N-heterocyclic carbenes functioning as monoligating clamps

Article abstract of DOI:10.1002/ejic.201300087

Benzimidazolium salts N,N′-disubstituted with 9-alkylfluorenyl groups (3a-e, alkyl = methyl, ethyl, propyl, butyl, benzyl) have been synthesised in high yields in three steps from o-phenylenediamine. This amine was treated with fluorenone in the presence of TiCl₄ and tetramethylethylenediamine (TMEDA) to form N,N′-bis(9H-fluoren-9-ylidene)benzene-1,2-diamine (1) in 91 % yield. Diamines 2a-e were then obtained in yields superior or equal to 77 % by reacting diimine 1 with the appropriate organolithium reagent. In the final step, diamines 2a-e were treated with ethylorthoformate under acidic conditions to afford benzimidazolium salts 3a-e. These were readily converted into the PEPPSI palladium complexes 4a-e (PEPPSI = pyridine-enhanced precatalyst preparation stabilisation and initiation). NMR and X-ray diffraction studies revealed that the flat fluorenylidene moiety orientates the alkyl groups towards the metal centre and because of its restricted rotational freedom makes the ligand bulkiness time independent. Thus, the metal centre is permanently confined between the two alkyl groups, and thereby forms a monoligating clamp with the carbenic centre. The CH₂ groups close to the palladium ion give rise to anagostic C-H···Pd interactions. Catalytic tests revealed that the palladium complexes 4a-e are highly efficient in Suzuki-Miyaura cross-coupling reactions; their activity is equal or superior to the best PEPPSI catalysts reported to date. N-Heterocyclic carbene ligands act as clamps with Pd centres to form highly active Suzuki-Miyaura cross-coupling catalysts. The remarkable performance displayed by these ligands relies on the presence of expanded 9-alkylfluorenyl substituents with a restricted rotational freedom, which results in a permanent, meridional confinement of the metal centre. Copyright

Full text of DOI:10.1002/ejic.201300087

FULL PAPER
DOI:10.1002/ejic.201300087
N-Heterocyclic Carbenes Functioning as Monoligating
Clamps
Matthieu Teci,[a] Eric Brenner,*[a] Dominique Matt,*[a] and
Loïc Toupet[b]
COVER PICTURE
Keywords: Palladium / Carbene ligands / Conformation analysis / Cross-coupling
Benzimidazolium salts N,NЈ-disubstituted with 9-alkylfluor-
enyl groups (3a–e, alkyl = methyl, ethyl, propyl, butyl,
benzyl) have been synthesised in high yields in three steps
from o-phenylenediamine. This amine was treated with
fluorenone in the presence of TiCl4 and tetramethylethyl-
precatalyst preparation stabilisation and initiation). NMR
and X-ray diffraction studies revealed that the flat fluorenyl-
idene moiety orientates the alkyl groups towards the metal
centre and because of its restricted rotational freedom makes
the ligand bulkiness time independent. Thus, the metal cen-
enediamine (TMEDA) to form N,NЈ-bis(9H-fluoren-9-ylid- tre is permanently confined between the two alkyl groups,
ene)benzene-1,2-diamine (1) in 91% yield. Diamines 2a–e
were then obtained in yields superior or equal to 77% by
reacting diimine 1 with the appropriate organolithium rea-
gent. In the final step, diamines 2a–e were treated with ethyl-
orthoformate under acidic conditions to afford benzimidazol-
ium salts 3a–e. These were readily converted into the PEPPSI
palladium complexes 4a–e (PEPPSI = pyridine-enhanced
and thereby forms a monoligating clamp with the carbenic
centre. The CH2 groups close to the palladium ion give rise
to anagostic C–H···Pd interactions. Catalytic tests revealed
that the palladium complexes 4a–e are highly efficient in
Suzuki–Miyaura cross-coupling reactions; their activity is
equal or superior to the best PEPPSI catalysts reported to
date.
may form a pocket about the metal centre. These are remote
from the heterocyclic moiety, and therefore any modifica-
Introduction
N-Heterocyclic carbenes (NHCs) have attracted con- tion of the ligand encumbrance has essentially no signifi-
siderable interest in transition-metal chemistry since the iso- cant effect on the electronic properties. Thus, steric fine-
lation of the first stable compound of this family in 1991[1] tuning of NHCs can be achieved straightforwardly without
and the discovery of the catalytic activity of their com- changing the ligand donor properties.
plexes.[2–6] Palladium-mediated cross-coupling reactions are
Many authors have attested to the pivotal role of steric
among the catalytic reactions for which NHCs have been
successfully applied.[3,7–13] In these transformations, elec-
tronic and steric ligand factors are crucial. Although their
strong σ-donor properties make NHCs particularly efficient
for promoting the oxidative addition step of the reaction,
their steric encumbrance may facilitate the reductive elimi-
nation step as well as increase the stability of catalytic inter-
mediates. As the carbenic centre of an NHC is part of a
planar moiety, the steric properties of these ligands are
mainly determined by the bulk of the N-substituents that
parameters in Suzuki–Miyaura cross-coupling reac-
tions.[7,14,15] A general belief is that to be efficient, NHCs
used in cross-coupling reactions must create strong hemi-
spherical encumbrance about the catalytic centre.
It has also been widely contended that high catalytic
activity requires that the sterically demanding N-substitu-
ents (typically o-substituted aryls[8,16,17] or spiroalkyl
groups[7,18]) possess sufficient structural flexibility to adapt
to the steric requirements of the individual steps of the
whole catalytic cycle. In the present study, we describe
highly active Suzuki–Miyaura catalysts based on NHCs,
which unlike the usual expanded NHCs, generate only
“planar” confinement in which encumbrance is created at
two virtual, trans-located positions. These NHCs form com-
plexes in which the metal centre is held within a clamp-like
ligand characterised by its restricted conformational flexi-
bility (Scheme 1). The new ligands, which were generated
from benzimidazolium precursors, all have 9-alkylfluorenyl
groups as N-substituents with such restricted rotational
freedom that the steric congestion at the metal centre can
undergo at most very minor fluctuations.
[a] Laboratoire de Chimie Inorganique Moléculaire et Catalyse,
Institut de Chimie UMR 7177 CNRS,
Université de Strasbourg,
67008 Strasbourg Cedex, France
E-mail: eric.brenner@unistra.fr
dmatt@chimie.u-strasbg.fr
Homepage: http://inorganics.online.fr/
[b] Institut de Physique de Rennes UMR 6251CNRS,
Université de Rennes 1,
Campus de Beaulieu – Bâtiment 11 A,
35042 Rennes Cedex, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300087.
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Scheme 1. Coordination of metal centres to clamp-like NHC li-
gands.
Results and Discussion
The alkylfluorenyl-substituted benzimidazolium salts 3a–
e were obtained in three steps according to Scheme 2. Their
syntheses began with that of 1,[19] which was obtained by
condensation of 1,2-diaminobenzene with fluorenone in the
presence of TiCl4.
Figure 1. ORTEP representation of the benzimidazolium salt 3b.
The chloride counterion as well as the cocrystallising methanol mo-
lecule are not shown.
The benzimidazolium salts 3a–e were readily converted
into the PEPPSI Pd complexes 4a–e by applying a standard
procedure (Scheme 3, PEPPSI = pyridine-enhanced precat-
alyst preparation stabilisation and initiation).[23] PEPPSI
complexes are regarded as belonging to the most efficient
cross-coupling catalysts.[15,16,23–25]
Scheme 2. Synthesis of benzimidazolium salts 3a–e. Reagents and
conditions: a) fluorenone, TiCl4, tetramethylethylenediamine
(TMEDA), toluene, reflux, 91%; b) RLi, tetrahydrofuran (THF),
–78 °C to room temp., 77–85%; c) HC(OEt)3, concd. HCl, 80 °C,
75–95%. Compound 1 has a C2-symmetric structure.
Diimine 1 was then reacted with five different organo-
lithium reagents RLi to afford the corresponding diamines
2a–e in high yields. We observed that the use of Grignard
reagents instead of alkyllithium compounds resulted in sig-
nificantly lower yields (Ͻ 50%), and the reactions led to
Scheme 3. Synthesis of palladium complexes 4a–e (conditions:
PdCl2, K2CO3, pyridine, 80 °C, 22–85%).
The 1H NMR spectra of complexes 4a–e revealed the
azophilic addition products. In the final step, the amines presence of a pyridine/NHC ratio of 1:1. The most remark-
were treated with ethylorthoformate under acidic condi- able feature of these spectra concerns the NCCH2 signals,
tions to result in the benzimidazolium chlorides 3a–e.[20,21] which are remarkably downfield shifted with respect to
In each of the corresponding 1H NMR spectra, the C-2 those of the corresponding diamines (Δδ = 2.26–3.38 ppm).
proton of the imidazolium ring appears as a singlet at δ ≈ X-ray diffraction studies carried out for 4a–c (R = Me, Et,
1
11.2 ppm. 2D H NMR ROESY experiments established a nPr) revealed that in the solid state the methylenic NCCH2
strong proximity between this proton and the NCCH2 atoms are very close to the metal dz2 orbital, and the carb-
atoms, but not with the aromatic H atoms of the benzimida- enic plane bisects the two CH2 groups. The methylenic CH
zolium moiety, and this is indicative of the fluorenyl planes atom is located only ca. 2.55 Å from the metal centre, and
folding back towards the central phenylene ring (see Sup- the NHC ligand can thus be viewed as a clamp that meridi-
porting Information). This structural feature was confirmed onally confines the metal centre (Figure 2). This feature is
by an X-ray diffraction study carried out for 3b (Figure 1). obviously imposed by the directional properties of the
Molecular modelling (Spartan)[22] further revealed that fluorenyl planes that push the two methylenic NCCH2
the alkylfluorenyl groups cannot rotate about the groups towards the dz2 orbital. As for 3a–e, there is no indi-
N–Calkylfluorenyl bond owing to the expanded structure of cation of alkyfluorenyl moieties freely rotating about the
the fluorenyl plane. Back-folding of the fluorenyl moieties corresponding N–Cquat bonds.[26] Thus, to the best of our
was also found in diamine 2a in the solid state (see Support- knowledge, among the bulky NHCs reported to date,[27] the
ing Information).
above ligands constitute the most confining NHCs with
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time-independent crowding. The downfield shift observed ployed in more challenging cross-coupling reactions. For
for the NCCH2 protons as well as the 1J(13C,1H) values (ca. example, in the reaction of 2,6-dimethoxyphenylboronic
130 Hz) are typically those of anagostic C–H···Pd bonds in acid with 4-chlorotoluene, 4b was twice as active as complex
which the bonding interactions display highly electrostatic 6 (see Supporting Information).
character.[28–30]
Table 1. Suzuki–Miyaura cross-coupling of phenylboronic acid
with p-tolyl chloride using 4–8.
Entry[a]
Complex
Yield[b]
1
2
3
4
5
6
7
8
9
Pd(OAc)2
0
4a
4b
4c
4d
4e
5
40
70
60
21
19
20
63
38
37
6
7
10
8
Figure 2. ORTEP representation of complex 4b, which bears an
NHC ligand that behaves as a clamp.
[a] Reaction conditions: p-tolyl chloride (1 mmol), phenylboronic
acid (1.5 mmol), Cs2CO3 (2 mmol), dioxane (3 mL). [b] Yields were
1
determined by H NMR spectroscopy with 1,4-dimethoxybenzene
To assess their catalytic potential, complexes 4a–e were
used in a Suzuki–Miyaura test reaction, namely that be-
tween p-tolyl chloride and phenylboronic acid. The catalytic
runs were carried out in dioxane at 80 °C with 1 mol-% of
catalyst precursor in the presence of 2 equiv. of Cs2CO3.
All complexes were remarkably active (Table 1). Under the
above conditions, the conversions ranged from 19 to 70%
after 1 h, and the highest reaction rates were observed with
4b (R = Et). Interestingly, the activity of 4a (R = Me) was
twice that of tBu-substituted complex 5 (Figure 3), despite
the fact that the corresponding NHCs have nearly the same
buried volume [36.9% (4a); 36.6% (5)].[31–33] This probably
reflects the restricted rotational freedom of the N-substitu-
ents of 4a, which permanently holds the NCMe groups
close to the metal centre and, therefore, increases the metal
protection. In other terms, the time-averaged steric crowd-
ing about the metal centre is lower in 5, as the tBu groups
freely rotate around the corresponding N–Cquat bonds.
Clearly, these observations show that the buried-volume pa-
rameter, although it is useful in catalytic studies, does not
take into account molecular dynamics that may result in a
transient modification of the real bulk of a given ligand.
The significant activity increase observed on going from 4a
to the sterically more crowded complexes 4b and 4c is poss-
ibly due to a lowering of the activation barrier of the re-
ductive elimination step. However, increasing the bulk of
as internal standard. Averaged over two runs. No homocoupling
products were detected.
Figure 3. Palladium complexes with bulky NHC ligands used in
cross-coupling reactions for comparison with 4a–e.
In conclusion, we have shown that owing to its substitu-
the R groups by using substituents larger than propyl (com- ent-orienting properties, the 9-fluorenylidene moiety consti-
plexes 4d and 4e) led to somewhat lower activities; these tutes a valuable tool for the creation of NHC clamps with
groups probably restrict access of the substrate to the metal time-independent crowding. The palladium derivatives 4a–
centre. We also found that the activity of clamp complex 4b e, which contain such monoligating NHC ligands, were
surpasses that of complexes 6–8 (Figure 3), which contain shown to display activities superior or equal to those ob-
the most efficient NHCs used in Suzuki–Miyaura cross- tained with the fastest Pd–NHC Suzuki–Miyaura cross-
coupling reactions.[8,9,11,15,17,23] Finally, preliminary tests coupling catalysts reported to date. Overall, the above re-
showed that the above clamps can also efficiently be em- sults suggest that “meridional confinement”[34] may be an
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combined organic layers were dried with Na2SO4, and the solvent
was removed under reduced pressure. The crude product was puri-
fied by flash chromatography (SiO2; AcOEt/petroleum ether,
0.5:99.5) to afford 2a as a pale brown solid (1.030 g, 83%); m.p.
interesting alternative to hemispherical encumbrance. Li-
gands of this type will be assessed in other catalytic reac-
tions.
1
3
178 °C. H NMR (CDCl3, 300 MHz): δ = 7.51 (d, J = 7.1 Hz, 4
H, ArH), 7.41–7.35 (m, 8 H, ArH), 7.30–7.25 (m, 4 H, ArH), 6.10
(m, 2 H, ArH), 5.63 (m, 2 H, ArH), 4.38 (br. s, 2 H, NH), 1.76 (s,
6 H, CH3) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ = 150.0
(arom. Cq), 139.1 (arom. Cq), 135.8 (arom. Cq), 128.1 (arom. CH),
127.8 (arom. CH), 123.2 (arom. CH), 120.4 (arom. CH), 119.8
(arom. CH), 117.2 (arom. CH), 65.4 (CqCH3), 30.8 (CH3) ppm.
C34H28N2 (Mr = 464.60): calcd. C 87.90, H 6.07, N 6.03; found C
87.68, H 6.14, N 5.85.
Experimental Section
General Procedures: All commercial reagents were used as supplied.
The syntheses were performed in Schlenk-type flasks under dry
nitrogen. Solvents were dried by conventional methods and distilled
immediately prior to use. Routine H and 13C{1H} NMR spectra
1
were recorded with an FT Bruker AVANCE 300 (1H: 300.1 MHz,
13C: 75.5 MHz) instrument at 25 °C. H NMR spectroscopic data
1
are referenced to residual protonated solvents (CHCl3, δ = 7.26;
DMSO, δ = 2.50 ppm), and 13C chemical shifts are reported relative
to deuterated solvents (CDCl3, δ = 77.16; [D6]DMSO, δ =
39.52 ppm). Data are represented as follows: chemical shift, multi-
plicity (s = singlet, d = doublet, t = triplet, q = quartet, m = mul-
tiplet, br = broad), coupling constant (J) in Hertz (Hz), integration
and assignment. In the NMR spectroscopic data given hereafter,
Cq denotes a quaternary carbon atom. Flash chromatography was
performed as described by Still et al.[35] by employing Geduran
SI (E. Merck, 0.040–0.063 mm) silica. Routine TLC analyses were
carried out by using plates coated with Merck Kieselgel 60 GF254.
Mass spectra were recorded either with a Bruker MicroTOF spec-
trometer (ESI-TOF) with CH2Cl2 or CH3CN as solvent or with a
Bruker MaldiTOF spectrometer with dithranol as matrix. Elemen-
tal analyses were performed by the Service de Microanalyse, Insti-
tut de Chimie (CNRS), Strasbourg. Melting points were deter-
mined with a Büchi 535 capillary melting-point apparatus. Precur-
sor salts of 5,[20,36] 6,[37,38] 7[37,38] and 8[37,38] were prepared by fol-
lowing procedures described in the literature.
N,NЈ-Bis(9-ethyl-9H-fluoren-9-yl)benzene-1,2-diamine (2b): A solu-
tion of ethyl bromide (1.850 g, 17 mmol) in pentane (15 mL) was
added to a suspension of lithium powder (25 wt.-% in mineral oil,
0.940 g, 34 mmol) in pentane (15 mL) at a rate to maintain a steady
reflux (about 1 h). The solution was maintained at reflux for 3 h
and then allowed to reach room temperature. After decantation,
the clear supernatant (25 mL, C ≈ 0.5 m,[39] 12.5 mmol) was re-
moved and added dropwise by syringe to a stirred solution of 1
(2.17 g, 5 mmol) in THF (17 mL) at –78 °C. The dark solution was
then allowed to reach room temperature. The mixture was stirred
for 1 h, water (40 mL) was slowly added, and the product was ex-
tracted with AcOEt (3ϫ 40 mL). The combined organic layers were
dried with Na2SO4, and the solvent was removed under reduced
pressure. The crude product was purified by flash chromatography
(SiO2; AcOEt/petroleum ether, 0.5:99.5) to afford 2b as a pale
brown solid (2.010 g, 82%); m.p. 130 °C. 1H NMR (CDCl3,
300 MHz): δ = 7.76 (d, 3J = 7.4 Hz, 4 H, ArH), 7.42–7.35 (m, 8 H,
ArH), 7.31–7.26 (m, 4 H, ArH), 6.10–6.07 (m, 2 H, ArH), 5,66–
3
5.63 (m, 2 H, ArH), 4.43 (br. s, 2 H, NH), 2.22 (q, J = 7.4 Hz, 4
N,NЈ-Bis(9H-fluoren-9-ylidene)benzene-1,2-diamine (1): A solution
of ortho-phenylenediamine (2.160 g, 20 mmol), fluorenone (7.630 g,
42.3 mmol) and TMEDA (36 mL, 240 mmol) in toluene (200 mL)
was heated at 80 °C. TiCl4 (6.7 mL, 60 mmol) was then added drop-
wise over 20 min before the temperature was increased to 110 °C.
After 5 h, the reaction mixture was cooled to room temperature
and filtered through Celite to remove insoluble materials. These
were washed with AcOEt (100 mL), and the filtrate was concen-
trated to dryness. The residue was dissolved in AcOEt (ca. 20 mL),
and the resulting solution was passed through a short pad of silica
gel and eluted with AcOEt until the product was completely reco-
vered. The dark orange filtrate was concentrated to afford a red
solid, which was purified by flash chromatography (SiO2; CH2Cl2/
petroleum ether, 60:40). Diimine 1 was obtained as an orange solid
(7.85 g, 91%); m.p. Ͼ 250 °C. 1H NMR (CDCl3, 300 MHz): δ =
H, CH2), 0.61 (t, 3J = 7.4 Hz, 6 H, CH3) ppm. 13C{1H} NMR
(CDCl3, 75 MHz): δ = 148.3 (arom. Cq), 140.2 (arom. Cq), 135.8
(arom. Cq), 128.1 (arom. CH), 127.6 (arom. CH), 123.5 (arom.
CH), 120.2 (arom. CH), 119.6 (arom. CH), 117.0 (arom. CH), 69.0
(CqCH2), 36.4 (CH2), 8.4 (CH3) ppm. C36H32N2 (492.65): calcd. C
87.77, H 6.55, N 5.69; found C 87.63, H 6.56, N 5.49.
N,NЈ-Bis(9-propyl-9H-fluoren-9-yl)benzene-1,2-diamine (2c): Propyl
bromide (0.516 g, 4.20 mmol) dissolved in pentane (7 mL) was
added to a suspension of lithium powder (25 wt.-% in mineral oil,
0.234 g, 8.4 mmol) in pentane (7 mL) at a rate to maintain a steady
reflux (about 1 h). The solution was maintained at reflux for 3 h.
After decantation at room temperature, the clear supernatant
(6.7 mL, C ≈ 0.5 m,[6] 3.30 mmol) was removed and added dropwise
by syringe to a stirred solution of 1 (0.562 g, 1.30 mmol) in THF
(6 mL) at –78 °C under nitrogen. The dark solution was allowed to
reach room temperature. The solution was stirred for a further 1 h,
water (20 mL) was slowly added, and the reaction mixture was ex-
tracted with AcOEt (3ϫ 20 mL). The combined organic layers were
dried with Na2SO4, and the solvent was removed under reduced
pressure. The crude product was purified by flash chromatography
(SiO2; AcOEt/petroleum ether, 0.5:99.5) to afford 2c as a pale
brown solid (0.521 g, 77%); m.p. 80 °C. 1H NMR (CDCl3,
300 MHz): δ = 7.77 (d, 3J = 7.4 Hz, 4 H, ArH), 7.43–7.37 (m, 8 H,
ArH), 7.31–7.27 (m, 4 H, ArH), 6.12–6.09 (m, 2 H, ArH), 5,68–
5.65 (m, 2 H, ArH), 4.44 (br. s, 2 H, NH), 2.18–2.13 (m, 4 H,
3
7.72 (d, J = 7.5 Hz, 2 H, ArH), 7.50–7.41 (m, 4 H, ArH), 7.35–
3
3
4
7.10 (m, 12 H, ArH), 7.00 (ddd, J = JЈ = 7.5 Hz, J = 0.9 Hz, 2
H, ArH) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ = 162.9 (C=N),
143.6 (arom. Cq), 141.7 (arom. Cq), 140.9 (2ϫ, arom. Cq), 137.6
(arom. Cq), 131.8 (arom. CH), 131.6 (arom. CH), 128.3 (arom.
CH), 127.7 (arom. CH), 127.0 (arom. CH), 125.0 (arom. CH),
123.5 (arom. CH), 120.1 (arom. CH), 119.8 (arom. CH), 119.4
(arom. CH) ppm. The spectroscopic data are in full agreement with
those of the literature.[19]
N,NЈ-Bis(9-methyl-9H-fluoren-9-yl)benzene-1,2-diamine (2a): To a
stirred solution of diimine
1 (1.150 g, 2.67 mmol) in tetra-
3
hydrofuran (THF, 10 mL) cooled to –78 °C, was added dropwise
MeLi (1.6 m in Et2O, 4 mL, 6.40 mmol). The red solution quickly
turned black. The reaction mixture was allowed to reach room tem-
perature and was stirred for 30 min. After slow addition of water
(30 mL), the mixture was extracted with AcOEt (3ϫ 40 mL), the
CqCH2), 1.01–0.90 (m, 4 H, CH2CH3), 0.86 (t, J = 7.4 Hz, 6 H,
CH3) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ = 148.7 (arom.
Cq), 140.0 (arom. Cq), 135.8 (arom. Cq), 128.0 (arom. CH), 127.6
(arom. CH), 123.5 (arom. CH), 120.1 (arom. CH), 119.6 (arom.
CH), 117.1 (arom. CH), 68.7 (CqCH2), 45.8 (CqCH2), 17.2
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(CH2CH3), 14.4 (CH3) ppm. C38H36N2 (520.71): calcd. C 87.65, H
6.97, N 5.38; found C 87.53, H 6.93, N 5.28.
2.96 (s, 6 H, CH3) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ =
145.1 (arom. Cq), 142.6 (NCHN), 139.0 (arom. Cq), 130.9 (arom.
Cq), 130.3 (arom. CH), 129.4 (arom. CH), 126.3 (arom. CH), 124.9
(arom. CH), 120.8 (arom. CH), 114.8 (arom. CH), 71.4 (CqCH3),
27.3 (CH3) ppm. C35H27ClN2·1.4H2O (511.06 + 25.22): calcd. C
78.39, H 5.60, N 5.22; found C 78.61, H 5.55, N 4.98.
N,NЈ-Bis(9-butyl-9H-fluoren-9-yl)benzene-1,2-diamine (2d): To
a
stirred solution of diimine 1 (1.020 g, 2.35 mmol) in THF (10 mL)
cooled to –78 °C was added BuLi (1.6 m in hexanes, 4.4 mL,
7.04 mmol). The red solution quickly turned black. The reaction
mixture was allowed to reach room temperature and was stirred
for 30 min. Water (30 mL) was slowly added, the reaction mixture
was extracted with AcOEt (3ϫ 40 mL), the combined organic lay-
ers were dried with Na2SO4, and the solvent was removed under
reduced pressure. The crude product was purified by flash
chromatography (SiO2; AcOEt/petroleum ether, 0.5:99.5) to afford
2d as a pale brown solid (1.010 g, 78%); m.p. 79 °C. 1H NMR
1,3-Bis(9-ethyl-9H-fluoren-9-yl)benzimidazolium Chloride (3b): Di-
amine 2b (0.400 g, 0.81 mmol) was dissolved under magnetic stir-
ring in HC(OEt)3 (3 mL). After the addition of HCl (12 m, 89 μL,
1.07 mmol), the mixture was heated at 80 °C for 15 h. The mixture
was cooled to room temperature, petroleum ether was added (ca.
20 mL). The precipitate was collected by filtration and washed with
petroleum ether (3ϫ 15 mL). Compound 3b (0.414 g, 95%) was
3
(CDCl3, 300 MHz): δ = 7.77 (d, J = 7.4 Hz, 4 H, ArH), 7.43–7.36
obtained as a white solid; m.p. 205 °C. 1H NMR (CDCl3,
3
(m, 8 H, ArH), 7.32–7.27 (m, 4 H, ArH), 6.14–6.11 (m, 2 H, ArH), 300 MHz): δ = 11.11 (s, 1 H, NCHN), 7.81 (d, J = 7.5 Hz, 4 H,
5.67–5.66 (m, 2 H, ArH), 4.43 (br. s, 2 H, NH), 2.19–2.14 (m, 4 H,
ArH), 7.79 (d, 3J = 7.5 Hz, 4 H, ArH), 7.48 (ddd, 3J = 3JЈ = 7.5 Hz,
CqCH2), 1.26 (tq, 3J = 3JЈ = 7.3 Hz, 4 H, CH2CH3), 1.03–0.92 (m, 4J = 0.9 Hz, 4 H, ArH), 7.34 (ddd, J = JЈ = 7.5 Hz, J = 0.9 Hz,
3
3
4
3
4 H, CqCH2CH2), 0.84 (t, J = 7.3 Hz, 6 H, CH3) ppm. 13C{1H} 4 H, ArH), 6.76–6.73 (m, 2 H, ArH), 6.25–6.21 (m, 2 H, ArH),
NMR (CDCl3, 75 MHz): δ = 148.8 (arom. Cq), 140.1 (arom. Cq), 3.68 (q, J = 7.0 Hz, 4 H, CH2), 0.51 (t, 3J = 7.0 Hz, 6 H, CH3)
3
135.8 (arom. Cq), 128.0 (arom. CH), 127.6 (arom. CH), 123.5 ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ = 143.2 (arom. Cq),
(arom. CH), 120.2 (arom. CH), 119.6 (arom. CH), 117.2 (arom. 142.2 (NCHN), 140.6 (arom. Cq), 131.6 (arom. Cq), 130.4 (arom.
CH), 68.6 (CqCH2), 43.3 (CqCH2), 26.0 (CqCH2CH2), 23.0 CH), 129.6 (arom. CH), 126.3 (arom. CH), 125.1 (arom. CH),
(CH2CH3), 14.1 (CH3) ppm. C40H40N2 (548.76): calcd. C 87.55, H
7.35, N 5.10; found C 87.21, H 7.39, N 5.39.
120.6 (arom. CH), 115.0 (arom. CH), 75.4 (CqCH2), 31.7 (CH2),
7.6 (CH3) ppm. C37H33ClN2O (3b·H2O, 557.13): calcd. C 79.77, H
5.97, N 5.03; found C 79.94, H 6.20, N 4.80.
N,NЈ-Bis(9-benzyl-9H-fluoren-9-yl)benzene-1,2-diamine (2e): To a
mixture of tBuOK (0.151 g, 1.34 mmol) and toluene (143 μL,
1.31 mmol) in THF (10 mL) cooled to –78 °C was added BuLi
(1.6 m in hexanes, 760 μL, 1.22 mmol). The suspension quickly
turned red and was kept at –78 °C for 20 min. Diimine 1 (0.200 g,
0.46 mmol) was then added portionwise, and the mixture quickly
turned black. The reaction mixture was allowed to reach room tem-
perature and was stirred for 30 min. Water (20 mL) was slowly
added, and the reaction mixture was extracted with AcOEt (3ϫ
20 mL). The combined organic layers were dried with Na2SO4, and
1,3-Bis(9-propyl-9H-fluoren-9-yl)benzimidazolium Chloride (3c): Di-
amine 2c (0.422 g, 0.81 mmol) was dissolved under magnetic stir-
ring in HC(OEt)3 (3 mL). After the addition of HCl (12 m, 89 μL,
1.07 mmol), the mixture was heated at 80 °C for 15 h. The solution
was cooled to room temperature, and petroleum ether was added
(ca. 20 mL). The resulting precipitate was collected by filtration
and washed with petroleum ether (3ϫ 15 mL). Compound 3c
(0.358 g, 78%) was obtained as a white solid; m.p. 195 °C. 1H
3
NMR (CDCl3, 300 MHz): δ = 11.03 (s, 1 H, NCHN), 7.82 (d, J
3
the solvent was removed under reduced pressure. The crude prod- = 7.5 Hz, 4 H, ArH), 7.80 (d, J = 7.5 Hz, 4 H, ArH), 7.47 (ddd,
3
4
3
3
uct was purified by flash chromatography (SiO2; AcOEt/petroleum
ether, 0.5:99.5) to afford 2e as a pale brown solid (0.241 g, 85%);
m.p. 103 °C. 1H NMR (CDCl3, 300 MHz): δ = 7.64 (d, 3J = 7.5 Hz,
4 H, ArH), 7.39–7.33 (m, 4 H, ArH), 7.28 (m, 8 H, ArH), 7.23–
7.14 (m, 6 H, ArH), 6.89 (dd, 3J = 7.7 Hz, 4J = 1.5 Hz, 4 H, ArH),
6.04–6.00 (m, 2 H, ArH), 5.58–5.55 (m, 2 H, ArH), 4.68 (br. s, 2
3J = JЈ = 7.5 Hz, J = 0.9 Hz, 4 H, ArH), 7.34 (ddd, J = JЈ =
7.5 Hz, 4J = 0.9 Hz, 4 H, ArH), 6.74–6.71 (m, 2 H, ArH), 6.23–
6.19 (m, 2 H, ArH), 3.62–3.57 (m, 4 H, CqCH2), 0.92 (t, 3J =
7.1 Hz, 6 H, CH3), 0.85–0.72 (m, 4 H, CH2CH3) ppm. 13C{1H}
NMR (CDCl3, 75 MHz): δ = 143.6 (arom. Cq), 141.9 (NCHN),
140.3 (arom. Cq), 131.0 (arom. Cq), 130.3 (arom. CH), 129.5
H, NH), 3.42 (s, 4 H, CH2) ppm. 13C{1H} NMR (CDCl3, 75 MHz): (arom. CH), 126.1 (arom. CH), 125.1 (arom. CH), 120.5 (arom.
δ = 147.9 (arom. Cq), 139.9 (arom. Cq), 135.7 (arom. Cq), 135.5
(arom. Cq), 130.9 (arom. CH), 128.2 (arom. CH), 127.6 (arom.
CH), 127.3 (arom. CH), 126.8 (arom. CH), 124.1 (arom. CH),
120.2 (arom. CH), 119.5 (arom. CH), 116.5 (arom. CH), 69.0
(CqCH2), 50.1 (CH2) ppm. C46H36N2 (616.79): calcd. C 89.58, H
5.88, N 4.54; found C 89.09, H 6.10, N 4.52. Despite several
recrystallisations the carbon content remained below the expected
value.
CH), 114.9 (arom. CH), 74.8 (CqCH2), 39.9 (CqCH2), 16.6
(CH2CH3), 14.1 (CH3) ppm. C39H35ClN2·0.7H2O (567.16 + 12.61):
calcd. C 80.79, H 6.33, N 4.83; found C 80.91, H 6.27, N 4.77.
1,3-Bis(9-butyl-9H-fluoren-9-yl)benzimidazolium Chloride (3d): Di-
amine 2d (0.912 g, 1.66 mmol) was dissolved under magnetic stir-
ring in HC(OEt)3 (5 mL). After the addition of HCl (12 m, 183 μL,
2.19 mmol), the mixture was heated at 80 °C for 15 h. The solution
was then cooled to room temperature, and petroleum ether was
1,3-Bis(9-methyl-9H-fluoren-9-yl)benzimidazolium Chloride (3a): added (ca. 20 mL). The precipitate was collected by filtration,
Diamine 2a (0.502 g, 1.08 mmol) was dissolved under magnetic washed with petroleum ether (3ϫ 15 mL) and dried in vacuo. Com-
stirring in HC(OEt)3 (3 mL). HCl (12 m, 120 μL, 1.40 mmol) was pound 3d (0.878 g, 89%) was obtained as a white solid; m.p.
1
then added, and the mixture was heated at 80 °C for 15 h. The
mixture was cooled to room temperature, and petroleum ether was
added (ca. 20 mL). The precipitate was collected by filtration and
washed with petroleum ether (3ϫ 15 mL). Compound 3a (0.553 g,
170 °C. H NMR (CDCl3, 300 MHz): δ = 11.10 (s, 1 H, NCHN),
3
3
7.83 (d, J = 7.5 Hz, 4 H, ArH), 7.82 (d, J = 7.5 Hz, 4 H, ArH),
7.49 (dd, 3J = 3JЈ = 7.5 Hz, 4 H, ArH), 7.35 (dd, 3J = 3JЈ = 7.5 Hz,
4 H, ArH), 6.75–6.70 (m, 2 H, ArH), 6.23–6.19 (m, 2 H, ArH),
86%) was obtained as a white solid; m.p. 210 °C. 1H NMR (CDCl3, 3.67–3.62 (m, 4 H, CqCH2), 1.40 (tq, 3J = 3JЈ = 7.3 Hz, 4 H,
3
300 MHz): δ = 11.24 (s, 1 H, NCHN), 7.89 (d, J = 7.5 Hz, 4 H, CH2CH3), 0.77–0.67 (m, 10 H, CqCH2CH2 and CH3) ppm.
ArH), 7.85 (d, 3J = 7.5 Hz, 4 H, ArH), 7.50 (ddd, 3J = 3JЈ = 7.6 Hz,
13C{1H} NMR (CDCl3, 75 MHz): δ = 143.7 (arom. Cq), 142.1
(NCHN), 140.4 (arom. Cq), 131.0 (arom. Cq), 130.3 (arom. CH),
3
3
4
4J = 0.9 Hz, 4 H, ArH), 7.35 (ddd, J = JЈ = 7.6 Hz, J = 0.9 Hz,
4 H, ArH), 6.82–6.78 (m, 2 H, ArH), 6.25–6.21 (m, 2 H, ArH), 129.5 (arom. CH), 126.1 (arom. CH), 125.1 (arom. CH), 120.5
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(arom. CH), 114.9 (arom. CH), 74.8 (CqCH2), 37.7 (CqCH2), 25.34 151.8 (arom. CH), 146.4 (arom. Cq), 140.3 (arom. Cq), 138.2
(CqCH2CH2), 22.59 (CH2CH3), 14.28 (CH3) ppm. (arom. CH), 134.6 (arom. Cq), 129.1 (2ϫ, arom. CH), 125.0 (arom.
C41H39ClN2·0.8H2O (595.21 + 14.41): calcd. C 80.78, H 6.71, N CH), 124.8 (arom. CH), 122.1 (arom. CH), 120.0 (arom. CH),
4.60; found C 80.59, H 6.62, N 4.71.
113.5 (arom. CH), 76.7 (CqCH2), 36.8 (CH2), 8.7 (CH3) ppm.
C42H35Cl2N3Pd (759.07): calcd. C 66.46, H 4.65, N 5.54; found C
66.73, H 4.99, N 5.16.
1,3-Bis(9-benzyl-9H-fluoren-9-yl)benzimidazolium Chloride (3e): Di-
amine 2e (0.450 g, 0.73 mmol) was dissolved under magnetic stir-
ring in HC(OEt)3 (3 mL) and then HCl 12 m (81 μL, 0.97 mmol) trans-[1,3-Bis(9-propyl-9H-fluoren-9-yl)benzimidazol-2-ylidene]-
was added. The mixture was heated at 80 °C for 15 h. The solution
was cooled to room temperature, petroleum ether was added (ca.
20 mL), and the resulting precipitate was collected by filtration and
washed with petroleum ether (3ϫ 15 mL). Compound 3e (0.362 g,
75%) was obtained as a white solid; m.p. 225 °C. 1H NMR (CDCl3,
300 MHz): δ = 11.49 (s, 1 H, NCHN), 8.01–8.07 (m, 4 H, ArH),
(pyridine)palladium(II) Dichloride (4c): Nitrogen was passed (2 min)
through a suspension of benzimidazolium salt 3c (0.352 g,
0.62 mmol), K2CO3 (0.427 g, 3.09 mmol) and PdCl2 (0.132 g,
0.74 mmol) in pyridine (3 mL). The suspension was then heated at
80 °C for 24 h under vigorous stirring. The solution was cooled to
room temperature, the mixture was filtered through Celite, and the
solid was washed with CH2Cl2 (ca. 20 mL). The solvent was re-
7.52–7.50 (m, 4 H, ArH), 7.42–7.35 (m, 8 H, ArH), 6.93 (t, 3J =
3
7.5 Hz, 2 H, ArH), 6.83–6.76 (m, 6 H, ArH), 6.61 (d, J = 7.5 Hz, moved in vacuo, and the residue was purified by flash chromatog-
4 H, ArH), 6.25–6.22 (m, 2 H, ArH), 5.06 (s, 4 H, CH2) ppm. raphy (SiO2; CH2Cl2/petroleum ether, 50:50) to afford 4c as a yel-
13C{1H} NMR (CDCl3, 75 MHz): δ = 143.9 (arom. Cq), 142.9
low solid (0.108 g, 22 %); m.p. Ͼ 240 °C. 1H NMR (CDCl3,
(NCHN), 142.2 (arom. Cq), 132.0 (arom. Cq), 131.1 (arom. Cq), 300 MHz): δ = 9.16–9.14 (m, 2 H, o-NC5H5), 7.87–7.72 (m, 9 H,
131.0 (arom. CH), 130.3 (arom. CH), 128.9 (arom. CH), 127.0 8H ArH and 1 H, p-NC5H5), 7.43–7.38 (m, 6 H, 4H ArH and 2H
(arom. CH), 126.6 (arom. CH), 126.3 (arom. CH), 125.6 (arom. m-NC5H5), 7.30–7.26 (m, 4 H, ArH), 6.35–6.32 (m, 2 H, ArH),
3
CH), 120.4 (arom. CH), 115.2 (arom. CH), 75.1 (CqCH2), 44.2 6.15–6.11 (m, 2 H, ArH), 5.46–5.41 (m, 4 H, CqCH2), 0.92 (t, J
3
3
(CH2) ppm. C47H39ClN2O (3e·2H2O, 699.28): calcd. C 80.73, H
5.62, N 4.01; found C 80.43, H 5.34, N 4.11.
= 7.2 Hz, 6 H, CH3), 0.62 (tq, J = JЈ = 7.2 Hz, 4 H, CH2CH3)
ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ = 160.7 (NCN), 151.9
(arom. CH), 147.0 (arom. Cq), 140.0 (arom. Cq), 138.2 (arom.
CH), 134.5 (arom. Cq), 129.1 (arom. CH), 129.1 (arom. CH), 124.9
(arom. CH), 124.8 (arom. CH), 122.4 (arom. CH), 120.0 (arom.
CH), 113.4 (arom. CH), 76.0 (CqCH2), 46.0 (CqCH2), 17.6
(CH2CH3), 14.5 (CH3) ppm. C44H39Cl2N3Pd·0.2CH2Cl2 (787.13 +
16.99): calcd. C 66.02, H 4.94, N 5.23; found C 66.06, H 4.96, N
5.14.
trans-[1,3-Bis(9-methyl-9H-fluoren-9-yl)benzimidazol-2-ylidene]-
(pyridine)palladium(II) Dichloride (4a): Nitrogen was passed (2 min)
through a suspension of benzimidazolium salt 3a (0.260 g,
0.51 mmol), K2CO3 (0.356 g, 2.58 mmol) and PdCl2 (0.110 g,
0.62 mmol) in pyridine (3 mL). The suspension was then heated at
80 °C for 15 h under vigorous stirring. The mixture was cooled to
room temperature, the mixture was filtered through Celite, and the
solid was washed with CH2Cl2 (ca. 20 mL). The solvent was re-
moved in vacuo, and the residue was purified by flash chromatog-
raphy (SiO2; CH2Cl2/petroleum ether, 50:50) to afford 4a as a yel-
trans-[1,3-Bis(9-butyl-9H-fluoren-9-yl)benzimidazol-2-ylidene]-
(pyridine)palladium(II) Dichloride (4d): Nitrogen was passed (2 min)
through a suspension of benzimidazolium salt 3d (0.503 g,
low solid (0.316 g, 85 %); m.p. Ͼ 240 °C. 1H NMR (CDCl3, 0.84 mmol), K2CO3 (0.590 g, 4.27 mmol) and PdCl2 (0.179 g,
300 MHz): δ = 9.19–9.12 (m, 2 H, o-NC5H5), 7.85–7.78 (m, 9 H,
1.01 mmol) in pyridine (3 mL). The mixture was then heated at
8H ArH and 1 H, p-NC5H5), 7.47–7.37 (m, 6 H, 4H ArH and 2H 80 °C for 15 h under vigorous stirring. The mixture was cooled to
3
3
m-NC5H5), 7.29 (dd, J = JЈ = 7.5 Hz, 4 H, ArH), 6.40–6.37 (m, room temperature and filtered through Celite, and the filtered solid
2 H, ArH), 6.08–6.05 (m, 2 H, ArH), 4.02 (s, 6 H, CH3) ppm. was washed with CH2Cl2 (ca. 20 mL). The solvent was removed in
13C{1H} NMR (CDCl3, 75 MHz): δ = 161.5 (NCN), 151.8 (arom.
vacuo, and the residue was purified by flash chromatography (SiO2;
CH), 149.2 (arom. Cq), 138.4 (arom. Cq), 138.2 (arom. CH), 134.4 CH2Cl2/petroleum ether, 50:50) to afford 4d as a yellow solid
1
(arom. Cq), 129.1 (2ϫ, arom. CH), 124.9 (arom. CH), 124.7 (arom. (0.279 g, 41%); m.p. Ͼ 240 °C. H NMR (CDCl3, 300 MHz): δ =
CH), 122.5 (arom. CH), 120.7 (arom. CH), 113.1 (arom. CH), 72.2
9.19–9.17 (m, 2 H, o-NC5H5), 7.83–7.72 (m, 9 H, 8H ArH and 1
(CqCH3), 33.8 (CH3) ppm. C40H31Cl2N3Pd·0.2CH2Cl2 (731.02 + H, p-NC5H5), 7.43–7.38 (m, 6 H, 4H ArH and 2H m-NC5H5),
16.99): calcd. C 64.55, H 4.23, N 5.62; found C 64.39, H 4.31, N
5.56.
7.30–7.25 (m, 4 H, ArH), 6.34–6.31 (m, 2 H, ArH), 6.15–6.11 (m,
2 H, ArH), 5.47–5.41 (m, 4 H, CqCH2), 1.34 (tq, 3J = 3JЈ = 7.3 Hz,
3
3
3
4 H, CH2CH3), 0.69 (t, J = 7.3 Hz, 6 H, CH3), 0.55 (tt, J = JЈ
= 7.3 Hz, 4 H, CqCH2CH2) ppm. 13C{1H} NMR (CDCl3,
75 MHz): δ = 160.5 (NCN), 151.9 (arom. CH), 147.0 (arom. Cq),
140.1 (arom. Cq), 138.2 (arom. CH), 134.6 (arom. Cq), 129.1
(arom. CH), 129.0 (arom. CH), 124.9 (arom. CH), 124.7 (arom.
CH), 122.3 (arom. CH), 120.0 (arom. CH), 113.4 (arom. CH), 76.1
(CqCH2), 43.6 (CqCH2), 26.1 (CqCH2CH2), 23.0 (CH2CH3), 14.1
(CH3) ppm. C46H43Cl2N3Pd·0.1CH2Cl2 (815.18 + 8.49): calcd. C
67.22, H 5.29, N 5.10; found C 67.37, H 5.39, N 4.99.
trans-[1,3-Bis(9-ethyl-9H-fluoren-9-yl)benzimidazol-2-ylidene]-
(pyridine)palladium(II) Dichloride (4b): Nitrogen was passed (2 min)
through a stirred suspension of benzimidazolium salt 3b (0.103 g,
0.19 mmol), K2CO3 (0.132 g, 0.96 mmol) and PdCl2 (0.041 g,
0.23 mmol) in pyridine (2.5 mL). The suspension was then heated
at 80 °C for 15 h under vigorous stirring. The solution was cooled
to room temperature, the mixture was filtered through Celite, and
the solid was washed with CH2Cl2 (ca. 20 mL). The filtrate was
concentrated under vacuum and purified by flash chromatography
(SiO2; CH2Cl2/petroleum ether, 50:50) to afford 4b as a yellow solid
(0.113 g, 78%); m.p. Ͼ 240 °C. H NMR (CDCl3, 300 MHz): δ = (pyridine)palladium(II) Dichloride (4e): Nitrogen was passed (2 min)
trans-[1,3-Bis(9-benzyl-9H-fluoren-9-yl)benzimidazol-2-ylidene]-
1
9.13–9.10 (m, 2 H, o-NC5H5), 7.84–7.76 (m, 5 H, 4H ArH and 1
through a suspension of benzimidazolium salt 3e (0.200 g,
0.30 mmol), K2CO3 (0.207 g, 1.5 mmol) and PdCl2 (0.065 g,
0.36 mmol) in pyridine (3 mL). The suspension was then heated at
80 °C for 15 h under vigorous stirring. The mixture was cooled to
room temperature and filtered through Celite, and the filtered solid
3
H, p-NC5H5), 7.72 (d, J = 7.5 Hz, 4 H, ArH), 7.45–7.35 (m, 6 H,
4H ArH and 2H m-NC5H5), 7.28 (ddd, 3J = 3JЈ = 7.5 Hz, 4J =
0.9 Hz, 4 H, ArH), 6.37–6.34 (m, 2 H, ArH), 6.18–6.15 (m, 2 H,
3
3
ArH), 5.45 (q, J = 7.3 Hz, 4 H, CH2), 0.42 (t, J = 7.3 Hz, 6 H,
CH3) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ = 160.4 (NCN), was washed with CH2Cl2 (ca. 20 mL). The solvent was removed in
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FULL PAPER
vacuo, and the residue was purified by flash chromatography (SiO2;
collected solid was washed with CH2Cl2 (ca. 20 mL). The filtrate
and the washings were evaporated to dryness, and the resulting
CH2Cl2/petroleum ether, 50:50) to afford 4e as a yellow solid
1
(0.177 g, 67%); m.p. Ͼ 240 °C. H NMR (CDCl3, 300 MHz): δ = residue was purified by flash chromatography (SiO2; CH2Cl2/petro-
9.01–8.98 (m, 2 H, o-NC5H5), 8.02–7.97 (m, 4 H, ArH), 7.66 (tt,
leum ether, 50:50) to afford 7 as a yellow solid (0.322 g, 70%); m.p.
3J = 7.7 Hz, 4J = 1.7 Hz, 1 H, p-NC5H5), 7.44–7.22 (m, 14 H, 12H
235 °C. 1H NMR (CDCl3, 300 MHz): δ = 8.58–8.51 (m, 2 H, o-
ArH and 2H m-NC5H5), 6.94 (tt, 3J = 7.4 Hz, 4J = 1.1 Hz, 2 H, NC5H5), 7.57–7.44 (m, 3 H, 2H ArH and 1 H, p-NC5H5), 7.35 (d,
ArH), 6.83–6.74 (m, 8 H, 4H ArH and 4H CH2), 6.41 (d, 3J = 3J = 7.6 Hz, 4 H, ArH), 7.14–7.05 (m, 4 H, 2H ArH and 2 H, m-
7.4 Hz, 4 H, ArH), 6.38–6.35 (m, 2 H, ArH), 6.16–6.12 (m, 2 H,
ArH) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ = 161.3 (NCN),
151.6 (arom. CH), 146.2 (arom. Cq), 140.1 (arom. Cq), 138.0
3
3
3
NC5H5), 3.18 (qq, J = JЈ = 6.7 Hz, 4 H, CHMe2), 1.49 (d, J =
3
6.7 Hz, 12 H, CHCH3), 1.12 (d, J = 6.7 Hz, 12 H, CHCH3) ppm.
13C{1H} NMR (CDCl3, 75 MHz): δ = 155.0 (NCN), 151.5 (arom.
(arom. CH), 134.6 (arom. Cq), 134.5 (arom. Cq), 131.2 (arom. CH) CH), 146.7 (arom. Cq), 137.5 (arom. CH), 135.2 (arom. Cq), 130.3
129.2 (arom. CH), 128.6 (arom. CH), 126.6 (arom. CH), 125.9 (arom. CH), 125.1 (arom. CH), 124.1 (2ϫ, arom. CH), 28.8
(arom. CH), 125.4 (arom. CH), 125.0 (arom. CH) 122.5 (arom.
(CHMe2) 26.4 (CH3) 23.4 (CH3) ppm. C32H41Cl2N3Pd (645.01):
CH), 119.9 (arom. CH), 113.8 (arom. CH), 76.3 (CqCH2), 50.0 calcd. C 59.59, H 6.41, N 6.51; found C 59.53, H 6.63, N 6.29. The
(CqCH2) ppm. C52H39Cl2N3Pd (883.21): calcd. C 70.71, H 4.45, N
4.76; found C 70.72, H 4.56, N 4.82.
spectroscopic data are in full agreement with those of the litera-
ture.[15]
trans-[1,3-Bis(tert-butyl)benzimidazol-2-ylidene](pyridine)pallad-
ium(II) Dichloride (5): Nitrogen was passed (2 min) through a sus-
pension of 1,3-bis(tert-butyl)benzimidazolium chloride (0.630 g,
2.36 mmol), K2CO3 (1.63 g, 11.8 mmol) and PdCl2 (0.415 g,
2.34 mmol) in pyridine (5 mL). The suspension was then heated at
80 °C for 15 h under vigorous stirring. The mixture was cooled to
room temperature and filtered through Celite, and the filtered solid
was washed with CH2Cl2 (ca. 20 mL). The combined washings and
the filtrate were evaporated to dryness. The residue was then puri-
fied by flash chromatography (SiO2; CH2Cl2/petroleum ether,
50:50) to afford 5 as a yellow solid (0.921 g, 80%); m.p. 229 °C. 1H
NMR (CDCl3, 300 MHz): δ = 9.03–8.98 (m, 2 H, o-NC5H5), 7.83–
7.79 (m, 2 H, ArH), 7.78 (tt, 3J = 7.6 Hz, 4J = 1.6 Hz, 1 H, p-
trans-[1,3-Bis(2,6-diisopropylphenyl)imidazolin-2-ylidene](pyrid-
ine)palladium(II) Dichloride (8): Nitrogen was passed (2 min)
through a suspension of bis(2,6-diisopropylphenyl)imidazolinium
chloride (0.303 g, 0.71 mmol), K2CO3 (0.496 g, 3.55 mmol) and
PdCl2 (0.152 g, 0.85 mmol) in pyridine (5 mL). The suspension was
then heated at 80 °C for 15 h under vigorous stirring. The mixture
was cooled to room temperature and filtered through Celite, and
the collected solid washed with CH2Cl2 (ca. 20 mL). The filtrate
was evaporated to dryness, and the resulting residue was purified
by flash chromatography (SiO2; CH2Cl2/petroleum ether, 50:50) to
afford 8 as a yellow solid (270 mg, 58%); m.p. 227 °C. 1H NMR
3
(CDCl3, 300 MHz): δ = 8.54–8.47 (m, 2 H, o-NC5H5), 7.52 (tt, J
4
= 7.6 Hz, J = 1.6 Hz, 1 H, p-NC5H5), 7.44–7.37 (m, 2 H, ArH),
NC5H5), 7.38–7.33 (m, 2 H, m-NC5H5), 7.24–7.18 (m, 2H ArH), 7.32–7.27 (m, 4 H, ArH), 7.10–7.04 (m, 2 H, m-NC5H5), 4.06 (s, 4
3
3
3
2.52 (s, 18 H, CH3) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ =
H, CH2), 3.60 (qq, J = JЈ = 6.7 Hz, 4 H, CHMe2), 1.57 (d, J =
3
157.2 (NCN), 151.6 (arom. CH), 137.9 (arom. CH), 135.1 (arom. 6.7 Hz, 12 H, CHCH3), 1.27 (d, J = 6.7 Hz, 12 H, CHCH3) ppm.
Cq), 124.7 (arom. CH), 121.6 (arom. CH), 115.3 (arom. CH), 61.3
(CqCH3), 31.9 (CH3) ppm. C20H27Cl2N3Pd (486.77): calcd. C
49.35, H 5.59, N 8.63; found C 48.81, H 5.64, N 8.43.
13C{1H} NMR (CDCl3, 75 MHz): δ = 186.2 (NCN), 151.3 (arom.
CH), 147.6 (arom. Cq), 137.4 (arom. CH), 135.5 (arom. Cq), 129.5
(arom. CH), 125.5 (arom. CH), 124.0 (arom. CH), 58.9 (CH2), 28.8
(CHMe2) 26.9 (CH3) 24.3 (CH3) ppm. C32H43Cl2N3Pd (647,03):
calcd. C 59.40, H 6.70, N 6.49; found C 59.15, H 6.98, N 6.01.
trans-[1,3-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene](pyridine)-
palladium(II) Dichloride (6): Nitrogen was passed (2 min) through
a suspension of bis(2,4,6-trimethylphenyl)imidazolium chloride
(0.511 g, 1.50 mmol), K2CO3 (1.04 g, 7.50 mmol) and PdCl2
(0.320 g, 1.80 mmol) in pyridine (5 mL). The suspension was then
heated at 80 °C for 15 h under vigorous stirring. The mixture was
cooled to room temperature and filtered through Celite, and the
collected solid washed with CH2Cl2 (ca. 20 mL). The filtrate was
evaporated to dryness, and the resulting residue was purified by
flash chromatography (SiO2; CH2Cl2/petroleum ether, 50:50) to af-
General Procedure for Palladium-Catalysed Suzuki–Miyaura Cross-
Coupling Reactions:
A mixture of the palladium complex
(0.01 mmol), phenylboronic acid (0.183 g, 1.50 mmol) and base
(2 mmol) was suspended in an appropriate solvent (3 mL). After
the addition of p-tolyl chloride (0.126 g, 1 mmol), the mixture was
vigorously stirred at 80 °C for a given period of time. The hot mix-
ture was filtered through Celite. 1,4-Dimethoxybenzene (0.069 g,
0.5 mmol; internal standard) was then added to the filtrate. The
solvent was removed under reduced pressure, and the crude mixture
was analysed by 1H NMR spectroscopy. The yields were deter-
mined by comparing the intensity of the methyl signal of the prod-
1
ford 6 as a yellow solid (0.689 g, 82%); m.p. Ͼ 240 °C. H NMR
3
(CDCl3, 300 MHz): δ = 8.54–8.48 (m, 2 H, o-NC5H5), 7.54 (tt, J
= 7.6 Hz, 4J = 1.5 Hz, 1 H, p-NC5H5), 7.12–7.07 (m, 2 H, m-
NC5H5), 7.06 (s, 2 H, ArH), 7.05 (s, 4 H, ArH), 2.37 (s, 6 H, p- uct [δ(Me) = 2.41 ppm] with that of the internal reference [δ(Me)
CH3), 2.36 (s, 12 H, o-CH3) ppm. 13C{1H} NMR (CDCl3,
75 MHz): δ = 152.9 (NCN), 151.6 (arom. CH), 139.2 (arom. Cq),
137.5 (arom. CH), 136.4 (arom. Cq), 135.2 (arom. Cq), 129.3
= 3.78 ppm]. In some experiments the product was isolated chro-
matographically. The isolated yield turned out to be very close (de-
viation less than 5%) to that determined by using the internal refer-
(arom. CH), 124.2 (arom. CH), 124.0 (arom. CH), 21.3 (p-CH3) ence.
19.2 (o-CH3) ppm. C26H29Cl2N3Pd (560.85): calcd. C 55.68, H
Crystal Data for Benzimidazolium Salt 3b·MeOH: Crystals suitable
5.21, N 7.49.f; found C 55.68, H 5.51, N 7.23.
for X-ray diffraction were obtained by slow evaporation of a deu-
terated chloroform solution of the complex: C38H35ClN2O, M =
571.13, monoclinic, space group P21/c, a = 13.2006(3), b =
16.0741(4), c = 14.5915(4) Å, β = 102.141(3), V = 3026.89(13) Å3,
Z = 4, μ = 0.160 mm–1, F(000) = 1208. Crystals of the compound
were mounted on an Oxford Diffraction CCD Sapphire 3 Xcalibur
trans-[1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene](pyridine)-
palladium(II) Dichloride (7): Nitrogen was passed (2 min) through
a suspension of bis(2,6-diisopropylphenyl)imidazolium chloride
(0.301 g, 0.71 mmol), K2CO3 (0.493 g, 3.55 mmol) and PdCl2
(0.151 g, 0.85 mmol) in pyridine (5 mL). The suspension was then
heated at 80 °C for 15 h under vigorous stirring. The mixture was
cooled to room temperature and filtered through Celite, and the
diffractometer. Data collection with Mo-Kα radiation (λ
=
0.71073 Å) was carried out at 140 K. 23233 reflections were col-
Eur. J. Inorg. Chem. 2013, 2841–2848
2847 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
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Crystal Data for Complex 4b: Crystals suitable for X-ray diffraction
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CCDC-811568 (for 3b·MeOH) and -811477 (for 4b) contain the
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be obtained free of charge from The Cambridge Crystallographic
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Supporting Information (see footnote on the first page of this arti-
cle): 2D 1H–1H ROESY NMR spectra for 3a and 4b, base and
solvent screening for 4-methylbiphenyl synthesis with palladium
complex 4b, general procedure for the cross-coupling of (2,6-di-
methoxyphenyl)boronic acid with 4-chlorotoluene, ORTEP repre-
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Acknowledgments
This work was supported by the Ministère de l’Enseignement
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Received: January 22, 2013
Published Online: March 8, 2013
Eur. J. Inorg. Chem. 2013, 2841–2848
2848
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim