Influence of first and second coordination environment on structural Fe(II) sites in MIL-101 for C-H bond activation in methane

Vitillo, J. G.; Lu, C. C.; Cramer, C. J.; Bhan, A.; Gagliardi, L.
ACS Catal. 2021, 11, 579 (doi:10.1021/acscatal.0c03906).

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. In analogy to metalloenzymes, 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. 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 indicate that varying substituents has a minimal electronic effect on the iron center in the 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 the rules for the selection of second coordination shell elements able 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 Å between the oxoferryl and any atom of the ring substituent; and (iii) low conformational flexibility of the added substituent.