Nanoparticles for Ligand-Free Metal Catalysis
FULL PAPER
size exclusion chromatography (ISEC) were purchased from Polymer
Standards Service, PSS (Germany). The microstructures of the monolith-
ic materials were investigated by the use of a Zeiss DSM 940A scanning
electron microscope (Carl Zeiss, Oberkochen, Germany).
the support was dried in vacuo for 4 h. The platinum content was deter-
mined by ICP-OES.
Suzuki-type cross-coupling reactions: All Suzuki-type coupling reactions
were carried out in 25 mL Schlenk tubes at 508C for 24 h. The reaction
mixtures were prepared as follows: Distilled water (8 mL) and THF
(8 mL) were placed in a 25 mL Schlenk tube and then the following sub-
strates were added: tert-Butylbenzene (internal standard for GC–MS,
100 mg, 0.746 mmol), phenylboronic acid (1.0 g, 8.2 mmol), the aryl bro-
mide (8.2 mmol), and KOtBu (1.5 g, 12.3 mmol). Finally, tetra-N-butyl-
Synthesis of monoliths: All monoliths were prepared as follows: Stainless
steel columns (100ꢆ4.6 mm i.d.) were cleaned, rinsed, and sonicated in a
1:1 mixture of ethanol and acetone. The columns were closed at one end
with frits and end fittings. Then the columns were filled with the polymer-
ization mixture, sealed at both ends, and irradiated. Unless stated other-
wise, a total dose of 22 kGy was applied. After irradiation, the columns
were directly connected to a HPLC pump and flushed with dichlorome-
thane for 4 h at a flow rate of 0.2 mLminÀ1, then with THF for 30 min at
a flow rate of 0.3 mLminÀ1, and finally with water for 30 min at a flow
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
THF was removed under reduced pressure and the residue was dissolved
in diethyl ether. The diethyl ether layer was washed with water (2ꢆ
50 mL) and saturated brine solution (2ꢆ50 mL), then dried over anhy-
drous Na2SO4, and concentrated under reduced pressure. The products
were characterized by GC–MS and NMR and IR spectroscopy.
rate of 0.3 mLminÀ1
.
Inverse size exclusion chromatography (ISEC): The volume fractions of
the inter-microglobule porosity (ez), the pore porosity (ep), the total po-
rosity (et), the pore volume (Vp), and the mean pore diameter (Fm) of
the monolithic columns were characterized by ISEC[7] as described else-
where.[6a,23]
Heck coupling reactions: All Heck-coupling experiments were carried
out in 25 mL Schlenk tubes at 1408C for 24 h. The reaction mixtures
were prepared as follows: N,N-Dimethylformamide (DMF; 10 mL) was
placed into a 25 mL Schlenk tube and then tert-butylbenzene (100 mg,
0.746 mmol), styrene (1.589 g, 15.28 mmol), the aryl bromide
(12.7 mmol), and NaOAc (1.252 g, 15.28 mmol) were added. Finally, the
monolithic material (13 mg, 0.331 mmol of palladium) was added and the
mixture was degassed twice. The mixture was then stirred at the desired
temperature. After completion of the reaction, the mixture was cooled to
room temperature and the organic products were extracted with diethyl
ether. The conversions were checked by GC—MS and NMR and IR
spectroscopy.
Hydrolysis of the epoxy groups within pores of >7 nm: The epoxide
groups of porous polymer rods were hydrolyzed by flushing the mono-
lithic column with
a
solution of poly(styrenesulfonic acid) (Mw =
flow rate of
69400 gmolÀ1
,
4.5 wt% in water) for 15 min at
a
0.3 mLminÀ1. Then the monolith was kept for 15 h at 658C. The hydro-
lyzed column was then washed for 2 h at a flow rate of 0.3 mLminÀ1 with
water/methanol (2:1) and THF. These columns were then again charac-
terized by ISEC.
Functionalization of pores of <7 nm: A typical experiment was as fol-
lows: The remaining epoxide groups within the small pores of the mono-
lith were allowed to react with 2. Thus, a solution containing 500 mg of 2
per 5 mL of 1,4-dioxane was introduced into the monolith, which was
then kept at 608C for 16 h. The thus modified column was then washed
with 55 mL of CH2Cl2 (flow rate 0.3 mLminÀ1 for 3 h). After this proce-
dure, the monoliths were ready for ROMP-based grafting.
TEM and EDXS analyses: The (crushed) monolithic material was dis-
persed in ethanol and one droplet of this suspension was applied to a
carbon-coated copper grid. Then the solvent was evaporated. For bright-
field TEM imaging as well as for energy-dispersive X-ray spectrometry
(EDXS), a Hitachi H-8100 transmission electron microscope (operated at
200 keV at a point-to-point resolution of 0.23 nm and equipped with a
LaB6 filament and a STEM unit) was used. EDXS data were acquired
with a Si(Li) detector with a spectral resolution of 138 eV and analyzed
by using the NORAN system SIX software.
Functionalization of pores of <7 nm by ROMP-based grafting: The ini-
tiator 1 (4.0 mg, 4.86 mmol) was dissolved in CH2Cl2 (1.5 mL) and intro-
duced into the monolith. The monolith was sealed and kept at room tem-
perature overnight. Then the monolith was flushed with CH2Cl2 for
30 min at a flow rate of 0.3 mLminÀ1 to remove any unattached catalyst
and then with argon to remove all solvent. A sample of each monomer
3–8 (100 mg) was dissolved in CH2Cl2 (1.5 mL) and introduced into the
monolith (Table 2). The monolith was sealed and kept at 408C overnight.
The following day, the monolith was flushed with a 10 vol% solution of
ethyl vinyl ether (EVE) in DMSO and then with THF and kept in vacuo
overnight. The amount of grafted monomer was determined by either
acid–base titration (monomers 3 and 4) or elemental analysis (nitrogen-
and phosphorus-containing monomers).
Determination of the ruthenium, palladium, and platinum content by
ICP-OES
Ruthenium: The monoliths were subjected to ROMP-based functionaliza-
tion as described above. EVE (20 vol-% in DMSO) was added to
remove the initiator and the effluent was collected, concentrated in
vacuo, and dissolved in aqua regia. The ruthenium content, which corre-
sponds to the total amount of initiator immobilized on the monolithic
surface, was quantified by ICP-OES. Ruthenium was quantified at l=
267.876 nm by using the average of at least three consecutive measure-
ments, the background was measured independently at l1 =267.759 and
l2 =267.998 nm. The limit of detection (LOD) was 0.08 mgLÀ1. For the
calibration, aqueous ruthenium standards (pH 1, nitric acid) containing 0,
0.004, 0.14, 0.5, and 12.0 mgRuLÀ1 were used.
Quantification of 3 and 4: Monoliths grafted with 3 or 4 were removed
from the stainless-steel column, ground, and dried in vacuo overnight.
Acid–base titrations were performed with three independent samples.
Each sample was stirred for 3 days in a mixture of a standard solution of
0.1m sodium hydroxide (f=1.000) and 1,4-dioxane (80:20 v/v, 10 mL).
Each sample was filtered using sintered glass crucibles and washed with
deionized water (15 mL). Unconsumed NaOH was back-titrated versus
phenolphthalein using 0.01m HCl (f=1.000).
Palladium and platinum: Samples (30–40 mg) of the monoliths of interest
were dissolved in a minimum amount of aqua regia (typically 5–7 mL) by
applying microwave irradiation. The digest was transferred into a volu-
metric flask and the volume of the solution was adjusted to 10 mL. Palla-
dium quantification was accomplished by ICP-OES at l=340.458 nm
(average of at least three consecutive measurements) and the background
was measured at l1 =340.458 and l2 =340.955 nm. The limit of detection
(LOD) was 0.014 mgLÀ1. For calibration, palladium-containing aqueous
standards (pH 1, nitric acid) with palladium concentrations of 0, 0.004,
0.14, 0.5, and 12.0 mgLÀ1 were used. Platinum was quantified by ICP-
OES at l=214.423 nm (average of at least three consecutive measure-
ments) and the background was measured at l1 =214.35 and l2 =
214.55 nm. The limit of detection (LOD) was 0.004 mgLÀ1. For calibra-
tion, platinum-containing aqueous standards (pH 1, nitric acid) with plati-
num concentrations of 0, 0.004, 0.14, 0.5, and 12.0 mgLÀ1 were used.
Preparation of palladium-loaded monoliths: A solution of [H2PdCl4] was
prepared by dissolving anhydrous PdCl2 (25 mg, 0.14 mmol) in a mini-
mum amount of HCl (37 wt%). The pH was adjusted to around 5 by the
dropwise addition of an aqueous 15 wt% solution of NaOH. Finally,
THF (0.2 mL) was added to enhance solvent compatibility. This solution
of [H2PdCl4] (1.5 mL) was introduced into the monolith modified with
poly-5. After introducing the [H2PdCl4] solution into the monolith, it was
washed with water/THF (80:20, 20 mL). Finally, the support was dried in
vacuo for 4 h and the palladium content was determined by ICP-OES.
Preparation of platinum-loaded monoliths: A solution of [PtCl4] (15 mg,
0.077 mmol) in THF (1.5 mL) was introduced into the monolith modified
with poly-5. Then the monolith was washed with THF (30 mL). Finally,
Determination of palladium leaching: To determine the palladium con-
tent in the reaction mixtures, clear filtrates were collected at the end of
the reactions for precise palladium analysis. After evaporation of the sol-
Chem. Eur. J. 2010, 16, 4650 – 4658
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4657