Methane Activation with Rhenium Catalysts. 1. Bidentate Oxygenated Ligands

Abstract

Trends in methane activation have been explored for rhenium-based catalysts in conjunction with bidentate oxygenated ligands of the form (L1)(L2)Re(OH)(OH2) [L1, L2 = acac, catechol, glycol]. When placed in acidic media, the equilibrium for this reference catalyst shifts to the protonated forms (L1)(L2)Re(OH2)(OH2) in almost all cases. In all cases the activation of the reference complex proceeds through a concerted metathesis type transition state, and only one of the 13 reference complexes proceeds with methane activation through a barrier of less than 35 kcal mol-1. Study of the identity complexes (L1 = L2) revealed that protonation of the ligand oxygens is unfavorable for acac and catechol, but favorable for glycol; however in only one case is the barrier for methane activation improved by this route. Electron density on the central rhenium is the best predictor for the magnitude of the methane activation barrier; namely, increased electron density (obtained by considering lower oxidation states) on the metal leads to lower barriers. Lower oxidation states form weaker Re−O bonds, which increase lability of the leaving groups and decrease the barrier to proton transfer from methane.