Hydrogen Atom Transfer Barriers by High-Valent Iron(IV)-Oxo Complexes: A DFT and Multireference Ab Initio Study.

Area of Science:

• Computational Chemistry
• Organometallic Chemistry
• Catalysis

Background:

• Hydrogen atom transfer (HAT) from strong C-H bonds to high-valent iron-oxo species is crucial for enzymatic and synthetic oxidation processes.
• Accurate prediction of activation barriers for these reactions is challenging due to the complex electronic structure of iron-oxo species and the presence of competing reaction pathways.

Purpose of the Study:

• To investigate methane hydrogen atom transfer (HAT) by a series of ten iron(IV)-oxo (Fe^IVO) complexes.
• To obtain accurate reference activation free energies for methane activation using high-level computational methods.
• To benchmark the performance of common density functional theory (DFT) functionals against multireference calculations for predicting Fe^IVO HAT reactivity.

Main Methods:

• Density functional theory (DFT) calculations were employed to study methane HAT by ten Fe^IVO complexes.
• Multireference ab initio calculations, specifically CASPT2, were used to obtain highly accurate reference activation free energies.
• Benchmarking of various DFT functionals was performed against the CASPT2 results to assess their accuracy in predicting reaction barriers.

Main Results:

• Activation free energies for methane HAT were found to correlate strongly with key distances along the hydrogen-transfer coordinate, but only weakly with the oxo radical character.
• The iron(IV)-oxo complex [Fe^IVO(^tBu₃tacn)]²⁺, featuring a low coordination number and an S = 2 ground state, is predicted to be a promising room-temperature methane activation catalyst.
• DFT functionals showed pathway-dependent accuracy, with several underestimating barriers for side pathways. A linear relationship between DFT mean absolute error and Hartree-Fock exchange fraction was identified, indicating higher errors for high-%HF functionals.

Conclusions:

• Accurate prediction of Fe^IVO HAT reactivity requires high-level computational methods, such as multireference calculations, due to the limitations of common DFT functionals.
• The study highlights the importance of considering specific structural and electronic features, like coordination number and spin state, in designing effective iron-oxo catalysts for methane activation.
• Findings emphasize the need for multireference data to improve data-driven predictions and guide the development of more accurate computational models for Fe^IVO HAT reactions.