Autonomous movement of silica and glass micro-objects based on a catalytic molecular propulsion system

Article abstract of DOI:10.1002/chem.200701227

A general approach for the easy functionalization of bare silica and glass surfaces with a synthetic manganese catalyst is reported. Decomposition of H₂O₂ by this dinuclear metallic center into H₂O and O₂ induce

Full text of DOI:10.1002/chem.200701227

DOI: 10.1002/chem.200701227
Autonomous Movement of Silica and Glass Micro-Objects Based on a
Catalytic Molecular Propulsion System
Christoph Stock, Nicolas Heureux, Wesley R. Browne, and Ben L. Feringa*[a]
Abstract: A general approach for the easy functionalization of bare silica and
glass surfaces with a synthetic manganese catalyst is reported. Decomposition of
H2O2 by this dinuclear metallic center into H2O and O2 induced autonomous
movement of silica microparticles and glass micro-sized fibers. Although several
mechanisms have been proposed to rationalise movement of particles driven by
H2O2 decomposition to O2 and water (recoil from O2 bubbles,[36,45] interfacial ten-
sion gradient[37–42]), it is apparent in the present system that ballistic movement is
due to the growth of O2 bubbles.
Keywords: autonomous movement ·
manganese
·
microparticles
·
molecular motor · surface analysis
Introduction
propulsion mechanisms including oxygen bubble formation,
change in surface tension or oxygen gradients.[42] Propulsion
of conductive carbon fibers, using a bio-electrocatalyst, able
to decompose glucose and oxygen into gluconolactone and
water, was reported by Mano and Heller.[43] More recently,
self-propulsion of millimeter-sized semiconductor diodes,
through an electric field, was reported by the group of
Velev.[44] In a molecular approach to autonomous move-
ment, we recently reported a synthetic oxygen evolving
Controlled movement is one of the most fascinating features
of living cells.[1] However, locomotion on the micro- and
nanoscale through a fluid environment, in synthetic systems,
remains one of the major challenges confronting nanotech-
nology today.[2–12] The bottom-up construction of molecular-
level motors has brought the prospect of synthetic molecular
based mechanical machines within sight. Artificial molecular
systems constructed in order to mimic aspects of mechanical
function include shutlles,[13–17] rotors,[18–21] muscles,[22–24]
switches,[25–28] elevators[29,30] and motors.[31–35]
An attractive approach to apply chemical transformations,
that is, using a chemical fuel, to induce translational motion
is based on a seminal contribution by Whitesides and co-
workers.[36] Rotational and translational movement of milli-
meter and micrometer-sized objects[37–41] has been achieved
using bimetallic systems and H2O2 as a fuel with proposed
complex as the catalyst for
a micro-scale propulsion
system.[45] In our design, the micro-sized object, for instance
a silica microparticle (40–80 mm), is linked, via a short
spacer, to a metal complex, able to catalyze H2O2 dispropor-
tionation at a molecular level (Scheme 1).
[a] Dr. C. Stock, Dr. N. Heureux, Dr. W. R. Browne,
Prof. Dr. B. L. Feringa
Organic Chemistry Laboratories
Stratingh Institute for Chemistry & Zernike Institute for Advanced
Materials
Scheme 1. Design of a system propelled by a catalytic molecular motor.
University of Groningen, Nijenborgh 4
9747 AG Groningen (The Netherlands)
Fax : (+31)50-363-4296
The use of a molecular catalytic complex was found to be
of a considerable advantage, as it can be easily modified,
tether length can be adjusted, and local catalyst density is
controllable. Moreover, we considered that a molecular
E-mail: b.l.feringa@rug.nl
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author: Movies of auto-
nomous movement of a silica microparticle and of glass fibers.
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FULL PAPER
system would be easily adaptable on different kinds of ma-
terials, giving the extensive background in surfaces function-
alization that exists in the literature.[46,47] Furthermore, it
offers the possibility to control oxygen evolution at the mo-
lecular level through the molecular catalyst function and
catalyst activity. Our first generation system was based on a
dinuclear manganese catalyst.[45] Designed as a functional
model for Mn–catalase enzymes,[48–50] ligand 1,[45,51] bearing a
phenol, a tertiary amine and two pyridine binding sites was
used to complex manganese. However, anchoring of our cat-
alytic system on silica surfaces relied on the use of fragile
imine linkers as well as the requiring the use of pre-func-
tionalized silica particles (Scheme 2).
the functionalization. In a shorter process, direct coupling of
terephthalic acid using DCC/DMAP also supplied the de-
sired material, that showed carboxylic acid absorption in IR
at 1682 cmÀ1. Complexation with Mn
(ClO4)2 in the presence
A
of tetradentate ligand 1 and extensive washing of the micro-
particles to remove any unbound ligand or complex provid-
ed the silica particles functionalized with the dinuclear man-
ganese catalyst 4. The elaboration of surface bound manga-
nese complexes was confirmed by IR spectroscopy and com-
parison with a non-bound manganese catalyst 5 synthesized
from p-formylbenzoic acid.[45] Disappearance of the carbox-
ylic acid band and appearance of the bridged carboxylate
absorption in the 1590–1600 cmÀ1 region was a strong indica-
tion that our manganese com-
plex was covalently immobi-
lized on the surface of the silica
particles. Furthermore, the high
activity for decomposition of
H2O2 by these particles, using a
5% H2O2 solution in acetoni-
trile was also good evidence for
the formation of the bound di-
nuclear manganese complex[53]
(Scheme 3)
While using an excess of 4-
(methoxycarbonyl)benzoic acid
in the elaboration of functional-
ized particles 3, a qualitative
Scheme 2. Structure of surface bound synthetic dinuclear manganese complex as an immobilized catalase
mimic.
Kaiser test indicated the pres-
ence of free primary amino
groups, as supported by the
strong purple color observed.[54]
Development of a general method for the functionaliza-
tion of different non-functionalized surfaces with a stable
synthetic catalytic motor would be of great interest. Here
we report a general method for the functionalization of bare
silica and glass microparticles with a synthetic manganese
catalyst, through an amide or ether linkage, and the autono-
mous movement of these microscopic objects.
Therefore a better description of the functionalized silica
microparticles obtained via this procedure is depicted in
Scheme 4 (method A).
The presence of reactive primary amino groups on the
surface of the silica particles could perturbfunctionalization
as well as characterization of more elaborated systems. In
order to avoid the presence of free amine functionalities, we
envisioned, as a key step towards our second generation
system, an alternative approach using trialkoxy silanes, fol-
lowing a procedure by Jones.[55] Conceptually, the ligand for
metal binding is first functionalized with the trialkoxy silane
spacer which allows anchoring to the surface (Scheme 4, B).
Moreover, this procedure should allow functionalization of
bare materials, extending greatly the flexibility of our meth-
odology. To enhance the robustness of the linkage between
the particles and the molecular motor, increasing the stabili-
ty of the system and avoiding plausible leakage of the man-
ganese catalyst, we designed two new spacers based on the
robust amide and ether links. To prevent competitive bind-
ing of the free carboxylic acid functionality to the silica sur-
face, the methylester derivatives were used. Starting from
terephthalic mono methyl ester 10, condensation of corre-
sponding acid chloride with 3-(triethoxysilyl)propan-1-amine
afforded the amide tethered benzoic ester 11. Alternatively,
allylic ether 12 was converted into trimethoxy silane 13 via a
Results and Discussion
The design of a molecular motor capable of generating ki-
netic energy from a chemical source comprises a linker be-
tween the object and the catalyst. As a first step towards a
second-generation system and in order to increase the stabil-
ity of our molecular system, we first designed a new amide-
based tether.
Pre-functionalized aminopropyl silica microparticles
2
(40–63 mm) were modified to benzoic acid coated particles
through the use of two different procedures. Acylation of
primary amines using methyl 4-(chlorocarbonyl)benzoate,[52]
followed by ester saponification afforded functionalized
silica particles 3 (Scheme 3). The characteristic C=O absorp-
tion in IR at 1729 cmÀ1 for ester functionalized particles and
at 1680 cmÀ1 for carboxylic acid groups was used to follow
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B. L. Feringa et al.
Bare silica microparticles
were
then
functionalized
through their suspension in a
solution of trialkoxy silane de-
rivative in toluene, using either
the amide 11 or the aromatic
ether 13. As expected, the
Kaiser test revealed the ab-
sence of free amino groups on
the surface of particles derivat-
ized with 11 or 13 as the sus-
pension remained yellow. Hy-
drolysis of ester moieties to
reveal the free carboxylic acid
groups, followed by complexa-
tion with Mn(ClO4)2 in pres-
G
ence of ligand 1, produced di-
nuclear manganese catalyst
functionalized particles 16 and
17 (Scheme 6).
The presence of the manga-
nese complex on the surface of
16 and 17 was confirmed by IR
Scheme 3. Synthesis of surface bound manganese catalyst anchored via an amide linkage. a) Methyl 4-(chloro-
carbonyl)benzoate, Et3N, toluene; b) KOH, H2O, EtOH; terephthalic acid, DCC, DMAP, toluene; d) Mn-
(ClO4)2, Et3N, 1, MeOH, MeCN.
Scheme 5. Preparation of trialkoxy silane linkers. a) SOCl2; b) 3-(tri-
ethoxysilyl)propan-1-amine, Et3N, DCM, 95%; c) HSiCl3, H2PtCl6, THF;
d) MeOH, Et3N, 65%.
spectroscopy, diffuse reflectance UV/Vis spectroscopy
(Figure 1), EPR spectroscopy (Figure 2) and by elementary
analysis. In the IR spectroscopy, as previously observed, dis-
appearance of the carboxylic acid band and appearance of
the bridged carboxylate absorption in the 1590–1600 cmÀ1
region was a strong indication that our manganese complex
was covalently immobilized on the surface of the silica parti-
cles. The presence of the dinuclear manganese catalyst was
also confirmed by UV/Vis spectroscopy, as exemplified for
the functionalized particles 16. Comparison of the spectrum
of the non-bound reference manganese catalyst 5 and the
spectrum of the silica supported manganese catalyst 16, ob -
tained by diffuse reflectance UV/Vis, revealed the same
characteristic absorption bands, at 222 nm, in the region be-
tween 245 and 275 nm and in the region between 285 and
295 nm. The stronger absorption for the silica-tethered man-
ganese catalyst 16 around 243 nm was attributed to the
structure of the amide benzoic ester linker, as supported by
Scheme 4. Two different approaches allowing functionalization of silica
particles: method A: Anchoring of benzoic acid moiety to amine func-
tionalized silica; method B: Attachment of trialkoxysilane functionalized
acid. a) Terephthalic acid, DCC, DMAO, toluene; b) CHCl3, D.
platinum-catalyzed hydrosilylation reaction with HSiCl3 fol-
lowed by methanolysis[56] (Scheme 5).
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Catalytic Molecular Propulsion System
FULL PAPER
a control the EPR spectrum
(Figure 2) of a ligand free silica
supported mononuclear MnII
species confirms that non-ligat-
ed manganese is not involved
to a significant extent in the
catalase activity of the silica
particles 16. Indeed addition of
H2O2 to MnII modified particles
(i.e., ligand free) results in im-
mediate flocculation and pre-
cipitation as a brown inactive
solid.
The manganese content of
the immobilized dinuclear cata-
lyst 16 was determined by ICP-
AES to be 0.51% by mass, and
hence the dinuclear catalyst
loading was calculated to be
about 0.05 mmolgÀ1.
Decomposition of H2O2 as a
solution in acetonitrile was ob-
served for suspensions of both
Scheme 6. Functionalization of bare silica microparticles with the dinuclear manganese catalyst. a) Toluene; b)
KOH, H2O, EtOH; Mn(ClO4)2, Et3N, 1, MeOH, MeCN.
A
the spectrum of the triethoxysilane derivative 11, which
shows a maximum absorption at 238 nm (Figure 1).
Figure 2. EPR spectra (X-band at 77 K) of c: silica supported manga-
nese catalyst 16 (suspension in DCM), a: non-bound manganese cata-
lyst 5 (2.10À3 m in DCM) and g: ligand free silica supported MnII. Re-
cording conditions: microwave power = 63.5 mW; modulation amplitude
= 0.1 mT; modulation frequency = 50 kHz; time constant = 40.96 ms;
T = 77 K at 9.47 GHz.
Figure 1. UV/Vis spectra of c: amide tethered benzoic ester 11
(2.10À5 m in CH3CN), a: non-bound manganese catalyst 5 (2.10À5 m in
CH3CN) and g: silica supported manganese catalyst 16 (DR UV/Vis,
suspension in glycerol).
EPR spectroscopy presents a powerful tool in the study of
manganese-based catalytic systems. The EPR spectrum of
ligand free silica supported MnII[57] is characteristic for a
mononuclear quasi-octahedral MnII species, which do not
show discernible hyperfine splitting. The EPR spectra of
both non-bound manganese catalyst 5 and silica supported
manganese catalyst 16 bear a remarkable similarity to that
of the dinuclear manganese(II) hydroxylation and epoxida-
tion catalysts previously reported by our group.[58] These
data strongly support the presence of the dinuclear manga-
nese(II) catalyst on the surface of the silica particles 16. As
materials (i.e., 16 and 17). In order to study the movement
induced by O2 formation, a thin liquid film, containing a
dilute suspension (in CH3CN) of the functionalized micro-
particles 16 or 17, mixed with non-functionalized microparti-
cles (40–63 mm) as reference, on a microscope slide, was
monitored after addition of a 5% H2O2 solution in CH3CN.
Oxygen evolution was observed as small oxygen bubbles ap-
pearing only on the catalyst-functionalized microparticles.
Translational movement of the functionalized silica particles
were observed (Figure 3; see also movie in the Supporting
Information).
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B. L. Feringa et al.
thetic manganese catalyst. De-
composition of H2O2 by this di-
nuclear metallic center into
H2O and O2 induced autono-
mous movement of silica micro-
particles and glass micro-sized
fibers. Although several mecha-
nisms have been proposed to
rationalise movement of parti-
cles driven by H2O2 decomposi-
tion to O2 and water (recoil
Figure 3. Translational movement of a catalyst-functionalized silica microparticle 17 at a) 0 s, b) 8 s, c) 19 s
(average speed=9.7 mmsÀ1). Dimension of the video frame is 500”360 mm.
This successful method for functionalization of silica sur-
faces with the catalyst system encouraged us to the prepara-
tion of other functionalized surfaces. In order to induce au-
tonomous movement of different micro-objects and to show
the generality of our second generation molecular approach,
we selected glass surfaces, giving that their functionalization
with trialkoxy silane derivatives is also known.[59,60] Prior to
functionalization, glass slides (0.5”0.5 cm) were activated
using the method described by Linker.[61] Activated glass
slides 18 were then coated via the condensation of trime-
thoxysilane 13. Hydrolysis of the ester moiety and synthesis
of the manganese catalyst were then performed as previous-
ly described (Scheme 7). Functionalization of the surface
was monitored by contact angle measurements, which
changed from 78–808 for ester functionalized glass slides to
55–608 for the more hydrophilic carboxylic acid functional-
ized glass slides and to 70–738 for the glass slides bearing
the manganese catalyst 19.
from O2 bubbles,[36,45] interfacial tension gradient[37–42]), it is
apparent in the present system that ballistic movement is
due to the growth of O2 bubbles. The exact mechanism by
which propulsion of the particles takes place in terms of the
dependence of bubble formation on catalyst distribution and
the presence of bubble nucleation sites is at present under
further investigation. Future work will focus on the applica-
tion of the methodology described here to achieve move-
ment of nano-sized objects.
Experimental Section
General methods: Chemicals were purchased from Acros, Aldrich, Fluka
or Merck. Solvents for extractions and chromatography were technical
grade. All solvents used in reactions, were freshly distilled from appropri-
ate drying agents before use. Flash chromatography was carried out using
Moreover, under microscope,
glass slides were shown to be
catalytically active in the de-
composition of a 5% H2O2 so-
lution in acetonitrile, giving us
a good evidence for the cova-
lent immobilization of the man-
ganese complex. However, the
glass slides were not suitable
for movement in a thin liquid
film, because of their large size.
Nevertheless, glass fibers (glass-
wool, 10–25 mm in diameter),
functionalized in the same
manner, showed translational
movement upon addition of
Scheme 7. Functionalization of bare glass surfaces. a) Toluene; b) KOH, H2O, EtOH; c) Mn
MeOH, MeCN.
A
aqueous H2O2 solution to
a
fibers suspension in acetonitrile
(Figure 4; see also movie in the
Supporting Information).
Conclusion
In conclusion, we have devel-
oped a general method for easy
functionalization of bare silica
Figure 4. Translational movement of glass fibers 19 at a) 0 s, b) 65 s, c) 145 s (average speed=1.6 mmsÀ1). Di-
and glass surfaces with a syn- mension of the video frame is 1000”720 mm.
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Catalytic Molecular Propulsion System
FULL PAPER
Merck silica gel 60 (230–400 mesh ASTM). NMR spectra were obtained
using a Varian Gemini-400. Chemical shifts are reported in d units (ppm)
relative to the signal of TMS. MS(EI) spectra were obtained with a Jeol-
600 spectrometer. FTIR spectra were recorded (as intimate mixtures
with KBr) using a Nicolet Nexus FTIR spectrometer. EPR spectra (X-
band, 9.46 GHz) were recorded in liquid nitrogen (77 K) on a Bruker
ECS 106 instrument, equipped with a Bruker ECS 041 XK Microwave
and dried under vacuum. Yield: 590 mg (0.61 mmol, 61%); IR: n˜ = 629,
760, 808, 883, 1014, 1097, 1159, 1203, 1276, 1403, 1454, 1483, 1558, 1602,
1660, 1760 cmÀ1; ES-MS: m/z: 867.5 [L2Mn2(O2CC6H4CHO)]+; elemental
analysis calcd (%) for C46H41ClMn2N6O9: C 57.12, H 4.27, Cl 3.67, Mn
11.36, N 8.69, O 14.89; found: C 56.98, H 4.38, N 8.51.
N-(3-(Triethoxysilyl)propyl)-terephthalamic acid methyl ester 11: Tereph-
thalic acid mono methyl ester 10 (2.04 g, 11.2 mmol) was suspended in
thionyl chloride (25 mL). The reaction mixture was heated under reflux
for 3 h followed by removal of the excess thionyl chloride by distillation.
The pale yellow solid obtained was used in the next step without further
purification.
bridge and
a Bruker ECS 080 magnet. Samples for measurement
(250 mL) were transferred to an EPR tube, which was frozen in 77 K im-
mediately. UV/Vis spectra were obtained using a diffuse reflectance at-
tachment of a JASCO V-570 UV/Vis-NIR spectrophotometer. Images
and movies of the moving particles and fibers were recorded using an
Olympus BX 60 microscope, equipped with a Sony 3CCD DXC 950P
digital camera, connected to a personal computer, using Matrox Inspec-
tor 2.1. imaging software. Contact angles were measured on a Krüss
Drop Shape Analysis System DSA 10 Mk2.
A solution of 3-(triethoxysilyl)propan-1-amine (2.01 g, 9.06 mmol) and
triethylamine (1.37 g, 13.6 mmol) in dichloromethane (20 mL) were
added to a solution of the acid chloride in dichloromethane (20 mL). The
solution was refluxed for 1.5 h followed by concentration in vacuo. The
crude reaction mixture was purified by column chromatography on silica
gel (pentane/ethyl acetate 2:1) yielding trialkoxysilane 11 as a pale
yellow solid (3.29 g, 10.7 mmol, 95%). 1H NMR (400 MHz, CD3Cl,
258C): d=8.09 (d, J=8.1 Hz, 2H), 7.83 (d, J=8.1 Hz, 2H), 6.66 (s, 1H),
3.94 (s, 1H), 3.84 (q, J=6.9 Hz, 6H), 3.48 (dt, J=6.6, 6.6 Hz, 2H), 1.82–
1.73 (m, 2H), 1.22 (t, J=6.9 Hz, 9H), 0.72 ppm (t, J=8.1 Hz, 2H);
13C NMR(100 MHz, CD3Cl, 258C): d=166.6, 166.2, 132.3, 129.5, 129.4,
126.9, 58.4, 52.2, 42.3, 22.6, 18.1, 7.7 ppm; IR (KBr): n˜ = 3315, 2983,
2886, 1727, 1641, 1544, 1436, 1278, 1103, 1079 cmÀ1; HRMS: m/z: calcd
for C18H29NO6Si: 383.1764, found: 383.1780.
Kaiser test: Three solutions were prepared: 1) 500 mg ninhydrin in
10 mL ethanol, 2) 80 g phenol in 20 mL ethanol, 3) 2 mL of an 1 mm
aqueous solution of KCN diluted to 100 mL with pyridine. A small
sample of the particles (2 to 5 mg) was placed in a test tube and three
drops of each of the three reagents were added. The tube was heated at
1008C for 2 min and the suspension turned purple (presence of free
amino groups) or remained yellow (absence of free amino groups).
Preparation of functionalized silica particles 4 according to Scheme 3
Method A: 3-Aminopropyl functionalized silica gel 2 (400 mg, 40–63 mm,
ꢀ1 mmolgÀ1 NH2 loading, Aldrich) was suspended in toluene (18 mL). 4-
(Methoxycarbonyl)benzoic acid (500 mg, 2.77 mmol), DCC (573 mg,
2.77 mmol) and DMAP (34 mg, 0.28 mmol) were then added and the sus-
pension was stirred at room temperature for 20 h. The silica microparti-
cles were filtered off, washed with ethanol (3”20 mL) and with dichloro-
methane (3”10 mL) and dried in vacuo. IR (KBr): n˜ =1729, 1591, 1548,
Methyl 4-(3-(trimethoxysilyl)propylcarbamoyl)benzoate (13): 4-Allyloxy-
benzoic acid methyl ester 12 (3.41 g, 17.8 mmol), trichlorosilane (3.61 g,
26.64 mmol) and hexachloroplatinic acid monohydrate (2 mg, 3.9 mmol)
were added to dry THF (5 mL) and the mixture stirred at room tempera-
ture for 24 h. The solvent was then removed in vacuo and to the crude
trichlorosilane adduct was added dropwise methanol/triethylamine 1:1
((15 mL). The reaction mixture was stirred at room temperature for 2 h
and subsequently diluted with diethyl ether (10 mL). The solvent were
removed in vacuum and the crude reaction mixture was purified by
column chromatography on silica gel (pentane/ethyl acetate 90:10) yield-
ing trialkoxysilane 13 as a pale yellow solid (4.80 g, 15.3 mmol, 86%).
1H NMR(400 MHz, CD3Cl, 258C): d=7.95 (dd, J=8.9, 2.1 Hz, 2H), 6.88
(dd, J=8.9, 2.1 Hz, 2H), 3.97 (td, J=6.5, 1.8 Hz, 2H), 3.86 (d, J=2.1 Hz,
3H), 3.57 (s, 9H), 1.93–1.82 (m, 2H), 0.77 ppm (td, J=8.1, 1.8 Hz, 2H);
13C NMR (100 MHz, CD3Cl, 258C): d=166.9, 131.5, 122.4, 115.5, 114.1,
69.8, 51.8, 50.5, 22.5, 5.2 ppm; MS(EI): m/z: 314 (4), 272 (4), 241 (11),
152 (26), 121 (100), 93 (11), 91 (9), 65 (10); HRMS: m/z: calcd for
C14H22O6Si: 314.1185, found: 314.1191.
1403, 1284 cmÀ1
.
The samples were suspended in a mixture of ethanol and water (20 mL,
1:2 ratio). Potassium hydroxide (1.20 g) was added and the suspension
was heated to reflux for 3 h. The silica particles were filtered off and
washed with ethanol (3”30 mL) and dried in vacuo. IR (KBr): n˜ =1680,
1579, 1508, 1369, 1085 cmÀ1
.
Method B: 3-Aminopropyl functionalized silica gel 2 (400 mg, 40–63 mm,
ꢀ1 mmolgÀ1 NH2 loading, Aldrich) was suspended in toluene (12 mL).
Terephthalic acid (100 mg, 0.60 mmol), DCC (124 mg, 0.60 mmol) and
DMAP (10 mg, 0.08 mmol) were added and the suspension was stirred at
room temperature for 17 h. The silica microparticles were filtered off,
washed with ethanol (3”20 mL) followed by dichloromethane (3”
Preparation of functionalized silica particles 16 and 17 according to
Scheme 6
10 mL) and dried in vacuo. IR (KBr): n˜
1087 cmÀ1
= 1682, 1575, 1511, 1372,
.
General procedure: Merck silica gel 60 (400 mg, 230–400 mesh ASTM)
was suspended in toluene (120 mL) and trialkoxysilane 11 or 13
(1.32 mmol) was added. The mixture was stirred at room temperature for
16 h then the silica microparticles were filtered off, washed with toluene
(3”30 mL) and with ethyl acetate (3”30 mL) and dried in vacuo.
Manganese perchlorate hexahydrate (121 mg, 0.33 mmol) and ligand 1
(102 mg, 0.33 mmol) were dissolved in methanol (2 mL) and the solution
was stirred for 30 min. The benzoic acid functionalized silica (200 mg)
was then added and the mixture was stirred for 15 min before the addi-
tion of triethylamine (0.07 mL, 0.50 mmol). After an additional 15 min,
acetonitrile (3 mL) was added and the mixture was left overnight. The
solid was then collected by filtration and subsequently washed with meth-
anol (3”5 mL) and acetonitrile (3”5 mL) and dried in vacuo. IR (KBr):
Amide-functionalized particles (14): IR (KBr): n˜ =1727, 1646, 1592, 1546,
1439, 1387, 1094 cmÀ1
.
Ether-functionalized particles (15): IR (KBr): n˜ =1716, 1608, 1513, 1438,
1105 cmÀ1
n˜ = 1562, 1502, 1446, 1376, 1096 cmÀ1
.
.
The microparticles were next suspended in a mixture of ethanol and
water 1:3 (30 mL). Potassium hydroxide (1.10 g) was added to the suspen-
sion and the mixture was refluxed for 4 h. The silica particles were then
filtered off and washed with water (3”20 mL) and ethanol (3”20 mL)
and dried in vacuo.
Preparation of reference non-bound manganese catalyst 5 according to
Scheme 3
Manganese perchlorate hexahydrate (724 mg, 2.0 mmol) and ligand 1
(610 mg, 2.0 mmol) were dissolved in methanol (10 mL) and the mixture
was stirred for 30 min. Subsequently, 4-carboxybenzaldehyde (150 mg,
1.0 mmol) was added. After all solids were dissolved, triethylamine was
added (0.42 mL, 3.0 mmol) at once and the solution turned pale green.
After stirring for about 5 min, a solid precipitated and stirring continued
for another 5 min. The solution was heated until boiling and gradually
15 mL of CH3CN were added, while keeping the mixture boiling. After
the last few mL of CH3CN were added, the solids suddenly dissolved and
the solution was left overnight. The supernatant was removed and the
pale green crystals were washed thrice with MeOH and thrice with Et2O
Amide-functionalized particles: IR (KBr): n˜ = 1680, 1581, 1511, 1381,
1090 cmÀ1
.
Ether-functionalized particles: IR (KBr): n˜ =1692, 1606, 1513, 1426,
1098 cmÀ1
.
Manganese perchlorate hexahydrate (121 mg, 0.33 mmol) and ligand 1
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was stirred for 30 min. The carboxylic acid functionalized silica (100 mg)
was then added and the mixture was stirred 15 min before the addition of
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B. L. Feringa et al.
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Preparation of functionalized glass slides and fibers 19
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Acknowledgements
This work was partially supported by the Deutsche Forschungsgemein-
schaft (DFG) through a postdoctoral fellowship, by the Netherlands Or-
ganization for Scientific Research (NWO-CW) and by the National Re-
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Received: August 7, 2007
Revised: November 14, 2007
Published online: February 7, 2008
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3153