Characterization of the minor cis isomer of a dipendant polyamino acid macrocycle as a dimeric nickel(II) complex

The dipendant-arm polyamino acid macrocycle 13-amino-13-methyl-1,4,8,11-tet raazacyclotetradecane-6-carboxylate (acammac), characterized earlier as the major trans isomer (3), has been isolated as the minor cis isomer (4) by c rystallization 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 wi th each carboxylate group bound both to its `parent' nickel atom and bridgi ng to the adjacent nickel atom. The macrocycle adopts a folded geometry, ea ch nickel atom appearing as a cis-NiN4O2 chromophore. The pendant primary a mine group and three of the four secondary amine groups are also coordinate d, whereas the fourth secondary amine group is not coordinated but stabiliz ed by a proton bridging within a superbase between the uncoordinated amine and one of the coordinated amines.