Synthesis, spectra and redox behavior of copper(II) complexes of curcumin diketimines as models for blue copper proteins

Abstract

Several new CuII complexes, having positive reduction potentials in an in situmanner, were synthesized by reaction of the Knoevenagel condensate, 4-salicylidene-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione (from curcumin) with 4-X-anilines and CuCl2. The solution electronic spectra of these complexes show intense absorption around 530 nm with unusual high extinction values (e, 1600--3600 m-1 cm-1) due to the low symmetry Lπ--dπ (Cu) LMCT transition. The e.s.r. spectral features with g || > g || > 2.0 having four g || values and a broadening of the g ||component with moderately low A || (110 × 10-4 cm-1) also suggest a lower symmetry around CuII. The cyclic voltammetric studies of the CuII complexes in MeCN show a positive reduction potential (E pc=502--138 mV) with high peak-to-peak separation (ΔE p=87--335 mV). The higher e and low A ||values together with positive reduction potentials for these CuII complexes suggest that they can mimic the functional properties of blue copper proteins, but have poor redox stability.