A combination computational and experimental studies (high-pressure and high-throughput techniques) have allowed us to elucidate the mechanism and suggest a rational design and optimization strategies of a bis(phosphine)-supported iridium-catalyst for methane monoborylation. The activation of the C–H bond in methane via oxidative addition using tris(boryl) iridium(III) complexes bearing bis-chelating supporting ligands has been modeled computationally. This model shows that the use of the soft Lewis base ligand such as 1,2-bis(dimethylphosphino)ethane (dmpe) lowers the activation barrier of the rate-determining step as it facilitates polarization of the metal-center, lowering the barrier of the oxidative addition to afford a seven-coordinate iridium(V) intermediate. The experimental optimization of this reaction using high-throughput methods shows that we can obtain up to 170 turnovers at 150 °C (500 psi) within 16 h using bis(pinacolato)diboron, a well-defined homogeneous and monomeric catalyst (dmpe)Ir(COD)Cl that is readily available from commercial precursors. In this work we also establish decent selectivity for the monoborylation product H<sub>3</sub>CBpinacolato. We have found that high-boiling cyclic aliphatic solvents such as decalin, cyclooctane, methylcyclohexane are also suitable for this reaction, while being inert toward borylation. In accordance with the lower calculated activation barrier, catalytic turnover is also observed at 120 °C but over a longer period of time. It was found that the borylation of methane is only achieved via one catalytic cycle, and buildup of pinacolborane, a side-product from methane borylation with bis(pinacolato)diboron, inhibits catalytic activity. New pre-catalysts, and high-pressure NMR spectroscopic studies have been further conducted to help understand the mechanism of this reaction.

The use of high-pressure and high-throughput experimentation, and theory to optimize the catalytic borylation of methane with well-defined iridium complexes

ACS Spring Meeting 2021

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