The origin of optical activity in complexes of the type [Co(diimine)2carbohydrate]2+

Abstract

Complexes of the type Delta- and Lambda-[Co(diimine)(2)carbohydrate](2+) were synthesized with the diimines: 1,10-phenanthroline and 2,2'-bipyridine and with the carbohydrates: aminosugars, hexoses, pentoses and sucrose, with the aim of finding out which are the determining factors of the resultant chirality in these compounds. The optical characteristics of the complexes obtained were determined through measurement of their absorption and circular dichroism spectra. All the complexes synthesized exhibited an excitonic effect in the UV spectral region. Assignment of the circular dichroism bands in the visible could be performed by applying ligand field theory treatment and assuming a D-2 local symmetry to the optically active bands in the region of the d-d* absorption bands corresponding to the (1)A(1) --> B-1(2) and (1)A(1) --> B-1(3) transitions. Rotatory strength of these bands can be ascribed to coupling of the magnetic moments of the d-d* bands with the electric moments of the pi --> pi* transitions, polarized along the short axis of the diimine molecules, which turned out to be almost parallel in the Delta- and Lambda-[Co(diimine)(2)carbohydrate](2+) complexes.