SYNTHESIS AND CHARACTERISATION OF MANGANESE(II) COMPLEXES WITH SEMICARBAZIDE AND THIOSEMICARBAZIDE BASED LIGANDS

SYNTHESIS AND CHARACTERISATION OF MANGANESE(II) COMPLEXES WITH SEMICARBAZIDE AND THIOSEMICARBAZIDE BASED LIGANDS

Ruchi Verma

Department of Education in Science and Mathematics, National Council of Education Research and Training, Sri Aurobindo Marg, New Delhi - 110016, India.

ABSTRACT: Mn(II) complexes with four semicarbazide and thiosemicarbazide based ligands such as 2-formyl pyridine semicarbazone (L₁), 2-formyl pyridine thiosemicarbazone (L₂), 5-methyl 2-formyl pyridine semicarbazone (L₃) and 5-methyl 2-formyl pyridine thiosemicarbazone (L₄) have been synthesized and characterized. All the ligands were characterized by elemental analyses, IR, ¹HNMR and mass spectral studies. The complexes were characterized by using various physicochemical techniques such as elemental analysis, molar conductance, magnetic susceptibility measurements and spectral studies such as IR, UV-visible and EPR. The complexes were found to have general compositions Mn(L)₂X₂ [where L = L₁, L₂, L₃ & L₄, X = Cl^–, NO₃^–]. The molar conductance data of chloro and nitrato complexes of L₁, L₂, L₃ and L₄ suggest nonelectrolytic nature of the complexes. Therefore they may be formulated as [Mn(L)₂X₂].Complexes show magnetic   moment corresponding to five unpaired electrons. On the basis of IR electronic and EPR spectral data distorted octahedral geometry of complexes has been suggested.

Mangenese(II) Complexes, Semicarbazones, Thiosemicarbazones

INTRODUCTION: Metal complexes of semicarbazones and thiosemicarbazones were found to show more biological activities as compared to the free semicarbazone and thiosemicarbazone ¹. In addition, the complexes can exhibit bioactivities which are not shown by the free ligands. Owing to the wide range of medicinal properties of semicarbazones and thiosemicarbazones, their metal complexes are emerging as a new class of experimenral studies. A lot of work is going on the synthesis and characterisation of transition metal complexes with semicarbazones and thiosemicarbazones ². The transition metal complexes of these ligands were synthesised mainly due to their pharmacological activities. Another important aspect to synthesize such complexes is to compare the coordination behaviour of   N      O   and   N     S   donor ligands.

The complexes of manganese(II) with sulphur containing ligands play an excellent role in catalytic property ^{3, 4}. Manganese and its compounds are widely used in analytical chemistry, metallurgical processes, paints and pigment industry.

Further, the coordination chemistry of manganese with a diverse range of ligands remains an area of considerable interest towards coordination compounds ^{5, 6, 7} in adsorption into soil because these are very abundant in soil and are essential for plant growth. In soil, these are formed by bio-degradation of lignin ^{8, 9, 10}. Mn(II) was found to be important for enzymatic systems with DNA. DNA and RNA polymerases ^{11, 18} catalyze the replication and transcription of DNA and have a specific requirements for Mn(II) ^{12- 19}. In view of the above applications of Mn(II) the synthesis and characterization of the Mn(II) complexes with semicarbazide and thiosemicarbazide based ligands are highly desirable.    In this paper we repeat the synthesis and spectral studies of Mn(II) complexes with semicarbazide and thiosemicarbazide based ligands: 2-formyl pyridine semicarbazone (L₁), 2-formyl pyridine thiosemicarbazone (L₂), 5-methyl-2-formyl pyridine semicarbazone (L₃) and 5-methyl-2-formyl pyridine thiosemicarbazone (L₄). The ligands were synthesised by the reactions as shown in Fig.1.

(X = O or S in L₁ and L₂ respectively)

(X = O or S in L₁ and L₂ respectively)

(X = O or S in L₃ and L₄ respectively)

FIG. 1: SYNTHESIS OF LIGANDS

MATERIALS AND METHODS:

Experimental procedure: All reagents and chemicals used were of Anala R grade. All solvents used were of standard/spectroscopic grade.

Preparation of Ligand: Ligand L₁ and L₃ were  prepared by adding hot ethanolic solution (20 mL) of 2-formyl pyridine/5-methyl-2-formyl pyridine ( 0.02 mol) to a hot aqueous solution of semicarbazide hydrochloride and 0.02 g sodium acetate. Mixture was refluxed on water bath for an hour at around 80°C. On cooling at »0 °C compounds were  precipitated out. They were filtered and washed with hot water and dried over P₄O₁₀.

Ligand L₂ and L₄ were synthesised by refluxing the mixture of thiosemicarbazide (0.02 mol) and 2-formyl pyridine (0.02 mol)/5-methyl-2-formyl pyridine in a round bottom flask at around 80 °C till clear solution is obtained. Ligand was precipitated out on cooling at 0 °C. It was filtered and washed with ethanol.

Preparation of Manganese (II) Complexes with ligands L₁ to L₄: The complexes were prepared by using semicarbazide and thiosemicarbazide based ligands and corresponding Mn(II) salts (MnCl₂.4H₂O and Mn(NO₃)₂.H₂O). A hot ethanolic (20 mL) solution of corresponding metal salts (0.001mol) was mixed with hot ehtanolic solution of the ligands L₁ to L₄ (0.002 mol). The mixture was refluxed for 3-4 h at 80°C (±5). The completion of the reaction was confirmed by Thin Layer Chromatography. On cooling the contents, coloured complexes were precipitated out. These were filtered, washed with 50% ethanol and dried in vacuum over P₄O₁₀. Purity of the complexes were checked by Thin Layer Chromatography.

Instrumentation: Carbon (C), Hydrogen (H) and Nitrogen (N) were analysed on a carlo-Erba 1106 elemental analyser. Molar conductance was measured on the ELICO (CM82T) conductivity bridge. Magnetic susceptibility was measured at room temperature on a Gony balance using CuSO₄.5H₂O as calibrant ¹HNMR spectra was recorded at room temperature on a Bruker Advance DPX-300 spectrometer using DMSO-d₆ as a solvent. IR spectra (KBr) were recorded in DMSO on a shimadza UV mini-1240 spectrophotometer. EPR spectra were recorded as polycrystalline samples and in a DMSO solution at liquid nitrogen temperature (LNT) and room temperature (RT) on an E₄-EPR spectrometer using DPPH as a g-marker.

RESULTS AND DISCUSSION:

Characterization of ligands: The semicarbazide and thiosemicarbazide based ligands (L₁–L₄) have been characterized by elemental analysis, IR ^20-26, UV, ¹HNMR and mass spectral studies.

Characterization of metal complexes: The formation of the complexes can be represented by the reaction :

L + CoX₂    [Co(L)₂X₂]

Where, L=L₁, L₂, L₃ and L₄

X=Cl^–, NO₃^–, ½SO₄^2–

On the basis of elemental analysis the complexes were found to have general compositions Co(L)₂X₂. The molar conductance data suggest the non electrolytic nature of the complexes. Hence, the complexes may be formulated as [Co(L)₂X₂]. The results of elemental analysis and molar conductance data  of complexes are shown in Table 1.

TABLE 1: ELEMENTAL ANALYSIS AND MOLAR CONDUCTANCE DATA OF Mn(II) COMPLEXES

IR Spectra of complexes: The important IR bands and their assignments of semicarbazide and thiosemicarbazide based ligands and their Mn(II) complexes are shown in Table 2.

TABLE 2: IMPORTANT IR BANDS (cm^–1) AND ASSIGNMENTS OF SEMICARBAZIDE AND THIOSEMCARBAZIDE BASED LIGANDS AND THEIR Mn(II) COMPLEXES

Abbreviations; s= strong, ms=medium strong, m = medium, mw=medium weak, w = weak, sh = sharp

FIG. 1: IR SPECTRUM OF [Mn(L₁)₂Cl₂)]

FIG. 2: IR SPECTRUM OF [Mn(L₃)₂Cl₂]

FIG. 3: IR SPECTRUM OF [Mn(L₄)₂Cl₂]

IR Spectral Bands due to Anions:

IR Spectra of Nitrato Complexes: The IR spectra of all the nitrato complexes show absorption bands in the region of 1464-1526 (n₅) n_a (NO₂, 1167-1199 (n₁), n_s(NO₂) and 778-806 cm^–1 (n₂) n (NO). This indicates that the nitrate group coordinates in a unidentate manner.

Magnetic Moment: The magnetic moment ^27-30 of all the Mn(II) complexes under study at room temperature lies in the range of 5.91 – 6.05 B.M., corresponding to five unpaired electrons (Table 3).

 Electronic spectra of Mn(II) complexes of L1, L2, L3 and L4: The electronic spectra ^31-52 of Mn(II) complexes were recorded using DMF/DMSO as a solvent and are depicted in (Fig. 3.4-3.9). The electronic spectral data of complexes are listed in Table 3.

Electronic spectra of complexes under study show bands in the region of 17637-18658 (n₁), 18976-24652 (n₂), 23211-29413 (n₃), 32678-39372 (n₄) cm^–1 which are characteristic of distorted octahedral geometry. The assignments are obtained by fitting the observed spectra to the Tanabe-Sugano diagram. Thus these bands may be assigned to following transitions:

⁶A₁g ® ⁴T_1g (⁴G) (n₁)

⁶A₁g ® ⁴E_g, ⁴A₁g (⁴G) (10B+5C) (n₂)

⁶A₁g ® ⁴E_g, (⁴D) (17B+5C) (n₃)

⁶A₁g ® ⁴T_1g, (⁴P) (n₄) respectively ^50-52.

TABLE 3: MAGNETIC MOMENT AND ELECTRONIC SPECTRAL DATA OF Mn(II) COMPLEXES

FIG. 4: ELECTRONIC SPECTRUM OF [Mn(L₁)₂Cl₂]

FIG. 5: ELECTRONIC SPECTRUM OF [Mn(L₁)₂(NO₃)₂]

FIG. 6: ELECTRONIC SPECTRUM OF [Mn(L₃)₂Cl₂]

FIG. 7: ELECTRONIC SPECTRUM OF [Mn(L₃)₂(NO₃)₂]

FIG. 8: ELECTRONIC SPECTRUM OF [Mn(L₄)₂Cl₂]

FIG. 9: ELECTRONIC SPECTRUM OF [Mn(L₄)₂(NO₃)₂]

Ligand Field Parameters: The experimentally observed transition energies are used to calculate the values of parameters B, C, Dq and β. The best set of values for parameters B and C could be obtained by using transitions.

6A1g ® 4Eg, 4A1g (G) (10B+5C) (n2)

6A1g ® 4Eg, (4D) (17B+5C) (n3)

This is due to the fact that the energies of these two transitions are independent of the crystal field splitting energy and depends only on parameters B and C ⁵³. The values for Dq could be evaluated with the help of the curve transition energy versus Dq, given by Orgel, using an energy level due to transition 6A1g ® 4T1g. The value of Dq parameter could not be obtained from 6A1g ® 4E1g 4A1g (4G) and 6A1g ® 4Eg (4D) transition because, they have almost zero or negative slope. These transitions are independent of the Dq value (Table 4).

TABLE 4: LIGAND FIELD PARAMETERS OF Mn(II) COMPLEXES

Racah and Slater Condon-Shortly parameters: The values of various parameters are listed in Table 4. Parameters B and C are linear combinations of certain coulomb’s and exchange integral and are generally treated as empirical parameters obtained from the spectra of the free ions. Slater Condon-Shortly parameters F₂ and F₄ are related to the Racah parameters B and C as follows:

B = F₂ –5F₄, C = 35F₄

The values of the parameters F₂ and F₄ are also listed in (Table 4). The electron-electron repulsion in the complexes is less than that in the free ion, resulting in an increased distance between electrons and thus an effective increase in the size of the orbitals. On increasing delocalization the value of b decreases and is less than 1 in the complexes. An estimate of b has been obtained from the nephelauxetic parameter for ligand (hx) and nephelauxetic parameters of the metal ion Km as (‘1-b) = hx X Km. The values of the parameter of the metal (hx) for the complexes have been calculated using the covalency contribution of Mn(II) ions i.e. Km = 0.07, while the B corresponds to 960 cm^–1 for the free Mn(II) ions has been used to calculate the values of b. The calculated value of b and hx indicate that the complexes under study have appreciable ionic character.

CONCLUSION: On the basis of elemental analysis, molar conductance measurements, magnetic moment, IR, electronic, and the subsequent discussion given above the following structure may be proposed for all the complexes (Fig. 10 – 13).

FIG. 10: Mn(II) COMPLEXES WITH LIGAND L₁

FIG. 11: Mn(II) COMPLEXES WITH LIGAND L₂

FIG. 12: Mn(II) COMPLEXES WITH LIGAND L₃

FIG. 13: Mn(II) COMPLEXES WITH LIGAND L₄

ACKNOWLEDGEMENT: The author expresses her sense of gratitude to The Principal, M. M. H. College, Ghaziabad (C.C.S. University, Meerut), N.C.E.R.T., Sri Aurobindo Marg, New Delhi and to SAIF, IIT, Mumbai, Department of Chemistry, University of Delhi for providing instrumental facilities.

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    Verma R: Synthesis and characterisation of manganese(II) complexes with semicarbazide and thiosemicarbazide based ligands. Int J Pharm Sci Res 2017; 8(3): 1504-13.doi: 10.13040/IJPSR.0975-8232.8(3).1504-13.

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