Water-soluble carbene complexes as catalysts for the hydrogenation of acetophenone under hydrogen pressure

Article abstract of DOI:10.1016/j.jorganchem.2012.01.001

The synthesis of water-soluble Rh(I), Ir(I), and Ru(II) N-heterocyclic carbene complexes is described. These complexes are applied as catalysts for aqueous phase hydrogenation reactions. Good hydrogenation activities under ca. 40 atm pressure H₂

Full text of DOI:10.1016/j.jorganchem.2012.01.001

Journal of Organometallic Chemistry 703 (2012) 56e62
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Water-soluble carbene complexes as catalysts for the hydrogenation
of acetophenone under hydrogen pressure
,
b
,
,b,
Hitrisia Syska a 1, Wolfgang A. Herrmann a, Fritz E. Kühn a
*
a Chair of Inorganic Chemistry, Catalysis Research Center and Department of Chemistry, Technische Universität München, Lichtenbergstraße 4,
D-85747 Garching b. München, Germany
b Molecular Catalysis, Catalysis Research Center and Department of Chemistry, Technische Universität München, Lichtenbergstraße 4, D-85747 Garching b. München, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of water-soluble Rh(I), Ir(I), and Ru(II) N-heterocyclic carbene complexes is described.
These complexes are applied as catalysts for aqueous phase hydrogenation reactions. Good hydroge-
nation activities under ca. 40 atm pressure H2 at room temperature are observed.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 28 October 2011
Received in revised form
22 December 2011
Accepted 2 January 2012
Keywords:
Homogeneous catalysis
Aqueous hydrogenation
Water-soluble N-heterocyclic carbenes
Acetophenone
1. Introduction
achieved by incorporating hydrophilic moieties in the ligands
[13e17]. A wide variety of polar functional substituents e.g. SO3Na,
Development of catalysts applicable in aqueous systems is very
important for a sustainable, environmentally friendly “green”
chemistry. Water as a solvent bears a number of attractive physi-
cochemical properties in comparison to traditional organic
solvents, which are in many cases flammable, explosive, toxic or
carcinogenic. Water in particular is one of the least expensive and
most easily accessible solvents [1]. Accordingly, organometallic
catalysis in aqueous media has attracted interest since the 1970s,
with pioneering work being carried out for example by Joo, Sasson,
and Sinou. A variety of catalytic reactions in aqueous phase have
been documented since then [1e10]. Sometimes reaction rates
even increase when water replaces organic solvents, e.g. in
DielseAlder and some coupling reactions [11,12]. However, a chal-
lenge often associated with catalysis in water is the need for water-
soluble and water stable ligand/catalyst systems and the decrease
in catalytic activity and/or stereoselectivity when going from
organic solvents to water. Water-solubility of catalysts is often
OSO3Li, CO2Na, OH, PMeþ3 , NMe3þ, P(O)(ONa)2, polyethers, etc. have
been incorporated in order to render the respective ligands water-
soluble [1,11,13]. Among these, the sulfonate moiety has been
applied particularly successfully and is often used.
Since N-heterocyclic carbenes (NHCs) had been recognized as
efficient ligands for catalytically applicable complexes in the mid
1990s [18], this group of ligands has experienced an almost
unprecedented growth in importance [18,19]. A multitude of cata-
lyst systems with NHC ligands, mostly as spectator ligands has been
described, particularly during the last 15 years [20e37]. NHCs
display bonding properties roughly comparable to phosphines
when applied as ligands [20e40]. Furthermore, NHC ligands are
two-electron s-donors with little p-accepting ability [41e46]. They
are considered to behave in several aspects similar to tertiary
phosphines, but bind more strongly to the metal center and are
additionally excellent electron donors. Therefore, NHCs display
some advantages over phosphine ligands in the design of water-
soluble catalysts. The advantages are: (i) NHCs may act as stabi-
lizing ligands for transition metal complexes; thus they might
improve the stability of catalysts in water; (ii) phosphines are easily
oxidized, have to be applied in considerable excess in many cases
and often generate undesirable byproducts or residues e most
likely avoidable with carbene ligands; and (iii) NHCs are known to
be less toxic than phosphines [47e51]. Based on the high interest in
* Corresponding author. Molecular Catalysis, Catalysis Research Center and
Department of Chemistry, Technische Universität München, Lichtenbergstraße 4, D-
85747 Garching b. München, Germany. Tel.: þ49 89 289 13081; fax: þ49 89 289
13473.
E-mail address: fritz.kuehn@ch.tum.de (F.E. Kühn).
Tel.: þ49 89 289 13081; fax: þ49 89 289 13473.
1
0022-328X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jorganchem.2012.01.001

H. Syska et al. / Journal of Organometallic Chemistry 703 (2012) 56e62
57
CH3
under solvent-free condition. After solidification, the mass was
washed 3 times with toluene and dried under high vacuum. The
white solid was readily soluble in water.
O
N
CH3
+
+
1H NMR (400 MHz, D2O, 25 ꢀC),
d[ppm]: 1.81 (pq, 2H, CH2), 2.14
N
CH3
N
N
H
(pq, 2H, CH2), 2.94 (t, 2H, CH2), 4.09 (s, 3H, CH3), 4.53 (t, 2H, CH2),
7.69 (d, 2H, CHCHarom), 7.83, 7.9 (d, 2H, CHCHarom), 9.28 (s, 1H,
NCHN). 13C NMR (100.5 MHz, D2O, 25 ꢀC),
d[ppm]: 21.13 (CH2), 27.06
CH3
CH3
(CH2), 32.78 (CH2), 46.26 (N-CH3), 49.98 (CH2), 112.89, 126.62,
130.86 (CHCHCHCH), 132.02 (NCCN), 141.07 (NCHN). EA calcd. for
(C12H16N2SO3) C: 53.71 H: 6.01 N: 10.44 S: 11.95, found: C: 53.14 H:
6.19 N: 10.15 S: 11.34. MS-FAB: m/z (%) ¼ 268.7 (Mþ).
(CH2)2SO3Na
O
N
+
ONa
CH
S
Br
N
2.2. General synthesis procedure of the carbene complexes (3e5)
O
(CH2)2SO3
1.6 ml of 1 M NaOEt/EtOH solution was dissolved in 10 ml
ethanol and slowly added to a suspension of 0.6 mmol (M(COD)Cl)2.
After the mixture was stirred for 30 min at room temperature, the
azolium precursor (1.2 mmol) was added. The suspension was
stirred at room temperature for 72 h. Ethanol was removed in
vacuo, and then the residue was washed with diethyl ether and
dried under high vacuum. The rhodium/iridium precursors
(Ir(COD)OEt)2/(Rh(COD)OEt)2 were prepared from (Rh(COD)Cl)2/
(Ir(COD)Cl)2 by reaction with sodium ethanolate in ethanol at
ambient temperature. Substitution of the chloro bridge of the
dimeric precursor (Rh(COD)Cl)2 by an ethoxy bridge allows the
coordinated base to deprotonate the azolium in situ, leading to the
desired carbene complexes.
1
Scheme 1. Synthesis of ligand 1.
NHCs as ligands for homogeneous catalysts, the interest in the
preparation of water-soluble derivatives is also increasing [49e58].
In this work, we report on the preparation and catalytic properties
of sulfoalkyl-substituted azolium-derived NHC complexes in the
hydrogenation of aromatic ketones in aqueous media.
2. Experimental
All reactions were carried out under an argon atmosphere using
standard Schlenk techniques. Solvents were dried by standard
procedures and distilled under argon prior to use. Chemicals
purchased from Alfa Aesar were used without further purification.
NMR measurements were carried out using a Bruker Avance DPX
400 spectrometer and Bruker Avance DPX 300 spectrometer.
Elemental analysis (C, H, N) was performed by the Microanalytical
Laboratory of TUM. Mass spectra were performed with a Finnigan
MAT 90 spectrometer using FAB technique. Catalytic runs were
monitored by GC methods on a Hewlett-Packard instrument HP
5890 series II equipped with a FID and DB23 column.
2.2.1. Complex 3 (C20H27ClN2NaO3RhS)
1H NMR: (400 MHz, D2O, 20 ꢀC, ppm),
d[ppm]: 1.91 (m, COD
allyl), 2,32 (pq, 2H, CH2), 2.45 (pq, 2H, CH2), 3.05 (t, 2H, CH2), 3.23,
3.58 (COD vinyl), 4.11 (s, 3H, CH3), 4.52 (t, 2H, CH2), 7.26 (d, 2H,
CHCHarom), 7.41 (d, 2H, CHarom). 13C-MAS NMR (75.47 MHz,
25 ꢀC),
d[ppm]: 24.3 (CH2), 31.4 (COD allyl), 35.8 (CH2), 51.1 (NCH3),
51.2 (CH2) 69.6 (COD allyl), 92.3, 99.5(COD vinyl), 115 (CHCHarom),
122.8 (CHCHarom),135.5 (NCCNarom), 195.5 (NCcarbN). EA calcd. for
(C20H27ClN2NaO3RhS): C: 44.74 H: 5.07 N: 5.22 S: 5.97, found: C:
42.96 H: 5.28 N: 5.60 S: 6.65. MS-FAB: m/z (%) ¼ 512.1 (Mꢁ).
2.1. Synthesis of 1-methyl-3 (butyl-4-sulfonate) benzimidazolium
betaine (2)
2.2.2. Complex 4 (C20H27ClIrN2NaO3S)
1H NMR: (400 MHz, D2O, 25 ꢀC),
d[ppm]: 1.79 (m, COD allyl), 2.11
(pq, 4H, CH2), 2.93 (t, 2H, CH2), 3.85 (COD vinyl), 4.06 (s, 3H, CH3),
4.34 (COD vinyl), 4.48 (t, 2H, CH2), 7.62 (d, 2H, CHCHarom), 7.77,
Benzimidazole (1.32 g, 10 mmol) was stirred with 1,4-
butanesultone (1.36 g, 10 mmol) at room temperature for 3 days
7.83 (d, 2H, CHarom). 13C-MAS NMR (75.47 MHz, 25 ꢀC),
d[ppm]:
23.8 (CH2),26.3(CH2), 30.7 (COD allyl), 34.1 (CH2), 48.3 (NCH3), 50.8
(CH2), 85.7 (CODvinyl), 111.4 (CHCHarom), 124.7 (CHCHarom), 135.5
(NCCNarom), 191.7 (NCcarbN). EA calcd. for (C20H27ClIrN2NaO3S): C:
38.36 H: 4.35 N: 4.47 S: 5.12, found: C: 37.89 H: 4.68 N: 4.97 S: 6.02.
MS-FAB: m/z (%) ¼ 602 (Mꢁ).
O
+
S
N
N
O
O
N
N
(CH2)4SO3
CH3
CH3
2.2.3. Complex 5 (C16H25N2RhClNaO3S)
2
1H NMR (400 MHz, D2O, 25 ꢀC),
d[ppm]: 1.71 (m, COD allyl), 2.42
Scheme 2. Synthesis of ligand 2.
(pq, 2H, CH2), 2.87 (pq, 2H, CH2), 3.55, 3.67 (COD vinyl), 3.98 (t, 2H,
Scheme 3. Rhodium e and iridium e NHC complexes 3e5.

58
H. Syska et al. / Journal of Organometallic Chemistry 703 (2012) 56e62
Et
O
Cl
M
M
M
M
2NaOEt
O
Cl
-2NaCl
Et
M = Rh, Ir
M= Rh, Ir
NHC
2(NHC-H)+X-
-2 EtOH
M
X
M= Rh, Ir
3 - 5
Scheme 4. Synthesis of rhodium e and iridium e NHC complexes 3e5.
2NaOEt
MeOH
N
N
N
N
(CH2)4SO3
(CH2)4SO3Na
..
RuCl2(p-cymene)
+
2
N
N
(CH2)4SO3Na
..
Cl
Ru
N
Cl
N
(CH2)4SO3Na
6
Scheme 5. Synthesis of the ruthenium e NHC complex 6.

H. Syska et al. / Journal of Organometallic Chemistry 703 (2012) 56e62
59
Table 1
Aqueous hydrogenation of acetophenone, catalyzed by water-soluble carbene complexes.
No Catalyst Catalyst loading (mol%)
Conversiona (%)
Yieldb (%)
Selectivityc (%)
(CH2)2SO3Na
N
a)
þ Rh (III) Acetate
2.5
92
74
81
78
95
CH
N
(CH2)2SO3
(CH2)2SO3Na
1
N
N
b)
þ [RuCl2 (p-cymene)]2
2.5
75
58
CH
(CH2)2SO3
(CH2)4SO3
N
N
2
þ [RuCl2 (p-cymene)]2
2.5
68
65
CH
CH3
N
3
2.5
95
89
94
N
Rh
(CH2)4
SO3Na
Cl
N
4
2.5
51
37
71
N
Ir
(CH2)4
SO3Na
Cl
N
N
5
2.5
98
46
47
Rh
(CH2)4
SO3Na
Cl
Cl
Ru
6
2.5
90
87
96
N
Cl
N
(CH2)4SO3Na
a
Conversion ¼ (moles acetophenone reacted)/(moles acetophenone fed).
Yield ¼ (moles 1-phenylethanol formed)/(moles acetophenone fed).
b
c
Selectivity ¼ (moles 1-phenylethanol formed)/(moles acetophenone reacted).

60
H. Syska et al. / Journal of Organometallic Chemistry 703 (2012) 56e62
CH2), 4.15 (s, 3H, CH3), 4.50 (t, 2H, CH2), 7.12 (1H, NCHN), 7.16, (1H,
NCHN). 13C-MAS NMR (75.47 MHz, 25 ꢀC),
[ppm]: 23.9 (CH2), 30.7
(CH2),31.6 (COD allyl), 39.0 (CH2), 51.9 (NCH3), 57.7 (CH2), 67.3, 89.6,
96.6 (COD vinyl), 124.0 (NCHCHN), 180.6 (NCcarbN). EA calcd. for
(C16H25N2RhClNaO3S): C: 39.48 H: 5.18 N: 5.75 S: 6.59, found: C:
38.04 H: 5.36 N: 6.18 S: 7.97. MS-FAB: m/z (%) ¼ 462.9 (Mꢁ).
(1C, CiPr, p-cymene), 112.87 (CHCHarom), 126.89 (CHCHarom),
131.53, 132. 15 (NCCNarom), 180.09 (NCcarbN). EA calcd. for
(C22H29Cl2RuN2NaO3S): C: 44.30 H: 4.90 N: 4.70 S: 5.38 Cl: 11.89
Na: 3.85, found: C: 44.07 H: 5.30 N: 5.03 S: 5.42 Cl: 12.13 Na: 3.5.
MS-FAB: m/z (%) ¼ 573 (Mꢁ).
d
2.4. General procedure for hydrogenation [59]
2.3. Synthesis procedure for of the carbene complexes (6)
In a Schlenk tube 0.06 mmol of ligand (1, 2) and 0.96 ml of 1 M
aqueous sodium hydroxide solution was added to 10 ml of water
under argon and the mixture was degassed. After complete disso-
lution of the ligand, 0.03 mmol of metal precursor (Rh (III) acetate,
[RuCl2(p-cymene)]2) was added. After stirring for 24 h at room
temperature, the solution was transferred to a 50 ml stainless steel
glass coated autoclave and 1.2 mmol of substrate was added. The
autoclave was purged with hydrogen and final pressure was
adjusted to 40 atm. The mixture was stirred at room temperature
for 21 h and then analyzed by gas chromatography to determine the
conversions and reaction yields using diethylene glycol dibutyl
ether as an internal standard.
1 ml of 1 M NaOEt/MeOH solution was slowly added to
a suspension of 1 mmol azolium precursor in 10 ml methanol. After
the mixture was stirred for 24 h at room temperature and free
carbene was formed, 0.5 mmol ruthenium precursor [RuCl2(p-
cymene)]2 in 5 ml methanol was added. The suspension was stirred
at room temperature for another 24 h. Methanol was removed
under vacuum. The residue was then washed with diethyl ether and
dried under high vacuum.
2.3.1. Complex 6 (C22H29Cl2RuN2NaO3S)
1H NMR (400 MHz, CD3OD, 25 ꢀC),
d[ppm]: 1.31 (m, 6H, CH
For comparison, the same reaction condition were applied for
the isolated NHC complex catalysts (3e6)
(CH3)), 1.85 (pq, 2H, CH2), 2.17(s, 3H, C(CH3)), 2.19(m, 2H, CH2), 2.76
(sept, 1H, CHCH3), 2.87 (t, 2H, CH2), 4.14 (s, 3H, N-CH3), 4.57(m,
NCH2), 5.29(m, 2H, C6H4), 5.54 (m, 2H, C6H4), 7.70(d, 2H,
CHCHarom), 7.92 (d, 2H, CHCHarom). 13C NMR (100.5 MHz, CD3OD,
3. Results and discussion
25 ꢀC),
d[ppm]: 17.35 (CH(CH3)), 20.90 (C(CH3)), 21.35 (CH2), 27.44
(CH2), 31.16 (CH(CH3)), 31.32 (CH2), 46.47 (NCH3), 49.84 (NCH2),
76.26 (2C, C6H4), 78.27 (2C, C6H4), 93.54 (1C, CMe, p-cymene), 97.28
For the synthesis of water-soluble NHC complexes, sulfonated
N-alkylazolium salts were applied to prepare the ligands. Ligand 1
Table 2
NMR results.
1H
13C
(CH ) SO
1.81 (pq, 2H, CH2), 2.14 (pq, 2H, CH2), 2.94 (t, 2H,
CH2), 4.09 (s, 3H, CH3), 4.53 (t, 2H, CH2), 7.69 (d,
2H, CHCHarom), 7.83, 7.9 (d, 2H, CHCHarom), 9.28
(s, 1H, NCHN).
21.13 (CH2), 27.06 (CH2), 32.78 (CH2), 46.26
(N-CH3), 49.98 (CH2), 112.89, 126.62, 130.86
(CHCHCHCHarom), 132.02 (NCCNarom), 141.07
(NCHN).
N
Ligand 2
CH
N
CH
1.91 (m, COD allyl), 2,32 (pq, 2H, CH2), 2.45 (pq,
2H, CH2), 3.05 (t, 2H, CH2), 3.23, 3.58 (COD vinyl),
4.11 (3H, CH3), 4.52 (t, 2H, CH2), 7.26 (d, 2H,
CHCHarom), 7.41 (d, 2H, CHCHarom).
24.3 (CH2), 31.4 (COD allyl), 35.8 (CH2), 51.1
(N-CH3), 51.2 (CH2) 69.6 (COD allyl), 92.3, 99.5(COD
vinyl), 115 (CHCHarom), 122.8 (CHCHarom), 135.5
(NCCNarom), 195.5 (NCcarbN).
N
Complex 3
N
Rh
(CH )
SO Na
Cl
1.79, (m, COD allyl), 2.11 (pq, 4H, CH2), 2.93 (t, 2H,
CH2), 3.85 (COD vinyl), 4.06 (3H, CH3), 4.34 (COD
vinyl), 4.48 (t, 2H, CH2), 7.62 (d, 2H, CHCHarom),
7.77 (d, 1H, CHarom), 7.83(d, 1H, CHarom).
23.8(CH2), 26.3(CH2), 30.7 (COD allyl), 34.1 (CH2),
48.3 (NCH3), 50.8 (CH2), 85.7 (CODvinyl), 111.4
(CHCHarom), 124.7 (CHCHarom), 135.5
(NCCNarom), 191.7 (NCcarbN).
N
Complex 4
Complex 5
N
Ir
(CH )
SO Na
Cl
N
1.71 (m, COD allyl), 2.42 (pq, 2H, CH2), 2.87 (pq,
2H, CH2), 3.55, 3.67 (COD vinyl), 3.98 (t, 2H, CH2),
4.15 (3H, CH3), 4.50 (t, 2H, CH2), 7.12 (d, 1H,
NCHN), 7.16 (d, 1H, NCHN).
23.9 (CH2), 30.7 (CH2), 31.6 (COD allyl), 39.0 (CH2),
51.9 (NCH3), 57.7 (CH2), 67.3, 89.6, 96.6 (COD vinyl),
124.0 (NCHCHN), 180.6 (NCcarbN).
N
Rh
(CH )
SO Na
Cl
17.35 (CH(CH3)), 20.90 (C(CH3)), 21.35 (CH2), 27.44
(CH2), 31.16 (CH(CH3), 31.32(CH2), 46.47 (NCH3),
49.84 (NCH2), 76.26 (2C, C6H4), 78.27 (2C, C6H4),
93.54 (1C, CMe, p-cymene), 97.28 (1C, CiPr, p-
cymene), 112.87 (CHCHarom), 126.89 (CHCHarom),
131.53, 132.15 (NCCNarom), 180.09 (NCcarbN).
Cl
1.31 (d, 6H, CH (CH3)), 1.85 (pq, 2H, CH2), 2.17 (s,
3H, C (CH3)), 2.19 (pq, 2H, CH2), 2.76 (sept, 1H,
CHCH3), 2.87 (t, 2H, CH2), 4.14 (s, 3H, NCH3), 4.57
(t, NCH2), 5.29 (m, 2H, C6H4), 5.54 (m, 2H, C6H4),
7.70 (d, 2H, CHCHarom), 7.92 (d, 2H, CHCHarom).
Ru
Complex 6
N
Cl
N
(CH ) SO Na

H. Syska et al. / Journal of Organometallic Chemistry 703 (2012) 56e62
61
(see Scheme 1) was synthesized by reaction of imidazole with
dimethylacetamide, triethylamine, and the sodium salt of 2-
bromoethanesulfonic acid similar to a previously published work
[60]. Ligand 2 (see Scheme 2) was synthesized via the reaction of
methyl benzimidazole and 1,4-butanesultone at room temperature,
similar to a previously published procedure, using a different azo-
lium salt, however (see experimental part and ref. [60])
The new complexes 3e5 (see Scheme 3) were prepared by in
situ deprotonation of azolium salts and subsequent reaction of
rhodium/iridium precursors (Rh(COD)Cl)2/(Ir(COD)Cl)2 with the
sulfonated ligands 2 at ambient temperature (see Scheme 4).
Complexes 3e5 are readily soluble and stable in H2O.
ring is seen at 31.16 ppm. Substituted C atoms of p-cymene ring
result in the signal at 76.26 and 78.27 ppm. The NCH3 signal is seen
at 46.47 ppm and the CH2 signals of the ligand are observed at
21.35, 27.44, 31.32 and 49.84 ppm respectively. For the better
understanding of NMR, Table 2 gives an overview of the signals of
the ligands and the complexes.
The catalytic activities of complexes 3e5 were examined in the
hydrogenation of acetophenone in water under a pressure of
40 atm H2 at room temperature. In order to be able to compare the
activity of the catalysts, a fixed reaction time of 21 h was applied.
Complexes 3e5 show good activities in basic media in the aqueous
hydrogenation of acetophenone under hydrogen pressure without
any phase-transfer agent. In situ formed complexes of Ru and Rh
with ligands 1e2 also show quite good activities for aqueous
hydrogenation of acetophenone. The catalytic reaction results are
summarized in Table 1.
The mechanism of these hydrogenations catalyzed by those
complexes is not well examined at present. Based on the commonly
accepted mechanism together with observations made in the
literature by Bujoli et al. [59] and Zhang et al. [61] (see Scheme 6),
the main role of the added base appears to be deprotonation of the
intermediate A, while water can protonate the alkoxide ligand. A is
obtained from the insertion of a ketone into one of the M-H bonds.
A push-pull process would favor the reductive elimination of A,
allowing the oxidative addition of hydrogen to generate B. More
research is currently under way to gain better insight into the
reaction mechanism.
With the same carbene ligand, the Rh complex 3 produces
a higher yield, conversion, and selectivity than compound 4 with
the larger Ir as central atom. Furthermore, 3, as benzimidazole
derived complex, leads to higher yield and selectivity, but almost
the same conversion, when compared to the imidazole derived
In situ deprotonation of azolium salt to form free carbene and
subsequent reaction of ruthenium precursor [RuCl2(p-cymene)]2
were used to prepare the new complex 6 (see Scheme 5).
In the 1H NMR spectrum of complex 3, pseudo-quintets are
observed at
d
(1H) ¼ 2.32 and 2.45 ppm and triplets at
d
(1H) ¼ 3.05
and 4.52 ppm. They are assigned to the non-equivalent CH2
protons. The signals at
ative for the COD protons. The signal at 4.11 ppm stems from the
CH3 methyl substituent. The CH signals of the benzimidazole ring
are observed at 7.26 and 7.41 ppm, respectively. In the 13C NMR
d
(1H) ¼ 1.91, 3.23 and 3.58 ppm are indic-
spectrum of complex 3, the signal at
d
(
13C) ¼ 195.5 ppm originates
from the Rh-C carbene carbon atom, whereas the signals at
d
(
13C) ¼ 31.4, 69.6, 92.3, and 99.5 ppm are caused by the COD
carbons. The NCH3 signal is observed at 51.1 ppm and the CH2
signals of the ligand are found at 24.3, 35.8, and 51.2 ppm.
For the 1H NMR spectrum of complex 4, the non-equivalent CH2
protons are observed at
pseudo-quintets and triplets, while the signals at
d
(1H) ¼ 2.11, 2.93, and 4.48 ppm, showed
d
(1H) ¼ 1.79, 3.85,
and 4.34 ppm are assigned to the COD protons. The signal at
4.06 ppm comes from the CH3 methyl substituent. The CH signals of
the benzimidazole ring are found at 7.62, 7.77, and 7.83 ppm,
respectively. In the 13C NMR spectrum of complex 4, the signal at
d
(
13C) ¼ 191.7 ppm stems from the Rh-C carbene carbon atom,
13C) ¼ 30.7 and 85.7 ppm are due to the
H
whereas the signals at
d
(
COD carbons. The NCH3 signal is observed at 48.3 ppm and the CH2
signals of the ligand can be seen at 23.8, 26.3, 34.1 and 50.8 ppm.
The 1H NMR spectrum of complex 5 shows pseudo-quintets at
L
L
S
H
S
S
H2
M
S
M
d
(1H) ¼ 2.42 and 2.87 ppm and triplets at 3.98 and 4.50 ppm, which
X
X
are assigned to the non-equivalent CH2 protons. The signals at
d
(1H) ¼ 1.71, 3.55 and 3.67 ppm are indicative for the COD protons.
(1H) ¼ 4.15 ppm stems from the CH3 methyl
B
The signal at
d
substituent. The CH signals of the benzimidazole ring are observed
H2O
at 7.12 and 7.16 ppm. For the 13C NMR spectrum of complex 5, the
13C) ¼ 180.6 ppm originates from the Rh-C carbene
OH
signal at d(
carbon atom, while the signals at
96.6 ppm come from the COD carbons. The NCH3 signal is seen at
51.9 ppm and the CH2 signals of the ligand are observed at 23.9,
30.7 and 39 and 57.7 ppm.
d(
13C) ¼ 31.6, 67.3, 89.6, and
O
S
OH-
H
H
Derived from a different metal precursor, complex 6 has NMR
signals differing from complexes 3e5. The non-equivalent CH2
L
L
S
H
S
S
protons for complex 6 are found at
d
(1H) ¼ 1.85, 2.19, 2.87 and
M
O
M
O
4.57 ppm. The signals at 1.85 and 2.19 ppm are seen as pseudo-
quintets, whereas the signals at 2.87 and 4.57 ppm are triplets.
The CH3 signals of the isopropyl group, being attached to the p-
+ S
X
X
cymene ring are seen at 1.31 ppm. The signal at
d
(1H) ¼ 2.17 ppm is
referred to the CH3 methyl substituent of p-cymene ring. The signal
at
d
(1H) ¼ 4.14 ppm stems from the CH3 methyl substituent of
benzimidazole. The CH signals of the benzimidazole ring are
A
observed at 7.70 and 7.92 ppm, respectively. For the 13C NMR
spectrum of complex 6, the signal at
from the RueC carbene carbon atom. The signals at
d
(
13C) ¼ 180.09 ppm stems
d(
13C) ¼ 17.35
S = H2O, M = Metal, X = Anion, L = Ligand
come from the isopropyl carbons. The CH3 methyl substituent of p-
cymene ring occurs at 20.90 ppm. The CH substituent of p-cymene
Scheme 6. Mechanism for the aqueous hydrogenation of acetophenone.

62
H. Syska et al. / Journal of Organometallic Chemistry 703 (2012) 56e62
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of the carbene ligand on directing the reaction. Particularly in
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follows
the
route:
acetophenone
/
1-
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Four new water-soluble complexes containing Ir, Rh, Ru as
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This work was supported by the Elitenetzwerk Bayern (PhD
grant for HS). Dr. Karl Öfele, Dr. Mirza Cokoja, Amylia Abdul Ghani,
Kevser Mantas Öktem are acknowledged for helpful discussions
and Fawzi Belmedjahed for technical support.
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