Ru(II)-polypyridine complexes with alkynyl Schiff base ligand: influence of π-conjugation, donor/acceptor substituents, and counter anions on electrochemical, luminescence, and catalytic properties

Article abstract of DOI:10.1080/00958972.2020.1757663

Ru(II)-polypyridine complexes of the general formula [Ru(L₁/L₂)(phen)₂]X₂ (1a–6a) and [Ru(L₁/L₂)(bipy)₂]X₂ (1b–6b) (where X = ClO₄, BF₄, PF₆; phen = 1,10-phenanthroline, bipy = 2,2′-bipyridine) were prepared by the reaction of [Ru(phen)₂Cl₂]·2H₂O and [Ru(bipy)₂Cl₂]·2H₂O with (E)-5-((4-methoxyphenyl)ethynyl)-N-(pyridin-2-ylmethylene)pyridin-2-amine (L₁) and (E)-5-((4-nitrophenyl)ethynyl)-N-(pyridin-2-ylmethylene)pyridine-2-amine (L₂) in the presence of NaBF₄, NaClO₄, and NaPF₆. The electrochemical properties of all the complexes indicate reversible redox behavior corresponding to Ru(II)–Ru(III) couple and are susceptible to variation of electron-donating/accepting properties of substituent group on L₁ and L₂. All complexes showed room temperature luminescence corresponding to π→π* intra-ligand charge-transfer (ILCT) transition with chelation enhanced fluorescence and is finely tuned by increasing π-conjugation, size of counter anions, and variation of substituent group with different electronic effects in the complexes. All the complexes worked as an effective catalyst for the oxidation of benzyl alcohol to corresponding benzaldehyde in good yield at room temperature. (Figure presented.).

Full text of DOI:10.1080/00958972.2020.1757663

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Ru(II)-polypyridine complexes with alkynyl Schiff
base ligand: influence of π-conjugation, donor/
acceptor substituents, and counter anions on
electrochemical, luminescence, and catalytic
properties
A. B. Patil-Deshmukh, S. S. Mohite & S. S. Chavan
To cite this article: A. B. Patil-Deshmukh, S. S. Mohite & S. S. Chavan (2020): Ru(II)-polypyridine
complexes with alkynyl Schiff base ligand: influence of π-conjugation, donor/acceptor substituents,
and counter anions on electrochemical, luminescence, and catalytic properties, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2020.1757663
To link to this article: https://doi.org/10.1080/00958972.2020.1757663
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1757663
Ru(II)-polypyridine complexes with alkynyl Schiff base
ligand: influence of p-conjugation, donor/acceptor
substituents, and counter anions on electrochemical,
luminescence, and catalytic properties
A. B. Patil-Deshmukh, S. S. Mohite and S. S. Chavan
Department of Chemistry, Shivaji University, Kolhapur, Maharashtra, India
ABSTRACT
ARTICLE HISTORY
Ru(II)-polypyridine complexes of the general formula [Ru(L
1
/
Received 4 December 2019
Accepted 5 April 2020
L
2
)(phen)
2 2 1 2 2 2
]X (1a–6a) and [Ru(L /L )(bipy) ]X (1b–6b) (where
0
X ¼ ClO , BF , PF ; phen ¼ 1,10-phenanthroline, bipy ¼ 2,2 -bipyri-
4
4
6
KEYWORDS
dine) were prepared by the reaction of [Ru(phen)
2 2 2
Cl ]ꢀ2H O and
Ru(II) complexes;
photoluminescence; cyclic
voltammetry; cata-
lytic activity
[
Ru(bipy) Cl ]ꢀ2H O with (E)-5-((4-methoxyphenyl)ethynyl)-N-(pyri-
2
2
2
1
din-2-ylmethylene)pyridin-2-amine (L ) and (E)-5-((4-nitrophenyl)e-
thynyl)-N-(pyridin-2-ylmethylene)pyridine-2-amine (L in the
2
)
presence of NaBF , NaClO , and NaPF . The electrochemical prop-
4
4
6
erties of all the complexes indicate reversible redox behavior cor-
responding to Ru(II)–Ru(III) couple and are susceptible to variation
of electron-donating/accepting properties of substituent group on
1 2
L and L . All complexes showed room temperature luminescence
ꢁ
corresponding to p!p intra-ligand charge-transfer (ILCT) transi-
tion with chelation enhanced fluorescence and is finely tuned by
increasing p-conjugation, size of counter anions, and variation of
substituent group with different electronic effects in the com-
plexes. All the complexes worked as an effective catalyst for the
oxidation of benzyl alcohol to corresponding benzaldehyde in
good yield at room temperature.
CONTACT S. S. Chavan
MS), India 416 004.
sanjaycha2@rediffmail.com
Department of Chemistry, Shivaji University, Kolhapur
(
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
A. B. PATIL-DESHMUKH ET AL.
1. Introduction
Over the past several decades, large number of ruthenium–polypyridine complexes has been
extensively studied due to their unique photophysical and electrochemical properties associ-
ated with strong metal–ligand charge-transfer absorption. Several ruthenium–polypyridine
complexes have been exploited owing to their potential applications in the synthesis of metal-
lodendrimers, supramolecular assemblies, in analytical chemistry, electrochemistry, ring-open-
ing metathesis, and polymerization [1–4]. They also act as efficient catalysts in a variety of
organic transformations such as hydrogenation of esters to alcohols, synthesis of imines from
amines and alcohols, the synthesis of amides from amines and esters, and the direct synthesis
of polyamides from diamines and diols [5–8]. In recent years, considerable research interest
has focused on the development of newer classes of mixed ligand ruthenium–polypyridine
complexes by incorporation of desired groups or other types of donor sites which alter the
redox and other physicochemical properties of the prepared complexes. The development of
these complexes becomes popular because of their potential applications in photochemical
conservation of solar energy, molecular electron devices, and as photoactive DNA cleavage
agents for therapeutic purposes [9–11]. Furthermore, the presence of counter anions can play
a crucial role in unique properties of these complexes, since the anions have many features
such as negative charge, size, and geometry. Many compounds of this type such as
2þ
2þ
[
Ru(bipy)2] or [Ru(phen)2] with various ligands like pyrroles [12], imidazole-4,5-dicarbox-
ylic acid or biquinoline derivatives [13], and chiral salicylixazolinate [14] have been reported.
The Schiff base ligands also have attracted considerable attention for the construction of
mixed-ligand transition metal complexes because of their diverse structure and potential
applications in various fields [15–17]. Because of great flexibility and diverse structural aspects,
a wide range of Schiff bases have been synthesized and their complexation behavior has been
studied. Some Schiff bases bearing alkynyl functionality are reported to exhibit excellent lumi-
nescence and optical properties [18, 19]. The structural modification of the complexes due to
alkynyl functionality causes a large p-delocalization over the coordinating ligand than the
regular complexes. Moreover, the rigidity of the structure and the dipole moment of the com-
plexes may thus increase.
In connection with such studies, the present article reports the synthesis of a series
of Ru(II)-polypyridine complexes of the type [Ru(L /L )(phen) ]X (1a–6a) and [Ru(L /
1
2
2
2
1
L )(bipy) ]X (1b–6b) (where L ¼ (E)-5-((4-methoxyphenyl)ethynyl)-N-(pyridin-2-ylme-
2
2
2
1
thylene)pyridin-2-amine; L2 ¼ (E)-5-((4-nitrophenyl)ethynyl)-N-(pyridin-2-ylmethylene)-
0
pyridine-2-amine; X ¼ ClO , BF , PF ; phen ¼ 1,10-phenanthroline, bipy ¼ 2,2 -
4
4
6
bipyridine) and the study of the effect of newly-introduced group L and L as well as
1
2
influence of p-conjugation, donor/acceptor substituents, and size of counter ions on
the physical and luminescence properties of the complexes. The electrochemical
behavior and catalytic properties of the complexes have also been reported.
2. Experimental
2.1. Materials and general methods
All chemicals used were of analytical reagent grade and used without purification.
Solvents used for the synthesis were distilled over appropriate drying reagents.

JOURNAL OF COORDINATION CHEMISTRY
3
[
Ru(phen) ]Cl [20] and [Ru(bipy) ]Cl [21] were prepared according to the literature
2 2 2 2
procedure. Caution! Perchlorate salts of metal complexes are potentially explosive.
Only small quantities of materials should be prepared and the samples should be
handled with care.
Elemental analyses (C, H, and N) were performed on a Thermo Finnigan FLASH EA-
1
3
12 CHNS analyzer. Electronic absorption spectra were recorded on a Shimadzu UV-
600 spectrophotometer. Infrared spectra were recorded on a Perkin-Elmer FTIR spec-
ꢂ1
1
13
trometer as KBr pellets in the 4000–400 cm spectral range. H- and C-NMR spectra
of the samples were measured on a Bruker-300 MHz instrument using TMS [(CH ) Si]
3
4
as an internal standard. ESI-MS spectra were recorded using a Finnigan LCQ spectrom-
eter. Thermal analysis of the complexes was carried out on a Perkin Elmer thermal
ꢃ
–1
analyzer in nitrogen atmosphere at a heating rate of 10 C min . Luminescence prop-
erties were measured using a Perkin Elmer LS 55 spectrofluorometer equipped with
quartz cuvette of 1-cm path length at room temperature. Cyclic voltammetry measure-
ments were performed on a CH-400A electrochemical analyzer. A standard three elec-
trode system consisting of Pt disk working electrode, Pt wire counter electrode, and
Ag/AgCl reference electrode was used. All measurements were carried out in CH CN
3
ꢂ1
solution at room temperature with scan rate 100 mV s using tetrabutylammonium
perchlorate (TBAP) as a supporting electrolyte.
2
.2. Synthesis of ligands (L and L )
1 2
To a solution of 5-((4-methoxyphenyl)ethynyl)pyridin-2-amine (1 mmol, 0.224 g) or 5-
(4-nitrophenyl)ethynyl)pyridin-2-amine (1 mmol, 0.239 g) in methanol (10 mL), the solu-
tion of pyridine-2-carboxyaldehyde (1 mmol, 0.107 g) in CH Cl (10 mL) was added and
(
2
2
refluxed for 2 h. Completion of reaction was checked by TLC. After evaporation of solv-
ent, the orange solid obtained was washed with petroleum ether, recrystallized from
ethanol, and dried in vacuo.
L : Yield: 90% (0.282 g). Elemental analysis: Calc. for C H N O: C, 76.66; H, 4.82; N,
1
20 15
3
ꢂ1
1
2
3.41%. Found: C, 76.59; H, 4.74; N, 13.44%. IR (KBr), vmax (cm ): 1631 v(C[dbond]N),
156 v(C[tbond]C), 1161 v(OCH ), 3014 v(C[sbond]H)arom, 1048 v(C[sbond]H)arom. H-
1
3
NMR (CDCl ; 300 MHz), d(ppm): 8.87 (s, 1H, HC[dbond]N), 3.73 (s, 3H, OCH ), 9.11–9.13
3
3
(d, 1H, Ar), 6.66–8.40 (m, 10H, Ar).
L : Yield: 87% (0.285 g). Elemental analysis: Calc. for C H N O : C, 69.51; H, 3.68; N,
2
19 12 4 2
ꢂ1
1
7.06%. Found: C, 69.41; H, 3.63; N, 17.09%. IR (KBr), vmax (cm ): 1637 v(C[dbond]N),
2
156 v(C[tbond]C), 1470 v(NO ), 3014 v(C[sbond]H)arom, 1048 v(C[sbond]H, arom.bend).
2
1
H-NMR (CDCl ; 300 MHz), d (ppm): 8.87 (s, 1H, HC[dbond]N), 9.11–9.13 (d, 1H, Ar),
3
6.35–8.39 (m, 10H, Ar).
2
.3. Synthesis of [Ru(L )(phen) ](X) (1a–3a)
1 2 2
To the solution of [Ru(phen) ]Cl (1 mmol, 0.532 g) in dry ethanol (10 mL), L (1 mmol,
2
2
1
0.313 g) in ethanol (10 mL) was added in anaerobic condition and refluxed for 4 h.
Then the solvent was evaporated to 5 mL and in that, saturated aqueous solution of

4
A. B. PATIL-DESHMUKH ET AL.
NaClO /NaBF /NaPF was added. On cooling the reddish brown solid product obtained
4
4
6
was filtered, washed well with 1:1 ethanol:water, and dried in vacuo.
a: Yield: 78% (0.682 g). Elemental analysis: Calc. for RuC H N ClO : C, 60.45; H, 3.57;
1
4
4
31
7
5
1
N, 11.21%. Found: C, 60.39; H, 3.46; N, 11.27%. H-NMR (DMSO; 300 MHz), d (ppm): 9.33 (s,
H, HC[dbond]N), 3.73 (s, 3H, OCH ), 6.61–8.92 (m, 27H, Ar.). IR (KBr), v
v(C[dbond]N), 2169 v(C[tbond]C), 1159 v(OCH ). K (CH CN, X cm mol ): 129.19. ESI
MS: 774 ([Ru(C H N ) (C H NHC[dbond]NC H NC[tbond]CC H OCH )] , 23), 413 ([Ru(C
H NHC[dbond]NC H NC[tbond]C C H OCH )] , 55), 282 ([C H NHC[dbond]NC H NC
tbond]CC H OCH ] , 100), 208 ([C H NC[tbond]CC H OCH ] , 49).
ꢂ1
1
(cm ): 1583
3
max
ꢂ1
ꢂ1
2
3
m
3
þ
1
2
8
2 2
5
4
5
3
þ
6
4
3
5
4
5
3
þ
6
4
3
5
4
5 3
þ
[
6
4
3
5
3
6
4
3
2
a: Yield: 78% (0.672 g). Elemental Analysis: Calc. for RuC H N OBF : C, 61.33; H,
44 31 7 4
1
3
.63; N, 11.38%. Found: C, 61.27; H, 3.54; N, 11.41%. H-NMR (DMSO; 300 MHz), d
(
v
X
ppm): 9.33 (s, 1H, HC[dbond]N), 3.73 (s, 3H, OCH ), 6.61–8.92 (m, 27H, Ar). IR (KBr),
3
ꢂ1
(cm ): 1583 v(C[dbond]N), 2169 v(C[tbond]C), 1159 v(OCH ). K (CH CN,
max
ꢂ1
3
m
3
2
ꢂ1
cm mol ): 129.03. ESI MS: 774 ([Ru(C H N ) (C H NHC[dbond]NC H NC
12 8 2 2 5 4 5 3
þ
þ
[tbond]CC H OCH )] , 21), 413 ([Ru(C H NHC[dbond]N C H NC[tbond]CC H OCH )] ,
6 4 3 5 4 5 3 6 4 3
þ
5
9), 282 ([C H NHC[dbond]NC H NC[tbond]CC H OCH ] , 100), 208 ([C H N C[tbond]
5
þ
4
5
3
6
4
3
5 3
CC H OCH ] , 43).
6
4
3
3
a: Yield: 78% (0.717 g). Elemental Analysis: Calc. for RuC H N OPF : C, 57.46; H,
44 31 7 6
1
3
.37; N, 10.66%. Found: C, 57.34; H, 3.28; N, 10.69%. H-NMR (DMSO; 300 MHz), d
(
v
X
ppm): 9.33 (s, 1H, HC[dbond]N), 3.73 (s, 3H, OCH ), 6.61–8.92 (m, 27H, Ar). IR (KBr),
3
ꢂ1
(cm ): 1583 v(C[dbond]N), 2169 v(C[tbond]C), 1159 v(OCH ). K (CH CN,
max
ꢂ1
3
m
3
2
ꢂ1
cm mol ): 129.34. ESI MS: 774 ([Ru(C H N ) (C H NHC[dbond]NC H NC
12 8 2 2 5 4 5 3
þ
þ
[tbond]CC H OCH )] , 26), 413 ([Ru(C H NHC[dbond]NC H NC[tbond]CC H OCH )] ,
6 4 3 5 4 5 3 6 4 3
þ
6
1), 282 ([C H NHC[dbond]NC H NC[tbond]CC H OCH ] , 100), 208 ([C H C[tbond]C
5 4 5 3 6 4 3 5 3
þ
3
C H OCH ] , 47).
6
4
2
.4. Synthesis of [Ru(L )(phen) ](X) (4a–6a)
2 2 2
Complexes 4a–6a were prepared similar to the procedure performed for the prepar-
ation of 1a–3a except that L was replaced by L (1 mmol, 0.328 g).
1
2
4
a: Yield: 69% (0.613 g). Elemental Analysis: Calc. for RuC H N ClO : C, 58.08; H,
43 28 8 6
1
3
(
.17; N, 12.60%. Found: C, 57.99; H, 3.11; N, 12.66%. H-NMR (DMSO; 300 MHz), d
ppm): 9.36 (s, 1H, HC[dbond]N), 6.46–8.92 (m, 27H, Ar). IR (KBr), v
v(C[dbond]N), 2169 v(C[tbond]C). Km (CH CN, X cm mol ): 129.14. ESI MS: 789
[Ru(C H N ) (C H NHC[dbond]NC H NC[tbond]CC H NO )] , 18), 428 ([Ru(C H NH
ꢂ1
(cm ): 1587
max
ꢂ1
2
ꢂ1
3
þ
(
1
2
8
2 2
5
4
5
3
6
4
2
5 4
þ
C[dbond]NC H NC[tbond]CC H NO ] , 53), 282 ([C H NHC[dbond]NC H NC[tbond]
5
3
6
4
2
5
4
5 3
þ
þ
CC H NO ] , 100), 223 ([C H NC[tbond]CC H NO ] , 48).
6
4
2
5
3
6
4
2
5
a: Yield: 69% (0.604 g). Elemental Analysis: Calc. for RuC H N O BF : C, 58.62; H,
43 28 8 2 4
1
3
(
22; N, 12.78%. Found: C, 58.88; H, 3.15; N, 12.85%. H-NMR (DMSO; 300 MHz), d
ppm): 9.37 (s, 1H, HC[dbond]N), 6.47–8.92 (m, 27H, Ar). IR (KBr), v
v(C[dbond]N), 2169 v(C[tbond]C). Km (CH CN, X cm mol ): 129.09. ESI MS: 789
[Ru(C H N ) (C H NHC[dbond]NC H NC[tbond]CC H NO )] , 17), 428 ([Ru(C H NHC[d
ꢂ1
(cm ): 1587
max
ꢂ1
2
ꢂ1
3
þ
(
1
2
8
2 2
5
4
5
3
6
4
2
5 4
þ
bond]NC H NC[tbond]CC H NO ] , 57), 282 ([C H NHC[dbond]NC H NC[tbond]CC H
6 4
5
3
6
4
2
5
4
5
3
þ
þ
NO ] , 100), 223 ([C H NC[tbond]CC H NO ] , 46).
2
5
3
6
4
2

JOURNAL OF COORDINATION CHEMISTRY
5
6
a: Yield: 69% (0.645 g). Elemental Analysis: Calc. for RuC H N O PF : C, 55.25; H,
43 28 8 2 6
1
3
(
.02; N, 11.99%. Found: C, 55.19; H, 2.89; N, 12.01%. H-NMR (DMSO; 300 MHz), d
ppm): 9.36 (s, 1H, HC[dbond]N), 6.46–8.92 (m, 27H, Ar). IR (KBr), v
v(C[dbond]N), 2169 v(C[tbond]C). Km (CH CN, X cm mol ): 129.13. ESI MS: 789
[Ru(C H N ) (C H NHC[dbond]NC H NC[tbond]CC H NO )] , 21), 428 ([Ru(C H NHC
ꢂ1
(cm ): 1587
max
ꢂ1
2
ꢂ1
3
þ
(
[
1
2
8
2 2
5
4
5
3
6
4
2
5 4
þ
dbond]NC H NC[tbond]CC H NO ] , 63), 282 ([C H NHC[dbond]NC H NC[tbond]
5
3
6
4
2
5
4
5 3
þ
þ
CC H NO ] , 100), 223 ([C H NC[tbond]CC H NO ] , 42).
6
4
2
5
3
6
4
2
2
.5. Synthesis of [Ru(L )(bipy) ](X) (1b–3b)
1 2 2
To the solution of [Ru(bipy) ]Cl (1 mmol, 0.484 g) in dry ethanol (10 mL), L (1 mmol,
2
2
1
0.313 g) in ethanol (10 mL) was added in anaerobic condition and refluxed for 4 h.
Then solvent was evaporated to 5 mL and in that, saturated aqueous solution of
NaClO /NaBF /NaPF was added. On cooling the reddish brown solid product obtained
4
4
6
was filtered, washed well with 1:1 ethanol:water and dried in vacuo.
b: Yield: 69% (0.570 g). Elemental Analysis: Calc. for RuC H N ClO : C, 58.15; H,
1
40
31
7
5
1
3
.78; N, 11.87%. Found: C, 58.11; H, 3.70; N, 11.93%. H-NMR (DMSO; 300 MHz), d
(
v
X
ppm): 9.33 (s, 1H, HC[dbond]N), 3.78 (s, 3H, OCH ), 6.61–8.92 (m, 27H, Ar). IR (KBr),
3
ꢂ1
(cm ): 1593 v(C[dbond]N), 2166 v(C[tbond]C), 1159 v(OCH ). K (CH CN,
max
ꢂ1
3
m
3
2
ꢂ1
cm mol ): 131.21. ESI MS: 726 ([Ru(C H N ) C H NHC[dbond]NC H NC[tbo
10 8 2 5 4 5 3
þ
þ
nd]CC H OCH ] , 31), 413 ([Ru(C H NHC[dbond]NC H NC[tbond]CC H OCH )] , 62),
2
H OCH ] , 42).
6
4
3
5
4
5
3
6
4
3
þ
82 ([C H NHC[dbond]NC H NC[tbond]CC H OCH ] , 100), 208 ([C H NC[tbond]CC
5 4 5 3 6 4 3 5 3 6
þ
3
4
2
b: Yield: 69% (0.561 g). Elemental Analysis: Calc. for RuC H N OBF : C, 59.05; H,
40 31 7 4
1
3
.84; N, 12.05%. Found: C, 58.98; H, 3.74; N, 12.12%. H-NMR (DMSO; 300 MHz), d
(
v
X
ppm): 9.33 (s, 1H, HC[dbond]N), 3.77 (s, 3H, OCH ), 6.61–8.92 (m, 27H, Ar). IR (KBr),
3
ꢂ1
(cm ): 1593 v(C[dbond]N), 2166 v(C[tbond]C), 1159 v(OCH ). K (CH CN,
max
ꢂ1
3
m
3
2
ꢂ1
cm mol ): 131.14. ESI MS: 726 ([Ru(C H N )C H NHC[dbond]NC H NC
10 8 2 5 4 5 3
þ
þ
[tbond]CC H OCH ] , 28), 413 ([Ru(C H NHC[dbond]NC H NC[tbond]C C H OCH )] ,
6 4 3 5 4 5 3 6 4 3
þ
6
7), 282 ([C H NHC[dbond]NC H NC[tbond]CC H OCH ] , 100), 208 ([C H NC[tbond]
5
þ
4
5
3
6
4
3
5 3
CC H OCH ] , 39).
6
4
3
3
b: Yield: 69% (0.601 g). Elemental Analysis: Calc. for RuC H N OPF : C, 55.11; H,
40 31 7 6
1
3
.58; N, 11.25%. Found: C, 55.07; H, 3.51; N, 11.29%. H-NMR (DMSO; 300 MHz), d
(
v
X
ppm): 9.33 (s, 1H, HC[dbond]N), 3.76 (s, 3H, OCH ), 6.61–8.92 (m, 27H, Ar). IR (KBr),
3
ꢂ1
(cm ): 1593 v(C[dbond]N), 2166 v(C[tbond]C), 1159 v(OCH ). K (CH CN,
max
ꢂ1
3
m
3
2
ꢂ1
cm
mol ): 131.21. ESI MS: 726 ([Ru(C H N )C H NHC[dbond]NC
10 8 2 5 4 5
þ
H NC[tbond]CC H OCH ] , 24), 413 ([Ru(C H NHC[dbond]NC H N C[tbond]CC H OC
H )] , 54), 282 ([C H NHC[dbond]NC H NC[tbond]CC H OCH ] , 100), 208 ([C H
NC[tbond]CC H OCH ] , 40).
3
6
4
3
5
4
5
3
6 4
þ
þ
3
5
4
5
3
6
4
3
5
3
þ
6
4
3
2
.6. Synthesis of [Ru(L )(bipy) ](X) (4b–6b)
2 2 2
Complexes 4b–6b were prepared similar to the procedure performed for the prepar-
ation of 1b–3b except that L was replaced by L (1 mmol, 0.328 g).
1
2

6
A. B. PATIL-DESHMUKH ET AL.
4
b: Yield: 63% (0.530 g). Elemental Analysis: Calc. for RuC H N ClO : C, 55.68; H,
39
28
8
6
1
3
(
.35; N, 13.32%. Found: C, 55.61; H, 3.30; N, 13.37%. H-NMR (DMSO; 300 MHz), d
ppm): 9.31 (s, 1H, HC[dbond]N), 6.46–8.71 (m, 27H, Ar). IR (KBr), v
v(C[dbond]N), 2166 v(C[tbond]C). Km (CH CN, X cm mol ): 129.51. ESI MS: 741
[Ru(C H N )C H NHC[dbond]NC H NC[tbond]CC H NO ] , 19), 428 ([Ru(C H NHC[db
ꢂ1
(cm ): 1599
max
ꢂ1
2
ꢂ1
3
þ
(
1
0
8
2
5
4
5
3
6
4
2
5 4
þ
ond]NC H NC[tbond]CC H NO ] , 49), 282 ([C H NHC[dbond]NC H NC[tbond]CC H
6 4
5
3
6
4
2
5
4
5
3
þ
þ
NO ] , 100), 223 ([C H NC[tbond]CC H NO ] , 51).
2
5
3
6
4
2
5
b: Yield: 64% (0.530 g). Elemental Analysis: Calc. for RuC H N O BF : C, 56.53; H,
39 28 8 2 4
1
3
(
.41; N, 13.52%. Found: C, 56.46; H, 3.30; N, 13.60%. H-NMR (DMSO; 300 MHz), d
ppm): 9.31 (s, 1H, HC[dbond]N), 6.49–8.71 (m, 27H, Ar). IR (KBr), v
v(C[dbond]N), 2166 v(C[tbond]C). Km (CH CN, X cm mol ): 129.28. ESI MS: 741
[Ru(C H N )C H NHC[dbond]NC H NC[tbond]CC H NO ] , 22), 428 ([Ru(C H NHC[d
bond]NC H NC[tbond]CC H NO ] , 51), 282 ([C H NHC[dbond]NC H NC[tbond]CC
H NO ] , 100), 223 ([C H NC[tbond]CC H NO ] , 40).
ꢂ1
(cm ): 1599
max
ꢂ1
2
ꢂ1
3
þ
(
1
0
8
2
5
4
5
3
6
4
2
5 4
þ
5
3
6
4
2
5
4
5
3
6
þ
þ
4
2
5
3
6
4
2
6
b: Yield: 63% (0.558 g). Elemental Analysis: Calc. for RuC H N O PF : C, 52.83; H,
39 28 8 2 6
1
3
(
.18; N, 12.64%. Found: C, 52.77; H, 3.09; N, 12.66%. H-NMR (DMSO; 300 MHz), d
ppm): 9.31 (s, 1H, HC[dbond]N), 6.47–8.71 (m, 27H, Ar). IR (KBr), v
v(C[dbond]N), 2166 v(C[tbond]C). Km (CH CN, X cm mol ): 129.57. ESI MS: 741
[Ru(C H N ) C H NHC[dbond]NC H NC[tbond]CC H NO ] , 27), 428 ([Ru(C H NHC[d
bond]NC H NC[tbond]CC H NO ] , 60), 282 ([C H NHC[dbond]NC H NC[tbond]CC
6
H NO ] , 100), 223 [C H NC[tbond]CC H NO ] , 38).
ꢂ1
(cm ): 1599
max
ꢂ1
2
ꢂ1
3
þ
(
1
0
8
2
5
4
5
3
6
4
2
5 4
þ
5
3
6
4
2
5
4
5
3
þ
þ
4
2
5
3
6
4
2
2.7. General procedure for catalytic oxidation of alcohol
The oxidation of alcohol catalyzed by Ru(II) complexes was carried out according to
the following procedure. To a solution of substituted benzyl alcohol (2 mmol) in DMF,
NMO (3 mmol) was added followed by addition of 1a–6a and 1b–6b (0.01 mmol). The
reaction mixture was stirred at room temperature for 3 h. The mixture was reduced in
vacuo, the residue was taken up in diethyl ether and was filtered through a bed of sil-
1
13
ica gel. The purified product obtained was then characterized by IR, H-NMR, and C-
NMR spectra.
3. Results and discussion
3.1. Synthesis and characterization
The starting material 5-(ethynyl)pyridin-2-amine was prepared by using Pd(II)/Cu(I) cat-
alyzed coupling of 5-iodo-2-aminopyridine followed by hydrolysis reaction with KOH in
MeOH by following the procedure reported earlier [21]. Further coupling of 5-(ethy-
nyl)pyridin-2-amine with 1-iodo-4-methoxybenzene and 1-iodo-4-nitrobenzene
afforded 5-((4-methoxyphenyl)ethynyl)pyridin-2-amine and 5-((4-nitrophenyl)ethynyl)-
pyridin-2-amine, respectively. The Schiff base ligands (E)-5-((4-methoxyphenyl)ethynyl)-
N-(pyridin-2-ylmethylene)pyridin-2-amine (L ) and (E)-5-((4-nitrophenyl)ethynyl)-N-(pyri-
1
din-2-ylmethylene)pyridin-2-amine (L ) were obtained by the condensation of equimo-
2
lar amount of 5-((4-methoxyphenyl)ethynyl)pyridin-2-amine and 5-((4-nitrophenyl)et
hynyl)pyridin-2-amine with pyridine-2-carboxyaldehyde in excellent yield (Scheme 1).

JOURNAL OF COORDINATION CHEMISTRY
7
Scheme 1. Synthetic route to the preparation of L /L .
1
2
(A)
(B)
Figure 1. Proposed molecular structure of (A) 1a–6a and (B) 1b–6b.
The mononuclear octahedral complexes of the type [Ru(phen) (NC H N[d
2
5 4
bond]CHC H C[tbond]CR)]X
(1a–6a) and [Ru(bipy) (NC H N[dbond]CHC H C[tb
2 5 4 5 3
5
3
2
ond]CR)]X2 (1b–6b) were synthesized by the reaction of ethanolic solution of
Ru(phen) Cl ]ꢀ2H O and [Ru(bipy) Cl ]ꢀ2H O with appropriate ligand L /L followed by
[
2
2
2
2
2
2
1
2
0
–
addition of aqueous NaX solution under N atmosphere (where bipy ¼ 2,2 -bipyridine;
2
–
–
phen ¼ 1,10-phenanthroline; R ¼ C H OCH , C H NO ; X ¼ ClO , BF , PF ) (Figure 1).
6
4
3
6
4
2
4
4
6
The new complexes prepared are air-stable, show great thermal stability, and are
moisture-insensitive in solid phase. They are soluble in common organic solvents such
as DMF, DMSO, acetonitrile, etc. and their stability in solvent is ensured by ESI-MS
spectral studies. Composition and identity of all new compounds were deduced from
1
13
satisfactory elemental analysis, FTIR, UV–vis, H-NMR, C-NMR, and mass spectroscopy.
The IR spectra of all the complexes displayed numerous bands of variant intensities.
ꢂ1
ꢂ1
The band at 1586–1599 cm in 1a–6a and 1593–1607 cm in 1b–6b represents the
V(C[dbond]N) stretching frequency, which is observed at lower frequency region as
compared to L and L , indicating coordination of (C[dbond]N) group to Ru(II) ion in
1
2
the complexes [22]. Further proof for the complexation of nitrogen in 1a–6a and
ꢂ1
1
b–6b is obtained from the appearance of a new band at ca. 491–517 cm which is

8
A. B. PATIL-DESHMUKH ET AL.
ꢂ1
assignable to v(M[sbond] N). The pyridine v(C[sbond]N) observed at ꢄ1603 cm in L1
ꢂ1
and L is also shifted to lower frequency by 29–37 cm in 1a–6a and 1b–6b [23]. It is
2
to be noted that, upon changing the nature of co-ligand, donor–acceptor groups in
Schiff base ligand and counter anions in the complexes appeared to have little effect
on the v(C[dbond]N) frequency. Complexes 1a–3a and 1b–3b exhibit an unsplit band
ꢂ1
at ꢄ1166 and ꢄ1160 cm which are due to v(OCH ) group and two bands at ꢄ1414
3
ꢂ1
ꢂ1
and ꢄ1567 cm for 4a–6a and ꢄ1410 and ꢄ1560 cm for 4b–6b, which can be
attributed to v
and v (NO ) modes, respectively [24]. The IR spectra of 1a–6a are
sym 2
asym
slightly denser as compared to 1b–6b due to more aromatic character of phen ligand
ꢂ1
ꢂ1
relative to bipy. The bands at 519–539 cm and 838–839 cm are ascribed to the for-
mation of Ru[sbond]N bond from imine and pyridine nitrogen, respectively [25]. The
ꢂ1
medium strong band at 2156 cm in L and L ascribed to stretching frequency of
1
2
V(C[tbond]C) was slightly shifted to lower energies after complexation in 1a–6a and
b–6b suggesting perturbation of the phenyl site leads to electron dissipation at the
triple bond through conjugation. The perchlorate complexes 1a, 4a, 1b, and 4 b
1
ꢂ1
ꢂ1
exhibit broad band at ꢄ1096 cm (ꢀ ) and unsplit band at ꢄ637 cm (ꢀ ) suggesting
3
4
–
the stretching vibration of non-coordinated ClO4 ion in the complexes [26]. In tetra-
ꢂ1
fluoroborate complexes 2a, 5a, 2b and 5b, the intense band at ꢄ1081 cm is attrib-
uted to the anti-symmetric ꢀ(B[sbond]F) stretching mode [27]. However, strong bands
ꢂ1
ꢂ1
at ꢄ849 cm and ꢄ563 cm in 3a, 6a, 3b, and 6b are consistent with the presence
–
of PF anion in the complexes [28].
6
1
The H-NMR spectra of L1 and L2 exhibit azomethine group resonated at
ꢄ8.87 ppm. In the complexes, the signal of azomethine proton is considerably shifted
to the downfield region and observed at 9.33–9.37 ppm in 1a–6a and 9.31–9.33 ppm
in 1b–6b as a consequence of electron donation to the metal center [29]. The singlet
at 3.73 ppm in L is representative of ([sbond]OCH ) group which did not affect much
1
3
during complexation. The apparent position of individual proton signals in the aro-
matic region is not possible because of the ring protons of Schiff base ligand and
phen/bipy moieties in 1a–6a and 1b–6b are overlapped in the region 6.46–8.92 ppm.
However, the proton counts in the NMR spectra of each of the complexes authenticate
the expected structural formulas.
1
3
The C-NMR spectra of 1a–6a and 1b–6b showed that the coordination also
affects the chemical shift of methine group of imine ligands and observed at
1
67.9–170.97 ppm, while for L and L , it appeared at 162.94 ppm and 160.92 ppm,
1 2
respectively. The signals due to carbon atoms of aromatic ring are observed in the
range 115.58–157.84 ppm. The signals of alkynyl group are observed in the range of
97.65–99.13 ppm. This confirms the formation of new Ru(II)-polypyridyl complexes.
3.2. Thermogravimetric analysis
To investigate the thermal stability of 1a–6a and 1b–6b, thermogravimetric analysis
ꢃ
(
1
TGA) was performed up to 800 C under flowing nitrogen at a heating rate of
ꢃ
ꢂ1
0 C min . The perchlorate complexes 1a, 4a, 1b, and 4b are potentially explosive
and hence are not studied for safety reasons. The TGA curves of 2a, 3a, 2b, and 3b
ꢃ
show that there is no mass loss up to ca. 187 C, revealing the absence of either water

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 2. Cyclic voltammograms of (A) 1a and (B) 1b.
ꢃ
or solvent molecule. From 187 to 635 C, these complexes underwent complicated
multiple weight loss with total mass loss corresponding to coordinated phen (2a and
3
a) or bipy (2b and 3b) ligand, counter anions, and coordinated Schiff base moiety (%
obsd. 38.78 (2a), 39.42 (3a), 36.12 (2b), 35.97 (3b); % calcd. 38.87 (2a), 39.58 (3a),
6.38 (2b), 35.74 (3b)). Complexes 5a, 6a, 5b, and 6b show very similar behavior to
3
the above; once again the absence of water or solvent molecule indicated and decom-
position follow the same stages. Complexes then underwent a rapid and significant
weight loss (% obsd. of 42.32 (5a), 42.78 (6a), 39.38 (5b), 38.89 (6b); % calcd. 42.57
ꢃ
(
5a), 42.88 (6a), 39.56 (5b), 38.69 (6b)) from 194 to 644 C, indicating loss of decom-
posed coordinated ligands along with counter anions.
3.3. Cyclic voltammetry
The electrochemical properties of 1a–6a and 1b–6b have been studied in CH CN solu-
3
ꢂ3
tion (10 M) by cyclic voltammetry using a platinum working electrode. All complexes
are electroactive and show one metal-centered oxidation couple as well as one ligand-
based and two coligand-based reductions in the potential range of ±2 V versus satu-
rated calomel electrode (SCE). The representative voltammograms of 1a and 1b are
shown in Figure 2 and the electrochemical data are depicted in Table 1. The com-
plexes displayed one reversible couple in the range 1.46–1.53 V for 1a–6a and
1.60–1.69 V for 1b–6b versus SCE, which is assigned to the oxidation couple of
Ru(II)–Ru(III). The involvement of one Schiff base ligand around Ru(II) ion in the pre-
sent set of complexes 1a–6a and 1b–6b shifted the Ru(II)–Ru(III) couple to higher val-
2
þ
2þ
ues than that of [Ru(phen)3]
and [Ru(bipy)3]
[30]. This implies that the donor
strength of L1 and L2 are lower than that of phen and bipy. In addition to the
Ru(II)–Ru(III) reversible couple, all complexes displayed three reversible one-electron
reductions. The first ligand reduction was at –0.87 to –0.93 V for L and –1.07 to –1.21
1
for L while the other two were at –1.55 to –1.78 V and –1.61 to –1.79 V for bipy/phen
2
moieties (all potentials are referenced to SCE) [31]. The electrochemical properties

1
0
A. B. PATIL-DESHMUKH ET AL.
a
Table 1. Electrochemical data of 1a–6a and 1b–6b .
Ligand reductions
II
III
b
b
Complex
Ru –Ru couple E1/2 (V)
E1/2 (V)
E1/2 (V)
E1/2 (V)
1
2
3
4
5
6
1
2
3
4
5
6
a
a
a
a
a
a
a
b
b
b
b
b
b
1.47
1.46
1.48
1.51
1.53
1.52
1.60
1.61
1.63
1.67
1.69
1.69
–0.92
–0.93
–0.93
–1.21
–1.17
–1.19
–0.87
–0.92
–0.91
–1.07
–1.14
–1.15
–1.35
–1.37
–1.36
–1.37
–1.38
–1.41
–1.52
–1.51
–1.54
–1.51
–1.54
–1.55
–1.55
–1.57
–1.54
–1.57
–1.56
–1.59
–1.68
–1.70
–1.71
–1.70
–1.69
–1.70
Solvent, acetonitrile; Supporting electrolyte, tetrabutylaminoperchlorate; Reference electrode, SCE; Working elec-
trode, Pt wire.
1/2 ¼ 0.5(Epa þ Epc), where Epa and Epc are the anodic and cathodic peak potentials, respectively.
b
E
correlate well with the nature of substituent groups on the azomethine moiety. The
electron donating OCH group cathodically shifts the redox potential in 1a–3a and
3
1
b–3b (Table 1). In contrast, the electron withdrawing NO group induces an appre-
2
ciable anodic shift in the redox potential of 4a–6a and 4b–6b (Table 1). These results
might be due to replacement of [sbond]NO by strong electron donating [sbond]OCH
2
3
0
group [32]. 2,2 -Bipyridine is a well-known potential electron-transfer center and each
bipy moiety can accept two electrons in one electrochemically accessible LUMO [33];
0
similarly in phen as well. Since 1a–6a and 1b–6b have two N,N -donor units, four one-
electron reductions are therefore expected. However, in practice, we have observed
two reductions within the ±2 V potential range from bipy and phen. The other
expected two reductions could not be seen, possibly due to solvent cut-off. In com-
parison of the electrochemical data of the prepared complexes it reveals that the
redox processes for 1a–6a appeared at slightly more positive potential as compared
to those for corresponding 1b–6b. This is attributed to better stabilization of phen-
based 1a–6a complexes compared to bipy-based complexes 1b–6b as a consequence
of its strong p-acidic character. It is also observed that the E1/2 values for 1a–6a are
slightly greater than 1b–6b which conclude that the more the p-acidic character the
higher the E1/2 values.
3.4. Absorption and emission behavior
The electronic absorption spectra of all the complexes were recorded from 200 to
ꢂ4
9
00 nm in CH CN solution (10 M) at room temperature. The spectral data are sum-
3
marized in Table 2 and the spectra are shown in Figure 3. All ruthenium complexes
are diamagnetic, indicating the presence of ruthenium in þ2 oxidation state. In 1a–6a,
one absorption band around 260 nm is found, which is assigned to the spin-allowed
p–pꢁ
transitions of the phen ligand. However, 1b–6b show two intense absorption
ꢁ
bands around 240 and 290 nm, which are assigned to the typical spin-allowed p–p
transitions of the bipy ligand [34]. For [Ru(phen)3]
2
þ
2þ
and [Ru(bipy)3] , the lowest
energy MLCT transition takes place around 440 and 450 nm, respectively, which is

JOURNAL OF COORDINATION CHEMISTRY
11
Table 2. Absorption and emission data of L , L , 1a–6a and 1b–6b.
1
2
Compound
k
Abs (nm)
303, 331
311, 342
k
Ex (nm)
k
Em (nm)
/
s (ns)
L
L
1
2
272
278
291
298
293
299
298
296
296
291
295
295
291
296
554
561
723
727
720
728
731
726
721
724
715
727
728
725
–
–
–
–
1
2
3
4
5
6
1
2
3
4
5
6
a
a
a
a
a
a
b
b
b
b
b
b
259, 344, 481
260, 347, 484
262, 349, 485
262, 348, 494
262, 349, 497
0.0714
0.0789
0.0653
0.0778
0.0823
0.0694
0.0686
0.0712
0.0624
0.0698
0.0731
0.0663
6.39
6.38
6.40
6.78
6.84
6.86
6.18
6.14
6.16
6.34
6.37
6.36
264, 349, 497
237, 290, 355, 470
241, 293, 357, 472
242, 296, 358, 473
241, 294, 359, 487
243, 295, 359, 489
244, 297, 363, 488
Figure 3. Electronic absorption spectra of (A) 1a–6a and (B) 1b–6b.
0
reasonably red shifted on substitution of one polypyridine moiety by another N,N -
donor ligand L and L . The most noticeable feature in the visible region of the elec-
1
2
tronic absorption spectra of the complexes is that, the lowest-energy MLCT maxima
2
þ
2þ
are red-shifted with respect to the MLCT of [Ru(phen)3] and [Ru(bipy)3] and the
amount of shift depends on the electronic properties of the substituted alkynyl moiety
of L [35]. Consequently, the bathochromic shift is observed for 4a–6a and 4b–6b rela-
tive to 1a–3a and 1b–3b, which is due to the presence of electron-withdrawing char-
acter of the nitro group. Furthermore, compared to 1a–6a, the absorption bands due
to MLCT transition appeared at higher wavelength in the spectra of 1b–6b, which
may be due to phen ligand showing a strong electron-donating potential than bipy
ligand [36]. Little effect of switching the counter ions is also observed on absorption
spectra of all the complexes.
Emission spectra of L and L as well as their mixed ligand Ru(II) complexes 1a–6a
1
2
ꢂ4
and 1b–6b were recorded in CH CN solution (10 M) at room temperature and spec-
3
tral data are displayed in Table 2. The imine alkynyl ligands L and L exhibit emission
1
2
at 554 nm and 561 nm upon excitation at 272 and 278 nm, respectively. Upon com-
plexation, the mononuclear complexes exhibit strong red-shifted emission at

1
2
A. B. PATIL-DESHMUKH ET AL.
Figure 4. Emission spectra of (A) 1a–6a and (B) 1b–6b.
7
6
1
20–731 nm for 1a–6a and 715–728 nm for 1b–6b with lifetime 6.39–6.86 and
.18–6.36 ns, respectively (Figure 4). These results confirm that the emission origins in
ꢁ
a–6a and 1b–6b are predominantly due to p!p intra-ligand transition and chela-
tion enhanced fluorescence (CHEF) caused by coordination of imine group to Ru(II) in
the complexes [37]. The presence of additional p-conjugation due to Schiff base ligand
in 1a–6a and 1b–6b might be one of the important contributing factor for this
intense emission, which confirm the chelation of L and L . The positive influence on
1
2
the emission wavelength observed in 4a–6a and 4b–6b is probably a result of the
higher electron-withdrawing ability of nitro group which offers an extra conjugated
backbone. It was also observed that the fluorescence efficiency of the phen complexes
(
(
1a–6a) appeared at longer wavelength with enhanced intensity as compared to bipy
1b–6b) complexes. This may be due to the non-radiative decay process which is
more effective in the bipy complexes. The presence of counter anion also shows the
pronounced effect on the emission behavior of the Ru(II) complexes. When the size of
the counter anion increases, the emission wavelength of the Ru(II) complexes
–
–
–
decreases and it follows the path BF4 < ClO4 < PF . These results could be attrib-
6
uted to the difference in coordinating ability of counter anions with Ru(II) ion as well
as difference in solubility of the complexes in solution [38].
The emission quantum yield (/) of all the complexes was determined with refer-
ence to quinine sulfate (/ ¼ 0.52) and are narrated in Table 2. The appeared quantum
yield values are 0.065–0.082 for 1a–6a and 0.062–0.073 for 1b–6b. Compared to
2
þ
[
Ru(phen/bipy)3] , the significant increase in / value observed for 1a–6a and 1b–6b
is probably due to increase in p-conjugation in the complexes. These results are in
good agreement with those values reported in the literature [39].
3.5. Catalytic oxidation of alcohol
The synthesized complexes were tested for the oxidation of alcohols to corresponding
aldehydes. In order to optimize the reaction conditions such as temperature, solvent,
and ratio of the reagents, the oxidation of benzyl alcohol to aldehyde was chosen as

JOURNAL OF COORDINATION CHEMISTRY
13
(
R = H, CH
3
, OCH )
3
Scheme 2. Catalytic oxidation of alcohol.
Table 3. Spectral data of oxidation product.
–1
1
13
Compound
IR (cm )
1702
H-NMR (ppm)
C-NMR (ppm)
Benzaldehyde
9.96 (s, [sbond]CHO),
.86–7.41 (m, 5H)
9.99 (s, [sbond]CHO), 1.16
s, [sbond]CH ),
.87–7.47 (m, 4H)
910.1 (s, [sbond]CHO), 3.78
s, [sbond]OCH ),
.86–7.49 (m, 4H)
192.5, 136.2, 134.4,
129.8, 128.9
192.7, 136.2, 134.4,
129.9, 128.9, 21.77
7
4
-Methyl-benzaldehyde
1701
(
7
3
4
-Methoxy-benzaldehyde
1698
193.2, 136.2, 134.4,
129.8, 129.1, 55.56
(
7
3
model reaction and the representative complexes 1a and 1b were used as a catalyst.
Increasing the temperature of the reaction, no positive effect was observed on the
increasing yield of the product. For this reason, all the reactions were carried out at
ambient temperature. Using 1a and 1b as a catalyst, some other solvents such as
CH CN and DMSO were examined other than DMF for conversions of alcohol into cor-
3
responding aldehydes. Using solvents CH CN and DMSO, yields were considerably low-
3
ered. Therefore, DMF was used as solvent for subsequent reactions. The ratio of the
substrate/catalyst was also optimized and was varied from 1/0.005 to 4/0.005. The
ratio of 1/0.005 was chosen because at higher ratios the reaction took a long time to
go to completion. Blank experiments were carried out under the same conditions and
we observed that the yield of benzaldehyde obtained without the addition of the
catalyst was less than 5%, confirming the role of Ru(II) complex catalyst in the catalytic
oxidation reaction. Under these optimized conditions, it was observed that 1a–6a and
1
b–6b in the presence of NMO as a co-oxidant (Scheme 2) significantly enhance the
conversion of benzyl alcohol to desired benzaldehyde in good yield and the product
1
13
obtained was characterized by IR, H-NMR, and C-NMR spectra (Table 3). In a typical
experiment, a DMF solution of an excess of NMO was added to a stirred solution of
an benzyl alcohol and a catalytic amount of Ru(II) complex in DMF. After stirring for
15 min, the red color of the reaction mixture turned brown, probably due to coordin-
ation of alcohol to the Ru(II) center. The results obtained for the oxidation of the alco-
hol by the present catalyst system are summarized in Table 4. By using 1a–6a the
oxidation yield of benzaldehyde with benzyl alcohol was observed up to 69%–80%.
The oxidation yield of 4-methyl-benzaldehyde with 4-methyl-benzyl alcohol was
75%–84%. However, the oxidation yield of 4-methoxy-benzaldehyde with 4-methoxy-
benzyl alcohol considerably increases up to 90%–95%. For 1b–6b, the oxidation yield
of benzaldehyde with benzyl alcohol reached up to 61%–67%. However, the oxidation
yield of 4-methyl-benzaldehyde with 4-methyl benzyl alcohol and 4-methoxy-benzal-
dehyde with 4-methoxy-benzyl alcohol was observed up to 67%–77% and 85%–90%,

1
4
A. B. PATIL-DESHMUKH ET AL.
a
Table 4. Catalytic oxidation of alcohol by 1a–6a and 1b–6b .
b
Substrate
Product
Catalyst
Yield
Turnover
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
79
80
77
69
70
70
84
84
83
77
76
75
93
95
94
90
91
91
64
67
66
61
62
61
76
77
74
69
67
67
89
89
90
87
85
85
158
160
154
138
140
140
162
168
166
154
152
150
186
190
188
180
182
182
128
134
132
122
124
122
152
154
148
138
134
134
178
178
180
174
170
170
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
a
–3
–3
-3
Reaction conditions: 0.01 ꢅ 10 M of the complex, 3.0 ꢅ 10 M of NMO and the substrate 2.0 ꢅ 10 M were all
3
added to 20 cm of DMF solvent and stirred for 3 h.
Turnover ¼ moles of product/moles of catalyst.
b
respectively. Following the above study, it was found that the oxidation of electron-
donating 4-methoxy-benzyl alcohol transformed readily into corresponding benzalde-
hyde in excellent yields compared to oxidation of benzyl alcohol and 4-methyl benzyl
–
–
4
–
6
alcohol [40]. Replacement of non-coordinating counter ions (ClO , BF , and PF ) has
4
no major impact on the catalytic activity of 1a–6a and 1b–6b. The donor/accepter
nature of imine ligand in the catalyst, however, affects the yield of an oxidation prod-
uct of benzyl alcohol.
3.6. The mechanism of catalytic oxidation of benzyl alcohol to benzaldehyde by
Ru(II) complex
The mechanism suggested here is a process for catalytic oxidation of benzyl alcohol
to benzaldehyde by Ru(II) complexes. Results of the present investigations suggest
that the complexes are able to react efficiently with NMO to yield a high valent ruthe-
nium-oxo intermediate species capable of oxygen atom transfer to alcohols [41, 42].

JOURNAL OF COORDINATION CHEMISTRY
IV
2þ
15
The Ru(II) complex reacts with NMO to produce [Ru O(phen) (L /L )]
and
2
1
2
IV
2þ
[
[
Ru O(bipy) (L /L )] , and the Ru(IV) complex reacts with benzyl alcohol to form
2 1 2
IV
2þ
IV
2þ
Ru (OH)(PhCH O)(phen) (L /L )]
and [Ru (OH)(PhCH O)(bipy) (L /L )] , respect-
2 2 1
2
2
1
2
2
–
–
ively. Afterwards, the hydride ion H is abstracted from benzylate anion PhCH O and
2
IV
2þ
then coordinated with ruthenium(IV) to form [Ru (H)(OH)(PhCHO)(phen) (L /L )] and
2
1
2
IV
2þ
[
Ru (H)(OH)(PhCHO)(bipy) (L /L )] , which is unstable, losing water and giving benzal-
2 1 2
dehyde according to the following equations:
II
IV
Ru þ NMO ! Ru O þ NMM
(1)
(2)
(3)
(4)
IV
IV
Ru O þ PhCH2OH ! Ru ðOHÞðPhCH2OÞ
IV
IV
Ru ðOHÞðPhCH2OÞ ! Ru ðHÞðOHÞðPhCHOÞ
IV
II
Ru ðHÞðOHÞðPhCHOÞ ! Ru þ PhCHO þ H2O
4. Conclusion
The mononuclear octahedral complexes of the type [Ru(phen) (NC H N[db
2
5 4
ond]CHC H C[tbond]CR)]X (1a–6a) and [Ru(bipy) (NC H N[dbond]CHC H C[tbond]
5
3
2
2
5
4
5 3
CR)]X (1b–6b) were synthesized by the reaction of ethanolic solution of [Ru(phen) C
2
2
l ]ꢀ2H O and [Ru(bipy) Cl ]ꢀ2H O with appropriate ligand L /L followed by addition of
2
2
2
2
2
1
2
0
aqueous NaX solution under N atmosphere (where bipy ¼ 2,2 -bipyridine; phen ¼
2
–
–
–
1
,10-phenanthroline; R ¼ C H OCH , C H NO ; X ¼ ClO , BF , PF ). The phen com-
6
4
3
6
4
2
4
4
6
plexes (1a–6a) are thermally more stable than the bipy complexes (1b–6b). All com-
plexes exhibit reversible redox behavior corresponding to Ru(II)/Ru(III) couple and are
susceptible to electron donating/accepting properties of the substituent group in the
complexes. Further, the redox processes for phen complexes appeared at more posi-
tive potential as compared to those for corresponding bipy complexes. Room tem-
ꢁ
perature luminescence is observed for all complexes as a result of intra-ligand p!p
transition and CHEF. The presence of extended p-conjugation, donor/acceptor sub-
stituent on L /L , p-acidic character of phen and bipy as well as size of counter anions
1
2
shows pronounced effect on emission properties of the complexes. The catalytic activ-
ity of all the complexes in the presence of NMO showed efficient catalytic activity for
the oxidation of benzyl alcohol to the corresponding aldehydes at room temperature.
Disclosure statement
No potential conflict of interest statement was reported by the authors.
Funding
One of the authors (ABP-D) is thankful to UGC-BSR-SRF, New Delhi, India for awarding UGC
research fellowship in science for meritorious students.

1
6
A. B. PATIL-DESHMUKH ET AL.
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