High-valent oxo-molybdenum and oxo-rhenium complexes as efficient catalysts for X-H (X = Si, B, P and H) bond activation and for organic reductions

High-valent oxo-complexes have recently emerged as powerful catalysts for the activation of X-H (X = Si, B, P and H) bonds and for the reduction of several functional groups. This new reactivity represents a complete reversal from the traditional role of these complexes as oxidation catalysts and opened a new research area for high-valent oxo-complexes. This tutorial review highlights the work developed using high-valent oxo-molybdenum and oxo-rhenium complexes as excellent catalysts for X-H (X = Si, B, P and H) bond activation and for organic reductions.     

Fluorescence study of tetracaine–cyclodextrin inclusion complexes

The steady-state fluorescence emission from the local anaesthetic tetracaine (TCA) in water–solvent mixtures and in the presence of α-, β- and γ-cyclodextrin (CD) was investigated at various pH values. Emission was observed from the locally and the intramolecular charge transfer excited states. The TCA–CD system was found to be characterised by 1:1 associate in every case. The association constants of each complex were determined.     

Coexistence of small and large amplitude limit cycles of polynomial differential systems of degree four

A class of degree four differential systems that have an invariant conic x ² + Cy ² = 1, C ∈ ℝ, is examined. We show the coexistence of small amplitude limit cycles, large amplitude limit cycles, and invariant algebraic curves under perturbations of the coefficients of the systems.     

Extraction and spectrophotometric determination of niobium by tetraphenylarsonium and phosphonium chloride from a hydrochloric acid solution

The extraction of niobium(V) in the form of a chloro complex has been studied. Radiometrical and spectrophotometrical studies show that niobium(V) is extracted practically completely from a solution containing more than 9 mol dm^?3 chloride in the range of 2–5 M hydrogen ion concentration by chloroform solutions of tetraphenylarsonium (TPA) and tetraphenylphosphonium (TPP) chloride and that niobium is not extracted at chloride concentrations less than 6 mol dm^?3. The mechanism of extraction is based on the formation of the ion-associated compounds that form between the onium cation and the oxotetra-chloroniobate(V) anion. The extracted complexes in chloroform have a maximum absorbance at 282 nm (TPA) and 285 nm (TPP); they obey Beer's law in the range of 1–10 μg Nb ml^?1, and are stable for at least 24 hr. The molar absorptivity of the method is 1.33 × 10⁴ dm³ mol^?1cm^?1. The composition of the extracted species [(C₆H₅)₄X] [NbOCl₄] where X = As or P was determined spectrophotometrically, radiometrically, and by characterization of the crystalline compounds isolated.     

Study of the effects of thermal treatment on chemical and physical characteristics of phosphates

Via the thermal treatment of natural phosphates and their analysis, it was proved that the decrease in their solubility in the interval 400–550°C is an indication of the degree of incorporation of OH groups into the apatite structure of phosphates, whereas the solubility at 950°C is an indication of the degree of incorporation of the non-volatile components. The higherR ₉₅₀, the more extensive this substitution, and the more reactive the natural phosphate.     

Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher-Order Cycloaddition Reactions

The structure of Fischer carbene complexes (FCCs) is electron deficient. If bearing an α,β-unsaturated system, it can generate a wide variety of compounds by undergoing many different transformations, including higher-order cycloadditions. The latter are described as pericyclic reactions in which more than six electrons participate. These reactions have been employed in various areas of organic synthesis, resulting in highly selective compounds with a broad range of scaffolds. The first studies on higher-order cycloadditions involving FCCs frequently yielded competing byproducts. Many groups have attempted to increase selectivity by exploring distinct reaction conditions, reagents and co-catalysts (e. g., metal-mediated cycloadditions). The present review is the first to focus exclusively on using higher-order cycloadditions involving FCCs to synthesize carbocycles and heterocycles. Based on two decades of reports, an analysis is made of the main aspects of the mechanisms proposed for higher-order cycloadditions and the structural diversity obtained by the substituent effect.     

Comparison of Phosphorescent Pt(II) Complexes with C^N^N vs N^N^N Chelators and Caffeine-Based NHC-co-ligands†

In this work, we explored coordination compounds featuring caffeine-based carbene co-ligands and tridentate dianionic pincer luminophores derived from 2,6-bis(1H-1,2,4-triazol-5-yl)pyridine (N), as well as from 2-phenyl-6-(1H-1,2,4-triazol-5-yl)pyridine (C), bearing either Ad (adamantyl) or tBu (tertiary butyl) substituents. The new 2-phenyl-6-(1H-1,2,4-triazol-5-yl)pyridine-based ligand precursors along with four Pt(II) complexes, namely Pt(C-tBu), Pt(C-Ad), Pt(N-tBu) and Pt(N-Ad) were characterized. Further on, the influence of the different substituents at the chelating luminophores and of the caffeine-based NHC-co-ligand on the photophysical properties (including photoluminescence quantum yields (Φ_L), excited-state lifetimes (τ), radiative (k_r), and non-radiative (k_nr) deactivation rate constants) was assessed in fluid solutions at room temperature (RT) and in frozen glassy matrices at 77 K. All four luminophores perform equivalently well within the experimental uncertainty. In deoxygenated fluid solutions at RT, photoluminescence quantum yields reaching up to 24 ± 2% and excited-state lifetimes of around 12 μs were found. The generally long excited-state lifetimes and only minor blue shift upon cooling to 77 K along with mostly well-resolved vibrational progressions point to metal-perturbed ligand-centered excited states. Notably, the yield of the complexation reaction in case of Pt(C-tBu) and Pt(C-Ad) was almost two times higher compared to Pt(N-tBu) and Pt(N-Ad). Cyclometallation is not an essential feature to achieve high photoluminescence quantum yields, but it can improve the synthetic efficiency. In summary, it can be observed that coordination chemical concepts based on natural products can lead to stable phosphorescent species with interesting excited-state properties.     

Carbon nanofiber paper cathode modification for higher performance of phosphoric acid fuel cells on polybenzimidazole membrane

Entire carbon nanofiber mats (carbon nanofiber paper) based on polyacrylonitrile pyropolymer composite were prepared by the preliminary oxidation (stabilization) of the initial polymer at 250–350°C in air and following pyrolysis at 800–1200°C under vacuum. The mats were tested as cathodes in a fuel cell on polybenzimidazole membrane. Properties of the pyropolymers which were obtained by polymer carbonization could be significantly changed by the addition of specific additives to polyacrylonitrile and also by changing thermal treatment. Particularly, the addition of Ketjen Black® or Vulcan® XC72 carbon blacks and polyvinyl pyrrolidone during electrospinning step resulted in increase of material electrical conductivity and inner porosity, which is important for improving fuel cell performance. Depending on oxidation and pyrolysis temperature, the physical properties of platinated carbon nanofiber paper and the efficiency of a fuel cell on polybenzimidazole membrane significantly change.