Self-Assembly of Charged Supramolecular Sandwiches Formed by Corannulene Tetraanions and Lithium Cations

The reduction of corannulene (C₂₀H₁₀) with excess lithium metal in a strong chelating O-donor solvent, diglyme, leads to the formation of the highly reduced C₂₀H₁₀^4– anion. However, in contrast to the formation of the sandwich-type supramolecular aggregate [Li₅(C₂₀H₁₀^4–)₂]^3– observed in THF, corannulene tetraanions and lithium counterions in diglyme form only contact ion pairs according to ⁷Li NMR spectroscopy. Furthermore, the slow dissociation of the premade sandwich [Li₅(C₂₀H₁₀^4–)₂]^3– in neat diglyme has been demonstrated by multinuclear NMR spectroscopy. In contrast, the [Li₅(C₂₀H₁₀^4–)₂]^3– sandwich can be crystallized from the THF/diglyme mixture as the new crystalline product [Li­(THF)₂(diglyme)]⁺­[Li₂(THF)­(diglyme)//Li₅(C₂₀H₁₀^4–)₂]⁻, showing a complex 1D hybrid architecture according to the single-crystal X-ray diffraction study

Silicon in a Negatively Charged Shell: Anions of Spirosilabifluorene

Mono- and dianions of a polycyclic compound with a central sp³-hybridized silicon atom, spirosilabifluorene (C₂₄H₁₆Si, <b>1</b>), were prepared by reduction with alkali metals. The salts containing <b>1</b>^•– and <b>1</b>^2– anions were isolated and studied by single-crystal X-ray diffraction. The lithium salt of the C₂₄H₁₆Si^•– radical monoanion ([Li­(THF)₄⁺]­[<b>1</b>^•–], <b>2</b>) exists as a solvent-separated ion pair in the solid state. Substantially different geometrical parameters were found for each of the fluorene groups within the C₂₄H₁₆Si^•– anion of <b>2</b> due to asymmetric charge distribution. The C₂₄H₁₆Si^2– dianion was isolated in the form of its sodium ([{Na­(THF)₃⁺}­{Na­(THF)⁺(<b>1</b>^2–)], <b>3</b>) or potassium ([{K­(THF)⁺}₂(<b>1</b>^2–)], <b>4</b>) salt. The environment at the central silicon atom in the dianion is flattened in comparison to the monoanion and neutral compound, with the angle between the two fluorene planes measured at 55° in <b>1</b>^2– vs 89<b>°</b> in <b>1</b>^•– and 83° in <b>1</b>⁰. The aggregation of dianions and alkali-metal counterions leads to the formation of dimeric units and 1D polymeric chains in the solid sodium and potassium salts, respectively. The structure of the cesium salt <b>5</b>, containing both mono- and dianions in the crystal lattice, was also studied by X-ray diffraction. Complexes <b>2</b>–<b>5</b> were investigated by ESR and variable-temperature multinuclear NMR spectroscopy. Theoretical investigations at the PBE0, MP2, and multireference NEVPT2 levels of theory for the C₂₄H₁₆Si^<i>n</i>– (<i>n</i> = 0–2) species revealed the conjugation of two fluorene units over the central silicon atom and a singlet ground state for the dianion

“Naked” Mono- and Dianions of Corannulene with Lithium Counterions

Controlled reduction of corannulene (C₂₀H₁₀, <b>1</b>) with lithium metal has been investigated in several O-donor solvent media (THF, DME, and diglyme). In contrast to the prior reduction studies performed <i>in situ</i>, the isolation of solid products of singly and doubly charged corannulene in the form of crystalline lithium salts has been accomplished. The single-crystal X-ray diffraction studies of two products containing monoreduced corannulene, [Li­(DME)₃⁺]­[<b>1</b>^–] (<b>2</b>) and [{Li­(15-crown-5)⁺}₂]­[<b>1</b>^–]₂ (<b>3</b>), revealed the presence of naked bowl-shaped monoanions. The corannulene dianions have been crystallized from two different media as corresponding salts, [{Li­(DME)⁺}­{Li­(DME)_1.5⁺}]₂[<b>1</b>^2–]₂ (<b>4</b>) and [Li­(diglyme)₂⁺]₂[<b>1</b>^2–] (<b>5</b>), illustrating the effect of solvents on the alkali metal coordination environment and binding to the curved π-carbon surfaces. The adduct <b>4</b> has Li⁺ ions bound to the five- and six-membered rings of the doubly reduced corannulene convex face. In contrast to the above contact ion pair obtained from the DME solution, crystallization of the C₂₀H₁₀^2– anion from the strongly chelating diglyme leads to the isolation of free <b>1</b>^2–, in which the geometrical parameters of the corannulene core are not affected by direct coordination of alkali metal ions. Notably, the latter represents the first example of the “naked” corannulene dianion and allows us to examine how charging the C₂₀H₁₀ bowl with two electrons affects its core geometry. The single-crystal X-ray structural data are complemented by full characterization of products <b>2</b>–<b>5</b> using NMR, ESR, and UV–vis spectroscopies

Molecular Structures of <i>N</i>,<i>N</i>′‑Dimethylbenzimidazoline-2-germylene and -stannylene in Solution and in Solid State by Means of Optical (Raman and UV–vis) Spectroscopy and Quantum Chemistry Methods

X-ray data obtained for germylene <b>1</b> evidence its monomeric structure, unlike that of stannylene <b>2</b>, which had been shown previously to form a coordination dimer. Raman spectra of solid and liquid <b>1</b> are identical, whereas the Raman spectra of solid <b>2</b> and its solution <b>2a</b> differ significantly. The spectrum of <b>2</b> is complicated and contains the lines corresponding to N → Sn coordination bonds forming a dimer. The spectrum of <b>2a</b> is simpler and close to that of monomeric <b>1</b>, thus pointing to the rupture of the dimer in solution. The UV–vis spectrum of solid <b>2</b> exhibits a band corresponding to a transition involving the N → Sn coordination bonds. Quantum theory of atoms in molecules data estimate the energy of this bond as ∼19 kcal/mol. The aromaticity of <b>1</b> and <b>2</b> with their 10 π-electron systems including divalent Ge or Sn atoms is confirmed by negative nucleus-independent chemical shift values