Alkane Dehydrogenation Catalyzed by a Fluorinated Phebox Iridium Complex

We report an iridium acetate complex with a fluorinated Phebox ligand (2,6-bis­(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3,5-bis­(trifluoromethyl)­phenyl) that is a highly effective catalyst for acceptorless dehydrogenation of alkanes. Under typical acceptorless dehydrogenation conditions, a high turnover frequency is obtained, which is limited by the rate of expulsion of H₂ from the reaction solution. Rates and turnover numbers for acceptorless dehydrogenation are significantly greater than found for the nonfluorinated analogue. As in the case of the nonfluorinated analogue, Na⁺ acts as a cocatalyst with the fluorinated catalyst again yielding greater rates and total turnovers. Computational studies shed light on the possible mechanistic pathways. The initial alkane activation is a net Ir–H/C–H bond metathesis leading to the formation of an Ir–alkyl bond and loss of H₂; this is the slowest chemical step in the cycle. The lowest-energy pathway is calculated to proceed via concerted metalated deprotonation (CMD) of the alkane. Pathways proceeding via transition states with oxidative addition (Ir­(V)) character, however, are calculated to be only slightly higher in energy. These transition states can lead either to Ir­(V) intermediates, which then lose H₂, or connect directly to a dihydrogen complex. The role of Na⁺ is largely to promote dechelation by coordinating to an acetate oxygen, opening a vacant coordination site that allows reaction with the alkane. This coordination by Na⁺ prevents the CMD mechanism from operating, but it significantly lowers the energy of the Ir­(V) TSs. NBO analysis shows a net transfer of charge from the alkane atoms to the metal complex in the Ir­(V) TSs, with and without coordinated Na⁺. Thus, the oxidative addition is actually reductive in nature, driven in part by electrophilicity of the metal center. The Na⁺ cation further increases electrophilicity in addition to promoting dechelation. The greater activity of the fluorinated catalyst compared with the parent complex can also be explained in terms of the electrophilic nature of the reaction. The fluorinated catalyst is also more resistant to decomposition than the nonfluorinated analogue.