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 ster ic interaction between the axial methyl substituent at the 5-position and t he water molecule coordinating to the nickel(II) ion. The bimacrocyclic cob alt(III) complex with L-3 (6,6'-bi[5R(S),7S(R)-dimethyl-1,4,8,11-tetraazacy clotetradecane], trans[(Co2LCl4)-Cl-3]Cl-2, was synthesized and its structu re was analyzed by X-ray crystallography, as well as the monomacrocyclic tr ans- 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 twis t-boat conformation. The steric interaction between the coordinated chlorid e anions on the cobalt(III) ion and the methyl substituents introduced at t he 5- and 7-positions in the six-membered chelate ring was discussed.