Pincer-ligated iridium complexes are the most active, robust, and selective catalysts for dehydrogenating alkanes to alkenes. These catalysts typically use sacrificial olefinic hydrogen acceptors and operate through an iridium(I/III) cycle with key involvement of a 14-electron, 3-coordinate iridium(I) fragment which effects C-H activation. Alkane dehydrogenation using iridium(III) might enable the use of oxygen or recyclable oxidants as hydrogen acceptors. The 5-coordinate, 16-electron, PCP pincer complex [3,5-bis[(di-t-butylphosphino)methyl]-4-pyridinyl] iridium chlorohydride, with a lutidine-based pincer scaffold, is found to undergo intramolecular addition of an sp3 C-H bond of a tert-butyl group upon exposure to oxygen, ferrocenium salts, or benzoquinone. The phenyl-based analog [3,5-bis[(di-t-butylphosphino)methyl]-phenyl]iridium chlorohydride, by contrast, does not react with chemical oxidants. Combined experimental and DFT investigations reveal that initial proton-coupled electron transfer occurs from an intermediate 4-coordinate, 16-electron iridium zwitterion. Formation of the zwitterion is enabled by the strong basicity of the metallated 4-pyridinyl nitrogen. This previously unknown mode of C-H activation through metal-ligand cooperativity may hold promise for alkane dehydrogenation via electron transfer pathways.

4-pyridinyl pincer iridium complex performs C-H addition with transfer of protons and electrons to chemical oxidant and base

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.