Understanding the subtleties of frustrated Lewis pair activation of carbonyl compounds by N-Heterocyclic carbene/alkyl gallium pairings

This study reports the use of the trisalkylgallium GaR3 (R=CH2 SiMe3 ), containing sterically demanding monosilyl groups, as an effective Lewis-acid component for frustrated Lewis pair activation of carbonyl compounds, when combined with the bulky N-heterocyclic carbene 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu) or 1,3-bis(tert-butyl)imidazolin-2-ylidene (SItBu). The reduction of aldehydes can be achieved by insertion into the C=O functionality at the C2 (so-called normal) position of the carbene affording zwitterionic products [ItBuCH2 OGaR3 ] (1) or [ItBuCH(p-Br-C6 H4 )OGaR3 ] (2), or alternatively, at its abnormal (C4) site yielding [aItBuCH(p-Br-C6 H4 )OGaR3 ] (3). As evidence of the cooperative behaviour of both components, ItBu and GaR3 , neither of them alone are able to activate any of the carbonyl-containing substrates included in this study NMR spectroscopic studies of the new compounds point to complex equilibria involving the formation of kinetic and thermodynamic species as implicated through DFT calculations. Extension to ketones proved successful for electrophilic α,α,α-trifluoroacetophenone, yielding [aItBuC(Ph)(CF3 )OGaR3 ] (7). However, in the case of ketones and nitriles bearing acidic hydrogen atoms, C-H bond activation takes place preferentially, affording novel imidazolium gallate salts such as [{ItBuH}(+) {(p-I-C6 H4 )C(CH2 )OGaR3 }(-) ] (8) or [{ItBuH}(+) {Ph2 C=C=NGaR3 }(-) ] (12)

Synthesis and Electronic Structure Determination of Uranium(VI) Ligand Radical Complexes

   Pentagonal bipyramidal uranyl complexes of salen ligands, N,N’-bis(3-tert-butyl-(5R)-salicylidene)-1,2-phenylenediamine, in which R = tBu (1a), OMe (1b), and NMe2 (1c), were prepared and the electronic structure of the one-electron oxidized species [1a-c]+ were investigated in solution. The solid-state structures of 1a and 1b were solved by X-ray crystallography, and in the case of 1b an asymmetric UO22+ unit was found due to an intermolecular hydrogen bonding interaction. Electrochemical investigation of 1a-c by cyclic voltammetry showed that each complex exhibited at least one quasi-reversible redox process assigned to the oxidation of the phenolate moieties to phenoxyl radicals. The trend in redox potentials matches the electron-donating ability of the para-phenolate substituents. The electron paramagnetic resonance spectra of cations [1a-c]+ exhibited gav values of 1.997, 1.999, and 1.995, respectively, reflecting the ligand radical character of the oxidized forms, and in addition, spin-orbit coupling to the uranium centre. Chemical oxidation as monitored by ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy afforded the one-electron oxidized species. Weak low energy intra-ligand charge transfer (CT) transitions were observed for [1a-c]+ indicating localization of the ligand radical to form a phenolate / phenoxyl radical species. Further analysis using density functional theory (DFT) calculations predicted a localized phenoxyl radical for [1a-c]+ with a small but significant contribution of the phenylenediamine unit to the spin density. Time-dependent DFT (TD-DFT) calculations provided further insight into the nature of the low energy transitions, predicting both phenolate to phenoxyl intervalence charge transfer (IVCT) and phenylenediamine to phenoxyl CT character. Overall, [1a-c]+ are determined to be relatively localized ligand radical complexes, in which localization is enhanced as the electron donating ability of the para-phenolate substituents is increased (NMe2 > OMe > tBu)

Determination of stress in thin bismuth layers on flexible polymer substrate with use of optical interferometry

Celem pracy było wyznaczanie naprężeń w cienkich warstwach bizmutu na giętkim podłożu polimerowym z użyciem interferometrii optycznej. W pomiarach wykorzystano czujnik składający się z siatki dyfrakcyjnej utworzonej bezpośrednio badanej powierzchni oświetlony wiązką laserową. Powstający obraz interferencyjny pozwala na określenie podłużnej i poprzecznej składowej deformacji.Porównano dwie metody wytwarzania siatek dyfrakcyjnych. Litografia interferencyjna wykorzystuje konwencjonalną fotolitografię, w której fotomaskę zastąpiono wzorem interferencyjnym dwóch wiązek światła padających na powierzchnię. Parametry procesu, takie jak: natężenie światła, czas naświetlania i czas wywołania fotorezystu zostały dobrane eksperymentalnie w specjalnie zbudowanym do tego celu układzie optycznym. Drugą metodą była bezpośrednia interferencyjna litografia laserowa wykorzystująca dwie wiązki światła z nanosekundowego lasera Nd:YAG do selektywnego usuwania materiału z powierzchni próbki. Technika ta pozwala na szybkie przygotowanie siatek dyfrakcyjnych o wysokiej jakości. Obie metody posłużyły do wytworzenia siatek dyfrakcyjnych na podłożu polimerowym, a uzyskane struktury zostały scharakteryzowane za pomocą skaningowej mikroskopii elektronowej.Pomiary odkształceń prowadzone były z wykorzystaniem układu eksperymentalnego składającego się z kamery CMOS, próbki z siatką dyfrakcyjną i lasera. Próbka montowana była we własnoręcznie zbudowanym urządzeniu rozciągającym pozwalającym na precyzyjną kontrolę odkształcenia. Wyznaczanie składowych odkształcenia dokonywane było przez analizę obrazu interferencyjnego z użyciem programu napisanego w środowisku LabVIEW. Porównano dwa algorytmy wyznaczania położeń prążków dyfrakcyjnych na obrazie. Pierwszy z nich wykorzystuje zaawansowane funkcje przetwarzania obrazu do wyodrębnienia obszarów odpowiadających maksimom i określenia ich środków mas. W tym przypadku duży wpływ na znalezione położenie prążka ma jego kształt. Drugi z wykorzystanych algorytmów, wyznaczanie centroidu, pozwala zmniejszyć ten problem poprzez uwzględnienie jasności pikseli tworzących dane maksimum na obrazie osiągając większą dokładność pomiarów.Uzyskane wyniki eksperymentalne potwierdzają skuteczność zaproponowanej metody pomiaru odkształceń pozwalając na wyznaczanie zarówno odkształceń w dwóch kierunkach jak i współczynnika Poissona. Dane te umożliwiają określenie naprężeń w badanej próbce. Opisana metoda może znaleźć zastosowanie w badaniach, w których użycie klasycznych tensometrów nie jest możliwe, na przykład przy charakteryzowaniu giętkiej elektroniki.Praca została przygotowana w ramach projektu LIDER V Narodowego Centrum Badań i Rozwoju (nr. : LIDER/008/177/L-5/13/NCBR/2014)The aim of this study was determination of stresses in thin bismuth layers on flexible polymer substrate with the use of optical interferometry. The measurements were made using a sensor consisting of a diffraction grating formed directly on examined surface illuminated by laser beam. Resulting interference pattern allows determination of longitudinal and transversal strain component.Two methods of diffraction grating preparation were examined. Interference lithography utilizes conventional photolithographic process in which photomask was replaced with interference pattern of two beams of light illuminating the surface. Processing parameters such as light intensity, exposure time and photoresist development time were tuned experimentally using constructed optical setup. Second employed method was direct laser interference patterning using two beams of nanosecond Nd:YAG laser to selectively remove material from sample surface. This technique allowed quick preparation of high quality gratings. Both methods were used to create diffraction gratings on polymer substrate. Obtained structures were characterized by means of scanning electron microscopy.Deformation measurements were carried out using experimental setup consisting of a CMOS camera, sample with diffraction grating and a laser. The sample was mounted in hand-made device allowing for precise deformation control. Determining strain components was performed by analyzing the interference image using program developed with LabVIEW environment.Different methods of determining positions of diffraction maxima on image were compared. The first one utilizes advanced image processing functions to extract areas corresponding to maxima and determines their center of mass. In this case shape of particular maximum has great impact on its reported location. The second algorithm used centroid finding to alleviate this problem by taking into account brightness of pixels forming the maximum. This allows for more precise accuracy of measurements.Experimental results obtained in this study confirm effectiveness of proposed strain measurement method, allowing the determination of both deformations in two directions as well as the Poisson ratio. These data allows calculation of stresses in examined sample. Described method is suitable in experiments in which the use of conventional tensometers is not possible, for example in the characterization of flexible electronics devices.This work was supported by the National Centre for Research and Development within LIDER V program, Poland (project Nr.: LIDER/008/177/L-5/13/NCBR/2014)

Multielectron Redox Reactions Involving C-C Coupling and Cleavage in Uranium Schiff Base Complexes

The reaction of U(III) with Schiff base ligands and the reduction of U(IV) Schiff base complexes both promote C-C bond formation to afford dinuclear or mononuclear U(IV) amido complexes, which can release up to four electrons to substrates through the oxidative cleavage of the C-C bond. © 2010 American Chemical Society

Cation–Cation Complexes of Pentavalent Uranyl: From Disproportionation Intermediates to Stable Clusters

International audienceAbstract Three new cation–cation complexes of pentavalent uranyl, stable with respect to the disproportionation reaction, have been prepared from the reaction of the precursor [(UO 2 py 5 )(KI 2 py 2 )] n ( 1 ) with the Schiff base ligands salen 2− , acacen 2− , and salophen 2− (H 2 salen= N , N ′‐ethylene‐bis(salicylideneimine), H 2 acacen= N , N ′‐ethylenebis(acetylacetoneimine), H 2 salophen= N , N ′‐phenylene‐bis(salicylideneimine)). The preparation of stable complexes requires a careful choice of counter ions and reaction conditions. Notably the reaction of 1 with salophen 2− in pyridine leads to immediate disproportionation, but in the presence of [18]crown‐6 ([18]C‐6) a stable complex forms. The solid‐state structure of the four tetranuclear complexes, {[UO 2 (acacen)] 4 [μ 8 ‐] 2 [K([18]C‐6)(py)] 2 } ( 3 ) and {[UO 2 (acacen)] 4 [μ 8 ‐]} ⋅ 2 [K([222])(py)] ( 4 ), {[UO 2 (salophen)] 4 [μ 8 ‐K] 2 [μ 5 ‐KI] 2 [(K([18]C‐6)]} ⋅ 2 [K([18]C‐6)(thf) 2 ] ⋅ 2 I ( 5 ), and {[UO 2 (salen) 4 ][μ 8 ‐Rb] 2 [Rb([18]C‐6)] 2 } ( 9 ) ([222]=[222]cryptand, py=pyridine), presenting a T‐shaped cation–cation interaction has been determined by X‐ray crystallographic studies. NMR spectroscopic and UV/Vis studies show that the tetranuclear structure is maintained in pyridine solution for the salen and acacen complexes. Stable mononuclear complexes of pentavalent uranyl are also obtained by reduction of the hexavalent uranyl Schiff base complexes with cobaltocene in pyridine in the absence of coordinating cations. The reactivity of the complex [U V O 2 (salen)(py)][Cp* 2 Co] with different alkali ions demonstrates the crucial effect of coordinating cations on the stability of cation–cation complexes. The nature of the cation plays a key role in the preparation of stable cation–cation complexes. Stable tetranuclear complexes form in the presence of K + and Rb + , whereas Li + leads to disproportionation. A new uranyl–oxo cluster was isolated from this reaction. The reaction of [U V O 2 (salen)(py)][Cp* 2 Co] (Cp*=pentamethylcyclopentadienyl) with its U VI analogue yields the oxo‐functionalized dimer [UO 2 (salen)(py)] 2 [Cp* 2 Co] ( 8 ). The reaction of the {[UO 2 (salen) 4 ][μ 8 ‐K] 2 [K([18]C‐6)] 2 } tetramer with protons leads to disproportionation to U IV and U VI species and H 2 O confirming the crucial role of the proton in the U V disproportionation