Crystallographic and molecular mechanics study of a cobalt(III) complex of a structurally reinforced macrocycle

Abstract

The complex [CoL⁶(Cl)][CoCl₄]·H₂O, where L⁶ is the reinforced macrocycle 7-amino-7-methyl-1,5,9,13-tetraazabicyclo[11.2.2]heptadecane, has been prepared and its crystal structure determined. The green-black crystals are monoclinic, space group P2₁/c, with a= 8.778(2), b= 13.344(4), c= 19.380(8)Å, β= 98.48(2)°, Z= 4, and a final conventional R= 0.0736. The cobalt is co-ordinated in a planar fashion in the cavity of the macrocycle, with one axial co-ordination site occupied by the pendant primary amine group, and the other by a chloride. The Co–N bond lengths to the tertiary nitrogens of the piperazine bridge of the macrocycle are 2.01(1) and 2.02(1)Å, distinctly longer than those [1.97(1)Å] to the secondary amines in the cavity of the macrocycle. The Co–N distance to the pendant primary amine is 2.01(1)Å, and Co–Cl to the axially co-ordinated chloride trans to the pendant amine group is 2.26(1)Å. Molecular mechanics calculations are used to analyse the highly distorted co-ordination sphere around the Co^III, where the N–Co–N angle involving the nitrogens of the piperazine bridge of the macrocycle is only 73.5(4)°. The calculations reproduce the co-ordination geometry around the Co^III well, including the variation in Co–N bond length and the highly distorted N–Co–N angles.