Influence of First and Second Coordination Environment on Structural Fe(II) Sites in MIL-101 for C-H Bond Activation in Methane

Divalent iron sites in tri-iron oxo-centered metal nodes in metal-organic frameworks (MOFs) catalyze light alkane oxidation. The first two steps of the reaction sequence, which are also the most energetically demanding ones, are the formation of the active species, Fe(IV)=O, by N2O decomposition and subsequent C-H bond cleavage. We have employed Kohn-Sham density functional methods to explore how modification of the microenvironment around the Fe(II) center can modulate its catalytic activity, akin to what noted in metalloenzymes. We have varied the substituents on the organic linker of the MIL-101(Fe) MOF, as a way to modulate the energy barriers associated with the first two steps of the methane to methanol reaction. The calculations show that varying substituents has a minimal electronic effect on the iron center and its first coordination shell. However, their proximity to the active site can modify the barriers by 20%. Hydrogen bond donors can lower both barriers, such that the resulting Fe(IV)=O species are simultaneously more stable and more reactive than those of the parent MOF. The screening of a large set of systems allowed us to establish rules for the selection of second coordination shell elements to improve the reactivity of oxoferryl-based catalysts: (i) functionality with a low pK(a) or large positive electrostatic potential, (ii) a distance around 1.5 angstrom between the oxoferryl and any atom of the ring substituent, and (iii) low conformational flexibility of the added substituent.

Automated summary

The study demonstrates that modifying the microenvironment around the divalent iron sites in metal-organic frameworks can modulate their catalytic activity for light alkane oxidation. By varying substituents and their proximity to the active site, the stability and reactivity of the Fe(IV)=O species can be improved to lower energy barriers for methane oxidation to methanol reaction.