Ligand-Exchange Processes on Solvated Lithium Cations: DMSO and Water/DMSO Mixtures

Selective Solid-Liquid Extraction of Lithium Cation Using Tripodal Sulfate-Binding Receptors Driven by Electrostatic Interactions

Owing to the important role of and increasing demand for lithium resources, lithium extraction is crucial. The use of molecular extractants is a promising strategy for selective lithium recovery, in which the interaction between lithium and the designed extractant can be manipulated at the molecular level. Herein, we demonstrate that anion receptors of tripodal hexaureas can selectively extract Li2SO4 solids into water containing DMSO (0.8% water) compared to other alkali metal sulfates. The hexaurea receptor with terminal hexyl chains displays the best Li+ extraction selectivity at 2-fold over Na+ and 12.5-fold over K+. The driving force underpinning selective lithium extraction is due to the combined interactions of Li+-SO42- electrostatics and the ion-dipole interaction of the lithium-receptor (carbonyl groups and N atoms); the latter was found to be cation size dependent, as supported by computational calculations. This work indicates that anion binding receptors could drive selective cation extraction, thus providing new insights into the design of receptors for ion recognition and separation.

From Bulk to Interface: Solvent Exchange Dynamics and Their Role in Ion Transport and the Interfacial Model of Rechargeable Magnesium Batteries

Multivalent battery chemistries have been explored in response to the increasing demand for high-energy rechargeable batteries utilizing sustainable resources. Solvation structures of working cations have been recognized as a key component in the design of electrolytes; however, most structure-property correlations of metal ions in organic electrolytes usually build upon favorable static solvation structures, often overlooking solvent exchange dynamics. We here report the ion solvation structures and solvent exchange rates of magnesium electrolytes in various solvents by using multimodal nuclear magnetic resonance (NMR) analysis and molecular dynamics/density functional theory (MD/DFT) calculations. These magnesium solvation structures and solvent exchange dynamics are correlated to the combined effects of several physicochemical properties of the solvents. Moreover, Mg2+ transport and interfacial charge transfer efficiency are found to be closely correlated to the solvent exchange rate in the binary electrolytes where the solvent exchange is tunable by the fraction of diluent solvents. Our primary findings are (1) most battery-related solvents undergo ultraslow solvent exchange coordinating to Mg2+ (with time scales ranging from 0.5 μs to 5 ms), (2) the cation transport mechanism is a mixture of vehicular and structural diffusion even at the ultraslow exchange limit (with faster solvent exchange leading to faster cation transport), and (3) an interfacial model wherein organic-rich regions facilitate desolvation and inorganic regions promote Mg2+ transport is consistent with our NMR, electrochemistry, and cryogenic X-ray photoelectron spectroscopy (cryo-XPS) results. This observed ultraslow solvent exchange and its importance for ion transport and interfacial properties necessitate the judicious selection of solvents and informed design of electrolyte blends for multivalent electrolytes.

Ion diffusion captures composition-dependent anomalies in water-DMSO binary mixtures

Aqueous dimethyl sulfoxide (Aq-DMSO) binary mixture exhibits many fascinating composition-dependent anomalies that are explained by using the peculiarities of the water-DMSO hydrogen bond. Ions can couple strongly to these composition-dependent anomalies to produce exotic dynamics of their own. We carry out theoretical studies using computer simulations to understand the structural and dynamical aspects of rigid monovalent cations (Li+, Na+, K+, Rb+, and Cs+) in aqueous DMSO solutions, with chloride as the counterion. We uncover a number of composition-dependent ion diffusion anomalies, which can be traced back to the interplay between the size-dependent charge density of the ion and the resulting difference in interactions of the ion with water and DMSO molecules. Size and composition dependence of the diffusion coefficients of the five ions exhibit fascinating variations that can be explained partially.

A Personal Account on Inorganic Reaction Mechanisms

The presented Review is focused on the latest research in the field of inorganic chemistry performed by the van Eldik group and his collaborators. The first part of the manuscript concentrates on the interaction of nitric oxide and its derivatives with biologically important compounds. We summarized mechanistic information on the interaction between model porphyrin systems (microperoxidase) and NO as well as the recent studies on the formation of nitrosylcobalamin (CblNO). The following sections cover the characterization of the Ru(II)/Ru(III) mixed-valence ion-pair complexes, including Ru(II)/Ru(III)(edta) complexes. The last part concerns the latest mechanistic information on the DFT techniques applications. Each section presents the most important results with the mechanistic interpretations.

Crystal structures of polymerized lithium chloride and dimethyl sulfoxide in the form of {2LiCl·3DMSO} n and {LiCl·DMSO} n

Two novel LiCl·DMSO polymer structures were created by combining dry LiCl salt with dimethyl sulfoxide (DMSO), namely, catena-poly[[chlorido-lithium(I)]-μ-(dimethyl sulfoxide)-κ² O:O-[chlorido-lithium(I)]-di-μ-(dimethyl sulfoxide)-κ⁴ O:O], [Li₂Cl₂(C₂H₆OS)₃] _n , and catena-poly[lithium(I)-μ-chlorido-μ-(dimethyl sulfoxide)-κ² O:O], [LiCl(C₂H₆OS)] _n . The initial synthesized phase had very small block-shaped crystals (<0.08 mm) with monoclinic symmetry and a 2 LiCl: 3 DMSO ratio. As the solution evaporated, a second phase formed with a plate-shaped crystal morphology. After about 20 minutes, large (>0.20 mm) octa-hedron-shaped crystals formed. The plate crystals and the octa-hedron crystals are the same tetra-gonal structure with a 1 LiCl: 1 DMSO ratio. These structures are reported and compared to other known LiCl·solvent compounds.

Effect of water on the behaviour of lithium and superoxide ions in aprotic solvents

An aprotic lithium-air battery is a promising candidate for next-generation energy storage systems, but its practical performance is still low. The addition of water to an electrolyte can substantially increase the capacity and round-trip efficiency of batteries. However, fundamental mechanisms of the water impact are still far from being fully understood. To contribute to this issue, we studied by molecular dynamics simulations the effect of water additives on the behaviour of discharge intermediates Li⁺ and O₂^- in two frequently used solvents: dimethoxyethane (DME) and dimethyl sulfoxide (DMSO). We have estimated the structures of the solvation shells around Li⁺ and O₂^- ions, and the residence times of various electrolyte components inside the solvation shells depending on the concentration of water additives. Furthermore, we have estimated the rate and the equilibrium of the Li⁺ and O₂^- association. Our results reveal that water additives in electrolytes shift the equilibrium of the association reaction toward soluble Li⁺ and O₂^- ions in both DME and DMSO. These data argue for the view that water promotes the solution discharge mechanism, thus increasing the capacity. Moreover, we show that water accelerates the kinetics of the association reaction due to the decrease of the stability of Li⁺ and O₂^- solvation shells. This may explain the reduced discharge overpotential when water is added.

Inorganic reaction mechanisms. A personal journey

This review covers highlights of the work performed in the van Eldik group on inorganic reaction mechanisms over the past two decades in the form of a personal journey. Topics that are covered include, from NO to HNO chemistry, peroxide activation in model porphyrin and enzymatic systems, the wonder-world of Ru^III(edta) chemistry, redox chemistry of Ru(iii) complexes, Ru(ii) polypyridyl complexes and their application, relevant physicochemical properties and reaction mechanisms in ionic liquids, and mechanistic insight from computational chemistry. In each of these sections, typical examples of mechanistic studies are presented in reference to related work reported in the literature.

Effect of Cation Size on Solvation and Association with Superoxide Anion in Aprotic Solvents

Influence of cation size on solvation strength, diffusion, and kinetics of the association reaction with anions O₂ ^- in aprotic solvents, such as acetonitrile and dimethyl sulfoxide, has been investigated by means of molecular dynamics simulations. The work is motivated by the need to understand the molecular nature of the solvent-induced changes in capacity of Li-air batteries. We have shown that the dependence of the solvation shell stability on the cation size has a maximum at a particular ion radius that corresponds to a solvent coordination number of 4. The shell stability maximum coincides with the diffusion coefficient minimum. The variation of the cation shell stability has a crucial impact on the kinetics of the cation-O₂ ^- association. We have demonstrated that profound inhibition of the association reaction for Li⁺ in dimethyl sulfoxide is a result of the lock-and-key effect that cannot be described in the framework of Hard Soft Acid Base theory.

Recent advances in understanding of the mechanism and control of Li2O2formation in aprotic Li–O2batteries

This review systematically summarizes the recent advances in the mechanism studies and control strategies of Li₂O₂formation in aprotic Li–O₂batteries.

Entropic electrolytes for anodic cycloadditions of unactivated alkene nucleophiles

Unactivated alkenes were previously found to be effective carbon nucleophiles for anodic cycloadditions in lithium perchlorate/nitromethane electrolyte solution.

Cluster-continuum quasichemical theory calculation of the lithium ion solvation in water, acetonitrile and dimethyl sulfoxide: an absolute single-ion solvation free energy scale

A solvation free energy scale excluding the net electrostatic potential inside the solute cavity is presented.

Quantum chemical investigations of the water exchange mechanism on [Al(III)(H₂O)₅(L)]²⁺ as a function of the donor strength of the anionic L

Water exchange reactions of the complexes Al(H₂O)₅(L)]²⁺·H₂O for L = →OCN⁻, F⁻, CF₃⁻, →NC⁻, →CN⁻, Cl⁻, Br⁻, H⁻, SH⁻, OH⁻, →NCO⁻, →NCS⁻, →SCN⁻, CF₃CH₂⁻, CH₃⁻, Et⁻, i-Pr⁻ and t-but⁻, were studied by DFT calculations (B3LYP/6-311+G**). The reactions follow a dissociative (D) pathway for Al(H₂O)₅(L)]²⁺·H₂O to form the five-coordinate intermediate [Al(H₂O)₄L]²⁺·2H₂O for the weaker donor ligands of the series. On increasing the donor strength of L, the five-coordinate intermediate becomes significantly more stable than the reactant state. At this point there is a mechanistic changeover to an associative (A) pathway for [Al(H₂O)₄L]²⁺·2H₂O as reactant to form a six-coordinate intermediate Al(H₂O)₅(L)]²⁺·H₂O. For some of the anionic ligands L = → NC⁻, →CN⁻, Cl⁻, OH⁻ and →SCN⁻) the energy gap between the reactant and intermediate states is small, such that the water exchange mechanism lies in the boarder of dissociative and associative pathways. The water exchange process involves cis- and trans-orientated transition states to form the product state that is similar to the reactant state.

Coordination of terpyridine to Li+ in two different ionic liquids

On the basis of (7)Li NMR experiments, the complex-formation reaction between Li(+) and the tridentate N-donor ligand terpyridine was studied in the ionic liquids [emim][NTf2] and [emim][ClO4] as solvents. For both ionic liquids, the NMR data implicate the formation of [Li(terpy)2](+). Density functional theory calculations show that partial coordination of terpyridine involving the coordination of a solvent anion can be excluded. In contrast to the studies in solution, X-ray diffraction measurements led to completely different results. In the case of [emim][NTf2], the polymeric lithium species [Li(terpy)(NTf2)]n was found to control the stacking of this complex, whereas crystals grown from [emim][ClO4] exhibit the discrete dimeric species [Li(terpy)(ClO4)]2. However, both structures indicate that each lithium ion is formally coordinated by one terpy molecule and one solvent anion in the solid state, suggesting that charge neutralization and π stacking mainly control the crystallization process.

Molecular dynamics simulation of LiTFSI-acetamide electrolytes: structural properties

The liquid structures of nonaqueous electrolytes composed of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and acetamide, with LiTFSI/acetamide molar ratios of 1:2, 1:4, and 1:6, were studied by molecular dynamics simulations. The simulations indicate that the Li+ cations prefer to be six-coordinate by the sulfonyl oxygen atoms of the TFSI- anions and the carbonyl oxygen atoms of the acetamide molecules, rather than by the most electronegative nitrogen atom of the TFSI- anion. Therefore, close Li+-TFSI- contact pairs exist in the system. The TFSI- anion prefers to provide only one of four possible oxygen atoms to coordinate to the same Li+ cation. Three conformations (cis, trans, and gauche) of the TFSI- anions were found to coexist in the liquid electrolyte. At high salt concentrations, the TFSI- anions mainly adopt the gauche conformation in order to provide more oxygen atoms to coordinate to different Li+ cations, while simultaneously reducing the repulsion among the Li+ cations. On the other hand, the fraction of TFSI- anions adopting the cis conformation is largest for the system with the molar ratio of 1:6, in which many clusters, mainly composed of the Li+ cations and the TFSI- anions, are immersed in the acetamide molecules. The size and charge distribution of clusters were also investigated. In the system with the molar ratio of 1:2, nearly all of the ions in the PBC (periodic boundary conditions) box aggregate into a bulky cluster that gradually disassembles into small clusters with decreasing salt concentration. The addition of acetamide molecules was found to effectively relax the liquid electrolyte structure, and the system with the molar ratio of 1:4 was found to exhibit a more homogeneous liquid structure than the other two electrolyte systems with molar ratios of 1:2 and 1:6.

Novel iron(III) porphyrazine complex. Complex speciation and reactions with NO and H2O2

The complex [iron(III) (octaphenylsulfonato)porphyrazine] (5-), Fe (III)(Pz), was synthesized. The p K a values of the axially coordinated water molecules were determined spectrophotometrically and found to be p K a 1 = 7.50 +/- 0.02 and p K a 2 = 11.16 +/- 0.06. The water exchange reaction studied by (17)O NMR as a function of the pH was fast at pH = 1, k ex = (9.8 +/- 0.6) x 10 (6) s (-1) at 25 degrees C, and too fast to be measured at pH = 10, whereas at pH = 13, no water exchange reaction occurred. The equilibrium between mono- and diaqua Fe (III)(Pz) complexes was studied at acidic pH as a function of the temperature and pressure. Complex-formation equilibria with different nucleophiles (Br (-) and pyrazole) were studied in order to distinguish between a five- (in the case of Br (-)) or six-coordinate (in the case of pyrazole) iron(III) center. The kinetics of the reaction of Fe (III)(Pz) with NO was studied as a model ligand substitution reaction at various pH values. The mechanism observed is analogous to the one observed for iron(III) porphyrins and follows an I d mechanism. The product is (Pz)Fe (II)NO (+), and subsequent reductive nitrosylation usually takes place when other nucleophiles like OH (-) or buffer ions are present in solution. Fe (III)(Pz) also activates hydrogen peroxide. Kinetic data for the direct reaction of hydrogen peroxide with the complex clearly indicate the occurrence of more than one reaction step. Kinetic data for the catalytic decomposition of the dye Orange II by H 2O 2 in the presence of Fe (III)(Pz) imply that a catalytic oxidation cycle is initiated. The peroxide molecule first coordinates to the iron(III) center to produce the active catalytic species, which immediately oxidizes the substrate. The influence of the catalyst, oxidant, and substrate concentrations on the reaction rate was studied in detail as a function of the pH. The rate increases with increasing catalyst and peroxide concentrations but decreases with increasing substrate concentration. At low pH, the oxidation of the substrate is not complete because of catalyst decomposition. The observed kinetic traces at pH = 10 and 12 for the catalytic cycle could be simulated on the basis of the obtained kinetic data and the proposed reaction cycle. The experimental results are in good agreement with the simulated ones.