Tuning the conversion of cyclohexane into cyclohexanol/one by molecular dioxygen, protons and reducing agents at a single non-porphyrinic iron centre and chemical versatility of the tris(2-pyridylmethyl)amine TPAFeIICl2 complex in mild oxidation chemistry

Abstract

We report that the oxygen sensitivity of some Fe(II) complexes with tripodal ligands can be used, with benefit, in the oxidation of cyclohexane under mild conditions. Depending on the solvent, two very different reaction pathways are involved, which share the coordination of O₂ to the metal as the common initial step. We have synthesized a series of α-chlorinated tripods in the tris(2-pyridylmethyl)amine series Cl_nTPA (n = 1–3) and fully characterized the corresponding FeX₂ complexes (X = Cl, CF₃SO₃). The single-crystal X-ray structure analyses of the FeCl₂ complexes are reported. In CH₃CN, the FeCl₂ complexes react smoothly with O₂, whereas the Fe(CF₃SO₃)₂ complexes are non-sensitive. In CH₃CN, the reaction of the oxygen-sensitive Cl_nTPAFeCl₂ (n = 0–3) with O₂, acetic acid and zinc amalgam, in the presence of cyclohexane, affords a mixture of cyclohexanol/one in an ≈ ol/one ratio of 3.1 and a selectivity of the C3°/C2° in the adamantane conversion that is consistent with a metal-oxo based oxidation. Limited efficiency (≈ 2 TON) was observed for the parent TPAFeCl₂ complex and Cl₁TPAFeCl₂, whereas both other complexes turned out to be poorly active. The TPAFeCl₂ complex was used to address mechanistic questions: when the reaction was carried out in pyridine, the ol/one ratio shifted to 0.15 while efficiency was improved by 7-fold. In pyridine and in the presence of a spin trap (DMPO), the radical-based character of the reaction was definitely established, by contrast with acetonitrile, where no oxygenated radicals were detected. Thus, the reactivity differences arise from involvement of two distinct active species. The dichotomous radical/biomimetic pathway is discussed to interpret these results.