INFLUENCE OF CHELATE-RING SIZE AND NUMBER OF SULFUR-DONOR ATOMS ON SPECTRA AND REDOX BEHAVIOR OF COPPER(II) BIS(BENZIMIDAZOLYL) TETRA-THIOETHER AND PENTA-THIOETHER COMPLEXES

The linear quadridentate R(CH2)mSCH2CH2S(CH2)nSCH2CH2S(CH2)mR, where m = 1, n = 2 (L1), or 3 (L2), m = 2, n = 2 (L3) or 3 (L4), and the pent adentate RCH2(SCH2CH2)4SCH2R (L5) (R = benzimidazol-2-yl) compounds fo rmed 1:1 copper(II) perchlorate complexes. Some of the quadridentate c ompounds also formed complexes of the type CuLX2 (X = Cl-, NO3- or BF4 -). All the complexes exhibited an absorption band around 30 000 cm-1 originating from a S(sigma)-->Cu(II) charge-transfer transition. In so lution the ClO4- and BF4- salts of [CuL1]2+ exhibit only one ligand-fi eld band (14 800 cm-1) while the other complexes show two bands (almos t-equal-to 11 000, 15 000-16 000 cm-1). The polycrystalline EPR spectr a of the former complexes are axial while those of the other complexes are rhombic. The cryogenic solution EPR spectra of the former complex es differ from those of the other complexes by exhibiting comparativel y low g(parallel-to) values and well resolved nitrogen superhyperfine structures. All these spectral features suggest a unique 'folded' geom etry for the [CuL1]2+ complex. Among the tetrathioether complexes, the Cu(II)-Cu(I) redox potential increases with increase in the number of six-membered chelate rings, implying an increase in preference for th e copper(I) over the copper(II) state. For copper(II) complexes of bis (benzimidazolyl) thioether ligands with all-five-membered chelate ring s the potential increases with increase in the number of thioether don ors.