BIMETALLIC REACTIVITY - GENERAL-SYNTHESIS OF BINUCLEATING MACROCYCLIC LIGANDS CONTAINING 6-COORDINATE AND 4-COORDINATE SITES

General methods are described for the synthesis of binucleating macrocyclic ligands containing 6- and 4-coordinate sites. The procedure involves the formation of a mononuclear complex containing two appropriately positioned aldehyde groups followed by acid-catalyzed condensation with a diamine to give the isolated complexes of the type [M(ligand)(H+)2]n+ where the 4-coordinate site is occupied by two protons. The following diamines were incorporated in the 6- and 4-coordinate sites; ethylenediamine, trimethylenediamine and S-1,2-propylenediamine. Eight of the diamine combinations were isolated and characterized. The monometallic complexes are ideal precursors for the synthesis of site-specific heterobimetallics under mild conditions. When S-1,2-propylenediamine is present at the 6-coordinate site only one chiral topology is induced in the complex but an S-1,2-propylenediamine link at the 4-coordinate site induces little or no diastereomeric selection. The stereochemistry of monometallic and bimetallic complexes was deduced by H-1 NMR data, X-ray crystallography, and circular dichroism spectra. The first two methods indicate that when the macrocycle contains ethylenediamine links in both the 6- and 4-coordinate sites, the ligand cannot accommodate two metals in a planar geometry. The ideal ring size for the incorporation of two metals in a C2 symmetric topology requires trimethylenediamine links in both sites. The circular dichroism spectra are generally not useful in assigning absolute configurations without information about the conformation of the ligand. The Co(III)/Co(II) redox couples of complexes of the type [Co(ligand)ZnCl]n+ indicate that the Co(III) state is readily accessible when the 6-coordinate site incorporates an ethylenediamine link but not when trimethylenediamine is present at this site. It is probable that this difference reflects, in the latter complexes, the inability of the ligand to accommodate the structural requirements of Co(III).