Synthesis, characterization, electrochemistry, catalytic, and antimicrobial studies of ruthenium(III) complexes containing ONO donor ligands

Article abstract of DOI:10.1080/00958971003735440

A series of ruthenium(III) complexes [RuX(EPh₃)₂L] (where X = Cl or Br; E = P or As; L = deprotonated dibasic tridentate ligand) were prepared by the reaction of [RuX₃(EPh₃)₃] with Schiff bases (H

Full text of DOI:10.1080/00958971003735440

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Synthesis, characterization,
electrochemistry, catalytic, and
antimicrobial studies of ruthenium(III)
complexes containing ONO donor
ligands
N. Thilagavathi a , A. Manimaran b & C. Jayabalakrishnan b
a Department of Chemistry , Surya Engineering College , Erode
638 107, Tamil Nadu, India
b Department of Chemistry , Sri Ramakrishna Mission Vidyalaya,
College of Arts and Science , Coimbatore 641 020, Tamil Nadu,
India
Published online: 26 Mar 2010.
To cite this article: N. Thilagavathi , A. Manimaran & C. Jayabalakrishnan (2010) Synthesis,
characterization, electrochemistry, catalytic, and antimicrobial studies of ruthenium(III)
complexes containing ONO donor ligands, Journal of Coordination Chemistry, 63:7, 1252-1262, DOI:
10.1080/00958971003735440
To link to this article: http://dx.doi.org/10.1080/00958971003735440
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Journal of Coordination Chemistry
Vol. 63, No. 7, 10 April 2010, 1252–1262
Synthesis, characterization, electrochemistry, catalytic,
and antimicrobial studies of ruthenium(III) complexes
containing ONO donor ligands
N. THILAGAVATHIy, A. MANIMARANz
and C. JAYABALAKRISHNAN*z
yDepartment of Chemistry, Surya Engineering College,
Erode 638 107, Tamil Nadu, India
zDepartment of Chemistry, Sri Ramakrishna Mission Vidyalaya,
College of Arts and Science, Coimbatore 641 020, Tamil Nadu, India
(Received 21 July 2009; in final form 17 November 2009)
A series of ruthenium(III) complexes [RuX(EPh3)2L] (where X ¼ Cl or Br; E ¼ P or As;
L ¼ deprotonated dibasic tridentate ligand) were prepared by the reaction of [RuX3(EPh3)3]
with Schiff bases (H2L1–H2L4). The ligands were prepared by the condensation of N-4 phenyl/
methyl semicarbazide with o-vanillin/o-hydroxy acetophenone. The complexes were character-
ized by elemental, physico-chemical, and electrochemical methods. Catalytic studies of these
complexes for the oxidation of alcohols and aryl–aryl coupling were carried out. Antimicrobial
experiments were also carried out.
Keywords: Ruthenium(III); EPR spectra; Oxidation of alcohols; N-methylmorpholine-N-
oxide; Schiff base
1. Introduction
Controlled oxidation of alcohols is one of the most important reactions in organic
chemistry, particularly the conversion of primary alcohols to aldehydes is crucial for the
synthesis of fine chemicals such as fragrances or food additives [1]. Numerous oxidizing
agents are available to influenace this key reaction; however, these reagents are usually
toxic inducing problems [2]. Owing to its central position in the periodic table,
ruthenium shows the properties that are common to both early and late transition
metals resulting in a confluence of desirable properties. Ru-based oxidation catalysis is
being rapidly developed as it affords economic and environmental benefits [3–5].
The utility of the biaryl structural motif has prompted research directed at discovering
efficient and high-yield methods for its preparation. Transition metal catalysis has
featured prominently in these efforts, leading to a range of useful cross-coupling
reactions. Transition-metal catalyzed carbon–carbon bond formation represented a
milestone in synthetic organic chemistry allowing the coupling of substrates that would
*Corresponding author. Email: cjbsrmvcbe@gmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958971003735440

Ruthenium(III) complexes
1253
have previously been thought impossible [6, 7]. Semicarbazones have a wide range of
biological activities and are versatile ligands. Biological properties of semicarbazones
are often related to coordination, constituting an interesting family of ligands for
medicinal inorganic research [8]. Ruthenium complexes of such ligands have been well
reported in literature [9–11]. In continuation of our work, we report here the synthesis,
characterization, catalytic utility, and antimicrobial studies of Ru(III) complexes
containing Schiff-base ligands derived from the condensation of N-4 phenyl/methyl
semicarbazide with o-vanillin/o-hydroxy acetophenone. The general structure of the
Schiff bases are given in scheme 1.
2. Experimental
2.1. Materials and methods
All reagents were analytical reagent grade. Solvents were purified and dried according
to standard procedures [12]. RuCl3ꢀ3H2O was purchased from Loba Chemie. CHN
analyzes were performed in a Vario EL III CHNS analyzer at Cochin University. IR
spectra of the complexes were recorded as KBr pellets with a Perkin Elmer 597 infrared
spectrophotometer from 4000 to 200 cmꢁ1. Electronic spectra were recorded in
dichloromethane with a Systronics double beam UV-Vis Spectrophotometer 2202
with an accuracy of ꢂ0.5 nm. EPR spectra of powdered samples at RT and LNT were
recorded with a Bruker model ER-200-D Spectrometer using DPPH as a g-marker at
X-band at the Indian Institute of Technology, Chennai. Cyclic voltammetric studies
were carried out in acetonitrile using a glassy-carbon working electrode and potentials
were referenced to a standard calomel electrode (SCE). [N(Bu4)BF4] was used as
R
OH
O
N
NH C NH R„„
R„
Ligand
R
R„
R„„
1
H L
2
OCH
OCH
H
H
C H
3
6
5
2
H L
2
H
CH
3
3
3
H L
2
CH
C H
3
3
6
5
4
H2L
H
CH
CH
3
Scheme 1. General structure of the ligands.

1254
N. Thilagavathi et al.
supporting electrolyte and the scan rate was 100 mV sꢁ1. Melting points were recorded
on a Raaga heating table and are uncorrected. Magnetic susceptibility measurements
were made with an EG and G-PARC vibrating sample magnetometer. Molar
conductivities of the freshly prepared solutions were measured using an Elico CM
180 conductivity meter. [RuCl3(PPh3)3] [13], [RuCl3(AsPh3)3] [14], [RuBr3(AsPh3)3] [15],
and ligands [16] were prepared by the reported methods.
2.2. Synthesis of [RuCl(PPh3)2L1]
The complex [RuCl(PPh3)2L1] was synthesized by refluxing a mixture containing H2L1
(0.029 g, 0.1 mmol) and [RuCl3(PPh3)3] (0.099 g, 0.1 mmol) in benzene (25 mL) for 6 h
ꢃ
and concentrated to 5 mL. Addition of 10 mL of petroleum ether (60–80 C) gave the
precipitate which was washed with petroleum ether and dried under vacuum. All other
complexes which were synthesized using this procedure are given in the following
sections (scheme 2).
2.2.1. [RuCl(PPh3)2L1]. Yield: 63%. m.p. 4 245ꢃC. Elemental Anal. Calcd (%):
C, 64.9; H, 4.6; N, 4.4. Found (%): C, 64.2; H, 4.7; N, 4.6. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1527; ꢀ(N¼C), 1592; ꢀ(N–N), 1087; ꢀ(Ph–C–O), 1297. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 587 (767), 486 (2687), 423 (2380).
R
OH
[RuX (EPh ) ]
+
3
3 3
O
C
N
HN
NH R„„
R„
Reflux in benzene
6h
R
EPh
3
O
X
Ru
O
3
N
EPh
R„
C
NH R„„
N
R = H or OCH3; R„ = H or CH3; R„„ = C6H5 or CH3; X = Cl or Br; E = P or As
Scheme 2. Preparation of the ruthenium(III) complexes.

Ruthenium(III) complexes
1255
2.2.2. [RuCl(PPh3)2L2]. Yield: 61%. m.p. 4 245ꢃC. Elemental Anal. Calcd (%):
C, 62.6; H, 4.7; N, 4.8. Found: C, 63.0; H, 4.7; N, 4.7. IR (KBr, cmꢁ1): ꢀ(C¼N), 1537;
ꢀ
(N¼C), 1595; ꢀ(N–N), 1083; ꢀ(Ph–C–O), 1296. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 592 (876), 490 (2885), 433 (3591).
2.2.3. [RuCl(PPh3)2L3]. Yield: 92%. m.p. 4 245ꢃC. Elemental Anal. Calcd (%):
C, 66.0; H, 4.7; N, 4.5. Found (%): C, 64.8; H, 4.4; N, 4.5. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1539; ꢀ(N¼C), 1652; ꢀ(N–N), 1081; ꢀ(Ph–C–O), 1288. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 598 (819), 490 (2854), 434 (2904).
2.2.4. [RuCl(PPh3)2L4]. Yield: 66%. m.p. 4 245ꢃC. Elemental Anal. Calcd (%):
C, 63.8; H, 4.8; N, 4.9. Found (%): C, 65.0; H, 4.5; N, 4.5. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1537; ꢀ(N¼C), 1587; ꢀ(N–N), 1083; ꢀ(Ph–C–O), 1288. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 588 (872), 498 (2698), 428 (2874).
2.2.5. [RuCl(AsPh3)2L1]. Yield: 71%. m.p. ¼ 216ꢃC. Elemental Anal. Calcd (%):
C, 59.3; H, 4.2; N, 4.1. Found (%): C, 59.7; H, 4.3; N, 4.5. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1525; ꢀ(N¼C), 1595; ꢀ(N–N), 1091; ꢀ(Ph–C–O), 1292. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 610 (842), 513 (2753), 424 (2954).
2.2.6. [RuCl(AsPh3)2L2]. Yield: 81%. m.p. ¼ 209ꢃC. Elemental Anal. Calcd (%):
C, 56.9; H, 4.3; N, 4.3. Found (%): C, 57.8; H, 4.5; N, 4.4. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1523; ꢀ(N¼C), 1593; ꢀ(N–N), 1089; ꢀ(Ph–C–O), 1294. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 613 (704), 513 (2652), 433 (3852).
2.2.7. [RuCl(AsPh3)2L3]. Yield: 82%. m.p. 4 245ꢃC. Elemental Anal. Calcd (%):
C, 60.3; H, 4.3; N, 4.1. Found (%): C, 60.2; H, 4.1; N, 4.4. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1541; ꢀ(N¼C), 1610; ꢀ(N–N), 1091; ꢀ(Ph–C–O), 1290. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 623 (719), 513 (2787), 434 (4450).
2.2.8. [RuCl(AsPh3)2L4]. Yield: 72%. m.p. 4 245ꢃC. Elemental Anal. Calcd (%):
C, 57.9; H, 4.3; N, 4.4. Found (%): C, 56.1; H, 4.1; N, 4.4. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1541; ꢀ(N¼C), 1605; ꢀ(N–N), 1089; ꢀ(Ph–C–O), 1292. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 623 (798), 524 (2851), 432 (4390).
2.2.9. [RuBr(AsPh3)2L1]. Yield: 69%. m.p. ¼ 210ꢃC. Elemental Anal. Calcd (%):
C, 56.9; H, 4.0; N, 3.9. Found (%): C, 57.9; H, 4.2; N, 4.3. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1515; ꢀ(N¼C), 1596; ꢀ(N–N), 1081; ꢀ(Ph–C–O), 1290. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 618 (876), 519 (2856), 441 (4298).
2.2.10. [RuBr(AsPh3)2L2]. Yield: 75%. m.p. ¼ 185ꢃC. Elemental Anal. Calcd (%):
C, 54.5; H, 4.1; N, 4.1. Found (%): C, 53.9; H, 4.0; N, 4.5. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1515; ꢀ(N¼C), 1600; ꢀ(N–N), 1081; ꢀ(Ph–C–O), 1294. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 639 (412), 529 (2397), 444 (6412).

1256
N. Thilagavathi et al.
2.2.11. [RuBr(AsPh3)2L3]. Yield: 84%. m.p. ¼ 200ꢃC. Elemental Anal. Calcd (%):
C, 57.7; H, 4.1; N, 4.0. Found (%): C, 57.6; H, 4.0; N, 4.0. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1539; ꢀ(N¼C), 1612; ꢀ(N–N), 1081; ꢀ(Ph–C–O), 1290. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 626 (387), 525 (2965), 418 (3564).
2.2.12. [RuBr(AsPh3)2L4]. Yield: 71%. m.p. ¼ 190ꢃC. Elemental Anal. Calcd (%):
C, 55.3; H, 4.1; N, 4.2. Found (%): C, 56.5; H, 4.3; N, 4.3. IR (KBr, cmꢁ1): ꢀ(C¼N)
,
1541; ꢀ(N¼C), 1608; ꢀ(N–N), 1083; ꢀ(Ph–C–O), 1292. UV-Vis (dichloromethane) ꢁ, nm
("; dm3 molꢁ1 cmꢁ1): 624 (497), 524 (2123), 439 (3476).
2.3. Catalytic oxidation experiments
2.3.1. NMO as oxidant. To a solution of the alcohol (0.07–0.13 mL, 1 mmol) in
dichloromethane (20 mL), N-methylmorpholine-N-oxide (NMO) (0.35 g, 3 mmol),
and the ruthenium complex (0.009 g, 0.01 mmol) were added and the solution was
heated under reflux for 3 h. The mixture was then filtered and the filtrate was dried over
anhydrous Na2SO4, evaporated to dryness, and extracted with diethyl ether. The diethyl
ether extract was filtered and evaporated to give the corresponding carbonyl compound
which was quantified as its 2,4-dinitrophenylhydrazone [12, 17].
2.3.2. Molecular oxygen as oxidant. To a solution of alcohol (0.07–0.13 mL, 1 mmol)
in dichloromethane (20 mL) a solution of the ruthenium complex in dichloromethane
(0.009 g, 0.01 mmol) was added, and the mixture was stirred under an oxygen
atmosphere at ambient temperature for 6 h. The mixture was evaporated to dryness and
extracted with diethyl ether (60–80ꢃC). The combined extracts were filtered and
evaporated to give the corresponding carbonyl compound, which was then quantified as
its 2,4-dinitrophenylhydrazone [12, 18].
2.4. Aryl–aryl coupling experiments
Magnesium turnings (0.320 g) were placed in a flask equipped with a CaCl2 guard tube.
A crystal of iodine was added. Bromobenzene [0.75 mL of total 1.88 mL] in anhydrous
diethyl ether (5 mL) was added with stirring. The remaining bromobenzene in ether
(5 mL) was added dropwise and the mixture was refluxed for 40 min. To this mixture,
1.03 mL (0.01 mol) of bromobenzene in anhydrous diethyl ether (5 mL) and the
ruthenium complex (0.05 mmol) chosen for investigation were added and heated under
reflux for 6 h. The reaction mixture was cooled, hydrolyzed with a saturated solution of
aqueous NH4Cl and the ether extract on evaporation gave a crude product which was
chromatographed to get pure biphenyl and compared with an authentic sample [19]
(m.p.: 69–72ꢃC).
2.5. Antibacterial activity
The ligands and their complexes were tested for in vitro growth inhibitory activity
against Escherichia coli and Bacillus subtilis using the disc diffusion method.

Ruthenium(III) complexes
1257
The bacteria were cultured in nutrient agar medium and used as inoculum for the study.
Bacterial cells were swabbed onto nutrient agar medium in Petri plates. Two different
concentrations of the ligands and the complexes (0.5 and 1.0 g of the substances in
100 mL of the solvent) were prepared in DMSO and soaked in filter paper discs (5 mm
diameter, 1 mm thick). These discs were placed on the already seeded plates and
incubated at 35 ꢂ 2ꢃC for 24 h. The diameter (mm) of the inhibition zone around each
disc was measured after 24 h. A common standard antibiotic, Ampicillin, was used as a
standard in the same solvent and at the same concentration [20]. A control test with the
solvent was also carried out under identical conditions.
3. Results and discussion
All complexes are quite stable in air and light. Analytical data for the complexes are in
good agreement with proposed formulas. The complexes are soluble in common
organic solvents, such as DMSO, dichloromethane, and chloroform. The molar
conductivities for complexes in DMSO (1.0 ꢄ 10ꢁ3 mol) are in the range of 10.5–
13.4 ohmꢁ1 cm2 molꢁ1. It is clear from conductivity measurements that the complexes
are non-electrolytes [21].
3.1. IR spectra
Considerable shift in the band positions of ꢀ(C¼N), ꢀ(N–N), and phenolic ꢀ(C–O) of the
complexes from the IR spectra of the ligands and the absence of phenolic ꢀ(O–H) in all
the complexes indicate azomethine nitrogen and deprotonated phenolic oxygen
coordination to ruthenium. Absence of amide ꢀ(C ¼ O) and ꢀ(N–H) vibrations and also
the absence of the enolic ꢀ(O–H) vibrations in the spectra of the complexes confirm
enolate oxygen coordination after deprotonation. Coordination modes of the ligands
were further confirmed by the presence of the characteristic peaks due to Ru–O, Ru–N,
Ru–PPh3, and Ru–AsPh3 [21–24]. In all complexes, the Schiff bases coordinate to
ruthenium as dibasic tridentate (ONO) donors that are in line with the recently reported
work involving a similar type of ligand [25].
3.2. Electronic spectra
Electronic spectra of the complexes show five types of bands assigned on the basis of the
wavelength of absorption and molar absorption coefficient. Bands in the UV region
were assigned to intraligand, ꢂ ! ꢂ*, and n ! ꢂ* transitions [26, 27]; less intense
shoulders observed in the visible region were assigned to d–d transitions. Ligand field
parameters confirm octahedral geometry around ruthenium [28].
3.3. Magnetic moment and EPR spectra
Effective magnetic moments of the ruthenium(III) complexes are from 1.78 to 1.96 BM,
indicating one unpaired electron and confirming that ruthenium is þ3 [23]. The room

1258
N. Thilagavathi et al.
temperature and liquid nitrogen temperature EPR spectra of powder samples
(Supplementary material) show two signals with g ¼ 2.41–2.69 and g|| ¼ 2.05–2.22,
?
indicating axial distortion. Axial distortion splits the t2 level into a and e and the energy
levels are in the order dxy 4 dxz, dyz with the unpaired electron in dxy. The trans binding
of the two PPh3 ligands is primarily responsible for the axial distortion [29, 30].
The EPR spectra of ruthenium(III) complexes showed no indication of hyperfine
interaction of Ru with N, As, P, Cl, and Br. Further, there exist no significant variation
in the EPR spectra of the complexes recorded at room temperature and liquid nitrogen
temperature, indicating octahedral geometry around ruthenium.
3.4. Electrochemical properties
All the complexes showed only a reversible oxidation wave in the anodic side of
SCE. A representative cyclic voltammogram and the data are provided in the
‘‘Supplemental material’’. Redox potentials were observed from 1.13 to 1.23 V with
peak to peak separations within the range of 50–120 mV. The observed oxidation is
assigned from ruthenium(III) to ruthenium(IV). The peak-to-peak separation value
(DEp ¼ 50–90 mV) for all the complexes except [RuCl(PPh3)2L3] and [RuCl(AsPh3)2L3]
indicates that the oxidation in these complexes are reversible and the corresponding
values for the above-mentioned complexes are 100 and 120 mV, respectively,
indicating quasi-reversible oxidation. The stability of the þ3 oxidation state and the
accessibility of þ4 state are mainly attributed to phenolate-O coordination.
For electron-withdrawing substituents, oxidation of the metal occurs at higher
potentials and for electron-releasing substituents, it occurs at lower potentials [31, 32].
3.5. Catalytic oxidation of alcohols
Oxidation of primary/secondary alcohols was carried out using the ruthenium
complexes as catalysts and NMO/molecular oxygen as the oxidant. Alcohols were
converted into their corresponding aldehydes/ketones (table 1). Among phosphine
complexes, the order of catalytic activity in terms of yield and turnover number was
[RuCl(PPh3)2L3] 4 [RuCl(PPh3)2L4] 4 [RuCl(PPh3)2L1]. In terms of substituents pre-
sent in the Schiff-base moiety, the order of activity is CH3, C6H5 4 CH3, CH3 4 OCH3,
C6H5. Hence, it is inferred that inductive (–CH3 and –OCH3) and mesomeric effects
(–C6H5) of the substituents play a major role in catalytic activity of their corresponding
complexes. Electron releasing substituents, –CH3 and –C6H5 in H2L3 make the complex
more active. In the complex with two –CH3 groups, the one which is directly attached to
the azomethine nitrogen contributes more to the electronic effect and hence to the
catalytic activity. Contribution from the other –CH3 group is less, owing to the short
distance of the inductive effect. With one –OCH3 and one –C6H5 as substituents, the
resultant contribution from the þM effect of the –C6H5 group and the – I effect of
the –OCH3 group makes this complex less active than the other two complexes. When
the activities of [RuCl(PPh3)2L1] and [RuCl(AsPh3)2L1] are compared, triphenylarsine
complex was more active, perhaps due to the high donor ability of the arsine. Hence,
electron-releasing groups increase the catalytic activity and electron-withdrawing
groups decrease the catalytic activity [33]. Oxidation of primary aliphatic alcohols was
moderate in conversion. When the results are compared with recently reported work,

Ruthenium(III) complexes
Table 1. Oxidation of alcohols by Ru(III) complexes.
1259
NMO oxidant
Yield (%)a TONb
Molecular oxygen oxidant
Complex
Substrate
Product
Yield (%)a
TONb
[RuCl(PPh3)2L1] Benzyl alcohol
Cyclohexanol
Benzaldehyde
Cyclohexanone
79
81
75
58
68
69
80
83
76
60
69
71
41
42
39
30
35
35
42
43
40
31
36
37
Cinnamyl alcohol Cinnamaldehyde
n-Butanol Butyraldehyde
Isobutyl alcohol Ethyl methyl ketone
n-Propanol
[RuCl(AsPh3)2L1] Benzyl alcohol
Cyclohexanol
Propionaldehyde
Benzaldehyde
Cyclohexanone
81
84
79
59
69
71
82
85
80
61
71
73
43
45
43
35
36
36
44
46
44
36
37
37
Cinnamyl alcohol Cinnamaldehyde
n-Butanol Butyraldehyde
Isobutyl alcohol Ethyl methyl ketone
n-Propanol
[RuCl(PPh3)2L3] Benzyl alcohol
Cyclohexanol
Propionaldehyde
Benzaldehyde
Cyclohexanone
83
85
79
62
70
73
84
87
80
63
72
76
44
44
41
35
38
39
45
45
41
36
39
40
Cinnamyl alcohol Cinnamaldehyde
n-Butanol Butyraldehyde
Isobutyl alcohol Ethyl methyl ketone
n-Propanol
[RuCl(PPh3)2L4] Benzyl alcohol
Cyclohexanol
Propionaldehyde
Benzaldehyde
Cyclohexanone
80
82
77
58
68
70
81
84
78
60
70
72
41
44
41
32
31
37
42
44
42
33
32
38
Cinnamyl alcohol Cinnamaldehyde
n-Butanol Butyraldehyde
Isobutyl alcohol Ethyl methyl ketone
n-Propanol Propionaldehyde
TON ¼ Turn over number.
aYield based on substrate.
bMoles of product per mole of catalyst.
involving a series of ruthenium(II) complexes containing similar donors, ruthenium(III)
complexes are better catalysts than ruthenium(II) complexes [34].
We tested the catalytic efficiency of our complexes with molecular oxygen as oxidant,
but the yields and turnover numbers were moderate when compared with NMO and
other currently employed oxidants. The new complexes react more efficiently with
NMO than with molecular oxygen. This is indicated by the low-product yield when
molecular oxygen is employed as the oxidant as previously observed [35, 36]. It has been
established that the oxidation of alcohols by the ruthenium(III)/molecular oxygen
system is quite slow, requiring high pressure and temperature [32].
Though many catalytic systems have been reported for the oxidation of alcohols,
most of them suffer from limitations. Chromium-based oxidations are effective but
have serious drawbacks generating stoichiometric amounts of heavy-metal waste, and
Cr(VI) is a proven carcinogen [37]. Despite being an effective oxidant, practical
applications of iodosylbenzene as oxidant are hampered by its low solubility in
non-reactive media, as well as low thermal stability and explosive properties upon
moderate heating [38]. As reported earlier, TEMPO-bleach oxidation did not work
well with unsaturated alcohols [39]. Ionic liquids, which are excellent clean solvents,
suffer from reduced activity due to residual chloride [40] and not effective for the
oxidation of aliphatic alcohols [36]. TPAP, the most widely used ruthenium catalyst,
requires low-temperature storage to retain its activity [41]. Other catalytic systems

1260
N. Thilagavathi et al.
Table 2. Aryl–aryl coupling of Ru(III) complexes.
Complex
Biphenyl yield (g)
Biphenyl yield (%)
[RuCl(PPh3)2L1]
[RuCl(PPh3)2L2]
[RuCl(PPh3)2L3]
[RuCl(PPh3)2L4]
[RuCl(AsPh3)2L1]
[RuCl(AsPh3)2L2]
[RuCl(AsPh3)2L3]
[RuCl(AsPh3)2L4]
[RuBr(AsPh3)2L1]
[RuBr(AsPh3)2L2]
[RuBr(AsPh3)2L3]
[RuBr(AsPh3)2L4]
0.42
0.32
0.42
0.50
0.49
0.50
0.40
0.41
0.39
0.44
0.38
0.41
28.51
21.72
28.51
33.94
33.26
33.94
27.15
27.83
26.47
29.86
25.79
27.83
(N-halo reagents) either require a longer reaction time or special conditions such as very
low temperatures or highly volatile solvent [42]. The complexes reported in this article
possess comparable catalytic activity with the currently employed catalytic systems such
as supported gold nanoparticles in aqueous H2O2 [43], Pd(OAc)2 oxygen/toluene [44],
and electrochemical oxidation [45] in terms of yield and selectivity. Dichloromethane
used in this study was reported to be a better solvent than the other common organic
solvents in terms of yield and it requires less reaction time [39].
3.6. Aryl–aryl coupling
The ruthenium complexes were used as catalysts for the coupling of phenyl
magnesium bromide with bromobenzene to give biphenyl as the product. The yield
of biphenyl is moderate (table 2). This may be due to the fact that the active species
(d7 configuration) derived from ruthenium complexes are less stable [46].
3.7. Biocidal studies
Inhibition activity of the test compounds was assessed from the zones of inhibition.
Error in the measurement is given in parentheses (table 3). Antimicrobial activities show
that the ruthenium chelates are more toxic than their parent ligands against the same
micro-organisms under identical experimental conditions. The four Schiff-base ligands
had the same activity against the organisms and the magnitude of their influence in their
corresponding complexes was also same. The inhibition activity was found to be slightly
augmented by bromide than the more electronegative chloride. The inhibition activity
was increased with the concentration. Though the complexes possess inhibition activity,
it did not reach the effectiveness of Ampicillin.
4. Conclusion
A series of ruthenium(III) complexes [RuX(EPh3)2L] were prepared by the reaction of
[RuX3(EPh3)3], with Schiff bases prepared by condensation of N-4 phenyl/methyl

Ruthenium(III) complexes
Table 3. Antibacterial activity of Ru(III) complexes.
1261
Zone of inhibition (mm)
B. subtilis
E. coli
Ligand/complex
0.5%
1.0%
0.5%
1.0%
H2L1
7 (ꢂ0.02)
11 (ꢂ0.06)
10 (ꢂ0.04)
14 (ꢂ0.03)
7 (ꢂ0.03)
10 (ꢂ0.02)
12 (ꢂ0.00)
15 (ꢂ0.05)
6 (ꢂ0.05)
9 (ꢂ0.03)
9 (ꢂ0.03)
12 (ꢂ0.04)
7 (ꢂ0.05)
10 (ꢂ0.05)
12 (ꢂ0.02)
14 (ꢂ0.05)
20 (ꢂ0.04)
10 (ꢂ0.05)
15 (ꢂ0.03)
13 (ꢂ0.05)
18 (ꢂ0.03)
11 (ꢂ0.05)
14 (ꢂ0.02)
17 (ꢂ0.05)
18 (ꢂ0.05)
9 (ꢂ0.05)
10 (ꢂ0.02)
13 (ꢂ0.05)
12 (ꢂ0.00)
15 (ꢂ0.03)
9 (ꢂ0.05)
12 (ꢂ0.04)
16 (ꢂ0.05)
15 (ꢂ0.03)
18 (ꢂ0.05)
12 (ꢂ0.05)
14 (ꢂ0.05)
17 (ꢂ0.03)
20 (ꢂ0.05)
11 (ꢂ0.05)
13 (ꢂ0.05)
15 (ꢂ0.05)
16 (ꢂ0.05)
12 (ꢂ0.05)
15 (ꢂ0.05)
15 (ꢂ0.05)
19 (ꢂ0.05)
26 (ꢂ0.00)
[RuCl(PPh3)2L1]
[RuCl(AsPh3)2L1]
[RuBr(AsPh3)2L1]
H2L2
[RuCl(PPh3)2L2]
[RuCl(AsPh3)2L2]
[RuBr(AsPh3)2L2]
H2L3
11 (ꢂ0.05)
13 (ꢂ0.03)
16 (ꢂ0.05)
7 (ꢂ0.05)
[RuCl(PPh3)2L3]
[RuCl(AsPh3)2L3]
[RuBr(AsPh3)2L3]
H2L4
13 (ꢂ0.05)
14 (ꢂ0.05)
16 (ꢂ0.00)
9 (ꢂ0.05)
10 (ꢂ0.04)
11 (ꢂ0.05)
13 (ꢂ0.00)
8 (ꢂ0.05)
[RuCl(PPh3)2L4]
[RuCl(AsPh3)2L4]
[RuBr(AsPh3)2L4]
Ampicillin
14 (ꢂ0.02)
17 (ꢂ0.03)
19 (ꢂ0.05)
24 (ꢂ0.00)
11 (ꢂ0.04)
12 (ꢂ0.04)
15 (ꢂ0.05)
22 (ꢂ0.05)
0.5% and 1.0% indicate 0.5 and 1.0 g of the substance in 100 mL of DMSO, respectively.
semicarbazide with o-vanillin/o-hydroxy acetophenone. The complexes were character-
ized by analytical, spectral, magnetic moment, and electrochemical data. The complexes
are catalysts for the oxidation of primary/secondary alcohols, better catalytic activity in
the presence of NMO than molecular oxygen as oxidant and moderate activity in the
case of aryl–aryl coupling. All the complexes possess better antibacterial activity against
B. subtilis and E. coli than their parent ligands.
We have introduced a new mild reagent for the oxidation of different types
of alcohols in refluxing dichloromethane. The stability, easy preparation, mild reaction
conditions, high-yields of products, and reaction under non-aqueous conditions make
this reagent useful for the oxidation of alcohols. The scope of our catalyst is illustrated
by the highly selective oxidation of non-activated aliphatic alcohols as well as activated
benzyl alcohol and also secondary alcohols in excellent yields.
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