Efficient and versatile catalysis for β-alkylation of secondary alcohols through hydrogen auto transfer process with newly designed ruthenium(II) complexes containing ON donor aldazine ligands

Article abstract of DOI:10.1080/00958972.2017.1381692

A new series of ruthenium(II) carbonyl complexes, [RuCl(CO)(EPh₃)₂(L_1-2)] (1–4) (E?=?P or As; H₂L₁?=?salicylaldazine, H₂L₂?=?2-hydroxynaphthaldazine), have been assembled from ruthenium(II) precursors [RuHCl(CO)(EPh₃)₃] and bidentate ON donor Schiff base ligands (H₂L_1-2). Both ligands and their new ruthenium(II) complexes have been characterized by elemental analyses, spectroscopic methods (UV, IR, NMR (¹H, ¹³C, ³¹P) as well as ESI mass spectrometry. The molecular structures of H₂L₁ and 1 have been confirmed by single crystal X-ray diffraction. Based on the above studies, an octahedral coordination geometry around the metal center has been proposed for 1–4. To investigate the catalytic effectiveness of 1–4, the complexes have been used as catalysts in β-alkylation of secondary alcohols with primary alcohols and synthesis of quinolines. The effect of solvent, time, base, catalyst loading, and substituent of the ligand moiety on the reaction was studied. Notably, 1 was a more efficient catalyst toward alkylation of a wide range of alcohols and quinolines synthesis. The reusability of the catalyst was checked and the results showed up to six catalytic runs without significant loss of activity.

Full text of DOI:10.1080/00958972.2017.1381692

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Efficient and versatile catalysis for β-alkylation
of secondary alcohols through hydrogen
auto transfer process with newly designed
ruthenium(II) complexes containing ON donor
aldazine ligands
Periyasamy Premkumar, Rajendran Manikandan, Muthukumaran Nirmala,
Periasamy Viswanathamurthi & Jan Grzegorz Malecki
To cite this article: Periyasamy Premkumar, Rajendran Manikandan, Muthukumaran Nirmala,
Periasamy Viswanathamurthi & Jan Grzegorz Malecki (2017): Efficient and versatile catalysis for
β-alkylation of secondary alcohols through hydrogen auto transfer process with newly designed
ruthenium(II) complexes containing ON donor aldazine ligands, Journal of Coordination Chemistry,
DOI: 10.1080/00958972.2017.1381692
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1381692
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Date: 19 September 2017, At: 08:36

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://doi.org/10.1080/00958972.2017.1381692
Efficient and versatile catalysis for β-alkylation of secondary alcohols through
hydrogen auto transfer process with newly designed ruthenium(II) complexes
containing ON donor aldazine ligands
PERIYASAMY PREMKUMAR†, RAJENDRAN MANIKANDAN#†, MUTHUKUMARAN NIRMALA†,
PERIASAMY VISWANATHAMURTHI*† and JAN GRZEGORZ MALECKI‡
†Department of Chemistry, Periyar University, Salem-636 011, India
‡Department of Crystallography, Silesian University, Szkolna 9, 40-006 Katowice, Poland
A new series of ruthenium(II) carbonyl complexes, [RuCl(CO)(EPh3)2(L1-2)] (1-4) (E = P or As;
H2L1 = salicylaldazine, H2L2 = 2-hydroxynaphthaldazine), have been assembled from
ruthenium(II) precursors [RuHCl(CO)(EPh3)3] and bidentate ON donor Schiff base ligands
(H2L1-2). Both ligands and their new ruthenium(II) complexes have been characterized by
elemental analyses, spectroscopic methods (UV, IR, NMR (1H, 13C, 31P) as well as ESI mass
spectrometry. The molecular structures of H2L1 and 1 have been confirmed by single crystal
X-ray diffraction. Based on the above studies, an octahedral coordination geometry around the
metal center has been proposed for 1-4. To investigate the catalytic effectiveness of 1-4, the
complexes have been used as catalysts in β-alkylation of secondary alcohols with primary
alcohols and synthesis of quinolines. The effect of solvent, time, base, catalyst loading and
substituent of the ligand moiety on the reaction was studied. Notably, 1 was a more efficient
catalyst towards alkylation of a wide range of alcohols and quinolines synthesis. The reusability
of the catalyst was checked and the results showed up to six catalytic runs without significant
loss of activity.
Keywords: Aldazine ligands; Ruthenium(II) carbonyl complexes; β-Alkylation of alcohols;
Synthesis of quinolines
#Present address: Department of Chemistry, K. S. Rangasamy college of Arts and Science (Autonomous)
Tiruchengode-637215, India
*
Corresponding author. Email: viswanathamurthi72@gmail.com
1

1. Introduction
Alcohols are one of the most important classes of organic compounds owing to their wide variety
of uses in industrial and laboratory chemistry. There are many methods for synthesis of alcohols
[1]. All of these methods have their own advantages; however, these methods also have
disadvantages such as side reactions, tedious work-up procedures, use of toxic reagents and
formation of large amounts of inorganic salts as waste. Hence, developing more efficient
procedures utilizing nonhazardous and easily available starting materials such as green alkylation
methodologies would be desirable for future sustainable processes [2]. The β-alkylation of
secondary alcohols with primary alcohols is an attractive tool and represents one of the most
convenient and straightforward methods for synthesis of coupled secondary alcohols through
tandem processes. Recently, less toxic and more readily available alcohols were used as
alkylating agents in a greener approach using a metal catalyzed borrowing-hydrogen strategy or
hydrogen auto transfer process [3]. This approach is atom economic, thermodynamically
favored, proceeds with only water as by-product and follows a cascade redox type pathway
involving in-situ dehydrogenation of the alcohol to the ketone or aldehyde, followed by aldol
reaction resulting in the loss of a water molecule and subsequent hydrogenation of the resulting
enone to yield the coupled secondary alcohol as product [4]. Both heterogeneous and
homogeneous catalysts promote the β-alkylation of secondary alcohols with primary alcohols. In
heterogeneous catalysis, cross-coupling reactions of secondary benzylic alcohols with primary
alcohols mediated by Pd/C [5], Ag/Al2O3 [6] and Au-Pd (hydrotalcite supported) [7] have been
investigated. However, the reactions produced coupled ketones as unwanted major products.
Various transition metal complexes such as Ir, Rh, Ru, Pd, Cu, Ni and Fe constitute a majority of
the homogeneous catalysts because of their high catalytic performance with high product
selectivity [8]. Among them ruthenium catalytic systems bearing phosphine ligands have been
reported to complete the β-alkylation of secondary alcohols with primary alcohols with good
yields and selectivity [9]. Heterocyclic compounds like quinoline and its derivatives are
extremely versatile compounds, used as building blocks in large number of natural product
functional materials, agrochemicals and pharmacological applications [10]. The synthesis of
quinoline and its derivatives involved transition metal catalyzed transformations [11].
Particularly ruthenium triphenylphosphine catalyst was used for the synthesis of quinolines in
2

oxidative coupling of 2-aminobenzyl alcohol with secondary alcohol [11c].
Ligand design is a significant part of synthetic activity because of the subtle control that
ligands exert on metal centers to which they are coordinated. Particularly, the choice of ligands is
important in stabilization of highly reactive species, unusual oxidation states and implementing
catalytic properties. Aldazines are very promising ligands in formation of different coordination
compounds because there is wide range of possible substituents. Mostly, they are capable of
forming alternate binding sites towards formation of metal complexes [12]. Especially, aldazines
containing ‒C=N‒N=C‒ linkages are versatile building blocks in organic synthesis [13].
In continuation of our research on the synthesis, characterization and catalytic
applications of Schiff base transition metal complexes [14, 15], we herein describe synthesis,
characterization and catalytic applications of ruthenium(II) complexes bearing Schiff base
ligands with ancillary ligands such as triphenylphosphine/triphenylarsine and carbon monoxide.
Considering the economic attractiveness and excellent functional group tolerance of ruthenium
in homogenous catalysis, we screened the catalytic activity of synthesized complexes in
alkylation of alcohols and synthesis of quinolines.
2. Experimental
2.1. Materials
All the chemicals were received as analar or chemically pure grade. Solvents were freshly
purified and dried according to their standard procedures. RuCl3·3H2O,
triphenylphosphine/triphenylarsine and salicylaldehyde/2-hydroxynaphthaldehyde were
purchased from Aldrich and used as received. The ruthenium precursors [RuHCl(CO)(PPh3)3]
and [RuHCl(CO)(AsPh3)3] [16] were prepared according to literature methods.
2.2. General methods
Microanalyses of carbon, hydrogen and nitrogen were carried out using a Vario EL III elemental
analyzer. Infrared spectra of the ligands and the metal complexes were recorded as KBr disks
from 4000-400 cm-1 using a Nicolet Avatar model FT-IR spectrophotometer. UV-Vis spectra
were obtained using a Shimadzu UV-1650 PC spectrometer from 800-200 nm. 1H, 13C and
31P NMR spectra of ligands and complexes were performed in DMSO-d6 or CDCl3 at room
temperature with a Bruker AV400 instrument with chemical shifts relative to tetramethylsilane
3

or o-phosphoric acid as reference. Electrospray ionization mass spectra were recorded by liquid
chromatography mass spectrometry quadrupole time-of-flight Micro Analyzer (Shimadzu) at
SAIF, Panjab University and Chandigarh. Melting points were checked on a Technico micro
heating table and are uncorrected.
2.3. Synthesis of aldazines (H2L1-2)
The ligands were prepared by literature method [17]. Typically hydrazine monohydrate (0.048
mL, 1 mmol) was dissolved in methanol (10 mL) and treated with two equivalents of
corresponding aldehydes (2 mmol) in methanol (10 mL). The reaction mixture was subsequently
refluxed for 2 h and then cooled to room temperature. A yellow crystalline precipitate was
formed, which was filtered, washed with methanol (20 mL) and air dried.
2.3.1. Salicylaldazine (H2L1). H2L1 was prepared from hydrazine monohydrate (0.048 mL, 1
mmol) and salicylaldehyde (0.212 mL, 2 mmol). Yield: 92% (0.270 g); M.P: 213 C; Anal. Calc.
for C14H12N2O2: C, 69.99; H, 5.03; N, 11.66%. Found: C, 69.75; H, 5.34; N, 11.78%. IR (KBr,
cm-1): 3451 (OH), 1620 (C=N). 1H NMR (300.13 MHz; CDCl3, ppm): 11.37 (s, 1H, OH), 8.50
(s, 1H, C=N), 6.93-7.39 (m, 8H, ArH). (ESI, m/z): 241.2 [M+H]+. Single crystals suitable for
X-ray determination were grown by slow evaporation of dichloromethane and chloroform (1:1,
v/v) solution of H2L1 at room temperature.
2.3.2. 2-Hydroxynaphthaldazine (H2L2). H2L2 was synthesized from hydrazine monohydrate
(0.048 mL, 1 mmol) and 2-hydroxynaphthaldehyde (0.344 g, 2 mmol). Yield: 89% (0.350 g);
M.P: 208 C; Anal. Calc. for C22H16N2O2: C, 77.63; H, 4.74; N, 8.23%. Found: C, 77.69; H,
4.54; N, 8.26%. IR (KBr, cm-1): 3462 (OH), 1630 (C=N). 1H NMR (300.13 MHz; CDCl3, ppm):
13.01 (s, 1H, OH), 8.62 (s, 1H, C=N), 7.38-8.17 (m, 10H, ArH). (ESI, m/z): 341.12 [M+H]+.
2.4. Synthesis of ruthenium(II) complexes (1-4)
All the new metal complexes were prepared according to the following general procedure.
[RuHCl(CO)(EPh3)3] (E = P or As) (1 mmol) in methanol (10 mL) was slowly added to
chloroform (10 mL) solution of aldazine ligands (H2L1-2) (1 mmol) and heated under reflux for
5-8 h, whereby the solution turned from pale yellow to orange. After reducing the content to half
volume and standing for a day, the complexes were obtained as orange precipitates, were filtered
and washed several times with ether and dried in vacuo.
4

2.4.1. Synthesis of [RuCl(CO)(PPh3)2L1]CH3CN (1). The complex was synthesized from
[RuHCl(CO)(PPh3)3] (0.952 g, 1 mmol) and H2L1 (0.240 g, 1 mmol). Yield: 94% (1.120 g);
M.P: 88 C; Anal. Calc. for C53H43N3O3ClP2Ru: C, 65.73; H, 4.48; N, 4.34%. Found: C, 65.75;
H, 4.36; N, 4.28%. IR (KBr, cm-1): 3420 (OH), 1964 (C≡O), 1617 (C=N), 1569 (C=N). 1H NMR
(300.13 MHz; CDCl3, ppm): 10.5 (s, 1H, OH), 9.4 (s, 1H, C=N), 8.9 (s, 1H, C=N), 7.84-7.78 (m,
15H, ArH), 7.34-7.27 (m, 15H, ArH), 6.93-6.89 (d, 1H, ArH), 6.85-6.82 (t, 2H, ArH), 6.78-6.72
(d, 1H, ArH), 6.68-6.54 (d, 1H, ArH), 6.48-6.42 (d, 1H, ArH), 6.23-6.15 (t, 2H, ArH). 13C NMR
(75.47 MHz; CDCl3, ppm): 205.5 (C≡O), 166.2 (C=N), 159.9 (C=N), 153.7 (C_O), 117.2-138.2
(ArC). 31P NMR (162 MHz; CDCl3, ppm): 47.12-49.03. (ESI, m/z): 893.4 [M-Cl, CH3CN]+.
Single crystals suitable for X-ray determination were grown by slow evaporation of acetonitrile
and chloroform (1:1, v/v) solution of 1 at room temperature.
2.4.2. Synthesis of [RuCl(CO)(PPh3)2L2] (2). The complex was synthesized from
[RuHCl(CO)(PPh3)3] (0.952 g, 1 mmol) and H2L2 (0.340 g, 1 mmol). Yield: 92% (1.188 g);
M.P: 94 C; Anal. Calc. for C59H46N2O3ClP2Ru: C, 71.83; H, 4.50; N, 2.72%. Found: C, 71.57;
H, 4.76; N, 2.53%. IR (KBr, cm-1): 3428 (OH), 1936 (C≡O), 1617 (C=N), 1564 (C=N). 1H NMR
(300.13 MHz; CDCl3, ppm): 13.1 (s, 1H, OH), 9.7 (s, 1H, C=N), 8.3 (s, 1H, C=N), 7.92-7.73 (m,
15H, ArH), 7.77-7.61 (m, 15H, ArH), 7.58-7.53 (d, 2H, ArH), 7.23-7.15 (d, 2H, ArH), 7.18-7.13
(d, 2H, ArH), 7.08-7.02 (t, 2H, ArH), 6.99-6.95 (t, 2H, ArH), 6.88-6.86 (d, 2H, ArH). 13C NMR
(75.47 MHz; CDCl3, ppm): 204.9 (C≡O), 164.6 (C=N), 159.5 (C=N), 154.3 (C_O), 118.1-139.2
(ArC). 31P NMR (162 MHz; CDCl3, ppm): 47.63-49.32. (ESI, m/z): 993.19 [M-Cl]+.
2.4.3. Synthesis of [RuCl(CO)(AsPh3)2L1] (3). The complex was synthesized from
[RuHCl(CO)(AsPh3)3] (1.084 g, 1 mmol) and H2L1 (0.240 g, 1 mmol). Yield: 87% (1.151 g);
M.P: 98 C; Anal. Calc. for C51H42N2O3ClAs2Ru: C, 62.22; H, 4.16; N, 2.75%. Found: C, 62.48;
H, 4.37; N, 2.93%. IR (KBr, cm-1): 3436 (OH), 1942 C≡O, 1625 (C=N) 1570 (C=N). 1H NMR
(300.13 MHz; CDCl3, ppm): 11.3 (s, 1H, OH), 10.4 (s, 1H, C=N), 8.7 (s, 1H, C=N), 7.59-7.49
(d, 1H, ArH), 7.64-7.75 (d, 1H, ArH), 7.49-7.35 (m, 15H, ArH), 7.35-7.16 (m, 15H, ArH) 7.16-
7.11 (d, 1H, ArH), 7.09-7.05 (d, 1H, ArH) 6.99-6.97 (d, 2H, ArH), 6.95-6.91 (d, 2H, ArH).
13C NMR (75.47 MHz; CDCl3, ppm): 203.8 (C≡O), 165.1 (C=N), 161.6 (C=N), 155.9 (C_O),
192.24-138.24 (ArC). (ESI, m/z): 980.16 [M-Cl]+.
5

2.4.4. Synthesis of [RuCl(CO)(AsPh3)2L2] (4). The complex was synthesized from
[RuHCl(CO)(AsPh3)3] (1.084 g, 1 mmol) and H2L2 (0.291 g, 1 mmol). Yield: 84% (1.155 g);
M.P: 110-112 C; Anal. Calc. for C59H46N2O3ClAs2Ru: C, 65.42; H, 4.15; N, 2.81%. Found: C,
65.72; H, 4.32; N, 2.84%. IR (KBr, cm-1): 3427 (OH), 1956 C≡O, 1620 (C=N), 1565 (C=N). 1H
NMR (300.13 MHz; CDCl3, ppm): 12.8 (s, 1H, OH), 10.1 (s, 1H, C=N), 9.0 (s, 1H, C=N), 8.70-
7.88 (m, 30H, ArH), 7.75-7.64 (m, 4H, ArH), 7.52-7.38 (m, 4H, ArH), 7.37-7.28 (m, 2H, ArH),
7.22-7.17 (t, 1H, ArH), 7.15-7.13 (d, 1H, ArH). 13C NMR (75.47 MHz; CDCl3, ppm): 203.4
(C≡O), 163.9 (C=N), 158.9 (C=N), 156.6 (C_O), 120.1-139.2 (ArC). (ESI, m/z): 1080.29
[M-Cl]+.
2.5. Crystal structure determination
Crystals of H2L1 and 1 were mounted on glass fibers for data collection. Crystal data were
collected at 295 K using a Gemini A Ultra Oxford Diffraction automatic diffractometer. Graphite
monochromated Mo-Kα radiation (λ = 0.71073 Å) was used throughout. The absorption
corrections were performed by the multi-scan method. Corrections were made for Lorentz and
polarization effects. The structures were solved by direct methods using SHELXS [18].
Refinement and all further calculations were carried out using SHELXL [18]. Hydrogens were
included in calculated positions and treated as riding using the SHELXL default parameters. The
non-hydrogen atoms were refined anisotropically using weighted full-matrix least squares on F2.
Atomic scattering factors were incorporated into the computer programs.
2.6. Catalysis
2.6.1. General procedure for β-alkylation of secondary alcohols with primary alcohols. A
two-necked 25 mL flask fitted with a reflux condenser and septum was charged with secondary
alcohol (2.5 mmol), primary alcohol (2.5 mmol), 1 mol% of ruthenium(II) catalyst, and base
KOH (2.5 mmol) in 1,4-dioxane (1 mL). The reaction mixture was heated at 100 °C for 8 h.
After completion of the reaction, the mixture was cooled to room temperature, diluted with
dichloromethane and hexane mixture and filtered. The filtrate was concentrated in vacuo and the
resulting residue was purified by column chromatography (hexane/ethyl acetate, 6:4, v/v) to
provide the desired product. The products were characterized by 1H and 13C NMR spectroscopy.
6

2.6.2. General procedure for quinoline synthesis. Catalytic conversion of alcohol to quinoline
was carried out using ruthenium(II) complexes as catalysts with the following procedure. The
reaction vessel was charged with 2-aminobenzyl alcohol (2.5 mmol), secondary alcohol
(2.5 mmol), 1 mol% of ruthenium(II) catalyst and KOH (2.5 mmol) in 1,4-dioxane (1 mL) and
the mixture was refluxed at 100 °C for 8 h. After completion of the reaction, the mixture was
cooled, diluted with dichloromethane and hexane mixture and filtered. The filtrate was
concentrated in vacuo, and the resulting residue was purified by column chromatography
(hexane/ethyl acetate, 1:1, v/v) to provide the desired product. The products were characterized
by 1H and 13C NMR spectroscopy.
3. Results and discussion
3.1. Synthesis of ligands and metal complexes
Aldazine ligands (H2L1-2) were synthesized by one step reaction of hydrazine mono hydrate with
corresponding aldehydes (scheme 1). The ruthenium complexes [RuCl(CO)(EPh3)2(L1-2)] (1-4)
were synthesized by reacting H2L1-2 with [RuHCl(CO)(EPh3)3] (E = P or As) in equimolar ratio
(scheme 2). The isolated complexes are stable in air and soluble in common solvents such as
dichloromethane, chloroform, benzene, acetonitrile, ethanol, methanol, DMF and DMSO. All the
complexes were characterized by elemental analyses, IR, electronic, NMR, ESI-mass spectral
methods and single crystal XRD. Complexes 1-4 generally showed the molecular ion peak with
loss of a chloride [M-Cl]+ (figures S3-S6).
HO
O
NH2NH2.H2O
N
N
OH
Methanol/Reflux
OH
Salicylaldehyde/
2-hydroxynaphthaldehyde
H2L1-2
Scheme 1. Synthesis of H2L1-2.
7

HO
HO
[RuHCl(CO)(EPh3)3]
Methanol/chloroform
N
N
N
N
Cl
O
CO
E
Ru
OH
E
H2L1-2
E = P/As
Scheme 2. Synthesis of ruthenium(II) complexes (1-4).
3.2. Structural evaluations with spectroscopic studies
3.2.1. IR spectroscopic analysis. The IR spectra provided significant information about the
metal ligand bonding. The peaks at 1630-1624 cm-1 in the ligands were assigned to azomethine
groups. In complexes, the peak due to νC=N(1) shifted to lower frequencies indicating the
participation of νC=N(1) group in bonding with ruthenium. However, no shift in the frequency of
νC=N(2) indicates non-participation of C=N(2) in bonding with ruthenium. A strong band at 1320–
1300 cm-1 in free ligands was assigned to phenolic C–O stretch. One C–O stretch shifted to
higher frequency (1390–1360 cm-1) in the complexes, showing coordination through the phenolic
oxygen. The complexes display a medium to strong band at 1964-1936 cm-1 from coordinated
carbon monoxide at slightly higher frequency than in the precursor complexes. Vibrations from
PPh3/AsPh3 were at 1435-1457 cm-1. IR spectra of the complexes confirm the coordination mode
of the aldazine ligands to ruthenium(II) via one azomethine nitrogen and phenyl oxygen along
with the presence of triphenylarsine and triphenylphosphine groups.
3.2.2. Electronic spectroscopic analysis. Electronic absorption spectra of the ligand and
complexes (figures S7-S10) have been recorded in dichloromethane solution. The complexes
showed intense absorptions from 220-570 nm. The absorption at 400-570 nm is probably due to
metal-to-ligand charge transfer. The high energy bands below 370 nm were assigned to ligand-
centered transitions. The pattern of the electronic spectra of new ruthenium(II) complexes
suggests presence of an octahedral environment around metal center [19].
3.2.3. 1H NMR spectroscopic analysis. 1H NMR spectra of the ligands and complexes
(figures S11-S14) show signals in the expected regions. Peaks at 8.6-8.5 ppm in the free ligands
are assigned to azomethine protons. In spectra of complexes, two singlets were assigned to
8

coordinated (10.4-9.4 ppm) and free (9.0-8.3 ppm) azomethine protons. Spectra of free ligands
have a peak at 13.1-10.5 ppm, characteristic of phenolic OH. Spectra of free ligands and 1-4
showed a series of overlapping multiplets for aromatic protons at 8.70-6.15 ppm.
3.2.4. 13C NMR spectroscopic analysis. The 13C NMR spectra exhibit the expected signals
(figures S15-S17). The complexes exhibit a peak at 205.50-203.4 ppm which is due to terminal
C≡O carbon [20]. Two azomethine carbons with a difference in chemical shift values of 5 ppm
are observed in all the complexes. The azomethine carbon with higher chemical shift values
(166.2-163.9 ppm) can be attributed to azomethine having nitrogen bound to ruthenium. The
other chemical shift (161.6-158.9 ppm) can be ascribed to unbound azomethine. The peak at
156.7–152.7 ppm can be attributed to coordinated carbonyl carbon. Peaks for aromatic carbons
were at 117.2-139.2 ppm.
3.2.5. 31P NMR spectroscopic analysis. 31P NMR spectra were recorded to confirm the presence
of triphenylphosphine groups and their configuration in the complexes. The observation of two
sharp singlets around 49.32-47.12 ppm for 1 and 2 indicated that the two triphenylphosphine
ligands were cis in these complexes (figures S18 and S19).
3.2.6. Single crystal X-ray diffraction studies. Single crystal X-ray diffraction studies of H2L1
and 1 confirm the structures suggested from the spectroscopic studies. Details of the data
collection, solution and refinement are gathered in the Experimental section and data are
presented in tables 1 and 2. The ORTEP view of H2L1 (50% probability ellipsoids) along with
partial atom numbering scheme is shown in figure 1 and important bond lengths and angles are
summarized in table 2.
The perspective view of 1 with atom numbering scheme is depicted in figure 2, while
selected bond lengths and angles are given in table 2. Complex 1 crystallized in the monoclinic
P21/c space group. The coordination geometry around ruthenium is six coordinate with distorted
octahedral geometry from coordination of N(1) and O(2) from aldazine ligand, P(1) and P(2)
from triphenylphosphine ligands, C(1) from carbonyl ligand and a chloride Cl(1). The
ruthenium(II) has a core RuCl(CO)ONP2 coordination environment along with one lattice
CH3CN molecule. In 1, the cis angles are [P(2)−Ru(1)−P(1)] 99.47(2), [O(2)−Ru(1)−Cl(1)]
9

83.20(5) and [O(2)−Ru(1)−N(1)] 86.78(7). The carbonyl group is trans to coordinated O(2)
[C(1)−Ru(1)−O(2)] with an angle of 177.86(9). The PPh3 ligands are mutually cis for better
π-interaction; the presence of O(2)−N(1), a stronger π-acidic ligand may have forced the bulky
PPh3 ligands to take cis position for steric reasons. The cis angles deviate from linearity and
O(2)−N(1) (six-membered ring) leads to small [O(2)−Ru(1)−N(1)] bite angle 86.78(7). The
ruthenium-ligand bond distances, C(1)‒Ru(1) 1.839(3), N(1)‒Ru(1) 2.110(2), O(2)‒Ru(1)
2.0759(17), P(1)‒Ru(1) 2.3808(7), P(2)‒Ru(1) 2.3594(7), Cl(1)‒Ru(1) 2.4459(7), in 1 agree well
with that reported for other ruthenium(II) complexes containing triphenylphosphine and the
carbonyl group (CO) [21].
3.3. Catalytic studies
3.3.1. Catalytic β-alkylation of secondary alcohols with primary alcohols. Numerous reports
have demonstrated that ruthenium(II) complexes are active catalysts for synthesis of organic
compounds. It has also been established that the introduction of Schiff base ligand to the metal
center enhances the catalytic activity. To investigate a promising catalytic system, a screen was
performed for a model reaction between 1-phenyl ethanol and benzyl alcohol [22]. In order to
ascertain optimal reaction conditions, the influence of solvent, base, time and the effect of
catalyst concentration on the yield were investigated (table 3). We are interested in exploring the
solvent-dependent differences in the activities of catalysts on carrying out the model reaction
using toluene, benzene, 1,4-dioxane, acetonitrile, EtOH, H2O, DMF and DMSO (table 3).
Aromatic hydrocarbon solvents such as toluene and benzene (table 3, entries 2 and 3) were better
reaction media than polar aprotic (CH3CN, DMF, DMSO; table 1, entries 5-7) or protic solvents
(table 3, entries 8 and 9). Reaction carried out in 1,4-dioxane offered rapid reaction and excellent
yield (up to 92%) [23] (table 3, entry 4). Reaction in the absence of base, lowered the reaction
yield for β-alkylation of alcohols (table 3, entry 10). Weak bases such as Na2CO3 and K2CO3
were ineffective (table 3, entries 11 and 12). The reaction was accelerated by the addition of
strong bases such as KOtBu, NaNH2 and NaOH (table 3, entries 13-15). When the reaction was
continually checked using KOH, the yield was excellent (table 3, entry 4 up to 92%). So we
choose KOH as the best choice of base for all the reactions. The control experiment was
performed for extended time and no further gain in conversion was obtained up to 24 h (table 3,
entries 16-18). The role of catalyst was checked without catalyst in the presence of solvent and
base. However, the yield of the product is very low (table 3, entry 19).
10

3.3.1.1. Effect of catalytic efficiency with co-ligands and catalyst loading. We also studied the
influence of the substituents and catalyst loading on the catalytic activity. The results are
summarized in table 4. Lower catalyst loadings lead to moderate yields and longer reaction times
are required to achieve maximum conversion. When the amount of catalyst was increased
(table 4, entries 1-3) the product yield increased. Likewise, lowering the catalyst loading had a
detrimental impact on reactivity. The ability to use small amounts of catalyst and still achieve
high conversions is of importance in alkylation reactions (table 4, entry 3). When the catalyst
loading was increased further, there is no improvement in yields (table 4, entry 4). However, it is
important to note that complexes with salicylaldehyde substituents and those containing PPh3
and AsPh3 co-ligands showed the best selectivity toward the synthesis of alcohol (table 4,
entries 3 and 5). Thus, electronic properties may also account for the catalytic activity. On the
basis of the above optimized conditions, we extended the catalytic alkylations for a variety of
secondary alcohols with primary alcohols (table 5) and the formation of alkylated products was
confirmed by 1H NMR and 13C NMR spectroscopy (figures S20-S28 and S29-S34).
3.3.1.2. Recyclability of catalyst. For any catalyst system, it is important to observe its ease of
partition, recoverability and reusability. The reusability of 1 was investigated using 1-phenyl
ethanol and benzyl alcohol as model substrates. After each run, the catalyst was recovered by the
addition of dichloromethane and hexane mixture. The catalyst was then thoroughly washed with
hexane and dried in air before using in the next run. As shown in figure 3, the catalyst can be
efficiently recycled and reused more than six times without significant loss of catalytic activity
or selectivity and after that the activity slightly decreases. This may be due to incomplete catalyst
recovery in the reaction mixture. When comparing the efficiency of our new catalyst with
previously reported literature [2b, 22a, 24], the present catalyst exhibits the finest activity in
terms of minimum catalyst loading, mild conditions and short reaction time. In addition the
complexes are easier to prepare and cheaper than others.
3.3.1.3. Mechanistic study. The reaction mechanism remains to be elucidated. Nevertheless,
some comments can be made from our results and also in accord with literature [25]. Initially,
the phosphine ligand is lost to make room for alkoxide attack; subsequent β-elimination yields
11

the anionic metal hydride complex. Although not observed, we propose this species to be the key
intermediate of the catalytic process. Anionic hydrides exhibit hydridic reactivity and have been
employed in ketone, aldehyde, alkyl halide and acyl chloride reduction. The initially formed
α,β-unsaturated ketone is first reduced. The reduction might proceed by an associative
mechanism in which the aldazine ligand undergoes ring-slippage, to allow substrate coordination
to the metal center and subsequent hydride transfer or the reduction might proceed via an
intermolecular hydride transfer process. In both processes, the α.β-unsaturated ketone is
activated toward hydride transfer by potassium ion. The hydride is added to the β-carbon of the
substrate to give the intermediate structure. Then, successive transfer hydrogenation of C=O
bond gives the corresponding product.
3.3.2. Quinoline synthesis. Encouraged by the above results, the previously optimized reaction
conditions were further applied to the synthesis of quinolines using various alcohols. As can be
seen from table 6, all the reactions proceed smoothly and give the corresponding quinolines in
good to high yield upon isolation (table 6, entries 1-9). The reaction of 1-phenylethanol and
tolylethanol with 2-amino benzylalcohol progressed smoothly to give the corresponding
quinoline products in excellent yields under optimized reaction conditions (table 6, entries 1
and 2). Moderate yields of products could be obtained for the reactions involving long chain
secondary alcohols (table 6, entries 8 (75%) and 9 (64%)). Unexpectedly, 2-furanylethanol
reacted to give the corresponding quinoline in a good yield (table 6, entry 6 (79%)). The reaction
of 2-aminobenzyl alcohol with 2-naphthanylethanol formed 2-(naphthalen-2-yl) quinoline in
89% yield (table 6, entry 5). All the products were confirmed by 1H NMR and 13C NMR
spectroscopy (figures S35-S37 and S38-S40).
4. Conclusion
Synthesis and characterization of ruthenium(II) carbonyl complexes bearing aldazine
functionalized Schiff base ligands are reported. The structures of these complexes were
determined by analytical and spectroscopic methods and X-ray single crystal analysis (for H2L1
and 1). Based on analytical and spectral results an octahedral structure was confirmed for the
complexes. The catalytic study of 1-4 for β-alkylation of alcohols was studied and solvent, time,
base and catalyst loading were optimized. The results also showed that steric and electronic
12

effects of the ligands play a role in the catalytic activity of the new complexes. In the
β-alkylation 1 is a versatile and efficient catalyst under moderate conditions in comparison to its
analogues and other transition metal complexes [26]. Also, 1 has high tolerance to functional
groups in the quinoline syntheses. These results demonstrate high versatility and potential of
aldazine functionalized Schiff base ligands with triphenylphosphine/triphenylarsine co-ligands in
homogeneous catalytic reactions. The two catalytic systems were resilient to the use of various
substrates and generate minimal waste.
Supporting information
CCDC 1469977 and 1524091 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
[Fax: +44–1223/150 336033; E-mail: deposit@ccdc.cam.ac.uk]. Representative ESI-MS, UV
and NMR (1H, 13C, 31P) spectra of coordination compounds, catalysis protocols, characterization
data for β-alkylation of alcohols and quinoline synthesized products and selected 1H and
13C NMR spectra of coupling products.
Acknowledgement
One of the authors (P.P.) expresses his sincere thanks to Periyar University, Salem, India for
financial support through University Research fellowship. The authors also thank UGC, India for
financial assistance to Department of Chemistry, Periyar University, Salem under SAP
(No. 540/20/DRS-I/2016(SAP-I)).
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15

Figure 1. Perspective view (50% probability ellipsoids) of H2L1 with atom numbering scheme.
16

Figure 2. Perspective view (50% probability ellipsoids) of 1 with atom numbering scheme.
17

Figure 3. Recyclability of 1 using 1-phenyl ethanol and benzyl alcohol.
18

Table 1. Crystal data and structure refinement for H2L1.
Compound
H2L1
1
C53H43N3O3ClP2Ru
969.37
Empirical formula
C14H12N2O2
240.26
Formula weight
T (K)
295
295 K
λ (Å)
0.71073
Monoclinic
P21/n
0.71073
Crystal system
Monoclinic
P21/c
Space group
Unit cell dimensions (Å, )
a (Å)
8.553(2)
6.3005(13)
11.825(3)
90
18.4179(7)
10.5542(4)
24.3652(8)
90
b (Å)
c (Å)
α ()
108.14(3)
90
101.896(4)
90
β ()
γ ()
Volume (Å3)
605.6(3)
2
4634.5(3)
4
Z
Calculated density (g cm-3)
Absorption coefficient (mm-1)
F(000)
1.318
1.389
0.090
0.512
252
1992.0
θ range for data collection (°)
Index ranges
3.521-29.422
-8<=h<=11
-5<=k<=8
-16<=l<=11
2739
3.372-29.488
-25<=h<=25
-14<=k<=14
-33<=l<=33
28559
Reflections collected
Independent reflections (Rint)
Data / restraints / parameters
Goodness-of-fit on F2
1430 (0.0503)
1430 / 0 / 84
1.061
11131 (0.0931)
11131 / 0 / 570
1.029
Final R indices [I > 2σ(I)]
R1 = 0.0726
R1 = 0.0444
wR2 = 0.1762
wR2 = 0.0820
R indices (all data)
R1 = 0.1318
R1 = 0.0767
wR2 = 0.2536
wR2 = 0.0931
19

Table 2. Selected bond lengths (A) and angles (°) for H2L1 and 1 with
the optimized geometrical values.
H2L1
X-ray
1
X-ray
O(1)‒C(2)
N(1)‒N(1)
N(1)‒C(7)
C(1)‒C(2)
C(1)‒C(6)
C(1)‒C(7)
C(2)‒C(3)
C(3)‒C(4)
C(4)‒C(5)
C(5)‒C(6)
1.346(3)
1.402(4)
1.280(3)
1.399(4)
1.393(3)
1.446(4)
1.389(4)
1.362(4)
1.390(4)
1.370(4)
C(1)‒Ru(1)
N(1)‒Ru(1)
O(2)‒Ru(1)
P(1)‒Ru(1)
P(2)‒Ru(1)
Cl(1)‒Ru(1)
1.839(3)
2.110(2)
2.0759(17)
2.3808(7)
2.3594(7)
2.4459(7)
P(1)‒Ru1‒Cl(1)
O(2)‒Ru(1)‒Cl(1)
O(2)‒Ru(1)‒P(1)
O(2)‒Ru(1)‒N(1)
N(1)‒Ru(1)‒Cl(1)
N(1)‒Ru(1)‒P(1)
N(1)‒Ru(1)‒P(2)
C(1)‒Ru(1)‒Cl(1)
C(1)‒Ru(1)‒P(1)
C(1)‒Ru(1)‒P(2)
C(1)‒Ru(1)‒O(2)
C(1)‒Ru(1)‒N(1)
C(1)‒Ru(1)‒Cl(1)
C(1)‒Ru(1)‒P(1)
C(1)‒Ru(1)‒P(2)
88.57(2)
83.20(5)
93.97(5)
86.78(7)
82.88(6)
171.28(6)
89.25(6)
97.26(8)
88.14(8)
94.32(8)
177.86(9)
91.19(10)
97.26(8)
88.14(8)
94.32(8)
C(2)‒C(1)‒C(7)
C(6)‒C(1)‒C(2)
C(6)‒C(1)‒C(7)
O(1)‒C(2)‒C(1)
O(1)‒C(2)‒C(3)
C(3)‒C(2)‒C(1)
C(4)‒C(3)‒C(2)
C(3)‒C(4)‒C(5)
C(6)‒C(5)‒C(4)
C(5)‒C(6)‒C(1)
N(1)‒C(7)‒C(1)
121.6(2)
118.5(2)
120.0(2)
122.3(2)
118.4(2)
119.3(3)
121.0(3)
120.5(3)
118.8(3)
121.9(3)
121.3(2)
20

Table 3. Screening and optimization of solvents, bases and time for β-alkylation of secondary
alcohols with primary alcohol catalyzed by 1a.
OH
OH
Ru Catalyst 1
+
R1
R2
R2
OH
R1
1,4-dioxane/KOH
5a
Entry
Solvent
Base (mol%)
Time (h)
Yield (%)b
1
---
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
---
18
21
22
8
19c
87
84
92
27
71
76
59
61
27d
56
64
68
72
84
92
93
93
17e
2
Toluene
3
Benzene
4
1,4-Dioxane
MeCN
5
15
12
10
17
16
24
13
19
10
12
11
10
12
14
12
6
DMF
7
DMSO
8
EtOH
9
H2O
10
11
12
13
14
15
16
17
18
19
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
Na2CO3
K2CO3
KOtBu`
NaNH2
NaOH
KOH
KOH
KOH
KOH
a All reactions were carried out at 100 C using R1 = 1-phenylethanol (2.5 mmol), R2 = benzyl
alcohol (2.5 mmol), catalyst 1 (1 mol%), and base (2.5 mmol) in 1,4-dioxane (1 mL).
b Yields were calculated after isolation of the product through column chromatography using
silica gel (200-400 mesh).
c The reaction was performed under solvent-free conditions.
d The reaction was performed under base-free conditions.
e The reaction was performed under without catalyst conditions.
21

Table 4. Effect of substitution and catalyst loading on β-alkylation of 1-phenyl ethanol with
benzyl alcohola.
Entry
Catalyst
(mol%)
TONb
TOFc
Yield (%)d
1
2
3
4
5
6
7
1
1
1
1
2
3
4
0.25
0.5
1.0
1.5
1.0
1.0
1.0
256
158
92
32
20
12
8
64
79
92
90
89
84
77
60
90
11
11
10
84
77
a Reactions were carried out using 1-phenyl ethanol (2.5 mmol), benzyl alcohol (2.5 mmol),
KOH (1 mol%), catalysts 1-4 at 100 C for 8 h in 1,4-dioxane (1 mL).
b (TON) = (mmol of product)/(mmol of catalyst) after time t.
c TOF = TON/Time.
d Yields were calculated after isolation of the alcohol product through column chromatography
using silica gel (200-400 mesh).
22

Table 5. β-Alkylation of different secondary alcohols with primary alcohols under optimized
conditionsa.
OH
OH
Ru Catalyst 1
+
R1
R2
R2
OH
R1
1,4-dioxane/KOH
Entry
R1
R2
Product
Yield (%)b
TOFc
1
C6H5
C6H5
5a
5b
5c
5d
5e
5f
92
82
71
76
84
89
78
68
73
67
84
80
73
69
62
12
10
9
2
C6H5
CH3(CH2)4
(CH3)2CHCH2
1-Naphthyl
4-CH3OC6H4
C6H5
3
C6H5
4
C6H5
10
11
11
9
5
C6H5
6
CH3CH2
CH3CH2
CH3CH2
CH3CH2
CH3CH2
CH3C6H4
CH3C6H4
CH3C6H4
CH3C6H4
CH3C6H4
7
CH3(CH2)4
(CH3)2CHCH2
1-Naphthyl
4-CH3OC6H4
C6H5
5g
5h
5i
8
9
9
9
10
11
12
13
14
15
5j
8
5k
5l
11
10
9
CH3(CH2)4
(CH3)2CHCH2
1-Naphthyl
4-CH3OC6H4
5m
5n
5o
9
8
a All reactions were performed using 2.5 mmol of each alcohols, catalyst 1 (1 mol%), and base
(2.5 mmol) in 1,4-dioxane (1 mL), at 100 C.
b Yields were calculated after isolation of the alcohol product through column chromatography
using silica gel (200-400 mesh).
c TOF = TON/Time, time fixed 8 h.
23

Table 6. Synthesis of quinolines from 2-aminobenzyl alcohol and secondary alcohola.
R2
OH
OH
Ru Catalyst 1
+
R1
R2
NH2
N
R1
1,4-dioxane/KOH
Entry
R1
C6H5
R2
Product
Yield (%)b
TOFc
1
2
3
4
5
6
7
8
9
H
6a
6b
6c
6d
6e
6f
95
94
82
90
89
79
89
75
64
12
12
10
11
11
10
11
9
CH3C6H4
CH3OC6H4
CH3CH2
Naphthyl
2-Furanyl
C6H5
H
H
H
H
H
CH3
6g
6h
6i
C6H5
C6H5CH2CH2
CH3(CH2)5
C6H5
8
a All reactions were performed using 2.5 mmol of each alcohol, catalyst 1 (1 mol%), and base
(2.5 mmol) in 1,4-dioxane (1 mL) at 100 C.
b Yields were calculated after isolation of the quinoline product through column chromatography
using silica gel (200-400 mesh).
c TOF = TON/Time, time fixed 8 h.
24

Graphical abstract
25