Coordinated Metal Centers Research Articles

Exploring the multifaceted roles of metal-organic frameworks in ecosystem regulation.

Achieving microecological balance is a complex environmental challenge. This is because the equilibrium of microecological systems necessitates both the eradication of harmful microorganisms and preservation of the beneficial ones. Conventional materials predominantly target the elimination of pathogenic microorganisms and often neglect the protection of advantageous microbial species. Metal-organic frameworks (MOFs) with excellent physicochemical properties (such as crystalline particles of various dimensions with highly porous network topology, variable local networking structures, diverse compositions with functional groups, high specific surface areas and pore volumes for surface and porous guest molecular adsorption/adhesion/affinity/binding and separation) have been extensively studied as a type of bactericidal material. However, only recently, studies on using MOFs to protect microorganisms have been reported. This review provides a comprehensive analysis of the mechanisms and applications of various MOFs (such as ZIF-8, ZIF-90, HKUST-1, MOF-5, and MIL-101) in both microbial eradication and protection. Insights into previous studies on MOF development, the material-bacteria interaction mechanisms, and potential clinical and environmental applications are also elucidated. MOFs with different framework structures/topologies (zeolite, sodalite, scaffolding, diamond, one-dimensional, and spherical/cylindrical cavities/pore networks), particle dimensions, polyhedral, cubic, rod and open/uncoordinated metal centers or fully coordinated metal centers, and ligand functional groups are discussed to understand the varying degrees of activation and interaction of microorganisms. This review holds potential in guiding future research on the design, synthesis, utilization, and integration of MOFs for the targeted eradication and protection of microorganisms and generating novel MOFs with selective antimicrobial and protective properties. Moreover, this review delivers a timely update and outlines future prospects for MOFs and their interaction with microorganisms, emphasizing their potential as a promising candidate among the next generation of smart materials in the field of ecosystem regulation.

Structure-activity relationship of small organic molecule functionalized Bi-based heterogeneous catalysts for electrocatalytic reduction of CO2 to formate.

Ligand engineering has proven to be an effective strategy for tuning and controlling the microenvironment of coordinated metal centers, highlighting the critical bridge between the activity and structural features of catalysts during electrocatalytic CO2 reduction reactions (eCO2RR). However, the limited availability of diverse organic ligands has hindered the development of novel high-performing electrocatalysts. In contrast, small organic molecules have been widely used in the fabrication of metal complexes due to their well-defined functionalities, low cost, and easy accessibility. Herein, functionalized small organic molecules were employed to prepare a new type of Bi-based heterogeneous molecular catalyst. These molecular catalysts enhance both electrical conductivity and catalytic activity for converting CO2 to formate in the eCO2RR. The relationship between the structure and electrochemical performance of organic-functionalized Bi-based heterogeneous catalysts was thoroughly investigated. Comprehensive characterization and kinetic studies demonstrated that the functional groups of the organic molecules construct bond pathways for electron transfer and promote the transformation of the active phase from Bi to Bi2O2CO3. In-situ Raman spectroscopy reveal that the organic molecules remain intact during the structural reorganization, which is beneficial for the sustained generation of Bi2O2CO3 active site during the eCO2RR process. Consequently, the organic functionalized Bi-based catalysts achieved a high formate Faradaic efficiency (FEHCOO-) of 89.8% and a high current density (jHCOO-) of 40.0mA cm-2 at a potential of -0.95V vs. RHE in an H-type cell. This work establishes a novel strategy for creating active heterogeneous catalysts using small organic molecules, opening new avenues for the development of efficient electrocatalysts for CO2 reduction.

Quantitative Determination of Metal Center Coordination Numbers in Resin‐Based Supported Catalysts via CO2 Adsorption

Quantitative Determination of Metal Center Coordination Numbers in Resin‐Based Supported Catalysts via CO₂ Adsorption

Construction of CeO2/Co(OH)2/FeS@NF nanosheet arrays for high-performance electrocatalytic oxygen evolution/urea oxidation, and overall water/urea splitting reactions

To develop non-noble electrocatalysts with high activity and stability for oxygen evolution reaction (OER)/urea oxidation reaction (UOR) is crucial to boost the efficiency of overall water/urea splitting reaction (OWS/OUS) for H2 production. Herein, we synthesized novel CeO2/Co(OH)2/FeS nanosheet arrays on nickel foam (CeO2/Co(OH)2/FeS@NF) through a simple two-step hydrothermal and following solvothermal routine. In this special structure, the ultra-thin 2D nanosheet morphology can adequately offer large contact area and thus abundant of active sites, facilitating significantly the mass transfer. Furthermore, the incorporation of FeS into CeO2/Co(OH)2 will not only modify the electron structure but also provide a number of phase interfaces, which can improve the inherent electron conductivity and promote the electron transport. More importantly, the rich oxygen vacancies (Ov) sites can vary the coordination of metal centers, modulating both the charge distribution and M−O bonding strength. As results, the CeO2/Co(OH)2/FeS@NF shows superior low overpotentials of 178 mV/1.30 V at 10 mA cm−2 to OER/UOR. The activity keeps its low value of 232 mV/1.375 V even when the current density was increased to as high as 300 mA cm−2. More importantly, the electrode of CeO2/Co(OH)2/FeS@NF//CeO2/Co(OH)2/FeS@NF just requires a low cell voltage of 1.41 V to realize OUS at a current density of 10 mA cm−2. Specifically, we also realized solar-driven H2 production. Plenty of H2 bubbles were continuously generated with a high speed of 600 L h−1 m−2 on the working electrode surface once the solar panel was placed under the natural sunlight.

Nitrogen reduction reaction enhanced by single-atom transition metal catalysts on functionalized graphene: A first-principles study

Ammonia production seeks alternatives to the conventional Haber-Bosch process, with nitrogen reduction reaction (NRR) emerging promising. Addressing the challenge of efficient catalysts, the functionalized graphene-based single atom catalysts (SACs) stand out. While prior studies have favored heteroatom-doped catalysts, the coordination of metal centers with nitrogen atoms remains underexplored. This work investigates transition metal (TM) SACs on nitrogen-doped graphene (N3G) using density functional theory (DFT) for electro-catalytic NRR. Results highlight the stability of V@N3G, Mo@N3G, W@N3G, with binding energies of −7.77, −5.43, and −3.89 eV, respectively. Insights into work function, d-band center, N–N bond, and IR stretching's role in N2 activation are gained through this study. Bader charge analysis reveals electron redistribution between the support and adsorbed N2. Employing Computational Hydrogen Electrode (CHE) method, comparative free energy diagrams for TM@N3G (V, Mo, W) via., enzymatic, consecutive, alternating, and distal pathways outline potential rate determining step (PDS) with and without the Implicit solvation method. Remarkably, W@N3G catalyst exhibits the lowest PDS in the presence of solvation energy, surpassing other catalysts. The multi-adsorption of N2 on W@N3G enhances NRR process, stabilizing intermediates for efficient ammonia production. This computational study sheds light on metal center SACs on functionalized graphene support as a potential electro-catalyst for efficient and stable NRR.

Precise Control of Metal Active Sites of Metal-Organic Framework Nanozymes for Achieving Excellent Enzyme-Like Activity and Efficient Pancreatitis Therapy.

Acute pancreatitis (AP) is a potentially life-threatening inflammatory disease that can lead to the development of systemic inflammatory response syndrome and its progression to severe acute pancreatitis. Hence, there is an urgent need for the rational design of highly efficient antioxidants to treat AP. Herein, an optimized Cu-based metal-organic framework (MOF) nanozyme with exceptional antioxidant activity is introduced, designed to effectively alleviate AP, by engineering the metal coordination centers in MN2Cl2 (M=Co, Ni, Cu). Specifically, the Cu MOF, which benefits from a Cu active center similar to that of natural superoxide dismutase (SOD), exhibited at least four times higher SOD-like activity than the Ni/Co MOF. Theoretical analyses further demonstrate that the CuN2Cl2 site not only has a moderate adsorption effect on the substrate molecule •OOH but also reduces the dissociation energy of the product H2O2. Additionally, the Cu MOF nanozyme possesses the excellent catalase-like activity and •OH removal ability. Consequently, the Cu MOF with broad-spectrum antioxidant activity can efficiently scavenge reactive oxygen species to alleviate arginine-induced AP. More importantly, it can also mitigate apoptosis and necrosis of acinar cells by activating the PINK1/PARK2-mediated mitophagy pathway. This study highlights the distinctive functions of tunable MOF nanozymes and their potential bio-applications.

Metalloporphyrin catalyzed self-switchable terpolymerization of propene oxide, phthalic anhydride and lactide for one-pot prepare block copolyesters

The copolymerization of polylactic acid with other monomers or polymers can enhance its performance and expand its range of applications. In this work, a range of metalloporphyrin catalysts featuring diverse substituents and metal coordination centers were synthesized and employed in conjunction with co-catalyst bis(triphenylphosphoranylidene) ammonium chloride (PPNCl) for the terpolymerization of phthalic anhydride (PA), propylene oxide (PO), and lactide (LA) ester without the use of initiators. The catalytic activity of the porphyrin ligand is significantly influenced by its substituents and different metal centers. Under optimal conditions, catalyst TOMePP-CoCl exhibits the most outstanding reactivity among all catalysts. In particular, this terpolymerization is a one-pot self-switching polymerization catalyzed by the metalloporphyrin/PPNCl system. PA selectively undergoes ring-opening copolymerization with PO first, while LA remains inert until PA totally consumed. Through continuous feeding experiments, a multiblock copolymer with fine structure controllability was successfully synthesized, which has been verified by DOSY NMR analysis.

Mono-, di- and trimetallic coinage nanoparticles prepared via the Brust-Schiffrin method.

The Brust-Schiffrin two-phase method is a facile way to prepare thiolate-protected metal nanoparticles, but its mechanism remains controversial. In this work, we demonstrate the use of the Brust-Schiffrin method based on coordination compound theory. We confirmed that the formation of stable complexes is the driving force for a series chemical reaction in the organic phase. We found that the stable Cu(I)-thiolate complex decreased the half-cell reduction potential of Cu(I)/Cu(0). Thus, when thiol ligands were in excess, thiolate-protected Cu(I) clusters formed rather than Cu(0)-cored nanoparticles. The thiolate-protected metal-hydride nanoclusters were the intermediate between the metal complexes and nanoparticles. The "metallophilic" interactions of the d10 closed-shell electronic configuration of the metal coordination centers were proposed as the driving force for nanocluster and nanoparticle formation. To confirm this mechanism, we synthesized Au, Ag, and Cu monometallic nanoparticles and bi- and trimetallic nanoparticles. We found that although thiolate-protected Cu(I) nanoclusters are not easily reduced, they can combine with Au and/or Ag nanoclusters to form nanoparticles. The proposed mechanism is expected to provide deeper insight into the Brust-Schiffrin method and further extend its application to metals other than Au, Ag and Cu.

PGM-free single atom catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells.

The progress of proton exchange membrane fuel cells (PEMFCs) in the clean energy sector is notable for its efficiency and eco-friendliness, although challenges remain in terms of durability, cost and power density. The oxygen reduction reaction (ORR) is a key sluggish process and although current platinum-based catalysts are effective, their high cost and instability is a significant barrier. Single-atom catalysts (SACs) offer an economically viable alternative with comparable catalytic activity for ORR. The primary concern regarding SACs is their operational stability under PEMFCs conditions. In this article, we review current strategies for increasing the catalytic activity of SACs, including increasing active site density, optimizing metal center coordination through heteroatom doping, and engineering porous substrates. To enhance durability, we discuss methods to stabilize metal centers, mitigate the effects of the Fenton reaction, and improve graphitization of the carbon matrix. Future research should apply computational chemistry to predict catalyst properties, develop in situ characterization for real-time active site analysis, explore novel catalysts without the use of platinum-based catalysts to reduce dependence on rare and noble metal, and investigate the long-term stability of catalyst under operating conditions. The aim is to engineer SACs that meet and surpass the performance benchmarks of PEMFCs, contributing to a sustainable energy future.

Metal Dipicolylamines and their Biomedical Applications: A Mini Review

Research in synthetic inorganic chemistry demonstrates that metal complexes are widely utilized as therapeutic as well as diagnostic agents. Due to the coordinative saturation, substitutional inertness, and unique redox characteristics, metal polypyridyl complexes have ignited attention and have been exploited in a number of biological and biomedical fields. The polypyridyl ligand, dipicolylamine (DPA) is a symmetrical secondary amine with two pyridyl rings. Delocalization of excited electrons occurs throughout the ligand system due to its conjugated character and the metal-to-ligand charge transfer transitions, resulting in a strong fluorescence signal. The high lipophilic nature of this ligand has proven to improve metal-DPA complex absorption by cell membranes. The N–H amine group in DPA and its analogues have the additional function of allowing hydrogen bonds to form, and deprotonation of this amine has allowed the creation of mononuclear and polynuclear species, with deprotonation of the bridging amine nitrogen also playing a significant role in the coordination sequence. DPA-appended metal complexes have garnered attention in the quest for linkages between magnetic, spectroscopic, structural, and coordination geometries. A number of mononuclear metal-DPA complex crystal structures with four, five, or six coordinated metal centers have been synthesized and explored for their biological properties. This study reviews DPA-derivatized ligand-linked metal complexes as promising cancer, microbial, and fungal inflammatory therapeutics, as well as potential diagnostic agents for fluorescence imaging and radiopharmaceuticals.
 Keywords: Dipicolylamine, metal complexes, coordination geometries, fluorescent agents

Synthesis, characterization and molecular docking of benz-imidazolium Se-adducts: Antimicrobial and anticancer studies

N-heterocyclic carbene (NHC) based compounds are remarkably known for astonishing biological potentials. Coordination of metal center with these compounds could substantially improve the biological potential for better efficacy. In this context, three binuclear azolium salts (S1–S3) and their selenium adducts (C1–C3) were synthesized and assured through various analytical tools. Compounds C1–C3 were confirmed through ESI-MS and mass spectra were found consistent with theoretical values and structural pattern. Synthesized compounds were simulated through computational approach and results were compared with experimental findings which confirm successful synthesis of designed compounds. Synthesized compounds were screened against bacterial strains and interestingly, the studied compounds showed good antimicrobial having IC50 8.41–21.55 and 11.85–20.84 µM against S.aureus and E.coli respectively. The studied compounds showed good antioxidant activity to scavenge DPPH free radicals among which azolium salts were found better in antioxidant potential (IC50 5.75–6.55 µg/mL) than their respective selenium compounds (IC50 12.55–6.12 µg/mL). Interestingly, selenium compounds showed better cytotoxicity against HEK-293 and HepG2 cell lines than that of azolium salts which confirmed the contributing role of selenium for biological potential of compounds. The haemolytic assay against red blood cells showed that selenium compounds are least toxic and can be further trailed for in vivo studies.

Unusual Effects of the Metal Center Coordination Mode on the Photophysical Behavior of the Rhenium(I) and Rhenium(I)-Iridium(III) Complexes.

Binuclear transition-metal complexes based on conjugated systems containing coordinating functions are potentially suitable for a wide range of applications, including light-emitting materials, sensors, light-harvesting systems, photocatalysts, etc., due to energy-transfer processes between chromophore centers. Herein we report on the synthesis, characterization, photophysical, and theoretical studies of relatively rare rhenium(I) and rhenium(I)-iridium(III) dyads prepared by using the nonsymmetrical polytopic ligands (NN2 and NN3) with the strongly conjugated phenanthroline and imidazole-quinoline/pyridine coordinating fragments. Availability of these different diimine chelating functions and targeted synthetic procedures allowed one to obtain a series of mononuclear (Re and Ir) and binuclear (Re-Re and Re-Ir) metal complexes with various modes of {Re(CO)3Cl} and {Ir(NC)2} metal fragment coordination. The obtained compounds were characterized by 1D 1H and 2D (COSY and NOESY) NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction crystallography. The photophysical study of the complexes (absorption, excitation and emission spectra, quantum yields, and excited-state lifetimes) showed that their emission parameters display strong dependence on the manner of metal center coordination to the diimine bidentate functions. The mononuclear complexes with an unoccupied imidazole-quinoline/pyridine fragment [Re(NN2), Re(NN3), and Ir(NC2)2(NN2)] or those containing a coordinated {Ir(NC)2} fragment in this position [Ir(NC2)2(NN1) and Re(NN2)Ir(NC1)2-Re(NN2)Ir(NC4)2] exhibit moderate-to-intense phosphorescence (quantum yields vary from 3% to 56% in a degassed solution), whereas the complexes containing a {Re(CO)3Cl} moiety in the imidazole-quinoline/pyridine position [Re2(NN2), Re2(NN3), and Ir(NC2)2(NN2)Re] demonstrate a strong reduction in the phosphorescence efficiency with a quantum yield of ≪0.1%. Quenching of the phosphorescence in the latter types of emitters is discussed in terms of a strong decrease in the radiative rate constants for these complexes compared to their analogues mentioned above, while the nonradiative constants remain nearly unchanged. Theoretical density functional theory (DFT) and time-dependent DFT (TD DFT) calculations, including evaluation of the radiative rate constants for the couple of structurally analogous complexes with and without a {Re(CO)3Cl} moiety coordinated to the imidazole-quinoline/pyridine chelating function, confirmed the observed trend in the variation of the emission intensity.

N-heterocyclic carbene based Bi-nuclear organoselenium compounds impart a mild biocidal potential compared to their ligands: Synthesis, characterization, computational studies

N-heterocyclic carbene (NHC) based compounds are remarkably known for astonishing biological potentials. Coordination of metal center with these compounds can substantially improve the biological potential for better efficacy. In this context, three binuclear azolium salts (L1-L3) and subsequent selenium adducts L1Se-L3Se were synthesized and assured through analytical techniques. Synthesized compounds were also simulated through computational approach and results were compared with experimental observations that also relatable with biological potentials. Synthesized compounds were screened against bacterial strains and interestingly, the studied compounds showed good antimicrobial potential with MIC values of 7.01, 10.7 and 10.5 µM for S. Aureus (gram positive bacteria) while 12.5, 11.75 and 14.5 µM against E. Coli (gram negative bacteria). The studied compounds showed good antioxidant activity to scavenge DPPH free radicals among which azolium salts were found better in antioxidant potential (IC50 5.75–6.55 µg/mL) than their respective selenium compounds (IC50 9.50–12.75 µg/mL). The hemolytic assay against red blood cells showed that ligands are least toxic comparative to their Se-adducts and can be further trialed for In Vivo studies.

Microfluidic one-step synthesis of a metal-organic framework for osteoarthritis therapeutic microRNAs delivery.

As a class of short non-coding ribonucleic acids (RNAs), microRNAs (miRNA) regulate gene expression in human cells and are expected to be nucleic acid drugs to regulate and treat a variety of biological processes and diseases. However, the issues with potential materials toxicity, quantity production, poor cellular uptake, and endosomal entrapment limit their further applications in clinical practice. Herein, ZIF-8, a metal-organic framework with noncytotoxic zinc (II) as the metal coordination center, was selected as miRNA delivery vector was used to prepare miR-200c-3p@ZIF-8 in one step by Y-shape microfluidic chip to achieve intracellular release with low toxicity, batch size, and efficient cellular uptake. The obtained miR-200c-3p@ZIF-8 was identified by TEM, particle size analysis, XRD, XPS, and zeta potential. Compared with the traditional hydrothermal method, the encapsulation efficiency of miR-200c-3p@ZIF-8 prepared by the microfluidic method is higher, and the particle size is more uniform and controllable. The experimental results in cellular level verified that the ZIF-8 vectors with low cytotoxicity and high miRNAs loading efficiency could significantly improve cellular uptake and endosomal escape of miRNAs, providing a robust and general strategy for nucleic acid drug delivery. As a model, the prepared miR-200c-3p@ZIF-8 is confirmed to be effective in osteoarthritis treatment.

Sensitizing photoactive metal–organic frameworks via chromophore for significantly boosting photosynthesis

Photosensitization related to energy/electron transfer process is of great importance to natural photosynthesis. Herein, we proposed a promising strategy to improve the sensitizing ability of the typical photoactive MOFs (UiO-Ir) by engineering its metal coordination center with NBI (1,8-naphthalenebenzimidizole) chromophore. The resulting MOFs (UiO-Ir-NBI) exhibited a strong sensitizing ability for significantly boosting photosynthesis. Impressively, the catalytic yield of 2-chloroethyl ethyl sulfoxide with UiO-Ir-NBI can reach 99%, over 6 times higher than that with UiO-Ir (16.4%). Moreover, UiO-Ir-NBI exhibited an excellent catalytic stability and a broad substrate tolerance, highlighting its great application prospect. Systematic investigations revealed that the strong visible light absorption, long excited state lifetime and efficient electron-hole separation of UiO-Ir-NBI greatly contributed to harvesting visible light and facilitating interface electron/energy transfer for efficient solar energy utilization. This work provides a new horizon to boost photosythesis of MOFs by engineering their metal sensitizing centers at a molecular level.

Conjugated Nonplanar Copper-Catecholate Conductive Metal–Organic Frameworks via Contorted Hexabenzocoronene Ligands for Electrical Conduction

Conductive metal-organic frameworks (c-MOFs) with outstanding electrical conductivities and high charge carrier mobilities are promising candidates for electronics and optoelectronics. However, the poor solubility of planar ligands greatly hinders the synthesis and widespread applications of c-MOFs. Nonplanar ligands with excellent solubility in organic solvents are ideal alternatives to construct c-MOFs. Herein, contorted hexabenzocoronene (c-HBC) derivatives with good solubility are adopted to synthesize c-MOFs. Three c-MOFs (c-HBC-6O-Cu, c-HBC-8O-Cu, and c-HBC-12O-Cu) with substantially different geometries and packing modes have been synthesized using three multitopic catechol-based c-HBC ligands with different symmetries and coordination numbers, respectively. With more metal coordination centers and increased charge transport pathways, c-HBC-12O-Cu exhibits the highest intrinsic electrical conductivity of 3.31 S m-1. Time-resolved terahertz spectroscopy reveals high charge carrier mobilities in c-HBC-based c-MOFs, ranging from 38 to 64 cm2 V-1 s-1. This work provides a systematic and modular approach to fine-tune the structure and enrich the c-MOF family with excellent charge transport properties using nonplanar and highly soluble ligands.

Copper‐Phthalocyanine‐based Covalent Organic Polymers for Electrocatalytic Ambient Ammonia Synthesis

AbstractAccurate structure‐designed single‐metal‐atom doped covalent organic polymers (COPs) are expected to be one of the most promising nitrogen reduction reaction (NRR) electrocatalysts due to their adjustable metal center coordination environment.[1–3] However, pyrolysis of the material is often required to overcome the intrinsically low conductivity of COPs materials, resulting in undesirable structural changes.[4,5] Moreover, the ligand portion connected to the monomer can modulate the electronic state of the catalytic center of the monomer to achieve enhanced kinetics parts of the reaction.[6–8] To overcome this shortcoming, this study reports hybrid electrocatalysts formed by self‐assembling pristine covalent organic polymers (COPs) with oxidized carbon nanotubes (O‐CNT). The electrical conductivity of the hybridized COP/O‐CNT materials can be increased by the O‐CNT. The catalyst attains a FE of 26.8 % and a large NH3 yield rate of 12.3 ug−1 h−1 cm−2 at −0.3 V versus a reversible hydrogen electrode. DFT calculations show that the NRR process tends to choose alternation paths. Therefore, the synthesized PCuPc/O‐CNT as a stable, high‐efficiency NRR catalyst offers a valuable reference for single‐atom COPs electrocatalyst research in electrochemical nitrogen fixation.

A combined experimental and theoretical study of covalent vs noncovalent dimer formation in vanadium(V) complexes with Schiff base ligands

This manuscript reports the synthesis, spectroscopic and X-ray characterization of three new vanadium(V) complexes, namely [VO2L1] (1), (μ-O)2[V(O)(L2)2]·2CH3CN (2) and (μ-O)2[V(O)(L3)2]·CH3CN (3) [HL1 (2-ethoxy-6-((quinolin-8-ylimino)methyl)phenol), HL2 (2-(1-(2-(methylamino)ethylimino)propyl)phenol) and HL3 (2-(1-(2-(ethylamino)ethylimino)propyl)phenol)]. Complex 1 is a mononuclear dioxovanadium(V) system containing a five coordinated metal center, whilst complexes 2 and 3 are centrosymmetric dinuclear systems with two [(L)V = O] moieties [(L2)- for 2 and (L3)- for 3], which are linked together through asymmetrically bridging oxygen atoms. DFT calculations have been used to rationalize the different behavior of complex 1, that is related to the lack of strong H-bond donors and strong ability to self-assemble via electrostatically enhanced π-stacking interactions. Moreover, the strength of π–π and CH–π interactions has been evaluated and the interactions have been characterized by several computational tools like MEP, QTAIM and NCI Plot.

Electron transfer catalysis mediated by 3d complexes of redox non-innocent ligands possessing an azo function: a perspective.

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes.

The Local Coordination Effects on the Reactivity and Speciation of Active Sites in Graphene-Embedded Single-Atom Catalysts over Wide pH and Potential Range.

Understanding the catalytic performance of different materials is of crucial importance for achieving further technological advancements. This especially relates to the behaviors of different classes of catalysts under operating conditions. Here, we analyzed the effects of local coordination of metal centers (Mn, Fe, Co) in graphene-embedded single-atom catalysts (SACs). We started with well-known M@N4-graphene catalysts and systematically replaced nitrogen atoms with oxygen or sulfur atoms to obtain M@OxNy-graphene and M@SxNy-graphene SACs (x + y = 4). We show that local coordination strongly affects the electronic structure and reactivity towards hydrogen and oxygen species. However, stability is even more affected. Using the concept of Pourbaix plots, we show that the replacement of nitrogen atoms in metal coordinating centers with O or S destabilized the SACs towards dissolution, while the metal centers were easily covered by O and OH, acting as additional ligands at high anodic potentials and high pH values. Thus, not only should local coordination be considered in terms of the activity of SACs, but it is also necessary to consider its effects on the speciation of SAC active centers under different potentials and pH conditions.