Synthesis and Characterization of Ruthenium(II) Complexes Bearing Benzimidazole Ligands: For Transfer Hydrogenation Catalysis

Article abstract of DOI:10.1080/15533174.2013.862655

Ru(II) complexes with the general formula [RuCl₂(p-cymene)(L1-3)], K1-3, (L1-3: monodentate benzimidazole ligands), [RuCl₂(L5-6)(S)], K4-7, (L5-6: tridentate benzimidazole ligands, S: Solvents [methanol or acetonitrile]), and [RuCl₂(L5)(L1-2)], K8-9, were synthesized from [RuCl₂(p-cymene)]₂ dimer and mono- and tridentate benzimidazole ligands. The compounds were characterized by elemental analysis, IR, UV-Vis, and NMR. The synthesized Ru(II) complexes (K1-9) were tested as catalysts for the transfer hydrogenation (TH) 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 60 minutes, yields of up to 97%).

Full text of DOI:10.1080/15533174.2013.862655

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Synthesis and characterization of Ruthenium(II)
complexes bearing benzimidazole ligands: for transfer
hydrogenation catalysis
Osman Dayana & Mustafa Yunus İnana
a Çanakkale Onsekiz Mart University, Department of Chemistry, Laboratory of Inorganic
Synthesis and Molecular Catalysis, 17020 Çanakkale, Turkey
Accepted author version posted online: 31 Dec 2014.
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To cite this article: Osman Dayan & Mustafa Yunus İnan (2014): Synthesis and characterization of Ruthenium(II) complexes
bearing benzimidazole ligands: for transfer hydrogenation catalysis, Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry, DOI: 10.1080/15533174.2013.862655
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Synthesis and characterization of Ruthenium(II) complexes bearing benzimidazole
ligands: for transfer hydrogenation catalysis
Osman Dayana*, Mustafa Yunus İnana
a Çanakkale Onsekiz Mart University, Department of Chemistry, Laboratory of Inorganic
Synthesis and Molecular Catalysis, 17020 Çanakkale, Turkey.
Abstracts
Ru(II) complexes with the general formula [RuCl2(p-cymene)(L1-3)], K1-3, (L1-3: monodentate
benzimidazole ligands), [RuCl2(L5-6)(S)], K4-7, (L5-6: tridentate benzimidazole ligands, S:
Solvents (methanol or acetonitrile)) and [RuCl2(L5)(L1-2)], K8-9, were synthesized from
[RuCl2(p–cymene)]2 dimer and mono- and tridentate benzimidazole ligands. The compounds
were characterized by elemental analysis, IR, UV-Vis, and NMR. The synthesized Ru(II)
complexes (K1–9) were tested as catalysts for the transfer hydrogenation (TH) 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 60 minutes, yields of up to 97%).
Keywords: Ru(II) complexes, transfer hydrogenation, benzimidazoles, acetophenone
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1. Introduction
Transition metal complexes of benzimidazole ligands have been frequently utilized for
their unique properties such as biological activity, high thermal stability and good catalytic
performance [1-4]. Because of the outstanding properties of these ligands in many applications,
there is increased interest in synthesis of their derivatives. Benzimidazole and their derivatives
can be easily modified due to its pyrol type nitrogen. Thus, steric and electronic parameters on
the metal center bearing benzimidazole derivatives can be tunable, and the complex, with best
properties, can be synthesized [5-16].
In particular, recent studies have shown that the transition metal complexes (such as Cu,
Pd) of 2,6-bis(benzimidazol-2-yl)pyridines (a tridentate benzimidazole ligand) are especially
good catalyst for different catalytic reactions [1-2, 17].
The reduction of C=Y (Y=N or O) bonds via catalytic transfer hydrogenation using Ru(II)
catalyst is very attractive because of its simplicity and safety. Thus, studies in this area have been
continuing with increasing interest in the synthesis of the best catalyst. Notably, when Noyori's
Ru(II) catalysts, containing N-Donor ligands, provided good performance, scientists focused on
N-donor ligands [18-19]. In particular, several studies indicate that Ru complexes containing 2,6-
bis(benzimidazol-2-yl)pyridine ligands are good catalysts for the transfer hydrogenation of
ketones [4, 20-21].
In this study, Ru(II) complexes containing benzimidazole ligands (monodentate and
tridentate) have been synthesized and characterized using different spectroscopic techniques. In
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addition, their catalytic performance for transfer hydrogenation of acetophenone was
investigated.
2. Experimental
The reactions were performed in air, unless otherwise stated. The reagents and solvents were
obtained from commercial suppliers and used without further purification. L1 [22], L2-L3 [23],
L4 [24]; L5-L6 [4]; [RuCl2(p-cymene)]2 [25]; and [RuCl2(DMSO)4] [26] were synthesized by
modification of the method in the published procedure. All ligands were obtained in good yields
(Supporting Information).
NMR spectra were recorded at 297 K on a Bruker 300 MHz Ultrashield TM NMR
spectrometer at 300 MHz (1H), 75.48 MHz (13C). Chemical shifts () are given in parts per
million. Infrared spectra were measured with a Perkin Elmer Spectrum One FTIR system and
recorded using a universal ATR sampling accessory within the range 550–4000 cm−1. UV-Vis
spectra were measured with a Perkin Elmer Lambda 25 system. The C, H and N analyses were
performed using a CHNS–932 (LECO) instrument. Melting points were determined in open
capillary tubes on a digital Stuart SMP10 melting point apparatus. GC measurements for
catalytic experiments were performed using a Younglin Acme 6100 GC instrument with a flame
ionization detector and an Optima 5MS capillary column. (The GC parameters were as follows:
Oven: 80 °C (isothermal); Carrier gas: H2 (Split ratio 15:1); Flow rate: 4 mL/min.; Injector port
temperature: 220 °C; Detector temperature: 280 °C; Injection volume: 6.0 µL.)
2.1.
Synthesis of Compounds
[RuCl2(p-cymene)(L1)], (K1): L1 (0.0385 g, 0.326 mmol) and [RuCl2(p-cymene)]2 dimer
(0.1 g, 0.163 mmol) were placed in the same Schlenk tube and methanol (10 mL) as solvent was
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added. Then the mixture was stirred under reflux for 1 day. At the end of this time, the mixture
was concentrated to 3 mL, precipitated by addition of diethyl ether, and filtered off. The residue
was crystallized from MeOH/Et2O.
Yield: 83%, 0.063 g. m.p.: 264 °C. Anal. Calcd. for C17H23Cl2N2Ru: C: 48.12; H: 4.75; N:
1
6.60 %; found: C: 47.89; H: 5.12; N: 6,55 %. H-NMR (CDCl3, δ ppm): 1.09-1.22 (m, 6H, -Ht),
1.91-2.20 (s, 3H, -Hp), 2.67-2.88 (m, 1H, -Hs), 5.47-6.24 (m, 4H, -Hq,r), 7.04-7.38 (m, 4H, -
Hb,c,d,e), 8.57 (s, 1H, -Ha), 13.39-13.82 (br, 1H, -NH). 13C{1H}-NMR (CDCl3, ppm): 18.4 (aliph.
–CH), 22.0 (aliph. –CH), 22.5 (aliph. –CH), 30.5 (aliph. –CH), 80.7 (arom. –CH), 83.4 (arom. –
CH), 86.0 (arom. –CH), 86.9 (arom. –CH), 97.6 (arom. –CH), 101.0 (arom. –CH), 101.4 (arom.
–CH), 106.8 (arom. –CH), 115.6 (arom. –CH), 122.8 (arom. –CH), 133.1 (arom. –CH), 140.9
(arom. –CH), 146.3. IR (cm-1): 3142, 3110, 3091, 2970, 2912, 2872, 1622, 1595, 1489, 1414,
1383, 1304, 1245, 1160, 1133, 1108, 1057, 1008, 965, 951, 891, 871, 804, 776, 741, 696, 676.
UV(nm): 216, 274, 325, 404.
[RuCl2(p-cymene) (L2)], (K2): K2 was synthesized according to the synthesis method of
K1 using L2 (0.0679 g, 0.326 mmol) and [RuCl2(p-cymene)]2 dimer (0.1 g, 0.163 mmol).
Yield: 84%, 0.115 g. m.p.: 248 °C. Anal. Calcd. for C24H26Cl2N2Ru: C: 56.03; H: 5.09; N:
1
5.45 %; found: C: 55.75; H: 5.21; N: 5.82 %. H-NMR (CDCl3, δ ppm): 1.19-1.21 (m, 6H, -Ht),
2.09 (s, 3H, -Hp), 2.80-2.89 (m, 1H, -Hs), 5.49-5.84 (m, 6H, -Hf,q,r), 7.19-7.37 (m, 9H, -
Hb,c,d,e,g,h,i), 8.43 (s, 1H, -Ha). 13C{1H}-NMR (CDCl3, ppm): 18.4 (aliph. –CH), 22.0 (aliph. –
CH), 22.5 (aliph. –CH), 48.1 (aliph. –CH), 86.0 (arom. –CH), 86.9 (arom. –CH), 97.8 (arom. –
CH), 100.6 (arom. –CH), 106.8 (arom. –CH), 110.2 (arom. –CH), 120.0 (arom. –CH), 122.1
(arom. –CH), 122.9 (arom. –CH), 127.9 (arom. –CH), 129.2 (arom. –CH), 133.7 (arom. –CH),
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137.5 (arom. –CH), 142,2 (arom. –CH), 142.7 (arom. –CH), 144.7 (arom. –CH). IR (cm-1): 3133,
3120, 3062 ; 3041, 2977, 2960, 2925, 2867, 1614, 1588, 1512, 1461, 1448, 1392, 1362, 1291,
1258, 1208, 1184, 1160, 1118, 1080, 1056, 1012, 1004, 952, 866, 802, 774, 741, 726, 698, 663.
UV(nm): 211, 268, 324, 403.
[RuCl2(p-cymene)(L3)], (K3): K3 was synthesized according to the synthesis method of
K1 using L3 (0.0862 g, 0.326 mmol) and [RuCl2(p-cymene)]2 dimer (0.1 g, 0.163 mmol).
Yield: 92%, 0.158 g. m.p.: 225 °C. Anal. Calcd. for C28H34Cl2N2Ru: C: 58.94; H: 6.01; N:
1
4.91%; found: C: 58.78; H: 6.25; N: 4.83%. H-NMR (CDCl3, δ ppm): 1.01-2.23 (m, 21H, –
Hg,h,p,t), 2.79-2.88 (m, 1H, -Hs), 5.40-6.03 (m, 6H, -Hf,q,r), 7.00-8.00 (m, 6H, -Ha,b,c,d,e,i). 13C{1H}-
NMR (CDCl3, ppm): 15.4 (aliph. –CH), 15.8 (aliph. –CH), 20.7 (aliph. –CH), 22.0 (aliph. –CH),
30.5 (aliph. –CH), 43.9 (aliph. –CH), 80.6 (arom. –CH), 83.2 (arom. –CH), 86.0 (arom. –CH),
86.9 (arom. –CH), 98.2 (arom. –CH), 100.6 (arom. –CH), 100.9 (arom. –CH), 106.8 (arom. –
CH), 111.0 (arom. –CH), 120.0 (arom. –CH), 122.1 (arom. –CH), 122.9 (arom. –CH), 127.9
(arom. –CH), 129,2 (arom. –CH), 133.7 (arom. –CH), 137.5 (arom. –CH), 142.2 (arom. –CH),
142.7 (arom. –CH), 144.7 (arom. –CH). IR (cm-1): 3154, 3071, 3004, 2970, 2965, 2866, 1611,
1515, 1460, 1402, 1348, 1325, 1255, 1204, 1057, 1025, 1011, 947, 903, 875, 851, 803, 770, 756,
685, 655. UV(nm): 210, 269, 324, 398.
[RuCl2(L5)(CH3OH)], (K4): L5 (0.150 g, 0.305 mmol) and [RuCl2(p-cymene)]2 (0.0931
g, 0.152 mmol) were placed in the same Schlenk tube and methanol (10 mL) was added. The
mixture was stirred under reflux for 1 day. At the end of time, the reaction mixture was
concentrated to 3 mL, precipitated by addition of Et2O, and filtered off.
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Yield: 80%, 0.17g. m.p.: >280 °C. Anal. Calcd. for C34H29Cl2N5ORu: C: 58.71; H: 4.20; N:
1
10.07%; found: C: 58.39; H: 4.12; N: 11.21%. H-NMR (CDCl3, δ ppm): 3.17 (d, 3H, CH3OH),
4.13-4.18 (m, 1H, –OH), 6.35 (s, 4H, -Hf), 7.11-7.32 (m, 10H, -Hg,h,i), 7.63-8.09 (m, 8H, -
Hl,m,n,o), 8,32 (d, 2H, –Hk), 8,47 (t, 1H, -Hj). 13C{1H}-NMR (CDCl3, ppm): 48.6 (aliph. –CH),
49.1 (aliph. –CH), 112.2 (arom. –CH), 113.2 (arom. –CH), 119.1 (arom. –CH), 122.8 (arom. –
CH), 124.4 (arom. –CH), 126.3 (arom. –CH), 126.6 (arom. –CH), 127.0 (arom. –CH), 128.4
(arom. –CH), 129.5 (arom. –CH), 135.7 (arom. –CH), 136.3 (arom. –CH), 136.9 (arom. –CH),
137.4 (arom. –CH), 141.5 (arom. –CH), 142.1 (arom. –CH), 152.2 (arom. –CH), 152.5 (arom. –
CH), 153.8 (arom. –CH). IR (cm-1): 3486, 3231, 3059, 3026, 2970, 2895, 2877, 2847, 2776,
1596, 1519, 1499, 1485, 1471, 1454, 1438, 1414, 1360, 1333, 1310, 1297, 1282, 1256, 1191,
1180, 1155, 1121, 1095, 1081, 1026, 1002, 944, 916, 877, 861, 814, 789, 761, 752, 742, 734,
699, 684. UV (nm): 205, 318, 360, 444.
[RuCl2(L5)(CH3CN)], (K5): L5 (0.150 g, 0.305 mmol) and [RuCl2(p-cymene)]2 (0.0931 g,
0.152 mmol) were placed in the same Schlenk tube and acetonitrile (10 mL) was added. The
reaction mixture was stirred under reflux for 1 day. At the end of this time, the mixture was
concentrated to 3 mL, precipitated by addition of Et2O, and filtered off.
Yield: 98%, 0.21g. m.p.: >280 °C. Anal. Calcd. for C35H28Cl2N6Ru: C: 59.66; H: 4.01; N:
1
11.93%; found: C: 60.18; H: 3.85; N: 12.35%. H-NMR (CDCl3, δ ppm): 2.09 (s, 3H, CH3CN),
6.34 (s, 4H, -Hf), 7.15-7.32 (m, 10H, -Hg,h,i), 7.63-8.07 (m, 8H, -Hl,m,n,o), 8.31 (d, 2H, –Hk), 8.46
13
(t, 1H, -Hj). C{1H}-NMR (CDCl3, ppm): 1.7 (aliph. –CH), 48.5 (aliph. –CH), 113.9 (arom. –
CH), 119.1 (arom. –CH), 124.4 (arom. –CH), 126.2 (arom. –CH), 126.6 (arom. –CH), 127.0
(arom. –CH), 128.4 (arom. –CH), 129.5 (arom. –CH), 135.7 (arom. –CH), 136.9 (arom. –CH),
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137.3 (arom. –CH), 141.4 (arom. –CH), 152.2 (arom. –CH), 152.5 (arom. –CH). IR (cm-1):
3488, 3102, 3061, 3027, 1657, 1619, 1599, 1498, 1483, 1454, 1439, 1362, 1334, 1301, 1260,
1202, 1177, 1156, 1120, 1094, 1077, 1029, 1001, 989, 947, 902, 862, 842, 816, 781, 765, 752,
726, 691, 679. UV (nm): 209, 317, 345, 361, 499.
[RuCl2(L6)(CH3OH)], (K6): K6 was synthesized according to the synthesis method of K4
using L6 (0.12 g, 0.199 mmol) and [RuCl2(p-cymene)]2 dimer (0.061 g, 0.0995 mmol).
Yield: 80%, 0.128 g. m.p.: >280 °C. Anal. Calcd. for C42H45Cl2N5ORu: C: 62.45; H: 5.61;
1
N: 8.67%; found: C: 61.48; H: 5.86; N: 8.42%. H-NMR (CDCl3, δ ppm): 2.11-2.24 (d, 24H, -
Hg,h), 3.17 (d, 3H, CH3OH), 4.15-4.20 (m, –OH), 6.36 (s, 4H, -Hf), 6.54 (d, 2H, -Hl), 7.17 (s, 2H,
-Hi), 7.25 (t, 2H, –Hm), 7.45 (t, 2H, –Hn), 8.28 (t, 1H, -Hj), 8.42 (d, 2H, -Hk), 8,92 (d, 2H, -Ho).
13C{1H}-NMR (CDCl3, ppm): 16.0 (aliph. –CH), 20.7 (aliph. –CH), 48.9 (aliph. –CH), 49.1
(aliph. –CH), 113.7 (arom. –CH), 119.2 (arom. –CH), 125.7 (arom. –CH), 126.2 (arom. –CH),
130.5 (arom. –CH), 131.0 (arom. –CH), 133.0 (arom. –CH), 134.5 (arom. –CH), 135.8 (arom. –
CH), 136.1 (arom. –CH), 137.1 (arom. –CH), 141.9 (arom. –CH), 142.6 (arom. –CH), 152.9
(arom. –CH), 153.5 (arom. –CH). 154.6 (arom. –CH). IR (cm-1): 3575, 3049, 2994, 2970, 2917,
2894, 2863, 2787, 1612, 1599, 1499, 1459, 1447, 1430, 1389, 1372, 1345, 1329, 1308, 1289,
1276, 1237, 1203, 1186, 1154, 1113, 1087, 1056, 1039, 1012, 927, 902, 876, 859, 839, 820, 802,
756, 740, 696, 683, 664. UV(nm): 214, 318, 357, 368, 441.
[RuCl2(L6)(CH3CN)], (K7): K7 was synthesized according to the synthesis method of K5
using L6 (0.15 g, 0.248 mmol) and [RuCl2(p-cymene)]2 dimer (0.076 g, 0.124 mmol).
Yield: 95%, 0.20 g. m.p.: >280 °C. Anal. Calcd. for C43H44Cl2N6Ru: C: 63.23; H: 5.43; N:
1
10.29%; found: C: 62.96; H: 5.62; N: 9.58%. H-NMR (CDCl3, δ ppm): 2.06-2.12 (m, 24H, -
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Hg,h), 2.24 (s, 3H, CH3CN), 6.33 (s, 4H, -Hf), 6.48 (d, 2H, -Hl), 7.16-8.84 (m, 11H, -Harom.).
13C{1H}-NMR (CDCl3, ppm): 1.2 (aliph. –CH), 15.4 (aliph. –CH), 20.1 (aliph. –CH), 48.2
(aliph. –CH), 106.2 (arom. –CH), 114.2 (arom. –CH), 117.7 (arom. –CH), 118.7 (arom. –CH),
122.3 (arom. –CH), 125.1 (arom. –CH), 125.6 (arom. –CH), 130.0 (arom. –CH), 132.5 (arom. –
CH), 134.1 (arom. –CH), 134.9 (arom. –CH), 135.5 (arom. –CH), 142.0 (arom. –CH), 151.6
(arom. –CH), 152.9 (arom. –CH). IR (cm-1): 3383, 3060, 2969, 2916, 2868, 1606, 1591, 1571,
1528, 1509, 1459, 1450, 1031, 1383, 1355, 1325, 1294, 1253, 1233, 1205, 1171, 1121, 1085,
1031, 1008, 994, 950, 911, 880, 851, 824, 804, 764, 743, 683. UV(nm): 315, 341, 356, 490.
[RuCl2(L5)(L1)] (K8): L5 (0.157 g, 0.32 mmol) and [RuCl2(DMSO)4] (0.155 g, 0.32
mmol) were placed in the same Schlenk tube and ethanol (15 mL) was added. The mixture was
heated under reflux for 1 day. Then, L1 (0.038 g, 0.32 mmol) was added to the reaction mixture
and reflux process continued for 1 day more. At the end of this time, the mixture was
concentrated to 3 mL, precipitated by addition of Et2O, and filtered off.
Yield: 80%, 0.2 g. m.p.: >280 °C. Anal. Calcd. for C40H31Cl2N7Ru: C: 61.46; H: 4.00; N:
1
12.54%; found: C: 61.83; H: 4.23; N: 11.96%. H-NMR (CDCl3, δ ppm): 6.33 (d, 4H, –Hf),
6.89-8.71 (m, 26H, -Harom.). 13C{1H}-NMR (CDCl3, δ ppm): 13C{1H}-NMR spectrum of K8
could not be recorded because of its low solubility. IR(cm-1): 3098, 3058, 3025, 2971, 1620,
1597, 1507, 1498, 1486, 1459, 1439, 1357, 1333, 1304, 1260, 1230, 1217, 1204, 1154, 1088,
1030, 1012, 965, 915, 860, 814, 786, 755, 743, 693, 678. UV(nm): 214, 318, 343, 357, 434.
[RuCl2(L5)(L2)], (K9): K9 was synthesized according to the synthesis method for K8
using L5 (0.165g, 0.32 mmol), [RuCl2(DMSO)4] (0.155 g, 0.32 mmol) and L2 (0.038 g, 0.32
mmol).
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Yield: 77%, 0.2 g. m.p.: >280 °C. Anal. Calcd. for C47H37Cl2N7Ru: C: 64.75; H: 4.28; N:
11.25%; found: C: 64.28; H: 4.65; N: 10.89%. 1H-NMR (CDCl3, δ ppm): 5.56 (s, 2H, –Hf), 6.33
(s, 4H, –Hf), 7.14-8.74 (m, 31H, -Harom). 13C{1H}-NMR (CDCl3, δ ppm): 13C{1H}-NMR
spectrum of K9 could not be recorded because of its low solubility. IR(cm-1) : 3635, 3465, 3103,
3085, 3059, 3023, 2971, 2922, 1629, 1599, 1507, 1497, 1486, 1460, 1456, 1438, 1355, 1332,
1308, 1260, 1198, 1156, 1121, 1085, 1053, 1030, 1002, 957, 916, 860, 812, 786, 756, 730, 697,
677. UV(nm): 317, 343, 358, 433.
2.2.
Transfer hydrogenation of acetophenone using Ru(II) complexes as catalysts:
In a typical catalytic reaction, a Ru(II) complex (0.0085 mmol) and acetophenone (0.849
mmol) was dissolved in isopropyl alcohol (4 mL). This mixture was stirred at 82 °C for 10
minutes. After cooling to room temperature, KOH (0.849 mmol) was introduced and the mixture
was stirred at 82 °C. Reactions were monitored by GC.
3. Result and discussion
In this study, Ru(II) complexes (K1-9) containing monodentate-, tridentate- and
monodentate+tridentate-benzimidazole ligands were synthesized, characterized and their
catalytic application for transfer hydrogenation of acetophenone was investigated. For this
purpose, benzimidazole (L1), in the form of light yellow crystals, was prepared in good yield
using o-phenylene diamine and formic acid in hot water. L2 and L3 were synthesized in good
yield by the reaction of L1, KOH and RX (RX= C6H5CH2Cl and 2,3,5,6-(CH3)4-C6HCH2Cl,
respectively) in refluxing acetone. Similarly, L4 as a cream solid was prepared in good yield
using pyridine-2,6-dicarboxylic acid and o-phenylene diamine in polyphosphoric acid at 190 °C.
Finally, L5 and L6, in the form of yellow solids, were synthesized in good yield by the reaction
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of L4, KOH and RX (RX= C6H5CH2Cl and 2,3,5,6-(CH3)4-C6H1CH2Cl, respectively) (Scheme
1). L1-6 were soluble in most organic solvents such as THF, MeOH, DMF, DMSO, CHCl3 and
CH2Cl2, and spectral properties and melting points of these compounds are compatible with the
literature (Supporting Information).
Six-coordinate ruthenium complexes, K1-9, were synthesized by the appropriate use of
these ligands. Heteroleptic [RuCl2(p-cymene)(L1-3)], (K1-3) type complexes which contain
monodenate benzimidazole ligands were synthesized by the reaction of [RuCl2(p-cymene)]2 and
L1-3 in MeOH at room temperature as yellow-orange solids. K1-3 complexes are soluble in
chlorinated solvents such as CHCl3 and CH2Cl2. K4-7, [Ru(L5-6)(Solvent)Cl2], type complexes
containing tridentate benzimidazole ligands were synthesized by the reaction of [(p-
cymene)RuCl2]2 and L5 or L6 in solvent (Solvent=CH3OH for K4 and K6; Solvent=CH3CN for
K5 and K7) at room temperature as purple-brown solids. Finally, [Ru(L5)(L1-2)Cl2] type
complexes which contain tridentate and monodentate benzimidazole ligands at the same time
were synthesized by the reaction of [Ru(DMSO)4Cl2], L5 and L1 or L2 in ethanol under reflux
conditions as brown solids (Scheme 2).
1
All complexes gave H– and 13C{1H}-NMR spectra corresponding to the proposed
formulations. From 1H-NMR spectra of heteroleptic aren-Ru(II) complexes containing
monodentate benzimidazole ligands (K1-3), the characteristic peaks belonging to –Hq and –Hr
protons were observed in the range 5.40-6.24 ppm. For K2 and K3, the peaks belonging to -Hf
protons were observed in the range 5.40-6.03 ppm together with –Hq and –Hr protons as
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multiplets. For these type complexes, the protons belonging to the benzimidazole fragment were
monitored above 7.00 ppm as various peaks. For K1, -NH proton was shown in the range 13.39-
13.82 ppm as a broad peak. From 1H-NMR spectra of [Ru(Cl)2(L5-6)(S)] type complexes (K4-
7), characteristic -Hk and -Hj protons were observed at 8.32 and 8.47 ppm for K4, at 8.31 and
8.46 ppm for K5, and at 8.42 and 8.28 ppm for K6. These -Hk and -Hj protons of K7 were
monitored in the range 7.16-8.84 ppm as multiples together with other aromatic peaks. From 1H-
NMR spectra of [Ru(Cl)2(L5)(L1-2)] type complexes (K8-9), the –Hf proton was observed at
6.33 ppm for K8, and at 5.56 and 6.33 ppm for K9. Other peaks for K8-9 were monitored in the
aromatic region as various multiplets with appropriate integral values. The total count of carbon
peaks for all complexes matched well with the composition of the complexes except for K8-9 in
13C{1H}-NMR spectra. 13C{1H}-NMR spectrum of K8-9 could not be recorded because of its
very low solubility. However, satisfactory elemental analyses were obtained for all complexes.
The characteristic C=N stretching vibration of the benzimidazole ring of L1-3 are
observed at 1620, 1613 and 1612 cm-1, respectively. Similarly, the C=C stretching vibration of
L1-3, another characteristic peak for benzimidazole derivatives, was observed at 1588, 1584 and
1586 cm-1, respectively. Upon coordination of Ru(II), K1-3, the C=N stretching vibration of the
benzimidazole ring slightly shifted to 1622, 1614 and 1611 cm-1. For 2,6-
bis(benzimidazolyl)pyridine derivatives (L4-6), the strong peaks at 1622, 1614 and 1611 cm-1
are attributed to the C=N stretching vibration of the benzimidazole ring. In the IR spectra of K4
and K6, which are derivatives of L5, C=N stretching vibration of the benzimidazole ring is
monitored at 1596 and 1612 cm-1, respectively. For K5 and K7, the peaks at 1619 and 1612 cm-1
are attributed to C=N stretching vibration of the benzimidazole ring, respectively. In the IR
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spectra of Ru(II) complexes bearing monodentate and tridentate benzimidazole ligands at the
same time, K8-9, C=N stretching vibration belonging to benzimidazole rings are monitored at
1620 and 1597 cm-1 for K8 and at 1629 and 1599 cm-1 for K9.
The UV-Vis absorption spectra were recorded in CH3OH at room temperature
(Supporting Information, Figure S1). The normalized UV-Vis absorption spectra of L1-3 and
their heteroleptic arene-Ru(II) complexes (K1-3) are shown in Figure S1a and b, respectively.
The L1-3 have three absorption bands in the range of 208-273 nm, which can be assigned to
ππ* transitions. The heteroleptic K1-3 complexes show two absorption bands in the range of
210-275 nm, which can be assigned to intra ligand ππ* transitions and another two absorption
bands in the range of 324-404 nm, which can be assigned to the spin-allowed metal to ligand
charge transfer transitions (MLCT). The normalized UV-Vis absorption spectra of L4-6 and their
Ru(II) complexes (K4-7) are shown in Figure S1c and d respectively. The L4-6 have two
absorption bands in the range of 215-316 nm, which can be assigned to intra molecular ππ*
transitions. The K4-7 complexes show three absorption bands in the range of 205-368 nm, which
can be assigned to intra ligand ππ* transitions and another one absorption band in the range of
441-499 nm, which can be assigned to the metal to ligand charge transfer transition (MLCT).
The UV-vis spectra of [Ru(L4-6)(S)Cl2] type complexes are very interesting. If S=CH3CN, the
maxima of the MLCT band are red shifted by approximately 50 nm compared to S=MeOH
analogues. The UV-Vis spectra of K8-9 are shown in Figure S1. The K8-9 complexes show four
absorption bands in the range of 214-358 nm, which can be assigned to intra ligand ππ*
transitions. The MLCT bands of K8-9 are very broad in the range of approximately 400-650 nm
with wavelength maxima at 434 and 433 nm.
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3.1.
Catalytic Studies
Transfer hydrogenation reactions catalyzed by transition metal complexes (especially Ru(II)
complexes) has attracted attention in both academic and industrial fields because multiple bonds
can be easily reduced under mild conditions. Due to this, new catalysts have been continuously
synthesized. In this study, the synthesized Ru(II) complexes (K1-9) were employed as catalyst
for transfer hydrogenation of acetophenone as model substrate of aryl ketones.
Catalytic reactions were performed under identical conditions to allow comparison of results and
optimization reactions were carried out using K7. First, catalytic reactions under inert
atmosphere were compared with open air. Results show that catalytic yields are good under inert
atmosphere. Second, a significant increase in the yield of the catalytic reaction was observed by
increasing the amount of base. Third, according to base survey results, the best catalytic yield
was provided by KOH. Finally, lowering the temperature of the catalytic reaction caused a
significant decrease in catalytic yield. Under thus optimized conditions, all Ru(II) complexes
were employed as catalyst for catalytic transfer hydrogenation of acetophenone. Results are
given in Table 1.
In short, the results show that K1-9 complexes are active catalysts under these conditions for
transfer hydrogenation of acetophenone. However, K4 and K5 are much more efficient catalysts
than the others. For [RuCl2(L5-6)(S)] type complexes, the complexes containing acetonitrile
ligand (K5 and K7) are slightly better catalysts than methanol containing analogues (K4 and
K6). [RuCl2(p-cymene)(L1-3)] type complexes showed relatively low catalytic activity
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compared to the complexes containing tridentate benzimidazole ligands except for K9. Lower
catalytic activity of K9 may be related to the low solubility of this complex.
4. Conclusions
In this work, Ru(II) complexes (K1-9) containing mono- and tri-dentate benzimidazole ligands
were synthesized, characterized and their catalytic efficiency for transfer hydrogenation of
acetophenone was investigated. All complexes are active catalysts under working conditions, and
the sequence of catalytic activity is K5>K4>K7>K6>K8>K1>K3=K9>K2. As expected, the
Ru(II) complexes containing tridentate benzimidazole ligands (K4-K7) are more active than
their arene containing analogues (K1-3).
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h
i
g
NH2
R
f
e
b
H
N
e
d
c
N
R
KOH
d
c
NH2
a
+
a
N
N
+
b
L1
Cl
L2-3
HCOOH
R
-H
L2, L5
L3, L6
-2,3,5,6-(CH3)4
NH2
NH2
HO
OH
+
N
O
O
R
R
j
k
R
H
N
H
N
KOH
N
N
N
N
+
l
N
N
N
N
m
Cl
o
n
L4
L5-6
Scheme 1. Synthesis of ligands and numbering scheme for the ligands
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R
R
[Ru(p-cymene)Cl2]2
+
L1-3
CH3OH
N
N
N
N
N
q
r
t
Cl Ru Cl
p
s
O
[Ru(p-cymene)Cl2]2
H3C
H
Cl
K4, K6
Ru
+
Cl
L5-6
R
R
N
N
N
N
N
CH3CN
N
N
R
Cl Ru Cl
K1-3
N
[Ru(DMSO)Cl2]
+
CH3
L5
K7
K5,
L1
L2
N
N
N
N
N
N
N
N
N
N
Cl Ru Cl
Cl Ru Cl
N
N
N
N
H
K8
K9
Scheme 2. Synthesis of complexes and numbering scheme for the ligands
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Table 1. Catalytic activity for transfer hydrogenation of acetophenone catalyzed by Ru(II)
complexes [a]
O
OH
OH
H
Ru(II), KOH
82 o
O
+
+
C
Entrie
Catalyst
Yielda, %
s
1
2
3
4
5
6
7
39
21
35
95
97
78
K1
K2
K3
K4
K5
K6
K7
85, 75b, 24c, 68d, 10e, 17f,
34g, 40h, 82i
8
9
68
35
K8
K9
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a TH of acetophenone under argon atmosphere. [Ru:KOH:Substrate][1:100:100]; 2–propanol (4
ml); 82 °C, 1h.
b TH of acetophenone under argon atmosphere. [Ru:KOH:Substrate][1:10:100]; 2–propanol (4
ml); 82 °C, 2h.
c TH of acetophenone under open air. [Ru:KOH:Substrate][1:10:100]; 2–propanol (4 ml); 82 °C,
2 h.
d TH of acetophenone. [Ru:KOH:Substrate][1:10:100]; 2–propanol (4 ml); 82 °C, 1h.
e TH of acetophenone. [Ru:Na2CO3:Substrate][1:100:100]; 2–propanol (4 ml); 82 °C, 1 h.
f TH of acetophenone. [Ru:K2CO3:Substrate][1:100:100]; 2–propanol (4 ml); 82 °C, 1 h.
g TH of acetophenone. [Ru:Cs2CO3:Substrate][1:100:100]; 2–propanol (4 ml); 82 °C, 1 h.
h TH of acetophenone. [Ru:NaOMe:Substrate][1:100:100]; 2–propanol (4 ml); 82 °C, 1 h.
i TH of acetophenone. [Ru:NaOH:Substrate][1:100:100]; 2–propanol (4 ml); 82 °C, 1 h.
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