Ruthenium(II) unsymmetrical N2O2 tetradentate Schiff-base complexes: Synthesis, characterization, and catalytic studies

Article abstract of DOI:10.1080/00958970903499651

A series of six-coordinate ruthenium(II) complexes [Ru(CO)(L^x)(B)] (B= PPh₃, AsPh₃ or Py; L^x = unsymmetrical tetradentate Schiff base, x= 5-8; L⁵=salen-2-hyna, L⁶=Cl-salen-2-hyna, L⁷=valen-2-hyna, L⁸=o-hyac-2-hyna) have been prepared by reacting [RuHCl(CO) (EPh₃)₂(B)] (E=P or As) with unsymmetrical Schiff bases in benzene under reflux. The new complexes have been characterized by analytical and spectroscopic (infrared, electronic, ¹H, ³¹P, and ¹³C NMR) data. An octahedral structure has been assigned for all the complexes. The new complexes are efficient catalysts for the transfer hydrogenation of ketones and also exhibit catalytic activity for the carbon-carbon coupling reactions.

Full text of DOI:10.1080/00958970903499651

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Ruthenium(II) unsymmetrical N2O2
tetradentate Schiff-base complexes:
synthesis, characterization, and
catalytic studies
S. Gowri a , M. Muthukumar a , S. Krishnaraj a , P.
Viswanathamurthi a , R. Prabhakaran b & K. Natarajan b
a Department of Chemistry , Periyar University , Salem 636 011,
Tamil Nadu, India
b Department of Chemistry , Bharathiar University , Coimbatore
641 046, Tamil Nadu, India
Published online: 04 Jan 2010.
To cite this article: S. Gowri , M. Muthukumar , S. Krishnaraj , P. Viswanathamurthi , R.
Prabhakaran & K. Natarajan (2010) Ruthenium(II) unsymmetrical N2O2 tetradentate Schiff-base
complexes: synthesis, characterization, and catalytic studies, Journal of Coordination Chemistry,
63:3, 524-533, DOI: 10.1080/00958970903499651
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Journal of Coordination Chemistry
Vol. 63, No. 3, 10 February 2010, 524–533
Ruthenium(II) unsymmetrical N2O2 tetradentate Schiff-base
complexes: synthesis, characterization, and catalytic studies
S. GOWRIy, M. MUTHUKUMARy, S. KRISHNARAJy,
P. VISWANATHAMURTHI*y, R. PRABHAKARANz and
K. NATARAJANz
yDepartment of Chemistry, Periyar University,
Salem 636 011, Tamil Nadu, India
zDepartment of Chemistry, Bharathiar University, Coimbatore 641 046,
Tamil Nadu, India
(Received 29 April 2009; in final form 27 August 2009)
A series of six-coordinate ruthenium(II) complexes [Ru(CO)(Lx)(B)] (B ¼ PPh3, AsPh3 or Py;
Lx ¼ unsymmetrical tetradentate Schiff base, x ¼ 5–8; L5 ¼ salen-2-hyna, L6 ¼ Cl-salen-2-hyna,
L7 ¼ valen-2-hyna, L8 ¼ o-hyac-2-hyna) have been prepared by reacting [RuHCl(CO)
(EPh3)2(B)] (E ¼ P or As) with unsymmetrical Schiff bases in benzene under reflux. The new
complexes have been characterized by analytical and spectroscopic (infrared, electronic, 1H,
31P, and 13C NMR) data. An octahedral structure has been assigned for all the complexes. The
new complexes are efficient catalysts for the transfer hydrogenation of ketones and also exhibit
catalytic activity for the carbon–carbon coupling reactions.
Keywords: Ruthenium(II) complexes; Unsymmetrical Schiff base; Spectroscopic characteriza-
tion; Catalytic activity
1. Introduction
Analytical, biological, and industrial applications of Schiff-base complexes have made
this field attractive [1–8]. These complexes exhibit catalytic activity in chemical
transformations such as Kharasch addition, enol ester synthesis, alkyne dimerization,
olefin metathesis, and atom transfer radical polymerization [9–13]. Transition metal
complexes have also been used as antimicrobials [14, 15] and sensor materials [16].
Unsymmetrical Schiff bases have advantages because of the possibility of fine-tuning
structures by introducing different electron withdrawing and/or donating groups on the
periphery of both imine units. Unsymmetrical Schiff bases have been reported to give
improved enantioselective behavior in various organic transformations as compared
with their symmetrical derivatives [17].
Ruthenium-mediated transfer hydrogenation (TH) of ketones under basic conditions
employing i-PrOH or other H2-donor solvents has emerged as an atom-economical
*Corresponding author. Email: viswanathamurthi@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ꢀ 2010 Taylor & Francis
DOI: 10.1080/00958970903499651

Unsymmetrical Schiff-bases
525
methodology for the synthesis of secondary alcohols [18]; ruthenium-based catalytic
systems are effective in TH of ketones [19]. A number of transition metal phosphine
complexes have been used as catalysts in TH of aldehydes [20] and ketones [21] because
of steric and electronic properties of such ligands [22].
Transition-metal-catalyzed coupling reactions are important processes in organic
chemistry and have been extensively studied as a powerful method for the formation of
carbon–carbon bonds, applied in the synthesis of many compounds, especially complex
natural products, supramolecular chemistry, and materials science [23–25]. Palladium-
or nickel-catalyzed Suzuki–Miyaura cross-coupling [26–28] is the most important
strategy for the construction of unsymmetrical biaryl compounds. Several reports are
available on the catalytic activity of ruthenium complexes, mostly for oxidation and TH
reactions, not for coupling reactions. To study the catalytic activity of ruthenium
complexes for TH and coupling reactions, we synthesized unsymmetrical Schiff bases,
salicylaldehyde ethylenediamine 2-hydroxy-1-naphthaldehyde (salen-2-hyna), 5-chloro
salicylaldehyde ethylenediamine 2-hydroxy-1-naphthaldehyde (Cl-salen-2-hyna),
o-vanillin ethylenediamine 2-hydroxy-1-naphthaldehyde (valen-2-hyna), o-hydroxy
acetophenone ethylenediamine 2-hydroxy-1-naphthaldehyde (o-hyac-2-hyna), and
their ruthenium(II) complexes containing PPh3/AsPh3 co-ligands and explored their
catalytic activities. The complexes were characterized by analytical and spectral
1
[infrared (IR), electronic, H, 31P, and 13C NMR] methods.
2. Experimental
All reagents used were Analar or chemically pure grade. Solvents were freshly distilled
according to standard procedure. Elemental analyses of carbon, hydrogen, and nitrogen
were performed on a Carlo Erba (Thermoquest, Italy) 1108 model analyzer at Central
Drug Research Institute (CDRI), Lucknow, India. FT-IR spectra were recorded in KBr
pellets with a Nicolet (Thermoscientific, US) FT-IR spectrophotometer from 400 to
4000 cmꢀ1. Electronic spectra were recorded on a Shimadzu (Japan) UV-Vis 1650 PC
spectrophotometer from 200 to 800 nm using CH2Cl2. NMR spectra (1H, 31P, and 13C)
were recorded on a Jeol (Japan) GSX-400 instrument in CDCl3 at room temperature.
1H NMR chemical shifts were referenced to tetramethylsilane (TMS) as an internal
standard and 13C NMR chemical shifts were referenced to the internal solvent
resonance. 31P NMR spectra of the complexes were obtained using ortho phosphoric
acid as reference. Gas chromatographic (GC) analyses were conducted on an ACME
6000 (Younglin instrument, Korea) series instrument equipped with a flame ionization
detector (FID) using a DP-5 column of 30 m length, 0.53 mm diameter, and 5.00 mm
film thickness. Melting points were recorded on a Technico micro heating table and are
uncorrected. The starting complexes [RuHCl(CO)(PPh3)3] [29], [RuHCl(CO)(AsPh3)3]
[30], and [RuHCl(CO)(Py)(PPh3)2] [31] were prepared according to the literature
methods.
2.1. Preparation of new unsymmetrical tetradentate Schiff-base ligands, Lx [x ^ 5–8]
The unsymmetrical tetradentate Schiff bases were synthesized by a two-stage process
(scheme 1). In the first step, half-units Lx (x ¼ 1–4) were prepared by condensing

526
S. Gowri et al.
R1
R2
OH
R1
R2
OH
Reflux
H2N
NH2
+
Solvent
NH2
C O
C N
R3
R3
Half unit Lx (x = 1–4)
HO
HO
R1
R2
OH
Lx
Reflux
+
O C
Solvent
N C
H
C N
R3
H
Tetradentate ligand Lx (x = 5–8)
R1 = R2 = R3 = H; L1 = salen; L5 = salen-2-hyna
R1 = R3 = H, R2 = Cl; L2 = Cl-salen; L6 = Cl-salen-2-hyna
R1 = OCH3, R2 = R3 = H; L3 = valen; L7 = valen-2-hyna
R1 = R2 = H, R3 = CH3; L4 = o-hyac; L8 = o-hyac-2-hyna
Scheme 1. Synthesis of half units and tetradentate unsymmetrical Schiff bases.
substituted benzaldehyde or o-hydroxy acetophenone with ethylenediamine in ethanol in
1 : 1 molar ratio [32]. In the second stage, to a well-stirred solution of Lx [x ¼ 1–4]
(4 mmol) in 50 cm3 chloroform, 2-hydroxy-1-naphthaldehyde (4 mmol) in 30 cm3
chloroform was added dropwise. After addition was complete, the mixture was stirred
for another 30 min and refluxed for 2–3 h. The resulting solution was concentrated under
vacuum to remove solvent and the product obtained was recrystallized from chloroform.
2.2. Synthesis of new ruthenium(II) unsymmetrical Schiff-base complexes
All complexes were prepared by the following procedure. To a solution of
[RuHCl(CO)(EPh3)2(B)] (0.10 g, 0.10 mmol) in benzene (20 cm3), the appropriate
unsymmetrical Schiff base (29–46 mg, 0.10 mmol) was added in 1 : 1 molar ratio in a
100 cm3 round-bottomed flask. The mixture was refluxed for 5 h on a water bath. The
reaction mixture gradually changed to deep color during heating. Then, the contents
were concentrated to 3 cm3 under reduced pressure, cooled, and the product was
separated by the addition of 10 cm3 of petroleum ether (60–80ꢁC). The product was
recrystallized from CH2Cl2–petroleum ether (60–80ꢁC). The compounds were dried
under vacuum and the purity of the complexes was checked by TLC.
2.3. Procedure for catalytic TH
Catalytic TH reactions were studied using ruthenium(II) unsymmetrical Schiff-base
complex as a catalyst, ketone as substrate, and KOH as base at 1 : 300 : 2.5 molar ratios
with the following procedure. A mixture containing ketone (3.75 mmol), ruthenium
complex (0.0125 mmol), and KOH (0.03 mmol) in 10 cm3 of i-prOH was refluxed on a
water bath for appropriate time at 95ꢁC. At completion the catalyst was removed from

Unsymmetrical Schiff-bases
527
the reaction mixture by the addition of petroleum ether, followed by filtration and
subsequent neutralization with 1 M HCl. Then, the ether layer was filtered through a
small bed (2 cm3) of silica gel by column chromatography, the filtrate was concentrated
to ꢂ1 cm3, subjected to GC analysis, and the hydrogenated product was identified and
determined with authentic samples.
2.4. Procedure for catalytic activity of carbon–carbon coupling reaction
Magnesium turnings (320 mg) were placed in a two-neck-round-bottomed flask with a
calcium chloride guard tube. A crystal of iodine was added to activate the surface of the
magnesium. Bromobenzene (0.50 cm3 of a total of 1.3 cm3, 12 mmol) in 5 cm3 of
anhydrous diethyl ether was added with stirring and heated under reflux. Turbidity
after 5 min indicated the initiation of the reaction. The remaining bromobenzene in
5 cm3 of ether was added dropwise and the reaction mixture was refluxed for 40 min. To
this reaction mixture, 1.03 cm3 (10 mmol) of bromobenzene in 5 cm3 of anhydrous
diethyl ether and the ruthenium complex (0.05 mmol) was added and the mixture was
refluxed for 6 h. The reaction mixture was cooled and hydrolyzed with a saturated
solution of aqueous ammonium chloride. The ether extract on evaporation of the
solvent gave the crude product, which was chromatographed to get pure biphenyl and
was compared with an authentic sample (m.p.: 69–72ꢁC).
3. Results and discussion
New six-coordinate ruthenium(II) unsymmetrical tetradentate Schiff-base complexes
[Ru(CO)(Lx)(B)] were obtained as shown in scheme 2. Analytical data of the new
HO
R1
R2
OH
[RuHCl(CO)(EPh3)2(B)]
+
N C
C N
H
R3
Reflux/
5 h
Benzene
CO
O
O
R1
R2
Ru
B
N C
C N
H
R3
(B = PPh3, AsPh3 or Py; R1 = H or OCH3, R2 = H or Cl, R3 = H or CH3)
Scheme 2. Formation of new ruthenium(II) unsymmetrical Schiff-base complexes.

528
S. Gowri et al.
complexes (table 1) agree well with the proposed molecular formulas. In all reactions,
Schiff bases are dianionic tetradentate ligands, replacing two molecules of triphenylpho-
sphine or triphenylarsine, a hydride and a chloride from [RuHCl(CO)(EPh3)2(B)].
The products obtained from reactions of [RuHCl(CO)(PPh3)3] with the Schiff bases
contained one triphenylphosphine, whereas the corresponding reactions with
[RuHCl(CO)(PPh3)2(py)] yielded new complexes with no triphenylphosphine ligands,
indicating labile nature of the Ru–P bond compared with the Ru–N bond in these
complexes. The difference in the strength of Ru–P and Ru–N bonds may be explained
on the basis of the better ꢀ donating ability of the nitrogen bases [33, 34] compared to
triphenylphosphine.
3.1. Infrared spectroscopic analysis
IR absorptions for the ligands and complexes are shown in table 2. A strong band at
1644–1634 cmꢀ1 in free Schiff base is characteristic of the azomethine. If the Schiff base
coordinates through nitrogen, there must be a reduction in the azomethine frequency
owing to the lowering of electron density upon coordination. In the spectra of
complexes, this band shifts to 1639–1611 cm ꢀ 1, indicating coordination through
nitrogen [35]. A strong band at 1303–1315 cm–1 in the free Schiff bases is assigned to
phenolic C–O stretch. In ruthenium complexes, this band shifts to 1355–1318 cmꢀ1
showing coordination through the phenolic oxygen [36]. This is further supported by
the disappearance of ꢁOH around 3401–3422 cmꢀ1 in the complexes. Hence, from IR
spectroscopic data carbonyl and phenolic oxygen are involved in coordination to
ruthenium. Strong absorption around 1945–1923 cmꢀ1 has been assigned to the
carbonyl group in the ruthenium complexes. For complexes containing heterocyclic
nitrogen [37], a medium intensity band appeared at 1030 cmꢀ1
.
3.2. Electronic spectroscopic analysis
The electronic spectra of the complexes in dichloromethane showed three to four bands
at 423–233 nm (table 2). Bands around 423–310 nm have been assigned to charge
Table 1. Analytical data of ruthenium(II) unsymmetrical Schiff-base complexes.
Calcd (found) (%)
Complex
Yield (%)
d.p. (ꢁC)
C
H
N
[Ru(CO)(PPh3)(L5)]
[Ru(CO)(PPh3)(L6)]
[Ru(CO)(PPh3)(L7)]
[Ru(CO)(PPh3)(L8)]
[Ru(CO)(AsPh3)(L5)]
[Ru(CO)(AsPh3)(L6)]
[Ru(CO)(AsPh3)(L7)]
[Ru(CO)(AsPh3)(L8)]
[Ru(CO)(Py)(L5)]
87
84
82
78
73
76
79
88
85
81
78
76
161–163
145–148
150–152
146–150
158–161
160–164
140–142
134–138
191–194
187–190
179–183
182–185
66.19(66.09)
63.11(63.02)
65.12(64.99)
66.56(66.49)
62.32(62.25)
59.58(59.44)
61.40(61.27)
62.74(62.69)
59.53(59.47)
55.86(55.81)
58.47(58.45)
60.21(60.13)
4.41(4.34)
4.07(4.01)
4.51(4.48)
4.60(4.51)
4.16(4.05)
3.85(3.78)
4.25(4.18)
4.14(4.06)
4.03(3.98)
3.60(3.53)
4.18(4.10)
4.30(4.25)
3.96(3.91)
3.77(3.69)
3.80(3.74)
3.88(3.80)
3.73(3.68)
3.56(3.49)
3.58(3.52)
3.66(3.59)
8.01(7.93)
7.51(7.41)
7.57(7.42)
7.80(7.73)
[Ru(CO)(Py)(L6)]
[Ru(CO)(Py)(L7)]
[Ru(CO)(Py)(L8)]

Unsymmetrical Schiff-bases
529
Table 2. IR absorption frequencies (cmꢀ1) and electronic spectroscopic data (nm) of free ligands and their
ruthenium(II) unsymmetrical Schiff-base complexes.
Compound
ꢁCꢃO
ꢁC¼N
ꢁC–O
ꢃmax (") (dm3 molꢀ1 cmꢀ1
)
L5
–
–
–
–
1636
1634
1642
1644
1626
1615
1611
1616
1622
1634
1635
1639
1623
1616
1614
1635
1303
1310
1316
1315
1353
1351
1345
1348
1343
1350
1353
1355
1342
1318
1348
1349
–
–
–
L6
L7
L8
–
[Ru(CO)(PPh3)(L5)]
[Ru(CO)(PPh3)(L6)]
[Ru(CO)(PPh3)(L7)]
[Ru(CO)(PPh3)(L8)]
[Ru(CO)(AsPh3)(L5)]
[Ru(CO)(AsPh3)(L6)]
[Ru(CO)(AsPh3)(L7)]
[Ru(CO)(AsPh3)(L8)]
[Ru(CO)(Py)(L5)]
[Ru(CO)(Py)(L6)]
[Ru(CO)(Py)(L7)]
[Ru(CO)(Py)(L8)]
1942
1938
1940
1938
1928
1937
1923
1945
1927
1938
1934
1936
390(8318), 310(11,630), 237(21,956)
395(8157), 311(10,886), 240(21,242)
400(7392), 311(10,868), 235(22,890)
396(7984), 310(11,630), 236(22,182)
401(7289), 308(12,238), 236(22,274)
419(6993), 402(7168), 311(10,849), 233(23,956)
421(6894), 401(7257), 311(10,916), 233(23,892)
420(6913), 401(7263), 311(10,864), 234(23,215)
423(6795), 310(11630), 235(22,890)
399(7458), 310(11,613), 292(18,213), 235(22,890)
422(6812), 401(7289), 272(15,968), 234(23,215)
419(6993), 310(11,642), 234(23,198)
transfer transitions from the metal t2g level to the unfilled ꢂ* molecular orbital of the
ligand [38]. Other high-intensity bands in the region below 300 nm are characterized as
ligand-centered transitions and have been designated as ꢂ–ꢂ* and n–ꢂ* transitions for
the electron localized on the azomethine of the Schiff bases [39]. This assignment is in
conformity with the assignment made for similar octahedral complexes.
3.3. 1H NMR spectroscopic analysis
Coordination of the unsymmetrical Schiff base to ruthenium is further confirmed by
1H NMR spectra (Supplementary material). A singlet at 8.91–9.22 ppm has been
assigned to the azomethine proton; the position of the azomethine signal in the
complexes is downfield compared to the free ligands, indicating deshielding of the
azomethine proton due to coordination [40]. A singlet at 2.36 ppm is assigned to methyl.
Multiplets around 6.98–7.52 ppm in the complexes are assigned to aromatic protons of
triphenylphosphine, triphenylarsine, pyridine, and Schiff-base ligand [35]. In addition,
methylene protons appeared in the region 3.59–3.83 ppm for all the complexes [37]. The
absence of a resonance around 12 ppm in the complexes indicates the deprotonation of
the phenolic group of the Schiff bases on complexation, and coordination to ruthenium.
3.4. 31P NMR spectroscopic analysis
To confirm the triphenylphosphine group present in the complexes, the 31P NMR
spectra were recorded for few complexes (Supplementary material). A signal
around 20–22 ppm in the spectra of [Ru(CO)(PPh3)(L5)] and [Ru(CO)(PPh3)(L6)]
revealed the presence of triphenylphosphine; [Ru(CO)(Py)(L6)] does not exhibit
such signal.

530
S. Gowri et al.
CO
O
O
R1
R2
Ru
B
N C
C N
H
R3
(B = PPh3, AsPh3 or Py; R1 = H or OCH3, R2 = H or Cl, R3 = H or CH3)
Figure 1. Proposed structure of new ruthenium(II) unsymmetrical Schiff-base complexes.
Table 3. Catalytic activity of the TH of ketones by [Ru(CO)(PPh3)(L8)] with i-PrOHa.
Substrate
Product
Time (h)
2
Conversion (%)b
O
OH
94
O
O
OH
4
6
4
92
53
87
OH
O
OH
aConditions: reactions were refluxed using 3.75 mmol of ketone (10 cm3 isopropanol);
catalyst : ketone : KOH ratio 1 : 300 : 2.5.
bYield of product was determined using a ACME 6000 series GC-FID with a DP-5 column of 30 m
length, 0.53 mm diameter, and 5.00 mm film thickness and by comparing with the authentic samples.
3.5. 13C NMR spectroscopic analysis
13C NMR spectra revealed the presence of an expected number of signals corresponding
to different types of carbon atoms present in the compounds. The spectral
(Supplementary material) data showed a strong resonance at 168.36–170.52 ppm
assigned as –C¼N. Multiplets around 126.27–143.31 ppm are assigned to aromatic
carbons. The methylene carbons appear in the region 42.45–44.58 and 38.53–40.82 ppm.
Sharp singlets at 55.62 and 21.97–22.04 ppm are assigned to methoxy and methyl
carbons, respectively. The spectra confirm the formation of new ruthenium(II)
unsymmetrical Schiff-base complexes.
Based on analytical and spectroscopic data, an octahedral structure (figure 1) is
proposed for all the ruthenium(II) unsymmetrical Schiff-base complexes.
3.6. Catalytic TH of ketones
Catalytic TH of various ketones in the presence of [Ru(CO)(PPh3)(L8)] has been studied
in isopropanol–KOH using a molar ratio 1 : 2.5 : 300 for the catalyst, KOH, and ketone
in 10 cm3 of isopropanol. The catalyst performed efficiently for both aliphatic and
aromatic ketones with high conversion and turnover (table 3). Cyclohexanone was

Unsymmetrical Schiff-bases
531
Table 4. Catalytic activity of ruthenium(II) unsymmetrical Schiff-base com-
plexes for coupling reaction.
Yield of biphenyla
Complex
(g)
(%)
[Ru(CO)(PPh3)(L7)]
[Ru(CO)(PPh3)(L8)]
[Ru(CO)(AsPh3)(L7)]
[Ru(CO)(AsPh3)(L8)]
[Ru(CO)(Py)(L6)]
[Ru(CO)(Py)(L7)]
No catalyst
0.289
0.251
0.248
0.251
0.302
0.246
0.021
15.7
13.6
13.4
13.6
16.3
13.3
01.2
aBased on phenylmagnesium bromide: 12 mmol.
converted into cyclohexanol in 94% yield and acetophenone was converted into
1-phenyl ethanol in 92% yield. Similarly, benzophenone and isobutyl methyl ketone
underwent TH to afford the corresponding alcohol in 53% and 87% yields. In the
absence of base, no TH of ketones was observed.
3.7. Catalytic coupling reactions
The ruthenium(II) unsymmetrical Schiff-base complexes were also tested as catalysts
for aryl–aryl coupling [41, 42]. The system chosen is the coupling of phenylmagnesium
bromide with bromobenzene to give biphenyl. Bromobenzene was first converted to the
corresponding Grignard reagent. Then bromobenzene, followed by the test complex,
was added and the mixture was refluxed for 6 h. After work up, the reaction mixture
gave biphenyl.
Yields of biphenyl formed from the coupling reactions varied from 13% to 16%
(table 4) depending upon the catalyst. Reactions without ruthenium catalyst produced
1.2% biphenyl, insignificant compared to the yields of biphenyl obtained from the
reactions catalyzed by ruthenium complexes. This shows the catalytic role of ruthenium
complexes in the coupling. The yields of biphenyl obtained from the reactions catalyzed
by the ruthenium(II) complexes are low due to (1) active species derived from
ruthenium complexes are less stable, as the effectiveness of the catalysts is directly
related to their ability to generate the corresponding active species [43] and
(2) bromobenzene undergoes dehalogenation to form benzene as side product. That
product yields are not much differed for different catalysts may be due to similar active
species formed in all reactions.
4. Conclusions
Several new ruthenium(II) unsymmetrical Schiff-base complexes were synthesized and
characterized by analytical and spectroscopic techniques. An octahedral structure has
been tentatively proposed for all complexes. The complexes showed efficient catalytic
activity for TH reactions and moderate activity for carbon–carbon coupling reactions.

532
S. Gowri et al.
Although some ruthenium complexes have been reported in recent literature [44–52],
ruthenium(II) unsymmetrical Schiff-base complexes have been rarely seen as catalysts
for this kind of TH and carbon–carbon coupling reactions. The presence of a labile
triphenylphosphine makes the complexes catalytically active and activity may be tuned
by changing the electronic environment around the metal by altering the substituents in
unsymmetrical Schiff-base ligand.
Supplementary material
1H and 31P NMR spectra of representative ruthenium(II) unsymmetrical Schiff-base
complexes have been provided as supplementary materials.
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