Mechanism,reactivity, and selectivity of the iridium-catalyzed C(sp3)–H borylation of chlorosilanes

The iridium-catalyzed C(sp³)–H borylation of methylchlorosilanes is investigated by means of density functional theory, using the B3LYP and M06 functionals. The calculations establish that the resting state of the catalyst is a seven-coordinate Ir(v) species that has to be converted into an Ir(iii)tris(boryl) complex in order to effect the oxidative addition of the C–H bond. This is then followed by a C–B reductive elimination to yield the borylated product, and the catalytic cycle is finally completed by the regeneration of the active catalyst over two facile steps. The two employed functionals give somewhat different conclusions concerning the nature of the rate-determining step, and whether reductive elimination occurs directly or after a prior isomerization of the Ir(v) hydride intermediate complex. The calculations reproduce quite well the experimentally-observed trends in the reactivities of substrates with different substituents. It is demonstrated that the reactivity can be correlated to the Ir–C bond dissociation energies of the corresponding Ir(v) hydride intermediates. The effect of the chlorosilyl group is identified to originate from the α-carbanion-stabilizing effect of the silicon, which is further reinforced by the presence of an electron-withdrawing chlorine substituent. Furthermore, the source of selectivity for the borylation of primary over secondary C(sp³)–H can be explained on a steric basis, by repulsion between the alkyl group and the Ir/ligand moiety. Finally, the difference in the reactivity between C(sp³)–H and C(sp²)–H borylation is investigated and rationalized in terms of distortion/interaction analysis.     

Leak testing of carbon–tin nanocomposites by thermal analysis methods

Electrode materials consisted of tin nanograins encapsulated in different origin carbon buffer matrix (starch or water soluble polymer) were obtained in a simple and inexpensive process. The tin precursor was synthesized using modified reverse nanoemulsion technique (w/o) and then coated by a source of carbon. The composites precursors were pyrolysed, affording formation of C/Sn anode materials. The resulting samples were investigated by powder X-ray diffraction studies in order to verify the structure and calculate crystallites sizes. The morphology of the nanocomposites was characterized by low-temperature nitrogen adsorption method (N₂-BET). Thermal analysis measurements (EGA-TG/DTG/DTA and DSC) allowed determining optimal conditions of preparation process and estimating carbon content in the obtained anode materials. Thermogravimetric studies also proved to be highly useful in establishing the leak behaviour of C/Sn nanocomposites. The electrochemical performance of the nanopowders was examined by charge–discharge tests in R2032-type coin cell. The thermal analysis results as well as low-temperature nitrogen adsorption data indicated that the origin of carbon precursor has major impact on morphology and leak behaviour of the obtained carbon buffer matrix. The electrochemical tests showed that better tightness of carbon–tin nanocomposites resulted in higher gravimetric capacity and better cell performance.     

Structure and thermal properties of the fritted glazes in SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–CaO–MgO–Na<Subscript>2</Subscript>O–K<Subscript>2</Subscript>O–ZnO system

In this work, the structure and thermal properties of aluminosilicate fritted glazes in SiO₂–Al₂O₃–CaO–MgO–Na₂O–K₂O–ZnO system with (4.0 mol%) and without addition of ZnO were examined by GIXRD, FTIR, MAS-NMR and thermal methods (DTA, DIL). It has been found that the all experimental glazes are amorphous material (transparent glazes). On the base of spectroscopic investigations, it was found that zinc ions exist in the network glazes in the octahedral coordination—Zn²⁺ ions play a network modifier role in structure of glazes. An analysis of the data obtained from thermal tests showed that addition of ZnO into chemical composition results in decrease in glass transition temperature value (T _g) for all glazes (DTA, DIL). The coefficient of thermal expansion (α) is decreased as the whole measurement range for one series of fritted glazes.     

Pointwise versus equal (quasi-normal) convergence via ideals

We prove a characterization showing when the ideal pointwise convergence does not imply the ideal equal (aka quasi-normal) convergence. The characterization is expressed in terms of a cardinal coefficient related to the bounding number b$\mathfrak{b}$. We also prove a characterization showing when the ideal equal limit is unique.     

Synthesis, structure and α-olefin polymerization activity of group 4 metal complexes with [OSSO]-type bis(phenolate) ligands

The tetradentate [OSSO]-type bis(phenol) ligands, [{2,2′-(HOC₆H₂-4,6-R₂)₂CH₂SCH₂CH₂SCH₂}] (R = ^tBu, 2; Br, 3) react with MBz₄ (M = Zr, Hf) to yield the corresponding dibenzyl complexes, [M{2,2′-(OC₆H₂-4,6-R₂)₂CH₂SCH₂CH₂SCH₂}Bz₂] (R = Br, M = Zr, 4^Br; Hf, 5^Br; R = ^tBu, M = Hf, 5) in a good to very good yield. Zirconium diamido complexes, [Zr{2,2′-(OC₆H₂-4,6-R₂)₂CH₂SCH₂CH₂SCH₂}(NMe₂)₂] (R = ^tBu, 6; Br, 6^Br) were prepared in a reaction of the corresponding disodium salt of 2 or 3 generated in situ with ZrCl₂(NMe₂)₂(THF)₂. Heating of 6 with TMSCl at 35 °C afforded zirconium dichloro complex, [Zr{2,2′-(OC₆H₂-4,6-^tBu₂)₂CH₂SCH₂CH₂SCH₂}Cl₂] (7), whereas the titanium analog 8 was prepared in a direct reaction with TiCl₄. While for complexes 4^Br, 5, 5^Br, 6, 6^Br and 7 single C₂-symmetric isomers were observed in solution at room temperature, as revealed by the NMR spectroscopic data, titanium complex 8 formed as a mixture of cis-α (8a) and cis-β (8b) isomers in a ratio of approx. 20:80% (measured in CD₂Cl₂). The VT NMR studies revealed a reversible conversion of 8a into 8b above 60 °C. The X-ray crystal structure determination of complexes 4^Br, 5^Br and 7 confirmed their C₂-symmetrical configuration in the solid state with cis-arranged benzyl/chloro groups and the trans-coordination of two bulky phenolato moieties. The zirconium dibenzyl complexes exhibit good catalytic activities in homopolymerization of 1-hexene (atactic poly(1-hexene), PDI = 1.5-1.7) and vinylcyclohexane (isotactic poly(vinylcyclohexane), PDI = 1.2-1.8) upon activation with a co-catalyst. In both polymerizations no increase of activity was observed for the complex 4^Br with electron-withdrawing substituents on phenolate rings. Moreover, polymerization of liquid propylene catalyzed by the titanium dichloro isomeric mixture 8 afforded at 5 °C ultrahigh molecular weight atactic/isotactic polypropylene mixtures.     

Lipophilicity of Tetraarylporphyrins. Part 1. Tetra-(Hydroxyphenyl)Porphyrins with Long Alkyl Chain in the Molecule

JPC – Journal of Planar Chromatography – Modern TLC - In the phototherapeutic use of photosensitizers, lipophilicity is an important parameter that allows predicting their biological...     

On Riesz Transforms Characterization of <Emphasis Type="Italic">H</Emphasis><Superscript>1</Superscript> Spaces Associated with Some Schrödinger Operators

Let \(\mathcal Lf(x)=-\Delta f (x)+V(x)f(x)\), V?≥?0, \(V\in L^1_{loc}(\mathbb R^d)\), be a non-negative self-adjoint Schrödinger operator on \(\mathbb R^d\). We say that an L ¹-function f is an element of the Hardy space \(H^1_{\mathcal L}\) if the maximal function

$ \mathcal M_{\mathcal L} f(x)=\sup\limits_{t>0}|e^{-t\mathcal L} f(x)| $

belongs to \(L^1(\mathbb R^d)\). We prove that under certain assumptions on V the space \(H^1_{\mathcal L}\) is also characterized by the Riesz transforms \(R_j=\frac{\partial}{\partial x_j}\mathcal L^{-1\slash 2}\), j?=?1,...,d, associated with \(\mathcal L\). As an example of such a potential V one can take any V?≥?0, \(V\in L^1_{loc}\), in one dimension.