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Ruthenium(II) complexes with pyridine-based Schiff base ligands: Synthesis, structural characterization and catalytic hydrogenation of ketones
Journal Pre-proof Ruthenium(II) complexes with pyridine-based Schiff base ligands: Synthesis, structural characterization and catalytic hydrogenation of ketones
Kenan Buldurun, Metin Özdemir PII:
S0022-2860(19)31375-4
DOI:
https://doi.org/10.1016/j.molstruc.2019.127266
Reference:
MOLSTR 127266
To appear in:
Journal of Molecular Structure
Received Date:
19 July 2019
Accepted Date:
21 October 2019
Please cite this article as: Kenan Buldurun, Metin Özdemir, Ruthenium(II) complexes with pyridinebased Schiff base ligands: Synthesis, structural characterization and catalytic hydrogenation of ketones, Journal of Molecular Structure (2019), https://doi.org/10.1016/j.molstruc.2019.127266
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Journal Pre-proof Ruthenium(II) complexes with pyridine-based Schiff base ligands: Synthesis, structural characterization and catalytic hydrogenation of ketones
Kenan Bulduruna, Metin Özdemira
aDepartment
of Chemistry Food Processing, Muş Alparslan University, 49250 Muş, TURKEY
*Corresponding
author: E-mail: [email protected]
ABSTRACT
This study investigated the synthesis and characterization of which was found as which was found as a result of the reaction of Schiff base ligands containing pyridine with [RuCl2(pcymene)]2. The spectroscopic techniques used for the characterization process were the elemental analysis, mass spectroscopy, FT-IR, UV-Vis and 1H and
13C
NMR. The results
showed that Ru(II) complexes occured through the coordination of azomethine N, carbonyl and hydroxyl O atoms of the ligands. Ru(II) complexes were utilized as catalysts for the transfer hydrogenation of a series of ketones in the i-propanol solution and in the presence of KOH. The catalytic reactions indicated that the Ru(II) complexes (1a-d) were effective catalysts in the transfer hydrogenation reaction.
Keywords: Schiff base, spectroscopic techniques, catalysis, Ru(II) complexes, transfer hydrogenation
Journal Pre-proof 1.
Introduction
Schiff bases are generally known as the imine (-C=N-) or azomethine (-HC=N-) group. Schiff bases attract significant attention due to their interesting and important properties (e.g., biological and catalytic activities, electroluminescent properties, fluorescence properties, and applications in organic photovoltaic materials and sensors) [1-3]. Schiff bases are among the most popular ligand systems in inorganic chemistry owing to their great σdonor and π-acceptor property. Due to the relationship between structure and reactivity, the electron donor and electron acceptor properties of the ligand affect the reactivity of the coordination compounds and also have an important impact on this activity [4-6]. Transition metal complexes derived from the Schiff base ligands have the versatile catalytic application for various fields [7]. In recent years, a wide range of half-sandwich ruthenium(II) Schiff base complexes (which present a variety of methods for the design and synthesis of novel compounds) have attracted considerable interest thanks to many interesting properties in the different area range from biological to catalysis chemistry [8,9]. A large number of Schiff bases and their Ru(II) complexes have been extensively investigated since they are effective catalysts in homogeneous and heterogeneous catalysis activity [10]. Moreover, half-sandwich ruthenium complexes containing Schiff base ligands with oxygen-nitrogen donor atoms of the catalytic activity play an important role in hydrogenation reactions [8]. Ruthenium complexes include a large number of ligands such as arlyazo ligands, acylthiourea ligand [11], NHC ligands [12], Schiff base ligands [4,13-15], Ndonor ligands [16-17], phenolate-oxazoline ligand. Particularly, pyridines ligand-based (Ru(II)-aren) catalysts are well-known for promoting transfer hydrogenation reaction in presence of variety base [8,18]. Half-sandwich ruthenium(II) Schiff base complexes also play
Journal Pre-proof a vital role as catalysts in the synthesis of valuable organic compounds. The Ru(II) complexes can be prepared from the easy access precursors under smooth reaction conditions. In this study, Schiff base ligands were prepared through the condensation of 6-tertbutyl 3-ethyl 2-amino-4,5-dihydrothieno[2,3-c]pyridine-3,6(7H)-dicarboxylate with different substituted benzaldehyde. The Schiff base reacted with metal salt [RuCl2(p-cymene)]2 (2:1) to give Ru(II) complexes (1a-d). The four new Ru(II) complexes were prepared, and their structure was characterized with elemental analysis, spectroscopy measurements (FT-IR, UVVis, mass spectroscopy and 1H-13C NMR) techniques. The Ru(II) complexes were used as catalysts in i-propanol for the transfer hydrogenation of various ketones in the presence of KOH.
2. Experimental
2.1. Material and physical measurements All reagents and solvents were commercially available and reagents were used without further purification. IR spectra were performed on a Perkin Elmer 65 instrument (KBr pellets) in the 4000-400 cm-1 range. Electronic spectra of the Ru(II) metal complexes (1a-d) were obtained in EtOH on a Shimadzu 1800 spectrophotometer in the range of 200-800 nm. 1H and 13C
NMR spectra were obtained with a Brucker 300 MHz. Microanalysis of C, H, N and S
were recorded on a Perkin-Elmer Model 240C analyzer. Mass spectra of the complexes were recorded on an Agilent 1100 MSD spectrophotometer. The catalytic conversions were monitored on an Agilent 6890N GC system using GC-FID by an HP-5 column of 30 m length, 0.32 mm diameter and 0.25 µm film thickness. Column chromatography was obtained through the silica gel 60 (70-230 mesh).
Journal Pre-proof 2.2. The general procedure to the synthesis of Ru(II) complexes(1a-d) [RuCl2(p-cymene)]2 (0.37 g, 0.6 mmol) was added into a solution of ligand (0. 50 g, 1.2 mmol) in 20 mL methanol. The reaction mixture was heated with stirring at 70 °C for 4-5 hours. At the end of the reaction, the precipitate of the orange powder product formed. Then, it was filtered and recrystallized from dichloromethane/diethyl ether (1:3) at room temperature. The product was washed several times with diethyl ether and subsequently dried in a vacuum.
C2 H5 O O
OC2 H5
N
S
Ru
Ru H C
O
O
Cl
Cl
Cl
+ 1/2
Cl
N
Cl
Methanol / 80 oC
Ru
O
N
S
HC
O
R
N
R
O 1a-d
Where, R= H, OCH3 , NO2, OH
+
O C2H5 O
H C
Ru O
N
OC2 H5
O
N
S
Cl-
Cl
O
N
N
S Cl
O
O
Ru
HC
O
1b
1a
+
+
C2 H5 O
C2 H5 O
O
O Ru
Ru O
N O
S
N
Cl-
Cl
O
HC
N
N S
HC
O
H3 CO
NO2 1c
Cl
1d
Scheme 1. Synthesis of the Ru(II) complexes (1a-d)
Cl . 2.5H2 O
Journal Pre-proof Synthesis of Ru(II) complex 1a. Yield: 78%, mp.: 242-245 °C. IR (KBr, ν cm-1): 3094, 3047 (-CH, Ar.), 2960, 2925 (-CH, Alip.), 1670 (C=O), 1577 (CH=N), 1549, 1490 (C=C, Ar.), 780 (C-S-C), 554, 525 (M-O), 500 (M-N), 463 (Ru-Cl). 1H NMR (300 MHz, CDCl3) δ: 9.51 (s, 1H, CH=N), 7.47-6.77 (m, 5H, Ar. -CH), 5.77-4.19 (m, 8H, p-cymene moiety, 2CH2), 4.02-3.14 (m, 4H,CH2 (pyridine moiety), 1.28-1.21 (d, 12H, -OC(CH3)3), 2.58-2.60 (m, 1H, -HC(CH3)2), 2.30 (s, 3H, -CH3), 1.21-1.28 (d, 6H, -HC(CH3)2). 13C NMR (75 MHz, CDCl3) δ: 165.00, 164.00 (C=O)2, 163.00 (CH=N), 137.00, 129.00, 130.00, 107.00, 78.88 (thionyl, p-cymene moiety, benzene ring, -OC(CH3)3, 58.96, 14.11 (-OC2H5), 44.50, 18.30 (pyridine moiety, p-cymene, -CH3), 28.40 (-OC(CH3)3), 30.40 (p-cymene, HC(CH3)2), 24.40 (p-cymene, HC(CH3)2). UV-Vis. (λmax, nm): π→π*, 213, 222, 233, 265; n→π*, 304, 397. Anal. Calc. for C32H40N2O4SRuCl2: C, 53.34; H, 5.55; N, 5.88; S, 4.44. Found: C, 53.40; H, 5.68; N, 3.90; S, 4.50. LC/MS/MS: m/z: 720.90 (Calc.), 720.97 (Found) [M+H ]+. Synthesis of Ru(II) complex 1b. Yield: 72%, mp.: 272-274 °C. IR (KBr, ν cm-1): 3055, 3028 (-CH, Ar.) 2925, 2869 (-CH, Alip.), 1686 (C=O), 1672 (CH=N), 1560, 11548 (C=C, Ar.), 1160 (C-O), 755 (C-S-C), 552, 525 (M-O), 499 (M-N), 463 (Ru-Cl). 1H NMR (300 MHz, CDCl3) δ: 8.83 (s, 1H, CH=N), 7.33-6.97 (m, 4H, Ar. -CH), 4.56-4.30 (m, 8H, pcymene moiety , -2CH2), 2.29 (s, 3H, -CH3), 3.35-2.87 (m, 4H, CH2 (pyridine moiety), 1.931.36 (d, 12H, -OC(CH3)3), 2-79-2.81 (m, 1H, -HC(CH3)2), 1.20-1.25 (d, 6H, -HC(CH3)2). 13C NMR (75 MHz, CDCl3) δ: 163.00, 155.00 (C=O)2, 162.00 (CH=N), 139.00, 133.00, 132.00, 130.00, 106.00, 87.00 (thionyl, p-cymene moiety, benzene ring, -OC(CH3)3), 60.00, 18.00 (OC2H5), 41.00, 17.00 (pyridine moiety), 29.00 (-OC(CH3)3), 30.40 (p-cymene, HC(CH3)2), 22.00-18.16 (p-cymene, -CH3, HC(CH3)2). UV-Vis. (λmax, nm): π→π*, 204, 209, 287; n→π*, 416. Anal. Calc. for C32H39N2O5SRuCl: C, 54.90; H, 5.75; N, 4.03; S,4.57. Found: C, 54.81; H, 5.53; N, 4.22; S,4.76. LC/MS/MS: m/z: 699.45 (Calc.), 699.49(Found) [M]+.
Journal Pre-proof Synthesis of Ru(II) complex 1c. Yield: 76%, mp.: 245-248 °C. IR (KBr, ν cm-1): 3093, 3047 (-CH, Ar.) 2970, 2955 (-CH, Alip.), 1669 (C=O), 1578 (CH=N), 1525, 1491 (C=C, Ar.), 1385 (C-NO2), 781 (C-S-C), 558 (M-O), 501 (M-N), 465 (Ru-Cl). 1H NMR (300 MHz, CDCl3) δ: 8.32 (s, H, CH=N), 7.46-6.80 (m, 4H, Ar. -CH), 4.20-4.01 (m, 6H, p-cymene moiety, -2CH2), 3.35-3.25 (m, 6H, p-cymene moiety, CH2 (pyridine moiety), 2.09-1.26 (d, 12H, -OC(CH3)3), 2.29-2.31 (m, 1H, -HC(CH3)2), 2.27 (s, 3H, -CH3), 1.20-1.26 (d, 6H, HC(CH3)2). 13C NMR (75 MHz, CDCl3) δ: 165.00, 163.00 (C=O)2, 159.00 (CH=N), 136.00132.00, 129.00-108.00, 90.00 (thionyl, p-cymene moiety, benzene ring, -OC(CH3)3), 49.00, 14.84 (-OC2H5), 41.00, 18.00 (pyridine moiety), 30.44 (-OC(CH3)3), 30.40 (p-cymene, HC(CH3)2), 22.08-18.24 (p-cymene, -CH3, HC(CH3)2). UV-Vis. (λmax, nm): π→π*, 216, 220, 258, 283; n→π*, 308, 398. Anal. Calc. for C32H39N3O6SRuCl2: C, 50.20; H, 5.09; N, 5.49; S, 4.18. Found: C, 50.22; H, 5.13; N, 5.58; S, 4.26. LC/MS/MS: m/z: 762.90 (Calc.), 762.63 (Found) [M-2H]-. Synthesis of Ru(II) complex 1d. Yield: 74%, mp.: 212-215 °C. IR (KBr, ν cm-1): 3411 (-OH), 3058 (-CH, Ar.) 2963, 2954 (-CH, Alip.), 1668 (C=O), 1585 (CH=N), 1531, 1492 (C=C, Ar.), 1247 (-CO3), 781 (C-S-C), 571, 510 (M-O), 486 (M-N), 461 (Ru-Cl). 1H NMR (300 MHz, CDCl3) δ: 8.89 (s, H, CH=N), 7.27-6.82 (m, 4H, Ar. -CH), 5.81-4.51 (m, 8H,pcymene moiety, -2CH2), 3.82-2.50 (m, 4H, CH2 (pyridine moiety), 1.33-1.44 (d, 12H, OC(CH3)3), 2.09-2.28 (m, 1H, -HC(CH3)2), 2.26 (s, 3H, -CH3), 1.19-1.22 (d, 6H, -HC(CH3)2). 13C
NMR (75 MHz, CDCl3) δ: 164.00, 163.00 ((C=O)2), 159.00 (CH=N), 133.00-132.00,
132.00-117.00, 77.23 (thionyl, p-cymene moiety, benzene ring, -OC(CH3)3), 56.00, 14.37 (OC2H5), 61.00, 18.00 (pyridine moiety), 28.43 (-OC(CH3)3), 29.00 (p-cymene, HC(CH3)2), 21.08-18.21 (p-cymene, -CH3, HC(CH3)2). UV–Vis. (λmax, nm): π→π*, 223, 239, 250, 269, 283; n→π*, 300, 333, 398, 411, 438. Anal. Calc. for C33H47N2O7.5SRuCl2: C, 49.81; H, 5.91;
Journal Pre-proof N, 3.52; S, 4.02. Found: C, 49.90; H, 6.01; N, 3.41; S, 4.16. LC/MS/MS: m/z: 794.90 (Calc.), 794.95 (Found) [M]+.
2.3. General procedure followed for the transfer hydrogenation of ketones
The catalytic transfer hydrogenations of ketones were studied via pre-catalyst Ru(II) (1a-d) (0.001 mmol). The base KOH (4.0 mmol) was dissolved in i-propanol (5 mL). Substrate ketone (2.0 mmol) was added into this solution, and the mixture was refluxed at 80 °C for 8 hours in an oil bath. Following the completion of the reaction, the mixture was cooled, filtered through silica gel or alumina bed and was eluted with the use of ethyl acetate/hexane (1:4) mixture. The purity of compounds was analyzed with GC and/or GCMS. All complexes (1a-d) were found tobe effective catalysts, providing conversions range of 70100%. The observed results were summarized in Table 3. In the current work, a series of novel Ru(II) complexes containing pyridine groupbased Schiff base were successfully synthesized, and they were employed as catalysts for the transfer hydrogenations of acetophenone derivatives. The performance of the catalysts in the transfer hydrogenations was monitored with the use of benzophenone as a model substrate.
O C CH3
+ Ru(II) Complex (1a-d)
80
R
H
i-PrOH, KOH o C,
8h
R
C CH3 OH
R= Br, H, OCH3, Ph Reaction conditions: Substrate (2.0 mmol), Ru(II) complex (0.001 mmol), KOH (4.0 mmol), i-PrOH (5 mL), 8h, 80 οC.
Journal Pre-proof Ru(II) complexes (0.001 mmol), ketone (2.0 mmol) and KOH (4.0 mmol) were placed in a Schlenk flask, and 5 mL of i-PrOH was added into the mixture. Then, the obtained mixture was refluxed at 80 °C for 8 hours. The samples periodically taken from the reaction medium were passed through the column and analyzed in GC in order to determine conversion rates. As a result, the optimum reaction time was found to be 8 hours. The solvent was removed, and then the residue was passed through the column in the ethylacetate/hexane (1:4) mixture. The product was purified by a column chromatography method and controlled by NMR and GC. After the optimum reaction conditions were established, the catalytic activities of Ru(II) complexes in transfer hydrogenations reactions were examined using substituted acetophenone derivatives. Table 3 shows the yields (%) determined according to substituted ketones. Catalytic activities in the transfer hydrogenations reaction were investigated in the presence of KOH base using various ketones (acetophenone, m-methoxyacetophenone, pbromoacetophenone, p-methoxyacetophenone and benzophenone) and using Ru(II)-Schiff base complexes as catalytic. Thus, the corresponding secondary alcohols were obtained as a result of the reaction of transfer hydrogenations with the use of different substituted ketones. Besides the optimization process, the influence of bases was examined through i-PrOH as a solvent and diverse bases. No reaction was observed in the absence of the base. However, with addition base, the transfer hydrogenations reactions completed within 8 hours. The use of strong inorganic bases (such as NaOH, KOH) provided the higher conversion of the corresponding alcohol products. On the other hand, weak inorganic bases (such as Cs2CO3, Na2CO3, KOButand K2CO3) were found to be less effective when compared to strong bases. KOH was observed to be a superior base. Considering the absence of catalyst, transfer hydrogenations was observed as 11% yield for 10 hours. (Table 1, entry 9).
Journal Pre-proof Table 1. Effect of various bases in the TH of acetophenone Entry 1 2 3 4 5 6 7 8 *9
Base KOH Cs2CO3 Na2CO3 K2CO3 KOBut NaOH KOH KOH
Time (h) 10 10 10 10 10 10 8 10 10
Yield (%) 95 51 27 85 68 57 95 11
*catalysts were not used.
Table 2. Conversion versus reaction time Entry 1
Solvent i-PrOH
Time (h) 12
Yield (%) 95
2 3
i-PrOH i-PrOH
10 8
95 86
Table 3. Transfer hydrogenation of ketones catalyzed by complexes (1a–d) Entry
Substrate
Base
1
Catalyst
Yield (%)
KOH
1a
95
KOH
1b
90
KOH
1c
83
4
KOH
1d
81
5
KOH
1a
86
KOH
1b
90
KOH
1c
79
KOH
1d
85
KOH
1a
84
2
O C CH 3
3
O C CH 3
6 7
H3CO
8 9
O Br
C CH3
Journal Pre-proof 10
KOH
1b
70
11
KOH
1c
75
12
KOH
1d
87
13
KOH
1a
73
KOH
1b
86
KOH
1c
71
16
KOH
1d
88
17
KOH
1a
80
KOH
1b
85
KOH
1c
100
KOH
1d
100
14 O H3CO
15
C CH3
18 O
19
C
20
Reaction Conditions: Ketone (2.0 mmol), catalyst (1a-d) (0.01 mmol), KOH (4.0 mmol), i-PrOH (5 mL), 8 h, 80 οC.
The yields in Table 3 indicate that complexes were generally effective catalysts in the transfer hydrogenation reactions. All Ru(II) complexes were found to show excellent catalytic activity against the aforementioned ketones. In general, catalytic products ranging from 70100% were obtained (Table 3). Such a highconversion may be ascribed to both the electronic effects of the groups in the structure of the ligand and the structures of the ketones used.
3. Results and discussion
3.1. NMR spectroscopic measurements
1H
NMR spectra of the complex 1b confirmed that OH group proton signals were lost,
and a set of signals appeared in the region of 4.81-4.32 ppm for the protons of the p-cymene
Journal Pre-proof rings in the complex. The disappearance of -OH protons may indicate that the ruthenium was coordinated by phenolic oxygen [7,19]. The 1H NMR spectra of the complexes 1a, 1c, 1d exhibited a singlet signal for the imine proton at 9.51-8.32 ppm, which was assigned to imine proton. In all the complexes, the methyl(isopropyl), methane(isopropyl) proton and methylgroup of the p-cymene showed doublets, multiplets around 1.19-1.28 and 2.09-2.81 ppm, singletin the region of 2.26-2.30 respectively. In all complexes aromatic protonsof the pcymene rings appeared in the region of 5.81-3.35 ppm. Furthermore, the 13C NMR spectra of complexes showed that the azomethine carbon and carbonyl carbon peaks were 163.30159.59 ppm. This confirmed that the ruthenium was coordinated through the imine nitrogen and carbonyl oxygen atom [20-28].
3.2. Infrared (IR) absorption spectroscopy
The IR spectral bands for the complexes (1a-d) were principally characterized by the strong bands at 3411 cm-1, 1686-1669 cm-1, 1672-1577 cm-1 and 781-755 cm-1, which were assigned to υ(OH, OH2), υ(C=O), υ(C=N) and υ(C-S-C), respectively. Moreover, some new bands with medium to weak band intensities did appear at 571-510 cm-1 and 501-486 cm-1, which were assigned to υ (M-O), υ (M-N). Anew band appeared at 465-461 cm-1. This was attributed to υ(Ru-Cl). The appearance of the broad strong absorption band (at about 3411 cm1)
in the complex (1d) was associated with the presence of lattice water molecules OH group.
It may be inferred from the infrared spectra data that CH=N (1a-d), C=O (1a, 1c, 1d) and ArOH (1b) coordinated with nitrogen, carbonyl group and phenolic oxygen atoms, respectively. These observations may confirm the coordination of the Schiff bases with Ru(II) metalion for all the complexes [21,28-31].
Journal Pre-proof 3.3. Electronic absorption spectra
For the absorption spectra of the Ru(II) complexes (1a-d) in ethanol, the medium intensity band at 304–398 nm and the high-intensity bands at 220-287 nm were observed, which were linked to the ligand-centered (LC) π→ π* and n→ π* transitions, respectively. The lowest energy absorption bands at 411-438 nm indicatedthe metal to ligand charge (MLCT) transitions [32,22]. The electronic spectra, FT-IR, NMR and elemental analysisof all the complexesmay suggest the presence of an octahedral (piano stool) geometry around ruthenium(II) metal ion [28-33]. Scheme 1 shows the proposed structure of the complexes.
3.4. Catalysis studies
The catalytic activities of synthesized Ru(II)-Schiff base complexes (1a-d) were investigated through the use of acetophenone derivatives. Consequently, these complexes were found to be active catalysts for transfer hydrogenation reactions. The evaluation of the yield values in Table 3 suggests that catalysts were usually effective in the hydrogenation reactions. From the comparison of the substituents, it was found that the complexes (1a-d) showed catalytic activity in the range of 95-81% when acetophenone was used as the substrate (Table 3). This may show that there was a limited effect of the groups coordinated to the Schiff base and particularly benzene ring on the activity of the catalysts. The complexes (1a-d) showed high catalytic activity ranging from 95-81% when mmethoxyacetophenone was used as the ketone (Table 3). This may indicate that the activity of the catalysts was not significantly affected by the electronic effects and position groups
Journal Pre-proof coordinated the ketone used and Ru(II) complexes. However, the activity of the catalysts (2b) was increased to some extent by theelectronic effects of coordinated groups (Table 3). When p-bromoacetophenone was used as the ketone, they were noted to show good catalytic activity ranging from 70-87%. This ratio shows that the yield of pbromoacetophenone was relatively lower in comparison with those of other ketones (e.g. mmethoxyacetophenone and acetophenone). This can be attributed to the electron-withdrawing group (-Br) coordinated to the ketone (Table 3). Particularly, the 1b complex was noticed with low catalytic activity (Table 3). It is clear in Table 3 that the 1b complex containing the –OH group aromatic ring of Schiff base exhibited low catalytic activity due to the electron-withdrawing group they possess. When p-methoxyacetophenone was used as the ketone, the complexes (1a-d) were found to show catalytic activity ranging from 71-88%. A yield of 88% was observed in the case of the complexes (1d) containing an electron donor group coordinated to the aromatic ring (methoxy), as seen in Table 3, entry: 16. The complexes (1c) containing the electronwithdrawing group bound to the aromatic ring exhibited a relatively low catalytic activity with a yield of 71% (Table 3, entry: 15). This may indicate that the electronic structure and position of the coordinated groups in the Ru(II) complex (1c) did affect the activity of the catalysts. The case in which benzophenone is used as the ketone, complexes (1a-d) showed high catalytic activity ranging from 80-100% (Table 3, entries: 17-20). The complexes (1c-1d), containing electron withdrawing-donating groups coordinated to the aromatic ring, exhibited very high catalytic activity with a yield of 100% (Table 3, entries: 19,20). This is probably due to the structure of ketone that was used as the substrate rather than the electronic effects of the groups present in the structures of the Ru(II) complexes used.
Journal Pre-proof The Schiff base-Ru(II) complexes (1a-d), which were used in the transfer hydrogenation reactions of ketones, yielded a good degree of catalytic conversion ranging from 70-100%. At the same time, the structure and position of the ligand-bound substituted groups increased their catalytic activity. It was found that the electronic structure of the Schiff base groups and the electronic effect of the substrate used did not significantly affect the activity of the catalysts. When the catalytic results of Ru(II) complexes were examined, it was observed that all complexes were active for transfer hydrogenation reactions and formed active catalytic systems under optimum conditions. Ru(II) complexes (1a-d) were found to be very good catalytic activities. Considering transfer hydrogenations reactions, the substituted ketones conversion into the corresponding secondary alcohol did occur at 80 °C and in ipropanol for 8 hours.
4. Conclusions
This study reported the synthesis of four novels Ru(II)-Schiff base complexes (1a–d) containing pyridine-based Schiff base ligands. The Ru(II)-Schiff base complexes were prepared through the reaction of the [RuCl2(p-cymene)]2 dimers with pyridine-based Schiff base ligand. The complexes were entirely characterized by the use of spectroscopy techniques. It was observed that the synthesized complexes were very effective catalysts in transfer hydrogenation of the ketones where KOH was used as the base. Catalytic activities of Ru(II) complexes were readily accessible, and such complexes were effective catalyst precursors for the transfer hydrogenation of ketones. The catalytic activities of these four pyridines based Ru(II)-Schiff base complexes did exhibiteexcellent activity in the transfer hydrogenation of ketones. Thus, to determine the effect of synthesized complex compounds
Journal Pre-proof as catalysts, more efficient, environmentally friendly and cost-effective methods have been developed by comparing them with literature studies.
Acknowledgment
Thank Scientific Research-Publications and Projects Research and Practice Center (BAYPUAM) under research project no BAP-18-TBMY-4902-01 for their support.
Journal Pre-proof References
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Journal Pre-proof Declaration of Interest Statement We (Kenan Buldurun, Metin Özdemir,) declare that there is no conflict of interests regarding the publication of this article.
Journal Pre-proof Highlights Ru(II) complexes containing pyridine-based Schiff base ligands were synthesized. Ru(II) complexes were characterized through different spectroscopic techniques. The catalytic activity of transfer hydrogenation (TH) was investigated. The catalytic activities of Ru(II) complexes were excellent in the TH of ketones.
Kenan Buldurun, Metin Özdemir PII:
S0022-2860(19)31375-4
DOI:
https://doi.org/10.1016/j.molstruc.2019.127266
Reference:
MOLSTR 127266
To appear in:
Journal of Molecular Structure
Received Date:
19 July 2019
Accepted Date:
21 October 2019
Please cite this article as: Kenan Buldurun, Metin Özdemir, Ruthenium(II) complexes with pyridinebased Schiff base ligands: Synthesis, structural characterization and catalytic hydrogenation of ketones, Journal of Molecular Structure (2019), https://doi.org/10.1016/j.molstruc.2019.127266
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Journal Pre-proof Ruthenium(II) complexes with pyridine-based Schiff base ligands: Synthesis, structural characterization and catalytic hydrogenation of ketones
Kenan Bulduruna, Metin Özdemira
aDepartment
of Chemistry Food Processing, Muş Alparslan University, 49250 Muş, TURKEY
*Corresponding
author: E-mail: [email protected]
ABSTRACT
This study investigated the synthesis and characterization of which was found as which was found as a result of the reaction of Schiff base ligands containing pyridine with [RuCl2(pcymene)]2. The spectroscopic techniques used for the characterization process were the elemental analysis, mass spectroscopy, FT-IR, UV-Vis and 1H and
13C
NMR. The results
showed that Ru(II) complexes occured through the coordination of azomethine N, carbonyl and hydroxyl O atoms of the ligands. Ru(II) complexes were utilized as catalysts for the transfer hydrogenation of a series of ketones in the i-propanol solution and in the presence of KOH. The catalytic reactions indicated that the Ru(II) complexes (1a-d) were effective catalysts in the transfer hydrogenation reaction.
Keywords: Schiff base, spectroscopic techniques, catalysis, Ru(II) complexes, transfer hydrogenation
Journal Pre-proof 1.
Introduction
Schiff bases are generally known as the imine (-C=N-) or azomethine (-HC=N-) group. Schiff bases attract significant attention due to their interesting and important properties (e.g., biological and catalytic activities, electroluminescent properties, fluorescence properties, and applications in organic photovoltaic materials and sensors) [1-3]. Schiff bases are among the most popular ligand systems in inorganic chemistry owing to their great σdonor and π-acceptor property. Due to the relationship between structure and reactivity, the electron donor and electron acceptor properties of the ligand affect the reactivity of the coordination compounds and also have an important impact on this activity [4-6]. Transition metal complexes derived from the Schiff base ligands have the versatile catalytic application for various fields [7]. In recent years, a wide range of half-sandwich ruthenium(II) Schiff base complexes (which present a variety of methods for the design and synthesis of novel compounds) have attracted considerable interest thanks to many interesting properties in the different area range from biological to catalysis chemistry [8,9]. A large number of Schiff bases and their Ru(II) complexes have been extensively investigated since they are effective catalysts in homogeneous and heterogeneous catalysis activity [10]. Moreover, half-sandwich ruthenium complexes containing Schiff base ligands with oxygen-nitrogen donor atoms of the catalytic activity play an important role in hydrogenation reactions [8]. Ruthenium complexes include a large number of ligands such as arlyazo ligands, acylthiourea ligand [11], NHC ligands [12], Schiff base ligands [4,13-15], Ndonor ligands [16-17], phenolate-oxazoline ligand. Particularly, pyridines ligand-based (Ru(II)-aren) catalysts are well-known for promoting transfer hydrogenation reaction in presence of variety base [8,18]. Half-sandwich ruthenium(II) Schiff base complexes also play
Journal Pre-proof a vital role as catalysts in the synthesis of valuable organic compounds. The Ru(II) complexes can be prepared from the easy access precursors under smooth reaction conditions. In this study, Schiff base ligands were prepared through the condensation of 6-tertbutyl 3-ethyl 2-amino-4,5-dihydrothieno[2,3-c]pyridine-3,6(7H)-dicarboxylate with different substituted benzaldehyde. The Schiff base reacted with metal salt [RuCl2(p-cymene)]2 (2:1) to give Ru(II) complexes (1a-d). The four new Ru(II) complexes were prepared, and their structure was characterized with elemental analysis, spectroscopy measurements (FT-IR, UVVis, mass spectroscopy and 1H-13C NMR) techniques. The Ru(II) complexes were used as catalysts in i-propanol for the transfer hydrogenation of various ketones in the presence of KOH.
2. Experimental
2.1. Material and physical measurements All reagents and solvents were commercially available and reagents were used without further purification. IR spectra were performed on a Perkin Elmer 65 instrument (KBr pellets) in the 4000-400 cm-1 range. Electronic spectra of the Ru(II) metal complexes (1a-d) were obtained in EtOH on a Shimadzu 1800 spectrophotometer in the range of 200-800 nm. 1H and 13C
NMR spectra were obtained with a Brucker 300 MHz. Microanalysis of C, H, N and S
were recorded on a Perkin-Elmer Model 240C analyzer. Mass spectra of the complexes were recorded on an Agilent 1100 MSD spectrophotometer. The catalytic conversions were monitored on an Agilent 6890N GC system using GC-FID by an HP-5 column of 30 m length, 0.32 mm diameter and 0.25 µm film thickness. Column chromatography was obtained through the silica gel 60 (70-230 mesh).
Journal Pre-proof 2.2. The general procedure to the synthesis of Ru(II) complexes(1a-d) [RuCl2(p-cymene)]2 (0.37 g, 0.6 mmol) was added into a solution of ligand (0. 50 g, 1.2 mmol) in 20 mL methanol. The reaction mixture was heated with stirring at 70 °C for 4-5 hours. At the end of the reaction, the precipitate of the orange powder product formed. Then, it was filtered and recrystallized from dichloromethane/diethyl ether (1:3) at room temperature. The product was washed several times with diethyl ether and subsequently dried in a vacuum.
C2 H5 O O
OC2 H5
N
S
Ru
Ru H C
O
O
Cl
Cl
Cl
+ 1/2
Cl
N
Cl
Methanol / 80 oC
Ru
O
N
S
HC
O
R
N
R
O 1a-d
Where, R= H, OCH3 , NO2, OH
+
O C2H5 O
H C
Ru O
N
OC2 H5
O
N
S
Cl-
Cl
O
N
N
S Cl
O
O
Ru
HC
O
1b
1a
+
+
C2 H5 O
C2 H5 O
O
O Ru
Ru O
N O
S
N
Cl-
Cl
O
HC
N
N S
HC
O
H3 CO
NO2 1c
Cl
1d
Scheme 1. Synthesis of the Ru(II) complexes (1a-d)
Cl . 2.5H2 O
Journal Pre-proof Synthesis of Ru(II) complex 1a. Yield: 78%, mp.: 242-245 °C. IR (KBr, ν cm-1): 3094, 3047 (-CH, Ar.), 2960, 2925 (-CH, Alip.), 1670 (C=O), 1577 (CH=N), 1549, 1490 (C=C, Ar.), 780 (C-S-C), 554, 525 (M-O), 500 (M-N), 463 (Ru-Cl). 1H NMR (300 MHz, CDCl3) δ: 9.51 (s, 1H, CH=N), 7.47-6.77 (m, 5H, Ar. -CH), 5.77-4.19 (m, 8H, p-cymene moiety, 2CH2), 4.02-3.14 (m, 4H,CH2 (pyridine moiety), 1.28-1.21 (d, 12H, -OC(CH3)3), 2.58-2.60 (m, 1H, -HC(CH3)2), 2.30 (s, 3H, -CH3), 1.21-1.28 (d, 6H, -HC(CH3)2). 13C NMR (75 MHz, CDCl3) δ: 165.00, 164.00 (C=O)2, 163.00 (CH=N), 137.00, 129.00, 130.00, 107.00, 78.88 (thionyl, p-cymene moiety, benzene ring, -OC(CH3)3, 58.96, 14.11 (-OC2H5), 44.50, 18.30 (pyridine moiety, p-cymene, -CH3), 28.40 (-OC(CH3)3), 30.40 (p-cymene, HC(CH3)2), 24.40 (p-cymene, HC(CH3)2). UV-Vis. (λmax, nm): π→π*, 213, 222, 233, 265; n→π*, 304, 397. Anal. Calc. for C32H40N2O4SRuCl2: C, 53.34; H, 5.55; N, 5.88; S, 4.44. Found: C, 53.40; H, 5.68; N, 3.90; S, 4.50. LC/MS/MS: m/z: 720.90 (Calc.), 720.97 (Found) [M+H ]+. Synthesis of Ru(II) complex 1b. Yield: 72%, mp.: 272-274 °C. IR (KBr, ν cm-1): 3055, 3028 (-CH, Ar.) 2925, 2869 (-CH, Alip.), 1686 (C=O), 1672 (CH=N), 1560, 11548 (C=C, Ar.), 1160 (C-O), 755 (C-S-C), 552, 525 (M-O), 499 (M-N), 463 (Ru-Cl). 1H NMR (300 MHz, CDCl3) δ: 8.83 (s, 1H, CH=N), 7.33-6.97 (m, 4H, Ar. -CH), 4.56-4.30 (m, 8H, pcymene moiety , -2CH2), 2.29 (s, 3H, -CH3), 3.35-2.87 (m, 4H, CH2 (pyridine moiety), 1.931.36 (d, 12H, -OC(CH3)3), 2-79-2.81 (m, 1H, -HC(CH3)2), 1.20-1.25 (d, 6H, -HC(CH3)2). 13C NMR (75 MHz, CDCl3) δ: 163.00, 155.00 (C=O)2, 162.00 (CH=N), 139.00, 133.00, 132.00, 130.00, 106.00, 87.00 (thionyl, p-cymene moiety, benzene ring, -OC(CH3)3), 60.00, 18.00 (OC2H5), 41.00, 17.00 (pyridine moiety), 29.00 (-OC(CH3)3), 30.40 (p-cymene, HC(CH3)2), 22.00-18.16 (p-cymene, -CH3, HC(CH3)2). UV-Vis. (λmax, nm): π→π*, 204, 209, 287; n→π*, 416. Anal. Calc. for C32H39N2O5SRuCl: C, 54.90; H, 5.75; N, 4.03; S,4.57. Found: C, 54.81; H, 5.53; N, 4.22; S,4.76. LC/MS/MS: m/z: 699.45 (Calc.), 699.49(Found) [M]+.
Journal Pre-proof Synthesis of Ru(II) complex 1c. Yield: 76%, mp.: 245-248 °C. IR (KBr, ν cm-1): 3093, 3047 (-CH, Ar.) 2970, 2955 (-CH, Alip.), 1669 (C=O), 1578 (CH=N), 1525, 1491 (C=C, Ar.), 1385 (C-NO2), 781 (C-S-C), 558 (M-O), 501 (M-N), 465 (Ru-Cl). 1H NMR (300 MHz, CDCl3) δ: 8.32 (s, H, CH=N), 7.46-6.80 (m, 4H, Ar. -CH), 4.20-4.01 (m, 6H, p-cymene moiety, -2CH2), 3.35-3.25 (m, 6H, p-cymene moiety, CH2 (pyridine moiety), 2.09-1.26 (d, 12H, -OC(CH3)3), 2.29-2.31 (m, 1H, -HC(CH3)2), 2.27 (s, 3H, -CH3), 1.20-1.26 (d, 6H, HC(CH3)2). 13C NMR (75 MHz, CDCl3) δ: 165.00, 163.00 (C=O)2, 159.00 (CH=N), 136.00132.00, 129.00-108.00, 90.00 (thionyl, p-cymene moiety, benzene ring, -OC(CH3)3), 49.00, 14.84 (-OC2H5), 41.00, 18.00 (pyridine moiety), 30.44 (-OC(CH3)3), 30.40 (p-cymene, HC(CH3)2), 22.08-18.24 (p-cymene, -CH3, HC(CH3)2). UV-Vis. (λmax, nm): π→π*, 216, 220, 258, 283; n→π*, 308, 398. Anal. Calc. for C32H39N3O6SRuCl2: C, 50.20; H, 5.09; N, 5.49; S, 4.18. Found: C, 50.22; H, 5.13; N, 5.58; S, 4.26. LC/MS/MS: m/z: 762.90 (Calc.), 762.63 (Found) [M-2H]-. Synthesis of Ru(II) complex 1d. Yield: 74%, mp.: 212-215 °C. IR (KBr, ν cm-1): 3411 (-OH), 3058 (-CH, Ar.) 2963, 2954 (-CH, Alip.), 1668 (C=O), 1585 (CH=N), 1531, 1492 (C=C, Ar.), 1247 (-CO3), 781 (C-S-C), 571, 510 (M-O), 486 (M-N), 461 (Ru-Cl). 1H NMR (300 MHz, CDCl3) δ: 8.89 (s, H, CH=N), 7.27-6.82 (m, 4H, Ar. -CH), 5.81-4.51 (m, 8H,pcymene moiety, -2CH2), 3.82-2.50 (m, 4H, CH2 (pyridine moiety), 1.33-1.44 (d, 12H, OC(CH3)3), 2.09-2.28 (m, 1H, -HC(CH3)2), 2.26 (s, 3H, -CH3), 1.19-1.22 (d, 6H, -HC(CH3)2). 13C
NMR (75 MHz, CDCl3) δ: 164.00, 163.00 ((C=O)2), 159.00 (CH=N), 133.00-132.00,
132.00-117.00, 77.23 (thionyl, p-cymene moiety, benzene ring, -OC(CH3)3), 56.00, 14.37 (OC2H5), 61.00, 18.00 (pyridine moiety), 28.43 (-OC(CH3)3), 29.00 (p-cymene, HC(CH3)2), 21.08-18.21 (p-cymene, -CH3, HC(CH3)2). UV–Vis. (λmax, nm): π→π*, 223, 239, 250, 269, 283; n→π*, 300, 333, 398, 411, 438. Anal. Calc. for C33H47N2O7.5SRuCl2: C, 49.81; H, 5.91;
Journal Pre-proof N, 3.52; S, 4.02. Found: C, 49.90; H, 6.01; N, 3.41; S, 4.16. LC/MS/MS: m/z: 794.90 (Calc.), 794.95 (Found) [M]+.
2.3. General procedure followed for the transfer hydrogenation of ketones
The catalytic transfer hydrogenations of ketones were studied via pre-catalyst Ru(II) (1a-d) (0.001 mmol). The base KOH (4.0 mmol) was dissolved in i-propanol (5 mL). Substrate ketone (2.0 mmol) was added into this solution, and the mixture was refluxed at 80 °C for 8 hours in an oil bath. Following the completion of the reaction, the mixture was cooled, filtered through silica gel or alumina bed and was eluted with the use of ethyl acetate/hexane (1:4) mixture. The purity of compounds was analyzed with GC and/or GCMS. All complexes (1a-d) were found tobe effective catalysts, providing conversions range of 70100%. The observed results were summarized in Table 3. In the current work, a series of novel Ru(II) complexes containing pyridine groupbased Schiff base were successfully synthesized, and they were employed as catalysts for the transfer hydrogenations of acetophenone derivatives. The performance of the catalysts in the transfer hydrogenations was monitored with the use of benzophenone as a model substrate.
O C CH3
+ Ru(II) Complex (1a-d)
80
R
H
i-PrOH, KOH o C,
8h
R
C CH3 OH
R= Br, H, OCH3, Ph Reaction conditions: Substrate (2.0 mmol), Ru(II) complex (0.001 mmol), KOH (4.0 mmol), i-PrOH (5 mL), 8h, 80 οC.
Journal Pre-proof Ru(II) complexes (0.001 mmol), ketone (2.0 mmol) and KOH (4.0 mmol) were placed in a Schlenk flask, and 5 mL of i-PrOH was added into the mixture. Then, the obtained mixture was refluxed at 80 °C for 8 hours. The samples periodically taken from the reaction medium were passed through the column and analyzed in GC in order to determine conversion rates. As a result, the optimum reaction time was found to be 8 hours. The solvent was removed, and then the residue was passed through the column in the ethylacetate/hexane (1:4) mixture. The product was purified by a column chromatography method and controlled by NMR and GC. After the optimum reaction conditions were established, the catalytic activities of Ru(II) complexes in transfer hydrogenations reactions were examined using substituted acetophenone derivatives. Table 3 shows the yields (%) determined according to substituted ketones. Catalytic activities in the transfer hydrogenations reaction were investigated in the presence of KOH base using various ketones (acetophenone, m-methoxyacetophenone, pbromoacetophenone, p-methoxyacetophenone and benzophenone) and using Ru(II)-Schiff base complexes as catalytic. Thus, the corresponding secondary alcohols were obtained as a result of the reaction of transfer hydrogenations with the use of different substituted ketones. Besides the optimization process, the influence of bases was examined through i-PrOH as a solvent and diverse bases. No reaction was observed in the absence of the base. However, with addition base, the transfer hydrogenations reactions completed within 8 hours. The use of strong inorganic bases (such as NaOH, KOH) provided the higher conversion of the corresponding alcohol products. On the other hand, weak inorganic bases (such as Cs2CO3, Na2CO3, KOButand K2CO3) were found to be less effective when compared to strong bases. KOH was observed to be a superior base. Considering the absence of catalyst, transfer hydrogenations was observed as 11% yield for 10 hours. (Table 1, entry 9).
Journal Pre-proof Table 1. Effect of various bases in the TH of acetophenone Entry 1 2 3 4 5 6 7 8 *9
Base KOH Cs2CO3 Na2CO3 K2CO3 KOBut NaOH KOH KOH
Time (h) 10 10 10 10 10 10 8 10 10
Yield (%) 95 51 27 85 68 57 95 11
*catalysts were not used.
Table 2. Conversion versus reaction time Entry 1
Solvent i-PrOH
Time (h) 12
Yield (%) 95
2 3
i-PrOH i-PrOH
10 8
95 86
Table 3. Transfer hydrogenation of ketones catalyzed by complexes (1a–d) Entry
Substrate
Base
1
Catalyst
Yield (%)
KOH
1a
95
KOH
1b
90
KOH
1c
83
4
KOH
1d
81
5
KOH
1a
86
KOH
1b
90
KOH
1c
79
KOH
1d
85
KOH
1a
84
2
O C CH 3
3
O C CH 3
6 7
H3CO
8 9
O Br
C CH3
Journal Pre-proof 10
KOH
1b
70
11
KOH
1c
75
12
KOH
1d
87
13
KOH
1a
73
KOH
1b
86
KOH
1c
71
16
KOH
1d
88
17
KOH
1a
80
KOH
1b
85
KOH
1c
100
KOH
1d
100
14 O H3CO
15
C CH3
18 O
19
C
20
Reaction Conditions: Ketone (2.0 mmol), catalyst (1a-d) (0.01 mmol), KOH (4.0 mmol), i-PrOH (5 mL), 8 h, 80 οC.
The yields in Table 3 indicate that complexes were generally effective catalysts in the transfer hydrogenation reactions. All Ru(II) complexes were found to show excellent catalytic activity against the aforementioned ketones. In general, catalytic products ranging from 70100% were obtained (Table 3). Such a highconversion may be ascribed to both the electronic effects of the groups in the structure of the ligand and the structures of the ketones used.
3. Results and discussion
3.1. NMR spectroscopic measurements
1H
NMR spectra of the complex 1b confirmed that OH group proton signals were lost,
and a set of signals appeared in the region of 4.81-4.32 ppm for the protons of the p-cymene
Journal Pre-proof rings in the complex. The disappearance of -OH protons may indicate that the ruthenium was coordinated by phenolic oxygen [7,19]. The 1H NMR spectra of the complexes 1a, 1c, 1d exhibited a singlet signal for the imine proton at 9.51-8.32 ppm, which was assigned to imine proton. In all the complexes, the methyl(isopropyl), methane(isopropyl) proton and methylgroup of the p-cymene showed doublets, multiplets around 1.19-1.28 and 2.09-2.81 ppm, singletin the region of 2.26-2.30 respectively. In all complexes aromatic protonsof the pcymene rings appeared in the region of 5.81-3.35 ppm. Furthermore, the 13C NMR spectra of complexes showed that the azomethine carbon and carbonyl carbon peaks were 163.30159.59 ppm. This confirmed that the ruthenium was coordinated through the imine nitrogen and carbonyl oxygen atom [20-28].
3.2. Infrared (IR) absorption spectroscopy
The IR spectral bands for the complexes (1a-d) were principally characterized by the strong bands at 3411 cm-1, 1686-1669 cm-1, 1672-1577 cm-1 and 781-755 cm-1, which were assigned to υ(OH, OH2), υ(C=O), υ(C=N) and υ(C-S-C), respectively. Moreover, some new bands with medium to weak band intensities did appear at 571-510 cm-1 and 501-486 cm-1, which were assigned to υ (M-O), υ (M-N). Anew band appeared at 465-461 cm-1. This was attributed to υ(Ru-Cl). The appearance of the broad strong absorption band (at about 3411 cm1)
in the complex (1d) was associated with the presence of lattice water molecules OH group.
It may be inferred from the infrared spectra data that CH=N (1a-d), C=O (1a, 1c, 1d) and ArOH (1b) coordinated with nitrogen, carbonyl group and phenolic oxygen atoms, respectively. These observations may confirm the coordination of the Schiff bases with Ru(II) metalion for all the complexes [21,28-31].
Journal Pre-proof 3.3. Electronic absorption spectra
For the absorption spectra of the Ru(II) complexes (1a-d) in ethanol, the medium intensity band at 304–398 nm and the high-intensity bands at 220-287 nm were observed, which were linked to the ligand-centered (LC) π→ π* and n→ π* transitions, respectively. The lowest energy absorption bands at 411-438 nm indicatedthe metal to ligand charge (MLCT) transitions [32,22]. The electronic spectra, FT-IR, NMR and elemental analysisof all the complexesmay suggest the presence of an octahedral (piano stool) geometry around ruthenium(II) metal ion [28-33]. Scheme 1 shows the proposed structure of the complexes.
3.4. Catalysis studies
The catalytic activities of synthesized Ru(II)-Schiff base complexes (1a-d) were investigated through the use of acetophenone derivatives. Consequently, these complexes were found to be active catalysts for transfer hydrogenation reactions. The evaluation of the yield values in Table 3 suggests that catalysts were usually effective in the hydrogenation reactions. From the comparison of the substituents, it was found that the complexes (1a-d) showed catalytic activity in the range of 95-81% when acetophenone was used as the substrate (Table 3). This may show that there was a limited effect of the groups coordinated to the Schiff base and particularly benzene ring on the activity of the catalysts. The complexes (1a-d) showed high catalytic activity ranging from 95-81% when mmethoxyacetophenone was used as the ketone (Table 3). This may indicate that the activity of the catalysts was not significantly affected by the electronic effects and position groups
Journal Pre-proof coordinated the ketone used and Ru(II) complexes. However, the activity of the catalysts (2b) was increased to some extent by theelectronic effects of coordinated groups (Table 3). When p-bromoacetophenone was used as the ketone, they were noted to show good catalytic activity ranging from 70-87%. This ratio shows that the yield of pbromoacetophenone was relatively lower in comparison with those of other ketones (e.g. mmethoxyacetophenone and acetophenone). This can be attributed to the electron-withdrawing group (-Br) coordinated to the ketone (Table 3). Particularly, the 1b complex was noticed with low catalytic activity (Table 3). It is clear in Table 3 that the 1b complex containing the –OH group aromatic ring of Schiff base exhibited low catalytic activity due to the electron-withdrawing group they possess. When p-methoxyacetophenone was used as the ketone, the complexes (1a-d) were found to show catalytic activity ranging from 71-88%. A yield of 88% was observed in the case of the complexes (1d) containing an electron donor group coordinated to the aromatic ring (methoxy), as seen in Table 3, entry: 16. The complexes (1c) containing the electronwithdrawing group bound to the aromatic ring exhibited a relatively low catalytic activity with a yield of 71% (Table 3, entry: 15). This may indicate that the electronic structure and position of the coordinated groups in the Ru(II) complex (1c) did affect the activity of the catalysts. The case in which benzophenone is used as the ketone, complexes (1a-d) showed high catalytic activity ranging from 80-100% (Table 3, entries: 17-20). The complexes (1c-1d), containing electron withdrawing-donating groups coordinated to the aromatic ring, exhibited very high catalytic activity with a yield of 100% (Table 3, entries: 19,20). This is probably due to the structure of ketone that was used as the substrate rather than the electronic effects of the groups present in the structures of the Ru(II) complexes used.
Journal Pre-proof The Schiff base-Ru(II) complexes (1a-d), which were used in the transfer hydrogenation reactions of ketones, yielded a good degree of catalytic conversion ranging from 70-100%. At the same time, the structure and position of the ligand-bound substituted groups increased their catalytic activity. It was found that the electronic structure of the Schiff base groups and the electronic effect of the substrate used did not significantly affect the activity of the catalysts. When the catalytic results of Ru(II) complexes were examined, it was observed that all complexes were active for transfer hydrogenation reactions and formed active catalytic systems under optimum conditions. Ru(II) complexes (1a-d) were found to be very good catalytic activities. Considering transfer hydrogenations reactions, the substituted ketones conversion into the corresponding secondary alcohol did occur at 80 °C and in ipropanol for 8 hours.
4. Conclusions
This study reported the synthesis of four novels Ru(II)-Schiff base complexes (1a–d) containing pyridine-based Schiff base ligands. The Ru(II)-Schiff base complexes were prepared through the reaction of the [RuCl2(p-cymene)]2 dimers with pyridine-based Schiff base ligand. The complexes were entirely characterized by the use of spectroscopy techniques. It was observed that the synthesized complexes were very effective catalysts in transfer hydrogenation of the ketones where KOH was used as the base. Catalytic activities of Ru(II) complexes were readily accessible, and such complexes were effective catalyst precursors for the transfer hydrogenation of ketones. The catalytic activities of these four pyridines based Ru(II)-Schiff base complexes did exhibiteexcellent activity in the transfer hydrogenation of ketones. Thus, to determine the effect of synthesized complex compounds
Journal Pre-proof as catalysts, more efficient, environmentally friendly and cost-effective methods have been developed by comparing them with literature studies.
Acknowledgment
Thank Scientific Research-Publications and Projects Research and Practice Center (BAYPUAM) under research project no BAP-18-TBMY-4902-01 for their support.
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Journal Pre-proof Declaration of Interest Statement We (Kenan Buldurun, Metin Özdemir,) declare that there is no conflict of interests regarding the publication of this article.
Journal Pre-proof Highlights Ru(II) complexes containing pyridine-based Schiff base ligands were synthesized. Ru(II) complexes were characterized through different spectroscopic techniques. The catalytic activity of transfer hydrogenation (TH) was investigated. The catalytic activities of Ru(II) complexes were excellent in the TH of ketones.