Some Novel Manganese(III) Mixed Ligand Complexes and its Decolourization Studies

Introduction

Synthesis, characterization, and applications of Mn(III) complexes had been the focus of global research interest due to their role in the chemical and biological system.^1-7 Mn(III) complexes supported on Fe₃O₄ nanoparticles had been used for selective oxidation of thiols to sulphides.⁸ Mn(III) porphyrin anchored onto multiwall carbon nanotubes had been used as an efficient and reusable catalyst for heterogeneous reduction of aldehyde and ketones.⁹ Mn(III)-Schiff base dicyanamide complexes had been used for checking the rhombicity effect in peroxidase studies.¹⁰ The application of encapsulated Salen- and Salhd-Mn(III) complexes in an AP- pillared clay for bicarbonate assisted epoxidation of cyclohexane had been demonstrated.¹¹ Mn(III) had been found to play an important role at the redox centers of biological systems, important among those are Mn containing catalase, Mn containing ribonucleotide reductase, and the oxygen evolving center of photosystem II (PS-II).¹² Attempts had been made to mimic the active site of these enzymes also.^13-15 In most of the cases, Mn(III) complexes have been synthesized using potassium permanganate or Mn(II) and Mn(III) acetate.^16-19 However, most of the reported Mn(III)-complexes have octahedral or square pyramidal geometry.^15,20-21

The above studies prompted the authors’ group to initiate the studies on the synthesis, characterization and application of Mn(III) complexes. In an earlier, study synthesis and characterization of novel complexes of Mn(III) with macrocyclic porphyrin ligand and ethylenediamine have been reported by this group.²² The depolymerization activity of these complexes towards humic acid as coal model had also been reported earlier.²³ Recently Mn(III)-porphyrin complexes with oxidation property have been reported by our group.²⁴ Another work on synthesis and characterization of a novel Mn(III)-(-diketone) complex with catalytic and antifungal activity have also been reported from our laboratory.²⁵ Tris (glycinato) Mn(III) complex has been synthesize for the first time in our laboratory.²⁶ There is a report in the literature²⁷ that coordination of imidazole to Mn(III) in a Mn(III)-porphyrin complex enhances the catalytic activity of the complex towards the oxidation of benzyl alcohol to benzaldehyde in presence of sodium periodate. Considering the above report, the authors have synthesized and characterized ethylenediamine mixed ligand complexes of Mn^(III)(gly)₂ Cl, Mn^(III)(gly)₂ Br, Mn^(III)(gly)₂NCS, and Mn^(III)(gly)₂N₃ and have studied the decolourization potential of these novel complexes towards decolourization of methyl red dye. It has been observed that substitution of ethylenediamine in Mn(gly)₂X complex enhances its efficiency as a heterogeneous catalyst towards the decolourization of Methyl Red in presence of H₂O₂.

Materials and Methods

Glycine was purchased from Mumbai (sigma-aldrich). All the reagents and solvents were of analytical grade and procured from Merck Ltd., Mumbai (India) and used without further purification. Mill-Q water had been used throughout the experiments.

Characterization

The purity of these complexes was checked in our laboratory by TLC method using Silica gel on glass plates. The mobile phase was dichloromethane and methanol 7:3 (v/v). Detection was made by keeping the TLC glass plates in iodine-chamber. The complexes were analyzed for C, H, and N element in Sophisticated Instrumentation Center of Cochin University of Science and Technology, Cochin. The UV-Vis spectra of the complexes were recorded on UV-Vis Spectrophotometer Hitachi (Japan) model U-2900 available in our laboratory. The FT-IR spectra of the complex were recorded on Perkin Elmer FT-IR spectrometer 2000 in KBr disk at the Indian Institute of Technology Kanpur. Magnetic susceptibility measurement data for all these Mn(III) complexes were recorded by the instrument VSM (vibrating sample magnetometer) at IIT Kanpur. DART mass spectra of complexes were recorded on Reservoir, JMS-T 100 LC at the Sophisticated Analytical Instrumentation Facility, Central Drug Research Institute, Lucknow.

Synthesis

Preparation of Ethylenediamine Complexes, [Mn(gly)₂X(en)]

All these [Mn(gly)₂X(en)] complexes were prepared according to the earlier reported procedure in the literature [26,28] where X= Cl‾, Br‾, N₃^‾, NCS‾.

For the preparation of azido complex, 0.82 g Mn(gly)₂N₃ complex was refluxed with 1 mL of ethylenediamine for 18 h. After refluxion the obtained product was cooled for1 h at room temperature (27ºC) resulting in dark black powder of Mn(gly)₂ X(en) complex, which was dried under vacuum over P₂O₅ and washed with mill Q water. The Cl‾, Br‾ and NCS‾ ethylenediamine complexes were prepared analogously.

The reaction scheme for all these synthesized complexes is given below.

Where gly = glycine, X = Cl, Br, NCS and N₃

Catalysis of Methyl Red by the Complexes

The reaction solution (2ml) consisted of 0.24% methyl red, 14.63% H₂O₂ and 0.5 mg of the complex in milli Q water Since the complexes were insoluble in the reaction medium hence acted as heterogeneous catalysts. The time of 100% decolourization of the dye was recorded with the help of a stopwatch by visual observations. The UV/Vis spectra of the reaction solution before and after the decolourization of dye was recorded.

Results and Discussion

The objective of this communication has been clearly stated in the introduction section. Here ethylenediamine added complexes of Mn(gly)₂Cl, Mn(gly)₂Br, Mn(gly)₂N₃ and Mn(gly)₂NCS have been synthesized to see if the catalytic properties of these novel mixed ligand complexes towards the decolourization of dye in presence of H₂O₂ is enhanced by the addition of ethylenediamine.

The purity of these complexes was checked by thin layer chromatography on silica gel glass plates. A Single spot for each of the complex indicates that these complexes are pure. All the physical properties of these complexes are summarized in Table 1. The colour is black in all, it indicates that the colour of the complexes is governed by the presence of Mn(III) ion.

Table 1: Physical properties of Mn(III) Complexes. Click here to View table

 

The UV/Vis spectra of the complexes were recorded in dimethyl sulphoxide (DMSO). One of the spectrum for Mn(gly)₂Cl(en) is shown in Fig.1 while the λmax values for these four complexes are given in Table 2. The λmax and the molar extinction coefficient values for this typical spectrum are 484 nm and 2.142×10³ M^-1 cm^-1 respectively. The molar extinction coefficient value ε₄₈₄ for this complex is in the range of spin allowed transition ⁵T₂g → ⁵Eg.²⁹ The λmax values for all these novel mixed ligand complexes are red shifted in comparison to their parent complexes, thus showing the decrease in the crystal field stabilization energies of these complexes. The crystal field stabilization energies of Mn(gly)₂Cl(en), Mn(gly)₂Br(en), Mn(gly)₂N₃(en) and Mn(gly)₂NCS(en) have been evaluated as 23674 cm^-1, 23466 cm^-1, 23854 cm^-1 and 2333 cm¹ respectively whereas the crystal field stabilization energies of Mn(gly)₂Cl, Mn(gly)₂ Br, Mn(gly)₂ N₃ and Mn(gly)₂NCS are 29,069cm-¹, 28409 cm^-1, 25201cm^-1 and 27173 cm^-1respectively.²⁶ In this way the ethylenediamine complexes are relatively less stable than their parent complexes. The ligand field parameters such as 10 Dq, B and β have also been calculated for these ethylendiamine Mn(III) complexes and are summarized in Table 2. The values of Racah interelectronic repulsion parameter (B) are below the free ion value for Mn(III) ion (1140 cm^-1)³⁰ it confirms the covalent nature of metal ligand bonds present in all four complexes while the covalency factor β is varying in the range of 0.75±0.88.

Figure 1: UV/Visible spectra of chlorobis (glycinato) Mn(III) ethylenediamine [Mn(gly)2 Cl (en)] complex. Click here to View figure

 

Table 2: λmax values and Ligand Field Parameter of all these mixed ligand complexes.

Complexes	λmax	10Dq(Cm1)	B(Cm-1)	β
Mn(gly)2N3(en)	482	23,854	883	0.77
Mn(gly)2NCS(en)	488	23,320	863	0 .75
Mn(gly)2Br(en)	486	23,496	870	0.76
Mn(gly)2Cl(en)	484	23,674	876	0.76

 

Figure 2: DART Mass spectra of azidobis (glycinato) Mn(III) ethylenediamine [Mn(gly)2N3(en)] complex. Click here to View figure

 

The magnetic moment values for all these manganese(III) complexes have been found in the range of 4.20- 4.90 BMat room temperature. It confirms the presence of four unpaired electrons and a high-spin d⁴ system in all the four novel complexes.^33,34 On the basis of all the above studies distorted octahedral structure has been proposed for all the novel mixed ligand complexes and it is given as in Fig 4.

Figure 3: Decolorizationof methyl red in presence of H2O2 by the glycinato complex. (a) Spectrum of 0.2% methyl red in 14.62% H2O2 in milli Q water, (b) Spectrum of decolourized 0.2% methyl red by Mn(gly)3 complex in 14.62% H2O2 in  milli Q water. Click here to View figure

 

Figure 4: Structure for all these synthesized complexes, [Mn (gly)2X(en)] Where X= Cl‾, Br‾, N3‾, NCS‾ Click here to View figure

 

Now a days oxidation and removal of synthetic dyes from the environment has become one of the important research areas to control environmental pollution. These novel complexes have been tested for their ability to decolourize methyl red in presence of H₂O₂. The UV spectrum of decolourization reaction of Methyl Red using the initial [Mn(gly)₃] is shown in Fig. 3 here Fig 3 (a) is the spectrum of 0.2% methyl red in 14.63% H₂O₂ in milliQ water and Fig. 3 (b) is the spectrum of the dye decolourized in presence of [Mn(gly)₃] complex. In separate experiments, relative efficiencies of [Mn(gly)₃], [Mn(gly)₂Cl] and [Mn(gly)₂Cl(en)] have been tested towards their ability to decolourize Methyl Red dye. The composition of reaction solution and time required for complete decolourization of the dye are given in Table 4.

Table 3: FT-IR spectra of all these complexes.

Mn(Gly)2 N3en	Mn(Gly)2NCSen	Mn(Gly)2Clen	Mn(Gly)2Bren	Assignment
604	599	613	665	ν(Mn-O)
487	497	510	486	ν (Mn-N)
1385	1320	1320	1320	ν (CH2)
1648	1635	1625	1651	ν (COO)
2040	—–	—–	—–	ν (N3)
—–	2090	—–	—–	ν (NCS)
3400	3400	3400	3368	ν(NH2)coordinated
3583	3583	3583	3583	ν(NH2)free

 

Table 4: The results of decolourization of methyl red by the complexes.

S.No.	Complexes	Quantity	Decolourization Time
1.	Mn(gly)3	0.5 mg	10 sec
2.	Mn(gly)2Cl	0.5mg	90 sec
3.	Mn(gly)2Clen	0.5mg	5 sec

 

The order of efficiency of decolourization for methyl red in presence of these complexes can be given as [Mn^(III) (gly)₂Cl(en)] [Mn(gly)₃] ˃ [Mn^(III)(gly)₂Cl]. The time required for the complete decoulourization of 0.24% Methyl Red in the presence of 14.63% H₂O₂ in case of Mn(gly)₂ Cl(en) complex is only five second while in case of Mn(gly)₃ it is 10 second and in case of Mn(gly)₂Cl it is 90 second. These complexes have been used for three cycles of catalysis without any apparent change in their these complexes are reusable efficient heterogeneous catalysts for dye decolourization and in this way may be used on the industrial scale for the removal of environmental pollution caused by organic dyes.

Conclusion

Four novel complexes [Mn^(III)(gly)₂Cl(en)], [Mn^(III)(gly)₂Br(en)], [Mn^(III) (gly)₂ SCN(en)], and [Mn^(III)(gly)₂N₃ (en)] have been synthesized and characterized by UV-Visible, FT-IR,  Mass spectrophotometry and magnetic susceptibility measurements. Their tentative structure has been proposed to be octahedral. These ethylenediamine mixed ligand complexes of Mn(III) are better reusable heterogeneous catalysts for the decolourization of Methyl Red in presence of H₂O₂.

Acknowledgments

The authors are thankful to CSIR New Delhi for the financial support through the grant no. 02(0261)/16/EMR II dated 28/04/16. The authors are also thankful to the Head Department of Chemistry, D.D.U. Gorakhpur University, Gorakhpur for administrative support. The authors acknowledge the services rendered by the Sophisticated Analytical Instrument Facility (SAIF), Cochin and Central Drug Research Institute (CDRI), Lucknow for recording FT-IR and DART Mass of the samples and IIT Kanpur (ACMS Department) for magnetic susceptibility measurements.

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