Formation and structure of a complex of sucrose with cobalt(III)bis(phenanthroline)

Sucrose forms a dicationic complex with cobalt(III)bis(phenanthroline) which can be isolated as the sparingly water-soluble triiodide salt or the water-soluble chloride. The circular dichroism (CD) spectrum demonstrates the Delta configuration at Co(III) and the presence of two phenanthroline and one sucrose residues in the complex. The O-2(g)--O-1(f) distance in crystalline sucrose permits strain-free coordination of these centers with Co(III) and in the complex the H-1,1'(f) singlet of sucrose separates into a pair of doublets. The 1H NMR spectrum in DMSO-d(6) shows that OH-2(g) is deprotonated and the signal of OH-1(f) is shifted strongly downfield by complexation with Co(III). Coordination involving glucose and fructose residues is consistent with neither alpha-methyl glucoside nor fructofuranose forming mixed complexes with phenanthroline. Structure simulation with the semi-empirical PM3(tm) basis set indicates that complexation by O-2(g) and OH-1(f) can give a Delta-complex with little structural distortion, whereas in hypothetical Lambda-complexes there is distortion of the sucrose residue. Observation of an NOE involving the sucrose and phenanthroline residues supports the postulated structure of the Delta-complex.