Anion-dependent formation of helicates versus mesocates of triple-stranded M2L3 (M = Fe2+, Cu2+) complexes.

A series of dinuclear triple-stranded complexes, [Fe2L3⊃X]X6 [X = BF4– (1), ClO4– (2)], [Fe2L3⊃SO4]2(SO4)5 (3), [Fe2L3⊃Br](BPh4)6 (4), Fe2L3(NO3)Br6 (5), and [Cu2L3⊃NO3](NO3)6 (6), which incorporate a central cavity to encapsulate different anions, have been synthesized via the self-assembly of iron(II) or copper(II) salts with the N,N′-bis[5-(2,2′-bipyridyl)methyl]imidazolium bromide (LBr) ligand. X-ray crystallographic studies (for 1–4 and 6) and elemental analyses confirmed the cagelike triple-stranded structure. The anionic guest is bound in the cage and shows remarkable influence on the outcome of the self-assembly process with regard to the configuration at the metal centers. The mesocates (with different configurations at the two metal centers) have formed in the presence of large tetrahedral anions, while helicates (with the same configuration at both metal centers) were obtained when using the relatively smaller spherical or trigonal-planar anions Br– or NO3–.