Water decontamination with hydrogen production using microwave-formed minute-made ruthenium catalysts

Article abstract of DOI:10.1039/c5gc01798j

A method for the decontamination of water, with concomitant hydrogen formation, is herein described. Formaldehyde is an impurity that is often present in industrial wastewater in significant quantities. The formaldehyde decomposition is possible with a series of ruthenium catalysts which are accessible within minutes via microwave-assisted synthesis.

Full text of DOI:10.1039/c5gc01798j

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Water decontamination with hydrogen production
using microwave-formed minute-made ruthenium
catalysts†
Cite this: DOI: 10.1039/c5gc01798j
Leo E. Heim, Simona Vallazza, Dominic van der Waals and Martin H. G. Prechtl*
Received 4th August 2015,
Accepted 6th November 2015
A method for the decontamination of water, with concomitant hydrogen formation, is herein described.
Formaldehyde is an impurity that is often present in industrial wastewater in significant quantities. The
formaldehyde decomposition is possible with a series of ruthenium catalysts which are accessible within
minutes via microwave-assisted synthesis.
DOI: 10.1039/c5gc01798j
www.rsc.org/greenchem
carbon dioxide.9 One of the key reasons for their ubiquitous
nature in catalysis is the robustness of the compounds
Introduction
Formaldehyde is a ubiquitous product in natural environ- formed; this is due to the strong arene–ruthenium bond. This
ments due to its high solubility in water (∼400 g L−1) and its stability coupled with a plethora of inexpensive and readily
production from the bio-degradation of organic matter. Whilst available aryl ligands, many of which are frequently sourced
formaldehyde occurs naturally, there are many industrial pro- from natural products, gives rise to a wide availability of versa-
cesses that can increase the levels of formaldehyde in the tile potential catalysts.
wastewater to dangerous levels including in particular; wood
Our current interest, within the field of homogeneous
processing, paint and resin industries and paper manufactur- ruthenium catalysis, was initiated by the discovery that the
ing.1,2 Removal of this formaldehyde contamination is essential commercially available catalyst, [Ru(p-cymene)Cl2]2, could be
to mitigate environmental damage and to ensure the availability used for the rapid release of hydrogen gas from aqueous for-
of safe, non-toxic water supplies. Challenges remain however to maldehyde solutions as depicted in Fig. 1.10 Of note from this
identify an inexpensive method for this water purification, this report was that methanediol, favourably produced from the
is especially important as formaldehyde has been shown to be addition of formaldehyde to water, could release a high quan-
particularly toxic to aquatic organisms.3
tity (8.4 wt%) of hydrogen. The requirement for novel and
Whilst many different routes to mitigate the impact of effective methods for hydrogen production and storage is an
formaldehyde release have been investigated including; sul- expanding research area due to the need for alternatives to
phite trapping4,5 heterogeneous catalytic wet oxidation6 as well fossil fuels in order to mitigate the damage of anthropologic
as aerobic7 and anaerobic2 bio-reactors; there has been rela- climate change. Reviews into the area of hydrogen storage-
tively little research into the use of homogeneous or supported and-release systems in general11–13 and others that focus
molecular catalysts. Investigating the ability to remove the specifically on the use of C1 organic compounds as suitable
toxic formaldehyde from wastewater using a highly active, and storage materials have recently been published.14–17 Whilst
ideally recyclable system, is therefore of great interest. The con- many of the methodologies covered in these reviews require
version of the hydrogen-rich waste effluence into the valuable harsh reaction conditions or have considerable technical
commodity of hydrogen gas would additionally be beneficial.
obstacles hindering the facile release of hydrogen; our pre-
Ruthenium arene complexes have been widely used in the viously reported catalytic system allows for hydrogen pro-
field of homogeneous catalysis for many years and have been
employed for applications as varied as asymmetric transfer
hydrogenation8 through to the functionalization of aqueous
Department of Chemistry, Institute of Inorganic Chemistry, University of Cologne,
Greinstraße 6, 50939 Cologne, Germany. E-mail: martin.prechtl@uni-koeln.de;
http://catalysis.uni-koeln.de; Fax: +49 221 4701788
†Electronic supplementary information (ESI) available: Experimental protocols
Fig. 1 Ruthenium catalysed hydrogen release from aqueous
and data. See DOI: 10.1039/c5gc01798j
formaldehyde.
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duction from aqueous formaldehyde solutions at low tempera-
tures without the need for any additives.10
Having noted the rapid dehydrogenation of aqueous form-
aldehyde, it has been speculated whether the methodology
could be expanded to become applicable for the catalytic
removal of formaldehyde contamination, in wastewater, with
the concomitant production of hydrogen. In order to identify
the optimum arene–ruthenium system however, a series of
analogous tailored ruthenium catalysts was needed. Current
established methodologies for the formation of these catalysts,
whilst effective, can require long reaction times and high
temperatures to achieve only moderate yields in some cases.18
This has encouraged us to investigate the advantages that can
be attained through the application of microwave irradiation
for the synthesis of these compounds.
Fig. 2 Synthetic routes for the formation of aryl ruthenium halide
dimers.
Microwave heating has long been used in organic synthesis
and is considered to be of synthetic value due to the consider-
ably shorter reaction times required. It has been used for
diverse reactions such as; nanoparticle formation,19,20 polymer
synthesis21 and application in various bio-reactions.22,23 The
use of microwave irradiation has the effect of significantly
reducing the energy required to conduct a reaction, an effect
that is reported to increase upon scale-up towards industrial-
level usage.24 In addition, further fulfilment of the green
chemistry principals25 can be achieved as the reactions are
generally cleaner, due to the short reaction times hindering
the progression of side reactions; this thus produces less waste
products in addition to requiring less purification.24
reductive diene addition methodology,18 through thermally
driven arene-exchange reactions and additionally through the
complexation of aryl ethers produced in situ.28
The chloro–ruthenium–arene complexes; entries 1–5 in
Table 1, have been synthesised in a microwave reactor by
heating the correspondent cyclohexadiene derivatives,
obtained commercially or via Birch-reductions, with ethanolic
hydrated ruthenium chloride. Good to excellent yields were
reported after short reaction times of four minutes. The syn-
thesis of analogous bromo- and iodo-versions of the [Ru-
(p-cymene)Cl2]2 catalyst was also attempted, entries 6 and 7 in
Table 1. For the bromo-compound, hydrated ruthenium
bromide was used as precursor and only a moderate yield of
the desired catalyst was recovered. Ruthenium iodide arene
complexes by comparison are commonly formed in a two step
process involving production of the ruthenium aryl chloride
The use of microwave enhanced reactions for the pro-
duction of ruthenium arene ligands is
a much under
researched field. An initial report in the area by Mingos et al.
described the production of only the [Ru(p-cymene)Cl2]2 aryl-
ruthenium compound in moderate yields with the best results
being reported with the use of a microwave autoclave.26
A
recent paper by Severin and co-workers has built upon the
work to describe a methodology for the formation of [Ru-
(p-cymene)Cl2]2 without the need for the degassing of the
solvent and with better reported yields.27 The production of
ruthenium arene complexes with alternative aromatic character
and modifications to the bridging functionalities has, however,
yet to be explored using microwave assisted heating techniques.
We report herein a rapid and efficient route for the micro-
wave assisted synthesis of a range of arene ruthenium catalysts
and the result of investigations into their activity towards the
decomposition of aqueous formaldehyde down to trace
amounts, <30 ppm.
Table 1 Reaction conditions and results for the microwave assisted
synthesis of [RuX2(arene)]2 complexes
Temp. Time Yield
Entry Complex
[°C]
[min] [%]
a,b
1
2
3
4
5
6
7
8
[RuCl2(benzene)]2
130
130
130
130
130
130
130
130
90
4
4
4
4
4
90
34
20
30
30
30
88
93
77
100
83
41
30
42
70
69
64
a,b
[RuCl2(toluene)]2
[RuCl2(mesitylene)]2
a,b
a,b
[RuCl2(C6H5-CH2CH2OH)]2
a,b
[RuCl2(p-cymene)]2
[RuBr2(p-cymene)]2
a,b
a,b,c
[RuI2(p-cymene)]2
[RuCl2(anisole)]2
a,d
a,e
9
10
11
[RuCl2(C6H5-OCH2CH2OH)]2
[RuCl2(1,2,4,5-tetramethylbenzene)]2
f
200
200
Results and discussion
Synthesis of [RuX2(arene)]2 complexes
f
[RuCl2(C6Me6)]2
a From
hydrated
ruthenium(III)–chloride
or
bromide
and
cyclohexadienes. b Ethanol as solvent, 10 equiv. 1,4-diene. c From
hydrated ruthenium(III)–chloride and phellandrene heated for 4 min,
subsequent addition of 10 equiv. of NaI and heating for a further
30 min. d Methanol as solvent, 10 equiv. 1-methoxycyclohexa-1,4-diene.
e Glycol as solvent, 10 equiv. 1-methoxycyclohexa-1,4-diene. f Ligand
exchange from [RuCl2(p-cymene)]2 with 10 wt equiv. of arene.
The microwave assisted synthetic methodology described
below, offers a considerable improvement for the synthesis of
a variety of [RuX2(arene)]2 complexes. It has been shown to
expedite the formation of the catalysts through three key
routes as depicted in Fig. 2, namely; via the well established
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from hydrated ruthenium chloride along with a cyclohexa- catalysts 1–11, their performance was initially examined at
diene; subsequent halogen exchange is then conducted using 95 °C with different catalyst loadings. With the exception of
an excess of sodium iodide to give the product.18 The use of the bromo- and iodo-substituted analogues, all the investi-
this two step process can be much expedited by microwave gated catalysts were shown to be able to reduce the formal-
irradiation as shown in Table 1, entry 7; reducing heating dehyde content of diluted solutions (5 wt%) to levels below
times from 5 h to 34 minutes. Attempts to produce the Ru- 45 ppm (conv. >99.9%) within 24 h, using catalyst loadings of
(p-cymene)I2]2 in a one-step procedure led only to intractable both 1.0 mol% and only 0.1 mol%. For better comparison of
mixtures.
the catalyst activities we conducted the decomposition reac-
An alternative route for the formation of functionalized tions catalysed by 1–11 at 70 °C (Table 2).
arene ruthenium complexes, first reported through conven-
The least efficient catalyst was seen to be the iodo-ruthe-
tional heating by White and co-workers, concerns the com- nium(p-cymene) analogue, Table 2 entry 7. This is thought to
plexation of aryl ethers. In this interesting paper they note the be due to solubility problems with the catalyst as even at
use of different alcohols, other than methanol, as the reaction 0.1 mol% loadings the complex was not completely soluble.
medium, when attempting to complex the reduced anisole With the reaction solution already saturated at 0.1 mol%, the
diene to ruthenium chloride. This led to the incorporation of addition of more catalyst does not have a significant effect;
the solvent alcohol into the aryl unit of the ruthenium catalyst precluding direct comparison between the bridging ligands.
in the place of the methoxy group. Entry 8 in Table 1 shows
Among the arene ligands it was noted that deviations, away
that the methyl–aryl ether complex can be formed in a med- from the standard p-cymene ligated complex, including both
iocre isolated yield. Interestingly when conducting the reaction increases and reductions in the number of substituents were
in glycol the 2-phenoxyethanol ligated complex was formed; seen to retard the reaction. The electronic nature of the arene
this leaves a versatile chemical handle for further potential substituent does not appear to play a significant role as there
modifications, such as immobilization, as well as improving are no notable differences in catalytic activity seen after the
the water solubility of the complex.
duration of the reaction between the complexes with the
Whilst the diene-route is the most commonly used toluene ligand and the ether–ligands. One potential benefit
methodology for arene–ligand attachment, it is not suitable from this wide reaction tolerance, with relation to ligand modi-
for complexation of ligands with a high degree of substitution. fications could be the ability to use the ligands for additional
This is due to the increased electron density of the aromatic purposes, such as catalyst immobilisation, without interfering
ring impeding the Birch-reduction. In some instances direct with the reaction.
arene ligand exchange reactions were instead required to form
To further investigate the reaction profiles of the catalysts
the catalytic complexes. Following an adapted protocol from the initial released gas-flow was monitored. From these initial
Bennett et al.,18 the [RuCl2(arene)]2 complexes seen in entries gas production rates the initial rate of formaldehyde decompo-
10 and 11 in Table 1, were produced by heating the [Ru(p- sition for each catalyst could be determined, allowing for
cymene)Cl2]2 substrate with 10 weight equivalents of the direct comparison as shown in Table 3.
respective ligands. All of the reactions were performed without
the use of protective gas atmospheres or dry solvents.
The turnover frequencies (TOFs) of the catalysts were calcu-
lated shortly after initiation of the reaction, the results of
Notably the required reaction times were substantially which along with the turnover numbers (TONs) after the reac-
shorter, than for the classic method of refluxing the reagents, tion had run to completion are given in Table 3. These clearly
whilst the yields were comparable or in many cases better
(‡comparative examples are given in the Notes and reference
section).18 An example, in which microwave irradiation can be
Table 2 Catalytic formaldehyde decomposition at 70 °C
seen to be beneficial is in the case of the formation of
Complex 8; this requires 30 hours of conventional heating
under reflux to give an isolated yield of 25% and by the for-
mation of the well known ruthenium (para-cymene) dichloride
dimer requiring 4 hours of heating, again under reflux, by tra-
ditional means, to yield only 64% of the isolated product.18
Conv. [%]
with 1 mol%
cat.a
Conv. [%] with
0.1 mol% cat.a
Entry Complex
1
2
3
4
5
6
7
8
9
[RuCl2(benzene)]2
[RuCl2(toluene)]2
[RuCl2(mesitylene)]2
85
90
85
50
50
50
60
70
70
20
50
50
[RuCl2(C6H5-CH2CH2OH)]2 85
The decomposition of aqueous formaldehyde solutions
[RuCl2(p-cymene)]2
[RuBr2(p-cymene)]2
[RuI2(p-cymene)]2
[RuCl2(anisole)]2
[RuCl2(C6H5-
90
85
20
85
85
With these compounds in hand, attention was turned towards
their application as catalysts for the release of hydrogen from
aqueous formaldehyde solutions. For the evaluation of the
OCH2CH2OH)]2
[RuCl2(1,2,4,5-
tetramethylbenzene)]2
[RuCl2(C6Me6)]2
10
11
80
70
20
50
‡Literature precedent for the thermal preparation of exemplary complexes and
the given reaction conditions (T [°C], t [h]) and yields (%): Complex 1 (78 °C, 4 h,
95%);18 3 (78 °C, 16 h, 90%);18 5 (78 °C, 4 h, 65%);18 6 (78 °C, 4 h, 53%);18
(65 °C, 30 h, 25%);18 9 (80 °C, 3 h, 66%);23 11 (185 °C, 2 h, 80%).30
8
a Measured after 24 h, starting concentration: 5 wt% HCHO.
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Table 3 Catalyst turnover numbers and frequencies for formaldehyde
Table 5 Effects of additives on the decomposition of formaldehyde
decomposition
solutions at 95 °C
TOFa
(h−1
TON with
0.1% cat.b
Entry
Additivea
Reaction time (h)
Conversion [%]
Entry
Complex
)
1
2
3
4
5
6
7
8
No additive
No additive
KCl
KCl
KBr
KBr
KI
2
24
2
24
2
24
2
24
50
1
2
3
4
5
6
7
8
[RuCl2(benzene)]2
[RuCl2(toluene)]2
[RuCl2(mesitylene)]2
[RuCl2(C6H5-CH2CH2OH)]2
[RuCl2(p-cymene)]2
[RuBr2(p-cymene)]2
[RuI2(p-cymene)]2
[RuCl2(anisole)]2
[RuCl2(C6H5-OCH2CH2OH)]2
[RuCl2(1,2,4,5-tetramethylbenzene)]2
[RuCl2(C6Me6)]2
1284
2694
2092
1546
3142
2132
613
1057
738
1612
1508
500
500
500
600
700
700
200
500
500
200
500
99.92–99.93b
40
99
10
98
10
82
c
KI
a Starting concentration: 5 weight% HCHO; 1.0 equiv. of additive used.
b Analysed with the most sensitive formaldehyde test (see ESI);
this conversion correspondence to trace amounts of 35–40 ppm
formaldehyde.
9
10
11
a Measured after 3 min. b Measured after reaction completion, starting
concentration: 5 wt% HCHO. c Purchased from Sigma Aldrich.
ability of the system to decompose aqueous formaldehyde.
The results, reported in Table 5, clearly show that whilst the
addition of potassium halides do impair the decomposition,
particularly in the case of potassium iodide; decomposition
can proceed to low concentrations of formaldehyde when
given sufficient reaction times. A highly important parameter
to be considered in the treatment of wastewater is the ability to
use the methodology at a variety of solution acidities. In
Table 6. the results for decontamination tests, conducted at
different pH-levels so as to determine both the versatility and
robustness of the reaction, are shown. The results report the
concentrations of formaldehyde remaining when the opti-
mised [Ru(p-cymene)Cl2]2 catalyst was used at 95 °C over both
short and longer reaction frames in various pHs. It is apparent
that the catalytic system is stable, even at the higher reaction
temperature, across a wide range of pH values although efficacy
decreases in more acidic conditions. The utilisation of the more
accurate method for determining contamination at very low
concentrations of formaldehyde showed that the system is both
highly efficient as well as robust enough to endure the pro-
longed reaction times in both acidic and basic medium.
shows that the Ru(p-cymene)Cl2]2 complex exhibits both the
highest turnover number along with the greatest turnover fre-
quency which, in addition to the benefit of the catalyst being
the least expensive, has lead to subsequent investigations focus-
ing primarily on the use of the [Ru(p-cymene)Cl2]2 catalyst.
Further optimisation for the [Ru(p-cymene)Cl2]2 catalyst
was undertaken, the results of which are reported in Table 4.
As might be expected it was seen that the higher reaction
temperatures yielded the greatest levels of formaldehyde
decomposition with even the lower catalyst loadings reducing
the contaminant to less than 0.5 mg mL−1 within 24 hours.
Conversely reducing the temperature increased the contami-
nation levels seen, particularly at low catalyst loadings. One
particularly salient result however is reported in entry 5 of
Table 4; this shows that almost complete removal of form-
aldehyde can be achieved at room temperature, both higher
catalyst loadings and longer reaction times were however
required.
As mentioned it was seen that the iodo-analogue of the
ruthenium(p-cymene) dichloride catalyst showed considerably
less reactivity for the decomposition of formaldehyde; this is
despite having a similar electronic configuration to the chlor-
ide catalyst. It was decided therefore to investigate the robust-
ness of the catalytic system and to determine whether the
addition of halogen ions had any negative effect upon the
Catalyst separation and recyclability
One of the significant issues to address within the field of
water decontamination is the immobilization or separation of
the catalyst. This can be important due to the potential toxicity
of the catalyst as well as the economical benefit of being able
to reuse the metal catalyst.
Table 4 [Ru(p-cymene)Cl2]2 catalysed decomposition of formaldehyde
solutions
Table 6 Formaldehyde decomposition reactions at various pH levels
Entry
Temperature [°C]
Catalyst [mol%]
Conversiona [%]
Conv. [%]
Entry pH after 2 h
Conv. [%] after
24 ha
HCHO traces after
24 ha
1
2
3
4
5
70
70
95
95
27
0.1
1.0
0.1
1.0
10
85
90
99.5
99.5
99.5b
1
2
3
4
10
7
60
60
99.95–99.98
99,92–99,95
99,92–99,95
99.6–99.7
10–25 ppm
25–40 ppm
25–40 ppm
200–300 ppm
4–5 85
50
3
a Measured after 24 h, starting concentration:
b Concentration of formaldehyde detected after 120 h.
5
wt% HCHO. a Monitored with the most sensitive formaldehyde test (see ESI);
starting concentration: 5 wt% HCHO.
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Two primary methods were used to analyse the recyclability
of the catalyst; namely sample recharge reactions and biphasic
separation.
Table 8 Biphasic catalysis recycling experiments with EMIM NTf2 IL
Catalyast cycle
Conversiona
Cycle TON
Cumulative TON
Recharge-experiments were employed to determine the
ability for the catalyst to maintain its efficacy after several uses
and at high catalyst turnover numbers. Table 7 details the
results obtained using the optimum catalytic system over mul-
tiple catalytic cycles with removal of the purified water and
recharging with formaldehyde solution. Whilst very high con-
versions could be seen over the initial three cycles; beyond this
extent of recycling the decomposition of the catalyst led to
unreliable inconsistencies between duplicated reactions. The
cycle TON increases in each cycle to account for the estimated
catalyst loss due to the removal of aliquots for analysis.
For the investigation into the applicability of a biphasic
system, numerous common organic solvents that are immisci-
ble with water were investigated including 2-Me-THF which
has been reported by Leitner et al. to enable the separation of
a cationic ruthenium triphos complex from the aqueous reac-
tion phase.29 Disappointingly in all these attempts significant
catalyst leaching, into the aqueous phase, was noted upon
cooling and separation. So as to avoid this ruthenium loss, the
use of ionic liquids as solvents for the catalyst was investigated.
Initial investigations revealed that the use of particularly
hydrophobic ionic liquids (ILs) was required and those with a
triflimide (−NTf2) anion showed the best separation from the
aqueous phase whilst remaining active. Imidazolium cations
with highly hydrophilic side chains, such as ones containing
vicinal alcohols did not, however, form biphasic mixtures upon
addition to water, regardless of the counterion. Four suitable
ionic liquids were compared for their applicability in the reaction
for the decomposition of formaldehyde, the results of which are
reported in the ESI† and reveal N-ethyl-N-methyl-imidazolium
triflimide (EMIM NTf2) to be the optimal ionic liquid.
1
2
3
4
5
6
85%
83%
88%
91%
80%
82%
43
41
44
46
40
41
43
84
128
174
214
255
a Measured after 24 h at 95 °C, starting concentration: 5 wt% HCHO.
Catalyst loadings of 0.0481 g (2.0 mol%) [Ru(p-cymene)Cl2]2 used.
A background reaction, with the exclusion of the ruthenium
catalyst, was conducted to ensure that the formaldehyde was
not simply leaching into the IL and it was found that in the
absence of catalyst the aqueous phase remains at 5 wt% for-
maldehyde solution after heating for 24 h. Table 8 shows the
high conversions seen throughout the repeat use of the catalyst
suspended in the ionic liquid phase. Further research is focus-
ing on the optimisation of the use of ionic liquids as well as
investigating alternative methods for catalyst immobilization
which allow for water purification down to the ppm levels seen
with the homogenous monophasic systems.
Conclusions
We have described herein a robust and efficient method for
the decontamination of formaldehyde containing wastewater
with the concomitant production of hydrogen gas. Form-
aldehyde impurities can be decomposed down to trace
amounts as low as 10–25 ppm, thus below hazardous levels.
The optimised system makes use of arene ruthenium halide
complexes which are rapidly formed through the use of micro-
wave irradiation in solvents from renewable sources. The toler-
ance of the system, with respect to halogen sources as well as a
variety of temperatures and pH values, has been demonstrated.
Further investigation into the recyclability of these molecular
catalysts has shown that repeated use through recharge experi-
ments result in the maintenance of high levels of catalyst
efficiency. Ionic liquids have been revealed to be highly
effective for creating a biphasic system for catalytic form-
aldehyde decomposition with negligible levels of catalyst
leaching seen even after multiple recycling cycles. Improved
immobilization of the catalysts through various routes is cur-
Taking the most effective biphasic catalyst system in hand,
recyclability experiments were conducted using a greater cata-
lyst loading to determine the potential for the repeated use of
the biphasic system. After pre-forming what is postulated to be
the active ruthenium-formiato catalyst system in the N-ethyl-N-
methyl imidazolium triflimide ionic liquid using formic acid,
the initial aliquot of 5 wt% formaldehyde solution was added.
This sample was then heated at 95 °C for 24 h, the aqueous
phase separated, analysed and replaced with a fresh aliquot of
formaldehyde solution.
Table 7 Recharge experiment for the catalytic decomposition of rently being investigated to determine the optimum method
formaldehyde
for catalyst retention with high catalytic efficiency.
Catalyst
cycle
Detected
FA conc.
Cycle
TONb
Cumulative
TON
Conversiona
Experimental
Catalyst synthesis
1
2
3
<300 ppm
<300 ppm
<300 ppm
>99%
>99%
>99%
99
103
108
99
202
310
Experimental procedure for the formation of catalysts entries
1–9 in Table 1. A 30 mL microwave vessel was filled with
0.96 mmol of hydrated ruthenium(III) halide, 10 equiv. of the
a Measured after 24 h at 95 °C, starting concentration: 5 wt% HCHO.
0.0240 g (1 mol%) [Ru(p-cymene)Cl2]2 catalyst used. b 0.1 mL aliquots
taken after each run.
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desired cyclohexadiene and 15.5 mL ethanol. For entries 8 & 9
in Table 1 1-methoxy-1,4-cyclohexadiene was used as the diene
along with the corresponding alcohol. The reaction mixture
was heated to 130 °C, except for 9 which required 90 °C, for
the reaction times indicated in Table 1. Afterwards the reaction
mixture was cooled down to −78 °C, the precipitated complex
was filtered off and washed with pentane (20 mL) and dried in
air. The yields for complexes 1–9 are 30–100%. For the catalysts
formed in entries 10 & 11; a 30 mL microwave vessel was filled
with 0.2 grams of [Ru(p-cymene)Cl2]2, 10 wt equiv. (2.0 grams)
of the desired arene was added without solvent and the reaction
mixture was heated to 200 °C for 30 minutes. Excess arenes
were removed by soxhlet extraction with pentane overnight and
the catalyst recovered from DCM in vacuo in 64–69% yield.
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Exemplary experiment: 4.0 μmol (0.1 mol%) [RuCl2(p-cymene)]2
was added along with 2.5 g (4 mmol) of an aqueous form-
aldehyde solution (5.0 wt% formaldehyde) into an open reac-
tion vessel. The reaction mixture was heated with stirring to
95 °C for 24 h. An aliquot (200 μL) of the reaction mixture was
used to determine the residual formaldehyde content after
dilution using the Merck Millipore formaldehyde tests. For the
biphasic reactions; 0.0481 g (0.08 mmol) of [Ru(p-cymene)Cl2]2
was dissolved in 1 mL of ionic liquid. 0.645 mL of formic acid
was added to the reaction vial and heated at 70 °C for 3 h to
form the active catalyst within the IL phase. 2.5 mL of 5 wt%
aqueous formaldehyde solution was added and the reaction
mixture heated to 95 °C for 24 h. Upon completion the
aqueous phase of the mixture was separated, once the reaction
had cool to room temperature, then submitted to the same
formaldehyde quantification procedure. Catalyst Turnover Fre-
quencies (TOFs) were measured using a flow meter to deter-
mine the initial volumes of gas release with time. These could
then directly be converted into catalyst TOFs as it has pre-
viously been seen that the initial gas release is solely due to
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Acknowledgements
We gratefully acknowledge financial support provided by the
Ministerium für Innovation, Wissenschaft und Forschung
(NRW-returnee award 2009 to M.H.G.P.), the Heisenberg-
Program (Deutsche Forschungsgemeinschaft), the COST
Action “Catalytic Routines for Small Molecule Activation
(CARISMA)” and the Ernst-Haage-Prize 2014 of the Max-Planck
Institute for Chemical Energy Conversion. A patent has been
filed under 10 2013 011 379.2 at the German Patent Office
DMPA and extended to PCT in 2014.
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K. M. Thenert, J. Kothe, T. Vom Stein, U. Englert,
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