Steric effects of axial methyl substituents on axial coordination of macrocyclic nickel(II) and cobalt(III) complexes

The nickel(II) complex with L-1 (5,5,7-trimethylcyclam: 5,5,7-trimethyl-1,4,8,11-tetrazacyclotetradecane), trans[NiL1(H2O)(2)]Cl-2. H2O, was isolated. An X-ray structural analysis gave strong evidence for the presence of steric interaction 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[(Co2LCl4)-Cl-3]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-[(Co2LCl4)-Cl-3](2+) was quite different from that of [Ni2L3](4+). In trans-[(Co2LCl4)-Cl-3](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.