STRUCTURALLY REINFORCED MACROCYCLIC LIGANDS

The coordinating properties of structurally reinforced macrocyclic ligands are discussed. The ligand B-12-aneN4 (1,4,7,10-tetraazabicyclo[8.2.2]tetradecane) has a very small cavity, and is able to compress the low-spin Ni(II) ion. In spite of the resulting high steric strain in [Ni(B-1 2-aneN4)]2+, the complex is of high stability, because the free ligand itself is also of very high steric strain. In complexes of Cu(II) with unreinforced macrocycles with cavities that are too large for the Cu(II), the Cu-N bonds are not stretched, but rather the coordination geometry of the Cu(II) Is distorted toward tetrahedral, and normal Cu-N bonds are found. In contrast, a large cavity reinforced macrocycle such as NE-3,3-HP (11-methyl-11-nitro-1,5,9,13-tetraazabicyclo-[11.3.2]tetradecane) is able to stretch the Cu-N bonds out to 2.09 angstrom, since the homopiperazine reinforcing bridge prevents buckling of the ligand. However, with further increase in macrocyclic ring size, buckling of the ligand and tetrahedral distortion of the Cu(II) occurs, showing the need for even more rigid reinforcing groups. The synthesis and properties of ligands containing the bispidine group, which has an adamantane-like structure, as a reinforcing group, is discussed.