Mesoporous silica as phase transfer agent in the biphasic oxidative cleavage of alkenes using triazole complexes of ruthenium as catalyst precursors

Article abstract of DOI:10.1016/j.apcata.2018.07.038

The oxidative cleavage of alkenes was performed using ruthenium triazole -arene complexes immobilized on mesoporous silica materials. These silica-organometallic hybrid materials were found to show enhanced activity when compared to conventional homogeneous systems even when operating at a relatively low catalyst loading. The enhanced catalytic performance of these heterogeneous systems can be attributed to the mesoporous silica acting as a phase transfer agent in the biphasic catalyst system.

Full text of DOI:10.1016/j.apcata.2018.07.038

Accepted Manuscript
Title: Mesoporous silica as phase transfer agent in the biphasic
oxidative cleavage of alkenes using triazole complexes of
ruthenium as catalyst precursors
Authors: L. Leckie, S.F. Mapolie
PII:
S0926-860X(18)30379-X
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.07.038
APCATA 16764
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Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
27-3-2018
24-7-2018
26-7-2018
Please cite this article as: Leckie L, Mapolie SF, Mesoporous silica as phase
transfer agent in the biphasic oxidative cleavage of alkenes using triazole
complexes of ruthenium as catalyst precursors, Applied Catalysis A, General (2018),
https://doi.org/10.1016/j.apcata.2018.07.038
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Mesoporous silica as phase transfer agent in the biphasic
oxidative cleavage of alkenes using triazole complexes of
ruthenium as catalyst precursors
L. Leckie a, S.F. Mapolie a
,*
a
DST-NRF Centre of Excellence in Catalysis (c*change), Department of Chemistry and Polymer Science,
Stellenbosch University, Private Bag 1, Matieland,7601 Stellenbosch, South Africa.
*Corresponding author: smapolie@sun.ac.za (S.F. Mapolie)
Graphical abstract
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1

Highlights
 A series of novel ruthenium complexes of pyridine-triazole and quinoline-triazole
ligands were immobilized on mesoporous silica, MCM-41 and SBA-15.
 The immobilized catalysts were shown to be highly active in the oxidative cleavage of
alkenes using IO(OH)5 as oxidant.
 The silica supports acts as phase transfer agents to transport catalyst precursors from
organic layer to aqueous layer.
 High selectivity to the aldehyde product is observed for all the catalyst precursors at
short reaction times.
Abstract
The oxidative cleavage of alkenes was performed using ruthenium triazole -arene complexes immobilized on
mesoporous silica materials. These silica-organometallic hybrid materials were found to show enhanced activity
when compared to conventional homogeneous systems even when operating at a relatively low catalyst loading.
The enhanced catalytic performance of these heterogeneous systems can be attributed to the mesoporous silica
acting as a phase transfer agent in the biphasic catalyst system.
Keywords
Immobilized complexes, mesoporous silica, oxidative cleavage of alkenes, ruthenium-arene complexes
1.
Introduction
Unsaturated hydrocarbons, such as alkenes can be converted to more valuable oxygenates via
oxidative cleavage of the carbon-carbon double bond. This is a useful synthetic process and has been
the topic of several reviews [1–5]. Oxidative cleavage of olefins is often performed industrially
employing non- catalytic processes such as ozonolysis. There are also other stoichiometric reactions
involving transition metal based oxidation reagents such as KMnO4 or CrO2Cl2 . The latter is known to
selectively transform olefins into aldehydes or carboxylic acids. The use of such systems is however
undesirable due to the toxic nature of the metal salts employed and the fact that the organic substrates
are usually not very soluble in the aqueous medium required for the permanganate ion [5].
2

Catalytic olefin oxidations using a transition metal catalyst, often based on group 7 or 8 metals, and
-
in conjunction with oxidants, such as IO4 or H2O2 have been investigated as alternatives [5,6].
Ruthenium, osmium, and tungsten catalysts have previously been reported to catalyze the oxidative
cleavage of various unsaturated fatty acids and alkenes. In addition Fe and Mn based catalytic systems
have also been reported. However, these are usually limited to activated alkenes, such as styrene.
Alkenes can selectively be cleaved to form aldehydes using osmium-based catalysts with NaIO4 as an
oxidant. A major drawback however is that many of the Os compounds are often toxic, which make
their usage not desirable [5]. Over recent years, ruthenium-based catalysts have gained increasing
interest due the versatility of the metal oxide, RuO4, which has been shown to be the active species in
these oxidative alkene cleavage reactions. Ruthenium tetroxide has been employed stoichiometrically
in the oxidation of alkenes to aldehydes [5]. There are several reports where even ruthenium trichloride
on its own has simply been employed to catalytically convert alkenes into carboxylic acids [7,8] in the
presence of an oxidant such as sodium hypochlorite or sodium periodate [9,10]. Ruthenium trichloride
in the presence of periodic acid, IO(OH)5, is known to convert alkenes to carboxylic acids however in
this case the reduction product, HIO is very soluble in water alleviating problems during the work-up of
the reaction [5]. Studies have shown that RuO4 exhibits high selectivity towards terminal oxygenated
cleavage products. This is due to the fact that neither dihydroxylated nor epoxidized intermediates are
involved in the reaction mechanism using these types of catalysts [11]. The primary products are
aldehydes with carboxylic acids obtained as a result of over-oxidation of the aldehyde [5]. Due to the
chemoselectivity of the Ru catalysts for terminally functionalized oxygenates, there have been several
attempts to develop new ruthenium-based catalysts. Some reports on the development of homogeneous
and heterogeneous ruthenium based catalytic systems for the oxidative alkene cleavage reaction have
appeared in the recent literature. For example, Shoair et al. discovered that a Ru-bipy complex in the
presence of IO(OH)5 oxidatively cleaved both terminal and internal alkenes to yield carboxylic acids
[9]. These authors also found that the incorporation of a bidentate nitrogen donor ligand such as
bipyridine, led to a catalyst capable of mediating the oxidative cleavage reaction at a much lower
(tenfold lower) catalyst loading than other ruthenium catalyst precursors, which do not possess a
nitrogen donor ligand.
In light of these developments, our group has recently embarked on a study to investigate the ability
of other N,N chelating ligands to mediate the Ru catalyzed oxidative cleavage of alkenes. Recently we
reported enhanced activity for the complexes based on pyridine-imine as well as quinoline-imine ligands
[12]. In the same publication, further enhancement in activity and selectivity was observed when these
complexes were immobilized on mesoporous silica supports. We have now elaborated on these systems
exploring the use of other complexes of Ru using novel nitrogen-based chelating ligands. The current
work is thus an attempt at exploring the use of pyridine-triazole, pyridine-N-oxide/triazole as well as
quinoline-triazole complexes of ruthenium in oxidative cleavage reactions. Both homogeneous as well
as immobilized complexes of this type were evaluated as catalyst precursors.
3

2.
Experimental
2.1. General remarks and instrumentation
All reactions were performed using standard Schlenk techniques under an inert atmosphere (Argon
or Nitrogen) unless stated otherwise. Highly air-sensitive materials were stored in a nitrogen-purged
glovebox and all manipulations with these materials were carried out in the glovebox to prevent
decomposition or oxidation. In the case of microwave-assisted reactions, a CEM Discover SP
Microwave reactor was used. Fourier transform infrared (FT-IR) spectra were recorded using an ATR
accessory on a Nicolet Avatar 330 FT-IR spectrometer equipped with a Smart Performer ATR
attachment with a ZnSe crystal. NMR spectra were recorded on a Varian Unity Inova instrument at 300,
1
13
400 and 600 MHz for H and C NMR spectroscopy. ESI-MS was performed using a Waters Synapt
G2 Mass Spectrometer. A Thermo Elemental Analyzer (CHNS-O) was used for the accurate
determination of the elemental composition of samples. BET nitrogen adsorption/desorption analysis
was done on a Micromeritics 3Flex Surface Characterization instrument (77 K). The samples were
degassed at 273 K for 18 hours prior to analysis. Powder XRD analysis was done using a BRUKER
AXS (Germany) D8 Advance Diffractometer. Transmission electron micrographs were collected using
an FEI Tecnai G2 20 field-emission gun (FEG) TEM, operated in bright field mode at an accelerating
voltage of 200 kV, on a Ni/Cu grid. Energy dispersive X-ray spectra were collected using an EDAX
liquid nitrogen cooled Lithium doped Silicon detector. A TA Instruments Q500 thermogravimetric
analyser was used to acquire TGA data. ICP-OES analysis was performed on a Spectro Arcos ICP-OES
with a Burgener T2100 and cyclonic spray chamber as nebulizer. The samples were prepared by
digesting between 20-30 mg of the immobilized catalyst in concentrated nitric acid (15 ml), at 100 °C
for 24 hours. The sample was then filtered and made up to a known volume and analyzed. A Radleys
12-stage carrousel parallel reactor equipped with a gas distribution system was used for the catalytic
reactions. GC analyses were performed using a Varian 3900 instrument equipped with a polar
Cyclosil-B column (30 m, 0.250 mm diam. and 0.25 μm film) using helium as carrier gas. p-xylene was
used as internal standard. HPLC-MS analyses were performed on a Waters Synapt G2 instrument fitted
with an Acquity BEH C18 2.1 x 50mm 1.7μm column. ESI mass spectra were recorded by direct
injection into a stream of acetonitrile and 0.1% formic acid employing a cone voltage of 15 V on a
Waters API Quattro Micro spectrometer.
2.2. Materials
All reagents were acquired from Sigma-Aldrich or Fluka and used without any further purification.
These include 2-ethynyl pyridine, 2-chloroquinoline, copper iodide, octyl bromide, 3-chloropropyl
triethoxysilane,
meta-Chloroperoxybenzoic
acid
(mCPBA),
α-terpinine,
RuCl3∙xH2O,
cetyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate (TEOS) and Poly(ethylene)-block-
poly(propylene)-block-poly(ethylene). All solvents were purchased from Sigma-Aldrich or Kimix
4

Chemicals. dichloromethane, diethyl ether, hexane, toluene and tetrahydrofuran were purified using a
Pure SolvTM Micro solvent purifier fitted with activated alumina columns. Ethanol and methanol were
purified by distillation over a mixture of magnesium filings and iodine. Acetonitrile was purified by
distillation over phosphorous pentoxide. CCl4 was used without further purification.
2.3 Synthesis of model and functionalized ligands and complexes
See Supplementary Information for experimental details and characterization of the model and
siloxane functionalized ligands and complexes.
2.4 Synthesis of immobilized catalysts IC1-IC6
The native silica, MCM-41 [13] and SBA-15 [14], were synthesized according to modified
literature procedures. The procedure for immobilization of the siloxane-functionalized complexes on
mesoporous silica, MCM-41 and SBA-15, as well as the characterization of these materials is discussed
in the Supplementary Information but follows a previously published approach for other ruthenium
complexes [12].
2.5 Typical procedure for the oxidative cleavage of alkenes
The catalyst, MC1 (5x10-4 mmol), was added to a mixture of CCl4 (1.25 mL) and MeCN (1.25 mL)
in a Radley’s parallel reactor tube. To the yellow mixture was added the appropriate alkene (0.5 mmol)
followed by a solution of oxidant, IO(OH)5, (0.57 g, 2.5 mmol) dissolved in distilled water (2.5 mL).
The biphasic reaction mixture was allowed to stir for the required time at 25 °C. The mixture was then
extracted with diethyl ether (9 mL) and the organic layer dried over anhydrous MgSO4. The mixture
was sampled (1 mL) and p-xylene (0.100 mL) added as internal standard for product quantification.
Conversions were determined via gas chromatography. The work-up of the reaction mixture for the
oxidative cleavage of cyclopentene was different to that of 1-octene and styrene. In the case of
cyclopentene as substrate, the mixture was extracted with DCM and the organic layer dried over MgSO4,
1 mL of the organic fraction was sampled for GC analysis. The solvent was then removed from the
aqueous layer remaining to obtain a white residue. MeCN (~ 5ml) was added to the white residue at
which point a large amount of the white solid remained undissolved. The mixture was syringe filtered.
The filtrate was dried over MgSO4 and 1 mL was sampled for GC analysis.
2.6. UV-Vis investigation to probe the formation of RuO4 in the oxidative cleavage of 1-octene
Reactions were carried out under the general reaction conditions employed for the oxidative
cleavage of 1-octene in the absence of substrate. At the appropriate time between 0.2-0.4 ml of the
organic layer of the reaction was sampled and this was then diluted to a total volume of 3 ml with a 1:1
(v/v) MeCN/CCl4 solvent mixture. The UV-Vis spectra of these solutions were recorded over the
wavelength range 200 nm to 800 nm.
5

2.7. Reaction of model complex, MC2, with periodic acid, with the aim of identifying possible
reaction intermediates in the oxidative cleavage reaction
Model complex, MC2, (20 mg, 0.02 mmol) was dissolved in MeCN (1 ml). To the golden yellow
solution was added IO(OH)5 (27 mg, 0.12 mmol) dissolved in water (0.25 ml). The solution turned dark
brown within 30 min and a small amount of brown precipitate formed. The mixture was filtered through
cotton wool and celite, to afford a golden yellow filtrate. The celite had been discoloured to brown. The
solvent was removed at 35 °C from the filtrate to obtain a golden yellow residue.
3.
Results and Discussion
3.1
Synthesis of ruthenium triazole arene pre-catalysts
Prior to the immobilization of the ruthenium triazole-arene complexes, we prepared the siloxane
functionalized precursors as well as the corresponding unfunctionalized model complexes by reacting
the [Ru(p-cymene)Cl2]2 dimer with the appropriate N,N or N,O ligand in a 1:2 mole ratio respectively
(Scheme 1). The immobilization of the different complexes is effected by simple condensation of the
siloxane functionalities with the silanol groups on the surface of the silica support. This is illustrated
for immobilized catalysts, IC1 and IC2 in Scheme 2. The full range of immobilized catalyst systems
prepared is illustrated in Chart 1. Also shown are the corresponding model complexes. For complete
details of the preparation of the precursor complexes, consult the Supporting Information.
The immobilized catalysts were characterized using a range of solid state analytical techniques including
infrared spectroscopy, nitrogen adsorption/desorption (BET) surface analysis, low-angle powder X-ray
diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermal
gravimetric analysis (TGA) and ICP-OES. TEM, SEM and powder XRD results confirm that very little
change in the morphology of the support is observed after immobilizing the catalysts. The TEM
micrographs of the immobilized catalysts IC1 (immobilized on MCM-41) and IC2 (immobilized on
SBA-15) are shown in Figure S62† and Figure S63† (ESI) respectively. These compare well with
what has been previously been observed for types of mesoporous silica materials [15,16]. The
immobilization of the complexes onto the supports was also confirmed by the decrease in surface area
as determined by means of BET (Brunauer Emmett Teller) surface analysis. In all cases , Type IV
isotherm plots [17-20] were obtained for the MCM-41 and SBA-15 based immobilized catalysts (Figure
S70† and Figure S71†, ESI respectively), confirming the mesoporous nature of the materials. The
surface area, pore diameter and total pore volume for MCM-41 based materials and SBA-15 based
materials calculated from BET analysis are summarized in Table 1. The results correspond well with
6

those reported previously [21-24]. Immobilization of the complexes onto the supports was also
confirmed by the increase in weight loss relative to the native silica obtained from TGA analysis and
the detection of ruthenium on the supports (Figures S66† and S67†, ESI)
3.2
Oxidative cleavage of alkenes
The model and immobilized catalysts depicted in Chart 1were evaluated in the oxidative cleavage of
various alkenes.
3.2.1 Oxidative cleavage of 1-octene
The model complexes (MC1-MC3) and immobilized ruthenium complexes (IC1-IC6) were employed
in the oxidative cleavage of 1-octene (Scheme 3) in the presence of periodic acid as oxidant. The
catalysis was performed in a biphasic solvent mixture consisting of an aqueous phase and an organic
phase of acetonitrile and carbon tetrachloride. The former being a coordinating solvent is required to
prevent the formation of insoluble ruthenium intermediates during the catalytic process [10a] while the
latter has been reported to play a role in solubilisation of the in-situ formed RuO4, which is generally
regarded as the active species [10b]. Other water immiscible solvents, including other chlorinated
solvents were either themselves susceptible to oxidation or gave little to no conversion of substrates.
The catalysts were found to successfully transform 1-octene to heptaldehyde and subsequently to
heptanoic acid, with the latter only being formed over extended reaction times. From these results, it is
clear that one can selectively produce aldehydes by simply controlling the reaction time. The efficiency
of the various catalysts were compared against each other, in terms of turnover numbers (TON’s) at
various reaction times (Table 2). Also shown are turnover frequencies (TOF’s), which gives some
insight into catalyst life times.
When comparing the model and immobilized catalyst systems, we see that all the immobilized
catalysts were significantly more active than their model counterparts when used at similar metal
concentrations. The immobilized catalyst systems gave good conversions at a relatively low metal
concentration of 0.1 mol%. This trend can for example be seen for the pyridine triazole N,N system
where the TON’s obtained for immobilized catalysts, IC1 and IC2, are significantly greater than the
those obtained for the model complex, MC1 (entries 1-6, Table 2). This phenomenon is also observed
for the pyridine-N-oxide/triazole (IC3 and IC4) and quinoline-triazole (IC5 and IC6) catalyst systems,
7

although in the case of the pyridine-N-oxide/triazole the difference in activity between the model and
the immobilized catalyst is less pronounced. It should be emphasized that the activities were compared
at the same metal loading and under similar reaction conditions. It is thus obvious that immobilization
significantly enhances the efficacy of the catalysts. The mode by which this enhancement occurs is
discussed later (vide infra).
Another observation that is evident from the results, is that the N,O (pyridine-N-oxide/triazole)
catalyst systems, model complex (MC2) and immobilized complexes (IC3 and IC4) were more
effective in the oxidative cleavage reaction than the N,N systems viz. the pyridine-triazole complexes
(MC1, IC1 and IC2) and quinoline-triazole complexes (MC3, IC5 and IC6). Thus for example the
pyridine-N-oxide/triazole model complex, MC2, achieved a reasonable TON of 250 compared to the
model N,N systems, MC1 and MC3, which showed TON’s of only 48 and 92 respectively, when
employing these catalyst at the same ruthenium concentration over a 3 hour reaction time. The
immobilized analogues of the pyridine-N-oxide/triazole catalyst was also more active than the
immobilized pyridine-triazole and quinoline-triazole analogues with the catalyst containing the
pyridine-N-oxide moiety achieving turnover numbers, which are almost double those for the other two
N,N complexes. All the immobilized catalysts, achieve conversions between 90-100% after about 9
hours reaction time. The homogeneous model complexes on the other hand with the exception of MC2,
show conversions of only 6-24% over a longer reaction time (~12 hour). MC2, which is the complex
with a pyridine-N-oxide moiety, shows a 100% conversion after about 6 hours reaction time. However,
in this particular case the reaction solution at this stage contains a mixture of aldehyde and carboxylic
acid unlike the case for immobilized analogues (IC3 and IC4) which show 100% selectivity towards
the carboxylic acid at shorter reaction times. Another noticeable observation is the fact that the
immobilized catalysts shows an activity profile which appears relatively stable over extended time. This
is evident when looking at the TOF values for these catalyst systems. The model complexes, MC1 and
MC3 show a significant decrease in TOF values over time, which is indicative of a dramatic decrease
in the concentration of active species in solution over time. The immobilized analogues on the other
hand display relatively steady TOF values indicating the steady generation of active species over
extended times.
3.3. Investigation into reaction pathway using immobilized catalyst systems
Having observed that the immobilized catalysts performed significantly better than their model
counterparts (Table 2) it was decided to investigate this in more detail.
3.3.1. Detection of the active species by UV-vis spectroscopy
Previous reports in the literature including work from our own group, indicate that the oxidative
cleavage of alkenes using Ru based catalysts is in fact mediated by RuO4, which is formed in-situ during
the reaction [9, 19, 25]. For the previously reported systems, the presence of RuO4 in the reaction
8

mixture could be confirmed using UV-Vis spectroscopy. The presence of RuO4 is indicated by two
characteristic absorption bands, one between 280-340 nm and the other in the region between 370-420
nm (Figure 1). In addition, these peaks also possess a distinctive fine structure [12,16]. These
characteristic peaks are also observed in reaction mixtures in which our immobilized catalysts had been
treated with the oxidant, IO(OH)5. (Figures 1 and 2) The concentration of this species was found to be
higher in solutions of those complexes which gave higher TON’s in the catalytic reactions.
3.3.2. Possible mechanism
Based on previous reports, a probable mechanism for the oxidative cleavage reaction is shown in
Scheme 4. A crucial step in the cycle involves the transformation of the ruthenium precursor to RuO4
in the presence of the oxidant, periodic acid. As indicated earlier, UV-Vis experiments confirm that
the Ru (II) complex is indeed the source of the tetroxide. It is proposed that the alkene substrate
subsequently coordinates to the RuO4 to form a ruthenium (VI) diester [12]. Unfortunately, in our case
the metal diester intermediate was not detected in reactions using our catalyst precursors. However, this
is possibly due to the fleeting nature this species. Although such metallo-diester species are widely
accepted as an important intermediate in these reactions, its detection has only been reported once
previously. Albarella et al. reported the detection of traces of such a metallo-diester when carrying out
a stoichiometric reaction between RuO4 and (-)-α-pinene performed in CCl4 [11]. To the best of our
knowledge, this is the only example where this type of particular metallo-ester has been detected.
Currently it is accepted that the rearrangement of this cyclic ruthenium diester ultimately affords the
aldehyde as product in conjunction with RuO2. The latter is then oxidized in the presence of oxidant to
reform RuO4. The aldehyde product formed initially, can subsequently undergo further oxidation to the
analogous carboxylic acid product [12]. This second oxidation process is however much slower,
resulting in the carboxylic acid product only being observed at extended reaction times. The re-oxidation
of RuO2 is a well-known reaction and is regarded a simple REDOX reaction. It has been invoked as a
crucial step in the oxidative alkene cleavage in several reports in the literature [5, 10a, 10b and 10e].
The immobilized triazole-based complexes reported in this paper were found to be effective
catalysts in the oxidative cleavage of alkenes giving high conversions at a relatively low catalyst loading
of 0.1 mol%. This is at much lower metal loading than conventional systems. For example the well-
known and widely employed Sharpless catalyst system give high conversions but requires catalyst
loadings around 2 mol% [10a]. A report by Yang et al. in which the oxidative cleavage of alkenes using
RuCl3 are reported uses Ru loading of 3.5% [10c]. The use of RuO2 as a catalyst precursor has also been
reported but once again high metal concentrations are required to ensure reasonable conversion of the
substrate [10d]. Homogeneous systems supported by other N,N chelating ligands have been reported
before and gave reasonable yield working at 0.5 mol % metal loading [9]. This however is five times
higher than the catalyst loading for our catalyst system.
9

3.3.3. Role of the mesoporous silica support on the catalytic process
From the results obtained during our catalytic studies, it is clear that the support has a significant
impact on the catalytic activity. It was initially thought that the silica played a role in stabilizing the
active oxidizing species, RuO4, and in so doing ensured a reasonable concentration of this species
present in solution resulting in faster conversion of the substrate when using the immobilized systems.
However closer investigation of this system, revealed that the enhanced activity is in fact not due to
catalyst stabilization by the support but rather that another phenomenon was at play. What we noticed
was that during the execution of the oxidative cleavage reactions with immobilized catalysts, IC1-IC6,
the supported catalysts displayed some hydrophilic-like characteristics in that almost all of the silica
material migrates to the water layer of the biphasic reaction mixture. On reflection, this is not entirely
unexpected as silica with its high concentration of silanol groups on its surface can reasonably be
expected to show some degree of hydrophilicity. Indeed scanning the literature, we came across some
reports related to this phenomenon [ 27-30]. Included amongst these are few instances where silica has
been employed as a support in biphasic catalysis and where it facilitates phase transfer of a catalyst
between two immiscible solvents [33]. However, to the best of our knowledge, this has not previously
been reported for biphasic oxidative
cleavage of alkenes.
3 depicts how the mesoporous silica, MCM-41 migrates to the aqueous phase in biphasic mixtures. In
mixture 1, we see that native MCM-41 prefers the aqueous phase (lower layer) to the organic layer
(diethyl ether) on top. Mixture 2 shows that the model complex MC1 dissolved in the organic phase in
a mixture of CCl4/MeCN/H2O, whereas in mixture 3 we see that immobilized analogue IC1 migrates to
the aqueous layer in the same solvent mixture. Finally, in mixture 4 we observe that native MCM-41
also prefers the aqueous phase in the CCl4/MeCN/H2O solvent system. Taking the above observations,
into consideration, it becomes apparent that the enhanced catalytic performance seen for the
immobilized complexes can largely be attributed to the hydrophilic nature of mesoporous silica. This
allows the silica-supported catalysts to be easily transported to the aqueous phase. In the case of the
immobilized catalysts, this facilitates the transfer of the ruthenium complex, immobilized on the silica,
from the organic CCl4/MeCN phase into the aqueous phase in which the oxidant, IO(OH)5, is present.
This ensures close contact between the ruthenium pre-catalyst and the oxidant in the aqueous layer, thus
promoting the formation of the active species, RuO4. The latter is insoluble in water but highly soluble
in CCl4. It therefore rapidly migrates back to the organic phase where it comes into contact with the
hydrocarbon substrate. The silica is thus acting as some sort of phase transfer agent. In contrast, in the
cases where pure model homogeneous complexes are used, the complexes are soluble in the organic
phase (CCl4/MeCN), and therefore the ruthenium pre-catalyst only comes into contact with the water
10

soluble IO(OH)5 at the interface of the aqueous and organic layers. In this case, a far lower
concentration of the catalyst precursor is exposed to the oxidant, resulting in a much lower rate of
formation of RuO4 (Error! Reference source not found.4).
When comparing the different types of support it can be seen that the SBA-15 based systems are
slightly more active than their MCM-41 counterparts. This is most likely due to the fact that SBA-15
has larger pore sizes than MCM-41. Assuming some of the catalyst precursors in addition to being on
the surface of the support are also encapsulated in the pores, it would be reasonable to suggest that access
by the oxidant to the encapsulated metal sites would be easier in the case of the larger sized SBA-15.
This would enhance rate at which the active species is generated.
Finally given the nature of the reaction where the Ru precursor is ultimately converted into soluble
RuO4, we decided not to embark on any recycling experiments.
3.3.
Oxidative cleavage of other alkenes with immobilized catalyst, IC4
With an effective catalyst system in hand, the most active of the immobilized catalysts, IC4, was
applied in the oxidative cleavage of other alkenes to assess the substrate scope of the immobilized
ruthenium pyridine-triazole based catalyst. Thus the oxidative cleavage of styrene (Scheme 5) and
cyclopentadiene (Scheme 5) was investigated using the above complex as catalyst precursor.
Table 3 shows that selective and complete conversion of the styrene to form benzaldehyde occurs within
a 30-minute reaction time. Over-oxidation to benzoic acid could only be observed if the reaction was
allowed to proceed for 1 hour. However, the oxidation of benzaldehyde to benzoic acid seems to be a
much slower process, with only 6.6% selectivity for benzoic acid being observed after a reaction period
of 1 hour.
We also evaluated the oxidative cleavage of a cyclic alkene, viz. cyclopentene. In the case of the
oxidative cleavage of a cyclic olefin, it can be expected that the reaction would result in the formation
of the corresponding bifunctional aldehyde and/or carboxylic acid (Scheme 6). This is indeed what was
observed in our system, where glutaraldehyde and glutaric acid were obtained as products after the
oxidative cleavage of cyclopentene using IC4 (Entries 3 and 4, Table 3). In addition to the bifunctional
acid and aldehyde, an intermediary, namely 5-hydroxypentanoic acid, was identified as a product using
HPLC-MS (Fig. S7†) Complete conversion of the substrate could be observed after a relatively short
reaction time of only 30 minutes, with glutaraldehyde as the only product. When the reaction time was
extended to 1 hour, the selectivity changed such that a mixture of glutaraldehyde, 5-hydoxypentanoic
acid and glutaric acid could be observed in the product stream. When comparing the oxidative cleavage
of the various substrates, we see that in the case of the oxidation of 1-octene, around only 50% of the
substrate was converted predominantly into the heptaldehyde after 3 hours. However, for the activated
alkenes such as styrene and cyclopentene, the cleavage reaction proceeded much faster, resulting in
11

100% conversion of the substrate into the aldehyde product within 30 minutes. This shows that the
immobilized catalyst IC4 can effectively be used for the selective conversion of different types of
alkenes into valuable oxygenated products.
3.4 Probing the catalyst behavior of the ruthenium triazole model complexes
To further understand the behavior of these ruthenium pre-catalysts in the alkene oxidative
cleavage, we employed the model complexes, MC1 and MC2, to investigate how these are converted
into the active species, RuO4. The role of the ligands in this process was also examined. UV-vis
spectroscopy was used to probe this.
3.4.1. Monitoring the transformation of the catalyst precursor into RuO4 using UV spectroscopy
The first point of departure was to detect the formation of ruthenium tetroxide in reactions in which
the above-mentioned model complexes are treated with oxidant. As expected, the analysis of these
mixtures was not straightforward due to the fact that some of the major peaks in the UV-Vis spectrum
of the model complex (pre-catalyst) overlap with those of RuO4. This results in the latter only being
detected once enough of the model complex had been transformed in the presence of the oxidizing agent.
In light of this, the rates of RuO4 formation from the model catalysts could not be compared directly
with those obtained for the immobilized catalysts. The UV-vis spectra of reaction mixtures obtained of
the model catalysts in the presence of the oxidant are depicted below in Figure 5. The UV-vis spectra
of the pure model complexes, MC1-MC3 show peaks between 250-270 nm associated with electronic
transitions in the ligand. We also see metal to ligand charge transfer (MLCT) bands between 285-300
nm and 310-370 nm for complexes MC1 and MC3.. These two complexes are supported by chelating
N,N chelating ligands, viz. pyridine-triazole (MC1) and quinoline-triazole (MC3). The UV-vis
spectrum of MC2, a complex that has a chelating ligand, which incorporates a pyridine-N-oxide moiety,
does not show any distinct metal to ligand charge transfer band. A band around 400 nm, associated with
the d-d transitions of the metal ion, can be observed for all three model complexes.
When the model complexes are exposed to the oxidant, we see a transformation in the spectra of
all three pre-catalysts. The appearance of a medium intensity peak between 290-320 nm and a low
intensity peak between 380-400 nm can be associated with the formation of ruthenium tetroxide. The
latter peak is however somewhat masked by the signal for the d-d transitions of the metal ion. The
transformation of model catalyst MC2 to RuO4 occurs over a relatively short period (5-hour ), whereas
for the model catalyst systems, MC1 and MC3, we only detect RuO4 at much longer reaction times (~24
hours).Thus it is clear that the pyridine-N-oxide moiety of MC2 facilitates the formation of RuO4. In
contrast, the precursors with N,N chelating systems (MC1 and MC3) are less able to do so. It is thought
that in the case of the pyridine-N-oxide complexes, the N-oxide functionality facilitates the transfer of
the oxo group to metal centre leading to the formation ruthenium oxide species. This type of oxygen
atom transfer process is not unusual and has been observed for late transition metal complexes. Thus,
12

for example Mei et al. proposed such an oxygen atom transfer process from coordinated pyridine-N-
oxide ligands to the metal in their study of catalytic C-H functionalization using mediated by iron
complexes [34]. Similar oxygen transfer atom reactions were investigated computationally by Pardue
et al. for Fe, Ru and Os complexes [35].
3.4.2. Activation of model ruthenium pre-catalyst, MC2
To further probe the behaviour of the catalyst precursor MC2, in solution, we reacted it with the
oxidant, periodic acid, with the aim of identifying the type of species present in solution. MC2, was
dissolved in MeCN and reacted with 5 mol equivalents of periodic acid dissolved in a small amount of
water. The mixture, which changed colour from golden yellow to brown almost immediately, was
filtered and the solvent removed from the filtrate to afford a brown residue. The residue was analysed
using proton NMR spectroscopy, FT-IR spectroscopy, ESI mass spectrometry, as well as thin layer
chromatography (TLC) in order to gain an understanding of the species being formed during the
reaction. One of the products detected, was the free pyridine-N-oxide/triazole ligand, albeit in a
relatively small amount. This was confirmed by TLC analysis using an authentic sample of the ligand.
Furthermore the crude mixture was also analysed by ESI mass spectrometry as well as proton NMR
spectroscopy. In addition to the free ligand, the proton NMR spectrum (Fig. S5†) shows the presence
of a ruthenium complex in which a pyridine-triazole ligand (7.11-7.71 ppm) as well as the p-cymene
ligand (5.43-5.69 ppm) are both co-ordinated to the ruthenium centre i.e. a metal complex resembling
the model complex MC1, which contains a pyridine-triazole ligand.
This is confirmed by ESI mass spectrometry (Figure 6) of the crude reaction mixture which show a peak
at m/z: 529.17 corresponding to this species. Also detected in the MS is a peak corresponding to some
unreacted pyridine-N-oxide/triazole model complex MC2 (m/z: 545.16 ). In addition a signal assigned
to the free pyridine-N-oxide/triazole ligand (m/z: 275.19 ) is also seen in the spectrum. A high mass
signal at m/z: 621.10 can be assigned to an analogue of the pyridine-triazole model complex MC1 in
which the original chloride is substituted by an iodide as an inner sphere ligand. The source of iodide is
believed to be the oxidant, IO(OH)5, which is thought to be reduced to the iodide ion during the reaction.
An important difference between the pyridine-triazole and pyridine-N-oxide/triazole complexes
employed as catalysts in this reaction, is that the complex containing the pyridine-N-oxide moiety, MC2,
has a bidentate ligand with an oxygen donor site, the origin of which is the pyridine-N-oxide
functionality. In this case, oxygen transfer from the N-oxide to the metal centre is an important step.
As was seen previously in the proposed mechanism for oxidative alkene cleavage (Scheme 4), the
transfer of oxygen atoms from an oxidant to ruthenium centre plays a crucial role in the formation of
the metal tetroxide. Therefore, the presence of an oxygen atom already coordinated to the metal centre,
13

as is the case in complex MC2 could facilitate oxygen transfer to the metal and thus accelerate the
formation of RuO4. This possibility does not exist for the simple pyridine-triazole complex MC1. In
the case of the MC2 system, transfer of the oxide ion from the pyridine-N-oxide/triazole ligand to metal
would result in the formation of the N, N pyridine triazole ligand. The transformation of the pyridine-
N-oxide/triazole complex MC2 to the pyridine triazole complex MC1 is indeed what was observed as
evidenced from mass spectral and 1H NMR analysis. This then suggests that there is facile oxygen atom
transfer from the pyridine-N-oxide/triazole ligand to the ruthenium centre when MC2 is employed as
catalyst precursor. This allows the pyridine-N-oxide containing complexes to mediate the oxidation
process more effectively.
4.
Conclusion
The model ruthenium catalysts (MC1-MC3) and immobilized analogues (IC1-IC6) employed in
the oxidative cleavage of 1-octene promoted the formation of heptaldehyde and heptanoic acid as
products. The immobilized catalysts were more active than their model counterparts at a relatively low
catalyst loading of 0.1 mol%. The pyridine-N-oxide/triazole catalysts showed enhanced activity when
compared with the N,N (pyridine -triazole and quinoline-triazole) catalysts. UV-Vis studies confirmed
RuO4 to be an important intermediate in the catalytic cycle in reactions for both the model and
immobilized catalyst systems. The immobilized catalysts were hydrophilic in nature, the result being
that the immobilization of the ruthenium complex on the silica support facilitates the transfer of the
ruthenium pre-catalyst from the organic phase to the aqueous phase, which contains the oxidant. This
promotes the oxidation of the precursor to RuO4, which is the actual active species. This phase transfer
allows the proposed active species, RuO4, to form at a faster rate leading to an overall enhancement of
reaction rates for the immobilized catalysts compared to their model analogues. The immobilized
catalyst IC4 was capable of the oxidative cleavage of other alkenes such as styrene and cyclopentene
where the catalyst selectively afforded the aldehyde products benzaldehyde and glutaraldehyde,
respectively, at shorter reaction times (0.5 hrs).
Investigation into the transformation of the pyridine-N-oxide model complex, MC2 in the presence
of oxidant, periodic acid, showed the formation of a complex with an N, N chelating ligand. This
suggests that oxygen atom transfer from the pyridine-N-oxide/triazole ligand to the ruthenium centre is
most likely occurring when MC2 is employed as catalyst precursor in the oxidative cleavage of alkenes.
The oxygen atom transfer from a pyridine-N-oxide moiety of the ligand in the case of MC2 is the likely
reason why this pre-catalysts showed superior catalytic activity compared to the N,N complexes, MC1
and MC3.
14

5.
Acknowledgements
The authors acknowledge the financial support by the National Research Foundation (NRF), grant no.
GUN 90493 and the c*change Centre of Excellence in Catalysis, the Department of Science and
Technology (DST), South Africa. The Division of Research Development at Stellenbosch University is
also thanked for their financial support.
15

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17

Figure captions
.
Figure 1: The UV-Vis spectrum indicating the formation of RuO4 in organic layer of reaction mixtures using various ruthenium
precursors (a) RuCl3·xH2O (b) IC1 (c) IC3 and (d) IC5
18

Figure 2 The UV-Vis spectrum indicating the formation of RuO4 in organic layer of reaction mixtures using various ruthenium
precursors (a) IC3 (c) IC4 and (d) IC6
Figure 3: Depiction of hydrophilic silica in the aqueous phase of biphasic solvent mixtures
(1 = MCM-41 + H2O/Ether; 2 = Model complex MC1 + CCl4/MeCN/H2O; 3 = Immobilized complex IC1 + CCl4/MeCN/H2O;
4 = MCM-41 + CCl4/MeCN/H2O).
19

Figure 4: Depiction of the formation of RuO4 from immobilized catalysts (left) versus model catalyst systems (right).
Figure 5: UV-Vis spectra of the organic layer of reaction mixtures using model catalysts, MC1-MC3.
20

Figure 6: ESI-MS (positive mode) spectrum of model complex MC2 after reaction with 5 mol eqvs. IO(OH)5.
21

Scheme captions
BPh4
C H Si(OEt)
3
6
3
C H Si(OEt)
BPh4
3
3
6
N
N
1/2[[Ru (p-cymene)Cl2]2
NaBPh4
N
N
N
N
N
N
Ru
Cl
SC1:
Scheme 1: Preparation of siloxane-functionalized complex, SC1
Scheme 2: Immobilization of siloxane-functionalized complex, SC1 on various types of silica
Ru cat.
H2O/MeCN/CCl4
CHO
AND / OR
IO[OH]5 ; 25 oC
COOH
Scheme 3: Typical reaction conditions and products formed in the oxidative cleavage of 1-octene.
22

Ru(arene)(triazole)
complex
XO
X
O
O
Ru
X
O
CR CR
O
XO
RuO2
O
O
Ru O
O
R
R
XO
X
2RCOOH
2RCHO
Scheme 4: Proposed mechanism for the oxidative cleavage of alkenes using a Ru(arene)(triazole) complex [25].
OH
Ru cat.
H O/MeCN/CCl
2
4
O
O
AND / OR
o
IO[OH] ; 25
C
5
Scheme 5: Typical reaction conditions and products formed in the oxidative cleavage of styrene.
O
O
OH
Ru cat.
H2O/MeCN/CCl4
O
IO[OH]5 ; 25 o
C
O
O
OH
OH
O
23

Scheme 6: Typical reaction conditions and products formed in the oxidative cleavage of cyclopentene.
24

Chart
MODEL COMPLEXES
-
C8H17
-
-
BPh4
BPh4
C8H17 BPh4
N
C8H17
N
N
N
N
N+
N
N
N
N
N
N
O-
Ru
Ru
Ru
Cl
Cl
Cl
MC1
MC2
MC3
IMMOBILIZED COMPLEXES
Chart 1: Catalyst precursors employed in the oxidative cleavage of alkenes.
25

Table
Table 1: Summary of BET surface area, average pore diameter and total pore volume of immobilized
catalysts IC1-IC6
Materiala
BET Surface Area
Pore Diameter (Å)
Total Pore Volume
(m2/g)
1079
1 058
1 043
1 041
708
(cm3/g)
0.77
0.69
0.72
0.71
0.72
0.62
0.60
0.61
28.6
26.1
27.4
27.2
40.6
43.8
42.3
42.0
MCM-41
IC1 (MCM-41)
IC3 (MCM-41)
IC5 (MCM-41)
SBA-15
568
IC2 (SBA-15)
IC4 (SBA-15)
IC6 (SBA-15)
567
575
a Support material indicated in brackets confirmed by TEM-EDS.
Table 2: Conversion of 1-octene and turnover numbers (TON) and turnover frequencies (TOF)
obtained for catalysts evaluated in the oxidative cleavage of 1-octene.a
Entry
Catalyst
Support
material
Reaction
Conversionb CHO/COOHc TON d
TOFe
time
(%)
(%)
(hours)
1
2
N/A
N/A
3
12
3
4.80 ± 0.20
6.76 ± 1.86
28.5 ± 5.03
84 ± 2.80
100/0
100/0
100/0
73/13
100/0
51/48
100/0
29/71
89/11
0/100
92/4
48
16.0
MC1
MC1
IC1
68
5.6
3
MCM-41
MCM-41
SBA-15
SBA-15
N/A
285
840
343
990
250
1000
468
1000
504
1000
920
241
238
95.0
4
9
93.3
IC1
5
3
34.3 ± 1.98
99.0 ± 2.10
25.0 ± 1.09
100± 1.29
46.8 ± 5.46
100± 1.20
50.4 ± 4.62
100± 1.79
9.20 ± 2.50
24.1 ± 1.54
23.8 ± 1.79
114.3
110.0
83.3
IC2
6
9
IC2
7
3
MC2
MC2
IC3
8
N/A
6
166.7
156.0
166.7
168.0
166.7
30.7
9
MCM-41
MCM-41
SBA-15
SBA-15
N/A
3
10
11
12
13
14
15
6
IC3
3
IC4
6
0/100
100/0
100/0
100/0
IC4
3
MC3
MC3
IC5
N/A
12
3
20.1
MCM-41
79.3
26

16
17
18
MCM-41
SBA-15
SBA-15
9
3
9
100± 1.19
27.0 ± 4.20
90.0 ± 1.60
80/20
100/0
83/17
1000
270
111.1
90.0
IC5
IC6
IC6
900
100.0
[a] Reaction conditions: 1-octene (0.5 mmol); IO(OH)5, (2.5 mmol); 25 °C; 0.1 mol% Ru catalyst loading [b] 1-octene
consumption. This is the average of at least two runs. [c] % heptaldehyde vs. heptanoic acid formed [d] TON = mmol octene
consumed /mmol catalysts [e] TOF = mmol octene consumed/mmol catalyst/hour
Table 3: Oxidative cleavage of other alkenes with immobilized catalyst, IC4.[a]
Entry
Substrate
Time (hrs)
Substrate conv.
Products (Selectivity %)
(%)
100
100
Styrene
Styrene
0.5
1
Benzaldehyde (100)
Benzaldehyde (93.4)
Benzoic acid (6.6)
1
2
Cyclopentene
Cyclopentene
0.5
1
100
100
Glutaraldehyde
3
4
Glutaraldehyde (37.7%),
5-hydroxypentanoic acid (47.3%)
and glutaric acid (15.0%)
1-octene
3
50
Heptaldehyde (91.7)
Heptanoic acid (8.3)
5
[a] Reaction conditions: Water (2.5 mL), CCl4 (1.25 mL) and MeCN (1.25 mL); 1-octene (0.5 mmol); IO(OH)5, (2.5 mmol);
25 °C; 0.1 mol% Ru catalyst loading
27