Substituent Effects on the Activity of Cp*Ir(pyridine-carboxylate) Water Oxidation Catalysts: Which Ligand Fragments Remain Coordinated to the Active Ir Centers?

Abstract

Identifying structure–property correlations for molecular water oxidation catalysts (WOCs) is strongly hampered by the complexity of the water oxidation (WO) reaction mechanism and catalyst in situ modifications. Here, the substitution effects for [Cp*Ir­(X-pic)­NO3] WOCs (pic = κ2-pyridine-2-carboxylate complexes) have been systematically explored by synthesizing several complexes differing for the nature of the X substituent on the pyridine-carboxylate ligand and testing them in WO driven by NaIO4. The activity data were correlated with the σ-Hammett parameter of the various substituents, which was proven to effectively reproduce the trends in electron donor properties of the ligands. The measured TOFmax values were found to roughly increase with the σ-Hammett parameter (i.e., with decreasing electron density at the metal center), reaching plateaux at activity values comparable to that of the simpler [Cp*Ir­(H2O)3]­(NO3)2 catalyst. This trend has been typically observed for WOCs following a water nucleophilic attack mechanism. However, nuclear magnetic resonance studies on the reaction between [Cp*Ir­(X-pic)­NO3] and NaIO4 suggest that the activity is actually determined by the ease of pyridine-carboxylate ligand loss, likely being a necessary step toward precatalyst activation. This indicates that the same active species is generated from all [Cp*Ir­(X-pic)­NO3] starting complexes, albeit at different rates and/or in different amounts. A somewhat clear picture therefore appears to emerge, partially in contradiction with some of the hypotheses previously proposed: the active species should contain some degradation fragments of the Cp* (based on previous literature studies) and no X-pic derived ligands (based on the results reported in this work).

Ligands modification strategies for mononuclear water splitting catalysts

“… Plot of σ-Hammett parameter versus (A) 1 H NMR chemical shift of ortho-proton on the pyridine ligands, data from Sato et al (2015) , (B) 13 C NMR chemical shift of quaternary carbon atom on the Cp, data from Rodriguez et al (2021) , (C) Redox couples Ru III /Ru II , adapt with permission from Angew. Chem.…”

“…Proceeding by a I2M pathway, an electron-withdrawing group on 1 causes destabilization of the Ru V = O species, favoring the O-O bond formation. For catalysts following WNA pathway, Rodriguez et al (2021) reported that increasing electron-withdrawing ability can facilitate the nucleophilic attack at the Ir V intermediate (a rate-determining step), therefore enhance the catalytic activity of [Cp*Ir (Xpic)NO 3 ], as indicated by the correlations between σ and the measured TOF max .…”

Highly Active Visible Light-Promoted Ir/g-C3N4 Photocatalysts for the Water Oxidation Reaction Prepared from a Halogen-Free Iridium Precursor

“…Nowadays, iridium­(III) complexes are highlighted as superior homogeneous catalysts for the OER. − Various organic ligands were utilized ,,, to modulate activity and stabilize the iridium active centers in such compounds, but high oxidation potential of oxygen species generated along the OER causes the degradation of the ligands. ,, In this context, the reorientation to the heterogeneous Ir-based catalyst is the natural initiative. − In a view of the rarity of iridium and its high cost, the maximization of metal utilization in heterogeneous catalysts is highly essential; however, typical turnover frequency (TOF) values of iridium heterogeneous OER catalysts are rather low compared to homogeneous analogues. A correct thorough adjustment of the active species size and morphology plays a pivotal role in gaining the highest activity with adequate stability.…”

“…Nowadays, iridium(III) complexes are highlighted as superior homogeneous catalysts for the OER. 8−19 Various organic ligands were utilized 9,14,15,20 to modulate activity and stabilize the iridium active centers in such compounds, but high oxidation potential of oxygen species generated along the OER causes the degradation of the ligands. 11,16,21 In this context, the reorientation to the heterogeneous Ir-based catalyst is the natural initiative.…”

On the importance of π-stacking interactions in the complexes of copper and zinc bearing pyridine-2,6-dicarboxylic acid N-oxide and N-donor auxiliary ligands

“…7 However, latter ligands including 1,10-phenanthrolin and 2,2′-bipyridine can form only chelate rings with metal ions thus preventing the formation of high dimensional frameworks so they are also known as terminators. 8 Among O-donor ligands, pyridine-based carboxylate ligands are responsible for creating thousands of compounds known not only for their structural diversity but also for their wide variety of applications in many fields from catalysts, 9,10 luminescence, 11,12 gas adsorption 13,14 to their antiproliferative activities. 15,16 For several years, pyridine-2,6dicarboxylic acid (H 2 pydco) has received considerable attention due to the symmetric position of its carboxylate groups on each side of the pyridine ring which can form 5-membered chelate rings or bridge with neighboring metal centres.…”