Effects of electronic structure on chiral induction in outer-sphere electron transfer reactions of complexes with the C3 symmetric ligand, 1,4,7-triazacyclononane-1,4,7-tris[2′( R)-2′-propionate](-3)

Abstract

The ligand 1,4,7-triazacyclononane-1,4,7-tris[2′( R)-2′-propionate](-3)(( R)-tacntp 3−), binds stereospecifically to transition metal ions. The structures of the complexes [Cr(( R)-tacntp)]·NaBr and [Fe(( R)-tacntp)]·H 2O have been determined by X-ray crystallography. Both complexes have the Λ-configuration but the conformation of the chelate rings in Λ-[Cr(( R)-tacntp)] is (λ,λ,λ) with a geometry close to octahedral while in Λ-[Fe(( R)-tacntp)] it is (δ,δ,δ) and the geometry is closer to that of a trigonal prism. Chiral induction in the electron transfer reactions of Λ-[Co(( R)-tacntp)], Λ-[Fe(( R)-tacntp)] and Λ-[Mn(( R)-tacntp)] with [Co(( RR,SS)-chxn) 3] 2+ has been investigated. All three reactions are outer-sphere and four isomeric [Co(( RR,SS)-chxn) 3] 3+ products are identified in each case. The oxidants Λ-[Fe(( R)-tacntp)] and Λ-[Mn(( R)-tacntp)] show very similar selectivities, quite different from those of Λ-[Co(( R)-tacntp)]. Reasons for this behavior are discussed.