Intricacies of Cation-Anion Combinations in Imidazolium Salt-Catalyzed Cycloaddition of CO2 Into Epoxides

The cycloaddition of CO2 into epoxides catalyzed by imidazolium and related salts continues to attract attention due to the industrial importance of the cyclic carbonate products. The mechanism of the imidazolium-catalyzed transformation has been proposed to require the participation of the acidic C2 proton. However, other simple salts without acidic protons, such as N,N,N,N-tetrabutylammonium chloride, are also efficient catalysts for the reaction. Hence, we decided to investigate the role of the ring protons of imidazolium salts in this reaction. To this end, we systematically studied the catalytic activity of a series of methyl substituted imidazolium cations, in the presence of various halide anions, both by experiment and in silico. Our results demonstrate that, while stabilization of intermediates by C2, C4, or C5 protons in imidazolium salts takes place, it is the nucleophilicity of the anion that governs the overall activity, which is intimately related to the strength of the interactions between the cation and anion. Consequently, the reactivity of the halide anion strongly depends on the nature of the cation and cosolvents. This study completes the (known) mechanism and should facilitate the development of highly efficient catalysts

Switching ion binding selectivity of thiacalix[4]arene monocrowns at liquid–liquid and 2D-confined interfaces

Understanding the interaction of ions with organic receptors in confined space is of fundamental importance and could advance nanoelectronics and sensor design. In this work, metal ion complexation of conformationally varied thiacalix[4]monocrowns bearing lower-rim hydroxy (type I), dodecyloxy (type II), or methoxy (type III) fragments was evaluated. At the liquid–liquid interface, alkylated thiacalixcrowns-5(6) selectively extract alkali metal ions according to the induced-fit concept, whereas crown-4 receptors were ineffective due to distortion of the crown-ether cavity, as predicted by quantum-chemical calculations. In type-I ligands, alkali-metal ion extraction by the solvent-accessible crown-ether cavity was prevented, which resulted in competitive Ag+ extraction by sulfide bridges. Surprisingly, amphiphilic type-I/II conjugates moderately extracted other metal ions, which was attributed to calixarene aggregation in salt aqueous phase and supported by dynamic light scattering measurements. Cation–monolayer interactions at the air–water interface were monitored by surface pressure/potential measurements and UV/visible reflection–absorption spectroscopy. Topology-varied selectivity was evidenced, towards Sr2+ (crown-4), K+ (crown-5), and Ag+ (crown-6) in type-I receptors and Na+ (crown-4), Ca2+ (crown-5), and Cs+ (crown-6) in type-II receptors. Nuclear magnetic resonance and electronic absorption spectroscopy revealed exocyclic coordination in type-I ligands and cation–π interactions in type-II ligands

Calorimetric and Spectroscopic Studies on the Solvation Energetics for H2 Storage in the CO2/HCOOH System

Solvents play a crucial role in many chemical reactions and additives can be used to shift the reaction equilibrium. Herein we study the enthalpy of mixing for selected solvents (aqueous, organic) and basic additives (amines, aqueous KOH) when mixed with formic acid with the aim to optimize hydrogen storage/delivery in the CO2/HCOOH system. Formic acid, resulting from carbon dioxide hydrogenation, reaches highest yields when effectively “removed” from the reaction equilibrium. In terms of energy efficiency, any heat release during CO2 hydrogenation has to be reinvested in the reverse reaction, during the production of hydrogen. In any scenario, the usage of basic chemicals, non innocent solvents causes higher energy release in the CO2 hydrogenation, which as to be reinvested in the hydrogen delivery process. Therefore, the enthalpy of mixing is a valuable parameter for designing hydrogen storage devices since it allows to estimate the energy balance for the CO2 hydrogenation/H2 liberation cycle. The highest formic acid concentrations in direct catalytic CO2 hydrogenation under acidic conditions were reached in DMSO. DMSO exhibits considerably stronger interactions with formic acid compared to water as was observed in calorimetric measurements. This difference can be ascribed, at least partly, to stronger hydrogen bonding of FA to DMSO than to water in the corresponding solutions, examined by a combination of IR spectroscopic and quantum chemical studies. Furthermore, the investigation of the DMSO/FA- and water/FA systems by 1H- and 13C-NMR spectroscopy revealed that only 1:1 aggregates are formed in the DMSO solutions of FA in a broad concentration range, while the stoichiometry and the number of the FA-water aggregates essentially depends on the concentration of the aqueous solutions

simple physical model for the simultaneous rationalisation of melting points and heat capacities of ionic liquids

The anal. of potential energy surfaces of ion pairs within the framework of an anharmonic oscillator model allows rationalization and prediction of m.ps. (Tmp) and heat capacities (Cp) of ionic liqs. (ILs) comprising di- and trialkylimidazolium or tetraalkylphosphonium cations and weakly coordinating BF4, PF6, or Tf2N anions. Multiple short contacts between the counterions are demonstrated to be typical for the imidazolium based ILs. Differences in the types of contacts result in moderate changes of m.ps. of the ILs, comparable with differences in Tmp exptl. detd. for the same crystal. The theor. evaluation of IL heat capacities addnl. requires a consideration of conformational behavior of the corresponding cations. A similar conformational compn. of 1-butyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate at ambient temp. is demonstrated by the combined DFT-vibrational spectroscopy approach. A rough proportionality of Cp to 1/Tmp of ionic liqs. is suggested, provided that the conformational compn. of the ILs does not change on crystal-to-liq. transition

A fresh look at participation of phosphorus atom in conjugation

<p>The strength of conjugation between the lone pair on phosphorus atom and aryl, alkenyl and styryl moieties of several phosphines has been quantitatively characterized by the use of Raman intensities of the bands of the above-mentioned substituents. It is shown that conjugation in the phosphines produces an intensification of Raman bands and a simultaneous bathochromic shift of the electronic absorption spectra for all the phosphines, except of phenylphosphine PhPH₂, where the lone pair on P^III does not conjugate with phenyl moiety. Nevertheless, the lone pair is able to conjugation within more expanded aromatic π-systems, and the conjugational effects are strongly enhanced as π-system of arylphosphines expands.</p

Delineation of the Critical Parameters of Salt Catalysts in the N-Formylation of Amines with CO2

N-formylation of amines with CO2 is base-catalyzed and studies of salt catalysts reveal that the reaction is efficiently catalyzed by "free" floating anions of high basicity, as represented in the cover image. More information can be found in the Full Paper by P. J. Dyson et al. (DOI: 10.1002/chem.201901686)

2,3-(Dibenzimidazol-2-yl)quinoxalines: Unexpected Dynamical Effect on Steady-State Electronic Absorption Spectra

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Structural Studies of the Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrafluoroborate in Dichloromethane Using a Combined DFT-NMR Spectroscopic Approach

DFT methods in combination with NMR spectroscopy are used to investigate possible variants of the mol. structure of the ion pairs of the ionic liq. (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF4], in dichloromethane. According to the computations of the chem. shifts, exptl. NMR spectra can be rationalized by an equil. between ca. 70-80% of structures with the anion positioned near to the C2 atom of the imidazolium ring and ca. 20-30% of structures with the anion close to the C5 and/or C4 atoms. The content of the latter structures, according to the computed Gibbs free energies, does not exceed 10%. Both the computations and the exptl. NMR data suggest that the ratio of the two above-mentioned types of structures of the imidazolium-based ILs is influenced by the concn./polarity of their dichloromethane solns

Leaching from Palladium Nanoparticles in an Ionic Liquid Leads to the Formation of Ionic Monometallic Species

Molecular dynamics simulations and DFT calculations suggest that leaching of palladium species from Pd nanoparticles in ionic liquids does not involve "naked" Pd(0) atoms or neutral ArPdX species formed by oxidative addition of arylhalides. Instead, the ionic liquid contributes largely to leaching of ionic PdX- or PdAr+ species