Cation Tunable Reactivity and Catalysis with Iridium Pincer-Crown Ether Complexes

Iridium 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 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, and the concentration of cation. Using the same complex, rapid, selective, and highly controllable iridium-catalyzed allylbenzene isomerization is described, enabled by tunable hemilability based on alkali metal cation binding with the macrocyclic pincer-crown ether ligand. An inactive chloride-ligated complex can be activated by halide abstraction with sodium salts, with the resulting catalyst [κ5-(15c5NCOPiPr)Ir(H)]+ exhibiting modest activity. Addition of Li+ provides a further boost in activity, with up to 1000-fold rate enhancement. Ethers and chloride salts dampen or turn off reactivity, leading to three distinct catalyst states with activity spanning several orders of magnitude. Mechanistic studies suggest that the large rate enhancement and high degree of tunability stem from control over substrate binding. Methods to convert CO2 to formate using iridium pincer-crown ether complexes are discussed. The synthesis of iridium dihydride pincer-crown ether complexes is attempted. Electrochemistry of pincer-crown ether complexes reveal very negative iridium reduction potentials. Lastly, CO2 hydrogenation to formate is accomplished in bicarbonate solutions with turn over numbers up to 1000.Doctor of Philosoph