Silica-Supported Molybdenum Alkylidyne N-Heterocyclic Carbene Catalysts: Relevance of Site Isolation to Catalytic Performance

Article abstract of DOI:10.1002/cctc.201701654

The immobilization of the molybdenum alkylidyne complex [(p-OMe-C₆H₄C≡)Mo(OCMe(CF₃)₂)₃?DME] (DME=1,2-dimethoxyethane) on silica using a silica-supported N-heterocyclic carbene (NHC), that is, 1,3-di(isopropyl)imidazol-2-ylidene, is reported. Two different types of silica, that is, silica 60 and partially dehydroxylated silica 60 obtained via heating to 650 °C (SiO_2-650) were used. In both cases, excess silanol groups were end-capped with trimethylsilyl chloride. Immobilization of both the NHC precursor and the metal alkylidyne was followed by ¹⁹F magic angle spinning (MAS) and ¹³C cross-polarization magic angle spinning (CPMAS) NMR and inductively-coupled optical emission spectroscopy (ICP–OES). The metal alkylidyne supported on both types of silica was active in alkyne metathesis and showed activity for a series of functional 2-alkynes. Most importantly, the metal alkylidyne supported on SiO_2-650 displayed a substantially higher alkyne metathesis activity than the homogeneous analog or the one supported on regular silica, which is attributed to a site isolation effect and, as a result, reduced bimolecular decomposition.

Full text of DOI:10.1002/cctc.201701654

Accepted Article
Title: Silica-Supported Molybdenum Alkylidyne N-Heterocyclic Carbene
Catalysts: Relevance of Site Isolation to Catalytic Performance
Authors: Philipp Hauser, Michael Hunger, and Michael Rudolf
Buchmeiser
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To be cited as: ChemCatChem 10.1002/cctc.201701654
Link to VoR: http://dx.doi.org/10.1002/cctc.201701654
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ChemCatChem
10.1002/cctc.201701654
Silica-Supported Molybdenum Alkylidyne N-Heterocyclic Carbene Catalysts:
Relevance of Site Isolation to Catalytic Performance
1
2
1
Philipp M. Hauser , Michael Hunger , Michael R. Buchmeiser*
1
2
Institute of Polymer Chemistry, Institute of Chemical Technology, University of Stuttgart,
Pfaffenwaldring 55, D-70569 Stuttgart, Germany
*
corresponding author: michael.buchmeiser@ipoc.uni-stuttgart.de
Abstract
The
immobilization
Cº)Mo(OCMe(CF
of
the
molybdenum
alkylidyne
complex
[(p-OMe-
6
C H
4
3 2 3
) )
×DME] (DME = 1,2-dimethoxyethane) on silica using a silica-
supported N-heterocyclic carbene (NHC), i.e. 1,3-di(i-propyl)imidazol-2-ylidene, is reported.
Two different types of silica, that is silica 60 and partially dehydroxylated silica 60 obtained
via heating to 650°C (SiO2-650) were used. In both cases, excess silanol groups were end
capped with trimethylsilyl chloride. Immobilization of both the NHC precursor and the metal
19
13
alkylidyne was followed by F magic angle spinning (MAS) and C cross polarization magic
angle spinning (CPMAS) NMR and inductively-coupled optical emission spectroscopy (ICP-
OES). The metal alkylidyne supported on both types of silica was active in alkyne metathesis
and showed activity for a series of functional 2-alkynes. Most important, the metal alkylidyne
supported on SiO2-650 displayed a substantially higher alkyne metathesis activity than the
homogeneous analog or the one supported on regular silica, which is attributed to a site
isolation effect and, as a result, reduced bimolecular decomposition.
Introduction
Alkyne metathesis catalyzed by well-defined metal alkylidynes was first reported by Schrock
[1]
[1a, 2]
[3]
[4]
[5]
[6]
et al. Seminal work by this group
and by Fürstner, Nuckolls, Veige, Tamm,
[4, 7]
[8]
[9]
Fischer,
Zhang and Moore contributed to the armor of catalysts and protocols relevant
1
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ChemCatChem
10.1002/cctc.201701654
to alkyne metathesis. Inspired by the high activity of Mo- and W-imido/oxo alkylidene NHC
[10]
complexes, we recently reported on a new class of formally 14-electron (not counting the
lone pairs of oxygen) metal alkylidynes bearing N-heterocyclic carbenes (NHCs) and their
[11]
catalytic activity in some 2-alkyne benchmark reactions In case NHCs such as 1,3-di(i-
[12]
propyl)imidazol-2-ylidenen (IPr) with a low Tolmann electronic parameter and sufficiently
high pK value, respectively, are used, the corresponding metal alkylidyne complexes bear
the NHC throughout the catalytic cycle. Based on the set of existing data, a cationic metal
a
[11]
alkylidene NHC intermediate has been proposed, yet was not unambiguously proved so
[11]
far. In view of the high stability of metal alkylidynes that bear the IPr-ligand,
we
immobilized the corresponding metal alkylidyne NHC complex on two types of silica to (i)
proof the integrity of the complex under catalytic conditions and (ii) to study the influence of
site isolation on catalytic performance. Here we present our results.
Results and Discussion
Synthesis of silica-supported Mo-alkylidyne NHC complexes cat-1 and cat-2
Immobilization of IPr on silica was accomplished using the synthetic protocol outlined in
Scheme 1. Starting from 4-hydroxymethylimidazole, the corresponding t-butyldimethylsilyl-
protected compound 1 was prepared and bisalkylated at the 1- and 3-postion, respectively,
using i-propyl iodide to yield 2. Compound 2 was converted into the corresponding
tetrafluoroborate salt 3 via reaction with NaBF . Reaction of 3 with dry silica at 130°C in 1,2-
4
dichloroethane yielded the corresponding silica-supported imidazolium salts 4 and 5.
t-Butyldimethylsilanol was detected by GC-MS, confirming immobilization via a newly formed
C-O-Si bond. Notably, two types of silica were employed, (i) dry silica 60 and (ii) partially
dehydroxylated silica prepared via heating silica 60 to 650°C under vacuum, referred to as
2
SiO2-650. The two silica materials had the same specific surface area (~ 565 m /g) and
average pore diameter (6.2 nm) but differed in the number of surface-silanol groups
2
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ChemCatChem
10.1002/cctc.201701654
(2.1 mmol SiOH/g for silica 60 and ~ 0.3 mmol SiOH/g for the partly dehydroxilated SiO2-650).
1
13
Reactions were also followed by H MAS NMR and C CPMAS NMR (Figures S6, S7, S.I.)
and unambiguously confirmed immobilization of the precursor. Thus, the imidazolium
13
moieties were observed at d = 119.5 and 133.8 ppm in the C CPMAS NMR spectrum and
1
13
at d = 7.21 ppm in the H MAS NMR spectrum. The C CPMAS NMR signal at d = 53.6 ppm
1
can be assigned to the CH
ppm was found. The CH(CH
spectrum and at d = 0.7 ppm in the H MAS NMR spectrum. The C CPMAS NMR signal at
d = −7.6 ppm was assigned to CH -Si groups, resulting from the reaction of intermediary
2
-O-silica group. Accordingly, a H MAS NMR signal at d = 4.65
13
3
)-groups were found at d = 22 ppm in the C CPMAS NMR
1
13
3
formed t-butyldimethylsilanol with the silica surface. Next, with both silica materials, excess
silanol groups were end capped with a large excess of trimethylsilyl chloride. Reaction with
lithium hexamethyldisilazide (LiHDMS) followed by the reaction with [(p-OMe-
C
6
H
4
Cº)Mo(OCMe(CF
3
)
2
)
3
×DME] resulted in the formation of cat-1 (on silica 60) and cat-2
on SiO2-650). F MAS NMR (Figures S8, S9, S.I.) confirmed the formation of the catalysts.
While with cat-1 some remaining imidazolium BF -salt could be detected at d = −148 ppm
19
(
4
along with the signals for the supported catalyst around d = −80 ppm, conversion to the
immobilized catalyst proceeded clearly for cat-2 using dehydroxylated silica. Thus, after
19
deconvolution, two F MAS NMR signals at d = −78.0 and −81.7 ppm, ratio 1:2, were
observed (Figure S10, S.I.). Both the shifts and the relative intensity ratios are in line with
those of the homogeneous analog, [(p-OMe-C
6
H
4
Cº)Mo(OCMe(CF
3
)
2
) (IPr)], for which the
3
[11]
alkoxides (in C ) are observed at d = −76.15 (m, 6F), −76.32 (m, 6F), −76.87 (s, 6F).
6
D
6
Together with molybdenum contents of 0.17 and 0.08 mmol Mo/gsilica for cat-1 and cat-2,
determined by treatment of the immobilized catalysts with aqua regia and followed by
analysis by ICP-OES, respectively, this strongly points towards immobilization of the metal
alkylidyne via the NHC. Notably, both cat-1 and cat-2 differ from the systems recently
[6g]
reported by Coperét and Tamm et al., in which one fluorinated alcohol was replaced by a
3
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ChemCatChem
10.1002/cctc.201701654
surface-silanolate, and from the system described by Moore et al., in which one amide was
[9g]
replaced by a surface-silanolate.
Scheme 1. Synthetic route for the synthesis of silica-supported Mo-alkylidyne NHC
complexes cat-1 and cat-2. TBDMSO = tbutyldimethylsilyloxy, TMS = trimethylsilyl, HMDS =
hexamethyldisilazide.
Alkyne metathesis reactions
A set of benchmark reactions was chosen to identify potential differences between the two
supported Mo-alkylidyne NHC catalysts cat-1 and cat-2 and the homogeneous analog, [(p-
OMe-C
6
H
4
Cº)Mo(OCMe(CF
3
)
2
) (IPr)]. Table 1 summarizes the results obtained; Figure 1
3
shows the structures of the substrates S1 – S7 and the corresponding products P1 – P7.
Table 1. Productivity (TON) in alkyne metathesis reactions using catalysts [(p-OMe-
C
6
H
4
C
º
)Mo(OCMe(CF
Reaction conditions: toluene, room temperature, 4 h.
catalyst S1 S2 S3 S4 S5 S6 S7
)Mo(OCMe(CF
380 160
3
)
2
)
3
(IPr)], cat-1 and cat-2. (substrate:catalyst
=
1000:1).
[
(p-OMe-C
6 4
H C
º
)
3 2
)
3
(IPr)] 990 990 790 670
0
4
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10.1002/cctc.201701654
Yield (%)
cat-1
99
990 990 230 440
99 99 23 44
99
79
67
0
0
0
38
-
16
-
Yield (%)
cat-2
-
-
990 990 990 960 260 970 800
99 99 99 96 26 97 80
Yield (%)
Figure 1. Structures of substrates S1 – S7 and the corresponding products P1 – P7 formed
along with 2-butyne.
While cat-1 is outrivaled by the homogeneous analog, cat-2 shows substantially higher
productivities, expressed in turnover numbers, TONs, for all substrates used. Compared to
the TONs for both the homogeneous, NHC free catalyst [MesCºMo(OCMe(CF
3
)
2
)
3
] (TON =
] (TON
54000), both the supported and homogeneous Mo-alkylidyne NHC catalysts show lower
1
85,000) and the corresponding supported catalyst [(ºSiO)Mo(ºCMes)(OCMe(CF
3
)
2 2
)
[6g]
=
productivities, yet the supported Mo-alkylidyne NHC catalyst showed higher TONs compared
to the homogeneous analog. Interestingly, also substrates that are either poorly converted
(S3, S4) or not converted at all (S5, a nitrile-containing 2-alkyne) by both [(p-OMe-
C
6
H
4
Cº)Mo(OCMe(CF (IPr)] and cat-1 undergo alkyne metathesis with the same catalyst
3
)
2 3
)
supported on partially dehydroxylated silica. To confirm that the products P1 – P7 were the
sole products to be generated, metathesis reactions were conducted in deuterated benzene
5
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ChemCatChem
10.1002/cctc.201701654
(for further information, see S.I.). No other products could be detected, neither by NMR nor
by GC-MS. In view of the lower catalyst loading in cat-2, which is approximately half the one
in cat-1, site isolation of the catalytic centers must occur and is made responsible for the
[13]
higher productivity observed for cat-2. In fact, similar effects have been proposed earlier.
The data presented here also proof the integrity of the complex under catalytic conditions.
Implications of the results presented here on the stability of the proposed intermediary
cationic Mo-alkylidynes are under investigations. To further compare the activities of the
different catalysts, the conversion of substrate S3 was measured over time for catalysts cat-
2
and the homogeneous analog. Figure 2 shows that after 30 minutes cat-2 already reaches
9
0 % conversion as compared to 40 % for the homogeneous catalyst.
Figure 2. Conversion-time diagram for the metathesis of S3 with [(p-OMe-
6 4
C H C
º
)Mo(OCMe(CF
3
)
2
) (IPr)] and cat-2 (substrate : catalyst = 1000 : 1, room temperature,
3
toluene, internal standard for GC-MS: t-butylbenzene).
On a final note, molybdenum leaching was below the limit of detection of ICP-OES (< 2
ppm). This supports the initial assumption that the corresponding Mo-alkylidyne NHC
complex remains fully integer throughout the catalytic cycle and no dissociation of the NHC
occurs.
6
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10.1002/cctc.201701654
Conclusions
The first silica-supported Mo-alkylidyne NHC complexes have been prepared. Using partially
dehydroxylated silica, catalyst loadings are lower by approximately a factor of two compared
to regular silica. The resulting site isolation effectively prevents bimolecular catalyst
decomposition, which could be responsible for the higher productivities and higher functional
group tolerance of catalysts supported on partially dehydroxylated silica. Both catalytic
activities and the low metal leaching strongly suggest a high structural integrity of the
supported catalyst.
Experimental
All reactions were performed under the exclusion of air and moisture by standard Schlenk
techniques unless otherwise noted. Reactions involving metal complexes were performed in
a nitrogen-filled glove box (MBraun Labmaster 130). Glassware was either stored at 120°C
over night and cooled in an evacuated antechamber or dried at 500°C under high vacuum
(
0.01 mbar). CH
2
Cl , diethyl ether, toluene and pentane were dried by using an MBraun SPS-
2
8
00 solvent purification system with alumina drying columns and stored over 4 Å Linde type
molecular sieves. THF was distilled from Na prior to use and stored over 4 Å Linde type
molecular sieves and Selexsorb®. Deuterated solvents were filtered over activated alumina
and stored over 4 Å Linde type molecular sieves inside the glove box. All liquid substrates
were dried by stirring over CaH
2
for several hours and subsequent distillation followed by
filtration over activated alumina. Substrates were degassed by several freeze-pump-thaw
cycles
and
stored
over
activated
4
Å
molecular
sieves.
[(p-OMe-
[11]
C
6
H
4
Cº)Mo(OCMe(CF
)
3 2
3
) (IPr)] was prepared according to the literature.
Silica 60 M
(Machery-Nagel, particle diameter 0.04-0.063 mm, pore diameter ~ 6.2 nm, specific surface
2
area ~ 565 m /g, 2.1 mmol SiOH/g) was used. Silica 650 prepared therefrom had the same
7
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ChemCatChem
10.1002/cctc.201701654
particle diameter, average pore diameter and specific surface area, but ca. 0.3 mmol SiOH/g.
[6d, 11, 14]
For the synthesis of substrates S1 – S7 refer to refs.
1
13
H and C NMR spectra were recorded on a Bruker Avance III 400 spectrometer at 400 and
00 MHz, respectively. Chemical shifts are reported in ppm from tetramethylsilane with the
solvent resonance resulting from residual solvent protons (CDCl : 7.26 ppm, C 7.16 ppm,
CD Cl 5.13 ppm) as reference. Data are reported as follows: chemical shift, multiplicity (s =
1
3
D
6 6
2
2
singlet, d = doublet, t = triplet, q = quartet, quint = quintet, sept = septet, br = broad, m =
multiplet), integration and coupling constants (Hz). GC-MS-data were obtained on an Agilent
Technologies 5975C inert MSD with triple-axis detector, a 7693 autosampler, and a 7890A
GC system equipped with a SPB-5 fused silica column (34.13 m×0.25 mm×0.25 µm film
thickness). The injection temperature was set to 150°C. The column temperature ramped
from 45 to 250°C within 8 min, and was then held for further 5 min. The column flow was
-
1
1
.05 mL min . Elemental analyses were carried out at the Institute of Inorganic Chemistry,
1
13
19
University of Stuttgart, Germany. H MAS NMR, C CPMAS NMR, and F MAS NMR
spectra were recorded on a Bruker Avance III 400 WB spectrometer at the Institute of
Chemical Technology, University of Stuttgart, Germany, under in situ conditions. A 4 mm
ZrO rotor with a gas-tight delrin cap and a sample spinning rate of 8 kHz were used. ICP-
2
OES data were recorded on a Spectro Acros 160 CCD, equipped with a Cetac ASX-260
Autosampler. Analysis of the samples was carried out with the Software Smart Analyzer
Vision 4.02.0834. For calibration, Mo-containing standard solutions (5% HNO , Chem-Lab,
3
.
-1
Zedelgem, Belgium) containing 0.00, 0.10, 1.00, and 5.00 mg kg of Mo were used. Mo was
measured at l = 202.095 nm, the background was measured at l = 430.01 nm. The
.
-1
. -1
detection range was 0.00205–6 mg L . The standard deviation was 0.0636 mgL . The
surface area and the pore size were calculated from N adsorption / desorption
measurements at −196°C using an Autosorb-1 Surface Area and Pore Size Analyzer
Quantachrome Instruments, Boynton Beach, Fl., USA) with the corresponding AS1 win
2
(
version 1.51 software. To remove adsorbed material from the surface, samples were first
heated in vacuo for several hours.
8
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10.1002/cctc.201701654
4
-(((t-Butyldimethylsilyl)oxy)methyl)-1,3-di(i-propyl)-1H-imidazolium
iodide
(2).
4
-(((t-Butyldimethylsilyl)oxy)methyl)-1H-imidazole (4.00 g, 18.9 mmol) was dissolved in
tetrahydrofuran (200 mL) and cooled to −30°C. To this solution NaH (0.68 g, 28.4 mmol) was
added in small portions and the mixture was stirred for 3 h at room temperature. Next, the
reaction mixture was filtered through a pad of celite and 2-iodopropane (8.00 mL, 80.0 mmol)
was added to the filtrate and the mixture was refluxed for 80 h. All volatiles were removed
under reduced pressure, the residue was dissolved in chloroform (200 mL) and filtered
through a pad of celite. The solvent was removed under reduced pressure and the residue
was purified by column chromatography on silica G60 using CHCl
3
:CH OH=95:5 as mobile
3
1
phase to yield the product as a white solid (1.48 g, 2.7 mmol, 14%): H NMR (400 MHz,
4
4
CDCl ) δ = 10.30 (d, JH-H = 1.7 Hz, NCHN, 1H), 7.43 (d, JH-H = 1.6 Hz, NCHC, 1H), 5.06
3
3
i
i
3
i
(
sept, JH-H = 6.7 Hz, Pr, 1H), 4.78 – 4.60 (m, CH
2
, Pr, 3H), 1.69 (d, JH-H = 6.8 Hz, Pr, 6H),
3
i
t
13
1
.60 (d, JH-H = 6.7 Hz, Pr, 6H), 0.86 (s, Bu, 9H), 0.10 (s, Me, 6H) ppm; C NMR (101 MHz,
i
i
CDCl ) δ = 135.2 (CImidazolium), 133.3 (CImidazolium), 117.4 (CImidazolium), 54.8 (Pr), 53.4 (Pr), 51.8
3
t
i
i
t
(
Me), 25.7 ( Bu), 23.6 (Pr), 23.3 (Pr), 18.2 ( Bu), −5.3 (Me) ppm; Anal. calcd. (%) for
16 2
C H33IN OSi: C, 45.28; H, 7.84; N, 6.60; found: C, 45.07; H 8.01; N, 6.70.
4
-(((t-Butyldimethylsilyl)oxy)methyl)-1,3-di(i-propyl)-1H-imidazolium tetrafluoroborate
(
3). 2 (1.10 g, 2.59 mmol) was dissolved in CH Cl (50 mL) and stirred with a solution of
2
2
sodium tetrafluoroborate (0.34 g, 3.11 mmol) in demineralized water (20 mL) at room
temperature for 18 h. The organic layer was separated, dried over sodium sulfate and all
volatiles were removed under reduced pressure to yield the product as a white solid (0.74 g,
1
4
1
.93 mmol, 75%): H NMR (400 MHz, CDCl
3
) δ = 9.51 (d, JH-H = 1.7 Hz, NCHN, 1H), 7.39 (d,
4
3
i
i
J
H-H = 1.6 Hz, NCHC, 1H), 4.87 (sept, JH-H = 6.7 Hz, Pr, 1H), 4.75 – 4.62 (m, CH
2
, Pr, 3H),
3
i
3
i
t
1
6
.64 (d, JH-H = 6.8 Hz, Pr, 6H), 1.57 (d, JH-H = 6.7 Hz, Pr, 6H), 0.87 (s, Me, 9H), 0.10 (s, Bu,
19
13
H) ppm; F NMR (376 MHz, CDCl ) δ = −151.06, −151.12 ppm; C NMR (101 MHz,
3
i
i
CDCl ) δ = 134.3 (CImidzolium), 133.6 (CImidazolium), 117.5 (CImidazolium), 54.7 (Pr), 53.5 (Pr), 51.7
3
9
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ChemCatChem
10.1002/cctc.201701654
t
i
i
t
(
CH
2
), 25.8 ( Bu), 23.2 (Pr), 23.1 (Pr), 18.2 ( Bu), −5.4 ppm (Me). HRMS (ESI) m/z calcd. for
+
C16
H
33
N
2
OSi : 297.2358; found: 297.2358.
Immobilization of 3 on silica and endcapping (4): Silica (200 mg, 2.1 mmol SiOH/g) and
-(((t-butyldimethylsilyl)oxy)methyl)-1,3-di(i-propyl)-1H-imidazolium tetrafluoroborate
240 mg, 0.62 mmol) were suspended in 1,2-dichloroethane (25 mL) and stirred for 18 h at
30°C. After cooling the reaction mixture to room temperature, the silica was filtered off,
washed several times with methylene chloride and diethyl ether and dried in vacuo. Next,
CH SiCl (1.5 mL, 11.8 mmol) was added to a suspension of this material (200 mg) in
CH and the mixture was stirred for 18 h at room temperature. Then the silica was filtered
4
(
1
(
)
3 3
Cl
2 2
off, washed with methylene chloride and diethyl ether and dried in vacuo.
Immobilization of 3 on partly dehydroxylized silica and endcapping (5): Silica650
,
(1.13 g, partly dehydroxylized in vacuo for 5 h at 500°C and another 10 h at 650°C, 0.3 mmol
SiOH/g)
and
4-(((t-butyldimethylsilyl)oxy)methyl)-1,3-di(i-propyl)-1H-imidazolium
tetrafluoroborate (100 mg, 0.26 mmol) were suspended in 1,2-dichloroethane (30 mL) and
stirred for 18 h at 130°C. The surface-modified silica was filtered off, washed several times
with methylene chloride and diethyl ether and dried in vacuo. To a suspension of this
material (200 mg) in CH
Cl
2 2
(20 mL) was added (CH
3
) SiCl (1.5 mL, 11.8 mmol) and the
3
mixture was stirred for 18 h at room temperature. Then the silica was filtered off, washed with
methylene chloride and diethyl ether and dried in vacuo.
Deprotonation of immobilized imidazolium salt (6):
A
solution of lithium
hexamethyldisilazide (104 mg, 0.62 mmol) in tetrahydrofuran (20 mL) was added slowly to a
suspension of 4 (200 mg) in the same solvent and stirred for 2 h at room temperature. The
silica was filtered off, washed with dry dimethylsulfoxide and diethyl ether and dried in vacuo.
Deprotonation of 5 was carried out in an analogous manner to yield 7.
Immobilization of [(p-OMe-C
6 4
H C
º
)Mo(OCMe(CF ) )
3 2 3
×
DME] (cat-1): 6 was suspended in
toluene (10 mL) and a solution of [(p-OMe-C
6
H
4
Cº)Mo(OCMe(CF ×DME] (340 mg,
3
)
2 3
)
1
0
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ChemCatChem
10.1002/cctc.201701654
0
.40 mmol) in toluene (8 mL) was added. The reaction mixture was stirred for 18 h at room
temperature. The silica was filtered off, washed with toluene and diethyl ether and dried in
vacuo. Immobilization of [Mo(CC OMe)(OC(CF Me) (DME)] via 7 to yield cat-2 was
carried out in an analogous manner.
H
6 4
)
3 2
3
Determination of the molybdenum content by ICP-OES: A known amount of cat-1 (8.5
mg) was dissolved in aqua regia (5 mL) by heating in a microwave oven for 1 h at 160°C.
After cooling to room temperature, the mixture was diluted with demineralized water to 25 mL
and filtered through a CHROMAFIL Xtra syringe filter (RC 0.20). The molybdenum content of
the sample was determined by ICP-OES to be 5.6 mg/L, which corresponds to 1.64 wt.-%
molybdenum or 0.17 mmol catalyst/gsilica. Determination of the molybdenum content of cat-2
was carried out in an analogous manner and revealed 0.08 mmol cat-2/gsilica
.
Representative procedure for the determination of the molybdenum content of the
-
3
metathesis products by ICP-OES: Cat-2 (20.9 mg, 1.7∙10 mmol) and 300 mg ground 5 Å
molecular sieves were suspended in toluene. A solution of 1-phenyl-1-propyne (21.1 mg,
0
.17 mmol) was added and the mixture was stirred for 4 h at room temperature. Then the
mixture was filtered and all volatiles of the filtrate were removed under reduced pressure.
Aqua regia (5 mL) was added to the residue and the mixture was heated for 1 h in a
microwave oven at 160°C. The resulting suspension was diluted to 25 mL with demineralized
water and filtered through a CHROMAFIL Xtra syringe filter (RC 0.20). No molybdenum was
-
1
detected by ICP-OES. The limit of detection was 0.95 μg∙L ; accordingly, catalyst bleeding
was < 0.15% (< 1.5 ppm Mo in the metathesis product).
General procedure for alkyne metathesis reactions: The substrate (20-40 mg) was
dissolved in toluene (3 mL), 0.1 mL t-butylbenzene and 300 mg ground 5 Å molecular sieve
were added. Then a solution of the respective catalyst in toluene was added and the reaction
mixture was stirred for 4 h. Conversion was determined by GC-MS referring to
t-butylbenzene as internal standard.
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10.1002/cctc.201701654
General procedure for alkyne metathesis reactions with immobilized catalysts: The
-
4
immobilized catalyst (for cat-2 and S1: 3.3 mg, 2.6∙10 mmol) and 300 mg ground 5 Å
molecular sieves were suspended in toluene (1 mL). A solution of the respective substrate
(for cat-2 and S1: 30.6 mg, 0.26 mmol) and 0.1 mL t-butylbenzene in toluene (2 mL) was
added and the mixture was stirred for 4 h at room temperature. Conversion was determined
by GC-MS referring to t-butylbenzene as internal standard.
Acknowledgement
Financial support provided by the Deutsche Forschungsgemeinschaft (DFG, project nr.: BU
2
174/22-1) and XiMo AG, Switzerland, is gratefully acknowledged. We wish to thank Tim
Lebherz for carrying out the nitrogen-adsorption measurements and Iris Elser and Maximilian
Koy for providing substrates.
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TOC Graphics
OMe
CF3
CF3
CF3
F3C
O
O
Mo
F3C
F3C
O
N
N
O
Key words: molybdenum; alkylidyne; alkyne metathesis; heterogeneous catalysis; N-
heterocyclic carbene (NHC)
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