Effect Of Alkali Metals Research Articles

Selective Hydrogen Isotope Exchange Catalysed by Simple Alkali-Metal Bases in DMSO.

Isotope Exchange processes are becoming the preferred way toprepare isotopically labelled molecules, avoiding the redesign of multistep synthetic protocols. In the case of deuterium incorporation, the most used strategy has employed transition metals, that offer high reactivity under mild reaction conditions. Despite their success, the trade-off is that these metals are precious, and often exhibit high toxicity. Therefore, alternative protocols using earth abundant catalysts would be a welcome addition to this field. Here, we show how the simple bases NaHMDS (HMDS = hexamethyldisilazide) and NaCH2SiMe3can efficiently and selectively promote deuteration of a wide range of C(sp2)-H and C(sp3)-H bonds in DMSO-d6, providing an easy and direct access to deuterated compounds. Heterocycles, fluoroarenes,N-heterocyclic carbenes, amides and other aromatic molecules could be deuterated under mild conditions using catalytic amounts of base. Mechanistic studies have flagged up the importance of the metalated substrate and metalated solvent in solution, establishing an equilibrium between these compounds, which is crucial for the success of this approach. An alkali-metal effect was observed, with heavier alkali-metal amides being more reactive at room temperature, but their lower stability at higher temperatures made sodium bases the optimal reagents for Hydrogen Isotope Exchange.

Sodium mediated deprotonative borylation of arenes using sterically demanding B(CH2SiMe3)3: unlocking polybasic behaviour and competing lateral borane sodiation.

The deprotonative metalation of organic molecules has become a convenient route to prepare functionalised aromatic substrates. Amongst the different metallating reagents available, sodium bases have recently emerged as a more sustainable and powerful alternative to their lithium analogues. Here we report the study of the sterically demanding electrophilic trap B(CH2SiMe3)3 for the deprotonative borylation of arenes using NaTMP (TMP = 2,2,6,6-tetramethylpiperidide) in combination with tridentate Lewis donor PMDETA (PMDETA = N,N,N',N'',N''-pentamethyldiethylenetriamine). Using anisole and benzene as model substrates, unexpected polybasic behaviour has been uncovered, which enables the formal borylation of two equivalents of the relevant arene. The combination of X-ray crystallographic and NMR monitoring studies with DFT calculations has revealed that while the first B-C bond forming process takes place via a sodiation/borylation sequence to furnish [(PMDETA)NaB(Ar)(CH2SiMe3)3] species, the second borylation step is facilitated by the formation of a borata-alkene intermediate, without the need of an external base. For non-activated benzene, it has also been found that under stoichimetric conditions the lateral sodiation of B(CH2SiMe3)3 becomes a competitive reaction pathway furnishing a novel borata-alkene complex. Showing a clear alkali-metal effect, the use of the sodium base is key to access this reactivity, while the metalation/borylation of the amine donor PMDETA is observed instead when LiTMP is used.

Assessing Alkali-Metal Effects in the Structures and Reactivity of Mixed-Ligand Alkyl/Alkoxide Alkali-Metal Magnesiates.

Advancing the understanding of using alkali‐metal alkoxides as additives to organomagnesium reagents in Mg−Br exchange reactions, a homologous series of mixed‐ligand alkyl/alkoxide alkali‐metal magnesiates [MMg(CH2SiMe3)2(dmem)]2 [dmem=2‐{[2‐(dimethylamino)ethyl]methylamino} ethoxide; M=Li, 1; Na, 2; (THF)K, 3] has been prepared. Structural and spectroscopic studies have established the constitutions of these heteroleptic/heterometallic species, which are retained in arene solution. Evaluation of their reactivity towards 2‐bromoanisole has uncovered a marked alkali‐metal effect with potassium magnesiate 3 being the most efficient of the three ate reagents. Studies probing the constitution of the exchange product from this reaction suggest that the putative [KMgAr2(dmem)]2 (Ar=o‐OMe−C6H4) intermediate undergoes redistribution into its single metal components [KAr]n and [MgAr(dmem)]2 (5). This process can be circumvented by using a different potassium alkoxide containing an aliphatic chain such as KOR’ (R’=2‐ethylhexyl) which undergoes co‐complexation with Mg(CH2SiMe3) to give [KMg(CH2SiMe3)2(OR’)]2 (7). This ate, in turn, reacts quantitatively with 2‐bromoanisole furnishing [KMgAr2(OR’)]2 (9) which is stable in solution as a bimetallic compound. Collectively this work highlights the complexity of these alkali‐metal mediated Mg−Br exchange reactions, where each reaction component can have a profound effect not only on the success of the reaction; but also the stability of the final metalated intermediates prior to their electrophilic interception.

Physical and chemical aspects at the interface and in the bulk of CuInSe2-based thin-film photovoltaics.

Chalcopyrite CuInSe2 (CISe)-based thin-film photovoltaic solar cells have been attracting attention since the 1970s. The technologies of CISe-based thin-film growth and device fabrication processes have already been put into practical applications and today commercial products are available. Nevertheless, there are numerous poorly understood areas in the physical and chemical aspects of the underlying materials science and interfacial and bulk defect physics in CISe-based thin-films and devices for further developments. In this paper, current issues in physical and chemical studies of CISe-based materials and devices are reviewed. Correlations between Cu-deficient phases and the effects of alkali-metals, applications to lightweight and flexible solar minimodules, single-crystalline epitaxial Cu(In,Ga)Se2 films and devices, differences between Cu(In,Ga)Se2 and Ag(In,Ga)Se2 materials, wide-gap CuGaSe2 films and devices, all-dry processed CISe-based solar cells with high photovoltaic efficiencies, and also fundamental studies on open circuit voltage loss analysis and the energy band structure at the interface are among the main areas of discussion in this review.

Polycrystalline CuGaSe2 thin film growth and photovoltaic devices fabricated on alkali-free and alkali-containing substrates

Abstract Polycrystalline thin films of ternary CuGaSe2 (CGS), which are one of the potential materials for wide-gap top cells in tandem structure photovoltaic devices, are grown on alkali-free and alkali-containing substrates. The effects of alkali-metals on CGS film morphology, grain size, and photovoltaic device properties are studied. High temperature (>600 °C) growth of CGS films is also investigated with a focus upon variations in alkali-metal distribution profiles in CGS films and the formation of a surface Cu-deficient layer (CDL), which plays an important role in p-n junction formation. The use of a high growth temperature is effective in realizing large grain sizes due to enhanced elemental migration during growth, whereas the formation of a surface CDL is diminished and the alkali-distribution profiles in CGS films are modified resulting in a decrease in carrier density. It is suggested that large CGS grain sizes contribute to obtaining high photocurrent values, but coping with modifications in surface/interface and carrier density with alkali-metals is important to enhance overall photovoltaic performance.

Effects of Alkali-Metal Ions and Counter Ions in Sn-Beta-Catalyzed Carbohydrate Conversion.

Alkali-metal ions have recently been shown to strongly influence the catalytic behavior of stannosilicates in the conversion of carbohydrates. An effect of having alkali-metal ions present is a pronounced increase in selectivity towards methyl lactate. Mechanistic details of this effect have remained obscure and are herein addressed experimentally through kinetic experiments and isotope tracking. The presence of alkali-metal ions has a differential effect in competing reaction pathways and promotes the rate of carbon-carbon bond breakage of carbohydrate substrates, but decreases the rates of competing dehydration pathways. Further addition of alkali-metal ions inhibits the activity of Sn-Beta in all major reaction pathways. The alkali-metal effects on product distribution and on the rate of product formation are similar, thus pointing to a kinetic reaction control and to irreversible reaction steps in the main pathways. Additionally, an effect of the accompanying basic anions is shown, supposedly facilitating the cation exchange and eliciting a different concentration-dependent effect to that of neutral alkali-metal salts.

First-principles study on alkali-metal effect of Li, Na, and K in CuInSe2 and CuGaSe2

The substitution energies and migration energies of the alkali metal atoms of Li, Na, and K in CuInSe2 (CIS) and CuGaSe2 (CGS) were investigated by first-principles calculations. The substitution energies of Li, Na, and K atoms in CIS and CGS were calculated for two different cationic atom positions of Cu and In/Ga in the chalcopyrite unit cell. In CIS and CGS, the substitution energies of NaCu are much lower than those of NaIn and NaGa. The substitution energies of the LiCu atoms in CIS and CGS are lower than those of NaCu, while the substitution energies of KCu atoms in CIS and CGS are much higher than those of NaCu. Therefore, it is difficult to form KCu in CIS and CGS. The migration energies of Li, Na, and K atoms in CIS and CGS are obtained by a combination of the linear and quadratic synchronous transit (LST/QST) methods and the nudged elastic band (NEB) method. The theoretical migration energies of a Na atom at the Cu site to the nearest Cu vacancy (NaCu → VCu) in CIS and CGS are much lower than those of (CuCu → VCu) in CIS and CGS. The mechanism underlying the alkali metal effect of Li, Na, and K in the CIGS film during the post-deposition treatment of LiF, NaF, and KF is discussed on the basis of the calculated substitution and migration energies.

Alkali-Metal Effect on Ternarization of Alkali Metal-GICs (AMC24) with Ethylene

Sorption behavior of ethylene with stage-2 binary GICs (AMC24: AM=K, Rb, Cs), and the structure and thecharacteristics of resulting ternary compounds (AMC24 (C2H4) n) were investigated. The differences of these propertieswere discussed in terms of alkali metal effect. It was confirmed that the easinessof ethylene sorption and the stabilityof resulting ternary compound are in parallel relation, in the order of Cs>Rb>K. In the case of KC24, co-intercalation of ethylene seemed to be doubtful in regards to XRD observations. The ethylene-sorption behavior by AMC24 isconsidered to be mainly controlled on the size of alkali metal ions. The difficulty in preparation of KC24 (C2H4) n maybe due to narrow interlayer space of KC24 and weak K+-C2H4 interaction. In conclusion, CsC24 (C2H4) n was found tobe most stable in air both in its structure and in its high electrical conductivity.

Molecular dynamics simulation of alkali-metal diffusion in alkali-metal disilicate glasses

Long-timescale molecular dynamics simulations of duration 150–250 ps have been performed on alkali-metal disilicate glasses at 1400 K with the general formula M 2 Si 2 O 5 (M=Na, K, or an equiatomic mixture of the two). The simulations are long enough to analyse the diffusive behaviour of the alkali-metal ions in detail. The calculated diffusion coefficients show the mixed alkali-metal effect when systems having different alkali-metal contents are treated at the same pressure, but not when they are treated at the same volume (and therefore at higher pressure). There is no evidence for curvature in the time-dependent mean-square displacements of the alkali-metal ions over the 10–60 ps range, which suggests the absence of long-time memory effects. The van Hove correlation functions confirm the hopping mechanism for diffusion, though there is only partial evidence for the existence of a selection effect in the alkali-metal mobility in the mixed glass.