A Germylene Supported by Two 2-Pyrrolylphosphane Groups as Precursor to PGeP Pincer Square-Planar Group 10 Metal(II) and T-Shaped Gold(I) Complexes

An efficient synthesis of 2-di-tert-butylphosphanylmethylpyrrole (HpyrmPtBu₂), by treating 2-dimethylaminomethylpyrrole (HpyrmNMe₂) with tBu₂PH at 135 °C in the absence of any solvent, has allowed the preparation of the new PGeP germylene Ge(pyrmPtBu₂)₂ ( 1 ), by treating [GeCl₂(dioxane)] with LipyrmPtBu₂, in which the Ge atom is stabilized by intramolecular interactions with one (solid state) or both (solution) of its phosphane groups. Reactions of germylene 1 with Group 10 metal dichlorido complexes containing easily displaceable ligands have led to [MCl{κ³P,Ge,P-GeCl(pyrmPtBu₂)₂}] [M=Ni ( 2 ), Pd ( 3 ), Pt ( 4 )], which have an unflawed square-planar metal environment. Treatment of germylene 1 with [AuCl(tht)] (tht=tetrahydrothiophene) rendered [Au{κ³P,Ge,P-GeCl(pyrmPtBu₂)₂}] ( 5 ), which is a rare case of a T-shaped gold(I) complex. The hydrolysis of 5 gave the linear gold(I) derivative [Au(κP-HpyrmPtBu₂)₂]Cl ( 6 ). Complexes 2 – 5 contain a PGeP pincer chloridogermyl ligand that arises from the insertion of the Ge atom of germylene 1 into a M−Cl bond of the corresponding metal reagent. The bonding in these molecules has been studied by DFT/NBO/QTAIM calculations. These results demonstrate that the great flexibility of germylene 1 makes it a better precursor to PGeP pincer complexes than the previously known germylenes of this type.     

Impact of C=C/B−N Replacement on the Diels–Alder Reactivity of Curved Polycyclic Aromatic Hydrocarbons

The influence of the replacement of C=C bonds by isoelectronic B−N moieties on the reactivity of π-curved polycyclic aromatic hydrocarbons has been computationally explored by means of density functional theory calculations. To this end, we selected the Diels–Alder cycloaddition reactions of the parent corannulene and its BN-doped counterparts with either cyclopentadiene or maleic anhydride. In addition, the analogous reactions involving larger buckybowls, such as BN-hemifullerene, BN-circumtrindene, and BN-fullerene, have been also considered. It has been found that whereas corannulene behaves as a dienophile, its BN counterpart better acts as a diene. In contrast, the larger BN-curved systems cannot be used as dienes in Diels–Alder reactions, but undergo facile (i.e., low barrier) cycloaddition reactions with cyclopentadiene. The observed trends in reactivity, which cannot be directly explained by using typical frontier molecular orbital arguments, are quantitatively described in detail by means of state-of-the-art computational methods, namely the activation strain model of reactivity combined with the energy decomposition analysis method. The results of our calculations highlight the crucial role of the curvature of the system on the reactivity and its influence on the strength of the orbital interactions between the deformed reactants during their transformations.     

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.     

Conformational space of tetrakis(2-naphthalenyl)ethene: a naphthologous AIE luminogen

The AIE luminogen tetrakis(2-naphthalenyl)ethene (2-NA ₄ E) was synthesized by Barton’s double extrusion diazo-thione coupling method from 2,2′-dinaphthyl thioketone and 2,2′-(diazomethylene)bisnaphthylene in 77 % yield. The structure of 2-NA ₄ E was confirmed by its ¹H NMR and ¹³C NMR spectra with full assignments. 2-NA ₄ E and its parent tetraphenylethene (Ph ₄ E) have been subjected to a comprehensive computational DFT study, in search of their conformational spaces. Seven conformers and two transition states of 2-NA ₄ E have been located. Four conformers and one transition state of Ph ₄ E have been located. The conformers of 2-NA ₄ E and Ph ₄ E are not overcrowded, as indicated by the contact distances in the fjord and cove regions. The relative free energies (ΔG ₂₉₈) of the six most stable conformers of 2-NA ₄ E are in the narrow range of 2.3 kJ/mol; they make comparable contributions (12–29 %) to the equilibrium mixture. The energy barriers for the diastereomerization D ₂-Z,Z,Z,Z $ \rightleftharpoons $ ? D ₂-E,E,E,E via the transition state C ₁-Z,E,E,Z and for the enantiomerization C ₂-Z,Z,E,E $ \rightleftharpoons $ ? C ₂-E,E,Z,Z via the transition state C _i -Z,E,Z,E are only 29.8 and 29.0 kJ/mol, respectively, indicating very rapid rates of diastereomerization and enantiomerization at room temperature. The values of naphthalenyl torsion angles and ethenic twist angles in 2-NA ₄ E are almost identical to those in the parent Ph ₄ E. The previously proposed “bulkiness” of the naphthalenyl substituents and the validity of the restriction of naphthalenyl rotation are challenged. The analysis of the AIE effect in 2-NA ₄ E should take into account the intermolecular homochiral and heterochiral interactions between the conformers.     

A three‐dimensional coordination polymer of 3‐(3,5‐dicarboxybenzyloxy)benzoic acid with zinc

The synthesis is reported of the tricarboxylic acid 3‐(3,5‐dicarboxybenzyloxy)benzoic acid (H₃L) and the product of its reaction under solvothermal conditions with Zn^II cations, namely poly[[μ₆‐3‐(3,5‐dicarboxylatobenzyloxy)benzoato](dimethylformamide)‐μ₃‐hydroxido‐dizinc(II)], [Zn₂(C₁₆H₉O₇)(OH)(C₃H₇NO)]_n, the formation of which is associated with complete deprotonation of H₃L. Its crystal structure consists of a single‐framework coordination polymer of the organic L³⁻ ligand with Zn^II cations in a 1:2 ratio, with additional hydroxide and dimethylformamide (DMF) ligands coordinated to the Zn^II centres. The Zn^II cations are characterized by coordination numbers of 5 and 6, being bridged to each other by hydroxide ligands. In the polymeric framework, the carboxylate‐ and hydroxy‐bridged Zn^II cations are arranged in coordination‐tessellated columns, which propagate along the a axis of the crystal structure, and each L³⁻ ligand links to seven different Zn^II centres via Zn—O bonds of two different columns. The coordination framework, composed of [Zn₂(L)(OH)(DMF)]_n units, forms an open architecture, the channel voids within it being filled by the zinc‐coordinating DMF ligands. This report provides the first structural evidence for the formation of coordination polymers with H₃L via multiple metal–ligand bonds through its carboxylate groups.<!?tpb=21.5pt>     

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...