CHIRAL INVERSION AROUND A 7-COORDINATED COBALT CENTER INDUCED BY AN INTERACTION BETWEEN SUGARS AND SULFATE ANIONS

Novel seven-coordinated cage-type cobalt(II) complexes containing N-gl ycosides 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.4 H(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)a mine, were prepared and characterized by elemental analysis, electroni c absorption and circular dichroism spectroscopies, and X-ray crystall ography. Crystal data are as follows. 2b.2CH(3)OH: C26H56N4O14Br2Co, m onoclinic, space group P2(1), a=11.045(2) Angstrom, b=17.283(6) Angstr om, 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:C25H 58N4O20SCo, orthorhombic, space group P2(1)2(1)2(1), a=14.836(2) Angst rom, 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 m etal. The complex cation of 3b has a monoface-capped octahedron coordi nated by a heptadentate (L-Rha)(3)tren ligand having pseudo-C-3 symmet ry with a Delta (lambda(3)-lel form) configuration. The other facial s ite of the complex cation is capped by the SO42- anion through hydroge n banding with the hydroxy groups of L-rhamnose residues. The C-3 heli cal-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 i ntensities 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-p air formation. On the basis of electrostatic theories of ion associati on, 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, wh ich 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.