Recyclable silica-supported iridium bipyridine catalyst for aromatic C-H borylation

Article abstract of DOI:10.1021/cs4009539

A mesoporous silica (SBA-15)-supported bipyridine iridium complex is prepared by grafting of bipyridine onto the silica support, followed by complexation of an iridium(I) precursor in the presence of HBpin and cyclooctene. Structural analyses by X-ray powder diffraction, nitrogen physisorption, FT-IR, and solid-state NMR spectroscopy demonstrate that the 3-dimensional, hexagonal pore structure of SBA-15 is maintained after the immobilization. In particular, as a heterogeneous catalyst, this silica-supported iridium complex shows moderate to good catalytic activity in the aromatic C-H borylation of a variety of substrates. More importantly, the heterogeneous catalyst is recovered easily and reused repeatedly by simple washing without chemical treatment and exhibits good recycling performance with a modest decrease in the catalytic rate, showing good potential for increasing the overall turnover number of this synthetically useful catalyst.

Full text of DOI:10.1021/cs4009539

Research Article
pubs.acs.org/acscatalysis
Recyclable Silica-Supported Iridium Bipyridine Catalyst for Aromatic
C−H Borylation
Fengshou Wu,†,‡,§ Yan Feng, and Christopher W. Jones*,†
†,§
†School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332,
United States
‡
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei, 430072, P. R. China
*
S Supporting Information
ABSTRACT: A mesoporous silica (SBA-15)-supported bipyr-
idine iridium complex is prepared by grafting of bipyridine onto
the silica support, followed by complexation of an iridium(I)
precursor in the presence of HBpin and cyclooctene. Structural
analyses by X-ray powder diffraction, nitrogen physisorption,
FT-IR, and solid-state NMR spectroscopy demonstrate that the
3
-dimensional, hexagonal pore structure of SBA-15 is
maintained after the immobilization. In particular, as a
heterogeneous catalyst, this silica-supported iridium complex
shows moderate to good catalytic activity in the aromatic C−H
borylation of a variety of substrates. More importantly, the
heterogeneous catalyst is recovered easily and reused repeatedly
by simple washing without chemical treatment and exhibits
good recycling performance with a modest decrease in the catalytic rate, showing good potential for increasing the overall
turnover number of this synthetically useful catalyst.
KEYWORDS: iridium, heterogeneous catalysis, recyclable, immobilization, mesoporous materials, arenes, C−H borylation
1
. INTRODUCTION
can be prepared, heterogeneous catalytic systems are attractive.
In 2007, Zhu et al. synthesized ionic liquid-stabilized iridium(0)
nanoparticles that could be used as catalysts for the C−H
Arylboronic acids and their derivatives are very important
intermediates in organic synthesis. Among the routes to
arylboronic acid derivatives, direct C−H borylation of arenes is
the most attractive. Since the first catalysts for the direct
borylation of arenes based on Cp*Ir complexes were reported
by Smith and co-workers, various iridium systems containing
phosphine- and nitrogen-based ligands have been studied. In
1
8
borylation of arenes by pinacolborane (HBpin). These
2
catalytic systems were reused six times; however, the catalyst
was difficult to recover because the products first had to be
separated from the reaction mixture by distillation. In 2009,
Nishida and co-workers obtained an insoluble iridium complex
by filtration from the reaction mixtures of 2,2′-bipyridine-4,4′-
dicarboxylic acid, [IrCl(cod)]2 and bis(pinacolato)diboron.
The iridium catalyst could be reused 10 times over 1 week in
aromatic C−H borylation reactions. However, the structure of
this iridium complex was unconfirmed because of its instability
3
4
2
002, Ishiyama, Hartwig, and co-workers reported the5
borylation of arenes catalyzed by bipyridine iridium complexes.
Concurrently, catalysts based on iridium phosphine complexes
2
were reported by Smith, Maleczka, and co-workers. Generally,
catalysts containing bipyridine derivatives are more reactive for
most borylations of arenes compared with those containing
9
under air and its insolubility in organic media. In the same
6
6
phosphine ligands. Studies of the precatalysts such as (η -
mesitylene)Ir(Bpin) and (dtbpy)Ir(Bpin) (coe) (dtbpy =4,4′-
year, Sawamura and co-workers reported a supported catalyst
10
3
3
for aryl borylation. Their studies showed that reaction of
di-tert-butyl-2,2′-bipyridine, coe = cyclooctene) support a
B pin with methylbenzoates catalyzed by the combination of
III
2
2
mechanism in which species of the formula L Ir Bpin are
2
3
5
,7
[Ir(cod)(OMe)] and a silica-supported phosphine ligand
2
the intermediates that mediate C−H cleavage. A valence
cycle between Ir and Ir was involved during the process of
the aryl borylation (Scheme 1). After much experimentation
by Hartwig and co-workers, (dtbpy)Ir(Bpin) (coe) was isolated
III
V
resulted in ortho-directed aromatic borylation in excellent
yields. Subsequently, similar reactions were studied with HBpin
7d
11
as the boron reagent using the same catalyst. However, the
3
I
7
c
supported Ir complex purported to form during the reaction
in 80−95% yield.
However, direct C−H borylation is usually catalyzed by
iridium complexes in a homogeneous mode of operation, which
shares common drawbacks, that is, catalyst recovery and
recycling is difficult. To this end, in cases that stable catalysts
Received: October 21, 2013
Revised: March 25, 2014
Published: March 25, 2014
©
2014 American Chemical Society
1365
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6,7d
Scheme 1. Proposed Mechanisms for Ir-Catalyzed Aromatic C−H Borylation
was not actually isolated because it is not air stable, especially
under moist conditions.
In parallel, they also reported a similar supported phosphine-
catalyst is highly stable in air and can be reused several times
without significantly affecting the structure or the texture of the
catalyst.
7d
Rh system based on [Rh(OH)(cod)] . This Rh catalysis was
2
effective for arenes with oxygen-based directing groups,
2
. MATERIALS AND METHODS
1
2
complementing the Ir-catalyzed orthoborylation. Like the
supported phosphine-iridium systems, recycling tests were not
studied for this rhodium catalyst. In related work, Westcott et
al. also reported a supported phosphine-Rh system for olefin
hydroboration in which reactions with simple vinyl arenes
1
2
.1. General. Solution H and 13C NMR spectra were
recorded on a Mercury VX 400 MHz instrument. Solid-state
13
29
C and Si cross-polarization magic angle spinning (CP-MAS)
NMR spectra and 11B MAS NMR (B = 9.40 T, ν ( B) =
11
0
L
1
4
7
28.38 MHz) spectra were collected on Bruker DSX 300 or
00 MHz instruments. Mass spectra were recorded with a VG
070 EQ-HF hybrid tandem mass spectrometer or an Applied
(
ArCHCH ) and catecholborane (HBcat) catalyzed by
2
Rh(acac)(coe) (acac = acetylacetonato)/silica-diphenylphos-
2
phine could give selective formation of the corresponding
1
3
Biosystems 4700 spectrometer. Iridium and silicon elemental
analyses were performed by Columbia Analytical Services
branched isomers [ArCH(Bcat)Me]. The immobilized
rhodium complex recovered by filtration from the reaction
mixtures could be reused, although little is known about the
structure of this supported catalyst. Thus, a more convenient
catalyst for borylation reactions that is air-stable and easier to
recover and reuse would be a useful advance.
(
Tucson, AZ, USA), using ICP-AES. Carbon, hydrogen, and
nitrogen contents were determined via CHN analysis, also by
Columbia Analytical Services. Thermogravimetric analysis
(
TGA) was performed on a Netzsch STA409, and the ligand
In this context, here, we describe a new silica-supported
iridium complex for C−H borylation catalysis. Specifically, we
have developed an immobilized bipyridine-iridium system
prepared from a silica-supported bipyridine ligand and an
loading was calculated from relative weight loss in the range
150−400 °C. X-ray powder diffraction was performed on a
PAnalytical X’Pert Pro diffractometer with a Cu Kα source. The
nitrogen (N ) adsorption−desorption isotherms were meas-
2
Ir(I) precursor [Ir(cod)(OMe)] . Mesoporous silica SBA-15
ured on a Micromeritics Tristar 2030 at 77 K. The samples
were outgassed at 110 °C overnight before the measurements.
The surface areas were determined on the basis of the
Brunauer−Emmett−Teller (BET) method, and the total pore
volumes and pore sizes were calculated using the Barrett−
Joyner−Halenda (BJH) method applied to the adsorption
isotherms. FTIR spectroscopy was performed using a Bruker
Vertex 80v.
2
was chosen as a support for the iridium complex because of its
advantageous properties, such as excellent chemical and
thermal stability, high porosity, large surface area, and high
surface concentration of silanols, that are useful for silane
grafting. The synthesized silica-supported iridium complex was
characterized in detail to elucidate its structure using a
combination of methods, such as NMR spectroscopy, IR
spectroscopy, nitrogen physisorption, XRD, elemental analysis,
and TGA. In parallel, an analogous unsupported iridium
complex was also prepared for comparison using a similar
method. The catalytic results showed that both the
unsupported catalyst (Ir-1) and supported catalyst (Ir-2)
exhibit moderate to good activity for the borylation of various
arenes containing different kinds of functional groups.
Furthermore, recycling studies revealed that the heterogeneous
2.2. Materials. Chloroform-d (CDCl , CIL; 99%), 4,4′-
3
dimethyl-2,2′-dipyridyl (dMBpy; Aldrich; 98%), n-butyllithium
(n-BuLi; Aldrich; 2.5 M in hexane), poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly-(ethylene glycol) (EO-
PO-EO; Aldrich), hydrochloric acid (HCl; ACS grade),
tetraethyl orthosilicate (TEOS; Acros; 98%), (3-chloropropyl)-
trimethoxylsilane (CPTMS; Aldrich; 95%), hexamethyldisila-
zane (HMDS; Adlrich; 99%), bis(pinacolato)diboron (B pin ;
2
2
1
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Aldrich; 99%), cis-cyclooctene (coe; Aldrich; 95%), 4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (HBpin; Aldrich; 97%),
methoxy(cyclooctadiene)iridium(I) dimer ([Ir(cod)(OMe)]2;
Alfa; Ir nominally 58%), 1,1,2,2-tetrachloroethane (TECE;
Aldrich; 98%), acetone (ACS grade, BDH), and cyclohexane
was filtered; washed with 80 mL each of toluene, hexane, and
1
3
DCM; and dried under vacuum overnight at 100 °C. C CP-
MAS NMR (100 MHz, solid state): δ = 149, 124, 47, 33, 28,
21, 12, 0.1. Elemental analysis (%): C, 13.93; H, 2.29; N, 1.19;
Si, 33.61.
(
Aldrich; 99.5%) were used as received. Arenes (Aldrich),
2.4. Preparation of the Unsupported Iridium Catalyst
(Ir-1). To a 100 mL Schlenk flask, 212 mg [Ir(cod)(OMe)]2
(0.32 mmol) and 20 mL cyclohexane were added, then 0.56 mL
of cis-cyclooctene (4.2 mmol) was added to this solution. The
yellow mixture was then kept stirring for 10 min before 0.56
mL pinacolborane (4.2 mmol) was added dropwise by syringe.
Modified bipyridine 1 (222 mg, 0.64 mmol) was added as a
single solid portion, and the solution was kept stirring for 45
min. Volatiles were removed by evaporation, and Ir-1 was
purified by trituration with hexane to give a yellow solid (62%
hexane (Aldrich; >99%), methylene chloride (CH Cl ; Aldrich;
2
2
>
(
99%), tetrahydrofuran (THF; Aldrich; >99%), and toluene
Aldrich, anhydrous) were dried and deoxygenated using a
purification system and stored under nitrogen in a glovebox.
.3. Preparation of Silica-Supported Bipyridine.
.3.1. Synthesis of Bare SBA-15. To a 2 L flask, 24.0 g of
2
2
EO-PO-EO, 120 mL of 38% aqueous HCl, and 635 mL of
distilled water were added. The reaction mixture was kept
stirring at 40 °C until the triblock copolymer template
completely dissolved. Then 52.6 g of TEOS was added, and
the solution was stirred overnight. The mixture was
subsequently kept quiescent and heated to 100 °C for 24 h.
The solid product was recovered by filtration and washed with
1
yield); decomposition at 140−145 °C. H NMR (400 MHz,
CDCl ): δ = 9.43 (d, J = 5.2 Hz, 2H), 7.84 (s, 2H), 7.21 (d, J =
3
5.2 Hz, 2H), 4.20 (s, 2H), 3.54 (s, 9H, OMe), 2.70 (t, J = 3.6
Hz, 2H, CH −Py), 2.41(s, 3H, Me), 1.72 (m, 2H, CH ), 1.56
2
2
copious amounts of DI H O, and air-dried overnight. The
(m, 2H, CH ), 1.45 (br s, 36H), 1.09 (br s, 12H), 0.73 (t, J =
2
2
13
calcination of the dried material was carried out by slowly
increasing the temperature from ambient to 550 °C, and then
cooling back to room temperature. The solid material was
further dried overnight under vacuum and stored in a glovebox.
3.2 Hz, 2H, CH −Si). C NMR (100 MHz, CDCl ) δ = 155.9,
2
3
148.9, 148.2, 130.1, 124.6, 122.0, 83.4, 82.9, 74.5, 52.5, 35.1,
29.1, 26.6, 26.1, 25.4, 24.9, 24.5, 21.1. 11B NMR (128 MHz,
CDCl ) δ = 22.5 (s). Elemental analysis (%): C, 48.94; H, 6.92;
3
2
.3.2. Synthesis of 4′-[4-(Trimethoxysilanyl)butyl]-4-meth-
N, 2.64; Si, 3.21; Ir, 20.38.
yl-2,2′-bipyridine (1). To a 250 mL flask, 1.476 g of
diisopropylamine and 40 mL of dry THF were added under
nitrogen; 5.43 mL of n-BuLi (2.5 M, 0.0132 mol) was then
added dropwise to this solution at 0 °C. After the reaction
mixture was kept stirring for 30 min at this temperature, a
solution of 2.42 g of 4,4-dimethyl-2,2′-bipyridine (0.0132 mol)
in dry THF (100 mL) was then added slowly. The remaining
mixture was then allowed to keep stirring for an additional 1 h
at 0 °C under nitrogen. Subsequently, a solution of 1.620 g of
2.5. Preparation of the Silica-Supported Iridium
Catalyst (Ir-2). The synthesis procedure was similar to that
of Ir-1 with a slight change. To a 100 mL Schlenk flask, 60 mL
of cyclohexane and 0.705 g of [Ir(cod)(OMe)] (1.06 mmol)
2
were added, then 2.0 mL of cis-cyclooctene (15 mmol) was
added. The yellow solution was kept stirring while 2.0 mL
pinacolborane (15 mmol) was added slowly. The solution
turned red immediately. Then 5.0 g silica-supported bipyridine
3 was added in one portion as a white solid. The solution
became dark red gradually. The flask was sealed with a rubber
septum and stirred for 30 min at room temperature and then at
40 °C overnight. The solution was filtered off, and the red solid
was washed with 60 mL each of cyclohexane, hexane, and DCM
and dried under vacuum overnight at 100 °C. C CP-MAS
NMR (100 MHz, solid state): δ = 153, 149, 124, 88, 82, 78, 68,
47, 34, 28, 21, 12, 0.1. 29Si CP-MAS NMR (79.5 MHz, solid
state): δ = 10.2, −63.2, −71.3, −113.6. Elemental analysis (%):
C, 14.56; H, 2.48; N, 1.10; Si, 27.60; Ir, 3.79.
(
3-chloropropyl)trimethoxylsilane (2.41 mL, 0.0132 mol) in 10
mL of dry THF was added slowly and the solution was stirred
for an additional 2 h. The reaction mixture was then allowed to
warm to room temperature and stirred overnight before
quenching with two drops of acetone. The remaining solvent
was removed by rotovap, yielding the main product,
alkoxysilane-modified 2,2′-bipyridine as a a pale yellow solid
1
3
(
1). It was dried under vacuum at room temperature for several
hours. In this step, the product should not be washed because
the alkoxysilane-modified product is easily hydrolyzed. mp
2.6. Direct Borylation of Arenes Catalyzed by Ir-1 and
Ir-2. In a glovebox, Ir catalyst (0.0035 mmol on the basis of
iridium), bis(pinacolato)diboron (64 mg, 0.25 mmol), arene (1
mmol), and anhydrous hexane (3 mL) were added in a 10 mL
glass tube with a magnetic stirring bar. The tube was then
sealed with a screw cap and removed from the glovebox. After
the resulting mixture was stirred at 70 °C for a designated time,
t, the mixture was cooled to room temperature and filtered. The
solvent was removed by rotovap. 1,1,2,2-Tetrachloroethane was
then added to the remaining mixture as an internal standard.
1
1
34−138 °C. H NMR (CDCl , 400 MHz): δ = 8.54 (m, 2H,
3
Py), 8.20 (s, 2H, Py), 7.12 (d, J = 4.0 Hz, 2H, Py), 3.55 (s, 9H,
OMe), 2.71 (t, J = 3.6 Hz, 2H, CH −Py), 2.43(s, 3H, Me), 1.73
2
(
m, 2H, CH ), 1.58 (m, 2H, CH ), 0.75 (t, 2H, J = 3.2 Hz,
2
2
CH −Si).
2
2.3.3. Grafting the Bipyridine onto SBA-15 (2). A 4.0 g
portion of dry SBA-15, 60 mL of toluene, and 1.6 g modified
bipyridine 1 (4.62 mmol) were added to a 100 mL flask. The
reaction mixture was stirred at reflux under nitrogen for 24 h.
The resultant powder was filtered; washed with 80 mL each of
toluene, hexane, and DCM; and dried under vacuum overnight
1
The yield of the product was determined by H NMR on the
basis of the protons in the boronate ester of the product and in
13
at 100 °C. C CP-MAS NMR (100 MHz, solid state): δ = 150,
25, 48, 34, 28, 22, 12.
.3.4. End-Capping the Functionalized SBA-15 (3). The
B pin . The pure product was isolated by flash silica gel column
2
2
1
chromatography (hexane/ethyl acetate = 100/0 to 90/10).
Reactions were run multiple times for some substrates to
minimize the impact of error in a single run, with the average
yields reported in each case.
2.7. Solvent, Temperature and Ir Loading Effects on
Aryl Borylation Catalyzed by Ir-2. In a glovebox, Ir-2
(designated Ir loading), bis(pinacolato)diboron (64 mg, 0.25
2
remaining accessible silanols on the bipyridyl-functionalized
SBA-15 were then capped using trimethysilyl (−SiMe ) groups
3
as follows: A suspension of 2 (2.0 g) and [(CH ) Si)] NH
3
3
2
(
HMDS) (8 mL, 0.038 mol) in 25 mL dry toluene were stirred
at room temperature overnight. The resultant white powder
1
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mmol), 1,2-dichlorobenzene (1 mmol), and anhydrous solvent
(
3 mL) were placed in a 10 mL glass tube with a magnetic
stirring bar. The tube was then sealed with a screw cap and
removed from the glovebox. After the resulting mixture was
stirred at reflux for a designated time, the mixture was cooled to
room temperature and filtered. The solvent was removed by
rotovap. 1,1,2,2-Tetrachloroethane was then added to the
remaining mixture as an internal standard. The yield of the
1
product was determined by H NMR on the basis of the
protons in the boronate ester of the product and in B pin .
2
2
2
.8. Recycling Studies. In a glovebox, Ir-2 (1.0 g, 0.20
mmol on the basis of iridium), bis(pinacolato)diboron (3.6 g,
4.28 mmol), 1,2-dichlorobenzene (57 mmol), and anhydrous
1
hexane (60 mL) were placed in a 100 mL glass tube with a
magnetic stirring bar. The tube was then sealed with a screw
cap and removed from the glovebox. After the resulting mixture
was stirred at 70 °C for 32 h, the catalyst was separated from
the reaction mixture by filtration in air. The recycled catalyst
was washed with CH Cl and hexane to remove residual
Figure 1. Powder XRD patterns of (a) SBA-15, (b) 2, (c) 3, (d) Ir-2,
and (e) recycled (3 times) Ir-2.
2
2
organics and then was dried under vacuum at 100 °C overnight.
2
7c
[
Ir(dtbpy)(η -coe)(Bpin) ] in high yield. Thus, our synthesis
3
2.9. Kinetic Analysis of the Catalyst Ir-1, Ir-2 and
procedure was similar to their previous publications with some
modification, in which HBpin and coe were added to the
solution of [Ir(cod)(OMe)]2 before the bipyridine ligand
Recycled Ir-2. In a glovebox, Ir catalyst (0.0035 mmol on the
basis of iridium), bis(pinacolato)diboron (64 mg, 0.25 mmol),
1,2-dichlorobenzene (1 mmol), and anhydrous hexane (3 mL)
(
Scheme 2).
were placed in a 10 mL glass tube with magnetic stirring bar.
The tube was then sealed with a screw cap and removed from
the glovebox. After the resulting mixture was stirred at 70 °C
for a designated time, the mixture was cooled to room
temperature and filtered. The solvent was removed by rotovap.
3
.2. Characterization of the Silica-Supported Ligands
and Complexes. Figure 1 shows the low-angle XRD patterns
of SBA-15 and immobilized ligands and catalysts. All the
materials displayed XRD patterns with one intense diffraction
peak and two weak peaks around 0.80°, 1.45°, and 1.62°
indexed to (100), (110), and (200) reflections, which were
characteristic of 2-D hexagonal (P6 mm) structure, suggesting
that the ordered mesostructure in pure SBA-15 was maintained
1
,1,2,2-Tetrachloroethane was then added to the remaining
mixture as an internal standard. The yield of the product was
determined by H NMR on the basis of the protons in the
1
boronate ester of the product and in B pin2
17
2
after the functionalization, catalysis, and recycling. However,
the intensity of the diffraction peaks was decreased after the
grafting and complexation. This is commonly observed with
such organic−inorganic hybrid materials because the presence
of organic or organometallic species within the pores reduces
the electron density contrast between the silicate walls and
3
. RESULTS AND DISCUSSION
3.1. Synthesis of the Iridium Catalysts. The immobiliz-
able bipyridine ligand was prepared homogeneously according
to the procedure described in the literature with some minor
modification. A dMBpy precursor was functionalized with
1
4
18
pores, reducing the intensity of the diffraction lines. Such a
one of the methyl groups converted with lithium diisopropyl-
amine to a CH2 anionic group, and the latter subsequently
decrease in reflection intensity also provides the evidence that
grafting occurs inside the mesopore channels.
−
reacted with the (3-chloropropyl)trimethoxylsilane to yield the
immobilizable dMBpy ligand 1.
The nitrogen adsorption−desorption isotherms of SBA-15
and silica-supported materials are shown in Figure 2. All the
isotherms exhibited type IV behavior with H1-type hysteresis
loops at high relative pressures according to IUPAC
classification, which is a characteristic of capillary condensation
The SBA-15 materials were synthesized according to the
1
5
procedure described by Brunelli et al. The calcined
mesoporous silicate was characterized by N physisorption to
2
19
determine the surface area and pore size of the silica support.
Powder X-ray diffraction was carried out to determine the
overall structure of SBA-15. The XRD patterns (Figure 1) were
consistent with a hexagonal arrangement of pores. The
functionalized bipyridine was then grafted onto the SBA-15
surface by reaction with the surface silanols at reflux in
within uniform pores. A sharp inflection in the P/P range
0
from 0.6 to 0.8 was found in all isotherms of the materials,
providing further proof on the maintenance of the mesoporous
20
structure after grafting and recycling. The textural properties
of the samples are summarized in Table 1. These results were in
good agreement with the XRD data, which demonstrates that
the mesoporous structure of SBA-15 prevailed after the
immobilization of the bipyridine and complexation with Ir.
The functionalized hybrid materials exhibited a considerable
decrease in the BET surface area, pore volume, and pore
diameter. Our results indicated that the BET surface area was
1
6
toluene. With heating at reflux, the −Si(OCH ) groups react
3
3
with the surface silanol groups on SBA-15 to yield the surface-
tethered ligand. Filtration and elution of soluble materials and
subsequent drying under vacuum afforded the bipyridine-
functionalized silica gel 2. The accessible surface silanols that
remained unreacted were then end-capped with a trimethysilyl
2
2
802 m /g for SBA-15 but decreased to 415 m /g for the
immobilized bipyridine, and correspondingly, the pore volume
(
−SiMe ) group to give 3. The iridium catalysts Ir-1 and Ir-2
3
3
were synthesized by coordinating the homogeneous and
immobilized bipyridine liands with an Ir precursor, respec-
shrank to 0.73 from 1.14 cm /g for the parent material, which
I
was mainly due to coverage of the pore surface with the ligand.
The 13C CP-MAS NMR spectra of silica-supported ligand 2
verified the presence of bipyridyl groups in the grafted material
tively. As mentioned by Hartwig and co-workers, the order of
addition of HBpin and bipyridine was important to form
1
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;
7c a
Scheme 2. Synthetic Routes for Creation of Homogeneous and Silica-Supported Iridium Complexes
a
Note that the iridium complex is depicted without the iridium interacting with oxygen lone pairs on the surface (which could occur if the bpy or coe
ligand were lost from some species) but such interactions cannot be ruled out on the basis of the data presented here.
(
Figure 3). The broad peaks at 150.3 and 125.4 ppm are readily
assigned to pyridyl ring carbons. The spectrum of 2 exhibited
one peak at 48.7 ppm attributed to residual methoxide groups
and four sharp peaks at 12.4, 22.5, 28.6, and 34.2 ppm
attributed to the methylene carbons of the surface-bound alkyl
spacer. All these observations are consistent with the successful
postgrafting of ligands onto the mesoporous support. The new
peak at 0.1 ppm in 3 does not appear in that of 2, which was
assigned to the trimethylsilane groups incorporated after silanol
capping, which further confirmed that the accessible residual
hydroxyl groups on the silica surface were protected
successfully. After complexation of the silica-supported ligand
with the iridium precursor, new signals around 80 ppm
appeared, as shown in the 13C NMR spectra of Ir-2, which are
attributed to the groups of the boronate esters and cyclooctene
coordinated with iridium. The presence of some boronate
esters and cyclooctene interacting with the silica surface rather
than the bipyridyl-ligated iridium cannot be ruled out from
these spectra. The peaks of the silica-supported Ir complex Ir-2
are marked in Figure 4.
Figure 2. Nitrogen adsorption−desorption isotherms of (a) SBA-15,
b) 2, (c) 3, (d) Ir-2, and (e) recycled Ir-2.
(
Table 1. Textural Parameters of SBA-15 and Silica-
Supported Materials Determined from N Isotherms at 77 K
2
a
b
b
29
surface area
pore volume
pore size
(nm)
The Si CP-MAS NMR spectrum (Figure 5) showed clearly
2
3
materials
SBA-15
(m /g)
(cm /g)
4
that the silica-supported iridium complex presented a strong Q
802
415
386
350
316
1.14
0.73
0.66
0.59
0.54
6.69
6.66
5.89
5.77
5.82
3
2
(
−113 ppm) and medium T (−71 ppm) and T (−63 ppm)
peaks. The strong Q signal in Ir-2 indicates that the material
2
4
3
possessed a highly condensed network structure of {Si(OSi) },
4
Ir-2
2
3
as expected. The relatively weak T and T signals suggest the
formation of {R(HO)Si(OSi) } and RSi(OSi) } (R = ligands or
recycled Ir-2
2
3
a
Determined by the BET method from the adsorption branch.
Determined by the BJH method from the adsorption branch.
metal−ligand complexes), which were due to a reaction of the
b
Si(OCH ) moiety with the −OH groups of the SBA-15
3
3
1
7
surface. These peaks give direct evidence of the covalent
grafting of the organosilane to the silica surface. In addition, the
1
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Figure 3. 13C CP-MAS NMR spectra of 2, 3, and Ir-2.
Figure 4. Comparison 13C CP-MAS NMR spectra of Ir-2 and recycled Ir-2.
1
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Figure 5. 29Si CP-MAS NMR spectra of Ir-2 and recycled Ir-2.
Figure 6. 11B MAS NMR spectra of Ir-2 and recycled Ir-2.
29Si spectrum exhibited a medium peak at 10 ppm, which was
assigned to the trimethylsilane group (Si(CH ) ) introduced by
elucidate the structure of the supported boron species are
warranted because multiple different boron environments may
exist in this material, and a more detailed study at multiple
fields and via application of multinuclear NMR may shed
additional light on the boron speciation.
3
3
21
the silanol protection step.
As shown in Figure 6, the 11B NMR spectrum of Ir-2 shows
two peaks in the solid state. The chemical shift around 20 ppm
falls within the expected range for boronic ester in the solid
Figure 7 displays FT-IR spectra of the SBA-15, the silica-
2
2
state, and this resonance is assigned to the boron ester group
coordinated with iridium according to the related spectrum of
Ir-1 in solution (Supporting Information Figure S3). Another
peak around 0 ppm that did not exist in the spectrum of Ir-1 in
immobilized ligands, and the supported iridium catalysts. For
−1
SBA-15 and 2, the bands at 3437 and 1632 cm are attributed
−
1
−1
to the stretching (3437 cm ) and bending (1632 cm )
vibrations of the surface silanol groups as well as the remaining
1
1
−1
solution is tentatively assigned to the four-coordinate B
physisorbed water molecules. In addition, a band at 965 cm is
−
resonance of a boron group (B(OR) ) formed due to the
also observed in these two materials. This band can be assigned
4
24
interaction between the boron atom of some boronate species
and the silica surface (Si−O−Si or Si−OH), since the oxygen
atoms in the silica surface can donate electron density through
to the Si−O stretching vibrations of the Si−O−H groups. All
these bands became very weak or disappeared in 3 and Ir-2,
indicating the hydroxyl groups on the silica surface were largely
removed by ligand grafting and protection by trimethysilyl
22
electron lone pairs into the vacant p orbital of the boron.
Such resonances around 0 ppm also exist in other silica-
−1
groups. In the framework region (400−1500 cm ), the bands
23
−1
supported boron materials. Additional experiments to further
at 1085 and 806 cm were assigned to the antisymmetric
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Table 3. Ligand and Iridium Loading of the Immobilized
Materials
a
b
c
d
L loading
L loading
Ir loading
(L/Ir)
(
mmol/g
(mmol/g
cat)
(mmol/g
cat)
(mole ratio
cat)
entry
samples
cat)
1
2
3
3
0.52
0.45
0.48
0.43
0.39
0.36
Ir-2 fresh
Ir-2 after
st use
Ir-2 after
nd use
0.20
0.19
1.95
1.89
1
4
0.44
0.37
0.19
1.94
2
a
b
Based on thermogravimetric analysis. Based on total nitrogen
c
content by CHN elemental analysis. Based on total iridium content
by ICP-AES elemental analysis. Molar ratio of ligand to iridium based
d
on element analysis.
Figure 7. FT-IR spectra of (a) SBA-15, (b) 2, (c) 3, (d) Ir-2, and (e)
recycled Ir-2.
with iridium also exist on the surface. In addition, the presence
of some iridium species interacting with the silica surface via
oxygen lone pairs (after loss of coe or bpy species) cannot be
ruled out on the basis of the B spectrum of heterogeneous
catalyst.
stretching vibration and symmetric stretching vibration of
−1
[
SiO ], and the band at 470 cm was assigned to rocking
4
25
vibrations of [SiO4]. The peaks indicative of ν(Si−C) in 2, 3,
11
−1
and Ir-2 should appear around 1100 cm ; however, they were
difficult to distinguish because of overlap by the strong
absorbance from the ν(Si−O) band. Weak bands of 2, 3, and
Ir-2 at 2800 cm− were assigned to asymmetric and symmetric
3
.3. C−H Borylation of Arenes. With the supported
iridium complex Ir-2 in hand, its ability to catalyze aromatic C−
H borylation reactions was examined. The unsupported iridium
complex Ir-1 was also used in these catalytic processes for
comparison. The borylation was carried out in hexane solution
1
stretching vibrations of −CH − bonds, and the bands around
2
1
1
620 cm− were attributed to breathing vibrations of CC
bonds of the aromatic rings. In addition, the weak bands at
at reflux, with a slight excess of substrate to B pin to avoid
−1
2
2
1
480 and 780 cm represent −CH − bending and −CH −
2
2
multiple borylation of the arenes. The catalytic results obtained
from the borylation of various arenes are summarized in Table
26
rocking modes, respectively. All these relatively weak bands
derived from the ligands or metal complexes suggest successful
incorporation of the intended species into the mesoporous
silica materials, which was further supported by the solid-state
NMR spectra discussed above.
4
. In all cases, the borylation occurred at C−H bonds located
meta or para to a substituent in preference to those located in
the ortho position. Thus, the reactions occurred regiospecifi-
cally with disubstituted arenes. Both 1,2- and 1,4- dichlor-
obenzenes yielded the corresponding boronate ester as a single
product (Table 2, entries 1 and 2). The yields for reaction of
the 1,4- isomer were lower than for 1,2- isomer, which was
probably due to the steric hindrance. Isomerically pure
products were obtained in moderate to good yields, even
with two distinct substituents on arenes (Table 4, entries 3−5).
In addition, electron-withdrawing substituents activated the
arene for the borylation process, as shown by the catalytic
results from entries 3−5 (Table 4). Functional group tolerance
of the borylation reaction is higher than in boronate synthesis
The elemental analysis data of the silica-supported ligand and
iridium complexes are listed in Table 2. These demonstrate
Table 2. Elemental Analysis Results for the Immobilized
Ligand and Complex
entry
samples
% C
% H
% N
% Si
% Ir
1
2
3
4
3
13.93
14.56
14.99
15.21
2.29
2.48
2.53
2.24
1.19
1.10
1.02
1.04
33.61
27.60
28.12
Ir-2 fresh
3.79
3.68
3.63
Ir-2 after 1st use
Ir-2 after 2nd use
a
24.07
27
through magnesium or lithium reagents. The reaction was
suitable for arenes possessing a wide variety of functional
groups, such as Cl, Br, CF , OMe, and CO Me. The aryl
a
This % Si value might be inaccurate, given the consistency of all the
other data.
3
2
bromide underwent borylation at the C−H bond (entry 3)
without C−X bond cleavage. In the case of indole, the
borylation selectively occurred at the α-position to form a
single isomer in 90% yield for Ir-1 and 72% for Ir-2 (Table 4,
entry 6).
A comparison of the catalytic activity of the homogeneous
catalyst (Ir-1) and its immobilized counterpart (Ir-2) showed
an activity reduction in most cases, which could be due to lower
accessibility of the active sites within the pores of SBA-15. For
example, for the substrates with electron-donating groups
(entry 5), the supported complex behaved poorly compared
with the homogeneous catalyst; however, the heterogeneous
catalyst (Ir-2) exhibited similar or even better catalytic
performance relative to the homogeneous analogue (Ir-1)
when choosing 1,2-dicholorobenzene (entry 1) or 1-trifluor-
omethyl-3-bromobenzene (entry 3) as the substrate. A high
turnover number (TON of 136) of the heterogeneous catalyst
very little change of metal content after recycling, suggesting
good stability of the supported complex. Moreover, the ligand
loading of the materials calculated from TGA (Supporting
Information Figure S1) was similar to that from the elemental
microanalysis, as shown in Table 3. The overestimation of the
ligand content via TGA is likely due to some loss of silanols (on
the surface or buried within the silica walls) via condensation at
high temperatures, with this weight loss falsely ascribed to
organic species. The L/Ir ratios (Table 3) suggest that
uncoordinated bipyridine ligands must exist and that about
half of the ligands were initially involved in the metalation with
Ir.
It should be noted that the elemental analysis and IR and
NMR spectroscopic results are consistent with the creation of
the intended supported species as depicted in Scheme 2,
although the presence of bipyridines that are not coordinated
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Table 4. Ir-Catalyzed Direct Borylation of Arenes with
B2pin2
Table 5. Aromatic Borylation Catalyzed by Ir-2 in Different
Solvents
a
a
b
c
entry cat. (mol %)
solvent
hexane
T (°C) time (h) yield (%)
1
2
3
4
5
6
7
8
9
0.5
0.9
1.4
1.8
1.8
1.8
1.8
1.4
1.4
70
70
70
70
40
70
80
25
45
48
48
24
24
48
48
48
48
48
24
77
95
95
hexane
hexane
hexane
DCM
THF
12
acetonitrile
hexane
hexane
4
28
a
A mixture of B pin (0.25 mmol), 1,2-dichlorobenzene (1.0 mmol),
2
2
and Ir-2 was stirred in solvent (3 mL) for the period shown in the
b
c
1
Table. The mole ratio of iridium to B pin . H NMR yields are based
2
2
on the protons of the Bpin group in the product and in B pin .
2
2
amount of catalyst led to higher activity. The lowest tested
catalyst loading was 0.5 mol %, and only low yields (24% after
4
8 h, entry 1) were observed with this small amount of catalyst.
The highest yield was attained when the iridium loading was
elevated to 1.4 mol %. However, there was no increase in the
yield if additional catalyst was added beyond this loading. All of
these trends are typical, showing no unexpected deviations of
behavior of this catalyst relative to homogeneous versions.
Although reaction yields at long times are useful information
for synthetic purposes, to assess the utility of the catalyst, it is
best to gather kinetic profiles for the reactions. To this end,
kinetic experiments were performed for the borylation of 1,2-
a
A mixture of B pin (0.25 mmol), an arene (1.0 mmol), and Ir
2
2
catalyst (0.0035 mmol on the basis of iridium) was stirred in hexane (3
b
1
dichlorobenzene with B pin to compare the activity of the
catalysts, as shown in Figure 8. The kinetic profiles revealed
mL) at 70 °C for the period shown in the Table. The average H
NMR yields are calculated on the basis of the protons of the Bpin
group in the product and in B pin . The data in parentheses are the
2
2
2
2
yields obtained in duplicate runs.
(
Ir-2) was attained, which was comparable to the homogeneous
5
catalyst 1/2[{Ir(OMe)(cod)} ]/dtbpy (TON of 67) or the
silica-SMAP-Ir(OMe)(cod) (TON of 200) under similar
conditions. A higher turnover number was reported by Hartwig
2
10
(
TON of 8000) and Sawamura (TON of 20000) using more
extreme conditions where the reactions were carried out in the
presence of a large excess of substrate to B pin at high
temperature (100 °C).
2
2
5
,10
The reaction solvent was reported by many groups to have
7d
an effect on the activity of the catalyst. For example, Ishiyama
and co-workers reported that the activity of their homogeneous
catalysts for borylations was much lower in polar solvents
28
relative to that in hexane. To verify whether the supported
catalyst showed similar behavior, other borylation experiments
were carried out with DCM, acetonitrile, and THF as solvents.
As shown in Table 5, the supported catalyst was not effective
for aryl borylation in polar solvents. No product was observed
when the experiment was run in DCM or acetonitrile solution.
Only 12% yield was obtained when THF was used, even if the
reaction was carried out for a prolonged period of time. Thus,
the order of reactivity in different solvents was hexane (95%) >
THF (12%) > DCM (0%) = acetonitrile (0%), which is
Figure 8. Kinetic analysis of the borylation of 1,2-dichlorobenzene
catalyzed by iridium complexes.
that the borylation catalyzed by the homogeneous Ir-1 (initial
−1
TOF = 17.2 h ) occurred at a higher rate than that of Ir-2
−1
(initial TOF = 6.5 h ), indicating a higher activity for the
unsupported catalyst. The decrease in the activity may be
because a fraction of the supported Ir is inactive in the reaction,
leading to TOFs being lower for the supported catalyst. This
may be due to unfavorable interactions of the iridium center
with the silica surface or possible decomposition of some
fraction of the iridium complexes tethered to the surface.
Indeed, 11B NMR evidence may suggest that the complex is
altered to some degree upon addition to the silica support.
However, a definitive explanation for the slower rates over the
heterogeneous catalyst will require a more extensive structural
28a
consistent with that of the typical homogeneous catalyst.
The influence of the temperature and catalyst loading on the
activity of the catalyst was also evaluated. The results of entries
3
, 8, and 9 in Table 5 demonstrate that an increased reaction
temperature led to improved yields (in hexane solution). The
data presented in entries 1−4 (Table 5) show that an increased
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investigation of the catalyst focusing on the coordination sphere
of the iridium and boron centers.
showed that the L/Ir ratio of the recycled catalyst was relatively
constant over the recycles, as shown in Table 3. Similarly, the
filtered solid from the recycling test was also characterized by
solid state NMR spectroscopy, IR spectroscopy, nitrogen
physisorption, XRD, and TGA (see above). All the results
were very similar to those of the fresh catalyst, with minor
reductions in porosity and increases in residual organic content,
indicating that the mesostructure of the SBA-15 and supported
metal complex were well preserved after the catalytic reaction.
Thus, the gradual decrease in the catalytic activity during the
recycle was associated not with leaching of iridium metal, but
rather, with other reasons, such as the loss of boronate ester
groups, loss of pore volume and surface area due to desposition
of organic species in the pores during the reactions, or both.
Future studies should focus on elucidating the mechanisms that
lead to loss of activity upon recycle.
After being simply filtered from the reaction media and
washed with CH Cl , Ir-2 could be used repetitively several
2
2
times for the aryl borylation with B pin , as shown in Table 6
2
2
Table 6. Recycling Studies of Ir-2 in Borylation of 1,2-
a
Dichlorobenzene
a
A mixture of B pin (0.25 mmol), 1,2-dichlorobenzene (1.0 mmol),
2
2
and Ir-2 (0.0035 mmol on the basis of iridium) was stirred in hexane
b
1
(
3 mL) at 70 °C. H NMR yields are based on the protons of the
Bpin group in the product and in B pin .
4. CONCLUSIONS
2
2
In summary, we have successfully prepared a mesoporous silica-
supported bipyridine iridium complex Ir-2 for direct C−H
borylation of arenes. This is the first reported supported
and Figure 8. Although the initial TOF of the catalyst was
gradually decreased upon recycling, which is typical of
supported catalysts, the recycled catalyst still afforded 94%
yield after two cycles when using 1,2-dichlorobenzene as the
substrate. The time required to achieve 94% yield increased
from 24 to 48 h over three runs. It should be noted that no
regeneration of the catalyst with an iridium precursor or boron
reagent was carried out between the individual runs. These
results indicate that the silica-supported iridium catalyst Ir-2
possessed good efficiency and recyclability in the aromatic C−
H borylation reaction.
analogue of Hartwig’s 1/2[{Ir(OMe)(cod)} ]/dtbpy catalyst.
2
Both the unsupported catalyst and supported catalyst showed
moderate to good activity for various arenes containing
different kinds of functional groups. The supported catalyst
exhibited a comparable TON relative to a typical homogeneous
catalyst 1/2[{Ir(OMe)(cod)} ]/dtbpy or silica-SMAP-Ir-
2
(
OMe)(cod) developed by Hartwig (homogeneous) and
Sawamura (supported) under similar conditions. More
importantly, the heterogeneous catalyst was stable in air and
could be recovered by simple filtration and used repetitively
with only a modest decrease in catalytic rate. For example, the
use of the recycled catalyst for C−H borylation of 1,2-
dichlorobenzene produced the desired product in very high
yield over multiple cycles. The structural characterization of the
fresh and recycled catalyst suggested that the integrity of both
the metal complex and the inorganic support were retained in
consecutive catalytic runs, showing good potential for
application in practical synthesis in which high TONs are
desired.
To verify that the observed catalysis was due to silica-
supported rather than leached iridium species in solution, a hot
filtration experiment was carried out. As shown in Figure 9, the
ASSOCIATED CONTENT
■
*
S
Supporting Information
Additional experimental details and material characterization
data as noted in text. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Figure 9. Hot filtration test for the Ir-2 catalyzed borylation reaction
of 1,2-dichlorobenzene and B pin with hexane at 70 °C. The solid
Corresponding Author
*E-mail: cjones@chbe.gatech.edu.
2
2
catalyst was filtered off after 10 h in the run represented by red
triangles.
Author Contributions
F.W. and Y.F. contributed equally to this work.
§
Notes
heterogeneous catalyst Ir-2 was filtered from the reaction
mixture after 10 h, and the reaction was continued for an
additional 20 h. It was found that there was no appreciable
change in the yield of the product after the solid catalyst was
removed, and the solid-free solution was colorless. This
observation eliminated the possible catalytic role of homoge-
neous iridium species, confirming that the recycle efficiency was
derived from the heterogeneous catalyst itself. This finding was
consistent with the results of the elemental analysis, which
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Science
Foundation under the Center for Chemical Innovation in
Stereoselective C−H Functionalization (CHE-1205646). F. Wu
gratefully acknowledges the financial support of the China
Scholarship Council (CSC).
1
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dx.doi.org/10.1021/cs4009539 | ACS Catal. 2014, 4, 1365−1375