Metal-Organic Framework Stabilizes a Low-Coordinate Iridium Complex for Catalytic Methane Borylation

摘要

Catalytic borylation has recently been suggested as a potential strategy to convert abundant methane to fine chemicals. However, synthetic utility of methane borylation necessitates significant improvement of catalytic activities over original phenanthroline- and diphosphine-Ir complexes. Herein, we report the use of metal-organic frameworks (MOFs) to stabilize low-coordinate Ir complexes for highly active methane borylation to afford the monoborylated product. The mono(phosphine)-Ir based MOF, Zr-P1-Ir, significantly outperformed other Ir catalysts in methane borylation to afford CH(3)Bpin with a turnover number of 127 at 110 degrees C. Density functional theory calculations indicated a significant reduction of activation barrier for the rate limiting oxidative addition of methane to the four-coordinate (P1)-Ir-III(Bpin)(3) catalyst to form the six-coordinate (P1)-Ir-V(Bpin)(3)(CH3)(H) intermediate, thus avoiding the formation of sterically encumbered seven-coordinate Ir-V intermediates as found in other Ir catalysts based on chelating phenanthroline, bipyridine, and diphosphine ligands. MOF thus stabilizes the homogeneously inaccessible, low-coordinate (P1)Ir(boryl)(3 )catalyst to provide a unique strategy to significantly lower the activation barrier for methane borylation. This MOF-based catalyst design holds promise in addressing challenging catalytic reactions involving highly inert substrates.