Effect of Ligand Fields on the Reactivity of O₂‐Activating Iron(II)‐Benzilate Complexes of Neutral N5 Donor Ligands

Abstract

Three new iron(II)-benzilate complexes [(N4Py)Fe II

Inspiration from Nature: Influence of Engineered Ligand Scaffolds and Auxiliary Factors on the Reactivity of Biomimetic Oxidants

“…There are limited reports on isolobal analogues of iron­(IV)-oxo species. − However, Borovik and co-workers observed that the iron­(III)-imido complexes could be synthesized by intermolecular C–H abstraction by an iron­(IV)-species (Scheme ). The system was spectroscopically characterized and crystallographically trapped.…”

“…The incorporation of steric groups in the second coordination sphere seems to be instrumental in defining a corridor for the approach of the substrates into the active sites. , For instance, iron picket-fence porphyrin complexes have bulky amide substituents positioned in a single facial orientation thereby constructing a cavity for the small molecules such as dioxygen to enter. − In addition, calix[6]­arene enriched systems were developed with interactions in the secondary coordination sphere to study biomimetic approaches of copper complexes. − These and subsequent studies established that the introduction of a steric effect influences the stability and reactivity of iron­(IV)-oxo systems. − In particular, in a distorted octahedral geometry, the substituents on the sixth position of a pyridine donor ligand markedly influence the energy levels of the iron-oxo core due to the weakening of the equatorial ligand field. This was shown to lead to improved electrophilicity of the iron­(IV)-oxo species. − In modified N4Py systems, the methyl groups appended to specified pyridine rings were shown to affect the stability and reactivity patterns of iron­(IV)-oxo species through guiding the substrate approach toward the active site . A systematic study using N4Py-derivatized iron­(IV)-oxo complexes with various ortho -substituted pyridine rings showed that the two sets of pyridine rings are unequal and lead to differences in stability and reactivity with substrates.…”

Which is the real oxidant in competitive ligand self-hydroxylation and substrate oxidation—a biomimetic iron(ii)-hydroperoxo species or an oxo-iron(iv)-hydroxy one?

“…50,51 Meanwhile, the finding of the nucleophilic reactivity also indirectly supported a putative iron(II)-hydroperoxo complex as the reactive species. 52 Up to now, there has been no direct experimental evidence of such an oxidant for the dioxygen activation case. In other words, 1 might either directly carry out the oxidation reactions or undergo O-O bond heterolysis to yield an oxo-iron (IV)-hydroxo intermediate (2 in Scheme 1), which works in the following oxidation reactions.…”

Aerobic oxidation of aldehydes to acids in water with cyclic (alkyl)(amino)carbene copper under mild conditions

“…25 Paine et al reported the oxidation of benzaldehyde with O 2 by iron(II)-benzilate complexes with a benzoic acid yield of only 15%. 26 Shi reported the combination of homogeneous N-hydroxyphthalimide (NHPI) and cobaltous ions for the oxidation of benzaldehyde into benzoic acid under a high O 2 pressure (2 MPa) at 90 1C. 27 N-Heterocyclic carbenes (NHCs) were found to be good catalysts for the aerobic oxidation of aldehydes at 55 1C.…”