Ternary Alkali Metal Transition Metal Acetylides

Understanding the solubilization of Ca acetylide with a new computational model for ionic pairs

The unique reactivity of the acetylenic unit in DMSO gives rise to ubiquitous synthetic methods. We theoretically consider CaC2 solubility and protolysis in DMSO and formulate a strategy for CaC2 activation in solution-phase chemical transformations. For this, we use a new strategy for the modeling of ionic compounds in strongly coordinating solvents combining Born-Oppenheimer molecular dynamics with the DFTB3-D3(BJ) Hamiltonian and static DFT computations at the PBE0-D3(BJ)/pob-TZVP-gCP level. We modeled the thermodynamics of CaC2 protolysis under ambient conditions, taking into account its known heterogeneity and considering three polymorphs of CaC2. We give a theoretical basis for the existence of the elusive intermediate HC[triple bond, length as m-dash]C-Ca-OH and show that CaC2 insolubility in DMSO is of thermodynamic nature. We confirm the unique role of water and specific properties of DMSO in CaC2 activation and explain how the activation is realized. The proposed strategy for the utilization of CaC2 in sustainable organic synthesis is outlined.

Velocity-Map Imaging and Magnetic-Bottle Photoelectron Spectroscopy of [SeCCH]-: Electronic Properties and Spin-Orbit Splitting

The recently synthesized acetylide compound KSeCCH containing the main group element selenium within the novel and in crystalline form unprecedented [SeCCH]^- anion was successfully investigated in the gas phase by high-resolution velocity-map imaging (VMI) and magnetic-bottle (MB) photoelectron spectroscopy coupled with an electrospray ionization source. Both VMI and MB spectra exhibited identical electron affinities (EA, 2.517 ± 0.002 eV), spin-orbit coupling (SOC) splittings (1492 ± 20 cm^-1), and Se-C stretching frequencies (573 ± 20 cm^-1) of the corresponding neutral tetra-atomic radical [SeCCH]^• with the VMI spectrum possessing six times higher spectral resolution compared with the MB spectrum. These experimental values were well reproduced by calculations at the CCSD(T) level, in which both the isolated [SeCCH]^- anion and the [SeCCH]^• radical adopted linear geometries. The simulated spectra based on the calculated Franck-Condon factors, the SOC splitting, and the experimental line width matched well with the measured spectra. Furthermore, comparisons of the EA and SOC splitting values with the previously reported isolobal species [SeCN]^• are also made and discussed. The decrease in the EA and SOC splitting of [SeCCH]^• is ascribed to the differences in the electronegativities between C and N atoms as well as the electron density on the Se atom in its singly occupied molecular orbital (SOMO).

Cs2Cd(C2H)2(C2): A Crystalline Acetylide with Bridging C2 Units

Cs₂Cd(C₂H)₂(C₂) was synthesized by heating known Cs₂Cd(C₂H)₄ either in a dry argon atmosphere at 200 °C or under ammonothermal conditions (130 °C, ∼ 100 bar). The crystal structure of the resulting dark orange-brown microcrystalline material was solved and refined from synchrotron powder diffraction data (Cmcm, Z = 4). Cs₂Cd(C₂H)₂(C₂) is composed of Cd²⁺ cations tetrahedrally coordinated end-on by four acetylide groups. Two of them are terminating C₂H^- groups, whereas the other two positions are occupied by bridging C₂^2- anions. Thus, a polymeric _∞¹[Cd(C₂H)₂(C₂)_2/2^2-] chain-like anion results and these chains are separated by Cs⁺ cations. So obviously Cs₂Cd(C₂H)₂(C₂) is formed from Cs₂Cd(C₂H)₄ by a condensation reaction of two of its four C₂H^- groups under the release of one acetylene (C₂H₂) molecule. This reaction mechanism is supported by DSC/TGA measurements, and the crystal structure of Cs₂Cd(C₂H)₂(C₂) is further supported by IR spectroscopic investigations.

AI SeC2 H (AI =K, Rb, Cs): Crystalline Compounds with the Elusive - Se-C≡C-H Anion

By reaction of alkali metal acetylides, A^I C₂ H (A^I =K, Rb, Cs), with elemental selenium in liquid ammonia highly crystalline powders of A^I SeC₂ H were obtained. The structure analysis based on the resulting synchrotron powder diffraction data revealed that all compounds crystallize in an orthorhombic unit cell (Cmc2₁ , Z=4) exhibiting the elusive ^- SeC₂ H anion, which is unprecedented in a crystalline compound up to now. Elemental analysis and IR spectroscopic data confirm this finding. Upon heating, A^I SeC₂ H compounds release acetylene based on DSC/TGA experiments resulting in powders with the proposed composition A^I ₂ Se₂ (C₂ ). The resulting powders were indexed with small cubic unit cells, but a reasonable structural model could not be developed up to now. Upon exposure of A^I SeC₂ H compounds to water elemental selenium is formed again.

The synthesis of ternary acetylides with tellurium: Li2TeC2 and Na2TeC2

Novel ternary acetylides Li₂TeC₂ and Na₂TeC₂ were synthesized via the robust direct reaction of tellurium powder and mono- or bialkali acetylides in liquid ammonia.

Ultrahigh energy density Li-ion batteries based on cathodes of 1D metals with -Li-N-B-N- repeating units in α-Li(x)BN₂ (1 ⩽ x ⩽ 3)

Ultrahigh energy density batteries based on α-Li(x)BN2 (1 ⩽ x ⩽ 3) positive electrode materials are predicted using density functional theory calculations. The utilization of the reversible LiBN2 + 2 Li(+) + 2 e(-) ⇌ Li3BN2 electrochemical cell reaction leads to a voltage of 3.62 V (vs Li/Li(+)), theoretical energy densities of 3251 Wh/kg and 5927 Wh/l, with capacities of 899 mAh/g and 1638 mAh/cm(3), while the cell volume of α-Li3BN2 shrinks only 2.8% per two-electron transfer on charge. These values are far superior to the best existing or theoretically designed intercalation or conversion-based positive electrode materials. For comparison, the theoretical energy density of a Li-O2/peroxide battery is 3450 Wh/kg (including the weight of O2), that of a Li-S battery is 2600 Wh/kg, that of Li3Cr(BO3)(PO4) (one of the best designer intercalation materials) is 1700 Wh/kg, while already commercialized LiCoO2 allows for 568 Wh/kg. α-Li3BN2 is also known as a good Li-ion conductor with experimentally observed 3 mS/cm ionic conductivity and 78 kJ/mol (≈0.8 eV) activation energy of conduction. The attractive features of α-Li(x)BN2 (1 ⩽ x ⩽ 3) are based on a crystal lattice of 1D conjugated polymers with -Li-N-B-N- repeating units. When some of the Li is deintercalated from α-Li3BN2 the crystal becomes a metallic electron conductor, based on the underlying 1D conjugated π electron system. Thus, α-Li(x)BN2 (1 ⩽ x ⩽ 3) represents a new type of 1D conjugated polymers with significant potential for energy storage and other applications.