SELF-ASSEMBLY OF DOUBLE AND TRIPLE HELICES CONTROLLED BY METAL-ION STEREOCHEMICAL PREFERENCE

The syntheses of the dinucleating bisbidentate ligand bis[5-(1-methyl-2-(6-methyl-2'-pyridyl)benzimidazolyl)]methane (bismbmp, 3) and its mononuclear analogue 6-methyl-2-(1-methylbenzimidazol-2-yl)pyridine (mbmp, 1) are reported. The ligand mbmp (1) reacts with copper(I) to give [Cu(mbmp)2]ClO4 whose crystal structure (CuC28H26N6ClO4, z = 7.877 (1) angstrom, b = 13.810 (2) angstrom, c = 25.090 (7) angstrom, beta = 95.29 (1)degrees, monoclinic, P2(1)/c, Z = 4) shows a mononuclear structure with Cu(I) pseudotetrahedrally coordinated by two bidentate alpha,alpha'-diimine donor groups. The same chromophore is found for each copper ion in the dinuclear complex [Cu2(bismbmp)2] (ClO4)2.H2O obtained by reaction of copper(I) with bismbmp. Conductivity measurements and UV-visible spectra show that the dinuclear structure is maintained in solution in polar aprotic solvents, and H-1-NMR measurements unambigously establish a double-helical structure for this complex. Mbmp reacts with cobalt(II) to give octahedral [Co(mbmp)3](ClO4)2.CH3CN. The same coordination sphere is obtained when bismbmp reacts with cobalt(II), to give the triple-helical dinuclear complex [Co2-(bismbmp)3] (ClO4)4.3H2O, selectively produced by self-assembly. Spectroscopic results show that the triple-helical structure (D3 symmetry) observed in the solid state is maintained in polar aprotic solvents. Spectrophotometric titrations show that bismbmp reacts with zinc(II) to give successively a triple-helical complex [Zn2(bismbmp)3]4+, a double-helical complex [Zn2(bismbmp)2]4+, and a third complex [Zn3(bismbmp)2]6+. The influence of metal ion stereochemical preferences on the self-assembly of dinuclear helical complexes is discussed.