Chiral inversion around a seven-coordinated cobalt center induced by an interaction between sugars and sulfate anions

Novel seven-coordinated cage-type cobalt(II) complexes containing N-glycosides from mannose-type aldoses and tris(2-aminoethyl)amine (tren), [Co((aldose)(3)tren)]X-2.nH(2)O (1a.5H(2)O, aldose=D-mannose (D-Man), X = Cl-; 1b.5H(2)O, aldose = 6-deoxy-L-mannose (L-Rha), X = Cl-; 2a.4H(2)O, aldose = D-Man, X = Br-; 2b.H2O, aldose = L-Rha, X = Br-) and [Co((aldose)(3)tren)]SO4.nH(2)O (3a.4H(2)O, aldose = D-Man; 3b.3H(2)O, aldose = L-Rha), where (aldose)(3)tren is tris(2-(aldosylamino)ethyl)amine, were prepared and characterized by elemental analysis, electronic absorption and circular dichroism spectroscopies, and X-ray crystallography. Crystal data are as follows. 2b.2CH(3)OH: C26H56N4O14Br2Co, monoclinic, space group P2(1), a=11.045(2) Angstrom, b=17.283(6) Angstrom, c=10.996(3) Angstrom, beta=117.371(6)degrees, V=1864(1) Angstrom(3), Z=2, R=0.072 for 2787 independent reflections. 3b.3H(2)O.CH3OH:C25H58N4O20SCo, orthorhombic, space group P2(1)2(1)2(1), a=14.836(2) Angstrom, b=22.489(2) Angstrom, c=12.181(3) Angstrom, V=4064(1) Angstrom(3), Z=4, R=0.077 for 2010 independent reflections. The complex cation of 2b consists of a cobalt atom coordinated by a heptadentate (L-Rha)(3)tren ligand to produce a mono-face-capped octahedron having pseudo-C-3 symmetry with-a Lambda (lambda(3)-ob form) configuration around the metal. The complex cation of 3b has a monoface-capped octahedron coordinated by a heptadentate (L-Rha)(3)tren ligand having pseudo-C-3 symmetry with a Delta (lambda(3)-lel form) configuration. The other facial site of the complex cation is capped by the SO42- anion through hydrogen banding with the hydroxy groups of L-rhamnose residues. The C-3 helical-configurational inversion around the cobalt(II) center, Delta < -- > Lambda, induced by an interaction between the sugars and the SO42- anion, was monitored by circular dichroism spectroscopy, for which the sign of the Cotton effect of 1 and 2, having halide counteranions, is opposite to that of 3, having sulfate counteranions, and was found to proceed reversibly upon addition and removal of sulfate anions. The intensities of circular dichroism spectra for [Co((L-Rha)(3)tren)](2+) and [Co((D-Man)(3)tren)](2+) were appreciably changed upon addition of sulfate ions. The spectral changes were interpreted in terms of ion-pair formation. On the basis of electrostatic theories of ion association, the closest distance between the sulfate ion and the [Co((D-Man or L-Rha)(3)tren)](2+) cation was estimated as similar to 5 Angstrom, which is consistent with the Co-S distance of 4.697(6) Angstrom observed in the crystal structure of 3b. The sulfate ion was suggested to fit into the cavity composed of the sugar hydroxyl groups through hydrogen bonds even in the solution state.