Potentiometric Study of Lanthanide Salicylaldimine Schiff Base Complexes

Article abstract of DOI:10.1007/s10953-012-9946-9

Formation of complexes between the lanthanide ions and N,N′- bis(salicylidene)-4-methyl-1,3-phenylenediamine ligand was studied in solution by pH potentiometry. The potentiometric titration was performed at 25.00 C in 0.1 mol·dm^-3 NaClO₄

Full text of DOI:10.1007/s10953-012-9946-9

J Solution Chem (2013) 42:18–26
DOI 10.1007/s10953-012-9946-9
Potentiometric Study of Lanthanide Salicylaldimine
Schiff Base Complexes
•
Małgorzata T. Kaczmarek Renata Jastrza˛b Wanda Radecka-Paryzek
•
Received: 13 October 2011 / Accepted: 19 March 2012 / Published online: 11 January 2013
ꢀ Springer Science+Business Media New York 2013
Abstract Formation of complexes between the lanthanide ions and N,N0-bis(salicylid-
ene)-4-methyl-1,3-phenylenediamine ligand was studied in solution by pH potentiometry.
The potentiometric titration was performed at 25.00 ꢁC in 0.1 molꢀdm-3 NaClO4 ionic
strength and in DMSO:water (30:70 v:v) solvent mixture. N,N0-bis(salicylidene)-4-methyl-
1,3-phenylenediamine ligand (H2L) occurs in three forms: fully or partially deprotonated
and unionized. Computer analysis of potentiometric data indicated that in solution the
lanthanide (Ln) complexes exist as LnL2, Ln(HL)2 and Ln(H2L)2 species. This observation
appears to be in contrast to the solid-state behavior of these complexes prepared in a self-
assembly process and structurally defined. Stability constants for La3?, Eu3?, Gd3?, Tb3?
,
Ho3? and Lu3? (Ln3?) complexes were determined. The order of stabilities of LnL2
species in terms of metal ions is La3? [ Eu3? & Gd3? = Tb3? \ Ho3? \ Lu3? with a
prominent ‘‘gadolinium break’’.
Keywords Schiff base complexes ꢀ Lanthanides ꢀ Salicylaldimines ꢀ Potentiometry ꢀ
Gadolinium break
1 Introduction
Schiff bases of the salen-type, derived from salicylaldehyde and diamines, are well known
as polydentate ligands that can coordinate metal ions in both protonated [1–3] and
deprotonated forms [4–6]. These compounds are often called ‘‘privileged ligands’’ because
they are easy to obtain and have numerous applications such as fluorgenic agent, pesti-
cides, herbicidal agents [7, 8], and as ionoselective electrodes for the determination of
anions in analytical samples [9]. Interest in Schiff base ligands has increased recently
because of their antitumor, antibacterial, antivirus as well as antifungal activities, which
are increased by coordinating the ligands to a metal ion [10, 11]. Transition metal com-
plexes of salen-type ligands have been applied in heterogenous and homogenous catalysis
M. T. Kaczmarek (&) ꢀ R. Jastrza˛b ꢀ W. Radecka-Paryzek
´
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
e-mail: gosiat@amu.edu.pl
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J Solution Chem (2013) 42:18–26
19
[12–14], diagnostic pharmaceuticals, and laser technology [15]. X-ray structural data
reveal that in the metal complexes of salen-type ligands, the Schiff bases generally act as
deprotonated tetradentate ligands with the N2O2 set of donor atoms capable of effective
coordination in a planar arrangement [16–20]. However, our recent findings extend the
unique series of structurally defined salen-type lanthanide complexes and provide rela-
tively rare examples of complexes in which the salicylaldimine Schiff base acts as a neutral
undeprotonated ligand [21–24]. Having in mind that the solid-state characteristics of the
complexes are not necessarily reflected in their solution behavior, we have decided to
examine the formation of complexes between lanthanide ions and a salicylaldimine ligand
by using pH potentiometry in solution. This paper describes potentiometric studies of the
Schiff base lanthanide complexes formed between La3?, Eu3?, Gd3?, Tb3?, Ho3? and
Lu3? ions and N,N’-bis(salicylidene)-4-methyl-1,3-phenylenediamine.
2 Experimental
2.1 Chemicals Used
Lantanum(III), europium(III), gadolinium(III), terbium(III), holmium(III) and lutetium(III)
nitrates, salicylaldehyde, and 4-methyl-1,3-phenylenediamine were obtained from Aldrich
Chemical Company. 4-Methyl-1,3-phenylenediamine was purified by recrystallization
from n-heptane.
2.2 Physical Measurements
IR spectra were recorded using KBr pellets in the range of 4,000–400 cm-1 on a Bruker
IFS 66v/S spectrophotometer. Mass spectra were measured using fast atom bombardment
(FAB) and electrospray ionization (ESI) techniques. FAB-mass spectra were obtained on a
AMD-604 mass spectrometer with nitrobenzyl alcohol as matrix. Electrospray mass
spectra were determined in methanol using a Waters Micromass ZQ spectrometer. The
concentrations of the compounds were about 10-4 molꢀdm-3. Sample solutions were
introduced into the mass spectrometer source with syringe pump at a flow rate of
40 lLꢀmin-1 with a capillary voltage of ?3 kV and desolvation temperature of 300.00 ꢁC.
The source temperature was 120.00 ꢁC. The cone voltage (Vc) was set to 30 V to allow
transmission of ions without fragmentation. Scanning was performed from m/z = 200 to
1
1,000 in 6 s, and 10 scans were summed to obtain the final spectrum. 13C NMR and H
NMR spectra were recorded in DMSO-d6 on a Varian Gemini 300 spectrometer with
chemical shift (ppm) reported relative to TMS as an internal reference. Electronic
absorption spectra were measured on a JASCO V-550 spectrophotometer in ethanol and
DMSO. Microanalyses (CHN) were obtained using a Perkin-Elmer 2400 CHN
microanalyzer.
2.3 Preparation of N,N0-Bis(salicylidene)-4-methyl-1,3-phenylenediamine (H2L)
To a solution of salicylaldehyde (24 mg, 0.2 mmol) in ethanol (10 mL), 4-methyl-1,3-
phenylenediamine (12 mg, 0.1 mmol) in ethanol (10 mL) was added dropwise with stir-
ring. The reaction was carried out for 24 h at 60 ꢁC. The yellow solution volume was then
reduced to 5 mL by roto-evaporation and yellow crystals formed. After 2 days at room
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J Solution Chem (2013) 42:18–26
temperature, yellow crystals suitable for X-ray were isolated and dried in air. Yield
89 %. MS (ESI?): m/z = 331 [C21H18N2O2 ? H]?. MS (FAB): m/z = 331
(C21H18N2O2 ? H)?. IR (KBr): m = 3490, 3448 (O–H), 2712 (O–H…N), 1619 (C=N),
1
1,499, 760, 747 (C=C), and 1282 (C–O) cm-1. H NMR (300 MHz, DMSO-d6) d = OH:
13.28, 13.14 (two s, 2H); two HC = N: 9.06, 9.03 (two s, 2 9 1H); aryls: 7.69 (d,
J = 7.96 Hz, 2H), 7.48 (d, J = 7.96 Hz, 2H), and 6.99 (t, J = 7.96 Hz, 4H); phen-H: 7.42
(d, J = 6.6 Hz, 2H), 7.32 (s, 1H), and 7.07 (d, J = 6.6 Hz, 2H); phen-CH3: 2.37 (s, 3H).
13C NMR (300 MHz, DMSO-d6): d = 163.74, 163.16, 160.41, 160.00, 147.77, 147.12,
133.43, 133.25, 132.68, 132.51, 131.34, 130.66, 119.68, 119.35, 119.26, 119.16, 116.63,
116.51, 111.13, and 17.45. UV–Vis (EtOH): kmax (e, Lꢀmol-1ꢀcm21): 211 (27,075), 233
(20,051), 270 (17,147), 302 (13,294), and 344 (16,668) nm. UV–Vis (DMSO); kmax (e,
Lꢀmol-1ꢀcm21): 274 (6,381), 306 (5,402), and 346 (6,375) nm. C21H18N2O2 (330) cal-
culated: C 75.93, H 5.10, N 8.86; found C 75.79, H 5.05, N 8.82.
2.4 Potentiometric Measurements
All experiments were carried out using DMSO:water 30:70 (v/v) solvent mixtures
(demineralized carbonate-free water was used). The concentrations of metal ions were
determined by inductively coupled plasma optical emission spectrometry (ICP OES). The
potentiometric titrations were carried out using a Titrino 702 Metrohm equipped with an
autoburette with a Metrohm Solvotrode combination pH glass electrode specially designed
for use with nonaqueous acid–base titrations, calibrated prior to each titration [25]. Prior to
each measurement series, a correction to the pH-meter reading was made and two standard
buffers were applied (pH = 4.002 and pH = 9.225). All potentiometric titrations were
made in an atmosphere of inert gas (helium) at the constant ionic strength (0.1 molꢀdm23
LiNO3), at 25.00 0.5 ꢁC, using CO2-free NaOH as a titrant at a concentration of
0.1799 molꢀdm23. The concentration of the ligand was 1 9 1023 molꢀdm23, and the
metal-to-ligand ratio investigated was 1:2 because solid complexes are formed only at this
ratio. The determined pKw for DMSO:water (30:70 v/v) is 14.501 [26]. The protonation
constants of N,N0-bis(salicylidene)-4-methyl-1,3-phenylenediamine, as well as the stability
constants of the complexes, were determinated using HYPERQUAD software [27] and the
corresponding distribution of respective forms was obtained by using the HALTFALL
program [28]. The calculations were performed using 150–350 points for each analysis,
taking into account only the part of the titration curves where no precipitate occurred.
Precipitation was observed above pH = 7 for all studied systems. In all cases the testing
began with the simplest hypothesis and then in the following steps the models were
expanded to include progressively more species, and the results were scrutinized to
eliminate the species that were rejected in the refinement procedures. The criteria used for
verification of results were given in an earlier paper [29].
3 Results and Discussion
The N,N0-bis(salicylidene)-4-methyl-1,3-phenylenediamine ligand (H2L) (Fig. 1) was
obtained by the condensation reaction of salicylaldehyde and 4-methyl-1,3-phenylenedi-
amine in a 2:1 molar ratio. Single crystals suitable for X-ray analysis were obtained by
slow evaporation of the solvent.
The crystal structure of the ligand N,N0-bis(salicylidene)-4-methyl-1,3-phenylenedi-
amine (H2L) (Fig. 1) has four symmetry-independent molecules. Hydrogen bonds are
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J Solution Chem (2013) 42:18–26
21
OH
N
N
HO
Fig. 1 Formula and crystal structure of ligand H2L
observed between the nitrogen atom on the imine group and oxygen atom of the hydroxy
group. Additionally, weak interactions are observed between C–H and the aromatic rings
[23, 30].
The formation of complexes between N,N0-bis(salicylidene)-4-methyl-1,3-phenylene-
diamine (H2L) ligand and La3?, Eu3?, Gd3?, Tb3?, Ho3? and Lu3? ions (Ln3?) was
studied by pH potentiometry in low water fraction solutions. Due to the insolubility of the
ligand in water, the potentiometric titrations were performed in a DMSO:water 30:70 (v/v)
solvent mixture (pKw = 14.501). For N,N0-bis(salicylidene)-4-methyl-1,3-phenylenedi-
amine, three protonation constant were determined from computer analysis of the titration
data: log10 K1 = 8.54 (7), log10 K2 = 6.36 (7) (protonation of the phenolic units) and log10
K3 = 4.52 (4) (protonation of an imine nitrogen) [31, 32].
Results of the computer analysis of the potentiometric titration data for the Ln3?/H2L
systems are listed in Table 1. The assumed model was verified by analysis of the standard
deviations, while the convergence of the experimental data with the curve obtained for the
model was evaluated by the Hamiltonian test and the Chi squared test.
Complex formation starts at pH = 2.5 for the Eu3? and Gd3? systems and at pH = 3.0
for the La3?, Tb3?, Ho3? and Lu3? systems. Complexes of the Ln(H2L)2, Ln(HL)2 and
LnL2 types are observed for all of the lanthanide ions (Fig. 2). All of these species exist in
solution up to pH = 7. Precipitation occurs above this pH value.
Ln(H2L)2 complexes with two fully protonated ligands start to form at pH = 2.5 for
La3?, Eu3? and Gd3? and at pH = 3.0 for Tb3?, Ho3? and Lu3?. The stability constants of
Ln(H2L)2 formation, determined using HYPERQUAD software, are 36.80, 37.39, 37.28,
36.39, 36.32 and 36.74 for La3?, Eu3?, Gd3?, Tb3?, Ho3? and Lu3?, respectively. The
equilibrium constants of Ln(H2L)2 formation, calculated according to the formula log10
KLn(H2L)2 = log10 bLn(H2L)2 - 2log10 bH2L, are 7.00, 7.59, 7.48, 6.59, 6.52, and 6.91 for
La3?, Eu3?, Gd3?, Tb3?, Ho3? and Lu3?, respectively (Table 1).
Table 1 Stability constants (log10 b) and equilibrium constants (log10 Ke) of complexes formed in the
Ln3?/H2L systems
La(III)
Eu(III)
Gd(III)
Tb(III)
Ho(III)
Lu(III)
Ln(H2L)2
Ln(HL)2
LnL2
log10
log10 Ke
log10
log10 Ke
log10
log10 Ke
b
36.80 (3)
7.00
37.39 (6)
7.59
37.28 (3)
7.48
36.39 (8)
6.59
36.32 (8)
6.52
36.74 (8)
6.91
b
27.89 (2)
10.81
28.05 (5)
10.97
27.91 (3)
10.83
27.46 (5)
10.38
27.54 (6)
10.46
27.97 (6)
10.89
b
17.72 (2)
17.72 (2)
17.19 (7)
17.19 (7)
17.30 (6)
17.30 (6)
17.30 (4)
17.30 (4)
17.53 (6)
17.53 (6)
18.26 (5)
18.26 (5)
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J Solution Chem (2013) 42:18–26
100
[%]
100
[%]
LaL
2
H
L
EuL
2
3
H
L
3
La
80
80
60
40
20
0
Eu
Eu(HL)
2
60
40
20
0
Eu(H L)
2
2
La(H L)
2
2
La(HL)
2
H
L
2
H
L
2
2
3
4
5
6
pH
7
2
3
4
5
6
pH 7
100
[%]
100
[%]
H
L
TbL
2
H
L
3
3
GdL
2
Tb
80
60
40
20
0
80
60
40
20
0
Gd
Gd(H L)
2
2
H
L
2
Gd(HL)
Tb(HL)
2
2
H
L
2
Tb(H L)
2
2
2
3
4
5
6
pH
7
2
3
4
5
6
pH 7
100
[%]
100
[%]
H
L
Ho
LuL
3
2
HoL
2
H
L
Lu
3
80
60
40
20
0
80
60
40
20
0
Lu(HL)
2
Ho(HL)
H
L
2
Lu(H L)
2
2
2
Ho(H L)
2
2
H
L
2
2
3
4
5
6
pH
7
2
3
4
5
6
pH 7
Fig. 2 Distribution diagram for the lanthanide/H2L systems; CLn = 0.001 molꢀdm23 and CH2L
0.002 molꢀdm23
=
Ln(HL)2 species with two monoprotonated ligands are formed at pHs at which the
ligand H2L is partly deprotonated and the lanthanide(III) ion is connected to this form,
according to H2L ꢀ HL þ H and 2HL þ Ln ꢀ LnðHLÞ2 equilibria, respectively.
Coordination to form this type of complexe begins at pH = 3.5 for Eu3? and Lu3? and
at pH = 3.75 for La3?, Gd3?, Tb3? and Ho3?. The stability constants of Ln(HL)2 for-
mation determinate using HYPERQUAD software are 27.89, 28.05, 27.91, 27.46, 27.54
and 27.97 for La3?, Eu3?, Gd3?, Tb3?, Ho3? and Lu3?, respectively. The equilibrium
constants of formation of Ln(HL)2, calculated according to the formula log10
KLn(HL)2 = log10 bLn(HL)2 - 2log10 bHL, are 10.81, 10.97, 10.83, 10.38, 10.46 and 10.89
for La3?, Eu3?, Gd3?, Tb3?, Ho3? and Lu3?, respectively, indicating similar stability of
these complex species with a slight decrease in stability for the complexes of Tb3? and
Ho3? (Table 1).
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J Solution Chem (2013) 42:18–26
23
The LnL2 complexes form by the result of total ligand deprotonation according to
HL ꢀ L þ H and 2L þ Ln ꢀ LnL2 equilibria. They start to form at pH = 4.0 for the Lu3?
complex, pH = 4.25 for the La3? complex, and pH = 4.5 for the Eu3?, Gd3?, Tb3? and
Ho3? complexes. The stability constants for LnL2 formation, determined using HYPER-
QUAD software, are 17.72, 17.19, 17.30, 17.30, 17.53 and 18.26 for La3?, Eu3?, Gd3?, Tb3?
,
Ho3? and Lu3?, respectively. A decrease in the stability constants of the LnL2 complex
species is observed around Eu3?. It is worth noting that the stability constants of lanthanide
complexes generally increase monotonically with increasing atomic number of the lantha-
nides due to the lanthanide contraction (i.e. decrease in ionic radii). However, for many
lanthanide complexes there is a marked deviation from the expected linear relation around
gadolinium. This phenomenon is known as the ‘‘gadolinium break’’ [33–40]. Our findings are
in accord with this behavior. The order of stabilities of the LnL2 complexes in terms of metal
ions is La3? [ Eu3? & Gd3? = Tb3? \ Ho3? \ Lu3? with an evident ‘‘gadolinium break’’.
Recently, we reported the synthesis, spectral characterization and definitive structural
identification of lanthanide complexes containing N,N’-bis(salicylidene)-4-methyl-1,3-
phenylenediamine [23, 24]. These complexes of formula Ln(H2L)2(NO3)3, where
Ln3? = La3?, Eu3?, Gd3? and Lu3?, were formed in a self-assembly process involving the
condensation reaction between salicylaldehyde and 4-methyl-1,3-phenylenediamine in the
presence of lanthanide nitrates acting as template agents [23]. The Eu3? and Gd3? com-
plexes were isolated as crystals, whereas the La3? and Lu3? complexes formed powder
solids. Contrary to the solution behavior of these complexes, in all of the solid complexes
the two salicylaldimine ligands remain un-deprotonated and exist exclusively in the neutral
form. Interestingly, comparison of the equilibrium constants observed for the same lan-
thanide ions along the Ln(H2L)2 series in solution (Fig. 3a) reveals that the highest values
11.0
7.6
(a)
(b)
10.9
10.8
10.7
10.6
10.5
10.4
10.3
7.4
7.2
7.0
6.8
6.6
6.4
Tb
Lu
Tb
Lu
La
Eu
Gd
Ho
La
Eu
Gd
Ho
19.0
18.5
18.0
17.5
17.0
(c)
Tb
Lu
La
Eu
Gd
Ho
Fig. 3 Comparison of equilibrium constants of complexes studied for a Ln(H2L)2, b Ln(HL)2, c LnL2
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J Solution Chem (2013) 42:18–26
are for complexes of La3? and Gd3? that are able to form well defined crystals. The
analogous La3? and Lu3? complexes, isolating as solid powders, correspond to lower
equilibrium constants in solution. The lowest values of the equilibrium constants are for
Tb3? and Ho3?, which accounts for the existence of their complexes in solution but not in
the solid phase.
4 Conclusions
Our potentiometric study reveals that, in solution, complexes of N,N’-bis(salicylidene)-4-
methyl-1,3-phenylenediamine with lanthanide ions (La3?, Eu3?, Gd3?, Tb3?, Ho3? and
Lu3?) appear to have 1:2 lanthanide ion-to-ligand stoichiometry. In contrast to the exis-
tence of exclusively neutral unionized ligands in the well-defined complexes in the solid
state, they form three different types of species in solution: with two neutral undeproto-
nated salicylaldimine ligands, with two monodeprotonated ligands and with two fully
deprotonated ligands, depending on pH. The stability constants of the complexes with two
fully deprotonated ligands display a minimum around europium and gadolinium as a
manifestation of the ‘‘gadolinium break’’. This discontinuity around gadolinium, observed
in a number of physicochemical properties of the lanthanide complexes with various
ligands, may be attributed to the half-filled 4f orbitals of the gadolinium ion.
These results, along with our earlier findings, confirm that the solid-state characteristics
of the complexes are not necessarily reflected in their solution behavior.
Acknowledgments This work was partially supported by the Polish Ministry of Science and Higher
Education (Grant N N204 127 039).
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