UC Berkeley

This dissertation discusses the effort towards the use of metal–organic frameworks for stabilizing iron(IV)-oxo species and the oxidation of hydrocarbons with dioxygen. Metal–organic frameworks consist of metal ions or clusters coordinated in multiple dimensions by multitopic organic linkers, and have been studied for various applications, including catalysis, sensing, gas storage and separation for the last decades. However, limited studies have been done to use metal–organic frameworks as candidates to mimic the function of metalloenzymes, especially nonheme iron enzymes.Chapter 1 gives an introduction to dioxygen activation and iron(IV)-oxo intermediates in biological systems. The synthetic efforts toward synthesizing similar iron(IV)-oxo species in homogeneous and heterogeneous systems are also discussed. In addition, metal–organic frameworks have been investigated as candidates for hydrocarbon oxidation catalysts while no iron(IV)-oxo intermediates have been characterized by spectroscopic methods. Chapter 2 describes nonheme iron(II) sites in a metal–organic framework that activates dioxygen at a low temperature of 100 K to form high-spin S = 2 Fe(IV)=O species. Such reactive sites are characterized by various spectroscopic methods, including in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), in situ Mössbauer spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS). Moreover, the potential use of the iron-containing framework as a catalyst and its high reactivity toward hydrocarbon oxidation are exemplified by catalytic oxygenation of cyclohexane and the conversion of ethane to ethanol with dioxygen as the terminal oxidant. Chapter 3 further extends the study of metal–organic frameworks with nonheme iron(II) sites for dioxygen activation and hydrocarbon oxidation. By changing the coordination ligands around the iron, the reactivity of the frameworks towards dioxygen varies drastically. The desolvated frameworks can selectively convert methane to methanol in the presence of dioxygen in a flow reactor under mild conditions. The reactive high-spin Fe(IV)=O intermediates are characterized by in situ DRIFTS, in situ Mössbauer spectroscopy, NRVS and x-ray absorption spectroscopy. Chapter 4 explores other approaches for dioxygen activation using metal–organic frameworks. Taking inspiration from Chapters 2 and 3, Fe(II) sites are introduced into a metal−organic framework with open binding sites through post-synthetic metalation. The iron site bears a similar coordination environment to that in the nonheme iron halogenases. Initial reactivity test shows that exposure of the framework to air in the presence of cyclohexane leads to the formation of chlorocyclohexane, cyclohexanol and cyclohexanone.