Cation-Modulated Reactivity of Iridium Hydride Pincer-Crown Ether Complexes

Complexes
of a new multidentate ligand combining a rigid, strongly
donating pincer scaffold with a flexible, weakly donating aza-crown
ether moiety are reported. The pincer-crown ether ligand exhibits
tridentate, tetradentate, and pentadentate coordination modes. The
coordination mode can be changed by Lewis base displacement of the
chelating ethers, with binding equilibria dramatically altered through
lithium and sodium cation–macrocycle interactions. Cation-promoted
hydrogen activation was accomplished by an iridium monohydride cation
ligated in a pentadentate fashion by the pincer-crown ether ligand.
The rate can be controlled on the basis of the choice of cation (with
lithium-containing reactions proceeding about 10 times faster than
sodium-containing reactions) or on the basis of the concentration
of the cation. Up to 250-fold rate enhancements in H/D exchange rates
are observed when catalytic amounts of Li+ are added.