Synthesis and Characterization of trans-[CoCl2(N4x)]+ (N4x = 1,4,7,10-Tetraazacycloheptadecane, -cyclooctadecane, -cyclononadecane, -cycloicosane, -cyclohenicosane, and -cyclodocosane)

Abstract

Abstract New macrocyclic ligands, 1,4,7,10-tetraazacyclooctadecane (N411), -cyclononadecane (N412), -cycloicosane (N413), -cyclohenicosane (N414), and -cyclodocosane (N415), and the Co(III) complexes, trans-[CoCl2(N4x)]+ (x = 10−15) were prepared and characterized. While two isomers, one has an RSSR(SRRS) and the other an RSSS(SRRR) configuration for the four chiral nitrogen atoms, were obtained for the N4x (x = 12−15) complexes, only the RSSR(SRRS) isomer was isolated for the N410 and N411 complexes. The ligand field strength of N4x (Δ(N)) decreases largely from x = 6 to 9 or 10, and then shows an increasing tendency with the increasing ring members. The reduction potential for the Co(III)/Co(II) couple (E1⁄2) becomes less negative with the increasing ring members, and the ΔEp(= Epa − Epc) values are very large for the N48–N411 complexes. The variations of these observed values with the number of ring members indicate that medium-sized chelate rings are unstable compared with other sized ones.