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

An equilibrium acidity (pKa) 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.

Brönsted Basicities and Nucleophilicities of N-Heterocyclic Olefins in Solution: N-Heterocyclic Carbene versus N-Heterocyclic Olefin. Which Is More Basic, and Which Is More Nucleophilic?

A Brönsted basicity scale comprising nine representative N-heterocyclic olefins (NHOs) was established by measuring the equilibrium acidities of their corresponding precursors in DMSO using an ultraviolet-visible spectroscopic method. The basicities (pKaHs) of the investigated NHOs cover a range from 14.7 to 24.1. The basicities of unsaturated NHOs are stronger than those of their N-heterocyclic carbene (NHC) analogues; however, the basicities for the saturated ones are much weaker than those of their NHC analogues, which is largely due to the aromatization effect that intrinsically influences the acidic dissociations of NHC and NHO precursors. The nucleophilicities of four NHOs were measured photometrically by monitoring the kinetics of reactions of these NHOs with common reference electrophiles for quantifying nucleophilic reactivities. In general, the nucleophilicity of the NHOs is much stronger than that of commonly used Lewis bases such as Ph3P or DMAP [4-(dimethylamino)pyridine] but weaker than that of their NHC analogues; however, caution should be taken when generalizing this conclusion to a wide range of electrophiles with distinctively electronic and structural properties.

The Brönsted Basicities of N-Heterocyclic Olefins in DMSO: An Effective Way to Evaluate the Stability of NHO-CO2 Adducts.

A Brönsted basicity scale (∼24 pK units) for 85 commonly seen imidazole-, imidazoline-, triazole-, and thiazole-based N-heterocyclic olefins (NHOs) in DMSO was established using a well-examined computational model. The influence of substituents on the Brönsted basicities of these NHOs was investigated through basicity comparisons and rationalized by geometric analyses. The Gibbs energy (ΔGr) of the reaction between NHO and CO2 was also calculated, which linearly correlates with the basicity of the corresponding NHO, suggesting that the stability of NHO-CO2 adducts can be evaluated by the basicity of NHOs and a stronger basicity leads to a more stable NHO-CO2 adduct.

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.

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.

Comprehensive Basicity Scales for N-Heterocyclic Carbenes in DMSO: Implications on the Stabilities of N-Heterocyclic Carbene and CO2 Adducts.

A very broad acidity scale (≈40 pK units) for about 400 N-heterocyclic carbene precursors (NHCPs) with various backbones and electronic features, including imidazolylidenes, 1,2,4-triazolylidenes, cyclic diaminocarbenes (CDACs), diamidocarbenes (DACs), thiazolylidenes, cyclic (alkyl)(amino)carbenes (CAACs) and mesoionic carbenes (MICs), was established in DMSO by a well examined computational method. Varying the backbone structure or flanking N-substituents can have different extent of acidifying effects, depending on both the nature and number of substituent(s). The Gibbs energies (ΔGr s) for the reactions between the corresponding NHCs and CO2 were also calculated. There is a good linear correlation between the pKa s of most NHCPs and ΔGr s, suggesting that a greater basicity of NHC leads to a more stable NHC-CO2 adduct. Interestingly, the nearby asymmetric environment has virtually no differential effect on the acidities of the chiral NHCP enantiomers, but has a pronounced effect on the ΔGr values.

Structural features of selected protic ionic liquids based on a super-strong base.

Protic ionic liquids (PIL) were prepared from a super-strong base 1,7-diazabicyclo[5.4.0]undec-7-ene (DBU) and super-strong acids, trifluoromethane sulfonic acid (TfOH), and (trifluoromethanesulfonyl)-(nonafluorobutylsulfonyl)imide, (IM14H), ([DBUH][TfO] and [DBUH][IM14], respectively; the latter for the first time) and their chemical and physical properties and structural features have been explored using a synergy of experimental and computational tools. The short range order in neat DBU, as well as the long range structural correlations induced by charge correlation and hydrogen bonding interactions in the ionic liquids, have been explored under ambient conditions, where these compounds are proposed for a variety of applications. Similar to other [DBUH]-based PILs, the probed ones behave as good ionic liquids. Molecular dynamics-rationalised X-ray diffraction patterns show the major role played by hydrogen bonding in affecting morphology in these systems. Additionally, we find further evidence for the existence of fluorous domains in [DBUH][IM14], thus potentially extending the range of applications for these PILs.

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.

Acidity Scale of N-Heterocyclic Carbene Precursors: Can We Predict the Stability of NHC-CO2 Adducts?

The acidity scale (∼14 p K units) of more than 90 triazolium, imidazolium, and imidazolinium based NHC precursors in DMSO was established systematically by a well-developed computational model. The substituent effects on the acidities of these NHC precursors were analyzed through acidity comparison and Hammett correlation. The binding energy (Δ G1) of the reaction between NHC and CO2 was also calculated and linearly correlates with the basicity of the corresponding NHC, which implies that the stability of the NHC-CO2 adduct is essentially dictated by the basicity of NHC.

A Systematic Theoretical Study on the Acidities for Cations of Ionic Liquids in Dimethyl Sulfoxide.

The acidities of 40 commonly seen cations of ionic liquids (CILs) in dimethyl sulfoxide (DMSO) were investigated by a well-established theoretical method, SMD/M06-2X/6-311++G (2df,2p)//B3LYP/6-31+G(d). The calculated p Ka's agree excellently with the available experimental data and range from 20.0 to 45.8 with an acidity order of 1,3-dialkylimidazolium > amidinium > pyridinium > tetraalkylphosphonium > morpholinium > pyrrolidinium ≈ piperidinium > guanadinim cations. The established acidity scale in this work provides a useful tool, as verified by the acidity comparisons, to assess the stability of ILs under various extent basic conditions, and also reveals the relative basicity of several classical N-heterocyclic carbenes and olefins as well as ylides.

Recent Advances and Advisable Applications of Bond Energetics in Organic Chemistry.

Most organic transformation involves cleavage and formation of various covalent bonds, and naturally, can be regarded as a process of bond reorganization, which should be intrinsically related to bond energies (e.g., p Ka, BDE, etc.). However, in many cases such as in C-H bond activation/functionalization, direct correspondence between the bond energy and reaction rate or other relevant properties is only occasionally observed when applying the bond data by simple rules like the Linear Free-Energy Relationships (LFERs) in handling intricate reaction systems. In this Perspective, we present examples to argue that the above-mentioned situation is not a consequence of a diminishing role of the bond energetics in research, but most likely, comes from an improper use of energetic strategy, or simply due to a faulty selection of the data from unsuitable sources. Some advisable applications of bond energies in unscrambling the problems in modern day chemistry are exemplified through representative recent advances of the researches in this connection. Some of the possible directions of future research endeavors in the field of bond energetics and its prudent applications are recommended.

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

Equilibrium acidities (pKas) of 42 organic acids were precisely determined in protic ionic liquid (PIL) [DBUH][OTf]. Surprisingly, the often seen homoassociation complication during the pKa 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 pKas 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.

CO2 Absorption by DBU-Based Protic Ionic Liquids: Basicity of Anion Dictates the Absorption Capacity and Mechanism.

PILs are promising solvent systems for CO2 absorption and transformations. Although previously tremendous work has been paid to synthesize functionalized PILs to achieve a high-performance absorption, the underlying mechanisms are far less investigated and still not clear. In this work, a series of DBU-based PILs, i.e., [DBUH][X], with anions of various basicities were synthesized. The basicities of the anions were accurately measured in [DBUH][OTf] or extrapolated from the known linear correlations. The apparent kinetics as well as the capacities for CO2 absorption in these PILs were studied systematically. The results show that the absorption rate and capacity in [DBUH][X] are in proportional to the basicity of PIL, i.e., a more basic PIL leads to a faster absorption rate and a higher absorption capacity. In addition, the spectroscopic evidences and correlation analysis indicate that the capacity and mechanism of CO2 absorption in [DBUH][X] are essentially dictated by the basicities of anions of these PILs.

The Essential Role of Bond Energetics in C-H Activation/Functionalization.

The most fundamental concepts in chemistry are structure, energetics, reactivity and their inter-relationships, which are indispensable for promoting chemistry into a rational science. In this regard, bond energy, the intrinsic determinant directly related to structure and reactivity, should be most essential in serving as a quantitative basis for the design and understanding of organic transformations. Although C-H activation/functionalization have drawn tremendous research attention and flourished during the past decades, understanding the governing rules of bond energetics in these processes is still fragmentary and seems applicable only to limited cases, such as metal-oxo-mediated hydrogen atom abstraction. Despite the complexity of C-H activation/functionalization and the difficulties in measuring bond energies both for the substrates and intermediates, this is definitely a very important issue that should be more generally contemplated. To this end, this review is rooted in the energetic aspects of C-H activation/functionalization, which were previously rarely discussed in detail. Starting with a concise but necessary introduction of various classical methods for measuring heterolytic and homolytic energies for C-H bonds, the present review provides examples that applied the concept and values of C-H bond energy in rationalizing the observations associated with reactivity and/or selectivity in C-H activation/functionalization.

Absolute pKas of Sulfonamides in Ionic Liquids: Comparisons to Molecular Solvents.

Absolute pKas of 25 sulfonamides in four ionic liquids (ILs) were measured spectroscopically with high precision and subsequently compared with those in conventional molecular solvents. It is found that the acidity order of these sulfonamides is as follows: in water > in DMSO > in ILs > in acetonitrile (ACN). The well-known solvent polarity index ε fails to explain the observed stronger bond-weakening effect of ILs in comparison to that of ACN, whose ε value is much greater. In addition, the regression analyses show that the pKas of sulfonamides determined in ILs linearly correlate with these in molecular solvents of distinct properties, but with various slopes. A rationale and related discussion on the effect of solvation in ILs are presented.

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.

Toward Prediction of the Chemistry in Ionic Liquids: An Accurate Computation of Absolute pK(a) Values of Benzoic Acids and Benzenethiols.

In contrast to the great success of computational methodologies in molecular solvents, effective and accurate calculation of the fundamental bond energetic properties in ionic liquids (ILs) is essentially absent. This is largely due to the unusual complexity of handling solvation quantities of ILs and the lack of precisely determined bond parameters to serve as the authentic benchmark to calibrate the modeling methodology. Herein, we report the first accurate calculations of absolute pK(a) values in a commonly used IL, [Bmim][NTf2], with the carefully developed "ion-biased" cluster-continuum model. Experimental pK(a) values of benzoic acids and benzenethiols in [Bmim][NTf2] were reproduced with mean unsigned errors of only 0.3 and 0.5 pK(a) units, respectively, which enables theoretical approaches with a suitable strategy as a powerful tool to handle complicated problems in ILs and to eventually realize the rational design of the IL chemistry.

Is Amine a Stronger Base in Ionic Liquid Than in Common Molecular Solvent? An Accurate Basicity Scale of Amines.

The equilibrium basicities of 21 frequently used amines in two room-temperature ionic liquids (RTILs) were measured precisely. The standard deviation was much superior to that sparsely reported elsewhere. The data comparisons revealed that amines are stronger bases in ionic ligquids than in DMSO and water but weaker base than in acetonitrile (AN). Interestingly, regression analyses demonstrate that the basicity scales obtained in two RTILs correlate well with that in AN but not with those in water and DMSO.