Selenium-containing N-heterocyclic carbenes and their first palladium(II) complexes: Synthesis, structure, and pendent alkyl chain length dependent catalytic activity for Suzuki-Miyaura coupling

Article abstract of DOI:10.1021/om400148c

Three new N-alkyl-N′-(2-ethyl-1-selenophenyl)imidazolium salts, L1-L3 (differing in the chain length of alkyl substituent of N), which are precursors to (Se, C_NHC) ligands and their N-heterocyclic carbene (NHC) complexes [Pd(L-HX)Cl₂] (1-3) [L = L1-L3, X = I/Br], have been synthesized and characterized by multinuclear NMR and HR-MS. For the synthesis of Pd complexes 1-3 from appropriate imidazolium salts (L1-L3), the route of the silver carbene transfer reaction has been adopted. Single-crystal structures of 1 and 2 have been established with X-ray diffraction. Thermally stable, moisture- and air-insensitive complexes 1-3 have been found to be efficient catalysts for Suzuki-Miyaura coupling reactions (yield up to 96% in 2 h at 80 C). Nanoparticles (NPs) (size of 80-85% particles ～2-5 nm) formed in the beginning of these reactions (i.e., when the temperature of the reaction mixture reaches 80 C) appear to be important for catalytic coupling, probably as dispensers of Pd(0), and contain Pd and Se. The Pd:Se is nearly 3:2, 4:5, and 1:1 in NPs formed from 1, 2, and 3, respectively. The catalytic activity of 3, having the longest alkyl chain containing (Se C_NHC) ligand among the three complexes, has been found to be higher relative to those of 1 and 2. The length of the alkyl chain present in the complex molecule probably controls the catalytic activity by influencing the dispersion of NPs (containing Pd and Se) generated in situ during the catalytic process. The two-phase test has indicated a homogeneous nature of catalytic process, which probably occurs via leaching of Pd(0) from these NPs. This is first example where Pd-Se NPs catalyze Suzuki coupling largely via a homogeneous process, which is probably combined with oxidative addition to Pd(0) at the surface. The effect of alkyl chain length on catalytic efficiency may be useful in designing ligands/Pd complexes that efficiently catalyze Suzuki-Miyaura coupling.

Full text of DOI:10.1021/om400148c

Article
pubs.acs.org/Organometallics
Selenium-Containing N‑Heterocyclic Carbenes and Their First
Palladium(II) Complexes: Synthesis, Structure, and Pendent Alkyl
Chain Length Dependent Catalytic Activity for Suzuki−Miyaura
Coupling
Kamal Nayan Sharma, Hemant Joshi, Alpesh K. Sharma, Om Prakash, and Ajai K. Singh*
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India
S
* Supporting Information
ABSTRACT: Three new N-alkyl-N′-(2-ethyl-1-selenophenyl)imidazolium salts,
L1−L3 (differing in the chain length of alkyl substituent of N), which are
precursors to (Se, CNHC) ligands and their N-heterocyclic carbene (NHC)
complexes [Pd(L-HX)Cl2] (1−3) [L = L1−L3, X = I/Br], have been synthesized
and characterized by multinuclear NMR and HR-MS. For the synthesis of Pd
complexes 1−3 from appropriate imidazolium salts (L1−L3), the route of the silver
carbene transfer reaction has been adopted. Single-crystal structures of 1 and 2 have
been established with X-ray diffraction. Thermally stable, moisture- and air-
insensitive complexes 1−3 have been found to be efficient catalysts for Suzuki−
Miyaura coupling reactions (yield up to 96% in 2 h at 80 °C). Nanoparticles (NPs)
(size of 80−85% particles ∼2−5 nm) formed in the beginning of these reactions
(i.e., when the temperature of the reaction mixture reaches 80 °C) appear to be
important for catalytic coupling, probably as dispensers of Pd(0), and contain Pd
and Se. The Pd:Se is nearly 3:2, 4:5, and 1:1 in NPs formed from 1, 2, and 3, respectively. The catalytic activity of 3, having the
longest alkyl chain containing (Se CNHC) ligand among the three complexes, has been found to be higher relative to those of 1
and 2. The length of the alkyl chain present in the complex molecule probably controls the catalytic activity by influencing the
dispersion of NPs (containing Pd and Se) generated in situ during the catalytic process. The two-phase test has indicated a
homogeneous nature of catalytic process, which probably occurs via leaching of Pd(0) from these NPs. This is first example
where Pd−Se NPs catalyze Suzuki coupling largely via a homogeneous process, which is probably combined with oxidative
addition to Pd(0) at the surface. The effect of alkyl chain length on catalytic efficiency may be useful in designing ligands/Pd
complexes that efficiently catalyze Suzuki−Miyaura coupling.
their better stability toward oxidation and the formation of
more stable metal complexes.16 Consequently NHCs have
made up a class of ligands able to challenge tertiary
phosphines13 used to design Pd complexes for cross-coupling
reactions for a long time. Recently chalcogen ligands have
emerged as alternatives to phosphines due to strong donor
properties and the air/moisture stability of their metal
complexes.17 Sulfur-containing NHCs in association with
palladium(II) precursors have been extensively studied in
catalysis,18 and in many research papers palladium complexes of
bidentate (S, CNHC) and (S, CNHC, S) pincer-type ligands have
been reported as efficient catalysts for cross-coupling reactions,
thus establishing the effectiveness of the sulfur−NHC
combination for these ligands.19 However, for catalysis of
Suzuki−Miyaura C−C coupling, use of palladium(II) com-
plexes of selenium-containing NHC ligands does not exist to
our knowledge. Thus it would be worthwhile to explore Pd(II)
complexes with a bidentate ligand framework constructed by a
INTRODUCTION
■
N-Heterocyclic carbenes (NHCs), which acquired importance
due to the pioneering work of Wanzlick,1 Lappert,2 and
Arduengo,3 have received intensive attention as ligands.4 They
constitute a class of compounds that have contributed
extensively to organometallic chemistry5 and homogeneous
catalysis.6 Their medicinal applications7 are also important.
Carbenes derived from five-membered heterocycles make up
the most common class of NHCs.3,8 However, their four-,9
six-,10 and seven-membered11 analogues have also been
reported. In organometallic chemistry and catalytic organic
transformations, NHCs have been found to bring stability to
the metal center and become ligands of spectacular choice in
place of costly and air-sensitive phosphines. The tunability of
NHCs has made a wide variety of these ligands feasible,12 and
thus their size is comparable to the range shown by
phosphines.13 Palladium(II)-monodentate NHC ligand com-
plexes have remarkable activities for palladium-catalyzed C−C
coupling reactions,14 and many research efforts have been
devoted to the synthesis of NHC-based polydentate ligands.15
NHCs are often used as alternatives to phosphines owing to
Received: February 22, 2013
© XXXX American Chemical Society
A
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combination of selenium and NHC for catalysis of Suzuki−
Miyaura C−C coupling. The catalytic activity of palladium
species having a dendritic macromolecular structure20,21
increases with the generation of dendrimers,21 and a significant
effect of alkyl chain length on catalytic efficiency for the
Suzuki−Miyaura coupling reaction has been recently reported
in the case of Pd(II) complexes of selenated Schiff bases.17a
The effect of the length of the pendent alkyl chain of an NHC
on catalytic activity of its Pd complexes has not been reported
so far and therefore is worth investigating. The NHCs linked to
a polymer or silica have been reported for catalyst designing.22
Thus the length of a pendent (noncoordinating) alkyl chain
present on a ligand framework of a palladium complex of a
selenium-containing NHC has been varied from C1 to C18 to
study its influence on catalytic properties for Suzuki−Miyaura
C−C cross-coupling reactions. Generally the mechanism23 of
Suzuki−Miyaura coupling with molecular complexes of
palladium(II) is believed to be through Pd(0), which may be
in nanoparticle (NP), colloid, and/or soluble form. The
catalytic process24 has been reported to be homogeneous,
heterogeneous, and a mixture of the two. However, in the case
of Pd(II) complexes of a chalcogenated ligand as a catalyst17a,d
mostly the formation of Pd- and chalcogen-containing NPs has
been observed. To understand how a catalytic process proceeds
via such NPs is a very interesting part of our research program.
In the present paper we report three new selenium-
containing imidazolium salts, differing in the alkyl chain length
(longest in L3) and counteranion present (see Scheme 1),
alkyl chain present on the ligand of the molecular complex
appears to influence the dispersion of these Pd−Se NPs, which
in turn affects the catalytic efficiency.
EXPERIMENTAL DETAILS
■
Physical Measurements. The C, H, and N analyses were carried
1
out with a Perkin-Elmer 2400 Series II C, H, N analyzer. The H,
13C{1H}, and 77Se{1H} NMR spectra were recorded on a Bruker
Spectrospin DPX-300 NMR spectrometer at 300.13, 75.47, and 57.24
MHz, respectively. IR spectra in the range 4000−400 cm−1 were
́
recorded on a Nicolet Protege 460 FT-IR spectrometer as KBr pellets.
High-resolution mass spectral (HR-MS) measurements were per-
formed with electron spray ionization (10 eV, 180 °C source
temperature) and using sodium formate as calibrant on a Bruker
MIcroTOF-Q II, taking samples in CH3CN. The diffraction data on
single crystals of 1 and 2 were collected on a Bruker AXS SMART
Apex CCD diffractometer using Mo Kα (0.71073 Å) radiations at
298(2) K. The software SADABS26 was used for absorption correction
(if needed) and SHELXTL for space group, structure determination,
and refinements.27 All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were included in idealized positions with
isotropic thermal parameters set at 1.2 times that of the carbon atom
to which they are attached. The least-squares refinement cycles on F2
were performed until the model converged. High-resolution TEM
characterization revealing particle sizes and shapes was performed with
a Philips Tecnai F20 operated at 200 kV. Samples for high-resolution
transmission electron microscopy (HR-TEM) were prepared by drying
a drop of dispersion of nanocrystals in cyclohexane or ethanol on a
carbon-coated copper grid. The elemental composition of NPs was
studied with a Carl ZEISS EVO5O scanning electron microscope
(SEM) and associated EDX system model QuanTax 200, which is
based on the SDD technology and provides an energy resolution of
127 eV at Mn Kα. The sample was mounted on a circular metallic
sample holder with a sticky carbon tape. The melting points
determined in an open capillary are reported as such. Yields refer to
isolated yields of compounds, which have a purity of ≥95%. Estimation
of palladium in NPs was carried out on an AA700 series flame atomic
absorption spectrometer (Lab India, New Delhi). Thermogravimetric
analysis (TGA) was carried out using a Perkin-Elmer system in a
flowing nitrogen atmosphere with a heating rate of 10 °C/min.
Chemicals and Reagents. Imidazole, diphenyl diselenide, sodium
borohydride, palladium(II) chloride, methyl iodide, 1-bromooctane,
and 1-bromooctadecane procured from Sigma-Aldrich (USA) were
used as received. 1-(2-Phenylselanylethyl)-1H-imidazole was prepared
by the method reported earlier.28 All the solvents were dried and
distilled before use by standard procedures.29 The common reagents
and chemicals available commercially within the country were used.
Synthesis of 1-Methyl-3-(2-ethyl-1-selenophenyl)-3H-imida-
zol-1-ium Iodide (L1). 1-(2-Phenylselanylethyl)-1H-imidazole
(0.251 g, 1.0 mmol) was taken in 10 mL of chloroform, and an
excess of methyl iodide (1.419 g, 10.0 mmol) was added to it. The
reaction mixture was stirred at room temperature for 1 day. After the
completion of the reaction, the solvent was evaporated off under
reduced pressure on a rotary evaporator to get L1 as an orange,
viscous oil.
Scheme 1. Synthesis of L1−L3 and Complexes 1−3
which are precursors of the selenium-containing NHC ligands.
The palladium(II) complexes have been prepared from these
imidazolium salts via typical silver carbene transfer reactions.25
The carbenic proton of an imidazolium salt is first
deprotonated by the basic ligand of the metal precursor,
Ag2O, and thereafter Ag is abstracted in favor of Pd. The
catalytic applications of these new palladium(II) complexes
(two of them are structurally characterized) in Suzuki−Miyaura
coupling reactions have been explored. The complexes act as
dispensers of Pd- and Se-containing nanoparticles and are
stable in solution as well as in solid state and probably carry out
catalytic coupling again by releasing Pd(0) (largely via a
homogeneous process, which is probably combined with
oxidative addition to Pd(0) at the surface). The length of the
1
Yield: 0.373 g, 95%. H NMR (CDCl3, 25 °C vs Me4Si): δ (ppm)
3.47 (t, 3JH−H = 6.59 Hz, 2H, H5), 3.99 (s, 3H, CH3), 4.63 (t, 3JH−H
=
6.6 Hz, 2H, H6), 7.27−7.28 (m, 3H, H2 and H1), 7.41 (s, 1H, H7),
7.49−7.52 (m, 2H, H3), 7.56 (s, 1H, H8), 9.74 (s, 1H, H9). 13C{1H}
NMR (CDCl3, 25 °C vs Me4Si): δ (ppm) 27.1 (C5), 36.9 (C10), 50.1
(C6), 122.6 (C8), 123.1 (C7), 127.5 (C4), 127.7 (C1), 129.4 (C2),
132.9 (C3), 136.4 (C9). 77Se{1H} NMR (CDCl3, 25 °C vs Me2Se): δ
(ppm) 279.0. HR-MS (CH3CN) [M − I]+ (m/z) = 267.0395;
calculated value for [C12H15N2Se]+ = 267.0395 (δ: 0.0 ppm). IR (KBr,
cm−1): 3081 (m; νC−H aromatic), 2944 (s; νC−H aliphatic), 1570 (m;
νCN aromatic), 1473 (m; νCC aromatic), 743 (m; νC−H aromatic).
Synthesis of 1-Octyl-3-(2-ethyl-1-selenophenyl)-3H-imida-
zol-1-ium Bromide (L2). 1-(2-Phenylselanyl-ethyl)-1H-imidazole
(1.255 g, 5.0 mmol) was placed in a Schlenk tube equipped with a
B
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Article
magnetic stirrer, and then 1-bromooctane (0.965 g, 5.0 mmol) was
added. The mixture was heated for 12 h at 85 °C under a N2
atmosphere and then allowed to cool to room temperature. The dark
orange oil so obtained was washed with dry CH3CN (2 × 40 mL) and
dried in vacuo.
1H, H6), 6.92 (s, 1H, H8), 7.09−7.27 (m, 4H, H1, 2, and 7), 7.66 (bs, 2H,
H3). 13C{1H} NMR (CDCl3, 25 °C vs Me4Si): δ (ppm) 14.1 (C17),
22.5 (C16), 26.5 (C15), 28.9 (C14), 29.0 (C13), 29.1 (C12), 30.7 (C5),
31.6 (C11), 50.9 (C10), 51.5 (C6), 122.5 (C8), 123.7 (C7), 128.1 (C4),
129.5 (C1), 130.0 (C2), 134.8 (C3). HR-MS (CH3CN) [M − Cl]+ (m/
z) = 505.0113; calculated value for [C19H28ClN2PdSe]+ = 505.0143
(δ: 6.0 ppm). IR (KBr, cm−1): 3048 (m; νC−H aromatic), 2922 (s;
νC−H aliphatic), 2854 (s; νC−H aliphatic), 1625 (m; νCN aromatic), 1468 (m;
νCC aromatic), 741 (m; νC−H aromatic).
1
Yield: 1.998 g, 90%. H NMR (CDCl3, 25 °C vs Me4Si): δ (ppm)
0.88 (t, 3JH−H = 6.9 Hz, 2H, H17), 1.26−1.33 (m, 10H, H12−16), 1.84−
3
3
1.89 (m, 2H, H11), 3.50 (t, JH−H = 5.1 Hz, 2H, H5), 4.22 (t, JH−H
=
3
6.9 Hz, 2H, H10), 4.68 (t, JH−H = 5.1 Hz, 2H, H6), 7.11 (s, 1H, H8),
7.27−7.30 (m, 3H, H2 and H1), 7.34 (s, 1H, H7), 7.49−7.50 (m, 2H,
H3), 10.45 (s, 1H, H9). 13C{1H} NMR (CDCl3, 25 °C vs Me4Si): δ
(ppm) 13.9 (C17), 22.4 (C16), 26.0 (C15), 27.2 (C5), 28.7 (C14), 28.8
(C13), 30.0 (C12), 31.5 (C11), 49.8 (C10), 49.9 (C6), 121.4 (C8), 122.7
(C7), 127.5 (C1), 127.7 (C4), 129.3 (C2), 132.8 (C3), 136.5 (C9).
77Se{1H} NMR (CDCl3, 25 °C vs Me2Se): δ (ppm) 276.1. HR-MS
3. Yield: 0.286 g, 84%. Anal. Calcd for C29H48Cl2N2PdSe: C, 51.15;
H, 7.10; N, 4.11. Found: C, 50.48; H, 6.78; N, 3.89. Mp: 215.0 °C. 1H
NMR (CDCl3, 25 °C vs Me4Si): δ (ppm) 0.88 (t, 3JH−H = 6.3 Hz, 3H,
H27), 1.26 (bs, 28H, H12−25), 1.78 (bs, 2H, H11), 3.47 (bs, 1H, H5),
4.05 (bs, 1H, H5), 4.41 (bs, 2H, H10), 4.83 (bs, 1H, H6), 5.16 (bs, 1H,
H6), 6.90 (s, 1H, H8), 7.12−7.27 (m, 4H, H1, 2, and 7), 7.62 (bs, 2H,
H3). 13C{1H} NMR (CDCl3, 25 °C vs Me4Si): δ (ppm) 14.1 (C27),
22.6 (C26), 26.6 (C25), 29.2 (C24), 29.3 (C23) 29.5 (C22), 29.6 (C21),
29.6 (b, C12−20), 30.9 (C5), 31.9 (C11), 51.9 (C10), 51.7 (C6), 122.5
(C8), 123.4 (C7), 128.4 (C4), 129.5 (C1), 130.0 (C2), 133.2 (C3). HR-
MS (CH3CN) [M − Cl]+ (m/z) = 645.1703; calculated value for
[C29H48ClN2PdSe]+ = 645.1711 (δ: 1.2 ppm). IR (KBr, cm−): 3041
(m; νC−H aromatic), 2916 (s; νC−H aliphatic), 2848 (s; νC−H aliphatic), 1566
(m; νCN aromatic), 1466 (m; νCC aromatic), 1225 (m; νC−N aromatic), 731
(m; νC−H aromatic).
(CH3CN) [M − Br]+ (m/z) = 365.1490; calculated value for
[C19H29N2Se]+ = 365.1491 (δ: 0.3 ppm). IR (KBr, cm−1): 3135, 3061
(m; νC−H aromatic), 2926, 2857 (s; νC−H aliphatic), 1566 (m; νCN aromatic),
1468 (m; νCC aromatic), 1262 (m; νC−N aliphatic), 741 (m; νC−H aromatic).
Synthesis of 1-Octadecyl-3-(2-ethyl-1-selenophenyl)-3H-
imidazolium Bromide (L3). 1-(2-Phenylselanylethyl)-1H-imidazole
(1.255 g, 5.0 mmol) and 1-bromooctadecane (1.665 g, 5.0 mmol)
were reacted as described for L2. The dark orange oil so obtained was
washed with dry CH3CN (2 × 40 mL) and dried in vacuo.
1
Yield: 2.628 g, 90%. H NMR (CDCl3, 25 °C vs Me4Si): δ (ppm)
Procedure for Suzuki−Miyaura Coupling Reaction. An oven-
dried flask was charged with aryl bromide (1.0 mmol), phenylboronic
acid (1.5 mmol), K2CO3 (2.0 mmol), DMF (3.0 mL), H2O (2.0 mL),
and complex 1, 2, or 3. The flask was fitted with a water condenser and
placed on an oil bath maintained at 80 °C. The reaction mixture was
stirred and the reaction monitored with TLC until maximum
conversion of aryl bromide to the product occurred. Thereafter the
mixture was extracted with diethyl ether (2 × 20 mL). The extract was
washed with water and dried over anhydrous Na2SO4. The solvent of
the extract was evaporated off on a rotary evaporator, and the residue
was purified by a column of silica gel using CHCl3−hexane as eluent.
0.89 (t, 3JH−H = 6.9 Hz, 3H, H27), 1.26−1.32 (m, 28H, H13−26), 1.82−
3
1.86 (m, 4H, H11 and H12), 3.50 (t, JH−H = 6.0 Hz, 2H, H5), 4.23 (t,
3
3JH−H = 7.5 Hz, 2H, H10), 4.68 (t, JH−H = 6.0 Hz, 2H, H6), 7.12 (s,
1H, H8), 7.28−7.30 (m, 3H, H2 and H1), 7.35 (s, 1H, H7), 7.48−7.52
(m, 2H, H3), 10.45 (s, 1H, H9). 13C{1H} NMR (CDCl3, 25 °C vs
Me4Si): δ (ppm) 14.0 (C27), 22.5 (C26), 26.1 (C25), 27.2 (C5), 28.8
(C24), 29.2 (C23), 29.2 (C22), 29.35 (C21), 29.43 (C20), 29.47 (C17−19),
29.51 (C13−16), 30.1 (C12), 31.7 (C11), 49.8 (C10), 50.0 (C6), 121.4
(C8), 122.8 (C7), 127.6 (C1), 127.7 (C4), 129.4 (C2), 132.9 (C3),
136.7 (C9). 77Se{1H} NMR (CDCl3, 25 °C vs Me2Se): δ (ppm) 275.7.
HR-MS (CH3CN) [M − Br]+ (m/z) = 505.3055; calculated value for
[C29H49N2Se]+ = 505.3057 (δ: 0.2 ppm). IR (KBr, cm−1): 3048 (m;
νC−H aromatic), 2922 (s; νC−H aliphatic), 2851 (s; νC−H aliphatic), 1565 (m;
νCN aromatic), 1468 (m; νCC aromatic), 1265 (m; νC−N aromatic), 732 (m;
νC−H aromatic).
General Procedure for the Synthesis of [Pd(Se, CNHC)Cl2]
Complexes (1−3). The imidazolium salt (L1/L2/L3) (0.196 g/0.222
g/0.292 g, 0.5 mmol) was dissolved in dry CH2Cl2 (50 mL), and solid
Ag2O (0.116 g, 0.5 mmol) added under nitrogen atmosphere. The
reaction mixture was stirred overnight in the dark at room temperature
and filtered through a Celite pad. The filtrate was concentrated to 20
mL on a rotary evaporator. Thereafter it was added to a suspension of
[Pd(CH3CN)2Cl2] (0.129 g,, 0.5 mmol) in CH3CN with continuous
stirring at room temperature in a N2 atmosphere. It was stirred further
for 8 h and filtered through a Celite pad. The filtrate was concentrated
on a rotary evaporator to about 5 mL and mixed with diethyl ether (15
mL) to obtain 1−3 as yellow solids, which were filtered and dried in
vacuo. The single crystals of 1 and 2 were grown by slow evaporation
of their solutions in CH3CN−Et2O mixtures (3:1).
1. Yield: 0.181 g, 82%. Anal. Calcd for C12H14Cl2N2PdSe: C, 32.57;
H, 3.19; N, 6.33. Found: C, 31.69; H, 2.81; N, 5.70. Mp: 190.0 °C. 1H
NMR (CD3CN, 25 °C): δ (ppm) 2.73 (bs, 1H, H5), 3.45 (bs, 1H,
H5), 3.96 (s, 3H, H10), 4.68 (bs, 2H, H6), 7.04 (s, 1H, H8), 7.18 (s,
1H, H7), 7.40−7.43 (m, 3H, H1 and 2), 7.58 (bs, 2H, H3). 13C{1H}
NMR (CD3CN, 25 °C): δ (ppm) 14.6 (C10), 29.5 (C5),, 50.4 (C6),
124.4 (C8 and 7), 128.2 (C4), 129.9 (C1), 131.9 (C2), 134.4 (C3). HR-
MS (CH3CN) [M − Cl]+ (m/z) = 406.9057; calculated value for
C12H14ClN2PdSe = 406.9045 (δ: −2.9 ppm). IR (KBr, cm−1): 3104
(m; νC−H aromatic), 1716 (m; νCN aromatic), 1471 (m; νCC aromatic), 743
(m; νC−H aromatic).
1
All coupling products were authenticated by H and 13C{1H} NMR
spectra.
Hg Poisoning Test. An excess of Hg (Hg:Pd, 400:1) was taken in
the reaction flask before the addition of reactants. Thereafter the
coupling reaction of 4-bromobenzaldehyde (1.0 mmol) with phenyl-
boronic acid (1.2 mmol) using 1, 2, or 3 (1.0 mol %) as catalyst was
carried out in the flask under optimum conditions. A 5−10%
conversion was observed after 5 h of reaction.
PPh3 Poisoning Test. To the coupling reaction of 4-
bromobenzaldehyde with phenylboronic acid, PPh3 (0.5 mol %) was
added under optimal conditions before addition of catalyst (1.0 mol
%). After 5 h of reaction cross-coupled products were obtained in 5−
10% yield.
Two-Phase Test. The 4-bromobenzoic acid-immobilized (as
amide) silica (0.20 g) prepared by standard procedure30 (details in
the Supporting Information), phenylboronic acid (0.36 g, 3 mmol), 4-
bromoacetophenone (0.20 g, 1 mmol), and K2CO3 (0.56 g, 4 mmol)
were taken in a mixed solvent DMF (8 mL) + water (4 mL), and the
mixture was heated at 80 °C for 12 h. Thereafter it was cooled to room
temperature and filtered through a G-4 crucible. The residue was
washed with 20 mL of H2O followed by diethyl ether (50 mL). The
filtrate and washings were collected together and mixed with 50 mL of
water. The resulting mixture was extracted with diethyl ether (50 mL).
The solvent of the extract was evaporated off on a rotary evaporator,
and the residue subjected to 1H NMR. The solid residue was
hydrolyzed with KOH (1.68 g dissolved in 10 mL of EtOH + 5 mL of
H2O) at 90 °C for 3 days. The resulting solution was neutralized with
aqueous 20% (v/v) HCl and extracted with dichloromethane, followed
by ethyl acetate. The solvent of the combined extract was evaporated
1
off, and the resulting residue was analyzed with H NMR.
2. Yield: 0.227 g, 84%. Anal. Calcd for C19H28Cl2N2PdSe: C, 42.20;
H, 5.22; N, 5.18. Found: C, 41.59; H, 4.46; N, 4.69. Mp: 210.0 °C. 1H
NMR (CDCl3, 25 °C vs Me4Si): δ (ppm) 0.88 (t, 3JH−H = 6.6 Hz, 3H,
H17), 1.19 (bs, 10H, H12−16), 1.81 (bs, 2H, H11 and 12), 3.71 (bs, 1H,
H5), 4.02 (bs, 1H, H5), 4.37 (bs, 2H, H10), 4.87 (bs, 1H, H6), 5.17 (bs,
Isolation of NPs Generated from Complexes 1−3 during
Suzuki−Miyaura Coupling. A mixture of Pd(II) complex 1/2/3
(0.5 mmol), phenylboronic acid (1.5 mmol), 4-bromobenzaldehyde
(1.0 mmol), and K2CO3 (2.0 mmol) in DMF (4.0 mL) and water (4.0
mL) was heated at 100 °C for 2 h and then cooled to room
C
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temperature. The solvent was decanted, and the black residue (NPs)
was washed with a water−acetone mixture (1:3) and dried in vacuo.
The NPs were characterized by SEM-EDX, HR-TEM, and TEM-EDX.
Procedure for Suzuki−Miyaura Coupling Catalyzed by NPs
Obtained from 1−3. The coupling reactions of 4-bromobenzalde-
hyde, 4-bromotoluene, and 4-bromoanisole in the presence of 5 mg of
NPs obtained from 1 and 3 (Pd mol %: 2.98 and 2.13, respectively)
were carried out under conditions optimized for complexes 1−3. All
1
products were authenticated by H and 13C{1H} NMR spectra.
RESULTS AND DISCUSSION
■
The preparations of carbene ligand precursors (L1, L2, and L3)
and palladium(II) complexes (1, 2, and 3) are shown in
Scheme 1. 1, 2, and 3 are the first examples of Pd(II) complexes
of selenium-containing carbene. They were synthesized by
silver carbene transfer reaction in which L1, L2, or L3 was first
treated in CH2Cl2 with Ag2O in a N2 atmosphere at room
temperature and in the dark, followed by addition to the
resulting solution of silver complex, a suspension of [Pd-
(CH3CN)2Cl2] in CH3CN. The yellow solids 1, 2, and 3 were
found to be stable and thermally robust. The ligands L1, L2,
and L3 were found to be soluble in common organic solvents.
The complex 1 shows good solubility in DMF, DMSO, and
CH3CN. It has been found sparingly soluble in CH2Cl2 and
CHCl3, and in CH3OH, diethyl ether, THF, and hexane
negligibly soluble. On the other hand complexes 2 and 3 have
been found sparingly soluble in DMF, DMSO, CH3CN,
CH2Cl2, CHCl3, and CH3OH and insoluble in diethyl ether,
hexane, and CH3OH.
Figure 1. Thermal ellipsoid diagram of 1 at the 30% probability level.
Bond lengths (Å): Pd(1)−Se(1) 2.3694(7), Pd(1)−C(9) 1.945(3),
Pd(1)−Cl(1) 2.3512(10), Pd(1)−Cl(2) 2.3156(11). Bond angles
(deg): Cl(1)−Pd(1)−Se(1) 86.18(3), Cl(2)−Pd(1)−Se(1)
170.50(3), C(9)−Pd(1)−Se(1) 89.30(10), C(9)−Pd(1)−Cl(1)
171.31(11), C(9)−Pd(1)−Cl(2) 91.35(10), Cl(2)−Pd(1)−Cl(1)
94.34(4).
The air-stable imidazolium salts (L1, L2, and L3) and Pd
complexes 1−3 were characterized with 1H, 13C{1H}, and
77Se{1H} NMR, HR-MS, and IR spectra. The 77Se{1H} NMR
spectra of L1, L2, and L3 are given in Figures S19−S21 of the
Supporting Information. A single signal was observed at 279.0,
276.1, and 275.7 ppm respectively in the 77Se{1H} NMR
spectra of L1, L2, and L3. The H and 13C{1H} NMR spectra
1
Figure 2. Thermal ellipsoid diagram of complex 2 at the 30%
probability level. Bond lengths (Å): Pd(1)−Se(1) 2.3909(15), Pd(1)−
C(9) 1.957(10), Pd(1)−Cl(1) 2.395(3), Pd(1)−Cl(2) 2.378(3).
Bond angles (deg): Cl(1)−Pd(1)−Se(1) 86.48(7), Cl(2)−Pd(1)−
Se(1) 173.61(8), C(9)−Pd(1)−Se(1) 87.9(3), C(9)−Pd(1)−Cl(1)
167.8(3), C(9)−Pd(1)−Cl(2) 91.5(3), Cl(2)−Pd(1)−Cl(1)
95.30(9).
(see Supporting Information, Figures S7−S18) of L1−L3 and
their complexes 1−3 have been found to be consistent with
their molecular structures (Scheme 1). In the 1H NMR spectra
protons attached to C-9 of L1, L2, and L3 have been observed
at 9.74, 10.45, and 10.46 ppm, respectively, and are in the range
reported for the related imidazolium halides (δ 9.0−12.0),31
1
suggesting the high acidity of these protons. In the H NMR
spectra of 1, 2, and 3 a signal around 9.0−11.0 ppm was found
to be absent, indicating the deprotonation of C-9 and hence the
coordination of NHC with Pd(II).
ring, and the imidazole ring is twisted, making an angle with the
coordination plane of the metal. The twisting causes inefficient
overlapping of metal dπ orbital with the pz orbital of the
carbenic atom and is consistent with the negligible π-accepting
property of NHC ligand.
In the mass spectrum of imidazolium salts L1, L2, and L3 the
peak appearing at 267.0395, 365.1491, and 505.3055,
respectively, may be ascribed to imidazolium cations (see
Supporting Information, Figures S1−S3). The mass spectra of
complexes 1, 2, and 3 show peaks at 406.9057, 505.0113, and
645.1703, which may be ascribed to the [M − Cl]+ cationic
fragment of the corresponding complex (see Supporting
Information, Figures S4−S6).
Crystal Structures. The crystal structures of complexes 1
and 2, the first examples of Pd(II) complexes of selenium-
containing carbene, have been solved. Their crystal and
refinement data are given in Table S1 of the Supporting
Information. The ORTEP diagrams of 1 and 2 are shown in
Figures 1 and 2 with selected bond lengths and angles (more
values are given in Table S2 of the Supporting Information). In
each of the two structures, the palladium center adopts a nearly
square planar geometry. The ligands are coordinated with Pd in
a bidentate (Se, CNHC) mode, forming a six-membered chelate
The Pd−Se bond lengths of complexes 1 and 2 are
2.3694(11) and 2.3909(15) Å, respectively, consistent with
the earlier reported values32 2.381(7)−2.385(5) Å for species
[PdCl2(Se, N)ligand] and [PdCl(N, Se, N)ligand]+, in which
also the six-membered chelate ring is present. The Pd−C bond
lengths of complexes 1 and 2 are 1.945(3) and 1.957(10) Å,
respectively, consistent with reported values of 1.978(4)−
1.989(4) Å for [PdCl2(S, CNHC)ligand] complexes.19a The Pd−
Cl bond lengths [2.3156(11)−2.315(3) Å] of 1 and 2 are
normal.19a In the crystals of 1 and 2, due to secondary
interactions (see Table S3 of the Supporting Information (SI)
for interatomic distances), several interesting supramolecular
structural patterns (Figures 3−5) have been observed. The
inter/intramolecular Cl···H−C [2.733(11)−2.856(2) Å] and
D
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Figure 3. Inter/intramolecular C−H···Cl and C−H···π interactions in complex 1.
Figure 4. Inter/intramolecular C−H···Cl interactions in complex 2.
C−H···π [2.733(1)−3.744(1) Å] interactions result in chain
structures for complexes 1 and 2.
reaction. Therefore, it is possible that 1−3 are precatalysts and
during catalytic coupling reactions black particles that form are
Pd-containing species, which have a role in the catalytic
process. This implies that variations observed in catalytic
efficiency with the alkyl chain length may arise due to some
difference in these species generated in situ in the course of
catalysis with 1−3. Thus the black precipitates obtained from 1,
2, and 3 in the course of catalysis of the coupling reaction of 4-
bromobenzaldehyde with PhB(OH)2 under optimum reaction
conditions have been isolated (details in the Experimental
Section) and analyzed in detail so that a possible correlation
among the length of the alkyl chain present on the ligand’s
framework, catalytic activity, and nature of the precipitate
(possibly true catalysts or having a role in the catalysis) can be
understood. The SEM-EDX (see SI; Figures S22−S24), TEM-
EDX (see SI; Figures S25−S27), and HR-TEM (Figures 6−8)
of the black precipitate suggest that it is made of nanosized
particles of spherical shape. These nanosized species contain Pd
Application in the Suzuki−Miyaura Coupling Reac-
tion. The catalytic potential of 1−3 for the Suzuki−Miyaura
coupling reaction has been investigated. The results given in
Table 1 suggest that the complexes 1−3 are efficient for
Suzuki−Miyaura coupling, as good yields were obtained even
with deactivated substrates such as 4-bromoanisole. With many
substrates a reaction time of 2 h was sufficient for good
conversion. The catalytic efficiency of 3 has been found to be
highest among 1−3. Complex 1 shows a significantly lower
efficiency than those of 2 and 3. As the pendent alkyl chain
length is the longest in the case of complex 3, the observed
difference in efficiency may be ascribed to its presence fully or
partly, as such an observation has been made in the case of
catalysis of Suzuki coupling by palladium(II) complexes of
Schiff bases.17a In the course of Suzuki reactions catalyzed with
1−3, black particles appear immediately at the start of the
E
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Figure 5. Inter/intramolecular C−H···π interactions in complex 2.
Table 1. Suzuki−Miyaura Coupling Reactions Catalyzed by 1−3
Figure 6. HR-TEM micrographs of NPs (50 nm scale bar) generated
from complexes 1, 2, and 3 during Suzuki−Miyaura coupling.
Figure 7. HR-TEM micrographs of NPs (20 nm scale bar) generated
from complexes 1, 2, and 3 during Suzuki−Miyaura coupling.
F
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Figure 8. Size distribution of NPs generated from complexes 1, 2, and 3 during Suzuki−Miyaura coupling.
and Se. The Pd:Se ratio on the basis of EDX is nearly 3:2, 4:5,
and 1:1 in NPs formed from 1, 2, and 3, respectively. They are
stable in solution as well as in the solid state. The stability is
imparted by Se and may be contributed by associated organic
fragments (see below). The size of 80−85% of these NPs
obtained from 1−3 is between 2 and 5 nm (see Figures 6−8).
The Pd, C, H, and N in these nanosized species were
determined (Pd by flame AAS). The results are given in Table
poisoning tests the possibility of involvement of surface Pd
atoms of nanoparticles in oxidative addition to form soluble
Pd(II) intermediate Ar−Pd−Br and driving the catalysis
homogeneously thereafter also exists.34 This test (called a
three-phase test when the catalyst is a solid phase), developed
by Rebek and co-workers,33 is considered to be more definitive
for the presence of metal species that are catalytically active
homogeneously. The test involves covalent immobilization of
one of the reaction partners; for example, 4-bromobenzoic acid
has been immobilized on silica in the present case. If the
catalyst behaves in a heterogeneous fashion, the immobilized
substrate is not expected to be converted to a coupled product.
When Pd is released (i.e., catalysis is homogeneous), the
anchored substrate can be converted to product. The addition
of a soluble aryl halide to the reaction mixture ensures the
presence of a catalytic process and its real active species. The
present two-phase test made with an immobilized aryl bromide
is shown in Scheme 3. 4-Bromoacetophenone and immobilized
Table 2. Suzuki−Miyaura Coupling Catalyzed with NP1 and
NP3
Scheme 2. Suzuki−Miyaura Coupling
Table 3. Data Analyses of Isolated NPs
Scheme 3. Two-Phase Test for Suzuki−Miyaura Coupling
analyses (%)
Pd (%
wt)
weight (%) loss in TGA
nanoparticle
C
H
N
(up to 200 °C)
NP1
NP2
NP3
63.6
54.9
45.2
4.26
6.36
7.55
1.36
1.39
1.36
2.06
1.80
1.82
9.98
14.52
16.73
3. The NPs were subjected to TGA, and weight loss up to 200
°C is also shown in Table 3. It appears that on the basis of
weight loss in TGA and C, H, and N analyses results that the
NPs are associated with organic matter (most likely the
dissociation fragment of ligands). As the percentage of C in
NPs follows the order NP3 > NP2> NP1 (obtained from 3, 2,
and 1, respectively), these fragments most probably have
pendent alkyl chain related species. To identify whether the
reaction is heterogeneous or homogeneous, Hg-poisoning17a
and triphenylphosphine17a tests were carried out. Both tests
suggest that the catalysis is not completely proceeding
heterogeneously via NPs containing Pd and Se. To answer
the heterogeneous vs homogeneous question, a two-phase
test33 has been carried out, as in the absence of clear-cut
4-bromobenzoic acid (as amide) have been reacted with
phenylboronic acid under optimum reaction conditions. The
soluble part has been separated by filtration and analyzed after
workup with 1H NMR. The yield of the cross-coupled product
(4-acetylbiphenyl) has been found to be ∼95%. The solid phase
has been hydrolyzed, and the resulting products after workup
1
have been analyzed with H NMR. Of the immobilized 4-
bromobenzoic acid (as amide), ∼95% has been converted to
the cross-coupled product (biphenyl-4-carboxylic acid). This
observation suggests that the catalytically active Pd atoms leach
out from the in situ generated NPs and are probably responsible
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for carrying out catalysis homogeneously. The leaching may
happen first and is followed by oxidative addition required for
coupling. The surface Pd atoms of nanoparticles undergo
oxidative addition to form soluble Pd(II) intermediate Ar−Pd−
Br, and the catalysis is driven thereafter (homogeneously). The
catalysis in the present case is thus a largely homogeneous
process and is probably combined with oxidative addition to
Pd(0) at the surface.
the alkyl chain present in the complex molecule probably
controls the catalytic activity by influencing the dispersion of
Pd- and Se-containing NPs generated in situ during the catalytic
process. The two-phase test has indicated the homogeneous
nature of the catalytic process, which probably occurs via
leaching of Pd(0) from these NPs. This is the first example
where Pd−Se NPs are catalyzing Suzuki coupling largely via a
homogeneous process and is probably combined with oxidative
addition at the surface. These results may pave the way to
design ligands/Pd complexes that catalyze Suzuki coupling
more efficiently.
The nanoparticles, formed in the case of 3, are much more
uniformly dispersed than those from 1 and 2 (see Figures
6−8). The −CH3 group is present on the ligand framework of
1, and the linear C18H37 group on that of 3. The large size of
the alkyl chain in the case of 3 appears to have made the
nanoparticles more uniformly dispersed (Figure 7), resulting in
its higher catalytic activity. The carbon analyses results
discussed above suggest that ligand fragments associated with
NPs appear to have the effect of the alkyl chain (full or in part)
of the original ligand. This corroborates with the effect of the
alkyl chain on the characteristics of NPs. The influence of alkyl
chain length on the catalytic activity and nanoparticle
dispersion both suggest that the real catalysts are NPs or
originating from them. This is shown independently by the
catalytic activity of nanoparticles for Suzuki coupling (Table 2).
The isolated NPs appear to be somewhat deactivated in
comparison to those generated in situ, as their amount needed
(>2.13 mol % of Pd) for comparable conversions is more than
those of 1 and 3. Some Pd may be leached in the reaction
mixture before isolation. This may result in lowering of
reactivity, which may be further reduced due to some
aggregation in the course of isolation. However the isolated
NPs are not recyclable. This is consistent with the inference
made above that the coupling reactions catalyzed with 1−3 are
largely homogeneous and probably combined with oxidative
addition at the surface. Thus 1−3 are dispensers of NPs, which
further generate probably homogeneous real Pd(0) catalysts.
The level of catalytic activity of 3 for coupling of aryl bromides
as substrates is generally good. The time required for activated
aryl bromides is short, while a relatively longer time is required
for deactivated aryl bromides. The effect of chain length of
selenated Schiff bases ligated with Pd(II) on catalytic efficiency
in Suzuki−Miyaura coupling reactions has been recently
reported by our research group,17a but similar effect of alkyl
chain length of N in the case of NHC has been observed for the
first time. Such an observation is important in the context of
designing efficient catalysts for Suzuki coupling.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystal and refinement data, bond lengths and angles, NMR
and mass spectra TGA, and CIFs of 1 (CCDC no. 923390) and
2 (CCDC no. 923391). This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com.
Fax: +91 11 26581102. Tel: +91 11 26591379.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.K.S. thanks Council of Scientific and Industrial Research
(CSIR) India [project 01(2421)10/EMR-II] and Department
of Science and Technology India [Project no. SR/S1/IC-40/
2010] for financial support. H.J. and O.P. thank the University
Grants Commission (UGC) for SRF. K.N.S. and A.S. thank
CSIR (India) for SRF and JRF, respectively.
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dx.doi.org/10.1021/om400148c | Organometallics XXXX, XXX, XXX−XXX