Synthesis, characterization, thermal stability, reactivity, and antimicrobial properties of some novel lanthanide(III) complexes of 2-(N-salicylideneamino)-3-carboxyethyl-4,5,6,7-tetrahydrobenzo[b]thiophene

Article abstract of DOI:10.1134/S1070328406080124

Two series of new lanthanide(III) complexes of the type [Ln(HSAT) ₂(H₂O)₃Cl₃] and [Ln(HSAT) ₂(NO₃)₃], where Ln = La, Pr, Nd, Sm, Eu, Gd, Dy, Tm, Yb, or Lu, and HSAT = 2-(N-salicy

Full text of DOI:10.1134/S1070328406080124

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2006, Vol. 32, No. 8, pp. 600–609. © Pleiades Publishing, Inc., 2006.
Synthesis, Characterization, Thermal Stability, Reactivity,
and Antimicrobial Properties of Some Novel Lanthanide(III)
Complexes of 2-(N-salicylideneamino)-3-carboxyethyl-4,5,6,7-
tetrahydrobenzo[b]thiophene*
a
b
K. Mohanan and S. N. Devi
a
Department of Chemistry, University of Kerala, Kariavattom Campus, Trivandrum, Kerala, 695581, India
b
Department of Chemistry, Government Women’s College, Trivandrum, 695014, Kerala, India
E-mail: drkmohanan@rediffmail.com
Received September 27, 2005
Abstract—Two series of new lanthanide(III) complexes of the type [Ln(HSAT) (H O) Cl ] and
2
2
3
3
[
Ln(HSAT) (NO ) ], where Ln = La, Pr, Nd, Sm, Eu, Gd, Dy, Tm, Yb, or Lu, and HSAT = 2-(N-salicylidene-
2 3 3
amino)-3-carboxyethyl-4,5,6,7-tetrahydrobenzo[b]thiophene, are synthesized by the reaction of LnCl or
3
Ln(NO ) with the title ligand in ethanol. The complexes are characterized by elemental analysis, magnetic
3
3
1
moment values, molar conductivity, IR, UV-Vis, and H NMR spectral data. Two selected complexes are sub-
ject to thermogravimetric analysis, and their kinetic parameters are estimated using Coats–Redfern equation.
The complex [La(HSAT) (NO ) ] underwent facile transesterification when refluxed in methanol. The ligand
2
3 3
and some selected complexes are screened for their antimicrobial properties. Antimicrobial activities of the
ligand increase on coordination with the metal ion.
DOI: 10.1134/S1070328406080124
Metal complexes of Schiff bases form an interesting phene (HSAT), containing ONO donor functions. Apart
class of compounds, which find extensive applications from providing extra stability to 2-aminothiophene, this
in various fields [1–4].Among these complexes, several method has also given new possibilities for coordina-
lanthanide(III) complexes containing Schiff bases have tion and reactivity. In addition to the synthesis and char-
acterization of two new series of complexes of the type
Ln(HSAT) (H O) Cl ] and [Ln(HSAT) (NO ) ], the
antibacterial properties, thermal behavior, and reactiv-
ity of some selected complexes are also reported in this
investigation.
also been reported. However, lanthanide(III) complexes
formed from heterocyclic Schiff bases, particularly
those containing thiophene ring system have received
comparatively less attention. Apart from the structural
diversities and bonding interactions, the multitude
applications of lanthanide complexes make it an excit-
ing arena in coordination chemistry [5–7]. There are
few reports on lanthanide(III) complexes of Schiff
bases derived from thiophene-2-aldehyde and primary
amines [8, 9]. However, lanthanide(III) complexes of
Schiff bases formed from 2- or 3-aminothiophene have
not been reported so far. This can be mainly explained
by instability of aminothiophenes, which made the syn-
thesis of such compounds difficult. However, 2-ami-
nothiophene has been made stable by suitable substitu-
tion and fusion with cyclohexane ring by the Gewald
synthesis [10]. The resulting compound, namely,
[
2
2
3
3
2
3 3
EXPERIMENTAL
Preparation of 2-(N-salicylideneamino)-3-carboxy-
ethyl-4,5,6,7-tetrahydrobenzo[b]thiophene (HL). The
ligand was prepared by the condensation of I with sali-
cylaldehyde in 1 : 1 molar ratio in ethanol medium.
To a solution of the amine (0.01 mol) in ethanol
30 ml) salicylaldehyde (0.01 mol) in ethanol (10 ml)
(
was added. The resulting mixture was refluxed on a
water bath for 2 h. The color of the solution changed to
deep yellow. The solution was concentrated to ~25 ml
and allowed to cool. Golden yellow crystals of the
ligand formed were filtered and dried. They were fur-
ther purified by recrystallization from ethanol.
Preparation of lanthanide(III) salts. The lan-
thanide nitrate (chloride) was prepared from the corre-
sponding oxide. A lanthanide oxide was dissolved in
50% nitric acid (hydrochloric acid), and the salt formed
2
-amino-3-carboxyethyl-4,5,6,7-tetrahydrobenzo[b]thio-
phene (I), which is fairly stable, has been condensed
with salicylaldehyde in an ethanol medium to obtain a
potentially tridentate Schiff base, viz., 2-(N-salicylide-
neamino)-3-carboxyethyl-4,5,6,7-tetrahydroben-zo[b]thio-
*
The text was submitted by the authors in English.
6
00

SYNTHESIS, CHARACTERIZATION, THERMAL STABILITY, REACTIVITY
601
Table 1. Elemental analysis data and other details of the lanthanide(III) chloride complexes of HSAT
Content (found/calcd), %
Λ, Ohm–1 cm mol
2
–1
Yield,
%
µ ,
eff
BM
Complex
nitro-
benzene
metal
C
H
N
S
DMF DMSO
[
[
[
[
[
[
[
[
[
[
La(HSAT) (H O) Cl ]
55 14.65/14.51 45.34/45.12 4.40/4.59 2.66/2.92 6.62/6.68 2.9
56 14.70/14.60 45.32/45.06 4.26/4.59 2.75/2.92 6.50/6.67 3.5
52 15.10/14.97 45.00/44.87 4.32/4.57 2.76/2.90 6.72/6.64 2.7
10.6 6.3
11.2 5.9
10.4 7.1
10.2 6.8
10.4 7.5
10.3 7.9
D
2
2
3
3
Pr(HSAT) (H O) Cl ]
3.45
3.52
1.52
3.46
7.85
2
2
3
3
Nd(HSAT) (H O) Cl ]
2
2
3
3
Sm(HSAT) (H O) Cl ] 60 15.70/15.52 44.72/44.58 4.43/4.54 2.70/2.88 6.52/6.60 2.6
2
2
3
3
Eu(HSAT) (H O) Cl ]
53 15.85/15.75 44.43/44.51 4.40/4.53 2.66/2.88 6.48/6.59 3.2
54 16.00/16.11 44.36/44.27 4.38/4.50 2.75/2.86 6.60/6.55 3.1
55 16.75/16.56 44.52/44.03 4.32/4.48 2.71/2.85 6.45/6.52 2.5
2
2
3
3
Gd(HSAT) (H O) Cl ]
2
2
3
3
Dy(HSAT) (H O) Cl ]
9.1 8.1 10.47
2
2
3
3
Tm(HSAT) (H O) Cl ] 55 16.85/17.10 44.00/43.74 4.37/4.45 2.69/2.83 6.56/6.48 2.8
10.3 8.4
10.1 6.8
10.3 7.9
7.21
4.56
D
2
2
3
3
Yb(HSAT) (H O) Cl ]
52 17.10/17.44 43.69/43.57 4.32/4.43 2.70/2.82 6.40/6.45 3.8
50 17.25/17.61 43.22/43.48 4.36/4.42 2.69/2.81 6.37/6.44 3.2
2
2
3
3
Lu(HSAT) (H O) Cl ]
2
2
3
3
was crystallized after concentrating the solution in a complexes were recorded using CsI discs on a Polytec
steam bath.
FIR 30 Fourier far-infrared spectrometer.
1H NMR spectra of the ligand and complexes were
Synthesis of complexes. All the complexes were
prepared by the following general procedure. To a mag- recorded on a JEDL EX90 FT NMR spectrometer
netically stirred and warmed ethanolic solution employing TMS as an internal reference and DMSO-d6
(
100 ml) of the ligand (0.01 mol, pH adjusted to ~6.0) as solvent.
a solution of the lanthanide nitrate (chloride)
0.005 mol) in a minimum quantity of rectified spirit
Thermal studies (TG and DTG) of the complexes
were carried out using a Du Pont 2000 thermal analyzer
system equipped with a 951 thermogravimetric ana-
lyzer at a heating rate of 10 K/min and a sample size of
(
was added. After 30 min, the entire mixture was trans-
ferred to a flask and refluxed for 8 h on a water bath.
The volume of the solution was then reduced to about
half of the original volume and left overnight. The com-
plex separated out as a powdery material was filtered
and washed successively with small amounts of ethanol
and ether. Finally, it was dried in vacuum over P O .
Elemental analyses (carbon, hydrogen, and nitro- with those from the TG data.
gen) were carried out using a Heraeus Carlo Erba 1108-
CHN analyzer and sulfur in the complex was estimated
gravimetrically as barium sulfate after oxidizing it with
concentrated nitric acid in the presence of a few drops
of bromine water. The lanthanide content was deter-
mined by the oxalate-oxide method [11].
Molar conductivity (L) of 10 M solutions of the
complexes in nitrobenzene, DMF, and DMSO was
measured at 300 ± 2 K using a Systronics direct reading
Conductivity Meter Type 305. Dip-type conductivity
cell with platinum electrode calibrated with a solution
of AR KCl was used for conductivity studies.
4
–5 mg in an atmosphere of static air. In all cases, the
complexes were subjected to independent pyrolysis
experiments in which the samples were heated for 2 h
in a crucible up to 800°C until a constant weight was
obtained. The observed mass losses were compared
2
5
The transesterification reaction was carried out by a
reported method [13].
RESULTS AND DISCUSSION
The analytical data indicated that the condensation
of I with salicylaldehyde occurred in an equimolar
ratio. The ligand formed well-defined complexes with
La(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III),
Dy(III), Tm(III), Yb(III), and Lu(III) salts.
All the complexes are stable at room temperature
and possess good keeping qualities. Analytical data of
the complexes are in good accord with their formula-
tion as in Tables 1 and 2. Formulation of these com-
plexes has been made on the basis of their elemental
analyses and molar conductance measurements. Molar
conductance values of the complexes measured in dif-
ferent solvents adequately confirm the nonelectrolytic
nature of these metal chelates [14]. The complexes
–
3
Magnetic measurements of the complexes were per-
formed on a Gouy-type magnetic balance, using
Hg[Co(NCS) ] as calibrant. Diamagnetic corrections
4
were computed using Pascal’s constants [12]. The
effective magnetic moments (m ) were calculated from
eff
corrected molar magnetic susceptibilities of the com-
plexes.
IR spectra of the ligand and the metal complexes exhibited the 1 : 2 metal-to-ligand stoichiometry and
were recorded using discs with KBr on a FTIR-8108 can be formulated as [Ln(HSAT) (H O) Cl ] and
2
2
3
3
Shimadzu spectrophotometer. Far-IR spectra of the [Ln(HSAT) (NO ) ] in which the ligand is coordinated
2
3 3
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 8 2006

6
02
MOHANAN, DEVI
Table 2. Elemental analysis data and other details of the lanthanide(III) nitrate complexes of HSAT
Content (found/calc.), %
Λ, Ohm–1 cm mol
2
–1
Yield,
%
µ ,
eff
BM
Complex
nitro-
benzene
metal
C
H
N
S
DMF DMSO
[
[
[
[
[
[
[
[
[
[
La(HSAT) (NO ) ]
60 14.27/14.13 44.20/43.95 3.55/3.86 7.02/7.12 6.48/6.51
3.8
3.1
3.7
3.1
3.3
4.8
4.6
3.8
3.9
3.6
12.1
10.3
9.8
6.9
7.2
6.8
6.6
7.1
5.9
D
2
3 3
Pr(HSAT) ((NO ) ] 62 14.40/14.23 44.00/43.89 3.70/3.86 7.00/7.11 6.56/6.50
3.47
3.53
1.44
3.42
7.83
2
3 3
Nd(HSAT) (NO ) ] 58 14.70/14.59 43.85/43.71 3.69/3.84 6.95/7.08 6.40/6.47
2
3 3
Sm(HSAT) (NO ) ] 58 15.38/15.12 43.80/43.44 3.53/3.82 6.92/7.03 6.30/6.43
9.3
2
3 3
Eu(HSAT) (NO ) ]
54 15.10/15.25 43.50/43.37 3.66/3.81 6.87/7.02 6.50/6.42
10.6
9.8
2
3 3
3 3
Gd(HSAT) (NO ) ] 55 15.87/15.70 43.40/43.15 3.62/3.79 6.83/6.99 6.45/6.39
2
Dy(HSAT) (NO ) ] 59 16.25/16.14 43.05/42.92 3.59/3.77 6.75/6.95 6.48/6.35
9.4
6.3 10.44
2
3 3
Tm(HSAT) (NO ) ] 56 16.80/16.67 42.80/42.64 3.60/3.75 6.82/6.91 6.22/6.31
9.3
7.8
6.4
7.8
7.22
4.50
D
2
3 3
Yb(HSAT) (NO ) ] 51 17.15/17.01 42.62/42.47 3.62/3.73 6.64/6.88 6.38/6.29
10.0
10.3
2
3 3
Lu(HSAT) (NO ) ]
54 17.31/17.17 42.60/42.39 3.61/3.72 6.67/6.86 6.39/6.28
2
3 3
to the metal ion without deprotonation in a tridentate
The spectra of compounds, which cannot acquire
mode and the nitrate group acted invariably as biden- quinonoid form, for example 3-hydroxybenzylidene
tate.
Structure of the ligand. The structure of the ligand
bases, lack the 400 nm band altogether. Systems, which
assume the quinonoid form, more easily show a prom-
inent band at ca. 400 nm. It has been reported that some
1
was established by UV, IR, and H NMR spectral data. amino acid Schiff bases derived from 2-hydroxy-1-na-
Introduction of a hydroxy group at the ortho position to phthaldehyde exist predominantly in the quinoneamine
the azomethine group raises the possibility of pheno- form, and the ligand is coordinated to the metal ion
through the azomethine nitrogen, one of the carboxy-
late oxygen atoms and the quinone oxygen [16].
limine–quinoneamine tautomerism. A bulk of informa-
tion in this regard has evolved from UV-visible spec-
troscopy, a technique well suited for such an investiga-
tion. It has been reported that the tautomeric
Recently, this phenomenon has been studied exten-
sively by Antonov and coworkers who explained the
equilibrium depends on the extent of conjugation, tautomerism exhibited by 2-hydroxy-1-naphthalde-
nature and position of the substituent, and polarity of hyde Schiff bases from theoretical and experimental
the solvent, and, hence, this phenomenon has received point of view [17]. There are also recent reports that the
considerable attention both from experimental and the- quinoneamine tautomeric form is typical of molecular
oretical point of view. Bands were assigned to the indi- complexes of o-hydroxyaryl Schiff bases [18, 19].
vidual tautomers. Thus, the UV spectrum of N-sali-
In the case of the present ligand, there is also a pos-
cylidenebutylamine showed strong bands (in an ethanol
solution) at 253 and 312 nm for the phenolimine form
sibility of phenolimine–quinoneamine tautomerism
Figs. 1a, 1b). However, HSAT exhibited intense bands
π* and n
respectively) and much weaker bands at 278 and 401 π* transitions of the phenolimine form (Fig. 1a). This
nm for the quinoneamine form [15]. also rules out the possibility of the quinoneamine form
(
(assignable to π
π* and n
π* transitions, at 256 and 330 nm characteristic of π
OC H
OC H
2
5
2
5
C
O
C
O
H
N
H
N O
S
O
S
(
a)
(b)
Fig. 1. Tautomeric structures of the ligand HSAT: phenolimine form (a) and quinoneamine form (b).
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 8 2006

SYNTHESIS, CHARACTERIZATION, THERMAL STABILITY, REACTIVITY
603
in the ligand. The data obtained from the UV spectral salicylidene bases, the interaction of the lanthanide ion
1
studies is further substantiated by the H NMR spec- with phenolic oxygen does not increase the acidity to be
trum of the ligand. The UV spectrum of the ligand sufficient for ionization of the proton [25]. Conse-
recorded in the solid state also exhibited the character- quently, the phenolic oxygen is coordinated to the lan-
istic bands (259 and 332 nm) for the phenolimine form. thanide ion without deprotonation. As a result of this, in
the metal complexes, the ν(OH) band becomes less
The IR spectrum of the free ligand exhibited a broad
–
1
–
1
broad showing a peak centered at 3200 cm (in the
nitrato complex) indicating that the OH group is coor-
dinated to the metal ion without deprotonation. Conse-
quently, the ν(C–O) stretching frequency shows a posi-
band ranging from 3300 to 2900 cm and centered
–
1
around 3100 cm assignable to ν(O–H) of the hydro-
gen-bonded phenolic group, and ν(C–O) was observed
–
1
–1
at 1280 cm . An intense band appearing at 1700 cm
can be attributed to ν(C=O) of the ester group, and a
–1
tive shift by about 20 cm . This type of coordination by
the phenolic oxygen without deprotonation has been
already reported [25, 26].
–
1
medium-intensity band appearing at 1640 cm can be
assignable to ν(C=N) of the azomethine group. Vibra-
tions characteristic of the thiophene ring [20] were
Apart from all the above frequencies, the vibrations
–1
observed at 1530, 1425, and 1360 cm . The ester car- characteristic of substituted thiophene nucleus remain
bonyl group is also involved in weak hydrogen bonding almost unaffected. This rules out the possibility of
with the phenolic OH group forming a sort of bifunc- bonding by the ring sulfur atom to the metal ion. How-
tional hydrogen bonding. However, in competition with ever, in the case of aquachloro complexes, the band due
the azomethine nitrogen for the phenolic OH, the ester to the phenolic OH was masked by that due to coordi-
carbonyl can manage only a meager share. This nated water, so that a broad band with an additional
–
1
explains why the ester carbonyl frequency in the Schiff peak centered at ~3500 cm is observed in these com-
base is relatively higher than that in the free amine plexes. The nitrate ion is capable of exhibiting mono-
–1
(
1660 cm ). On the basis of these observations, the dentate, bidentate, and bridging bidentate bonding
ligand has been represented as in Fig. 1a.
modes toward metal ions. Generally, lanthanides prefer
bidentate coordination of a nitrate group. This can be
ascribed to the favorable metal–nitrate interaction for a
given degree of nitrate–nitrate repulsion [27], forming
complexes with higher coordination numbers.
In conformity with the UV and IR spectral data, the
H NMR spectrum of the ligand recorded in DMSO-d6
exhibited a downward shift of the OH proton resonating
at 12.86 ppm, indicative of strong intramolecular
hydrogen bonding [21]. Signals for aromatic protons
1
Crystallographic studies also reveal that nitrate is
are observed in the range of 6.21–7.91 ppm, and the invariably bidentate towards the lanthanide ion [28].
signal appearing at 8.14 ppm was assigned to the “Short bite” ligands like nitrate (bidentate) minimize
hydrogen of the azomethine moiety. The signals for the ligand–ligand repulsion and, hence, are ideally suited
methyl protons and methylene protons of the ester to lanthanides, which show a pronounced tendency to
function have been observed at 1.20 and 3.70 ppm, attain relatively high coordination numbers [28–30].
respectively. However, these methyl proton signals Although symmetry of the nitrate ion remains the same
merged with the signals due to the ring protons of the in all the above modes of bonding, monodentate and
cyclohexane moiety.
bidentate bonding modes are yet distinguishable. IR
spectra of the nitrato complexes revealed two additional
strong bands at 1485 and 1270 cm , which are absent
in the free ligand. These bands can be assignable to the
characteristic vibrations of the coordinated nitrate ion.
Structure of the metal complexes. The UV spec-
tral bands characteristic of the phenolimine form of the
ligand are only marginally redshifted in the spectra of
the metal complexes, indicating that the same structural
form of the ligand persists in the metal complexes also.
–
1
The magnitude of separation between these bands is
–
1
IR spectrum of the free Schiff base was compared with >200 cm , which indicates bidentate coordination by
that of the metal complexes in order to confirm the the nitrate group [31]. The nonligand bands of medium
coordinating sites of the ligand to the metal ion. The IR intensity appearing in the regions 430–440, 360–370,
–
1
spectra of the metal chelates are closely similar among and 310–320 cm can be assigned to the ν(Ln–O),
themselves. In the spectra of the metal chelates, the ν(Ln–N), and ν(Ln–Cl) modes, respectively [29, 32–
–
1
band at 1700 cm is shifted to lower frequency by 34]. The diagnostic IR and far-IR spectral bands of the
–
1
about 40 cm , indicating coordination by the ester car- complexes are presented in Tables 3 and 4.
bonyl. This type of coordination by ester carbonyl has
already been reported by several investigators [22, 23].
The azomethine ν(C=N), appearing at 1640 cm in the
free ligand, registered a negative shift by about 20–25 cm−
in the metal complexes, indicating the involvement of
the azomethine nitrogen in bond formation [24].
In agreement with the UV and IR spectral data, the
1H NMR spectra of [La(HSAT) (NO ) ] and
–1
2
3 3
[
Lu(HSAT) (NO ) ] recorded in DMSO-d also exhib-
1
2 3 3 6
ited signals for the OH proton at 12.10 and 12.64 ppm,
respectively, indicating that the OH group is coordi-
nated to the metal ion without deprotonation. The sig-
However, the phenolic oxygen coordinates to the nals for the –CH=N– proton are shifted to 7.96 and
metal ion without deprotonation. It has been reported 7.98 ppm, respectively, in the above metal complexes.
that, with respect to lanthanide complexes of certain The signals due to other protons are found in the
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 8 2006

6
04
MOHANAN, DEVI
–
1
Table 3. IR and far-IR spectral data of the lanthanide(III) chloride complexes of HSAT (cm )
Complex
ν(O–H)
ν(C=O)
ν(C=N)
ν(C–O)
ν(Ln–O)
ν(Ln–N)
ν(Ln–Cl)
[
[
[
[
[
[
[
[
[
[
La(HSAT) (H O) Cl ]
3500,3210
3550,3200
3541,3220
3550,3200
3520,3210
3545,3195
3535,3200
3520,3190
3535,3190
3548,3210
1654
1655
1655
1660
1655
1650
1654
1655
1658
1656
1615
1618
1614
1617
1618
1620
1615
1616
1615
1618
1310
1305
1307
1308
1310
1302
1301
1300
1305
1304
438
434
432
436
438
431
433
435
430
432
368
363
366
368
364
362
361
364
383
360
320
315
318
314
318
309
316
314
314
313
2
2
3
3
Pr(HSAT) (H O) Cl ]
2
2
3
3
Nd(HSAT) (H O) Cl ]
2
2
3
3
Sm(HSAT) (H O) Cl ]
2
2
3
3
Eu(HSAT) (H O) Cl ]
2
2
3
3
Gd(HSAT) (H O) Cl ]
2
2
3
3
Dy(HSAT) (H O) Cl ]
2
2
3
3
Tm(HSAT) (H O) Cl ]
2
2
3
3
Yb(HSAT) (H O) Cl ]
2
2
3
3
Lu(HSAT) (H O) Cl ]
2
2
3
3
–
1
Table 4. IR, far-IR and UV spectral data of the lanthanide(III) nitrate complexes of HSAT (cm )
Complex
ν(O–H)
ν(C=O)
ν(C=N)
ν (NO )
ν (NO ) ν(Ln–O) ν(Ln–N) λmax, nm
1 3
5
3
[
[
[
[
[
[
[
[
[
[
La(HSAT) (NO ) ]
3200
3210
3210
3190
3215
3200
3210
3200
3220
3210
1650
1652
1654
1650
1655
1656
1654
1654
1658
1655
1622
1624
1622
1619
1624
1621
1623
1619
1620
1620
1483
1280
430
432
436
434
438
433
434
438
439
436
368
363
364
362
361
366
364
368
368
362
314
318
316
319
320
320
315
320
318
320
2
3 3
Pr(HSAT) ((NO ) ]
1482
1482
1478
1485
1489
1480
1482
1483
1485
1280
1278
1275
1280
1280
1282
1278
1282
1280
2
3 3
Nd(HSAT) (NO ) ]
2
3 3
Sm(HSAT) (NO ) ]
2
3 3
Eu(HSAT) (NO ) ]
2
3 3
Gd(HSAT) (NO ) ]
2
3 3
Dy(HSAT) (NO ) ]
2
3 3
Tm(HSAT) (NO ) ]
2
3 3
Yb(HSAT) (NO ) ]
2
3 3
Lu(HSAT) (NO ) ]
2
3 3
expected region and shifted by about 0.10–0.20 ppm in shift of the band due to alteration in the interelectronic
the spectra of the metal complexes. The changes in proton repulsion among the 4f-electrons. Since the Nujol-mull
signals for [La(HSAT) (NO ) ] and [Lu(HSAT) (NO ) ] spectrum of each complex resembles that in solution, it
2
3 3
2
3 3
are presented in Table 5.
appears that the coordination number remains unaltered
in the solid state also. On the basis of all the above spec-
tral evidences, a coordination number of 12 was pro-
posed (Figs. 2, 3). This type of a higher coordination
number has been reported for the lanthanide(III) com-
plexes with similar type of ligands [29, 30].
Visible spectral bands of the lanthanide complexes
are hypersensitive to stereochemistry. In the case of the
praseodymium(III) and neodymium(III) complexes, an
enhancement of intensity of certain hypersensitive
bands compared to the aquated ions was noticed. This
can be ascribed to quadrupolar effects due to an inho-
mogeneous electrostatic field and changes in symmetry
around the lanthanide(III) ion. Taking the aqua ion as
standard, the ligands were found to cause a slight red-
Thermogravimetric studies. The complexes
[
La(HSAT) (H O) Cl ] and [La(HSAT) (NO ) ] were
2 2 3 3 2 3 3
subjected to thermogravimetric analysis with a view to
study their thermal behavior. The schemes of decompo-
sition of the aquachloro complex and nitrato complex
are given in Table 6. The thermogram of the aquachloro
complex indicated three-stage decomposition involving
a loss of coordinated water molecules, oxidation of the
organic moieties, while that of the nitrato complex
showed only a two-stage decomposition pattern. The
former complex was stable up to 408 K. At this temper-
ature, the first stage of decomposition started and com-
pleted at 420 K. The mass loss corresponding to this is
equivalent to three moles of coordinated water per mole
of the complex (weight loss found, 5.50%; calculated
1
Table 5. H NMR spectral data of HSAT and its diamagnetic
metal complexes
1H chemical shift, ppm
Compound
OH –HC=N– aromatic protons
HSAT
12.86
8.14
7.96
7.98
6.21–7.91
6.11–7.82
6.10–7.82
[
[
La(HSAT) (NO ) ] 12.10
2
3 3
Lu(HSAT) (NO ) ] 12.64
2
3 3
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SYNTHESIS, CHARACTERIZATION, THERMAL STABILITY, REACTIVITY
605
culated value, 43.87%) corresponding to the loss of
three nitrate ions and the salicylaldehyde moiety, and
this was observed in the temperature range of 453–613 K.
The second stage of decomposition of the complex
occurred in the temperature range 683–873 K with a
mass loss of 40.50% (calculated value, 40.82%) and is
consistent with the complete decomposition of the
complex, elimination of the aminothiophene moiety,
and oxidation to La O . These two-stage decomposi-
HC
HO
H O
Cl
S
2
Cl
Cl
N
OC H
2
5
H O
2
Ln
OH
O
2
3
O
OH2
tion profiles are denoted by two different exothermic
peaks at 560 and 823 K, respectively (Figs. 5a, 5b).
N
OC H
2
5
CH
S
The kinetic and thermodynamic parameters were
estimated using the Coats–Redfern equation [35] and
are presented in Table 7. The order of the different
stages of thermal decomposition was estimated using a
computer program developed for this study. The value
of order parameter (n) obtained from the above method
was substituted in the Coats–Redfern equation. The
values of activation energy (E) were obtained from the
Fig. 2. Structure of the aquachloro complex
Ln(HSAT) (H O) Cl ].
[
2
2
3
3
weight loss, 5.64%). This stage involves only the dehy- slope and that of A from the intercept of the straight
dration of the metal chelate, and, hence, the kinetic line. The entropy of activation (DS) was also estimated.
parameters were not estimated for this particular stage.
The different stages observed in the thermal decom-
The second stage of decomposition took place in the
position of the metal complexes were separately dealt
temperature range from 453 to 583 K. A mass loss of
with to estimate the different kinetic parameters. The
3
6.44% (calculated, 36.72%) was observed and corre-
value of γ ≈ 1 obtained for the plot of the left hand of
the Coats–Redfern equation versus 1/T is in good
agreement with the experimental data. The negative
values of ∆S show that the complexes are more ordered
in the activated state than the reactants, and the reac-
tions are slow as evidenced by a slow mass loss in the
thermogram. The thermogravimetric profiles and their
corresponding kinetic data are in good agreement with
a multistage decomposition/oxidation of the metal che-
lates. In both cases, it is observed that the activation
sponds to the loss of three chloride ions and the salicy-
laldehyde moiety. The third stage of decomposition
originates at 603 K and completes at 890 K (Figs. 4a,
4
4
b). The mass loss of 41.81% (calculated value,
2.02%), which occurs in this region, accounts for the
elimination of the thiophene moiety and formation of
the stable oxide La O . In the DTG profile, these three
2
3
stages of decomposition are represented by three differ-
ent peaks at 413, 553, and 803 K, respectively.
However, the nitrato complex decomposed in two energies for the final stage of thermal decompositions
stages; the first stage with a mass loss of 43.56% (cal- are higher than those for all the other earlier stages of
O
HC
HO
S
O N
N
O
O
O
OC H
2 5
Ln
O
N
N
O
O
O
O
OH
N
O
OC H
2
5
CH
S
Fig. 3. Structure of the nitrato complex [Ln(HSAT) (NO ) ].
2
3 3
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 8 2006

6
06
MOHANAN, DEVI
Table 6. Scheme of thermal decomposition of [La(HSAT) (H O) Cl ] and [La(HSAT) (NO ) ]
2
2
3
3
2
3 3
Stage
Decomposi- Peak tem-
Mass loss
Probable mode of decomposition
and fragments lost
Complex
of decom- tion tempera- perature,
position ture range, K
K
found, % calc., %
[
[
La(HSAT) (H O) Cl ] First
408–420
453–583
603–890
453–613
683–873
413
5.50
36.44
41.81
43.56
40.50
5.64 Loss of three coordinated water mole-
cules
2
2
3
3
Second
Third
553
803
560
823
36.72 Loss of salicylaldehyde moiety and
three chlorine atoms
42.02 Loss of aminothiophene moiety and ox-
idation of La to La O
2
3
La(HSAT) (NO ) ]
First
43.87 Loss of salicylaldehyde moiety and
2
3 3
three NO group
3
Second
40.82 Loss of aminothiophene moiety and ox-
idation of La to La O
2
3
thermal decomposition. This can be attributed to the
Also, the stretching frequency of the ester of the car-
different types of decomposition mechanisms operat- bonyl group observed for the methyl derivative at
–
1
1
644 cm is a direct indication for the occurrence of
ing in different temperature ranges.
transesterification. Substitution of the ethyl group by
the methyl group is further supported by the NMR
Transesterification. There are several reports that
metal complexes of carboxylic esters undergo facile spectrum. Although several mechanisms concerning
transesterification on refluxing with alcohol [36, 37]. transesterification were suggested, it appears that an
Most of such studies have been carried out on transi- increased nucleophilicity of the acyl carbon atom
induced by the azomethine group is of prime impor-
tance. That is, the alkoxy carbonyl group immediately
attached to the α-carbon atom can be transesterified
easily [38]. In view of the difficulty of preparing metal
chelates of esters, this general method of synthesis by
transesterification should prove beneficial.
tion-metal complexes with different types of ligands
38]. However, such studies on lanthanide complexes
are rare. The complex [La(HSAT) (NO ) ] has been
subjected to transesterification reaction in methanol
medium [13]. The crystallinity, appearance, and solu-
bility behavior of the product obtained after transesteri-
fication are distinctly different from those of
[
2
3 3
Antimicrobial properties. The antimicrobial prop-
erties of the lanthanide(III) complexes have been
[
La(HSAT) (NO ) ].
2
3 3
ln(g(α)/T2)
ln(g(α)/T2)
(
a)
(b)
–
–
–
–
12
13
14
15
–
–
–
–
–
–
–
–
10
11
12
13
14
15
16
17
–16
17
–
1
.6
1.7
1.8
1.9
2.0
2.1
1.15
1.20
1.25
1.30
1.35
3
3
1
/T × 10
1/T × 10
Fig. 4. Decomposition of [La(HSAT) (H O) Cl ]: stage 1 (a) and stage 3 (b).
2
2
3
3
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SYNTHESIS, CHARACTERIZATION, THERMAL STABILITY, REACTIVITY
607
ln(g(α)/T2)
ln(g(α)/T2)
(
a)
2
0
2
4
6
8
–
–
–
–
–
–
10
11
12
13
14
15
(b)
–
–
–
–
–
–
–
10
12
14
1
.5
1.6
1.7
1.8
1.9
2.0
1.15
1.20
1.25
1.30
1.35
1.40
3
3
1
/T × 10
1/T × 10
Fig. 5. Decomposition of [La(HSAT) (NO ) ]: stage 1 (a) and stage 2 (b).
2
3 3
reported by several investigators [39, 40]. Allured by as a diffusion barrier and also provide suitable sites for
these observations, biological experiments for evaluat- binding. Apart from this, a decrease in polarity
ing the antibacterial and antifungal activities of the increases the lipophilic character of the chelate, and the
ligand and some of their selected complexes have been interaction between the metal ion and lipid is thus
performed using reported method [41, 42]. The screen- favored. This can lead to the breakdown of the perme-
ing data obtained for some pathogenic bacteria and ability barrier of the cell resulting in interference with
fungi are presented in Table 8. It has been observed that the normal cell processes. If the geometry and charge
the ligand is physiologically active and chelation distribution around the molecule is incompatible with
enhances its activity.
the geometry and charge distribution around pores of
the bacterial cell wall, penetration through the wall by
a toxic agent cannot take place, and this prevents the
toxic reaction within the pores [45]. In addition to this,
anion coordination has also got some influence on the
antibacterial activity as revealed by the metal com-
plexes under investigation.
A possible mode of toxicity can be speculated in the
light of chelation theory [43, 44]. Chelation reduces the
polarity of the metal ion considerably, mainly because
of the partial sharing of its positive charge with the
donor group and a possible π-electron delocalization
over the whole chelate ring. The lipids and polysaccha-
rides are some important constituents of cell walls and
Chelation is not the only reason for antibacterial
membranes, which are preferred for metal ion interac- activity. The higher toxicity of the metal complexes can
tion. In addition to this, cell walls also contain many be attributed to the involvement of a metal ion in the
amino phosphates, carbonyl and cysteinyl ligands, normal cell processes [45]. The widespread interaction
which maintain the integrity of the membrane by acting of metal ions with cellular components is due to the fact
Table 7. Kinetic parameters of the lanthanam(III) complexes of HSAT
Order
parameter, n
Activation energy Preexponential
Entropy of activation
Correlation
coefficient, r
Complex
–1
–1
–1
–1
(E), kJ mol
factor (A), sec
(∆S), J K mol
[
La(HSAT) (H O) Cl ]
1.3
1.1
1.5
1
129.05
19.21 × 106
–110.64
–144.06
–179.66
–37.90
0.9922
0.9904
0.9914`
0.9918
2
2
3
3
(
2nd stage)
[
La(HSAT) (H O) Cl ]
175.09
94.07
5.01 × 105
4.84 × 103
2
2
3
3
(
3rd stage)
[
(
La(HSAT) (NO ) ]
2
3 3
1st stage)
[
(
La(HSAT) (NO ) ]
225.12
1.80 × 1011
2
3 3
2nd stage)
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 8 2006

6
08
MOHANAN, DEVI
Table 8. Antibacterial and antifungal activities of HSAT and some of its selected lanthanide(III) complexes
Zone of inhibition, mm
Compound
E. coli
S. aureus
S. caure
C. albicans
A. niger
HSAT
9
14
11
18
14
17
14
10
19
16
20
17
21
18
10
20
17
24
19
16
14
11
19
16
21
17
18
15
12
21
18
23
18
23
18
[
[
[
[
[
[
La(HSAT) (H O) Cl ]
2 2 3 3
La(HSAT) (NO ) ]
2
3 3
Nd(HSAT) (H O) Cl ]
2
2
3
3
Nd(HSAT) (NO ) ]
2
3 3
Sm(HSAT) (H O) Cl ]
2
2
3
3
Sm(HSAT) (NO ) ]
2
3 3
that all the above structures contain a variety of func- geometry of the complex, concentration, hydrophilic-
tional groups that can act as metal-binding ligands. The ity, lipophilicity, presence of other coligands, steric and
problem is how to obtain these interactions in cells and pharmacokinetic factors, and other environmental fac-
organisms, whose nonpolar membranes hinder the tors.
movement of charged metal ions into the cell, where a
myriad of metal binding sites exist to compete for the
metal ion and where specificity of cellular interaction
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complex must have high enough thermodynamic stabil-
ity to reach the site without being dissociated and (c)
then react as metal complexes with cells to produce a
selective effect.
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