Iron(ii)-α-keto acid complexes of tridentate ligands on gold nanoparticles: the effect of ligand geometry and immobilization on their dioxygen-dependent reactivity

Abstract

Two mononuclear nonheme iron(II)-benzoylformate (BF) complexes [(6Me₂-Me-BPA)Fe(BF)](ClO₄) (1a) and [(6Me₃-TPMM)Fe(BF)](ClO₄) (1b) of tridentate nitrogen donor ligands, bis((6-methylpyridin-2-yl)methyl)(N-methyl)amine (6Me₂-Me-BPA) and tris(2-(6-methyl)pyridyl)methoxymethane (6Me₃-TPMM), were isolated and characterized. The structural characterization of iron(II)-chloro complexes indicates that the ligand 6Me₂-Me-BPA binds to the iron(II) centre in a meridional fashion, whereas 6Me₃-TPMM behaves as a facial ligand. Both the ligands were functionalized with terminal thiol for immobilization on gold nanoparticles (AuNPs), and the corresponding iron(II) complexes [(6Me₂-BPASH)Fe(BF)(ClO₄)]@C₈Au (2a) and [(6Me₃-TPMSH)Fe(BF)(ClO₄)]@C₈Au (2b) were prepared to probe the effect of immobilization on their ability to perform bioinspired oxidation reactions. All the complexes react with dioxygen to display the oxidative decarboxylation of the coordinated benzoylformate, but the complexes supported by 6Me₃-TPMM and its thiol-appended ligand display faster reactivity compared to their analogues with the 6Me₂-Me-BPA-derived ligands. In each case, an electrophilic iron–oxygen oxidant was intercepted as the active oxidant generated from dioxygen. The immobilized complexes (2a and 2b) display enhanced O₂-dependent reactivity in oxygen-atom transfer reactions (OAT) and hydrogen-atom transfer (HAT) reactions compared to their homogeneous congeners (1a and 1b). Furthermore, the immobilized complex 2b displays catalytic OAT reactions. This study supports that the ligand geometry and immobilization on AuNPs influence the dioxygen-dependent reactivity of the complexes.