Minor Modifications to the Ligands Surrounding a Ruthenium Complex Lead to Major Differences in the Way in which they Catalyse the Hydrogenation of Arenes

Article abstract of DOI:10.1002/adsc.200390014

The hydrogenation of benzene and other arenes under aqueous-organic biphasic conditions is evaluated using the ruthenium complexes Ru(η ⁶-C₁₀H₁₄)(pta)Cl₂ (pta = 1,3,5-triaza-7-phosphaadamantane), Ru(η⁶-C₁₀H ₁₄)(tppts)Cl₂ (tppts = tris-3-sulfonatophenylphosphine trisodium salt) and [Ru(η⁶-C₁₀H₁₄)(pta) ₂Cl]⁺. The active catalysts formed during the hydrogenations correspond to a trinuclear cluster, a colloid and a mononuclear complex, respectively.

Full text of DOI:10.1002/adsc.200390014

FULL PAPERS
Minor Modifications to the Ligands Surrounding a Ruthenium
Complex Lead to Major Differences in the Way in which they
Catalyse the Hydrogenation of Arenes
a,
b
a
¬
Paul J. Dyson, * David J. Ellis, Gabor Laurenczy
a
À
¬
¬ ¬
Institut de chimie moleculaire et biologique, Ecole polytechnique federale de Lausanne, EPFL BCH, 1015 Lausanne,
Switzerland
Fax: ( ‡ 41)-21-693-98-85, e-mail Paul.Dyson@epfl.ch.
b
Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
Received: June 3, 2002; Accepted: October 5, 2002
‡
Abstract: The hydrogenation of benzene and other C10H14)(pta)2Cl] . The active catalysts formed during
arenes under aqueous-organic biphasic conditions is the hydrogenations correspond to a trinuclear cluster,
evaluated using the ruthenium complexes Ru(h6- a colloid and a mononuclear complex, respectively.
C10H14)(pta)Cl2 (pta ˆ 1,3,5-triaza-7-phosphaadaman-
tane), Ru(h6-C10H14)(tppts)Cl2 (tppts ˆ tris-3-sulfona- Keywords: arenes; biphasic catalysis; cluster catalysis;
tophenylphosphine trisodium salt) and [Ru(h6- hydrogenation; P-ligands; ruthenium
‡
Introduction
Cl)Ru(h6-C6Me6)] are effective catalysts, but catalyst
decomposition is observed and this is often indicative of
The reduction of arenes is an important reaction in colloid formation.[9] The bis-hexamethylbenzene com-
organic synthesis, the preparation of cleaner fuels and in plex, Ru(h6-C6Me6)(h4-C6Me6) is also an active catalyst
paper production.[1] While stoichiometric reagents are for arene hydrogenation, but whether or not the catalyst
often used arene hydrogenation catalysts are becoming is homogeneous remains a matter of some doubt.[10] The
increasingly employed and they are dominated by the stereoselectivity was not particularly high for the hydro-
platinum group metals, especially the elements ruthe- genation of xylenes and cyclohexenes were obtained in
nium and rhodium. The majority of catalysts are some quantity from the hydrogenation of benzene. One
heterogeneous, and although a number of molecular of the most recent homogeneous arene hydrogenation
compounds have been reported to homogeneously catalysts based on ruthenium is RuH2(H2)2(PCy3)2,
catalyse the hydrogenation of arenes, many have since however, the addition of mercury to the reaction
been found to undergo transformations to colloids reduced the activity to zero, thereby suggesting that
during reaction. As such, the whole concept of homoge- the active catalyst is colloidal.[11] The complex [Ru(h6-
‡
neous arene hydrogenation catalysis has been called C10H14)(h2-triphos)Cl] has been shown to catalyse the
into question.[2]
hydrogenation of arenes in dichloromethane and ionic
It would appear that the majority of rhodium catalysts, liquids.[12] It would appear to act in a homogeneous
including [Rh(h5-C5Me5)Cl2]2,[3] [Rh(h4-C6H8-1,4)Cl]2,[4] fashion although no decomposition or loss of activity is
Rh(h4-C8H10-1,5)(Ph2PCH2COO)
and observed in the ionic liquid and some decomposition
Rh(CO)2(Ph2PCH2COO)[5] form colloids which cata- takes place in dichloromethane. In this paper we show
lyse the hydrogenation of arenes. These compounds are that closely related water-soluble ruthenium complexes
either not active for the hydrogenation of polymer- can catalyse the hydrogenation of arenes in very differ-
bound benzene derivatives,[6] or turnovers vary with ent ways.
time;[7] both phenomena are indicative of colloidal
catalysis. As a prominent example, [RhCl4]À was initially
believed to be a homogeneous arene hydrogenation Results and Discussion
catalyst, but later catalytic activity was shown to be
based on rhodium nanoclusters.[8]
The compounds Ru(h6-C10H14)(pta)Cl2 1 (pta ˆ 1,3,5-
The possibility that ruthenium complexes act as triaza-7-phosphaadamantane), Ru(h6-C10H14)(tppts)Cl2
homogeneous arene hydrogenation catalysts seems 2 (tppts ˆ tris-3-sulfonatophenylphosphine trisodium
‡
more
C6Me6)(PPh3)(H)Cl
promising.
The
and
complexes
Ru(h6- salt) and [Ru(h6-C10H14)(pta)2Cl] 3 (Scheme 1) were
[(h6-C6Me6)Ru(m-H)2(m- prepared for evaluation as catalysts for the hydrogena-
Adv. Synth. Catal. 2003, 345, No. 1+2
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1615-4150/03/3451+2-211 215 $ 17.50+.50/0
211

Paul J. Dyson et al.
FULL PAPERS
rise to two further singlet resonances, a peak at 4.53 ppm
may be assigned to the CH2 protons in the nitrogen
heterocycle and the peak at 4.32 ppm to the CH2 protons
in the phosphorus-nitrogen heterocycle.
Ru
Ru
Ru
Cl
Cl
Cl
Cl
Cl
N
N
P
P
P
SO3Na
P
N
N
N
N
N
N
N
NaO3S
NaO3S
1
2
3
Catalytic Evaluation of 1 3
Scheme 1.
Compounds 1 3 are highly water-soluble and were
immobilised in water and tested for activity as pre-
tion of arenes under biphasic conditions. A number of catalysts for the hydrogenation of arenes. The results
ruthenium-pta[13] and -tppts[14] complexes have shown to from these studies are listed in Table 1. The two
catalyse the reduction of other substrates in water. complexes with pta ligands, viz. 1 and 3, exhibit similar
Compound 1 is known,[15] whereas 2 and 3 have not been turnover frequencies, although 1 is more active than 3,
reported previously. Compound 2 was prepared in an whereas the tppts complex 2 is considerably more active
analogous manner to 1, viz. from the direct reaction of than either of the pta complexes.
the dimer [Ru(h6-C10H14)Cl2]2 with two equivalents of
The reactions were repeated in the presence of
tppts in methanol. The negative ion electrospray mass mercury and with 1 and 3 no significant change in
spectrum of 2 exhibits a single peak at m/z 851 which activity was observed. In contrast, the addition of
corresponds to the ion [Ru(h6-C10H14)(tppts)Cl2 Na]À mercury to the hydrogenations using 2 result in signifi-
due the loss of a sodium ion from the tppts ligand. The cantly reduced turnovers (see Table 2), which suggests
31P{1H} NMR spectrum of 2 in D2O shows a singlet that the activity is nearly entirely colloidal. 31P NMR
resonance at 34.81 ppm which corresponds to co- spectroscopy shows that the tppts dissociates, and since
ordinated tppts. The 1H NMR spectrum of 2 in tppts can act as a surfactant,[17] it is not unreasonable to
CD3OD shows a broad multiplet at 8.30 7.60 ppm, assume that it stabilises the colloids in aqueous solution.
which can be attributed to the tppts ligand phenyl rings. In other hydrogenations, e.g., CO2 reduction in aqueous
A further multiplet at 5.90 5.30 ppm corresponds to solution under similar conditions, there was no evidence
the aromatic ring protons of the p-cymene ring. The for colloid formation using the same complex.[18]
distinctive septet resonance at 2.55 ppm (J ˆ 7.32 Hz) is
Compounds 1 and 3 would appear to act as homoge-
readily assigned to the single CH proton of the isopropyl neous catalysts for the hydrogenation of arenes. Exami-
grouping of the p-cymene ligand. A doublet at 0.99 ppm nation of 1 after reaction by NMR spectroscopy and
(J ˆ 7.11 Hz) can be assigned to the methyl group electrospray mass spectrometry shows that it undergoes
proton of the same group. One final singlet resonance a transformation to a triruthenium cluster that does not
at 1.77 ppm occurs due to the methyl group attached contain any pta. The 1H NMR spectrum of the resulting
directly to aromatic ring.
complex shows that only the p-cymene ring remains
Compound 3 is derived from 1 by reaction with one attached. Loss of the phosphine is confirmed by
equivalent of pta together with one equivalent of silver 31P{1H} NMR spectroscopy, which shows that the phos-
tetrafluoroborate in dichloromethane. It is not too
dissimilar from the tppts complexes [Ru(h6-are-
Table 1. The hydrogenation of various arene substrates using
3 immobilised in water.[a]
‡
ne)(tppts)2H] described previously, although prepared
1
by a different route.[16] The positive ion electrospray
mass spectrum of 3 contains a peak at m/z 585 which
Substrate
Catalyst
Turnover
‡
corresponds to the cation [Ru(h6-C10H14)(pta)2Cl] and
(mol molÀ1 hÀ1)
a lower intensity peak at m/z 428 due to the presence of
C6H6
1
1
1
2
2
2
2
3
3
3
170
130
122
488
365
245
98
150
129
72
‡
[Ru(h6-C10H14)(pta)Cl] derived from the molecular ion
C6H5Me
C6H5Et
C6H6
by loss of a pta group. The 31P{1H} NMR spectrum of 3 in
CDCl3 contains a singlet at À 32.97 ppm, which corre-
sponds to the co-ordinated pta ligands. Two sets of well-
defined peaks corresponding to the p-cymene ring and
the pta ligand are observed. A quartet at 5.45 ppm (J ˆ
6.42 Hz) can be assigned to the aromatic ring protons of
the co-ordinated p-cymene ring. The distinctive septet
of the p-cymene ligand at 2.79 ppm (J ˆ 6.55 Hz). The
methyl group protons of the isopropyl group give rise to
a doublet at 1.28 ppm (J ˆ 6.55 Hz) and the singlet
resonance at 2.08 ppm can be assigned to the remaining
methyl groupattached to the ring. The pta ligand gives
C6H5Me
C6H5Et
i
C6H5 Pr
C6H6
C6H5Me
C6H5Et
[a]
Reaction conditions: catalyst (30 mg) in water (10 mL),
substrate (1 mL), H2 (60 atm) 90 8C, 1 h. Products formed
were completely hydrogenated cyclohexane analogues of
the respective substrate.
212
Adv. Synth. Catal. 2003, 345, 211 215

Modifications to the Ligands Surrounding a Ruthenium Complex
FULL PAPERS
Table 2. The hydrogenation of various arene substrates using
2 immobilised in water with mercury added as a colloidal
poison.[a]
Substrate
Turnover (mol molÀ1 hÀ1)
C6H6
32
4
16
2
C6H5Me
C6H5Et
C6H5-i-Pr
[a]
Catalyst (64 mg 0.0733 mmol), water (10 ml), arene sub-
strate (1 mL), mercury (1 mL). Reaction time of 1 hour.
Products formed were completely hydrogenated cyclohex-
ane analogues of the respective substrate. Turnovers are
quoted in number of moles of substrate converted per
mole of catalyst per hour.
phine ligand is no longer present. The positive ion
electrospray mass spectrum exhibits a significant peak
centred at m/z 743 which could correspond to a cluster of
Figure 1. The proposed structure of the catalyst derived from
‡
1, viz. [Ru3(h6-C10H14)3(m3-Cl)] ; and the structures of (b)
‡
formula [Ru3(h6-C10H14)3(m3-Cl)] (in negative ion mode
‡
[Ru3(h6-C6Me6)2(h6-C6H6)(m3-O)(m2-H)3] and (c) [Ru3(h6-
‡
C6Me6)2(h6-C6H6)(m3-O)(m2-H)2(m2-OH)] .
no peaks containing the ruthenium isotopomer are
observed). Electrospray ionisation mass spectrometry
has previously been shown to be very reliable for the
characterisation of water-soluble organometallic clus-
ters.[19] The valance electron count for a cluster of this not correlate to those of the trinuclear clusters reported
formula is 46, two fewer than the number expected for a by S¸ss-Fink.[20] However, it is well known that build-up
trinuclear (triangular) cluster. The electronic situation of clusters in water from related ruthenium species can
here, however, is not too dissimilar from that observed lead to a variety of different products depending upon
for the tetranuclear clusters [H4Ru4(h6-arene)4]2‡, which the conditions.[23]
are also active arene hydrogenation catalysts following
addition of dihydrogen.[20] Unlike the tetraruthenium
cluster, the p-cymene ligands present in the starting
material do not exchange with the arene substrate. Such Conclusions
a situation is more typical of the clusters [Ru3(h6-
‡
C6Me6)2(h6-C6H6)(m3-O)(m2-H)3] ,
and
[Ru3(h6- Homogeneous arene hydrogenation catalysts are few
In these and far between and molecular ruthenium clusters
C6Me6)2(h6-C6H6)(m3-O)(m2-H)2(m2-OH)] .
‡ [21]
clusters it has been proposed that the arene substrate probably represent the most thoroughly characterised
interacts with the face of the triruthenium unit opposite examples. There has also been much discussion about
to the m3-capping ligand, in a way not too dissimilar from the effectiveness of homogeneous cluster catalysis.
the well characterised m3-h2:h2:h2-arene co-ordinated Generally, clusters are not deemed to be as useful as
mode,[22] but involving much weaker interactions. The mononuclear complexes in homogeneous catalytic ap-
proposed structure for the cluster generated from 1 is plications. This is partly due to the complexity of the
shown in Figure 1. We do not think that the cluster cluster reaction mechanism, for example, only very
generated from 1 is the same as those mentioned above recently has direct evidence been presented which
as the turnover numbers differ, and we do not observe shows that catalysis by intact clusters takes place.[24]
hydrides on the material recovered after reaction. In an Whatever the problems associated with cluster catalysis,
attempt to confirm the identity of the active arene for arene hydrogenation reactions small metal clusters
hydrogenation catalyst generated from 1 an in situ NMR have proven to be very effective and they are superior to
experiment was conducted. Compound 1, benzene and their mononuclear counterparts. It is even possible that
water were pressured under 100 atmospheres of hydro- many of the mononuclear complexes that have been
1
gen in an NMR tube. The H NMR spectrum immedi- shown to hydrogenate arenes actually do so after
ately showed an intense doublet centred at À 9.33 (J ˆ transforming into a cluster. While compound 3 des-
57.6 Hz). With time, this doublet decreases in intensity cribed in this paper possibly represents an example of a
and a number of new hydride signals are observed mononuclear homogeneous arene hydrogenation cata-
between À 10 and À 20 ppm. While the identity of these lyst further studies are required in order to confirm this
hydride containing species remains unknown, they do hypothesis.
Adv. Synth. Catal. 2003, 345, 211 215
213

Paul J. Dyson et al.
FULL PAPERS
Experimental Section
Hydrogenation Reactions
The compounds 1,3,5-triaza-7-phosphaadamantane (pta)[25]
All hydrogenations were carried out in a Parr stainless steel
autoclave (300 mL) fitted with a PTFE liner. The catalyst was
added directly to the autoclave and the solvent was added. The
autoclave was then sealed and purged with nitrogen and the
reaction substrate was then added through the liquid inlet port
via a syringe. The autoclave was then purged thoroughly with
hydrogen gas (99.9995% purity) and the appropriate reaction
pressure was then set at room temperature. The autoclave was
then heated to the required reaction temperature and stirring
was commenced for the period required. Once the desired time
period had elapsed, the stirring was stopped and the autoclave
allowed to cool before releasing the pressure. Substrate and
catalyst layers were separated in a separating funnel. In
hydrogenations where colloidal poisoning was conducted
10 drops of mercury were added to the autoclave, otherwise
all other parameters remained unchanged.
and Ru(h6-C10H14)(pta)Cl2
were made according to the
[15]
literature procedures. All other reagents were supplied by
Aldrich and were used without further purification.
Electrospray mass spectra were obtained a VG Autospec
instrument. NMR spectra were recorded on a Bruker DRX-
1
400 spectrometer with H at 400.13, 31P at 161.98 and 13C at
100.1 MHz. 1H NMR chemical shifts are reported in ppm
relative to residual 1H signals in the deuterated solvents
(CDCl3 d ˆ 7.29), 31P{1H} NMR spectra are reported in ppm
downfield of an external 85% solution of phosphoric acid. All
hydrogenation products were analysed by NMR and/or gas
chromatography using a Varian gas chromatograph with a
capillary carbowax column (30 m) using injection, oven and
detector temperatures 10 30 8C above the boiling points of
the substrate/product being studied.
Acknowledgements
Synthesis of Ru(h6-C10H14)(tppts)Cl2 (2)
We would like to thank the Royal Society for a University
Research Fellowship (PJD) and the University ofYork ofr
financial support (DJE).
A solution of [Ru(h6-C10H14)Cl2]2 (153 mg, 0.25 mmol) and
tppts (284 mg, 0.5 mmol) in methanol was heated to reflux for
5 h during which the solution changed from orange to deepred
in colour. Removal of the solvent under reduced pressure
followed by washing in dichloromethane (2 Â 10 mL) and
further drying on a vacuum line resulted in a dark red
microcrystalline solid; yield: 400 mg (0.458 mmol, 92%). The
product was used without further purification for the inves-
tigation of its catalytic properties.
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1
34.81 (s), 29.92 (s); H NMR (CD3OD): d ˆ 8.30 7.60 (m,
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¬
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‡
‡
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Modifications to the Ligands Surrounding a Ruthenium Complex
FULL PAPERS
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Adv. Synth. Catal. 2003, 345, 211 215
215