Synthesis, characterization, and electrochemical properties of a new series of inorganic and organometallic Co(III) complexes with a Schiff base ligand derived from tyrosine

A new series of inorganic and organometallic cobalt Schiff base complexes containing the tetradentate ligand [Z-Tyr(3-Ac)-OMe]2en derived from the condensation of two equivalents of protected 3-acetyl- L-tyrosine with one equivalent of ethylenediamine has been synthesized. As the amino acids have L configuration, the resulting ligand is chiral. The complexes have been characterized by 1H and 13C NMR, ESI-MS spectrometry and U.V.-visible spectroscopy. The U.V.-visible spectra recorded in CH2Cl2 show an absorption band at about 650 nm, characteristic of penta-coordinated alkylcobalt(III)(salen), but an evident deviation from the Lambert Beer law in the concentration range 0.20  2.0  10-3 M suggests some kind of association in this solvent. In coordinating solvent the complexes are hexa-coordinated. The equilibrium constants for pyridine ligation in CH3OH decrease with increasing electron-donor power of the axial alkyl group. Cyclic voltammetry studies indicate that after the reduction RCo(III)/RCo(II), the complexes decompose giving a Co(I) species that is stable on the CV time scale and can be reoxidized to Co(II) and Co(III) species. Both the Epa and the Epc values show that the peak potentials are shifted to more negative values with increasing inductive effect of the axial alkyl group.