Studies on the Effect of Picolines on the Stereochemistry of Lanthanide(III) Nitrate Coordination Compounds of 4[N-Furfural)amino]antipyrine Semicarbazone and Antibacterial Activities

4[N-Furfural)amino]antipyrine Semicarbazone의 질산 란탄(III) 배위화합물의 입체화학에 미치는 Picolines의 영향과 항박테리아 활성

Abstract

The effect of ${\alpha}$-, ${\beta}$- and ${\gamma}$-picolines on the stereochemistry of the coordination compounds of lanthanide(III) nitrates derived from 4[N-(furfural)amino]antipyrine semicarbazone (FFAAPS) has been studied. The general composition of the present coordination compounds is [Ln(FFAAPS)$(NO_3)_3$Pic] (Ln=La, Pr, Nd, Sm, Gd, Tb, Dy or Ho and Pic=${\alpha}$-, ${\beta}$- or ${\gamma}$-picolines). All these coordination compounds have been characterized by elemental analyses, molecular weight, molar conductance, magnetic susceptibility, infrared and electronic spectra. The infrared studies suggest that the FFAAPS behaves as a neutral tridentate ligand with N, N, O donor while ${\alpha}$-, ${\beta}$- or ${\gamma}$-picoline is coordinated to the lanthanide(III) ions via heterocyclic N-atom. Nitrates are bicovalently bonded in these compounds. From the electronic spectral data, nephelauxetic effect (${\beta}$), covalence factor ($b^{1/2}$), Sinha parameter (${\delta}%$) and the covalence angular overlap parameter (${\eta}$) have been calculated. Thermal stabilities of these complexes have been studied by thermogravimetric analysis. The coordination number of lanthanide(III) ions in the present compound is found to be ten. The antibacterial studies screening of the primary ligand FFAAPS and the complexes showed that the present complexes have moderate antibacterial activities.

4[N-(furfural)amino]antipyrine semicarbazone(FFAAPS)와 질산 란탄(III)이 형성하는 배위화합물의 입체화학에 미치는 ${\alpha}$-, ${\beta}$- 및 ${\gamma}$-picolines의 영향을 연구하였다. 이들 배위화합물의 일반적 조성은 [Ln(FFAAPS)$(NO_3)_3$Pic] (Ln=La, Pr, Nd, Sm, Gd, Tb, Dy 또는 Ho 및 Pic=${\alpha}$-, ${\beta}$- or ${\gamma}$-picolines)이다. 모든 배위화합물은 원소분석, 분자량, 몰전기전도도, 자기수자율, 적외선 및 자외선 스펙트럼으로 특성을 조사하였다. 적외선 연구결과 FFAAPS는 N, N, O 주개를 갖는 중성 삼배위 리간드로 행동하는 반면, ${\alpha}$-, ${\beta}$- 또는 ${\gamma}$-picoline는 헤테로 N-원자를 통하여 란탄(III) 이온에 배위된다. 질산 음이온은 이들 화합물에서 이배위로 결합한다. 자외선 스펙트럼 결과로부터 전자구름 퍼짐 효과(${\beta}$), covalence factor($b^{1/2}$), Sinha parameter (${\delta}%$) 및 covalence angular overlap parameter(${\eta}$)를 계산하였다. 이들 착물의 열적성질을 열무게 분석법에 의해 연구하였다. 본 화합물에 있어서 란탄(III)의 배위수는 10으로 조사되었다. 기본 리간드인 FFAAPS와 착물의 항박테리아 선별조사 결과, 이들 착물은 중간 정도의 항박테리아 활성을 보였다.

INTRODUCTION

Initially coordination chemistry of lanthanides was limited to strongly chelating ligands with oxygen as donor atoms.1-3 With the development of new complexing compounds, a significant number of lanthanide complexes with various types of ligands were synthesized and characterized. The chemistry of metal complexes with heterocyclic compounds containing nitrogen, sulfur and oxygen as complexing ligands has attracted increasing attention. It is well known that heterocyclic compounds are widely distributed in nature and essential to many biochemical processes. These compounds are of worth attention for many reasons due to their biological activities while many drugs involve heterocycles, sulfur, oxygen, nitrogen, amino-nitrogen, azomethine nitrogen and alcoholic or phenolic oxygen are some of the donor atoms of interest. Pyrazolone (N-heterocyclic compounds) is an active moiety as a pharmaceutical ingredient, especially in non-steroidal antiinflammatory agents used in the treatment of arthritis and other musculoskeletal and joint disorders. Earlier work reported that some drugs showed increased activity when administered as metal chelates rather than as organic compounds. The coordinating behavior of 4-aminoantipyrine has been modified into a flexible ligand system by condensation with a variety of reagents like aldehyde, ketone,semicarbazide 4 thiosemicarbazide, etc.5-7 Several biological effects of lanthanides have been developed and recognized for decades and used as tools in biomedical studies in the last century.8-17 In recent years, new experimental methods have been developed due to which new data on the role of lanthanides in the biochemical processes operating in cellular membranes, organelles and cytoplasm have been obtained.8-17 In pursuit better of understanding of the chelating behavior of some N, N, O and N, N, S donor semicarbazones and thiosemicarbazones in metal complexes our 4-7,18-29 and other 30-31 research groups have acquired more information about their nature of coordination behavior, structures, spectral 4-7,18-29 and biological properties.6,24-29 In continuation of our study, in present communication we studied the effect of picolines on the stereochemistry of lanthanide(III) nitrates coordination compounds of 4[N-(furfural)amino]antipyrine semicarbazone (Fig. 1) and their biological properties.

Fig. 1.Structures of 4[N-(furfural)amino]antipyrine semicarbazone (FFAAPS) and α-, β- and γ-picolines.

 

EXPERIMENTAL

Materials

The lanthanide(III) nitrates were obtained from Rare Earth Products Ltd. (India) and were used without further purification. α-, β- and γ-picolines, 4-aminoantipyrine and furfural were purchased from S.D. Fine Chemicals (India). The ligand 4[N-(furfural)amino]antipyrine semicarbazone was synthesized by reported method.4,18 All the solvents were obtained from BDH, E-Merck and S.D. Fine Chemicals (India). These solvents were used as such or after distillation if felt necessary.

Synthesis of the complexes

All the complexes of lanthanide(III) nitrato complexes were synthesized by mixing the methanolic solution of respective lanthanide(III) nitrate, FFAAPS and picolines in the molar ratio 1:1:1 and refluxing the reaction mixture for ca. 2 h. The resulting solution was concentrated by evaporation on a water bath. It was then washed repeatedly with methanol and then extracted with anhydrous diethyl ether to get the solid complex. The coloured complexes were filtered and dried in vacuo over P4O10.

Composition and analytical estimations

The lanthanide content in the complexes was estimated as their oxides by direct combustion in a platinum crucible. The estimation was further confirmed by dissolving the product of direct combustion in dil. HCl. The acid solution of the decomposed complexes was transferred into a flask. Its pH was adjusted to 5.8-6.4 using acetic acid-sodium acetate buffer and was then titrated against 0.1 M EDTA using xylenol orange indicator. The results from both the methods were in excellent agreement within the experimental error. The nitrogen contents in the compounds were estimated by Kjeldahl method. The molecular weights of the complexes were determined cryoscopically in freezing nitrobenzene (PhNO2) using a Beckmann thermometer of accuracy ± 0.01 ℃.

Physical measurements and antibacterial studies

The conductivity measurements were carried out at room temperature in PhNO2 using a Toshniwal conductivity Bridge (type CL01/01) and a dip type cell operated at 220 volts. All the measurements were done at room temperature. A Gouy’s balance was used for the magnetic measurements at room temperature where Hg[Co(SCN)4] was used as a calibrant. The infrared spectra of the lanthanide(III) complexes were recorded, on a Perkin-Elmerspectrophotometer model, in KBr in the range of 4000-200 cm-1. The visible spectra of different Pr3+ , Nd3+ and Sm3+ complexes were recorded using a Hilger Uvispek spectrophotometer with 1 cm quartz cell. The red shift of the hypersensitive bands has been utilized to calculate the nephelauxetic effect (β) in these complexes. From the β-values the covalence factors (b½), Sinha paramter i.e. metal-ligand covalency percent (δ%) and the covalency angular overlap parameter(η) have been calculated by following equations.32,33

The thermogravimetric analysis of lanthanide(III) complexes was carried out in static air with open sample holder and a small platinum boat, the heating rate was 6 ℃/min.

The antibacterial activity of some of the lanthanide(III) complexes were screened by agar-cup method in DMF solvents at a concentration of 50 μg mL-1 and the results were checked against gram positive bacteria Bacillus subtilis (B. subtilis) and Staphylococcus aureus (S. aureus) and gram negative bacteria Escherichia coli (E. coli) and Salmonella typhi (S. typhi).6,28

 

RESULTS AND DISCUSSION

The reaction of methanolic solutions of lanthanide(III) nitrates with 4[N-(furfural)amino]antipyrine semicarbazone (FFAAPS) and α-, β- or γ-picolines (α, β, γ-Pic) resulting in the formation of complexes of the general composition [Ln(FFAAPS)(NO3)3Pic] where Ln = La, Pr, Nd, Sm, Gd, Tb, Dy or Ho. The analytical data of the present compounds are given in Table 1. The complexes are generally stable and could be stored for a long time and are nonhygroscopic in nature. Majority of the complexes do not possess sharp melting point and decomposed around 250 ℃. The analytical data presented in Table 1 indicate that the complexes are generally pure and need no further purification.

The molar conductance values (Table 1) in PhNO2 are too low to account for any dissociation, therefore these coordination compounds are considered to be non-electrolytes.34 The ratio of molecular weight observed to that calculated was ~ 0.98. It clearly shows that the complexes are monomeric in solution. The magnetic moment values calculated in 4f-metal complexes are also given in Table 1 which shows that the lanthanum complexes are diamagnetic in nature. The same is expected from its closed shell electronic configuration and absence of unpaired electrons. The remaining tripositive lanthanide(III) complexes are paramagnetic due to the presence of 4f-electrons, which are effectively shielded by 5s2 and 5p6 electrons. The comparison of these observed values with those observed for 8-hydrated sulphate35 and those calculated for uncomplexed ions, indicates that the 4f-electrons do not participate in any coordination bond formation in these complexes. The magnetic moments of these complexes are well within the range predicted and observed in the compounds of paramagnetic ions as reported earlier.36-38

Table 1.Analytical, conductivity, molecular weight and magnetic moment data of mixed ligand complexes of lanthanide(III) nitrate with FFAAPS as primary ligand and α-, β- and γ-picolines as secondary ligand

Infrared spectra

The main infrared bands of the free ligands (FFAAPS and picolines) and their lanthanide(III) nitrate complexes are given in Table 2. As expected, the ν(NH2) of the hydrazinic nitrogen of semicarbazide (~1622 cm-1) is absent in the infrared spectra of FFAAPS.37,38 It has also been observed that the amide-II band is shifted towards the lower energy side compared to that of the semicarbazone. The effect is due to the electron density drift from the hydrazinic nitrogen.37,38 The characteristics absorption of the carbonyl group in FFAAPS is observed at ca. 1700 cm-1.39 In the complexes, this band is shifted to lower energy region 1655-1645 cm-1 as shown in Table 2. The amide-II band in FFAAPS has been observed at 1565 cm-1, this band is also shifted towards lower wave numbers by ~35 cm-1 i.e. 1630 cm-1. This observation suggests that the coordination is through the carbonyl oxygen atom. The strong band at ca. 1600 cm-1 in the present semicarbazone apparently has a large contribution from the ν(C=N) band in all the complexes as compared to the free ligand. Another strong band was observed at 1610 cm-1 due to azomethine (C=N) absorption. On complexation, this band is shifted towards the lower frequency region, clearly indicating the coordination through the azomethine N-atom.37,38 In far infrared region the bands due to ν(Ln-N)/ν(Ln-O) are also observed.37-41

Table 2.Key infrared spectral bands (cm-1) of mixed ligand complexes of lanthanide(III) nitrate with FFAAPS and α-, β- and γ-picolines

In the spectra of picolines, strong absorptions occur in range 1650-1435 cm-1 due to C=C, C=N stretching and ring vibrations.42,43 Out of these, the absorptions associated with the cyclic ring are apparently unaffected on complexation, while those arising from the heterocyclic rings are shifted to higher frequencies due to tightening of the ring on coordination. This is suggestive of the view that the picoline is bonded with Ln3+ ion through the hetero-N atom.42-44

In all the nitrato complexes, the occurrence of two strong absorptions at 1525-1500 cm-1 and 1300-1285 cm-1 region (Table 3) is attributed to D4 and D1 modes of vibration of the covalently bonded nitrate group, respectively, suggesting that the nitrate groups lie inside the coordination sphere.45 Other absorptions associated with the covalent nitrate groups are also observed in the spectra of the metal complexes. If the (ν4-ν1) difference is taken as an approximate measure of the covalency of the nitrate group,45 a value of ~200 cm-1 for the complexes studied herein suggest strong covalency for the metal-nitrate bonding. According to Lever et al., 46 bidentate coordination involves a greater distortion from D3h symmetry than monodentate coordination. To identify the monodentate or bidentate nature, we applied Lever separation method which states that a separation of 40-50 cm-1 in the combination bands in the 1800-1700 cm-1 region conclude the bidentate nitrate coordination.46 The bidentate character of the nitrate groups has been established by X-ray47 and neutron diffraction studies.48 Hence the nitrate groups in the present complexes are of bidentate nature.

Table 3.Infrared absorption frequencies (cm-1) of NO3 - ion in La(FFAAPS)(NO3)3 with α-, β- or γ-picolines

Electronic spectra studies

The electronic spectral data of the complexes in acetonitrile were recorded and the data for some typical complexes are shown in Table 4. The electronic spectral data for an aqueous salt solution are also given in Table 4. Lanthanum (III) does not show any significant absorption in the visible region. However, praseodymium (III), neodymium( III), samarium(III), gadolinium(III) and dysprosium(III) has the absorption bands in the visible and near infrared region. These bands appear due to transitions from the ground levels 3H4, 4I9/2, 6W5/2, 8S7/2 and 6H15/2 to the excited J-levels of 4f-configuration, respectively. These red shifts or nephelauxetic effect are also observed in acetonitrile solution of the com lexes. The red shift is usually accepted as evidence of a higher degree of covalency than existing in the aquo compounds.32 The marked enhancement in the intensity of the band in all the complexes has been observed which were utilized to calculate the nephelauxetic effect (β) in the complexes. Using the β-values the covalence factor (b½), Sinha parameter i.e. metalligand covalency percent (δ%) and the covalency angular overlap parameter(η) have been calculated (cf. vide infra). The positive value for (1-β) and δ% in these chelate complexes (Table 4) suggest that the bonding between the lanthanide(III) and the ligands is more covalent with respect to the bonding in the lanthanide(III) aquo ion. The positive values of b½ and η support covalent bonding.

Table 4.Electronic spectral data (cm-1) and related bonding parameters of mixed ligand complexes of lanthanide(III) nitrate with FFAAPS andα-, β- and γ-picolines

Thermogravimetric studies

Thermoanalytical results of typical [Ln(NO3)3(FFAAPS)α-Pic] (Ln=La, Pr, Nd, Sm or Gd) complexes are shown presented In Table 5. The pyrolysis curves of these complexes indicate virtually no change in weight up to 120 ℃. At 125-165 ℃, a loss of 11.87-12.20% has been observed, which corresponds to the evaporation of one α-picoline ligand. Further, a loss of 53.49-54.60% in 280-340 ℃ temperature region shows the loss of FFAAPS ligand. The lanthanide oxide (La2O3, Pr6O11, Nd2O3, Sm2O3 or Gd2O3) is the final product at ~850 ℃ as shown in the following equations.49

Table 5.aCalculated for loss of α-Pic. bCalculated for loss of FFAAPS. cCalculated for lanthanide oxides (La2O3, Pr6O11, Nd2O3, Sm2O3, Gd2O3).

Antimicrobial studies

Several biological activities of lanthanide(III) complexes have been identified and used as tools in biomedical studies. Their potential pharmaceutical values have received attention as well.8-17 The antibacterial activities of the present Lanthanide(III) complexes and tetracycline standard drug were also screened by agar-cup method in DMF solvent at a concentration of 50 μg mL-1.6,28 The results were checked against gram positive bacteria B. subtilis and S. aureus and gram negative bacteria E. coli and S. typhi and reported in Table 6. The diameters of zone of inhibition (in mm) of the standard drug tetracycline against gram positive bacteria B. subtilis and S. aureus and gram negative bacteria E. coli and S. typhi were found to be 18, 17, 21 and 22, respectively. Under identical conditions, Table 6 shows that the Lanthanide(III) complexes of FFAAPS and α-picoline have moderate antibacterial activities against these bacteria.

Table 6.Antibacterial activity of FFAAPS and [Ln(FFAAPS)(NO3)3α-Pic]

 

CONCLUSION

The isolated complexes have been characterized by elementalanalyses, molar mass, molar conductance, magnetic susceptibility, infrared and electronic spectral studies. The conductance, molecular weight determination of these coordination compounds in nitrobenzene indicates their non-ionic nature. Hence all the three nitrate groups are present inside the coordination sphere. Infrared data reveals the bidentate (O, O donor) nature of NO3−. FFAAPS is acting as neutral tridentate (N,N,O-donor) and α-, β- and γ-picolines is coordinating via its heterocyclic N-atom. The overall experimental evidence showed that these metal ions display a coordination number ten. The lanthanide(III) ions are surrounded by 7-oxygen atoms (6-oxygens of 3-bidentate nitrate ions, 1-oxygen from amide groups of FFAAPS), 2-nitrogen atoms of azomethine groups of FFAAPS and 1-nitrogen atom of heterocyclic ligand and thus produces a coordination number of ten for the central lanthanide(III) ion (Fig. 2).50

Fig. 2.Proposed structure of [Ln(FFAAPS)(NO3)3 γ-Pic], Ln = La, Pr, Nd, Sm, Gd, Tb, Dy or Ho) and Pic = α-, β- or γ-picolines (C.N. = 10).