Identifying and Evading Olefin Isomerization Catalyst Deactivation Pathways Resulting from Ion-Tunable Hemilability

Hemilabile ligands are found in many
leading organometallic catalysts,
but it can be challenging to tune the degree of hemilability in a
particular catalyst. This work explores the impact of cation-tunable
hemilability on the speciation of iridium­(III) pincer-crown ether catalysts during high-activity
olefin isomerization. Under conditions where strong cation–macrocycle
interactions are fostered and terminal olefin has been consumed, labilization
of the aza-crown ether group leads to an η6-arene
complex, wherein the pincer ligand is metallated at a different position.
Arene complexes of styrene, naphthalene, and mesitylene were independently
synthesized and found to exhibit diminished catalytic activity for
allylbenzene isomerization. In response to these findings, a previously
unreported catalyst bearing a synthetically modified pincer ligand
was designed, resulting in a refined system that maintains high activity
even when arene complexes are formed.