Ru(II) xantphos complex as an efficient catalyst in transfer hydrogenation of carbonyl compounds

Article abstract of DOI:10.1016/j.inoche.2011.04.013

In the transfer hydrogenation of aromatic/aliphatic carbonyl compounds using iso-propanol-potassium hydroxide mixture as hydrogen donor, a mixed ligand Ru(II) complex, prepared from xantphos ligand and RuCl₂(dmso) ₄, was examined. Besides, the solid state structure of the Ru (II) complex was determined using X-ray single crystallography. The catalytic reactivity of the complex was remarkable (up to 99%) and the catalyst showed more efficiency in the reduction of ketones than aldehydes.

Full text of DOI:10.1016/j.inoche.2011.04.013

Inorganic Chemistry Communications 14 (2011) 1161–1164
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Ru(II) xantphos complex as an efficient catalyst in transfer hydrogenation of
carbonyl compounds
⁎
Ali Nemati Kharat , Abolghasem Bakhoda, Bahareh Tamaddoni Jahromi
School of chemistry, University College of science, University of Tehran, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In the transfer hydrogenation of aromatic/aliphatic carbonyl compounds using iso-propanol–potassium
hydroxide mixture as hydrogen donor, a mixed ligand Ru(II) complex, prepared from xantphos ligand and
RuCl2(dmso)4, was examined. Besides, the solid state structure of the Ru (II) complex was determined using
X-ray single crystallography. The catalytic reactivity of the complex was remarkable (up to 99%) and the
catalyst showed more efficiency in the reduction of ketones than aldehydes.
Received 20 February 2011
Accepted 15 April 2011
Available online 23 April 2011
Keywords:
Ruthenium complex
Xantphos
© 2011 Elsevier B.V. All rights reserved.
Transfer hydrogenation
X-ray crystallography
Hydrogen bonding
C―H…π interaction
Phosphines and their derivatives have aroused much interest in
the recent years because of their reactivity, structural novelty and
catalytic activity [1–6]. Recent understanding within the catalytic
systems and the relationship between ligand properties and catalytic
performance will give access to more catalysts via rational design of
the ligands. An important aspect to design the ligands is based on their
natural bite angle, introduced by Casey and Whiteker [7] and is one of
the most extensively applied parameter for diphosphines. The metal
complexes containing large bite angle ligands such as xantphos, [9, 9-
dimethyl-4, 5-bis (diphenylphosphino) xanthene], and others with a
relatively similar moiety exhibit noticeable coordination chemistry
along with catalytic activity [8–10]. Over the last decades, the
ruthenium catalyzed reactions directed to organic synthesis have
got much attention and a large number of highly efficient synthetic
approaches are well documented in literature [11–17]. Recently, ex-
tensive research has been assigned to the improvement of ruthenium-
catalyzed hydrogenation of unsaturated polar bonds such as alde-
hydes and ketones [18–20]. Ru-P^N^P and Ru-P^O^P systems were
found to be active and selective catalytic systems for wide range of
substrates. Looking for an efficient and easily accessible complex, bis
(phosphine) and mixed donor phosphine-amine complexes RuII(P^N)
and RuII (P^N^N) have showed highly efficient catalysts for the
hydrogenation of esters to alcohols (or conversely, the dehydrogena-
tive coupling of alcohols to form esters)[21,22]. More recently,
xantphos pincer complexes of the Ru [23] have also been reported
to catalyze the transfer hydrogenation of carbonyl bonds with a high
efficiency. As part of our ongoing study, we report here the synthesis
and characterization of a ruthenium (II) complex of xantphos 1,
containing dimethyl sulfoxides and the catalytic activity of 1 was also
investigated in transfer hydrogenation reaction of variety of carbonyl
compounds.
1 was synthesized by simple reaction between xantphos and RuCl2
(dmso)4 in methylene chloride ([24] See Scheme 1).
Complex 1 crystallizes in a monoclinic system with space group
P21/n where the Ru atom is located at the center of a distorted
octahedral environment formed by two P donor and one O atom of
xantphos ligand, one S atom of dmso ligand and two Cl atoms. The
selected bond lengths and bond angles are presented in Fig. 1.
The longer bond length of Ru1―P1 (2.3298(6) Å) compared with
Ru1―P2 (2.2991(7) Å) may point to a weaker interaction in the
former and is likely to be dissociated during catalytic reaction. How-
ever, Ru―S (2.1919(7) Å) is another alternative for cleavage in the
catalytic cycle. The shorter Ru―O bond length (2.1587(17) Å) is the
hardest choice for cleavage during catalytic cycle. The natural bite
angle, 111.7° of the free xantphos ligand reduces to 105.48° (bite
angle, P1―Ru1―P2) upon coordination. The complex 1 exhibits some
interesting intermolecular interactions which stabilized the packing
of 1. The interactions between these molecules are presented by
⁎
Corresponding author at: PO. Box 13145–1357, Tehran, Iran. Tel.: +98 21
61112499; fax: +98 21 66495291.
E-mail address: alnema@khayam.ut.ac.ir (A.N. Kharat).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.04.013

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A.N. Kharat et al. / Inorganic Chemistry Communications 14 (2011) 1161–1164
Scheme 1. Synthetic route for synthesis of complex 1.
dotted lines and summarized in Fig. 2. The monomeric units of 1 show
intermolecular C―H…O, C―H…π and C―H…Cl interactions. These
are considered as weak to medium hydrogen bonds [25,26]. The
hydrogen bondings are principally observed between one of the H
atoms of the phenyl ring of the xantphos ligand and the Cl atom of the
complex. Also, an additional hydrogen bonding detected between one
hydrogen atom of methylene chloride solvent and Cl atoms of the
complex. Another hydrogen bonding between two units of the
complex by means of two dmso ligands plays a significant role in
stabilization of the packing of 1 (see Fig. 2).
Notably, the catalytic efficiency of 1 decreased in presence of the
large substituents on the substrate. This might be due to the steric
hindrance caused by the bulky xantphos ligand around the metal
center through substrate binding. These consequences can be justified
according to the proposed mechanism represented in Scheme 3. As
can be seen, the substrate should be coordinated to the metal center
for further migratory insertion step. Ketones have more electron
density on their oxygen due to the existence of alkyl groups.
Satisfactorily, ketones with small alkyl groups would be better
coordinated to the Ru center and can be reduced remarkably.
Such interactions provide an additional stabilization to crystal
consistency and are basically attributable to the supramolecular
nature of most inorganics. The catalytic activity of 1 was investigated
for transfer hydrogenation of carbonyl compounds. The complex 1
was found to be highly efficient in the reduction of aldehyde and
ketones to their corresponding alcohols (Scheme 2). Even though the
large number of Ru(II) catalysts reported for this particular trans-
formations, the use of RuII (P^O^P) systems are quite rare [27].
In the present study, the catalytic conversions of selected aldehydes
and ketones are found in the range 84–N99% within the reaction time of
8–24 h (See Table 1). Although the catalytic activity of the complex
displays slightly lower efficiency for the hydrogenation of aldehydes, it,
however, shows higher activity for hydrogenation of ketones with
respect to similar reported catalysts [23].In addition, iso–PrOH/KOH was
used as hydrogen source because of the higher solubility of KOH in iso–
PrOH rather than the cheaper sodium hydroxide.
On the whole, the catalytic activity of 1 is higher than other
structurally similar Ru (II) complexes, which may be due to the
presence of the dmso ligand. In conclusion, the synthesis and
characterization of the novel Ru(II) complex 1 has been performed.
Fig. 2. MERCURY view of C―H…Cl hydrogen bonding (blue dashed line) and C―H…π
interactions (magenta dashed line) in the packing of complex 1.
Fig. 1. The coordination-sphere labeled diagram of [Ru (P,O,P-xantphos) (S-dmso) Cl2] .
Thermal ellipsoids are displayed at 50% probability level. Hydrogen atoms and the
methylene chloride solvent molecule are omitted for clarity.
Scheme 2. Transfer hydrogenation of aldehydes and ketones using basic iso-propanol
catalyzed by complex 1.

A.N. Kharat et al. / Inorganic Chemistry Communications 14 (2011) 1161–1164
1163
Table 1
Catalytic transfer hydrogenation of carbonyl substrates by complexes 1a.
Entry
1
Substrate
O
Conversion (%)
Reaction Time (h)
88
8
H
O
2
3
89
83
8
H
H
MeO
Cl
O
10
O
4
5
81
10
H
H
H
86
97
8
S
O
O
6
7
12
Me
Et
O
94
12
Scheme 3. Proposed catalytic cycle for the reduction of carbonyl compounds.
O
8
9
Me
96
93
12
24
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Ne
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Appendix A. Supplementary material
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S bonded dmso). 1H NMR (CDCl3, ppm): 1.66 (s, 3H, CH3), 1.71 (s, 3 H, CH3), 3.13
(s, 3H, CH3, S bonded dmso), 3.15 (s, 3H, CH3, S bonded dmso). 31P {1H} NMR
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