Structures of potassium encapsulated within the 1,3-alternate conformation of calix[4]arenes

Abstract

5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis(diethylcarbamoylmethoxy)calix[4]arene (L¹) has been synthesised in the 1,3-alternate conformation for the first time. Proton NMR complexation studies suggest that this molecule forms 1:1 and 2:1 complexes with potassium cations. A crystal structure determination has been performed on the 2:1 complex [K₂L¹][CuCl₂][I₃], [monoclinic, space group P2₁/n, a= 13.972(6), b= 23.373(12), c= 25.271(12)Å, β= 102.5(1)°, Z= 4, R= 0.092 for 6501 ‘observed’ reflections]. The structure of the 1:1 potassium complex of the related diamide, 5,11,17,23-tetra-tert-butyl-25,27-bis(diethylcarbamoylmethoxy)-26,28-dihydroxycalix[4]arene (L²), [KL²][I₃], has also been elucidated [triclinic, space group P, a= 14.160(8), b= 16.326(10), c= 16.540(10)Å, α= 65.83(1), β= 67.37(1), γ= 88.87(1)°, Z= 2, R= 0.091 for 4976 ‘observed’ reflections]. Remarkably, the calixarene is again found in the 1,3-alternate conformation, although ¹³C NMR studies suggest that, in this case, the conformation is not maintained in solution. The co-ordination spheres of the potassium cations in both structures are similar, with a hitherto unobserved combination of close contacts to four oxygen atoms, and to two phenyl rings of the respective calix[4]arenes, Molecular mechanics calculations have been carried out to investigate the relative stabilities of the calixarene conformations.