Indian Journal of ChemistryVol. 29A, October 1990, pp. 1024-1026

Studies on some antifungal transition metalchelates of 2-(2' -hydroxybenzylidene)-

aminobenzimidazole

R S Srivastava

Department of Chemistry, M.G. Degree College,Gorakhpur 273 001

Received 31 December 1987; revised 15 November 1989;revised and accepted 4 April 1990

Some new complexes of the type Mtbenzimidazole),.xH20 [M= Mn{II), Co{II), Ni{II) and Cu{II)] and'Mfbenzimidazolel, [M=Cr{III) and Fe{III)]have beenprepared and characterized on the basis of their elemen-tal analysis, IR spectra, electronic spectra and magneticmoments. The ligand and its metal chelates have beencompared with Dithane M-45, a commercial fungicide,for their fungicidal action against Aspergillus niger andRhizopus nigracans.

The benzimidazole nucleus appears to be especiallytoxic to fungi. Recently, a large number ofbenzimid-azole derivatives have been developed and patentedas fungicides 1-5 • 2-(Benzylthiomethyl)benzimida-zoles have been found to be carcinostatic and fungi-cidal", Methyl-l-butyl carbamoyl- 2-benzimidazolecarbamate is a novel fungicide which has been suc-cessfully used to control losse smut of wheat andbarley 7. Benzimidazole carbamates possess fungi-cidal, acaricidal and nematocidal properties". How-ever, very little is known about metal chelatesof 2-(2' -hydroxybenzylidene )aminobenzimidazole.Recently it has been observed that the metal com-plexes are more fungitoxic than the parent com-pounds", This prompted us to synthesise and char-acterize it and evaluate the fungitoxic properties ofsome of its complexes with transition metal ions.

ExperimentalAll the chemicals used were of AR grade or

equivalent quality, 2-Aminobenzimidazole was pre-pared by the method of Phillips!", Its schiff basewith salicylaldehyde was prepared by the usual con-densation method. The metal complexes were pre-pared by the following method.

The requisite metal salt and sodium salt of ligandwere refluxed in 1 : 2 molar ratio in 50% aq. ethanol(v/v) for 5-6 hrs. The compounds which precipitat-ed on cooling were washed several times with coldwater followed by 50% aq. ethanol (v/v) and driedin vacuo.

1024

H

OC>-N=CHN H0-6

(I)

The complexes were analysed for their metal con-tents employing standard literature procedures afterdestroying the organic residue with a mixture ofconcentrated hydrochloric and nitric acids. The an-alytical results are given in Table 1.

Physical measurements were carried out by themethods reported earlier"; fungicidal activity was al-so studied by the earlier reported method", The wa-ter content of the complexes were determined byheating them in the range of 150-3QO°C and deter-mining the loss in weight.

Results and discussionThe elemental analysis (Table 1) indicate that the

stoichiometry of the complexes is Mfbenzimida-ZOle)2.xH20 except for the Cr(III) and Fe(III) com-plexes. These complexes show stoichiometry ofMfbenzimidazolelj. The low conductance values( - 2 mho em? x 1 mol- 1) showed their non-electro-lytic nature in dioxane.

The infrared spectrum of the ligand shows twoabsorption bands, one at 1640 em - 1 due to strech-ing mode of acyclic N = C group and the other at1610 em - 1 due to - N = C of the heterocyclicring. A band appearing at 3440 em - 1 can be at-tributed to vN - H mode. The band in the region3640-3480 em -I in the spectra of the complexes isattributed to vOH of the water molecule. The ligand(I) with four potential donors can interact with me-tals in a number of ways. However, there is a posi-tive shift of 10-20 em -Ion complexation in thevNH and -c = N (heterocyclic ring) modes. Thisrules out the possibility ot coordination through thenitrogens of these groups as well as its behaviour asmonodentate ligand 11. A negative shift of - 40em - 1 in the acyclic - N = C band and disappear-ance of - OH band in the IR spectra of the com-plexes indicate bonding through the nitrogen ofacyclic - N = C group and oxygen of hydroxylgroup. Thus, the ligand behaves as a bidentate li-gand bonding through only one nitrogen. In addi-tion the complexes exhibited two bands (rockingand wagging) in the region 650-g80 em -r I assign-

Table 1- Analytical data, melting points and magnetic momentsof the complexes

Complex Found (Calc.), %(m.p.,OC)

Metal Nitrogen Jl.eff

(BM)

Cr(Benzimidazole h 6.78 16.62 3.70(>250d) (6.81) (16.51)

Mn(Benzimidazole)2 (HP)2 10.36 15.76 6.0(> 250d) (10.38) (15.88)

Fe(Benzimidazole h 7.30 16.40 5.82(>250d) (7.28) (16.43)

Cotfsenzimidazole), (HPh 11.00 15.78 4.42

(> 250d) (11.05) (15.76)Ni(Benzimidazoleh (H2Oh 10.56 15.82 3.20

(>250d) (10.51 ) (15.85)Cu(Benzimidazole h 11.80 15.64 1.82

(>250d) (11.82) (15.62)

Satisfactory carbon and hydrogen analyses have been obtainedfor all the complexes.

Table 2- Fungicidal screening data

Compound Average percentage inhibition after 96 hrs(serial as in

Table 1) A. niger R. nigracanscone. used (ppm) cone. used (ppm)

1000 100 10 1000 100 10

Ligand 61.6 42.0 28.6 62.2 40.6 26.81 68.5 48.6 32.8 70.0 46.5 30.62 97.0 80.8 70.0 95.5 78.0 68.53 96.5 81.7 72.8 90.6 72.5 62.64 65.0 50.2 30.0 63.2 42.2 28.65 66.6 48.2 28.8 64.2 42.8 30.76 98.0 82.5 69.5 88.6 80.0 70.5

Dithane M-45(A commercial 100.0 88.2 72.6 98.0 86.8 75.0

fungicide)

able to the coordinated water which is absent in thechelating agent 12 • The loss of water at relatively hightemperature (160-21O°C) indicated that these arecoordinated.

Analytical data and magnetic moments of the che-lates are recorded in Table 1.

From the assignment of the electronic spectralbands, their positions and spectral parameters, it isclear that all the complexes are octahedral exceptCufll) complex which shows planar environmentaround the Cutll) ion. The values of Dq, and B'have been calculated from V2 and V3 bands using thematrices of Tanabe and Sugano".

Cr(ill) has 3d3 electronic configuration with 4A2g,gTound dlltE'. Three hands observed at 18000,

NOTES

25000 and 36000 em -I for Cr(Ill) complexes areassigned to the transitions:4A2g -+ 4T2iF), 4A2g -+ 4T1iF) and 4A2g -+ 4T1g(P) re-spectively.

For Mn(ll) complexes, three d-d transitions ob-served at 19000,24200 and 31800 cm-I are as-signed to 6A1g -+ 4 T1g(G), 6A1g -+ 4 Eg (G) and6AI -+ 4E (P) respectively. A band at 31800 cm-I isdue

gto thi charge transfer. This suggests a distorted

~ symmetry for Mn(ll) complex.In the case of Fe(Ill) complex, three bands are ob-

served at 13000, 21000 and 30000 em - I. Thesebands may be assigned to the transitions:6Aig -+ 4 TIg, 6A1g -+ 4 T2g and 6Aig -+ 4 T1g respectively.This shows octahedral geometry for Fe(Ill) com-plex.

The electronic spectral bands for Co(ll) com-plexes are observed at 8000 and 20540 em -I whichare assigned to the transitions: 4 T1g-+ 4 T2g (V I) and4T1g -+ 4T2g (P) (v3) respectively. However, we couldnot observe the band arising from the transition4 TI -+ 4 A2 probably because of it being a two elec-

g g . . ahdaltron transition. These data indicate an oct e renvironment around Co(ll) ion'". From the valuesof V3 and vI> values of Dq, B' and v2 work out as800, 836 and 16000 em -I respectively. The v/ VIratio 2.0 lies well within the limit reported for oc-tahedral structure ofCo(II) complexes".

Three bands were observed for Ni(II) complexat 13000, 18500 and 26000 em - I. These are as-signed to the transitions: 3A2g -+ 3 T2g (VI), 3Azg -+ 3 Tig(F) (v2) and 3A2g -+ 3 T1g (P) (v3) respectively. The ra-tio of V/VI (1.42) usually observed for Oh Ni(II)complexes". From these frequencies we have calcu-lated the values of Dq (1300 em -I), B ' (866 ern - 1)and tJ(19.8%).

The electronic spectral data of the Cu(II) complexare characterized by a broad band in the region15600 em -I and a strong band near 22000 em - I.

The d-dbands for complex for which a planar struc-ture has been proposed'? occur in the range 14000-1800 em - I. Therefore, the above observations areconsistent with an essentially planar structure.

As expected, all the metal chelates had significantfungitoxicity at 1000 ppm but their toxicity dec-reased markedly on dilution (viz. at 100 and 10ppm). The metal chelates were more potent againstboth the fungi, viz., A. niger and R. nigracans, thanthe parent compound and its sodium salt (Table 2).This can be explained on the basis of chelation theo-ry",

AcknowledgementThe author thanks the authorities of M.G. Degree

College, Gorakhpur for providing laboratory facilit-

1025

INDIAN J CHEM, SEC A, OcrOBER 1990

ies and the Director, CDRI, Lucknow for spectralanalyses. This work is partly supported by acontin-gent grant administered by UGC, New Delhi.

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