Optimization of Synthetically Versatile Pyridylidene Amide Ligands for Efficient Iridium-Catalyzed Water Oxidation

The synthetic versatility of pyridylidene amide (PYA) ligands has been exploited to prepare and evaluate a diverging series of iridium complexes containing C,N-bidentate chelating aryl-PYA ligands for water oxidation catalysis. The phenyl-PYA lead structure 1 was modified (i) electronically through introduction of one, two, or three electron-donating methoxy substituents on the aryl ring, (ii) by incorporating long aliphatic chains to the pyridyl fragment of the PYA unit, and (iii) by altering the PYA positions from para-PYA to its ortho- and meta-isomers. Electrochemistry indicated no substantial electronic effect of the aliphatic chains, and only minor changes of the electron density at iridium when modifying the aryl ligand site, yet substantial alteration if the PYA ligand is the ortho- (E_1/2 =+0.72 V), para- (E_1/2 =+0.64 V), or meta-isomer (E_1/2 =+0.56 V vs. saturated calomel electrode; SCE). In water oxidation catalysis, the long alkyl chains did not induce any rate enhancement compared with the phenyl-PYA lead compound, whereas MeO groups incorporated in the aryl group enhanced the catalytic activity from a turnover frequency (TOF_max )=1600 h^-1 in the original Ph-PYA system gradually as more MeO groups were introduced up to a TOF_max =3300 h^-1 for a tris(MeO)-substituted aryl-PYA system. The variation of the PYA substitution had only a minor impact on catalytic activity and revealed only a weak trend in the sequence ortho>meta>para. The high activity of the tris(MeO) system and the ortho-PYA isomer were attributed to efficient hydrogen bonding, which assists O-H bond activation and proton transfer. Remarkably, merging of the two optimized motifs, that is, an aryl unit with three MeO substituents and the PYA as the ortho isomer, into a single new aryl-PYA ligand system failed to improve the catalytic activity. Computational analysis suggests too much congestion at the active site, which hinders catalytic turnover. These results illustrate the complexity of ligand design and the subtle effects at play in water oxidation catalysis.