(49d) Tailoring the Coordinating Environment Around Active Iron Centers within MOFs for Liquid-Phase Oxidation Reactions

Selective hydrocarbon oxidation processes are sensitive to metal active site nature that can be modified by designing their surrounding coordination environment. Isometallic Fe-carboxylate MOFs (MIL-100, MIL-101, NH₂-MIL-101) are synthesized to elucidate how differing coordination environments imparted by different organic ligand identities around active Fe centers influence oxidation rates and oxygenate selectivities for a probe liquid aryl (styrene) oxidation by H₂O₂. Both frameworks exhibit MTN zeotype frameworks with mesoporous cages, but (NH₂-)MIL-101 has larger pore aperture diameters (1.6 nm, 1.2 nm) than MIL-100 (0.86 nm, 0.55 nm). Systematic in situ thiophene titrations quantify active Fe centers, allowing rigorous definitions of kinetic metrics, with MIL-101 showing the highest oxidation rates, followed by NH₂-MIL-101 and MIL-100. Radical trapping agent (nitroblue tetrazolium, methanol) and initial rate studies assert that all Fe frameworks are characterized by the same surface-mediated oxidation mechanism, despite the orders of magnitude differences between oxidation rates. Here, oxidation rates trend with Fe(II) fractions as measured by ex situ Mossbauer spectroscopy after reaction, with Fe(II) formation enabled by the reductive elimination of halide capping ligands unique to the MIL-101 family. Densities of defective undercoordinated Fe sites from missing linkers inversely correlate with oxidation rates and result in higher fractions of unproductive oxidant utilization since defective Fe sites face microporous voids rather than mesopores like native Fe sites, disfavoring productive metallocycle transition state formation. While Fe valency and geometry influence reactivity, primary oxygenate selectivities are dictated by non-covalent interactions. NH₂-MIL-101 demonstrates 100% selectivity for benzaldehyde over styrene oxide at differential styrene conversions compared to a maximum of 59 % for MIL-101, owing to stabilizing hydrogen-bonding interactions between N—H donors and the postulated benzaldehyde metallocycle transition state structure. Overall, this work constructs structure-function relationships between material characteristics and observed oxidation activities.