pi.-Groups in ion pair bonding. Stabilization of the dianion of naphthalene by lithium tetramethylethylenediamine

On the existence of low-valent magnesium-calcium complexes

DFT-Calculations predict that a low-valent complex (BDI)Mg-Ca(BDI) with bulky β-diketiminate (BDI) ligands is thermodynamically stable. It was attempted to isolate such a complex by salt-metathesis between [(^DIPePBDI*)Mg^-Na⁺]₂ and [(^DIPePBDI)CaI]₂ (^DIPePBDI = HC[C(Me)N-DIPeP]₂; ^DIPePBDI* = HC[C(tBu)N-DIPeP]₂; DIPeP = 2,6-CH(Et)₂-phenyl). Whereas in alkane solvents no reaction was observed, salt-metathesis in C₆H₆ led to immediate C-H activation of benzene to give (^DIPePBDI*)MgPh and (^DIPePBDI)CaH, the latter crystallizing as a THF-solvated dimer [(^DIPePBDI)CaH·THF]₂. Calculations suggest reduction and insertion of benzene in the Mg-Ca bond. The activation enthalpy for the subsequent decomposition of C₆H₆^2- into Ph^- and H^- is only 14.4 kcal mol^-1. Repeating this reaction in the presence of naphthalene or anthracene led to heterobimetallic complexes in which naphthalene^2- or anthracene^2- anions are sandwiched between (^DIPePBDI*)Mg⁺ and (^DIPePBDI)Ca⁺ cations. These complexes slowly decompose to their homometallic counterparts and further decomposition products. Complexes in which naphthalene^2- or anthracene^2- anions are sandwiched between two (^DIPePBDI)Ca⁺ cations were isolated. The low-valent complex (^DIPePBDI*)Mg-Ca(^DIPePBDI) could not be isolated due to its high reactivity. There is, however, strong evidence that this heterobimetallic compound is a fleeting intermediate.

Polycyclic Aromatic Hydrocarbons as Anode Materials in Lithium-Ion Batteries: A DFT Study

The structure and energetics of the interactive behavior of Li⁺ and Li with polycyclic aromatic hydrocarbons (PAHs) have been studied at the wB97XD/6-311G(d,p) level of DFT. The electron distribution in the PAHs, analyzed using the topology of the molecular electrostatic potential (MESP), led to the categorization of their aromatic rings into five types, viz R_s, R_n, R_d, R_b, and R_e. Among the different rings, sextet-type R_s and naphthalene-type R_n rings showed the highest interaction with Li⁺. The change in MESP at the nucleus of Li⁺ (ΔV_Li⁺) due to the formation of the complex Li⁺...PAH is found to be proportional to the adsorption energy (E₁). In Li...PAH, the spin density on Li is close to zero, suggesting the formation of Li⁺...PAH^•- due to the electron transfer from Li to PAH. The adsorption energy (E₂) for Li...PAH does not correlate with the change in MESP at the nucleus of Li, whereas the dissociation energy (E₃) of Li⁺...PAH^•- to yield Li⁺ and PAH^•- correlates well with the MESP data, ΔV_Li. The study confirms that the change in MESP at the nucleus of Li⁺ due to complex formation gives a quantitative measure of the electronic effect of the cation-π binding. The cell potential (V_cell) is predicted for the lithium ion battery (LIB) using the Li⁺...PAH and Li...PAH adsorption energies. On the basis of the V_cell data, "carbon nanoflake"-type systems, viz coronene, circumbiphenyl, C₄₂H₁₆, and C₅₀H₁₈ are suggested as good anode materials for LIBs.

Diazazethrene bisimide: a strongly electron-accepting π-system synthesized via the incorporation of both imide substituents and imine-type nitrogen atoms into zethrene

The development of highly electron-accepting π-systems is a fundamentally challenging issue despite their potential applications as high-performance n-type organic semiconductors, organic rechargeable batteries, and stable redox-active organocatalysts. Herein, we demonstrate that the incorporation of both imide substituents and imine-type nitrogen atoms into zethrene affords the strongly electron-accepting π-system diazazethrene bisimide (DAZBI). DAZBI has a low-lying LUMO (-4.3 eV vs. vacuum) and is readily reduced by the weak reductant l-ascorbic acid to afford the corresponding dihydro species. The injection of two electrons into DAZBI provides the corresponding dianion. These reduced species display remarkable stability, even under ambient conditions, and an intense red fluorescence. A DAZBI dimer, which was also synthesized, effectively accommodated four electrons upon electron injection.

Chemical Reduction of a Nanosized [6]Cyclo-2,7-naphthylene Macrocycle

Chemical reduction of a naphthylene macrocycle, [6]cyclo-2,7-naphthylene ([6]CNAP, 1), with alkali metals, Li and K, revealed the accessibility of the doubly-reduced state of 1. The macrocyclic 1^2- anion was isolated in different coordination environments and crystallographically characterized. The single-crystal X-ray diffraction confirmed the formation of contact-ion complexes with one Li⁺ and two K⁺ ions in THF, and a "naked" dianion in the solvent-separated ion product with K⁺ ions in the presence of 18-crown-6 ether. The detailed structural analysis of 1^2- showed that the π-conjugation over the biaryl linkages between naphthylene panels were enhanced upon two-fold reduction, which was rationally explained by theoretical calculations.

The rational design of Li-doped nitrogen adsorbents for natural gas purification

Separation of nitrogen (N2) and methane (CH4) is one of the most challenging and energy-intensive processes in the natural gas industry, due to their close physico-chemical properties. The quest for an effective N2-selective adsorbent has long been the focus of research; however, the results have been sparse. In this work, a first-principle study has been used to construct and investigate Li-doped polycyclic aromatic hydrocarbons (PAHs) for N2 rejection in natural gas purification. We doped lithium on a series of linear/nonlinear PAHs consisting of two to six benzene rings. The adsorption affinity of the Li-doped organic molecular systems toward N2 and CH4 was evaluated by calculating the interaction energy using density functional theory. From the gas adsorption selectivities for different Li-doped PAHs, Li-doped phenanthrene and chrysene showed the highest N2 over CH4 equilibrium selectivities, with values of 119.7 and 80.8, respectively. It was found that the Li atom enabled the π bond of the aromatic substrate to interfere with the N2 lowest unoccupied molecular orbital, resulting in strong physisorption of N2. These results indicate the high potential of Li-doped phenanthrene and chrysene for N2 removal from natural gas.

The Chatt reaction: conventional routes to homoleptic arenemetalates of d-block elements

Joseph Chatt was the first to discover in the early 1960s that previously unknown transition metal compounds were accessible and isolable via the reactions of alkali metal arene radical anions with transition metal precursors containing good leaving groups, such as weakly basic neutral or anionic ligands, especially halides. Later Peter Timms confirmed the importance of these early studies with the synthesis of several new bis(arene)metal(0) sandwich compounds by a variant of Chatt's route. Following a brief historical survey of alkali metal arene compounds, first examined in some detail by Wilhelm Schlenk, use of these reagents in the conventional syntheses of anionic homoleptic arene metal complexes of the d-block elements will be described. In several cases these species are quite useful because they function as storable "naked" atomic metal anion reagents, especially in their reactions with carbon monoxide and isocyanides. In view of Chatt's seminal contributions to an often unique route to organometallic and inorganic compounds, it is proposed that this valuable synthetic method be named the "Chatt reaction" in honor of a giant of chemistry.

Doubly zwitterionic, di-reduced, highly electron-rich, air-stable naphthalenediimides: redox-switchable islands of aromatic-antiaromatic states

The di-reduced state of the naphthalene moiety and its congeners have long captivated chemists as it is elusive to stabilize these intrinsically reactive electron-rich π-systems and for their emergent multifaceted properties. Herein we report the synthesis and isolation of two-electron (2e^-) reduced, highly electron-rich naphthalenediimides (NDIs). A doubly zwitterionic structure is observed for the first time in a naphthalene moiety and validated by single crystal X-ray crystallography and spectroscopic methods. The synthesis avoids hazardous reducing agents and offers an easy, high-yielding route to bench-stable di-reduced NDIs. Notably, we realized high negative first oxidation potentials of up to -0.730 V vs. Fc/Fc⁺ in these systems, which establish these systems to be one of the strongest ambient stable electron donors. The study also provides the first insights into the NMR spectra of the di-reduced systems revealing a large decrease in diatropicity of the naphthalene ring compared to its 2e^- oxidized form. The NICS, NICS-XY global ring current, gauge-including magnetically induced current (GIMIC) and AICD ring current density calculations revealed switching of the antiaromatic and aromatic states at the naphthalene and the imide rings, respectively, in the di-reduced system compared to the 2e^- oxidized form. Notably, the substituents at the phosphonium groups significantly tune the antiaromatic-aromatic states and donor ability, and bestow an array of colors to the di-reduced systems by virtue of intramolecular through-space communication with the NDI scaffold. Computational studies showed intramolecular noncovalent interactions to provide additional stability to these unprecedented doubly zwitterionic systems.

Ca(ii), Yb(ii) and Tm(iii) complexes with tri- and tetra-anions of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene

Sandwich-like structures derived from organic polyanions and multi-centered cation assemblies.

Lithium Enolates Derived from Pyroglutaminol: Mechanism and Stereoselectivity of an Azaaldol Addition

A lithium enolate derived from an acetonide-protected pyroglutaminol undergoes a highly selective azaaldol addition with (E)-N-phenyl-1-[2-(trifluoromethyl)phenyl]methanimine. The selectivity is sensitive to tetrahydrofuran (THF) concentration, temperature, and the presence of excess lithium diisopropylamide base. Rate studies show that the observable tetrasolvated dimeric enolate undergoes reversible deaggregation, with the reaction proceeding via a disolvated-monomer-based transition structure. Limited stereochemical erosion stems from the intervention of a trisolvated-monomer-based pathway, which is suppressed at low THF concentrations and elevated temperature. Endofacial selectivity observed with excess lithium diisopropylamide (LDA) is traced to an intermediate dianion formed by subsequent lithiation of the monomeric azaaldol adduct, which is characterized as both a dilithio form and a trilithio dianion-LDA mixed aggregate.

Carbon-Rich Active Materials with Macrocyclic Nanochannels for High-Capacity Negative Electrodes in All-Solid-State Lithium Rechargeable Batteries

A high-capacity electrode active material with macrocyclic nanochannels is developed for a negative electrode of lithium batteries. With appropriate design of the molecular and crystal structures, a ubiquitous chemical commonly available in reagent stocks of any chemistry laboratories, naphthalene, was transformed into a high-performance electrode material for all-solid-state lithium batteries.

Secondary diphosphine and diphosphido ligands: synthesis, characterisation and group 1 coordination compounds

Two types of secondary diphosphines, 1,8-(ArPH)2C14H8 (1a: Ar = Tripp, 2,4,6-triisopropylphenyl; 1b: Ar = Mes, 2,4,6-trimethylphenyl) and 1,3-((t)BuPHCH2)2C6H4 (2), based on rigid 1,8-anthracene and flexible m-xylyl frameworks, respectively, have been synthesized using different strategies. Compounds 1a and 1b were formed by nucleophilic aromatic substitution of a potassium organophosphido salt onto 1,8-difluoroanthracene, while compound 2 was obtained by addition of the Grignard reagent [1,3-(ClMgCH2)2C6H4]x to a dichloroorganophosphine, followed by reduction to the diphosphine. These compounds were isolated as ca. 1 : 1 mixtures of rac and meso diastereomers as determined by multinuclear NMR spectroscopy. Borane and selenide derivatives of 2, 1,3-((t)BuPH(BH3)CH2)2C6H4 (3) and 1,3-((t)BuPH(Se)CH2)2C6H4 (4), were obtained. Preferential crystallization of one diastereomer of 3 and 4 was observed; X-ray crystallographic studies identified this as the rac isomer for diselenide 4. Metallation studies of compounds 1a and 2 yielded several alkali metal salts. The reaction of KH or K metal with 1a yielded the compounds 1,8-(TrippPK)2C14H8·xTHF (5) and 1,8-(TrippPK)2C14H10·xTHF (6), respectively; in complex 6 the central aromatic ring has been reduced to yield a bent dihydroanthracene backbone. A crown ether derivative of 6, [K(18-crown-6)(THF)2]2[1,8-(TrippP)2C14H10] (7), was characterised crystallographically. Double deprotonation of compound 2 with (n)BuLi/TMEDA (TMEDA = N,N,N',N'-tetramethylethylenediamine) afforded the yellow dilithium complex 1,3-((t)BuPLiCH2)2C6H4·TMEDA (8), which crystallized as a dimer featuring lithium-arene π interactions.

Enediolate-dilithium amide mixed aggregates in the enantioselective alkylation of arylacetic acids: structural studies and a stereochemical model

A combination of X-ray crystallography, (6)Li, (15)N, and (13)C NMR spectroscopies, and density functional theory computations affords insight into the structures and reactivities of intervening aggregates underlying highly selective asymmetric alkylations of carboxylic acid dianions (enediolates) mediated by the dilithium salt of a C2-symmetric chiral tetraamine. Crystallography shows a trilithiated n-butyllithium-dilithiated amide that has dimerized to a hexalithiated form. Spectroscopic studies implicate the non-dimerized trilithiated mixed aggregate. Reaction of the dilithiated amide with the dilithium enediolate derived from phenylacetic acid affords a tetralithio aggregate comprised of the two dianions in solution and the dimerized octalithio form in the solid state. Computational studies shed light on the details of the solution structures and afford a highly predictive stereochemical model.

New modes of reactivity in the threshold of the reduction potential in solution. Alkylation of lithium PAH (polycyclic aromatic hydrocarbon) dianions by primary fluoroalkanes: a reaction pathway complementing the classical birch reductive alkylation

Some of the most highly reduced organic species in solution, such as the dianions of PAHs (polycyclic aromatic hydrocarbons) display unexpected reactivity patterns when they react with an appropriate counterpart. As seen before in their reaction with propene and other alkenes, PAHs(-2) apparently react with fluoroalkanes in a nucleophilic fashion in spite of being generally regarded as powerful electron-transfer reagents in their reactions with haloalkanes. This methodology complements the current methodologies on reductive alkylation of polycyclic arenes by allowing access to a new set of regioisomers, the regiochemistry of which can be easily predicted by simple MO calculations.

Reactions of Li- and Yb-coordinated N,N′-bis(trimethylsilyl)-β-diketiminates: one- and two-electron reductions, deprotonation, and C–N bond cleavage

The synthesis and characterisation of novel Li and Yb complexes is reported, in which the monoanionic beta-diketiminato ligand has been (i) reduced (SET or 2 [times] SET), (ii) deprotonated, or (iii) C-N bond-cleaved. Reduction of the lithium beta-diketiminate Li(L(R,R'))[L(R,R')= N(SiMe(3))C(R)CHC(R')N(SiMe(3))] with Li metal gave the dilithium derivative [Li(tmen)(mu-L(R,R'))Li(OEt(2))](R = R'= Ph; or, R = Ph, R[prime or minute]= Bu(t)). When excess of Li was used the dimeric trilithium [small beta]-diketiminate [Li(3)(L(R,R[prime or minute]))(tmen)](2)(, R = R'= C(6)H(4)Bu(t)-4 = Ar) was obtained. Similar reduction of [Yb(L(R,R'))(2)Cl] gave [Yb[(mu-L(R,R'))Li(thf)](2)](, R = R[prime or minute]= Ph; or, R = R'= C(6)H(4)Ph-4 = Dph). Use of the Yb-naphthalene complex instead of Li in the reaction with [Yb(L(Ph,Ph))(2)] led to the polynuclear Yb clusters [Yb(3)(L(Ph,Ph))(3)(thf)], [Yb(3)(L(Ph,Ph))(2)(dme)(2)], or [Yb(5)(L(Ph,Ph))(L(1))(L(2))(L(3))(thf)(4)] [L(1)= N(SiMe(3))C(Ph)CHC(Ph)N(SiMe(2)CH(2)), L(2)= NC(Ph)CHC(Ph)H, L(3)= N(SiMe(2)CH(2))] depending on the reaction conditions and stoichiometry. The structures of the crystalline complexes 4, 6x21/2(hexane), 5(C(6)D(6)), and have been determined by X-ray crystallography (and have been published).

Ammonia free partial reduction of aromatic compounds using lithium di-tert-butylbiphenyl (LiDBB)

The reduction of a series of hetero- and carbocyclic aromatic compounds under ammonia free conditions is described. By using LiDBB as a source of electrons, bis(methoxyethyl)amine (BMEA) as a protonating agent, and THF as a solvent, we were able to accomplish reductions more usually performed under Birch type conditions. Moreover, the use of these conditions was further enhanced by the tolerance of the reducing system toward reactive electrophiles that cannot be used successfully in ammonia.

Probing alkali metal-pi interactions with the side chain residue of tryptophan

Feeble forces play a significant role in the organization of proteins. These include hydrogen bonding, hydrophobic interactions, salt bridge formation, and steric interactions. The alkali metal cation-pi interaction is a force of potentially profound importance but its consideration in biology has been limited by the lack of experimental evidence. Our previous studies of cation-pi interactions with Na(+) and K(+) involved the side arms of tryptophan (indole), tyrosine (phenol), and phenylalanine (benzene) as the arene donors. The receptor system possesses limiting steric constraints. In this report, we show that direct interactions between alkali metals and arenes occur at or within the van der Waals contact distance.