Structure, spectra and redox behaviour of copper(II) complexes of bis(benzimidazolyl)diamine ligands

The linear quadridentate ligand N,N′-bis(benzimidazol-2-ylethyl)ethane-1,2-diamine (L1) and its 1-methylbenzimidazole analogue (L2) and homologues form 1 : 1 complexes with Cu(ClO4)2; L1 also forms complexes of the types CuL1X2 where X = NO3, PF6, Br or Cl and CuL1(X)Y where X = Cl or Br and Y = ClO4 or Br. Deep blue CuL1Br2·2H2O crystallizes in the monoclinic space group C2/c with Z= 4, a= 9.919(2), b= 16.626(3), c= 14.102(3)A and β= 94.39(2)°. The structure was solved by Patterson and Fourier difference methods and refined by the least-squres technique to R= 0.064 for 2195 independent reflections with I > 1.5σ(I). The molecule lies on a two-fold axis symmetrically around CuII. The co-ordination around CuII is found to be square planar with two amino nitrogens and two benzimidazole nitrogens forming the equatorial plane [Cu–N 1.983(3) and 2.037(4)A]. The bromides are at longer distances [3.349(1)A] in axial sites. Ligand field and EPR spectra indicate that one bromide or chloride ion is axially co-ordinated to CuII in [CuL1]2+. This ion exhibits quasi-reversible redox behaviour. Electrochemical studies of the dihalides in methanol have established the presence of [CuL1X2], [CuL1(X)]+ and [CuL1]2+ in equilibrium. In complexes with 565 [CuL4]2+[L4=N,N′-bis(benzimidazol-2-ylmethyl)ethane-1,2-diamine] and 555 [CuL3]2+[L3=N,N′-bis(1-methylbenzimidazol-2-ylmethyl)propane-1,3-diamine] chelate rings, CuII does not seem to lie in the N4 square plane, as revealed by their low A∥ values and irreversible electrochemical behaviour. The CuII–CuI redox potentials in methanol are in the order [CuL1]2+ < [CuL3]2+ < [CuL4]2+; this illustrates that six-membered chelate rings are suitable to stabilize CuII, when Cu–N σ interactions are favourable.