Studies of Some Lanthanide ( III ) Nitrate Complexes of Schiff Base Ligands

The studies of 16 new lanthanide(III) nitrate complexes of Schiff base ligands are discussed. Schiff bases were obtained by the condensation of 2–methyl–4–N,N–bis–2'–cyanoethyl aminobenzaldehyde with aniline and 3 different substituted anilines. Lanthanide(III) nitrates, viz. gadolinium(III) nitrate, lanthanum(III) nitrate, samarium(III) nitrate and cerium(III) nitrate were chosen to synthesize new complexes. The complexes were characterized on the basis of physicochemical studies viz. elemental analysis, spectral, viz. IR and electronic spectral and magnetic studies. TGA studies of some of the representative complexes were also done. Some of the representative complexes were also screened for the anti microbial studies.


Introduction
Lanthanides or lanthanons form a longest series of the periodic table.It is 4f-inner transition series.Lanthanide(III) ions, because of their size and charge are the best ions to form stable complexes with high coordination number [1][2][3][4][5] .Coordination compounds of lanthanide in which lanthanide ions exhibit coordination number 6 to 10 are reported [6][7] .
Schiff base metal complexes have played a major role in the development of coordination chemistry [8][9][10][11] .In this work, we wish to report lanthanide(III) nitrate complexes with some Schiff base ligands shown in the Figure 1.

Preparation of ligands
Schiff base ligands were prepared by either of the following methods [12][13] .

Method (A)
A solution of the aldehyde (1 mmol) in absolute alcohol (50 mL) was mixed with corresponding amine i.e. (i) aniline (ii) p-fluoroaniline (1:1 mmol) in the same solvent and two drops of piperidine were added.The mixture was refluxed for 4½ h.On cooling dark coloured solids separated, which were filtered under suction and recrystallised from ethanol as pale/ yellow solids.

Method (B)
A mixture of the aldehyde (1 mmol) and corresponding amine i.e. (i) p-chloroaniline (ii) p-toludine (1:1 mmol) was taken in a round bottom flask to which two drops of piperidine were added.

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The contents were heated for 4½ h in an oil bath maintained at 105-110 0 C. The contents first melted and then solidified.The Schiff bases were purified by recrystallisation from ethanol as pale/yellow solids.Physical characteristics of the Schiff base ligands are listed in Table 1.

Synthesis of complexes
Complexes were prepared by treating corresponding lanthanide(III) nitrate (1 mmol) in isopropanol with ligand solution (1:4 mmol) in the same solvent.In some cases complexes were isolated immediately in cold while in other cases in hot solution.In some cases resulting solution were refluxed on the water bath for 1-3 h (Ca 70-75 0 C).The resulting complexes were collected after filtration washed with isopropanol and finally with ether and dried in vacuo over CaCl 2 14 .The elemental analysis of these complexes are given in Table 2. New complexes are fairly stable and can be stored for a long period.All are sufficiently soluble in common organic solvents and are non hygroscopic in nature.These complexes show almost negligible electrical conductivity in PhNO 2 medium which shows their non electrolytic nature 15 .

Results and Discussion
The molecular weight data of the complexes also support this fact.Magnetic moment data show that lanthanum(III) nitrate complexes are essentially diamagnetic in nature while all other complexes are paramagnetic due to presence of 4f-electrons which are effectively shielded by 5s 2 p 2 electrons 16 .This shows that 4f-electrons do not participate in the bond formation 17 .Magnetic moment data of the complexes are assembled in Table 2.

Spectroscopic studies Infrared Spectral studies
The I.R. Spectra were recorded in the 4000-400 cm -1 range using KBr pallets.IR spectra of the Schiff base ligands were analysed by comparison of the spectra of substituted benzene and the corresponding assignments were listed with comparing the IR spectra of previously reported data 18 .

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In case of complexes a notable peak 1599-1584 cm -1 of ligands shows negative shift and appears in the range of [1530-1505 cm -1 ] This notable peak is attributed to azomenthine ν(C= N) [19-20] stretching and its shift indicates that in complexes coordination of ligands in complexes is through azo methinic nitrogen atom [21][22] .The bands assigned to benzene ring vibration compared well with those of mono-substituted derivatives of benzene 23 several other modes associated with C-H out of plane deformation appear in the region [764-754 cm -1 ].Some new medium to weak bands were also observed in the range (496 to 470 cm -1 ) in case of complexes ligands have no absorption.These new medium to weak bands are assigned as ν (Ln -N) modes observed for various lanthanide(III) nitrate complexes.The partial IR data for various ligands and their corresponding complexes are given in Table 3.It has been found 24 that the separation of (ν 2 +ν 5 ) and (ν 2 +ν 3 ) is greater in bi-dentate (about 30-40 cm -1 ) than in monodentate (about 10 cm -1 ) nitrate complexes.The separation of (ν 2 +ν 1 ) and (ν 2 +ν7 4 ) frequencies is greater for bi-dentate nitrate anions (about 200-300 cm -1 ) as compared to mono-dentate nitrate anions (about 10-200 cm -1 ) complex containing ionic nitrate groups show only one band in this region assignable to (ν 1 +ν 3 ).The occurrence of two absorptions at 1525-1480 cm -1 and 1325-1283 cm -1 region is attributed to ν 4 and ν 1 modes which suggest that nitrate anions in the complexes under study are covalently bonded and are present inside the coordination sphere 25 If (ν 4 -ν 1 ) difference is taken as an approximate measure of covalence of nitrate groups a value of [~200 cm -1 ] for the complexes under study suggest a strong covalency nature of nitrate in the complexes.Lever separation rule 26 states that bi-dentate coordination of nitrate anion involves a greater distortion from D 3 h symmetry than uni-dentate coordination.In case of spectra of complexes, under study it has been found that the separation of (ν  +ν 4 ) is good enough and is in the order [~200 cm -1 ] this S206 KISHOR ARORA et al.
shows that according to Lever separation rule, nitrate ion may be considered as covalently bonded to metal ions in bi-dentate fashion.Infrared absorption frequencies (cm -1 ) of nitrato groups of lanthanide(III) nitrate complexes of Schiff base are listed in Table 4. Table 4. Infrared absorption frequencies (cm -1 ) of nitrato groups of lanthanide(III) nitrate complexes 1012 cm -1 810 cm -1 754 cm -1 665 cm -1

Electronic spectral studies
Electronic spectral studies of lanthanide(III) metal complexes are significant and are important tool for the measurement of covalency in complexes.The line like spectra of lanthanide(III) metal compound appearing in the UV-visible and near IR regions arise from electronic transitions within the 4f-levels which are normal forbidden (27-28) but may become allowed after removal of degeneracy of 4f-orbitals by external crystal field [29][30] .Lanthanum(III) complexes has no significant absorption in visible region.The absorption bands of samarium(III) in visible and near IR region may appear due to transitions from ground levels 4 H 5/2 to the excited J. levels.
The shift of hypersensitive bands has been utilized to calculate the nephelauxetic effect (β), Sinha's covalency parameter (δ%) (metal ligand covalency percentage) and the covalency factor (b½) along with covalency angular overlap parameter (η) these parameters have been calculated using the following expressions 31 The electronic spectral studies of lanthanide(III) metal complexes yield positive value for (1-β) and (δ%) which suggest that the bonding between metal and ligand is covalent in the complexes.The values of parameter of bonding (β½) and angular overlap parameter (η) are also found to be positive indicating covalent bonding complexes.The electronic spectral data are presented in Table 6.

Thermogravimetric analysis
Thermal studies of two representative complexes have been done successfully and their thermal decomposition patterns in different stages have also being studied.The decomposition data for the complexes is presented in Table 6 and 7.The complexes decompose by loosing ligand in parts in different heating stags and ultimately in the final step metal oxide are found in both the cases at high temperature [32][33] .

Anti microbial studies
Antimicrobial viz.antifungal and antibacterial studies of some of the representative complexes were carried out successfully.Results of these studies are included in Table 8 and 9.The studies were carried out on, Aspergillus fumigatus, Aspergillus niger and Mucor species among fungi and E.coli species in bacteria using paper disc method on appropriate nutrient medium.The results are included in Table 8 and 9.The colony diameters were measured after 48 h of incubation [La(MCEABAB) 4 (NO 3 ) 3 ] and [Ce(MCEABAB) 4 (NO 3 ) 3 ] were found to be effective against E.coli and [Sm(MCEABFAB) 4 (NO 3 ) 3 ] were found to be active against fungal species 14 .

Configuration of the complexes
The preferred coordination number for lanthanide(III) metal ions is either 6, 8 or 10.These Coordination numbers depend upon the nature of the anions present.Conductance and molecular weight data show that all the nitrate ions are present inside the coordination sphere in the complexes.IR spectral data also reveal that all nitrate ions are bi-dentately covalently bonded to lanthanide metal ions in all the complexes studied.Hence coordination number 10 is suggested for metal ion in these lanthanide(III) nitrate complexes [34][35] .The possible structure of the complexes is given

Species
et al.Studies of Some Lanthanide(III) Nitrate Complexes S209

Table 1 .
Physical characteristics of the ligands.

Table 2 .
Analytical, conductance, molecular weight and magnetic moment data for lanthanide(III) nitrate complexes of Schiff base ligands under Study.

Table 3 .
Partial IR spectral data for lanthanide(III) nitrate complexes of Schiff base ligands

Table 8 .
Antifungal activities of some of the representative complexes of lanthanide(III) metal ions with Schiff base ligands (After 48 hours of incubation).