Standard and Absolute pKa Scales of Substituted Benzoic Acids in Room Temperature Ionic Liquids

Thermoswitchable catalysis to inhibit and promote plastic flow in vitrimers

Acid-base catalysis is a common strategy to induce covalent bond exchanges in dynamic polymer networks. Strong acids or strong bases can promote rapid network rearrangements, and are simultaneously preferred catalysts for chemical reactions where maximum efficiency at the lowest possible temperature is aimed for. However, within the context of dynamic polymer networks, the incorporation of highly active catalysts can negatively affect the longer term application potential. Network dynamicity can diminish through catalyst ageing or quenching and highly active catalysts may prematurely activate bond exchanges, leading to dimensional instability and thus low creep resistance of the polymer networks. Herein, we present several examples where we explicitly explored weak acids (carboxylic acids) as catalysts for dynamic bond exchanges, using vinylogous urethanes (VU) as a well-understood protic acid catalysed vitrimer chemistry. Surprisingly, we have found that the sought-after long-term stability offered by a weak acid does not necessarily bring lower activity at high temperature. In fact, the weak acids show a remarkable thermoswitchable catalytic behaviour, going from an inactive hydrogen bonded state to an active state where the polymer matrix is protonated, with a profound impact on the network reactivity and rheology. Carboxylic acids with different electronic or steric environments show clear reactivity trends and their fine-tuning resulted in the most thermally responsive VU vitrimers studied to date. Our findings point out that catalyst choice and design for vitrimers is only poorly informed by catalyst performance in more traditional chemical reactions (in solvent), and that a more tailored catalyst design holds great promise for the field of vitrimers.

Acidity Scale in a Choline Chloride- and Ethylene Glycol-Based Deep Eutectic Solvent and Its Implication on Carbon Dioxide Absorption

An equilibrium acidity (pK_a) scale that comprises 16 Brönsted organic acids, including phenols, carboxylic acids, azoles, and phenylmalononitriles, was established in a choline chloride/EG-based deep eutectic solvent (DES) ([Ch][Cl]:2EG) by ultraviolet-visible (UV-Vis) spectroscopic methods. The established acidity scale spans about 6 pK units in the DES, which is similar to that for these acids in water. The acidity comparisons and linear correlations between the DES and other solvents show that the solvent property of [Ch][Cl]:2EG is quite different from those of amphiphilic protic and dipolar aprotic molecular solvents. The carbon dioxide absorption capabilities as well as apparent absorption kinetics for a series of anion-functionalized DESs ([Ch][X]:2EG) were measured, and the results show that the basicity of comprising anion [X] of choline salt is essential for the maximum carbon dioxide absorption capacity, i.e., a stronger basicity leads to a greater absorption capacity. The possible absorption mechanisms for carbon dioxide absorption in these DESs were also discussed based on the spectroscopic evidence.

A ferrocene-containing analogue of the MCU inhibitor Ru265 with increased cell permeability

An analogue of the mitochondrial calcium uniporter (MCU) inhibitor Ru265 containing axial ferrocenecarboxylate ligands is reported. This new complex exhibits enhanced cellular uptake compared to the parent compound Ru265.

Azide modification forming luminescent sp2 defects on single-walled carbon nanotubes for near-infrared defect photoluminescence

Azide functionalization produced luminescent sp²-type defects on single-walled carbon nanotubes, by which defect photoluminescence appeared in near infrared regions (1116 nm). Changes in exciton properties were induced by localization effects at the defect sites, creating exciton-engineered nanomaterials based on the defect structure design.

How to Predict the pK a of Any Compound in Any Solvent

Acid-base properties of molecules in nonaqueous solvents are of critical importance for almost all areas of chemistry. Despite this very high relevance, our knowledge is still mostly limited to the pK a of rather few compounds in the most common solvents, and a simple yet truly general computational procedure to predict pK a's of any compound in any solvent is still missing. In this contribution, we describe such a procedure. Our method requires only the experimental pK a of a reference compound in water and a few standard quantum-chemical calculations. This method is tested through computing the proton solvation energy in 39 solvents and by comparing the pK a of 142 simple compounds in 12 solvents. Our computations indicate that the method to compute the proton solvation energy is robust with respect to the detailed computational setup and the construction of the solvation model. The unscaled pK a's computed using an implicit solvation model on the other hand differ significantly from the experimental data. These differences are partly associated with the poor quality of the experimental data and the well-known shortcomings of implicit solvation models. General linear scaling relationships to correct this error are suggested for protic and aprotic media. Using these relationships, the deviations between experiment and computations drop to a level comparable to that observed in water, which highlights the efficiency of our method.

Interplay of Acidity and Ionic Liquid Structure on the Outcome of a Heterocyclic Rearrangement Reaction

The study of suitable probe reactions is a powerful tool to investigate the properties of nonconventional solvents such ionic liquids (ILs). In particular, we studied the acid-catalyzed mononuclear rearrangement of heterocycles (MHR) of the Z-phenylhydrazone of 5-amino-3-benzoyl-1,2,4-oxadiazole into the relevant 1,2,3-triazole, in solution of ILs by means of kinetic measurements. We chose as solvents six ILs differing both in the cation and anion, in the presence of five carboxylic and sulfonic acids as catalysts. For a useful comparison, the reaction was also performed in 1,4-dioxane and methanol. In general, the reaction occurs faster in ILs, compared to conventional solvents, according to the weaker solvation interactions operating in the former media. The effect of IL anion and cation on the reactivity and on the acidic strength of the catalysts was analyzed. To this aim, we measured the acidic strength of the sulfonic acids in each IL, estimated by the equilibrium formation constant of each acid with 4-nitroaniline. We found that the trend of reactivity as a function of the IL anion mainly reflects the larger difference in acidic strengths of the catalyst. Conversely, acidic strength spans a narrower range as a function of the IL cation. As a result, other factors come into play, such as the π-π interactions involving aromatic IL cations, substrate, and transition states, leading to a more articulate trend.

Counterintuitive solvation effect of ionic-liquid/DMSO solvents on acidic C-H dissociation and insight into respective solvation

How would acidic bond dissociation be affected by adding a small quantity of a weakly polar ionic liquid IL (the "apparent" or "measured" dielectric constant ε of the IL is around 10-15) into a strongly polar molecular solvent (e.g., ε of DMSO: 46.5), or vice versa? The answer is blurred, because no previous investigation was reported in this regard. Toward this, we, taking various IL/DMSO mixtures as representatives, have thoroughly investigated the effects of the respective solvent in ionic-molecular binary systems on self-dissociation of C-H acid phenylmalononitrile PhCH(CN)₂ via pK _a determination. As disclosed, in this category of binary media, (1) no linear correspondence exists between pK _a and molar fractions of the respective solvent components; (2) only ∼1-2 mol% of weakly polar ILs in strongly polar DMSO make C-H bonds even more dissociative than in neat DMSO; (3) a small fraction of DMSO in ILs (<10 mol%) can dramatically ease acidic C-H-dissociation; and (4) while the DMSO fraction further increases, its acidifying effect becomes much attenuated. These findings, though maybe counterintuitive, have been rationalized on the basis of the precise pK _a measurement of this work in relation to the respective roles of each solvent component in solvation.

Unexpected Strong Acidity Enhancing the Effect in Protic Ionic Liquids Quantified by Equilibrium Acidity Studies: A Crucial Role of Cation Structures on Dictating the Solvation Properties

The equilibrium acidities for several series of structural and electronic different organic acids were measured in 3-pyrrolidinium-based aprotic and protic ionic liquid (IL) analogues, that is, [Bmpy][NTf₂], [BpyH][NTf₂], and [PyH₂][NTf₂], by the UV-Vis method. The acidities of neutral acids are found to be much stronger in the protic ILs (PILs) than aprotic ILs (AILs), and the acidifying effect in the two PILs roughly increases proportionally to the number of protons in the cation of the PIL. On the other hand, interestingly, the cationic N⁺-H acids exhibit similar acidities in [Bmpy][NTf₂] and [BpyH][NTf₂] but much weaker than those in [PyH₂][NTf₂]. The Hammett ρ values for the acidic dissociation of para-substituted benzoic acids in two PILs are about the same as that in water (1.00) but significantly smaller than that in the AIL [Bmpy][NTf₂] (2.66). The correlations between the acidities in the PILs and water show double-linear relationships with different slopes and intercepts for the neutral and cationic acids. These, together with previous observations in the PILs [DBUH][OTf] (DBU = 1,5-diazabicyclo(5.4.0)-5-undecene) and EAN (ethylammonium nitrate), clearly indicate that the structure of the cation plays a subtle but a crucial role on sensing the electronic nature of solutes and strongly affects the solvation behaviors of PILs.

Proton Thermodynamics in a Protic Ionic Liquid, Ethylammonium Nitrate

In order to investigate the proton solvation state in protic ionic liquids (PILs), ten acid dissociation enthalpies and entropies of eight compounds were determined in ethylammonium nitrate (EAN). Regardless of the nature of the compound, 24 kJ mol^-1 larger enthalpy and 65 J mol^-1  K^-1 larger entropy than those in water, respectively, were observed. These values were reasonably explained by the differences in the proton solvation structure in EAN and water. Namely, protons in EAN exist as HNO₃ , having a higher reaction energy than that of H₃ O⁺ in water, undergo entropic stabilization as a result of the less-structured solvation. As such, the entropic effect of the proton solvation structure on the acid-base property is possibly applicable to all PILs. In addition, based on these proton thermodynamics, enthalpy and entropy windows were proposed as a novel perspective for the characterization of solvents. Use of this concept enabled the visualization of similarities and differences between EAN and water.

Cleaving DNA-model phosphodiester with Lewis acid-base catalytic sites in bifunctional Zr-MOFs

Organophosphates exist in many biomolecules. The design of artificial nucleases for efficient P-O bond cleavage is essential for the fields of genetic engineering and molecular biology. Herein, metal-organic frameworks (MOFs) with cooperatively isolated multi-catalytic active sites were utilized as heterogeneous catalysts for the hydrolytic cleavage of bis(p-nitrophenyl) phosphate (BNPP).

Equilibrium Acidities of Nitroalkanes in an Ionic Liquid

The acidity ladder scale in [BMPY][NTf₂] was successfully expanded toward the weak acidity region for about five more p K units compared to the previously established one. This allows the acidities of a series of 13 aliphatic and aromatic nitroalkanes to be determined accurately by the UV-vis spectroscopic method. The acidity of nitroalkane in [BMPY][NTf₂] covers ∼8 p K units and is significantly weaker than those in DMSO and water. The Hammett plot for 4-substituted phenylnitromethanes shows an excellent linearity with a slope of 2.06, which is rather close to that in DMSO but significantly larger than that in water (0.80). The regression analyses reveal that the solvation behavior of [BMPY][NTf₂] on the acidic dissociations of C-H acids is similar to that of DMSO.

Validation of pH Standards and Estimation of the Activity Coefficients of Hydrogen and Chloride Ions in an Ionic Liquid, Ethylammonium Nitrate

We selected and validated the pH values of three standard materials that function in the protic ionic liquid, ethylammonium nitrate (EAN). The pH values of 0.05 mol kg^-1 phthalate, oxalate, and phosphate buffers were 4.93 (0.04), 2.12 (0.04), and 7.13 (0.06), respectively (the values in the parentheses denote the standard deviation). Because the pH of EAN ranges from 0 to 10, with a neutral pH of 5, these materials are usable as acidic, basic, or neutral standards. The standard electrode potential of silver-silver chloride in EAN was 127.2 (1.7) mV. The activity coefficients of hydrogen and chloride ions remain equal to unity in EAN of a wide concentration range, which indicates that the effective ionic strength is independent of the solute ion concentration. In addition, the estimated value of the transfer activity coefficient of chloride ion suggests a weaker solvation in EAN compared with water in spite of a ubiquitous cation (C₂H₅NH₃⁺). These behaviors of ions in EAN can be explained by the unique solvation in the ionic liquid through direct ion-ion electrostatic interactions.

Voltammetric Perspectives on the Acidity Scale and H+/H2 Process in Ionic Liquid Media

Nonhaloaluminate ionic liquids (ILs) have received considerable attention as alternatives to molecular solvents in diverse applications spanning the fields of physical, chemical, and biological science. One important and often overlooked aspect of the implementation of these designer solvents is how the properties of the IL formulation affect (electro)chemical reactivity. This aspect is emphasized herein, where recent (voltammetric) studies on the energetics of proton (H⁺) transfer and electrode reaction mechanisms of the H⁺/H₂ process in IL media are highlighted and discussed. The energetics of proton transfer, quantified using the p K_a (minus logarithm of acidity equilibrium constant, K_a) formalism, is strongly governed by the constituent IL anion, and to a lesser extent, the IL cation. The H⁺/H₂ process, a model inner-sphere reaction, also displays electrochemical characteristics that are strongly IL-dependent. Overall, these studies highlight the need to carry out systematic investigations to resolve IL structure and function relationships in order to realize the potential of these diverse and versatile solvents.

Unexpected solvation-stabilisation of ions in a protic ionic liquid: insights disclosed by a bond energetic study

Equilibrium acidities (pK_as) of 42 organic acids were precisely determined in protic ionic liquid (PIL) [DBUH][OTf]. Surprisingly, the often seen homoassociation complication during the pK_a measurement of O-H acids in DMSO was not detected in [DBUH][OTf], implying that the incipient oxanion should be better solvation-stabilized by the PIL, although its "apparent" dielectric constant is much lower than that of the conventional molecular solvent DMSO. Evidence showing that the solute ions in the PIL are also free from other specific ion associations like ion-pairing is further demonstrated by the identical pK_as of protic amine salts bearing largely different counter-anions. Correlations between the RO-H, N-H, N⁺-H and RCOO-H bond acidities in [DBUH][OTf] and in water revealed different slopes and intercepts for each individual series, suggesting far superior properties of the DBUH⁺-based PIL for differentiating the solvation effect of various species in structural analysis to the well applied EAN that is known for leveling out differential solvation.

Experimental Insight into the Thermodynamics of the Dissolution of Electrolytes in Room-Temperature Ionic Liquids: From the Mass Action Law to the Absolute Standard Chemical Potential of a Proton

Room-temperature ionic liquids (ILs) are a class of nonaqueous solvents that have expanded the realm of modern chemistry, drawing increasing interest over the last few decades, not only in terms of their own unique physical chemistry but also in many applications including organic synthesis, electrochemistry, and biological systems, wherein charged solutes (i.e., electrolytes) often play vital roles. However, our fundamental understanding of the dissolution of an electrolyte in an IL is still rather limited. For example, the activity of a charged species has frequently been assumed to be unity without a clear experimental basis. In this study, we have discussed a standard component-based scheme for the dissolution of an electrolyte in an IL, supported by our observation of ideal Nernstian responses for the reduction of silver and ferrocenium salts in a representative IL, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim⁺][NTf₂ ^-] or [emim⁺][TFSI^-]). Using this scheme, which was also supported by temperature-dependent measurements with ILs having longer alkyl chains in the imidazolium ring, and the solubility of the IL in water, we established the concept of Gibbs transfer energies of "pseudo-single ions" from the IL to conventional neutral molecular solvents (water, acetonitrile, and methanol). This concept, which bridges component- and constituent-based energetics, utilizes an extrathermodynamic assumption, which itself was justified by experimental observations. These energies enable us to eliminate inner potential differences between the IL and molecular solvents (solvent-solvent interactions), that is, on a practical level, conditional liquid junction potential differences, so that we can discuss ion-solvent interactions independently. Specifically, we have examined the standard electrode potential of the ferrocenium/ferrocene redox couple, Fc⁺/Fc, and the absolute intrinsic standard chemical potential of a proton in [emim⁺][NTf₂ ^-], finding that the proton is more acidic in the IL than in water by 6.5 ± 0.6 units on the unified pH scale. These results strengthen the progress on the physical chemistry of ions in IL solvent systems on the basis of their activities, providing a rigorous thermodynamic framework.

Weakly Polar Aprotic Ionic Liquids Acting as Strong Dissociating Solvent: A Typical "Ionic Liquid Effect" Revealed by Accurate Measurement of Absolute pKa of Ylide Precursor Salts

Absolute pKas of selected salts with different counter-anions were measured with high precision in four aprotic ionic liquids (AILs), which enables a detailed examination of solvation effect of ILs on salts. Interestingly, the counter-anions of the ylide precursor salts, protic amine, and phenol salts of this study, though differing dramatically in size and electron dispersion, were found to have no effect on the respective pKas of the substrates. This indicates that the ionic species generated upon acidic dissociation of the salts in weakly polar AILs of low dielectric constant (ε: 10-15) are not ion-paired, or in other words, behave like "free ions" as if in strongly dissociating molecular solvents of high polarity (e.g., DMSO). This suggests that the widely assumed ion-pairing phenomenon, an issue of much debate, is not important in the AILs under our experimental conditions, presenting a typical "ionic-liquid effect" on the solvation of charged species in AILs.

Ionisation effects on the permeation of pharmaceutical compounds through silicone membrane

Silicone membrane is frequently used as an in vitro skin mimic whereby experiments incorporate a range of buffered media which may vary in pH. As a consequence of such variability in pH there is a corresponding variability in the degree of ionisation which in turn, could influence permeation through the mainly hydrophobic-rich membrane structure. This study reports the effect of pH on the permeation of five model compounds (benzoic acid, benzotriazole, ibuprofen, ketoprofen and lidocaine). For the five compounds analysed, each at three distinct percentages of ionisation, it was found that the greater extent of permeation was always for the more 'neutral', i.e. more greatly unionised, species rather than the anionic or cationic species. These findings fit with the theory that the hydrophobic membrane encourages permeation of 'lipid-like' structures, i.e. the more unionised form of compounds. However, results obtained with an Inverse Gas Chromatography Surface Energy Analyser (iGC SEA) indicate the membrane surface to be an electron dense environment. In the knowledge that unionised forms of compounds permeate (rather than the charged species) this negatively charged surface was not anticipated, i.e. the basic membrane surface did not appear to affect permeation.

Ionic liquid effects on a multistep process. Increased product formation due to enhancement of all steps

An ionic liquid is shown to increase the rate of all three steps in this imine formation and the microscopic origins of such are investigated. The magnitude of this enhancement varies with the nature of the substituent, though in all cases the rate of imine formation is increased.