Quantum chemical DFT study of the interaction between molecular oxygen and FeN4 complexes, and effect of the macrocyclic ligand

Abstract

Density functional theory (DFT) was used to examine the interaction between molecular oxygen (O₂) and macrocyclic iron complexes of the type FeN₄ during the formation of FeN₄--O₂ adducts. In order to understand how this interaction is affected by different macrocyclic ligands, O₂ was bonded to iron-tetraaza[14]annulene (FeTAA), iron-tetramethyl-tetraaza[14]annulene (FeTMTAA), iron-hexamethyl-tetraaza[14]annulene (FeHMTAA), iron dibenzotetraaza[14]annulene (FeDBTAA), and two iron-tetramethyl-dibenzotetraaza[14]annulene complexes (FeTMDBTAA1, FeTMDBTAA2). The ground state for FeN₄-O₂ adducts was the open-shell singlet. Analysis of the factors influencing the O₂ bonding process showed that different macrocyclic ligands yielded adducts with differences in O--O and Fe--O₂ bond lengths, total charge over the O₂ fragment, O--O vibrational frequency, and spin density in the O₂ fragment. A smaller energy gap between the α-HOMO of the FeN₄ complexes and the β-LUMO of O₂ increased the interaction between the complex and the O₂ molecule. The order of activity was FeDBTAA < FeTMDBTAA2 < FeTMDBTAA1 < FeTAA < FeTMTAA < FeHMTAA.

MIL-101 as Reusable Solid Catalyst for Autoxidation of Benzylic Hydrocarbons in the Absence of Additional Oxidizing Reagents

“…We propose that the activity of MIL-101 to generate benzyl radicals promoting autoxidation derives from the interaction of oxygen with the metal nodes of the lattice forming iron or chromium metalloperoxides with some radical character on the oxygen atom that will be able to abstract one hydrogen atom from the benzylic position. In support of our proposal, it should be indicated that there are precedents in the literature describing for other metal complexes the formation of metal-superoxo species, M-O 2 •‑ of Cr­(III) or Fe­(III) as metals. − These species exhibit activity to perform C–H bond activation in hydrocarbons. , In addition, hemo groups containing Fe 2+ are the natural O 2 carriers in blood and biological systems. − In the literature, there are also some precedents using MIL-101­(Cr) as catalyst for the benzylic oxidation of hydrocarbonsalways using TBHP as oxidizing reagent. Herein, we have shown that this oxidation can be promoted in the absence of TBHP.…”

Theoretical Study of the Electrocatalytic Reduction of Oxygen by Metallocorroles

“…Although the proposed four-electron mechanism can partially account for the experimentally observed reactivity of metallocorrole-based catalysts, [M­(tpfcBr8)], the exact ORR mechanism that could account for the excellent experimentally observed performance of these catalysts is unknown . The O 2 adsorption energies for transition metal macrocyclic complexes were discussed as a viable ORR reaction descriptor. ,, The computed O 2 adsorption free energies to [M­(tpfcBr8)] are negative for MMn, Fe, and Co but positive for MNi and Cu. When plotting the experimentally observed onset potentials as a function of the computed O 2 adsorption free energies, a volcano-like plot is observed (Figure b), indicating that for the best catalyst, [M­(tpfcBr8)] MCo, a negative binding free energy is observed, yet with an intermediate value between the strong binders, like MMn, and the weak binders, like MCu.…”

“…11 The O 2 adsorption energies for transition metal macrocyclic complexes were discussed as a viable ORR reaction descriptor. 35,58,59 The computed O 2 adsorption free energies to [M(tpfcBr8)] are negative for MMn, Fe, and Co but positive for MNi and Cu. When plotting the experimentally observed onset potentials as a function of the computed O 2 adsorption free energies, a volcano-like plot is observed (Figure 4b), indicating that for the best catalyst, [M(tpfcBr8)] MCo, a negative binding free energy is observed, yet with an intermediate value between the strong binders, like MMn, and the weak binders, like MCu.…”

Assembly of a mononuclear ferrous site using a bulky aldehyde-imidazole ligand

“…The lowest unoccupied molecular orbital (LUMO, α-267) is primarily distributed over the imidazole ring and the aldehyde oxygens. The HOMO-LUMO gap for 1 is large (4.49 eV) indicating high stability [61,62]. …”