Chelate ligand-driven structural diversity and modeling studies on the phenoxazinone synthase-like activity of cobalt(iii) complexes

Abstract

In the study of biomimetic catalytic activity, substrate–catalyst interactions control the rate of the catalytic process. This fact depends on the selection of model compounds. The presence of labile groups or the availability of space around the metal centre is essential for metal–substrate interactions. This could be achieved by the rational choice of the metal centre and chelate ligands. To gain insights into the above-mentioned aspect, we studied the biomimetic catalytic activity of an oxidase metalloprotein, phenoxazinone synthase (PHS), using two Co(III) complexes: complex 1 is mononuclear, whereas compound 2 is dinuclear. We used tetradentate (H₂L1, 6,6′-((1E,1′E)-(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(4-allyl-2-methoxyphenol)) and tridentate (HL2, (E)-4-allyl-2-(((2-aminoethyl)imino)methyl)-6-methoxyphenol) chelate ligands for complex 1 [Co(L1)(NCS)(H₂O)] and 2 [Co₂(L2)₂(µ_1,1-OMe)₂(NCS)₂]. We used the same coligand, i.e., thiocyanate ion (SCN⁻), in order to study the effect of only the chelate ligands. In the case of complex 1, the tetradentate N₂O₂ ligand coordinates equatorially with the Co(III) centre, leaving two axial positions available for coordination with labile groups (SCN⁻ and H₂O), resulting in a mononuclear complex. In complex 2, the tridentate N₂O chelate ligand facilitates the formation of a μ_1,1-methoxido-bridged dinuclear Co(III) complex with a rigid framework. In compound 2, the labile SCN⁻ group coordinated with the Co(III) centre in a rigid platform making its replacement by the substrate relatively difficult. Thus, the presence of available coordination sites in a flexible framework makes complex 1 a more suitable model catalyst for phenoxazinone synthase activity.