(304h) Probing Structural Dynamics and Selectivities during MOF Oxidation Reactions

Metal-organic frameworks (MOFs) offer adaptable synthetic protocols that result in morphological and chemical tunability, including pore sizes and chemical functionalities that can alter confinement of guest substrates and their corresponding transition-state structures. In this work, iron-based MIL-100(Fe) is examined for alkene oxidation reactions, which are relevant transformations for pharmaceutical, plastics, and food industries. Liquid-phase, batch reactions in acetonitrile were conducted with varying alkene (1-octene, styrene) and oxidant (hydrogen peroxide, tert-butyl hydroperoxide (TBHP)) identities, demonstrating different reactivities, product distributions, and extents of deactivation. Generally, the product pathways depended on the identity of the reactive, surface-bound intermediate formed, with hydroxyl radicals leading to aldehydes and hydroperoxyl radicals leading to epoxide-analogs. At low extents of reaction, styrene oxidation by TBHP led to selective formation of styrene oxide over benzaldehyde with retention of the long-range crystal structure, whereas use of hydrogen peroxide resulted in additional secondary reactions. This higher extent of secondary reactions (conversion of 1,2-epoxyoctane and octanal to hydroperoxides) was observed for 1-octene oxidation even using TBHP, suggesting varied stability of aryl versus alkyl metallocycle transition states. Titrations of MIL-100(Fe) acid sites paired with in-situ Fourier-transform infrared spectroscopy (FTIR) were used to understand the relative contribution of each site (i.e., Brønsted versus Lewis) to alkene oxidation reactivity and oxygenate product distributions. Further, MIL-100(Fe) thermal preactivation procedures were employed to modulate the local coordination environment and valency of the Fe node, inducing reductive elimination of the anionic capping ligand to generate open, reactive, coordinatively unsaturated Fe²⁺ sites. These pretreatment protocols promoted higher oxidation reactivity and resulted in changes to the oxygenate product distributions. Altogether, this work aims to construct rigorous structure-function relationships in alkene oxidation chemistry to facilitate design of selective and stable MOF catalysts.