Complexation of N′-[1-(2,4-dihydroxyphenyl)ethylidene] isonicotinohydrazide with lanthanide ions

Article abstract of DOI:10.1021/je200370d

The complexation of N′-[1-(2,4-dihydroxyphenyl)ethylidene] isonicotinohydrazide (DEH) with trivalent Pr, Nd, Gd, Tb, and Ho ions were studied employing potentiometric and spectroscopic techniques. The potentiometric studies carried out at different temper

Full text of DOI:10.1021/je200370d

ARTICLE
pubs.acs.org/jced
Complexation of N0-[1-(2,4-Dihydroxyphenyl)ethylidene]isonicotino-
hydrazide with Lanthanide Ions
Yuimi Varam* and Lonibala Rajkumari†
Department of Chemistry, Manipur University, Canchipur 795003, Manipur, India
S Supporting Information
b
ABSTRACT: The complexation of N0-[1-(2,4-dihydroxyphenyl)ethylidene]isonicotinohydrazide (DEH) with trivalent Pr, Nd,
Gd, Tb, and Ho ions were studied employing potentiometric and spectroscopic techniques. The potentiometric studies carried out
at different temperatures, (293.15, 303.15, and 313.15) K, in aqueous dioxane (30 %) medium at a constant ionic strength of I =
0.1 mol dmꢀ3 KNO3 indicate the presence of two ionizable protons in the ligand. The formation of Ln(III) complexes having 1:1
3
and 1:2 metalꢀligand stoichiometry has been inferred from the metalꢀligand formation curves and the elemental analysis data. The
trend in the stability of the Ln(III) complexes follows the order: Pr < Nd < Gd > Tb < Ho. Thermodynamic parameters (ΔG, ΔH,
and ΔS) of both protonation and complexation reactions are negative for all of the systems suggesting that the reactions are
spontaneous, exothermic, and enthalpy-driven. A photoluminescence study reveals that the complexes are not luminescent due to
the quenching effect of the ligand or aqueous or OH ions. The Tb(III) complex of the ligand isolated from ethanolic medium is
paramagnetic, behaves as a 2:1 electrolyte, and is of 1:2 stoichiometry.
their interactions have served as model systems for the study of
many bimolecular and metalloproteins. However, to understand
the interactions and activities of these Schiff bases and their
complexes, it is necessary to havedetailed knowledge about the
extent to which they bind to metal ions, the concentration of
metal complexes in equilibrium mixture, and thermodynamic
and solution equilibria involved in the reactions. These are
conveniently obtained through the determination of stability
constants.20,21
Thus, as part of our ongoing research on the coordination
properties of hydrazonic ligands, we report here about the
chelating abilities of a novel Schiff base, N0-[1-(2,4-dihydrox-
yphenyl)ethylidene]isonicotinohydrazide (DEH; Figure 1)
with Pr3+, Nd3+, Gd3+, Tb3+, and Ho3+ ions through the
determination of the stability constants potentiometrically
and the calculation of thermodynamic data associated with the
complexation reactions. Moreover, the ever-increasing use of
lanthanides as potential luminescent materials22ꢀ29 and, for
relatively simple instrumentation, to study the spectral prop-
erties of any novel lanthanide complex would be worthwhile.
Therefore, we have also extended our studies to the lumines-
cent behavior of the Tb(III) complex. We also discuss the
possible coordinating behavior of the ligand toward the Ln3+
ions based on spectral data.
1. INTRODUCTION
Schiff bases of aroylhydrazones derived from pyridine car-
boxylic acid hydrazides have attracted much attention from the
chemists. The acid hydrazides RꢀCOꢀNHꢀNH2 and their
corresponding aryolhydrazones RꢀCOꢀNHꢀNdCHR have
remarkable biological activity and a strong tendency to form
chelates with transition, lanthanide, and main group metals.
Their mode of chelation with metal ions have aroused interest
in the past due to possible biomimetic applications.1,2 Hydra-
zones containing an active hydrogen component, when used as
intermediates, can synthesize coupling products by using the
azomethine ꢀCONHNdCHꢀ group.3,4 Of the pyridine car-
boxylic acid hydrazides, isonicotinic acid hydrazide is a drug of
proven therapeutic5,6 importance, and in many cases, an increase
of such therapeutic effect was observed when the hydrazide was
complexed with a metal ion.7 It has been reported8ꢀ10 that its
hydrazones derived from condensation with aromatic aldehydes
or ketones showed better antitubercular/antitumor activity.
Metal complexes including lanthanide complexes of this and
other hydrazones not only have interesting spectral and magnetic
properties but also possess a diverse spectrum of biological and
pharmaceutical activities.9,11ꢀ15
The chelating behavior of Schiff bases formed by the con-
densation of isonicotinic acid hydrazides with heteroaromatic
aldehydes and ketones have been studied16ꢀ19 in our laboratory
to observe the influence that coordination exerts on their
conformation or configuration, in connection with the nature
of the metal, lanthanide ions in particular, and of the counterion.
The interactions between such polydentate oxygen and nitrogen
donor Schiff bases and trivalent lanthanide ions have always been
related with anionic or proton ionizability of these ligands as they
have the ability to displace more effectively the hydration sphere
of the metal cation, providing more kinetically stable complexes.
They have interested bioinorganic chemists for decades because
2. EXPERIMENTAL SECTION
2.1. Materials and Solutions. Isonicotinic acid hydrazide, 2,4-
dihydroxyacetophenone, NaOH, KNO3, and 1,4-dioxane were
obtained from E. Merck, Mumbai, and the metal salts, LnCl3 6
3
Received: April 15, 2011
Accepted: July 19, 2011
Published: August 10, 2011
r
2011 American Chemical Society
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Figure 1. N0-[1-(2,4-Dihydroxyphenyl)ethylidene]isonicotinohydrazide (DEH) and its mass spectrum.
H2O (Ln = Pr, Nd, Gd, Tb, Ho) were from Sigma-Aldrich, USA.
All other chemicals used in this study were of analytical reagent
(A.R.) grade. The metal salt solutions used for the potentio-
metric titrations were prepared in double-distilled water. Potas-
sium hydroxide solution prepared in doubly distilled water was
standardized with standard oxalic acid solution (5 10ꢀ2 mol dmꢀ3),
xylenol orange as an indicator.33 The IR spectra were recorded
in KBr medium on a Shimadzu FTIR-8400. The mass spectrum
was obtained on a JEOLSX 102/Da-6000 mass spectrometer.
NMR spectra were recorded on a JEOL AL 300 FT NMR
spectrometer in DMSO. A Perkin-Elmer LS 55 fluorescence
spectrophotometer was used for luminescent studies of Tb(III)
and DEH. Molar conductance was measured on a CON 510
Conductivity/TDS/°C/F meter, Eutech Instruments, while the
magnetic measurement was carried out using a magnetic suscept-
ibility balance, Sherwood Scientific Ltd., UK, and thermal
analysis on a Perkin-Elmer, STA 6000 simultaneous thermal
analyzer purged with nitrogen gas.
2.4. Preparation and Characterization of Ligand. The
ligand was prepared by refluxing ethanolic solutions of isonico-
tinic acid hydrazide (1.00 g in ∼20 mL) and 2,4-dihydroxyace-
tophenone (1.65 g in ∼30 mL) for ∼4 h. The yellow precipitates
obtained on cooling were filtered, washed with ethanol, and dried
at room temperature. Yield = 55 %; mp, 272 °C; M+1 peak
appears at m/z = 272 as the base peak in the mass spectrum.
(Figure 1).
3
3
which was again used for the standardization of HNO3. The
ligand solution (5 10ꢀ3 mol dmꢀ3) was prepared in a 30 %
3
3
aqueous dioxane mixture.
2.2. Apparatus and Procedures. Potentiometric Titration.
Potentiometric titration was carried out at three different tem-
peratures, (293.15, 303.15 and 313.15) K, on a Cyberscan pH
1100 model with a glass calomel electrode, standardized with
standard buffer solution of pH 4, 7, and 9 obtained from Eutech
Instrument, Singapore. All of the titrations were carried out in a
Dewar container and thermostatted with water circulated from a
Circular D8-G Haake Mess Techinik to maintain the reaction
mixture at the desired temperature.
Three sets of reaction mixtures were prepared, and the volume
of each set was made up to 25 mL with double-distilled water.
Solution (a): 5 mL of KNO3 (0.10 mol dmꢀ3) + 2.5 mL of
Elemental Analysis of Ligand: Found % (Calcd % for
C14H13N3O3). C, 61.68 (61.64); H, 4.67 (4.76); N, 15.43
(15.49); N2H4, 11.98 (11.80).
3
HNO3 (1 10ꢀ3 mol dmꢀ3
)
3
3
Solution (b): Solution (a) + 3 mL of DEH (6 10ꢀ4 mol dmꢀ3
)
)
3
3
3
3
Solution(c):Solution(b) +5mLofLn(III) (3 10ꢀ4 mol dmꢀ3
IR (υ, cmꢀ1). 1660 (amide I), 1562 (amide II), 1610 (CN),
987 (NN), 3265(NH), 3165 (OH), 2940 (υas CH3), 2893 (υs
CH3), 1226 (CꢀOH).
Each set was titrated potentiometrically, adding aliquots of
20 μL of standard 5 10ꢀ2 mol dmꢀ3 KOH solution at constant
3
3
1H NMR (δ). 13.384 (s, C20-OH), 11.418 (s, C40ꢀOH), 10.928
(s, NHCO), 2.416 (s, CH3), 8.782ꢀ7.825 (m, pyridine ring
protons), 7.48ꢀ6.275 (m, phenyl ring protons).
ionic strength maintained by 0.10 mol dmꢀ3 KNO3 solution,
3
keeping the metalꢀligand molar ratio at 1:2. The titrations
were repeated three times for each set. The equations of Irving
and Rossotti30,31 were used to determine the stability constants
of DEH and consequently used for the computation of forma-
tion constants of the complexes. The calculations were done by
a personal computer using Microsoft Excel.
13C NMR (ppm). 170.52, (s, CO), 166.55 (s, C20), 152.94 (s,
C40), 149.48 (NC), 143.85(s, C4), 136.85, 131.34, 124.25, 119.65
(C60, C10, C50, C30 of phenyl ring, respectively), 128.32, 127.24
(C2, C3 of pyridine ring, respectively), 65.21 (s, CH3).
Synthesis of the Tb(III)-DEH Complex. The complex was
synthesized by refluxing the ethanolic solutions of the metal
chlorides (1 g in ∼20 mL) and DEH (2.06 g in ∼40 mL) in 1:2
molar proportions for ∼6 h and isolated from the concentrated
reaction mixture using acetonitrile. It was then suction-filtered
and dried in a desiccator. Yield = 60 %.
Luminescence Studies. The ethanolic solutions of TbCl3 6
3
H2O and ligand were prepared. The concentration of metal ion
solution was kept constant (5 10ꢀ4 molꢀ3), while the concen-
3
tration of the ligand was varied according to the desired molar
proportion in each recording. The spectra were recorded at room
temperature.
2.3. Physical Measurements. C, H, and N were microana-
lyzed using a Perkin-Elmer model 240C, and the hydrazine
content was estimated volumetrically, after subjecting the com-
pound to acid hydrolysis for 4 h. Cl was estimated with a standard
method.32 Tb(III) was estimated complexometrically using
3. RESULTS AND DISCUSSION
3.1. Titration Curves. Potentiometric titrations of the ligand
with Pr(III), Nd(III), Gd(III), Tb(III), and Ho(III) were carried
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concentrations of acid and ligand in the reaction mixture,
respectively.
The protonꢀligand formation curves extending from 0.3 to
1.9 on the nH scale for the systems at temperatures of (293.15,
303.15, and 313.15) K (Figure 3a) obtained by plotting nH versus
pH show that two protons are dissociating from the ligand. The
protonation constants, log KH1(DEH) and log K2H(DEH), at different
temperatures were evaluated from the curves using Bjerrum's half
integral method34 and are collected in Table 2. Hydrazones
containing azomethine are reported9,12,35 to exhibit ketoꢀenol
tautomerism and deprotonate the amido proton at high pH.
Therefore, DEH is also expected to exhibit ketoꢀenol tautomer-
ism, and the amido proton would be dissociated from the ligand
through enolization as illustrated (Supporting Information,
Figure 2). DEH loses the enolic proton at higher pH, and the
first protonation constant, log KH1(DEH), may correspond to its
dissociation. Of the two phenyl protons, the deprotonation of the
ortho-OH proton of the phenyl ring will be favored in the
presence of some electron-withdrawing substituent, and its
dissociation may correspond to log KH2(DEH). The dissociation
of the para-OH proton of the phenyl ring is ruled out in the
working medium and at the working pH range and may be
feasible only at higher pH. The deprotonation of DEH is
dependent on temperature and decreases with an increase in
temperature, showing that the process is more favorable at low
temperatures.36
Figure 2. Titration curves of Ln(III) DEH complexes at 303.15 K and
I = 0 10 mol dmꢀ3 KNO3; (a) 5 mL of 0 10 mol dmꢀ3 KNO3 +
3
3
3
3
2.5 mL of 1 10ꢀ3 mol dmꢀ3 HNO3; (b) solution (a) + 3 mL of 6
3
3
3
10ꢀ4 mol dmꢀ3 DEH; (c) solution (b) + 5 mL of 3 10ꢀ4 mol dmꢀ3
3
3
3
Pr(III); (d) solution (b) + 5 mL of 3 10ꢀ4 mol dmꢀ3 Nd(III);
3
3
(e) solution (b) + 5 mL of 3 10ꢀ4 mol dmꢀ3 Tb(III); (f) solution
3
3
(b) + 5 mL of 3 10ꢀ4 mol dmꢀ3 Ho(III); (g) solution (b) + 5 mL
3
3
of 3 10ꢀ3 mol dmꢀ3 Gd(III).
3
3
out with standard KOH at I = 0.10 mol dmꢀ3 KNO3 and at three
3.3. MetalꢀLigand Formation Constant. The average num-
ber of ligands attached per metal ion (n) and the free ligand
exponent (pL) were calculated from the potentiometric data
using relations 2 and 3.
3
different temperatures, (293.15, 303.15, and 313.15) K, in 30 %
aqueous dioxane solution. Figure 2 represents the potentiometric
equilibrium curves of the acid titration curve (a), ligand titration
curve (b), and the complex curves (cꢀg) at 303.15 K. Similar
curves were obtained for titrations at other temperatures. The
acid titration curve started from pH 3.3 and extended to pH 10.9.
At the initial stage of the titration curve (pH range 3.2 to 4.2), the
ligand curve is shifted to the left of the acid curve due to the basic
properties of the ligand. The ligand may accept a proton in the
strongly acidic medium. The acid curve and the ligand curve
converged and started diverging from pH 4.5 to 4.7 which is due
to the liberation of the proton from the ligand at these pH ranges.
It is observed that the complex curves almost coincided with
the ligand curve in the initial stage and started diverging at the pH
range 6.0 to 6.5. This indicates the liberation of proton from
DEH due to the formation of the respective metal complexes.
The decrease in pH for the metal titration curves relative to the
ligand titration curve can be attributed to the formation of metal
ligand bonding.34 The color of the titrates after complex forma-
tion was found to be different from that of the ligand at the same
pH. In the titrations, low concentrations of the metals have been
used to preclude the formation of polynuclear complexes. The
complex curves show two inflections corresponding to the
formation of two types of complexes.
ðVM ꢀ VLÞðN þ E0Þ
n ¼
ð2Þ
̅
ðV0 þ VAÞTM0 nH
̅
and pL, the free ligand exponent, was given by
2
!
3
n
n¼j
1
β H
6
6
7
7
∑
n
antilog pH
n¼0
V0 þ VM
6
6
6
4
7
7
7
5
pL ¼ log10
ð3Þ
3
0
TL ꢀ nTM0
V0
̅
VM is the volume of alkali added to metal solution at the same pH
reading as that of VA, βnH is the overall protonation constant of
the ligand, and TM0 is the concentration of Ln(III) in the metal
solutions. Other symbols have their usual significance as in eq 1.
The metal ligand formation curves were obtained by plotting the
values of n against pL for different temperatures, and the
formation curves at 303.15 K are represented in Figure 3b.
The stepwise formation constants, log K
Ln
2(DEH)
and
Ln
3.2. ProtonꢀLigand Formation Constant. The average
number of protons, nH, bound to the ligand was calculated using
relation 1
logK
, were evaluated from the formation curves employing
2(DEH)
Bjerrum's half integral method34 and are given in Table 1. The
values of n (0.20 < n < 1.5) indicate that the complexes formed
are of 1:1 and 1:2 metalꢀligand stoichiometry. There is a
gradual decrease of the values of the stability constants with
increasing temperature, showing that the complex formation
process is exothermic and more favorable at lower tempera-
ðVL ꢀ VAÞðN þ E0Þ
nH ¼ Y ꢀ
ð1Þ
̅
ðV0 þ VAÞTL0
Ln
Ln
2(DEH)
where Y is the number of dissociable protons present in the
ligand, VL and VA are the volumes of KOH added to ligand and
acid solutions, respectively, for the same pH reading, V0 is the
initial volume of the reaction mixture, and E0 and TL0 are the
tures. The difference between log K
and log K
in
1(DEH)
Table 1 are all positive, meaning that the coordination of the
first ligand to the metal is more favorable than that of the
second ligand molecule.
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Figure 3. (a) Protonꢀligand formation curves of DEH at different temperatures. (b) Metalꢀligand formation curves of Ln(III) DEH complexes at 303.15 K.
Ln
DEH
Table 1. Stepwise MetalꢀLigand Formation Constants, log KL1(nDEH) and log K2L(nDEH), and Overall Formation log β
of Ln(III)
DEH Complexes and I = 0.10 mol dmꢀ3 KNO3 at Different Temperatures in Aqueous Dioxane Mediaa
3
T
metal ions
Gd
formation constant
K
Pr
Nd
Tb
Ho
Ln
1(DEH)
293.15
303.15
313.15
log K
7.81 ( 0.06
4.97 ( 0.06
12.78 ( 0.06
5.92 ( 0.05
4.97 ( 0.05
10.89 ( 0.05
4.58 ( 0.11
4.08 ( 0.11
8.67 ( 0.11
8.40 ( 0.09
6.30 ( 0.09
14.70 ( 0.09
6.04 ( 0.04
5.72 ( 0.04
11.76 ( 0.04
5.18 ( 0.11
4.85 ( 0.11
10.03 ( 0.11
8.65 ( 0.08
8.04 ( 0.08
16.69 ( 0.08
6.98 ( 0.05
6.66 ( 0.05
13.64 ( 0.05
5.79 ( 0.02
5.29 ( 0.02
11.08 ( 0.02
8.71 ( 0.06
7.60 ( 0.06
16.31 ( 0.06
6.59 ( 0.08
6.00 ( 0.08
12.60 ( 0.08
5.46 ( 0.02
5.12 ( 0.11
10.57 ( 0.11
8.74 ( 0.05
7.68 ( 0.05
Ln
2(DEH)
log K
Ln
DEH
log β
16.42 ( 0.05
6.87 ( 0.1
6.48 ( 0.1
13.34 ( 0.1
5.56 ( 0.1
5.17 ( 0.1
10.74 ( 0.1
Ln
1(DEH)
log K
Ln
2(DEH)
log K
Ln
DEH
log β
Ln
1(DEH)
log K
Ln
2(DEH)
log K
Ln
DEH
log β
a ( values are the standard deviations.
Figure 4. van't Hoff plot of pK for dissociation of (a) DEH and (b) Ln(III)-DEH complexes against 1/T.
At constant temperature, the trend in the formation con-
3.4. Effect of Temperature. The change in free energy
(ΔG) was calculated from the formation constant values
(log K) at various temperatures using the following
equation:
Ln
DEH
stants, log β
, of the metal chelates is: Pr(III) < Nd(III) <
Gd(III) > Tb(III) < Ho(III). The higher stability of Gd(III)
complex is not uncommon for the complexes of the lanthanide
series.37ꢀ39 A plot of log β
versus 1/r (Figure 3 of the
Ln
DEH
Supporting Information) indicates that the stability constants of metal
chelates generally increase with the increase in atomic number.
ΔG ¼ ꢀ 2:303RT log K
ð5Þ
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Table 2. Thermodynamics Parametersa of Dissociation of DEH in 30 % (v/v) Aqueous Dioxane Medium in the Presence of
0.10 mol dmꢀ3 KNO3 Solution at 303.15 K
3
T/K
log KH1(DEH)
log KH2(DEH)
ꢀΔG1
ꢀΔG2
ꢀΔH1
ꢀΔH2
ꢀΔS1
ꢀΔS2
293.15
303.15
313.15
10.07 ( 0.05
9.39 ( 0.01
8.52 ( 0.02
8.95 ( 0.0
7.27 ( 0.01
6.68 ( 0.02
56.52
54.47
51.08
50.23
42.18
40.05
135.87 ( 0.08
200.46 ( 0.43
0.63 ( 0.08
0.80 ( 0.43
a ΔG and ΔH = (kJ molꢀ1); ΔS = (kJ Kꢀ1 molꢀ1).
3
3
3
Table 3. Thermodynamics Parametersa of Ln(III) DEH Complexes at 303.15 K and I = 0.10 mol dmꢀ3 KNO3 in 30 % (v/v)
3
Aqueous Dioxane Medium
Ln(III)-DEH complexes
ꢀΔG1
ꢀΔG2
ꢀΔH1
ꢀΔH2
ꢀΔS1
ꢀΔS2
Pr
34.33
35.04
40.49
38.23
39.86
28.85
33.18
38.64
34.81
37.59
284.16 ( 0.17
284.29 ( 0.56
251.59 ( 0.04
286.38 ( 0.35
279.00 ( 0.18
77.18 ( 0.37
126.98 ( 0.14
241.42 ( 0.03
218.49 ( 0.09
220.24 ( 0.08
1.05 ( 0.17
1.05 ( 0.56
0.96 ( 0.04
1.07 ( 0.35
1.05 ( 0.18
0.34 ( 0.37
0.53 ( 0.14
0.92 ( 0.03
0.83 ( 0.09
0.85 ( 0.08
Nd
Gd
Tb
Ho
a ΔG and ΔH = (kJ molꢀ1); ΔS = (kJ Kꢀ1 molꢀ1). ΔH and ΔS were calculated using the linear fit program.
3
3
3
7
7
where R (ideal gas constant) = 8.314 J Kꢀ1 molꢀ1; K =
The emissions of Tb(III) corresponding to 5D4ꢀ F6, 5D4ꢀ F5,
3
3
7
5
7
protonation constant of DEH or stability constant of the
complexes; T = absolute temperature. The enthalpy change
(ΔH) for the dissociation of DEH and complexation process
5D4ꢀ F4, and D4ꢀ F3 transitions29 were clearly observed when
excited at 220 nm in ethanolic medium. They are not significant when
the ion is excited at (250, 280) nm (Figure 5II) and (270, 330) nm
(not shown in the figure). In the presence of DEH, the
excitation and emission spectra of the Tb(III) ion show a
significant difference (Figures 5I,II and 6), and this indicates
complexation or some kind of interaction between them.
Encapsulation of Tb(III) ion by various possible chelators of
DEH would have increased the emission quantum yield when
compared to the free metal ion. But all of the emission peaks of
terbium get quenched, and a broad band in the (485 to 500) nm
of low intensity appears in the spectrum which may be the
remnant D4ꢀ F6 peak. Though observed almost at the same
frequency [(485 to 500) nm], the intensity of the bands are
different for varying metalꢀligand molar ratio, which is at a
maximum for a 1:0.0 M/L molar ratio and becomes weaker for
1:2 to 1:3 M/L ratios.
was evaluated from the slope of the plot log KHn(DEH) or log
Ln
DEH
β
versus 1/T, (Figure 4a,b) using the graphical repre-
sentation of van't Hoff's eq 6, and the change in entropy
(ΔS) could then be calculated using relationship 7:
ΔG ¼ ΔH ꢀ TΔS
ð6Þ
ð7Þ
ΔS ¼ ðΔH ꢀ ΔGÞ=T
5
7
The data thus calculated for protonation and complexation
reactions of DEH are given in Tables 2 and 3, respectively. All
of the thermodynamic parameters are negative indicating that
all the reactions are spontaneous, exothermic, and enthalpy-
driven. The unfavorable change of entropy (negative ΔS) may
be due to extensive solvation of metal chelates in aqueous
organic medium.40 It is also likely to be contributed by the loss
of rotational entropy of the phenyl ring with the rest of the
molecule, >NꢀN< and NꢀC bond linkage of DEH, once
complexation had taken place. The thermodynamic para-
meters of the complexes when correlated with the reciprocal
ionic radii of the lanthanides, 1/r, show that generally free
energy (ΔG) and enthalpy (ΔH) increase, while entropy (ΔS)
decreases with a decrease in ionic radii of the lanthanides. It is
therefore apparent that the Ln(III) complexes of DEH are
stabilized by favorable enthalpy.
The quenching may be due to the presence of OH oscillators
in the coordinated water molecules, ligand, and in the solvent
that are generally more effective nonradiative relaxers of the 5D4
excited states than are other ligand or ligand donor groups.42ꢀ44
The quenching effect is directly proportional to the number of
water molecules (OH oscillator) both in the inner coordination
sphere (to a larger extent) and outer coordination sphere45 (to a
smaller extent) and to NꢀH vibrators.45,46 Some of the water
molecules which were hitherto present in the metal chlorides as
lattice water must have coordinated to the central metal atom in
the complex. Thus, through the presence of the high frequency
OH, NH, or CH (contribution negligible) oscillators present in
the complex, deactivation of the emissive levels of the metal ion
by vibrionic coupling (intensity and lifetime)47 may have taken
place. This will greatly diminish terbium luminescence. The
ligand, therefore, has no sensitizing effects but rather quenches
Tb(III) luminescence in ethanolic solution.
4. PHOTOLUMINESCENCE STUDY OF THE TB(III)-DEH
COMPLEX
Tb(III) emission can be greatly enhanced upon complexation or
when bound to a chelator. If the chelator contains suitable
chromophore, an emission enhancement of several orders of
magnitudes can be observed due to favorable energy transfer. Unlike
the extremely low absorptivity of the lanthanides, the sensitizers
(chromophore) typically have high molar extinction coefficients.41
4.1. Outer Versus Inner Sphere Complex. Choppin et al.48
proposed the use of thermodynamic parameters for the evaluation
of inner sphere versus outer sphere complexation. High negative
ΔH and negative ΔS obtained for complexation reactions would
suggest the formation of outer sphere complex. However, on the
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7
Figure 5. (I) λex = 545 nm, 5D4ꢀ F5, excitation spectra of the 5 10ꢀ4 mol dmꢀ3 Tb(III) ion (concentration constant) and DEH (concentration
3
3
variable) at different M: L molar proportions. Inset: enlargement of the spectra excluding 1:0 M/L molar ratios. (II) Emission spectra of the
5 10ꢀ4 mol dmꢀ3 Tb(III) ion monitored at different excitations of 220 nm (black line), 250 nm (red line), and 280 nm (green line) in ethanolic
3
3
medium.
Figure 6. Emission spectra of the 5 10ꢀ4 mol dmꢀ3 Tb(III) ion (concentration constant) and DEH (concentration variable) at different
3
3
metalꢀligand molar ratios monitored at (I) 220 nm and (II) 280 nm. Insets: enlargement of spectra excluding 1:0 molar ratios.
Table 4. Analytical Data and Physical Properties of DEH and Tb[(DEH)2(H2O)2Cl]Cl2 3H2O
3
found (calcd)%
ligand/complex
color
mp (°C)
Tb
N2H4
C
H
N
Cl
DEH
yellow
yellow
272
116a
11.98 (11.80)
9.04 (8.69)
61.68 (61.64)
38.23 (38.22)
4.67 (4.76)
3.98 (3.86)
15.43 (15.45)
9.49 (9.56)
Tb[(DEH)2(H2O)2Cl]Cl2 3H2O
a Decomposition temperature.
18 (18)
12.18 (12.16)
3
basis of the changes in the excitation and luminescence spectra, the
formation of both the inner and outer sphere complexes is highly
probable.49 The interaction between the ligand and Tb(III) may be
of both coordinated and weak interactions and most probably exist
as a mixture of inner sphere and outer sphere complex at room
temperature in ethanolic medium.
soluble in water and common organic solvents. The room tem-
perature magnetic moment of complex is 9.22 μB which is in the
expected range for paramagnetic f8 configuration of the Tb3+ ion.
The molar conductance value, 86 Ω cm2 molꢀ1, indicates that the
3
3
complex behaves as a 2:1 electrolyte in 1 10ꢀ3 mol dmꢀ3
3
3
ethanol.50
The thermogravimetric (TGA) curve of the Tb(III) com-
plex with DEH indicates the presence of five water molecules,
as shown by the decomposition curve in the (100 to 200) °C
range. Although decomposed fragments of the ligand could
not be estimated due to continuous weight loss, the complete
5. TB(III)-DEH COMPLEX
TheanalyticaldataofTb(III)-DEHcomplexaregiveninTable4.
The complex is yellow-colored, stable, and nonhygroscopic. It is
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7. CONCLUSION
A potentiometric study reveals the presence of two dissociable
protons in DEH. The protonꢀligand and metalꢀligand forma-
tion constants decrease with the increase in temperature. At a
particular temperature, the trend in the stability constants of the
metal complexes is in the order: Pr(III) < Nd(III) < Gd(III) >
Tb(III) < Ho(III). Complexation reactions are spontaneous,
exothermic, and of unfavorable entropy. The ligand has no
sensitizing effect on Tb(III) luminescence, and it coexists as a
mixture of inner and outer sphere complexes in ethanolic
medium. The potentiometric study indicates the formation of
both 1:1 and 1:2 metalꢀligand complexes, while the photolu-
minescence study shows that there is interaction between DEH
and the Ln(III) ions, which is confirmed by the isolation and
characterization of the Tb(III) complex.
Figure 7. Structure of [Tb(DEH)2(H2O)2Cl]Cl2 3H2O.
3
’ ASSOCIATED CONTENT
decomposition of ligand occurs at ∼900 °C. The gradual loss
in weight with increasing temperature from (200 to 900) °C
corresponds to the decomposition of the two ligand molecules
and chlorine present in the complex. The zone beyond 900 °C
or the weights that still remain suggest the ultimate product,
which in the nitrogenized atmosphere corresponds to TbN3.
The differential thermal (DTA) curve shows one endothermic
peak at 116 °C, tallying with the loss of water molecules from
the complex. An exothermic peak at 560 °C may be due to the
phase change such as crystallization or the transition between
different crystal structures or because of some chemical
reactions.
The IR spectrum of DEH in KBr medium exhibits an absorp-
tion band at 1660 cmꢀ1 due to (CdO) of the pyridyl moiety.
The band shifts to 1654 cmꢀ1 in the complex. This indicates the
coordination of the ligand with the metal ion through the
carbonyl group of the pyridyl moiety. Shifts of the CꢀOH
stretching vibration of the phenolic part from (1226 to
1203) cmꢀ1 support the coordination through the phenolic
oxygen which is also favorable due to the possible formation
of highly stable six membered chelates. The ligand band at
987 cmꢀ1 due to (NꢀN) undergoes a (20 to 30) cmꢀ1
hypsochromic shift upon complexation to 1008 cmꢀ1. The
magnitude of this positive shift indicates the monodentate
linking of the > NꢀN< residue.51 The bonding through oxygen
and nitrogen are further supported by the appearance of new
bands at (580 and 416) cmꢀ1 and assignable to (MꢀO) and
(MꢀN) vibrations, respectively.51 Thus, the bonding sites can be
inferred in the Tb(III)-DEH complex.
S
Supporting Information. Values of n and pL of Ln(III)
b
DEH at 303.15 K and I = 0.1 mol dmꢀ3 KNO3 and FTIR data of
3
DEH and the Tb(III)-DEH complex (Tables 1 and 2, re-
spectively). A thermogram of the complex is shown in Figure
1, keto enol tautomerism of DEH in Figure 2, and a plot of log
Ln
DEH
β
vs 1/r in Figure 3. This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: yuivah@gmail.com. Tel.: 09402880425. Fax: 0385-2435145.
Funding Sources
Special thanks to University Grants Commission, New Delhi for
providing financial assistance to Y.V. under RGNFS.
Notes
†E-mail: lonirk@yahoo.co.uk.
’ ACKNOWLEDGMENT
The authors also acknowledge Central Drug Research Insti-
tute, Lucknow for recording NMR and mass spectra of the ligand.
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