Isomerization and Racemization of the Tris(4,4′-dimethoxy- or 4,4′-diethoxy-2,2′-bipyridine 1,1′-dioxide)chromium(III) Complexes

• Published in: Bulletin of the Chemical Society of Japan 1996-03, Vol.69 (3), p.665-671
• Main Authors: Kanno, Hideaki, Yamamoto, Junko, Utsuno, Shunji, Fujita, Junnosuke
• Format: Article
• Language: English
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.69.665

Two new chromium(III) complexes, [Cr(4,4′-X2bpdo)3]3+ (X = MeO or EtO; bpdo = 2,2′–bipyridine 1,1′-dioxide), were prepared. These complexes form two diastereomers, lel3(Δ(λλλ), Λ(δδδ)) and lel2ob(Δ(λλδ), Λ(δδλ)), which were separated and resolved by an SP-Sephadex column chromatographic method using achiral eluting agents, such as Na2SO4 or NaCl. The isomers isomerized to each other (lel3 lel2ob) fairly rapidly in water along with changes in the absorption spectra, and then racemized very slowly (Δ Λ). These reactions, including conformational (δ λ) and configurational (Δ Λ) changes, are independent of H+ and free ligand concentrations, and are supposed to proceed by an intramolecular mechanism. For a series of [Cr(4,4′-X2bpdo)3]3+ (X = H, Me, MeO, EtO), both the rates of isomerization and racemization are remarkably reduced along with an increase in the electron-releasing ability of the 4,4′-X2 substituents on bpdo.