Copper(II) complexes of new pentadentate bis(benzimidazolyl)-dithioether ligands: synthesis, structure, spectra and redox properties

Abstract

Copper(II) complexes of a series of linear pentadentate ligands containing two benzimidazoles, two thioether sulfurs and a amine nitrogen, viz. N,N'-bis{4'-(2'-benzimidazolyl)(methyl)-3'-thiabutyl}amine(L1), N,N'-bis{4'-(2"-benzimidazolyl)(methyl)-3'-thiabutyl}N-methylamine (L²), 2,6-bis{4'-(2"-benzimidazolyl)(methyl)-3'-thiabutyl}pyridine(L³), N,N'-bis{4'-(2"-benzimidazolyl)-2'-thiabutyl}amine (L⁴), N,N'-bis{4'-(2"-benzimidazolyl)-2'-thiabutyl}N-methylamine (L⁵) and 2,6-bis{4'-(2"-benzimidazolyl)-2'-thiabutyl}-3pyridine (L⁶) have been isolated and characterized by electronic absorption and EPR spectroscopy and cyclic and differential pulse voltammetry. Of these complexes, [Cu(L¹)](BF₄)₂ (1) and [Cu(L²)](BF₄)₂ (4) have been structurally characterized by X-ray crystallography. The coordination geometries around copper(II) in 1 and 4 are described as trigonal bipyramidal distorted square based pyramidal geometry (TBDSBP). The distorted CuN₃S basal plane in them is comprised of amine nitrogen, one thioether sulphur and two benzimidazole nitrogens and the other thioether sulfur is axially coordinated. The ligand field spectra of all the complexes are consistent with a mostly square-based geometry in solution. The EPR spectra of complexes [Cu(L¹)](BF₄)₂ (1), [Cu(L¹)](NO₃)₂ (2), [Cu(L²)](BF₄)₂ (4) and [Cu(L³)](ClO₄)₂ (6) are consistent with two species indicating the dissociation/disproportionation of the complex species in solution. All the complexes exhibit an intense S(σ ) → Cu(II) CT band in the range 305-395 nm and show a quasireversible to irreversible Cu^II/Cu^I redox process with relatively positive E_1/2 values, which are consistent with the presence of two-coordinated thioether groups. The addition of N-methylimidazole (mim) replaces the coordinated thioether ligands in solution, as revealed from the negative shift (222-403 mV) in the Cu^II/Cu^I redox potential. The present study reveals that the effect of incorporating an amine nitrogen donor into CuN₂S₂ complexes is to generate an axial copper(II)-thioether coordination and also to enforce lesser trigonality on the copper(II) coordination geometry. Mononuclear copper(II) complexes of a series of new linear pentadentate bis(benzimidazolyl)-dithioether ligands bearing N₃S₂ donor set have been isolated. The effect of incorporating an amine nitrogen donor into CuN₂S₂ chromophore is to generate an axial Cu-S bond. The electronic and EPR spectral and electrochemical properties of the new complexes have been discussed.