Alkali Metal Countercations Research Articles

Palladium-Catalyzed Enantioselective β-C(sp3)-H Activation Reactions of Aliphatic Acids: A Retrosynthetic Surrogate for Enolate Alkylation and Conjugate Addition.

Enolate alkylation and conjugate addition into an α,β-unsaturated system have served as long-standing strategic disconnections for the installation of α- or β-substituents on carbonyl-containing compounds. At the onset of our efforts to develop C-H activation reactions for organic synthesis, we set our eye toward developing asymmetric β-C-H activation reactions of aliphatic acids with the perspective that this bond-forming event could serve as a more flexible retrosynthetic surrogate for both canonical carbonyl-related asymmetric transformations.In this Account, we describe our early efforts using strongly coordinating chiral oxazolines to probe reaction mechanism and the stereochemical nature of the C-H cleavage transition state. The characterization of key reactive intermediates through X-ray crystallography and computational studies suggested a transition state with C-H and Pd-OAc bonds being approximately coplanar for optimum interaction. We then moved forward to develop more practical, weakly coordinating monodentate amide directing groups, a necessary advance toward achieving the β-C-H activation of weakly coordinating native carboxylic acids. Throughout this journey, gradual deconvolution between a substrate's directing effect and its intimate interplay with ligand properties has culminated in the design of new ligand classes that ultimately allowed the competency of native carboxylic acids in β-C-H activation. These efforts established the importance of ligand acceleration in Pd-catalyzed C-H activation, where the substrate's weak coordination is responsible for positioning the catalyst for C-H cleavage, while the direct participation from the bifunctional ligand is responsible for enthalpically stabilizing the C-H cleavage transition state.Building upon these principles, we developed five classes of chiral ligands (MPAA, MPAQ, MPAO, MPAThio, MPAAM) to enable enantioselective β-C-H activation reactions, including carbon-carbon and carbon-heteroatom bond formation. The accumulated data from our developed enantioselective C-H activation reactions indicate that ligands possessing point chirality are most effective for imparting stereoinduction in the C-H activation step, the application of which enabled the desymmetrization and subsequent C-H functionalization of enantiotopic carbon and protons across a range of weakly coordinating arylamides and, more recently, free carboxylic acids. Progress in ligand design, in conjunction with the enabling nature of alkali metal countercations, led to the realization of a suite of β-methyl and now methylene C(sp3)-H activation reactions. These advancements also enabled the use of economical oxidants, such as peroxides and molecular oxygen, to facilitate catalyst turnover. In the future, continued progress in designing more efficient bifunctional chiral ligands is likely to provide a myriad of enantioselective β-C-H activation reactions of readily available native substrates.

Hexaniobate Cluster Anion Monolayers on Gold Nanoparticles: A New Structural Role for Alkali Metal Countercations.

Monolayer shells of polyoxotungstate cluster anions on gold nanoparticles in water were electrostatically stabilized by structurally integrated countercations, with formation constants, K, increasing in the order: Li+ < Na+ < K+ < TMA+ < Cs+ (TMA+ = tetramethylammonium). We now report that for hexaniobate cluster anions, K values increase in the same order, with the notable exception of TMA+, which is effectively unable to induce monolayer formation. These findings point to a new structural model in which hexaniobate anions form a spherical coordination polymer at the gold surface with alkali metal countercations serving as single-atom structural building units between hexaniobate linkers.

Chemical shifts of phenolic monomers in solution and implications for addition and self‐condensation

Alkali metal counter-cations alter the electron density of phenolates in solution by electrostatic interactions. This change in electron density affects their reactivity toward formaldehyde, hydroxymethylphenols, and isocyanates during polymerization. The electronic perturbation of phenolic model compounds in the presence of alkali metal hydroxides was investigated with (13)C and (1)H nuclear magnetic resonance in polar solvents relative to non-ionic controls, altering the chemical shifts of the model compounds, thus indicating changes in electron density using the chemical shift as a proxy. These shifts were attributed to Coulombic electrostatic interactions of the counter-cation with the phenolate anion that correlated to hydrated ionic radius and solvent dielectric constants. The predicted relative reaction rates for formaldehyde addition based on electron density ranking from (13)C nuclear magnetic resonance of the phenolic models was compared with the literature values. Predictions for condensation reactions of 2- and 4-hydroxymethylphenol from chemical shifts were consistent with published results. The results permit predictions for the reaction of phenolic compounds for the formation of thermosetting polymeric materials.

Thermal characterization of Flemion® membranes substituted by alkali metal cations

The thermal behavior of perfluorosulfonated membranes of three equivalent mass (EW=910, 1000 and 1100 g eq−1) has been studied for membranes in acid form and in the alkali metal countercations substituted samples. The water contents of the membranes decrease progressively with increasing EW and the countercations charge density. The monovalent cations substitutions increase the membranes thermal stability. DSC curves show a single endothermic peak around 120°C that give low peak temperature for low EW and high peak temperature for large cations size. The membrane mechanical properties changed for different EW and temperatures of membranes. Stress-strain analysis showed that K+ substituted membranes at both temperatures present a highest YM compared to the other alkali cation substitutions. The thermal properties of perfluorosulfonated membranes depend on the water contents, cation size, temperature and also on EW value.

Triplet dimers of silylbenzene anion radicals in low-temperature glass matrices

Anion radicals of o- and p-bis(trimethylsilyl)benzenes were found by ESR spectroscopy to form the corresponding triplet dimers in 2-methyltetrahydrofuran (MeTHF) glass matrices at low temperatures. The temperature dependence of the signal intensity of a triplet dimer has shown that the dimer has the triplet ground state. The spin–spin distances estimated with the D values increased with the increasing size of alkali metal countercations, indicating that the triplet dimer has a π −–M +–π − type sandwich structure. The stability of the triplet dimer depended remarkably on the coordination ability of the solvent; no triplet dimer was observed in tetrahydrofuran (THF).

Redox and magnetic switching in 1,3,5-acceptor-substituted benzenes: reversible formation of radical anions, dianions and trianions in doublet, triplet, and quartet spin states

1,3,5-Tris(dicyanovinyl)-substituted benzene 1 can accept one, two, and three electrons stepwise, as shown by (spectro)electrochemical methods. When the corresponding redox stages are attained by K-metal reduction in THF and 2-methyltetrahydrofuran, paramagnetic resonance and optical techniques can identify equilibria between adjacent redox states and different (para)magnetic stages. It can be shown that the dianion can adopt a triplet state whereas the trianion is present in a doublet and quartet spin multiplicity. Similar findings are established for the methoxy-substituted derivative 2. The formation of the different paramagnetic stages is closely connected to the association of the anions with alkali-metal countercations.

Synthesis of the Square-Planar Gallium Species K(2)

Unlike the reduction of [Ga-2,6-Trip2C6H3]Cl2 with sodium which produces the controversial “gallyne” Na2[(Ga-2,6-Trip2C6H3)2], reduction with potassium gives K2[Ga4(2,6-Trip2C6H3)2] (see structure). This suggests that the alkali metal countercations play an important role in stabilizing these species. Trip=2,4,6-iPr3C6H2.

Synthesis and Characterization of Large Single Crystals of Silicon and Germanium Clathrate-II Compounds and a New Tin Compound with Clathrate Layers

ABSTRACTWe have synthesized large single crystals of clathrate-II compounds with frameworks of silicon and germanium by employing mixed alkali metal countercations. The combinations of alkali metals are rationally selected in order to fit the different cages of the clathrate-II structure. This approach leads to the following stoichiometric and fully “stuffed” compounds: Cs8Na16Si136, Cs8Na16Ge136, Rb8Na16Si136 and Rb8Na16Ge136. The structures and the corresponding Si-Si and Ge-Ge distances are elucidated and established with high accuracy from extensive single crystal X-ray diffraction work. The compounds are stoichiometric, metallic, and are very stable at a variety of extreme conditions such as heat, concentrated acids, hydrothermal treatment etc. No evidence was found for vacancies in the silicon and germanium networks or partial occupancies of the alkali metal sites. The stoichiometry of these fully “stuffed” clathrates is consistent with the measured temperature independent Pauli paramagnetism, supported also by the conductivity measurements on single crystals and thermopower measurements on pellets. A new compound with novel clathrate-like structure forms when small and large cations are combined with tin. The new materials, A6Na18Sn46 (A = K, Rb, Cs), are made of clathrate layers and the interlayer space filled with Sn4-tetrahedra and alkali-metal cations. Its formula can be rationalized as A6Na6Sn34 + 3·Na4Sn4 (one clathrate layer and three tin tetrahedra). The compound is stable in air and is being currently tested at other conditions. Detailed measurements of its transport properties are under way.

Synthesis and Characterization of the Water-Soluble, All-Inorganic Composition, Keggin-Type Triniobium(V)-Substituted SiW9Nb3O407− Heteropolyoxoanions with Alkali Metal Countercations (Li+, Na+, K+, and Cs+)

Abstract The Keggin-type triniobium(V)-substituted tungstoheteropolyanion, A-β-SiW9Nb3O407−, was synthesized as its hepta alkali-metal salts of Li+, Na+, K+, and Cs+countercations. These alkali-metal salts were useful for allowing the crystallization of SiW9Nb3O407−, itself, and/or the SiW9Nb3O407−-supported organometallic complexes, as well as controlling the water solubility of these complexes. The analytically pure compounds, obtained as homogeneous colorless solids via a stoichiometric reaction of the (Bu4N)6H2Si2W18Nb6O77, (Nb–O–Nb)3-bridged anhydride with the corresponding alkali tetrafluoroborates, and then with the alkali hydroxides, were compositionally characterized by complete elemental analyses, TG/DTA and FABMS spectra. They were structurally characterized by FT-IR and solution 183W NMR measurements, as well as solid-state CP/MAS and GHD/MAS 29Si NMR and solution 29Si NMR measurements. The cation-dependent properties of these heteropolytungstates in the solid state were observed in the thermal stabilities, the amounts of hydrated water and adsorbed water, the solubilities in water and in organic solvents, and in the ease of crystallization.

Solvent-Shared and Solvent-Separated Ion Multiples of Perylene Radical Anions and Dianions: An Exemplary Case of Alkali Metal Cation Solvation

The hydrocarbon perylene (C20H12) has been crystallized, and several of its radical anions and dianion salts have been prepared by reduction with various alkali metal mirrors under argon and aprotic conditions in different ether solvents. The single-crystal structures determined at low temperature are discussed in terms of single and double negative charge perturbation based on geometry-optimized density functional calculations at the B3LYP level with 6-31G* basis sets. All perylene radical anion salts [C20H12•-][M+solv] and [C20H12•-···C20H12][M+solv] crystallize as solvent-separated ion pairs, whereas the structures of the perylene dianion salts [C20H122 -(M+solv)2] or [C20H122 -][M+solv]2 strongly depend on the optimum solvation of the alkali metal countercations. The experimental results allow the rationalization of essential facets of the network of redox equilibria in solution.

Synthesis and Characterization of Water-Soluble, All-Inorganic Composition, Dawson-Type Trisubstituted Heteropolytungstates. Effect of Alkali Metal Countercations (Li, Na, K, and Cs) on the P2W15Nb3O629− Polyoxoanion

Abstract The water-soluble form of the Dawson-type trisubstituted tungstoheteropolyanion, B-α-P2W15Nb3O629−, was synthesized as its nona alkali-metal salts of Li, K, and Cs. Analytically pure compounds, obtained as homogeneous colorless solids, had nonahydrate for the nonalithium (Li9) salt, tetrahydrate for the nonasodium (Na9) salt, trihydrate for the nonapotassium (K9) salt and trihydrate for the nonacaesium (Cs9) salt, respectively, after each was dried overnight at room temperature under 10−3—10−4 Torr. TG/DTA analyses show that they can be made with 25 water for the Li9 salt, 23 water for the Na9 salt, 9 water for the K9 salt, and 8 water for the Cs9 salt under atmospheric conditions. A compositional characterization was accomplished by complete elemental analyses and TG/DTA, and structural characterization was achieved by a combination of FT-IR and variable-temperature solid-state 31P NMR and by both 31P and 183W NMR measurements at room temperature in D2O and DMSO-d6 solutions. The amount of hydrated waters, solubility in solvents and ease of crystallization were significantly influenced by the alkali-metal countercations. These cation-dependent properties are useful for selecting polyoxoanion precursor favorable to the crystallization of P2W15Nb3O629−-supported organometallic complexes. The effects of the cations (Li, K, Na, and Cs) on the P2W15Nb3O629− were observed as different hydrated structures of the polyoxoanion and its cation-dependent thermal stability in the solid state by a combination of TG/DTA and FT-IR measurements, and also as a spectral change of the downfiled 31P signal due to interactions of the polyoxoanion with the countercations and/or with hydrated waters by variable-temperature (VT) solid-state GHD/MAS 31P NMR spectroscopy.

Study on the reduction species of sulfur by alkali metals in nonaqueous solvents

Rechargeable cells with a sulfur cathode, a lithium anode and a nonaqueous electrolyte operating at ambient temperature have been extensively studied as a possible application of sulfur combined with nonaqueous solvent. However, the behavior of the chemical species formed by the reduction of sulfur with alkali metals in nonaqueous solvent has not yet been clarified. In this report, sulfur is reduced in tetrahydrofuran (THF) by alkali metals (M: Na, K, Li and Cs), by anthracene radical anions or by diphenylethylene dimer dianions with alkali metal counter cations mainly under the experimental condition of [S 8] [M] > 0.25 . The system produced by the reduction is the yellow solution consisting of polysulfide dianions ( S m 2−, m > 1) and radical anions ( S n −, n > 1) which changes into a blue solution by the addition of hexamethylphosphoric triamide with a much higher polarity than THF. The influence of the kind of alkali metal, the temperature and the addition of solvents such as crown ether on the behavior of the chemical species formed are examined by ultraviolet ( uv)-visual electronic spectra, electron spin resonance (ESR) spectra and Raman spectra measurements. In addition, γ-irradiation with 60Co was carried out to reduce sulfur in glassy solid 2-methyl-THF at — 196 °C. Furthermore, the reaction between the yellow solution and aliphatic bromide is investigated. As a result, it is revealed that the yellow species is S 4 2−, the blue species is S 3 − and several polysulfide dianions and radial anions are in a state of equilibrium.

Irregular photoinduced electron transfer between pyrene dianion and alkali metal cations studied by Fourier-transform electron paramagnetic resonance

Polarized FT-EPR spectra of photolyzed pyrene/alkali metal (Li, Na, K, Rb, Cs)/THF solutions were found to be time and alkali metal dependent. The polarization effects are initiated by electron transfer (ET) reactions between the doubly- and singly-charged pyrene (Py 2- and Py .- ) and the alkali metal countercations (M + ). With the exception of Py/K/THF solution, the spin polarizations of all other systems, including those with a chelating agent added (cryptand), originate with a triplet precursor. The polarization is due to the action of several mechanisms: the correlated radical pair mechanism, S-T-1 radical pair (RP) mixing, and the radical-triplet pair mechanism

Control of axial–equatorial interactions in (dimethylglyoximato)cobalt(III) complexes. Part 1. Counter cation effect on the strength of the intermolecular hydrogen bond, OH ⋯ O–, of trans-bis(dimethylglyoximato)dinitrocobaltate(III) anions and crystal structure of the tetramethylammonium salt

The infrared frequencies observed for the particular functional group of both the axial and the equatorial ligands of the complex anion [Co(Hdmg)2(NO2)2](H2dmg = dimethylglyoxime) have been found to be widely different upon changing the counter cations M+(M = Li, Na, K, Rb or Cs) or NR4+(R = H, Me, Et or Bu). In particular, the O–H stretching frequency of the intramolecular hydrogen bond, O–H ⋯ O–, changes markedly depending on the kind of counter cation. Alkali-metal counter cations, M+, would exert a co-ordination effect upon the donor atom in the complex anion, while tetraalkylammonium counter cations would give a predominantly long-range potential effect only. In order to investigate the difference in the counter cation effects on the strength of the in-plane intramolecular hydrogen bond, crystal and the molecular structures of [NMe4][Co(Hdmg)2(NO2)2] and Na[Co(Hdmg)2(NO2)2]·2H2O were compared. An unusual conformation around the Co–NO–2 moiety was observed in [NMe4][Co(Hdmg)2(NO2)2]. The orientation of the axial ligands, NO2–, is restricted in such a way that they are located over the five-membered chelate ring formed by the Hdmg– moiety and the CoIII atom. On the other hand, Na[Co(Hdmg)2(NO2)2]·2H2O has a usual conformation which would lead to minimum non-bonded interaction between the Hdmg– moietry and the two axial ligands, NO2–. In this conformation, two NO2– are positioned over the pseudo-six-membered chelate rings. A further important structural feature in [NMe4][Co(Hdmg)2(NO2)2] is a very long Co–NO–2 distance [1.974(3), as compared with 1.949(3) and 1.943(3)A in the sodium salt]. This difference is ascribed to the counter cation effect. The stretching frequencies of O–H ⋯ O– and CN in [NMe4][Co(Hdmg)2(NO2)2] are quite different from those in Na[Co(Hdmg)2(NO2)2]·2H2O. These facts suggest that the counter cations play an important role in controlling the axial–equatorial interactions in [Co(Hdmg)2(NO2)2]–.

Grafting of Polyesters on Carbon Black V. Preparation of Polyester-Grafted Carbon Black with a Higher Grafting Ratio by the Copolymerization of Epoxide with Cyclic Acid Anhydrides Using COOK Groups on Carbon Black as the Initiator

The ring-opening copolymerization of epoxides with cyclic acid anhydrides was found to be initiated by carbon black containing potassium carboxylate (COOK) groups to give alternating polymers, i.e., polyesters. Polyesters were propagated from COOK groups and effectively grafted on carbon black; for example, the grafting ratios of polyesters from glycidyl methacrylate and succinic anhydride and from styrene oxide and maleic anhydride were 126.2 and 51.5%, respectively. The rate of copolymerization increased with increasing temperature, whereas the grafting ratio of polyesters decreased. The initiating activity of alkali metal carboxylate groups increased, depending on the type of alkali metal countercations, in the following order: COOLi < COONa < COOK < COORb < COOCs. This order was in agreement with that of increasing electropositivity of the countercations. In addition, the increasing electropositivity of the counter-cation caused an increase in the rate of chain transfer reaction, leading to a decrease in the grafting ratio and molecular weight of ungrafted polyesters. The effects of solvent on the copolymerization and grafting ratio are discussed.