Hard Soft Acid Base Research Articles

Imparting gas selective and pressure dependent porosity into a non-porous solid via coordination flexibility

Using a simple hard–soft acid–base concept we have deliberately designed gas-specific and pressure dependent porosity into a non-porous solid via coordination flexibility.

Correlating Ionic Conductivity, Oxygen Transport and ORR with Structure of Dialkylacetamide-Based Electrolytes for Lithium-Air Batteries

Chemical structures of lithium and tetrabutylammonium (TBA) salt solutions in N,N-dimethylacetamide (DMAc) and N,N-diethylacetamide (DEAc), two high Donor Number organic solvents, have been studied. In LiX salt solutions (where X = PF6−, CF3SO3−, ClO4− and NO3−), solvation occurs when the Li+ bonds with the solvent's carbonyl group forming Li+[O=C(CH3)N(CH3)2]nX− ion pairs. Infrared and 13C-NMR spectra are consistent with the ion pair being solvent-separated when the anion is PF6−, ClO4− or NO3−, and a contact ion pair in the case of CF3SO3−. Chemical interactions between TBA+ and the solvents to form conducting solutions appeared to be dipolar in nature. Ionic conductivities of TBA+ and Li+ electrolytes were measured and correlated with their viscosities. In 0.1M TBAPF6/DMAc, the O2 solubility and diffusion coefficient (3.09 × 10−6 mol/cm−3 and 5.09 × 10−5 cm2s−1, respectively) measured using microelectrode technique are typical of values measured in several TBA+ solutions. Microelectrode voltammetry revealed steady-state limiting current behavior for oxygen reduction reactions (ORR) in TBAX/DMAc electrolytes indicating a reversible ORR process. Conversely, microelectrode current-voltage data for ORR in LiX/DMAc electrolytes revealed irreversible behavior mainly ascribed to the blockage of the electrode surface by insoluble ORR products. The ORR in DMAc correlated with its high Donor Number and the overall process conformed to the Hard-Soft Acid-Base theory.

Experimental and Computational Studies of the Adsorption of Furan, Pyrrole, and Thiophene on Hydroxyapatites in a Single and Ternary Component

Non-doped (Hap) and doped hydroxyapatites with Cu2+ and Fe2+ were synthesized and used to study the adsorption of furan (Fur), pyrrole (Pyr), and thiophene (Thi) in single and ternary component systems. Spectroscopic data and N2 isotherms indicated there were no structural alterations in Hap with the incorporation of Cu2+ and Fe2+. For both single and ternary components, the incorporation of the metal ions resulted in an adsorption increment consistent with the hard-soft acid-base concept. When comparing the systems, there is an inversion with Thi (5.41 mg g-1) and Fur (5.51 mg g-1) being the most adsorbed species in the single and ternary components, respectively. Monte Carlo simulations helped explain the observed experimental trends. While the electrostatic effect seems to prevail in the single system, in the ternary one the mass transport phenomenon is also operative implying greater availability of Fur on surface leading to its higher adsorption efficiency.

Sorption of different metal ions on magnetic nanoparticles and their effect on nanoparticles settlement

The present work describes the sorption of Cs+, Sr2+, La3+ and Th4+ on Fe3O4 nanoparticles (NPs). The selection of metal ions was done on the basis of different charges to see its effect on sorption. Experiments were carried out at two different pH, 6 and 9 and in presence of NaCl salt to see the effect of these factors on metal ion removal rate. Interestingly no sorption of metal ions were observed at pH ∼ 6 while at pH ∼ 9 only Sr2+ and La3+ ions can be selectively sorbed on NPs surface. Experiments in presence of high concentration of salt suggest that the presence of salt does not have an appreciable effect on sorption suggesting the formation of an inner sphere complex of these metal ions on NPs surface. Sr2+ and La3+ sorption over NPs surface facilitates the enhancement of settlement of NPs. NPs settlement rate was monitored in presence of different metal ions and interestingly was found to vary with metal ions. The results were explained on the basis of DLVO theory, HSAB (hard soft acid base) principle, hydrogen bonding and different charges on metal ions.

Nanostructured Semiconducting Metal Oxides for Ammonia Sensors. A Novel HSAB Sensing Paradigm

The paper demonstrates how the Hard-Soft Acid Base (HSAB) theory can be used as a valuable criterion in the selection process of semiconducting metal oxides (MOX) suitable as sensing layers for ammonia detection. Six different cases of ammonia detection performed by chemiresistive sensors employing MOX and related nanocomposites as sensing layers are identified and discussed. The role of HSAB as an efficient selection tool for appropriate sensing layer (any type of gas), is further reinforced by analyzing and discussing literature results on MOX-based trimethylamine sensing layers. By analyzing the operation of a fiber-optic ammonia sensor, we demonstrate that the HSAB principle can be also successfully applied to the selection of sensing layers for detectors employing other sensing principles, different than the chemiresistive one. Changing the sensing paradigm (i.e., the amino groups-based compounds are part of the sensing layer, rather than part of the analyte), the paper shows that these types of molecules (polymers, carbon nanotubes, ionic liquids) are appropriate constituents of a CO2 sensing layer, in full accordance to the HSAB criteria.

Design of a Highly-Stable Pillar-Layer Zinc(II) Porous Framework for Rapid, Reversible, and Multi-Responsive Luminescent Sensor in Water

The hard–soft-acid–base (HSAB) and dual-ligand strategies have been used to construct a pillar-layer luminescent metal–organic framework (MOF) {[Zn2(TRZ)2(DBTDC-O2)]·DMAc}n (1), in which H2DBTDC-O2 = S,S-dioxodibenzothiophen-3,7-dicarboxylic acid and HTRZ = 1,2,4-triazole. Thanks to its high stability in water and quadrangle microporous channels, MOF 1 can be served as a platform of recyclable multi-responsive luminescent sensor for Fe3+, Al3+, SiF62–, Cr2O72–, nitrofurantoin (NFT), and nitrofurazone (NFZ) in water. This is the first MOF-based sensor for detecting SiF62– anion via luminescent enhancement effect. Significantly, 1 also represents a unique example to probe NFZ or NFT by using transition metal MOFs in water, which exhibits the most sensitive sensing ability for NFZ (Ksv = 5.2 × 104 M–1) and NFT (Ksv = 1.8 × 105 M–1) via luminescent quenching.

Fabrication of xanthate-modified chitosan/poly(N-isopropylacrylamide) composite hydrogel for the selective adsorption of Cu(II), Pb(II) and Ni(II) metal ions

A novel xanthate-modified chitosan/poly(N-isopropylacrylamide) (X-CS/NIPAAm) composite hydrogel was fabricated by a facile two-step method for the selective adsorption of Cu(II), Pb(II) and Ni(II) metal ions from aqueous solutions. The equilibrium adsorption data fitted well to the Langmuir model and the adsorption kinetic data followed the pseudo-second-order model. The maximum equilibrium adsorption capacities of composite hydrogel to Cu(II), Pb(II) and Ni(II) at 293K were about 115.1, 172.0 and 66.9mgg−1, respectively. The thermodynamics parameters showed that the adsorption processes to Cu(II), Pb(II) and Ni(II) were spontaneous and exothermic. The interactions between metal ions for multi-components systems were evaluated by the values of Qb/Qs and Qt/Qs. The adsorption mechanism was analyzed by XPS spectra and the Hard–Soft, Acids–Bases (HSAB) theory. The X-CS/NIPAAm composite hydrogel could be regenerated and repeatedly utilized at least five cycles without obvious adsorption capacity loss.

Controlling Reversible Expansion of Li2O2 Formation and Decomposition by Modifying Electrolyte in Li-O2 Batteries

Summary The aprotic lithium-oxygen (Li-O2) battery has attracted worldwide attention because of its ultrahigh theoretical energy density. However, its practical application is critically hindered by cathode passivation, large polarization, and severe parasitic reactions. Here, we demonstrate an originally designed Ru(II) polypyridyl complex (RuPC) though which the reversible expansion of Li2O2 formation and decomposition can be achieved in Li-O2 batteries. Experimental and theoretical results revealed that the RuPC can not only expand the formation of Li2O2 in electrolyte but also suppress the reactivity of LiO2 intermediate during discharge, thus alleviating the cathode passivation and parasitic reactions significantly. In addition, an initial delithiation pathway can be achieved when charging in turn; thus, the Li2O2 products can be decomposed reversibly with a low overpotential. Consequently, the RuPC-catalyzed Li-O2 batteries exhibited a high discharge capacity, a low charge overpotential, and an ultralong cycle life. This work provides an alternative way of designing the soluble organic catalysts for metal-O2 batteries.

Lewis Acid-Promoted Enantioselective Dearomative Spirocyclizations of Allenes.

A chiral oxazaborolidine combined with SnCl4 has been found to promote the dearomative spirocyclization of electron-rich benzyl allenyl ketones. The reaction outcome is sensitive to the nature of activating acid, which was rationalized using hard-soft acid-base (HSAB) theory. The spirocyclic product was obtained with up to 72% ee, which is the best result reported to date for these substrates. The formation of cross-conjugated or conjugated products is readily controlled by changing the oxygen-protecting groups.

A Solvent-Controlled Oxidation Mechanism of Li2O2 in Lithium-Oxygen Batteries

Summary Long-unresolved discrepancies in the oxidation mechanism of lithium peroxide (Li2O2) impede electrode/electrolyte development for Li-O2 batteries. In this study, we report direct evidence of the formation of soluble LiO2 upon the oxidation of Li2O2 and reveal a strong solvent-controlled Li2O2-oxidation reaction mechanism. We exploit a thin-film rotating ring-disk electrode to show that soluble LiO2 is generated when oxidizing Li2O2 in high-donicity solvent but is absent in low-donicity glyme-based solvent. Synchrotron-based X-ray absorption near-edge structure spectroscopy further proved the presence of LiO2 upon Li2O2 oxidation in high-donicity solvent but not in low-donicity solvent. We show that preferential formation of soluble LiO2 in the high-donicity solvent could account for poor cycling stability, which suggests that strategies that bypass the formation of soluble LiO2 upon Li2O2 oxidation are critical. Our work offers new insights in resolving the discrepancy in Li-O2 charging mechanism and design strategies to achieve efficient and long-life Li-O2 batteries.

Elucidating the Structure and Electrochemical Properties of Dimethylacetamide-Based Electrolytes for Lithium-Air Batteries

Improving the performance and rechargeability of Lithium-air batteries, which offer up to ten times more energy density than Li-ion batteries, has been the focus of much research in the last decade.1, 2 One of the ways to improve these batteries is to gain a deeper understanding of the oxygen reduction reactions (ORR) in non-aqueous electrolytes in order to find an optimum combination of Li salt and solvent that provides chemical stability and rechargeability. In this paper, we report the results of a study of the physical and electrochemical properties of N,N-dimethylacetamide (DMAc)-based electrolytes containing either lithium or tetrabutylammonium (TBA) salts (LiX and TBAX, where X= PF6 -, CF3SO3 -, ClO4 - and NO3 -). The chemical structures of the salt solutions were determined from NMR and FTIR spectroscopic data. The spectra revealed that DMAc (CH3CON(CH3)2) solvates the lithium salts via bonding between its carbonyl (C=O) group and Li+ to form ion pairs. In the case of Li salts with the anions PF6 -, ClO4 - or NO3, the ion pairs are solvent separated while in the case of CF3SO3 -, it is a contact ion pair. On the other hand, the nature of the interactions between DMAc and the TBA+ ions appeared to be dipolar in agreement with its soft Lewis acidity.3 The conductivities of these electrolytes were measured as a function of temperature from -20 to 40 oC and correlated with their concentrations and viscosities. The conductivities plotted versus temperature showed a linear fit for Vogel-Tamman-Fulcher (VTF) relationship indicating a solvent-assisted ion conduction mechanism. Diffusion coefficient and solubility of O2 in the various electrolytes measured using a microelectrode technique are reported in Table 1. Cyclic voltammograms (CV) for ORR on microelectrode in TBA/DMAc electrolytes (Figure 1A) showed steady-state limiting current behavior indicating that the ORR process is electrochemically reversible. In all Li/DMAc electrolytes the CVs (Figure 1B) on microelectrodes showed an electrochemical irreversible behavior. The irreversibility of the ORR process in Li/DMAc electrolytes is primarily caused by the deposition of insoluble reduction products on the electrode surface. The results obtained in this study correlates with the high donor number of DMAc (27.8) and comply with the Hard Soft acid base (HSAB) theory.3, 4

Visualization of Phase-Transfer Catalysis through Charge-Transfer Complexes

Unlike the large number of laboratory experiments on phase-transfer catalysis (PTC) that focus on a catalyzed chemical process, the present laboratory emphasizes the mechanism of phase-transfer catalysis itself. Students can follow visually the actual transfer of the reagent by the catalyst, and its consumption in the organic phase, in a step-by-step process that employs a colored charge-transfer (CT) complex as the transferred reacting species. Consumption of the transferred nucleophile is followed by discoloration of the organic phase. This transfer-consumption cycle can be repeated several times by shaking the two separated phases again, allowing students to finally grasp the whole process as a continuous repetition of these cycles. The experiment also links, through visualization, the important synthetic tool of PTC with abstract concepts based on molecular-orbital and hard–soft acid–base theories, invoked in the formation of a CT complex.

Construction of heterometallic M2Pd3 supramolecular cages via a metalloligand strategy as heterogeneous catalyst for Suzuki–Miyaura coupling reaction

On the basis of hard soft acid base theory, two neutral heterometallic M2Pd3 supramolecular cages, [Al2Pd3(L)6Cl6] (1) and [Fe2Pd3(L)6Cl6] (2) (where HL is 1-(4-(1H-imidazol-1-yl)phenyl)butane-1,3-dione), have been constructed by using the tripodal M(L)3 as metalloligands in combination with PdCl2 or Pd(MeCN)2Cl2. The supramolecular cage can be described as a trigonal bipyramid with two M3+ ions occupying at the two axial sites in ΔΔ or ΛΛ configurations and three Pd2+ ions sitting in the three equatorial positions with square planar geometry in trans configuration. The supramolecular cages exhibit high catalytic activity for the Suzuki–Miyaura coupling with arylbromide or arylchrolide and phenylboronic acid in biorenewable solvent glycerol under mild conditions. Moreover, the catalyst can be readily recycled and reused at least five times without loss of catalytic activity. The high catalytic performance may be attributed to the distinct properties of the cage structure with uniformly distributive and well-defined Pd active centres on the cage surfaces. The high stability in the cage structure also prevents Pd leaching out and agglomeration.

Biosorption of beryllium from aqueous solutions onto modified chitosan resin: Equilibrium, kinetic and thermodynamic study

ABSTRACTThe goal of the present work is the recovery of beryllium ion from their solution by modified chitosan hydrogel. Chitosan was chemically cross-linked with glutaraldehyde to prevent its dissolution in aqueous acidic solutions. The obtained chitosan/glutaraldehyde adsorbent was reacted with chloroacetic acid to produce carboxymethyl chitosan (CMC), which was converted into sodium form by reaction with sodium hydroxide solution to increase its hydrophilic properties. The chemically synthesized chitosan adsorbent contains carboxylate group that expected to have a strong affinity to beryllium ions according to the hard-soft acid-base concept by Pearson because that beryllium ion is a hard acid and has smaller ionic radii. The synthesized adsorbent was characterized and its affinity towards beryllium ions was tested. The different experimental parameters including pH, beryllium concentration, agitation period and temperature were studied to optimize the biosorption process. The maximum biosorption values of beryllium species on the investigated biosorbent are 44.96 and 36.72 mg/g at pH 1 and 5, respectively. Kinetics and thermodynamic parameters of the biosorption process were evaluated from kinetic and biosorption experiments. The adsorbed beryllium species were eluted with a 3 M H2SO4 solution.

Enhanced visible-light photocatalytic activity to volatile organic compounds degradation and deactivation resistance mechanism of titania confined inside a metal-organic framework

Poor visible-light-driven activity and deactivation property as well as wide band gap of the most common TiO2 photocatalyst significantly limits its practical application in volatile organic compounds (VOCs) purification. In this study, tiny TiO2 nanoparticles incorporated into a typical metal-organic framework (MOF), NH2-UiO-66, with controllable TiO2 content and size, were synthesized based on the hard-soft acid-base (HSAB) principle and applied to VOCs purification. Compared to bare TiO2, the TiO2@NH2-UiO-66 composites could extend the optical absorption to the visible light range and accelerate the photogenerated electrons-holes separation, due to the excellent interface contact between TiO2 and NH2-UiO-66. Moreover, the abundant interconnected 3D cavities of the outer MOF allowed for VOCs to easily diffuse into the pores, producing a concentration microenvironment around the encapsulated TiO2. The TiO2@NH2-UiO-66 composites exhibited a markedly improved photocatalytic efficiency and a good resistance to deactivation during the photocatalytic degradation of gaseous styrene under visible light illumination, which were associated with the synergetic effects between the TiO2 and MOF. The TiO2@NH2-UiO-66 with 5 wt% TiO2 could efficiently mineralize styrene to CO2 to some extent companying with the removal ratio >99% within 600 min, whereas the removal efficiency over the bare TiO2 only 32.5%.

Non‐classical Design of High‐Efficiency Sensitizers for Dye‐Sensitized Solar Cells

The dye regeneration step in a dye‐sensitized solar cell (DSSC) affects significantly the device efficiency. To be able to predict the dye regeneration efficiency by the electrolyte this paper provides a facile way to design high‐efficiency sensitizers for DSSC. This paper proposes, for the first time, a simple and ingenious way to identify the dye regeneration sites and their relative efficiencies when a specific electrolyte is used. Two steps are proposed to identify the dye regeneration sites and their relative regeneration efficiencies: (1) drawing all the resonance structures of the oxidized dye to determine the regeneration sites, and (2) choosing the most favored site for dye regeneration as the chemically softest (when the redox couple used is soft I−/I3− pair) and the least spatially hindered site. The regeneration sites identified by the resonance structures are consistent with the β‐LUSO (β lowest unoccupied spin orbital) distribution, which is generally used for identifying the dye regeneration sites, calculated with DT‐DFT theory. The relative dye regeneration efficiency and photovoltaic performance of both ruthenium and metal‐free organic dyes predicted by the method reported here are supported by experimental data and the proposed dye regeneration mechanism. Several types of dye molecules are used to demonstrate the correctness of this new tool. This non‐classical tool, which uses the well‐known chemical knowledge of the resonance structure and hard–soft acid–base principle, without any computer calculation or physicochemical measurement, provides a very simple and powerful tool to quickly conceive high‐efficiency sensitizers for DSSCs.

Optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals.

Optically-active nanocrystals (such as quantum dots and plasmonic noble metal nanoparticles) have received great attention due to their size-tunable optical properties. The indicator displacement assay (IDA) with optically-active nanocrystals has become a common practice for optical sensor development, since no sophisticated surface functionalization of nanoparticles is required. Among the IDA-based optical sensors, the use of metal ions as receptors seems to be attractive. Therefore, in this review, the research progress of optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals has been summarized. In particular, metal ion-mediated selective recognition has been summarized here based on the classical Hard-Soft-Acid-Base (HSAB) principle, which has been seldom mentioned before. Most of the references were therefore categorized according to their located place based on the HSAB theory. Besides, several metal ion modulation strategies that were not related to the HSAB theory (e.g., redox modulation) were also included. Finally, due to the cross-talk of metal ions in selective recognition, we have also summarized sensor array development based on multiple metal ion receptors in IDA sensing with optically-active nanocrystals. Several interesting applications of the IDA sensing with metal ions as receptors and optically-active nanocrystals as indicators are presented, with specific emphasis on the design principles and photophysical mechanisms of these probes.

Unraveling the Correlation between Solvent Properties and Sulfur Redox Behavior in Lithium-Sulfur Batteries

Systematic understanding of how solvent property influences Li-S redox chemistry is required to develop an effective electrolyte for Li-S batteries. In this study, we investigate the correlation between solvent property and Li-S redox chemistry in nine non-aqueous electrolyte solvents that cover a wide range of three main solvent physiochemical properties, namely dielectric constant (ɛ), Gutmann donor number (DN), and acceptor number (AN). We exploit various analytical techniques including cyclic voltammetry, rotating ring disk electrode technique, UV-Vis spectroscopy and galvanostatic measurement in a two-compartment cell. We show that the potential of S8-reduction increases with increasing AN and that the polysulfide-reduction/oxidation is strongly influenced by the DN. The common discrepancy in the literature on the role of dielectric constant and donor number is addressed by examining the redox reactions, polysulfide stability, and the effect of salt concentration in acetonitrile - a solvent with high dielectric constant and low DN. We show that the DN is the primary descriptor for polysulfide redox reactions, as it controls the effective charge density of the solvated cation (Li+), which affects the stability of polysulfides with different charge density via Pearson's Hard Soft Acid Base theory.

The HSAB principle from a finite-temperature grand-canonical perspective

We provide a new proof for Pearson’s hard/soft acid/base (HSAB) principle. Unlike alternative proofs, we do not presuppose a simplified parabolic dependence on the energy of the system with respect to changes in its number of electrons. Instead, we use the more physically grounded finite-temperature formulation of the grand-canonical ensemble. We show that under the usual assumptions regarding the chemical potentials and hardnesses of the involved species, the HSAB rule holds for a wide range of temperatures.

Influence of counterion on the methylation of some ambident nucleophiles: DFT study

The reactions of ambident anions YCHX− and nucleophiles YCHXNa (X, Y = CH2, O, and S) with CH3F were investigated in the gas phase and THF solution. For this purpose the M06-2X/6-311+G(d,p) theoretical model was engaged in combination with the CPCM solvation model. The goal of the work was to examine the influence of counterion (Na+ in this case) to the reaction systems. It was found that the order of reactivity of non-ambident anions in both media is as follows: $${\text{CH}}_{2} {\text{CHCH}}_{2}^{ - }$$ > OCHO− > SCHS−. On the other side, the reactivity of non-ambident nucleophiles follows different order: CH2CHCH2Na > SCHSNa > OCHONa. The latter order of reactivity is in agreement with chemical intuition as it is inversely proportional to the order of hardness of the respective nucleophilic atoms: C < S < O. Ambident nucleophiles behave differently in the two media, thus indicating that solvents also influence the examined reactions. Namely, HOMO–LUMO gaps are smaller in the gas-phase, whereas charge distribution is more favorable in THF solution. Owing to these facts, C-methylation is more favorable than S- and O-methylations, and S-methylation is less unfavorable than O-methylation in the gas-phase. In THF solution O- and S-methylations are kinetically controlled, and competitive C-methylations are thermodynamically controlled. In this way, the applicability of the hard–soft acid–base theory to the examined reaction systems is confirmed. Furthermore, the findings of this work offer robust confirmation to our approach which involves counterions in examinations. Counterions do exist in reaction systems, and their influence should not be neglected.