No abstract

Three chiral ligands with variable denticity, H2L2-H2L4, conjugated by N,N'-ethylenebis[N-methyl-(S)-alanine] and an ortho-heterosubstituted aromatic amine, were newly synthesized as analogues of previously reported H2L1. Four contracted-Λoxo cobalt(III) complexes [Co(L)](+) with left-handed helical structure of Λ4Δ2 configuration were prepared by one-electron oxidation of the corresponding contracted-Λred cobalt(II) complexes [Co(L)], which were generated from chiral ligands and Co(ClO4)2·6H2O or Co(CF3SO3)2·5.2H2O in the presence of an organic base. Although the prepared cobalt(III) complexes were very inert and kinetically stable against protonation and NO3(-) complexation, cobalt(III) reduction in the presence of CF3SO3H and/or Bu4NNO3 allowed immediate changing of their three-dimensional structures from the contracted-Λoxo form to the extended-Λ [Co(H2L)Y2](n+) (Y = solvent and/or anion, n = 0-2) form with left-handed helicity or to the extended-Δ [Co(H2L)(NO3)](+) form with right-handed helicity via N- to O-amide coordination switching. Both extended forms were contracted to the original Λoxo form by oxidation of the cobalt(II) center in the presence of an organic base. Thus, redox reactions triggered dynamic helicity inversion of the chiral cobalt complexes, via multiple molecular motions consisting of relaxation/compression, extension/contraction, and helicity inversion motions in combination with deprotonation/protonation of amide linkages and NO3(-) anion complexation.

Co filled carbon nanocapsules, which are formed by a heat treatment of the mixture of Co and diamond nanoparticles, have been studied by in situ transmission electron microscopy (TEM), x-ray diffraction, and Raman spectroscopy. Raman studies show that the heat treatment reduces the surface native oxide (Co3O4) of Co nanoparticles. The reduction is accompanied by graphitization of diamond nanoparticles, indicating that diamond nanoparticles being in contact with the metallic Co are transformed into graphitic coating. The in situ TEM studies show that the graphitic coating is formed in the heating process, not in the cooling process. Furthermore, once the coating is completed, the number of the graphitic layers is almost constant on further heating and cooling. These results allow us to conclude that metallic Co particles simply act as templates for graphitic coating.

Dynamic and consecutive molecular motions such as stretching, winding, and rotation are observed in nature. The ATP-driven F1 part of ATP synthase [1a] and the bacterial flagellar motor [1b] are typical examples, in which some external stimuli kick-off such events through conformational changes of biopolymers. Several molecular machines such as molecular rotors, gears, and shuttles have recently been developed, in which metal-coordination linkage isomerizes dynamically to offer single mode motion.[2] Since the planar amide linkage (-CO-NH-) has two preferred structures (cistrans isomers) and two different metal coordination modes (O-coordination and N-coordination), [3] its isomerism is often used to alter the three-dimensional structures of biological proteins. Herein, we develop a chemical device based on a chiral Co II complex that exhibits dual mode motions. The ligand employed here (H 2 L1) includes 2,5-dimethoxy benzene moieties attached through amide linkages to both terminals of a helical tetradentate ligand. The acidbase reaction of the corresponding cobalt complex triggered the interconversion of coordinating atoms between amide nitrogen atoms and amide oxygen atoms, giving rise to a stretching (extension/contraction) molecular motion. Since we previously demonstrated that the helicity of the Co II complex with H 2 L2 was dynamically inverted from the L cis-a form to the D cis-a form by adding achiral NO 3 À ions, [4][5][6] the employed H 2 L1-Co II complex was designed to work as a novel type of molecular machine that exhibits coupled stretching and inverting motions. Several types of helical ligands have shown extension/contraction molecular motion on metal complexation/decomplexation [2d-e, 7] and/or protonation/deprotonation, [7, 8] but the present type of kinetically labile Co II complex allows a dual molecular motion in a highly dynamic fashion, as would be required for a sophisticated supramolecular switching device.As established for the H 2