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 H 2 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 H 2 ; 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 H 2 , 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 ...