Characterization of the Minor cis Isomer of a Dipendant Polyamino Acid Macrocycle as a Dimeric Nickel(II) Complex

Abstract

The dipendant-arm polyamino acid macrocycle 13-amino-13-methyl-1,4,8,11-tetraazacyclotetradecane-6-carboxylate (acammac), characterized earlier as the major trans isomer (3), has been isolated as the minor cis isomer (4) by crystallization as the [Ni(4)](ClO₄) complex. The complex has been examined by an X-ray crystal structure analysis, and the cation exists as a dimer with each carboxylate group bound both to its ‘parent’ nickel atom and bridging to the adjacent nickel atom. The macrocycle adopts a folded geometry, each nickel atom appearing as a cis-NiN₄O₂ chromophore. The pendant primary amine group and three of the four secondary amine groups are also coordinated, whereas the fourth secondary amine group is not coordinated but stabilized by a proton bridging within a superbase between the uncoordinated amine and one of the coordinated amines.