Bioinspired iron(II)-β-diketonate and iron(II)-α-hydroxy ketone complexes of a carbanionic N3C ligand: Oxidation of metal center vs C[sbnd]C bond cleavage of co-ligand with dioxygen

Abstract

Two bioinspired low-spin iron(II)-β-diketonate complexes, [(L1)FeII(BA)] (2) and [(L1)FeII(DBM)] (3), and an iron(II)-α-hydroxy ketone complex [(L1)FeII(HAP)] (4) (L1 = tris(2-pyridylthio)methanido anion, BA = monoanionic benzoylacetone, DBM = monoanionic dibenzoylmethane and HAP = monoanionic 2-hydroxyacetophenone) of a tripodal carbanionic ligand were prepared from an iron(II) precursor complex [(L1)FeII(CH3CN)2](ClO4) (1). The dioxygen reactivity of the complexes (2, 3 and 4) was investigated to evaluate the effect of ligand geometry and of the spin state of iron on the CC bond cleavage of the co-ligands. Complexes 2 and 3 react with dioxygen to yield benzoic acid as a minor product. The major pathway involves the formation of the corresponding iron(III)-β-diketonate complexes, [(L1)FeIII(BA)](ClO4) (2ox-ClO4) and [(L1)FeIII(DBM)](ClO4) (3ox-ClO4). Complex 4, however, undergoes the CC bond cleavage of the iron-coodinated HAP to form benzoic acid as the major product. In the reaction, phenylglyoxal is formed as the minor product with concomitant generation of an iron-oxygen oxidant. The oxidant is able to transfer an oxygen atom to thioanisole and can exchange its oxygen atom with water. The reactivity patterns of the low-spin iron(II) complexes reported here are distinctly different from that of the corresponding high-spin complexes supported by a monoanionic facial N3 ligand. The results emphasize the role of spin-state of iron, the denticity of the supporting ligand, and the nature of co-ligand in affecting the CC bond cleavage reaction.