NCN-Pincer palladium complexes immobilized on MCM-41 molecular sieve: Application in Α-arylation reactions

Article abstract of DOI:10.1016/j.mcat.2018.10.024

Aromatic para-functionalized NCN pincer compounds tethered to a triethoxysilane moiety through a carbamate linkage were immobilized on ordered MCM-41 molecular sieve using a grafting process. The acquired immobilized organometallic pincer complexes were synthesized in high yields with no complex degradation and characterized by IR sprctroscopy, elemental content analysis. Nitrogen physisorption, XRD and TEM revealed that the mesoporous structure was retained during the immobilization process. The hybrid materials were applied as Lewis acid catalysts in the α-arylation reaction between aryl ketones and aryl halides, with the yield up to 95%. The tendency to form mono- or di-arylated products was investigated and the catalyst could be easily recovered and reused in five runs without a significant loss in its activity.

Full text of DOI:10.1016/j.mcat.2018.10.024

MolecularCatalysis462(2019)85–91
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
NCN-Pincer palladium complexes immobilized on MCM-41 molecular sieve:
Application in α-arylation reactions
Kai Wang, Qian Hua⁎, Liu Dabin, Ye Zhiwen
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, Jiangsu, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Aromatic para-functionalized NCN pincer compounds tethered to a triethoxysilane moiety through a carbamate
linkage were immobilized on ordered MCM-41 molecular sieve using a grafting process. The acquired im-
mobilized organometallic pincer complexes were synthesized in high yields with no complex degradation and
characterized by IR sprctroscopy, elemental content analysis. Nitrogen physisorption, XRD and TEM revealed
that the mesoporous structure was retained during the immobilization process. The hybrid materials were ap-
plied as Lewis acid catalysts in the α-arylation reaction between aryl ketones and aryl halides, with the yield up
to 95%. The tendency to form mono- or di-arylated products was investigated and the catalyst could be easily
recovered and reused in five runs without a significant loss in its activity.
α-arylation reaction
Immobilization
MCM-41
Pincer complexes
Catalyst recycling
1. Introduction
widely used in various carbon–carbon or carbon–hetero bond formation
and material science [9]. The functionalization of the main backbone of
Palladium catalyzed carbon–carbon bond formation reactions are
powerful tools in organic synthesis, providing mild methods for the
synthesis of valuable chemicals [1]. An important example of such re-
action is α-arylation reaction of aryl ketones with aryl halides. The first
practical researches of palladium-catalyzed α-arylation of carbonyl
compounds were reported independently by Hartwig [2], Buchwald
[3], and Miura [4]. Driven by their pioneering work, numerous alter-
native methods have emerged, and are now widely applied to synthe-
size pharmaceutical compounds [5]. A variety of ligands are tried in
these reactions, including phosphines [6], carbenes [7], modified fer-
rocenes phosphine-based pincer ligands [8], to broaden the substrates,
increase the reaction selectivity and provide high turnover frequency.
However, it is intricate to separate catalyst from the product solution,
which is an ineluctable aspect of homogeneous catalytic processes.
In view of economical and ecological respect, heterogeneous cata-
lysts possess obvious merits on account of the easy-separation after the
completion of reactions. Combination of high selectivity and activity of
homogeneous catalysts with easy-separation, catalyst recovery of het-
erogeneous catalysts would be expectable to reach an ideal catalyst
system.
the pincer compounds mainly took place on the para position of the
aromatic ring, and the ideal catalyst system is of great possibility to be
achieved by immobilizing complexes on an inorganic support ulteriorly
[10]. Various supports have already been attempted to immobilize
pincer-based metal catalysts, including hyperbranched polymers [11],
oligo(ethylene glycol) [12], dendronized polymers [13], carbosilane
dendrimers [14], cartwheel molecules [15], and polycationic den-
drimers [16], and the synthesized hybrid catalysts are further applied
and recycled from reaction products. Notably, in many cases it has been
observed that functionalization in the para position maintains the in-
tegrity of the pincer compounds both in structure and chemical re-
activity and only generates anchoring points.
The actual immobilization of a homogeneous catalyst on a hetero-
geneous support can be achieved mainly by two different approaches. A
transition metal is either complexed to ligands already chemically
bonded to a support, or a metal complex containing an appropriate
coupling agent is tethered to a support. In the first approach, the co-
ordination sphere around the metal center changes during im-
mobilization via various ligand exchange processes and the average
structural properties of this immobilized complex could be quite dif-
ferent from those of the homogeneous complex. In the second approach,
such changes may be minimal provided that the metal ion is strongly
complexed to the ligand and that this configuration remains intact
during grafting [17].
Pincer organometallic complex is depicted as a terdentate ligand
bound through a covalent CeM bond, and the metal center is clamped
by two hetero atom containing substituents via ortho-chelation. As a
subclass of cyclo-palladated compounds, pincer complex has been
⁎ Corresponding author.
E-mail address: iem_liu@163.com (H. Qian).
https://doi.org/10.1016/j.mcat.2018.10.024
Received 1 August 2018; Received in revised form 26 October 2018; Accepted 31 October 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

W. Kai et al.
MolecularCatalysis462(2019)85–91
Table 1
2.4. Grafting procedure
Comparison of previous methods and present work for the α-arylation reaction.
In a typical procedure [24], the complex 2 (49.8 mg, 0.1 mmol) was
dissolved in a CH2Cl2/MeOH mixture (1:1, 10 mL) and stirred over an
ion-exchange resin (DOWEX 50WX8) for 6 h. The resulting solution was
filtered and the resulting solid was dissolved in CH2Cl2 (20 mL) with
NEt3 (3 mL), DMAP (5 mg, 0.04 mmol), and 3-isocyanatopropyl-1-
MCM-41 (1 g) and was stirred at reflux for 24 h. The mixture was fil-
tered hot, then washed with EtOH, CH2Cl2, and pentane, and subse-
Entry
Catalyst system
Yield/%
1
2
3
4
5
6
7
[PdCl2(dppf)] [18]
[Ni{(iPr2Ph)2NHC}(PPh3)Cl2] [19]
(SIPr)Pd(Py)Cl2 [20]
[Pd(cin)Cl]2/DalPhos [21]
[t-BuPheBoxMe2]PdBr [22]
Bisamide(NCN)PdBr [23]
61
65
85
88
96
95
95
quently dried under vacuum. Brown powder. IR: νSiO-H 3520 cm−1
νcarbamate 1725 and 1550 cm−1,νN-H 3310 and 1441 cm−1
,
Immobilized bisamide(NCN)PdBr (present work)
.
For this study, we have used palladium complex of so-called ‘pincer’
ligands of the NCN type which can be subsequently immobilized onto a
MCM-41 support, and to study whether the heterogenized system
formed has retained its initial metal–ligand configuration. The im-
mobilized complexes were characterized by various techniques, such as
IR spectroscopy and elemental content analysis. The activity and re-
cyclability in catalyzing the α-arylation reaction between aryl ketones
and aryl bromides were also investigated. Table 1 illustrates several
reported literature for the α-arylation reactions. In most cases acet-
ophenone and its derivatives were used as substrate to afford good
yields of the arylated products, this study also took a further insight to
see the suitability of the catalyst system for aliphatic ketones and its
recyclability after the catalytic runs.
2.5. Protection of free silanol groups with trimethylsilyl groups
In a typical procedure, silica 3 (0.5 g) was treated with hexam-
ethyldisilazane (HMDS) (5 g, 31 mmol) in hexane (25 mL). The mixture
was heated at reflux for 24 h. The mixture was filtered, washed with
hexane (2 × 25 mL) and dried under vacuum to yield brown powder 5.
IR: νcarbamate 1750 and 1562 cm−1, νC-H 2901 cm−1, νN-H 3312 and
1450 cm−1
2.6. Synthesis of cationic organopalladium hybrid materials
In typical procedure, silica (0.5 g) and AgBF4 (9.7 mg,
.
a
3
0.05 mmol) were stirred in dry dichloromethane (10 mL) at room
temperature for 1 h in the absence of light, and the reaction mixture
was filtered through Celite eluting with dichloromethane. The filtrate
was evaporated to dryness to give the cationic organopalladium ma-
terial as a brown solid 4. IR: νcarbamate 1725 and 1550 cm−1, νC-H
2. Experimental section
2.1. General procedures
2893 cm−1, νN-H 3310 and 1451 cm−1
.
Synthetic procedures were conducted under a dry nitrogen atmo-
sphere using standard Schlenk techniques. All reagents were obtained
from commercial sources with reagent grade and were used without
further purification. The IR spectra were recorded on a Nicolet IS-10
spectrometer and expressed in cm−1 (KBr). XRD tests of the ordered
mesoporous silicas and hybrid material were carried out in Bruker D8
X-ray powder diffractometer (Bruker, Germany), using Cu Kα
(λ = 0.154 nm) radiation at a scanning rate of 5°/min with an angle
ranging from 0 to 10° of 2θ. Elemental analysis by ICP was performed
using a JICP-PS-1000 UV spectrophotometer (Teledyne Leeman Lab.,
U.S.A.) Specific surface area and pore size distribution of the hybrid
materials were measured by recording the nitrogen adsorption/deso-
rption isotherms at liquid N2 temperature (77 K), with the use of a
Micromeritics ASAP 2000 system. OH-functionalized NCN-pincer metal
complexes [PdBr(NCN−OH)] 2 were prepared as described previously
[23].
2.7. Catalysis and recycling
The catalytic experiments were carried out using potassium tert-
butoxide (1.5 mmol) as a base, functionalized silica materials (approx.
Pd content 1.0 mol%) as catalyst, and ketone (1.0 mmol), and aryl ha-
lide (1.2 mmol) as reagents in acetonitrile (10 mL) at reflux tempera-
ture. After cooling the reaction mixture, the reaction vessel was cen-
trifuged to settle the silica and the catalyst was separated from the
liquid product by decanting the supernatant carefully and then filtra-
tion. The filtrate was analyzed by gas chromatography to determine the
yield of the product. The recovered catalyst was washed twice with
CH2Cl2 and dried in vacuo. It was then used for the next run.
3. Results and discussion
3.1. Immobilization of NCN-Pincer palladium complexes on MCM-41
The immobilization process of NCN-Pincer Palladium Complexes on
MCM-41 was shown in Schemes 1 and 2, and the synthesis of [(OH-
NCN)PdBr] pincer complex was achieved according to our previous
work [23]. The conversion of amino-propyl groups bounding to MCM-
41 to active functionalities was successful and isocyanate-functiona-
lized material 1 was synthesized from alkyl amines with excess of tri-
phosgene in toluene. The reaction was found to be high-yielding with
byproduct that can easily be removed. The material 1 and an organo-
metallic fragment was successfully connected though a carbamate
linker, however, this reaction requires relatively extreme condition,
including sufficient stir in ion-exchange resin, large excess trimethyla-
mine, catalytic amount of N, N-dimethylamino pyridine, reflux tem-
perature, to ensure conversion of the isocyanate groups. To investigate
the effect of remained silanol groups on catalysis, the synthesized hy-
brid material 3 was further treated with hexamethyldisilazane in
hexane for 24 h at room temperature to obtain material 5, in which the
remaining silanol groups were capped by trimethylsilyl groups. Finally,
the material 3 and 5 were treated with AgBF4 in CH2Cl2 to afford
2.2. Synthesis of 3-aminopropyl-1-MCM-41
MCM-41 (10.00 g, calcined at 120 °C for 8 h and cooled to room
temperature) and 3-aminopropyl-1-(triethoxy)silane (9.29 g, 0.04 mol)
were placed in dry toluene (150 mL, 1.41 mol) and heated at reflux for
24 h. The mixture was filtered hot, washed with toluene, EtOH,
acetone, and CH2Cl2, and dried under vacuum to afford 10.1 g white
powder. IR: νSiO-H 3522 cm−1
.
2.3. Synthesis of isocyanate-functionalized MCM-41
3-aminopropyl-1-silica (5.00 g) was placed in a Schlenk with to-
luene (30 mL, 0.28 mol). Triphosgene (2.03 g, 6.8 mmol) was added,
and the resulting mixture was heated at reflux for 24 h. The mixture
was filtered hot, then washed with toluene, acetone, CH2Cl2, and
pentane, and subsequently dried under vacuum affording 4.88 g white
powder. IR: νCNO 2276 cm−1, νSiO-H 3522 cm−1
.
86

W. Kai et al.
MolecularCatalysis462(2019)85–91
Scheme 1. Synthesis of isocyanate-functionalized material 1: (i) 3-aminopropyl trimeth- oxysilane, toluene, reflux,24 h; (ii) triphosgene, toluene, reflux, 24 h.
cationic Pd-aquo complexes, which are believed to be the active species
in catalytic process.
3.2. Characterization
3.2.1. IR studies
FTIR experiments were performed to demonstrate the successful
loading of the pincer complex on isocyanate-functionalized MCM-41
(Fig. 1). The peak at 3425 cm−1 in FTIR spectrum of MCM-41 (Fig. 1a)
was characteristic of νSiO-H stretching vibrations. The bands at 1086 and
803 cm−1 could be attributed to the Si − O stretching vibrations, and
these peaks can also be observed in the FTIR spectrum of isocyanate-
functionalized MCM-41 sample (Fig. 1b), with a characteristic νNCO at
2276 cm-1, indicating the successful functionalization of MCM-41.
Comparison of the IR spectra of the isocyanate-functionalized MCM-41
(Fig. 1b), complex 2 (Fig. 1c) with those of hybrid materials 4 (Fig. 1d)
pointed to a strong decrease in the number of isocyanate groups in the
latter silicas, suggesting that not all isocyanate groups are tethered to a
pincer complex. In addition, the complex 2 displayed absorption bands
at 3450, 1649 and 1578 cm-1 that correspond to NeH stretching, C]O
stretching vibrations, and NeH bending vibrations of the amide groups,
respectively. These peaks were partial overlapped with those of iso-
cyanate-functionalized MCM-41 and the new formed characteristic
Fig. 1. FT-IR spectra of MCM-41 (a), isocyanate-functionalized MCM-41 (b),
stretches of carbamate group, which were observed at 1725 and 1550
cm-1. The features of this spectrum showed consentient results with
previous reports on carbamate-tethered silica species 25. The intensity
of the signal for isolated free silanol groups in 4 is decreased in the
spectrum of 6 (Fig. 1e), which is due to the substitution the TMS group
in 6.
complex 2 (c), hybrid material 4 (d) and hybrid material 6 (e).
hybrid materials and the MCM-41 indicated that the anchoring process
did not show significant influence on the mesoporous silica material.
The pattern of blank MCM-41 revealed three well-resolved peaks lo-
cated at 2.0, 3.7 and 4.5 degrees 2θ, that could be assigned to the (100),
(110) and (200) diffraction lines, respectively, associated with a hex-
agonal symmetry that is typical for MCM-41 [26]. Although small
variations between the d-spacing values of the (110) diffraction line of
3.2.2. XRD analysis
Fig. 2 shows the XRD patterns of MCM-41 and the corresponding
hybrid materials. The similar XRD patterns between the prepared
Scheme 2. Immobilization and activation of 2 on isocyanate-functionalized material: (i) CH2Cl2/ MeOH, then CH2Cl2/NEt3, DMAP, reflux, 24 h; (ii) AgBF4 in CH2Cl2;
(iii) HMDS, hexane, 24 h.
87

W. Kai et al.
MolecularCatalysis462(2019)85–91
catalytic process had little effect on the textural properties within five
runs.
3.2.5. TEM analysis
Fig. 4 shows two typical TEM images of the MCM-41 and the hybrid
material 6, which further confirmed the XRD results. Although it is
observed that few amount of channels in hybrid material partially
collapsed, in some degree, into disordered and wormhole-like packings
(Fig. 4b), the hexagonal periodicity of the mesophase of MCM-41 was
basically maintained after functionalization and immobilization pro-
cess, as displayed in the TEM images (Fig. 4a).
3.3. Catalysis
NCN-pincer palladium complexes are known to be excellent pre-
catalysts for the formation of Lewis acid catalysts for the α-arylation
reaction between aryl ketones and aryl halides [8,27]. In this study, we
generated the corresponding cationic-aquo complexes, which are be-
lieved to be the real active catalytic species, to investigate how these
hybrid materials work in the reaction. Table 3 shows the influences of
catalyst loadings on α-arylation coupling reaction of bromobenzene
with acetyl benzene over the two cationic-aquo complexes immobilized
on MCM-41. The model reaction was carried out at 81 °C for 2 h using
potassium tert-butoxide as base in acetonitrile solvent. The reaction
mediated by material 6 with 1.0 mol% Pd catalyst loadings occurred
with the highest yield (95%) (entry 5). Lower catalyst loadings (0.5 mol
%) offered a 90% yield with 85% monoarylated product after only 2 h
(enter 3). Finally, reducing the catalyst loading to 0.2 mol% delivered
an 83% yield after 6 h (entry 6).
Table 4 shows typical results of the reactions of bromobenzene with
various ketones over material 6. The reaction of propiophenone gave
monoarylated product in 89% yield (entry 1). Aryl ketones with an
electron donating group such as 4-methyl acetophenone, 4-methoxy-
acetophenone and 1-(4-dimethylamino-phenyl)-ethanone gave corre-
sponding cross-coupling products with high yields, although trace
amounts of diarylated coupling product were accompanied as a by-
product (entries 2–4). On the other hand, reaction of 4-fluorine acet-
ophenone and 1-(4-trifluoromethyl-phenyl)-ethanone having an elec-
tron-withdrawing group gave only 70% and 71% monoarylated yields,
respectively (entries 5 and 6). It was noteworthy that the coupling of
hindered ketones, such as 1-ferrocenylethanone and 1′-acetonaphthone,
could also give the desired mono-products in good yields (entries 7 and
8). When 3-pentanone was taken as a substrate, trace amount of dia-
rylated coupling product was obtained. However, when we change the
solvent to toluene, the reaction could afford a 77% diarylated yield
without any monoarylated product (entry 6). This unexpected result
caught our attention and we further increased the proportion of ketone/
bromobenzene to 1:1.5, the diarylated coupling product was conse-
quently enhanced to 80%. When the ratio was 1:2, the diarylated yield
was decreased to 55%, suggesting that the optimal proportion to obtain
diarylated product was 1:1.5. This is probably because the mono-pro-
ducts have a tendency to form a thermodynamic stable enolates in polar
solvent, while they tend to further transform into dynamic stable en-
olates in nonpolar solvent, which are the crucial intermediates to yield
the diarylated product.
Fig. 2. XRD patterns of MCM-41 (a), hybrid material 3 (b), hybrid material 6
(c) and hybrid material 6 after 5 runs (d).
the different samples were found, the XRD results demonstrated that
the hexagonal symmetry was retained for the functionalized material
during both the grafting treatment and after five runs of catalysis.
3.2.3. Elemental analysis
Carbon, hydrogen, oxygen and silicon analysis gives little informa-
tion because they are present in large excess with respect to the orga-
nometallic species (C as TMS capping groups, Si and O as polymeric
support). The grafting result of hybrid material 3 showed a marked
increase in nitrogen content, which is due to the introduction of a
pincer organometallic complex. Notably, large quantity of free iso-
cyanate groups still exist on the surface, suggesting that not all iso-
cyanate groups are tethered to a pincer complex. This can be deduced
via the disproportionate ratio of N/Pd, the nitrogen loading is expected
to be seven times as much as Pd content (Pd, 0.31%) if all isocyanate
groups had been substituted with carbamate groups during the im-
mobilization process. However, the observed N/Pd ratio was 22:1,
which was much larger than the expected proportion.
3.2.4. Nitrogen physisorption
The textural properties of MCM-41 and changes in surface proper-
ties at different stages of hybrid materials were further investigated
using N2 sorption analysis, and the result was shown in Fig. 3. The
isotherms of these four materials showed Type IV curves with a high
onset at low relative pressures originating from the intra-wall micro-
porosity, and hysteresis at higher values due to the ordered mesoporous
channels. The relative pressure region (p/p0 = 0.3-0.5) in which the N2
adsorption capacity increased sharply along with the emergence of
hysteresis ring is due to the condensation of nitrogen molecules in the
mesoporous pores. The nitrogen physisorption results indicated that the
textural properties of hybrid material 4 and 6, together with recycled
material after 5 runs of catalysis (Fig. 3b–d) were well retained.
The BET surface area and pore volume of the samples are given in
Table 2. Compared with the pure MCM-41, both materials 4 and 6
showed slight decrease in pore volumes, average pore size and specific
surface area, which indicates that the nature of the mesoporous silica
material was not significantly affected during the anchoring process of
organometallic moiety. The observed large pore volumes of hybrid
materials indicate adequate accessibility of catalyst site and further
ensure that the support is unlikely to influence much on the mass
transport of the catalytic reactions. The recycled material remained
nearly the same physisorption results with material 6, indicating the
3.4. Plausible mechanism
Based on the previous efforts [28], a plausible mechanism was
shown in Scheme 3 (eOH2 and eBF4 groups were omitted for clarity).
The first step is the oxidative addition of the aryl halide to Pd° species
generated from the precatalyst to form the organopalladium species.
Reaction with base and ketones gives the corresponding enolates, which
further affording the desired product via reductive elimination. The
availability of a vacant site on a species generated after oxidative ad-
dition could possibly facilitateβ-hydrogen elimination.
88

W. Kai et al.
MolecularCatalysis462(2019)85–91
Fig. 3. Nitrogen physisorption isotherms of MCM-41 (a), hybrid material 4 (b), hybrid material 6 (c) and hybrid material 6 after 5 runs (d).
3.5. Recycling
The recyclability of solid catalysts is a major problem for their
Table 2
Textural data of MCM-41 and hybrid materials samples.
[a]
[c]
Samples
Vmeso (cm3/g)
Pore size[b] (nm)
SBET (m2/g)
utilities. To investigate the reusable properties of catalyst, recycle ex-
periments were performed and the results were illustrated in Fig. 5.
After cooling the reaction mixture, the reaction vessel was centrifuged
to settle the silica and the catalyst was separated from the liquid pro-
duct by decanting the supernatant carefully and then filtration. The
recovered catalyst was washed twice with CH2Cl2, dried in vacuo, and
then used for the next run. It was found that the reactivity of the re-
cycled material was preserved at a high level within five runs, although
MCM-41
1.06
0.95
0.82
0.81
3.28
2.84
2.68
2.64
998
920
912
910
Hybrid material 4
Hybrid material 6
6 after 5 runs
[a] Vmeso=mesopore volume. [b] Average pore diameter by BJH. [c] SBET=BET
surface area.
Fig. 4. Nitrogen physisorption isotherms of MCM-41 (a), hybrid material 6 (b).
89

W. Kai et al.
MolecularCatalysis462(2019)85–91
Table 3
Optimization of catalyst species and loadings for the cross-coupling between
acetyl benzene and bromobenzene[a].
Entry Catalyst No. Pd loading (mol
%)
Yield[b] Percentage of mono-
product
1
2
3
4
5
6
None
—
0.5
0.5
1
1
0.2
0
0
4
6
4
6
6
86
90
93
95
83
78
85
86
88
77
[a] Reaction details: acetyl benzene (1.0 mmol), bromobenzene (1.2 mmol),
solvent (10 mL) at 81 °C for 2 h. [b] GC yield.
Table 4
α-arylation reaction of various ketones with bromobenzene over material 6[a].
Entry Ketones
Solvent
Yield[b]
mono-
di-product
product
Fig. 5. Recycling of catalysts in α-arylation of acetyl benzene and bromo-
benzene.
1
2
3
4
propiophenone
CH3CN
CH3CN
CH3CN
CH3CN
89
92
92
88
0
4
3
4-methyl acetophenone
4-methoxy acetophenone
1-(4-dimethylamino-phenyl)-
ethanone
4-fluorine acetophenone
1-(4-trifluoromethyl-phenyl)-
ethanone
1-ferrocenylethanone
1'-acetonaphthone
3-pentanone
designed experiments were carried out. It is possible that leaching is
produced by catalyst decomposition during catalytic process, so we
simply added one more equivalent substrate to the reaction mixture
without seperating the catalyst after the first run. The result showed no
significant decrease in reactivity, suggesting that the active palladium
sites did not change over the whole catalytic process. Other possible
reasons for catalyst deactivation were that small amout of palladium
sites were removed during the recycling peocedure or inseparable
species were formed. To test this possibility, ICP analysis was used for
the supernatants and washings of two runs, and the result showed that
the molar ratio of Si/Pd in the washings was 6.4 and 8.8. It is worth
noting that the theoretical value of Si/Pd is expected to be 1 for the
leached but intact catalyst [25], supposing the deactivation is only
caused by cleavage of SieOeSi bonds between silica and the organio-
metalic fragment. In other words, small and inseparable species/parti-
cles are most likely to be formed via surface reconstitution of molecular
sieve during recycling procedure. These results indicated that the sur-
face reconstitution should account for the main decease of the catalyst
activity during the recycling runs.
Trace
5
6
CH3CN
CH3CN
70
71
6
Trace
7
8
9
10
11[d]
12[e]
CH3CN
CH3CN
CH3CN
Toluene
Toluene
Toluene
57
55
93
—
—
—
Trace
Trace
Trace
77[c]
80[c]
55[c]
3-pentanone
3-pentanone
3-pentanone
[a] Reaction details: ketone (1.0 mmol), bromobenzene (1.2 mmol), solvent
(10 mL) at 81 °C for 2 h. [b] GC yield. [c] Based on bromobenzene. [d] 1.0 : 1.5
ratio of 3-pentanone: bromobenzene. [e] 1.0 : 2.0 ratio of 3-pentanone: bro-
mobenzene.
4. Conclusions
In summary, we have successfully synthesized hybrid materials
immobilized with aromatic para-functionalized NCN pincer compounds
tethered to a triethoxysilane moiety through a carbamate linkage. The
immobilized organometallic pincer complexes were synthesized in high
yields, both the inorganic support and organometallic moiety retained
their structure integrity during immobilization process. It was found
that hybrid material 6 showed a high catalytic activity in α-arylation
reaction of ketones, affording a 95% yield of product. The mono- and
di-arylated product could be selectively obtained through changing the
solvent and ratio of substrates. The anchoring of the active organo-
metallic catalyst on a stable support results in an excellent hetero-
geneous catalyst, which can be recycled and reused up to five times
without a significant loss of activity and nicely reach the purpose of
combining positive properties of homogeneous and heterogeneous
catalytic systems.
Scheme 3. Plausible mechanism of the reaction.
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