Self-assembly of tetrareduced corannulene with mixed Li–Rb clusters: dynamic transformations, unique structures and record 7Li NMR shifts

Abstract

Self-assembly processes of the highly reduced bowl-shaped corannulene generated by the chemical reduction with a binary combination of alkali metals, namely Li–Rb, have been investigated by variable-temperature ¹H and ⁷Li NMR spectroscopy. The formation of several unique mixed metal sandwich products based on tetrareduced corannulene, C₂₀H₁₀⁴⁻ (1⁴⁻), has been revealed followed by investigation of their dynamic transformations in solutions. Analysis of NMR data allowed to propose the mechanism of stepwise alkali metal substitution as well as to identify experimental conditions for the isolation of intermediate and final supramolecular products. As a result, two new triple-decker aggregates with a mixed Li–Rb core, [{Rb(THF)₂}₂]//[Li₃Rb₂(C₂₀H₁₀)₂{Li⁺(THF)}] (2) and [{Rb(diglyme)}₂]//[Li₃Rb₃(C₂₀H₁₀)₂(diglyme)₂]·0.5THF (3·0.5THF), have been crystallized and structurally characterized. The Li₃Rb₂-product has an open coordination site at the sandwich periphery and thus is considered transient on the way to the Li₃Rb₃-sandwich having the maximized intercalated alkali metal content. Next, the formation of the LiRb₅ self-assembly with 1⁴⁻ has been identified by ⁷Li NMR as the final step in a series of dynamic transformations in this system. This product was also isolated and crystallographically characterized to confirm the LiRb₅ core. Notably, all sandwiches have their central cavities, located in between the hub-sites of two C₂₀H₁₀⁴⁻ decks, occupied by an internal Li⁺ ion which exhibits the record high negative shift (ranging from −21 to −25 ppm) in ⁷Li NMR spectra. The isolation of three novel aggregates having different Li–Rb core compositions allowed us to look into the origin of the unusual ⁷Li NMR shifts at the molecular level. The discussion of formation mechanisms, dynamic transformations as well as unique electronic structures of these remarkable mixed alkali metal organometallic self-assemblies is provided and supported by DFT calculations.