Oxygen atom transfer with manganese-exchanged metal-organic frameworks

Abstract

Oxygenates represent some of the most versatile commodity chemicals, justifying continued interest in the discovery of new selective oxidation catalysts from both a fundamental and applied perspective. Metal-organic frameworks (MOFs) are an attractive platform for catalysis because they enable access to unique coordination environments and reactivities; this is due in part to their tunability combined with the site isolation offered by their solid state. In one example, partial substitution of Zn[superscript II] by Mn[superscript II] in Zn₄O(terephthalate)₃ (MOF-5) leads to a distorted all-oxygen ligand field supporting a single Mn[superscript II] site, whose structure was confirmed by Mn K-edge X-ray absorption spectroscopy. Upon exposure to [superscript t]BuSO₂PhIO, Mn-MOF-5 produces a putative Mn[superscript IV]-oxo intermediate, which upon further reaction with adventitious hydrogen is trapped as a Mn[superscript III]-OH species.

Most intriguingly, the intermediacy of the high-spin Mn[superscript IV]-oxo species is likely responsible for catalytic activity of the Mn[superscript II]-MOF-5 precatalyst, which in the presence of [superscript t]BuSO₂PhIO catalyzes oxygen atom transfer reactivity to selectively form epoxides from cyclic alkenes. In a second study, partial substitution of Zn[superscript II] by Mn[superscript II] in Zn₅(OAc)₄(bibenzotriazolate)₃ (CFA-1) yields a material in which manganese is supported by a ligand environment reminiscent of that found in molecular scorpionates. Unlike molecular analogs, Mn-CFA-1 is capable of activating molecular oxygen to convert substrates with sufficiently weak C-H bonds, such as cyclohexene, to alcohol and ketone products. In-situ spectroscopies including Mn K-edge X-ray absorption, DRIFTS, and Diffuse Reflectance UV-vis indicate that reactivity proceeds through a high valent Mn-peroxo species.

These results demonstrate that MOF secondary building units serve as competent platforms for accessing high-valent metal-oxygen species that consequently engage in catalytic oxygen atom transfer chemistry owing to the ligand fields and site isolation provided by the material.