PCP-pincer and related iridium complexes are the most effective molecular catalysts known for the dehydrogenation of alkanes.1 Key to this and related catalyses is facile C-H addition to the three-coordinate (PCP)Ir fragment. This 14e Ir(I) fragment in the presence of CO forms a very stable square planar carbonyl complex. In contrast to the three-coordinate Ir(I), C–H additions to square planar Ir(I) complexes, and carbonyl complexes in particular, are essentially unprecedented. However, our group has reported that Bronsted acids will catalyze the addition of the alkynyl C–H bond of phenylacetylene to (PCP)Ir(CO).2 We find that the microscopic reverse, C-H reductive elimination from (PCP)Ir(CO)(R)(H), is catalyzed by Bronsted acids for R = sp3 and sp2 hydrocarbyl fragments. Although the addition is therefore endoergic for such hydrocarbons, we envisioned that the use of acid-catalyzed C-H bond addition under CO atmosphere might be employed to transform alkanes to or alcohols or other oxygenates. (PCP)Ir(CO)(Et)(H) (a) was found to react under CO and H2 atmosphere to give formation of n-propanol. This suggested that addition of ethane to (PCP)Ir(CO) under such conditions could be employed in a catalytic hydrocarbonylation cycle. Although we have not achieved such catalytic alkane carbonylations, studies of the related chemistry have yielded insights which we believe give direction toward the successful catalyst design. The chemistry reported herein thus offers a new archetype for alkane carbonylation via acid catalysis serving as an active pre-catalyst to promote C-H bond additions/eliminations to “inert” (PCP)Ir(CO) complexes followed by CO insertion and C-H bond functionalization.

"Progress towards alkane carbonylation by PCP-pincer iridium carbonyl complexes"

ACS Spring Meeting 2021

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