SELF-ASSEMBLY OF HETERONUCLEAR SUPRAMOLECULAR HELICAL COMPLEXES WITH SEGMENTAL LIGANDS

The segmental bidentate-tridentate ligand l-2-(6''''-methylpyridin-2'' '-yl)-1H-benzimidazole (L(1)) reacts with Fe(II), Zn(II), and Co(II) i n acetonitrile to give mononuclear head-to-head complexes [M(L(1))(2)] (2+) where the two tridentate units of the ligands are pseudo-octahedr ally coordinated to M(II). Detailed electrospray-mass spectrometeric ( ES-MS), spectrophotometric, and H-1-NMR studies show that a second met al ion can react with [M(L(1))(2)](2+) to give homodinuclear C-2-symme trical head-to-head double helical complexes [M(2)(L(1))(2)](4+) where the second cation is pseudotetrahedrally bound by the two remaining b identate units of the ligands. When M = Fe(II), thermodynamic data sho w that the second metal ion is only weakly coordinated, while for M = Zn(II), Co(II), which display greater affinities for tetrahedral coord ination, stable homodinuclear [M(2)(L(1))(2)](4+) are obtained in acet onitrile. Reaction of [Fe(L(1))(2)](2+) with Ag(I) produces the self-a ssembled C-2-symmetrical heterodinuclear double-helical complex [FeAg( L(1))(2)](3+) where Fe(II) occupies the pseudo-octahedral site defined by the two tridentate units of the ligands and Ag(I) lies in the rema ining pseudotetrahedral site. Similarly, the trileptic ligand midazole -2,5-diyl)methylene]}bis[1H-benzimidazole] (L(2)) reacts with Fe(II) a nd Ag(I) (stoichiometric ratio 1:2) to give the first self-assembled D -2-symmetrical heterotrinuclear complex [FeAg2(L(2))(2)](4+) where Fe( II) is located in the central pseudo-octahedral site (bistridentate) a nd Ag(I) occupy the two capping pseudotetrahedral sites. H-1-NMR measu rements are compatible with a double-helical or a catenate structure f or [FeAg2(L(2))(2)](4+). The selectivity of the self-assembly processe s is discussed together with thermodynamic and structural factors requ ired for the formation of helical heteropolynuclear complexes.