Coordination patterns for biliverdin-type ligands. Helical and linked helical units in four-coordinate cobalt and five-coordinate manganese(III) complexes of octaethylbilindione

Spectroscopic and X-ray diffraction experiments help characterize three complexes of cobalt and manganese with the octaethylbiindione ligand. In cobalt complex, four pyrrole nitrogen atoms of the helical ligand surround a cobalt ion exhibiting planar coordination. Temperature-dependent NMR spectrum suggests thermal population of low-lying paramagnetic state in the complex. Magnetic susceptibility studies of a dimeric five-coordinate Mn(III) complex reveal a high spin d4Mn(III). These dimers in pyridine solution undergo cleavage producing monomeric five-coordinate complex, and the Mn in this complex exhibits coordination with four pyrrole nitrogen and one pyridine ligand.