An Investigation of the Multiple Binding Modes and Secondary Interactions of N-Arylated PNArP Ligands on Iridium

The activation of inert bonds is often of significance in synthetic chemistry and allows for the facile formation of chemically valuable synthons. Reaction conditions are often quite harsh, and transition metal catalysts need additional stability to avoid decomposition. Sterically bulky polydentate ligands have been employed to aid in the stabilization of reactive catalytic centers. However, the size of these species can sometimes interfere with catalytic efficiencies by overcrowding reactive sites at the metal center. One approach that synthetic organometallic chemists are employing to resolve this issue is designing ligand scaffolds that play a role in the catalytic cycle. Some ligand frameworks merely provide weak secondary interactions that are reversible in order to accommodate incoming substrates. Other ligands have been designed to actively participate in the activation of small molecules by undergoing structural changes.

We have taken a well studied PNP pincer ligand and created a series of frameworks in which an ancillary aryl substituent has been added to the central nitrogen atom. By doing so, we hoped to take advantage of the reactivity observed previously with PNP chelates in addition to secondary interactions from the pendant aryl substituent. Ligands of the formula H(PNArP) (Ar = naphthyl, tolyl, anisyl, and phenyl) were synthesized and fully characterized. Upon metalation, we observed selectivity in the intramolecular activation of C(sp3)-H, C(sp2)-H, and Calkyl-O bonds of the ancillary aryl substituents. Via multiple routes, a series of iridium(III)-dihydride complexes was synthesized: (PNnaphP)Ir(H)2, (o-methyl-PNtolP)Ir(H)2, (o-aryl-PNtolP)Ir(H)2, (PNPhenoxP)Ir(H)2, [(PNPhenolP)Ir(H)2][BArF], (o-aryl-PNtolP)Ir(H)2CO and (PNanisP)Ir(H)2CO. Thermal stability and reactivity studies demonstrated that some of the backbones underwent decomposition to new anionic pincer ligands, some of which demonstrated metal-ligand cooperativity in the activation of H2 and iso-propanol. Complexes containing an oxygen in the ancillary substituent participated in interesting transformations, such as the ROMP of nobornene.