DFT Virtual Screening Identifies Rhodium–Amidinate Complexes As Potential Homogeneous Catalysts for Methane-to-Methanol Oxidation

Abstract

In the search for new organometallic catalysts for low-temperature selective conversion of CH 4 to CH 3 OH, we apply quantum mechanical virtual screening to select the optimum combination of ligand and solvent on rhodium to achieve low barriers for CH 4 activation and functionalization to recommend for experimental validation. Here, we considered Rh because its lower electronegativity compared with Pt and Pd may allow it to avoid poisoning by coordinating media. We report quantum mechanical predictions (including implicit and explicit solvation) of the mechanisms for Rh III (NN) and Rh III (NN F ) complexes [where (NN) = bis( N -phenyl)benzylamidinate and (NN F ) = bis( N -pentafluorophenyl)pentafluorobenzylamidinate] to catalytically activate and functionalize methane using trifluoroacetic acid (TFAH) or water as a solvent. In particular, we designed the (NN F ) ligand as a more electrophilic analogue to the (NN) ligand, and our results predict the lowest transition state barrier (Δ G ‡ = 27.6 kcal/mol) for methane activation in TFAH from a pool of four different classes of ligands. To close the catalytic cycle, the functionalization of methylrhodium intermediates was also investigated, involving carbon–oxygen bond formation via S N 2 attack by solvent, or S R 2 attack by a vanadium oxo. Activation barriers for the functionalization of methylrhodium intermediates via nucleophilic attack are lower when the solvent is water, but CH 4 activation barriers are higher. In addition, we have found a correlation between CH 4 activation barriers and rhodium–methyl bond energies that allow us to predict the activation transition state energies for future ligands, as well.