Indian Journal of Chemistry Vol. 40A, August 2001, pp. 896-900 Synthesis, spectral characterization and electrochemical properties of osmium(II) complexes of 1-ethyl-2-(arylazo)imidazoles Prithwiraj By abarlta, Sanjib Pal, Umasankar Ray & Chittaranjan Sinha • Department of Che mi stry , Univers it y of Burdwan, Burdwan7 13104, In dia Received 4 Dece111 ber 2000: revised 29 May 200 1 Title complexes have been synthesised on react in g 1 -e thyl-2- (ary lazo)i mida zo le s ( RL , where R = H, Me, OMe, Cl ) with (N H 4 h[OsCir,l. Two isomers, blue-violet (isomer-A) a nd red- violet (isomer-B) of the compos iti on OsCI 2 (RLh have been c hromatographically separated. IR spectra show the presence of two v(Os-CI) (3 00-310 and 320-330 en,-') and suggest the co mpl exes wi th cis-OsC I 2 configuration. Three geometrical iso mers with cis-OsCI 2 configuration are possibl e; with consideration of co-ordination pairs in th e order of Cl, N 1 N(imida zole)] a nd 1 IN (azo)] they are cis-trans-cis (etc), cis-cis- trans (ee l) and cis-cis-cis (ccc). The 1 H-NMR spectra identify isomer-A and isomer-B as etc- and ccc- configurations, respectively. Absorption spectra exhibit multiple MLCT lt 2 (0s)-m'(RL)I transiti o ns in th e vi s ibl e reg ion and the energy of tran sition follow s th e order ccc > etc. Redox studies show Os (lll )/Os( ll ) and Os( IV )/Os( lll ) couples at 0.4-0.6V and 1.6-1.7 V versus SCE, re spect ive ly along with lig a nd reductions . It is becau se of the biological and chemical ubiquity of imidazoles 1 that we have been engaged for the last seve ral years to explore the che mi st ry of 2- (arylazo)imidazoles2-6. The exobidentate character of imidazole moiety is eliminated by !-alkylation to sy nthesi se 1-alkyl-2-(arylazo)imidazole. The ruthenium chemistry of these li gands are known in some detail 3 .4. Progress in ruthenium chemistry with this li ga nd ha s encou raged us to explore the corresponding osmium chemistry. In this work we describe the synthesis, spectra and redox properties of osmium(II) comp lex es of 1-ethyl-2- (arylazo)imidazole complexes. Experimental 2-(Arylazo )imidazoles and 1-ethyl-2- (arylazo )imidazo le s were prepared by reported procedure 3 . Osmium tetroxide was obtained from Johanson Matthey & Co, U. K. It was converted to (NH4h[OsCI 6 ] according to reported procedure 7 • Solvent purification a nd reagent synthesis for electrochemkal works were done as before 8 . Commercia lly available silica gel (6 0-1.20 mes h) from SRL was used for chromatographic purification. All other solvents and chemicals were of reagent grade and were used without further puri fication. Microanalytical data (C,H,N) were co ll ected using Perkin Elmer 2400 CHN elemental ana ly ser. UV- Vi s spectra were recorded by JAS CO Mode l V-570 UV- VIS-NIR spectrophotometer. Infrared (IR ) spectra were obta in ed using FfiR JASCO Model 420 spectrophotometer (KBr disk, 4000 - 200 cm- 1 ); 1 H NMR spectra were collected in CDCI 3 using Brucker 300 MHz Ff NMR spectrometer. The solution electr ic al cond uctivity was measur ed using a Systronics 304 conductivity meter with a olute concentration of -I o- 3 M in nitromethane. Electrochemical measuremen ts were carried out under a dinitrogen environment with EG & G PARC Model 270 computer contro ll ed, VE SAST AT usin g Pt-di sk milli working electrode. All re sults were collected at 298 K with the saturated calomel elec trode (SCE) as reference. The reported potentials are uncorrect ed for junction contribution. Preparation of etc- and ccc-dichloro-bis-f 1-eth yl -2- (ary/azo )imi da zole ]osmium( 1 !) , Os Cl 2( RLh Nitrogen gas was passed for 15 min through a brown-red solution of (NH 4h [OsCI 6 ] (0.5 g, 1.14 mmol) in 2-methoxyethanol (50 ml) . The so lut ion was refluxed on oil-bath with continuos stirring for half-an hour. 1-Ethyl-2-(arylazo)imidazoles (RL, where R = H, Me, OMe, Cl ) (2.28 mmol) was added pinchwise to thi s refluxing solution over another half-an hour. The mixture was refluxed under nit rogen and stirred magnetically for 8 h. During thi s period the solution turned brown-violet to blue- violet. This was concentrated slowly by bubbling N 2 gas under hot condition to about 20 ml and kept in the refrigerator for 12 h. The shining dark coloured crystalline precipitate was collected by filtration and washed with EtOH-H 2 0 (l:l,v/v) and again dried over P40 10 . The dry solid was dissolved in a sma ll volume of CH 2 Ch and was chromatographed on a si lica-gel column. A sma ll portion of the orange-yellow band was eluted with benzene and rejected. The blue-violet ba nd was eluted MeCN-C 6 H 6 (I :4, v/v) and th e red- violet band was eluted by MeOH. A violet mass