Bioinspired Diiron Complex with Proton Shuttling and Redox-Active Ligand for Electrocatalytic Hydrogen Evolution

A μ-oxo diiron complex, featuring the pyridine-2,6-dicarboxamide-based thiazoline-derived redox-active ligand, H2L (H2L = N2,N6-bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide), was synthesized and thoroughly characterized. [FeIII-(μ-O)-FeIII] showed electrocatalytic hydrogen evolution reaction activity in the presence of different organic acids of varying pKa values in dimethylformamide. Through electrochemical analysis, we found that [FeIII-(μ-O)-FeIII] is a precatalyst that undergoes concerted two-electron reduction to generate an active catalyst. Fourier transform infrared spectrum of reduced species and density functional theory (DFT) investigation indicate that the active catalyst contains a bridged hydroxo unit which serves as a local proton source for the Fe(III) hydride intermediate to release H2. We propose that in this active catalyst, the thiazolinium moiety acts as a proton-transferring group. Additionally, under sufficiently strong acidic conditions, bridged oxygen gets protonated before two-electron reduction. In the presence of exogenous acids of varying strengths, it displays electro-assisted catalytic response at a distinct applied potential. Stepwise electron-transfer and protonation reactions on the metal center and the ligand were studied through DFT to understand the thermodynamically favorable pathways. An ECEC or EECC mechanism is proposed depending on the acid strength and applied potential.

Mechanistic Investigations into the Selective Reduction of Oxygen by a Multicopper Oxidase T3 Site-Inspired Dicopper Complex

Understanding how multicopper oxidases (MCOs) reduce oxygen in the trinuclear copper cluster (TNC) is of great importance for development of catalysts for the oxygen reduction reaction (ORR). Herein, we report a mechanistic investigation into the ORR activity of the dinuclear copper complex [Cu₂L(μ-OH)]³⁺ (L = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine). This complex is inspired by the dinuclear T3 site found in the MCO active site and confines the Cu centers in a rigid scaffold. We show that the electrochemical reduction of [Cu₂L(μ-OH)]³⁺ follows a proton-coupled electron transfer pathway and requires a larger overpotential due to the presence of the Cu-OH-Cu motif. In addition, we provide evidence that metal-metal cooperativity takes place during catalysis that is facilitated by the constraints of the rigid ligand framework, by identification of key intermediates along the catalytic cycle of [Cu₂L(μ-OH)]³⁺ . Electrochemical studies show that the mechanisms of the ORR and hydrogen peroxide reduction reaction found for [Cu₂L(μ-OH)]³⁺ differ from the ones found for analogous mononuclear copper catalysts. In addition, the metal-metal cooperativity results in an improved selectivity for the four-electron ORR of more than 70% because reaction intermediates can be stabilized better between both copper centers. Overall, the mechanism of the [Cu₂L(μ-OH)]³⁺ -catalyzed ORR in this work contributes to the understanding of how the cooperative function of multiple metals in close proximity can affect ORR activity and selectivity.

Selective Detection of Picric Acid and Pyrosulfate Ion by Nickel Complexes Offering a Hydrogen-Bonding-Based Cavity

This work describes the synthesis and characterization of three mononuclear nickel complexes supported with amide-based pincer ligands. All three complexes presented an H-bonding-based cavity due to the migration of amidic protons to the appended heterocyclic rings that formed H-bonds with the metal-ligated solvent molecule(s). These complexes functioned as the nanomolar chemosensors for the detection of picric acid and pyrosulfate ion as inferred by the detailed absorption and emission spectral studies while further supported with FTIR, NMR, and mass spectra of the isolated products. We also illustrate a few practical detection methods for the sensing of picric acid in the solution state as the naked-eye colorimetric methods and in the solid state by employing polystyrene films.

Half-Sandwich Ruthenium Complexes of Amide-Phosphine Based Ligands: H-Bonding Cavity Assisted Binding and Reduction of Nitro-substrates

We present synthesis and characterization of two half-sandwich Ru(II) complexes supported with amide-phosphine based ligands. These complexes presented a pyridine-2,6-dicarboxamide based pincer cavity, decorated with hydrogen bonds, that participated in the binding of nitro-substrates closer to the Ru(II) centers, which is further supported with binding and docking studies. These ruthenium complexes functioned as the noteworthy catalysts for the borohydride mediated reduction of assorted nitro-substrates. Mechanistic studies not only confirmed the intermediacy of [Ru-H] in the reduction but also asserted the involvement of several organic intermediates during the course of the catalysis. A similar Ru(II) complex that lacked pyridine-2,6-dicarboxamide based pincer cavity substantiated its unique role both in the substrate binding and the subsequent catalysis.

Cobalt mediated N-alkylation of amines by alcohols: role of hydrogen bonding pocket

Cobalt complexes of amide-based pincers provide a H-bonding pocket that binds a reagent in the vicinity of the metal center. These complexes function as catalysts for the N-alkylation of amines using alcohols via a borrowing hydrogen strategy.

Synthesis and Characterization of Cu(II) and Mixed-Valence Cu(I)Cu(II) Clusters Supported by Pyridylamide Ligands

A group of copper complexes supported by polydentate pyridylamide ligands H₂bpda and H₂ppda were synthesized and characterized. The two Cu(II) dimers [Cu^II₂(Hbpda)₂(ClO₄)₂] (1) and [Cu^II₂(ppda)₂(DMF)₂] (2) were constructed by using neutral ligands to react with Cu(II) salts. Although the dimers showed similar structural features, the second-sphere interactions affect the structures differently. With the application of Et₃N, the tetranuclear cluster (HNEt₃)[Cu^II₄(bpda)₂(μ₃-OH)₂(ClO₄)(DMF)₃](ClO₄)₂ (3) and hexanuclear cluster (HNEt₃)₂[Cu^II₆(ppda)₆(H₂O)₂(CH₃OH)₂](ClO₄)₂ (4) were prepared under similar reaction conditions. The symmetrical and unsymmetrical arrangement of the ligand donors in ligands H₂bpda and H₂ppda led to the dramatic conformation difference of the two Cu(II) complexes. As part of our effort to explore mixed-valence copper chemistry, the triple-decker pentanuclear cluster [Cu^II₃Cu^I₂(bpda)₃(μ₃-O)] (5) was prepared. XPS examination demonstrated the localized mixed-valence properties of complex 5. Magnetic studies of the clusters with EPR evidence showed either weak ferromagnetic or antiferromagnetic interactions among copper centers. Due to the trigonal-planar conformation of the trinuclear Cu(II) motif with the μ₃-O center, complex 5 exhibits geometric spin frustration and engages in antisymmetric exchange interactions. DFT calculations were also performed to better interpret spectroscopic evidence and understand the electronic structures, especially the mixed-valence nature of complex 5.

Selective sensing of ATP by hydroxide-bridged dizinc(ii) complexes offering a hydrogen bonding cavity

This work illustrates the highly selective fluorescence detection of ATP in the presence of other competing anions, such as AMP, ADP, PPi and other phosphates by using a set of hydroxide-bridged dizinc(ii) complexes offering a cavity lined with hydrogen bonds and other interactive forces. ATP, as a whole, was recognized by the synergic combination of Zn-phosphate bonding, ππ stacking between the adenine ring of ATP and the pyridine ring of the dizinc complex and hydrogen bonding interactions that modulate the cavity structure of the dizinc complexes.

The vital role of ditopic N–N bridging ligands with different lengths in the formation of new binuclear dioxomolybdenum(vi) complexes: synthesis, crystal structures, supramolecular framework and protein binding studies

Role of bis-(pyridyl) and bis-(imidazole) auxiliary ligands in the formation of supramolecular architectures and BSA binding with new binuclear dioxomolybdenum(vi) complexes.

Exploring catecholase activity in dinuclear Mn(ii) and Cu(ii) complexes: an experimental and theoretical approach

Two dinuclear Mn(ii) and Cu(ii) complexes were prepared, characterised and assessed for non-covalent interactions and catecholase oxidase properties. The catecholase activity of2is further corroborated by theoretical calculations using DFT.

A merged copper(I/II) cluster isolated from Glaser coupling

Ubiquitous copper-oxygen species are pivotal in enabling multifarious oxidation reactions in biological and chemical transformations. We herein construct a macrocycle-protected mixed-valence cluster [(^tBuC≡CCu^I₃)-(μ₂-OH)-Cu^II] by merging a copper acetylide cluster with a copper-oxygen moiety formed in Glaser coupling. This merged Cu(I/II) cluster shows remarkably strong oxidation capacity, whose reduction potential is among the most positive for Cu(II) and even comparable with some Cu(III) species. Consequently, the cluster exhibits high hydrogen atom transfer (HAT) reactivity with inert hydrocarbons. In contrast, the degraded [Cu^II-(μ₂-OH)-Cu^II] embedded in a small macrocyclic homologue shows no HAT reactivity. Theoretical calculations indicate that the strong oxidation ability of Cu(II) in [(^tBuC≡CCu^I₃)-(μ₂-OH)-Cu^II] is mainly ascribed to the uneven charge distribution of Cu(I) ions in the ^tBuC≡CCu^I₃ unit because of significant [d_Cu(I) → π*_(C≡C)] back donation. The present study on in situ formed metal clusters opens a broad prospect for mechanistic studies of Cu-based catalytic reactions.

Copper-oxygen species in organometallic complexes and enzymes are involved in many oxidation reactions. Here, the authors synthesize a macrocycle-protected mixed valence Cu(I/II) cluster with an unusually strong oxidation capacity and apply it to hydrogen atom transfer reactions with inert hydrocarbons.

Oxo-bridged trinuclear and tetranuclear manganese complexes supported with nitrogen donor ligands: syntheses, structures and properties

This work illustrates syntheses, structures, redox and magnetic properties as well as catalase activities of rare μ₃-oxo bridged mixed-valent trinuclear Mn^IIMn^III complexes (1 and 2) and a μ₄-oxo bridged tetranuclear MnII4 complex (3) supported with nitrogen donor ligands.

Cu(ii) complexes of a tridentate N,N,O-donor Schiff base of pyridoxal: synthesis, X-ray structures, DNA-binding properties and catecholase activity

DNA binding and catecholase activities of two Cu(ii) complexes [Cu(L₁)X] (X = N₃, NCS) of pyridoxal derived ligands are reported.