METAL-COMPLEXES WITH MACROCYCLIC LIGANDS .45. AXIAL COORDINATION TENDENCY IN REINFORCED TETRAAZAMACROCYCLIC COMPLEXES

The Cu2+ and Ni2+ complexes of three reinforced tetraazamacrocycles, c ontaining a piperazine subunit and one or mio alkyl substituents al th e other two N-atoms have been prepared and their structural properties studied. In solution, the Ni2+ complexes are square-planar and show n o tendency to axially coordinate a solvent molecule or an N-3(-) ion. In contrast, the Cu2+ complexes change their geometry depending upon t he donor properties of the solvent, being square-planar in MeNO2 and p entacoordinate in DMF. They also easily react in aqueous solution with N-3(-) to give ternary species with pentacoordinate geometry, the sta bilities 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 s quare pyramidal geometry. The difference behavior of Ni2+ and Cu2+ ste ms from the fact that the structural change from square-planar to squa re-pyramidal can easily be accomplished for Cu2+, whereas, for Ni2+, i t is accompanied by an electronic rearrangement from the low-spin to t he high-spin configuration. The relatively rigid ligands cannot adapt to the somewhat larger high-spin Ni2+ ion.