Methane functionalization by an Ir(III) catalyst supported on a metal–organic framework: an alternative explanation of steric confinement effects

Abstract

A highly selective Ir catalyst supported on the metal–organic framework (MOF) UiO-67 for the catalytic borylation of methane has recently been synthesized. The high chemoselectivity of the catalyst toward monoborylated methane (CH3Bpin, Bpin = pinacolborane) instead of diborylated methane (CH2Bpin2) was speculated to be caused by the steric confinement of MOF UiO-67. In this study, we applied quantum mechanical methods to determine: (1) the steric effect of the UiO-67 framework in promoting the chemoselectivity of the Ir catalyst toward CH3Bpin and (2) the borylation mechanisms over the Ir catalyst supported on UiO-67. Our results show that UiO-67 framework sterically obstructs the diffusion of the larger CH2Bpin2 molecule within the MOF while allowing the smaller CH3Bpin molecule to pass through with little energy penalty. The diffusion of CH2Bpin2 from the tetrahedral pore to the tetragonal pyramidal pore within modified UiO-67 with coordinated Ir(Bpin)3 complex has an estimated barrier of 24.7 kcal/mol and is 14.2 kcal/mol higher than the diffusion of CH3Bpin. The electronic and steric effects of the support at the Ir catalytic center are much smaller than this confinement effect on diffusion, and the catalytic center behaves similarly to the homogeneous Ir catalyst. We determined an overall free energy of activation of 34.6 kcal/mol for the CH4 borylation reaction using the Ir(III) catalyst. We also determined that the turnover-determining step for the catalytic methane borylation is the isomerization of seven-coordinated Ir(V) complex instead of the commonly assumed C–H bond activation by oxidative addition.