Controlled loadings in a mesoporous material: Click-on silica

Article abstract of DOI:10.1021/ja804237b

Hybrid mesoporous SBA-15 silicas were synthesized directly with variable alkylazide loading representing 2-50% surface coverage. These hybrid silica materials retain the favorable physical attributes of the parent SBA-15 materials and allow efficient covalent attachment of ethynylated organic moieties through a copper catalyzed 3 + 2 Huisgen cycloaddition reaction. Three distinctly different examples are provided demonstrating the efficiency and robust nature of this attachment synthetic strategy. The direct syntheses provide predefined loadings of randomly distributed organics within the materials, from site-dense to site-isolated. Such control over loadings along with simply implemented analytic procedures should facilitate the translation of homogeneous chemistries to heterogeneous supports. Copyright

Full text of DOI:10.1021/ja804237b

Published on Web 10/11/2008
Controlled Loadings in a Mesoporous Material: Click-on Silica
Jun Nakazawa and T. Daniel P. Stack*
Department of Chemistry, Stanford UniVersity, Stanford, California 94305
Received June 4, 2008; E-mail: stack@stanford.edu
Scheme 1. Preparation of SBA-15-R-x (x ) 0.2-8 mol %)
Surface modification of mesoporous silica materials with orga-
nosilane groups holds considerable potential for many applications,
including heterogenization of discrete metal catalysts and even
enzymes.1 Beyond recycling and separation advantages, site-
isolation of oxidation catalysts in such hybrid materials provides
potential activity enhancements, as deleterious bimolecular catalyst
interactions can be attenuated significantly.2 Though conceptually
simple, a dearth of straightforward and robust synthetic procedures
are available that provide systematic loadings of catalysts within
mesoporous materials. Reported here is a procedure that yields
predictable and randomly distributed loadings of an organoazide3
by a “direct synthesis”4 of the mesoporous silica (ca. 2-50%
surface coverage). Subsequent coupling of the organoazide with
an ethynlated molecule through a Cu-catalyzed Huisgen [3 + 2]
cycloaddition reaction, a “click” reaction,5 yields expected loadings
of the desired entities. Significant differences in reactivity with O2
are demonstrated qualitatively between site-isolated and site-dense
Cu(I) complexes.
Mesoporous SBA-15 silicas are attractive as a support for
oxidation catalysts with large surface areas (700-900 m2 g-1), large
pores sizes (6-9 nm) and robust silica walls (3-6 nm).6 Under
forcing grafting conditions of excess (3-azidopropyl)-triethoxysilane
(Si-N3; >20 equiv) in refluxing toluene over several days, a near
fully loaded surface (ca. 1 mmol g-1) of the organoazide is possible
with calcined SBA-15.7,8 Trace amounts of water are critical, as
rigorously dry conditions only yield a maximum of ca. 0.1 mmol
g-1. If a random distribution of organosilanes is desired at less
than full coverage, water is problematic, as the monomeric
organotriethoxysilanes precondense in solution, leading to clustering
on the surfaces.9
feature of each sample decreased to ca. 20% of its original intensity
(Figure 1a), suggesting that not all organoazides integrated by a
direct synthesis are accessible for reaction.13 The loadings of the
Fc, pyrene and TPA materials were determined by other analytical
techniques and the loadings correlated closely with the initial Si-N3
mixing ratio (Figure 1b).8 The combination of a direct synthesis
of the hybrid SBA-15 material followed by click attachment allows
for the efficient preparation of controlled loadings of ethynlated
molecules.
The fluorescence of the SBA-15-pyrene-x materials provided
critical information related to the distribution and density of the
organic groups.7,14 Using our lowest loaded sample (x ) 0.2), only
appreciable monomeric pyrene fluorescence is observed at 400 nm
(Figure 2). Higher loaded samples produced significantly more
excimer fluorescence at 480 nm resulting from interaction of two
closely positioned pyrene entities with a corresponding decrease
in monomer fluorescence. A direct correlation of loading and
excimer fluorescence is indicated.
Assuming a 15 Å tether length of each surface immobilized
pyrene along with the ca. 700 m2 g-1 surface area, a loading of
ca. 0.2 mmol g-1 is possible with idealized hexagonal packing;
only monomer fluorescence would be anticipated from this ordered
distribution. Assuming both a random distribution and excimer
fluorescence at any level of pyrene overlap, a 10-fold greater surface
area per pyrene is needed to ensure ca. 90% site-isolation.8 The
Imono/Iexc ratio of SBA-15-pyrene-0.2 (0.03 mmol g-1), shows
By contrast, reproducible and predictable loadings of Si-N3
within an SBA-15 were accessible by a “direct” synthetic route,10
in which various molar amounts of Si-N3 are co-condensed with
tetraethoxy-orthosilicate (TEOS) in the SBA-15 preparation (Scheme
1).8 Key to this preparative method is that the templating P123
organic polymer can be removed under mild Soxhlet extraction
conditions rather than calcination, leaving the covalently attached
organoazides in the templated pores.
The pore size and surface area of each material, SBA-15-N3-x,
as assessed by power X-ray diffraction and N2 absorption isotherm
measurements, are consistent fully with the ordered structure of
the original SBA-15 material.6,8 The azido-propyl groups are evident
from both the CP-MAS 13C solid-state NMR spectra and the IR
spectra.8 The ratio of the azide (2110 cm-1) to surface-bonded H2O
bending mode (1640 cm-1 11
features in the IR spectra provides a
)
simple ratiometric assessment of the azide content. Incorporation
of the Si-N3 into the materials clearly scales in a linear fashion
with the initial mol % used in the direct synthesis (Figure 1a).
Ethynylferrocene (R ) Fc), 1-ethynylpyrene (R ) pyrene), or
(5-ethynyl-2-pyridylmethyl)bis(pyridylmethyl)-amine (R ) TPA)
were clicked to the organoazides of SBA-15-N3-x to yield SBA-
15-R-x (Scheme 1).8,12 Ratiometric assessment of the azide IR
Figure 1. (a) IR peak area of azide (2110 cm-1 normalized to the signal
at 1640 cm-1) of the SBA-15 materials as a function of the mixing ratio x
of Si-N3. (b) Loading amounts (mmol g-1) of SBA-15-R-x (R: Fc, Pyrene
and TPA. x: mol % mixing ratio of Si-N3). Fc loading was determined by
ICP Fe analysis of digested samples. Pyrene and TPA loadings were
determined by UV-vis analysis of digested samples.
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2008 American Chemical Society

C O M M U N I C A T I O N S
loadings and highlights heterogenization as a strategy for site-
isolation of reactive, monomeric species.
We have synthesized and characterized a series of mesoporous
silica materials (SBA-15-N3-x) with controlled, randomly distributed
azide loadings for quantitative click modification. Given the
simplicity and predictability of these material preparations, we
anticipate the materials will be widely adopted in applications
needing to heterogenize homogeneous chemistry.
Acknowledgment. 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
Fellowship. We thank Dr. G. Li, Dr. A. Vailionis, Mr. L. D. Lowe,
Jr., and Dr. S. Lynch at Stanford University for the ICP, XRD,
fluorescence, and solid NMR measurements, respectively.
Figure 2. Fluorescence spectra of SBA-15-pyrene-x (x ) 0.2, 0.5, 1, 2, 3,
4, 8 mol %) suspended in CHCl3 (λex ) 330 nm). For comparison the spectra
were normalized at 415 nm.
Scheme 2. Dioxygen Adducts of [CuITPA]1+
Supporting Information Available: Sample preparation and char-
acterization of SBA-15 materials. This material is available free of
charge via the Internet at http://pubs.acs.org.
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predominant monomer fluorescence. The correlation of the decrease
of the Imono/Iexc ratio with loading is also consistent with a
cumulative density function for randomly distributed objects.8 These
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complexes will be achieved at loadings in the 0.03 mmol g-1 range
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[CuI(TPA)]+ to form a purple dimeric peroxo complex (Scheme
2).8,15 This rapid dimerization precludes any investigation of the
superoxide species reactivity. Site-isolation of [CuITPA]1+ allows
the formation of this mononuclear species; SBA-15-TPA-0.5, loaded
with 0.9 equiv of [CuI(MeCN)4](SbF6) per TPA ligand, turns
brilliant green upon oxygenation at -90 °C in EtCN. The more
densely loaded material, SBA-15-TPA-4, oxygenates to form a
purple material. These color changes are consistent with the
homogeneous chemistry. Unfortunately, the instability of both
complexes at -90 °C precludes reactivity studies and suggests that
passivation of the silica surface may be needed. These initial results
indicate, however, that sensitive metal-based homogeneous chem-
istry can be translated efficiently into mesoporous silicas at variable
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