutions
–
suspended in H2O, and the mixture was extracted with CH2Cl2
(3ϫ 25 mL). The combined organic extracts were washed with H2O
(75 mL). The solvent was removed in vacuo, and the residue was
dissolved in toluene (30 mL). Benzyl bromide (3.5 mL, 29.4 mmol)
was added to this solution, and the mixture was heated under reflux
for 3 d. The brown precipitate was collected by filtration and
washed with toluene to yield 2 (0.780 g, 1.65 mmol, 69%). 1H
NMR (300 MHz, [D6]DMSO): δ = 8.33 (1 H, Ar-H) 7.92 (d, 1 H, were combined. Stirring was continued for 2 h at ambient tempera-
Ar-H), 7.79 (d, 1 H, Ar-H), 7.37 (br s, 10 H, Ar-H), 5.83 (s, 4 H, ture. The mixture was then filtered and concentrated under reduced
CH2), 2.98 (s, 3 H, CH3) ppm. 13C{1H} NMR (75.5 MHz, [D6]- pressure. The solid residue was washed with diethyl ether (30 mL)
DMSO): δ = 153.6 (NCN), 133.8, 132.3, 130.4, 129.4, 129.0, 128.9,
and dried in vacuo. Purification by column chromatography (silica,
128.4, 127.3, 127.2 (Ar-C), 118.8 (C-Br), 116.1, 115.2 (Ar-C), 48.5, dichloromethane + 5% ethanol) gave the product as a pale yellow
48.4 (CH2), 11.3 (CH3) ppm. C22H20Br2N2 (472.22): calcd. C 55.96,
H 4.27, N 5.93; found C 55.85, H 4.21, N 6.12. MS (ESI): m/z =
391 [M – Br]+, 79 [Br]–.
solid (83 mg, 19%). 1H NMR (300 MHz, CDCl3): δ = 7.70–7.31
(m, 25 H, Ar-H), 7.21–6.89 (m, 19 H, Ar-H), 5.05 (s, 4 H, CH2)
ppm. 19F{1H} NMR (170 MHz, CDCl3): δ = –77.49 (s, 10BF4),
–77.54 (s, 11BF4) ppm. 31P{1H} NMR (158 MHz, CDCl3): δ = 21.3
(s, 2 P, PPh3) ppm. C57H48BBrF4N2P2Pd (1096.07): calcd. C 62.46,
H 4.41, N 2.56; found C 62.41, H 4.18, N 2.19. MS (ESI): m/z =
1009 [M – BF4]+.
5-Bromo-1,3-dibenzyl-2-methylbenzimidazolium Tetrafluoroborate
(3): Compound 2 (0.472 g, 1.0 mmol) was dissolved in methanol
(35 mL). NaBF4 (1.098 g, 10 mmol) was added to the solution, and
the mixture was stirred overnight at ambient temperature. The sol-
vent was then removed in vacuo, and CH2Cl2 was added to the
reaction mixture. The suspension was filtered, and the filtrate was
collected. The solvent of the filtrate was removed under reduced
pressure to yield 3 (0.293 g, 0.61 mmol, 61%). Crystals suitable for
X-ray diffraction studies were obtained by the slow evaporation of
General Procedure for the Sonogashira Coupling Reaction: In a typi-
cal run, a Schlenk tube was charged with a mixture of degassed
N,N-dimethylformamide (DMF, 1 mL), CuI (5 mol-%), aryl halide
(1 mmol), phenylacetylene (2 mmol), triethylamine (1.2 mmol) and
the catalyst (1 mol-%) under an inert atmosphere of nitrogen. The
reaction mixture was then stirred at 80 °C for 24 h unless otherwise
indicated. The reaction mixture was cooled, and CH2Cl2 (15 mL)
was added to the reaction mixture. The mixture was washed with
water (3ϫ20 mL). The solvent was allowed to evaporate, and the
1
a concentrated methanol solution. H NMR (300 MHz, CD2Cl2):
δ = 7.77 (s, 1 H, Ar-H), 7.66–7.62 (m, 1 H, Ar-H), 7.53–7.50 (m, 1
H, Ar-H), 7.37–7.36 (m, 6 H, Ar-H), 7.22–7.18 (m, 4 H, Ar-H),
5.63, 5.61 (s, 4 H, CH2), 2.87 (s, 3 H, CH3) ppm. 13C{1H} NMR
(75.5 MHz, CD2Cl2): δ = 153.1 (NCN), 132.7, 132.7, 132.5, 130.9,
130.8, 129.8, 129.7, 127.1, 127.0 (Ar-C), 120.7 (C-Br), 116.2, 114.8
1
residue was analyzed by H NMR spectroscopy.
Crystallography: X-ray data were collected with a Bruker AXS
(Ar-C), 49.9, 49.8 (CH2), 11.5 (CH3) ppm. 19F{1H} NMR SMART APEX diffractometer with Mo-Kα radiation at 223(2) (for
(282.4 MHz, CD2Cl2): δ = –76.74 (s, 10BF4), –76.79 (s, 11BF4) ppm. 3) or 100(2) K (for 4) with the SMART program suite.[27] Data were
C22H20BBrF4N2 (479.12): calcd. C 55.15, H 4.21, N 5.85; found C processed and corrected for Lorentz and polarisation effects with
55.44, H 4.22, N 5.79. MS (ESI): m/z = 391 [M – BF4]+, 87
SAINT[28] and for absorption effects with SADABS.[29] Structural
solution and refinement were performed with the SHELXTL pro-
gram suite.[30] The structures were solved by direct methods to lo-
cate the heavy atoms, and difference maps were used for the light,
non-hydrogen atoms. All hydrogen atoms were placed at calculated
positions. All non-hydrogen atoms were generally given anisotropic
displacement parameters in the final model. A summary of the
most important crystallographic data is given in Table 1.
[BF4]–.
trans-[PdBr(Bn2-2-Me-bimy)(PPh3)2]+BF4
(4): Compound
3
–
(0.095 g, 0.2 mmol) and [Pd(PPh3)4] (0.230 g, 0.2 mmol) were
added to a two-neck flask fitted with a reflux condenser under an
atmosphere of nitrogen. Anhydrous CH2Cl2 (10 mL) was added to
the flask, and the reaction mixture was heated to reflux under ni-
trogen overnight, shielded from light. The reaction mixture was
then concentrated down to a few mL and added dropwise into
diethyl ether (100 mL) with vigorous stirring. The precipitate was
collected by filtration and washed with diethyl ether (2ϫ50 mL).
THF (15 mL) was added to the precipitate, and the solution was
left to stand for a few minutes. The white precipitate formed upon
standing was collected and washed with small amounts of THF to
CCDC-892727 (for 4) and -892728 (for 3) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Computational Methods: Following our previous study on various
carbene analogues,[19] geometry optimizations of 5–10 and their
mono- and diprotonated analogues have been performed at the
B3LYP[31,32] level with the Gaussian 09 program.[33] This functional
1
yield 4 (0.180 g, 0.16 mmol, 80%). H NMR (300 MHz, CD2Cl2):
δ = 7.47–7.31 (m, 21 H, Ar-H), 7.23–7.18 (m, 15 H, Ar-H), 7.09–
7.06 (m, 2 H, Ar-H), 6.98–6.92 (m, 3 H, Ar-H), 6.70 (s, 1 H, Ar- gives accurate results for proton-affinity calculations.[34] For 6–10,
H), 6.51–6.48 (m, 1 H, Ar-H), 5.35 (s, 2 H, CH2), 5.06 (s, 2 H,
all atoms were described with a aug-cc-pVTZ basis set.[35,36] The
smaller aug-cc-pVDZ basis set was used for the larger system 5, as
well as for 6 for comparison, and only minor deviations were ob-
served compared to the values obtained with the triple-ζ basis set.
A vibrational analysis performed after optimization confirms that
these molecules represent minima on the potential energy surface
CH2), 2.78 (s, 3 H, CH3) ppm. 13C{1H} NMR (75.5 MHz,
2
CD2Cl2): δ = 159.5 (t, JP,C = 4.42 Hz, C–Pd), 147.8 (NCN), 134.9
2/3
1
[
J
P,C
= 6.36 Hz, Ar-C], 133.4, 133.2 (Ar-C), 131.0 [t, JP,C =
23.8 Hz, Ar-C], 130.6, 129.6, 129.5, 129.3, 129.2 (Ar-C), 128.3 [t, 2/
2/3
3JP,C = 5.25 Hz, Ar-C], 127.2, 127.1 (Ar-C), 118.5 [t,
J
=
P,C
5.80 Hz, Ar-C], 110.4 (Ar-C), 49.0 (CH2), 48.9 (CH2), 11.1 (CH3) (PES). Proton affinities computed by this procedure have been
ppm. 19F{1H} NMR (282.4 MHz, CD2Cl2): δ = –76.64 (s, 10BF4), compared with values obtained with the M06–2X[34] and BMK[37]
–76.68 (s, 11BF4) ppm. 31P{1H} NMR (121 MHz, CD2Cl2): δ = functionals or with the def2-TZVPP basis set[38] (see Supporting
25.1 (s, 2 P, PPh3) ppm. C58H50BBrF4N2P2Pd (1110.10): calcd. C
62.75, H 4.54, N 2.52; found C 62.89, H 4.49, N 2.59. MS (ESI):
m/z = 1023 [M – BF4]+, 761 [M – BF4 – PPh3]+.
Information) and confirm the reliability of the B3LYP/aug-cc-
pVTZ level. For metal–ligand bonding analyses, the energy decom-
position analysis (EDA) of Morokuma[39] and Ziegler and
Eur. J. Inorg. Chem. 0000, 0–0
7
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Date: 17-07-13 16:54:05
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FULL PAPER
Rauk[40,41] has been performed on trans-[PdBrL(PPh3)2]+ (L = 5–
10) by using ADF2010.02[42–44] at the BP86/TZ2P level with in-
clusion of the zero-order regular approximation (ZORA) scalar rel-
ativistic scheme based on optimized geometries obtained at the ma-
rij-BP86/def2-SVP level by using Turbomole v6.3.1.[45] The core
electrons [1s for C and N, (He)2s2p for P, (Ne)3s3p for Br, (Ne)-
3s3p3d for Pd] were treated with the frozen-core approximation.
The interaction energy ΔEint between L and the [PdBr(PPh3)2]+
fragment is decomposed into electrostatic, Pauli repulsion and or-
bital-mixing components as in Equation (1).
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ΔEint = ΔEelstat + ΔEPauli + ΔEorb
(1)
This methodology, at the BP86/TZ2P level of calculation, has been
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firm the accuracy of the metal–ligand bonding analysis (see Sup-
porting Information). Geometry optimisation of trans-
[PdBr(6)(PPh3)2]+ at the M06L/def2-SVP level[47] by using
Gaussian 09 gives a similar molecular structure to that of the marij-
BP86/def2-SVP calculation and confirms the reliability of the geo-
metries obtained at this level of calculation. The interaction ener-
gies obtained at the M06L/TZ2P or TPSSTPSS/TZ2P levels, by
using ADF2013.01 and an all-electron basis set, are similar
(TPSSTPSS) or slightly larger (M06L) in all cases than those ob-
tained at the BP86/TZ2P level, which supports our conclusions on
the relative bonding capabilities of 5–10.
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The authors thank the National University of Singapore and the
Singapore Ministry of Education for financial support (WBS R-
143-000-410-112). Technical support from the staff at the CMMAC
of our department is appreciated. In particular, we thank Ms Geok
Kheng Tan, Ms Yimian O. and Prof. Lip Lin Koh for determining
the X-ray molecular structures. This work was granted access to the
HPC resources of CINES (allocation 2012-c2012086894) by Grand
Equipement National de Calcul Intensif (GENCI).
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Published Online: ᭿
Eur. J. Inorg. Chem. 0000, 0–0
9
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim