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Unmasking the Iron-Oxo Bond of the [(Ligand)Fe-OIAr]^2+/+ Complexes.

ABSTRACT: ArIO (ArI = 2-(^tBuSO₂)C₆H₄I) is an oxidant used to oxidize Fe^II species to their Fe^IV-oxo state, enabling hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions at low energy barriers. ArIO, as a ligand, generates masked Feⁿ═O species of the type Fe^(n-2)-OIAr. Herein, we used gas-phase ion-molecule reactions and DFT calculations to explore the properties of masked iron-oxo species and to understand their unmasking mechanisms. The theory shows that the I-O bond cleavage in [(TPA)Fe^IVO(ArIO)]²⁺ (1²⁺, TPA = tris(2-pyridylmethyl)amine)) is highly endothermic; therefore, it can be achieved only in collision-induced dissociation of 1²⁺ leading to the unmasked iron(VI) dioxo complex. The reduction of 1²⁺ by HAT leads to [(TPA)Fe^IIIOH(ArIO)]²⁺ with a reduced energy demand for the I-O bond cleavage but is, however, still endothermic. The exothermic unmasking of the Fe═O bond is predicted after one-electron reduction of 1²⁺ or after OAT reactivity. The latter leads to the 4e^- oxidation of unsaturated hydrocarbons: The initial OAT from [(TPA)Fe^IVO(ArIO)]²⁺ leads to the epoxidation of an alkene and triggers the unmasking of the second Fe═O bond still within one collisional complex. The second oxidation step starts with HAT from a C-H bond and follows with the rebound of the C-radical and the OH group. The process starting with the one-electron reduction could be studied with [(TQA)Fe^IVO(ArIO)]²⁺ (2²⁺, TQA = tris(2-quinolylmethyl)amine)) because it has a sufficient electron affinity for electron transfer with alkenes. Accordingly, the reaction of 2²⁺ with 2-carene leads to [(TQA)Fe^IIIO(ArIO)]²⁺ that exothermically eliminates ArI and unmasks the reactive Fe^V-dioxo species.

SUBMITTER: Tripodi GL

PROVIDER: S-EPMC9460779 | biostudies-literature | 2022 Sep

REPOSITORIES: biostudies-literature

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Unmasking the Iron-Oxo Bond of the [(Ligand)Fe-OIAr]2+/+ Complexes.

Tripodi Guilherme L GL Roithová Jana J

Journal of the American Society for Mass Spectrometry 20220803 9

ArIO (ArI = 2-(tBuSO2)C6H4I) is an oxidant used to oxidize FeII species to their FeIV-oxo state, enabling hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions at low energy barriers. ArIO, as a ligand, generates masked Fen═O species of the type Fe(n-2)-OIAr. Herein, we used gas-phase ion-molecule reactions and DFT calculations to explore the properties of masked iron-oxo species and to unders ...[more]

PMID: 35920859

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Project description:The thermodynamic properties of structurally similar Mn(III) and Mn(IV) complexes have been reinvestigated to understand their reactivity with substrates having C-H bonds. The complexes have the general formula [MnH(3)buea(O)](n-), where [H(3)buea](3-) is the tripodal ligand, tris[(N'-tert-butylureaylato)-N-ethylene]aminato. These complexes are unique because of the intramolecular hydrogen-bonding (H-bond) network surrounding the Mn-oxo units. The redox potentials for the Mn(III/IV)(O) couple was incorrectly assigned in earlier reports: the corrected value is -1.0 V vs Cp(2)Fe(+)/Cp(2)Fe in DMSO, while the Mn(IV/V)(O) process is -0.076 under the same conditions. The oxo ligand in the Mn(III)(O) complexes is basic with a pK(a) of 28.3; the basicity of the terminal oxo ligand in the Mn(IV)(O) complex is estimated to be approximately 15. These values were used to re-evalulate the O-H bond dissociation energy (BDE(OH)) of the corresponding Mn(II/III)-OH complexes: BDE(OH) values of 89 and 77 kcal/mol were determined for [Mn(III)H(3)buea(OH)](-) and [Mn(II)H(3)buea(OH)](2-), respectively. Both Mn(O) complexes react with 9,10-dihydroanthracene (DHA) to produce anthracene in nearly quantitative yields. This is surprising based on the low redox potiental of the complexes, suggesting the basicity of the oxo ligand is a major contributor to the observed reactivity. In contrast to the thermodynamic results, a comparative kinetic investigation found that the Mn(III)(O) complex reacts nearly 20 times faster than the Mn(IV)(O) complex. Activation parameters, determined from an Eyring analysis, found that the entropy of activation is significantly different between the two systems (DeltaDeltaS(++) = -35 eu, where DeltaDeltaS(++) = DeltaS(++)(Mn(IV)(O)) - DeltaS(++)(Mn(III)(O)). This unusual kinetic behavior can be explained in the context of the basicity of the oxo ligands that leads to different mechanisms: for [Mn(III)H(3)buea(O)](2-) a proton transfer-electron transfer mechanism is proposed, whereas for [Mn(IV)H(3)buea(O)](-) a hydrogen-atom transfer pathway is likely.

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Dataset Information

Unmasking the Iron-Oxo Bond of the [(Ligand)Fe-OIAr]^2+/+ Complexes.

Publications

Unmasking the Iron-Oxo Bond of the [(Ligand)Fe-OIAr]<sup>2+/+</sup> Complexes.

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