Metal Complexes with Macrocyclic Ligands. PartXLV Axial coordination tendency in reinforced tetraazamacrocyclic complexes

The Cu2+ and Ni2+ complexes of three reinforced tetraazamacrocycles, containing a piperazine subunit and one or two alkyl substituents at the other two N‐atoms have been prepared and their structural properties studied. In solution, the Ni2+ complexes are square‐planar and show no tendency to axially coordinate a solvent molecule or an N ion. In contrast, the Cu2+ complexes change their geometry depending upon the donor properties of the solvent, being square‐planar in MeNO2 and pentacoordinate in DMF. They also easily react in aqueous solution with N to give ternary species with pentacoordinate geometry, the stabilities of which have been determined. In the solid state, the X‐ray crystal structures of three Cu2+ complexes also show both geometrical arrangements, two having a square‐planar, the other one a distorted square pyramidal geometry. The difference behavior of Ni2+ and Cu2+ stems from the fact that the structural change from square‐planar to square‐pyramidal can easily be accomplished for Cu2+, whereas, for Ni2+, it is accompanied by an electronic rearrangement from the low‐spin to the high‐spin configuration. The relatively rigid ligands cannot Adapt to the somewhat larger high‐spin Ni2+ion.