Discrete complexes immobilized onto click-SBA-15 silica: Controllable loadings and the impact of surface coverage on catalysis

Article abstract of DOI:10.1021/ja210400u

Azidopropyl functionalized mesoporous silica SBA-15 were prepared with variable azide loadings of 0.03-0.7 mmol g^-1 (～2-50% of maximal surface coverage) through a direct synthesis, co-condensation approach. These materials are functionalized selectively with ethynylated organic moieties through a copper-catalyzed azide alkyne cycloaddition (CuAAC) or "click" reaction. Specific loading within a material can be regulated by either the azide loading or limiting the alkyne reagent relative to the azide loading. The immobilization of ferrocene, pyrene, tris(pyridylmethyl)amine (TPA), and iron porphyrin (FeTPP) demonstrates the robust nature and reproducibility of this two-step synthetic attachment strategy. Loading-sensitive pyrene fluorescence correlates with a theoretically random surface distribution, rather than a uniform one; site-isolation of tethered moieties ～15 A in length occurs at loadings less than 0.02 mmol g ^-1. The effect of surface loading on reactivity is observed in the oxygenation of SBA-15-[Cu^I(TPA)]. SBA-15-[Mn^II(TPA)]- catalyzed epoxidation exhibits a systematic dependence on surface loading. A comparison of homogeneous, site-isolated and site-dense complexes provides insight into catalyst speciation and ligand activity.

Full text of DOI:10.1021/ja210400u

Article
pubs.acs.org/JACS
Discrete Complexes Immobilized onto Click-SBA-15 Silica:
Controllable Loadings and the Impact of Surface Coverage on
Catalysis
Jun Nakazawa,†,‡ Brian J. Smith,† and T. Daniel P. Stack*
Department of Chemistry, Stanford University, Stanford, California 94305, United States
S
* Supporting Information
ABSTRACT: Azidopropyl functionalized mesoporous silica SBA-
15 were prepared with variable azide loadings of 0.03−0.7 mmol
g−1 (∼2−50% of maximal surface coverage) through a direct
synthesis, co-condensation approach. These materials are
functionalized selectively with ethynylated organic moieties
through a copper-catalyzed azide alkyne cycloaddition (CuAAC)
or “click” reaction. Specific loading within a material can be
regulated by either the azide loading or limiting the alkyne reagent
relative to the azide loading. The immobilization of ferrocene,
pyrene, tris(pyridylmethyl)amine (TPA), and iron porphyrin
(FeTPP) demonstrates the robust nature and reproducibility of
this two-step synthetic attachment strategy. Loading-sensitive pyrene fluorescence correlates with a theoretically random surface
distribution, rather than a uniform one; site-isolation of tethered moieties ∼15 Å in length occurs at loadings less than 0.02 mmol
g−1. The effect of surface loading on reactivity is observed in the oxygenation of SBA-15-[CuI(TPA)]. SBA-15-[MnII(TPA)]-
catalyzed epoxidation exhibits a systematic dependence on surface loading. A comparison of homogeneous, site-isolated and site-
dense complexes provides insight into catalyst speciation and ligand activity.
immobilized species,22,23 and the final coverage is highly
dependent on the hydration of the surface. A more patterned
1. INTRODUCTION
Modification of high surface area silica materials with
distribution, which assures a minimum separation between
organosilane functional groups holds considerable potential
for many applications,1−6 particularly heterogenization of
immobilized sites, can be achieved by grafting primary amines
discrete small-molecule catalysts.7−19 Beyond simplifying
with large protecting groups that are subsequently re-
moved.24,25 With this approach, the percentage of amine sites
catalyst removal and recycling, heterogenization of molecular
that are subsequently functionalized is sensitive to both the
catalysts is a promising strategy to combine the selectivity of
protecting-group size and the species immobilized; varying the
homogeneous catalysts with the stability of heterogeneous
surface loading requires various protecting groups of different
catalysts and probe reaction mechanisms. Moreover, hetero-
genization allows for manipulation of surface-species interaction
by controlling surface loadings, from site-dense to site-isolated.
sizes.
Co-condensation of functionalized monomers during silica
preparation is an alternative technique that avoids clustering
Site-isolation of immobilized catalysts provides potentially
found with postsynthetic grafting.22,23 In such direct synthesis,
enhanced activity through improved substrate accessibility to
the catalytic active species; the species can be constructed with
less sterically demanding ligands as potential intermolecular
a functionalized organosilane (RO)3SiR′ is co-condensed with
(RO)4Si around a templating organic polymer. The maximum
concentration of functionalized organosilane that does not
deactivation pathways are attenuated by limited mobility of the
disrupt long-term order of the silica is dependent on the nature
catalysts with regard to one another. Comparison of the
of the monomer.26 By systemically controlling the mole fraction
reactivity of site-isolated, site-dense, and homogeneous catalysts
of organosilane, the surface loading can be varied from site-
should provide mechanistic information about the reaction of
dense to site-isolated, and the impact of density on speciation
interest. A reliable synthetic method to prepare a controllable
range of surface loadings would advance the rationalized
and catalysis can be investigated.
In 2008 we reported the preparation of mesoporous silica co-
screening of heterogenized, discrete catalysts.
condensed with organoazide functional groups.27 The organic
Mesoporous silica SBA-15 are attractive candidates for
catalyst supports with high surface areas (700−900 m2 g−1),
azide functionality attached to the mesoporous silica allows for
immobilizing complexes to the surface via the high-yielding and
large pores sizes (6−9 nm), and robust silica walls (3−6
nm).20,21 Traditional functionalization involves hydrolytic
grafting of organosilanes onto a precondensed silica surface;
however, this synthetic strategy often results in clustering of the
Received: November 4, 2011
Published: January 24, 2012
© 2012 American Chemical Society
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highly efficient copper-catalyzed azide alkyne cycloaddition
reaction (CuAAC).28 Azides have also been introduced into
SBA materials via postsynthetic grafting.29 CuAAC on materials
prepared by both grafted and co-condensed azide-silica has
been used for the immobilization of numerous biomole-
cules30,31 and catalysts.32−37
Here we report a more detailed examination of variably
loaded azide materials prepared by a direct synthesis procedure
and the characterization of mesoporous SBA-15-type materials
with variable and well-controlled organoazide loadings (0.2−8
mol % organoazide; 2−50% surface coverage). Attachment of
four different ethynylated species to the surface azides allow
various aspects of the species distributions to be investigated.
2.1.2. Direct Synthesis and Characterization of SBA-15-N3-
x. Variably loaded organoazide SBA-15 materials (SBA-15-N3-
x) were prepared by a direct synthesis procedure involving the
co-condensation of defined molar ratios (x = 0.2−8%) of Si−
N3 and tetraethoxyorthosilicate (TEOS) in the presence of the
templating P123 polymer in aqueous acidic (HCl) media
(Scheme 2).5,21,44,45 The template polymer was removed by
Soxhlet extraction with ethanol over 24 h,21,45 though DSC
analysis and CP-MAS 13C solid-state NMR indicate that
polymer removal is not complete under these conditions.
The ordered pore structure of the SBA-15-N3-x materials was
characterized by powder X-ray diffraction (XRD) (Figure S1 in
Supporting Information [SI]). The diffraction pattern of a low
Si−N3 loaded material, SBA-15-N3-0.5, exhibits one intense and
two weak diffraction peaks at 2θ = 0.85, 1.47, and 1.70°,
2. RESULTS
2.1. Synthesis. 2.1.1. Organic Precursors. The monomer
Si−N3, prepared from commercially available 3-chloropropyl-
triethoxysilane and NaN3 (eq 1),6,38,39 was stored in a glovebox
to prevent polysiloxane formation, which could lead to a
nonrandom distribution on the SBA-15 surface. EthynylTPA
was synthesized by reductive amination of 2-formyl-5-
(trimethylsilylethynyl)pyridine with dipicolylamine, followed
by trimethylsilyl (TMS) deprotection of the ethynyl moiety
using K2CO3/MeOH (eq 2).40−42 Fe-ethynylTPPCl was
respectively. The observed peaks correspond to the d100, d110
,
d200 spacing of the hexagonal structure (p6mm) of the SBA
material,20 and are similar to that of unmodified SBA-15 (2θ =
0.85, 1.49, and 1.71°), prepared without Si−N3. This indicates
little change in bulk morphology by introduction of the
azidosilane. With increasing Si−N3 content, the d100 diffraction
peak broadens and shifts slightly. The d110 and d200 diffraction
peaks are absent with the highest Si−N3 loading attempted
(SBA-15-N3-8), indicating loss of long-term order. Similar
diffraction changes are reported for other modified SBA-15-
type materials prepared by the co-condensation method.21,45
The hexagonal lattice parameter a0 of each SBA-15-N3-x
sample, calculated from the d100 diffraction peak (Table 1),
show similar a0 values of ∼12−13 nm.
prepared in a three-step synthesis (Scheme 1), in which the
porphyrin ligand ethynylH2TPP was synthesized from 4-(3-
hydroxy-3-methyl-1-butynyl)-benzaldehyde (1 equiv), benzal-
dehyde (3 equiv), and pyrrole (4 equiv) in refluxing propionic
acid.43 The isopropanol alkyne-protecting group dramatically
changes the ligand polarity, aiding in separation via column
chromatography. Deprotection occurs under a refluxing KOH/
toluene suspension.
Pyrene-Tz-tBu (eq 3) and TPA-Tz-tBu (eq 4) were prepared
via CuAAC of the ethynylated precursors with tert-butyl azide
as homogeneous analogues to their surface-immobilized species
for UV−vis spectrometry and reactivity.
N2 adsorption−desorption isotherms of the SBA-15-N3-x
materials provide pore diameter and surface area information. A
type-IV isotherm was observed for all SBA-15-N3-x, character-
istic of mesoporous materials with narrow pore size
distributions (SBA-15-N3-4, Figure S2 in SI).21,45 The average
a
Scheme 1
a
Reagents and conditions: a) Refluxing propionic acid, 3 h, followed by KOH in refluxing toluene for 4 h (11% overall yield); b) FeCl2 in DMF, 1 h
(85% yield).
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Scheme 2. Preparation of Surface-Modified Mesoporous Silicas, SBA-15-R-x
Table 1. XRD Diffraction Peaks 2θ, Lattice Parameters a0,
and Wall Thickness dw of Unfunctionalized SBA-15 and
SBA-15-N3-x Samples
The organic functionality in the SBA-15-N3-8 material was
characterized by CP-MAS 13C solid-state NMR spectroscopy
(Figure 1a).21,45 The intense signals at 54.7, 23.0, 9.5 ppm were
assigned to three carbons of the azidopropyl moiety. Also
observed were small signals that correspond to residual P123
template (71, 17 ppm) and EtO-groups (59 and 17 ppm).
The azide moiety of the SBA-15-N3-x materials was
characterized by IR spectroscopy. The IR spectrum of SBA-
15-N3-4 in Figure 2a shows an intense azide stretching signal at
2110 cm−1, in addition to Si−O−Si νsym (800 cm−1), Si−OH ν
(960 cm−1), Si−O−Si νas (1080 cm−1) and surface-bonded H2O
δ (1640 cm−1) modes of the SBA-15 silica matrix.16,46,47 The
absorbance of the azide normalized to surface-bonded water
provides a spectroscopic indication of the amount of azide
present in the material, and this value scales with the Si−N3
molar ratio (Figure 3).
2.1.3. Synthesis and Characterization of SBA-15-R-x.
Ethynylferrocene (R = Fc), 1-ethynylpyrene (R = pyrene),
ethynylTPA (R = TPA), and Fe-ethynylTPPCl (R = FeTPP)
were attached covalently to SBA-15-N3-x materials via CuAAC
to yield SBA-15-R-x, using an excess (2−30 equiv) of the
ethynylated species (Scheme 2).6,15,28,39,50,51 After the reaction,
the silicas were washed with a Et2NCS2Na solution to remove
any adventitious bonded copper catalyst, then rinsed with
sodium acetate solution to remove excess Et2NCS2Na.
a
b
(100) (deg) (110) (deg) (200) (deg) a0 (nm) dw (nm)
SBA-15
0.851
1.486
1.711
12.0
5.4
SBA-15-N3-x
x = 0.2
0.5
0.765
0.850
0.805
0.846
0.856
0.856
1.330
1.470
1.400
1.471
1.491
1.535
1.680
1.570
1.686
1.736
13.3
12.0
12.7
12.1
11.9
11.9
10.6
6.7
6.9
6.6
6.0
6.9
6.7
5.9
1.0
2.0
3.0
4.0
1.496
1.726
c
c
8.0
0.965
-
-
a
b
a0 = 2d100/√3. dw = a0 − Dp, where Dp is the mesopore average
c
diameter. Too broad to identify.
surface area (BET), pore volume, pore diameter (Barrett−
Joyner−Halenda (BJH) adsorption) and wall thickness of the
SBA-15-N3-x materials only slightly vary with varying azide
loading: (areas: 730 30 m2 g−1, pore volumes: 0.91 0.1 cm3
g−1, pore diameters: 5.7 1 nm, wall thicknesses: 6.2 1 nm)
(Tables 1−4).
The organic functionalities in the SBA-15-R-x materials were
characterized spectroscopically. CP-MAS 13C solid-state NMR
was employed for SBA-15-R-8 samples (R = Fc, pyrene, and
TPA) (Figure 1b−d).21,45 The spectrum of SBA-15-Fc-8 shows
an intense Cp ring peak at 70.6 ppm and its spinning sideband
signals at 134 and 9 ppm, in addition to the propyl chain with
signals at 53.6, 23.7, and 9.2 ppm. The signals at 148.8 and
123.0 ppm are associated with the triazole ring. The solid state
13C NMR resonances of SBA-15-pyrene-8 and SBA-15-TPA-8
samples agree well with the solution 13C NMR spectra of the
homogeneous analogues pyrene-Tz-tBu and TPA-Tz-tBu,
respectively. SBA-15-FeTPP-x formation was confirmed via
UV−vis spectroscopy of fluoride-digested silica samples with a
Table 2. BET Surface Areas SBET (m2 g−1) of Functionalized
SBA-15 Materials
a
x =
SBA-15-R
0
0.2
0.5
1.0
2.0
3.0
4.0
8.0
b
N3-x
710
740
480
523
639
670
742
572
598
399
676
704
502
450
444
436
750
483
507
389
759
404
404
340
378
734
386
269
288
731
273
198
151
Fc-x
pyrene-x
TPA-x
FeTPP-x
c
c
c
-
-
-
a
Determined by the BET method48 from the adsorption branch.
Unfunctionalized SBA-15. The sample was not prepared.
b
c
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a
Table 3. Pore Volumes Vp (cm3 g−1) of Functionalized SBA-15 Materials
x =
SBA-15-R
0
0.2
0.5
1.0
2.0
3.0
4.0
8.0
b
N3-x
0.84
1.01
0.87
1.07
0.84
0.80
0.98
0.75
0.73
0.79
0.81
0.97
0.87
0.84
0.73
0.54
0.96
0.73
0.72
0.83
0.50
0.52
0.49
0.30
0.84
0.47
0.38
0.82
0.29
0.21
Fc-x
pyrene-x
TPA-x
FeTPP-x
0.63
0.41
0.18
c
c
c
-
-
-
a
b
c
Determined by the BJH method49 from the adsorption branch. Unfunctionalized SBA-15. The sample was not prepared.
Table 4. Pore Diameters Dp (nm) of Functionalized SBA-15
a
Materials
x =
SBA-15-
0
0.2
0.5
1.0
2.0
3.0
4.0
8.0
b
N3-x
6.6
6.6
6.7
7.0
6.6
6.7
5.1
4.7
5.0
5.3
5.8
6.1
6.4
6.7
5.5
6.5
6.1
5.3
5.2
5.0
5.1
5.2
5.1
5.6
5.2
5.0
5.1
4.7
3.6
3.5
Fc-x
pyrene-x
TPA-x
FeTPP-x
5.3
4.7
3.8
c
c
c
−
−
−
Determined by the BJH method49 from the adsorption branch.
Unfunctionalized SBA-15. The sample was not prepared.
a
b
c
Figure 2. IR spectra of (a) SBA-15-N3-4, and (b) SBA-15-pyrene-4 in
a KBr pellet.
Figure 3. IR peak area of azide (2110 cm−1 normalized to the signal at
1640 cm−1) of the SBA-15-N3-x and SBA-15-R-x materials as a
function of the mixing ratio x of Si−N3.
Ratiometric assessment of the azide IR feature of SBA-15-R-x
samples decreased to ∼20% of its original intensity after
immobilization (Figures 2 and 3). An exception is SBA-15-
FeTPP-3, in which the IR feature remains ∼40% of its original
intensity. By comparison, only minor residual azide (less than
5%) was detected by IR following CuAAC immobilization of 1-
ethynylpyrene onto an SBA-15 material that was postsyntheti-
cally grafted with azide at 0.1 mmol g−1.
The quantification of immobilized functional groups is
summarized in Table 5 and Figure 4. ICP analysis was
performed on SBA-15-Fc-x and SBA-15-FeTPP-x materials,10,52
while UV−vis spectroscopy was used for digested samples of
SBA-15-pyrene-x and SBA-15-TPA-x, referenced to the
homogeneous analogues pyrene-Tz-tBu and TPA-Tz-tBu. For
both detection methods, silica materials were digested using
strongly basic conditions or in situ prepared HF.53 The amount
of residual copper contamination from the CuAAC was <0.005
mmol g−1 in all samples by ICP analysis. For all methods of
detection, the loading of organic species increased with higher
Si−N3 percentage. With the exception of SBA-15-FeTPP-3,
Figure 1. CP-MAS 13C solid-state NMR of (a) SBA-15-N3-8, (b) SBA-
15-Fc-8, (c) SBA-15-pyrene-8, and (d) SBA-15-TPA-8. (†): spinning
sideband signals, (‡): residual P123 template and solvent EtOH/EtO-Si
groups, (Fc): ferrocene ring, (Py): pyridine group, (Tz): triazole
group.
395 nm soret (B) band matching that observed with Fe-
ethynylTPPCl under acidic conditions.
Porosimetry measurements of the SBA-15-R-x materials were
performed to determine the impact of the immobilized
functionality on the morphology of the bulk silica material. A
representative N2 adsorption−desorption isotherm of SBA-15-
pyrene-4 (Figure S2 in the Supporting Information) shows
lower gas adsorption in comparison to the SBA-15-N3-4
precursor, but maintains the same type IV profile. The BET
surface areas, pore volumes, and pore diameters of each
modified SBA-15-R-x (Tables 2−4) systematically decrease
with increased loading, consistent with occupancy of the pores
by the immobilized moieties.
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a
Table 5. Loading of Compounds and [residual Cu] (mmol g−1) of Functionalized SBA-15 Materials
x =
SBA-15-R
0.2
0.5
1.0
2.0
3.0
4.0
8.0
Fc-x
0.02
0.05
0.09
0.17
0.26
0.34
0.62
[0.003]
0.03
[0.003]
0.06
[0.003]
0.10
[0.002]
0.19
[0.002]
0.31
[0.005]
0.38
[0.003]
0.64
pyrene-x
TPA-x
[0.002]
0.02
[0.001]
0.06
[0.001]
0.10
[0.002]
0.18
[0.001]
0.26
[0.001]
0.33
[0.001]
0.53
[0.001]
0.02
[0.001]
0.04
[0.001]
0.07
[0.002]
[0.001]
0.15
[0.001]
[0.003]
b
b
b
FeTPP-x
−
−
−
[0.002]
[0.004]
[0.006]
−
[0.004]
−
−
a
b
Fc, FeTPP, and Cu loadings were determined by ICP analysis. Pyrene and TPA loadings were determined by UV−vis spectrometry. The sample
was not prepared.
Pyrene surface loading was also controlled by CuAAC onto a
high azide loading SBA with the alkyne species as the limiting
reagent. SBA-15-N3-4 was treated with different amounts of 1-
ethynylpyrene to target three different loadings: high, medium,
and low (0.32, 0.12, and 0.03 mmol g−1 SBA, respectively,
Figure 5b); in all three target loadings, greater than 90% of the
added pyrene is attached covalently to the surface. The high,
medium, and low samples gave similar fluorescence profiles to
fully treated SBA-15-pyrene-x of comparable loading.
The plots of the Imonomer/Ieximer ratio versus surface loading
for samples prepared by either method show the same trend
(Figure 6). From control experiments with unfunctionalized
SBA-15, simple surface physisorption can be excluded from
significant contribution to the fluorescence signal with even the
lowest loading samples. A low quantity of surface-adsorbed
pyrene (≤0.005 mmol g−1) was observed after copious washing.
2.2.2. Theoretical Pyrene Distribution. The maximum
loading of fully site-isolated pyrene can be calculated, assuming
an idealized hexagonal packing of circles.
Figure 4. Loadings (mmol g−1) of SBA-15-R-x (R = Fc, pyrene, TPA,
and FeTPP) as a function of the mixing ratio x of Si−N3. Fc and
FeTPP loadings were determined by iron ICP analysis of digested
samples. Pyrene and TPA loadings were determined by UV−vis
spectrometry of digested samples.
materials from the same SBA-15-N3-x gave similar loadings of
organic species.
2.2. Intermolecular Interaction As a Function of
Loading. 2.2.1. SBA-15-Pyrene-x: Monomer/Eximer Fluo-
rescence. The fluorescence of SBA-15-pyrene-x materials
provides information related to the distribution and density
of the pyrene groups.24,54−57 Pyrene is an ideal probe for
intermolecular interactions due to the difference in fluorescence
observed for the monomeric and excimer species. At a low
loading (x = 0.2), fluorescence is observed primarily due to the
monomeric pyrene species (400 nm, Figure 5a). Higher pyrene
surface loading yields increased excimer fluorescence (480 nm)
relative to the monomer fluorescence, illustrating an increase in
the number of intermolecular pyrene interactions. A direct
correlation of loading and excimer fluorescence is observed.
S
BET
2
maximum isolated loading =
2 3 l × N
(5)
A
where l is the length of the tethered pyrene (15 Å), SBET is the
total surface area (700 m2 g−1), and NA is the Avogadro
constant. A random distribution can be estimated by the
nearest neighbor method.58,59 The probability of nearest
neighbor distances of randomly located objects within a certain
area is given by the following Poisson distribution function.
probability density function: f(r)
2
= 2πδr exp( − δπr )
(6)
Figure 5. Fluorescence spectra of (a) SBA-15-pyrene-x (x = 8, 4, 3, 2, 1, 0.5, 0.2 mol %) and (b) limited alkyne SBA-15-pyrene-4. (Inset) Loadings
of pyrene. SBAs were suspended in CHCl3 with continuous stirring (λex = 330 nm). For visual comparison, the spectra were normalized at 415 nm.
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Figure 6. Comparison of a computed random distribution and
observed pyrene fluorescence monomer/eximer ratio. Open circle
values are estimates of monomer amount (%, left) using a nearest
neighbor method (SBET = 700 m2 g−1 and tether length l = 15 Å).
Filled shape values are experimental results of the Imonomer/Ieximer ratio
of SBA-15-pyrene-x (filled square, right), limited alkyne SBA-15-
pyrene-4 (filled diamond, right), and unfunctionalized SBA-15
exposed to 1-ethynylpyrene (filled triangle, right).
Figure 7. Color changes upon oxygenation of SBA-15-TPA-4 (a−c)
and SBA-15-TPA-0.5 (d−f) with 0.9 equiv of [CuI(MeCN)](SbF6) in
EtCN at −90 °C. Characteristic colors of intermediates in the
corresponding homogeneous solutions are displayed to the left.
cumulative distribution function: F(r)
2.3. Effect of Surface Loading on Catalysis. 2.3.1.
SBA-15-TPA-x: Epoxidation of 1-Octene. To characterize the
catalytic epoxidation of terminal olefins with the immobilized
SBA-15-TPA-x manganese complex and low acidity peracetic
acid,63 SBA-15-TPA-0.5 was first compared with the homoge-
neous analogue TPA-Tz-tBu. In both cases, at 1:1 metal/ligand
ratio and 0.1 mol % catalyst loading, epoxide yields <20% were
obtained (Figure 8). Most product formation is observed in the
2
= 1 − exp( − δπr )
(7)
1
2 δ
mean distance: r =
̅
(8)
where r is the distance between the centroids of two objects (30
Å) and δ is the average surface loading (object/Å2). Percent
site-isolated can be calculated from eq 9.
2
site‐isolated(%) = exp( − δπr ) × 100
(9)
2.2.3. SBA-15-TPA-x: Dioxygen Adduct Monomer/Dimer of
CuTPA Complexes. To probe the impact of site distribution on
metal reactivity, CuI complexes of SBA-15-TPA-x were treated
with dioxygen (Figure 7). In homogeneous solutions, oxygen-
ation of [CuI(TPA)]+ generates distinctly colored species,
depending on the dioxygen adduct formed: a superoxo
(monomeric) or peroxo (dimeric) species.60−62 In propionitrile
(EtCN), the yellow-colored solutions of [CuI(TPA)]+ oxygen-
ate rapidly at −90 °C to form a purple, dicopper(II), trans-μ-
1,2-peroxo species [(TPA)CuII−O−O−CuII(TPA)]2+.60 At
such temperatures, the monomeric superoxo species [(TPA)-
CuII−O−O•]+ is detected only transiently as a green
intermediate by stopped flow methods or isolated by slow
CO/O2 exchange.61,62
The heterogenized TPA species shows similar behavior to
the observed homogeneous solutions at specified loadings.
Complexation of the SBA-15-TPA-x materials in EtCN
solutions of [CuI(MeCN)4](SbF6) (0.9 equiv per TPA)
resulted in a color change in the materials from colorless to
yellow. Oxygenation of the SBA-15-[CuI(TPA)]+1-x materials
at low temperatures afforded different observations depending
on surface loading. For high loadings (x = 4, 0.33 mmol g−1),
the material turned from yellow to purple, indicating the
formation of a trans-peroxo, presumably due to the densely
packed complexes. At low loadings (x = 0.5, 0.06 mmol g−1), a
green color is observed, suggesting that the copper superoxide
is formed and stabilized by site-isolation.
Figure 8. Epoxidation of 1-octene, 0.25 M in HOAc, RT in 5 min. In
all cases, manganese content is 0.1 mol %, and the mol % of ligand is
varied.
first 2 min, and the reaction is effectively complete in 5 min.
Fixing the Mn concentration at 0.1 mol % and systematically
varying excess ligand dramatically improves the epoxidation
rate and yields in 5 min; a 10-fold excess of TPA-Tz-tBu (1 mol
%) provides greater than 90 TON per ligand or 900 TON per
manganese center.64 The manganese loaded SBA-15-TPA-0.5
gives reactivity nearly identical to that of the homogeneous
electronic analogue: 93% epoxide formation in 5 min with 1
mol % ligand and 0.1 mol % metal. The improved activity
found with higher ratios of ligand to metal is not due to the
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presence of additional basic pyridine groups, as 0.1 mol %
manganese, 0.1 mol % TPA, and 2,6-lutidine (1−4 mol %) did
not improve the epoxide yield. No appreciable epoxidation is
observed in the absence of manganese in 5 min.
reaction times were necessary (2 h). All catalyst loadings of
SBA-15-FeTPP-x gave similar reaction yields; no significant
relation between surface loading and reactivity was observed.
All SBA-15-TPA-x materials were evaluated for epoxidation
catalysis at 0.1 mol % MnII salt and 1 mol % ligand to
determine the effect of surface loading on the catalytic activity
per ligand (Figure 9). SBA-15-TPA-0.5 gave the best results
3. DISCUSSION
Controlled loading of mesoporous silica materials via CuAAC
was achieved with 3-azidopropyltriethoxysilane (Si−N3) as a
surface tether. The propyl azide tether was chosen to allow
immobilized ligands flexibility but still attenuate intermolecular
interactions with neighboring ligands at higher loadings. The
pore size and surface area of each material, as assessed by
powder X-ray diffraction and porosimetry measurements, are
fully consistent with the ordered hexagonal structure of the
original SBA-15 material at azide loadings less than 8%. The
loss of some long-term order in SBA-15-N3-8 indicates a
maximum amount of Si−N3 that can be co-condensed into the
material and still retain the SBA-15 long-range structure. The
presence of Si−N3 is evident from both the CP-MAS 13C solid-
state NMR spectra21,45 and the IR spectra. The ratio of the
azide IR feature (2110 cm−1) to surface-bonded H2O bending
mode (1640 cm−1) provides a simple and rapid ratiometric
assessment of the azide content.16,46,47 These results indicate
the direct synthesis of SBA-15-N3-x provides a predictable and
predefined loading of an anchor for “click” attachment onto
SBA-15.
Ethynylferrocene (R = Fc), 1-ethynylpyrene (R = pyrene),
ethynylTPA (R = TPA), and Fe-ethynylTPPCl (R = FeTPP)
were immobilized to the surface via a CuAAC reaction with
SBA-15-N3-x to yield functionalized SBA-15-R-x. Gas uptake
measurements of SBA-15-R-x samples indicate that the
mesoporous structure is retained with pore size reduction
due to pore occupancy by the immobilized species. ICP
evidence of only trace residual Cu (<0.005 mmol g−1) indicates
that the copper catalysts are readily removed by Et2NCS2Na
washing.
Figure 9. (a) Time dependent epoxidation of 1-octene with
Mn(CF3SO3)2 (0.1 mol %), SBA-15-TPA-x (1 mol %), and peracetic
acid (2 equiv). (b) Epoxide yield at 2 min relative to TPA surface
loading (boxed data points in a).
with ∼85% conversion to epoxide within 2 min. Epoxide yield
decreases with increasing surface loading, with SBA-15-TPA-8
yielding ∼35% epoxide in the same time period. Ten minutes
are required for SBA-15-TPA-8 to catalyze epoxide yields
similar to that obtained within one min using SBA-15-TPA-0.5.
A plot of epoxide yield at 2 min versus surface loading reveals a
near-linear dependence on surface loading (Figure 9b).
2.3.2. SBA-15-FeTPP-x: Carbene insertion. Iron porphyrin-
catalyzed carbene insertion into a N−H bond was surveyed
using piperidine and ethyl diazoacetate (EDA) (Figure 10).65
The loading of immobilized species is controlled by the
amount of azide within SBA-15-N3-x materials, and is fairly
independent of the nature of the species being attached.
Although the Fc, pyrene, TPA, and FeTPP materials were
prepared independently under different CuAAC conditions and
their surface loadings were determined by different analytical
techniques, the observed loadings all correlated closely to the
Si−N3 mixing ratio. Ratiometric assessment of the azide IR
feature of clicked SBA-15-R-x samples showed ∼80% decrease
in azide signal, suggesting that not all alkyl azides are accessible
for the CuAAC reaction when integrated into materials by co-
condensation (Figures 4 and 5), possibly due to azide
incorporation within inaccessible mesopores and micropores.
This is consistent with the control experiment of postsynthetic
Si−N3 grafting, which shows >95% azide conversion by IR. The
residual 20% azide content under co-condensation preparation,
however, is consistent across the materials prepared, and can be
anticipated if a given loading is targeted. Only with high
loadings of a bulky porphyrin species, SBA-15-FeTTP-3, do the
ligand steric demands appear to have a significant impact.
Loadings can also be controlled on a dense azide SBA-15-N3
surface by limiting the alkyne reagent, an attractive method if
the availability of the alkyne is limited. The alkyne loadings of
limited SBA-15-pyrene-4 illustrate the near quantitative nature
of the CuAAC reaction on the surface, relative to alkyne
substrate (Figure 5b inset). The combination of direct synthesis
of SBA-15-N3-x materials followed by CuAAC attachment
Figure 10. Time dependent formation of ethyl 1-piperidineacetate
from EDA (1.0 mmol) and piperidine (1.2 mmol) with homogeneous
and heterogeneous FeTPP catalysts (0.02 mol % in all cases).
The reaction was carried out with EDA (1.0 mmol) and a slight
excess of piperidine (1.2 mmol) in CH2Cl2 with homogeneous
FeTPPCl or SBA-15-FeTPP-x as catalysts (0.2 μmol, 0.02 mol
%). In homogeneous solution, FeTPPCl catalyzed full EDA
consumption over 1 h and afforded ethyl 1-piperidineacetate in
yields greater than 95%. Heterogeneous SBA-15-FeTPP
systems also provided equivalently high yields, although longer
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Figure 11. Graphical representations of theoretical surface distributions: (a) clustered, (b) uniform, (c) random.
allows for efficient synthesis of controlled loadings of
ethynlated molecules.
lowest limited alkyne loading (0.03 mmol g−1) displays
predominantly monomer fluorescence. Therefore, we can
conclude that the CuAAC reaction randomly distributes species
across the azide-decorated SBA-15 surface. This allows for a
range of accessible surface densities with a singly prepared high
azide loaded material.
The distribution of immobilized species can be characterized
spectroscopically. The fluorescence of pyrene is a sensitive
probe into the degree of site-isolation.24,54−57 From the
experimental fluorescence observed for the SBA-15-pyrene-x
materials, the Imonomer/Ieximer ratio dramatically changes in the
0.02−0.1 mmol g−1 SBA range from site-isolated to site-dense.
Surface distribution can be classified following three idealized
arrangements: (A) clustered, (B) uniform, (C) random (Figure
11).
(A) A clustered distribution is a dense condensation of
functionalized monomer during mesoporous silica formation.
Clustering of the sites during direct synthesis would result in
strong excimer fluorescence even at low loadings, but that
behavior is not observed.
(B) A uniform distribution maximizes the interspecies
distance of the repeating object (i.e., pyrene); this distribution
minimizes object overlap. Excimer fluorescence would be
expected only in loadings above an overlapping limit, calculated
∼0.2 mmol g−1, assuming a 15 Å length of each surface-
immobilized pyrene entity and a 700 m2 g−1 surface area (eq 5).
SBA-15-pyrene-1 (0.10 mmol g−1) and SBA-15-pyrene-2 (0.19
mmol g−1) both show significant excimer fluorescence,
indicating that a uniform distribution is unlikely.
The impact of surface loading on reactivity was demonstrated
through the exposure of SBA-15-[CuITPA]-x to dioxygen. The
color change of the site-dense SBA-15-[CuITPA]-4 suspension
(yellow to purple to pale blue) corresponds to the
homogeneous [CuI(TPA)]+ system. The localization of the
color change on the silica material and a colorless solution
indicate that color originates from the surface-bonded [CuI
(TPA)] species and not from leaching into solution. From the
homogeneous oxygenation studies of various CuI complexes,
thus far only two trans-μ-1,2- and μ-η2:η2-style peroxo
copper(II) dimers have shown a distinct purple color.67 In
[Cu(TPA)] systems, the trans-μ-1,2-peroxo copper(II) dimer,
[(TPA)CuII−O−O−CuII(TPA)]2+, is the only known purple-
colored species.60 From this, we conclude that a dioxygen
molecule can coordinate to two closely located CuI ions on the
site-dense SBA-15-[CuI(TPA)]-4 material to form trans-μ-1,2-
peroxo species. In contrast, the transient intermediate species of
the site-isolated SBA-15-[CuI(TPA)]-0.5 system is green in
color. Due to many CuII complexes with green appearance, the
oxygenated SBA-15-[Cu(TPA)]-0.5 species cannot definitively
be assigned from the color information alone to the
mononuclear end-on superoxo species [(TPA)CuII−O−O•]+,
which is short-lived in homogeneous solution.61,62 However,
the thermal sensitivity does point toward a reactive Cu−O2
intermediate. While more extensive spectroscopic and reactivity
characterization of these intermediates species is needed, these
qualitative results do indicate that sensitive metal-based
homogeneous chemistry can be translated into the mesoporous
silicas through CuAAC attachment and highlight a distinct
advantage of heterogenization at site-isolated loadings, namely
the stabilization of reactive monomeric species.
The SBA-15-TPA-x materials with Mn(CF3SO3)2 were
evaluated as rapid catalysts for the epoxidation of terminal
alkenes.8,68−70 In a previous catalyst screening study,
homogeneous [MnII(TPA)]2+ was found to be a good catalyst
for the epoxidation of 1-octene at 1 mol % (relative to
substrate) using a low acidity peracetic acid as the oxidant,
albeit a poor catalyst at 0.1 mol %.63 Oxidative ligand
degradation was hypothesized as limiting overall turnover. To
test this hypothesis, we have examined the yield of epoxide
when keeping the manganese concentration constant at 0.1 mol
% and varying only the amount of ligand (Figure 8). The
epoxide yield scales linearly with additional TPA-Tz-tBu,
achieving >90% epoxide with 1.0 mol % TPA ligand (10-fold
ligand excess to metal, 90 ligand TON, 900 Mn TON). This
(C) A theoretical random distribution can be estimated by
the nearest neighbor method58,59 as a function of surface
loading, using a Poisson distribution function (Figure 6). The
model predicts a significant increase in monomeric species as
the surface loading drops from 0.1 to 0.01 mmol g−1. The
I
monomer/Ieximer ratio of SBA-15-pyrene-0.2 (0.03 mmol g−1), the
lowest loading sample prepared, shows predominantly
monomer fluorescence, suggesting a loading with site-isolation
for a majority of the pyrene moieties. The correlation of a
decreasing Imonomer/Ieximer ratio with increasing surface loading is
also consistent with the theoretical trend of randomly
distributed objects. Assuming an experimental detection limit
of 10% of interacting species within a material, a loading of
∼0.02 mmol g−1 (∼10× the area swept by a single tethered
pyrene) is needed to observe only monomer fluorescence (cf.
∼0.2 mmol g−1 for a patterned surface). It is important to note
that these estimates ignore the overestimation of BET surface
area obtained from N2 isotherms and that silica surfaces are
irregular not flat.66 From the above distribution study we
conclude that pyrene, and therefore the original organoazide, is
distributed randomly and that spectroscopically measurable
site-isolation of all attached moieties (length ∼15 Å) will be
achieved at loadings <∼0.02 mmol g−1.
Limited alkyne conditions probe the selection of azide sites
during the CuAAC reaction. The Imonomer/Ieximer ratio tracks
identically with that of excess alkyne prepared materials; the
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enhanced reactivity is not due to a simple base effect from
additional pyridine moieties from TPA, as addition of 2,6-
lutidine did not improve the epoxide yield. Increasing yield with
excess ligand illustrates the value in characterizing metal TON
and ligand TON independently of one another.
limiting either azide impregnation during material synthesis or
alkyne concentration during CuAAC immobilization. Various
immobilized functional groups provided probes to study the
packing of species on the surface and the variation in nearest-
neighbor as a function of surface loading.
Both metal complex speciation and catalytic epoxidation
yield are sensitive to the surface loading. The ability to achieve
site-isolated species is advantageous under conditions in which
metal lability and ligand stability are a concern. In the case of
Mn(TPA) epoxidation, the similar activity of homogeneous and
site-isolated species provides insight into catalytic speciation, in
a way unachievable through purely homogeneous character-
ization. Overall, the CuAAC attachment of homogeneous
catalysts to SBA-15-N3-x is a simple, reproducible and widely
applicable strategy to prepare randomly distributed, solid-
supported catalysts.
Low surface-loading SBA-15-TPA-0.5 performs nearly
identical to TPA-Tz-tBu, illustrating the viability of surface-
immobilized TPA ligands. Both pyrene fluorescence and
Cu(TPA) dioxygen adduct formation suggest a significant
proportion of species on SBA-15-R-0.5 are isolated from
neighboring sites, precluding interaction with neighboring
complexes. The similar yields obtained with SBA-15-TPA-0.5
and TPA-Tz-tBu ligands under identical concentrations suggest
that Mn(TPA) dimers are not a necessary species in the
catalytic cycle. Moreover, since SBA-15-TPA-0.5 shows similar
ligand TON to homogeneous conditions, Mn(TPA) dimeriza-
tion is unlikely the major cause of ligand deactivation at this
surface loading and in solution. These observations illustrate
the power of this methodology to mechanically probe a
reaction mechanism. Future modification of this ligand should
therefore focus on oxidative stability over dimerization
inhibition.
ASSOCIATED CONTENT
* Supporting Information
Experimental section and Figures S1 and S2. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
When the range of SBA-15-TPA-x materials are compared at
0.1 mol % metal and 1 mol % ligand, the epoxide yield is
impacted significantly by the surface loading of the immobilized
species. SBA-15-TPA-0.5 provides yields and rates most similar
to those of homogeneous conditions, and ligand TON
decreases linearly with higher surface loading. Surface crowding
may impede access to the catalytic sites, and the high local
concentration of TPA ligand may accelerate intermolecular
deactivation pathways. It is also possible that the loss of long-
term order in the 8% material results in poor accessibility of
immobilized TPA on the time scale of the reaction. The cause
for lower yields with SBA-15-TPA-0.2 is unclear. Since
enhanced reactivity with excess ligand presumes significant
metal lability and transfer, it is possible a minimum surface
density is required for a maximum turnover frequency.
Increased interaction with surface silanol groups may also be
a deactivating factor. Overall, these observations illustrate the
importance of a rationally targeted surface loading versus the
simple goal of maximized coverage.
To contrast the loading sensitivity of epoxidation catalysis,
carbene insertion catalyzed with immobilized iron porphyrins
was also examined. Compared to alkene epoxidation, carbene
insertion employs mild reaction conditions, occurs at a much
slower rate, and uses the FeTPP catalyst, a precomplexed
species with high stability. With homogeneous and heteroge-
neous FeTPP catalysts, EDA was fully consumed within 2 h at
all surface densities with near quantitative yields and high
TON. Under these conditions the impact of metal lability,
ligand decomposition, and site accessibility are minimized; a
corresponding lack of dependence on surface loading is
observed.
AUTHOR INFORMATION
Corresponding Author
stack@stanford.edu
■
Present Address
‡Department of Material and Life Chemistry, Kanagawa
University, Yokohama 221-8686, Japan
Author Contributions
†These authors are equal contributors to this work.
ACKNOWLEDGMENTS
■
This work was financially supported in part by NIH (Grant
GM70050730) and the Stanford Global Climate and Energy
Project (GCEP). J.N. acknowledges a JSPS Research Fellow-
ship. We thank Dr. G. Li, Dr. A. Vailionis, Dr. R. D. Lowe, Jr.,
and Dr. S. Lynch at Stanford University for the ICP, XRD,
fluorescence, and solid-state NMR measurements, respectively.
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