Synthesis of a ruthenium(II) bipyridyl complex coordinated by a functionalized Schiff base ligand: Characterization, spectroscopic and isothermal titration calorimetry measurements of M2+ binding and sensing (M2+ = Ca2+, M

Article abstract of DOI:10.1016/j.saa.2009.01.015

Bis-[methylsalicylidine-4′benzoic acid]-ethylene (LH₂) complexed with cis-Ru(bpy)₂Cl₂·2H₂O provides a complex of composition [Ru(bpy)₂L]·2NH₄PF₆ (1), which has been characterized

Full text of DOI:10.1016/j.saa.2009.01.015

Spectrochimica Acta Part A 73 (2009) 29–34
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis of a ruthenium(II) bipyridyl complex coordinated by a functionalized
Schiff base ligand: Characterization, spectroscopic and isothermal titration
calorimetry measurements of M2+ binding and sensing (M2+ = Ca2+, Mg2+)
Namrata Dixita, Lallan Mishraa,∗, Sourajit M. Mustafib, Kandala V.R. Charyb, Hirohiko Houjouc
a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi- 221 005, India
b Department of Chemical Sciences, Tata Institute of Fundamental Research, Mumbai- 400 005, India
c Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
a r t i c l e i n f o
a b s t r a c t
Bis-[methylsalicylidine-4ꢀbenzoic acid]-ethylene (LH2) complexed with cis-Ru(bpy)2Cl2·2H2O provides
a complex of composition [Ru(bpy)2L]·2NH4PF6 (1), which has been characterized spectroscopically. Its
binding behaviour towards Mg2+ and Ca2+ ions is monitored using 1H NMR titration, isothermal titration
calorimetry (ITC) and luminescence microscopy. The luminescent ruthenium complex binds Ca2+ in a
Article history:
Received 1 October 2008
Received in revised form 17 December 2008
Accepted 14 January 2009
more selective manner as compared to Mg2+
.
Keywords:
Schiff base
© 2009 Elsevier B.V. All rights reserved.
Ruthenomacrocycle
Binding studies
Isothermal titration calorimetry
Luminescence
1. Introduction
importance. Additionally, the selection of aldehydes has been made
in view of its role in macrocyclisation [16] where as p-aminobenzoic
acid is selected owing to its biological relevance besides its bifunc-
tional nature [17].
Supramolecular construction through non-covalent interactions
like van der Waals force, hydrogen bonding, CH–␲ interactions,
␲-stacking and hydrophobic effects for the study of molecular
recognition have become the focal point of studies in the con-
temporary chemistry, biology and physiology [1–3]. The design
and synthesis of artificial receptors that can achieve recognition
at molecular level are demanding [4,5]. Polydentate ligands, such
as Schiff bases, assisted by metal ions, provide highly organized
supramolecular metal complexes. Such complexes possess binding
sites and cavities for various cations, anions and organic molecules
2. Experimental
2.1. Materials and methods
Ruthenium(III)chloride, 2,2ꢀ-bipyridine and p-aminobenzoic
acid were purchased from Sigma–Aldrich, while solvents were pur-
chased from E. Merck and used without any further purification.
The aldehyde was obtained as a gift sample from Dr. H. Hojou, IIS,
Tokyo University, Japan. Elemental analysis and FAB mass measure-
ments were carried on Carbo-Erba elemental analyzer 1108, JEOL
SX-102 mass spectrometer respectively. IR spectra were recorded
as KBr pellets on a JASCO FT-IR 5300 spectrometer whereas 1H
NMR spectra were recorded on a JEOL AL 300 MHZ spectrometer
using DMSO-d6 as solvents and TMS as internal references. How-
ever, 1H NMR spectra with the metal titration were recorded on
an 800 MHz spectrometer. UV/visible and luminescence measure-
ments are made in the range 200–700 nm on a Shimadzu UV-1601
spectrometer and Perkin Elmer LS-45 Luminescence spectrometer
respectively. Time-resolved fluorescence measurements are carried
out by time-correlated single-photon counting (TCSPC) method. A
CW Nd: Vanadate (Millennia, Spectra Physics, USA) pumped mode-
2+
[6–10]. In this context, Ru(bpy)2 containing systems have been
interesting owing to their involvement in efficient intramolecu-
lar electron or energy transfer processes [11]. The choice of such a
building block is justified by its favorable excited states, redox prop-
erties as well as chemical properties. The advantage of molecular
fluorescence for sensing and switching on/off is well documented
in the literature [12–15]. In this backdrop, we set out to append a
Ru(bpy)22+ moiety to a Schiff base ligand (LH2), as the luminescence
property of such a system could then be exploited to monitor the
binding of cations such as Mg2+ and Ca2+, which are of biological
∗
Corresponding author. Tel.: +91 542 2307321; fax: +91 542 2368174.
E-mail address: lmishrabhu@yahoo.co.in (L. Mishra).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.01.015

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N. Dixit et al. / Spectrochimica Acta Part A 73 (2009) 29–34
Scheme 1.
DMSO, Me4Si); 10.9 (2H, s, –OH); 9.6b,w(N–H from ammonium ion);
9.2 (2H, s, –N CH); 10.0 (2H, bpy); 8.7 (8H, m, Ar + bpy); 8.1 (11H,
m, Ar + bpy); 7.2 (7H, m, Ar); 6.5 (2H, m, Ar); 4.2 (2H, CH2); 3.0
(4H, –CH2–). 13C NMR: ıC (300 MHz, DMSO, Me4Si); 189.59, 187.50
(–C O); 168.37, 167.43 (CH N); 158.71, 158.40 (C–OH); 157.48,
157.26 153.25 (bpy); 152.88 (–CH–C*); 149.74, 149.51, 137.36, 137.12,
136.46 (bpy); 135.5 (N–C); 135.02 (COOH–C*); 135.0 (N–C *CH);
133.52 (COOH–C *CH); 131.88, 127.46, 126.25, 125.74, 125.27,
123.56, 123.276 and 119.559 (Ar); 113.757 (*C CH); 110.897 ( CH);
34.225 (–CH2–). b = broad, w = weak and * for C atom assigned; mass
(m/z)+413 [Ru(bpy)2]2+, 534 [L], 947 [Ru(bpy)2L]. Elemental analy-
sis (%): Calculated for [Ru(bpy)2L]·2NH4PF6, C 49.0, H 3.9 and N
8.8; Found C 48.9, H 3.7 and N 8.4. UV absorptions: ꢁmax (ace-
tonitrile)/nm (ε/dm3 mol−1 cm−1) 298 (38,500), 323 (11,200), 489
(9600), conductance: ꢂM (DMSO, 10−3 M) 133 (ꢃ−1 cm2 mole−1).
Emission: ꢁmax (acetonitrile)/nm (ε/dm3 mol−1 cm−1) 600.08 nm
(9.3 a.u.).
locked Ti-Sapphire laser with doubled tunable output was used for
exciting the samples at 456 nm. Isothermal titration calorimetry
(ITC) measurement was carried out using MicroCal VP-ITC titration
micro-calorimeter at 25 ◦C.
2.2. Preparation of bis-[methylsalicylidine-4ꢀbenzoic
acid]-ethylene (LH2)
Bis-[methyl salicyaldehyde]-ethylene (0.536 g, 1 mmol) was dis-
solved in methanol and added drop-wise to a methanolic solution
of p-aminobenzoic acid (0.272 g, 2 mmol) with stirring. Stirring was
continued for 2 h, resulting in formation of a yellow precipitate.
This precipitate was filtered off and washed several times with
methanol followed by washing with ether and dried in vacuo. Yield:
90%, M.P. > 250 ◦C, IR absorptions: ꢀmax/cm−1 3076 (C CH); 2934
(–CH2); 1680 (> O); 1599 (CH N); 1192 (–OH); 1577 and 1452
(C C, Ar). 1H NMR: ıH (300 MHz, DMSO, Me4Si); 13.37 (2H, s, –OH);
11.11 (2H, s, –COOH); 9.01 (2H, s, –N CH); 8.01 (4H, m, Ar); 7.52
and 7.35 (8H, m, Ar); 6.97 (2H, m, Ar); 4.72 (2H, s, CH2); 2.5 (4H,
s, –CH2–). 13C NMR: ıC (300 MHz, DMSO, Me4Si) 166.833 (–COO);
165.77 (CH N); 158.77 (C–OH); 151.43 (–CH–*C); 134.52 (N–C);
131.65 (COOH–*C); 131.20 (N–C *CH), 128.92 (COOH–C *CH);
121.54, 118.85 and 118.51 (Ar); 112.55 (*C CH); 112.33 ( CH);
34.97 (–CH2–) *assigned C atom, mass (m/z)+ 535 (molecular ion
peak), elemental analysis (%): calculated for C32H26N2O6, C 71.9,
H 4.8 and N 5.2; Found C 69.5, H 4.9 and N 4.7:UV absorptions:
ꢁmax(DMSO)/nm (ε/dm3 mol−1 cm−1) 280 (27,750), 298 (29,580),
403 (17,780).
3. Results and discussion
3.1. Characterization of ligand and complex
Complex 1 is found thermally stable and soluble in DMSO. Its
composition has been assigned on the basis of its elemental anal-
ysis and spectral data. The repeated purification then elemental
analyses measurements showed that complex contains two moles
of NH4PF6. It also emphasizes that ligand is getting de-protonated
during its complexation with metal ion. Though the possibility of
isomeric complexes does exist but their separation could not be
made at this stage of the study even using repeated column chro-
matography of the Complex 1.
2.3. Preparation of RuII Complex 1
LH2 (0.536 g, 1 mmol) was dissolved in DMF and triethylamine-
(∼2 mmol) was added to it with stirring followed by drop-wise
addition of a solution of Ru(bpy)2Cl2·2H2O (0.520 g, 1 mmol) in
DMF. The reaction mixture was then refluxed on a sand bath for 26 h
under N2 atmosphere. After cooling, an aqueous solution of NH4PF6
was added slowly to it and the resulting solution was cooled in a
refrigerator for 24 h. A dark colored solid appeared, which was then
filtered and washed several times with water and dried in vacuo.
The complex was further purified on alumina column using acetoni-
trile as eluent (Scheme 1). Yield: 40%, M. P. > 250 ◦C, IR absorptions:
ꢀmax/cm−1 3444b (–OH + N–H from NH4+); 2925 (–CH2); 1602
(CH N); 840.32 (PF6−); 762 (pyridyl). 1H NMR: ıH (300 MHz,
IR spectrum of the free ligand shows major peaks at 1687 and
1599 cm−1 assigned to ꢀ COOH and ꢀCH vibrations respectively.
N
These peaks shift to 1711 and 1602 cm−1 upon complexation. The
significant change observed in case of carboxylate group vibra-
tion supports its coordination with the metal ion. Another peak
observed at 841 cm−1 is assigned to ꢀ PF6 vibrations.
−
The above observations have further been supported by 1H and
13C NMR spectra of the metal complex as compared to correspond-
ing NMR spectra of free ligand. The peak observed at ı 13.3 ppm in
the spectrum of free ligand is assigned to OH protons and is con-
sidered intramolecularly H– bonded with imine N in view of earlier
report [16]. This peak shifts at ı 10.9 ppm in the spectrum of the

N. Dixit et al. / Spectrochimica Acta Part A 73 (2009) 29–34
31
Fig. 2. UV/vis spectrum of Complex 1 in acetonitrile (1 × 10−3 M).
and significant decrease in emission intensity is observed upon
addition of Ca2+ as compared to Mg2+. This observation is fur-
ther supported by the fluorescence micrographs taken using a
NIKON-ECLIPSE TS 100-F fluorescence microscope with and with-
out Ca2+ and Mg2+ (Fig. 4). These studies now prompted us
to go for deeper investigation on the binding properties of the
Complex 1.
Fig. 1. Proposed structure of Complex 1 with 1H-NMR assignment.
complex supporting that un-protonated OH groups are involved in
the formation of metal complex. This observation could indirectly
be considered to support that ligand is getting de-protonated from
its carboxyl groups. It is further corroborated by disappearance of
COOH protons peak of free ligand (observed at a ı of 11.1 ppm) in
the spectrum the metal Complex 1. Additional peaks observed in
the region ı 7.5–8.8 ppm are assigned to phenyl protons overlapped
with bpy protons. However, single peak observed at ı 10.1 ppm is
assigned to bpy protons only. In 13C NMR spectrum of free ligand,
peaks observed at ı 166.83 (–COO carbon) and 165.77 (CH N cabon)
ppm shift at 189.59 and 168.37 ppm respectively in the spectrum
of the Complex 1. Larger deshielding observed for COO groups also
support their coordination with the metal ion.
Absorption spectrum of the Complex 1 (Fig. 1) shows major
peaks at ꢁmax 489 nm assigned to MLCT transition in addition to
the intra-ligand peak observed at ꢁmax 323 nm. The complex emits
at ꢁmax 600 nm upon excitation at wavelength ꢁmax 489 nm (MLCT
transition from Ru(II) to bipyridine). The values of absorption and
emission maxima are in good agreement with earlier reports [18].
Thus, in view of these spectral studies along with mass and elemen-
tal analysis data, the tentative structure proposed for the complex
is shown in Fig. 2.
High resolution 1H NMR titration of the metal complex in the
presence of different amount of calcium chloride using 800 MHz
spectrometer was recorded. Upon addition of Ca2+ to the complex,
changes in the 1H NMR spectrum were noticed, which indicated
that the binding of Ca2+ to the complex occurs. As depicted in Fig. 5,
the resonance at ı 10.9 ppm (assigned for –OH proton) and at ı
9.2 ppm (assigned for imine proton) are affected by successive addi-
tions of Ca2+ 1H NMR spectra also show changes in the position
.
of phenyl (Ph) and bipyridyl (bpy) protons. Thus addition of Ca2+
affects OH, CH, Ph and bpy protons (Fig. 6), showing probable sites
for Ca2+ binding in these regions. However, as depicted in Fig. 5,
stoichiometry of the Complex 1 to Ca2+ turns out to be 1:1 only
from NMR titration data since further addition of calcium ions does
not cause any significant change in these groups. However, from the
cavity size ∼3.8 Å (calculated tentatively using reported parameters
of bond lengths and covalent radii of individual atoms) it could be
thought that more than one Ca2+ could be bound inside the cavity.
3.2. Binding studies
Preliminary investigation thus suggested that the complex for-
mation provides a metallomacrocycles which could be exploited as
receptor for guest binding studies specially with biologically rele-
vant cations like Mg2+ and Ca2+. The selection of this type of ligand
framework has been made in view of the structure of standard
compound known as receptor and bear COOH group [19].
Therefore, the study of cation binding behaviour of the complex
is initially started by the observation of changes in the UV/vis spec-
tral pattern of the Complex 1 upon addition of same amount of both
Ca2+ and Mg2+. Fig. 3 shows that more hypochromic shift in the peak
at ꢁmax 489 nm occurs upon addition of Ca2+ as compared to Mg2+
.
Fig. 3. Overlay UV/vis spectrum of Complex 1 with Ca2+ and Mg2+ in acetonitrile
Steady state fluorescence measurement also shows that the
Complex 1 emits at ꢁmax 600.08 nm upon excitation at ꢁmax 489 nm
(1 × 10−3 M), (a) Complex 1, (b) Complex 1 + Mg2+ and (c) Complex 1 + Ca2+
.

32
N. Dixit et al. / Spectrochimica Acta Part A 73 (2009) 29–34
Fig. 4. Overlay fluorescence spectrum of 1 in acetonitrile (1 × 10−3 M), with Ca2+ and Mg2+, (b) fluorescence micrograph of 1, (c) fluorescence micrograph of 1 + Mg2+ and (d)
fluorescence micrograph of 1 + Ca2+
.
Thus, NMR titration in this study was found ambiguous. Hence it
was thought to go for more sophisticated technique.
of binding (ꢄH) and the stoichiometry of binding (n). The heat
of dilution was determined in parallel control experiments by
injecting 10 mM guest ions in DMSO placed in calorimeter cell.
The heat of dilution was subtracted from the corresponding
cation binding experiment before fitting the data. The ITC data
of the complex with both Mg2+ and Ca2+ as titrants are shown
in Fig. 7. Each peak in the binding isotherm represents a single
injection of cation. The negative deflections from the baseline
upon addition of cation indicate that the reaction is exothermic.
The enthalpy change associated with each injection is plotted
against the cation molar ratio (lower panel of Fig. 6), and the ꢄH,
ꢄS and Ka (association constant) were determined from the plots.
The titration behaviour with Mg2+ indicates finite heat changes in
Therefore, we planned to use ITC [20] technique for such binding
studies. ITC measurement was carried out using MicroCal VP-ITC
titration micro-calorimeter at 25 ◦C. The solution of the Complex 1
(1 × 10−3 M in DMSO) was taken in a 1.43 ml calorimeter cell and
the guest ion (Mg2+ and Ca2+) solution (10 mM taken in DMSO)
was added sequentially in 2 ␮l aliquots, for a total of 128 injec-
tions, with an interval of 2 min. The ITC data were analyzed using
the software ORIGIN (supplied with Omega Microcalorimeter).
The heat of reaction per injection (micro-calories per second)
was determined by the integration of peak areas using Origin
software. This software provides the best-fit values for the heat
Fig. 5. 1H NMR spectrum of complex showing changes in the OH and CH N proton(s) resonance upon successive addition of Ca2+ to the Complex 1, recorded on a 800-MHz
NMR spectrometer.

N. Dixit et al. / Spectrochimica Acta Part A 73 (2009) 29–34
33
Fig. 8. Modelled space fill diagram of Ca2+ bound complex showing two overlapping
aromatic rings in cavity.
Fig. 6. 1H NMR spectrum of Complex 1 showing changes in bipyridyl and aromatic
protons upon successive addition of Ca2+, recorded on a 800-MHz NMR spectrome-
ter.
Since single crystal of the Complex 1 could not be obtained,
hence, the complex structure containing Ca2+ is modelled using
space filling diagram [22] and is shown in Fig. 8. It could be seen
from this figure that two aromatic rings are getting overlapped
together which might restrict the incorporation of more Ca2+ as
expected from calculated cavity size (∼3.8 Å). Again it could be con-
sidered in this context that smaller ionic radii of Mg2+ (0.65 Å) as
compared to ionic radii of Ca2+ (0.99 Å), might lead the selective
the system. However, the corresponding data could not be fitted
to any specific binding model. On the other hand, the titration
with Ca2+ could be fitted with a set of two sequential binding
sites. The associated thermodynamic parameters thus obtained
are ꢄH1 = −533.8 kJ mol−1, ꢄS1 = 13.9 kJ mol−1 K−1, K1 (first asso-
ciation constant) = 2.64 × 103 M−1 and ꢄH2 = −1318 kJ mol−1
,
binding of Ca2+ in comparison to Mg2+
.
ꢄS2 = 4.46 kJ mol−1 K−1 and K2 (second association con-
Time-resolved fluorescence measurements have also been car-
ried out by time-correlated single-photon counting method. A CW
Nd: Vanadate (Millennia, Spectra Physics, USA) pumped mode-
locked Ti-Sapphire laser with doubled tunable output was used
for exciting the samples at 456 nm. The decay in fluorescence
gives similar fluorescence lifetime for both the solutions i.e. with
and without Ca2+. This behaviour confirmed weak binding of
Ca2+ as the system is showing dynamic quenching [23]. Dynamic
quenching is observed when the guest does not bring out any
configurational change in the system, however when guest reacts
with the system and changes its configuration, static quenching is
observed.
stant) = 87.2 M−1
. These data clearly shows differential metal
binding characteristics upon titration with Ca2+ and Mg2+. It is
interesting here to note that the binding of Ca2+ though weak
as compared to that observed in EF-hand Ca2+ binding proteins
(K ∼ 104–106 M−1) [21], it is still substantially higher than observed
during Mg2+ binding, as shown in the ITC data. Thus, the Mg2+
binding to the Complex 1 can be considered insignificant. Further,
the Complex 1 could be considered to serve as a good carrier of
Ca2+ in comparison with Mg2+
.
Therefore, it is inferred with this experiment that Ca2+ binding
indeed fits to two sequential binding sites but they are not equiva-
lent. The one with smaller value of binding constant (K2 = 87.2 M−1
)
Hence, it supports that Ca2+ is weakly bound to the metal com-
plex without bringing any configurational change in metal complex.
The time-resolved fluorescence data fitted with three successive
perhaps not translated into observable changes in the 1H NMR spec-
trum is also observed in this experiment.
Fig. 7. Titration curves of the Complex 1 with (a) Ca2+ and (b) Mg2+ showing the calorimetric response upon successive injections of guest ions, which are added to the reaction
cell. Panel B depicts the binding isotherm of the calorimetric titration shown in panel A. The continuous line represents the least square-fit of the data to a two-sequential
binding-sites model.

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N. Dixit et al. / Spectrochimica Acta Part A 73 (2009) 29–34
decay profiles indicated a lifetime of 4.5 ns for the complex and
no observable changes were seen in the fluorescence data upon
addition of Ca2+ (4.2 ns). Such observation further substantiates the
weak binding of Ca2+ to the metal complex.
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Acknowledgements
We thank DAE-BRNS (Grant No. 2005/37/33/BRNS/1807 to L. M.),
Mumbai, India for Financial support and CDRI, Lucknow, India for
analytical data. The facilities provided by the National Facility for
High Field NMR, supported by Department of Science and Technol-
ogy (DST), Department of Biotechnology (DBT), Council of Scientific
and Industrial Research (CSIR), and Tata Institute of Fundamental
Research, Mumbai, India, are gratefully acknowledged. SMM is a
recipient of the TIFR Alumni Association Scholarship (2003–2005)
for career development. K. Chandra for his help in recording NMR
data.
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