Synthesis and catalytic properties of a heterocyclic-carbene complex of palladium

Article abstract of DOI:10.3184/174751911X12983924042406

The palladium N-heterocyclic carbene complex trans-[PdCl ₂{1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene}(NC ₅H₅)] has been prepared and its X-ray molecular structure is reported. The complex is highly catalytically active in the cross-coupling reaction of phenylhalides with phenylthiol to afford diphenyl sulfide and is efficient for the coupling of chlorobenzene.

Full text of DOI:10.3184/174751911X12983924042406

JOURNAL OF CHEMICAL RESEARCH 2011
MARCH, 161–162
RESEARCH PAPER 161
Synthesis and catalytic properties of a heterocyclic–carbene complex of
palladium
a
a
b
b
b
Yanhui Shi *, Zhengyuan Cai , Yu Peng , Zhan Shi and Guangsheng Pang
a
College of Chemistry and Chemical Engineering and Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials,
Xuzhou Normal University, Xuzhou, Jiangsu 221116, P. R. China
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, P. R. China
ThepalladiumN-heterocycliccarbenecomplextrans-[PdCl {1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene}(NC H )]
2
5
5
has been prepared and its X-ray molecular structure is reported. The complex is highly catalytically active in the
cross-coupling reaction of phenylhalides with phenylthiol to afford diphenyl sulfide and is efficient for the coupling
of chlorobenzene.
Keywords: NHC carbene, palladium complex, X-ray structure, C–S coupling
Palladium-catalysed cross coupling has become the principal
method for forming aromatic carbon–heteroatom bonds. The
The Pd–C (carbene) distance in 1 is 1.971(7) Å, shorter than
that shown by its saturated analogue 2 (1.993(7) Å).
1
8
reaction forming carbon–sulfur bonds has received less
attention, even though aryl thioethers are valuable synthetic
The molecular diagram of 1 is shown in Figure 2. The struc-
ture of the complex consists of a pseudo-square-planar mole-
cule with a palladium centre surrounded by imidazolylidene,
two chloro-ligands in a trans-configuration, and a pyridine
molecule. The Pd–N (pyridine) distance of 1 is 2.129(6) Å,
which is longer than that shown for its saturated analogue 2
2
intermediates frequently found in biologically- and pharma-
3
–5
ceutically-active molecules. Since the first isolation of a
stable, free N-heterocyclic carbene (NHC) by Arduengo in
6
1
991, NHCs have emerged as an extremely useful class of
8
ligands for transition-metal catalysis. NHC–palladium com-
(2.106(6) Å), indicating the strong trans-influence of the
7
,8
plexes have been successfully used in many processes. We
now report the synthesis and crystal structure of an NHC–
ligated palladium (NHC–Pd–Py) complex and test its catalytic
activity for the C–S coupling reaction.
unsaturated N-heterocyclic carbene ligand. All other distances
8
and angles lie in the expected range.
The synthesis of NHC–Pd–Py complex (1) was achieved by
refluxing 1,3-bis(2,6-diisopropylphenyl)imidazolinium chlo-
ride with palladium chloride in presence of K CO in pyridine.
2
3
This complex was characterised by NMR spectroscopy and
gave satisfactory elemental analyses. The complex is air and
moisture stable and can be stored in air in the solid state for
more than 6 months without any noticeable decomposition.
1
The H NMR complex of complex 1 shows two distinctive
signals for the eight CH groups, confirming that the molecule
3
has binary symmetry. However, either the isopropyl groups or
the phenyl rings cannot rotate freely, so there must be steric
1
3
hindrance around the Pd centre. C NMR provides direct
evidence of the metalation of the ligand, as seen by the signal
at 152.4 ppm, which is assigned to the Pd–Ccarbene resonance
shifted upfield relative to that of the imidazolium NCHC peak
1
1
of the starting ligand precursor (160.0 ppm). The resonance
of carbene carbon in 1 is significantly upfield compared to
that in its analogue with a C4–C5 saturated imidazole ring
namely, trans-[PdCl2{1,3-bis(2,6-diisopropylphenyl)imidazol
8
in-2-ylidene}(NC H )] (2) (184.2 ppm), which means that the
5
5
carbene in unsaturated compound 1 is more electron rich than
that in its saturated analogue 2 (Figure 1). The more electron
rich carbene should lead to the shorter Pd–C (carbene) dis-
tance, which was confirmed by an X-ray diffraction study.
Fig. 2 ORTEP structure of complex 1 with the probability
ellipsoids drawn at the 30% level.
(Hydrogen atoms have been omitted for clarity).
Fig. 1 Complexes of the C4–C5 unsaturated and saturated imidazole-based N-heterocyclic carbenes.
Correspondent. E-mail: yhshi_2000@126.com
*

1
62 JOURNAL OF CHEMICAL RESEARCH 2011
Scheme 1
The crystal is monoclinic, space group P2 /C with a = 1.491
which time the mixture was filtered through Celite and washed with
CH Cl . The solvent was removed under vacuum and the crude prod-
1
7
(3) nm, b = 1.714 9(3) nm, c = 1.490 5(3) nm, β = 102.20(3)°,
2
2
3
3
uct was washed with diethyl ether (15 mL). The pure compound was
obtained as a yellow solid by recrystallization from CH Cl /ether in
1% yield (1.04 g). The single crystal for the X-ray study was obtained
V = 3.726 89(13) nm , Dc = 1.314 g/cm , Mr = 737.11, Z = 4,
1
2
2
2
F(000) = 1536, R = 0.0485, R = 0.1225. Selected bond
lengths (Å) and angels (deg): Pd(1)-C(1) 1.971(7), Pd(1)–N(3)
8
by diffusion of toluene into saturated solution of the complex in
CH Cl .
2
.129(6), Pd(1)–Cl(1) 2.295(2), Pd(1)–Cl(2) 2.304(2), C(1)–
Pd(1)–N(3) 175.5(3), Cl(1)–Pd(1)–Cl(2) 179.29(8), N(3)–
Pd(1)–Cl(1) 89.69(18), C(1)–Pd(1)–Cl(2) 92.3(2).
2
2
1
HNMR (400MHz, CDCl ) δ: 8.56 (d, J = 5.2 Hz, 2H, o-NC H ),
3
5
5
7
.56–7.47 (m, 1H, p-NC H and 2H, p-C H ), 7.35 (d, J = 7.6 Hz, 4H,
5 5 6 3
The complex has been tested in the cross-coupling reaction
of phenyl halides (iodide, bromide, chloride) with phenylthiol
to afford the thioether (Scheme 1 and Table 1). It was found
that 1 was highly catalytically active in this C–S reaction, the
coupling reaction with chlorobenzene is also very efficient
m-C H ), 7.13–7.08 (m, 4H, 2H, m-NC H and 2H, CH), 3.16 (sept,
6
3
5
5
J = 6.4 Hz, 4H, CH(CH
(d, J = 6.8 Hz, 12H, CH
(
) ), 1.49 (d, J = 6.4 Hz, 12H, CH
3 2 3
13
), 1.12
). C NMR (100MHz, DMSO) δ: 152.4
NCN-Pd), 150.5, 146.2, 138.4, 135.0, 129.7, 126.3, 124.4, 123.5,
3
2
8.1, 25.8, 22.9. Anal. Found (Calcd for C H Cl N Pd): C 59.64
32 41 2 3
(
59.59), H 6.55 (6.41), N 6.38 (6.51)%.
(isolated yield: 91%).
Procedure for the C–S coupling reaction
Experimental
An oven-dried 4 mL vial containing a stirrer bar was charged with an
aryl halides (0.5 mmol), 1 (2 mol %) and KO Bu (84.2 mg, 0.75 mmol)
in a glove box and sealed with a cap containing a PTFE septum. After
taking the vial out of glove box, toluene (1 mL) and thiols (0.60 mmol)
were injected sequentially. The mixture was stirred at 100 °C for 1–36
h. The crude product was purified by column chromatography on
t
Imidazolium salt was synthesised according to the literature proce-
9,10
dure. All operations were performed under an inert atmosphere of
argon using standard Schlenk-line or glovebox techniques. Toluene
was distilled from sodium benzophenone ketyl under argon. Pyridine
was distilled from calcium hydride under argon. All other chemicals
were commercially available and used as received unless otherwise
silica gel with hexane giving diphenyl sulfide as a colourless liquid.
1
1
13
Diphenyl sulfide: H NMR (400MHz, CDCl ) δ: 7.21–7.35 (m,
stated. H and C NMR spectra were recorded on a Bruker AV400
3
13
1
0H); C NMR (100MHz, CDCl ) δ: 135.8, 131.0, 129.1, 127.0.
3
+
spectrometer, using CDCl or DMSO as solvents. GC-MS was per-
formed on an Agilent 6890-5973N system with electron ionization
3
EIMS m/z: 186 (M ).
(EI) mass spectrometry. Elemental analyses were performed on a
EuroVektor Euro EA-300 elemental analyser. X-ray crystallography
was conducted with a Bruker P4 CCD diffractometer using graphite-
momnochromated Mo Kα radiation (λ = 0.71073 Å). The structure
was solved by direct methods and was refined using the SHELXTL
We are grateful to the foundation of Xuzhou Normal
University (KY2008103 and KY2008104), Qing Lan Project
(
08QLT001 and 08QLD006) of Jiangsu Education Committee,
SRF for ROCS, SEM NSFC (21071121) and the State Key
Laboratory of Inorganic Synthesis and Preparative Chemistry
at Jilin University (2009–06) for financial support.
6
.1 software package. The linear absorption coefficient was 0.671,
and the range for collecting data (h, k, l and θ), –19≤ h ≤16, –17≤ k
22, –19≤ l ≤19, 3.04≤ θ ≤27.47. The total number of reflections that
were collected were 25350, in which the independent reflections were
519 and the number used in the structure determination were 7033.
≤
8
Received 5 December 2010; accepted 1 February 2011
Paper 1000463 doi: 10.3184/174751911X12983924042406
Published online: 23 March 2011
Synthesis of palladium complex
To a mixture of 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride
(
2
(
0.854 g, 2.0 mmol), PdCl (0.390 g, 2.2 mmol), and K CO (2.764 g,
0 mmol) in a 25 mL round bottom flask was added pyridine
10.5 mL). The reaction mixture was heated at 80 °C for 16 h, after
2
2
3
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