STRUCTURES OF POTASSIUM ENCAPSULATED WITHIN THE 1,3-ALTERNATE CONFORMATION OF CALIX[4]ARENES

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 P1, 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