The complexation behaviour between salts of Li+-Rb+ in CD3CN and tris(benzo
crown ether)s 2a,b X=P(NMeN=CH-B15C5)(3) (X = O, S) and tri[bis(benzocrown
ether)] [N=P(NMeN=CH-B15C5)(2)](3) 3 was investigated by C-13 NMR spectrosc
opy. Using the program RMNSTAB, the complexation constants for the differen
t possible complexes (M2L, ML and ML2 were L represents one benzo-15-crown-
5) were obtained and were compared with those of the corresponding monomer
material. A remarkable "biscrown effect'' for compounds 2a,b and 3 was foun
d, especially for potassium and rubidium by the predominant formation of st
able ML2 complexes. The strong chelate effect make these ligands highly eff
icient extracting agents for alkali metal picrate salts of K+, Rb+ and Cs+,
as shown by UV-Vis spectroscopy.