View of THE SPECTRAL STUDIES OF IRON (III) COMPLEXES OF A TRIDENTATE 1, 5-BIS (4-METHYL, BENZIMIDAZOL-2-YL)-3-THIAPENTANE LIGAND

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Vol. 02, Issue 12,December 2017 Available Online: www.ajeee.co.in/index.php/AJEEE

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THE SPECTRAL STUDIES OF IRON (III) COMPLEXES OF A TRIDENTATE 1, 5-BIS (4- METHYL, BENZIMIDAZOL-2-YL)-3-THIAPENTANE LIGAND

Darpan Singh¹, Vishrut Chaudhary² and Meghraj Singh³

1Lecturer Department of chemistry DIET Meerut (U.P.) India

2(Corresponding Author) Assistant Professor Department of Chemistry D.N. College Meerut (U.P.), India

3Lecturer Department of Chemistry ASIC Mawana Meerut (U.P.) India

Abstract - The mononuclear iron(III) complexes with stoichiometry (Fe(BMBES)X3].nH2O have been synthesized and characterized utilizing a tridentate ligand. 1, 5-bis(4- methyl,benzimidazol-2-yl)-3-thiapentane with anionic ligand like Cl-, NO3-, H3O-.

Mössbauer data for Fe(III) complexes indicate that the isomer shift values lie in the range typically observed for high spin Fe(III) complexes.

1 INTRODUCTION

The presence of oxo-bridged diiron centers in a variety of non-heme proteins and enzymes has resulted in the synthesis and characterization of a large number of oxo and hydroxo (alkoxo or phenoxo) bridged diiron complexes in an effort to model such active sites. These complexes display antiferromagnetic exchange coupling comparable to the biological systems. The size and extent of magnetic exchange parameter J with the nature of bridge and the geometries of the bridging atoms in the cyrstalline form can be correlated. These complexes are also investigated for their interesting electronic and redox properties and their use as catalysts for the oxidation of hydrocarbons.

Synthesis of dialkoxo bridged diiron (III) complexes with unique coordination mode of the ligands and structure, spectra and redox properties of these complexes derived from the flexible 1,3-bis(salicylamino) propan-2-ol, along with the structure and properties of dimeric complexes obtained from the analogous trianionic, pentadentate ligands are reported(H.Aneetha et.

al,1999). Crystal structure contains distorted octahedral iron (III) centers having N2O4 coordination cores with amine nitrogens, bridged by two alkoxo or two phenoxo oxygen atoms. Electronic spectra are characterized by high intensity charge-transfer transitions.

Model Systems that mimic the active sites of metalloenzymes are important not only for the understanding of enzyme mechanism, but also for the development of small molecular weight biomimetic catalysts. Superoxide dismutases (SOD) are metalloenzymes that disproportionate

superoxide into molecular oxygen and hydrogen peroxide through a cyclic oxidation-reduction mechanism (Fridovich et. al,1995). The active site structures of the Mn-and Fe-SODs have a distorted trigonal bipyramidal geometry in both the oxidized and the reduced resting states. It has been proposed that the coordination number of the active sites changes from five coordinate in the resting states to six- coordinate in both the oxidized and the reduced intermediate sates. The change in coordination number and geometry was accompanied by a change in charge at the active site through deprotonation of the coordinated water and protonation of the coordinated hydroxide and through the oxidation and reduction of the metal center. Metal complexes possessing ligands capable of being in multiple charge states might yield useful information regarding the relationship between the charge of the ligand and coordination geometry. Recently, Dohyun moon and myoung soo Lah reported a mononuclear octahedral Fe (III) complex containing the tripodal tetradentate ligand tris (2-benzimidazolyl-methyl) amine (H3ntb) in which all three amines of the benzimidazolyl groups are protonated(Kwak et. al,1999). The ligand may have different charges depending on the number of deporotonated secondary amine groups.

2 EXPERIMENTAL

All chemicals used were of AR grade and used as received. ⁵⁷Fe Mössbauer spectra were obtained using a ⁵⁷Co in Rh source, mounted on a constant acceleration spectrometer at the IIT Roorkee and CDRI Lucknow. The velocity scale was

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calibrated using a foil of natural -Fe at the room temperature and all isomer shifts are referred to the centre of the a-Fe spectrum. The ligand 1, 5-bis(4- methyl,benzimidazol-2-yl)-3-thiapentane

(BMBES) was characterized by elemental analysis ¹HNMR and electronic spectroscopy.Room temperature magnetic susceptibility measurements were made on Gouy balance.

3 METHOD OF PREPARATION OF COMPLEXES

[Fe(C20H22N4S)Cl3] :

To a solution of ligand 1:5-Bis(4-methyl, benzimidazol-2-yl)-3-thiapentane (2mmol 1.65 g. in 5 ml, MeOH) was added a solution of FeCl3 (2 mmol 0.55 g. in 5 ml) and was stirred overnight. Resulting brown solution was concentrated to approximately 5 ml on a rotatory evaporator and refluxed for one hour, on cooling. Addition of ether to this solution immediately resulted in formation of yellowish brown product. This was filtered and washed by a mixture of 1:2 MeOH:CH3CN solution and dried in vacuo over P4O10. Yield : 75%.

[Fe(C20H22N4S) (NO3)3] :

Ligand 1:5-Bis(4-methyl, benzimidazol-2- yl)-3-thiapentane (2 mmol 0.70g.) was suspended in 10 ml CH3CN) and Fe(NO3)3

(2 mmol 0.70g in 5 ml MeOH), was added.

On stirring and little heating a yellowish brown product was obtained. Product was filtered and washed with a mixture of 1:3 MeOH:CH3CN and finally with CH3CN and stored over P4O10 Yield : 70%.

[Fe(C20H22N4S) (HCOO)3] :

To an aqueous solution of FeCl3 (2 mmol 0.55g) an aqueous solution of NaOH was added and precipitated Ferric hydroxide (2 mmol) was washed and suspended in methanol. A dilute solution of formic acid in methanol (1:1) was added dropwise till the precipitate dissolved to give a clear solution. This was then added to a solution of ligand (C20H22N4S) (2 mmol in 10 ml MeOH) and resulting dark brown solution was refluxed for 6 hrs. On addition of ether, light brown product was obtained. This was filtered and washed with 1:2 MeOH: ether mixture, dried and stored over P4O10. Yield: 70%.

Table 1 Micro-analytical and Magnetic moment data of Iron (III) complexes

% Found (Calculated)

μeff (BM)⁺

Complexes C H N Fe Solid

[Fe(C20H22N4S)Cl3].3H2O 43.0 4.90 9.90 9.70 5.46 (43.13) (5.03) (10.06) (9.86) [Fe(C20H22N4S)(NO3)3].3H2O 37.01 4.23 15.10 8.50 5.37

(37.16) (4.33) (15.17) (8.64) [Fe(C20H22N4S)(HCOO)3] 50.92 4.50 10.22 10.20 5.32

(51.03) (4.62) (10.35) (10.32) 4 RESULT AND DISCUSSION

4.1 Electronic Spectroscopy:

Electronic spectra of all Fe(III) complexes were recorded in DMF. The study of absorption spectra of present benzimidazole ligands in DMF reveals two prominent absorption bands in ultraviolet region. These bands could be assigned to

  * transition within the benzimidazole nucleus. Present Fe(III) complexes also show UV spectral bands characteristic of benzimidazolyl group in the range of 270- 290 nm . These bands are blue shifted by a few nm with respect to the ligand (C20H22N4S) whereas for binuclear complexes with ligand. These bands are red shifted by a few nm. This supports the

binding of imidazolyl ring imine nitrogen to metal center(V.Mckee et. al,1985).

Fe(III) is high spin in most of its complexes accept with very strong field ligands. In complexes of Fe(III), a d⁵ case with weak field ligands each (d) orbital is singly occupied with parallel spins, any transition within (d) level must involve reversal of spin so there are no spin allowed (d-d) transitions; all are both multiplicity and Laporte forbidden, therefore bands may be usually very weak. Here ground state term is ⁶S, of the eleven excited states, four quartets i.e. ⁴G,

4F, ⁴D, and ⁴P involve the reversal of only one spin whereas other seven states are

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doublets and are doubly spin forbidden and are unlikely to be observed. So in octahedral ligand field extremely weak bands may be observed with molar extinction coefficients  ~ 0.02 – 0.03 M^¹ cm^¹. However Fe(III) has high charge and is able to polarize ligands giving intense charge transfer bands in near UV region with strong low energy wings in visible region, that will mask the very weak spin forbidden d-d bands. (Duta R.L et.

al,1992)

4.2 Cyclic Voltammetry:

Cyclic voltammetric experiments were carried out in a solvent mixture of DMSO:

CH3CN (9:1) at room temperature with 0.1 M NaClO4 as supporting electrolyte.

Solutions were deoxygenated by purging with N2 gas for 10 minutes prior to measurements.

All Fe(III) complexes with ligand ( 1:5-Bis(4-methyl, benzimidazol-2-yl)-3- thiapentane) show (Table 3, Fig. 1a-c) reversible/quasi reversible redox waves in the range of -0.205 V to + 0.136V, with E1/2 values ranging from – 0.138 V to + 0.104, corresponding to one electron Fe^(III)/Fe^(II) reduction couple. E1/2 data reveals that formato complex show, most negative potential followed by chloride and then nitrato complexes. Therefore, binding of HCOO^ anion considerably stabilizes the Fe(III) center whereas NO3

destabilizes the Fe(III) center.

Table 3 CV data of Fe(III) complexes at 298 K :

Complex Epc(v) Epa(v) E1/2(v)

[Fe(C20H22N4S)Cl3] - 0.096 +0.002 -0.048

[Fe(C20H22N4S](NO3)3] + 0.068 +0.136

+0.104

[Fe(C20H22N4S)(HCOO) 3] - 0.204 -0.070 -0.138 BMBES-1:5-Bis(4-methylbenzimidazol-2-yl)-3-thiapentane (C20H22N4S).

The variation in E1/2 observed supports that the anionic ligands remain bound to Fe(III) center in solution, an assumption which has been confirmed from other spectral studies.

(Knoop P. et. al,1990)

Fig. 1: Cyclic voltammograms of (a) [Fe(C20H22N4S)Cl3], at the scan rate 100 mVs^-1; (b) [Fe(C20H22N4S)(NO3)3], at the scan rate 100 mVs^-1

(c) [Fe(C20H22N4S)(HCOO)3], at the scan rate 100 mVs^-1 1:5-Bis(4-methylbenzimidazol-2-yl)-3-thiapentane (C20H22N4S).

4.3 Mössbauer spectroscopy:

Mössbauer spectra of Fe(III) complexes were obtained by using a ⁵⁷Co in Rh source, mounted on a constant acceleration(Dodwa S.S. et. al,1989) spectrometer. The velocity scale was calibrated using a foil of natural -Fe at

room temperature and all isomer shifts are referred to center of the -Fe spectrum. Experimental spectra were fitted with Lorentzian lines which are allowed to vary independently and were matched into a doublet, using a least- square computer programme of the

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Mössbauer parameters it is primarily the isomer shift which exhibits the considerable dependence on the covalency of the bond between the metal and donor atom and increase in covalency is accompanied by an increase in the s- electron density on the central atom, and sometimes by decrease in d-electron density, both changes result in the increase in electron density at the metal nucleus. In iron compounds the isomer shift decreases with an increase in the electron density at the nucleus. The more

covalent complexes have lower isomer shifts. The Mössbauer spectra of [Fe(C20H22N4S)X3] complexes at room temperature. Isomer shift values (δ) (Table 4) lie in the range of 0.4 to 0.45 mm s^-1 with respect to natural iron as is typically observed for other high spin iron(III) complexes. The isomer shift of 0.45-0.55 mm s^-1 with respect to metal ion is indicative of high-spin iron complexes having large ionic character to Fe(III)- ligand bond.

Table 4: Mössbauer data for [Fe(C20H22N4S)X3] complexes:

Complexes Isomer Shift Quadrupole Splitting

 ( mm/sec ) Eq

[Fe(C20H22N4S]Cl3 0.42 0.72

[Fe(C20H22N4S)(NO3)3] 0.42 0.86 [Fe(C20H22N4S)(HCOO) 3] 0.46 0.79 (C20H22N4S) - 1:5 – Bis ( 4 – methylbenzimidazol – 2 – yl ) – 3 -thiapentane

It has also been shown with the studies on iron compound, that quadrupole splitting decreases with increase in the symmetry of the central iron atom i.e. larger is the quadrupole splitting more unsymmetrical is the central iron atom.

[Fe(C20H22N4S)(NO3)3] complex shows quite a large quadrupole splitting value (0.82) suggesting that nitrate complex is more distorted than chloro or formato (ΔEq = 0.68, 0.78) complexes. Similar values are obtained for structurally characterized, rhombically distorted iron(III) complexes as well as in certain iron tyrosinate proteins. The values of isomer shift and quadropole splitting are consistent. With formulation of iron atom as high spin Fe(III).

Fig. 2 (a) : Mössbauer spectrum of [Fe(C20H22N4S)Cl3] ,(b) : Mössbauer spectrum of [Fe(C20H22N4S)(NO3)3] & (c) : Mössbauer spectrum of [Fe(C20H22N4S)(HCOO)3] 4.4 Magnetic susceptibility:

Magnetic susceptibility of Fe(III) complexes were determined by using CAHN 2000 balance at room temperature ( 296 K ) in solid state. The diamagnetic correction for each complex(Ray R.K. et.

al,1990) was estimated using pascal

constants and has been incorporated in the experimental susceptibility. Iron (III) is high spin in nearly all its octahedral complexes except for those with very strong field ligands. In high spin complexes, magnetic moment values are always very close to spin only value of 5.9 Velocity (mm / sec)

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BM because the ground state is ⁶Alg

(derived from ⁶S state of free ion and has no orbital angular momentum whereas low spin complexes with t2g⁵configuration usually have considerable orbital contribution to magnetic moment so values of about 2.3 BM at room temperature are obtained which are also temperature dependent (at liquid nitrogen temperature these values decrease to ~ 1.9 BM). There is evidence of high covalence and electron delocalization in low spin complexes. Five coordinate complexes may be high or low spin depending on ligands, mostly high spin complexes are encountered in oxo-bridged dinuclear complexes. (Anthonisamy et.

al,1999)

4.5 IR Spectroscopy:

IR spectra of free ligand (C20 H22 N4S) in KBr show a strong band at K 1435 cm^-1 along with a weaker band at 1470 cm^-1. In analogy with assigned bands for

imidazole, bands at 1430 or 1440 cm^-1 is attributed to C=N-C=C- while the other band is an overtone/or combination band(Ragona P.J. et. al,2003). The shift of this band is around 15-20 cm^-1 in complexes which implies direct coordination of all imine nitrogen atoms to Mn(II) center. This is the preferred nitrogen atom for coordination as found for other metal complexes with benzimidazoles.

In thiocyanato complexes, a strong band present at 2110 cm^-1 suggests presence of N bonded thiocyanato group.

In IR spectrum of Mn(C20H22N4S)(HCOO)2

band at 1390 cm^-1 and 1580 cm^-1 indicate the presence of unidentate formate group(Frontier et. al,2005 & Alexandra Janczyk et. al,2006).

On the basis of above studies the proposed structures of Iron(III) complexes with ligand (C20H22N4S) is shown in Fig 3.

Fig. 3: Proposed structure of [Fe((C20H22N4S))X3] complexes;(C20H22N4S) -1:5-Bis(4- methylbenzimidazol-2-yl)-3-thiapentane.

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N N

S

H

CH3 N

H N

H₃C

2

8 9

7 4

6 5

Fe X X X