Structures and electrochemical properties of the Co(III) ternary complexes containing NO3-type tripodal tetradentate ligands and amino acids:: effect of the outer coordination sphere on the electrochemical properties

With a view to understanding the effect of the outer coordination sphere of metal complexes on the redox potential, the electronic absorption spectra, X-ray crystal structure and cyclic voltammograms were investigated of aqueous and ethanol solutions containing the complexes [Co(X) (aa)], where X denotes an NO3-type ligand such as bcmpa (N,N-bis(carboxymethyl)-L-phenylalanine), nta (nitrilotriacetic acid), bcmga (N,N-bis(carboxymethyl)-L-glutamic acid), bcmle (N,N-bis(carboxymethyl)-L-leucine), and aa is an amino acid. Examination of the solution and crystalline structures revealed that the outer coordination sphere of the Co(III) complexes did not exert any influence on the inner coordination sphere of the Co(III) ion. The Co(III) complex systems showed irreversible redox behavior in both the aqueous and ethanol solutions. This is discussed on the basis of the relationship between the hydrophobicity of the outer coordination sphere and the electron transfer reaction by use of reduction potentials. (C) 1998 Elsevier Science S.A. All rights reserved.