Complexation studies of N′-[(1E)-1-phenylethylidene]isonicotinohydrazide: An aroylhydrazone Schiff base and lanthanides

Article abstract of DOI:10.1016/j.molliq.2016.11.098

The interaction of N′-[(1E)-1-phenylethylidene]isonicotinohydrazide (PHeH) with trivalent Pr, Nd, Gd, Tb, and Ho ions was studied with spectroscopic and potentiometric techniques. Photoluminescence studies of Tb^3?+ and PHeH in ethanol and in so

Full text of DOI:10.1016/j.molliq.2016.11.098

Journal of Molecular Liquids 227 (2017) 127–138
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Journal of Molecular Liquids
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Complexation studies of N′-[(1E)-1-phenylethylidene]isonicotinohydrazide:
An aroylhydrazone Schiff base and lanthanides
⁎
Yuimi Varam , Lonibala Rajkumari
Department of Chemistry, Manipur University, Canchipur 795003, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2016
Received in revised form 23 November 2016
Accepted 24 November 2016
Available online 02 December 2016
The interaction of N′-[(1E)-1-phenylethylidene]isonicotinohydrazide (PHeH) with trivalent Pr, Nd, Gd, Tb, and
Ho ions was studied with spectroscopic and potentiometric techniques. Photoluminescence studies of Tb3+
and PHeH in ethanol and in solid Tb3+-PHeH complex reveal that PHeH has no sensitizing effect on Tb3+ lumi-
nescence. The absorption spectral bands of PHeH under different conditions and that of Nd3+- PHeH at different
molar ratios were investigated. Potentiometric titrations of 1:4 Ln/PHeH molar ratio carried out at different temper-
ature at a constant ionic strength of I = 0.1 mol·dm−3 KNO3 in aqueous-dioxane medium reveal the moderate ther-
modynamic stability constants of the complex formation equilibria which follow the order: Pr b Nd b Gd b Tb b Ho.
The thermodynamic parameters (ΔG, ΔH, and ΔS) of protonation and complexation reactions are all negative sug-
gesting the spontaneous, exothermic, of unfavorable entropy and enthalpy-driven reactions. Actual synthesis and
characterization of Tb3+-PHeH complex, confirmed the veracity of the complex formation equilibria. Finally, the in-
formation accrued from the studies is used in the structural elucidation of the complexes.
Keywords:
N′-[(1E)-1-
phenylethylidene]isonicotinohydrazide
Lanthanides
Potentiometric
Thermodynamic parameters
Absorption
Photoluminescence
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
the pyridine nitrogen, the azomethine nitrogen and the carbonyl oxy-
gen [9], complexes of N-(2-propionic acid)-salicyloyl hydrazone [10],
Lanthanides and their complexes, due to the arrays of applications
and potential uses have not only aroused immense interest in chemistry
but on all branches of sciences. Due to their photoluminescence in the
visible and near-infrared region under UV excitation, lanthanide organic
complexes are of both fundamental and technological interests. A vast
treasure of knowledge on their complexes and applications are available
in the literature. Among the diverse classes of ligands which constitute
the Ln3+ complexes, N and O donor molecule like hydrazones and its
analogue constitute an important class of ligand [1,2]. Known as
privileged ligands [3], the Schiff bases, especially aroylhydrazones
have not only simple synthetic process but also interesting spectral
and biological activities [4].
In comparison to arrays of literature available with respects to
aroylhydrazones transition metal complexes derived from INH or isoniazid,
fewer aroylhydrazones lanthanide complexes such as complexes of 2.6-
diacetylpyridine-2-thenoyl isonicotinoyl dihydrazone [5], homo binuclear
complexes of N,N′-diisonicotinoyl-2-hydroxy-5-methylisophthalaldehyde
dihydrazone [6], nine-coordinate helix like structured complex of 1-phe-
nyl-3-methyl-5-hydroxy-4-pyrazolyl PH ketone isonicotinoyl hydrazone
(H2O) and its interaction with DNA [7], Ln-complexes of hexadentate
2.6-diacetylpyridine-2-thenoylpicolynoyl dihydrazone [8], complexes
of 2-pyridinecarboxaldehyde benzoyl-hydrazone coordinated through
complexes of 2-carboxybenzaldehyde isonicotinoylhydrazone and it
DNA-binding properties [11],complexes of 7-methoxychrom-one-3-
carbaldehyde-isonicotinoyl hydrazone and their interactions with calf-
thymus DNA [12], tetranuclear Ln3+-complexes in a highly unusual see-
saw geometry of 2-methoxy-6-(pyridine-2-ylhydrazonomethyl)phenol
[13], a series of 2.6-pyridinedicarboxaldhydebis(p-hydroxyphenylimine)
coordinated equatorially with six-coordinate octahedral Ln3+ ions
orthogonally oriented [14], nine-coordinated La3+ complexes of a
Schiff-base afforded from 1-phenyl-3-methyl-4-formyl-2-pyrazolin-
5-one and isoniazid [15], and a series of Ln4 clusters with 2.6-
(picolinoylhydrazone)pyridine with overall metal core of each clus-
ter comprising of two edge-sharing triangular Ln3 units linked by μ3-
OH bridges [16] are some of the more notable reported complexes.
The knowledge of Ln3+ and Schiff bases interactions, their stability
constants and the associated thermodynamics [17–20] would therefore
be of paramount importance in pharmaceuticals industry and in under-
standing the complex biological reactions like transamination, racemi-
zation and decarboxylation [2]. The complexation process, in all the
Ln3+ complexes generally involve displacement of protons (H+) of
the free chelating ligands by metal ions (Ln3+) where Ln3+ ions being
hard acids inevitably and preferably bind hard bases, like charged li-
gands or neutral O and N donors generally coordinated with eight to
nine donor atoms when available forming complexes that are kinetical-
ly and thermodynamically more stable. However, the solution equilib-
ria, the determination of the stability constants [21–23] and the
⁎
Corresponding author.
E-mail address: Yuivah@gmail.com (Y. Varam).
http://dx.doi.org/10.1016/j.molliq.2016.11.098
0167-7322/© 2016 Elsevier B.V. All rights reserved.

128
Y. Varam, L. Rajkumari / Journal of Molecular Liquids 227 (2017) 127–138
associated thermodynamics of the complexation reactions of lantha-
nides with Schiff bases are limitedly reported.
2.3. Potentiometric titration
The coordination behavior of the Schiff bases afforded by condensa-
tion of acid hydrazides with hetero aromatic aldehydes or ketones with
the transition and lanthanides metals have been studied [24–26]
through spectroscopic and pH metric methods in our laboratory. In
this work, we have undertaken a series of studies on N′-[(1E)-1-
phenylethylidene]isonicotinohydrazide, PHeH in solution. The spectral
properties of the ligand and its interaction with selected lanthanides in-
vestigated. The lanthanides were so selected representatively two each
from lighter and heavier lanthanides with Gd in the middle so as to ar-
rive at comprehensive generalisation of complexation with the ligands
for all the Ln3+ series. Nd3+ absorption help probe the coordination en-
vironment while Tb3+ is convenient for photoluminescence studies of
the metal complexes. The thermodynamics stability constants of the
ligand was evaluated and used for determination of the complex for-
mation equilibria with selected lanthanides and the chemical ther-
modynamics involved were evaluated. To confirm the veracity of
the complex formation equilibria, a terbium complex of PHeH is iso-
lated and characterized and the thermogram investigated. Finally,
from the information accrued through the spectral, analytical and
formation equilibria studies, the structure of the complexes is
proposed.
The Potentiometric titrations of PHeH with the Ln 3+ions Pr, Nd, Gd,
Tb, and Ho were carried out at a constant ionic strength I = 0.1 M KNO3
solution at three different temperatures 293.15 K, 303.15 K and
313.15 K in 40% aqueous-dioxane medium. The solutions titrated were
as follows.
0
1
0
1
A
−3
‐3
‐3
Ç
Ç
Ç
@
A
@
Solution ðaÞ
Solution ðbÞ
Solution ðcÞ
:
:
:
5 mL KNO3 0:10 moldm
þ 2:5 mL HNO3 110 moldm
0
1
‐3
‐4
Ç
Ç
@
A
Solution ðaÞ þ 4:69 mL PHeH 1 ꢀ 210 moldm
0
1
‐3
Ç
Ç
Solution ðbÞþx mL Lnþ3 310‐‐4 moldm
@
A
where x mL of Ln3+ taken is of the required strength from the stocks so-
lutions. The titrations were done at 1:4 Ln/ PHeH molar ratio to ensure
coordinative saturation of the metals during complexation reactions.
The titrations were repeated twice for each set.
2.4. UV–Visible absorption studies
For the UV–vis spectra of PHeH as a function of pH, 25 mL PHeH
2. Materials and methods
(8·10−5 mol·dm−3
) was titrated with freshly prepared KOH
(5·10−2 mol·dm−3) solution at variable pH in aqueous-dioxane
medium. The preparations of solutions follow the same procedures as
in potentiometric titrations. After each aliquots addition of KOH, the
pH was allowed to stabilize and 4 mL taken in a cuvette to record the
spectra. For absorption spectra of PHeH and Nd3+ complex,
1·10−3 mol·dm−3 Nd3+ (constant) and PHeH (variable) prepared in
absolute ethanol were taken for spectral recordings.
2.1. Physical measurements
The C, H and N of PHeH and Tb3+-PHeH were micro analyzed using
CHNS (O) Analyzer Model: FLASH EA 1112 series made by Thermo
finnigan, Italy at IIT Bombay. Chlorides ions and Tb3+ were estimated
as in our earlier work [25]. To estimate hydrazine content, volumetric ti-
trations of PHeH and the complex were done, after subjecting the com-
pounds to acid hydrolysis for 4 h. Shimadzu FTIR-8400 was used for
recording the IR spectra in KBr medium. The FAB mass spectrum was
obtained on a JEOLSX 102/Da-.6000 mass spectrometer. The 1H and
13C NMR spectra were recorded on a JEOL AL 300 FT NMR spectrometer
in DMSO. A Perkin Elmer LS 55 Fluorescence Spectrophotometer in
phosphorescence mode was used for room temperature luminescent
2.5. Luminescence studies
The luminescent spectra of TbCl3.6H2O and solid Tb3+-PHeH com-
plex were recorded. Solution studies of luminescence were done as
outlined earlier [25].
studies of Tb3+ and PHeH. Conductivity Measurement of the Tb3+
-
3. Calculations
PHeH complex was done with CON 510, Conductivity/TDS/°C/F meter,
Eutech Instruments. Magnetic Measurement was carried out with Mag-
netic Susceptibility Balance, Sherwood Scientific Ltd., UK. The thermal
analysis of the complex was done with Perkin Elmer, STA 6000 Simulta-
neous Thermal Analyzer purged with Nitrogen gas, heated at the rate of
10 °C/min. All the UV–Vis spectra were recorded on a Perkin–Elmer
Lambda 25 spectrophotometer keeping the slit width at 2 and scan
speed of 120, for all the measurements using glass cell of 1 cm path
length, equipped with sample holders, maintained at constant temper-
atures 303.15 K. Cyberscan pH 1100 model measures the pH of the solu-
tions after calibration using SOP. An Equibath Refrigerated Circulating
Bath, from Medica Instrument Mfg.co was used as thermostat during
potentiometric titrations
3.1. Proton ligand and metal ligand formation constants
The protonation constant of PHeH and the formation constants of
Ln3+-PHeH complexes (where Ln3+ = Pr3+, Nd3+, Gd3+, Tb3+ and
Ho3+) were calculated from the titration curves using Bjerrum's half in-
tegral method [27] and as modified by Irving and Rossotti [28,29]. The
average number of protons associated with HpEH (nH) at various pH
meter readings was determined from the acid and ligand titration
curves using Eq. (1)
ꢀ
ꢁ
ðVL−VAÞ N þ E0
nH ¼ Y−
ð1Þ
ðV0 þ VAÞT0L
2.2. Materials and solutions
where Y is the number of ionisable protons present in PHeH, VL and VA
are the volumes of KOH (5·10−2 mol·dm−3) consumed by solutions
(a) and (b), respectively, for the same pH reading, and (VL − VA) mea-
sures the difference in position between the ligand curve and the acid
curve. V0 is the initial volume of the reaction mixtures (25 mL), and E0
and T0L are the concentrations of nitric acid (1·10−3 mol·dm−3) and
PHeH (variables) in the reaction mixtures, respectively.
Isonicotinic acid hydrazide, acetophenone, KOH, KNO3 and 1, 4 Diox-
ane were obtained from E. Merck, Mumbai, and LnCl3.6H2O (Ln = Pr,
Nd, Gd, Tb, Ho) from Sigma-Aldrich, USA. All the chemicals used are of
A.R. grade. Solutions containing water were prepared with double dis-
tilled water. KOH solution standardized with standard oxalic acid solu-
tion (5·10−2 mol·dm−3), was used for standardization of HNO3. Since
the ligand is not soluble in water, stock solution (5·10−3 mol·dm−3
)
The average number of PHeH attached per Ln3+ ions (n), and the free
ligand exponent, pL, were calculated from the experimental titration
was prepared in 40% aqueous–dioxane mixture.

Y. Varam, L. Rajkumari / Journal of Molecular Liquids 227 (2017) 127–138
129
Fig. 1. Mass spectrum of N′-[(1E)-1-phenylethylidene]isonicotinohydrazide, PHeH.
curves using Eqs. (2) and (3).
refluxed for approximately 4 h. The precipitates obtained on cooling is
filtered, washed with ethanol and recrystallized from ethanol. Molecu-
lar Formula = C14H13N3O, Formula Weight = 240, % yield, 60%; mp,
178 °C. M+ peak in the mass spectrum (Fig. 1), m/z 240.
Elemental analysis of ligand: Found % (cald % for C14H13N3O) C 70.25,
(70.28%); H 5.33 (5.48%); N 17.32, (17.56%); N2H4 13.37, (13.35).
The ligand may exist in different tautomeric form I and II as shown in
Scheme A and may undergo E/Z isomerization as reported for similar li-
gand [31].
ꢀ
ꢁ
ðVM−VLÞ N þ E0
n ¼
ð2Þ
ð3Þ
ðV0 þ VAÞT0MnH
And pL, the free ligand exponent by using
ꢂ
ꢃ
2
3
n
X
1
n¼j βnH
n¼0
6
6
4
7
7
5
V0 þ VM
anti logpH
TL0−nT0M
pL ¼ log10
ꢁ
:
V0
4.2. Crystallography
VM is the volume of KOH added to metal solution at the same pH reading
as that of VA and βnH is the overall protonation constant of PHeH, and TM0
is the concentration of Ln3+ in the metal solutions. Other symbols have
their usual significance as in eq. (1). The metal ligand formation curves
were obtained by plotting n Vs pL for different temperatures. The pL
values at n = 0.5, 1.5, 2.5 etc. correspond to the first log K1(Ln), second
log K2(Ln), third log K3(Ln) formation/stability constants respectively.
The calculations were done by a personal computer using Microsoft
Excel.
In N′-[(1E)-1-phenylethylidene]isonicotinohydrazide, [32] the mol-
ecules are linked through hydrogen bonding of the amido nitrogen
into chains in which C-HO interactions stabilize the packing. The dihe-
dral angle between the pyridine ring and phenyl ring is noted as 12.21
(2)°
4.3. IR (υ, cm−1
)
The C _O stretching, amide I band of PHeH is observed at
1689 cm−1 while the amide II band of δ NH+ υ N\\C_ O at
1599 cm−1. The stretching bands of NC_ N, NN\\Nb, NH, are observed
at 1649, 995 and 3195 cm−1 respectively while the aromatic C\\H
stretch are observable at 3041, 3005 (prominent) cm−1. The υas (CH3)
and υs (CH3) at 2914 and 2837 cm−1, aromatic C\\N stretch at1548, ar-
omatic C\\C stretch at 1498, in plane and out-of-plane ring C\\C bend
of pyridine ring at 653 and 420 cm−1 respectively. The presence of a
band at 1278 cm−1 is attributed to stretching of OH in the tautomeric
form II.
3.2. Oscillator strength
The oscillator strength of the complexation reactions evaluated [30]
from the integrated band areas of Nd3+ hypersensitive peak, 4I9/2 to
4G5/2, 2G7/2 transition by using the following equations.
Pe ¼ 4:3198 ꢁ 10−9∫εðvÞdv
ð4Þ
where ε is the molar absorptivity in the energy υ (cm−1). The integral
have been approximated by the equation
∫εðvÞdv ¼ S=½NdðIIIÞꢂc
ð5Þ
H3C
where S is the area of the hypersensitive peak in unit of absorbance
cm−1, c is the path length of the cell in centimetres.
H3C
N N
N
HN N
4. Results and discussion
OH
N
O
4.1. Synthesis and characterization of the ligand
Form II
Form I
To the ethanolic solution of isonicotinic acid hydrazide (2.74 g in
40 mL), acetophenone (2 mL) was added and the reaction mixtures
Scheme A. Possible tautomeric form of PHeH.

130
Y. Varam, L. Rajkumari / Journal of Molecular Liquids 227 (2017) 127–138
pH 3.54
5
4
nitrogen [33–35] at low pH resulting in the fading of п–п* and n–п* tran-
Decreasing pH
sitions of carbonyl group. At higher near neutral pH, with deproton-
ation, the reconstruction of the aforesaid t ransitions will render PHeH
neutral. The azomethine containing hydrazones reportedly exhibit
keto-enol tautomerism [31] and the amido proton deprotonate at still
higher pH. The variation of pH has little effect on the pyridine and
phenyl ring structure and hence the least perturbation on the absorp-
tion band at 253 and 275 nm. An isosbestic point observed at
around 284 nm will illustrates [36–38] the single deprotonation con-
stant, logkH1(PHeH) (Section 4.7.2.) at high pH which is in accord with
the existence of a dissociable proton in PHeH.
3
2
A
Isosbestic point (284 nm)
pH 12
Increasing pH
1
240
260 280
300
320
340
360
380
400
In ethanol, the qualitative absorption spectra of PHeH (Fig. 3) shows
absorption band at 255 nm with a shoulder at 272 nm and a weak inten-
sity band at 330 nm when recorded at 303.15 K. These bands may be at-
tributed to the п–п* transitions of benzene ring, pyridine ring or
azomethine and n–п* transition of the carbonyl group respectively
[31,39]. On adddition of 2·10−3 mol·dm−3 Nd3+ consecutively, the ab-
sorption band of free PHeH at 330 nm shows bathochromic shift to
334 nm while the peak at 255 nm and 272 nm remain unaffected. This
suggest participation of C _O group but non participation of pyridyl
and phenyl ring in the complexation reactions. Even as the metal ligand
molar ratio reaches 1:1, the change in the absorption spectra remain in-
significant despite the slight shift of absorption peak at 330 nm. This
would imply that PHeH absorbs energy from the same chromophoric
groups and retains it structural identity in the complex [40]. The slight
variation in the absorption spectra of PHeH seen under different sol-
vents and different conditions may be attributed to solvent effects
such as existence of differently solvated species, replacement of metal
hydration sphere by solvent molecules, inner and outer complex forma-
tion etc.
Wavelength (nm)
Fig. 2. Absorption spectra of PHeH as a function of pH recorded under same condition as in
potentiometric titrations. Purple, pH 12.00; blue, pH 10.02; brown, pH 9.00; red, pH 7.00;
green, pH 5.20; light green, pH 4.00 and pink, pH 3.54. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this article.)
4.4. 13C NMR (ppm) proton off-resonance decoupling spectra
The singlet signals at 169.04 and 162.47 are assigned to the carbon of
CO and NC. The singlet at 156.99, doublet at 150–149.42 and 142.54–
141.14 are assigned to C4, C 2 and C 3 of the pyridine ring respectively
while the singlet at 137.85, doublet at 126.55 and 122.99, multiplet at
129.72, are assigned to C1′, C 2′, C 3′ and C4′ of the phenyl ring respec-
tively. The doublet at 14.82 is assigned to the carbon of methyl group.
4.5. 1H NMR (δ)
The singlet at 11.227 and 11.03 are attributed to tautomeric form II
and form I corresponding to OH and NH proton which look like a dou-
blet in the spectrum. The singlet at 8.766 is attributed to C 2 and doublet
at 7.871 to C 3 of the pyridine ring protons. The multiplet at 7.817–7.356
is attributed to C2′, C3′ and C′4 of the phenyl ring protons while the
multiplet at 2.339 to methyl protons.
All the characteristic f-f Nd3+ absorption bands are vividly seen in
the spectra in absolute ethanol (Fig. 4). The calculated oscillator
strength [30] of the hypersensitive bands [41,42] of Nd3+, Pe =
33.48 × 10−6 (value from this study) which being a polar coordinating
solvent is different from its oscillator strength in aqueous solution (7.7–
8.60 × 10−6) and its complexes. The oscillator strength increases as the
concentration of PHeH increases as given in Table 1. These finding sup-
port the ‘pseudoquodrupole’ mechanism of intensification likely due to
the more asymmetric nature of Nd3+ ions in ethanol and it complex
with PHeH than in aqueous solution [43–45].
4.6. Spectral studies of PHeH and its Ln3+ complexes
4.6.1. UV–Visible absorption spectra of PHeH and Nd3+
In aqueous – dioxane media, the absorption spectra of PHeH obtain-
ed in the pH range of 3.5–12 are shown in Fig. 2. The structural changes
in the ligand noted through Uv-vis absorption bands is correlated as in
Scheme 1. At low pH 3.54, PHeH shows absorption peak at
253 nm and a weak shoulder at 275 nm. The band at 328 nm shows
intensification at higher pH but vanish at lower pH while the absorption
band at 253 and 275 nm remain least affected by the variation in pH.
These observations may suggest the protonation of azomethine
Upon qualitative inspection of the hypersensitive absorption bands
of Nd3+ for all the Nd3+/PHeH molar ratios, the shapes remain the
same. This suggests the same coordination number for NdCl3.6H2O
and its complex with PHeH in absolute ethanol. By applying similar
concept and analogy as done in other lanthanide complexes [46,47],
the absorption spectra of Nd3+–PHeH complex qualitatively suggest
ten coordination numbers for Nd3+
.
H3C
HN NH
N
O
H3C
N
O
H3C
N N
O
HN
H
H
HO
N
N
CH3
N
HN
Neutral PHeH,
pH= 5.5-8
Deprtonated
PHeH, pH>8
N
Protonated
pH< 5.5
Scheme 1. PHeH structures as a function of pH.

Y. Varam, L. Rajkumari / Journal of Molecular Liquids 227 (2017) 127–138
131
1.6
1.2
0.8
Table 1
255 nm
Oscillator strength (Pe × 10−6) of 1·10−3 mol·dm−3 Nd3+ hypersensitive transitions 4I9/2
to 4G5/2 2G7/2 and PHeH complex in absolute ethanol.
,
272 nm
Dark yellow (Top) - PHeH
Pe × 10−6
3+
Blue (Bottom) - Nd + PHeH
Species
Area under the peak λmax
Oscillator strength
Nd
0.0775
0.0865
0.0955
0.1167
33.48
37.37
41.25
50.41
330 nm
A
Nd-PHeH (1:1)
Nd-PHeH(1:2)
Nd-PHeH(1:3)
0.4
0.0
260
280
300
320
340
360
380
400 420
quench out when Tb3+/PHeH reach 1:3 molar ratio. Interactions or
complexation reaction is therefore apparent. All the characteristic terbi-
um emissions are visible though of lesser intensity to that of solid
TbCl3.6H2O in solid Tb3+-PHeH complex as shown in Fig. 6 (II). The
quenching of the Tb3+ luminescence is evident in solution as well as
in solid.
Wavelength (nm)
Fig. 3. Experimental absorption spectra of 8·10−3 mol·dm−3 PHeH in ethanol on addition
of 2·10−3 mol·dm−3 NdCl3·6H2O.
Same excitation spectra are observed in solid TbCl3.6H2O and Tb3+
-
PHeH complex, at 270 nm with the excitation spectrum a little red
shifted in the complex as shown in Fig. 6 (I). While in ethanol,
TbCl3.6H2O gives the same excitation spectra at 270 nm which consider-
ably changed on addition of PHeH that look more like intensified Tb3+
absorption bands but slightly red shifted (Fig. 5A*). This imply the inef-
ficiency of the photo physical pathways or energy transfer from PHeH to
terbium [47,48].
4.6.2. Photoluminescence studies of Tb3+ and PHeH at different molar
ratios
For Ln3+ complexes to be highly luminescent, coordinative satura-
tion of the Ln3+ ions by encapsulating ligands that shield them from
the solvents to avoid radiationless deactivation; high thermodynamic
and kinetic inertness of the complex in solution; ability to overcome
the weak f-f electronic absorption of Ln3+ through chromophoric
groups which efficiently absorb energy and pass on to their emitting
state; efficient ISC with appropriate energy gap to avoid back energy
transfer etc. are some of the conditions needed [47,48]. Tb3+ generally
gives stable, water-soluble and luminescent complexes with long life-
time, large Stokes' shift, narrow emission band in the visible and near
infra-red region largely unaffected by the environment around it.
The photoluminescence of Tb3+-PHeH in both solid and solution
were investigated. The absorbance points in PHeH are theoretically lo-
cated at the appropriate energy, above 27,027 cm−1 to transfer energy
to the excited state of terbium [47,48] at 20400 cm−1. On excitation at
220 nm (metal centered luminescence) in ethanol, Tb3+ emissions cor-
responding to 5D4-7F6, 5D4-7F5, 5D4-7F4 and 5D4-7F3 transitions [47–49]
were observed but become less visible on excitation at 255, 270 and
330 nm (not shown in the figure) in the ligand centered luminescence,
as shown in Fig. 5(A), (B) and (C) respectively. No emission band of
PHeH is observable in the complex. The presence of PHeH however pro-
nouncedly affects Tb3+ emissions. Only a remnant low intensity 5D4-7F6
emission band at 485–500 nm remains while the other bands vanished.
As the concentration of PHeH increases, the intensity of the band,
though observed at the same wavelength decrease and consequently
The quenching of Tb3+ emissions encountered in the complex is at-
tributed to the presence of coordinated and lattice water molecules, OH,
NH and CH oscillators of the solvents and the ligand which provide ef-
fective non-radiative deactivation routes to the 5D4 excited states of ter-
bium [50,51]. Quenching being directly proportional to their presence in
the coordination environment, the emissive levels deactivate through
vibronic coupling [52,53]. Another probable mechanism in ethanol is
through photo induced electron transfer (PET) facilitated by site selec-
tive hydrogen bonding [54]. However, this route though common in
Eu3+ complex is unlikely in terbium complex due to the high reduction
potential of Tb3+ (Ered = −3.5 V) [36] rendering it difficult to form
redox-pair for electron transfer. Furthermore, the inability of PHeH to
encapsulate all the coordination sites of terbium renders some sites to
be occupied by other species in solution. Besides, the energy transfer
from PHeH could have been more efficient had the absorbing species,
pyridine and phenyl ring not separated by many bonds [55] with the lu-
minescent Tb3+ during coordination. It will be noteworthy, that in all
terbium complexes where pyridine ring is coordinated to it, the lumi-
nescence yield is enhanced greatly [56–59]. Therefore, it is speculated
and suggested that pyridine ring of PHeH do not participate in
coordination.
9
4
4
2
I9/2
to
G5/2
,
G7/2
hypersensitive band
7
5
3
A
1
500
540
580
620
660
700
740
780
820
860
900
Wavelength (nm)
Fig. 4. Absorption spectra of 1·10−3 mol·dm−3 Nd3+ (constant) and PHeH (variable conc.) in ethanol at different Nd3+/ PHeH molar ratio at 303.15 K. Red line, NdCl3 6H2O; green line,
1:1; Pink line, 1:2 and blue line, 1:3 Nd3+/ PHeH molar ratio. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

132
Y. Varam, L. Rajkumari / Journal of Molecular Liquids 227 (2017) 127–138
5
7
D - F
40
4
5
i
(1:0.0)
5
7
D - F
ii (1:0.5)
iii (1:1.0)
iv (1:1.5)
4
6
30
20
10
0
v
(1:2.0)
vi (1:2.5)
vii (1:3.0)
5D4-7F4
5D4-7F3
(A)
400
450
500
550
600
650
Wavelength (nm)
14
12
10
8
14
i
(1:0.0)
i
(1:0.0)
12
10
8
ii (1:0.5)
iii (1:1.0)
iv (1:1.5)
ii (1:0.5)
iii (1:1.0)
iv (1:1.5)
5D4-7F6
v
(1:2.0)
5D4-7F6
v
(1:2.0)
vi (1:2.5)
vii (1:3.0)
vi (1:2.5)
vii (1:3.0)
5D4-7F5
6
6
5D4-7F5
4
4
5D4-7F4
2
2
5D4-7F3
5D4-7F4
5D4-7F3
0
0
(B)
(C)
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 5. A*Excitation spectra (λem = 545 nm) of Tb3+ (constant) and PHeH (variables) at different Tb3+/PHeH molar ratios in ethanol: (a) TbCl3.6H2O; (b) 1:1; (c) 1:2 and (d) 1:3. Emission
spectra of Tb3+ and PHeH at different Tb3+/PHeH molar ratios monitored at (A) 220 nm (metal centered luminescence) (B) 255 nm (ligand centered) and C) 270 nm (ligand centered
luminescence).
4.7.
Potentiometric
studies
on
N′-[(1E)-1-phenylethylidene]
For all the titrations, the curves start at about 3.5 and extend to about
11.5 on the pH scale. At the initial stage, the acid curve lies below the li-
gand and the metal curves due to the more acidic nature. The ligand is
protonated in this acidic media and as the alkali is consecutively
added, the acid curve and the ligand curve intersect and diverge at
5.9–6.5 pH ranges signifying the liberation of proton from the protonat-
ed ligand. The ligand curves also show one inflection points correspond-
ing to the deprotonation in the ligand. The lowering of pH for the metal
isonicotinohydrazide, PHeH and it lanthanides complexes
4.7.1. Titration curves
Three sets of curves representing the titration curves namely acid
curve (a), ligand curve (b) and the complex curves (c-g) obtained for
the titrations of Ln3+ (Ln = Pr, Nd, Gd, Tb, Ho) and PHeH at 303.15 K
is shown in Fig. 7. Titration at other temperature gives similar curves.
180
140
5
7
D4- F5
1000
800
600
400
100
60
5
7
D4- F6
(I)
(II)
5
7
D4- F4
200
20
5
7
D4- F3
0
260
300
340
380 420
220
400
440
480
520
560
600
640
Wavelength (nm)
Wavelength (nm)
Fig. 6. (I) Excitation spectra (λem = 545 nm) of Tb3+ and Tb3+-PHeH in solid: red line, TbCl3.6H2O; green line, solid Tb3+-PHeH complex. (II) Emission spectra of solid Tb3+ pink line
monitored at 255 nm) and Tb3+-PHeH monitored at 255 (light green), 220 nm (red line) and 330 nm (blue line). (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)

Y. Varam, L. Rajkumari / Journal of Molecular Liquids 227 (2017) 127–138
133
Table 2
Protonation constant logk1H(PHeH) of PHeH at a constant ionic strength of
12
10
8
I = 0.10 mol·dm−3 KNO3 at different temperatures.
Temp (K)
Protonation constant, logk1H(PHeH)
293.15
303.15
313.15
11.37
10.66
10.54 0.01
0.01
0.01
(a)
(b)
(c)
(d)
(e)
(f)
HNO
3
PHeH
Pr
6
Nd
attached to the Ln3+ ions are inferred from the metal ligand formation
curves given in Fig. 8 (b). The curves for different Ln3+ at 303.15 K and
similar curves at other temperatures were obtained by plotting
the values of n against pL. The stepwise formation constants and the
Gd
Tb
4
(g)
Ho
overall formation constants were evaluated [27–29] from the pL values
Ln
at n = 0.5, 1.5 and 2.5 which correspond to the first logK
second
1(PHeH)
2
Ln
Ln
logK
and third logK
formation constants respectively along
0.0
0.5
1.0
1.5
2.0
2.5
2(PHeH)
3(PHeH)
with the overall formation constant logβLPnHeH given in Table 3. The values
of n (0.05 bnN 3.3) on the n scale indicate that three complexes of ML1,
ML2 and ML3 metal–ligand stoichiometries have been formed. The
formation constants at a particular temperature follow the order:
Pr3+ b Nd3+ b Gd3+ b Tb3+ b Ho3+. The calculated formation constant
of ML1, ML2, ML3 and MLn show trend that is a function of the size of
lanthanide cations. This is evident in the plot of logβLPnHeH vs. 1/r where
formation constants increase with increase atomic number and de-
crease ionic radii (Fig. SI 1). This is a typical trend [61,62] in which the
interaction between Ln3+ and the ligand is mainly electrostatic. The
strength of this interaction increases as the charge density on the lan-
thanide cation increases, thereby increasing LogβLPnHeH values from Pr3+
to Ho3+. The positive values of the difference between the first and
the second and the second and the third stepwise formation constants
indicate that coordination of the first PHeH to the Ln3+ is more favor-
able than the second and the third PHeH. This is obviously due to the
generated steric hindrance of the previous PHeH on the incoming
PHeH. The formation constants also increase with decreasing tempera-
ture implying that complexation is more favorable at lower tempera-
ture [60].
5.10-2mol.dm-3 KOH/cm-3
Fig. 7. Titration curves of Ln3+- PHeH complexes at 303.15 K in 1:4 metal ligand molar
ratios at
a constant ionic strength at I = 5 mL
0·10 mol·dm−3 KNO3. (a)
0·10 mol·dm−3 KNO3 + 2.5 mL 1·10−3 mol·dm−3 HNO3; (b) Solution (a) + 4.69 mL
1.2·10−3 mol·dm−3 PHeH; (c) Solution (b) + 6.25 mL 3·10−4 mol·dm−3 Pr3+; (d)
Solution (b) + 3.26 mL 3·10−4 mol·dm−3 Nd3+ ; (e) Solution (b) + 6.8 mL
3·10−4 mol·dm−3 Gd3+; (f) Solution (b) + 3.72 mL 3·10−4 mol·dm−3 Tb3+; and (g)
Solution (b) + 1.67 mL 3·10−3 mol·dm−3 Ho3+
.
curves relative to ligand curve is attributed to the deprotonation of the
ligand on formation of metal ligand bonding [27]. The metal curves cor-
responding to the respective metals show inflections due to the forma-
tion of various complex species in equilibrium.
4.7.2. Proton- ligand and metal ligand formation constants
The proton-ligand formation curves, obtained by plotting nH vs pH,
extend from 0.03 to about 1.2 on thenH scale give the number of proton
dissociated at different temperatures 293.15, 303.15 and 313.15 K as
shown in Fig. 8(a). These curves show that one proton dissociate from
PHeH in equilibrium in the system. From the curves, proton- ligand for-
mation or protonation constants logkH1(PHeH) at different temperatures
were evaluated [27–29] which are given in Table 2. The constants are
temperature dependent with higher values at lower temperature
which indicates that the process is favored at lower temperature [60].
The addition of PHeH to the Ln3+ ions takes place in a stepwise pro-
cess. The best evidence of complexation and the number of ligand
4.7.3. Thermodynamics of the protonation and complexation of PHeH and
lanthanides
The thermodynamic parameters ΔG, ΔH and ΔS associated with the
protonation of PHeH and its complexation with Ln3+ were calculated
using the same equations outlined earlier [24,25] which are given in
Table 4. Free energy (ΔG) decrease at higher temperature and increase
1.2
1.0
3.5
3.0
2.5
0.8
Pr
293.15 K
2.0
Nd
0.6
Gd
1.5
Tb
Ho
0.4
1.0
303.15 K
0.5
0.2
313.15 K
(a)
(b)
0.0
0.0
5
6
7
8
9
10
11
12
4
5
6
7
8
9
pH
pL
Fig. 8. (a) Proton-ligand formation curves of PHeH at different temperature. (b) Metal-ligand formation curves of Ln3+-PHeH complexes at 303.15 K at a constant ionic strength
of I = 0·10 mol·dm−3 KNO3.

134
Y. Varam, L. Rajkumari / Journal of Molecular Liquids 227 (2017) 127–138
Table 3
and overall formation constant logβPLnHeH of Ln3+-PHeH complexes at I = 0.10 mol·dm−3 KNO3 at different
Ln
3(PHeH)
Ln
1(PHeH)
Ln
2(PHeH)
Stepwise metal-ligand formation constants, logK
temperatures in aqueous-dioxane media.
, logK
logK
Ln3+
Temp.
(K)
Formation constants
Pr
Nd
Gd
Tb
Ho
Ln
293.15
logK
logK
logK
6.79
5.87
4.77
17.43
6.56
5.73
3.94
16.23
6.13
5.06
3.51
14.70
0.06
0.06
0.06
6.96
5.97
5.07
18.06
6.71
5.88
4.17
16.76
6.24
5.35
3.86
15.45
0.06
0.06
0.06
7.14
6.62
5.74
19.50
6.95
6.61
5.72
19.28
6.77
6.10
4.97
17.84
0.04
0.04
0.04
7.52
7.11
6.65
21.28
7.24
6.82
6.28
20.34
6.82
6.22
5.55
18.59
0.10
0.10
0.10
7.58
7.19
6.77
21.54
7.27
6.89
6.39
20.55
6.92
6.39
5.78
19.09
0.11
0.11
0.11
1(PHeH)
Ln
2(PHeH)
Ln
3(PHeH)
Ln
logβ
0.06
0.06
0.04
0.10
0.11
PHeH
Ln
303.15
313.15
logK
logK
logK
0.05
0.05
0.05
0.05
0.05
0.05
0.07
0.07
0.07
0.18
0.18
0.18
0.21
0.21
0.21
1(PHeH)
Ln
2(PHeH)
Ln
3(PHeH)
Ln
logβ
0.05
0.05
0.07
0.18
0.21
PHeH
Ln
logK
logK
logK
0.08
0.08
0.08
0.07
0.07
0.07
0.19
0.19
0.19
0.18
0.18
0.18
0.18
0.18
0.18
1(PHeH)
Ln
2(PHeH)
Ln
3(PHeH)
Ln
PHeH
logβ
0.08
0.07
0.19
0.18
0.18
Values are the standard deviations.
as the ionic radii of Ln3+ decrease while enthalpy of complex formation
(ΔH) obtained from graphical representation of Van’t Hoff equation
shown in Fig. SI 2 increase from ΔH1 to ΔH3. The overall enthalpy re-
mains more or less the same for all the lanthanide complexes. Entropy
(ΔS) follows similar trend as the enthalpy trends. The unusual low en-
thalpy and higher entropy of Gd3+ w. r. t. other Ln3+ complexes
under study may be related to the zero field stabilization energy, un-
availability of the well-shielded 4f electrons for bond formation [63].
The negative values of all the thermodynamic parameters imply that
the reactions are spontaneous, exothermic and of unfavorable entropy.
The unfavorable change of entropy (negative ΔS) may be due to struc-
tural change in the hydration sphere of lanthanide ions [64,65] and sol-
vation of metal chelates [66,67] in the presence of dioxane which being
an aprotic solvent is feasible. A small contribution to the negative entro-
py may also come from the loss of rotational entropy of NN\\Nband
N\\C bond linkages of PHeH after complexation. Apparently the driving
force of the complexation reactions in Ln3+ and PHeH is the favorable
enthalpy of formation.
be moisture or solvent impurities. From the TGA curves, the presence
of seven water molecules is estimated (Found, 11.36%, calculated
10.93%) of which three is assigned within and four outside the coordina-
tion sphere. The depression in the DTA curve at 98 °C and 108 °C are
attributed to the loss of these water molecules. Although the decompo-
sition of specific molecules or ions cannot be estimated due to the con-
tinuous weight loss from 203 to 790 °C and beyond, we conclude that
the weight loss seen from 203 °C and thereon is due to the loss of
three PHeH molecules, three chlorine atoms and Tb3+. Found, 88.62
(calculated 88.99%. The decomposition pattern thus correspond to the
complex Tb[(PHeH)3Cl(H2O)3]Cl2.4H20.
4.8.2. FTIR and structure of Tb+3- PHeH complex
In the absence of crystal, FTIR provide and reveals the actual coordi-
nation sites of Tb3+-PHeH complex when studied relatively with pure
PHeH. (Fig. SI 1). PHeH absorb at 1689 cm−1 attributed to stretching vi-
bration of the carbonyl (C_ O) group red shifted to 1654 cm−1 in the
complex. This signifies a decrease in the stretching energy of C _O dou-
ble bond character inevitably due to co-ordination. The stretching band
of NH, on account of keto-enol tautomerism [31] and hydrogen bonding
occur at 3195 cm−1. On complexation, tautomerism destroyed, NH
stretching vibration band returns to 3321 cm−1. NH bending modes
plus N\\C_ O stretching at 1599 cm−1, amide-II bands remains rela-
tively unaffected. The stretching of the azomethine nitrogen (NC_ N)
at 1649 cm−1 shift to 1595 cm−1 in the complex and the band due to
(NN–Nb) at 995 cm−1 indicates the monodentate linking of
N N\\N b residue confirming azomethine nitrogen coordination to ter-
bium [69]. The pyridine ring skeletal stretching vibrations of C _N
and C _C at 1548, 1498 cm−1, in- plane and out- of- plane deformation
modes at 653, 420 cm−1 in PHeH remains relatively unaffected in the
complex suggesting non- participation of the pyridine ring in the coor-
dination [71,72]. The bonding through oxygen and nitrogen are sup-
ported by the appearance of new bands at 536 cm−1 and 520 cm−1 of
medium intensities [73]. Tb3+-Cl stretching band is tentatively assigned
to a new band at 347 cm−1. The broad band in 3400–3200 cm−1 range
denotes υ(OH) of water molecules. Besides the ρrH2O and ρωH2O modes
absorb at 848–891 and 557 cm−1 respectively [74].
4.8. Tb3+ − PHeH complex.
The ethanolic solutions of the terbium chlorides (0.5 g in ~10 mL)
and PHeH (1.3265 g in ~40 mL) were refluxed in 1:4 molar proportions
for ~6 h and the volume reduced to half. The brownish colour of PHeH
solution instantly turns yellow on addition of terbium chlorides on com-
plexation. To the concentrated reaction mixture, acetonitrile was added
and the gummy solid formed is macerated and redissolved in ethanol.
This process was repeated until a yellow amorphous precipitate was ob-
tained. It was then suction-filtered and dried in a desiccator. The com-
plex is stable but hygroscopic on exposure to air. It is soluble in water
and common organic solvents. The room temperature magnetic mo-
ment of complex is 9.76 BM which is expected of the paramagnetic f8
electrons of Tb3+. The molar conductance value, 73 Ω cm2 mol−1 for
1·10−3 mol·dm−3 complex at 30 °C shows that the complex behaves
as 2:1 electrolyte [68]. Yield = 55%. Colour, Yellow; Mp N90 °C; Analyt-
ical data of Tb[(PHeH)3(H2O)3Cl]Cl2.4H2O: Found % (cald) %: Tb, 14.52
(14.34); N2H4, 3.00 (2.88); C, 42.31 (42.24); H, 3.48 (3.52); N, 11.50
(11.37); Cl,10.08 (9.59).
Potentiometric studies, micro elemental analysis of C, N, H, metal,
chlorine and hydrazine estimation, magnetic susceptibility measure-
ments and molar conductance value of Tb+3-PHeH complex, all point
to the formation of 1:3 metal ligand stoichiometric complex. The pres-
ence of coordinated water in metal complexes is evident from qualita-
tive luminescence quenching, solid IR spectrum and TG/DT analysis of
the complex. The ten coordination number, deduced from the shape
of the hypersensitive absorption band of Nd3+ and its PHeH complex,
is taken to be similar in solution and solid. The actual coordination
4.8.1. Thermal analysis
The nature of Tb3+-PHeH complex is evident in TGA/DTA studies
(Fig. 9). Heated at the rate of 10 °C/min in a nitrogenized atmosphere,
the weight loss below 110 °C but above 80o C is assigned to lattice
water and above 110 °C till 210 °C to coordinated water [69,70]. Tb3+
-
PHeH is hygroscopic with water molecules as shown by TGA curves in
the temperature range of 80-203 °C. The weight loss below 80 °C may

Table 4
Thermodynamic parameters of protonation and complexation reactions of Ln3+ and PHeH at 303.15 K and at constant ionic strenght I = 0.10 mol·dm−3 KNO3 in aqueous – dioxane medium.
PHeH/
Ln3+
PHeH
-
Temp
K
−ΔG1
−ΔG2
−ΔG3
−ΔGn
−ΔH1
−ΔH2
−ΔH3
−ΔH
−ΔS1
−ΔS2
−ΔS3
−ΔS
PHeH
293.15
303.15
313.15
293.15
63.82 0.01
62.97 0.01
–
–
–
–
–
–
63.54 0.01
63.55 0. 12
–
–
63.55 0. 12
0.01 0.00
0.01 0.00
63.85
0.01
Pr
38.11 0.06
32.94
0.06
33.26
0.05
30.34
0.08
33.51
0.06
34.13
0.05
32.08
0.07
37.16
0.04
38.36
0.07
36.57
0.19
40.36
0.10
39.58
0.02
37.29
0.18
40.36
0.11
39.99
0.21
38.31
0.18
26.77 0.06
97.82 0.06
94.19 0.05
88.13 0.08
101.03 0.06
97.27 0.05
92.63 0.07
109.46 0.04
111.89 0.07
106.96 0.19
119.98 0.10
118.04 0.02
111.46 0.18
120.90 0.11
119.27 0.21
114.46 0.18
57.73 0.09
62.93 0.10
32.49 0.00
61.31 0.07
57.89 0.02
70.54 0.22
53.96 0.19
45.11 0.21
77.80 0.14
70.02 0.09
111.05 0.14
112.60 0.22
66.82 0.26
96.18 0.16
86.67 0.11
239.3
0.17
0.06 0.09
0.08 0.10
0.02 0.00
0.06 0.07
0.05 0.02
0.13 0.22
0.29 0.14
0.47 0.17
0.46 0.04
0.12 0.50
0.39 0.37
0.32 0.22
303.15
313.15
293.15
303.15
313.15
293.15
303.15
313.15
293.15
303.15
313.15
293.15
303.15
313.15
38.07 0.05
36.65 0.08
39.06 0.06
38.94 0.05
37.41 0.07
40.04 0.04
40.34 0.07
40.59 0.19
42.21 0.10
42.02 0.02
40.89 0.18
42.54 0.11
42.19 0.21
22.82 0.05
21.04 0.08
28.46 0.06
24.20 0.05
23.14 0.07
32.22 0.04
33.20 0.07
29.80 0.19
37.32 0.10
36.45 0.02
33.28 0.18
37.99 0.11
37.08 0.21
34.66 0.18
Nd
Gd
Tb
Ho
229.12 0.04
144.42 0.50
235.28 0.37
214.58 0.22
0.07 0.19
0.02 0.21
0.13 0.14
0.10 0.09
0.09 0.22
0.01 0.26
0.19 0.16
0.16 0.11
41.49
0.18
ΔH = (kJ mol−1); ΔS = (kJ K−1 mol−1
)

136
Y. Varam, L. Rajkumari / Journal of Molecular Liquids 227 (2017) 127–138
Fig. 9. TGA/DTA of Tb3+-PHeH recorded at a heating rate of 10 °C/min. Inset: Calculations.
sites in the complex are inferred through IR spectra and ably supported
by other spectral studies that suggest an adduct forming ten coordinat-
ed metal complex [75] where PHeH coordinates as neutral bidentate li-
gand through carbonyl group in the keto form and the azomethine
nitrogen.
Funding sources
Special thanks to University Grants Commission, New Delhi for
providing financial assistance to YV under RGNFS (F.14-2(ST)/(2008)
(SA-III)).
Acknowledgment
5. Conclusion
The authors also acknowledge Central Drug Research Institute, Luck-
now for recording NMR and Mass spectra of the ligand and IIT Bombay
for C H N analysis of the ligand and the complex.
Ln3+ and PHeH form 1:3 metal ligand stoichiometric complexes
having moderate stability constants of the order: Pr3+ b Nd3+ b Gd3+-
Tb3+ b Ho3+ and their thermodynamics of protonation and complexa-
tion are spontaneous, exothermic and of unfavorable entropy. Actual
synthesis, characterization and studies on solid Tb3+-PHeH complex
Appendix A. Supplementary data
re-inferred and confirm these facts. Taking Tb+3-PHeH and Nd+3
-
Fig. SI 1 showing Plot of logβPLnHeHvs 1/r; Fig. SI 2, depicting Van't Hoff
plot of pK dissociation of (A) PHeH and (B) Ln3+ complexes of PHeH
against I/T and Fig. SI 3 showing FTIR of PHeH and Tb3+-PHeH complex
are available. SI Tables 1 containnand pL values of the corresponding pH
of Ln3+- PHeH complexes at 303.15 K carried out at a constant ionic
strength of I = 0.1 mol·dm−3 KNO3 in 1:4. Ln3+/PHeH molar ratio. Sup-
plementary data associated with this article can be found in the online
version, at http://dx.doi.org/10.1016/j.molliq.2016.11.098.
PHeH as isostructural-inferred from the stepwise formation equilibria
which shows formation of similar stoichiometric complex, Ln3+ more
specifically Tb+3 forms ten coordinated complex coordinated to three
PHeH molecules through carbonyl group and azomethine nitrogen,
three water or ethanol molecules and a chlorine atom in the inner coor-
dination sphere with two chlorine atoms as counter ion in the outer co-
ordination sphere as shown in Fig. 10.
N
CH3
NH
N
O
H2O
H3C
N
H2O
Tb
HN
Cl2 4H2O
Cl
O
O
O2H
N
N NH
N
CH3
Fig. 10. Suggested structures of Tb3+-PHeH complex (drawn and generated in Chemdraw).

Y. Varam, L. Rajkumari / Journal of Molecular Liquids 227 (2017) 127–138
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