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

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 CuII. 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)A 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–Nbzim and Cu–Namine distances are 2.011(4) and 2.597(6)A 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 max and high E½ for the CuII–CuI couple. Most of the 1 : 1 complexes are less reversible but exhibit E½ values more positive than those of the corresponding 1 : 2 complexes.