Steric Effects of Axial Methyl Substituents on Axial Coordination of Macrocyclic Nickel (II) and Cobalt (III) Complexes

摘要

The nickel (II) complex with L~1 (5, 5, 7-trimethylcyclam: 5, 5, 7-trimethyl-1, 4, 8, 11-tetrazacyclotetradecane), trans-[NiL~1 (H_2O)_2]Cl_2 · H_2O, was isolated. An X-ray structural analysis gave strong evidence for the presence of steric inter-action between the axial methyl substituent at the 5-position and the water molecule coordinating to the nickel (II) ion. The bimacrocyclic cobalt (III) complex with L~3 (6, 6'-bi[5R (S), 7S (R)-dimethyl-1, 4,8, 11-tetraazacyclotetradecane], trans-[Co_2L~3 Cl_4] Cl_2, was synthesized and its structure was analyzed by X-ray crystallography, as well as the monomacrocyclic trans- and cis-dichlorocobalt (III) complexes with L~2 (5R (S), 7S (R)-dimethyl-1, 4,8, 11-tetraazacyclotetradecane). The structure of trans-[Co_2L~3Cl_4]~(2+) was quite different from that of [Ni_2L~3]~(4+). In trans-[Co_2L~3Cl_4]~(2+), the bridged six-membered chelate rings adopted an approximately twist-boat conformation. The steric interaction between the coordinated chloride anions on the cobalt (III) ion and the methyl substituents introduced at the 5- and 7-positions in the six-membered chelate ring was discussed.