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 pKa 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.