Capped trigonal prism coordination geometry in a cobalt(II) chelate with a 24-membered macrocyclic ligand in which two ethylenediaminetetraacetato and two ethylenediamine groups are linked by amide bonds

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An X-ray crystal analysis was carried out on the cobalt(II) compound of a 24-membered macrocyclic ligand with four pendant acetato groups, Co 2(C 24H 36N 8O 12)·8H 2O: 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) Å 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 Å), an oxygen atom from an amide group (CoO=2.101 Å), two amine nitrogen atoms (CoN=2.229 and 2.300 Å) and an oxygen atom from a water molecule (CoO = 2.058 Å). 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 Å 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.