Formation Of Acid-base Adducts Research Articles

Asymmetric catalysis by chiral FLPs: A computational mini-review.

Steric hindrance in Lewis acid (LA) and Lewis base (LB) obstruct the Lewis acid-base adduct formation, and the pair was termed as frustrated Lewis pair (FLP). In the past 16years, the field of enantioselective catalysis by chiral FLPs has been slowly growing. It was shown that chiral LAs are significant as they are involved in the hydrogen transfer (HT) step to the imine, resulting in enantioselectivity. After H2 activation, the borohydride can exist in a number of plausible conformations and their stability is governed by the presence of noncovalent interaction through C-H····π and π····π interactions. However, LBs are not ideal for asymmetric induction as they compete with the imine substrate as a counter LB. Further, the proton transfer from chiral LB to the imine does not induce any chirality as chirality develops in the HT step. However, intramolecular FLPs with chiral scaffold are very efficient as they possess an optimum distance between LA and LB, which facilitates the H2 activation but precludes the adduct formation of the small molecules substrate with the LA component. This mini-review summarizes computational investigation involving chiral LA and LB, and discusses intramolecular FLPs in the enantioselective catalysis.

Lewis Acid-Base Adducts of α-Amino Acid-Derived Silaheterocycles and N-Methylimidazole

In chloroform solution, the reaction of bis(tert-butylamino)dimethylsilane ((tBuNH)2SiMe2) and an α-amino acid (α-amino isobutyric acid, H2Aib; D-phenylglycine, H2Phg; L-valine, H2Val) in the presence of N-methylimidazole (NMI) gave rise to the formation of the pentacoordinate silicon complexes (Aib)SiMe2-NMI, (Phg)SiMe2-NMI and (Val)SiMe2-NMI, respectively. Therein, the amino acid building block was a di-anionic bidentate chelator at the silicon atom. In solution, the complexes were involved in rapid coordination–dissociation equilibria between the pentacoordinate Si complex (e.g., (Aib)SiMe2-NMI) and its constituents NMI and a five-membered silaheterocycle (e.g., (Aib)SiMe2), as shown by 29Si NMR spectroscopy. The energetics of the Lewis acid-base adduct formation and the competing solvation of the NMI molecule by chloroform were assessed with the aid of computational methods. In CDCl3 solution, deuteration of the silaheterocycle NH group proceeded rapidly, with more than 50% conversion within two days. Upon cooling to −44 °C, the chloroform solvates of the adducts (Aib)SiMe2-NMI and (Phg)SiMe2-NMI crystallized from their parent solutions and allowed for their single-crystal X-ray diffraction analyses. In both cases, the Si atom was situated in a distorted trigonal bipyramidal coordination sphere with equatorial Si–C bonds and an equatorial Si–N bond (the one of the silaheterocycle). The axial positions were occupied by a carboxylate O atom of the silaheterocycle and the NMI ligand’s donor-N-atom.

Enhancing the stability of the polymeric Lewis-base-assisted dual-phase 3D CsPbBr3–Cs4PbBr6 perovskite by molecular engineering and self-passivation

Self-ordered polymeric Lewis acid–base adduct formation for a dual role of passivated CsPbBr3–Cs4PbBr6 perovskite grain morphology modulation and exciton dielectric confinement, thereby exhibiting high performances and stability in PeLEDs.

Functionalization of antimonene and bismuthene with Lewis acids.

Elemental 2D pnictogens (group 15) are an interesting class of materials with tunable band structures and high carrier mobilities. Heavier pnictogens (Sb and Bi) are stable under ambient conditions compared to lighter members (P and As) and are emerging as interesting candidates for various electronic and optoelectronic applications. The reactivity of these materials is due to the presence of a lone pair which can be effectively utilized to tune material properties via different functionalization strategies. In this work, we have synthesized antimonene and bismuthene nanosheets by liquid exfoliation which are emissive in the visible range and functionalized these nanosheets with group 12 and 13 Lewis acids (ZnCl2, CdCl2, BCl3, GaCl3, AlCl3, and InCl3). Interaction of these Lewis acids with the lone pairs on Sb/Bi leads to the formation of Lewis acid-base adducts with the corresponding changes in the bonding environment along with lattice distortion and rehybridization of the band structure. Interestingly, the changes in band structure upon functionalization were realized as a blue shift in the emission of few-layered Sb and Bi. This is the first report on the functionalization of heavier pnictogens by the formation of Lewis acid-base adducts and opens a path for tuning their properties for integration in electronic and optoelectronic devices.

Towards understanding the doping mechanism of organic semiconductors by Lewis acids.

Precise doping of organic semiconductors allows control over the conductivity of these materials, an essential parameter in electronic applications. Although Lewis acids have recently shown promise as dopants for solution-processed polymers, their doping mechanism is not yet fully understood. In this study, we found that B(C6F5)3 is a superior dopant to the other Lewis acids investigated (BF3, BBr3 and AlCl3). Experiments indicate that Lewis acid-base adduct formation with polymers inhibits the doping process. Electron-nuclear double-resonance and nuclear magnetic resonance experiments, together with density functional theory, show that p-type doping occurs by generation of a water-Lewis acid complex with substantial Brønsted acidity, followed by protonation of the polymer backbone and electron transfer from a neutral chain segment to a positively charged, protonated one. This study provides insight into a potential path for protonic acid doping and shows how trace levels of water can transform Lewis acids into powerful Brønsted acids.

Model for Metal Extraction from Chloride Media with Basic Extractants: A Coordination Chemistry Approach

The metal extraction mechanism of basic extractants is typically described as an anion exchange process, but this mechanism does not correctly explain all observations. This paper introduces a novel model for the extraction of metals by basic extractants from chloride media supported by experimental data on methyltrioctylammonium chloride and Aliquat 336 chloride systems. This model relies on the hypothesis that the metal species least stabilized in the aqueous phase by hydration (i.e., the metal species with the lowest charge density) is extracted more efficiently than the more water stabilized species (i.e., species with higher charge densities). Once it is transferred to the organic phase, the extracted species can undergo further Lewis acid-base adduct formation reactions with the chloride anions available in the organic phase to form negatively charged chloro complexes, which than associate with the organic cations. Salting-out agents influence the extraction, most likely by decreasing the concentration of free water molecules, which destabilizes the metal complex in the aqueous phase. The evidence provided includes (1) the link between extraction and transition-metal speciation, (2) the trend in extraction efficiency as a function of the concentration of different salting-out agents, and (3) the behavior of HCl in the extraction system. The proposed extraction model better explains the experimental observations in comparison to the anion exchange model and allows the prediction of optimal conditions for metal extractions and separations a priori, by selecting the most suitable salting-out agent and its concentration.

Crystallization and grain growth regulation through Lewis acid-base adduct formation in hot cast perovskite-based solar cells

The precise control of perovskite crystallization process that could provide smooth films with large grains is a prerequisite for solar cell fabrication to achieve higher efficiencies. To regulate this crystallization process and to achieve it multiple times remains a humongous challenge. This process has been reproducibly demonstrated here through efficient molecular exchange via a Lewis acid-base adduct formation. In this work, a Lewis acid-base adduct approach in concurrence with hot casting technique was employed to efficiently control the perovskite crystallization by judiciously adding dimethyl sulfoxide (DMSO) to a precursor solution of lead iodide (PbI 2 ) and methyl ammonium chloride (MACl) in dimethylformamide (DMF). High quality perovskite films were fabricated by precisely controlling the volume of DMSO which resulted in low recombination rate and which had better light harvesting ability. Uniform films of MAPbCl x I 3-x having large grain size were formed reproducibly on addition of 1.5 equivalents (eq.) DMSO to the precursor solution. Perovskite solar cells fabricated using this solution resulted in maximum power conversion efficiency (PCE) of 14.11% with enhanced stability as compared to the reduced PCE values obtained with other DMSO ratios and pure DMF solution cast films. • Hot casting using Lewis acid-base approach resulted in large grain perovskite. • 1.5 equivalents of Lewis base (DMSO) gave largest grain size and smooth film. • A power conversion efficiency >14% achieved in inverted perovskite solar cell. • This method also improved the overall stability of the solar cells.

Interfacial Lewis Acid-Base Adduct Formation Probed by Vibrational Spectroscopy.

Understanding Lewis pair (LP) interactions at heterogeneous environments is important for controlling surface reactions. We report the formation of interfacial Lewis adducts with tris(pentafluorophenyl)borane as the Lewis acid and 4-mercaptobenzonitrile attached to gold as the Lewis base. We use the nitrile vibrational frequency as a probe of adduct strength, with stronger adducts leading to larger frequencies. The vibrational frequency shifts of the surface adducts were measured via sum frequency generation spectroscopy and compared to the frequency shifts of bulk adducts. Our results show a distinctly smaller frequency shift for the surface adducts compared to the bulk, indicating a weaker Lewis acid-base interaction near the surface. We explore three possible origins of this difference: interfacial frustration, surface electric fields, and electronic energy level alignment. We highlight the relevance of each and note that likely more than one of them affect the observed surface LP interactions.

First-principles investigation of the Lewis acid-base adduct formation at the methylammonium lead iodide surface.

We have here performed a campaign of ab initio calculations focusing on the anchoring mechanism and adduct formation of some Lewis bases, both aliphatic and aromatic, on a PbI2-rich flat (001) methylammonium lead iodide (MAPI) surface. Our goal is to provide theoretical support to the recently reported experimental techniques of MAPI surface passivation via Lewis acid-base neutralization and similarly of MAI·PbI2·(Lewis base) adduct formation. We tested several X-donor bases (X = :N, :O, :S), paying attention to the thermodynamic stability of the final MAPI·base adducts and to their electronic properties. Factors that impact on the passivation mechanism are the directionality of the Lewis base lone pair and its enhanced/reduced overlap with MAPI Pb p orbitals, the dipole moment of the base and, similarly, the electronegativity of the X donor atom. Also non-covalent interactions, both at the surface side (intra, MAPI) and at the very interface (inter, MAPI·Lewis base), seem to contribute to the stability of the final adducts. Here we show that the thermodynamic stability does not necessarily correspond to the most effective base → acid dative bond formation. Starting from a low coverage (12.5% of the undercoordinated Pb atoms available at the surface are passivated) this paper paves the way towards the study of cooperative and steric effects among Lewis bases at higher coverages representing, to the best of our knowledge, one of the very first studies focusing on the molecular anchoring on the surfaces of this very important class of materials.

Communication—A Phosphorus Pentafluoride Scavenger to Suppress Solid Electrolyte Interphase Damage at Moderately Elevated Temperature

We demonstrate that lithium/silicon monoxide (SiO) cell failure can be hindered by utilizing a Lewis-basic tris(2,2,2-trifluoroethyl) phosphite (TTFP), as an electrolyte additive. In the absence of TTFP, the solid electrolyte interphase (SEI) on the SiO electrode is attacked by Lewis-acidic phosphorus pentafluoride (PF5) generated by thermal decomposition of lithium hexafluorophosphate (LiPF6) at moderately elevated temperature. The SEI damage is followed by electrolyte decomposition/film deposition on the damaged electrode surface causing film growth, which leads to severe cell polarization and eventual failure. Conversely, effective PF5 scavenging by TTFP through Lewis acid-base adduct formation decreases SEI damage and thus hinders cell failure.

Evidence for Cation-Controlled Excited-State Localization in a Ruthenium Polypyridyl Compound.

The visible absorption and photoluminescence (PL) properties of the four neutral ruthenium diimine compounds [Ru(bpy)2(dcb)] (B2B), [Ru(dtb)2(dcb)] (D2B), [Ru(bpy)2(dcbq)] (B2Q), and [Ru(dtb)2(dcbq)] (D2Q), where bpy is 2,2'-bipyridine, dcb is 4,4'-(CO2(-))2-bpy, dtb is 4,4'-(tert-butyl)2-bpy, and dcbq is 4,4'-(CO2(-))2-2,2'-biquinoline, are reported in the presence of Lewis acidic cations present in fluid solutions at room temperature. In methanol solutions, the measured spectra were insensitive to the presence of these cations, while in acetonitrile a significant red shift in the PL spectra (≤1400 cm(-1)) was observed consistent with stabilization of the metal-to-ligand charge transfer (MLCT) excited state through Lewis acid-base adduct formation. No significant spectral changes were observed in control experiments with the tetrabutylammonium cation. Titration data with Li(+), Na(+), Mg(2+), Ca(2+), Zn(2+), Al(3+), Y(3+), and La(3+) showed that the extent of stabilization saturated at high cation concentration with magnitudes that scaled roughly with the cation charge-to-size ratio. The visible absorption spectra of D2Q was particularly informative due to the presence of two well-resolved MLCT absorption bands: (1) Ru → bpy, λmax ≈ 450 nm; and (2) Ru → dcbq, λmax ≈ 540 nm. The higher-energy band blue-shifted and the lower-energy band red-shifted upon cation addition. The PL intensity and lifetime of the excited state of B2B first increased with cation addition without significant shifts in the measured spectra, behavior attributed to a cation-induced change in the localization of the emissive excited state from bpy to dcb. The importance of excited-state localization and stabilization for solar energy conversion is discussed.

Silaimidazolium and silaimidazolidinium ions

2-Silaimidazolium ions and 2-silaimidazolidinium ions were synthesized by Lewis acid-base adduct formation between N-heterocyclic silylenes and silyl arenium ions. They were isolated in high yields as their [B(C(6)F(5))(4)](-) salts. These salts are stable at room temperature and were characterized by NMR spectroscopy supported by the results of density functional computations of molecular structures and NMR chemical shifts. NICS calculations suggest for the imidazolium ions only a modest degree of aromaticity. Despite the bulkiness of the used substituents R(1) and R(2), silylium ions and behave as classical Lewis pairs and show no unexpected reactivity.

Tuning Lewis acidity using the reactivity of “frustrated Lewis pairs”: facile formation of phosphine-boranes and cationic phosphonium-boranes

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.

Boron–Nitrogen Adducts of 1,3,5‐Triaza‐7‐phosphaadamantane (PTA): Synthesis, Reactivity, and Molecular Structure

AbstractAddition of BH3 to 1,3,5‐triaza‐7‐phosphaadamantane (PTA) leads to the formation of the first coordination complex of PTA in which a nitrogen and not the phosphorus of PTA is involved in the bonding. Multinuclear NMR spectroscopy, X‐ray crystallography, and computational chemistry are utilized in the characterization of the products. Crystal structures of PTA–BH3 (1) and O=PTA–BH3 (2) adducts are described. In addition a coordination polymer, [CpRu(PTA)2Cl(Ph3B3O3)]n (3), resulting from Lewis acid–base adduct formation between the PTA ligands of CpRu(PTA)2Cl and the product of the cyclocondensation of phenylboronic acid is described. The polymeric compound 3 was also characterized by X‐ray crystallography. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Heterobimetallic bismuth–transition metal coordination complexes as single-source molecular precursors for the formation of advanced oxide materials

Abstract In this manuscript we outline recent results on the synthesis of heterobimetallic coordination complexes involving bismuth and the transition elements using either bifunctional ligands or Lewis acid–base adduct formation as a synthetic strategy. Significant structural features in the molecular heterobimetallic species resulting from these two approaches are highlighted. The ability of these complexes to act as single-source precursors for the formation of advanced oxide nanomaterials has been investigated, and the potential of these complexes versus traditional methods for the formation of advanced ceramics is discussed. To cite this article: T. Ould-Ely et al., C. R. Chimie 8 (2005) .

New heterometallic copper zinc alkoxides: synthesis, structure properties and pyrolysis to Cu/ZnO composites

The copper compound [(THF)KCu(O t Bu) 3] ∞ 1 was obtained by interaction of a 1:1 mixture of ZnCl 2/CuCl 2 with KO t Bu. Bi- and trifunctional aminoalcohols were used to synthesize the intramolecularly donor stabilized Cu(II) alkoxides Cu(OCH(R)CH 2NMe 2) 2 ( 3: R=Me, 4: =CH 2NMe 2) where 4 was structurally characterized. Lewis acid–base adduct formation with (Me 3Si) 3CZnCl gave the heterodinuclear compounds (Me 3Si) 3CZnCl · Cu(OCH(R)CH 2NMe 2) 2 ( 5: R=Me, 6: R=CH 2NMe 2), which were characterized by X-ray single-crystal structure analysis. The two metal centers Cu and Zn of 5 and 6 are bridged by two oxygen atoms to form a Cu–O–Zn core. Pyrolysis of compounds 5 and 6 in dry argon or a H 2/N 2 mixture at atmospheric pressure forms metallic copper and zinc oxide, whereas pyrolysis under O 2/Ar forms additionally oxidized copper species. Elemental analysis of the pyrolysis products showed carbon and nitrogen contamination. Scanning electron microscopy and energy dispersive X-ray analysis were performed to get information on the morphology and the chemical composition of the pyrolysis products.

Ligand additivity in 47-electron clusters. Electrochemistry of ( μ-H)Ru 3( μ3- η3-XCCRCR′)(CO) 9− n(PPh 3) n and reactions with electrophiles: one-electron oxidation and adduct formation

Reactions of clusters HRu 3( μ 3- η 3-XCCRCR′)(CO) 9− n (PPh 3) n (X=OMe, R=R′=Me, n=1, 2, 3; X=MeO, R=H, R′=EtO, n=2, 3; X=Et 2N, R=H, R′=Me, n=1, 2) with electrophilic reagents proceed either by 1-electron transfer or by Lewis acid-base adduct formation. The HOMO for the cluster series is Ru–Ru bonding with contributions from all three Ru atoms. Cyclic voltammograms of HRu 3( μ 3- η 3-XCCRCR′)(CO) 9− n (PPh 3) n ( n=2, 3) display in each case an electrochemically reversible to quasi-reversible, 1-electron oxidation, followed by an irreversible, 1-electron oxidation at a significantly more positive potential. The potential for the first oxidation is lowered both by an increasing degree of PPh 3 substitution and an increasing pi donor capability of the allylidene substituents. The dependence of the oxidation potential upon substitution of the metal and carbon framework atoms is analyzed as an example of ligand additivity in cluster systems. Radical cations derived from the di- and tri-substituted 1,3-dimetalloallyl clusters can be generated by oxidation with tris(4-bromophenyl)aminium hexachloroantimonate. These radical cations decompose within a few minutes at room temperature but are stable for long periods at temperatures below −40°C. Electrophilic addition of E=H 1+, Ag 1+ or Au(PPh 3) 1+ to the Ru–Ru bonds is observed. Adducts [ERu 3H( μ 3- η 3-XCCRCR′)(CO) 9− n (PPh 3) n ] 1+ have been characterized by IR and 1H- and 31P-NMR spectroscopies. The Au(PPh 3) moiety is found to bridge two or three Ru–Ru bonds in these adducts. Substitutional isomerism is induced by electrophilic addition.

HYDROLYSIS PATHWAY LEADING TO THE FORMATION OF NOVEL OXO- AND HALO-BRIDGED TIN(IV) LADDER COMPOUNDS. THE MOLECULAR STRUCTURE OF THE DIMERIC DISTANNOXANES [Ph2(CI)SnOSnPh2(OH)]2.2(CH3)2CO AND [Ph2(CI)SnOSnPh2(CI]2

Abstract Acid-base adduct formation occurs in benzene solutions of Ph2SnCl2 with quinuclidine, Dabco, and acridine as ligands. By use of the latter base, the dichlorodistannoxane [Ph2CISnOSnPh2Cl]2 (6) also was isolated. In more polar solvents, hydrolysis reactions of Ph2SnCl2 occur more readily, leading to dimeric distannoxanes. The hydroxystannoxane [Ph2C1SnOSnPh2OH]2.2(CH3)2CO (5) is isolated from the reaction of Ph2SnCl2 and quinuclidine in acetonitrile solution. It is also obtained by the analogous reaction of Dabco in acetone. Further characterized from these reactions were anionic complexes [Ph2SnC14][base H]2 (base = quinuclidine, Dabco, or acridine). The isolation and structural characterization of successive members in the hydrolysis lead to a mechanism of formation of the dimeric distannoxanes 5 and 6. X-ray analysis shows they both possess “ladder” structures with tin atoms in trigonal-bipyramidal conformations. This is the first structural study of distannoxanes containing directly bonded aryl groups. 5 crystallizes in the monoclinic space group P21/n with a = 11.758 (2) Å, b = 19.327 (3) Å, c = 11.649 (1) Å, β= 92.87 (1)°, and Z = 2. The structure was refined to R = 0.028 and Rw = 0.039. For 6, the monoclinic space group P211c was obtained with a = 10.284 (2) Å, b = 10.513 (3) Å, c = 22.143 (4) Å, β = 100.75 (2)°, and Z = 2. The structure was refined to R = 0.023 and Rw = 0.034.

XPS investigations of acid–base interactions in adhesion. Part 4.—Use of trichloromethane as a molecular probe for the quantitative assessment of polymer basicity

Trichloromethane has been used as a molecular probe for the assessment of acid–base properties of polymers in the solid state. It is shown in this paper that when CHCl3 is sorbed in polymers it undergoes a negative XPS chemical shift as a result of acid–base interactions. Following the FTIR approach of Fowkes, we determined ΔHAB, the enthalpy of acid–base adduct formation, for the different polymer–CHCl3 adducts, and found that the XPS binding energy (Eb) of the Cl 2p3/2 line is linearly related to ΔHAB. The relationship is of the form –ΔHAB/kcal mol–1= 228.8–1.14 Eb/eV.

Acid-Base Interactions in Adhesion: The Characterization of Surfaces & Interfaces by XPS

Abstract The consideration of acid-base interactions now plays an important role in the field of adhesion science. In this paper we describe ways in which the surface specificity of XPS can be employed to gauge the acidity or basicity of inorganic and organic surfaces, using a number of novel preparation methods. The methods described rely on the interaction of probe species, in either the gas, liquid or solid phase with the material of interest. For the determination of the properties of inorganic surfaces we describe ways in which the IEPS (isoelectric point of the solid surface) of hydroxylated iron surfaces can be determined by XPS used in conjunction with an ion exchange method. For polymer surfaces we show how the diffusion of Na+ from a soda-lime glass substrate provides an indicator of polymer acidity and illustrate how solvent vapour uptake contains valuable information relating to polymer acidity and acid-base adduct formation. Finally we indicate the manner in which XPS can be used to identify ...