CAPPED TRIGONAL PRISM COORDINATION GEOMETRY IN A COBALT(II) CHELATE WITH A 24-MEMBERED MACROCYCLIC LIGAND IN WHICH 2 ETHYLENEDIAMINETETRAACETATO AND 2 ETHYLENEDIAMINE GROUPS ARE LINKED BY AMIDE BONDS

An X-ray crystal analysis was carried out on the cobalt(II) compound of a 24-membered macrocyclic ligand with four pendant acetato groups, Co2(C24H36N8O12) . 8H2O: the ligand molecule was prepared by a condensation reaction between ethylenediaminetetraacetic dianhydride and ethylenediamine. The cobalt(II) compound crystallized in the hexagonal space group P6(2) (or P6(4)) with a=17.957(1), c=10.274(1) angstrom and Z=3. Each ligand molecule is coordinated to two cobalt ions, which are crystallographically equivalent to each other. Each metal ion has six well-defined coordination bonds with two oxygen atoms from carboxylato groups (Co-O=2.062 and 2.177 angstrom an oxygen atom from an amide group (Co-O=2.101 angstrom), two amine nitrogen atoms (Co-N=2.229 and 2.300 angstrom) and an oxygen atom from a water molecule (Co-O=2.058 angstrom). These six Co-X bonds form a quasi-trigonal prism around the Co atom. When an additional amide oxygen atom located at a distance of 2.720 angstrom from the Co atom is included in the coordination sphere, the coordination geometry is described as a capped trigonal prism. This less common coordination geometry is due to the inflexibility of the ligand molecule. A weak d-d band was observed at 515 nm in aqueous solution.