Polyhedron
Heteroleptic ruthenium(II) complexes of 2-(2-pyridyl)benzimidazoles: A
study of catalytic efficiency towards transfer hydrogenation of
acetophenone
a
b
Osman Dayan a, , Selin Demirmen , Namık Özdemir
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a Çanakkale Onsekiz Mart University, Department of Chemistry, Laboratory of Inorganic Synthesis and Molecular Catalysis, 17020 Çanakkale, Turkey
b Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Samsun, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Six ruthenium(II) complexes ([RuCl2(p-cymene)(L1–6)], SD1–6, (L1–6: bidentate pyridyl-benzimidazole
ligands)) were synthesized from [RuCl2(p-cymene)]2 dimer and bidentate pyridyl-benzimidazole ligands.
The compounds were characterized by elemental analysis, IR, UV–Vis, NMR and X-ray diffraction. The
synthesized Ru(II) complexes (SD1–6) were tested as catalysts for the catalytic transfer hydrogenation
(CTH) of acetophenone to secondary alcohols in the presence of KOH using 2-propanol as a hydrogen
source at 82 °C. All complexes were active catalysts for TH of acetophenone with good yields under mild
conditions (after 15 min, yields of up to 91%).
Received 26 August 2014
Accepted 15 October 2014
Available online 24 October 2014
Keywords:
Ru(II) complexes
Transfer hydrogenation
Benzimidazoles
Acetophenone
Pyridines
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
there are many attempts with poly-pyridyl ligands, the catalytic
studies of Ru(II) complexes bearing pyridyl-benzimidazoles was
Nitrogenous donor multidentate ligands are very attractive in
coordination chemistry because of their simplicity and availability
[1]. The steric and electronic properties around metal center easily
changed with these ligands. By the way, catalytically active metal
complexes having special properties have been synthesized. One
of those type ligands is a pyridine based bidentate ligand 2-(pyri-
din-2-yl)-1H-benzimidazoles. This chelator ligand can be easily
derivatized and strongly bind metal center.
not found for CTH. In this work, we have synthesized six ionic het-
eroleptic Ru(II) complexes containing bidentate pyridyl-benzimi-
dazoles and tasted as catalyst for the CTH of acetophenone. The
results showed that CTH of acetophenone has achieved with good
yields using simple ligands under mild conditions.
2. Experimental
On the other hand, the chemistry of ruthenium plays an impor-
tant role in many applications [2–9]. For example, Ru(II)-pyridyl
complexes has been attention due to potential applications in the
field of optoelectronic devices, medicinal chemistry, catalysis, etc.
[10–18]. Success depends on complex geometries and ligand mod-
ification in these applications.
The reactions were performed in air, unless otherwise stated.
The reagents and solvents were obtained from commercial suppli-
ers and used without further purification. [RuCl2(p-cymene)]2 [24]
were synthesized by modification of the method in the published
procedure. For other general experimental conditions, please see
supporting materials.
Because of easy working and relatively low cost, catalytic trans-
fer hydrogenation (CTH) of carbonyl compounds is preferred in
industrial and laboratory scale. Since discovered by Noyori, Ru(II)
complexes are the most popular catalysts for the CTH [19]. Since
that day, many studies show that catalytic efficiencies of com-
plexes are directly affected by ligand properties. For this purpose,
Ru(II) complexes including N-donor ligands have aimed to identify
the best catalyst for CTH of carbonyl compounds [20–23]. Although
2.1. Synthesis of compounds
2.1.1. General procedure for the ligands (L1–6)
L1 [25], L2 [26], L6 [27] were synthesized by modification of the
method in the published procedure.
A
solution of L1 (0.3 g, 2.54 mmol) and KOH (0.167 g,
2.54 mmol) was refluxed for 24 h in EtOH for L2–4 and in DMF
for L5–6. At the end of this time, the mixture was cooled room tem-
perature and added appropriate alkyl halide (2.54 mmol). Solution
was refluxed for another 24 h. The volatiles were removed under
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0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.