Accessible bidentate diol functionality within highly ordered composite periodic mesoporous organosilicas

Article abstract of DOI:10.1039/c6nj00401f

Periodic mesoporous organosilica (PMO) materials containing methoxy- or methoxyethoxymethyl-ether protected biphenol dopants were synthesized and the protecting groups were cleaved to liberate reactive C₂-symmetric diols within the solid-state material. Treatment of the deprotected PMO materials with phosphoryl chloride yielded phosphate ester functional groups predominantly at the exposed biphenol sites, while protected materials showed only surface phosphate species. Since biphenols are closely related to common ligands for asymmetric catalysis, these results open up significant possibilities for the design of chiral recoverable catalysts with chiral groups embedded in the walls of the material.

Full text of DOI:10.1039/c6nj00401f

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Accessible bidentate diol functionality within
highly ordered composite periodic mesoporous
Cite this:DOI: 10.1039/c6nj00401f
organosilicas†
a
a
ab
Lacey M. Reid, Gang Wu and Cathleen M. Crudden*
Periodic mesoporous organosilica (PMO) materials containing methoxy- or methoxyethoxymethyl-ether
protected biphenol dopants were synthesized and the protecting groups were cleaved to liberate
2
reactive C -symmetric diols within the solid-state material. Treatment of the deprotected PMO
Received (in Montpellier, France)
5
th February 2016,
materials with phosphoryl chloride yielded phosphate ester functional groups predominantly at the
exposed biphenol sites, while protected materials showed only surface phosphate species. Since
biphenols are closely related to common ligands for asymmetric catalysis, these results open up
significant possibilities for the design of chiral recoverable catalysts with chiral groups embedded in the
walls of the material.
Accepted 11th May 2016
DOI: 10.1039/c6nj00401f
www.rsc.org/njc
5
functionality, for example, by sulfonation, Diels–Alder rearrange-
ment, thiol–ene Click reaction or Friedel–Crafts alkylation
Introduction
6,7
8
9
Periodic mesoporous organosilica (PMO) materials have fascinated of ethylene-bridged PMOs, or by modification of phenylene-
1
0,11
12,13
materials and organic chemists for more than a decade since they bridged PMOs to yield amine
or sulfonic acid
functional
are highly ordered combinations of organic cores and polymerized groups. These new functional groups can be modified to yield
silicas. Due to the availability of monomers with highly stable even more complex organic functionality without sacrificing
5
,14,15
C–Si bonds, organic and inorganic regions are perfectly separated, the high degree of order within the PMO material.
While
and when condensation is performed in the presence of these reports describe efficient methods to achieve high
surfactants, high-surface-area materials are produced with loadings of functional groups, the original ‘‘R’’ organic back-
1
highly ordered arrays of pores. These materials have been bone is limited to simple phenyl, alkyl or alkenyl groups
studied as prospective candidates for a wide variety of applications rather than biaryls whose functionalization would be ideal
from catalysis and light-harvesting to chromatography and to prepare two-coordinate complexes. And in many cases,
1–3
sensing, owing to their unique surface properties. In particular, controlling the positional selectivity of the post-synthesis
1
6
manipulation of the central ‘‘R’’ unit in the bridged organosilane modification is challenging.
0
0
monomers [(R O)
powerful aspects of these materials. However, in practice, few defined, functional materials with high levels of functionality at
00% PMO materials have been reported with any significant specific sites. By preparing dopants monomers with protected
complexity in this R group, since complex central cores are known functionality whose structures are designed to pack effectively
3
Si-R–Si(OR )
3
], is proposed as one of the most
Herein we report a unique method for the preparation of well-
1
1,4
to disrupt ordering and stability of the material. This leads to the with the bulk of the material, we are able to incorporate high
use of only low levels of complex organosilane dopant monomers levels of functional dopants in PMO materials without any
that have little effect on the material, or to materials with poor loss of order. We then demonstrate, using traditional organic
order, in which case some of the key properties that make PMOs chemistry techniques, how reactive functional groups can be
interesting are lost.
liberated after the material condensation is complete. Finally,
Several groups have reported the post-synthesis modification we demonstrate that the liberated functional groups (diols)
of organic bridging groups in PMOs can be used to introduce can be reacted in a second stage to produce precursors to
catalytically active species. Through detailed CP MAS NMR
a
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L3N6,
studies, we show that diols can be selectively functionalized in
the presence of surface silanols. Overall this work provides the first
detailed assessment of the strategy of incorporated functionally
Canada. E-mail: cruddenc@chem.queensu.ca
b
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University,
Nagoya 464-8602, Japan
active monomers in PMO materials without destruction of local
or long-range order.
†
Electronic supplementary information (ESI) available: Additional solid-state NMR
data and NMR characterization of PMO precursors. See DOI: 10.1039/c6nj00401f
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Experimental
Materials and methods
High-resolution mass spectroscopy analysis (HRMS) was
performed by the Mass Spectroscopy and Proteomics Unit at
Queen’s University, Kingston, Ontario, Canada. Spectra were
measured on a Micromass GCT GC-EI TOF mass spectrometer
All chemicals were purchased from Sigma Aldrich, TCI, Fisher
Scientific or Acros Organics and used without further purification
unless otherwise noted. Catalyst (1,5-cyclooctadiene)chloro-
rhodium(I) dimer was generously provided by Johnson-Matthey.
Anhydrous DMF was purchased from EMD Chemicals Inc. in
a DrySolv container. Anhydrous ethanol was purchased from
Commercial Alcohols and used without further treatment.
Triethylamine and dichloro-methane were distilled over CaH2
before use. THF and hexanes were distilled from sodium metal
and benzophenone ketyl before use. Pyridine was distilled and
stored over sodium hydroxide.
+
by EI fragmentation.
XRD analysis was performed using a SMART6000 2D detector
equipped with a fixed chi, 3 circle goniometer, parallel beam
optics, and Rigaku Cu rotating anode operating at 50 kV and
90 mA. Powder samples were loaded between Mylar films and
spectra were corrected for Mylar background signals. Data was
collected in transmission mode over two frames at 600 s each, at
a measuring angle of a 2y range of 1–401. Measurements were
recorded by the McMaster Analytical X-ray Diffraction Facility
(MAX) in Hamilton, Canada.
Solution NMR were recorded on a Bruker Avance 300 with a
TEM analysis was performed using a JEOL 2010 scanning
1
13
QXI probe ( H: 300.1, C: 75.5 MHz), Bruker Avance 400 with a
transmission electron microscope operating at 200 keV. The
powder sample, dispersed in ethanol, was dropped onto a
porous carbon grid to fix the sample to the grid upon drying.
The measurements were performed at Microscopy and Micro-
analysis facility at the University of New Brunswick, Fredericton,
New Brunswick, Canada. Nitrogen adsorption experiments were
performed using a Micromeritics ASAP 2010 physisorption
analyser with nitrogen as the adsorbate gas. Powder samples
were degassed at 80 1C for at least 8 hours before analysis.
1
13
31
BBFO probe ( H: 400.1, C: 101, P: 162 MHz) or Bruker
1
13
Avance 500 with a BBFO probe ( H: 500.0, C: 125.5 MHz).
Shifts are reported in parts per million (ppm) with J coupling
constants indicated in Hertz. H and C spectra were calibrated
from the solvent residual peak in CDCl
1
13
3
1
3
or DMSO-d
6
and
P
spectra were calibrated externally against 85% H
3
PO
4
in water.
Signal multiplicity is indicated by the abbreviations: s = singlet,
d = doublet, t = triplet, q = quartet, m = multiplet.
1
3
29
31
Solid-state C, Si and P NMR measurements were
recorded on a Bruker Avance 600 NMR spectrometer using a
Bruker 5 mm CP MAS probe, at a frequency of 151, 119 and
Synthesis of organosilica monomers
0
0
Synthesis of precursor (6). The 4,4 -dinitro-2,2 -dimethoxy-
1
3
29
243 MHz, respectively. The C and Si CP MAS spectra were
0
1
,1 -biphenyl 6 was prepared using a literature procedure, and
referenced to adamantane, tetramethylsilane and ammonium
spectral data for the prepared compound matched the reported
13
1
7
dihydrogen phosphate, respectively. D1 delay time was 2 s for
C
characterization data. The product was used as prepared for
and Si spectra, and 3.5 s for P. The rotor was spun at a frequency the next step in the synthesis.
29
31
0
0
0
of 10–12 kHz and the number of scans was typically 800–2000,
4,4 -Diamino-2,2 -dimethoxy-1,1 -biphenyl (7). Tin(II) chloride
1
3
31
29
0
0
0
1
000–2000, and 2000–4000 for C, P and Si, respectively.
dihydrate (25.7 g) and the 2,2 -dimethoxy-4,4 -dinitro-1,1 -
High-field H MAS, P CP MAS and P– H HETCOR NMR biphenyl 6 (3.45 g) were dissolved in anhydrous ethanol (120 mL)
experiments were run by Dr Victor V. Terskikh on a Bruker and the reaction was stirred for 24 h at 70 1C. The mixture was
1
31
31
1
1
31
NMR ( H = 899.8 MHz, P = 364.2 MHz) at the High-Field NMR poured over crushed ice and the pH was adjusted to 7–8 with
1
31
Facility in Ottawa, Ontario, Canada. H and P spectra were sodium bicarbonate. In a large separatory funnel, the thick
referenced to adamantane (10 kHz MAS) and ammonium aqueous mixture was gently agitated with ethyl acetate to extract
dihydrogen phosphate, respectively, at 21.1 T using a 2.5 mm the product. The organic phases were combined, washed with
1
H/X MAS Bruker probe. H MAS NMR spectra of PMO materials brine and dried over magnesium sulfate. Removal of the solvent
were recorded with a D1 delay time of 10 s with 90–90 rotor-sync in vacuo gave a light tan powder (2.63 g, 95% yield). The crude
echo, spinning at 20 kHz MAS for 32 scans. BINOL hydrogen product matched the literature spectral data and was used in
1
phosphate reference spectra were recorded with a D1 delay time the next step without further purification. H NMR (400 MHz,
3
1
of 30 s with 16 scans. P MAS NMR spectra of PMO materials CDCl , d ppm): 6.99 (d, 2H, J = 8.3 Hz), 6.31–6.29 (m, 4H), 3.70
3
1
3
were recorded with direct polarization of phosphorous, H–P (s, 6H), 3.68–3.59 (bs, 4H), C NMR (101 MHz, CDCl , d ppm):
3
decoupled with a D1 delay time of 5 s and P1 width of 2 ms, 158.1, 146.7, 132.4, 118.3, 107.1, 99.1, 55.6. TOF HRMS (EI+)
3
1
spinning at 20 kHz for at least 6000 scans. P MAS NMR of found 244.1204 m/z (calcd for C H N O : 244.1212).
1
4
0
16 2 2
0
0
BINOL hydrogen phosphate reference spectra were recorded
with a D1 delay time of 30 s, P15 contact time of 5 ms, spinning was carried out according to a literature procedure. The
at 20 kHz for 32 scans. P– H HETCOR of PMO samples were product was obtained in 40% yield after purification by silica
4,4 -Diiodo-2,2 -dimethoxy-1,1 -biphenyl (8). Iodination of 7
1
7
3
1
1
recorded with no homonuclear decoupling, with a D1 delay gel flash chromatography (98 : 2 petroleum ether : ethyl acetate)
1
time of 3 s, P15 contact time of 500 ms, td2 ꢀ td1 = 1000 ꢀ 48,
H NMR (300 MHz, CDCl , d ppm): 7.26 (dd, 2H, J = 7.9, 1.5 Hz),
3
3
1
1
13
spinning at 20 kHz MAS for 2048 scans. P– H HETCOR of 7.19 (s, 2H), 6.84 (d, 2H, J = 6.8 Hz), 3.68 (s, 6H), C NMR (101
BINOL hydrogen phosphate reference spectra were recorded MHz, CDCl , d ppm): 157.4, 132.6, 129.7, 126.5, 120.5, 93.6,
3
+
with a D1 delay time of 30 s, P15 contact time of 200 ms, td2 ꢀ 55.9. TOF HRMS (EI) m/z: [M + H] calcd for C H I O :
1
4
12 2 2
td1 = 1000 ꢀ 192 with 8 scans collected (20 kHz MAS).
465.8927; found 465.8939 m/z.
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0 0 0
,4 -Diiodo-2,2 -dihydroxy-1,1 -biphenyl (9). Compound 8 80 1C under high-vacuum. The brown residue was triturated
4
(
1.33 g) was dissolved in dry dichloromethane (5.8 mL) in a with diethyl ether (3 ꢀ 5 mL) and these combined volumes were
flame-dried 50 mL round-bottom flask under argon gas. The concentrated down to B0.5 mL and loaded onto a silica gel
solution was cooled to ꢁ78 1C and a 1 M solution of BBr in column. The product was purified by flash chromatography
3
dichloromethane (8.6 mL) was added cautiously. The reaction (7 : 1 petroleum ether : ethyl acetate) to give a yellow oil (113 mg,
1
was held at ꢁ78 1C for 2 h then allowed to warm to room 45% yield). By H NMR spectroscopy, the product contained a
temperature overnight. The flask was placed on ice and the small amount of mono-silylated monomer that could not be
reaction was quenched slowly with methanol and subsequently removed. The product was carried on to materials synthesis.
1
diluted with distilled water. Aqueous sodium thiosulfate was
3
H NMR (300 MHz, CDCl , d ppm): 7.34–7.27 (m, 6H), 3.94 (q,
1
3
added and the product was extracted with ethyl acetate. The 12H, J = 7.0 Hz), 3.81 (s, 6H), 1.30 (t, 18H, J = 7.0 Hz), C NMR
combined organic phases were washed with aqueous sodium (75.5 MHz, CDCl , d ppm): 156.5, 131.3, 131.1, 129.9, 127.0,
3
+
thiosulfate, water and brine, and dried over magnesium sulfate. 117.0, 59.2, 55.7, 18.3. TOF HRMS (EI) m/z: [M + H] calcd for
The solvent was removed in vacuo to give a tan powder. The C H O Si : 538.2418; found 538.2433 m/z.
2
6
42
0
8
2
0
0
crude was recrystallized with ethyl acetate/hexanes (1 : 1) to give
4,4 -Bis(triethoxysilyl)-2,2 -di(2-methoxyethoxymethyl)-1,1 -
1
fluffy off-white needles (723 mg, 58% yield). H NMR (300 MHz, biphenyl (3). The bis-iodo compound 10 was prepared using
CDCl , d ppm): 7.41 (d, 4H, 7.2 Hz), 6.97 (d, 2H, 8.1 Hz). identical conditions to the dimethoxy monomer 2. The brown
H NMR (300 MHz, DMSO-d d ppm): 9.66 (s, 2H), 7.23 (d, residue remaining after Kugel Rohr distillation was triturated
H, J = 1.7 Hz), 7.16 (dd, 2H, J = 8.0 Hz, 1.7 Hz), 6.89 (d, 2H, J = with THF and these combined fractions were concentrated and
3
1
6
2
1
1
1
3
.7 Hz). C NMR (101 MHz, DMSO-d d ppm): 156.2, 133.5, loaded onto a silica gel column. The product was purified by
6
+
27.8, 125.1, 124.8, 93.7. TOF HRMS (EI) m/z: [M + H] calcd for flash chromatography (1 : 1 petroleum ether : ethyl acetate) to
1
C H I O : 437.8614; found 437.8619 m/z.
give a yellow oil in 41% yield. H NMR (400 MHz, CDCl , d ppm):
1
2
8 2
0
2
3
0
0
4,4 -Diiodo-2,2 -di(2-methoxyethoxymethyl)-1,1 -biphenyl (10). 7.51 (s, 2H), 7.38 (dd, 2H, J = 7.5 Hz, 0.9 Hz), 7.27 (d, 2H, J =
0
0
Compound 9 (888 mg) was dissolved in THF (10 mL) in a dried 7.5 Hz), 5.16 (s, 4 H), 3.93 (q, 12H, J = 7.0 Hz), 3.63 (AA BB , 4H,
0 0
5
0
0 mL round-bottom flask under argon. The flask was cooled to J = 12.1 Hz, 5.7 Hz, 3.8 Hz), 3.46 (AA BB , 4H, J = 12.1 Hz, 5.7 Hz,
1
3
1C in an ice bath. NaH 60% in mineral oil was weighed out 3.8 Hz), 3.53 (s, 6H), 1.30 (t, 18H, J = 7.0 Hz), C NMR (101 MHz,
, d ppm): 154.47, 131.75. 131.36, 131.03, 128.39, 121.84,
(490 mg) and added to the reaction by quickly removing the CDCl
3
rubber septum. The mixture bubbled vigorously and the yellow 94.50, 71.56, 67.53, 58.96, 58.82, 18.27. TOF HRMS (EI) m/z:
+
suspension became a tan slurry. The mixture was stirred for [M + H] calcd for C H O Si : 686.3154; found 686.3145 m/z.
3
2
54 12
2
0 0 0
1
hour at 0 1C and another portion of NaH in mineral oil
4,4 -Diiodo-1,1 -biphenyl-2,2 diyl hydrogen phosphate. The
(100 mg) was added. MEMCl was subsequently added dropwise bis-iodo compound 9 (100 mg) was dissolved in dry pyridine
at 0 1C and the reaction was allowed to slowly warm to room (0.5 mL) in a dried 3 mL vial purged with argon gas. Phosphoryl
temperature. After 18 h the reaction was quenched dropwise chloride (0.05 mL, 2 eq.) was added at room temperature and
with water. When the mixture ceased bubbling it was diluted the vial was sealed with a Teflon-llined cap and heated to 80 1C.
with water and ethyl acetate. The aqueous phase was extracted The clouded mixture gradually became a clear dark amber
with ethyl acetate and the combined organic phases were solution. After 4 h stirring at 80 1C, the reaction was cooled
washed with brine and dried over sodium sulfate. The solution and the vial placed on ice. Distilled water was added cautiously
was concentrated under vacuum to a volume of B1 mL and (0.2 mL) and vigorous bubbling was observed. When bubbling
then loaded onto a silica gel column. The crude was purified by had ceased, the vial was capped and heating at 80 1C was
flash chromatography (3 : 1 petroleum ether : ethyl acetate) to resumed. The reaction was stirred for 48 h then cooled and
1
yield the product as a yellow oil (724 mg, 58%). H NMR diluted with ethyl acetate and poured into dilute HCl. The
(400 MHz, CDCl
3
, d ppm): 7.61 (d, 2H, J = 1.6 Hz), 7.41 (dd, aqueous phase was extracted with ethyl acetate until clear. The
2
H, J = 8.0 Hz, 1.6 Hz), 6.91 (d, 2H, J = 8.0 Hz), 5.15 (s, 4H), 3.69 combined organic phase was dried over sodium sulfate and the
0
0
0
0
13
(AA BB , 4H), 3.52 (AA BB , 4H), 3.40 (s, 6H). C NMR (101 solvent was removed in vacuo to give an off-white solid that was
MHz, CDCl , d ppm): 155.3, 132.5, 131.1, 127.7, 124.6, 94.2, triturated with dichloromethane. Recrystallization of the solid
3
+
1
9
3.4, 71.5, 67.8, 59.1. TOF HRMS (EI) m/z: [M + H] calcd for with 1: 1 ethyl acetate/methanol gave a tan solid (33 mg). H NMR
C
20
H
24
0
I
2
O
6
: 613.9662; found 613.9650 m/z.
6
(400 MHz, DMSO-d , d ppm): 7.72 (d, 2H, J = 7.6 Hz), 7.58 (s, 2H),
0
0
13
4,4 -Bis(triethoxysilyl)-2,2 -dimethoxy-1,1 -biphenyl (2). Com- 7.37 (d, 2H, J = 7.9 Hz). C NMR (101 MHz, DMSO-d
6
, d ppm):
3
1
pound 8 (218 mg), chloro(1,5-cyclooctadiene)rhodium(I) dimer 134,9, 131.5, 130.6, 128.3, 95.6. P NMR (162 MHz, DMSO-d
catalyst (10 mol%, 33 mg) and tetrabutylammonium iodide d ppm): 3.34.
6
,
(
330 mg) solids were dissolved in anhydrous DMF (2.3 mL) in
a dried, 20 mL glass vial under argon flow. Cyclooctadiene
0.06 mL), triethoxysilane (0.34 mL) and triethylamine (0.39 mL) Materials were prepared according to a reported procedure for
General procedure for synthesis of PMO materials
(
1
8
were added by syringe through the rubber septum capping the vial. biphenylene-bridged PMO materials. In a typical procedure,
The septum was quickly replaced with a Teflon-lined lid and the a mixture of the organosilane precursor 2 or 3 with bulk
capped vial was heated to 80 1C for 48 hours. The solvent and monomer 1,4-bis(triethoxysilyl)biphenyl (BTESBp) 1, was added
excess volatile reagents were removed by Kugel Rohr distillation at dropwise to a stirring alkaline solution (0.51 M aq. NaOH) of
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octadecyltrimethylammonium chloride surfactant (1.3 eq. to with distilled water, ethanol and ethyl acetate. The material was
total organosilane) at room temperature. The sol was stirred at dried at 80 1C under vacuum.
room temperature for 24 hours at 600 rpm, then subjected to
Mesoporous silica MCM-41 was prepared from tetraethyl-
static aging at 95 1C for 24 hours. The white solid was collected orthosilicate (10.5 mL) added to a stirring solution of C TAB
1
6
by vacuum filtration and washed with several portions of surfactant (2.4 g) and 30% ammonium hydroxide (10.5 mL) in
distilled water. The surfactant template was removed by stirring distilled water (120 mL). The sol was stirred for 2 hours at room
the powder in either ethanol or acidic ethanol (0.05 M in HCl) temperature then filtered over vacuum, washing with distilled
at 60 1C. The powder was again collected by vacuum filtration water. The material was dried for 24 h at 80 1C under vacuum
and washed with ethanol, then dried overnight in a vacuum before further treatment.
oven at 80 1C.
The MCM-41 (40 mg) was suspended in dry pyridine (0.7 mL)
To passivate the surface of the PMO materials, the solid was in a dried 3 mL vial purged with argon gas, then phosphoryl
ꢁ
1
suspended in dry hexanes (50 g L ) and hexamethyl-disilazane chloride (0.12 mL) was added at room temperature. The vial
ꢁ
1
(
0.003 L g PMO) was added dropwise to the stirring solution was sealed with a Teflon-lined cap and the reaction stirred for
at room temperature. The suspension was heated to 65 1C for 1 hour at room temperature. After 1 h the reaction was cooled
8 h and then cooled down and filtered over vacuum. The on ice and quenched cautiously with a few drops of distilled
1
powder was washed with ethyl acetate, anhydrous ethanol and water, then continued to stir at room temperature for 4 h before
then acetone before being dried overnight at 80 1C under the solid was filtered over vacuum and washed with distilled
vacuum to give the TMS-capped PMO.
water and ethyl acetate. The powder was dried overnight at
0 1C under vacuum to give the HOP(O)-material as a white
8
powder (35 mg).
Deprotection of PMO materials
HOP(O)-MCM was suspended in 2 M HCl and stirred at
room temperature for 18 h before filtering over vacuum and
washing with 2 M HCl, distilled water, then ethyl acetate. The
material was recovered as a white solid (30 mg) after drying
overnight at 80 1C under vacuum.
Methoxy ether deprotection of solid-state materials was
achieved with boron tribromide. The PMO material (40 mg)
was suspended in dry dichloromethane (1.5 mL) and cooled to
ꢁ
78 1C. A solution of BBr 1 M in dichloromethane (1.5 mL)
3
was added dropwise and the suspension was stirred for a
further 2 hours at ꢁ78 1C before slowly warming to room
temperature. After 48 hours, the reaction was quenched with
distilled water and the material was collected by vacuum
filtration. The powder was washed with sodium thiosulfate,
water, and ethanol, before being dried overnight at 80 1C under
vacuum to recover 34 mg white powder.
Results and discussion
PMO materials functionalized with a biphenol-type dopant
In ground breaking work, Inagaki and co-workers have shown
that simple aromatic monomers such as bis(triethoxysilyl)-
biphenyl (BTESBp, 1) can form highly ordered materials with
crystal-like ordering of the monomers within the PMO pore
For deprotection of MEMO-PMOs, the solid-state material
(47 mg) was suspended in 15 mL acidified ethanol (0.05 M in
HCl) and stirred for 24 h at 60 1C. The solids were filtered over
vacuum and washed with ethanol then dried at 80 1C under
vacuum to give 31 mg PMO.
1
8
walls. These unique structures may result in interesting
surface properties including confinement effects that would
1
9
be advantageous in catalysis.
Structurally related biphenols and binapthols are widely
General procedure for preparation of phosphate ester PMOs
20,21
employed as ligands for metal-catalyzed reactions
and thus
PMO solids (40–50 mg) were suspended in dry pyridine (B1 mL) we chose to prepare protected versions of these ligands and
1
8
in a dried 3 mL vial purged with argon. Phosphoryl chloride attempt their incorporation in crystalline BTESBp (1)-based
0.05–0.1 mL) was added at room temperature. The suspension PMOs. We anticipated that removal of the ether protecting
(
was stirred at room temperature or 80 1C for 18–24 h before groups would be readily accomplished post-synthesis to generate
cooling to 0 1C in an ice bath and quenching by a few drops of reactive diols (Fig. 1). The reactivity of the liberated C -symmetric
2
22
distilled water. When the bubbling had ceased, the PMOs were diol would then be tested by the formation of a phosphate ester.
left to stir for 9 h at room temperature after which they were Functional monomers 2 and 3 were thus targeted as precursors
diluted with distilled water. The materials were filtered by to the final deprotected biphenol species. In 2, the phenol
vacuum, washing with 2 M HCl, water, ethanol and ethyl acetate, functional groups are protected as methyl ethers, chosen for
then dried overnight at 80 1C under vacuum. PMOs withstood the their small size to be minimally disruptive to the crystal-like
treatment in pyridine at 80 1C without any degradation, but the ordering of the PMO pore walls. Monomer 3, bearing a methoxy-
reaction proceeded to the same degree at room temperature over ethoxymethyl ether (MEM) substituent, was also prepared since
31
2
4 h by P NMR, therefore the reported data of HOP(O)-PMOs this protecting group is typically removed under milder conditions
herein describe materials treated with POCl at room temperature. than the simple methyl ether in 2. The synthesis of both mono-
3
To determine extent of phosphate ester cleavage, the mers begins with commercially available precursor 4 through
0
0
0
HOP(O)-PMO was suspended in 2 M HCl and stirred at room modification of a known route to 4,4 -diiodo-2,2 -dimethoxy-1,1 -
1
7
temperature for 18 h before filtering over vacuum and washing biphenyl 8 (Scheme 1). From 8, MEM-protected precursor 10
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Scheme 1 Synthesis of methoxy- and MEM-protected monomers.
P P
Table 1 Surface area (SA), pore diameter (D ) and pore volume (V )
characterization of PMO materials doped with 2 or 3. Dopants are
expressed in molar% by formulation
a
b
c
Dopant
(mol%)
SA
D
P
V
P
2
ꢁ1
3
ꢁ1
PMO
(m g
)
(Å)
(cm g )
1
00%-BTESBp-PMO
MeO-cPMO
0
(0)
711
591
647
631
723
636
764
20.5
20.6
20.7
20.6
20.4
19.2
19.3
0.521
0.407
0.421
0.433
0.628
0.499
0.613
1
0
2 (10)
2 (20)
2 (30)
3 (10)
3 (20)
3 (30)
Fig. 1 (a) Functional monomers are combined with BTESBP 1 to prepare
2
MeO-cPMO
(b) PMO materials with dopants embedded in the pore walls; (c) removal
30
10
20
MeO-cPMO
of protecting groups liberates the free biphenol functionality within the
solid-state PMO. MEM = methoxyethoxymethyl.
MEMO-cPMO
MEMO-cPMO
MEMO-cPMO
30
a
b
c
Calculated from BET isotherm. Calculated by BJH method. Single
0
point total pore volume at P/P 0.9999 mmHg.
could be obtained in two steps. The final silylated monomers 2
and 3 were prepared by rhodium-catalyzed cross-coupling of the
2
3
respective bis-iodo precursor 8 or 10 with triethoxysilane.
PMO materials were then prepared via base-catalyzed up to 30% loading of the functional monomer.
co-condensation of bulk monomer 1 and dopant 2 or 3 at up Incorporation of dimethoxybiphenyl monomer 2 in the
remained virtually identical to the parent PMO material, even
1
3
to 30% loading, employing an alkylammonium surfactant composite PMO (cPMO) was confirmed by solid-state C cross-
as the structure-directing agent according to the method polarization magic angle spinning (CP MAS) NMR of the solid
previously described for biphenylene-bridged PMOs. A mild powders. In addition to resonances at ca. 135 ppm due to
solution extraction with ethanol at 60 1C was employed to aromatic carbons in both dopant and bulk monomers, signals
1
8
3
remove the templating surfactant. This procedure was used to at 52 and 156 ppm representing the O–CH and (Ar)C–O carbons
X
X
prepare composite PMOs (cPMOs) named MeO- and MEMO- were observed, increasing in magnitude as the loading of the
cPMOs, where X is the mol% loading of dopant 2 or 3, dopant in the PMO material increased (Fig. 2a).
respectively. These materials exhibited mesoporous capillary
condensation isotherms with average pore sizes of approximately with the major Si signals representing T (R–SiO OH) and T
3
MeO-cPMOs showed a high degree of monomer condensation
2
9
2
2
3
21 Å, similar to 100% BTESBp materials (Table 1). Surface areas (R–SiO ) sites (Fig. 2b). The integrity of the organosilane monomers
in the doped materials were typically slightly lower compared was preserved in all of the cPMO materials, as confirmed by the lack
29
to PMOs prepared solely from BTESBp, indicating small of Si CP MAS NMR signals at ꢁ100 ppm (Q-sites) that would
morphological changes within the PMO, however pore sizes indicate Si–C bond cleavage (Fig. 2b).
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13
29
Fig. 2 Solid-state (a) C and (b) Si CP MS NMR of doped MeO-cPMO materials, with MAS side bands indicated with an asterisk (*).
Solid-state NMR also confirmed passivation of surface sizes was calculated from the adsorption isotherm data using
silanols after treatment with hexamethyldisilazane, which was the Barrett–Joyner–Halenda model and showed remarkably
employed prior to cleavage of the methoxy ether protecting similar pore sizes for all doped materials, at 20 ꢂ 0.7 Å.
groups (Fig. 2a and b).‡ The surface trimethylsilyl groups
Transmission electron microscopy (TEM) images of the
1
3
were visible at ꢁ1 ppm in the solid-state C NMR, and the MeO-cPMOs were similar for both 20%- and 30%-doped materials.
2
9
corresponding Si signal for the silicon environment of TMS While the layering of the thick, amorphous particles tended to
appeared at 5 ppm. obscure the mesoscopic ordering patterns by TEM, images
The MeO-functionalized cPMO materials retained a high focused on the thinner edges of particles clearly show 2D
degree of order in the pore walls even at high dopant loadings. hexagonal packing of mesopores (Fig. 3c and d).
In addition to the parent signal at 2y = 1.98 due to the 45 Å d100
A related series of MEMO-cPMO materials were prepared by
spacing of the hexagonally ordered mesopores, powder X-ray doping 10–30% of monomer 3 into BTESBp. Surprisingly, the longer-
diffraction (pXRD) analysis in the medium-angle region showed chain MEM protecting groups were also well tolerated and showed
strong signals at d spacings of 11.8, 5.92, 3.95, 2.96 and 2.37 Å no disruption to long-range ordering of the monomers in cPMO
(Fig. 3a). These higher order reflections indicate a lamellar materials at up to 30% of the MEMO dopant by formulation§
ordering of monomers in a crystal-like lattice within the pore (Fig. 4a). The MEMO-cPMOs displayed high surface areas of
2
ꢁ1
walls with a molecular spacing of d = 11.8 Å (2y = 7.48), 4600 m g with pore diameters of 19 Å (Fig. 4b), which are slightly
corresponding to the length of the bridging biphenylene unit. smaller than the MeO-cPMOs. As the dopant loading was increased
This is in agreement with previously reported 100% biphenylene- across the set of cPMO materials, similar to the MeO-cPMO
18
bridged PMOs. Remarkably, composite PMO materials prepared materials, no large differences were observed at different loadings of
with 30 mol% of dopant 2 showed no change in the number or dopant, consistent with the fact that these dopants are not disruptive
intensity of higher order diffraction peaks consistent with long to the overall structure of the material (Table 1 and Fig. 4b).
range ordering,§ suggesting that the dopant does not disrupt the
highly ordered structure of the parent biphenylene-bridged PMO. Deprotection within the PMO: accessing the free biphenol
Attempts to reach even higher loadings of 2 resulted in no poly-
condensation and precipitation of the PMO solid from solution.
With several families of highly ordered BTESBp PMO materials
doped with 2 or 3 in hand, the next step was to cleave the
The MeO-cPMO materials showed Type-IV nitrogen adsorption
methoxy or MEM ethers to liberate free diol groups within the
isotherms similar to 100% BTESBp PMOs, and displayed
material. Deprotection of the MeO-cPMOs to produce the corre-
isotherms characteristic of ordered porous materials even at
sponding HO-cPMO materials was carried out with boron
30 mol% loading of dopant 2 (Fig. 3b). The distribution of pore
3
tribromide (BBr ) under conditions typically used for the
24
deprotection of simple organic compounds. Despite the relatively
harsh conditions, the composite PMO materials were stable during
deprotection, showing no indication of decomposition in the solid-
‡
TMS-passivation of the surface was not a requirement for the deprotection, but
a larger excess of boron tribromide (up to 10 eq.) was necessary without
passivation.
29
13
state Si NMR spectra (Fig. S1, ESI†). Solid-state C NMR spectra
were recorded before and after treatment of the material to
investigate the extent of deprotection (Fig. 5a). The aliphatic
§
Actual incorporation of the dopant within the materials can be roughly
13
estimated by line fitting of solid-state C NMR data. These are not without
notable errors. See ESI† for details.
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Fig. 4 (a) Medium and (inset) small angle XRD diffractogram of PMOs with
2
0% and 30% dopant loadings of monomer 3, with the diffractogram for
MEMO-cPMO offset by 50 000 counts for ease of viewing; (b) nitrogen
3
0
adsorption isotherms and pore size distributions of MEMO-cPMO materials.
Although there have been some reports of apparently complete
25
deprotection of embedded methyl ethers, typically more easily
26,27
removed protecting groups need to be employed,
and it is
Fig. 3 (a) Medium and (inset) small angle XRD diffractogram of MeO-cPMOs,
30
often observed that some embedded protecting groups are never
with the diffractogram for MeO-cPMO offset by 200 000 counts for ease of
viewing; (b) nitrogen adsorption isotherms and pore size distributions for removed, a factor likely dependent on morphology.
9
20
30
MeO-cPMOs; and TEM images of (c) MeO-cPMO and (d) MeO-cPMO
As expected, removal of the MEM groups could be affected
under milder conditions than those required with the methyl
ether: considerable cleavage was even observed during acidic
extraction of the surfactant from the PMO pores (Fig. S2, ESI†),
and prolonged heating in the presence of acidic ethanol
allowed the majority of the MEM groups to be deprotected
in situ.
with scale bar indicating 50 nm.
peak at 52 ppm originating from the methoxy carbon decreased
significantly after the deprotection. The aromatic carbon signal
at 156 ppm was observed to shift upfield to 152 ppm as the free
hydroxyl group was liberated. However, even treatment with
a large excess of boron tribromide over 48 hours at room
temperature, did not result in complete deprotection of all
methoxy groups. These remaining protected monomers can be
13
20
Fig. 5b shows the solid-state C NMR profile of a MEMO-
cPMO material first treated with pure ethanol to remove the
surfactant, and then with acidified ethanol to cleave the
MEM ether groups to give hydroxyl functionalized materials
1
3
observed in the solid-state C NMR as residual peaks at 52 and
56 ppm, however it is clear that a large number of methoxy
(HO-cPMO).¶ As observed with the MeO-cPMO materials, a
1
small proportion of inaccessible MEM groups were observed,
as evidenced by the small aliphatic signals at 67 and 70 ppm
shown in Fig. 5b. Larger signals at 15 and 57 ppm are thought
to come from the byproducts of the MEM hydrolysis that may
also become trapped in the organosilica matrix.
groups have been removed by the process.
Passivation of the cPMO surface with trimethylsilyl groups
prior to the Lewis-acid assisted deprotection was also examined,
and also showed residual signals from the methoxy-protected
monomer (Fig. S1, ESI†). Repeated treatment of the deprotected
PMO under the same conditions did not appear to promote
further deprotection, suggesting that some dopant monomers
are buried within the organosilica framework and are not
Interestingly, in the case of the MEMO-cPMOs, TMS-
passivation actually hindered the deprotection, possibly by
¶
Due to the weaker ability of neutral ethanol as an extraction solvent, some
3
accessible to BBr .
surfactant is still observed in the pores.
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P P
Table 2 Surface area (SA), pore diameter (D ) and pore volume (V )
characterization of surface-passivated and deprotected PMO materials
prepared with dopant 2 or 3. Dopants are expressed in molar% by
formulation
a
b
c
Dopant
(mol%)
SA
D
P
V
P
2
ꢁ1
3
ꢁ1
PMO
(m g
)
(Å)
(cm g )
20
30
30
20
30
20
20
d
MeO-cPMO-TMSd
2 (20)
2 (30)
2 (30)
3 (20)
3 (30)
3 (20)
3 (20)
858
716
712
738
755
710
777
17.4
17.4
17.7
19.1
19.2
19.3
18.8
0.522
0.495
0.401
0.452
0.447
0.506
0.489
MeO-cPMO-TMS
d,e
MeO-cPMO-TMS
MEMO-cPMO
MEMO-cPMO
f
MEMO-cPMO-H
MEMO-cPMO-H
d, f
a
b
c
Calculated from BET isotherm. Calculated by BJH method. Single
d
point total pore volume at P/P 0.9999 mmHg. TMS-passivated PMO
materials. Treated with boron tribromide. Treated with acidified ethanol.
0
e
f
To prepare supported organophosphate derivatives, deprotected
materials were suspended in pyridine and treated with phosphoryl
chloride (POCl ) overnight at room temperature before quenching
3
the reaction with water to produce the hydrogen phosphate
derivative 11 (Fig. 6). The reaction mixture was filtered and
excess reagents were further removed by washing successively
with 2 M HCl, distilled water, ethanol, and organic solvent. The
resultant PMO powders were dried under vacuum to give the
corresponding HOP(O)-cPMOs, which showed no loss of material
1
3
30
Fig. 5 Solid-state C CP MAS NMR spectra of (a) MeO-cPMO with
zoomed aromatic (inset, left) and aliphatic (inset, right) regions, and; (b)
MEMO-cPMO with zoomed aliphatic region (inset), before (black) and
after (red) deprotection of the dopant with boron tribromide or acid,
2
9
20
integrity by Si NMR (Fig. S4, ESI†).
3
1
As seen in Fig. 6a and b, the solid-state P NMR spectra of
respectively, with MAS side bands and residual surfactant signals¶ indicated. HOP(O)-cPMOs exhibit a strong signal at 0 ppm. This P chemical
shift is comparable to the analogous organophosphate diester 11
31
31
prepared from the iodo-precursor 9, whose P signal was observed
at 3.3 ppm in solution (see ESI†). The P spectra also showed a
increasing the hydrophobicity of the surface, which then would
hinder interactions between the ethanolic solution and the
PMO pores. Similar to the case of passivated MeO-cPMO
materials, the introduction of TMS groups coating the surface
generally resulted in decreased pore size and pore volume
31
small shoulder at about ꢁ10 ppm as well as a small, broad
signal at ꢁ22 ppm. The identities of these additional signals
were investigated by control experiments described below.
Subjecting a MeO-cPMO material to phosphoryl chloride
(
Table 2). Because of the increased hydrophobicity, the MEM
groups remained intact after treatment with the acidified
31
before deprotection resulted in major P signals at ꢁ10 and
ꢁ
22 ppm and a minor signal at 0 ppm (Fig. 6c). Previous
P NMR studies on silicophosphate glasses, xerogels and
porous materials have reported Si–O–P resonances at ꢁ10
ethanol for 6 hours at 55 1C, but could be cleaved under the 31
same conditions over several days (Fig. S3, ESI†).8 The facile
removal of the MEM groups under mildly acidic conditions
makes the MEM dopant a convenient and practical precursor to
and ꢁ24 ppm for phosphate species [O = P–(OSi)(OH) ] and
2
34–37
[O = P–(OSi)
2
(OH)], respectively.
In support of this assign-
the biphenol moiety inside PMO materials. The monomer can
even be deprotected in a single step during the surfactant
extraction.
Having liberated the free biphenol moiety from both MeO-
and MEMO-cPMO materials, it was then critical to determine
whether the diol group could be further manipulated within
the HO-cPMO framework. Preparation of hydrogen phosphate
ester derivatives from biphenol- and BINOL-type compounds
ment, these signals were also observed in a 100%-BTESBp-derived
PMO material subjected to phosphoryl chloride under identical
conditions (Fig. 6d). Thus it is likely that these signals originate
from surface phosphate esters resulting from the reaction of
phosphoryl chloride with exposed silanols. However, these ‘non-
diol’ materials also posses a signal at 0 ppm that remained to be
assigned.**††
is synthetically straightforward and can be corroborated by ** Chemical shifts of phosphate esters can vary significantly depending on: (1)
3
1
28,29
P NMR.
Furthermore, phosphate ester derivatives have the number and electronic characteristics of attached aryl groups; (2) the
particular phosphorus environment (mono-, di-, and tri-ester); and (3) the
been reported recently as Brønsted acid catalysts for a variety of
interesting transformations,
are highly effective ligands for asymmetric catalysis.
2
9–32
geometry of the O–P–O bond (acyclic, cyclic 5-membered or 6-membered rings).
See D. G. Gorenstein, Phosphorus-31 NMR: Principles and Applications, Academic
Press, Inc., Orlando, 1984.
and related phosphoramidates
3
2,33
†
† These signals could also be attributed to condensed pyrophosphates having
8
Prolonged acidic treatment also removed the surface TMS groups. The MEMO-
resonances at ca. ꢁ20 ppm, or short-chain polyphosphates with middle versus
terminal groups typically appearing at ꢁ10 and ꢁ6 ppm. See ref. 22.
PMOs were not subjected to the harsher boron tribromide deprotection conditions.
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Fig. 7 (a) Comparison of phosphate diester species within HOP(O)-cPMOs
31
versus surface species in MCM materials; and solid-state P CP MAS NMR
spectra of HOP(O) materials prepared from (b) MCM-41 or (c) HO-cPMO,
before and after treatment with 2 M HCl acid, with MAS side-bands indicated
by an asterisk (*).
3
1
Fig. 6 Solid-state P CP MAS NMR spectra of final composite PMO
materials obtained by treating the following starting cPMO materials
with phosphoryl chloride at room temperature for 18 h: (a) HO-cPMO
20
20
prepared from MEMO-cPMO; (b) HO-cPMO prepared from MeO-cPMO; from surface phosphate ester species. However, signals at
20
(
c) MeO-cPMO, and (d) BTESBp-PMO.
0 ppm in the two sets of materials must arise from different
species, with HOP(O)-cPMOs revealing an acid-resistant species
compared to the acid-susceptible surface phosphoric acid in the
latter materials. The acid-resistant phosphate species resonating
at ꢁ6 ppm is attributed to the monoester phosphate resulting
from incomplete phosphorylation of the diol.§§
A phosphorylated MCM derivative HOP(O)-MCM was prepared
by treating MCM-41 with phosphoryl chloride then quenching
with water. Since MCM-41 is purely silicaceous, all phosphorous
species observed on the surface should be derived from silanols.
These materials also showed a similar P NMR profile to the
treated MeO- and BTESBp-PMOs, supporting our interpretation
that the signals at ꢁ10 and ꢁ22 ppm likely originate from surface
Si–O–P phosphate esters (Fig. S5, ESI† and Fig. 7b),‡‡ and they
also posses a signal at 0 ppm.
High-field solid state NMR analysis of the HOP(O)-cPMO
phosphorus species was employed to confirm the assignment
of the large signal at 0 ppm as resulting from the desired aryl
31
31 13
phosphate diester. Although P– C HETCOR NMR was not
sensitive enough to detect coupling of the phosphate phos-
phorus to its attached biphenylene carbon in the solid state, it
was possible to observe long-range phosphorus–proton dipolar
coupling with the biphenylene protons on the aromatic ring
The phosphorylated cPMO and MCM materials described above
were subjected to aqueous acid. Based on the stability of phosphate
38,39
diesters,
biphenyl-diyl phosphate diesters within the HOP(O)-
31
1
by P– H 2D HETCOR NMR experiments on a 900 MHz
cPMOs should be more resistant to acid-catalyzed hydrolysis than
surface silyl phosphate esters. In the event, when HOP(O)-MCM was
treated with 2 M HCl for 18 hours, complete hydrolysis of all
phosphate species was observed, including the small signal at
31
1
instrument over several days. Thus, P– H 2D HETCOR NMR
spectra of a HOP(O)-cPMO powder sample showed long-range
coupling of the phosphate species resonating at 0 ppm to the
aromatic biphenylene protons appearing as a broad signal
3
1
0
ppm, as indicated by the total loss of P signals in the solid-
31
1
centered at 7 ppm (Fig. 8). The P– H 2D HETCOR NMR spectrum
state NMR spectrum (Fig. 7b). However, when PMO-derived material
HOP(O)-cPMO, was treated with 2 M HCl under the same
31
1
of commercial BINOL hydrogen phosphate showed similar P– H
correlation peaks (Fig. S6, ESI†).¶¶ A weak correlation peak was
3
1
conditions, the P NMR spectrum showed no apparent loss of
the signal at 0 ppm while the signals at ꢁ10 and ꢁ22 ppm were
substantially decreased (Fig. 7c). Removal of the signal at ꢁ10 ppm §§ While we would expect this phosphate monoester to be somewhat susceptible
to hydrolysis, the hydrophobicity of the cPMO bridging groups might afford
after the acid treatment revealed a small, sharp signal at ꢁ6 ppm.
protection to these organophosphate species in comparison to surface silyl
3
These results indicate that HOP(O)-cPMOs and POCl -
phosphate esters.
¶ The acidic proton was absent in the HOP(O)-cPMO, or perhaps shifted upfield and
treated non-diol materials share a set of signals originating
¶
1
overlapped by the aromatic signals. The H signal at 4.5 ppm in the HOP(O)-cPMO is
thought to arise from water, and residual aliphatic signals from uncleaved OMEM
groups or byproducts of the cleavage.
‡
‡ The absence of terminal phosphate resonances at ꢁ6 ppm also confirmed that
these signals do not originate from polyphosphate species.
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Acknowledgements
The authors thank the Natural Sciences and Engineering
Research Council (NSERC) for support through the Chiral
Materials Collaborative Research and Training Experience
(CREATE) Program. C. M. C thanks NSERC for Discovery and
Accelerator awards, as well as the Canada Foundation for
Innovation (CFI) for infrastructure awards. L. M. R thanks
NSERC, the Marie Mottashed foundation, and the Walter C. Sumner
Memorial foundation for awards. Additionally the authors thank Dr
Francoise Sauriol (Queen’s University) for solid-state NMR analysis,
Dr Victor V. Terskikh (High-field NMR Facility, Ottawa) for high-field
solid-state NMR analysis, Victoria Jarvis (McMaster University) for
XRD analysis, and Dr Louise Weaver (University of New Brunswick)
for TEM analysis.
3
1
1
Fig. 8 High-field solid-state P– H 2D HETCOR spectrum of HOP(O)-
3
1
1
cPMO along with the regular P CP MAS (horizontal) and H (vertical)
MAS spectra. All spectra were obtained at 21.1 T with a sample spinning of
2
0 kHz.
Notes and references
also observed between the minor phosphorus signal at ꢁ6 ppm
and the biphenylene protons of the HOP(O)-cPMO supporting
the presence of a biphenyl-diyl monoester phosphate species
that would engage in coupling with the neighboring aromatic
protons through a single Ar–O–P bond (Fig. 8).
These results are highly encouraging for post-synthetic
transformations of functional groups within these types of
solid-state materials. They demonstrate that biphenol units
liberated by removal of methoxy or MEM groups are accessible
within the PMO pore walls and can be transformed by simple
chemical manipulation.
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