Synthesis, photophysical and electrochemical properties of a mixed bipyridyl-phenanthrolyl ligand Ru(II) heteroleptic complex having trans-2-Methyl-2-butenoic acid functionalities

Article abstract of DOI:10.3390/molecules16108353

In this work, two ligands: 4-(trans-2-Methyl-2-butenoic acid)-2,2'-bipyridine) (L1) and 5-(trans-2-methyl-2-butenoic acid)-1,10-phenanthroline (L₂), with the corresponding mixed-ligand heteroleptic Ru(II) complex were synthesized and characterized by FT-IR, ₁H-, ₁₃C-NMR spectroscopy and elemental analysis. The influence of the mixed functionalized polypyridyl ruthenium(II) complex on the photophysical and electrochemical properties were investigated and compared to individual single-ligand homoleptic complexes. Interestingly, the mixed-ligand complex formulated as [RuL1L₂(NCS)₂] exhibits broad and intense metal-to-ligand charge transfer (MLCT) absorption with a high molar extinction coefficient (λ_max = 514 nm, ε = 69,700 M _-1 cm_-1), better than those of individual single-ligand complexes, [Ru(L1)₂(NCS)₂] and [Ru(L₂) ₂(NCS)₂], and a strong photoluminescence intensity ratio in the red region at λ_em = 686 nm. The electrochemical properties of the complex indicated that the redox processes are ligand-based.

Full text of DOI:10.3390/molecules16108353

Molecules 2011, 16, 8353-8367; doi:10.3390/molecules16108353
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis, Photophysical and Electrochemical Properties of a
Mixed Bipyridyl-Phenanthrolyl Ligand Ru(II) Heteroleptic
Complex Having trans-2-Methyl-2-butenoic Acid Functionalities
Adewale O. Adeloye
Department of Chemistry, Faculty of Science and Agriculture, University of Fort Hare, P.M.B. X1314,
Alice 5700, South Africa; E-Mail: aadeloye@ufh.ac.za; Tel.: +27-761-423-313; Fax: +27-406-022-366
Received: 15 August 2011; in revised form: 24 September 2011 / Accepted: 28 September 2011 /
Published: 30 September 2011
Abstract: In this work, two ligands: 4-(trans-2-Methyl-2-butenoic acid)-2,2'-bipyridine) (L
and 5-(trans-2-methyl-2-butenoic acid)-1,10-phenanthroline (L ), with the corresponding
mixed-ligand heteroleptic Ru(II) complex were synthesized and characterized by FT-IR,
1
)
2
1
13
H-, C-NMR spectroscopy and elemental analysis. The influence of the mixed functionalized
polypyridyl ruthenium(II) complex on the photophysical and electrochemical properties were
investigated and compared to individual single-ligand homoleptic complexes. Interestingly,
1 2 2
the mixed-ligand complex formulated as [RuL L (NCS) ] exhibits broad and intense
metal-to-ligand charge transfer (MLCT) absorption with a high molar extinction coefficient
−1
−1
(λ
max = 514 nm, ε = 69,700 M cm ), better than those of individual single-ligand
complexes, [Ru(L ) (NCS) ] and [Ru(L ) (NCS) ], and a strong photoluminescence
1
2
2
2 2
2
intensity ratio in the red region at λem = 686 nm. The electrochemical properties of the
complex indicated that the redox processes are ligand-based.
Keywords: mixed-ligand Ru(II) complex; extended-π-bond conjugation; spectroscopy;
molar extinction coefficient; electrochemistry
1
. Introduction
Tremendous interest has been attracted to ruthenium(II) polypyridyl complexes because of their
potential applications in molecular electronic devices [1], as DNA structural probes or new therapeutic
agents [2,3], and as photosensitizers in the conversion of solar energy to chemical or electrical
energy [4]. It has been shown that one of the best way to enhance both the absorption coefficient and

Molecules 2011, 16
8354
red-shift of the metal-to-ligand charge transfer (MLCT) band in a ruthenium-based photosensitizer was
to extend the π-conjugation length of the colorant’s ancillary [5] or anchoring [6] ligands. Other
classes of ligands such as carboxylated terpyridine and phenanthroline showed enhanced UV-Vis
absorption over a broad range due to their large conjugated backbone structure [7]. Over the past
decade quite a large amount of data has accumulated on changes in the electrochemical and
photophysical properties of complexes effected by substitution in the 2,2'-bipyridine (bpy) or
1
,10-phenanthroline (phen) rings, or by replacement of one or both of the pyridine rings with other
nitrogen-containing heterocycles [8-13]. It has been shown that redox behaviour of these complexes
depends on the nature of the ligands, and there is a correlation between their spectroscopic properties
II
III
and Ru /Ru redox potential. Further studies of ruthenium(II) complexes are essential to
understanding the relationship between structure and function [14].
Based on our earlier studies and recent findings on ruthenium(II) heteroleptic complexes containing
mono- and/or oligo- anthracenyl functionalized polypyridine ligands, it was shown that the molar
absorption coefficient of ruthenium(II) complexes are not solely dependent on the extended
π-conjugation of the ancillary and anchoring ligands, but other factors such as the spatial geometry and
molecular aggregation in the molecules also play major roles [15-17]. Herein, a further attempt to gain
insight into the effects of ligand functionalization and/or mixed-ligand coordination on the
photophysical and electrochemical properties of ruthenium(II) polypyridyl complexes is undertaken
and discussed.
2
. Results and Discussion
2
.1. Chemistry
1 2
Scheme 1 (routes a and b) show the two step synthetic pathways for L and L , and outline the
chemistry of the present study. An initial mono-bromination of the starting materials (2,2'-bipyridine
and 1,10-phenanthroline) to afford 4-bromo-2,2'-bipyridine and 5-bromo-1,10-phenanthroline was
performed using environmentally benign conditions as reported by Vyas and co-workers [18]. A one-
step nucleophilic aromatic substitution reaction of the aryl bromide compounds and trans-2-methyl-2-
butenoic acid in a basic reaction condition using a stochiometric ratio of triethylamine and potassium
hydroxide under palladium-carbide catalysis afforded L
methane/chloroform in good yield. The synthesis of the metal precursor [RuCl
complexes [RuL (NCS) ], [Ru(L (NCS) ] and [Ru(L (NCS) ] followed the general synthetic
1
and L
2
after extraction into dichloro-
2
(dmso) ] [19], and the
4
1
L
2
2
1
)
2
2
2
)
2
2
routes shown in Scheme 1 (routes c–e). Sequential substitution of the DMSO coordinating ligand from
the metal precursor with stoichiometric ratio of L , L , and combination of L and L led to formation
1
2
1
2
of intermediate complexes [Ru(L
1
)
2
Cl
2
], [Ru(L
2
)
2
Cl
2
1 2 2
] and [RuL L Cl ], respectively, which were not
isolated. Final reactions were carried out by chloride exchange with thiocyanate groups [20].

Molecules 2011, 16
8355
Scheme 1. Synthetic pathways for Ligands L , L , and complexes [RuL L (NCS) ],
1
2
1
2
2
1 2 2 2 2 2
[Ru(L ) (NCS) ] and [Ru(L ) (NCS) ].
COOH
CH3
H3C
Br
(a)
CH3
N
N
H
N
N
N
N
COOH
L1
HBr/H O , (1/1)
(i)
2
2
H3C
MeOH, rt, 20 h
Br
MeOH, Pd/C,
(ii)
H3C
COOH
CH3
(b)
(
iii) KOH, Et N
3
N
N
N
N
o
(
i ) ref lux, 100 C, 10 min
(
c) [RuCl (H O)] + 4 (CH ) SO
[RuCl ((CH ) SO) ]
N
N
3
2
3 2
2
3 2
4
(ii) washed in acetone
L2
(yellow powder)
COOH
H3C
CH3
o
DMF, reflux, 120 C
N
(
d) [RuCl2((CH3)2SO)4] + L + L + 2NH NCS
1
2
4
N
N
N
8
h, Chromatography
Ru
N
CH3
COOH
C
N
C
S
S
H3C
HOOC
CH3
2
NH NCS
4
o
H3C
DMF, reflux, 120 C
8 h, Chromatography
2
(L1)
N
N
H3C
(
e) [RuCl ((CH ) SO) ]
2
3 2
4
COOH
N
Ru
N
CH3
N
N
C
C
2
(L2)
S
S
2
NH NCS
S
S
4
o
C
C
DMF, reflux, 120 C
h, Chromatography
N
N
8
N
Ru
N
H3C
H3C
HOOC
COOH
N
N
CH3
CH3
2
.2. Infrared Spectra Studies of the Ligands and Complex
The infrared absorption spectra of the ligands L and L and the complex [RuL L (NCS) ] share as
1
2
1
2
2
−1
common characteristic features a series of weak-to-moderate absorptions in the 3,390–3,032 cm region,
which were assigned to aromatic C–H stretching and/or considered indicative of the presence of O–Hstr
−1
groups in the compounds. The vibrational bands at 2,927, 2,938 cm in L and L , respectively, show
1
2

Molecules 2011, 16
8356
the presence of the aliphatic C–H bonds of the methyl, methylene and/or the methine groups in the
str
−1
compounds [21]. A slight difference shift to lower frequency (~10 cm ) may be due to spatial
arrangement and/or substitution pattern of the alkyl groups in the [RuL (NCS) ] complex. The
1
L
2
2
−1
strong intense band at 2,102 cm present in the complex (Figure 1), was assigned to the stretch
vibrational modes due to N-coordinated υ(CN) of the thiocyanate group. This band is conspicuously
absent in the spectra of the ligands.
Figure 1. FT-IR spectrum of [RuL L (NCS) ] complex.
1
2
2
The spectra of the two ligands and the complex display asymmetric carboxylate stretching bands
−
−1
−1
ν
asym(COO ) at 1,965 cm for L
but, more intense vibrational frequency band at 1,969 cm . It is interesting to note that upon addition
of a 1 mL volume of 20% conc. sulphuric acid to 20 mg of [RuL (NCS) ] complex, a strong band at
,745 cm was observed; this is indicative of the protonation and/or deprotonation of the carboxylate
group on either the bipyridine and phenanthroline ligands. If the ligands were partially deprotonated,
1 2
, and 1,973 cm for L , while the complex showed a very similar
−1
1
L
2
2
−
1
1
−
−1
ν
asym(COO ) would be expected at ~1,600 cm [22]. However, C=C and C=N stretching bands of the
−1
bipyridyl and phenanthrolyl ligands also appear near 1,600 cm making it difficult to determine if the
H-bpy and/or H-phen ligand is fully or only partially protonated on this basis. Aside from the small
−
variations in the νasym(COO ) bands, the IR spectra of the L
1
, L
2
ligands and [RuL
1
L
2
(NCS)
2
] complex
, L with
; 843,
are in very good agreement. In the fingerprint region, comparison of the infrared spectra of L
1
2
−1
that of the [RuL
1
L
2
(NCS)
2
] complex shows vibrational frequency bands at 757, 619 cm for L
1
−1
−1
7
34 cm for L
2
, and four bands of almost equal intensity at 843, 803, 765 and 728 cm for the
[
RuL (NCS) ] complex. These bands are associated with the mono-substitution pattern of the
1
L
2
2

Molecules 2011, 16
8357
anchoring trans-2-methyl-2-butenoic acid on the bipyridyl and phenanthrolyl ligands, and also
confirmed the mixed-ligand coordination of the [RuL L (NCS) ] complex.
1
2
2
1
13
2
.3. H and C-NMR Spectroscopic Studies
1
The aromatic region of the H-NMR spectrum of L gave six peaks at δ 8.66 (d, 1H), 8.41 (d, 1H),
1
1
7
.86 (dd, 1H), 7.36 (dd, 1H), 1.73 (s, CH
3
) and 1.66 (d, CH
3
) ppm. The H peaks are very similar to
those of the bromobipyridine starting material. The main difference is due to the appearance of the
1
3
methyl resonance in the aliphatic region of the spectrum. The C-NMR spectrum gave the anticipated
peaks at 169.76, 156.21, 149.98, 137.86, 136.88, 129.74, 124.81, 121.32, 14.81, and 12.71. The
bipyridine peaks, due to chemical equivalency, were observed in the 156–128 ppm range. The peak at
1
69.76 ppm was assigned to the carbonyl carbon; the two peaks at 124.81 and 121.32 were assigned to
the alkenyl carbons, while the trans-methyl group peaks were found at 14.81 and 12.71 ppm. The
1
H-NMR of L
.45 (s, 1H), and 7.35 (2d, 2H), that were assigned to the phenanthroline protons with indication of the
presence of a water molecule. The two signals in the aliphatic region at δ 1.68 (s, CH ) and 1.59
2
shows six signals in the aromatic region at δ 11.32 (br, OH), 8.98 (t, 2H), 7.92 (t, 2H),
7
3
1
3
(
d, CH
was similar to that of L
72.83 ppm. Other signals in the spectrum correspond to that of a phenanthroline and trans-2-methyl-
-butenoic acid moieties [16,17]. The proton NMR spectrum of the [RuL (NCS) ] complex in
3
) were assigned to the trans-methyl protons of the carboxylic acid group. The C spectrum of
L
2
1
, except for the downfield deshielding effect on the carbonyl carbon to
1
2
1
L
2
2
CDCl
3
(Figure 2) is consistent with the structure shown in Scheme 1.
1
Figure 2. H-NMR spectrum of [RuL
1
L
2
(NCS)
2
] in CDCl
3
, (δ) ppm.

Molecules 2011, 16
8358
Figure 2. Cont.
The presence of two different ancillary coordination ligands, such that all the protons are found in
electronically different environments, complicated both the aromatic and the aliphatic regions of the
spectrum, which showed ten distinct proton signals between δ 9.48–7.53 ppm integrating for fourteen
2
protons. By comparison with the L ligand, the phenanthroline protons were observed as doublets at
δ 9.25 and 9.48 ppm for H-2 and H-9 respectively. Two multiplet peaks at δ 8.01 and 7.53 ppm,
integrating for four protons, were assigned to H-3,8; and H-4,7 respectively, while the broad signal at
1
δ 8.08 ppm, integrating for one proton, was assigned to H-6. The unsubstituted arm of the L ligand
showed a doublet signal at 8.47 ppm, assigned to the H-3', H-4' protons, while the H-5' and H-6' proton
signals were found at 8.82 ppm. A doublet peak at δ 8.70 ppm was assigned to H-6, and H-5, H-3 were
assigned to a triplet at δ 8.19 ppm and a singlet at 7.98 ppm respectively. In the aliphatic region, four
broad singlet peaks at δ 2.20, 2.08, 1.29 and 0.92 ppm were unambiguously assigned to the
trans-methyl groups in the complex. In comparison to the individual ligands, the proton peaks in the
complex were shifted downfield. The deshielding pattern may be ascribed to the lone pair-lone pair
1
3
electron donation of the nitrogen to the d-orbital of the ruthenium metal. The C-NMR spectrum data
of the complex could not be adequately assigned to individual carbon atoms due to its complexity and
poor signal resolutions.

Molecules 2011, 16
8359
2
.4. Electronic Absorption and Emission Spectra
2
.4.1. Electronic Absorption Spectroscopy
The UV-Vis absorbance and emission spectra of complex [RuL L (NCS) ] were recorded at room
1
2
2
temperature in aerated DMF solution and are shown in Figures 3 and 4. The complex exhibits a very
broad and intense metal-to-ligand charge (MLCT) absorption band throughout the visible region of the
spectrum (400–700 nm), characteristic of many other ruthenium(II) polypyridyl complexes, and which can
II
be assigned to electronic transitions from the Ru based t g orbital to the ligand based π* orbitals. The
2
4
−1
−1
molar extinction coefficient of complex [RuL
1
L
2
(NCS)
2
] at its maximum (514 nm) is 6.97 × 10 M cm ,
−1
−1
4
−1
−1
precisely 48,950 and 21,552 M cm larger than that of [Ru(L
1
)
2
(NCS)
2
] (2.04 × 10 M cm ) at
4
−1
−1
5
08 nm and [Ru(L ) (NCS) ] (4.82 × 10 M cm ) at 446 nm, respectively. The mixed-ligand nature
2
2
2
of complex [RuL
1
L
2
(NCS)
and Ru→L
enhancing the oscillator strength in the blue and green portions of the spectrum relative to
Ru(L ) (NCS) ]. These features have been found as a means of improving light absorption
2
] unlike [Ru(L
2 2 2
) (NCS) ], produces multiple MLCT transitions in the
4
−1
−1
visible, i.e., Ru→L
1
2
with broad absorption at 352 nm (4.81 × 10 M cm ), thus
[
1
2
2
cross-sections at higher energy in similar complexes [23]. The intense absorption band in the UV
region around 280–300 nm (not shown) were assigned to the intraligand π→π* transitions of L and L
ligands. The lower-energy absorption in complex [RuL (NCS) ], was enhanced due to the presence
1
2
1
L
2
2
of the electron withdrawing nature of the carboxylic groups, which lowers the energy of the π* orbital
of the bipyridine and phenanthroline ligands, and a further stability of the complex provided by the
trans nature of the methyl groups.
Figure 3. Comparison of the UV-Vis absorption spectrum of [RuL
with [Ru(L (NCS) ] (dash line) and [Ru(L (NCS) ] (dot line) at a concentration of
× 10 M in DMF solution showing effect of extended π-conjugation and mixed-ligand
coordination.
1 2 2
L (NCS) ] (solid line)
1
−1
)
2
2
2
)
2
2
−3
1

Molecules 2011, 16
.4.2. Emission Study
The emission spectrum of complex [RuL L (NCS) ] compared with those of complexes
8360
2
1
2
2
1
[
Ru(L
1
)
2
(NCS)
2 2 2 2
] and [Ru(L ) (NCS) ] is displayed in Figure 4. Upon excitation into the LC and
1
MLCT bands, (λexc. = 500 nm), the complex displays appreciable luminescence at room temperature.
An emission wavelength maximum was found at 686 nm which is blue-shifted and of lower intensity
at 682 nm for complex [Ru(L (NCS) ], whereas a red-shift with enhanced intensity at 700 nm was
observed for complex [Ru(L ) (NCS) ]. It is well known that conjugated functional organic molecules
1
)
2
2
2
2
2
are useful for the study of electron transport at the molecular scale and that the use of fused-ring
systems is a powerful and practical approach [24,25].
Figure 4. Comparison of emission spectral of [RuL L (NCS) ] with [Ru(L ) (NCS) ] and
1
2
2
1 2
2
2 2 2
[Ru(L ) (NCS) ] in DMF solution.
The luminescent properties of a complex as well as its ability to play the role of excited state
reactant or product are related to the energy ordering of its low energy excited state and, particularly,
to the orbital nature of its lowest excited state. It could be seen in the complex that the choice of
ligands, has significant effects, which in turn influence the energy positions of the metal centre (MC)
and ligand centre (LC), as well as the metal-to-ligand transfer (MLCT). The energy of the MC excited
state depends on the ligand field strength, which in turn depends on the σ-donor and π-acceptor
properties of the ligands, the steric crowding around the metal (that can preclude a sufficiently close
approach between metal and ligand) [26-28], and the bite angle of the polydentate ligands (which in
some cases cannot be optimized because of molecular constraint) [29]. The energy of the LC excited
state depends on the intrinsic properties of the ligands, such as the HOMO-LUMO energy gap and the
singlet-triplet splitting. It has been shown that the energy of the MLCT excited state depends on the
reduction potential of the ligand involved in the MLCT transition, the oxidation potential of the metal
in the complex (which is affected by the electron donor and acceptor properties of all the ligands), and
by the charge separation caused by the transition. In the complex [RuL L (NCS) ], the intense
1
2
2

Molecules 2011, 16
8361
emission is a significant contribution to the excited state from an interaction between the metal
d-orbital and the ligand π-systems [30].
2
.5. Electrochemical Study
The cyclic and square wave voltammograms of the complex [RuL L (NCS) ] were examined in the
1
2
2
−1
potential range +1.5 to −1.5 V and at a scan rate 50 mV s using Ag|AgCl electrode in DMF solvent
with 0.1 M tetrabutylammonium hexafluorophosphate as supporting electrolyte (Figures 5 and 6).
−3
Figure 5. Cyclic voltammogram for [RuL L (NCS) ] complex at 1 × 10 M in freshly
1
2
2
distilled DMF containing 0.1 M TBABF
4
supporting electrolyte. Step potential = 5 mV,
−1
amplitude = 50 mV vs. Ag|AgCl, frequency = 10 Hz. Scan rate = 100 m Vs vs. Ag|AgCl.
−3
Figure 6. Square wave voltammogram for [RuL
distilled DMF containing 0.1 M TBABF supporting electrolyte. Step potential = 5 mV,
amplitude = 50 mV vs. Ag|AgCl, frequency = 10 Hz. Scan rate = 100 m Vs vs. Ag|AgCl.
1 2 2
L (NCS) ] complex at 1 × 10 M in freshly
4
−1

Molecules 2011, 16
8362
The voltammograms display the Ru(III)/Ru(II) couple at positive potential and ligand based
reduction couples at negative potentials. The quasi-reversible one-electron oxidation process IV at
E1/2 = +0.85 V, was assigned to the metal centre Ru(III)/Ru(II) wave couple [31]. Some unidentified
oxidation peak processes were observed, but were most pronounced in process III (E1/2 = +0.52 V)
which were tentatively ascribed to the ring oxidation of the bipyridine and/or phenanthroline ligands
and, as well as the carboxylate ions present in the complex. Two irreversible waves were observed at
−
0.92 and −0.70 V for processes I and II respectively. The more positive reduction potential may be
assigned to the contribution from L due to an additional (C=C) conjugative π-bond as found in the
2
structure of phenanthroline since both ligands contain the electron withdrawing COOH group being
responsible for lowering the LUMO levels.
3. Experimental
3.1. Materials and General Physical Measurements
All chemical and reagents were analytically pure and used without further purification. 4-Bromo-
,2'-bipyridine and 5-bromo-1,10-phenanthroline were synthesized as described in the literature [18].
2
4
-(trans-2-Methyl-2-butenoic acid)-2,2'-bipyridine (L ) and 5-(trans-2-methyl-2-butenoic acid)-1,10-
1
2
phenanthroline (L ) were synthesized following the literature procedure [15] with slight modifications
(
Scheme 1). All thin layer chromatography (TLC) analyses were done on aluminium sheets
precoated with normal phase silica gel 60 F254 (Merck, 0.20 mm thickness), unless otherwise stated.
The TLC plates were developed using any of the following solvent systems: Solvent system A:
dichloromethane-methanol (9:1); Solvent system B: dichloromethane-methanol (7:3); Solvent system
C: diethyl ether-methanol (1:1). Gel filtration was performed using Sephadex LH-20 previously
swollen in specified solvent (s) prior to loading of extract onto the column (3.5 cm × 8.5 cm).
Melting points were determined using a Gallenkamp electrothermal melting point apparatus.
Microanalyses (C, H, N, and S) were carried out with a Fisons elemental analyzer and infrared spectra
were obtained with KBr discs on a Perkin Elmer System 2000 FT-IR spectrophotometer. UV-Vis and
fluorescence spectra were recorded in a 1 cm path length quartz cell on a Perkin Elmer Lambda 35
1
13
spectrophotometer and Perkin Elmer Lambda 45 spectrofluorimeter, respectively. H- and C-NMR
1
13
spectra were run on a Bruker EMX spectrometer operating at 400 MHz for H and 100 MHz for C.
The chemical shift values were reported in parts per million (ppm) relative to (TMS) as internal
standard. Chemical shifts were reported for the ligands and complex with respect to CDCl
3
at δc 77.00
and δ CDCl at 7.26. Electrochemical experiment was performed using a PGSTAT 302 Autolab
H
3
potentiostat (EcoChemie, Utrecht, The Netherlands) driven by the general purpose Electrochemical
System data processing software (GPES, software version 4.9). A conventional three-electrode system
was used. The working electrode was a bare glassy carbon electrode (GCE), Ag|AgCl wire and
platinum wire were used as the pseudo reference and auxiliary electrodes, respectively. The potential
response of the Ag|AgCl pseudo-reference electrode was less than the Ag|AgCl (3 M KCl) by
0
.015 ± 0.003 V. Prior to use, the electrode surface was polished with alumina on a Buehler felt pad
and rinsed with excess millipore water. All electrochemical experiments were performed in freshly
distilled dry DMF containing TBABF as supporting electrolyte.
4

Molecules 2011, 16
.2. Synthesis of 4-(trans-2-Methyl-2-butenoic acid)-2,2'-bipyridine (L1)
8363
3
4
-Bromo-2,2'-bpy (1.05 g, 3.38 mmol) and trans-2-methyl-2-butenoic acid (0.34 g, 3.38 mmol)
were dissolved in MeOH (40 mL) in a 250 mL flask. Et N (1.0 mL and palladium-carbide (0.050 g)
3
were added and the mixture was reflux for 8 h at a temperature between 110−120 °C. The reaction was
allowed to cool to room temperature, and filtered, and the solvent removed under reduced pressure.
The residue was dissolved in degassed water and then extracted with chloroform. The chloroform
extract was concentrated in vacuo to give a brilliant colourless liquid which solidified after about 48 h
at room temperature. The resultant residue was recrystallized from Et O (30 mL). White crystalline
2
−1
solid; yield: 0.90 g (67%); melting point: ND; IR (KBr, νcm ): 3,054, 2,927, 2,676, 1,965, 1,690,
1
,648, 1,581, 1,559, 1,456, 1,419, 1,346, 1,251, 1,141, 1,089, 1,040, 992, 893, 757, 653, 631, 619, 555.
1
H-NMR (DMSO): δ 8.66 (d, J = 4.0 Hz, H-6, 6'), 8.41 (d, J = 8.0 Hz, H-3, 3'), 7.86 (dd, J = 7.6, 8.0 Hz,
13
H-5, 5'), 7.36 (dd, J = 5.2, 7.2 Hz, H-4'), 1.73 (s, CH
56.21, 149.98, 137.86, 136.88, 129.74, 124.81, 121.32, 14.81, 12.71. Elemental Analysis: Calculated
for C H N O : H 5.55, C 70.85, N 11.02; required H 5.55, C 70.60, N 11.43.
3 3
), 1.66 (d, CH ). C-NMR (DMSO): δ 169.76,
1
1
5
14
2
2
2
3.3. Synthesis of 5-(trans-2-Methyl-2-butenoic acid)-1,10-phenanthroline (L )
5-Bromo-1,10-phenanthroline (1.0 g, 3.86 mmol) and trans-2-methyl-2-butenoic acid (0.39 g,
3
3.86 mmol) were dissolved in MeOH (40 mL) in a 250 mL flask. Et N (1.0 mL) and palladium-carbide
(0.050 g) were added and the mixture was refluxed for 8 h at a temperature between 110−120 °C. The
reaction workup was as reported for L above. White-pink crystalline solid; yield: 0.74 g (53%); melting
1
−1
point: ND; IR (KBr, νcm ): 3,419, 3,032, 2,929, 1,694, 1,652, 1,619, 1,589, 1,561, 1,506, 1,420, 1,385,
1
1
,343, 1,256, 1,219, 1,140, 1,093, 1,080, 1,037, 1,015, 843, 766, 734, 769, 625, 530. H-NMR
(
CDCl
3
): δ 11.33 (br, OH), 8.98 (t, J = 4.0 Hz, 2H, H-2, 9), 7.92 (d, J = 8.0 Hz, 2H, H-4, 7), 7.45 (s, 1H,
1
3
H-6), 7.35 (2d, J = 4.4 Hz, 2H, H-3,8), 1.68 (s, 3H, CH
3
), 1.59 (d, J = 6.8 Hz, 3H, CH
): δ 172.83, 150.39, 150.30, 146.30, 146.21, 138.79, 136.13, 128.81, 128.72, 126.68, 126.59,
23.27, 123.19, 14.74, 12.14. Elemental Analysis: Calculated for C H N O : H 5.07, C 73.37, N
3
). C-NMR
(CDCl
3
1
1
1
7
14
2
2
0.07; required H 4.98, C 73.64, N 10.23.
3
.4. Synthesis of cis-Dithiocyanato-4-(trans-2-methyl-2-butenoic acid)-2,2'-bipyridyl-5-(trans-2-
(NCS)
methyl-2-butenoic acid)-1,10-phenanthrolyl-ruthenium(II) complex [RuL
1
L
2
2
]
In a 250 mL flask, [RuCl
40 mL) followed by the addition of ligands L
mixture was refluxed initially at 120 °C for 2 h in the dark and an excess of NH
2
(dmso)
4
] (0.44 g, 0.92 mmol) was dissolved in N,N-dimethylformamide
(0.23 g, 0.92 mmol) and L (0.26 g, 0.92 mmol). The
NCS (0.70 g, 10.40 mol)
(
1
2
4
was then added. The reaction was left to reflux for an additional 10 h. The solution was allowed to cool
to room temperature and filtered to remove unreacted starting material. The filtrate was concentrated to
dryness and 0.05 M NaOH solution (40 mL) was added to give a dirty brown precipitate which was
3
filtered off. The pH of the resulting solution was adjusted to 3 with 0.5 M HNO . The solution was left
to stand in the fridge (−2 °C) for 12 h before being filtered and concentrated in vacuo. The crude
residue was adsorbed onto Sephadex LH-20 adsorbent in a glass column and eluted using solvent
system C (250 mL). Dark brown solid; melting point: >250 °C; yield: 0.37 g (22%); IR (KBr)

Molecules 2011, 16
8364
−1
νmax/cm : 3,048, 2,918, 2,850, 2,102, 1,969, 1,745, 1,600, 1,585, 1,539, 1,514, 1,463, 1,442, 1,425,
4
−1
−1
1
,342, 1,309, 1,263, 1,153, 1,091, 1,019, 843, 803, 765, 728. UV-Vis (λmax/nm, ε = ×10 M cm ,
1
DMF): 352 (3.55), 514 (6.97). Emission wavelength: (λexc. = 500 nm, λem = 686 nm). H-NMR (CDCl
3
):
δ 9.48 (d, J = 4 Hz, 1H, H-9), 9.25 (d, J = 4 Hz, 1H, H-2), 8.82 (d, J = 4 Hz, 2H, H-5',6' ), 8.70 (d,
bpy
J = 8.0 Hz, 1H, H-6bpy), 8.47 (d, J = 8.0 Hz, 2H, H-3',4'bpy), 8.19 (d, J = 8.0 Hz, 1H, H-5bpy), 8.08 (s,
1
H, H-6phen), 8.01 (d, 2H, H-3,8phen), 7.98 (s, 1H, H-3bpy), 7.53 (m, 2H, H-4,7phen), 2.20 (s, 2H), 2.08 (s,
13
2+
3+
3H), 1.29 (s, 3H), 0.92 (s, 3H). C-NMR (CDCl ): ND. Cyclic voltammetry Data: Ru /Ru = +0.85 V;
3
Eanodic = +0.53; Ecathodic = −0.70, −0.92 V. Elemental Analysis: Calculated for RuC34
28 6 4 2
H N O S : H 3.76,
C 54.46, N 11.21, S 8.55; required H 3.87, C 54.79, N 11.56, S 8.44.
3
.5. Synthesis of cis-Dithiocyanato-bis-4-(trans-2-methyl-2-butenoic acid)-2,2'-bipyridyl)-
ruthenium(II) complex [Ru(L ) (NCS) ]
1
2
2
In a 250 mL flask, [RuCl
2 4
(dmso) ] (0.25 g, 0.51 mmol) was dissolved in N,N-dimethylformamide
(
40 mL) followed by the addition of ligand L (0.30 g, 1.02 mmol). The mixture was refluxed initially
1
4
at 120 °C for 2 h in the dark and then an excess of NH NCS (0.70 g, 10.40 mol) was added and the
same procedure as described above was followed. Dark brown solid; melting point: >250 °C, yield:
−
1
0
.07 g (51%); IR (KBr) νmax/cm : 3,550, 3,479, 3,237, 2,923, 2,851, 2,106, 1,638, 1,617, 1,460, 1,443,
,420, 1,309, 1,265, 1,243, 1,155, 1,084, 1,022, 800, 760, 728, 620, 474, 421. UV-Vis (λmax/nm,
1
4
−1
−1
ε = ×10 M cm , DMF): 346 (2.52), 497 (2.06), 665 (1.09), 740 (0.73). Emission wavelength:
1
(
λ
exc. = 500 nm, λem = 682 nm). H-NMR (CDCl
3
): δ 8.93 (br, d, 1H), 8.89 (br, s, 2H), 8.50 (d, 2H),
8
.11 (dt, 3H), 8.12 (s, 1H), 8.04 (d, 1H), 7.97 (d, 1H), 7.66 (br, s, 2H), 7.53 (br, s, 1H), 2.17 (br, s, 3H),
1
3
and 1.25 (s, 9H). C-NMR (CDCl
3
): ND. Elemental Analysis: Calculated for RuC32
28 6 4 2
H N O S : H
3.89, C 52.96, N 11.58, S 8.83; required H 4.12., C 52.57, N 11.55, S 8.69.
3
.6. Synthesis of cis-Dithiocyanato-bis-5-(trans-2-methyl-2-butenoic acid)-1,10-phenanthrolyl-
(NCS)
ruthenium(II) complex [Ru(L
2
)
2
2
]
In a 250 mL flask, [RuCl (dmso) ] (0.44 g, 0.90 mmol) was dissolved in N,N-dimethylformamide
2
4
(
40 mL) followed by the addition of ligand L
at 120 °C for 2 h in the dark and then an excess of NH
same procedure as described above was followed. Orange solid; melting point: 201–204 °C; yield:
2
(0.50 g, 1.79 mmol). The mixture was refluxed initially
4
NCS (0.27 g, 3.59 mol) was added and the
−1
0
1
4
.38 g (38%); IR (KBr) νmax/cm : 3,423, 3,058, 2,926, 2,856, 2,086, 2,058, 1,978, 1,631, 1,427, 1,384,
4
−1
−1
,221, 1,205, 1,147, 1,056, 879, 844, 773, 721, 621, 559. UV-Vis (λmax/nm, ε = ×10 M cm , DMF):
1
16 (4.49), 446 (4.82). Emission wavelength: (λexc. = 500 nm, λ = 700 nm). H-NMR (CDCl ): δ 8.77
em
3
(
d, J = 8.0 Hz, 4H), 8.38 (s, 2H), 8.08 (d, J = 4.8 Hz, 4H), 7.76 (dd, J = 5.2, 8.0 Hz, 4H), 3.40 (s, 6H).
1
3
Selected aromatic C-NMR data (CDCl
3
): 153.62, 148.13, 137.69, 131.33, 128.92 and 127.17
Elemental Analysis: Calculated for RuC H N O S : H 3.65, C 55.88, N 10.86, S 8.29; required H
3
6
28
6
4 2
3.32, C 55.95, N 10.51, S 8.59.

Molecules 2011, 16
. Conclusions
8365
4
In conclusion, an easy to synthesize mixed-ligand heteroleptic ruthenium(II) complex
RuL L (NCS) ] with broad spectral bandwidth and visible light absorption properties was designed,
[
1
2
2
synthesized and its photophysical and electrochemical properties investigated. It was shown that a
synergy effect brought about by the individual ligands has great influence on the photophysical and the
electrochemical properties of the complex, which are better than those of the individual single-ligand
homoleptic complexes: [Ru(L ) (NCS) ] and [Ru(L ) (NCS) ]. Particularly for this molecule, further
1
2
2
2 2
2
work to establish the solar-to-electrical energy conversion efficiency (σ) in its dye-sensitized solar
cells (DSSCs) is ongoing in our laboratory. However, in terms other characteristics of ruthenium(II)
polypyridine complexes, the high redox potentials, broad visible absorption and high luminescence
intensity render the new complex has great potential for use in biomedical applications, involving
electrochemical or photochemical detection methods, and/or therapeutic applications. In this context,
the carboxylic acid group allows for further functionalization as required to attach the complexes to
various substrates and biomolecules.
Acknowledgments
This work was financially supported by the Govan Mbeki Research Centre and University of Fort
Hare, Alice, South Africa. The author gratefully thanks A.I. Akinbulu, for the running of the
electrochemical experiment and P.A. Ajibade for the provision of some laboratory reagents. The
author also acknowledges Obafemi Awolowo University, Ile-Ife, Nigeria.
Conflict of Interest
The author declares no conflict of interest.
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Sample Availability: Samples of the compounds L
1
, L and [RuL (NCS) ] are available from the author.
2
1
L
2
2
©
2011 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
http://creativecommons.org/licenses/by/3.0/).
(