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.

本文报道了一类由新型多齿配体构筑的配合物，该配体将刚性强给电子的钳形（pincer）骨架与柔性弱给电子的氮杂冠醚（aza-crown ether）片段相结合。该钳形-冠醚配体可展现出三齿、四齿及五齿三种配位模式。其配位模式可通过螯合醚的路易斯碱置换进行调控，配体的结合平衡可通过锂离子、钠离子与大环的相互作用发生显著改变。由该钳形-冠醚配体以五齿配位方式配位的一氢化铱阳离子，可实现阳离子促进的氢气活化过程。反应速率可通过选择阳离子（含锂离子的反应速率约为含钠离子反应的10倍）或调控阳离子浓度进行控制。当加入催化量的锂离子时，H/D交换速率最高可获得250倍的提升。