Synthesis and characterization of cobalt(ii) complexes with tripodal polypyridine ligand bearing pivalamide groups. Selective formation of six- and seven-coordinate cobalt(ii) complexes

Abstract

The reactions of CoX₂ (X = Cl⁻, Br⁻, I⁻ and ClO₄⁻) with the tripodal polypyridine N₄O₂-type ligand bearing pivalamide groups, bis(6-(pivalamide-2-pyridyl)methyl)(2-pyridylmethyl)amine ligand (H₂BPPA), afforded two types of Co(II) complexes as follows. One type is purple-coloured Co(II) complexes, [CoCl₂(H₂BPPA)] (1^Cl) and [CoBr₂(H₂BPPA)] (1^Br) which were prepared when X = Cl⁻ and Br⁻, respectively. The other type is pale pink-coloured Co(II) complexes, [Co(MeOH)(H₂BPPA)](ClO₄⁻)₂ (2·(ClO₄⁻)₂) and [Co(MeCN)(H₂BPPA)](I⁻)₂ (2·(I⁻)₂), which were obtained when X = I⁻ and ClO₄⁻, respectively. From the reaction of 1^Cl and NaN₃, a purple-coloured complex, [Co(N₃)₂(H₂BPPA)] (1^azide), was obtained. These Co(II) complexes were characterized by X-ray structural analysis, IR and reflectance spectroscopies, and magnetic susceptibility measurements. All these Co(II) complexes were shown to be in a d⁷ high-spin state based on magnetic susceptibility measurements. The former Co(II) complexes revealed a six-coordinate octahedron with one amine nitrogen, three pyridyl nitrogens, and two counter anions, and one coordinated anion, Cl⁻, Br⁻ and N₃⁻, forming intramolecular hydrogen bonds with two pivalamide N–H groups. On the other hand, the latter Co(II) complexes showed a seven-coordinate face-capped octahedron with one amine nitrogen, three pyridyl nitrogens, two pivalamide carbonyl oxygens and MeCN or MeOH. In these structures, intramolecular hydrogen bonding interaction was not observed, and the metal ion was coordinated by the pivalamide carbonyl oxygens and solvent molecule instead of the counter anions. The difference in coordination geometries might be attributable to the coordination ability and ionic radii of the counteranions; smaller strongly binding anions such as Cl⁻, Br⁻ and N₃⁻ gave the former complexes, whereas bulky weakly binding anions such as I⁻ and ClO₄⁻ afforded the latter ones. In order to demonstrate this hypothesis, the small stronger coordinating ligand, azide, was added to complexes 2·(ClO₄⁻)₂ to obtain the dinuclear cobalt(II) complex in which two six-coordinate octahedral cobalt(II) species were bridged with azide, 3·(ClO₄⁻). Also, the abstraction reaction of halogen anions from complexes 1^Cl by AgSbF₆ gave a pale pink Co(II) complex assignable to 2·(SbF₆⁻)₂.