Influence of chelate-ring size and number of sulfur-donor atoms on spectra and redox behaviour of copper(II) bis(benzimidazolyl) tetra- and penta-thloether complexes

Abstract

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 pentadentate RCH₂(SCH₂CH₂)₄SCH₂R (L⁵)(R = benzimidazol-2-yl) compounds formed 1 : 1 copper(II) perchlorate complexes. Some of the quadridentate compounds 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(σ)→ Cu^II charge-transfer transition. In solution the ClO₄^– and BF₄^– salts of [CuL¹]²⁺ exhibit only one ligand-field band (14 800 cm^–1) while the other complexes show two bands (≈ 11 000, 15 000–16 000 cm^–1). The polycrystalline EPR spectra of the former complexes are axial while those of the other complexes are rhombic. The cryogenic solution EPR spectra of the former complexes differ from those of the other complexes by exhibiting comparatively low g_‖ values and well resolved nitrogen superhyperfine structures. All these spectral features suggest a unique ‘folded’ geometry 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 the copper(I) over the copper(II) state. For copper(II) complexes of bis(benzimidazolyl) thioether ligands with all-five-membered chelate rings the potential increases with increase in the number of thioether donors.