Scope and Mechanism of the C−H Bond Activation Reactivity within a Supramolecular Host by an Iridium Guest:  A Stepwise Ion Pair Guest Dissociation Mechanism

A chiral self-assembled supramolecular M4L6 assembly has been shown to be a suitable host for a series of reactive monocationic half-sandwich iridium guests 1, 3, and 4 that are capable of activating C−H bonds. Upon encapsulation, selective C−H bond activation of organic substrates occurs. Precise size and shape selectivity are observed in the C−H bond activation of aldehydes and ether substrates. The reactions exhibit significant kinetic diastereoselectivities. Thermodynamic studies have shown that the iridium starting materials and products are bound strongly by the host assembly. The encapsulation process is largely entropy-driven. Kinetic investigations with water-soluble phosphine traps and added salts have provided evidence for a unique stepwise mechanism of guest dissociation for [4 ⊂ Ga4L6]. Iridium guest 4 first dissociates from the host cavity to form an ion pair with the host exterior. This species then fully dissociates from the host exterior into the bulk solution. Model ion pair intermediates were characterized directly with 1H NMR NOESY techniques. The rate of iridium guest dissociation is slower than the rate observed for the C−H bond activation processes, indicating that the selective C−H bond activation reactivity occurs within the cavity of the supramolecular host.