N4 Complexes Research Articles

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.

Axial coordination tuning Fe single-atom catalysts for boosting H2O2 activation

Precisely regulating the coordination microenvironment of single-atom catalysts (SACs) to achieve enhanced reactivity is significant and desired but still in its infancy. Herein, a coordination-tuned and pyrolysis-free strategy is reported for the fabrication of a Fenton-like SAC containing the axial five-coordinated configuration (Fe–N5). The N species on the N-doped graphene act as anchoring points for iron phthalocyanine (a typical Fe–N4 complex) to obtain isolated Fe–N5 sites, which significantly modulates the electronic state of Fe atoms and lowers the H2O2 activation barrier for •OH production. Moreover, the enriched pyridinic N serve as contaminant adsorption sites shortening •OH diffusion distance, establishing a dual-site reaction mechanism with Fe–N5 sites. As such, the Fe–N5 catalyst exhibits exceptional Fenton activity towards catalytic oxidation of phenol (k = 0.180 min−1). Our work unravels the dependence of Fenton activity on the single-atom coordination environment and provides a platform for precise engineering of SACs.

Large specific surface area S-doped Fe–N–C electrocatalysts derived from Metal–Organic frameworks for oxygen reduction reaction

It is highly desired but challenging to develop platinum group metal-free electrocatalysts for oxygen reduction reaction (ORR), which can promote the commercialization of fuel cell technology. To achieve this target, we report a one-step doping method to prepare S-doped Fe–N–C catalysts using zeolite imidazole framework (ZIF-8) and iron (III) thiocyanate (Fe(SCN)3) as precursor. Different from conventional doping approach, i.e. physical mixing, Fe(SCN)3 is in-situ added during ZIF-8 formation which would encapsulate Fe(SCN)3 molecules inside ZIF-8 to avoid structure destruction and create potential replacement of Zn ions by Fe ions to form uniform Fe–N4 complexes. As a result, the prepared S-doped Fe–N–C catalysts own large specific surface areas with a maximum value of 1326 ​m2 ​g−1 and a dual-scale porous structure that benefits mass transport. Significantly, the composition-optimized catalyst exhibits superior ORR activity in both 0.1 ​M HClO4 electrolyte and 0.1 ​M KOH electrolyte, in which the half-wave potential reaches 0.81 ​V and 0.92 ​V (vs. RHE), respectively. Remarkable stability is also attained, which loses 2 ​mV only after 10000 potential cycles in O2-saturated 0.1 ​M HClO4 and remains almost constant in O2-saturated 0.1 ​M KOH, surpassing commercial Pt/C catalyst in both acidic and alkaline medium.

Chiral Cu(II)-N4 Complex Catalyzed Asymmetric Aza-Henry Reaction and Its Application in the Synthesis of β -Diamine

Chiral Cu(II)-N₄ Complex Catalyzed Asymmetric Aza-Henry Reaction and Its Application in the Synthesis of <i>β</i> -Diamine

Spectral characterization and in vitro microbiological activity of new bis-1,8-naphthalimides and their Cu(II) complexes

Two novel bis-1,8-naphthalimide derivatives with different substituents at C-4 position, termed L1 and L2, were synthesized and their photophysical properties in organic solvents with different polarity were investigated. Their Cu(II) complexes were also synthesized to be used as antimicrobial agents. The structural properties of [Cu(L1) (NO3)2] and [Cu(L2) (NO3)2] complexes were studied by nuclear magnetic resonance (NMR) and Fourier transform – infrared (FT-IR) spectroscopies. Cu(II) – ligands complexation at different Cu(II)/ligand molar ratios was studied by means of UV–Vis, fluorescence and electron paramagnetic resonance (EPR) spectroscopies. The results indicate the formation of Cu–N4 and Cu–N2O2 coordinations with different structure and stability conditions for the L1 and L2 derivatives. A different solvent at different polarity also affects the structural properties and the range of stability. L1 provided more stable Cu–N4 complexes than L2. [Cu(L1) (NO3)2] also showed stronger antibacterial and antifungal abilities than those of [Cu(L2) (NO3)2]. These results indicate that [Cu(L1) (NO3)2] works as antimicrobial agent to be used in biomedical and agrochemical applications.

Atomic Mechanism of Electrocatalytically Active Co-N Complexes in Graphene Basal Plane for Oxygen Reduction Reaction.

Superior catalytic activity and high chemical stability of inexpensive electrocatalysts for the oxygen reduction reaction (ORR) are crucial to the large-scale practical application of fuel cells. The nonprecious metal/N modified graphene electrocatalysts are regarded as one of potential candidates, and the further enhancement of their catalytic activity depends on improving active reaction sites at not only graphene edges but also its basal plane. Herein, the ORR mechanism and reaction pathways of Co-N co-doping onto the graphene basal plane have been studied by using first-principles calculations and ab initio molecular dynamics simulations. Compared to singly N-doped and Co-doped graphenes, the Co-N co-doped graphene surface exhibits superior ORR activity and the selectivity toward a four-electron reduction pathway. The result originates from catalytic sites of the graphene surface being modified by the hybridization between Co 3d states and N 2p states, resulting in the catalyst with a moderate binding ability to oxygenated intermediates. Hence, introducing the Co-N4 complex onto the graphene basal plane facilitates the activation of O2 dissociation and the desorption of H2O during the ORR, which is responsible for the electrocatalyst with a smaller ORR overpotential (∼1.0 eV) that is lower than that of Co-doped graphene by 0.93 eV. Our results suggest that the Co-N co-doped graphene is able to compete against platinum-based electrocatalysts, and the greater efficient electrocatalysts can be realized by carefully optimizing the coupling between transition metal and nonmetallic dopants in the graphene basal plane.

Synthesis of imidazole-terminated hyperbranched polymers with POSS-branching points and their pH responsive and coordination properties

An imidazole-terminated hyperbranched polymer with octafunctional POSS branching units denoted as POSS-HYPAM-Im was prepared by the polymerization of excess amounts of tris(2-aminoethyl)amine with the first-generation methyl ester-terminated POSS-core poly(amidoamine)-typed dendrimer, reacting with methyl acrylate, and ester-amide exchange reaction with 3-aminopropylimidazole. The imidazole-terminated hyperbranched poly(amidoamine) denoted as HYPAM-Im was also synthesized with 1-(3-aminopropyl)imidazole from a methyl ester-terminated hyperbranched poly(amidoamine) by the ester-amide exchange reaction. The transmittance of the POSS-HYPAM-Im solution drastically decreased when the solution pH was greater than 8.2. On the other hand, the transmittance of the HYPAM-Im solution gradually decreased when the solution pH at 8.5 and was greater than 9. Spectrophotometric titrations of the hyperbranched polymer aqueous solutions with Cu2+ ions indicated the variation of the coordination modes of POSS-HYPAM-Im from the Cu2+–N4 complex to the Cu2+–N2O2 complex and the existence of the only one complexation mode of Cu2+–N4 between Cu2+ ion and HYPAM-Im with increasing the concentrations. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2695–2701

Synthesis of first- and second-generation imidazole-terminated POSS-core dendrimers and their pH responsive and coordination properties

Imidazole-terminated first- and second-generation poly(amidoamine)-type oligomeric silsesquioxanes (POSS)-core dendrimers (denoted as POSS-Im16 and POSS-Im32, respectively) were synthesized by the ester–amide exchange reaction from first- and second-generation methyl ester-terminated POSS-core dendrimers, respectively. Transmittances of the aqueous POSS-Im32 solution drastically decreased at pH>7.2, but the change was reversible without hysteresis. The results of pH titrations for POSS-Im16 and POSS-Im32 showed well-defined one-step titration curves identical to those of monomeric imidazole compounds, such as 1-methylimidazole. Such simple acid–base behavior caused phase transition in the aqueous dendrimer solution to be highly sensitive to pH changes. The pKa values of 1-methylimidazole, POSS-Im16 and POSS-Im32 were 7.3, 7.0 and 6.7, respectively. Spectrophotometric titrations of the aqueous dendrimer solutions with Cu2+ ions indicated that coordination modes of POSS-Im16 changed from the Cu2+–N2O2 complex to the Cu2+–N4 complex as the concentration of the dendrimers increased; however, only one complexation mode (Cu2+–N4) existed between Cu2+ ions and POSS-Im32.

“A fighter is a lion”. Neuropsychological indexes in comprehending frozen metaphors

This paper considers the cognitive and linguistic processes underlying the comprehension of frozen metaphors, by analyzing both neuropsychological and behavioral data. In order to explore the dichotomy between the pragmatic models (i.e. the metaphor as a semantic anomaly) and the direct comprehension view (the metaphor as a “standard” meaning) on the one hand, and between an iconic (more imagery-based) vs. not iconic (linguistic) representational modality on the other hand, the paper analyzed the event-related potentials (ERPs) and the behavioral data (response time) of subjects submitted to verbal stimuli. 36 participants were required to comprehend metaphoric and literal sentences, half congruous (semantically not anomalous) and half uncongruous (semantically anomalous). The morphological analysis of the electroencephalographic (EEG) profile allowed to observe two negative deflections (the N3–N4 complex), differently distributed on the scalp surface. Repeated measures analysis of variance (ANOVA) applied to peak amplitude and latency variables underlined the different role of these deflections in metaphoric decoding. That is, the N4 effect appears to detect the presence of a semantic anomaly of the utterance (uncongruous condition) more localized on the posterior (Pz) area, but not sensitive to the metaphoric/literal content of the stimulus. On the contrary, N3 effect shows an ampler negativity for the metaphoric decoding, increased on the occipital area, likely a marker of a specific iconic representational format. Thus, though not equivalent, the two comprehension processes are not differentiated as a function of the way of the meaning access (direct or indirect), nor for their cognitive complexity (no response time variation) but for the typology of the representational format, more imagery-based for metaphoric stimuli, as it was showed by an increased N3 on the occipital visual area.

THIP, a Selective Gamma-Aminobutyric Acid Receptor Agonist, Alters Flash-Evoked Potentials in Rats

This study examined the effects of the GABA A agonist THIP on flash-evoked potentials (FEPs) recorded from the primary visual cortex (VC) and superior colliculus (SC) of chronically implanted hooded rats. Animals were given IP injections of saline, and of 8, 16, and 24 mg THIP/kg body weight on separate days. Evoked potentials were recorded at 5, 20, and 35 min following injection. Animals were tested at a standard (22.5°C) room temperature. Most significant effects were observed at the 20- and 35-min recording intervals for both the 16 and 24 mg/kg doses, with effects at the 24 mg/kg dose the most pronounced. VC P1 amplitude remained unchanged, while N1 was reduced to such an extent that it became positive, ultimately blending into the rising phase of a positive component appearing between N1 and P2. This positive component had a latency of about 6 ms longer than N1, and became larger in amplitude than P1 at the 24 mg/kg dose. P2 amplitude was drastically reduced, becoming negative. In contrast, components N2 and P3 were augmented, while the amplitude of N3 was unchanged. In the SC, P1 was augmented while P3 was reduced in amplitude. A biphasic (increase/decrease) effect was observed in the N4 complex. In both the VC and SC, latencies of most components were increased, with the late components in the VC increased to the greatest extent. A mild hypothermia was observed at 16 and 24 mg/kg. The results suggest that the GABA A receptor plays an important role in the elaboration of the middle (N1–P2) components of FEPs recorded from the rat VC, and that GABAergic mechanisms can influence other components in the VC and SC as well.

Heteronuclear μ-1- methylcytosinato- N3, N4 complexes containing very short Pt → Cu dative bonds

A series of mixed Pt 11,Cu 11 complexes derived from trans-[ a 2 Pt(1- MeC- N3) 2] 2+ (a = NH 3 or CH 3NH − ( 1,2), OH − ( 3), NH 3,H 2O ( 4), coordinated to Pt via N3) and of general formula trans-[ a 2 Pt(1- MeC −- N3, N4) 2 CuL] n+ with L = μ-CO 3 2− ( 1,2), OH − ( 3), NH 3,NH 3H 2O ( 4) , 1-MeC-N3 ( 5), 9-EtGH-N7,H 2O ( 6) have been isolated and characterized by X-ray analysis. In all compounds a trans- a 2 Pt II entity is bound to two cytosine nucleobases via N3 sites each, whereas a Cu II is bound to two monodeprotonated N4 sites. In all cases Pt II represents a ligand of Cu II, which either displays a distorted square-planar ( 1,2,3,5) or a pentacoordinate environment ( 4,6). Pt IICu II distances are very short, ranging from 2.49 to 2.56 Å in the six compounds. EPR spectra of frozen solutions have been examined in several cases. They appear, at least for 4a and 5, to be consistent with PtCu hyperfine interactions rather than the presence of two different Cu-only species.

Characterization of starburst dendrimers by the EPR technique. 1. Copper complexes in water solution

The structure of Cu(I1) complexes formed with anionic starburst dendrimers (n.5 G-SBD) in aqueous solution has been investigated by the electron paramagnetic resonance (EPR) technique. The line shapes of the EPR spectra of the complexes at room temperature show a distinction between earlier (n < 3) and later (n 1 3) generations and are consistent with a change of the dendrimer shape, which supports the results of molecular simulation of the dendrimer morphology as a function of generation. The earlier generations appear to possess a more open structure, which leads to a greater mobility of the copper complexes. Three different complexes of copper with groups composing the dendrimer structure are identified by analyzing the spectra as a function of the dendrimer size (generation), the pH, and the temperature. The magnetic parameters, evaluated at low temperature with the aid of spectral computation, indicate that the copper ions form monomeric carboxylate complexes at low pH (signal C). With an increase of pH, the ions interact with nitrogen centers in the internal porous structure of the dendrimers. The complex formed at intermediate pH is identified as a Cu(II)-N202 complex (signal A). Such a complex, which involves both the carboxylic groups at the dendrimer interface and the internal nitrogen centers, is preferentially formed by Iow-generation dendrimers. This result is consistent with the morphology of the dendrimer structure. The higher generation dendrimers present a wide number of internal sites in which the tightly packed structure appears to facilitate the interaction with more than two nitrogens. This Cu(II)-N30 or Cu(I1)-N4 complex gives a third signal (termed signal B), which increases its intensity at the expense of signal A, both with the increase of generation and with the increase of pH. However, the interaction with nitrogen centers in both cases is not strong enough to give superhyperfine structure in the EPR spectra. For freshly prepared samples, formation of the complex with the larger nitrogen coordination corresponds to enhancement in the EPR room temperature spectra of a signal of a nitrogen-centered radical species, which is also observed in the spectra of pure dendrimers. The spectral features of this radical are identified, by spectral computation, as resulting from an unpaired electron which couples with one nitrogen and four protons from two slightly nonequivalent CHz groups. Aging of the samples leads to an increase in intensity of signal B, simultaneously with the disappearance of the radical signal. This allows the identification of one of the coordinating sites in the internal dendrimer structure. Heating of the 6.5 and 7.5 G-SBD at high pH causes decomposition of the dendrimers, whereas lower generation dendrimers show good thermal stability. The evaluation of the Cu(I1) bonding parameters, a2, d2, and 8lz, indicates substantial covalency of the in-plane bonds, with the covalent character increasing respectively for the species corresponding to signal C, signal A, and signal B.

ChemInform Abstract: The Crystal Structure and Electronic Properties of the Bis(1,3‐dimethyl‐5‐diazophenyl‐6‐amino uracilato) Copper(II) Complex, Dimethyl Sulfoxide Solvate ‐ A New Pseudotetrahedral CuN4 Complex.

Abstract The crystal structure of the title complex has been determined by X-ray diffraction technique. The crystals are monoclinic, space group P21/n, with a = 12.878(2), b= 14.791(1), c = 17.268(2) A, β = 109.79(1)°, Z = 4, R = 0.057 for 2514 reflections. There are two ligands of 1,3-dimethyl-5-diazophenyl- 6-amino uracilato anions in bidentate coordination to a copper(II) ion per asymmetric unit. The ligands are in cis-arrangement to each other. The copper environment is pseudotetrahedral, with a dihedral angle φ of 39.7° between the two CuN4 ligand planes. This geometry seems to result from steric interaction between the ligands. The coordination of each molecule of the ligand takes place through the nitrogen atom of the deprotonated 6-amino group and a nitrogen of the azo group. The structure is stabilised by extensive stacking interactions between the phenyl rings as well as the pyrimidine rings. A pair of hydrogen-bonded interactions between the solvent molecule and the two ligands further stabilise the structure. The magnetic moment, ESR and electronic reflectance spectrum are in accordance with the observed molecular structure. A linear correlation between the highest energy d—d transition and the dihedral angle for several pseudotetrahedral N4 complexes has been proposed.

Interaction of (dien)Pd(II) complexes with the amino group of cytidine: a kinetic and NMR study

Interaction between cytidine and (dien)Pd(II) has been studied at high pH by spectrophotometry and by 1H, 13C and 15N NMR. With increasing pH interaction of (dien)Pd(II) with the exocyclic NH 2 group is observed, and this (dien)Pd-N4 complex is more stable at high pH than the (dien)Pd-N3 complex, which exists at neutral pH. Furthermore, at the higher concentrations used in the NMR spectroscopy (0.1-0.2 M) a binuclear complex implying both the N3 and N4 (exocyclic amino) sites is present, pH- jump experiments revealed two kinetically distinct reactions. The faster ‘Reaction I’ is catalysed by OH − and is attributed to the formation of the (dien)Pd- N4 complex. This reaction proceeds by a mechanism in which fixation of Pd(II) at the N3 site is required for the deprotonation of the NH 2 group by OH −. This process is followed by an intramolecular transfer of (dien)Pd(II) from N3 to the deprotonated N4. The slower ‘Reaction II’ is attributed to dissociation of both Pd-N3 and Pd-N4 complexes at high pH and leads to the thermodynamically stable (dien)Pd- (OH) +. Kinetic data for this process are in good agreement with those observed for the formation of (dien)Pd-N3.

The Reduction of Cobalt(III) Complexes by Ascorbic Acid. II. The Kinetics and Mechanisms of the Reactions of Diaqua and Aquahydroxo Macrocyclic N4 Complexes

Abstract The kinetics of the reductions of several macrocyclic N4 cobalt(III) complexes by l-ascorbic acid (H2A, have been studied at 25.0 °C, at an ionic strength of 0.10 mol dm−3 (LiClO4), and over the pH range from 1.64 to 3.47. The second-order rate constants in dm3 mol−1 s−1 are 3.4±0.6 (Me6[14]4,11-dieneN4), (4.2±0.2)×10 (Me4[14]tetraeneN4), and (4.2±0.1)×10 (ms-Me6[14]aneN4) for the reactions of [Co(N4)(H2O)2]3+ with HA− ions and (1.1±0.1)×102 (Me6[14]4,11-dieneN4), (1.3±0.1)×103 (Me4[14]tetraeneN4), (5.5±0.1)×10 (ms-Me6[14]aneN4), and (2.9±0.6)×10 ([14]aneN4) for the reactions of [Co(OH)(N4)(H2O)]2+ with HA− ions. The mechanisms of these reactions are discussed, and it is concluded that the lack of saturation of macrocyclic ligands facilitates the electron-transfer reaction between cobalt(III) and ascorbic acid.