Donor‐Flexible Nitrogen Ligands for Efficient Iridium‐Catalyzed Water Oxidation Catalysis

Navarro, Miquel; Li, Mo; Müller‐Bunz, Helge; Bernhard, Stefan; Albrecht, Martin

doi:10.1002/chem.201600875

Cited by 55 publications

(55 citation statements)

References 73 publications

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“…Bond lengths and angles around the iridium center are similar to those of the related iridium complexes 1-5, [18,19] and 8 (see above). The complex reveals the classical threelegged piano-stool geometry with apseudo-tetrahedral iridium center (Figure4).…”

Section: Modulation Of the Pyad Onor Propertiessupporting

confidence: 67%

“…[11,12] The use of stronger donor ligandsl ike Cp* (C 5 Me 5 À )f urthere nhanced the catalytic activity (B), [13] and even though strongs upport is available foragradual oxidationo ft he Cp* unit under catalytic conditions, [14] [Ir(Cp*)] has emerged as ap rivileged synthon for designing water oxidationc atalysts. [18] This complex shows excellent performance in water oxidation catalysis, providing maximum turnoverf requencies (TOF max )o f1 600h À1 and up to 86 000 turnovers.T his high activity might be attributed to the considerable donor flexibility of the PYAl igand, and the ensuing stabilizationo fd ifferent metal oxidations tates during the catalytic process (see limiting resonance structures Aa nd Bf or 1 in Figure 1). [15,16] Recently,w eh ave introduced pyridylidene amides (PYAs) as strong donor ligands [17] to the [Ir(Cp*)] iridium complex 1 (Figure 1).…”

Section: Introductionmentioning

confidence: 98%

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

Navarro

Smith

et al. 2018

Chemistry A European J

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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 =+0.72 V), para- (E =+0.64 V), or meta-isomer (E =+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 )=1600 h in the original Ph-PYA system gradually as more MeO groups were introduced up to a TOF =3300 h 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.

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Section: Modulation Of the Pyad Onor Propertiessupporting

confidence: 67%

Section: Introductionmentioning

confidence: 98%

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

Navarro

Smith

et al. 2018

Chemistry A European J

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“…Mechanistic investigations of homogeneous water oxidation catalysis with molecular sacrificial oxidant such as oxone (KHSO 5 ), NaIO 4 , or cerium ammonium nitrate {CAN, (NH 4 ) 2 [Ce(NO 3 ) 6 ]} unveiled a prominent role of the sacrificial oxidation, and catalyst performance varies considerably when modulating the terminal oxidant from e.g. NaIO 4 to CAN , . This variation has been attributed in parts to the fact that some oxidants such as oxone or NaIO 4 are also oxygen donors, and in other parts to direct interactions between the metal center and the oxidant,, which limits the usefulness of the oxidant as a proxy to short‐cut the water reduction cycle.…”

Section: Introductionmentioning

confidence: 99%

Relevance of Chemical vs. Electrochemical Oxidation of Tunable Carbene Iridium Complexes for Catalytic Water Oxidation

et al. 2020

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Based on previous work that identified iridium(III) Cp* complexes containing a C,N‐bidentate chelating triazolylidene‐pyridyl ligand (Cp* = pentamethylcyclopentadienyl, C5Me5–) as efficient molecular water oxidation catalysts, a series of new complexes based on this motif has been designed and synthesized in order to improve catalytic activity. Modifications include specifically the introduction of electron‐donating substituents into the pyridyl unit of the chelating ligand (H, a; 5‐OMe, b; 4‐OMe, c; 4‐tBu, d; 4‐NMe2, e), as well as electronically active substituents on the triazolylidene C4 position (H, 8; COOEt, 9; OEt, 10; OH, 11; COOH, 12). Chemical oxidation using cerium ammonium nitrate (CAN) indicates a clear structure‐activity relationship with electron‐donating groups enhancing catalytic turnover frequency, especially when the donor substituent is positioned on the triazolylidene ligand fragment (TOFmax = 2500 h–1 for complex 10 with a MeO group on pyr and a OEt‐substituted triazolylidene, compared to 700 h–1 for the parent benchmark complex without substituents). Electrochemical water oxidation does not follow the same trend, and reveals that complex 8b without a substituent on the triazolylidene fragment outperforms complex 10 by a factor of 5, while in CAN‐mediated chemical water oxidation, complex 10 is twice more active than 8b. This discrepancy in catalytic activity is remarkable and indicates that caution is needed when benchmarking iridium water oxidation catalysts with chemical oxidants, especially when considering that application in a potential device will most likely involve electrocatalytic water oxidation.

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“…Accordingly, the donor properties of this ligand can be modulated by external factors such as the coordinated metal and its spectator ligands and by the polarity of the solvent. [44][45][46] The flexibility imparts catalytic activity, and is further modulated upon alteration of the pyridine substitution pattern from para-PYAs (A, B) to meta-PYA scaffolds (C, Scheme 1). 47 Of note, such meta-PYA ligands cannot be represented by a neutral resonance structure and feature only zwitterionic structures.…”

Section: Introductionmentioning

confidence: 99%

Modular Pincer-type Pyridylidene Amide Ruthenium(II) Complexes for Efficient Transfer Hydrogenation Catalysis

2018

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A set of bench-stable ruthenium complexes with new N,N,N-tridentate coordinating pincer-type pyridyl-bis(pyridylideneamide) ligands was synthesized in excellent yields, with the pyridylidene amide in meta or in para position ( m-PYA and p-PYA, respectively). While complex [Ru( p-PYA)(MeCN)] is catalytically silent in transfer hydrogenation, its meta isomer [Ru( m-PYA)(MeCN)] shows considerable activity with turnover frequencies at 50% conversion TOF = 100 h. Spectroscopic, electrochemical, and crystallographic analyses suggest considerably stronger donor properties of the zwitterionic m-PYA ligand compared to the partially π-acidic p-PYA analogue, imparted by valence isomerization. Further catalyst optimization was achieved by exchanging the ancillary MeCN ligands with imines (4-picoline), amines (ethylenediamine), and phosphines (PPh, dppm, dppe). The most active catalyst was comprised of the m-PYA pincer ligand and PPh, complex [Ru( m-PYA)(PPh)(MeCN)], which reached a TOF of 430 h under aerobic conditions and up to 4000 h in the absence of oxygen. The presence of oxygen reversibly deactivates the catalytically active species, which compromises activity, but not longevity of the catalyst. Ligand exchange kinetic studies by NMR spectroscopy indicate that the strong trans effect of the phosphine is critical for high catalyst activity. Diaryl, aryl-alkyl, and dialkyl ketones were hydrogenated with high conversion, and α,β-unsaturated ketones produced selectively the saturated ketone as the only product due to exclusive C═C bond hydrogenation, a distinctly different selectivity from most other transfer hydrogenation catalysts.

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Donor‐Flexible Nitrogen Ligands for Efficient Iridium‐Catalyzed Water Oxidation Catalysis

Cited by 55 publications

References 73 publications

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

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

Relevance of Chemical vs. Electrochemical Oxidation of Tunable Carbene Iridium Complexes for Catalytic Water Oxidation

Modular Pincer-type Pyridylidene Amide Ruthenium(II) Complexes for Efficient Transfer Hydrogenation Catalysis

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