A mesoionic nitrogen-donor ligand: structure, iridium coordination, and catalytic effects

Development of Imidazo[1,2-a]pyridines Containing Sulfonyl Piperazines as Potential Antivirals against Tomato Spotted Wilt Virus

Mesoionic structures have become important advancements in recent agrochemical design. However, their potential beyond serving as excellent insecticides remains unexplored with limited reports available. Herein, a series of imidazo[1,2-a]pyridine mesoionics were developed by structurally incorporating sulfonyl piperazine moieties into imidazo[1,2-a]pyridines. Many of the obtained derivatives demonstrated bioactivity against tomato spotted wilt virus (TSWV) in bioassays. In particular, compound Z40, identified via three-dimensional quantitative structure activity relation models, displayed encouraging protective performance (half-maximal effect concentration = 252 μg/mL) compared to the positive controls ningnanmycin (332 μg/mL) and vanisulfane (523 μg/mL). Through defense enzyme assays, real-time quantitative polymerase chain reaction, and proteomics analysis, Z40 was identified as a plant immunomodulator that promotes defense enzyme activity and the mediates oxidative phosphorylation pathway, enabling plants to resist TSWV. We expect this study to help expand the possibilities of mesoionic compounds.

Pincer Platinum(II) Hydrides: High Stability Imparted by Donor-Flexible Pyridylidene Amide Ligands and Evidence for Adduct Formation before Protonation

Donor-flexible ligands are an emerging class of noninnocent ligands. Their ability to adapt their donating strength toward a metal center has had numerous catalytic advantages yet has never been utilized to stabilize and isolate intermediate complexes within these processes. We demonstrate through the use of a pincer ligand containing two donor-flexible pyridylidene amide (PYA) arms in coordination with platinum(II) that this ligand adaptability revealed remarkably stable hydride and formate complexes. These are typically fleeting catalytic intermediates within formic acid dehydrogenation and CO₂ hydrogenation catalytic cycles. The PYA platinum hydride complexes are indefinitely stable in air, while formate complexes show no sign of β-hydrogen elimination. This robustness allowed us to investigate hydride protonation as a seemingly simple reaction, though in-depth kinetic analysis reveals a pre-equilibrium step prior to platinum hydride protonation. This initial step has been attributed to adduct formation and is slower than the protonation, and therefore a relevant aspect when designing catalytic cycles for hydrogen release and its microscopic reverse, viz., hydrogen uptake.

Sterically and Electronically Flexible Pyridylidene Amine Dinitrogen Ligands at Palladium: Hemilabile cis/trans Coordination and Application in Dehydrogenation Catalysis

Ligand design is crucial for the development of new catalysts and materials with new properties. Herein, the synthesis and unique hemilabile coordination properties of new bis-pyridylidene amine (bis-PYE) ligands to palladium, and preliminary catalytic activity of these complexes in formic acid dehydrogenation are described. The synthetic pathway to form cationic complexes [Pd(bis-PYE)Cl(L)]X with a cis-coordinated N,N-bidentate bis-PYE ligand is flexible and provides access to a diversity of PdII complexes with different ancillary ligands (L=pyridine, DMAP, PPh3 , Cl, P(OMe)3 ). The 1 H NMR chemical shift of the trans-positioned PYE N-CH3 unit is identified as a convenient and diagnostic handle to probe the donor properties of these ancillary ligands and demonstrates the electronic flexibility of the PYE ligand sites. In the presence of a base, the originally cis-coordinated bis-PYE ligand adopts a N,N,N-tridentate coordination mode with the two PYE units in mutual trans position. This cis-trans isomerization is reverted in presence of an acid, demonstrating a unique structural and steric flexibility of the bis-PYE ligand at palladium in addition to its electronic adaptability. The palladium complexes are active in formic acid dehydrogenation to H2 and CO2 . The catalytic performance is directly dependent on the ligand bonding mode, the nature of the ancillary ligand, the counteranion, and additives. The most active system features a bidentate bis-PYE ligand, PPh3 as ancillary ligand and accomplishes turnover frequencies up to 525 h-1 in the first hour and turnover numbers of nearly 1000, which is the highest activity reported for palladium-based catalysts to date.

Pincer and Macrocyclic Pyridylidene Amide (PYA) AuIII Complexes

Gold-based homogeneous catalysis is dominated by redox neutral Au^I systems. Redox-active gold-based catalysts are less common, principally because of redox cycles between Au^I and Au^III being hampered by unfavorable potentials. We report gold(III) complexes containing pincer-based, donor-flexible pyridylidene amide (PYA) ligands to address these issues. These complexes act as electron reservoirs through two limiting resonance structures consisting of either soft, imine coordination sites or harder, zwitterionic amide donors. We further alter the donor properties by using the ortho-, meta-, and para-pyridylidene amide variants of the PYA pincer arms. These bis-PYA pincer ligands exhibited a high contribution of amide coordination in the solid-state of the gold(III) complexes; however, the solution data suggests a high contribution from the neutral L-type resonance forms. This L-type contribution, primarily shown through cyclic voltammetry studies, prevents reversible gold(III) reduction and also disfavors abstraction of the ancillary chloride ligand. Furthermore, a novel macrocyclic-PYA ligand is introduced, which shows secondary metal-ligand interactions.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

Modulation of N^N'-bidentate chelating pyridyl-pyridylidene amide ligands offers mechanistic insights into Pd-catalysed ethylene/methyl acrylate copolymerisation

The efficient copolymerisation of functionalised olefins with alkenes continues to offer considerable challenges to catalyst design. Based on recent work using palladium complexes containing a dissymmetric N^N'-bidentate pyridyl-PYA ligand (PYA = pyridylidene amide), which showed a high propensity to insert methyl acrylate, we have here modified this catalyst structure by inserting shielding groups either into the pyridyl fragment, or the PYA unit, or both to avoid fast β-hydrogen elimination. While a phenyl substituent at the pyridyl side impedes catalytic activity completely and leads to an off-cycle cyclometallation, the introduction of an ortho-methyl group on the PYA side of the N^N'-ligand was more prolific and doubled the catalytic productivity. Mechanistic investigations with this ligand system indicated the stabilisation of a 4-membered metallacycle intermediate at room temperature, which has previously been postulated and detected only at 173 K, but never observed at ambient temperature so far. This intermediate was characterised by solution NMR spectroscopy and rationalises, in part, the formation of α,β-unsaturated esters under catalytic conditions, thus providing useful principles for optimised catalyst design.

Modulating the water oxidation catalytic activity of iridium complexes by functionalizing the Cp*-ancillary ligand: hints on the nature of the active species

A comparative study on the behavior of a series of iridium dimeric WOCs with modified Cp* ligands reveals the key role played by the variable substituent.

Molecular and heterogenized dinuclear Ir-Cp* water oxidation catalysts bearing EDTA or EDTMP as bridging and anchoring ligands

The development of efficient water oxidation catalysts (WOCs) is of key importance in order to drive sustainable reductive processes aimed at producing renewable fuels. Herein, two novel dinuclear complexes, [(Cp*Ir)₂(μ-κ³-O,N,O-H₄-EDTMP)] (Ir-H₄-EDTMP, H₄-EDTMP^4- = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir)₂(μ-κ³-O,N,O-EDTA)] (Ir-EDTA, EDTA^4- = ethylenediaminetetraacetate), were synthesized and completely characterized in solution, by multinuclear and multidimensional NMR spectroscopy, and in the solid state, by single crystal X-Ray diffraction. They were supported onto rutile TiO₂ nanocrystals obtaining Ir-H₄-EDTMP@TiO₂ and Ir-EDTA@TiO₂ hybrid materials. Both molecular complexes and hybrid materials were found to be efficient catalysts for WO driven by NaIO₄, providing almost quantitative yields, and TON values only limited by the amount of NaIO₄ used. As for the molecular catalysts, Ir-H₄-EDTMP (TOF up to 184 min^-1) exhibited much higher activity than Ir-EDTA (TOF up to 19 min^-1), likely owing to the higher propensity of the former to generate a coordination vacancy through the dissociation of a Ir-OP bond (2.123 Å, significantly longer than Ir-OC, 2.0913 Å), which is a necessary step to activate these saturated complexes. Ir-H₄-EDTMP@TiO₂ (up to 33 min^-1) and Ir-EDTA@TiO₂ (up to 41 min^-1) hybrid materials showed similar activity that was only marginally reduced in the second and third catalytic runs carried out after having separated the supernatant, which did not show any sign of activity, instead. The observed TOF values for hybrid materials are higher than those reported for analogous systems deriving from heterogenized mononuclear complexes. This suggests that supporting dinuclear molecular precursors could be a successful strategy to obtain efficient heterogenized water oxidation catalysts.

Ambidentate bonding and electrochemical implications of pincer-type pyridylidene amide ligands in complexes of nickel, cobalt and zinc

Pincer-type tridentate pyridyl bis(pyridylidene amide) (pyPYA2) ligand systems were coordinated to the Earth-abundant first row transition metals nickel, cobalt and zinc. A one-pot synthesis in water/ethanol afforded octahedral homoleptic bis-PYA complexes, [M(pyPYA2)2](PF6)2, whereas five-coordinate mono-PYA dichloride complexes, [M(pyPYA2)Cl2], were obtained upon slow addition of the ligand to the metal chlorides in DMF. Electrochemical measurements further revealed a facile oxidation of the metal centers from Ni2+ to Ni4+ and Co2+ to Co3+, respectively, while the Zn2+ system was redox inactive. These experiments further allowed for quantification of the much stronger electron donor properties of neutral N,N,N-tridentate pyPYA2 pincer ligands as compared to terpy. Remarkably, ortho-PYA pincer ligands feature amide coordination to the metal center via oxygen or nitrogen. This ambidentate ligand binding constitutes another mode of donor flexibility of the PYA ligand system, complementing the resonance structure dynamics established previously. NMR spectroscopic and MS analysis reveal that the meta-PYA ligand undergoes selective deuteration when coordinated to cobalt. This reactivity suggests the potential of this ligand as a transient proton reservoir for HX bond activation and, moreover, indicates the relevance of several resonance structures and therefore supports the notion that meta-PYA ligands are mesoionic.

Donor Strength Determination of Pyridinylidene-amide Ligands using Their Palladium-NHC Complexes

Pyridinylidene-amides (PYAs) are a relatively new type of N-donor ligands that can exist in three isomeric forms and adopt various resonance structures. This makes them electronically flexible, and in order to evaluate their electronic profile using the Huynh electronic parameter (HEP), seven structurally diverse mixed N-heterocyclic carbenes (NHCs)/PYA palladium complexes of the type trans-[PdBr₂(ⁱPr₂-bimy)(PYA)] were prepared and fully characterized by various spectroscopic and spectrometric methods. This study shows that PYAs are among the strongest, formally neutral N-donors, but they are still weaker than phosphines and organometallic ligands such as NHCs. Notably, the donating abilities of isomeric PYAs are distinct and can be further fine-tuned by the choice of two substituents making them structurally and electronically versatile. These characteristics and the ease of their preparation hold promise for a wide applicability in coordination chemistry.

The Influence of the Ligand in the Iridium Mediated Electrocatalyic Water Oxidation

Electrochemical water oxidation is the bottleneck of electrolyzers as even the best catalysts, iridium and ruthenium oxides, have to operate at significant overpotentials. Previously, the position of a hydroxyl on a series of hydroxylpicolinate ligands was found to significantly influence the activity of molecular iridium catalysts in sacrificial oxidant driven water oxidation. In this study, these catalysts were tested under electrochemical conditions and benchmarked to several other known molecular iridium catalysts under the exact same conditions. This allowed us to compare these catalysts directly and observe whether structure–activity relationships would prevail under electrochemical conditions. Using both electrochemical quartz crystal microbalance experiments and X-ray photoelectron spectroscopy, we found that all studied iridium complexes form an iridium deposit on the electrode with binding energies ranging from 62.4 to 62.7 eV for the major Ir 4f_7/2 species. These do not match the binding energies found for the parent complexes, which have a broader binding energy range from 61.7 to 62.7 eV and show a clear relationship to the electronegativity induced by the ligands. Moreover, all catalysts performed the electrochemical water oxidation in the same order of magnitude as the maximum currents ranged from 0.2 to 0.6 mA cm^–2 once more without clear structure–activity relationships. In addition, by employing ¹H NMR spectroscopy we found evidence for Cp* breakdown products such as acetate. Electrodeposited iridium oxide from ligand free [Ir(OH)₆]^2– or a colloidal iridium oxide nanoparticles solution produces currents almost 2 orders of magnitude higher with a maximum current of 11 mA cm^–2. Also, this deposited material contains, apart from an Ir 4f_7/2 species at 62.4 eV, an Ir species at 63.6 eV, which is not observed for any deposit formed by the molecular complexes. Thus, the electrodeposited material of the complexes cannot be directly linked to bulk iridium oxide. Small IrO_x clusters containing few Ir atoms with partially incorporated ligand residues are the most likely option for the catalytically active electrodeposit. Our results emphasize that structure–activity relationships obtained with sacrificial oxidants do not necessarily translate to electrochemical conditions. Furthermore, other factors, such as electrodeposition and catalyst degradation, play a major role in the electrochemically driven water oxidation and should thus be considered when optimizing molecular catalysts.