Spectroscopic and electrochemical studies on the chromotropism of ternary copper(II) complexes

Abstract

Ternary complexes of copper(II) with β-ketoesters [β-keto = ethyl acetoacetate (HETAA) and ethyl benzoylacetate (HETBA)] and diamines [diam =  N,N,N′,N′-tetramethylethylenediamine (Me₄en), N,N,N′-trimethylethylenediamine (Me₃en), N,N,N′-triethylethylenediamine (Et₃en) and N-methyl-1,4-diazacycloheptane (medach)] of general formula Cu(β-keto)(diam)X, where X⁻  = ClO₄, BPh₄, NO₃, Cl or Br, have been prepared. Their structure and chromotropicity have been characterized using spectral analyses, electrochemical and magnetic susceptibility measurements. Complexes having perchlorate, tetraphenylborate and nitrate as anions show a remarkable change from reddish violet to green in different solvents and anions with increasing donor strength of the solvent or anion. Complexes having chloride and bromide as anions are highly influenced by the acceptor property rather than the donor property of the solvent. The spectral studies revealed the possibility of using these complexes as Lewis acid–base color indicators for both solvent and anion donor strength. Cyclic voltammetric measurements on the complexes in different solvents showed that the reduction process is mainly diffusion controlled and quasi-reversible or irreversible. Such behavior has been explained according to the ECE mechanism (E = electrochemical step, C = chemical step). The solvent effect has been rationalized in terms of the thermodynamics and kinetics of the redox processes. A correlation has been found between the copper(II) reduction potential and the spectral data in different solvents.