Copper(II) complexes of novel tripodal ligands containing phenolate and benzimidazole/pyridine pendants: synthesis, structure, spectra and electrochemical behaviour

Abstract

Mononuclear copper(II) complexes of tri- and tetra-dentate tripodal ligands containing phenolic hydroxyl and benzimidazole or pyridine groups have been isolated. They are of the type [CuL(X)]·nH₂O, [CuL(H₂O)]X·nH₂O or [CuL]·nH₂O where X = Cl^–, ClO₄^–, N₃^– or NCS^– and n= 0–4. The electronic spectra of all the complexes exhibit a broad absorption band around 14 000 cm^–1 and the polycrystalline as well as the frozen-solution EPR spectra are axial, indicating square-based geometries. The crystal structure of [CuL(Cl)][HL =(2-hydroxy-5-nitrobenzyl)bis(2-pyridylmethyl)amine] revealed a square-pyramidal geometry around Cu^II. The mononuclear complex crystallises in the triclinic space group P with a= 6.938(1), b= 11.782(6), c= 12.678(3)Å and α= 114.56(3), β= 92.70(2), γ= 95.36(2)°. The co-ordination plane is comprised of one tertiary amine and two pyridine nitrogens and a chloride ion. The phenolate ion unusually occupies the axial site, possibly due to the electron-withdrawing p-nitro group. The enhanced π delocalisation involving the p-nitrophenolate donor elevates the E_½ values. The spectral and electrochemical results suggest the order of donor strength as nitrophenolate < pyridine < benzimidazole in the tridentate and nitrophenolate < benzimidazole < pyridine in the tetradentate ligand complexes.