Indian Jo urn al of Chemistry Vol. -+2A. October 2003, pp. 253 1-2535 Two pentacoordinate copper(IJ) complexes that display accessible Cu(ITIIU) couples Michael G B Drew Department of Chemistry. University of Reading. Whit e kni g ht s. Reading RG6 6AD, UK and Jnan P Naskar. Shubhamoy Chowdhury & Dipankar Datta* . Department of In orga ni c Chemi str y, Indi an Association for th e Cultivati on of Sc ien ce. Calculla 700032, India Received /3 Feb ru ary 2003 Two copper(lI) complexes of th e type CuL 2 .imidazole (1) and (2). wh ere LH = l-nitroso-2- naphthol and 4.4' -b py = 4,4'- bipyridine. are characte ri sed by X- ray crys tall og raphy . In 2. th e two copper atoms are linked by 4,4'- bpy . In both the complexes. copper is found to have a di storted sq uare pyramida l N,0 2 coordination sphere. The axial pos iti on in J is occ upi ed by an oxygen atom while th ose in 2 by the nitrogen of 4.4'-bpy. The two complexes display quasircvers ibl e Cu( lIl1l1 ) co upl es around 0.68 V V.I'. saturat ed calomel electrode in cyclic voltammetry in di c hl oromethane. Examp les of pentacoordinate copper(Il) complexes are many' ·1 . But none of them is known to display any Cu( IIIIII) couple. Only one st ructurally characterised ex ample of a pentacoordinate copper(lIl) complex is known so far), fo r which no electrochemical dat a are avai lable. In thi s complex the copper(lII) center has a square pyramidal N 4 0 coordination sp here . Herein we desc ribe two pentacoordinate copper(II) complexes which display quasireversible Cu(lJIlII) couples. One of them is mononuclear and the other dinuclear. Experimental I -Nitroso-2-naphthol (L H; H, dissociable proton) \Va purchased from Loba Chemie Pvt. Ltd . (India) a nd 4.4'-bipyridi ne (4,4'-bpy) from Aldrich (USA). Tetrab uty lammonium perchlorate (TBAP) was prepared by the action of perchloric acid on tet rabutylammonium bromide (Spec trochem , India ). All other c hemicals and so lvents used were of analytical grade. The complex CuL:! and its imidazole (im) adduct, CuL 2 .im (1), were pre pared as reported previousl/. Micro-analyses were performed using a Perk in-Elmer 2400 11 analyzer. lR spec tra (KBr disc ; 4000- 400 cm- I ) were recorded o n a Perkin-Elmer 783 spectrophotometer and UV-vis spect ra on a Shimadzu UV -160A spectrophotometer. Cyclic vo lt ammetry was performed using an EG&G PARe electrochemical analysis sys tem (Mode l 250/5/0) under dry nitrogen atmosphere in conventional thr ee' electrode configurations with TBAP as the supporting electrolyte. A planar EG&G PARC G0229 glassy carbon milJi electrode was used as the working electrode 111 cyclic vo lt ammerry . Under th e experimental co nditions emp lo yed her e. th e ferrocene-ferrocenium couple a pp ears 111 dichloromethane at 0.47 V vs. SCE (saturated c al o mel el ectrode) with a peak-to-peak separation of 0.09 V at a scan rate (v) of 0.050 V S- I. Synthesis and characterisation of CII 2Ll -l .4'- bpy).2H 2 0.C 6 H/ .J (2) CuL 2 , 0.20 g (0.5 mmol ), was dissolved in 75 em ; of dichloromethane and filtered. To the filtrate. O.-W g (0.25 mmol) of so lid 4,4'-bpy was added. The resultant dark brown solution was refluxed for 24 h. Then the reacti o n mixture was left in the air for 3 h. The s hining brown precipitate wa s fi lt ered. washed with 10 cm ) of diethy lether and dried ill vacl/o over fused CaCho It was recrystallised from 1:3 mixture of n-hexane and dichloromethane ; yield, 0.02 g (7 CJc). Elemental analyses were co nsistent with th e stoic hiometry [Found: C. 61.37: H. 4.50 ; N, 7.78. Calc: C, 61.45; H. 4.61: . 7.6 8<7c J. lR data (cm- I ): 3436 (s), 1 607 (s). 1525 split), 1481 (w), 1454 (w), 1368 (s). 1335 (m). . 12 6::! (s), 1222 (s), 1169 (m), 1090 (w), 1003 (w). 844 (s). 751 (s), 652 (m) , 586 (m), 5 I 3 (m, sp lit), 433 (w). !lett/)la, 2.58 (at 300 K). UV/vis (MeCN): A/nm (E/ dm 1 mor l cm- I ): 596 sh (2 600) , 491 ( 12 000 ). 398 900), 284 (32 600), 261 (38 500) . X-ray crystallography Single crystals of 1 were o btained by aerial evaporation of a dilute acetone solution of th e complex and those of 2 were grown by direct diffusion of n-hexane into a moderately conc entrated dichloromethane solution of the complex. Data for I and 2 were measured with MoKa radiation Llsing th e MARresearch Image Plate System. The crystals were positioned at 70 mm from the ima ge plate . 100 frames were measured at 2° intervals with a countin g time of 2 mins. Data analysis was carried out with the XDS