Bipyridine periodic mesoporous organosilica: A solid ligand for the iridium-catalyzed borylation of C-H bonds

Article abstract of DOI:10.1002/adsc.201400125

With the goal to obtain a molecularly defined iridium(I) heterogeneous C-H functionalization catalyst, a periodic mesoporous organosilica (PMO) containing bipyridylene moieties in a matrix of biphenylene units as ligand platform was designed and fully characterized. The material exhibits a high surface area with small mesopores in a vermicular arrangement, and the pore walls show a highly crystal-like character. After surface passivation with chlorotrimethylsilane and functionalization with chloro(1,5-cyclooctadiene)iridium(I) dimer [{IrCl(COD)}₂] a molecularly defined Ir(I) surface complex was obtained according to EXAFS and UV-Vis spectroscopy. This functionalized material catalyzes the direct C-H borylation of arenes to yield the corresponding boronic esters and can be reused without significant loss of activity.

Full text of DOI:10.1002/adsc.201400125

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DOI: 10.1002/adsc.201400125
Bipyridine Periodic Mesoporous Organosilica: A Solid Ligand for
À
the Iridium-Catalyzed Borylation of C H Bonds
Wolfram R. Grꢀning,a Georges Siddiqi,a Olga V. Safonova,b
and Christophe Copꢁreta,*
a
Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland
Fax : (+41)-44-633-1325; phone: (+41)-44-633-9394; e-mail: ccoperet@ethz.ch
Paul Scherrer Institute, 5232 Villigen, Switzerland
b
Received: February 3, 2014; Published online: February 20, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400125.
Abstract: With the goal to obtain a molecularly de-
fined iridium(I) heterogeneous C H functionaliza-
als.[3] Post-synthetic grafting of organic ligands on the
oxide support has often led to deceiving results.[4]
However major improvements have been achieved
through the controlled incorporation of ligands in
SBA-type materials by co-condensation of functional
organotrialkoxysilanes and tetraethoxysilane in the
presence of structure directing agents, giving rise to
a more regular distribution of ligands within the pores
of the material.[5] These materials enabled the genera-
tion of highly efficient and supported well-defined
metal-based catalysts.[6] Towards the manufacture of
heterogeneous catalysts with molecular precision,
a promising class of materials has recently appeared:
periodic mesoporous organosilicas (PMO).[7] Robust
siloxane bridges hold together the organic linkers that
constitute both the surface and the bulk of the materi-
al. PMOs are advantageous as they are highly ordered
on the nanoscale: possessing small mesopores typical-
ly in a 2D hexagonal framework and exhibiting in
many instances crystal-like ordering in the pore walls,
especially when the organic linkers are arenes.[8] They
thus provide a well-defined immediate environment
making them ideal candidates as catalyst supports.
Most PMOs prepared for catalytic applications so far
rely on monodentate ligands[7e,9] with a few examples
À
tion catalyst, a periodic mesoporous organosilica
(PMO) containing bipyridylene moieties in a matrix
of biphenylene units as ligand platform was de-
signed and fully characterized. The material exhib-
its a high surface area with small mesopores in a ver-
micular arrangement, and the pore walls show
a highly crystal-like character. After surface passi-
vation with chlorotrimethylsilane and functionaliza-
tion chloro(1,5-cyclooctadiene)iridium(I)
with
dimer [{IrClACHTUNGTRENNUNG(COD)}2] a molecularly defined Ir(I)
surface complex was obtained according to EXAFS
and UV-Vis spectroscopy. This functionalized mate-
À
rial catalyzes the direct C H borylation of arenes
to yield the corresponding boronic esters and can
be reused without significant loss of activity.
À
Keywords: C H activation; heterogenous catalysis;
iridium; organic-inorganic hybrid materials; period-
ic mesoporous organosilicas; surface chemistry
Combining the advantages of homogeneous and het- describing more elaborate systems such as phenylpyri-
erogeneous catalysts has been a major research effort dine[10] or BINAP.[11,12]
in the past forty years, necessitating the elaboration
of heterogeneous catalysts with definition on the mo- borylation of C H bonds
In homogeneous catalysis, the Ir(I)-catalyzed direct
[13]
À
has been recognized as
lecular level.[1] One successful approach, called sur- a mild and versatile reaction generating high value
face organometallic chemistry, has consisted in the building blocks[14] directly from easily available start-
controlled grafting of transition metal complexes di- ing materials.[15] To increase lifetime, ease product
rectly on the surface of oxide materials.[2] In many in- separation and avoid product contamination with
stances, this approach has led to catalysts with per- metal traces, immobilization strategies such as a phos-
formances close to or sometimes surpassing those of phine ligand bound to a silica surface have been pro-
their homogeneous analogues. An alternative ap- posed, however this system is limited to the directed
[16]
À
proach is the development of supported homogeneous activation of C H bonds. A second approach uses
catalysts, in which the metal center is coordinated to poorly soluble carboxylic acid bipyridine deriva-
an organic ligand bound to high surface area materi- tives[17]
.
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Wolfram R. Grꢀning et al.
Scheme 1. a) Synthesis of precursor 1a a) 1) n-BuLi, Et2O, À958C, 1 h, 2) MgCl2, À958C, 0.5 h, 3) ClSi
258C, 16 h, 40%; b) Ni(PPh3)2Br2, Zn*, NBu4I, THF, 708C, 16 h, 53%. b) Synthesis of materials c) n-C18H37NMe3, H2O,
NaOH, 258C, 24 h, 978C, 21 h; d) TMSCl, NEt(i-Pr)2, THF, 258C, 16 h; e) [{IrCl(COD)}2], CH2Cl2, 30 min.
A
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Since one of the most promising homogeneous cat- Information) gives rise to a single broad peak at 1.98
À
alysts for C H borylation is based on [{IrX
G
(X=OMe, Cl) activated by bipyridine ligands, we ing a vermicular rather than a perfectly 2D hexagonal
have developed a heterogeneous catalyst based on structure. The combined values for pore size and pe-
the coordination of an Ir(I) molecular precursor to bi- riodicity give a pore wall thickness of ca. 2 nm; a rela-
pyridine moieties in the walls of a PMO. Towards this tively large value, as PMOs of this type typically give
goal a PMO containing bipyridylene units site dis- rise to walls of a few molecular layers.[8,10] In the pore
persed in a biphenylene matrix, for improved site iso- walls the material shows a high degree of ordering as
lation,[18] was prepared and successfully functionalized evident by powder X-ray diffraction (Figure 1, b).
by [{IrCl
G
Five sharp peaks are observed at 2V/8=7.5, 15, 23,
In order to prepare the bipyridine-containing PMO, 31, 38, they can be assigned to a periodic structure of
5,5’-bis(triethoxysilyl)-2,2’-bipyridine (1a) was synthe- 1.16 nm and the corresponding higher order reflec-
tized in two steps via a regioselective lithiation of 2,5- tions. This value is in good agreement with the length
dibromopyridine followed by a nickel(II)-catalyzed of the biaryl fragments and values for similar materi-
reductive coupling (Scheme 1, a). The material als reported earlier.[8b,10a] Transmission electron mi-
bpy1/10-PMO was then synthesized by the hydrolysis croscopy (Figure 1 c, d) confirms the presence of
and co-condensation of 1a (1 equiv.) and 4,4’-bis(tri- a porous structure and also shows molecular scale pe-
ACHTUNGTRENNUNGethoxysilyl)-1,1’-biphenyl 2 (10 equiv.) in dilute aque- riodicity perpendicular to the pore direction. The
ous sodium hydroxide solution in the presence of tri- structural features are homogenously distributed
methylstearylammonium chloride as structure direct- throughout the material (see the Supporting Informa-
ing agent (SDA) as outlined in Scheme 1, b.
tion).
The infrared spectrum of the material (see the Sup-
The parent material bpy1/10-PMO exhibits good
structuration as shown in Figure 1. Nitrogen adsorp- porting Information, Figures S3–S5) shows a sharp
tion and desorption experiments give rise to a type band at 3720 cmÀ1 characteristic of isolated silanol
À
IV isotherm with a small hysteresis loop (Figure 1, a) groups, as well as aromatic C H bands at 3070 and
typical for materials with small mesopores.[18,19] Analy- 3024 cmÀ1. Furthermore the characteristic vibrations
sis according to the Brunau–Emmett–Teller model[20] of the aromatic rings are observed at 1604 cmÀ1 with
yields a surface area of 690 m2 gÀ1, and the maximum a shoulder at 1580 cmÀ1 corresponding to the most in-
of the pore-size distribution was calculated to be ca. tense ring deformation bands of 2 (1600 cmÀ1) and 1a
2.4 nm at the limit of Barrett–Joyner–Halenda[21] anal- (1580 cmÀ1), respectively.
ysis. Small angle X-ray diffraction (see the Supporting
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Bipyridine Periodic Mesoporous Organosilica: A Solid Ligand
Figure 1. Textural properties of bpy1/10-PMO: a) nitrogen adsorption/desorption isotherm at 77 K; b) p-XRD pattern showing
reflections corresponding to a distance of 1.2 nm; c) high magnification showing TEM showing molecular scale and pore
structure periodicity; d) lower magnification image showing the porous structure.
Bpy1/10-PMO was also characterized by solid state T3-sites of the biphenylene units and bipyridylene
NMR spectroscopy; the obtained spectra are shown units. A second peak at À70 ppm corresponds to in-
in Figures S1 and S2 (Supporting Information). The completely condensed T2 sites, probably residing at
13C-cross-polarization magic angle spinning (CP- the surface of the material. No Q-sites are observed
À
MAS) NMR spectrum of the parent material bpy1/10
PMO in Figure S1 (Supporting Information) is domi-
-
emphasizing the stability of the C Si bond.
The surface of the material was passivated with
nated by the four resonances of the biphenylene chlorotrimethylsilane (Scheme 1, b) to give TMS-
groups at 142, 135, 131, and 126 ppm.[8b] A fifth lower bpy1/10-PMO, prior to coordination of the Ir(I) molec-
intensity resonance, well separated, at 155 ppm is ob- ular precursor on the bipyridylene moieties to mini-
served and is characteristic for the a-carbons in the mize unwanted surface interactions. The IR spectrum
bipyridine. The remaining resonances of the bipyridy- (Figures S3 and S4, Supporting Information) shows
lene fragment are buried under the biphenylene com- the disappearance of the isolated silanol band at
À1
À
ponent.
3720 cm and the appearance of alkyl C H bands at
The 29Si CP-MAS spectrum (Figure S2, Supporting 2975 cmÀ1. NMR spectroscopy also shows the appear-
Information) shows exclusively T-sites, the major ance of characteristic TMS resonances at À2 ppm in
peak is observed at À77 ppm and corresponds to the the 13C spectrum (Figure S1, Supporting Information)
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Wolfram R. Grꢀning et al.
Table 1. Illustration of the surface complex in Ir-bpy1/10
-
and at 14 ppm in the 29Si spectrum (Figure S2, Sup-
porting Information). TMS-bpy1/10-PMO was then re-
acted with chloro(1,5-cyclooctadiene)iridium(I) dimer
PMO and summary of EXAFS analysis: N is the number of
centers, R is the interatomic distance, s2 is the Debye-
Waller factor, E0 is the energy shift. The passive electron re-
duction factor SO2 =0.86 was fixed to the value obtained for
[{IrClACHTUNGTRENNUNG(COD)}2] to give a light blue green material Ir-
bpy1/10-PMO. The iridium content of the material was
found to be 1.5% by elemental analysis, which corre-
sponds to a functionalization of ca. 30% of the bipyri-
dylene units in the framework of the material. It is
noteworthy that dark-field transmission electron mi-
croscopy (Supporting Information) shows isolated iri-
dium centers spread over the whole material. The ab-
sence of Ir rich patches is thus consistent with a good
distribution of the bipyridylene units within the mate-
rial matrix. The 29Si NMR spectrum (Figure S2, Sup-
porting Information) remains unchanged, showing
that complexation does not lead to any modification
of the structure of the material. In addition, all textur-
al properties such as surface area and pore size of
bpy1/10-PMO are well preserved in the materials TMS-
bpy1/10-PMO and Ir-bpy1/10-PMO (Supporting Infor-
mation).
[{IrClACHTNUGTRENNUNG
N
R [ꢃ] s2 [ꢃ2]
E0 [eV]
1st shell
2nd shell
N
1-bpy
a-COD
–
2-bpy
b-COD
6-bpy
2
4
1
2
4
2
2.05(1) 0.0047(6) 5.7(6)
2.10(1) 0.0047(6) 5.7(6)
2.34(1) 0.0047(6) 5.7(6)
2.61(4) 0.010(5) 5.7(6)
2.98(3) 0.010(5) 5.7 (6)
2.77(4) 0.010(5) 5.7 (6)
C
Cl
C
C
C
No major changes are observed in the 13C NMR
spectra of Ir-bpy1/10-PMO, mainly because of the high
dilution of functionalized species. The surface func-
tionalization in Ir-bpy1/10-PMO was also monitored by
UV-Vis spectroscopy (Figure 2 and Supporting Infor- ing Information, Figure S7) show nearly identical ab-
mation, Figure S12). The parent material shows a flat sorption edges, confirming the conservation of the ox-
base line up to 320 nm before saturation. The func- idation state of Ir(I) upon complexation. Further-
tionalized material however shows a distinct band at more,
EXAFS
fitting
(Table 1;
Supporting
620 nm that is in good agreement with the analogous Information, Figures S8 and S10) is consistent with iri-
homogeneous complex 3 showing an absorption maxi- dium coordinated to a bipyridine unit by two nitrogen
mum at 600 nm (the preparation and structural char- centers at 2.05 ꢃ, to a COD ligand by four carbon
acterization of 3 is given in the Supporting Informa- centers at 2.10 ꢃ, and a chlorine center at 2.34 ꢃ in
tion).
the first coordination shell. Excluding Cl from the
The structure of the surface complex was estab- first coordination sphere of Ir is detrimental to the fit
lished by X-ray absorption near edge spectroscopy quality (Figure S11, Supporting Information). Further
(XANES) and extended X-ray absorption fine-struc- improvement of the fit was achieved by inclusion of
ture (EXAFS) at the Ir L3 edge.[22] The XANES spec- second neighbor shell consisting of four COD carbons
tra for [{IrClACHTUNGTRENNUNG
(COD)}2] and Ir-bpy1/10-PMO (Support- and four bipyridine carbons (Figure S8, Supporting
Information). These results confirm the formation of
molecularly defined, pentacoordinate
[Ir(I)Cl
(COD)bpyPMO] surface complex. These com-
a
AHCTUNGTRENNUNG
plexes are homogenously distributed within the mate-
rials (Supporting Information).
We then examined the catalytic performance of Ir-
bpy1/10-PMO for the C H bond functionalization of
À
a
series of arenes with bis(pinacolato)diboron
(B2pin2) as shown in Table 2. In the presence of
0.8 mol% of Ir, B2pin2 reacts quantitatively with ben-
zene in 24 h to give exclusively PhBpin, similarly to
the corresponding homogeneous catalysts.[13b] No re-
action (<1%) was observed in the absence of Ir or
bpy1/10-PMO (Table 2, entries 2 and 3). We attribute
the catalytic activity to the presence of both the bipyr-
idine containing material and the iridium surface
functionalization. It is also noteworthy that following
an induction period the reaction is pseudo 0 order in
Figure 2. UV-Vis spectra; left axis absorbance of materials
based on diffuse reflectance spectra and conversion with
Abs=logACHTUNGTRENNUNG 3 (1.8 mM/
(RÀ1). Right axis absorbance of
CH2Cl2).
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Bipyridine Periodic Mesoporous Organosilica: A Solid Ligand
À
Table 2. Catalytic tests for C H activation and borylation.
Entry
Catalyst
Arene
Time [h]
Conversion[a]
Yield[b]
1
2
3
4
5
6
7
8
Ir-bpy1/10-PMO
benzene
benzene
benzene
benzene
benzene-d6
toluene
24
24
24
16
21
24
24
24
95%[c]
0%
<1%
n.r.[f]
>99%
99%
76%[c]
–
–
95%
78%
87%
64%
10%
bpy1/10-PMO
N
E
A
(COD)}2]/bpy[e]
Ir-bpy1/10-PMO
Ir-bpy1/10-PMO
Ir-bpy1/10-PMO
Ir-bpy1/10-PMO
anisole
nitrobenzene
90%
40%
[a]
Conversion of B2pin2 based on GC.
[b]
[c]
[d]
[e]
[f]
GC yield based on hexamethylbenzene as internal standard.
Corresponds to a TOF=5 hÀ1 and 4 hÀ1.
8 mol% catalyst loading was used.
3 mol%, taken from ref.[13b]
Not reported.
benzene, due to its large excess. Under these condi- number of 640. Note however that the catalytic activi-
tions, the turnover frequency (TOF) reaches 4 hÀ1, ty decreased non-linearly hinting towards the partial
which is similar to or somewhat higher than that of deactivation of the catalyst at this loading. The cata-
the originally reported homogeneous system (TOF= lyst wash-off showed no significant iridium content (ꢀ
2 hÀ1).[13b] Ir-bpy1/10-PMO also shows good conversion 1 ppm), except for nitrobenzene where about 5 ppm
with substituted and electron-rich arenes (Table 2, en- are detected, consistent with the much lower observed
tries 6 and 7), both toluene and anisole lead to almost conversion and the fast deactivation of the catalyst.
full conversion (>90%) of B2pin2 after 24 h. Howev-
In conclusion, the synthesis of a periodic mesopo-
er, the very electron-poor nitrobenzene (Table 2, rous organosilica containing bipyridylene units in the
entry 8) gave only a conversion of 40% under identi- pore walls and its subsequent functionalization was
cal conditions, probably due to deactivation of the achieved to yield a molecularly-defined surface iridi-
catalyst.
um complex. The synthesized materials display good
À
For mono-substituted arenes, the regioselectivity is catalytic performances for the C H bond functionali-
close to what is expected from a statistical activation zation of arenes to arylboronic esters, in close resem-
of the C H bond of the meta- and para-positions of blance to the homogenous catalyst based on
À
the arenes. The selectivity patterns (%: o/m/p) ob-
ACHUTGTNNERNUG[{IrClACHTUNTGERN(NGUN COD)}2] and 4,4’-bis-tert-butyl-2,2-bipyridine.
served are very similar for toluene (0/64/36), anisole These materials open new avenues in the develop-
(0/71/29), and nitrobenzene (0/71/29) as it was also ment of well-defined heterogeneous catalysts, and we
observed in the homogeneous systems (0/69/31) and are currently exploring methods to enhance their ac-
(1/74/25) for toluene and anisole, respectively.[13b] The tivity and stability through rational design.
similar activity and regioselectivity of the molecular
and the surface complexes indicate the molecular
nature of the active sites in these materials. The sta- Experimental Section
bility of the catalytic system was evaluated, the cata-
Full experimental details can be found in the Supporting In-
formation.
lyst (0.8 mol%) was used in three consecutive runs
with the same loadings of reactants, which led each
time to greater than 90% conversion with no appar-
ent loss of activity for both toluene or anisole. In fact,
the reaction profile (0 order in benzene with similar
TOF) showed virtually identical behavior upon re-ad-
dition of diborane (Supporting Information). Further-
more, the catalyst loading in the case of benzene
could be reduced to 0.1 mol% yielding 64% conver-
Synthesis of bpy1/10-PMO
In a 100-mL PTFE cylinder trimethylstearylammonium
chloride (0.715 g, 2.05 mmol) was dissolved in water
(44.2 mL) and NaOH (6M, 0.74 mL, 4.4 mmol) was added.
The solution was stirred for 1 hour and then a mixture of
1 (275 mg, 0.57 mmol, 1.0 equiv.) and 4,4’-bis(triethoxysilyl)-
sion after 15 days corresponding to a turnover 1,1’-biphenyl 2 (2.72 g, 57.0 mmol, 10 equiv.) was added
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COMMUNICATIONS
Wolfram R. Grꢀning et al.
dropwise. The solution was stirred at 258C for 24 h and then
heated to 978C without stirring for 24 h. The resulting white
powder was filtered off and washed with water (120 mL ꢄ
3), acetone (120 mL ꢄ 5), and Et2O (120 mL). The material
was dried under vacuum (10À5 mbar) at 858C. The material
was then suspended in ethanol (200 mL) and hydrochloric
acid (2M, 3.8 mL) and stirred for 16 h. The material was fil-
tered off and washed with water (100 mL ꢄ 3), acetone
(100 mLꢄ3), and Et2O (20 mL ꢄ 3). After drying under
vacuum (10À5 mbar) at 858C bpy1/10-PMO was obtained as
white powder; yield: 1.14 g.
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Surface Passivation TMS-bpy1/10-PMO
bpy1/10-PMO (320 mg) was suspended in THF (5 mL) and
NEtACHTUNGTRENNUNG(i-Pr)2 (0.42 mL, 2.4 mmol) and TMSCl (0.25 mL,
2.0 mmol) were added. The suspension was stirred for 16 h
at 258C. Methanol (0.10 mL, 0.25 mmol) was added and stir-
ring was continued before the suspension was filtered and
the filter cake washed with THF/NEtACTHNUTRGEN(UNG i-Pr)2 9:1 (5 mLꢄ2),
THF (10 mLꢄ3), acetone (10 mLꢄ3), and Et2O (10 mLꢄ3).
The material was dried under vacuum (10À5 mbar) at 1408C
to give TMS-bpy1/10-PMO; yield: 280 mg.
Synthesis of Ir-bpy1/10-PMO
The material TMS-bpy1/10-PMO (210 mg) was charged into
a two-chambered Schlenk flask connected by a sintered
glass frit. To the material a solution of [{IrClACTHUNRGTNEUNG(COD)}2]
(40 mg, 0.06 mmol) in CH2Cl2 (6 mL) was added to the ma-
terial. The suspension was stirred for 30 min, filtered and
then washed by 3 cycles of solvent vacuum transfer and fil-
tration. The material was subsequently dried at 258C under
high vacuum (10À5 mbar) to give Ir-bpy1/10-PMO; yield:
170 mg. Care should be taken in handling this material as it
is highly air-sensitive.
Catalytic Test: General Procedure
In a glovebox under an argon atmosphere a 2-mL HPLC
vial was charged with Ir-bpy1/10-PMO (10 mg, 0.8 mmol Ir,
0.8 mol%) and B2pin2 (24 mg, 0.10 mmol, 1.0 equiv.). The
corresponding substrate (60 equiv.) was then added neat
with a syringe. The resulting suspension was immersed into
a sandbath heated to 808C and stirred for 24 h. Aliquots
were taken, diluted in Et2O and analyzed by GC (Agilent
HP-1MS, He). The retention times were calibrated with
known standards. The relative intensity was calibrated with
hexamethylbenzene as internal standard.
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Acknowledgements
We thank Dr. Alexey Fedorov for fruitful discussions and Dr.
Maryna Bodnarchuk and Dr. Frank Krumeich for TEM mi-
crographs.
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