Heterogeneous Ru(iii) oxidation catalysts: Via 'click' bidentate ligands on a periodic mesoporous organosilica support

Article abstract of DOI:10.1039/c6gc01494a

A 100% monoallyl ring-type Periodic Mesoporous Organosilica (PMO) is prepared as a novel, versatile and exceptionally stable catalytic support with a high internal surface area and 5.0 nm pores. Thiol-ene 'click' chemistry allows straightforward attachment of bifunctional thiols (-NH₂, -OH, -SH) which, exploiting the thioether functionality formed, give rise to 'solid' bidentate ligands. [Ru(acac)₂(CH₃CN)₂]PF₆ is attached and complex formation on the solid is studied via density functional theory. All resulting solid catalysts show high activity and selectivity in alcohol oxidation reactions performed in green conditions (25 °C/water). The PMO catalysts do not leach Ru during reaction and are thus easily recuperated and re-used for several runs. Furthermore, oxidation of poorly water-soluble (±)-menthol illustrates the benefits of using hydrophobic PMOs as catalytic supports.

Full text of DOI:10.1039/c6gc01494a

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DOI: 10.1039/C6GC01494A
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Heterogeneous Ru(III) oxidation catalysts via ‘click’ bidentate
ligands on a Periodic Mesoporous Organosilica support
Received 00th January 20xx,
Accepted 00th January 20xx
Sander Clericka, Els De Cancka, Kevin Hendrickxa,b, Veronique Van Speybroeckb and Pascal Van Der
Voorta
DOI: 10.1039/x0xx00000x
A 100% monoallyl ring-type Periodic Mesoporous Organosilica (PMO) is prepared as a novel, versatile and exceptionally
stable catalytic support with a high internal surface area and 5.0 nm pores. Thiol-ene ‘click’ chemistry allows
straightforward attachment of bifunctional thiols (-NH2, -OH, -SH) which, exploiting the thioether functionality formed,
give rise to ‘solid’ bidentate ligands. [Ru(acac)2(CH3CN)2]PF6 is attached and complex formation on the solid is studied via
Density Functional Theory. All resulting solid catalysts show high activity and selectivity in alcohol oxidation reactions
performed in green conditions (25°C/ water). The PMO catalysts do not leach Ru during reaction and are thus easily
recuperated and re-used for several runs. Furthermore, oxidation of poorly water-soluble (±)-menthol illustrates the
benefits of using hydrophobic PMOs as catalytic supports.
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hydrolysable group and Rf the bridging organic functionality of
interest, around a structure directing template. An extensive
amount of Rf-groups have been used, however different
Introduction
The use of selective catalysts and easy recycling thereof,
preferably with full recovery, is a major trend in green
chemistry. Heterogenization of the active site on a solid
support is an elegant method, often explored nowadays, to
obtain a catalyst that can be easily separated from the
medium by filtration. Herein, it is of paramount importance
that the active site does not leach during catalysis. Discovered
in 1999, Periodic Mesoporous Organosilicas (PMOs) belong to
the most novel and advanced porous silica materials.1-3 These
hybrid silicas are promising support materials as they bear
covalently bound organic functionalities (e.g. organocatalysts,
ligands) embedded in the pore walls of the silsesquioxane
framework, which overcomes leaching issues as compared to
classical silanol functionalization methods.4 Furthermore,
synthesis conditions are required as each functionality
interacts differently with the reaction medium. Also, larger
groups need to be co-condensed or “diluted” with small, non-
interacting precursors (e.g. bis(triethoxysilyl)ethane, -benzene)
or tetraethylorthosilicate (TEOS) to ensure structural stability.
Finally, fixation of Rf in between two Si-framework atoms was
described to induce faulty orientation and conformation of the
bridged organic functionality, which is especially undesired for
bidentate and chiral ligands.8
Given its modularity, ‘click’-chemistry9 is a very useful tool for
attaching chelating agents via one single covalent bond to a
support material, thus allowing free rotation of the ligand.
Such covalent attachment is inherently more stable (in water)
than conventional but leach-prone grafting of a trialkoxy-
organosilane, (RO)3SiRf, onto free silanol groups of
mesoporous silicas (MPS) or PMOs. Moreover, ‘click’ chemistry
yields a more homogeneous distribution of the functionalities
within the porous scaffolds, resulting in augmented catalytic
activity.10,11 Specially designed alkyn- or azide organosilicas,
either PMOs or MPSs co-condensed with (RO)3Si(CH2)3N3, can
easily be transformed by Cu(I) catalyzed azyde-alkyne
cycloaddition (CuAAC), into a desired functionality, e.g., dyes,
adsorbent moieties or catalysts.10-14
PMOs have ordered mesopores,
a
high degree of
hydrophobicity and a high surface area, which contributes to
improved mass transfer of organics, hydrolytic stability and the
amount of accessible functional groups. Following these
properties, the applications of PMOs are not limited to
catalysis, as they are also regularly employed in adsorption,
controlled drug release, chromatography and as low-k
materials.5-7
In general, PMOs are synthesized by condensation of a
silsesquioxane precursor (RO)3-Si-Rf-Si-(OR)3, with OR
a
Another option is the use of thiol-ene ‘click’ chemistry15, which
can be performed solely in water at room temperature using
an appropriate UV-active radical initiator (as compared to the
water/THF mixture used in CuAAC). Thiol-ene chemistry is both
readily performed on ethenylene-bridged organosilica
precursors in solution16 as on condensed materials having
accessible thiol groups. The latter ‘post-modification’ was
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shown in the attachment of catalytically active chiral quinine, applications. The allyl groups are one-step post-modified with
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proline or vanadyl-Salen complexes on 3-mercaptopropyl reagents of the form HS-(CH2)2-X, withDXOI:=10N.1H0329,/CO6HG,CS0H14,94tAo
functionalized SBA-1517-19, in the modification of a thiol obtain a heterogeneous bidentate ligand in a green and facile
bearing PMO with Rose Bengal20, in organosilane coatings21 manner (Fig. 1). A Ru(III)-complex is anchored onto these
and in the attachment of organosilica nanoparticles on glass.22 chelating ligands inside the PMO pores. The resulting well-
Only recently, our group has modified ethenylene-bridged defined heterogeneous Ru-catalyst is then tested in the
PMOs23 by thiol-ene reactions for the first time. Such PMOs, oxidation of carefully selected alcohols in water at room
clicked with cysteine and cysteamine, were successfully used temperature and a recycling and catalyst-leaching assessment
in the aldol condensation of 4-nitrobenzaldehyde and is performed.
acetone.24
In 2015, we developed an allyl-functionalized interconnected
[CH2Si]3 ring-type PMO (Fig. 1) with spherical morphology via
Experimental
spray-drying and applied it as a HPLC packing after thiol-ene
modification of the allyl groups with a C18-chain.25 We also
showed its exceptional hydrolytic stability (>pH 12 and
>150°C), in analogy with similar ring-structured PMOs.26,27 In
order to develop this ultra-stable material into a catalytic
support a different synthesis approach is required to improve
the pore morphology. Furthermore, the ‘dangling’ allyl
moieties of this material are easily accessible for post-
modification. To our knowledge, there are no reports
exploiting the thioether functionality inherently created by a
thiol-ene ‘click’ reaction, although the sulphur atom is
generally known to interact strongly with late transition metals
e.g. Ru, Rh, Pd.
The main focus of alcohol oxidation reactions using ruthenium,
either homo- or heterogeneously, lies on the usage of O2 or air
as green oxidants.28-35 Unfortunately, in almost all reports the
reaction is performed at elevated temperatures, in toluene or
a halogenated solvent (e.g. trifluorotoluene) and often a co-
oxidant is used. High yields are witnessed for aromatic
substrates in which the oxidation is driven by expansion of the
aromatic system (e.g. oxidation of benzylalcohol to
benzaldehyde). Switching to non-aromatic substrates, the
yield, however, becomes moderate. Catalytic systems have
been described where this Ru-oxidation is performed in water
and at room temperature.36 The trade-off is that periodic acid,
H5IO6 is used as sacrificial oxidant. However, this cheap, non-
toxic oxidant is safe and easy to handle and can be recovered
via electrochemistry in a separate process similar to its
Following chemicals were used as received: 1,1,3,3,5,5-
hexaethoxytrisilacyclohexane (HETSCH, 95%, ABCR), tBuLi
(1,7M in pentane, Sigma-Aldrich), allylbromide (99%, Sigma-
Aldrich), Pluronic P123 (Mn = 5800, Sigma-Aldrich), KCl
(≥99.5%, Carl Roth), HCl (37%, Carl Roth), 2-hydroxy-4’-(2-
hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959, 98%,
Sigma-Aldrich), 2-aminoethanethiol (>95%, TCI Europe), 2-
mercaptoethanol (99%, Acros), 1,2-ethanedithiol (≥98%,
Sigma-Aldrich), Ruthenium(III)acetylacetonate (99%, STREM),
acetonitrile (anhydrous, 99.8%, Sigma-Aldrich), H2SO4 (96%,
Carl Roth), n-pentane (≥99%, Carl Roth), CH2Cl2 (≥99.5%, Carl
Roth) toluene (anhydrous, 99.8%, Sigma-Aldrich), ammonium
hexafluorophosphate (99%, STREM), H5IO6 (99%, ABCR),
benzylalcohol (99-100.5%, Sigma-Aldrich), cyclohexanol (99%,
Sigma-Aldrich), (±)-menthol (>98%, TCI Europe).
Synthesis of 2-allyl-1,1,3,3,5,5-hexaethoxytrisilacyclohexane
(AHETSCH)
In a procedure adapted from Ide et al.25, 10 ml of HETSCH is
dissolved in 30 ml of anhydrous THF under Ar. This solution is
heavily stirred at -78.5°C; 1 eq. of t-BuLi is added dropwise
over 30 min and the mixture is stirred for another 30 min. By
means of a CO2 cooled syringe, the HETSCH solution is added
over 30 min to a separate flask containing 2.2 ml allylbromide
(1.07 eq.) in 20 ml of anhydrous THF, cooled to -78.5°C. The
reaction is left to stir overnight with temperature gradually
increasing to room temperature. The resulting yellow solution
is subsequently washed with 25 ml of a 0.2 m% NaHCO3
solution and 2x50 ml of water. Thereafter, the THF-fraction is
recuperated and the solvents are evaporated under reduced
pressure until a faint yellow oil is obtained (Yield GC: AHETSCH:
52%; HETSCH: 48%; trace impurity of THF adduct). Further
purification is performed via column chromatography (silica,
hexane:ethylacetate 20:1). Finally, AHETSCH is obtained as a
colorless oil.
industrial
preparation.
In
the
same
report,
[Ru(CH3CN)2(acac)2]PF6 is heterogenized on MPS via an NH2-
group and used for the selective oxidation of alcohols. Many
substrates are investigated, but leaching and recycling tests
are not considered.
Here, we present the hydrothermal synthesis of a novel 100%
monoallyl ring-type PMO (mAR) optimized for catalytic
mAR
Fig. 1: Schematic representation of the thiol-ene ‘click’ post-modification and anchoring of [Ru(acac)2(CH3CN)2]PF6 onto the ‘solid’ bidentate thioether ligands.
2 | J. Name., 2012, 00, 1-3
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1H NMR (300 MHz, CDCl3, Fig. S1) δ = 6.01 (ddt, J=17.0, 9.9, The solids (mAR-SX-Ru) are obtained as purple powders after
View Article Online
DOI: 10.1039/C6GC01494A
7.0, 1H, CH2CH=CH2), 5.00 (ddd, J=17.0, 3.6, 1.4, 1H, CH=CH2), filtration, washing with minimal amounts of CH2Cl2 and
4.87 (ddt, J=10.0, 2.1, 1.0, 1H, CH=CH2), 3.85 – 3.71 (m, 12H, vacuum drying overnight at 30°C.
OCH2), 2.41 – 2.32 (m, 2H, CHCH2CH=CH2), 1.26 – 1.16 (m,
Catalytic procedure for the oxidation of alcohols
18H, OCH2CH3), 0.38 (t, J=6.4, 1H, CH(Si)2(CH2CH=CH2)), 0.20
– 0.02(m, 4H, SiCH2Si). 13C NMR (400 MHz, CDCl3, Fig. S2) δ = 1 mmol of substrate (benzylalcohol, cyclohexanol, (±)-menthol)
141.38 (CH2CH=CH2), 113.45 (CH=CH2), 58.40 - 58.23 - 58.21 - is weighed off in a 15 ml reactor; 10 ml of H2O and the
58.11 (OCH2), 28.03 (CHCH2CH=CH2), 18.41
(OCH2CH3), 14.22 (CH(Si)2(CH2CH=CH2)), -1.44 (SiCH2Si).
-
18.37 supported catalyst (0.06 mol% Ru) are added, together with
0.1 g of toluene as a standard with similar low solubility as the
analytes in water. An extraction efficiency factor is determined
for all analytes to compensate for extraction losses. To start
Hydrothermal synthesis of mono-allyl ring-PMO (mAR)
In a 50 ml flask, a mixture is made with molar composition the reaction, 1.1 eq. of H5IO6 is added while stirring and the
AHETSCH:H2O:P123:HCl:KCl 1:500:0.0517:8.62:23.5. First, reaction is kept at 25°C for 3 h. Then, the solid catalyst is
0.375 g of Pluronic P123 is dissolved in 11.25 ml H2O. filtered off and the filtrate is extracted 3 times with 25 ml of
Subsequently, 0.9 ml of HCl (37%) and 2.19 g of KCl is added diethylether. Finally, the samples are analysed by means of gas
and the solution is stirred (800 RPM) until a clear blue solution chromatography. For recycling tests, the catalyst is washed
is obtained. Under continued stirring, the reaction mixture is with H2O and acetone and dried at 30°C overnight. Catalytic
heated to 45°C after which 0.5625 g AHETSCH is added at profiles are constructed by taking 1 ml aliquots at set times. To
once. 3 h later, stirring is switched off and the temperature is test the heterogeneity of the catalyst, the solid is filtered off
raised to 95°C in order to promote further condensation of the (0.45 µm membrane) after 10 min of reaction time and the
AHETSCH precursor for 24 h (‘ageing’ step). A white precipitate catalytic activity of the filtrate is further evaluated (hot
is filtered off and washed with 3 x 25 ml H2O and 3 x 25 ml filtration test).
acetone. The template (P123) is removed during a 6 h Soxhlet
Characterization and analysis
extraction in acetone and afterwards, the powder is dried
overnight at 120°C under vacuum. The amount of allyl groups 1H NMR spectra of AHETSCH are recorded in CDCl3 on a Bruker
is determined gravimetrically by bromination of the double 300 MHz AVANCE spectrometer with chemical shifts (δ)
bonds.37
expressed in ppm relative to a tetramethylsilane standard. 13
C
NMR was performed on a Bruker Avance III 400 MHz
spectrometer. X-ray powder diffraction (XRPD) patterns of all
‘Click’ post-modification of mAR to mAR-SX (X = NH2, OH, SH)
The accessible allyl groups in the pores of mAR react with 2- mAR-PMOs are recorded on a Thermo Scientific ARL X’TRA X-
aminoethanethiol, 2-mercaptoethanol or 1,2-ethanedithiol to ray diffractometer using Cu Kα radiation of 40 kV and 30 mA. A
obtain solid bidentate ligands, i.e., an amine- (mAR-SNH2), Micromeretics Tristar II is used for N2-sorption experiments at
hydroxyl- (mAR-SOH) or thiol- (mAR-SSH) functionalized 77 K to obtain the internal surface area (SBET) and pore size
thioether, respectively. In a general procedure, 3 eq. of thiol distribution (dp,BJH) making use of the BET and BJH theory,
per double bond are mixed with 0.75 eq. of Irgacure 2959 in 20 respectively. Diffuse Reflectance Infrared Fourier Transform
ml of H2O and flushed with Ar. 0.5 g of mAR is added and the Spectroscopy (DRIFTS) is performed using a Thermo Nicolett
suspension is stirred at room temperature in a home-made UV 6700 FT-IR spectrometer equipped with a Greasby-Specac
reactor (λ = 360 nm) for 1h. The products are filtered as off- diffuse reflectance cell, modified to measure samples at 20 –
white powders and resuspended in H2O at reflux temperature 300°C under vacuum. For CHNS elemental analysis, a Thermo
to remove any leftover reagents. Finally the powders are dried Flash 200 elemental analyser is used with V2O5 as catalyst.
at 110°C for 24 h and loading is determined via CHNS Transmission electron microscopy (TEM) images are taken on a
elemental analysis.
JEOL JEM 2200-FS TEM and scanning electron microscopy
(SEM) images on a JEOL JSM 7600F FEG SEM. Ru loadings (Kα)
are determined by X-ray fluorescence (XRF) on a Rigaku NEX
Synthesis of [Ru(acac)2(CH3CN)2]PF6 and anchoring onto mAR-SX
As adapted from literature38, [Ru(acac)2(CH3CN)2]PF6 is CG with an Al source and compared to Sr-Kα as internal
prepared by dissolving 1 g of Ru(acac)3 in 100 ml of a 1.5% standard. All catalytic tests are analyzed with an ultrafast
H2SO4 solution in anhydrous acetonitrile and stirring this at TRACE GC (Thermo, Interscience) equipped with a flame
room temperature until the red solution turns deep blue ionisation detector and
a 5% diphenyl / 95% poly-
(approx. 5 h). Next, 90% of the solution is evaporated and dimethylsiloxane column (10 m x 0.10 mm) using He as carrier
cooled to 0°C. NH4PF6 (0.409 g, 1.5 eq.) in 10 ml of H2O is gas at 0.8 mL/min.
added under stirring and the solution is left to stand for 30
Computational Methodology
min. [Ru(acac)2(CH3CN)2]PF6 (85%) is obtained as a blue
precipitate which is filtered, washed (2 x 10 ml of ice water, 2 x All calculations are performed within the Gaussian09 (G09)
10 ml of pentane) and dried for 24 h under vacuum.
package39 using Density Functional Theory (DFT). Calculations
Subsequently, [Ru(acac)2(CH3CN)2]PF6 is heterogenized by are performed using the B3LYP40,41 and OPBE42-45 functionals
stirring mAR-SX with 0.5 eq. of complex per bidentate ligand and a Def2-TZVP polarized split-valence triple-ζ basis set.46,47
in toluene (100 ml/g mAR-SX) for 24 h at room temperature. Dispersion corrections are added using Grimme’s DFT-D3
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version48 with Becke-Johnson damping.49 The coefficients for
the OPBE calculations are taken from Goerigk et al.50 (S8 =
3.3816, a1 = 0.5512 and a2 = 2.9444) and defined manually in
the Gaussian program. Furthermore, a correction energy for
the Basis Set Superposition Error (BSSE) is calculated using the
Boys and Bernardi Counterpoise correction.51
View Article Online
DOI: 10.1039/C6GC01494A
Results and Discussion
mAR-PMO as a stable and multifunctional catalytic support
The synthesis and work-up of AHETSCH as the PMO-precursor
is successfully optimized, yielding a pure compound and no
longer a mixture of mono-, bis- and trisallylated HETSCH (Fig.
S3) as described before.25 The importance of this purification is
seen in the X-Ray diffractograms (Fig. 2) of mAR and a PMO,
denoted mixAR, synthesized in the same conditions as mAR
but using the unpurified precursor mixture. For mAR, an
intense (100) reflection peak is distinguished, together with
less intense second order (110) and (200) peaks, which are
indicative for the 2D hexagonal (P6mm) ordered pore
structure of the material. The diffractogram of mixAR is
however lacking the second order peaks, indicating the loss of
long-range ordering. These results are clearly confirmed in the
TEM images of both materials (Fig. 2) where ordering is found
throughout the entire rod-shaped mAR particles (SEM image:
Fig. S4). The mixAR-PMO only shows patches of ordered pores
with variable alignment. The major enhancement of pore
Fig. 3: N2-sorption isotherms of mAR and mixAR. Inset: BJH pore size distribution
plot of mAR.
ordering for mAR is ascribed to a more uniform rate of
hydrolysis and condensation of purified AHETSCH.
Approximately the same unit cell parameter (a0) for mAR is
seen in the images as calculated via XRD (Table 1).
In terms of porosity similar differences are also observed. N2-
sorption experiments (Fig. 3) show type IV isotherms with
sharp H1 hysteresis for mAR, typical for highly ordered
mesoporous materials with uniform cylindrical pores (SBA-15
like). mAR shows a high internal surface area (SBET = 536 m2/g)
with 5.0 nm pores (dp,BJH) in accordance with TEM images.
Comparable results are obtained for mixAR (SBET = 472 m2/g,
dp,BJH = 5.1 nm) but the pore size distribution is broadened and
increased macroporosity, attributed to irregular, disordered
areas, is witnessed. Given that the slightest disorder of
mesopores already has a significant impact on the diffusion of
molecules within these pores52, results of this structural
assessment clearly indicate the superior quality of mAR as a
catalytic support.
mAR
mAR
100 nm
20 nm
mixAR
Furthermore, we have tested the hydrolytic stability of mAR-
PMOs by stirring the material in a 1M HCl solution and a 0.1M
NaOH solution at room temperature. XRD and N2 sorption (Fig.
S5, Table S1) show no structural change for the acid treated
samples. Also, the material remains unaffected after 3h in
strong basic medium. Only after 24h at pH 13 mAR starts to
show the first signs of deterioration, indicated by a small drop
in surface area and pore size. This exceptional hydrolytic
stability compared to other silicas makes mAR highly suitable
50 nm
Fig. 2: XRD diffractograms of the 100% monoallyl PMO (mAR, top) and the mixed
precursor PMO (mixAR, bottom). TEM images confirm the ordered pore geometry
of mAR.
Table 1: Structural properties of the synthesized PMO materials.
mAR
536
0.48
5.0
mAR-SNH2
400
mAR-SNH2-Ru
mAR-SOH
343
mAR-SOH-Ru
mAR-SSH
304
mAR-SSH-Ru
208
SBETa (m2/g)
271
0.32
4.8
309
0.39
5.0
b
Vp (ml/g)
0.45
0.44
0.46
0.28
dp,BJHc(nm)
5.1
5.0
5.1
5.1
d
a0 (nm)
12.1
12.1
12.0
12.0
11.9
11.9
11.8
a Specific surface area determined via Brunauer-Emmett-Teller theory. b Pore volume determined from adsorption branch at P/PO = 0.99. c Pore size calculated from
desorption branch following Barrett-Joyner-Halenda theory. d XRD unit cell parameter (a0 = 2d100/ √3) for P6mm 2D hexagonal ordering.
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for reactions requiring extreme pH in water. The
hydrophobicity of mAR not only explains its stability as we
, but might also enhance mass
transport of organics towards the catalytic support in water
(see catalytic tests).
Table 2: Functional loading after ‘click’ post-modification and Ru anchoring.
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DOI: 10.1039/C6GC01494A
mAR-SNH2-Ru
mAR-SOH-Ru
mAR-SSH-Ru
1.72
experienced before25,26
Lig.a (mmol/g)
Rub (mmol/g)
0.73
0.204
27
1.70
0.049
2.9
0.040
2.3
c
%
a Ligand loading determined from S-content in CHNS elemental analysis. b Ru
loading determined via XRF. c Amount of ligands functionalized with Ru(III).
‘Click’ post-modification of the allyl functional handles
The allyl groups of mAR are readily transformed into any
organic functionality of interest via thiol-ene ‘click’ chemistry.
The amount of accessible double bonds is gravimetrically
effective (0.73 mmol/g). For all post-modification steps, a drop
in SBET is observed as a result of the mass increase of the
material and the decoration of the pore walls with the
functionalities (Table 1). Furthermore, no structural
degradation is observed in the XRD diffractograms after
functionalization. (Fig. S6)
estimated at 2.4 mmol/g by performing
a gas-phase
bromination reaction. The functional loading of the resulting
materials (CHNS) after ‘click’ reaction with 2-aminoethanethiol
(mAR-SNH2), 2-mercaptoethanol (mAR-SOH) and 1,2-
ethanedithiol (mAR-SSH, after reduction of possible disulfide
bridges) on mAR is found in Table 2. Treatment of 1h in the UV
reactor already leads to high functional loading (ca. 1.7
mmol/g) for mAR-SOH and mAR-SSH. No further increase of
functionality is observed for a reaction time of 3h or even 24h.
The ‘click’ reaction of 2-aminoethanethiol is shown to be less
The infrared spectrum (DRIFTS) of mAR is given in Fig. 4. Next
to C-H and 2 Si-O-Si stretch vibrations, in the region 2950-2800
cm-1, 1200-1000 cm-1 and 800 cm-1, respectively. Distinct peaks
show up at 3070, 1640 and 890 cm-1 testimonial for the allyl-
groups (olefin C-H stretch, C=C stretch and olefin C-H out of
plane deformation). Vibrations in the 1475-1280 cm-1 region
are typical for the modified ring structure. After bromination
reaction (mAR-Br2), it is clear that all C=C vibrations (1640
cm-1) have disappeared (Fig. 4, zoom), implying that Br2 gas,
due to its small size, reacts with allyl groups in the micropores
and/or penetrates inside the pore walls. This makes us believe
that the gas-phase bromination reaction overestimates the
amount of reachable double bonds inside the material. During
the functionalization process in water, the thiols cannot reach
all of these allyl groups, indictated by the persisting C=C peaks
in the IR spectrum. In accordance with a lower loading (CHNS),
a less drastic decrease is observed for mAR-SNH2, however,
the primary amine gives rise to the appearance of new
vibration peaks at 1607 and 1502 cm-1 (NH2 bend and C-N
stretch). O-H stretches (mAR-SOH) are not observed due to
overlap with residual adsorbed water. S-H vibrations (mAR-
SSH) are distinguished at 2570 cm-1 in the RAMAN spectrum
(Fig. S7).
Table 2 also shows the amount of [Ru(acac)2(CH3CN)2]PF6
anchored onto the bidentate ligands created on the PMO. In
mAR-SNH2, Ru is attached to up to 27% of the SNH2 ligands. In
DRIFTS, the characteristic peaks of [Ru(acac)2(CH3CN)2]PF6, e.g.
acac carbonyl stretches at 1544 and 1523 cm-1, are
superimposed on the mAR-SNH2 spectrum to form mAR-SNH2-
Ru (Fig. 5). Given the meticulous washing step, this indicates
that anchoring of the Ru-complex is successful. As a 10 fold
less Ru gets anchored to the SOH and SSH ligands, the Ru-
complex peaks can no longer be identified. After anchoring,
the PF6 counter ion remains present as confirmed by XRF.
Unfortunately, DRIFTS does not provide sufficient evidence of
the exact complex formation once the Ru-complex is
heterogenized. Furthermore, other techniques (e.g. XPS)
proved insufficient because of the low Ru loading compared to
the PMO matrix. Therefore, we conducted a computational
study on a simplified ligand—Ru(III) model to explain the
nature of complex formation.
Fig. 4: DRIFT spectrum of mAR. Inset: Zoom of the region of interest for reaction
on the allyl groups.
Fig. 5: DRIFT spectrum of the homogeneous [Ru(acac)2(CH3CN)2]PF6 complex, mAR,
mAR-SNH2 and mAR-SNH2-Ru.
Computational study of complex formation
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DOI: 10.1039/C6GC01494A
Fig. 6: Computational model with two fragments indicated. Total charge of the system is +1 and it has a doublet spin state
To study the complexation process of the Ru catalyst to the walls of these PMOs are relatively hydrophobic, all catalysts
PMO anchored ligands, simplified model system is are homogeneously dispersed in water courtesy of the
a
constructed (Fig. 6). This model comprises the Ru centre, remaining silanol groups. In our initial experiments, we
surrounded by two acac groups and the ligand of interest, performed the oxidation of benzylalcohol, as this is the
terminated by a methyl group. We assume that the large pore substrate of choice in many reports on Ru catalyzed oxidation
diameter of 5 nm gives a large curvature and hence limited reactions35-36. However, in our system, we already observe full
interactions with the pore wall are expected. Therefore, the conversion of benzylalcohol to benzaldehyde without addition
PMO environment is neglected during the calculations.24 This of
a
Ru-catalyst (blank reaction). We assume that
simple model system allows us to investigate whether the benzylalcohol and similar substrates are easily oxidized as
complexation to the ligand is thermodynamically favoured, expansion of the π-system pushes the reaction towards full
compared to the original [Ru(acac)2(CH3CN)2]+ complex, and conversion.
hence corroborate that the complex is anchored to the PMO.
Therefore, we selected cyclohexanol as non-aromatic
Open shell transition metal systems are notoriously difficult for substrate, which does not benefit from conjugation after
DFT methods to predict correct spin states and geometries. oxidation. Catalytic results are represented in Table 3. All
Recent studies have shown that the OPBE functional performs considered Ru-catalysts show full conversion of cyclohexanol
very well for transition metal complexes.53,54 For comparison, after 180 min, whereas for the blank reaction only 12% is
B3LYP calculations are also performed, since this widely tested observed. This reaction is roughly four times faster compared
functional still proves to be very robust for organic systems.55 to [Ru(acac)2(CH3CN)2]PF6 anchored on an aminopropyl grafted
More details on the computational method can be found in SI. silica36, which nicely illustrates the enhanced diffusion of
Both functionals gave the same qualitative understanding of organics in water towards the hydrophobic PMO materials.
the system and predict complexation energies that are in the The results, however, show
a
discrepancy between
same range. The calculations (Fig. 6 and Table S2) show that [Ru(acac)2(CH3CN)2]PF6 and the solid PMO catalysts in terms of
the bidentate linkers can effectively bind the complex with selectivity. A similar loss in selectivity is seen if homogeneous
large interaction strength for every type of ligand.
Ru(acac)3 is used as catalyst. Here, a vacant reaction site must
be created by the expulsion of an acac-ligand, which causes a
tendency for overoxidation (by-products were determined as
cyclohexenone and hydroquinone). Overreaction does not
occur when the weakly bound CH3CN ligands of
[Ru(acac)2(CH3CN)2]PF6 are removed. This fact thus suggests
In conclusion, theoretical calculations support that the
complex formation with the bidentate ligand is
thermodynamically favourable compared to the starting
complex. Furthermore, the amine ligand has the strongest
interaction with the Ru complex, which supports the higher Ru
loading of mAR-SNH2-Ru. In order to have a more physically
correct point of view, we compared the anchored ruthenium
to the original complex as a reference system, which is
schematically represented in Fig. 6. Here, we first calculate the
energy cost to ‘remove’ both acetonitrile groups and the
consecutive stabilization caused by the recomplexation to the
new ligand, representative for the actual experimental
exchange process. Using this approach, we find that the
anchoring to the new ligand is thermodynamically favoured by
2 - 22 kcal/mol depending on the ligand (an order of NH2 > SH
> OH is found).
3 h
Table 3: Oxidation of cyclohexanol
Ru
(mol%)
5
2.5
0.06
Conv.a
(%)
>99
93
>99
>99
>99
12
Yielda
(%)
>99
88
95
81
Entry
Catalyst
1
2
3
4
5
6
[Ru(acac)2(CH3CN)2]PF6
Ru(acac)3
mAR-SNH2-Ru
mAR-SOH-Ru
mAR-SSH-Ru
Blank
Catalytic alcohol oxidation in water
80
12
For catalytic tests, we retained three different materials: mAR-
SOH-Ru, mAR-SSH-Ru with relatively low Ru loading and mAR-
NH2-Ru with a relatively high Ru loading. Although the pore
0
aGC-determined with toluene as internal standard
6 | J. Name., 2012, 00, 1-3
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that Ru is indeed anchored to mAR-SX by exchange of the
weak acetonitrile ligands corroborating with the assumption
made in the computational study.
View Article Online
DOI: 10.1039/C6GC01494A
Next, we constructed reaction profiles for mAR-SOH-Ru, mAR-
SSH-Ru and mAR-SNH2-Ru and performed ‘hot-filtration’ tests
(Fig. 7) to determine whether the catalysis takes place
exclusively on the PMO surface. Given the reaction profiles of
mAR-SOH-Ru and mAR-SSH-Ru, the optimum reaction time is
40 min with a high TOF10min of 1.53 s-1 and 1.47 s-1,
respectively. At this optimum, all cyclohexanol is selectively
oxidized into cyclohexanone and only thereafter further
oxidation products are formed as a result of continuing β-
elimination reactions.56 The dashed line in Fig. 7 shows the
reaction profile during the hot filtration (HF) test. For both
materials, separation of the catalyst from the reaction mixture
occurred after 10 min. No further reaction is witnessed in the
Fig. 8: Catalyst recycling experiment for mAR-SOH-Ru, with conversion of
cycyclohexanol (dark grey), yield of cyclohexanone (grey) and formation of the
byproducts, cyclohexenone and hydroquinone (white)
filtrates but the blank reaction. This, combined with the fact blank reaction. This indicates that the Ru-complex is detached
that no leaching of Ru was observed in the filtrate at ppm level from the support and partially leaches into the medium, where
(XRF) shows that the reaction occurs on the pore surface of the it can no longer be recovered. Via XRF, the Ru-leaching is
PMO and that we have developed non-leaching, fully determined at 30% after 3 catalytic runs. To ensure this
heterogeneous and recyclable catalysts. The latter statement leaching is not loading related, we prepared a mAR-SNH2-Ru
is demonstrated in Fig. 8, where catalyst-recycling experiments sample for which the anchoring was executed in acetone, to
for mAR-SOH-Ru show a similar activity for 3 consecutive obtain a catalyst with similar Ru-loading (0.037 mmol/g) to
catalytic runs of 180 min, without loss of structural ordering of mAR-SOH-Ru and mAR-SSH-Ru. Again, the hot filtration test
the support (Fig. S8). The total reactant conversion is similar was unsuccessful. The very different behaviour of mAR-SNH2-
for all runs. However, as seen in the reaction profiles, the Ru can be explained by taking into account the reaction
occurring reaction is sequential (-ol
à
-one
à
medium. H5IO6 not only acts as the oxidant, it can also readily
enone/hydroquinone). Therefore, the lower yield of protonate the NH2-group of the ligand, whereas –OH and –SH
cyclohexanone in Run 1 and 3 is accompanied by an increase remain unaltered.
of the overoxidation products. We believe that this slight This is confirmed in our computational study. The acidic
variability in the yield of cyclohexanone is due to small changes environment created by the periodic acid (pKa 3.29) is
in the kinetics of the reaction that could be caused by external sufficiently strong to protonate the amine group of the ligand
factors.
(approx. pKa 10). This leads to a subsequent change in the
However, when immersing mAR-SNH2-Ru in the reaction conformation of the complex as is shown in Fig. 9. The now
medium and filtering off the catalyst after 10 min, we positively charged amine group turns away from the Ru centre,
observed further reaction in the filtrate. The conversion of and only a weak interaction of the sulphur lone pair (about 6
cyclohexanol in the filtrate increased from 35% at the time of kcal/mol) remains, not sufficient to retain the complex in that
filtration to 83%, significantly more than expected for the position.
The hydroxyl group (approx. pKa -2) and the thiol group
(approx. pKa -7) of the other ligand chains cannot be
protonated in this reaction environment and therefore stay in
the same conformation. These results explain why ruthenium
leaching is only observed for the amine-based ligand,
notwithstanding that this ligand has the strongest affinity to
bind the ruthenium complex.
Fig. 9: In an acidic environment, the amine group can be easily protonated. The
Fig. 7: Reaction profiles of mAR-SOH-Ru (top) and mAR-SSH-Ru (bottom). A hot
filtration test (HF) is performed with filtration at t = 10 min (dashed).
remaining S—Ru interaction is rather weak and as a consequence, the complex
leaches out.
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As a control experiment, we prepared an aminopropyl grafted
SBA-15 and subsequently anchored [Ru(acac)2(CH3CN)2]PF6
(details in SI). For this catalyst, the Ru is no more stabilized by
the sulphur atom after protonation of the amine group. Here,
up to 50% of Ru is leached after 1 catalytic run, whereas for
mAR-SNH2-Ru 30% of Ru leaching is observed after 3 runs.
These results prove that the amine group indeed causes
leaching by protonation in the acidic medium. Also, the small
stabilizing effect of the lone sulphur atom is witnessed given
the less pronounced leaching for mAR-SNH2-Ru.
View Article Online
DOI: 10.1039/C6GC01494A
Fig. 11: Reaction profile for (±)menthol oxidation with mAR-SOH-Ru as catalyst.
Mechanistic insights
In Fig. 10, a mechanism for this catalytic oxidation is proposed.
In our system at 25°C, the acidity of H5IO6 is needed to expel
an acac-ligand and to create vacant Ru-sites as indicated by
the catalytic activity of Ru(acac)3. Furthermore, we did not
observe conversion if NaIO4 is used as neutral oxidant at room
temperature, demonstrating that the acidity is decisive for
catalytic activity. It is generally accepted that alcohols
undergo β-elimination in the presence of late transition
metals, which is often regarded as the rate-determining
step.28,30,57,58 Here, the resulting Ru-H2 is readily oxidized by
the periodate anion. Although the latter is consumed during
reaction, electrochemical regeneration is straightforward. By-
products of the cyclohexanol oxidation, cyclohexenone and
hydroquinone, are the result of extended β-elimination. This
side-reaction is far less favourable as these products are only
formed after full conversion to cyclohexanone. We believe the
selectivity of the reaction arises from selective coordination of
cyclohexanol to vacant Ru sites followed by ß-elimination.
Given the reaction profile, cyclohexanone, the reaction
product, must only be able to coordinate to the vacant sites
after all cyclohexanol is consumed to yield cyclohexenone.
Finally, we attempted cyclohexanol oxidation with either H2O2
or O2 as the oxidant at low pH to promote the removal of the
acac group. No conversion of the substrate was observed after
180 min. This results from the lower oxidation potential of
H2O2 and O2 compared to H5IO6, which is insufficient to
regenerate the active Ru(III)-species.
solution within the same medium via an electrochemical
procedure in a separate cycle. This, combined with the catalyst
recycling, the use of water as a solvent and performing the
reaction at room temperature, provides
a sustainable
alternative to current aerobic oxidation reactions at elevated
temperature in organic solvents.
Oxidation of poorly water-soluble alcohols
As a final experiment, we selected (±)-menthol as a sterically
hindered, poorly water-soluble substrate. Such substrates are
notoriously hard to oxidize in water/RT and no selective,
complete conversion is reported even after reaction for
>12h.31,36 Albeit the poor solubility of (±)-menthol in water,
we obtain full conversion after 6 hours, with >99% selectivity
towards menthone with our PMO catalyst (mAR-SOH-Ru) (Fig.
11). This remarkable behaviour must be attributed to the
properties of the mAR-PMO support. Sequential
hydrophobic/hydrophilic ‘zones’ in the PMO material,
originating from the organic bridges and siloxane/silanol
functionalities, create an ideal reaction environment for the
reaction of hydrophobic molecules in water.24, 59-63 Therefore,
reactants are locally enriched and/or reaction products are
repelled which, combined with an ordered pore structure,
results in high catalytic activity (TOF30min = 0.54 s-1). No further
β-elimination products are found, which implies that steric
hindrance of the isopropyl and methyl groups prevent
overreaction.
Arguably, in all performed catalytic reactions, H5IO6 is used as
a sacrificial oxidant. However, after filtration of the Ru
catalysts and subsequent extraction of the reaction products,
only an aqueous phase containing HIO3 remains. In potential
applications, this latter can be reconverted into a H5IO6
Conclusion
We have developed a novel and exceptionally stable 100%
monoallyl ring-type PMO (mAR) of which structure and
porosity (536 cm2/g, 5.0 nm pores) are optimized to serve as a
catalytic support material. Via thiol-ene ‘click’ chemistry, the
allyl groups protruding in the pores are transformed into three
distinct bidentate thioether ligands (SNH2, SOH, SSH).
Experiments and theoretical calculations confirm the
heterogenization of a Ru(III)-complex onto these solid ligands.
Although protonation of the amine group in the acidic catalytic
medium causes leaching for mAR-SNH2-Ru, mAR-SOH-Ru and
mAR-SSH-Ru are successfully applied as selective, truly
heterogeneous catalysts in the oxidation of cyclohexanol in
water at 25°C. Moreover, the hydrophobic/hydrophilic
reaction environment and ordered pores of the mAR-support
Fig. 10: Schematic mechanism of H5IO6 heterogeneous cyclohexanol
oxidation. X = OH, SH coming from mAR.
8 | J. Name., 2012, 00, 1-3
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enable high catalytic activity for a poorly water-soluble and 18
sterically very challenging substrate such as (±)-menthol.
19
Q. Chen, C. Xin, L.-L. Lou, K. Yu, F. Ding and S. Liu, Catal.
Lett., 2011, 141, 1378-1383.
View Article Online
DOI: 10.1039/C6GC01494A
C. Baleizao, B. Gigante, H. Garcia and A. Corma, J.
Catal., 2003, 215, 199-207.
J. Gehring, B. Trepka, N. Klinkenberg, H. Bronner, D.
Schleheck and S. Polarz, J. Am. Chem. Soc., 2016, 138,
3076-3084.
Acknowledgements
20
The authors would like to thank T. Planckaert for CHNS
elemental analysis and K. Haustraete for TEM imaging. The
21
22
23
A. K. Tucker-Schwartz, R. A. Farrell and R. L. Garrell, J.
Am. Chem. Soc., 2011, 133, 11026-11029.
J. Gehring, D. Schleheck, B. Trepka and S. Polarz, Appl.
Mat. & Interf., 2015, 7, 1021-1029.
C. Vercaemst, M. Ide, B. Allaert, N. Ledoux, F. Verpoort
and P. Van Der Voort, Chem. Comm., 2007, 22, 2261-
2263.
J. Ouwehand, J. Lauwaert, D. Esquivel, K. Hendrickx, V.
Van Speybroeck, J. Thybaut, P. Van Der Voort, Eur. J.
Inorg. Chem., 2016, 13, 2144-2151.
Research Board of Ghent University (BOF, UGent GOA Grant
01G00710), and the European Research Council (FP7(2007-
2013) Grant Agreement 240483) are acknowledged for their
financial support. Computational resources and services were
provided by the Stevin Supercomputer Infrastructure
of
Ghent University and by the Flemish Supercomputer
Center (VSC), funded by the Hercules Foundation and the
Flemish Government-Department EWI.
24
25
26
M. Ide, E. De Canck, I. Van Driessche, F. Lynen and P.
Van der Voort, RSC Adv., 2015, 5, 5546-5552.
F. Goethals, B. Meeus, A. Verberckmoes, P. Van Der
Voort and I. Van Driessche, J. Mater. Chem., 2010, 20,
1709-1716.
Notes and references
1
2
3
4
5
T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin,
Nature, 1999, 402, 867-871.
S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O.
Terasaki, J. Am. Chem. Soc., 1999, 121, 9611-9614.
B. J. Melde, B. T. Holland, C. F. Blanford and A. Stein,
Chem. Mater., 1999, 11, 3302-3308.
M. Ferre, R. Pleixats, M. W. C. Man and X. Cattoen,
Green Chem., 2016, 18, 881-922.
P. Van der Voort, D. Esquivel, E. De Canck, F. Goethals,
I. Van Driessche and F. J. Romero-Salguero, Chem. Soc.
Rev., 2013, 42, 3913-3955.
27
28
29
30
K. Landskron, B. D. Hatton, D. D. Perovic and G. A. Ozin,
Science, 2003, 302, 266-269.
K. Yamaguchi, K. Mori, T. Mizugaki, K. Ebitani and K.
Kaneda, J. Am. Chem. Soc., 2000, 122, 7144-7145.
G. Csjernyik, A. H. Ell, L. Fadini, B. Pugin and J. E.
Backvall, J. Org. Chem., 2002, 67, 1657-1662.
A. Dijksman, A. Marino-Gonzalez, A. M. I. Payeras, I.
Arends and R. A. Sheldon, J. Am. Chem. Soc., 2001, 123,
6826-6833.
6
N. Mizoshita, T. Tani and S. Inagaki, Chem. Soc. Rev.,
2011, 40, 789-800.
31
32
33
W. H. Fung, W. Y. Yu and C. M. Che, J. Org. Chem.,
1998, 63, 2873-2877.
7
F. Hoffmann, M. Cornelius, J. Morell and M. Froeba,
Angew. Chem., Int. Ed., 2006, 45, 3216-3251.
C. Baleizao, B. Gigante, D. Das, M. Alvaro, H. Garcia and
A. Corma, Chem. Comm., 2003, 1860-1861.
H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew.
Chem., Int. Ed., 2001, 40, 2004-2021.
A. Hanyu, E. Takezawa, S. Sakaguchi and Y. Ishii,
Tetrahedron Lett., 1998, 39, 5557-5560.
B. Karimi, D. Elhamifar, O. Yari, M. Khorasani, H. Vali, J.
H. Clark and A. J. Hunt, Chem.-Eur. J., 2012, 18, 13520-
13530.
I. E. Marko, P. R. Giles, M. Tsukazaki, I. Chelle-Regnaut,
C. J. Urch and S. M. Brown, J. Am. Chem. Soc., 1997,
119, 12661-12662.
8
9
34
10
11
12
J. Nakazawa, B. J. Smith and T. D. P. Stack, J. Am. Chem.
Soc., 2012, 134, 2750-2759.
J. Y. Shi, C. A. Wang, Z. J. Li, Q. Wang, Y. Zhang and W.
Wang, Chem.-Eur. J., 2011, 17, 6206-6213.
K. Buerglova, A. Noureddine, J. Hodacova, G. Toquer, X.
Cattoen and M. W. C. Man, Chem.-Eur. J., 2014, 20,
10371-10382.
35
36
K. Yamaguchi and N. Mizuno, Angew. Chem., Int. Ed.,
2002, 41, 4538-4541.
S.
Ganesamoorthy,
M.
M.
Tamizh,
K.
Shanmugasundaram and R. Karvembu, Tetrahedron
Lett., 2013, 54, 7035-7039.
13
A. Schachtschneider, M. Wessig, M. Spitzbarth, A.
Donner, C. Fischer, M. Drescher and S. Polarz, Angew.
Chem., Int. Ed., 2015, 54, 10465-10469.
37
38
E. De Canck, I. Dosuna-Rodriguez, E. M. Gaigneaux and
P. Van der Voort, Materials, 2013, 6, 3556-3570.
T. Koiwa, Y. Masuda, J. Shono, Y. Kawamoto, Y.
Hoshino, T. Hashimoto, K. Natarajan and K. Shimizu,
Inorg. Chem., 2004, 43, 6215-6223.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J.
L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
14
15
16
J. Gao, X. Zhang, S. Xu, F. Tan, X. Li, Y. Zhang, Z. Qu, X.
Quan and J. Liu, Chem.-Eur. J., 2014, 20, 1957-1963.
C. E. Hoyle and C. N. Bowman, Angew. Chem., Int. Ed.,
2010, 49, 1540-1573.
D. Esquivel, O. van den Berg, F. J. Romero-Salguero, F.
Du Prez and P. Van Der Voort, Chem. Comm., 2013, 49,
2344-2346.
39
17
O. Bortolini, L. Caciolli, A. Cavazzini, V. Costa, R. Greco,
A. Massi and L. Pasti, Green Chem., 2012, 14, 992-1000.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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Please do not adjust margins
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Page 10 of 11
ARTICLE
Journal Name
O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. 63
Peralta, F. Ogliaro, M. J. Bearpark, J. Heyd, E. N.
Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S.
S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M.
Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J.
Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L.
Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.
Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,
Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and
D. J. Fox, Gaussian, Inc., Wallingford, CT, USA, 2009.
A. D. Becke, J. Chem. Phys., 1993, 98, 5648-5652.
C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37,
785-789.
A. Karam, J. C. Alonso, T. I. Gerganova, P. Ferreira, N.
BIon, J. Barrault and F. Jerome, Chem. Commun., 2009,
7000-7002.
View Article Online
DOI: 10.1039/C6GC01494A
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
N. C. Handy and A. J. Cohen, Mol. Phys., 2001, 99, 403-
412.
W.-M. Hoe, A. J. Cohen and N. C. Handy, Chem. Phys.
Lett., 2001, 341, 319-328.
J. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett.,
1997, 78, 1396-1396.
J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev.
Lett., 1996, 77, 3865-3868.
F. Weigend, Phys. Chem. Chem. Phys., 2006, 8, 1057-
1065.
F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys.,
2005, 7, 3297-3305.
S. Grimme, J. Antony, S. Ehrlich and H. Krieg, The J.
Chem. Phys., 2010, 132, 154104.
S. Grimme, S. Ehrlich and L. Goerigk, J. Comput. Chem.,
2011, 32, 1456-1465.
L. Goerigk and S. Grimme, Phys. Chem. Chem. Phys.,
2011, 13, 6670-6688.
S. F. Boys and F. Bernardi, Mol. Phys., 1970, 19, 553-
566.
A. Zuerner, J. Kirstein, M. Doeblinger, C. Braeuchle and
T. Bein, Nature, 2007, 450, 705-709.
T. Bogaerts, S. Wouters, P. Van der Voort and V. Van
Speybroeck, ChemCatChem, 2015, 7, 2711-2719.
M. Swart, A. W. Ehlers and W. Koop, Mol. Phys., 2004,
102, 2467-2474.
Y. Minenkov, Å. Singstad, G. Occhipinti and V. R.
Jensen, Dalton Trans., 2012, 41, 5526-5541.
J. Zhang, Q. Jiang, D. Yang, X. Zhao, Y. Dong and R. Liu,
Chem. Sci., 2015, 6, 4674-4680.
S. E. Davis, M. S. Ide and R. J. Davis, Green Chem., 2013,
15, 17-45.
Z. Shi, C. Zhang, C. Tang and N. Jiao, Chem. Soc. Rev.,
2012, 41, 3381-3430.
W. Wang, J. E. Lofgreen, G. A. Ozin, Small, 2010, 23,
2634-2642.
D. Esquivel, C. Jimenez-Sanchidrian an F. J. Romero-
Salguero, Mater. Lett., 2011, 65, 1460-1462.
C. M. Li, H. Yang, X. Shi, R. Liu and Q. H. Yang,
Microporous Mesoporous Mater., 2007, 98, 220-226.
G. Morales, G. Athens, B. F. Chmelka, R. van Grieken
and J. A. Melero, J. Catal., 2008, 254, 205-217.
10 | J. Name., 2012, 00, 1-3
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Green Chemistry
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DOI: 10.1039/C6GC01494A
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