Ruthenium(II)-2, 2′-bipyridine/1, 10-phenanthroline complexes incorporating (E)-2-(((5-((4-methoxyphenyl)ethynyl)pyridin-2-yl)imino)methyl)-4-((4-nitro phenyl)ethynyl)phenol as a ligand

Article abstract of DOI:10.1007/s11243-020-00384-x

A new series of hexa-coordinated Ru(II) complexes of the type [Ru(L)(phen)₂]X (1a–d) and [Ru(L)(bipy)₂]X (2a–d) (where phen = 1,10-phenanthroline, bipy = 2,2′-bipyridine, X = NO₃, BF₄, ClO₄, PF₆) have been prepared by the reaction of (E)-2-(((5-((4-methoxyphenyl)ethynyl)pyridin-2-yl)imino)methyl)-4-((4-nitrophenyl)ethynyl)phenol (L) with [Ru(phen)₂]Cl₂·2H₂O and [Ru(bipy)₂]Cl₂·2H₂O. The complexes were characterized by physico-chemical and spectroscopic methods. All complexes are 1:1 conducting and diamagnetic in nature. In acetonitrile solution, the complexes displayed one reversible Ru(II)–Ru(III) oxidation couple and one irreversible Ru(III)–Ru(IV) oxidation and are sensitive to π-acidic character of phen and bipy ligands. The complexes show room-temperature luminescence originated from the lowest energy metal-to-ligand charge transfer excited state and are sensitive to difference in size of the counter anions. All the complexes displayed second harmonic generation by Kurtz-powder technique indicating their potential for the application as a useful NLO material.

Full text of DOI:10.1007/s11243-020-00384-x

Transition Metal Chemistry
https://doi.org/10.1007/s11243-020-00384-x
Ruthenium(II)‑2, 2′‑bipyridine/1, 10‑phenanthroline complexes
incorporating (E)‑2‑(((5‑((4‑methoxyphenyl)ethynyl)pyridin‑2‑yl)
imino)methyl)‑4‑((4‑nitro phenyl)ethynyl)phenol as a ligand
A. B. Patil‑Deshmukh1 · S. S. Mohite1 · S. S. Chavan1
Received: 19 December 2019 / Accepted: 25 March 2020
© Springer Nature Switzerland AG 2020
Abstract
A new series of hexa-coordinated Ru(II) complexes of the type [Ru(L)(phen)2]X (1a–d) and [Ru(L)(bipy)2]X (2a–d)
(where phen=1,10-phenanthroline, bipy=2,2′-bipyridine, X=NO3, BF4, ClO4, PF6) have been prepared by the reaction of
(E)-2-(((5-((4-methoxyphenyl)ethynyl)pyridin-2-yl)imino)methyl)-4-((4-nitrophenyl)ethynyl)phenol (L) with [Ru(phen)2]
Cl2·2H2O and [Ru(bipy)2]Cl2·2H2O. The complexes were characterized by physico-chemical and spectroscopic methods.
All complexes are 1:1 conducting and diamagnetic in nature. In acetonitrile solution, the complexes displayed one reversible
Ru(II)–Ru(III) oxidation couple and one irreversible Ru(III)–Ru(IV) oxidation and are sensitive to π-acidic character of phen
and bipy ligands. The complexes show room-temperature luminescence originated from the lowest energy metal-to-ligand
charge transfer excited state and are sensitive to diference in size of the counter anions. All the complexes displayed second
harmonic generation by Kurtz-powder technique indicating their potential for the application as a useful NLO material.
Introduction
charge-transfer, ligand substitution, stereochemical isom-
erisation process and to probe metal–ligand interactions. In
recent years, considerable research interest has been paid
on the development of newer class of ruthenium-polypyri-
dyl complexes either by introducing selective groups or
by using other types of donor sites to generate new mixed
ligand ruthenium complexes with the perspective of tuning
their spectroscopic and photo-redox properties [7]. Further-
more, 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 geom-
etry. These complexes possess their potential applications
in diverse area such as photosensitizers for photochemical
conservation of solar energy, molecular electron devices and
as photoactive DNA cleavage agents for therauptic purposes
[8–10]. Many compounds of this type such as [Ru(bipy)2]2+
or [Ru(phen)2]2+ with various ligands like pyrroles [11], imi-
dazole-4,5-dicarboxylic acid or biquinoline derivatives [12]
and chiral salicylixazolinate [13–15] have been reported.
In this paper, we report the introduction of alkynyl-func-
tionalized N, O donor Schif base ligand L in the [Ru(bipy)2]
Cl2 and [Ru(phen)2]Cl2 core to prepare new mixed ligand
complexes of the kind [Ru(L)(bipy)2]X (1a–d) and [Ru(L)
(phen)2]X (2a–d) (where L=2-((E)-(4-(2-(4-methoxyphe-
nyl)ethynyl)phenylimino)methyl)-4-(2-(4-methoxyphenyl)
ethynyl)phenol; phen = 1,10 phenanthroline, bipy = 2,2′
The chemistry of ruthenium-polypyridine complexes has
gained enormous interest during the past few years due to
their rich electrochemical and photophysical properties. Fac-
ile electron-donor transfer properties, strong metal-to-ligand
charge transfer character and long-lived MLCT excited states
of these complexes make them attractive for the construc-
tion of photochemical and electrochemical devices [1, 2].
They are also important in energy conversion, lumines-
cence sensors, electroluminescence display and biotechnol-
ogy [3–5]. The most extensively explored complexes are
[Ru(bipy)2]2+ (bipy = 2,2′-bipyridine) and [Ru(phen)2]2+
(phen=1,10-phenanthroline) because they show relatively
long-lived excited state life time and are readily fuorescent
at room temperature [6]. Their unique properties have led
them being used to investigate photochemically induced
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-020-00384-x) contains
supplementary material, which is available to authorized users.
* S. S. Chavan
sanjaycha2@redifmail.com
1
Department of Chemistry, Shivaji University, Kolhapur,
MS 416004, India
Vol.:(0123456789)
1 3

Transition Metal Chemistry
−
−
−
−
bipyridine X = NO3 , BF4 , ClO4 , PF6 ). The efects of
newly introduced group L as well as infuence of counter
anion on the spectroscopic and luminescence properties
of the complexes have been studied. The electrochemical
behaviour and second harmonic generation (SHG) properties
of the complexes have also been reported.
room temperature for about 24 h. The reaction mixture was
fltered and concentrated under vacuo. The residue obtained
was purifed by column chromatography on neutral alumina
using dichloromethane: petroleum ether (1:3) as eluent. An
of-white coloured product was aforded by removal of sol-
vent under vacuo and identifed as 5-((trimethylsilyl)ethynyl)
pyridin-2-amine.
Yield: 85% (0.853 mmol, 0.162 g). (Elemental analy-
sis (C, H, N, w/w%) Anal. Calc. For C10H14N2Si1: C,
63.1; H, 7.3; N, 14.7. Found: C, 63.0; H, 7.3; N, 14.7; IR
(KBr, cm−1): 2167 (C≡C). 1H NMR (300 MHz, CDCl3): δ
7.23 (s, 1H, ArH), 6.53 (d, 2H, ArH), 3.62 (br s, 2H, NH2),
0.24 (s, 9H, Si(CH3)3). 13C NMR (CDCl3; 300 MHz): δ
156.1, 148.3, 131.8, 120.9, 119.8, 98.7, 97.3, 29.7. MS
(ESI): m/z 190 [M+].
Experimental
Materials and general methods
All chemicals used were of analytical reagent grade pur-
chased from Sigma-Aldrich and used without further purif-
cation. Solvents used for synthesis were distilled over appro-
priate drying reagents. [Ru(phen)2]Cl2 [16], [Ru(bipy)2]Cl2
[17] and 5-((4-nitrophenyl)ethynyl)salicylaldehyde were
prepared according to the literature procedure [18].
Synthesis of 5‑(ethynyl)pyridine‑2‑amine (2)
Elemental analyses (C, H and N) were performed on a
Thermo Finnigan FLASH EA-112 CHNS analyzer. Elec-
tronic absorption spectra were recorded on a Shimadzu
UV-3600 spectrophotometer. Infrared spectra were recorded
on PerkinElmer FT-IR spectrometer as KBr pellets in the
Powdery KOH (1 mmol, 0.056 g) was added to a solution
of 5-((trimethylsilyl)ethynyl)pyridin-2-amine (1 mmol,
0.190 g) in 20 ml of methanol, and the resulting solution
was stirred at room temperature for 2–3 h. The solvent
was removed under vacuo, and residue was extracted with
CH2Cl2/H2O. The organic layer was collected, dried and
passed through a neutral alumina column using 1:3 dichlo-
romethane: petroleum ether as eluent. The removal of sol-
vent under vacuum aforded a yellow coloured solid.
1
4000–400 cm−1 spectral range. H NMR spectra of the
samples were measured on Bruker-300 MHz instrument
using TMS [(CH3)4Si] as an internal standard. Lumines-
cence properties were measured using a PerkinElmer LS
55 spectrofluorometer equipped with quartz cuvette of
1 cm path length at room temperature. Cyclic voltamme-
try measurements were taken on CH-400A electrochemical
analyzer. A standard three-electrode system consisting of Pt
disk working electrode, Pt wire counter electrode and Ag/
AgCl reference electrode were used. All measurements were
taken in CH3CN solution at room temperature with scan
rate 100 m Vs−1 by using tetrabutylammonium perchlorate
(TBAP) as a supporting electrolyte. Thermal analysis of the
complexes was carried out on a PerkinElmer thermal ana-
lyzer in nitrogen atmosphere at a heating rate of 10 °C/min.
The SHG efciency of all the complexes was measured with
respect to urea by powder technique developed by Kurtz
and Perry using Q switched Nd-YAG laser (Lab-170 spectra
physics) 10 ns laser with frst harmonic output of 1064 nm
at the pulse repetition rate 10 Hz. The homogeneous pow-
der was mounted in the path of laser beam of pulse energy
2.2 mJ obtained by split beam technique.
Yield: 81% (0.814 mmol, 0.096 g). Elemental analysis
(C, H, N, w/w%) Anal. Calc. For C7H6N2: C, 71.1; H, 5.1;
N, 23.7. Found: C, 71.1; H, 5.1; N, 23.7. IR (KBr, cm−1):
2198, (C≡C). 1H NMR (300 MHz, CDCl3): δ 7.26 (d, 2H,
Ar–H), 6.56 (d, 2H, Ar–H), 3.76 (br s, 2H, NH2), 3.12 (s,
1H, C≡C–H). 13C NMR (CDCl3; 300 MHz): δ 156.3, 149.3,
129.8, 121.7, 120.5, 99.3, 97.9. MS (ESI): m/z 118 [M+].
Synthesis of 5‑((4‑methoxyphenyl)ethynyl)
pyridine‑2‑amine (3)
To a mixture of 1-iodo-4-methoxybenzene (1 mmol,
0.234 g), 5-(ethynyl)pyridin-2-amine (1 mmol, 0.118 g),
PdCl2(PPh3)2 (0.01 mmol, 0.056 g), CuI (0.02 mmol,
0.016 g), 30 ml of Et3N was added. The resulting mixture
was stirred for 24 h at 40 °C. Then the reaction mixture
was fltered of and dried under vacuo. The residue obtained
was purifed by column chromatography on silica gel using
dichloromethane: petroleum ether (1:1) as eluent. The brown
coloured solid was aforded by removal of solvent under
vacuo.
Synthesis of 5‑((trimethylsilyl)ethynyl)
pyridin‑2‑amine (1)
Yield: 88% (0.879 mmol, 0.197 g). Elemental analysis (C,
H, N, w/w%) Anal. Calc. For C14H12N2O1: C, 74.9; H, 5.3;
N, 12.5. Found: C, 74.9; H, 5.3; N, 12.5. IR (KBr, cm−1):
2209, (C≡C); 1171, υ(OCH3). 1H NMR (300 MHz, CDCl3):
7.75 (d, 2H, Ar–H), 7.68 (d, 2H, Ar–H), 7.65 (d, 1H, Ar–H),
To a mixture of 5-iodo-pyridin-2-amine (1 mmol, 0.22 g),
trimethylsilylacetylene (1 mmol, 0.13 ml), PdCl2(PPh3)2
(0.01 mmol, 0.032), CuI (0.02 mmol, 0.017 g), 30 ml of
Et3N was added. The resultant mixture was then stirred at
1 3

Transition Metal Chemistry
7.47 (dd, 1H, Ar–H), 7.00) (d, 7.75 (d, 2H, Ar–H), 7.68
(d, 2H, Ar–H), 7.65 (d, 1H, Ar–H), 7.47 (dd, 1H, Ar–H),
7.00) (d, 1H, Ar–H), 3.85 (s, 3H, OCH3), 3.76 (s, 2H, NH2).
13C NMR (CDCl3; 300 MHz): δ 156.3, 149.7, 147.1, 140.1,
135.9, 131.4, 129.3, 127.6, 99.1, 97.7, 55.3. MS (ESI): m/z
473. MS (ESI): m/z 224 [M+].
mol−1): 25.16. IR (KBr, cm−1) vmax: 2056 (C≡C), 1647
(C=N), 1319 (C–O), 3054 (C–H, Ar strech), 1055 (C–H,
1
Ar bend). H NMR: ꢀ 9.29 (HC=N), ꢀ 8.89–8.92 (d, 4H,
phen), ꢀ 7.17–8.07 (m, 26H, Ar.),ꢀ 3.87 (s, 3H, OCH3).
MS (ESI): m/z 933 [M-BF4]+. 1c: Yield: 69% (0.69 mmol,
0.713 g). Elemental Analysis: Anal. Calc. for Found:
RuC53H34N7O8Cl: C, 61.6%, H, 3.3%, N, 9.4%. Found:
C, 61.3%, H, 2.9%, N, 9.6%. Λm (CH3CN, Ω−1cm2mol−1):
25.11. IR (KBr, cm−1) vmax: 2056 (C≡C), 1649 (C=N),
1327 (C–O), 3054 (C–H, Ar strech), 1055 (C–H, Ar bend).
1H NMR: ꢀ 9.40 (HC=N), ꢀ 8.89–8.92 (d, 4H, phen), ꢀ
7.17–8.07 (m, 26H, Ar.),ꢀ 3.87 (s, 3H, OCH3). MS (ESI):
m/z 933 [M-ClO4]+. 1d: Yield: 67% (0.67 mmol, 0.722 g).
Elemental Analysis: Anal. Calc. for RuC53H34N7O4PF6: C,
59.0%, H, 3.1%, N, 9.0%. Found: C, 58.9%, H, 3.0%, N,
9.0%. Λm (CH3CN, Ω−1cm2mol−1): 24.91. IR (KBr, cm−1)
vmax: 2056 (C≡C), 1650 (C=N), 1328 (C–O), 3054 (C–H,
Ar strech), 1055 (C–H, Ar bend). 1H NMR: ꢀ 9.42 (HC=N),
ꢀ 8.89–8.92 (d, 4H, phen), ꢀ 7.17–8.07 (m, 26H, Ar.),ꢀ 3.87
(s, 3H, OCH3). MS (ESI): m/z 933 [M-PF6]+.
Synthesis of (E)‑2‑(((5‑((4‑methoxyphenyl)ethynyl)
pyridin‑2‑yl)imino)methyl)‑4‑((4‑nitro phenyl)
ethynyl)phenol (L)
To a solution of 5-((4-nitrophenyl)ethynyl)salicyaldehyde
(1 mmol, 0.267 g) in methanol (10 ml), the solution of
5-((4-methoxyphenyl)ethynyl)pyridin-2-amine (1 mmol,
0.224 g) in CH2Cl2 (10 ml) was added and refuxed for 2 h.
Completion of reaction was checked by TLC. After evapo-
ration of solvent the obtained orange coloured solid was
washed with diethyl ether, recrystallized from methanol and
dried in vacuo.
Yield: 89% (0.89 mmol, 0.421 g). Elemental analysis (C,
H, N, w/w%) Anal. Calc. For C29H19N3O4: C, 73.5; H, 4.0;
N, 8.8. Found: C, 73.3; H, 3.9; N, 9.0. IR (KBr, cm−1) vmax
:
Synthesis of [Ru(L)(bipy)2]X (2a–d)
3268 (OH), 2062 (C≡C), 1635 (C=N), 1288 (C–O), 1161
(OCH3), 1470 (NO2), 3014 (C–H, Ar strech), 1048 (C–H,
Ar bend). 1H NMR (CDCl3; 300 MHz): ꢀ 10.9 (s, 1H, OH),
ꢀ 8.34 (s, 1H, HC=N), ꢀ 7.11–8.11 (m, 14H, Ar.), ꢀ 3.87 (s,
3H, OCH3). 13C NMR (CDCl3; 300 MHz): δ 162.9, 158.2,
156.7, 154.9, 152.5, 149.9, 149.0, 147.7,142.3, 140.5, 139.3,
135.9, 131.5, 130.5, 129.1, 127.7, 120.4, 119.7, 118.2, 99.1,
99.0, 98.1, 97.6, 55.3. MS (ESI): m/z 473 [M+].
To a solution of Ru(bipy)2Cl2 (1 mmol, 0.484 g) in dry
ethanol (10 ml), the ligand L (1 mmol, 0.473 g) in ethanol
(10 ml) was added under nitrogen atmosphere. The reac-
tion mixture in anaerobic condition was refuxed for 12 h,
and completion of reaction was checked by TLC. Then the
reaction mixture was allowed to cool and saturated aqueous
solution of NaX (where X=NO3, BF4, ClO4, PF6) was added
in 1:1 molar proportion. The product obtained was collected
by fltration, washed with ethanol and dried in vacuo.
2a: Yield: 68% (0.68 mmol, 0.645 g). Elemental Analy-
sis: Anal. Calc. for RuC49H34N8O7: C, 62.0%, H, 3.6%, N,
11.8%. Found: C, 62.0%, H, 3.2%, N, 11.9%. Λm (CH3CN,
Ω−1cm2mol−1): 27.44. IR (KBr, cm−1) vmax: 2056(C≡C),
1643 (C=N), 1306 (C–O), 3054 (C–H, Ar strech), 1055
Synthesis of [Ru(L)(phen)2]X (1a–d)
To the solution of Ru(phen)2Cl2 (1 mmol, 0.532 g) in dry
ethanol (10 ml), the ligand L (1 mmol, 0.473 g) in ethanol
(10 ml) was added under nitrogen atmosphere. The reac-
tion mixture in anaerobic condition was refuxed for 12 h,
and completion of reaction was checked by TLC. Then, the
reaction mixture was allowed to cool, and saturated aqueous
solution of NaX (where X=NO3, BF4, ClO4, PF6) was added
in 1:1 molar proportion. The product obtained was collected
by fltration, washed with ethanol and dried in vacuo.
1a: Yield: 67% (0.67 mmol, 0.667 g). Elemental Analy-
sis: Anal. Calc. for RuC53H34N8O7: C, 63.9%, H, 3.4%, N,
11.2%. Found: C, 63.8%, H, 3.33%, N, 11.3%. Λm (CH3CN,
Ω−1cm2mol−1): 26.02. IR (KBr, cm−1) vmax: 2056 (C≡C),
1643 (C=N), 1321 (C–O), 3054 (C–H, Ar strech), 1055
1
(C–H, Ar bend). H NMR: ꢀ 9.11 (HC=N),ꢀ 8.69–8.72
(dd, 4H, bipy), ꢀ 6.44–8.25 (m, 26H, Ar.), ꢀ 3.87 (s, 3H,
OCH3). MS (ESI): m/z 885 [M-NO3]+. 2b: Yield: 61%
(0.61 mmol, 0.593 g). Elemental Analysis: Anal. Calc. for
RuC49H34N7O4BF4: C, 60.5%, H, 3.5%, N, 9.4%. Found:
C, 60.3%, H, 3.2%, N,9.5%. Λm (CH3CN, Ω−1cm2mol−1):
26.47. IR (KBr, cm−1) vmax: 2056(C≡C), 1644 (C=N),
1314 (C–O), 3054 (C–H, Ar strech), 1055 (C–H, Ar bend).
1H NMR:ꢀ 9.13 (HC=N),ꢀ 8.69–8.72 (dd, 4H, bipy), ꢀ
6.44–8.25 (m, 26H, Ar.), ꢀ 3.87 (s, 3H, OCH3). MS (ESI):
m/z 885 [M-BF4]+. 2c: Yield: 64% (0.64 mmol, 0.63 g).
Elemental Analysis: Anal. Calc. for RuC49H34N7O8Cl: C,
59.7%, H, 3.4%, N, 9.9%. Found: C, 59.6%, H, 3.0%, N,
10.0%. Λm (CH3CN, Ω−1cm2mol−1): 26.01. IR (KBr, cm−1)
vmax: 2056(C≡C), 1646 (C=N), 1315 (C–O), 3054 (C–H, Ar
strech), 1055 (C–H, Ar bend). 1H NMR:ꢀ 9.21 (HC=N), ꢀ
1
(C–H, Ar bend). H NMR: ꢀ 9.18 (HC=N), ꢀ 8.89–8.92
(d, 4H, phen), ꢀ 7.17–8.07 (m, 26H, Ar.), ꢀ 3.87 (s, 3H,
OCH3). MS (ESI): m/z 933 [M-NO3]+. 1b: Yield: 67%
(0.67 mmol, 0.683 g). Elemental Analysis: Anal. Calc.
for RuC53H34N7O4BF4: C, 62.3%, H, 3.3%, N, 9.6%.
Found: C, 62.2%, H, 3.2%, N, 9.6%. Λm (CH3CN, Ω−1 cm2
1 3

Transition Metal Chemistry
8.69–8.72 (dd, 4H, bipy), ꢀ 6.44–8.25 (m, 26H, Ar.), ꢀ 3.87
(s, 3H, OCH3). MS (ESI): m/z 885 [M-ClO4]+. 2d: Yield:
61% (0.61 mmol, 0.629 g). Elemental Analysis: Anal. Calc.
for RuC49H34N7O4PF6: C, 57.0%, H, 3.3%, N, 9.5%. Found:
C, 56.9%, H, 3.1%, N, 9.5%. Λm (CH3CN, Ω−1cm2mol−1):
25.12. IR (KBr, cm−1) vmax: 2056(C≡C), 1650 (C=N),
1324 (C–O), 3054 (C–H, Ar strech), 1055 (C–H, Ar bend).
1H NMR:ꢀ 9.29 (HC=N),ꢀ 8.69–8.72 (dd, 4H, bipy), δ
6.44–8.25 (m, 26H, Ar.), ꢀ 3.87 (s, 3H, OCH3). MS (ESI):
m/z 885 [M-PF6]+.
characterization of the complexes are in good agreement
with the proposed formula (Fig. 1).
In the IR spectrum of L, a broad band appeared at
3268 cm−1 is ascribed to ʋ(OH). This band is diminished in
the spectra of 1a–d and 2a–d indicating deprotonation of the
phenolic proton before coordination. The medium–strong
bands centred at 2062 and 1161 cm−1 in L due to stretch-
ing frequency of ʋ(C≡C) and ʋ(OCH3) remain unchanged
in 1a–d and 2a–d. The ʋ(C=N) stretching frequency at
1635 cm−1 of free ligand L shifted to 1643–1650 cm−1
in 1a–d and 1643–1650 cm−1 in 2a–d is in accordance with
the coordination of (HC=N) group to ruthenium ion in the
complexes [20]. It is to be noted that changing the nature
of co-ligands and counter ions in the complexes appears to
have little efect on the ʋ(C=N) frequency. A strong band
observed at 1288 cm−1 in L has been assigned to phenolic
C–O stretching. On complexation, this band shifted to higher
frequency in the range 1328–1306 cm−1 showing that the
other coordination is through the phenolic oxygen atom [21,
22]. Bands in the 519–539 cm−1 and 465–490 cm−1 region
are ascribed to the formation of Ru–N and Ru–O bonds,
respectively [23]. This is further supported by the coordi-
nation of azomethine nitrogen and the phenolic oxygen to
ruthenium ion. In 1b and 2b, the intense band at~1080 cm−1
is attributed to the anti-symmetric stretching mode due
Results and discussion
The starting material 5-(ethynyl)pyridine-2-amine was pre-
pared by using Pd(II)/Cu(I) catalyzed coupling of 5-iodo-
2-aminopyridine followed by reaction with KOH in MeOH
by following the procedure reported earlier [19]. Further
coupling of 5-(ethynyl)pyridine-2-amine with 1-iodo-
4-methoxybenzene aforded 5-((4-methoxyphenyl)ethynyl)
pyridine-2-amine. The ligand L (E)-4-((4-methoxyphenyl)
ethynyl)-2-(((6-((4-nitrophenyl)ethynyl)pyridine-2-yl)imino)
methyl)phenol was obtained by the reaction of equimolar
amount of 5-((4-methoxyphenyl) ethynyl)pyridine-2-amine
and 2-hydroxy-5-((4-nitrophenyl)ethynyl)benzaldehyde in
excellent yield (Scheme 1). To understand the infuence of
π-conjugation and size of counter anions, the mononuclear
complexes [Ru(phen)2(L)]X (1a–d) and [Ru(bipy)2(L)]
X(2a–d) were prepared by the reaction of L with
[Ru(phen)2Cl2]·2H2O and [Ru(bipy)2Cl2]·2H2O followed
by addition of NaNO3, NaBF4, NaClO4 or NaPF6 under
N2 atmosphere (where bipy = 2, 2′-bipyridine; phen = 1,
−
to BF4 ion [24]. The perchlorate complexes 1c and 2c
exhibit broadband at ~ 1100 cm−1, and the unsplit band
at~637 cm−1 suggests the stretching vibration of non-coor-
dinated ClO4− ion [25]. However, the strong band exhibited
at 847 cm−1 and 568 cm−1 in 1d and 2d has been consistent
with the presence of PF6− anion in the complexes [26].
1
The H NMR spectrum of free ligand L exhibits aro-
−
−
−
−
10-phenanthroline; X=NO3 , ClO4 , BF4 , PF6 ). All the
complexes are soluble in polar solvents such as DMSO, ace-
tonitrile, and DMF. The results of elemental analyses and
matic hydroxyl (−OH) group resonated at δ 10.9 ppm,
which is absent in all the complexes. The singlet appeared
at δ 8.34 and δ 3.87 ppm due to the azomethine (–HC=N–)
Scheme 1 Synthetic route to
the preparation of (L)
1 3

Transition Metal Chemistry
I (1a-d)
II (2a-d)
-
-
-
-
(Where X = NO3 , BF4 , ClO4 , PF6 )
Fig. 1 Proposed molecular structure of the complexes I (1a–d) and II (2a–d)
and methoxy (–OCH3) protons in the free ligand L, respec-
Table 1 Electrochemical data of 1a–d and 2a–d
tively. In 1a–d and 2a–d, the signal of azomethine proton is
considerably deshielded to the range δ 9.11–9.25 ppm as a
consequence of electron donation to the metal centre [27].
The singlet of methoxy protons is not much afected during
complexation. The ring protons of L and phen/bipy moi-
eties in 1a–d and 2a–d are overlapped in the region δ
6.68–8.92 ppm. So the unambiguous assignment of indi-
vidual proton signal was not possible. However, the intensity
of the aromatic signals with that of the observable azome-
thine (−CH=N−) and methoxy (−OCH3) protons reveals
the presence of expected number of aromatic protons for all
the complexes. The proton counts in the NMR spectrum of
each of the complex corroborated with the formulae of the
products.
Complex
RuII–RuIII
RuIII–RuIV
Ligand
coupleaE1/2 (V) couple Epa(V)
coupleaE1/2 (V)
1a
1b
1c
1d
2a
2b
2c
2d
0.43
0.47
0.59
0.56
0.39
0.41
0.51
0.52
1.46
1.61
1.62
1.57
1.15
1.27
1.33
1.29
−1.37
−1.34
−1.21
−1.19
−1.57
−1.54
−1.51
−1.45
−1.68
−1.64
−1.62
−1.51
−1.78
−1.74
−1.72
−1.71
Solvent, acetonitrile; Supporting electrolyte, tetrabutylaminoperchlo-
rate; Reference electrode, SCE; Working electrode, Pt wire
aE1/2 =0.5(Epa+Epc), where Epa and Epc are the anodic and cathodic
peak potentials, respectively
Electrochemistry (CV)
than that of the previous Ru(II)–Ru(III) couple. The irrevers-
ibility of this oxidation in 1a–d and 2a–d indicates that the
[RuIV(L)(phen)2]3+ and [RuIV(L)(bipy)2]3+ generated dur-
ing the anodic scan are not stable in solution. This instability
of [RuIV(L)(phen)2]3+ and [RuIV(L)(bipy)2]3+ may be due
to their high reduction potential which makes them potential
oxidant [29]. In addition to one reversible and one irrevers-
ible oxidation processes, 1a–d and 2a–d display two succes-
sive reversible one-electron reduction at −1.60 to −1.79 V
and −1.50 to −1.75 V which are assigned to reduction of
two N, N′- donor ligands in the complexes [7]. 2, 2′-bipyri-
dine and 1, 10-phenanthroline are well-known potential elec-
tron-transfer centre, and each bipyridine and phenanthroline
can accept two electrons in one electrochemically accessible
LUMO [30]. Since the complexes 1a–d and 2a–d have two
N, N′-donor units, four one-electron reductions are therefore
Electrochemical proper ties of t he com-
plexes 1a–d and 2a–d have been examined by cyclic voltam-
metry in CH3CN solution (10−3 M L−1). The electrochemical
data are listed in Table 1, and representative voltammo-
gram is illustrated in Fig. 2. All the complexes display one
reversible couple in the range 0.43–0.56 V for 1a–d and
0.39–0.52 V for 2a–d which are assigned to Ru(II)–Ru(III)
process. Compared to [Ru(phen)3]2+ and [Ru(bipy)3]2+, the
overall decrease in Ru(III) to Ru(II) potential by 0.7 to 0.9 V
in 1a–d and 2a–d is due to incorporation of σ-donating ani-
onic ligand L− in [Ru(phen)2]2+ and [Ru(bipy)2]2+ core [28].
The complexes 1a–d and 2a–d exhibit a second irreversible
oxidation process in the range 1.15–1.73 V and 1.15–1.73 V
versus SCE which is assigned to Ru(III)–Ru(IV) process.
The current height of this process is found to be greater
1 3

Transition Metal Chemistry
Fig. 2 Cyclic voltammogram of
A (1c) and B (2c)
Table 2 Absorption and emission data of L, 1a–d and 2a–d
Compound λAbs (nm)
λEx (nm) λEm (nm)
ϕ
τ (ns)
L
274, 384
290
312
312
312
312
487
679
676
672
668
669
666
664
661
–
–
1a
1b
1c
1d
2a
2b
2c
2d
260, 392, 501
267, 494, 508
273, 402, 515
276, 411, 519
241, 292, 389, 501 308
247, 296, 393, 502 308
261, 299, 398, 509 308
267, 303, 400, 513 308
0.080 6.78
0.073 6.62
0.077 6.71
0.079 6.66
0.072 6.67
0.062 6.49
0.067 6.62
0.069 6.39
expected. But in practice, we have observed two reductions
within the 2 V potential range. The other expected two
reductions could not be seen possibly due to solvent cut-of.
In comparison of electrochemical data of the prepared com-
plexes, the redox processes for 1a–d appear at slightly more
positive potential than those for the corresponding 2a–d.
This is attributed to better stabilization of phen in 1a–d as
compared to bipy-based 2a–d as a consequence of its strong
π-acidic character [31]. It is also observed that the E1/2 val-
ues for 1a–d are slightly greater than 2a–d which concludes
that the more the π-acidic character higher the E1/2 values.
Fig. 3 Electronic absorption spectra of ligand (L)
spin-allowed π–π* transition of the ligands 1, 10-phenan-
throline and L, whereas 2a–d shows two intense absorp-
tions at 240 and 290 nm which are assigned to typical spin-
allowed π–π* transitions due to 2, 2′-bipyridine and L [32].
Another weak broad absorption at 392–411 nm in 1a–d and
389–400 nm in 2a–d may be due to n–π* transition of
ligand L. In addition to these bands, a weak broad absorp-
tion at 501–519 nm in 1a–d and 501–513 nm in 2a–d are
assigned to dπ(Ru) → L MLCT transition [33]. It may be
noted that the lowest-energy MLCT maxima in 1a–d and
2a–d are redshifted with respect to the MLCT of Ru(phen/
bipy)2Cl2 precursors. It also may be noted that the lowest
energy MLCT transition observed at 440 nm in Ru(phen)23+
and 450 nm in Ru(bipy)23+ is reasonably redshifted on substi-
tution of one of the polypyridyl ligand by anionic Schif base
ligand L− [34–36]. Little bathochromic shift is observed in
all complexes due to changing the size of the counter anions
from large to small. Moreover, compared to1a–d, the MLCT
transitions appear at higher wavelengths in the spectra of
Absorption and emission properties
Electronic absorption spectra of L and its correspond-
ing complexes 1a–d and 2a–d were recorded in acetoni-
trile solution (10−4 M L−1). The spectral data are listed in
Table 2, and the spectra of L and its complexes are illus-
trated in Figs. 3 and 4, respectively. The free ligand L
exhibits high energy band at λmax 274 and 384 nm which
are assigned to π → π* and n → π* transitions inside L.
On the other hand, the spectra of 1a–d display single
absorption at around 260 nm which can be assigned to
1 3

Transition Metal Chemistry
Fig. 4 Electronic absorption spectra of complexes A (1a–d) and B (2a–d)
the 2a–d which may be due to strong electron-donating abil-
ity of phen than the bipy ligand [37].
excitation at λmax =290 nm, free ligand L exhibits emission
at λmax =487 nm assigned to ligand-centred π–π* transition.
The complexes 1a–d show emission at 668–679 nm when
excited upon 312 nm (Fig. 5A), whereas 2a–d shows emis-
sion at 661–669 nm when excited upon 308 nm (Fig. 5B).
All complexes exhibit relatively strong emissions which
The emission properties of the ligand L and its Ru(II)
complexes 1a–d and 2a–d have been investigated at ambi-
ent temperature in acetonitrile solution (10−5 M L−1),
and relevant data are summarized in Table 2. Upon
Fig. 5 Emission spectra of A (1a–d) and B (2a–d)
1 3

Transition Metal Chemistry
originate from dπ(Ru)→L MLCT transition. The presence
of additional π-conjugation of L in complexes might be
one of the important contributing factors for this intense
emission which confirm the chelation of anionic form
of L− (Schif base ligand) to ruthenium ion that efectively
increases the ligand conformational rigidity and thus reduc-
ing the non-radiative loss [38]. It was observed that the
emission efciency of 1a–d appears at a longer wavelength
with enhanced intensity as compared to 2a–d (Table 2).
This might be due to non-radiative decay process which
is more efective in the bipy complexes (2a–d) as com-
pared to phen complexes (1a–d) [39]. It was also observed
that the emission energy of all complexes is sensitive to
the size of the counter anion. As a size of counter anion
increases, the emission wavelength in both the series of
Ru(II) complexes decreases and it follows the sequence
These results may be due to redshifted emission and reduced
emission intensity observed in 2a–d as compared to 1a–d.
Thermogravimetric analysis
To investigate the thermal stability of 1a–d and 2a–d, ther-
mogravimetric analysis (TGA) was performed at the tem-
perature range of 25–800 °C under nitrogen atmosphere.
The perchlorate complexes 1c and 2c are potentially explo-
sive and hence are not studied for safety reasons. At the
beginning of the thermal decomposition process, all the
complexes show loss of the lattice water molecules at low
temperature (30–150 °C). The TGA weight loss profles of
76% of the weight of the complexes are stretched over the
193–657 °C temperature range including two decomposi-
tion processes. From the frst decline in the temperature
range 196–460 °C, complexes underwent complicated
multiple weight loss with total mass loss corresponding
to cleavage of loosely connected metal-counter anion and
metal-nitrogen bonds from phen/bipy co-ligands (% Obs.
1a; 36.21, 1b; 35.36, 1d; 33.41 and % Calc. 1a; 36.19,
1b; 35.31, 1d; 33.40, % Obs. 2a; 33.01, 2b; 32.16, 2d;
30.41 and % Calc. 2a; 32.95, 2b; 32.11, 2d; 30.30). After
the frst decomposition step, remaining substance keeps
losing weights from 430 to 657 °C; this second dissocia-
tion is probably due to detachment of Schif base anion
L− from ruthenium metal ion which is in good agreement
with calculated C, H, N analysis (% Obs. 1a; 47.41, 1b;
46.24, 1d; 43.79 and % Calc. 1a; 47.34, 1b; 46.19, 1d;
43.69, % Obs. 2a; 49.79, 2b; 48.49, 2d; 45.81 and % Calc.
−
−
NO3− <BF4 <ClO4−<PF6 . These results could be attrib-
uted to efect on the lowest unoccupied molecular orbital
(LUMO), and highest occupied molecular orbital (HOMO)
of Ru(II) complexes leads to increase the energy gap from
NO3− to PF6− as a counter anions [40].
The quantum yield of 1a–d and 2a–d was determined
regarding quinine sulphate (ϕemr =0.52) and is found to be
0.073–0.080 for 1a–d and 0.062–0.072 for 2a–d (Table 2).
The luminescence lifetime plots of representative com-
plexes 1b and 2b are depicted in Fig. 6A, B, respectively.
The observed decay of 1a–d and 2a–d ft well with the
monoexponential nature of the complexes and is found to be
6.62–6.78 ns for 1a–d and 6.39–6.67 ns for 2a–d (Table 2).
Compared to 2a–d, the average lifetime of 1a–d is longer.
Fig. 6 The luminescence lifetime plots of A (1a) and B (2a)
1 3

Transition Metal Chemistry
2a; 49.74, 2b; 48.47, 2d; 45.73). Finally, beyond this tem-
perature, a black residue of metal oxide is obtained as an
end product. From the TGA plots of the complexes, we can
conclude that the phen-based complexes 1a–d are found
to be thermally more stable compared to the bipy-based
complexes 2a–d and the results were found to be in good
agreement with the composition of all complexes which
further supported by elemental analysis. Along with that
for 1a–d and 2a–d (Excluding ClO4− counter ion contain-
ing complexes 1c and 2c), the mass loss follows the order
through the molecule which favours the large second-order
polarizability of the complexes.
Conclusion
We have thus observed the efect of the incorporation of
alkynyl-functionalized Schif base ligand as a third ligand
in the [Ru(phen)2]2+ and [Ru(bipy)2]2+ core with respect
to spectroscopic and photophysical aspects. The pres-
ence of ligand L− in complexes 1a–d and 2a–d facilitates
the successive reversible Ru(II)–Ru(III) and irreversible
Ru(III)–Ru(IV) oxidation. The environment around Ru(II)
is more susceptible to undergo the lowest energy metal-to-
ligand charge transfer (MLCT) band. The emission proper-
ties of the complexes originate from the MLCT excited state
and vary considerably on the π-acidic character of the phen
and bipy ligand as well as the size of the counter anion. The
complexes 1a–d and 2a–d were found to show second har-
monic generation (SHG) activities and possess a promising
potential for the application as useful NLO materials due
to the existence of acidic character, counter anions system,
conjugation and potential to vary oxidation state.
−
of NO3− < BF4− < PF6 . These results are in accordance
with the increase in ions molecular weight.
SHG efciency
Over the last few decades, there has been considerable
interest in the nonlinear optical materials (NLO). These
materials not only play an important part in solid-state
laser techniques as frequency conversion materials but
also have promising applications in high-density optical
recording, laser printing, optical measurement system
and display [41, 42]. Among the various NLO materi-
als, metal–organic coordination network has attracted
much more attention to evaluate their potential applica-
tion as a second-order NLO material. To determine the
NLO response of 1a–d and 2a–d, we measured the SHG
efciency with the Kurtz–Perry method using a polycrys-
talline sample relative to reference urea. Comparison of
the area of SHG signal emitted by 1a–d and 2a–d with
the standard urea showed that the complexes 1a–d are
1.26–1.28 and 2a–d are 1.19–1.22 times more SHG
active than that of urea (Table 3). This may be due to bet-
ter hyper conjugation and π-donor capacity of Ru(II) in
the complexes. Compared to 2a–d, complexes 1a–d are
more SHG active as a consequence of higher aromaticity
due to strong π-acidic character of 1,10-phenanthroline
than that of 2,2′-bipyridine. From the results obtained we
may conclude that the substituents on coordinated ligand
L and counter anion play a major role in charge transfer
Acknowledgements 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.
Compliance with ethical standards
Conflict of interest The authors declare that they have no confict of
interest.
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