Homogeneous hydrogenation of arenes catalyzed by the bis(dihydrogen) complex [RuH2(H2)2(PCy3)2]

Article abstract of DOI:10.1016/S1381-1169(01)00187-X

The reaction of benzene, naphthalene and anthracene with dihydrogen under a pressure of 3 and 20 bar at 80°C in the presence of [RuH₂(H₂)₂(PCy₃)₂] (1) leads to cyclohexane, tetralin and a mixture of 1,2,3,4-tetrahydroanthracene (4H-An) and 1,2,3,4,5,6,7,8-octahydroanthracene (8H-An), respectively. Surprisingly, the increase of dihydrogen pressure lowers the yield of hydrogenation products. Product 1 may be used directly as a catalyst precursor or may be prepared in situ from [RuH{(η³-C₆H₈)PCy₂} {(η²-C₆H₉)PCy₂}] (2) under dihydrogen at room temperature. A number of functionalized arenes (toluene, chlorobenzene, durene) as well as tetralin, phenanthrene and 9,10-dihydroanthracene are not reduced under 3 bar of dihydrogen. The reaction of 1 with arenes, either in neat benzene or in cyclohexane in the case of naphthalene and tetralin at 80°C leads to the formation of respectively, η⁶-bound benzene (3), naphthalene (4) and tetralin (5) complexes that are also present in very small quantities in the final catalytic mixtures. Reaction of 1 with anthracene at room temperature yields the complex [Ru(η⁴-C₁₄H₁₀)(PCy)₂] (6) containing an η⁴-bound anthracene. Product 6 shows a catalytic activity for anthracene hydrogenation and can be regarded as an intermediate in the catalytic cycle.

Full text of DOI:10.1016/S1381-1169(01)00187-X

Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
Homogeneous hydrogenation of arenes catalyzed by the
bis(dihydrogen) complex [RuH2(H2)2(PCy3)2]
Andrzej F. Borowskia,∗, Sylviane Sabo-Etienneb,1, Bruno Chaudretb
a
Institute of Coal Chemistry, Polish Academy of Sciences, 5, Sowinskiego Street, 44-121 Gliwice, Poland
b
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France
Received 19 October 2000; accepted 14 March 2001
Abstract
The reaction of benzene, naphthalene and anthracene with dihydrogen under a pressure of 3 and 20 bar at 80◦C in the
presence of [RuH2(H2)2(PCy3)2] (1) leads to cyclohexane, tetralin and a mixture of 1,2,3,4-tetrahydroanthracene (4H-An)
and 1,2,3,4,5,6,7,8-octahydroanthracene (8H-An), respectively. Surprisingly, the increase of dihydrogen pressure lowers the
yield of hydrogenation products. Product 1 may be used directly as a catalyst precursor or may be prepared in situ from
[RuH{(␩3-C6H8)PCy2}{(␩2-C6H9)PCy2}] (2) under dihydrogen at room temperature. A number of functionalized arenes
(toluene, chlorobenzene, durene) as well as tetralin, phenanthrene and 9,10-dihydroanthracene are not reduced under 3 bar of
dihydrogen. The reaction of 1 with arenes, either in neat benzene or in cyclohexane in the case of naphthalene and tetralin at
80◦C leads to the formation of respectively, ␩6-bound benzene (3), naphthalene (4) and tetralin (5) complexes that are also
present in very small quantities in the final catalytic mixtures. Reaction of 1 with anthracene at room temperature yields the
complex [Ru(␩4-C14H10)(PCy)2] (6) containing an ␩4-bound anthracene. Product 6 shows a catalytic activity for anthracene
hydrogenation and can be regarded as an intermediate in the catalytic cycle. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Hydrogenation; Arenes; Ruthenium; Dihydrogen complexes
1. Introduction
complexes in biphasic aqueous solutions [8–10] and
colloidal rhodium as a suspension in water [11] or in
aqueous/supercritical fluid biphasic media [12]. For
some years, the use of molecular catalysts for arene
hydrogenation was stimulated by the studies of arenes
as models for products resulting from coal liquefac-
tion [13–16]. Early works on arene hydrogenation
catalysts concentrated on their activities [14–25],
but in some cases the homogeneity of these systems
was uncertain. However, a number of systems were
proven to be homogeneous and their studies have led
to a better understanding of the mechanisms of arene
hydrogenation [26–37].
Hydrogenation of arenes with soluble molecular
catalysts has not been studied very extensively since
it generally requires rather severe conditions and be-
cause heterogeneous catalysts are remarkably active
in these reactions [1–3]. Recently, several research
groups have reported efficient arene hydrogenation
catalytic systems based on rhodium and palladium
complexes tethered to a solid support [4–6], ruthe-
nium cluster catalyst in an ionic liquid [7], ruthenium
∗ Corresponding author
E-mail address: sabo@lcc-toulouse.fr (A.F. Borowski).
1 Tel.: +33-5-61-333-177; fax: +33-5-61-553-003.
In the 1980s,
a homogeneous process, the
“Dimersol” process, developed by the Institut Français
1381-1169/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00187-X

70
A.F. Borowski et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
du Pétrole, has been applied successfully for the hy-
drogenation of benzene to cyclohexane [1,38]. More
recently, a variety of arene substrates have been hy-
drogenated by hydride derivatives of niobium and tan-
talum [39–44]. There is, however, only one example
of a dihydrogen complex quoted as an effective cata-
lyst for hydrogenation of 9-methylanthracene [45].
Our recent observations that the bis(dihydrogen)
complex [RuH2(H2)2(PCy3)2] (1) [46,47] and some
of its derivatives (e.g. [RuH{(␩3-C6H8)PCy2}{(␩2-
C6H9)PCy2}] (2)) undergo H/D isotopic exchange
reactions in deuterated arene solvents (d6-benzene,
d8-toluene) are indicative of a mechanism that in-
volves coordination of an arene ring followed by a
C–H bond activation [48]. Moreover, the cyclohexyl
rings of the PCy3 ligands in 1 undergo reversible,
intramolecular dehydrogenation/hydrogenation reac-
tions in the presence of a hydrogen acceptor or dihy-
drogen, respectively. These findings prompted us to
study 1 as a potential catalyst for arene hydrogenation.
2.1. Hydrogenation under 3 bar H2
[RuH{(␩3-C6H8)PCy2}{(␩2-C6H9)PCy2}]
(2)
(0.02 g, 0.03 mmol) was placed into a Fisher–Porter
flask under argon atmosphere. The system was evacu-
ated and flushed with dihydrogen. A required amount
(normally 10 ml) of a degassed solvent was then
introduced. The reactor was pressurized to 3 bars
of dihydrogen and stirred at room temperature for
0.5–1 h, depending on the solvent, to allow total con-
version of 2 to 1, visualized by complete dissolution
and formation of a colorless homogeneous solution.
The reactor was then depressurized and a suitable
amount (normally, 1.5 mmol) of a degassed substrate
was introduced under a dihydrogen stream. The re-
action mixture was again pressurized to 3 bar of H2
and stirred at room temperature until all substrate dis-
solved (anthracene dissolves only partly under these
conditions). Then the reactor was placed into an oil
bath and heated at 80◦C while stirring magnetically
for the required amount of time. The Fisher–Porter
flask was then cooled down to room temperature,
depressurized and the hydrogenation products were
analyzed by GC.
2. Experimental
Microanalysis were performed by The Laboratoire
de Chimie de Coordination Microanalytical Service.
Proton and phosphorus spectra were recorded on a
Bruker AC 200 (at 200.132 and 81.015 MHz, re-
spectively) and on an AMX 400 (at 400.133 and
161.985 MHz, respectively) while 13C NMR spectra
were obtained by using AM 250 (50.323 MHz) and
AMX 400 (100.624 MHz) spectrometers, all oper-
ating in the Fourier transform mode. All manipula-
tions were carried out under argon using standard
Schlenk-line techniques. All solvents were freshly
distilled from standard drying agents and thoroughly
degassed under argon prior to use. RuCl3·3H2O
was purchased from Johnson Matthey Ltd. and all
other reagents were purchased from Aldrich and
degassed before use. Reaction products were ana-
lyzed by GC on a Hewlett Packard 5890 apparatus
fitted with a FID detector using a capillary column
(30 mm × 0.32 mm) packed with cross-linked methyl
silicone. Homogeneity of the reaction mixtures has
been confirmed by the test with liquid mercury which
is known to inhibit colloidal catalysis [49,50]. Prod-
ucts 1 and 2 have been prepared by published methods
[48].
2.2. Hydrogenation under 20 bar H2
2.2.1. Hydrogenation of neat benzene
A catalyst precursor (1) (0.075 g, 0.112 mmol) was
placed into a stainless steel autoclave (50 ml volume)
fitted with gas inlet and outlet tubings, externally
driven stir bar, manometer and a heating mantle. The
system was evacuated and flushed with argon. This
procedure was repeated three times using dihydrogen
in the place of argon. A total of 20 ml degassed ben-
zene was then introduced via a PTFE transfer tubing.
The system was then pressurized to 20 bar at 20◦C
with vigorous stirring and the reactor was heated up to
80◦C within 15 min, after which the internal pressure
in the reactor reached 24 bars. On termination of tests,
the reactor was cooled down to room temperature and
the products were analyzed by GC.
2.2.2. Hydrogenation of naphthalene and anthracene
Product 1 (0.02 g, 0.03 mmol) and a solid substrate
(1.5 mmol) were placed into the autoclave and the
same procedure was used as above but using cyclo-
hexane (20 ml) in place of benzene as solvent.

A.F. Borowski et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
71
2.3. Preparation of the complexes
3. Results and discussion
[RuH2(η6-C6H6)PCy3] (3): An amount of 0.085 g
(0.13 mmol) of 1 was dissolved in benzene (4 ml)
and refluxed for 3 h. The resulting orange solution
was evaporated to dryness giving a pale yellow solid
very soluble in most common solvents. The solid was
characterized as [RuH2(␩6-C6H6)PCy3] contaminated
3.1. Hydrogenation of benzene
Under mild conditions (80◦C, 3 bar H2) methylcy-
clohexane solutions of 1 catalyze the hydrogenation of
benzene to cyclohexane with a low turnover number
of 2.8 in 24 h (Table 1). After increasing the reaction
time to 45 h, no sign of metal precipitation is observed.
When similar experiments are carried out in neat ben-
zene, in which the substrate to catalyst ratio increased
ca. 15 times as compared with the reaction performed
in a methylcyclohexane solution, the turnover number
increased to 32 after 20 h, hence revealing a significant
increase in the turnover frequency. This activity seems
comparable to that of [␩6-C6(CH3)6Ru-␩4-C6(CH3)6]
under comparable reaction conditions (90◦C, 2–3 bar
H2); however, the substrate to Ru ratio has not been
specified in that work [30]. However, when the reac-
tion in neat benzene was carried out in the presence
of mercury as an homogeneity test, no conversion was
observed [49,50]. This result might be due to the for-
mation of colloids in this case and will be further in-
vestigated.
1
with free PCy3 by H, 31P and 13C NMR spectra in
C6D6.
[(PCy3)H2Ru(µ:η,η-C10H8)RuH2(PCy3)] (4): To a
suspension of 1 (0.140 g, 0.21 mmol) in cyclohexane
(5 ml) an excess of naphthalene (0.08 g, 0.63 mmol)
was added and the mixture was stirred for ca. 3 min
before heating at reflux temperature for 2 h. On cool-
ing, the solvent was removed under vacuum resulting
in a brown–red sticky mass from which unreacted
napthalene was removed by sublimation at 60◦C.
To the remaining brown solid, pentane (6 ml) was
added and the yellow precipitate formed was filtered
off, washed with pentane (2 ml × 1.5 ml) and dried
in vacuo. Yield: ca. 0.055 g (59%). Recrystallization
from benzene/pentane gave yellow, air sensitive crys-
tals. Anal. Calcd. for C46H78P2Ru2: C, 61.72; H,
8.78. Found: C, 61.81; H, 8.58.
When toluene has been used as a substrate in cyclo-
hexane, only traces of methylcyclohexane have been
found in the reaction mixture after 24 h. Other func-
tionalized arenes such as 1,2,4,5-tetramethylbenzene
(durene) and chlorobenzene do not undergo reduction
in the same conditions.
[RuH2(η6-C10H12)PCy3] (5): To a suspension of 1
(0.100 g, 0.15 mmol) in cyclohexane (6 ml) degassed
tetralin (61 ␮l, 0.45 mmol) was added and refluxed for
5 h. The resulting red solution was filtered off and
kept overnight under vacuum to give a red–brown
sticky mass. Pentane (8 ml) was added and the mix-
ture was filtered off and passed through a chromato-
graphic column filled with Al2O3. The orange phase
was eluted using benzene followed by evaporation un-
der vacuum to form a film on the surface of the flask.
Since we were unable to obtain the product in another
3.1.1. Synthesis of [RuH2(η6-C6H6)PCy3] (3)
NMR spectra recorded after a 45 h reaction of hy-
drogenation of d6-benzene used as solvent, display
almost exclusively signals characteristic of 1 (e.g. δ
−7.83 (Ru–H) and δ 76.2 (PCy3) for 1H and 31P NMR,
respectively) which suggests the constant presence of
this compound during the course of the catalytic re-
action. However, a very small doublet (δ −10.5, J =
42.3 Hz) and two weak signals of equal intensity at ca.
1
form, H and 31P NMR spectra were run after disso-
lution of the yellow film in C6D6. The product is con-
taminated with large amount of PCy3 and is free of
tetralin.
78 and 10 ppm can also be found in the H and 31P
NMR spectra, respectively.
The reaction of 1 in refluxing benzene under an ar-
gon atmosphere leads to a new product that we were
unable to isolate in pure form due to its high solubility
in most commonly used solvents. Nevertheless, a H
NMR spectrum (Table 2) of the pale yellow solid ob-
tained by evaporation to dryness of the reaction mix-
1
[RuH2(η4-C14H10)(PCy3)2] (6): A suspension of
1 (0.100 g, 0.15 mmol) and anthracene (0.0535 g,
0.3 mmol) in diethyl ether (8 ml) was stirred overnight
yielding a fine yellow precipitate that was col-
lected, washed with Et2O (3 ml × 5 ml) and vacuum
dried. Yield ca. 0.075 g (60%). Anal. Calcd. for
C50H78P2Ru: C, 71.31; H, 9.33. Found: C, 71.18; H,
1
9.2. IR ν(Ru–H), 2013 and 1975 cm−1
.

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A.F. Borowski et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
Table 1
a
Hydrogenation of arenes at 80◦C under 3 bar H2
Substrate
Time (h)
Conversion (%)
Products (%)
TONb
Benzene (neat)c
Benzened
Toluene
Chlorobenzene
Durene
Naphthalene
Tetralin
20
24
24
15
24
24
24
4
4.3
5.6
<0.1
None
None
25
Cyclohexane (100)
Cyclohexane (100)
Methylcyclohexane (traces)
32.1
2.8
Tetralin (100)
Cis- and trans-dekalin (traces)
4H-Ane (93)
13
<0.1
100
Anthracene
50f
3.5f
50f
8f
50f
13.5f
50f
5f
8H-Ang (7)
Anthracene
Anthracene
Anthraceneh
20
24
24
100
100
100
4H-Ane (84)
8H-Ang (16)
4H-Ane (73)
8H-Ang (27)
4H-Ane (90)
8H-Ang (10)
Phenanthrene
9,10-2H-Ani
17
4
None
None
a Unless stated otherwise, 2 is the catalyst precursor, solvent: cyclohexane (10 ml), [Ru] = 3×10−3 M, [arene] = 0.15 M, arene/Ru = 50,
products were analyzed by GC.
b TON is defined as mole of product formed per mole of Ru.
c [Ru] = 15 × 10−3 M, benzene/Ru = 750.
d Solvent: methylcyclohexane (10 ml).
e 4H-An: 1,2,3,4-tetrahydroanthracene.
f Calculations based on the assumption that 8H-An is formed in a two stage reaction. It involves hydrogenation of anthracene to 4H-An
(stage 1) followed by its hydrogenation to 8H-An (stage 2).
g 8H-An: 1,2,3,4,5,6,7,8-octahydroanthracene.
h [RuH2(␩4-C14H10)(PCy3)2] used as a catalyst precursor.
i 9,10-2H-An: 9,10-dihydroanthracene.
ture shows (in C6D6) a sharp singlet at δ 5.25 assigned
to ␩6-bound benzene and a doublet in the hydride
resonance region at δ −10.50 (JP–H = 42.5 Hz). In
31P{1H} NMR, a singlet at δ 78.3 transforms into a
triplet upon selective decoupling of the PCy3 protons.
A loss of one phosphine ligand in the reaction of 1
with benzene is confirmed by the presence of a sharp
singlet for free PCy3 at δ 10.0 in the 31P{1H} NMR
spectrum.
of 1 in the presence of boiling benzene are iden-
tical, hence indicating the formation of the same
product (3), a ␩6-bound benzene ruthenium dihy-
dride containing one tricyclohexylphosphine ligand:
[RuH2(␩6-C6H6)PCy3] (3) (Scheme 1). The H and
1
31P NMR data for 3 are very close to those re-
ported earlier for the triisopropyl phosphine analogue
[51].
The presence of an ␩6-coordinated benzene
molecule in this compound is further supported by
its 13C NMR spectrum showing a doublet at δ 83.4
3.2. Hydrogenation of naphthalene
As expected, in the same reaction conditions, naph-
thalene, a condensed arene, is easier to hydrogenate
than monoaromatic hydrocarbons. Hydrogenation of
the first double bond in naphthalene is thermodynam-
ically more favorable than the addition of the first
dihydrogen molecule to benzene [52]. It produces
1,2,3,4-tetrahydronaphthalene (tetralin) in 25% yield
(JC–H = 170.4 Hz) coupled to phosphorus (JC–P
=
2.5 Hz).
The 1H and 31P NMR resonances assigned to
the minor product observed in the reaction of 1
with benzene under hydrogenation conditions and
the respective data obtained for the reaction product

A.F. Borowski et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
73
Table 2
Selected NMR data (ppm) for compounds 3–6, anthracene, 4H-anthracene and 8H-anthracene
Compound
1H NMRa
31P NMRa
78.3 (s)b
13C NMRa
[RuH2(␩6-C6H6)PCy3] (3)
5.25 (s, 6H, ␩6-C6H6)6
83.4 (d, JC–H = 170.4 Hz; d,
JC–P = 2.5 Hz, C1–6
)
c
−10.4 (d, 2H, JP–H = 42.3 Hz, Ru–H)
5.03 (brm, ␩6-C10H8
)
68.9 (s)b
78.6 (s)b
67.9 (s)h
95.8 (s, C9,10
)
1,4,5,8
d
e
[(PCy3)RuH2]2(␮:␩,␩-C10H8) (4)
[RuH2(␩6-C10H12)PCy3] (5)
[RuH2(␩4-C14H10)(PCy3)2] (6)
4.19 (brm, 4H, ␩6-C10H8
)
78.0 (d, JC–H = 169.2 Hz, C1,4,5,8
)
)
2,3,6,7
−10.18 (d, 4H, JP–H = 29.3 Hz, Ru–H)
64.7 (brd, JC–H = 158 Hz, C2,3,6,7
5.35 (brm, 2H, ␩6-C10H12
5.20 (brm, 2H, ␩6-C10H12
)
)
1,4
2,3
d
−10.45 (d, 2H, JP–H = 42.3 Hz, Ru–H)
7.54 (dd, 2H, H5,8, Jvic = 6.0 Hz,
Jall = 3.3 Hz)f,g
79.6 (d, JC–H = 166 Hz, C2,3
)
e
7.23 (dd, 2H, H6,7, Jvic = 6.1 Hz,
Jall = 3.2 Hz)
60.8 (C1,4
)
i
6.99 (s, 2H, H9,10
5.63 (m, 2H, H1,4
4.20 (m, 2H, H2,3
)
)
)
−11.25 (t, 2H, JP–H = 28.5 Hz, Ru–H)
8.15 (s, 2H, H9,10
)
d
Anthracene
7.80 (dd, 4H, H1,4,5,8
,
Jvic = 6.5 Hz, Jall = 3.3 Hz)g
7.25 (dd, 4H, H2,3,6,7
,
Jvic = 6.6 Hz, Jall = 3.2 Hz)
7.66 (dd, 2H, H5,8, Jvic = 6.2 Hz,
Jall = 3.3 Hz)d,g
4H-anthracenej
7.40 (s, 2H, H9,10
)
7.28 (dd, 2H, H6,7, Jvic = 6.3 Hz,
Jall = 3.2 Hz)
2.74 (m, 4H, H1,4
)
ca. 1.62 (m, 4H, H2,3 and 8H,
H2,3,6,7 for 8H-An)
8H-anthracenek
6.74 (s, 2H, H9,10
)
d
2.63 (m, 8H, H1,4,5,8
)
ca. 1.62 (m, 8H, H2,3,6,7 and 4H,
H2,3 for 4H-An)
a C6D6, 293 K.
b 81.015 MHz.
c 50.323 MHz.
d 200.132 MHz.
e 100.624 MHz.
f 400.133 MHz.
g vic = vicinal, all = allylic.
h 161.985 MHz.
1
i Only 13C{ H} NMR.
j 4H-anthracene: 1,2,3,4-tetrahydroanthracene.
k 8H-anthracene: 1,2,3,4,5,6,7,8-octahydroanthracene.
after 24 h and no evidence for a further hydrogenated
product has been found (turnover number: ca. 13).
Tetralin has also been found (GC and H NMR) to
thalene in C6D6, in agreement with the much higher
reactivity of naphthalene compared to benzene. An
attempted hydrogenation of tetralin in cyclohexane
revealed only traces of cis- and trans-decalin after
1
be the sole product of the hydrogenation of naph-

74
A.F. Borowski et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
Scheme 1.
a comparable reaction time. The hydrogenation of
naphthalene is, however, catalyzed much faster by
niobium complexes, as described by Rothwell. In that
case, tetralin has been obtained in >95% yield after
24 h, but under a much higher pressure [40].
3.3. Synthesis of [(PCy3)H2Ru(µ:η,η-C10H8)-
RuH2(PCy3)] (4) and [RuH2(η6-C10H12)PCy3] (5)
In order to get more information concerning the
course of naphthalene hydrogenation, we have per-
formed several reactions with 1 and naphthalene in
a 1:3 molar ratio, using d12-cyclohexane as solvent.
The progress of the reaction was followed by 1H
NMR. Two products were observed with relative
ratios depending on the reaction duration. Shorter
reaction times (several hours) favored the formation
of a complex 4 (see below) showing two multi-
plets at δ ca. 4.7 and 3.8 and a doublet at δ −10.6
(JP–H = 29 Hz). Longer reaction times (above 10 h)
revealed the predominant formation of a compound
The bis(dihydrogen) complex 1 has been character-
ized as the main complex present in d6-benzene so-
1
lution after 20 h naphthalene hydrogenation. The H
NMR spectra of this reaction mixture reveal the pres-
ence of two additional complexes in very small quan-
tities. One is the complex characterized above as 3
while the other shows two multiplets at δ 5.35 and
5.20 and a doublet at δ ca. −10.5 (J = 42.4 Hz) in
1
the H NMR spectrum and a single peak at δ 78.6 in
the 31P NMR spectrum.

A.F. Borowski et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
75
(5) (see below) displaying two multiplets at δ ca. 5
and 4.9 and a doublet at δ ca. −11 (JP–H = 43 Hz).
Additionally, in all these experiments, we have ob-
served the formation of a small amount of the com-
plex [RuH{(␩3-C6H8)PCy2}{(␩2-C6H9)PCy2}] (2)
in which both phosphine ligands contain a dehy-
drogenated cyclohexyl ring bound to the metal in
an ␩3- and ␩2-mode, respectively [48]. As reported
previously, this complex may be formed upon re-
action of 1 with functionalized alkenes serving as
hydrogen acceptors [48] or as a product of thermal
decomposition of 1 under an helium atmosphere
[53].
singlet at δ 78.6 in the 31P{1H} NMR spectrum into
a triplet (JP–H = 42 Hz) upon selective decoupling
of PCy3 protons. The signals of the H1,4 protons of
the saturated ring of tetralin appear as a multiplet at
δ ca. 2.60, while the signals of the H2,3 protons are
probably hidden under the resonances of the PCy3
protons between 1.9 and 1.0 ppm. Finally, integra-
tion of the hydrides against the arene ring protons
indicate a 1:2 ratio, in agreement with the proposed
structure. Product 5 is therefore, a ruthenium(II) com-
plex in which the metal atom coordinates one tetralin
ligand through an ␩6-bound arene ring along with
two hydrides and one tricyclohexylphosphine ligand.
The same product has been found in small quantities
on termination of the hydrogenation tests of naph-
thalene.
The synthesis of [(PCy3)H2Ru(␮:␩,␩-C10H8)Ru-
H2(PCy3)] (4) was achieved by reacting 1 with naph-
thalene in boiling cyclohexane for 2 h (Scheme 1).
Product 4 was isolated as yellow crystals in 59% yield.
The 1H NMR spectrum of 4 (Table 2) shows two broad
multiplets at δ 4.19 and 5.03 characteristic of the pres-
ence of a ␲-bonded naphthalene ring. The two hydride
ligands resonate at δ −10.18 as a doublet due to cou-
pling with one phosphorus atom (JP–H = 29.3 Hz).
The ratio of two hydrides per one phosphorus is con-
firmed by a 31P NMR spectrum where the singlet at δ
68.9 splits into a triplet upon selective decoupling of
PCy3 protons. Finally, the arene protons to hydride in-
tegration ratio of 2:1 is in agreement with a dinuclear
formulation involving one naphthalene ligand for four
hydrides. The 13C NMR spectrum confirms the pres-
ence of coordinated naphthalene. A singlet at δ 95.8
has been assigned to the C9,10 carbons, while two dou-
blets at δ 78.0 and at δ 64.7 have been assigned to
the C1,4,5,8 and C2,3,6,7 carbons, respectively, of an
␩6-coordinated naphthalene molecule. Bubbling dihy-
drogen through a d6-benzene solution of 4 did not lead
3.4. Hydrogenation of anthracene
Anthracene is reduced in the standard conditions in
less than 4 h into a mixture of 1,2,3,4-tetrahydroanth-
racene (4H-An) (93%) and 1,2,3,4,5,6,7,8-octahydro-
anthracene (8H-An) (7%) (Table 1). The total
conversion of anthracene and the presence of these
two products suggest that after hydrogenation of
the first ring, the reaction proceeds on prolonged
reaction time to the hydrogenation of the second
ring but with a much slower rate. Interestingly, no
9,10-dihydroanthracene was detected. This compound
is the typical product of catalysis proceeding through
a radical mechanism [54]. The mixture of hydro-
genation products of anthracene has been isolated as
a white product after a 24 h reaction and analyzed
1
by GC and H NMR in C6D6. The assignments for
4H-An and 8H-An are given in Table 2.
1
to any changes in the H NMR spectrum. This may
The observation that 4H-An is an intermedi-
ate in the formation of 8H-An contrasts to the
results reported by Crabtree et al. [55], Caulton
et al. [56], and Halpern and Linn [45] for poly-
hydrido triphenylphosphine complexes of Ir, Re
and Ru, respectively. However, the almost exclu-
sive production of 4H-An prior to the formation of
8H-An, without detectable 9,10-dihydroanthracene,
has also been observed by Rothwell et al. for
[Nb(OC6H3Ph2-2,6)2(CH2C6H4-4Me)3] and [Ta-
(OC6H3Cy2-2,6)2(H)3(PMe2Ph)2] under comparable
conditions except for the considerably higher pressure
(83 bar H2) [40–44].
suggest the absence of involvement of 4 in the hydro-
genation catalytic cycle.
The second compound [RuH2(␩6-C10H12)PCy3]
(5), was produced in a 5 h reaction of (1) with tetralin
in boiling cyclohexane but could not be isolated in a
1
pure form due to its contamination by PCy3. The H
NMR spectrum of 5 in C6D6 shows two multiplets
centered at δ 5.35 and 5.20 assignable to the ␩6-bound
arene ring of tetralin. The presence of two hydrides
coupled to one phosphorus atom is evidenced by a
doublet at δ −10.45 (JP–H = 42.3 Hz) and this find-
ing is supported by the transformation of the sharp

76
A.F. Borowski et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
Finally, we have checked that 9,10-dihydroanthra-
cene is not hydrogenated under our standard condi-
tions.
observation supports this hypothesis. Bubbling dihy-
drogen through a C6D6 solution of 6 in an NMR tube
leads to rapid and total restoration of 1 along with
the liberation of anthracene and formation of some
amounts of 4H-An and 8H-An.
3.4.1. Synthesis of [RuH2(η4-C14H10)(PCy3)2] (6)
Reaction of 1 with a small excess of anthracene in
cyclohexane or diethyl ether under argon yields a yel-
low product analyzed as [RuH2(␩4-C14H10)(PCy3)2]
(6). The 1H NMR spectrum of 6 (Table 2) shows two
resonances for the H1,4 and H2,3 protons of the co-
ordinated anthracene ring as multiplets at δ 5.62 and
4.20, respectively. The hydrides resonate as a triplet at
δ −11.25 (JP–H = 28.5 Hz) due to coupling with two
equivalent trans-phosphorus atoms which display a
singlet at δ 67.9 in 31P NMR. The proton signal trans-
forms into a singlet upon phosphorus decoupling. The
relatively low value of proton–phosphorus coupling
found for 6 (28.5 Hz) suggests an ␩4-coordination
mode for anthracene since previous data concern-
ing comparable hydrido phosphino arene ruthenium
complexes report values below 30 Hz [57], whereas
the coupling constant JP–H is found above 40 Hz
for complexes accommodating ␩6-coordinated arenes
[51,58].
When considering these findings, we can propose
a very simple catalytic cycle for the hydrogenation of
anthracene to 4H-An (Scheme 1) which starts with the
dissociation of two H2 molecules creating a vacancy
allowing anthracene coordination in an ␩4-mode. The
rest of the catalytic cycle, although not investigated is
presumably straightforward. At the end of the catalytic
reaction, either a new anthracene molecule may coor-
dinate to Ru forming 6 and starting a new catalytic
cycle or the bis(dihydrogen) complex 1 regenerates
upon coordination of two H2 molecules. Reduction of
the second external ring of 4H-An begins presumably
when most (or all) of the anthracene is consumed.
3.5. Hydrogenation of naphthalene and
anthracene under 20 bar H2
The results of naphthalene and anthracene hydro-
genation performed under a pressure of 20 bar of di-
hydrogen (Table 3) cannot be compared directly with
those obtained under a lower pressure (3 bar, Table 1)
since the volumes of the reaction mixtures and the
catalyst concentrations differ considerably. A general
trend can nevertheless be assessed. Naphthalene has
been hydrogenated to tetralin at considerably slower
rate in the presence of 20 bar of dihydrogen leading to
a turnover number of only 1.5 after 24 h. The hydro-
genation of anthracene has been less affected by the
The H1,4 and H2,3 proton resonances are consider-
ably shifted upfield as compared with the analogous
signals for free anthracene (cf. Table 2). The reso-
nances of the H5,8 and H6,7 protons appear as dou-
blets of doublets and are only slightly shifted. The
integration ratio between the hydride signal and the
signals for protons H1,4, H2,3, H5,8, H6,7 and H9,10 is
1:1:1:1:1:1. The pattern found for 6 resembles that of
[Fe(CO)3(␩4-C14H10)] [59]. ␩4-Anthracene coordina-
6
tion rather than ␩ [60] is additionally supported by
the 13C NMR spectra of 6 in which the C2,3 and C1,4
carbon resonances are shifted upfield to δ 79.6 and
60.8, respectively. Finally, ␩4-coordination ensures an
18-valence-electron configuration for this complex.
Table 3
a
Hydrogenation of arenes at 80◦C under 20 bar H2
Substrate
Time
(h)
Conversion
(%)
Products
(%)
TONb
1.5
3.4.2. Hydrogenation of anthracene with 6 used
as a catalyst precursor
Naphthalene 24
Anthracene
Anthracene
3
47
93
Tetralin (100)
4
24
4H-Anc (100) 23.5
4H-Anc (91)
8H-And (2)
46.5
1
Product 6 also acts as a hydrogenation catalyst lead-
ing to a total conversion of anthracene into a mixture
of 4H-An (90%) and 8H-An (10%) after 24 h (80◦C,
3 bar H2, see Table 1). Since 6 readily forms from 1
by substitution of two dihydrogen molecules, it is very
likely that 6 may be involved, as a transient form, in a
catalytic cycle for anthracene hydrogenation. Another
a Catalyst precursor: [RuH2(H2)2(PCy3)2] (1) in 20 ml cy-
clohexane solvent [Ru] = 1.5 × 10−3 M, [arene] = 0.075 M,
arene/Ru = 50.
b TON is defined as mole of product formed per mole of Ru.
c 4H-An: 1,2,3,4-tetrahydroanthracene.
d 8H-An: 1,2,3,4,5,6,7,8-octahydroanthracene.

A.F. Borowski et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
77
increase of hydrogen pressure. After 4 h of reaction,
47% conversion of anthracene into exclusively 4H-An
was observed which corresponds to a turnover number
of 23.5. After 24 h, anthracene conversion to 4H-An
reached 91% and only 2% 8H-An could be found in
the reaction mixture.
considerably less active for benzene hydrogenation
than [(␩6-C6Me6)Ru(␮-H)2(␮-Cl)Ru(␩6-C6Me6)]Cl
in neat benzene [21] or [H4Ru4(␩6-C6H6)4][BF4] in
the ionic liquid [7] (0.11 mol mol−1 h−1 against 246
and 364 mol mol−1 h−1, respectively). Furthermore,
the absence of hydrogenation products in reactions
performed in neat benzene in the presence of mercury
may suggest the involvement of metal particles.
A better conversion has, however, been achieved for
naphthalene which has been reduced re-gioselectively
to tetralin, and for anthracene which is hydro-
genated even more readily into tetrahydro- and
octahydro-anthracene in consecutive reactions. Func-
tionalized arenes (toluene, chlorobenzene, durene),
tetralin, phenanthrene and 9,10-dihydroanthracene
have not been reduced in these reactions. In the latter
case, the reason may be steric (sp3 hybridisation of C9
and C10 carbons cause that this three-ring molecule is
not planar); the high resonance energy of the isolated
external aromatic ring may also account for these
observations. However, 1 is almost as efficient as a
catalyst precursor for hydrogenation of naphthalene
and anthracene as Nb and Ta complexes incorporating
aryloxide ligands [40,44].
In these arene hydrogenation reactions catalyzed by
1, we have observed that the increase of dihydrogen
pressure from 3 to 20 bar reduces the reaction rates.
This trend is rather exceptional in hydrogenation reac-
tions. In general, an increase of dihydrogen pressure
results in an increase of the reaction rate or has no
influence on it. The results of our studies allow us to
propose that similarly to many other catalytic systems
involving dihydrogen complexes [61], it is the dis-
sociation of dihydrogen molecules that provides free
coordination sites for substrate coordination. This
dissociation is disfavored at high pressure of dihydro-
gen. As a result, a considerably lower conversion of
arenes to hydrogenation products is observed when
increasing dihydrogen pressure.
4. Conclusions
Complex 1 can be used directly for hydrogenation
reactions or can be generated in situ from 2. Product
1 is the major species detected all along the catalytic
reaction. After termination of the hydrogenation
We have described in this paper the catalytic
activity of [RuH2(H2)2(PCy3)2] (1) for arene hy-
drogenation. In methylcyclohexane solution, 1 is
Scheme 2.

78
A.F. Borowski et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 69–79
reactions of benzene and naphthalene, 1 has been
found by NMR to be virtually the only complex
present in the reaction solution. A very small amount
of other complexes (3 in the case of benzene hydro-
genation, or 4 and 5 in the case of hydrogenation of
naphthalene) have been found in the reaction mix-
tures. They seem to be the products of side reactions.
These complexes have been isolated in separate exper-
iments by direct reaction of 1 with the corresponding
arene in a dihydrogen free atmosphere (Schemes 1
and 2). The isolated complex 6 shows a catalytic
activity for anthracene hydrogenation which suggests
its intermediary role in the catalytic cycle.
Somewhat surprisingly, the increase of hydrogen
pressure up to 20 bar, lowers conversion to the ex-
pected hydrogenation products in all the cases. This
can be best explained by the reduced accessibility of
free coordination sites for large arene molecules due
to an increased stabilization of the bis(dihydrogen)
complex 1.
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Acknowledgements
Financial support from the Polish Academy of Sci-
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