N4 Ligand Research Articles

Copper(II) complex of pentadentate N5 ligand as catalyst for 2-aminophenol oxidation

Exploring the catalytic activity of the copper(II) complex of pentadentate ligand in oxidizing o-aminophenol (OAP) has not been extensively studied. To scrutinize the least explored domain, we synthesized and characterized the copper(II) complex of pentadentate ligand, 3-(bis(pyridin-2-ylmethyl)amino)-N-(quinolin-8-yl)propanamide. Further, the complex was allowed to react with OAP to form 2-aminophenoxazin-3-one (APX) via oxidative coupling reaction, the resultant product is a functional analogue of phenoxazinone synthase (PHS), an active enzyme, relevant to the industrial production of a widely recognized antibiotic called Actinomycin D. Furthermore, kinetic studies were performed to comprehend the path of the reaction which unveiled the formation of the complex-substrate adduct. It is worth noticing that the complex displayed an impressive turnover number of 6.4 × 104 h−1 under atmospheric conditions in methanol. To the best of our knowledge, the present complex is the first copper(II) complex with pentadentate N5 ligand showing PHS mimicking catalytic activity.

Co(II)-N4 Catalysts for the Coupling of CO2 with Epoxides into Cyclic Carbonates: Catalytic Activity, Computational and Kinetic Studies.

In this study, we synthesized and characterized a series of cobalt(II) complexes bearing linear tetradentate N4 ligands. These Co(II)-N4 complexes proved to be efficient catalysts for the cycloaddition reaction between carbon dioxide and epoxides even at room temperature and 1 bar pressure of carbon dioxide without the need for solvents or cocatalysts. Furthermore, when combined with (triphenylphosphoranylidene)ammonium chloride (PPNCl) as a cocatalyst, the Co-N4 catalysts exhibited an impressive turnover frequency of up to 41,000 h-1 for coupling of epichlorohydrin/CO2. These Co(II)-N4 catalysts were found to have excellent stability and reusability, retaining their catalytic activity after they were recycled seven times. Density functional theory (DFT) calculations provided a comprehensive mechanism for the cycloaddition reaction, indicating that the rate-determining step is the epoxide ring opening, in both the presence and absence of PPNCl. Further kinetic studies allow us to determine the activation parameters (ΔH‡, ΔS‡, and ΔG‡ at 25 °C) of the coupling reaction using the Eyring equation. The Gibbs free activation energy obtained from the kinetic studies was in close agreement with that of the DFT calculations. The substituent effect on the cycloaddition reaction of CO2 with various substituted styrene oxides was also examined for the first time.

Modulation of Fe-Fe distance and spin in diiron complexes using tetradentate ligands with different flanking donors.

Here we report the synthesis and characterization of diiron complexes containing triaryl N4 and N2S2 ligands derived from o-phenylenediamine. The complexes display significant differences in Fe-Fe distances and magnetic properties that depend on the identity of the flanking NMe2 and SMe donor groups.

Phenoxazinyl Zn(II) diradical complex formed via redox-driven cyclization of a 2-aminophenol-based N3O ligand. Isolation of the modified N3 ligand radical and its Ni(II) complex.

Aerobic reaction between the pyridine-2-carboxamide-2-aminophenol N3O ligand (H3L1) and Zn(ClO4)2·6H2O in CH3CN affords an N3 phenoxazinylate coordinated Zn(II) complex; its diradical electronic structure [ZnII{(L1*)˙-}2] has been elucidated from redox, spectroscopic (UV-VIS and EPR), and magnetic measurements and DFT calculations. Isolation and characterization of the metal-assisted redox-driven modified N3 ligand as a radical cation (H2L1*)˙+ and its Ni(II)-diradical complex [NiII{(L1*)˙-}2] have also been achieved.

Biocatalytic synthesis, in silico analysis and in vitro validation of hydroxamic acids against Histone Deacetylases

Hydroxamic acids with functional group R-CONHOH, have been used as possible inhibitors of metalloenzymes because of their innate ability to strongly bind with metal ions. The synthesis from amides to hydroxamic acids is commonly carried out via a synthetic route wherein hazardous chemicals like acids and extreme reaction conditions are used. Hence, the present study aims to develop a biocatalytic process for the synthesis of a series of hydroxamic acids using the resting cells of Bacillus smithii IIIMB2907. Further, through in silico analysis of the synthesized compounds using molecular docking, molecular dynamics pharmacophore analysis, and density functional theory (DFT) calculations, it was observed that ligand N1-(3-(trifluoromethyl)phenyl)-N8-hydroxyoctanediamide (2c) showed better interaction with class I histone deacetylases (HDACs) on comparisons with FDA approved anti-cancer drug Suberoylanilide hydroxamic acid (SAHA). Furthermore, in vitro HDAC activity of 2c in C2C12 skeletal muscle cell lines validates the potency of this molecule as an HDAC inhibitor, as it shows 33 % HDAC inhibition on comparison with SAHA which showed only 14 % HDAC inhibition. Therefore, this study proposes a novel process for the synthesis of a potential drug candidate that may act as an inhibitor of class I HDAC’s.

The role of anagostic interactions in the Pd−Superoxo formation in aerobic Pd catalysed C–C coupling reaction: What do DFT computations reveal?

Aerobic oxidative CC coupling catalyzed by dimethyl PdII center (L–[Pd]) bearing tetradentate pyridinophane (N4) ligand L (L = N,N-dimethyl-2,11-diaza[3.3]-(2,6)pyridinophane) has been investigated using density functional theory (DFT) computations. In Pd-catalyzed aerobic oxidation processes, the interaction of dioxygen with PdII center is a crucial step. Computations reveal that the reactant dimethyl PdII complex L–[Pd] has anagostic interactions (4 × Pd···H ∼ 2.60 Å) in total agreement with reported experimental NMR downshift values and these interactions play a significant role in the formation of PdIII–Superoxo intermediate which is less probed. While oxygen approaches the reactant, PdII center becomes PdIII allowing axial amine coordination. This pushes two protons (Csp3–H) away from Pd center weakening the two anagostic interactions. The other two anagostic interactions are intact at long ranges and at closer distances they weaken facilitating Pd–O bonding. The formation of the PdIII center in the PdIII–Superoxo were confirmed by reported EPR measurements. To understand this process, this key step has been modelled. Calculations have been performed with fourteen different density functionals (M06L, B3LYP, PBE0, M06, M11 to name a few) and different basis sets both in gas phase and in solvent medium. It is found that the range separated functionals, particularly M11 functional describe the process accurately and this is attributed to their efficiency of handling electron correlation at different ranges by including different percentage of Hartree-Fock exchange. The role of anagostic interactions M⋯H (M = Ni, Pd and Pt) in metal–superoxo species bearing symmetric and asymmetric substituted N4 ligands have been investigated. These results clearly explain asymmetrically substituted N4 ligands assist the simultaneous creation of M–O bonds at medium ranges and the weakening of bis anagostic (repulsive) M∙∙∙H interactions at short ranges. These findings suggest that intermediate stabilization in CC coupling processes mediated by transition-metal catalysts containing N4 ligands depends critically on the strength of anagostic interactions.

Solvent Coligand-Induced Spin State Variation in Iron(II) Complexes Bearing Pentadentate N5 Ligand Toward the Construction of an N5O Spin Crossover Center

Three iron(II) complexes having a 1,2,3-triazole-containing linear pentadentate N5 ligand L3-Me-3Me and different monodentate solvent coligand X, [Fe(L3-Me-3Me)X](BPh4)2 [L3-Me-3Me = bis(N,N′-1-methyl-1H-1,2,3-triazol-4-yl-methylideneaminopropyl)methylamine; X = MeCN (1), dimethylformamide (DMF) (2), and dimethyl sulfoxide (DMSO) (3)] have been selectively prepared by the choice of reaction solvent. Along the well-known spectrochemical series, the strong-field solvent ligand MeCN generates a low-spin (LS) complex 1, the weak-field ligand DMSO generates a high-spin (HS) complex 3, and the complex 2 coordinated by the medium-field ligand DMF exhibits gradual and reversible one-step HS↔LS spin crossover (SCO) with T1/2 = 173 K in the temperature range of 400–5 K. This spin state variation among LS 1, HS 3, and SCO 2 has been investigated by the combination of temperature-dependent magnetic susceptibilities measurements, single-crystal X-ray diffraction (SCXRD) studies, Hirshfeld surface analyses, and density functional theory calculations. In addition, the details of structural change between HS and LS states of an N5O iron(II) center upon SCO are revealed for the first time by the variable-temperature SCXRD studies for 2 at nine different temperatures in which the difference of Fe–O bond length between the two spin states is shorter than that of any other Fe–N bond lengths.

Tuning of Dimensionality and Nuclearity as a Function of Ligand Field Modulation Resulting in Field-Induced Cobalt(II) Single-Ion Magnet

The present study reveals that predetermined diverse topology and desired properties can be achieved by strategically designing the ligand and it confirms that the structural and magnetic features of cobalt(II) complexes have a significant impact of the ligand modulation and proper utilization of a coligand. In this work, tactically, two new Pyclen-based macrocyclic N4 ligands have been designed to demonstrate the effect of the ligand field on structure and magnetic properties. Using these macrocyclic N4 ligands, a series of cobalt complexes, [Co(L1–N4)(μ-NCS)]2·[Co(NCS)4] (1), [Co4(L1–N4)2{μ-N(CN)2}8]n·2CH2Cl2 (2), and [Co(L2–N4)(NCS)2] (3), have been synthesized and characterized by different physicochemical techniques, including single-crystal X-ray diffraction, magnetic, and various spectroscopic methods. The dinuclear plus mononuclear (2 + 1) cobalt complex (1) and 2D cobalt coordination polymer (2) are synthesized using the same macrocyclic N4 ligand: 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane (L1–N4) with a variation of coligand from thiocyanato to dicyananamido. A penta-coordinated mononuclear cobalt complex (3) is synthesized by substituting one methyl group of L1–N4 with a carboxaldehyde group, which leads to a [3,9-dimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-6-carbaldehyde (L2–N4)] ligand. Single-crystal X-ray diffraction studies reveal the impact of strategic modification of ligands and variation of coligand on the coordination environment, dimensionality, and topology of Co(II) complexes. Whereas, detailed magnetic studies disclose the occurrence of Co(II) ions in a high-spin electronic state for all three complexes, yet, only complex 3 displays exciting slow relaxation of magnetization using an external dc magnetic field and acts as a single-ion magnet, while complexes 1 and 2 display no such property. Detailed theoretical studies have been carried out to better understand and justify the experimentally obtained result.

Stable CO2/water foam stabilized by dilute surface‐modified nanoparticles and cationic surfactant at high temperature and salinity

AbstractCO2 enhanced oil recovery and storage could see widespread deployment as decarbonization efforts accelerate to meet climate goals. CO2 is more efficiently distributed underground as a viscous foam than as pure CO2; however, most reported CO2 foams are unstable at harsh reservoir conditions (22 wt% brine, 2200 psi, and 80°C). We hypothesize that silica nanoparticles (NP) grafted with (3‐trimethoxysilylpropyl)diethylenetriamine ligands (N3), to improve colloidal stability, and dimethoxydimethylsilane ligands (DM), to improve CO2‐phillicity, combined with the cationic surfactant N1‐alkyl‐N3, N3‐dimethylpropane‐1,3‐diamine (RCADA), will develop viscous, stable CO2 foams at reservoir conditions. We grafted NP with N3 and DM ligands. We verified NP stability at reservoir conditions with measurements of zeta potential, amine titration curves, and NP diameter. We measured NP water contact angles (θw) at the water–air and water–liquid CO2 interfaces. In a high‐temperature, high‐pressure flow apparatus, we calculated the viscosity of CO2 foams across a beadpack and determined static foam stability with microscope observations. Modified NP were colloidally stable at reservoir conditions for 4 weeks, and had higher θw in liquid CO2 than in air. Addition of at least 0.5 μmol/m2 DM silane (0.5DM) greatly improved foam stability. RCADA‐only foam coarsening rates (dDSM3/dt) decreased 16–17× after adding 1 wt/vol% 8N3 + 1.5DM NP, and 5–10× with a 0.1–1 vol/vol% increase in RCADA concentration (with or without NP). 1 vol/vol% RCADA foam exhibited coarsening rates of 900 and 2400 μm3/min with 1 and 0.2 wt/vol% 8N3 + 1.5DM NP, respectively. These results demonstrate impressive foam stabilities at harsh reservoir conditions.

Flavonol dioxygenation catalysed by cobalt(II) complexes supported with 3N(COO) and 4N donor ligands: a comparative study to assess the carboxylate effects on quercetin 2,4-dioxygenase-like reactivity.

Two new cobalt(II)-acetato complexes, [CoII(L3NCOO)(OAc)]·0.5H2O (1OAc·0.5H2O) and [CoII(L4N)(OAc)](PF6) (2OAc(PF6)), were synthesised using ligands L3NCOO- (Li+L3NCOO- = lithium 2-(benzyl((6'-methyl-[2,2'-bipyridin]-6-yl)methyl)amino)acetate) and L4N (N-benzyl-1-(6'-methyl-[2,2'-bipyridin]-6-yl)-N-(pyridin-2-ylmethyl)methanamine), respectively, to mimic the functional activity of cobalt(II)-quercetin-2,4-dioxygenase (CoII-2,4-QD). Additionally, Co(II)-flavonolato ternary complexes, [CoII(L3NCOO)(fla)]·H2O (1fla·H2O) and [CoII(L4N)(fla)](PF6) (2fla(PF6)), were synthesised as enzyme-substrate models. All four complexes were thoroughly characterised by elemental analyses and spectroscopic methods. Structural characterisation was performed for 1OAc·0.5H2O, 2OAc(PF6)·CH2Cl2 and 2fla+ with a perchlorate counter anion, 2fla(ClO4)·1.5H2O. Furthermore, density functional theory (DFT) calculations, time-dependent DFT (TD-DFT) and molecular orbital (MO) analysis were performed for the flavonolato adducts 1fla and 2fla+. The catalytic activities of complexes 1OAc·0.5H2O and 2OAc(PF6) in the oxygenative degradation of flavonol (multiple-turnover reactions) were investigated at 70 °C in DMF to determine the effect of the carboxylate substituent over a pyridyl donor residue on reactivity. Complex 1OAc·0.5H2O showed a higher catalytic rate than complex 2OAc(PF6). The same reactivity order was observed for single-turnover dioxygenation reactions with ternary complexes (1fla > 2fla+). The formation constants (Kf) of 1fla and 2fla+ species are comparable, implying that catalyst-substrate adduct formation occurs in similar amounts for both catalytic reactions. Therefore, the Kf values have a similar impact on reactivities. However, the oxidation potential of the bound fla-/fla˙ couple in 1fla is considerably lower than that in 2fla+. DFT calculations predicted that the negatively charged carboxylate group of ligand L3NCOO- determines the higher reactivity of 1fla with dioxygen by decreasing the oxidation potential of the bound fla-/fla˙ couple. During the dioxygenation process, the reactive Co(II)-bound flavonoxy radical was generated via single-electron transfer from the coordinated fla- to dioxygen, simultaneously forming a superoxide ion. The anionic carboxylate group improves the stability of the bound flavonoxy radical by providing substantial electron density to the electron-deficient fla˙ through the Co(II) centre, allowing the reactive fla˙ species to accumulate at an optimal concentration for effective catalysis. EPR spectroscopy successfully detected the cobalt-bound fla˙ species formed through the dioxygenation of 1fla. NBT2+ and EPR spin-trapping experiments confirmed superoxide formation during the dioxygenation process. So, the present work describes CoII-2,4-QD model studies and clarifies the function of carboxylate in quercetinase-like reactivity.

Electrochemical generation of high-valent oxo-manganese complexes featuring an anionic N5 ligand and their role in O-O bond formation.

Generation of high-valent oxomanganese complexes through controlled removal of protons and electrons from low-valent congeners is a crucial step toward the synthesis of functional analogues of the native oxygen evolving complex (OEC). In-depth studies of the water oxidation activity of such biomimetic compounds help in understanding the mechanism of O-O bond formation presumably occurring in the last step of the photosynthetic cycle. Scarce reports of reactive high-valent oxomanganese complexes underscore the impetus for the present work, wherein we report the electrochemical generation of the non-heme oxomanganese(IV) species [(dpaq)MnIV(O)]+ (2) through a proton-coupled electron transfer (PCET) process from the hydroxomanganese complex [(dpaq)MnIII(OH)]ClO4 (1). Controlled potential spectroelectrochemical studies of 1 in wet acetonitrile at 1.45 V vs. NHE revealed quantitative formation of 2 within 10 min. The high-valent oxomanganese(IV) transient exhibited remarkable stability and could be reverted to the starting complex (1) by switching the potential to 0.25 V vs. NHE. The formation of 2via PCET oxidation of 1 demonstrates an alternate pathway for the generation of the oxomanganese(IV) transient (2) without the requirement of redox-inactive metal ions or acid additives as proposed earlier. Theoretical studies predict that one-electron oxidation of [(dpaq)MnIV(O)]+ (2) forms a manganese(V)-oxo (3) species, which can be oxidized further by one electron to a formal manganese(VI)-oxo transient (4). Theoretical analyses suggest that the first oxidation event (2 to 3) takes place at the metal-based d-orbital, whereas, in the second oxidation process (3 to 4), the electron eliminates from an orbital composed of equitable contribution from the metal and the ligand, leaving a single electron in the quinoline-dominant orbital in the doublet ground spin state of the manganese(VI)-oxo species (4). This mixed metal-ligand (quinoline)-based oxidation is proposed to generate a formal Mn(VI) species (4), a non-heme analogue of the species 'compound I', formed in the catalytic cycle of cytochrome P-450. We propose that the highly electrophilic species 4 catches water during cyclic voltammetry experiments and results in O-O bond formation leading to electrocatalytic oxidation of water to hydrogen peroxide.

Electrocatalytic production of hydrogen using nickel complexes with tridentate N3 ligands

The octahedrally coordinated nickel(II) complexes of the type [Ni(L)2](ClO4)2 (1, 2) have been isolated and characterised, where L is (pyridin-2-ylmethyl)amine (L1, 1) and 1-(1-methyl-1H-imidazol-2-yl)-N-(pyridin-2-ylmethyl)methanamine (L2, 2). Further, the electrochemical hydrogen evolution reaction (HER) was performed in acetonitrile and proved that NiI is an active redox centre in strong acids like p-toluenesulfonic acid. Unlike the analogous Ni complexes, the present complexes exhibit higher catalytic rates (∼ 104 M−1s−1) at relatively low overpotential (600 mV) and show significant TONs (1, 15; 2, 32). In the presence of weak acid like benzoic acid, electrocatalysis occurs only from the formal Ni0 state at the expense of reduced ligand backbone at the higher cathodic potential. In striking contrast, the overpotentials even decrease further (∼300 mV) by using weak acid compared to the TsOH. The differences in the HER activity arise mainly from the pyridine and imidazole donors in 1 and 2, respectively.

Alkane hydroxylation by m-chloroperbenzoic acid catalyzed by nickel(II) complexes of linear N4-tetradentate ligands

In this study, nickel(II) complexes with a series of linear tetradentate N4 ligands consisting with a cyclic diamine framework and (2-pyridyl)methyl or (2-pyridyl)ethyl sidearms were synthesized and characterized, and the oxidation of alkanes with m-CPBA catalyzed by the nickel(II) complexes was investigated. In this system, all nickel(II) complexes showed efficient catalytic activity (yield ∼ 80 %), where the nickel(II) complex of 6-membered cyclic diamine with (2-pyridyl)methyl sidearms (Ni(L6Pym2)(AN)2) showed the highest yield (88 %) and high alcohol-product selectivity (A/K = 9.8) for cyclohexane oxidation. Kinetic analysis showed that the rate-determining step in this catalytic reaction is the CH bond activation from the substrate. Counter anion of complexes affected the initial reaction rate, indicating that NiII-m-CPBA adduct is a key intermediate in catalytic cycle. In this system, ArC(O)O• (aroyloxyl radical), which is generated by homolytic cleavage of OO bond of the m-CPBA adduct intermediate, abstracts hydrogen atom from the substrate. However, radical trapping experiment using CCl4 showed that a Ni-based oxidant is also involved in addition to ArC(O)O•.

Synthesis, Structures, and Photoluminescence of Two Novel Zinc(II) Compounds Containing 2-Acetylpyridine-aminoguanidine

In the reaction of zinc(II) sulfate and the chloride salt of 2-acetylpyridine-aminoguanidine, two types of complex were obtained, i.e., [Zn(H2O)6](H2L)2(SO4)3·3H2O and [Zn(L)H2O(SO4)]·H2O, depending on the presence of LiOAc as the deprotonating agent. The physicochemical, structural, and photoluminescence properties of the complexes were examined. In the first complex, obtained in the absence of LiOAc, the Schiff base had the role of a counter-ion in its doubly protonated form, while in the presence of LiOAc, upon deprotonation, coordination takes place, and thus the Schiff base acts as a tridentate N3 ligand. In the latter complex, the ligand is coordinated through pyridine, azomethine, and the imino nitrogen of the aminoguanidine residue, leading to formation of two fused five-membered chelate rings. Both the examined complexes, as well as the ligand itself, show high photoluminescence.

Understanding the chemoselectivity switch in CO2 reduction catalyzed by Co and Fe complexes bearing a pentadentate N5 ligand

The Co and Fe complexes bearing a pentadentate N5 ligand could efficiently catalyze the photochemical CO2 reduction with excellent chemoselectivity favoring CO (Co catalysis) and formate (Fe catalysis). Density functional calculations were carried out to gain mechanistic insights and uncover the origin of the selectivity of these reactions. The pentadentate N5 ligand was delineated to be redox noninnocent that can accept electrons and protons, and the oxidation state of Co and Fe remained at +2 during the reduction. The theoretically established catalytic mechanisms suggest that, for the Co (1) catalysis, a nucleophilic attack on CO2 takes place by the double-reduced catalyst (formally Co0) to generate a CoII-carboxylate complex. Then, a proton is transferred from Et3NH+ to CoII-carboxylate to generate a CoII-COOH intermediate, which was further protonated on its hydroxyl group to cleave the C-O bond and release the CO molecule. For the Fe (2) catalysis, the nucleophilic attack on CO2 by a protonated and double-reduced species (formally Fe0-N-H) generates HCOO-. In this step, two electrons and one proton are transferred to the CO2 moiety from the N5 ligand. Such an orthogonal hydride (electron/proton) transfer mechanism is similar to the formate dehydrogenase metalloenzyme catalysis. The CO formation pathway is favored over the formic acid formation pathway in the Co-catalysis. However, the chemoselectivity reverses when changing the metal of the catalyst from Co to Fe, which promotes the formation of formic acid over CO by the orthogonal hydride transfer mechanism.

Cobalt Quaterpyridine Complexes for Highly Efficient Heterogeneous CO2 Reduction in Aqueous Media

AbstractLigands play a critical role in the electrocatalytic CO2 reduction reaction (CO2RR) based on heterogeneous molecular catalysts. Previous research on heterogeneous molecular electrocatalysis has mainly dealt with N4 ligands with pyrrole as subunits (porphyrin, phthalocyanine, etc.), while ligands constructed from pyridine subunits remain uncommon. The examples for comparing active configurations are few and far between. Herein, the development of new N4 cobalt complexes based on pyridine subunits is explored. After anchoring onto carbon nanotubes, they can exhibit CO2RR activity at a low overpotential of 140 mV, and high activity from ‐0.30 to ‐0.60 V versus reference hydrogen electrode with a selectivity of above 98%. Excellent performance at large current densities can also be observed in a flow cell. In situ attenuated total reflectance‐Fourier transform infrared spectroscopy proves that such electrocatalysts exhibit CO production at lower overpotential and moderate CO adsorption ability over a wide potential range. From density functional theory calculations, it is shown that a pyridine‐based cobalt complex on a carbon substrate can reduce the Gibbs free energy for reactions further than its counterpart pyrrole‐based ones. Further analysis proves that the semimetal behavior of optimized d‐orbitals may facilitate charge transfer and increase the activity. This provides a new insight for understanding catalytically active moieties in heterogeneous molecular catalysts with ligands constructed from pyridine subunits.

C-H Bond Activation by a Mononuclear Nickel(IV)-Nitrate Complex.

The recent focus on developing high-valent non-oxo-metal complexes for late transition metals has proven to be an effective strategy to study the rich chemistry of these high-valent species while bypassing the synthetic challenges of obtaining the oxo-metal counterparts. In our continuing work of exploring late transition metal complexes of unusually high oxidation states, we have obtained in the present study a formal mononuclear Ni(IV)-nitrate complex (2) upon 1-e- oxidation of its Ni(III) derivatives (1-OH and 1-NO3). Characterization of these Ni complexes by combined spectroscopic and computational approaches enables deep understanding of their geometric and electronic structures, bonding interactions, and spectroscopic properties, showing that all of them are square planar complexes and exhibit strong π-covalency with the amido N-donors of the N3 ligand. Furthermore, results obtained from X-ray absorption spectroscopy and density functional theory calculations provide strong support for the assignment of the Ni(IV) oxidation state of complex 2, albeit with strong ligand-to-metal charge donation. Notably, 2 is able to oxidize hydrocarbons with C-H bond strength in the range of 76-92 kcal/mol, representing a rare example of high-valent late transition metal complexes capable of activating strong sp3 C-H bonds.

Dioxygen Activation and Mandelate Decarboxylation by Iron(II) Complexes of N4 Ligands: Evidence for Dioxygen-Derived Intermediates from Cobalt Analogues.

The isolation, characterization, and dioxygen reactivity of monomeric [(TPA)MII(mandelate)]+ (M = Fe, 1; Co, 3) and dimeric [(BPMEN)2MII2(μ-mandelate)2]2+ (M = Fe, 2; Co, 4) (TPA = tris(2-pyridylmethyl)amine and BPMEN = N1,N2-dimethyl-N1,N2-bis(pyridin-2-yl-methyl)ethane-1,2-diamine) complexes are reported. The iron(II)- and cobalt(II)-mandelate complexes react with dioxygen to afford benzaldehyde and benzoic acid in a 1:1 ratio. In the reactions, one oxygen atom from dioxygen is incorporated into benzoic acid, but benzaldehyde does not derive any oxygen atom from dioxygen. While no O2-derived intermediate is observed with the iron(II)-mandelate complexes, the analogous cobalt(II) complexes react with dioxygen at a low temperature (-80 °C) to generate the corresponding cobalt(III)-superoxo species (S), a key intermediate implicated in the initiation of mandelate decarboxylation. At -20 °C, the cobalt(II)-mandelate complexes bind dioxygen reversibly leading to the formation of μ-1,2-peroxo-dicobalt(III)-mandelate species (P). The geometric and electronic structures of the O2-derived intermediates (S and P) have been established by computational studies. The intermediates S and P upon treatment with a protic acid undergo decarboxylation to afford benzaldehyde (50%) with a concomitant formation of the corresponding μ-1,2-peroxo-μ-mandelate-dicobalt(III) (P1) species. The crystal structure of a peroxide species isolated from the cobalt(II)-carboxylate complex [(TPA)CoII(MPA)]+ (5) (MPA = 2-methoxyphenylacetate) supports the composition of P1. The observations of the dioxygen-derived intermediates from cobalt complexes and their electronic structure analyses not only provide information about the nature of active species involved in the decarboxylation of mandelate but also shed light on the mechanistic pathway of two-electron versus four-electron reduction of dioxygen.

Electrochemical Formation and Reactivity of a Mn‐Peroxo Complex Bearing an Amido N5 Ligand

AbstractHerein we report the electrochemical generation of a mononuclear MnIII(OO) (peroxo) complex supported on a dpaq ligand (dpaq=2‐(bis(pyridin‐2‐ylmethyl)amino)‐N‐(quinolin‐8‐yl)acetamide) for the first time, and its reactivity in N,N‐dimethylformamide. The formation of the MnIII(dpaq)(OO) complex is probed by low temperature electronic absorption spectro‐electrochemistry experiments. An analysis of the reduction of the MnIII(dpaq)(OO) complex is carried out combining cyclic voltammetry and simulations. The involvement of a MnII(dpaq)(OOH) complex is proposed based on CV data and is corroborated by DFT computations.

Fluorescent Ligand-Based Discovery of Small-Molecule Sulfonamide Agonists for GPR120.

As a critical member of G protein-coupled receptors (GPCRs), G protein-coupled receptor 120 (GPR120) is a potential target for many physiological diseases, such as type 2 diabetes mellitus, inflammation, and obesity. Considering that small-molecule fluorescent ligands can combine the advantages of visualization, high sensitivity and selectivity, we initially undertook an effort to develop a series of fluorescent ligands to track GPR120 and establish a method to screen GPR120 agonists. The representative fluorescent ligand N1 possesses suitable optical property, equitable biological activity, and high fluorescence imaging feasibility, therefore, based on compound N1, we subsequently founded a bioluminescence resonance energy transfer (BRET) competition binding assay to screen three series of sulfonamide GPR120 agonists we developed herein. The activity evaluation results revealed that compound D5 was a potent GPR120 agonist with high activity and selectivity. Moreover, compound D5 exhibited a significant glucose-lowering effect in db/db mice, which indicates its potential application in the treatment of type 2 diabetes mellitus in vivo. It is anticipated that our fluorescent ligand-based method is a useful toolbox and will find broad applications in the discovery of small-molecule agonists for GPR120.