Synthesis, structure, spectra and redox of Cu(II) complexes of chelating bis(benzimidazole)-thioether ligands as models for electron transfer blue copper proteins

Abstract

The tridentate ligand 1,5-bis(benzimidazol-2-yl)-3-thiapentane (L¹) with N₂S donor set forms the complex [Cu(L¹)(H₂O)Cl]Cl 1a and the linear quadridentate ligand 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (L²) with N₂S₂ donor set forms the complexes [Cu(L²)](ClO₄)₂·2H₂O 2a and [Cu(L²)(NO₃)]NO₃2b. The linear pentadentate ligand 1,11-bis(pyrid-2-yl)-3,6,9-trithiaundecane (L³) with N₂S₃ donor set forms the complex [Cu(L³)](ClO₄)₂3. The perchlorate complex [Cu(L⁴)](ClO₄)₂·2CH₃CN 4 of the pentadentate ligand, N,N-bis(benzimidazol-2-ylmethylthioethyl)methylamine (L⁴) with N₃S₂ donor set has also been isolated. In 1a Cu(II) is coordinated to the two benzimidazole nitrogens and thioether sulfur of the ligand L¹, a chloride ion and a water molecule. The coordination geometry around copper is intermediate between trigonal bipyramidal and square pyramidal geometries and is better described as trigonal bipyramidal distorted square based pyramidal (TBDSBP) with the sulfur and nitrogen atoms and the chloride ion in the equatorial positions and the oxygen of water in the apical position. The coordination geometry around copper(II) in 2b is best described as trigonal bipyramidal, with both the thioether sulfur atoms [Cu-S(1), 2.529(5) and Cu-S(2), 2.438(6) Å] and one of the oxygen atoms of the nitrate ion [Cu-O(1), 2.066(13) Å] constituting the trigonal plane and both the benzimidazole nitrogens [Cu-N, 1.985(14) and 1.953(13) Å] occupying the axial positions. The bulky benzimidazole moieties of the ligand prevent the other nitrate ion from coordinating and favours trigonal bipyramidal geometry in spite of the presence of two six-membered chelate rings. In 4 the coordination plane of Cu(II) is comprised of two benzimidazole nitrogens, one thioether sulfur and N-methyl substituted amine nitrogen atom with the other thioether sulfur atom coordinated axially. The coordination geometry is best described as trigonal bipyramidal distorted square based pyramidal (TBDSBP). The ligand field and EPR spectra of 1a, 2a and 2b are consistent with trigonal bipyramidal geometry in the solid state, whereas two ligand field bands in solution and an axial EPR spectrum in frozen solution were observed suggesting a change in coordination geometry to a square-based one on dissolution. The complexes 3 and 4 exhibit only one ligand field band in the solid state and axial EPR spectrum consistent with a square based geometry. All the complexes exhibit an intense S(σ ) → Cu(II) CT band in the range 330-380 nm and a high positive Cu^II/Cu^I redox potential.