Polymetallated 4‐ tert ‐Butylcalix[4]arene Complexes of Sodium and Potassium Stabilized by Crown Ethers: Syntheses, Structures and Activation of Carbon Dioxide

4‐ tert ‐Butylcalix[4]arene and 18‐crown‐6 react with NaH in THF to form the complex [(18‐crown‐6)(THF)Na 4 (4‐ tert ‐butylcalix[4]arene‐4H)] 2 ·3THF ( 1 ). Analogously, the reaction with 4‐ tert ‐butyltetrathiacalix[4]arene in DMF results in the formation of [(18‐crown‐6)Na(dmf) 1.5 ] 2 [(4‐ tert ‐butyltetrathiacalix[4]arene‐4H) 2 Na 6 (dmf) 2 ] ( 2 ) The X‐ray analysis of 1 shows an Na 6 O 6 skeleton consisting of two fused open heterocubanes. Two Na + ions are encapsulated in the cavities of the calix[4]arene units. Additionally, two peripheral Na + ions, each surrounded by a crown ether and one THF ligand, are connected to the central Na 6 O 6 skeleton by one bridging oxygen of the calix[4]arene. In contrast, 2 contains the highly symmetrical anion [(4‐ tert ‐butyltetrathiacalix[4]arene‐4H) 2 Na 6 (dmf) 2 ] 2− and crown‐ether‐stabilized cations. While 1 shows a dynamic process in solution above 40 °C, the structure of the highly symmetrical compound 2 remains stable at higher temperatures. The solid‐state structure of the potassium complex [(dibenzo‐18‐crown‐6)K 4 (4‐ tert ‐butylcalix[4]arene‐4H)(THF) 3 ] 2 ·4THF ( 4 ) is similar to 1 ; however, the calix[4]arene units in the open heterocubane structure of 4 are shifted less than half a diameter to one another than in the sodium complex 1 . According to the 1 H NMR spectra complex 3a , containing 18‐crown‐6, has a very similar structure to the related complex 4 . Both 1 and 4 absorb carbon dioxide in THF and are active CO 2 ‐transfer reagents towards 2‐fluoropropiophenone; complex 2 is much less active. This clearly demonstrates the subtle influence of the structure of the alkali metal complexes on the CO 2 reactivity in these types of compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)