Synthesis and Structural Studies of Cr(III), Mn(II) and Fe

ISSN: 0973-4945; CODEN ECJHAO

E-Journal of Chemistry

http://www.e-journals.net 2011, 8(1), 421-426

Synthesis and Structural Studies of

Cr(III), Mn(II) and Fe(III) Complexes of

N(2-Benzimidazolyl)acetylacetohydrazone

G. H. ANURADHA* and A.V. CHANDRA PAL

*Department of Chemistry

R.B.V.R.R Women’s College, Hyderabad- 500027, India

Department of Chemistry

Osmania University, Hyderabad-500007, India

Received 9 February 2010; Revised 25 June 2010; Accepted 17 July 2010

Abstract: The ligand N(2-benzimidazolyl)acetylacetohydrazone (BAAH) have

been synthesized and characterized. Coordination complexes of Cr(III), Mn(II)

and Fe(III) have been synthesized with the ligand BAAH. These complexes

were characterized on the basis of analytical, conductance, thermal, magnetic

data and infrared and electronic spectral data. The ligand BAAH is behaving as

a neutral tridentate NNO donar employing two azomethine nitrogens (ring and

side chain) and carbonyl oxygen. The ligand and it’s metal complexes were

tested for anti microbial activity on the gram positive S. Aureus, E. coli

and Proteus.

Keywords: BAAH, Metal complexes, Physiological activity.

Introduction

The chemistry of benzimidazole and it’s derivatives reported in the literature is interesting

due to their chelating ability and physiological properties such as antiviral, fungicidal,

bactericidal in addition to analytical applications1,2

. 2-Hydrazinobenzimidazole have been

proved to be very active against the transition metal ions3-5

. Some derivatives of

2-substituted benzimidazoles show anti microbial6, anti influenza

7 and hypertensive

activities8. In view of this it was planned to make a study on the complexing abilities of

compounds (Schiff bases) derived from 2-hydrazinobenzimidazole with acetylacetone and

their antimicrobial activity also tested.

Experimental

Metal salts and solvents used were E-Merk products. All other chemicals used were of AR

grade. Elemental analysis (C, H. & N) was carried out using micro analytical techniques on

422 G. H. ANURADHA et al.

Heraens -CHN rapid analyzer. Metal estimations were done on a Perkin Elmer 2380 atomic

absorption spectrometer, conductivity measurements were carried on Elico digital

conductivity meter model No. CM-180, the magnetic susceptibility measurements were done

on a Faraday balance (CAHN-7600) using HgCo(SCN)4 as standard. DTA data was

established by using leeds and Northrup – USA instrument. Infrared spectra were recorded

on a Perkin - Elmer 1600. Fourier transforms infrared spectrophotometer in nujol mull. The

electronic spectra were recorded on a shimdazu UV - 160 spectrophotometer.

Synthesis of ligand (BAAH)

Acetylacetone (1.2 mL) was added to a solution of 2-hydrazinobenzimidazole (1.48 g) in

methanol (100 mL). The mixture was heated under reflux for three hours and then poured

into ice cold water. The white needles separated out were filtered and washed with a small

amount of aqueous alcohol. The compound was dried in vacuo and recrystallized from

aqueous methanol. Yield 60% m.p. 130 0C.

Characterization of the ligand

The molecular formula of the ligand is C12H14N4O. The 1H NMR spectrum of BAAH was

recorded in DMSO-d6 solution. The spectrum revealed surprisingly only two methyl

signals9, one at δ 6 characteristic of an olefinic proton and a broad peak at δ 10.6 reminiscent

of an intra molecular bonded enolic OH appeared10

. The usual A2B2 pattern of the aromatic

protons in 2-hydrazinobenzimidazole is also absent. All the information conforms to the

enol tautomeric structure of the compound.

The IR spectrum of BAAH shows a peak at 3060 cm-1

assigned11

to stretching vibration

of free νN-H. A band due to ring νN-H vibration is observed at 2923 cm-1

. Two sharp bands

observed at 1626 and 1551 cm-1

are ascribed to free νC=N and ring νC=N respectively12

.

The νN-N stretching frequency is observed13

at 1033 cm-1

. Surprisingly νC=O is not

observed, this confirms enol tautomerism.

The highest ion peak in the mass spectrum of the compound BAAH is at m/e 212.

Obviously the molecule looses elements of water under thermal conditions and hence the

molecular ion is not recorded. The ion at m/e 212 is also the base peak in the spectrum and is

represented by 2-pyrazolylbenzimidazole derivative. The molecular ion and the ion at m/e

212 appears to undergo extensive skeletal rearrangements giving ions at m/e 195, 184, 170,

144 and 133. The peak at m/e 133 corresponds to 2-aminobenzimidazole. The next

prominent amongst the ions is m/e 118 corresponding to benzimidazole and m/e 105

corresponding to benzazetidine moiety.

The electronic spectrum of BAAH ligand shows absorptions at 42918 cm-1,

33333 cm-1

,

26737 cm-1, 23355 cm

-1 and 21978 cm

-1. The high energy band 42918 cm

-1 is attributed to Π → Π*

transition of benzimidazole. The band at 33333 cm-1 is attributed to benzene moiety. The other

bands are assigned to the transitions possible with carbonyl group and azomethine groups.

The data are consistent with the expected structure (Figure 1) and confirms the enol

tautomeric structure of the compound.

N

N

N H N = C --- C H = C ---- CH 3

lOH

lCH3

H Figure 1. N(2-Benzimidazolyl)acetylacetohydrazone

Synthesis and Structural Studies of Cr(III), Mn(II) and Fe(III) Complexes 423

Synthesis of metal complexes

The ligand (0.01 mole) was dissolved in methanol (25 mL.) and to this solution of divalent /

trivalent metal chloride (0.01 mole) was added. The pH of the mixture was adjusted to 7

with 1% alcoholic ammonia and the reaction mixture was refluxed for 3-4 h. The coloured

product thus obtained was filtered in hot condition, washed successively with methanol and

petroleum ether and dried in vacuo. The purity of complexes was checked by TLC using

methanol and solvent mixtures.

Results and Discussion

Characterization of the metal complexes

Analytical data of C,H,N,Cl and metal are in good agreement with 1:1 (Table 1) metal-

ligand stoichiometry for all the three complexes. All the complexes are coloured and

stable, they neither melt nor decompose up to 300 0C. Except Cr(III) complex the

remaining two Mn(II) and Fe(III) complexes show negligible conductivity thus proving

that they are non- electrolytes. The conductance value of Cr(III) is 60 mhos indicating to

be 1:1 electrolyte. Hence one chloride ion is present in the ionization sphere of Cr(III)

complex. The differential thermal analysis of BAAH Cr(III) complex indicates that it

undergoes thermal decomposition in the range of 60 0C to 540

0C. There is a broad

endotherm in the range of 60 0C to 300

0C which is attributed to the deaquation of both

lattice water and coordinated water14,15

.

Table 1. Analytical data of complexes of BAAH

Calculated (Found) % ΩM Complex

stochiometry Carbon Hydrogen Nitrogen Chloride Metal mho cm

2

mol-1

Cr(III) complex

(CrC12H20N4O4Cl3)

33.91

(33.89)

4.24

(4.22)

13.18

(13.15)

25.08

(25.07)

12.24

(12.20) 60

Mn(II) complex

(MnC12H18N4O4)

42.72

(42.68)

5.34

(5.30)

16.61

(16.59) -

16.31

(16.25) 15

Fe(III) complex

(FeC12H17N4O4)

42.72

(42.70)

5.04

(5.01)

16.61

(16.58) -

16.61

(16.56) 24

Where L = C12H14N4O

Infrared spectra

The IR spectral data of the BAAH complexes are given in Table 2. The ligand seems to

employ the keto form in metal binding process. The strong band observed in the range of

1600-1620 cm-1

is due to νC=0 which is characteristic of keto form. In view of its

appearance at relatively low frequency it can be concluded that the carbonyl function is

involved in coordination. Below 1600 cm-1

the spectra of the complexes are more elaborate.

Invariably there is a sharp band around 1560 cm-1

and another similar band in the range of

1523 cm-1

. These bands are assigned to νC=N free and νC=N ring respectively which are

involved in coordination16,17

. Based on these observations it may be concluded that the

BAAH is behaving as a neutral tridentate NNO donar.

The νN-N appearing at 1033 cm-1

in free ligand spectra is located at a higher frequency

(up to 1060 cm-1

) in the spectra of all the complexes18

. It further supports the involvement of

azomethine nitrogen of hydrazine side chain in coordination. The peak at 740 cm-1

which is

assigned to the N-H out of plane deformation vibration in free ligand is not altered on

chelation, since coordination does not take place through the imino nitrogen atom.


Table 2. Characteristic IR frequencies of BAAH metal complexes

Complex νOH

νNH νC=O

νC=N

(free)

νC=N

(ring) νN-N New bands

BAAH 3060-

2924 - 1626 1552 1033

Cr(III)

complex

3000-

3400 1608 1560 1522 1060

920, 670, 565,

524, 480, 300, 270

Mn(II)

complex

3200-

3420 1600 1578 1523 1045

1119, 620, 502,

485, 390, 320

Fe(III)

complex

3100-

3365 1620 1560 1523 1043 1121, 567, 532

The position of broad bands in the spectra of the complexes at higher frequencies in

comparison with the band in the free ligand (2424 - 3060 cm-1

) can be attributed to νOH

of either hydroxide or water molecules. The spectra of metal complexes show a broad

strong band in the region 3000 to 3450 cm-1

which can be attributed19

to νNH and νOH.

In Mn(II) and Fe(III) complexes the presence of hydroxyl ions in coordination is further

supported by non ligand bands in the range of 1110-1121 cm-1

which are due to M-OH

bending modes.

The presence of coordinated water in Mn(III) complex is concluded by rocking and

wagging modes observed in the range of 880 - 930 cm-1

and 670 - 650 cm-1

respectively20

.

In the far IR spectra of Cr(III) complex non ligand bands at 270-300 cm-1

which are assigned

to coordinated chloride ions. The non ligand bands in the range of 485 - 670 cm-1

in all the

complexes are assigned to νM-O and νM-N vibrations21

.

Magnetic data

Magnetic susceptibilities are recorded at room temperature on faraday balance and the

magnetic moments (BM) calculated show that central metal ions are invariably in high spin

configurations. The magnetic moments were presented in Table 3. The magnetic moment of

Cr(III) complex is 3.82 BM which corresponds to a d3 system. The magnetic moment values

of Mn(II) complex is 6.1 BM, which is to be expected for d5 system

22. The Fe(III) complex

has a magnetic moment value of 5.9 BM, which corresponds to high spin d5 configuration

23.

Electronic spectra

The electronic spectral data of the complexes are given in Table 3. The absorption spectra of

Cr(III) complex exhibit three bands at 12,345 19,047 and 33,333 cm-1

. These are assigned in

sequence to 4A2 →

4T2,

4A2 →

4T1 (F) and

4A2 →

4T1 (P) transitions respectively. Octahedral

geometry is proposed24

. The absorption spectra of Mn(II) complex show bands at 19,047,

21,505, 23752 and 26,737 cm-1

. These bands can be attributed to doubly forbidden

transitions25

. The absorption spectrum of Fe(III) complex exhibits weak absorptions at

38,461, 12,453 and 11,614 cm-1

and can not be assigned to specific transitions.

Table 3. Magnetic and Electronic spectral data of BAAH metal complexes

Complex µeff (BM) Electronic spectral bands, cm-1

Cr(III) complex 3.82 12345, 19047, 33333

Mn(II) complex 6.1 19047, 21505, 23752, 26737

Fe(III) complex 5.9 11614, 12453, 38461

Synthesis and Structural Studies of Cr(III), Mn(II) and Fe(III) Complexes 425

Physiological activity

The culture was prepared by the addition of 1.5 g of Beef extract, 1.5 g of yeast extract,

5.0 g of peptone, 1.0 g dextrose, 3.5 g of NaCl, 3.68 g of dipotassium hydrogen phosphate,

1.32 g of mono potassium dihydrogen phosphate and 15 g of agar agar in one litre of water.

The pH of the medium was adjusted to 7.0. The medium was sterilized at 121 0C in an

autoclave at 15 lbs pressure for 15-20 minutes. The medium was cooled to 45-50 0C. 20 mL of

medium was poured in each of the Petri dishes. All the apparatus used were sterilized.

E.coli, S. aureus and proteus were introduced into the Petri plates and were grown.

Bacteriological testing

Bacteriological testing was done using paper disc method. Paper discs were soaked in the

solutions of ligand and complexes in DMF. Solutions were prepared in 100 µg/mL in DMF.

Paper discs were soaked in this solution and were dried. The dried paper discs were

introduced into the fully grown culture plates of E.Coli, S. aureus and Proteus.

The ligand BAAH is active against E.coli and inactive against S.aureus and Proteus.

Cr(III) complex is active against E.coli, S. aureus and Proteus. Mn(II) and Fe(III)

complexes are active against E.coli and S. aureus and inactive against Proteus.

Conclusion

BAAH is behaving as a neutral tridentate NNO system employing two azomethine nitrogens

(ring and side chain) and carbonyl oxygen. All the complexes are homonuclear and forming

1:1 complexes. From IR spectra it is suggested that ligand seems to employ the keto form in

metal binding process. Based on analytical, thermal, conductivity, magnetic and spectral

data octahedral geometries have been proposed to Cr(III), Mn(II) and Fe(III). The proposed

structures are given in Figure 2.

Figure 2. Probable structures of Cr(III), Mn(II) and Fe(III) complexes of the ligand BAAH


Acknowledgment

The authors are thankful to NCL Pune, Department of Chemistry, Pune University, CIL,

Hyderabad University and IICT, Hyderabad for providing spectral and library facilities.

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Synthesis and Structural Studies of Cr(III), Mn(II) and Fe