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

An equilibrium acidity (p) 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 p 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.

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 (ps) of the investigated NHOs cover a range from 14.7 to 24.

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

A Brönsted basicity scale (∼24 p 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 (Δ) of the reaction between NHO and CO was also calculated, which linearly correlates with the basicity of the corresponding NHO, suggesting that the stability of NHO-CO adducts can be evaluated by the basicity of NHOs and a stronger basicity leads to a more stable NHO-CO adduct.

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 of the IL is around 10-15) into a strongly polar molecular solvent (, of DMSO: 46.5), or ? 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) p determination.

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][NTf], [BpyH][NTf], and [PyH][NTf], 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][NTf] and [BpyH][NTf] but much weaker than those in [PyH][NTf].

Comprehensive Basicity Scales for N-Heterocyclic Carbenes in DMSO: Implications on the Stabilities of N-Heterocyclic Carbene and CO 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 (ΔG s) for the reactions between the corresponding NHCs and CO were also calculated.

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.

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

PILs are promising solvent systems for CO 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.

Equilibrium Acidities of Nitroalkanes in an Ionic Liquid.

The acidity ladder scale in [BMPY][NTf] 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][NTf] covers ∼8 p K units and is significantly weaker than those in DMSO and water.

Acidity Scale of N-Heterocyclic Carbene Precursors: Can We Predict the Stability of NHC-CO 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 G) of the reaction between NHC and CO was also calculated and linearly correlates with the basicity of the corresponding NHC, which implies that the stability of the NHC-CO adduct is essentially dictated by the basicity of NHC.

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 K, BDE, etc.

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 K's agree excellently with the available experimental data and range from 20.0 to 45.

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

Equilibrium acidities (ps) of 42 organic acids were precisely determined in protic ionic liquid (PIL) [DBUH][OTf]. Surprisingly, the often seen homoassociation complication during the p 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 ps of protic amine salts bearing largely different counter-anions.

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.

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

Absolute pKs 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.

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.

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.

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).

Double-line Hammett relationship revealed through precise acidity measurement of benzenethiols in neat ionic media: a typical "ionic liquid effect"?

Equilibrium acidities (pKa) of 14 aromatic thiols in four pure room temperature ionic liquids (RTILs) were precisely determined and the corresponding acidity scales were established for the first time. Regression analyses show a distinct double-line Hammett relationship with similar slopes and excellent linearity (R(2) of 0.996-0.

Standard and absolute pKa scales of substituted benzoic acids in room temperature ionic liquids.

Equilibrium acidity (pKa) scales of 15 substituted benzoic acids in four room temperature ionic liquids (RTILs), BmimOTf, BmimNTf2, BmpyNTf2, and Bm2imNTf2, were established under standard conditions using a modified indicator overlapping method. The effect of homo hydrogen bonding on equilibrium acidity was calibrated, and the derived pKa values were evidenced to be free from ion-paring complication. Regression analyses demonstrated that all of the pKa scales obtained in four RTILs are linearly correlated to each other with an R value better than 0.

Copper catalysed azide-alkyne cycloaddition (CuAAC) in liquid ammonia.

Copper(I) catalysed azide-alkyne cycloaddition reactions (CuAAC) occur smoothly in liquid ammonia (LNH(3)) at room temperature to give exclusively 1,4-substituted 1,2,3-triazoles with excellent yields (up to 99%). The CuAAC reactions in liquid ammonia require relatively small amounts of copper(I) catalyst (0.5 mole%) compared with that in conventional solvents.

Copper(I)-catalyzed amination of aryl halides in liquid ammonia.

The amination of aryl halides in liquid ammonia (LNH(3)) is catalyzed by a copper(I) salt/ascorbate system to yield primary aromatic amines in good to excellent yields. The low concentrations of catalyst required and the ease of product isolation suggest that this process has potential industrial applications. Commonly used ligands for analogous metal-catalyzed reactions are not effective.