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
U. Sivagnamam et M. Palaniandavar, 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, Journal of the Chemical Society. Dalton transactions, (15), 1994, pp. 2277-2283
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.