Indian Journal of ChemistryVol. 21A, July 1982, pp. 743-745

Complexes of Cobalt (II) with SubstitutedBenzimidazoles

N SHASHIKALA, E G LEELAMANI & G K N REDDY·

Department of Chemistry, Central College. Bangalore University,Bangalore 560001

Received 14 December 1981; reused 10 February 1982;accepted 27 March 1982

2-Ethylbenzimidazole (Et Szl), 2-n-propylbenzimidazole (n-Pr Bzl),2-isopropylbenzimidazole (i-PrBzl) and 2·n-butylbenzimidazole (n-

BuBzl) react with cobalt halides to give tetrahedral complexes of theformula CoX2L 2 (X =CI, Br or I; L ,= substituted benzimidazole),while 2-p-pyridylbenzimidazole (p-PyElzl) gives halo-bridgeddimeric complexes of the formula [CoX2(P-PyBzl)]2' The bidentateligand 2-o-aminophenylbenzimidazolc (AmphBzI), however, givesboth tetrahedral and octahedral complexes.

Transition metal complexes of imidazoles andbenzimidazoles have been reported by variousworkers 1 - 8. Study of these complexes has gainedimportance because of their biological significance andinteresting spectral and magnetic properties. We havebeen interested in metal complexes containing nitrogenheterocycles for investigating their ability to catalysehydrogenation of organic substrates. In thisconnection a number of new complexes of cobalt (II)with substituted benzimidazoles have now beenisolated and characterised. A few complexes of cobalt(II) of this type have been reported earlier 1.2.4.

The ligands were prepared according to theliterature methods". Hydrated cobalt (II) chloride wasa BDH product. Cobalt (II) bromide. xH20 and cobalt(II) iodide. xH20 were prepared by reacting cobaltcarbonate with the required halogen acid andevaporating the resulting solution to dryness undervacuum.

Dihalo bis(substituted benzimidazole)cobalt (II)[CoX zL2, X =Cl, Br or I; L= EtBzl, n-PrBzl, i-PrBzl,n-BuBzl]-Solution of cobalt (II) halide and the ligand(1:2 mol ratio) in n-butanol was refluxed (2 hr) andconcentrated to a small volume. Addition of petroleumether 1. and cooling (O°C), deposited the requiredcompound, which was washed with alcohol and driedin vacuo.

Halo(2-{3-pyridylbenzimidazole)cobalt (ll)-J-L-dihalo-halo{2-{3-pyridylbenzimidazole)cobalt (II) [CoX 2({3-PyBzl)]z (X = CI, Br or I)-Solutions of cobalt (II)halide in n-butanol and of ligand (1: 1 mol ratio) in n-butanol were mixed and refluxed (1 hr). The desiredcompound got deposited and was washed with alcoholand dried il1 vacuo.

Dihalo bis(2-0-aminophenylbenzimidazole)cobalt (II)[CoXz(AmPhBzl)z, X=Cl or Br]-Ethanolic so-lutions of cobalt (II) halide and ligand (1:2 mol ratio)were mixed and refluxed (2 hr). The compound whichseparated was washed with ethanol and dried in vacuo.

Dichloro(2-0-aminophel1ylbenzimidazole)cobalt (II)[CoC/2(AmPhBz/)]-It was prepared as above, using1: 1 metal halide-ligand ratio.

The substituted benzimidazoles of the formulaC6H4NC(NH)R(L), where R is 2-ethyl, 2-n-propyl, 2-isopropyl or 2-n-butyl, react with cobalt (II) halides togive tetrahedral complexes of the formula CoX2L z (X= Cl, Br or I). 2-o-Aminophenylbenzimidazole whichcan act as a bidentate ligand reacts with cobalt (II)halides in the mol ratio 2: 1 to give complexes of theformula CoXz(AmPhBzl)z with an octahedralgeometry. In the case of cobalt chloride, a fourcoordinate tetrahedral complex CoClz(AmPhBzl)could be isolated when the reaction was carried out inthe mol ratio 1: 1.

The compounds are non-electrolytes in acetonitrileand give characteristic vibrational bands in theinfrared spectra] assignable to the coordinated ligands(Table 1). In all the complexes the vNH of the ligandappears in the 3175-3375 cm -I region and the ligand isexpected to be coordinated through the pyridinenitrogen to the metal. Aminophenylbenzimidazolecontaining complexes show two bands, at 3175 and3100 cm-I, assignable to vNH of NHz of theaminophenyl group and of benzimidazole group!"respectively.

The solid state electronic spectra] (mujol mull) of thetetrahedral complexes show multiple absorptionbands around 4000, 6500 and 16000 ern -I. These areassigned to the transitions, 4Az->4Tz(F) (vd, 4Az-> 4T.(F)(V2) and 4 Az -> 4 T.(P)(V3) respectively! I. Theelectronic spectra (nujol mull) of the CoXz(AmPhBzl)z(X = Cl,Br) complexes of octahedral geometry show twobands one in the 900 cm - I region and another in the20,000 ern -I region. The former may be assigned to theVI transition 4 TI9(F)->4TziF) and the latter to the V3

transition 4TI9(F)->4T1iP) arising from an octahedralgeometry of the complexes. The V2band correspondingto the transition 4T.g(F)->4 A2g has not been observedprobably because of low intensity as it is a two-electrontransition. Further using the relationship Vz = VI

tlnfrared spectra were recorded in nujol on a Carl-ZeissSPECORD 75 instrument; vmol in ern -, throughout the discussion.

tRecorded on a Carl-Zeiss UV-Vis SPECORD and DMR 21 UV-Vis near IR spectrophotometer.

743

INDIAN J. CHEM., VOL. 21A, JULY 1982

-----_._--------Table I-Physical Properties and Analytical Data of Cobalt (II) Complexes

Compound Colour m.p. or /lei!. (B.M.) vNH cm " ' Found (calc.) %dec. p. per aloma

eC) C H N

CoCl2(EtBzl), Blue 189 4.61 3190 51.10 4.6(300) (51.20) (4.8) (13.27)

CoBrz{EtBzlh Blue 210 4.68 3233 42.13 4.08(299) (42.31) (3.94) (10.96)

CoI,(EtBzl), Greenish 240 4.69 3275 35.39 3.20 ~.98Blue (302) (35.73) (3.33) (9.25)

CoCI,{n-PrBll), Blue 244 4.48 3188 53.70 5.39 12.06(298) (53.55) (5.37) (12.44)

CoBr,(n-PrBzl), Blue 236-38 4.51 3211 45.01 4.48 10.08(303) (44.54) (4.49) (10.39)

Colin-PrBzl), Greenish 197-99 4.55 3284 37.73 3.79 8.66Blue (301) (3"7.94) (3.82) (8.84)

CoCI,(i-PrBzl), Blue 216 4.62 3175 53.40 6.00(300) (53.50) (5.40) (12.44)

CoBr,(i-PrBzl), Blue 190 4.48 3200 45.23 5.46(300) (44.54) (4.49) (10.39)

CoI2{i-PrBzl), Green >250 4.53 3375 :1"7.82 3.67 8.45(301) (3.82) (8.84)

CoCl,(n-BuBzl)2 Blue 171 4.51 3315 55.21 5.75 11.66(302) (55.24) (5.90) ( 11.73)

CoBr2(n-BuBzl), Blue 179 4.53 3210 46.83 4.93 9.89(304) (46.57) (4.99) (9.87)

Col,(n-BuBzl), Greenish 177 4.70 3245 40.07 4.06 8.28Blue (301) (39.96) (4.27) (8.47)

[CoCI,(P-PyBzl)], Blue >250 4.56 3195 44.30 2.70 12.70(304) (44.30) (2.80) (12.90)

[CoBrlP-PyBzl)Jz Blue >250 4.50 3189 35.40 2.10 10.00(303) (34.80) (2.20) (10.10)

[Col,(P-PyBzl)]2 Green >250 4.58 3316 28.61 1.66 8.17(304) 3195 (28.43) ( 1.79) (8.27)

CoCI,(AmPhBzl) Blue >250 4.60 3217-3116* 45.89 3.27 12.34(302) (46.01l (3.27) (12.39)

CoCi2(AmPhBzl), Purple >250 4.91 3168-3100* 56.40 3.80 15.40(302) (57.00) (4.00) (15.30)

CoBr 2(AmPhBzl), Straw >250 5.02 3179-3100* 48.50 3.40 13.10(302) (49.00) (3.50) (13.20)

C~P-PyBzI)4 (OCIO), Pink >250 4.89 3265 55.39 3.46 16.10(301) (55.49) (3.50) (16.17)

"Temperature in OK is in parentheses.·vNH and vNH2 overlap.

+ IODq, the Vl band would be expected to occuraround 19,000 ern -I, which would be very close to theV3band. A very weak shoulder does occur on the lowenergy side of the V3 band. This cannot be assignedunequivocally to the V2transition as it may also arisefrom spin orbit or a low symmetry splitting of the V3

band. The purple coloured CoX2(AmPhBzlhcomplexes give blue colourcd solutions in dimethyl-formamide and the electronic spectra of these solutionsshow a band around 16,000 em -I characteristic oftetrahedral complexes!". This suggests that when thecomplexes dissolve in dimethylformamide, the metal islocated in a tetrahedral environment arising out of arearrangement of the ligands or substitution of theligand molecules by solvent molecules.

744

The magnetic moments of tetrahedral cobalt (II)complexes are normally expected to be in the range 4.2to 4.7 B.M.13. The magnetic moments (Gouy method,Co[Hg(CNS)4] as cali brant; diamagnetic correctionscomputed from Pascal's constants) of all the newcomplexes isolated have been measured and these arein the range 4.4 to 4.7 B.M. (Table I), supporting theassignment of tetrahedral structure to the complexes.The complexes, CoX2(AmPhBzlh however, havemagnetic moments around 4.9 B.M. (Table 1) inconformity with an octahedral geometry suggested forthese complexes 14.

2-/1-Pyridylbenzimidazole complexes [CoX2(/1-PyBzl)h are probably halogen bridged dimers with thecobalt atom located in essentially tetrahedral

environment as suggested by magnetic and electronicspectral measurements. The metal-halogen bands arefound at 342 and 302 em - 1 in the case of the chlorocomplex and at 270 and 231 em - 1 in the case of thebromo complex in their far infrared spectra. Of thesethe low frequency band may be attributed to thebridging halogens, while, the high frequency one maybe attributed to the terminal halogens, in each complexgiving v bridging/v terminal ratio of 0.8815

. That thebridging may not be via the 2-p-pyridylbenzimidazoleligand is supported by the fact that the latter ligandreacts with cobalt perchlorate to give a complex of theformula, Co(fJ-Py8zl)4 (OCl03h- The infrared spectrum(nujol mull) of this complex exhibits the V3 band of theperchlorate as spli t band (1030 and 1140 ern - 1) and thenormally forbidden VI band appears as a sharp butweak band at 920 cm -1, suggesting coordination ofperchlorate to the metal through one of itsoxygensl6,17. Thus, the complex has an octahedralgeometry with the fJ-pyridylbenzimidazole moleculeacting as a monodentate ligand. The spectral(electronic) and magnetic properties also support anoctahedral structure for the perchlorate coordinatedcomplex.

NOTES

We thank Dr G.A. Webb, University of Surrey,England, for microanalyses and the CSIR, New Delhi,for financial support.

ReferencesI Sundberg R J & Bruce Martin R, Chern Rev, 74 (1974) 471.2 Goodgame M & Cotton F A,I Am chern Soc, 84 (1962) 1543.3 Ghosh S P & Banerjee A K, Indian 1 Chern, 2 (1964) 292.4 Goodgame D M L. Goodgame M & Rayner Canham G W,lnorg

chim Acta, 6 (1972) 245.5 Goodgame D M L & Machado A A SC,lnorgchim Acta,6(1972)'

317.6 Ghosh S P, Bhattacharjee P & Mishra L K,I I ndi.an chern Soc, 51

(1974) 308.7 Dash K C & Panchanan Pujari,J inorg nucl Chern, 37 (1975) 2061.8 Reddy G K N & Ramesh B R, Indian 1 Chern, 15A (1977) 621.9 Hein IW, Album R I& Liquitt J J,1 Am chern Soc, 79 (1957) 427.

10 Biradar N S & Goudar T R,1 inorg nucl Chern, 39 (1977) 358.II Lever A B P, Inorganic electronic spectroscopy (Elsevier. New

York) 1968.12 BulTagni S & Dunn T M, 1 chern Soc. (1961) 5105.13 Barefield & Bush D H. Quart Rev, 22 (1968) 457.14 Lewis J, Sci Progr, 51 (1962) 452.15 Nakamoto Kazua, Infrared spectra of inorganic and coordination

compounds (Wiely-Interscience, New York) 1970.16 Hathaway B J & Underhill A E,1 chem Soc. (1961) 3091.17 Naikar S B, Nanje Gowda N M & Reddy G K N, lndian J Chern,

20A (1981) 436.

745