New cobalt(III)-1,4,7,10-tetraazacyclotetradecane (N47), -pentadecane (N48), and -hexadecane (N49) complexes, trans-[CoCl2(N4x)]+ (x=7 (two isomers), 8 (two isomers), 9 (one isomer)), trans-[Co(NO2)2(N4x)]+ (x=7, 8, 9, one isomer), cis-[Co(NO2)2(N47)]+ (one isomer), and [Co(en)(N4x)]3+ (x=7, 8, one isomer) were synthesized and characterized. For analogous 1,4,7,10-tetraazacycloundecane (N46) complexes, two isomers for each of cis-[Co(NO2)2(N46)]+ and [Co(en)(N46)]3+ were also obtained and were found to isomerize to each other in acetonitrile. The ligand field parameters of nitrogen Δ(N) and chlorine Δ(Cl) estimated from the observed ligand field bands of trans-[CoCl2(N4x)]+ show a correlation between them; with an increase in ring size of N4x, the Δ(N) value decreases, while the Δ(Cl) value increases. The reduction potentials for each series of the dichloro, dinitro, and en complexes become less negative with an increase in ring size of N4x.
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