Catechol oxidase and phenoxazinone synthase mimicking activities of mononuclear Fe(III) and Co(III) complexes of amino-bis(phenolate)-based mixed ligands: Synthesis, spectral and electrochemical studies

A new class of aminophenol ligands, bis(5-(tert-butyl)-2-hydroxybenzyl)glycine (H3L1) and bis(2-hydroxy-5-methylbenzyl)glycine (H3L2) were synthesized by the Mannich reaction. The mixed ligand mononuclear iron(III) and cobalt(III) complexes have been synthesized by using these ligands and bipy/phen. All the complexes were characterized by IR, UV, EPR and mass spectral analysis. In the electronic spectra, the higher electron donating nature of the p-substituent -C(CH3)(3) (H3L1) compared with the -CH3 (H3L2) of the phenolic ring causes a bathochromic shift in the LMCT-charge transfer band. EPR studies revealed the paramagnetic nature of the complexes. Cyclic voltammograms showed three characteristic reduction peaks for iron(III) complexes and two reduction peaks for cobalt(III) complexes in the cathodic direction. There is an anodic shift in the reduction of the metal centre when the electron donating nature of the p-substituent of the phenolic ring decreases or the flexibility of alpha-diimine ligands increases. The synthesized complexes were effectively used as models for mimicking the activities of catechol oxidase and phenoxazinone synthase. The catalytic activity of the complexes are compared and discussed on the basis of the p-substituent of the phenolic ring and the flexibility of the exogenous ligands. The structural properties of the complexes are also discussed by using molecular modeling studies.