The segmental bidentatetridentate ligand 5-{2”[6”-( 1”'-(3,5- imethoxybenzyl)- 1'”H-benzimidazol-2”'- yl)-pyridin-2”-yl] - 1' -methyl- 1' H-benzimidazol-5'-ylmethy1l- 1 - methyl-2-(6””- ethylpyridine-2””-yl)- 1H -b enzimidazole (Ll) reacts with Fe(II), Zn(II), and Co(I1) in acetonitrile to give mononuclear head-to-head complexes [M(L1)2]2+ where the two tridentate units of the ligands are pseudo-octahedrally coordinated to M(I1). Detailed electrospray-mass spectrometeric (ES-MS), spectrophotometric, and ‘H-NMR studies show that a second metal ion can react with [M(L1)2]2+ to give homodinuclear C2-symmetrical head-to-head double helical complexes [M2(L1)2]4+ where the second cation is pseudotetrahedrally bound by the two remaining bidentate units of the ligands. When M = Fe(II), thermodynamic data show that the second metal ion is only weakly coordinated, while for M = Zn(II), Co(II), which display greater affinities for tetrahedral coordination, stable homodinuclear [ M2(L1)2I4+ are obtained in acetonitrile. Reaction of [Fe(L1)212+ with Ag(1) produces the self-assembled C2-symmetrical heterodinuclear double-helical complex [FeAg(L1)2I3+ where Fe(I1) occupies the pseudo-octahedral site defined by the two tridentate units of the ligands and Ag(1) lies in the remaining pseudotetrahedral site. Similarly, the trilepticligand l,l'-dimethyl-2,2'-bis[6-(methylpyridin- 2-yl)-S,S'-{pyridine-2,6-diylbis[( 1-methyl-1 H -benzimidazole-,5-diyl)methylene])bis[1 H -benzimidazole] (L2) reacts with Fe(1I) and Ag(1) (stoichiometricratio 1:2) to give the first self-assembled D2-symmetrical heterotrinuclear complex [FeAgz(L2)2]4+ where Fe(I1) is located in the central pseudo-octahedral site (bistridentate) and Ag(1) occupy the two capping pseudotetrahedral sites. 1H-NMR measurements are compatible with a double-helical or a catenate structure for [FeAgz(L2)2l4+. The selectivity of the self-assembly processes is discussed together with thermodynamic and structural factors required for the formation of helical heteropolynuclear complexes.
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Piguet et al. (1994) studied this question.