Modulation of Phenolate-Functionalized Donor-Flexible PYE Ligands for Iridium-Catalyzed Formic Acid Dehydrogenation

Formic acid (FA) is a promising hydrogen carrier. Recently, we developed a formally underligated iridium(III) complex [Ir(Cp*)(O,N)]⁺ (Cp* = pentamethylcyclopentadienyl, C₅Me₅^–; O,N = pyridylidene amine (PYE)-phenolate) 1a as a high performing catalyst for FA dehydrogenation. Here, we exploited the facile synthetic accessibility of the PYE-phenolate ligand to investigate the influence of the ligand on FA dehydrogenation. Comparison of the activity of 1a with related iridium complexes containing ligands derived either from hydroxy-quinoline or N-aryl-aminophenol (complexes 2 and 3) indicate that neither a static imine nor a static anionic amide bonding of the nitrogen site induces catalytic activity, in contrast to the donor-flexible PYE nitrogen in 1a. Introduction of substituents on the phenolate donor (−OMe, −Me, −H, −Cl, and −NO₂, complexes 4–7), and on the PYE heterocycle (−Me, −H, and −Cl, complexes 8–10), as well as comparison of the ortho-PYE with its meta- and para-isomers (complexes 11–13) indicate a narrow window of electronic and steric tolerance before catalytic activity decreases. Evaluation of electron density at the iridium center via cyclic voltammetry (CV) as well as chemical shift and half-life time of the iridium-hydride intermediate derived from complexes 4–7 reveals a direct correlation between iridium electron density, hydride stability, and Hammett σ_p parameters, though not with the catalytic activity of these complexes. This work indicates therefore that catalytic activity is not governed by maximizing hydricity or iridium electron density but by optimizing these parameters.