Importance of counter-ions in alkali and alkaline-earth cation extraction by 18-crown-6: molecular dynamics studies at the water/sc-CO₂interface

Abstract

We present a molecular dynamics study of concentrated solutions of K + and Sr 2+ cations and their complexes with 18C6 at the water/supercritical-CO 2 interface, and compare picrate '' Pic À '' vs. perfluorooctanoate '' PFO À '' counter-ions at 305 and 350 K. The results allow to understand why K + is poorly extracted in conditions where Sr 2+ is transferred to the supercritical fluid. In all cases, the solutes are found to concentrate near the interface, with interesting differences, depending on the cation, counter-ion and extractant. With the two anions, the SrX 2 salts are surface active, as a result of cation-anion pairing, while the distribution of the K + ions depends on X À . The inclusive K + &18C6 complexes partially decomplex, and decomplexation is more effective at 350 K than at 305 K. The most remarkable result is the extraction of Sr 2+ &18C6,2PFO À complexes in which the anions axially co-complex the cation, while retaining CO 2 -philic interactions via their perfluorinated chain. This Sr 2+ extraction is confirmed by several computational tests which point to the importance of concentration effects and agree with experiment. It contrasts with the formation of an interfacial film with Sr 2+ &18C6,2Pic À complexes, assembled via bridging Pic À Á Á ÁSr 2+ Á Á ÁPic À and stacking Pic À Á Á ÁPic À interactions.

Response of Crown Ether Functionalized Polythiophenes to Alkaline Ions

“…The solvation of crown ethers and their complexes with metallic cations in water and different ionic liquids has been investigated using MD simulations by Wipff and co-workers. − Thus, polarity of water and the presence of solvent ions play a crucial role in the ability and efficiency of simple crown ether molecules (i.e., macrocycles not fused to a polymer chains) to extract alkali and alkaline cations. − However, simulations reported in this work were carried out in dichloromethane solution because of the PThs functionalized with crown ether macrocycles are not soluble in water or any other polar solvent. Therefore, examination of the effects produced by this organic solvent on the systems under study is necessary.…”

Supramolecular ChemistryIn Silico

“…expensive and probably best used to probe potential structural effects, such as the emergence of hydrogen bonded networks, using molecular mechanics methods [32]. A study of calix [4]arene conformational interconversion using a continuum solvent approach has proved successful although it is unclear how well this method would translate to model the behaviour of a multicomponent host-guest system [33]. We attempted to apply explicit solvation to crown ether complexes but without success [27].…”

“…Applications of computational modelling in the field of supramolecular chemistry are broad in scope. Molecular dynamics simulations have been used to investigate synthetic ionophores and their cation complexes [2,3], the importance of counter ions in alkali and alkaline earth cation extraction by 18-crown-6 at the water/sc-CO 2 interface [4], metal ion selectivity by calix [4]tubes [5], guest binding by calix [4]sulfonate in water [6] and molecular recognition by crown ethers, cryptands and cryptates [7]. Molecular mechanics methods have been applied to oxacalix [3]arene [8], the nature of the macrocyclic effect with regard to [18]crown-6 and pentaglyme complexes with K + [9] and investigations into anion complexation by redox active podands [10].…”

“…Frontier orbital maps calculated by PM3 have been used to predict the direction of self-assembly of a novel class of self-organising cucurbituril nanotubes ushering in the possibility of de novo design for molecular wires in nanoelectronics [14]. At a higher level, ab initio quantum mechanical studies have been used to give insights into alkali metal and alkaline earth cation selectivity of [18]crown-6 [15,16], transient alkali metal inclusion in oxacalix [3]arenes [17], the influence of crystal packing on the host-guest interactions of calix [4]arene crystal structures [18], proton tunnelling in calix [4]arenes [19], the impact of cation-π interactions on alkali metal binding by calix [4]arenes [20] and ammonium binding by resorcarenes [21]. Also at this higher level of theory, comparisons have been made between GIAO-DFT and experimentally derived complexation-induced chemical shifts of calix [4]arene-solvent inclusion complexes [22].…”

Decomplexation and complexation of alkali metal cations by a crown-ether-type podand in dichloromethane: a steered molecular dynamics study

“…Consequently, it is currently the method of choice to model systems as ours, not only because it allows the explicit inclusion of hundreds of solvent molecules (comprising thousands of atoms), but also because they are extremely fluxional and, as such, not adequate to a high-level quantum calculation. It is noteworthy that alkali metal cation complexation by crownethers was one of the first problems to be studied by MD [8,9], and most published work in this area has been on them [8][9][10][11][12][13][14], on spherands [15][16][17][18][19][20][21] or on cryptands [14,[21][22][23][24][25][26], with only a few studies involving podands [3,4,[27][28][29].…”