Facial co-ordination in bis[bis(benzimidazol-2-ylmethyl)-amine]copper(II) perchlorate dihydrate. Synthesis, structure, spectra and redox behaviour

Abstract

The 1 : 1 and 1 : 2 copper(II) complexes with the tridentate compound bis(benzimidazol-2-ylmethyl)amine (L¹) and its benzimidazole (L²) and amine (L³)N-methyl-substituted derivatives have been prepared and their spectroscopic properties studied. While the 1 : 1 complexes are of the type CuLX₂·nH₂O (X = ClO₄^-, NO₃^-, Cl^- or Br^-), the 1 : 2 complexes are of the type CuL₂(ClO₄)₂·nH₂O (L = L¹ or L³, n= 0-4). In all these complexes L acts as a tridentate ligand with the amine nitrogen and both the benzimidazole nitrogens co-ordinating to Cu^II. The complex [CuL¹₂][ClO₄]₂·2H₂O crystallises in the monoclinic space group P2₁/c with a= 9.828(2), b= 9.546(2) and c= 19.906(2)Å and β = 95.71(1)° , for Z= 2. The R value is 0.0635 for 2180 significant reflections. The copper(II) ion has an elongated octahedral geometry with four equatorial benzimidazole and two long-distance axial amine N donors. The Cu-N_bzim and Cu-N_amine distances are 2.011(4) and 2.597(6)Å respectively. Factors favouring facial co-ordination of tridentate ligands are discussed. The 1 : 1 complexes involve meridional co-ordination of the ligands, with square-based geometry as revealed by ligand-field and EPR spectral properties. The NMe substitution as in CuL³(ClO₄)₂ confers low v_max and high E_½ for the Cu^II-Cu^I couple. Most of the 1 : 1 complexes are less reversible but exhibit E_½ values more positive than those of the corresponding 1 : 2 complexes.