Mechanistic Insights into C–H Borylation of Arenes with Organoiridium Catalysts Embedded in a Microporous Metal–Organic Framework
Zoha H. Syed, Zhihengyu Chen, Karam B. Idrees, Timothy A. Goetjen, Evan C. Wegener, Xuan Zhang, Karena W. Chapman, David M. Kaphan, Massimiliano Delferro, Omar K. Farha
Abstract
Organometallic iridium catalysts can be used in conjunction with bispinacolatodiboron (B2Pin2) to effect the borylation of a variety of substrates such as arenes, alkanes, heteroarenes, and oxygenates. Recently, efforts have also focused on integrating these catalysts into porous supports, such as metal–organic frameworks (MOFs). While the mechanism of homogeneous borylation systems has been thoroughly investigated experimentally and computationally, analogous studies in MOF-supported iridium catalysts have not been conducted. Herein, we report the mechanistic investigation of a phenanthroline-iridium catalyst immobilized in the organic linker of Universitetet i Oslo (UiO)-67 (Zr6O4(OH)4(BPDC)4(PhenDC)2, BPDC = biphenyl-4,4′-dicarboxylate, PhenDC = 1,10-phenanthroline-4,4′-dicarboxylate). By using benzene as a model substrate, variable time normalization analysis (VTNA) of the kinetic data suggested a rate law consistent with zero-order in B2Pin2, and first-order in arene. A primary kinetic isotope effect (KIE) in the time course of benzene-d6 borylation also provided complementary information about the role of the arene in the rate-determining step of the reaction. Characterization by techniques such as X-ray absorption spectroscopy (XAS) confirmed the presence of Ir(III), while pair distribution function (PDF) analysis suggested structures containing an Ir–Cl bond, further substantiated by X-ray photoelectron spectroscopy (XPS). Analysis of postcatalysis materials by inductively coupled plasma–optical emission spectroscopy (ICP-OES) revealed low boron accumulation, which may indicate an absence of boron in the resting state of the catalyst. Finally, in comparing borylation of benzene and toluene, a slight selectivity for benzene is observed, which is similar to the analogous homogeneous reaction, indicating the influence of substrate sterics on reactivity.