Reductive Activation of O₂ by Non-Heme Iron(II) Benzilate Complexes of N₄ Ligands: Effect of Ligand Topology on the Reactivity of O₂‑Derived Oxidant

A series of iron­(II) benzilate complexes (<b>1</b>–<b>7</b>) with general formula [(L)­Fe^II(benzilate)]⁺ have been isolated and characterized to study the effect of supporting ligand (L) on the reactivity of metal-based oxidant generated in the reaction with dioxygen. Five tripodal N₄ ligands (tris­(2-pyridylmethyl)­amine (TPA in <b>1</b>), tris­(6-methyl-2-pyridylmethyl)­amine (6-Me₃-TPA in <b>2</b>), <i>N</i>¹,<i>N</i>¹-dimethyl-<i>N</i>²,<i>N</i>²-bis­(2-pyridylmethyl)­ethane-1,2-diamine (<i>iso</i>-BPMEN in <b>3</b>), <i>N</i>¹,<i>N</i>¹-dimethyl-<i>N</i>²,<i>N</i>²-bis­(6-methyl-2-pyridylmethyl)­ethane-1,2-diamine (6-Me₂-<i>iso</i>-BPMEN in <b>4</b>), and tris­(2-benzimidazolylmethyl)­amine (TBimA in <b>7</b>)) along with two linear tetradentate amine ligands (<i>N</i>¹<i>,N</i>²-dimethyl-<i>N</i>¹<i>,N</i>²-bis­(2-pyridylmethyl)­ethane-1,2-diamine (BPMEN in <b>5</b>) and <i>N</i>¹,<i>N</i>²-dimethyl-<i>N</i>¹,<i>N</i>²-bis­(6-methyl-2-pyridylmethyl)­ethane-1,2-diamine (6-Me₂-BPMEN in <b>6</b>)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations of <b>1</b>–<b>3</b> and <b>5</b> contains a mononuclear six-coordinate iron­(II) center coordinated by a monoanionic benzilate, whereas complex <b>7</b> contains a mononuclear five-coordinate iron­(II) center. Benzilate binds to the iron center in a monodentate fashion via one of the carboxylate oxygens in <b>1</b> and <b>7</b>, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral hydroxy oxygen in <b>2</b>, <b>3</b>, and <b>5</b>. All of the iron­(II) complexes react with dioxygen to exhibit quantitative decarboxylation of benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting iron–oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and sulfone. Furthermore, complexes <b>2</b> and <b>4</b>–<b>6</b> react with alkenes to produce <i>cis</i>-diols in moderate yields with the incorporation of both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of interception studies, nucleophilic iron­(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O₂-derived iron–oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron­(II) hydroperoxides are energetically favorable for the complexes of 6-Me₃-TPA, 6-Me₂-<i>iso</i>-BPMEN, BPMEN, and 6-Me₂-BPMEN ligands, while high spin end-on iron­(II) hydroperoxides are favorable for the complexes of TPA, <i>iso</i>-BPMEN, and TBimA ligands