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Tuning the H-Atom Transfer Reactivity of Iron(IV)-Oxo Complexes as Probed by Infrared Photodissociation Spectroscopy.

ABSTRACT: Reactivities of non-heme iron(IV)-oxo complexes are mostly controlled by the ligands. Complexes with tetradentate ligands such as [(TPA)FeO]²⁺ (TPA=tris(2-pyridylmethyl)amine) belong to the most reactive ones. Here, we show a fine-tuning of the reactivity of [(TPA)FeO]²⁺ by an additional ligand X (X=CH₃ CN, CF₃ SO₃ ^- , ArI, and ArIO; ArI=2-(^t BuSO₂ )C₆ H₄ I) attached in solution and reveal a thus far unknown role of the ArIO oxidant. The HAT reactivity of [(TPA)FeO(X)]^+/2+ decreases in the order of X: ArIO > MeCN > ArI ≈ TfO^- . Hence, ArIO is not just a mere oxidant of the iron(II) complex, but it can also increase the reactivity of the iron(IV)-oxo complex as a labile ligand. The detected HAT reactivities of the [(TPA)FeO(X)]^+/2+ complexes correlate with the Fe=O and FeO-H stretching vibrations of the reactants and the respective products as determined by infrared photodissociation spectroscopy. Hence, the most reactive [(TPA)FeO(ArIO)]²⁺ adduct in the series has the weakest Fe=O bond and forms the strongest FeO-H bond in the HAT reaction.

SUBMITTER: Tripodi GL

PROVIDER: S-EPMC8048595 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Iron(iv)-oxo species have been identified as the active intermediates in key enzymatic processes, and their catalytic properties are strongly affected by the equatorial and axial ligands bound to the metal, but details of these effects are still unresolved. In our aim to create better and more efficient oxidants of H-atom abstraction reactions, we have investigated a unique heteroleptic diiron phthalocyanine complex. We propose a novel intramolecular approach to determine the structural features that govern the catalytic activity of iron(iv)-oxo sites. Heteroleptic ?-nitrido diiron phthalocyanine complexes having an unsubstituted phthalocyanine (Pc1) and a phthalocyanine ligand substituted with electron-withdrawing alkylsulfonyl groups (PcSO2R) were prepared and characterized. A reaction with terminal oxidants gives two isomeric iron(iv)-oxo and iron(iii)-hydroperoxo species with abundances dependent on the equatorial ligand. Cryospray ionization mass spectrometry (CSI-MS) characterized both hydroperoxo and diiron oxo species in the presence of H2O2. When m-CPBA was used as the oxidant, the formation of diiron oxo species (PcSO2R)FeNFe(Pc1)[double bond, length as m-dash]O was also evidenced. Sufficient amounts of these transient species were trapped in the quadrupole region of the mass-spectrometer and underwent a CID-MS/MS fragmentation. Analyses of fragmentation patterns indicated a preferential formation of hydroperoxo and oxo moieties at more electron-rich iron sites of both heteroleptic ?-nitrido complexes. DFT calculations show that both isomers are close in energy. However, the analysis of the iron(iii)-hydroperoxo bond strength reveals major differences for the (Pc1)FeN(PcSO2R)FeIIIOOH system as compared to (PcSO2R)FeN(Pc1)FeIIIOOH system, and, hence binding of a terminal oxidant will be preferentially on more electron-rich sides. Subsequent kinetics studies showed that these oxidants are able to even oxidize methane to formic acid efficiently.

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Tuning the H-Atom Transfer Reactivity of Iron(IV)-Oxo Complexes as Probed by Infrared Photodissociation Spectroscopy.

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