Hydro­gen‐bonded supramolecular lattice of the 1:3:4 complex between [5,10,15,20‐meso‐tetrakis(4‐hydroxy­phenyl)­porphyrinato‐κ4N]­zinc(II), dibenzo‐24‐crown‐8 and methanol

The title compound, [5,10,15,20‐meso‐tetrakis(4‐hydroxy­phenyl)­porphyrinato‐κ⁴N]­zinc(II) tris(dibenzo‐24‐crown‐8) methanol tetrasolvate, [Zn(C₄₄H₂₈N₄O₄)]·3C₂₄H₃₂O₈·4CH₄O, was synthesized and its molecular structure precisely charac­terized by low‐temperature single‐crystal analysis. All the components are involved in hydrogen bonding with each other, thus forming an extensively hydrogen‐bonded supramolecular lattice. The functionalized porphyrin moiety coordinates both equatorially and axially to the neighboring species.     

Isolable acetylene complexes of copper and silver

Copper and silver play important roles in acetylene transformations but isolable molecules with acetylene bonded to Cu(i) and Ag(i) ions are scarce. This report describes the stabilization of π-acetylene complexes of such metal ions supported by fluorinated and non-fluorinated, pyrazole-based chelators. These Cu(i) and Ag(i) complexes were formed readily in solutions under an atmosphere of excess acetylene and the appropriate ligand supported metal precursor, and could be isolated as crystalline solids, enabling complete characterization using multiple tools including X-ray crystallography. Molecules that display κ²-or κ³-ligand coordination modes and trigonal planar or tetrahedral metal centers have been observed. Different trends in coordination shifts of the acetylenic carbon resonance were revealed by ¹³C NMR spectroscopy for the Cu(i) and Ag(i) complexes. The reduction in acetylene _{C C} due to metal ion coordination is relatively large for copper adducts. Computational tools were also used to quantitatively understand in detail the bonding situation in these species. It is found that the interaction between the transition metal fragment and the acetylene ligand is significantly stronger in the copper complexes, which is consistent with the experimental findings. The C C distance of these copper and silver acetylene complexes resulting from routine X-ray models suffers due to incomplete deconvolution of thermal smearing and anisotropy of the electron density in acetylene, and is shorter than expected. A method to estimate the C C distance of these metal complexes based on their experimental _{C C} is also presented.

Gaseous acetylene can be trapped on copper(i) and silver(i) sites supported by pyrazole-based scorpionates to produce isolable molecules for detailed investigations and the study of metal-acetylene bonding.     

Why Do Cycloaddition Reactions Involving C60 Prefer [6,6] over [5,6] Bonds?

The origin of the experimentally known preference for [6,6] over [5,6] bonds in cycloaddition reactions involving C₆₀ has been computationally explored. To this end, the Diels–Alder reaction between cyclopentadiene and C₆₀ has been analysed by means of the recently introduced activation strain model of reactivity in combination with the energy decomposition analysis method. Other issues, such as the aromaticity of the corresponding transition states, have also been considered. These results indicate that the major factor controlling the observed regioselectivity is the more stabilising interaction between the deformed reactants in the [6,6] reaction pathway along the entire reaction coordinate.     

Characterization of Polymers by Direct Pyrolysis/Chemical Ionization Mass Spectrometry

The characterization of polymers by pyrolysis directly in the ion source of a double focusing magnetic sector mass spectrometer, operating in the chemical ionization mode, is described. Pyrolysis is achieved by two different probe techniques. A low temperature, slow heating rate direct insertion probe (DIP) is used at 400°C, and a specifically constructed high temperature, fast heating rate, high temperature pyrolysis (HTP) probe is used at 1000°C. This probe is capable of achieving pyrolysis temperatures of 1200°C at controlled heating rates up to 20,000°C/s. The mass spectrometric analysis of the pyrolysis products was achieved under chemical ionization (CI) conditions utilizing methane, isobutane, and ammonia as reagent gases. Under CI conditions the molecular ions formed in the mass spectrometer show little tendency to fragment. The CI mass pyrograms are very simple, with each peak in the spectra ascribable to a particular component in the pyrolysis product mixture. The results of the two probe pyrolysis techniques are compared and the utility of each technique for the characterization of polymers is demonstrated using the vinyl polymers polymethyl methacrylate, polyvinyl chloride, and polystyrene.     

Oxidation of Ascorbate by Ni(III) Complexes with Tetraaza-macrocyclic Ligands in Neutral Aqueous Solutions. A Pulse-Radiolysis Study

Abstract

The mechanisms and kinetics of oxidation of ascorbate, AH^?, by Ni(III)Lⁱ _aq and by LⁱNi(III) (HPO₄)₂ ^? complexes (L¹ = meso-(5,12)-7,7,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; L² = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated.

The oxidation of ascorbate by the LⁱNi(III) (HPO₄)₂ ^? and Ni(III)L¹ _aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A^.? radical anion and the corresponding Ni(II) complex. The oxidations by the LⁱNi(III)(HPO₄)₂ ^? complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L¹ _aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LⁱNi(III)(HPO₄)(H₂O)⁺ and LⁱNi(III)OH²⁺ are stronger oxidizing agents than LⁱNi(III)(HPO₄)^? ₂.

The oxidation of ascorbate by Ni(III)L² _aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L²(H₂O)Ni(II)(A^.?)²⁺, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A^.? anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance.     

How Strain Affects the Reactivity of Surface Metal Oxide Catalysts

Highly dispersed molybdenum oxide supported on mesoporous silica SBA‐15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2–2.5 Mo atoms nm^?2). X‐ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO₄ units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature‐programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O‐K‐edge at high Mo loadings are explained by distorted MoO₄ complexes. Limited availability of anchor silanol groups at high loadings forces the MoO₄ groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.     

Contactless conductivity detector array for capillary electrophoresis

A CE system featuring an array of 16 contactless conductivity detectors was constructed. The detectors were arranged along 70 cm length of a capillary with 100 cm total length and allow the monitoring of separation processes. As the detectors cannot be accommodated on a conventional commercial instrument, a purpose built set‐up employing a sequential injection manifold had to be employed for automation of the fluid handling. Conductivity measurements can be considered universal for electrophoresis and thus any changes in ionic composition can be monitored. The progress of the separation of Na⁺ and K⁺ is demonstrated. The potential of the system to the study of processes in CZE is shown in two examples. The first demonstrates the differences in the developments of peaks originating from a sample plug with a purely aqueous background to that of a plug containing the analyte ions in the buffer. The second example visualizes the opposite migration of cations and anions from a sample plug that had been placed in the middle of the capillary.     

Synthesis and Antifungal Activity of New Valine-Azole Hybrids

Russian Journal of General Chemistry - Herein, the synthesis of six new derivatives via molecular hybridization of imidazole or triazole and valine methyl ester as a rational design for accessing...