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

Abstract

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)Å 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 Cu^II. The co-ordination around Cu^II 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)Å]. The bromides are at longer distances [3.349(1)Å] in axial sites. Ligand field and EPR spectra indicate that one bromide or chloride ion is axially co-ordinated to Cu^II 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, Cu^II does not seem to lie in the N₄ square plane, as revealed by their low A_∥ values and irreversible electrochemical behaviour. The Cu^II–Cu^I 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 Cu^II, when Cu–N σ interactions are favourable.