49 research outputs found
Modulation of chiroptical and photophysical properties in helicenic rhenium(I) systems: the use of an Nâ(aza[6]helicenyl)âNHC ligand
The photophysical and chiroptical properties of a novel, chiral helicene-NHCâRe(I) complex bearing an N-(aza[6]helicenyl)-benzimidazolylidene ligand are described, showing its ability to emit yellow circularly polarized luminescence. A comparative analysis of this new system with other helicene-Re(I) complexes reported to date illustrates the impact of structural modifications on the emissive and absorptive properties
Synthesis, structural characterization, and chiroptical properties of planarly and axially chiral boranils
International audience2-Amino[2.2]paracyclophane reacts with salicylaldehyde or 2-hydroxyacetophenone to yield imines that then give access to a new series of boranils (8bâd) upon complexation with BF2. These novel boron-containing compounds display both planar and axial chiralities and were examined experimentally and computationally. In particular, their photophysical and chiroptical properties were studied and compared to newly prepared, simpler boranils (9aâd) exhibiting axial chirality only. Less sophisticated chiral architectures were shown to demonstrate overall stronger circularly polarized luminescence (CPL) activity
Stereospecific synthesis of chiral P-containing polyaromatics based on 7-membered P-rings
International audienceWe present the stereospecific synthesis of helicenoid-based phosphepines (7-membered P-rings) as well as chiral P-containing polycyclic aromatic hydrocarbons. In these systems, an axial to central chirality transfer takes place from the BINAP moiety to the P-atom. The impact of the molecular design on the structure, the (chir)optical (including circularly polarized luminescence) and redox properties are investigated
Evaluation of Band-Selective HSQC and HMBC: Methodological Validation on the Cyclosporin Cyclic Peptide and Application for Poly(3-hydroxyalkanoate)s Stereoregularity Determination
Band-selective (bs) HSQC, improving spectral resolution by restriction of the heteronuclear dimension without inducing spectral folding, has been recently used for polymer tacticity determination. Herein is reported an evaluation of various bs-HSQC and bs-HMBC sequences, first from a methodological point of view (selectivity, dependence to INEPT interpulse delay or relaxation delay), using the cyclic peptide cyclosporin selected as a model compound, and then from an applicative approach, comparing tacticity determined from bs-HSQC and bs-HMBC experiments to the one obtained from 1D 13C{1H} on poly(3-hydroxyalkanoate)s samples. For HSQC sequences, the 13C selectivity scheme consisting in substituting a 13C broadband refocalization by a selective one revealed itself problematic, with unwanted aliased signals, whereas the insertion of double pulsed field gradients spin-echo (DPFGSE) or the use of opposite sign gradients bracketing a selective refocalization gave satisfactory results. Determination of the probability of syndiotactic enchainments, Ps, by bs-HSQC is fully consistent and no precision loss was observed when decreasing acquisition time (37 min vs. 106 min for 1D 13C{1H}). Bs-HMBC, although not straightforwardly applicable for tacticity determination, could provide (after a calibration step) an alternative for compounds of which only 13C carbonyl signals are resolved enough for discriminating between syndiotactic and isotactic configurations
Détermination de la distribution intramoléculaire du 13C en abondance naturelle par RMN quantitative
Les conditions expérimentales pour la détermination de la distribution intramoléculaire du 13C par RMN quantitative ont été étudiées. Dans un premier temps, des molécules doublement marquées 13C sont utilisées comme outil pour évaluer les performances des méthodes d'analyse (justesse, précision). Deux stratégies sont ensuite proposées pour la réduction de la durée de l'analyse : L'emploi d'un agent relaxant permet de diminuer le temps de répétition, et l'utilisation de la séquence à transfert de polarisation INEPT permet une meilleure sensibilité. L'effet des imperfections des impulsions RF sur la séquence INEPT a été modélisé, et la séquence a été optimisée grùce à l'utilisation d'impulsions adiabatiques et à la modification du cycle de phase. Les méthodes développées permettent de réaliser des analyses quantitatives précises par RMN 13C, pour l'étude de phénomÚnes physiques entraßnant un fractionnement isotopique ainsi que pour l'authentification de produits naturels.Experimental conditions for the determination of the intramolecular distribution of 13C by NMR have been studied. In the first instance, 13C bi-labelled molecules were used to evaluate the performance of analytical methods (accuracy, precision). Two strategies were then set up for reducing the duration of the analysis. The use of relaxation agents enables a reduction of the repetition delay, and polarisation transfer sequences, such as INEPT, enhance sensibility. The effects of RF pulses imperfections on the INEPT sequence have been modelled, and the INEPT sequence optimised by use of adiabatic pulses and modifications to phase cycling. The methods developed make possible the precise quantitative analysis by 13C NMR of 13C/12C isotopic ratios, which can be applied in studies of isotopic fractionation and in the authentication of natural products.NANTES-BU Sciences (441092104) / SudocSudocFranceF
Structure of SilverâN-Heterocyclic Carbenes in Solution: Evidence of Equilibration in DMSO at Very Different Time Scales by <sup>1</sup>H NMR Experiments
AgâN-heterocyclic carbene
(NHC) complexes can occur as neutral
[(NHC)ÂAgX] and/or ionic [(NHC)<sub>2</sub>Ag]Â[AgX<sub>2</sub>] species
in solution. Herein, the behavior of four complexes bearing common
bulky <i>N</i>,<i>N</i>âČ-diaryl NHCs was
examined in DMSO. <sup>1</sup>H NMR experiments performed systematically
in CDCl<sub>3</sub> and in DMSO-<i>d</i><sub>6</sub> were
used to detect DMSO or chloride ion induced equilibration between
the neutral and ionic forms at very different time scales. We observed
that solubilization of IMesAgCl and SIMesAgCl in DMSO at 300 K led
in less than a few minutes to a complete equilibration. Here, magnetization
exchange rate constants were determined from EXSY experiments. In
contrast, IPrAgCl and SIPrAgCl, which possess more bulky NHCs, were
found to exhibit much lower exchange rates (or no exchange) at 300
K. IPrAgCl was shown to slowly generate an increasing amount of homoleptic
cationic complex in DMSO for several hours before reaching equilibrium.
In this case, the initial rate of formation of the cationic complex
was determined
Acid-base controlled multiple conformation and aromaticity switches in tren-capped hexaphyrins
International audienceUpon protonation, a tren-capped hexaphyrin undergoes successive rectangular-to-Möbius and Möbius-to-triangular conformational isomerizations, with concomitant antiaromaticity-to-aromaticity reversal. This affords different cage environments leading ultimately to a "crypto-bowl-shape" hexaphyrin hosting a trifluoroacetate counterion
Evaluation of Band-Selective HSQC and HMBC: Methodological Validation on the Cyclosporin Cyclic Peptide and Application for Poly(3-hydroxyalkanoate)s Stereoregularity Determination
Band-selective (bs) HSQC, improving spectral resolution by restriction of the heteronuclear dimension without inducing spectral folding, has been recently used for polymer tacticity determination. Herein is reported an evaluation of various bs-HSQC and bs-HMBC sequences, first from a methodological point of view (selectivity, dependence to INEPT interpulse delay or relaxation delay), using the cyclic peptide cyclosporin selected as a model compound, and then from an applicative approach, comparing tacticity determined from bs-HSQC and bs-HMBC experiments to the one obtained from 1D 13C{1H} on poly(3-hydroxyalkanoate)s samples. For HSQC sequences, the 13C selectivity scheme consisting in substituting a 13C broadband refocalization by a selective one revealed itself problematic, with unwanted aliased signals, whereas the insertion of double pulsed field gradients spin-echo (DPFGSE) or the use of opposite sign gradients bracketing a selective refocalization gave satisfactory results. Determination of the probability of syndiotactic enchainments, Ps, by bs-HSQC is fully consistent and no precision loss was observed when decreasing acquisition time (37 min vs. 106 min for 1D 13C{1H}). Bs-HMBC, although not straightforwardly applicable for tacticity determination, could provide (after a calibration step) an alternative for compounds of which only 13C carbonyl signals are resolved enough for discriminating between syndiotactic and isotactic configurations
Detection of Isotopic Atropisomerism Based on ortho-H/D Discrimination
International audienceRacemic and optically active 3-(2-deuteriophenyl)-2-(1-phenylpropan-2-yl)quinazoline-4-thiones were prepared. The nuclear magnetic resonance spectra clearly show that they exist as a 1:1 mixture of diastereomers due to the isotopic atropisomerism based on ortho-H/D discrimination (N-C axial chirality) and a chiral carbon
Ï Ligands in Alkaline Earth Complexes
International audiencePi ligands such as olefins and alkynes bind intramolecularly to the metal atom in d0 complexes of the large alkaline earths (Ae) calcium and strontium supported by fluoroalkoxo ligands with dangling unsaturated C=C or CâĄC bonds, and having the amide N(SiMe2H)2â as the co-ligand. These O-bridged dinuclear complexes are further stabilized by secondary C-FâAe and beta-Si-HâââAe interactions. In a set of structurally related Ca-olefin complexes, the strength of these interactions gradually increases as the coordination of the olefin onto Ca2+ becomes weaker (from eta-2-coordinated to eta-1 to fully dissociated) upon increasing steric congestion, thus ensuring that overwhelming electronic depletion does not occur at calcium. NMR data imply that the olefins are metal-bound in [D8]toluene solutions. The Ae···C(pi), C-FâAe and beta-Si-HâââAe non-covalent interactions are also strong in the parent Ae-alkyne complexes, the first examples of non-acetylide Ae-alkynes compounds. Calcium-arene complexes could not be made, as the aromatic tether did not bind to the metal atom. Instead, a trinuclear complex with non-interacting C6H5 groups was obtained. It exhibits exceptionally strong C-FâCa and beta-Si-HâââCa interactions. NMR data indicate that the congeneric calcium-allene complex can be made, but it spontaneously isomerizes towards the more stable Ca-alkyne via an unusual 1,3-hydride shift intramolecular process