Reactivity of an Iron–Oxygen Oxidant Generated upon Oxidative Decarboxylation of Biomimetic Iron(II) α‑Hydroxy Acid Complexes

Three biomimetic iron­(II) α-hydroxy acid complexes, [(Tp^Ph2)­Fe^II(mandelate)­(H₂O)] (1), [(Tp^Ph2)­Fe^II(benzilate)] (2), and [(Tp^Ph2)­Fe^II(HMP)] (3), together with two iron­(II) α-methoxy acid complexes, [(Tp^Ph2)­Fe^II(MPA)] (4) and [(Tp^Ph2)­Fe^II(MMP)] (5) (where HMP = 2-hydroxy-2-methylpropanoate, MPA = 2-methoxy-2-phenylacetate, and MMP = 2-methoxy-2-methylpropanoate), of a facial tridentate ligand Tp^Ph2 [where Tp^Ph2 = hydrotris­(3,5-diphenylpyrazole-1-yl)­borate] were isolated and characterized to study the mechanism of dioxygen activation at the iron­(II) centers. Single-crystal X-ray structural analyses of 1, 2, and 5 were performed to assess the binding mode of an α-hydroxy/methoxy acid anion to the iron­(II) center. While the iron­(II) α-methoxy acid complexes are unreactive toward dioxygen, the iron­(II) α-hydroxy acid complexes undergo oxidative decarboxylation, implying the importance of the hydroxyl group in the activation of dioxygen. In the reaction with dioxygen, the iron­(II) α-hydroxy acid complexes form iron­(III) phenolate complexes of a modified ligand (Tp^Ph2*), where the ortho position of one of the phenyl rings of Tp^Ph2 gets hydroxylated. The iron­(II) mandelate complex (1), upon decarboxylation of mandelate, affords a mixture of benzaldehyde (67%), benzoic acid (20%), and benzyl alcohol (10%). On the other hand, complexes 2 and 3 react with dioxygen to form benzophenone and acetone, respectively. The intramolecular ligand hydroxylation gets inhibited in the presence of external intercepting agents. Reactions of 1 and 2 with dioxygen in the presence of an excess amount of alkenes result in the formation of the corresponding cis-diols in good yield. The incorporation of both oxygen atoms of dioxygen into the diol products is confirmed by ¹⁸O-labeling studies. On the basis of reactivity and mechanistic studies, the generation of a nucleophilic iron–oxygen intermediate upon decarboxylation of the coordinated α-hydroxy acids is proposed as the active oxidant. The novel iron–oxygen intermediate oxidizes various substrates like sulfide, fluorene, toluene, ethylbenzene, and benzaldehyde. The oxidant oxidizes benzaldehyde to benzoic acid and also participates in the Cannizzaro reaction.