Endohedral Mixed Aggregates: Sodium Alkoxide Cages with Organic or Inorganic Central Anions and Variable Hull

Abstract

Alkali metal alkoxides are widely used in chemistry due to their Brønsted basic and nucleophilic properties. Potassium alkoxides assist alkyllithium in the metalation of hydrocarbons in Lochmann-Schlosser-bases. Both compounds form mixed aggregates, which enhance the thermal stability, solubility, and the basic reactivity of these mixtures. A very unusual spherical mixed alkoxy aggregate was discovered by Grützmacher et al., where a central dihydrogen phosphide anion is surrounded by a highly dynamic shell of thirteen sodium atoms and a hull of twelve tert-butoxide groups. This structural motif can be reproduced by a reaction of trimethylsilyl compounds of methane, halogens, or pseudo-halogens with excess sodium tert-butoxide. A nucleophilic substitution releases the corresponding anion, which is then encapsulated by the sodium alkoxide units. The compounds are soluble in hydrocarbon solvents, enabling studies of solutions by high-resolution NMR spectroscopy and IR/Raman studies of the crystalline materials.

Keywords: aggregation; alkoxide; cage compounds; isotopic labeling; sodium.

Scheme 1

a) formation of heavier alkali metal compounds by reaction of alkyllithium with alkali metal alkoxide; b) formation of a potassium superbase from neopentyl potassium and potassium alkoxide; c) unsuccessful reaction of trimethylsilylmethyl potassium with potassium alkoxide; d) formation of CH₃@1 from trimethylsilylmethyl sodium with excess sodium alkoxide.

Figure 1

Molecular structure of compounds of the composition X@1, X=CH₃, CN, Cl, Br, I, OCN, SCN, N₃, or NO₃. The structure of I@1 is shown as a representative structure. Hydrogen atoms and minor‐disordered units are omitted and only representative atom names are shown for clarity. The dark gray shaded areas emphasize the pentagonal dodecahedron formed by sodium atoms; the light grey shaded areas emphasize the corners of the icosahedron formed by OtBu oxygen atoms.

Figure 2

¹H–¹³C HSQC NMR spectrum of CH₃@1 in C₆D₆, the traces of the 2D plot are separately recorded ¹H (top) and ¹³C (left) NMR spectra. The inset shows a magnified view of the methide (CH₃) resonance in the high‐field area with traces of the corresponding spectra. The methide signal clearly shows a 1 : 3 : 3 : 1 quartet structure with a coupling constant of 96 Hz in the 13 C NMR spectrum, consistent with a CH₃ unit.

Scheme 2

The outcomes of reactions of α‐deuterated LiCHDSiMe₃ or α‐ and γ‐deuterated LiCHDSi(CH₂D)₃. In the first case the deuterium content of CH₂D@1 was depleted, the latter reaction led to a deuterium‐rich product.

Scheme 3

Synthesis of a methylcyclohexanolate (OMeCyc) analogue of CH₃@1 and alternative introduction of methylcyclohexanolate groups into the alkoxy shell of CH₃@1.

Figure 3

Evolution of a mixture of CH₃@1 and CH₃@Na₁₃(OMeCyc)₁₂ in deuterated benzene observed by ¹H NMR spectroscopy at 50 °C. The two initial singlet resonances are transformed into a group of roughly equidistant signals; from bottom to top: a) without additional NaOtBu, 0 min, 39 min, 87 min; b) with additional NaOtBu, 0 min, 60 min, 450 min.

Scheme 4

Synthesis of X@1 and 2‐F by reaction of functionalized trimethylsilyl compounds with excess NaOtBu. Trimethylsilylcyanide, trimethylsilylazide, and trimethylsilyl nitrate were also used enriched with ¹³C and ¹⁵N, respectively.

Figure 4

Graphical representation of the measured cell volumes of the compounds X@1 and the calculated volumes of the corresponding anions. The halide compounds are shown as full circles with a fitted line (determination coefficient 0.9972), all other compounds as open circles.

Figure 5

Molecular Structure of compound 2‐F. Selected minor disordered units and hydrogen atoms are omitted for clarity. The two shaded areas emphasize the two squares and the triangles formed by sodium atoms. The sodium atoms of the sodium triangles are disordered, the alternative positions and the resulting triangle are shown by dotted circles and dashed lines.