COORDINATION PATTERNS FOR BILIVERDIN-TYPE LIGANDS - HELICAL AND LINKED HELICAL UNITS IN 4-COORDINATE COBALT AND 5-COORDINATE MANGANESE(III) COMPLEXES OF OCTAETHYLBILINDIONE

The bilindione tetrapyrrole unit that is found in biliverdin, an intermediate in heme catabolism, is widely distributed in nature especially as a pigment, but its ability to act as a ligand toward metal ions and the range of complexes that it can form have received little attention. Because of the presence of the two keto groups at opposite ends, this type of tetrapyrrole cannot readily form planar structures analogous to those of porphyrins. Rather this tetrapyrrole ligand adopts a helical arrangement that minimizes contact between its terminal keto groups. Three complexes, monomeric {(OEB)Co}, dimeric {(OEB)Mn-III}(2), and monomeric {(OEB)Mn-III(py)} of octaethylbilindione (H(3)OEB), have been characterized by X-ray diffraction and spectroscopic techniques. The cobalt complex consists of a cobalt ion with approximately planar coordination that is surrounded by four pyrrole nitrogen atoms of the helical ligand. The H-1 NMR spectrum has marked temperature dependent changes. These changes are most prominent in the meso resonances and indicate that there is thermal population of a low-lying paramagnetic state. This behavior may result from low-spin/high-spin equilibrium for Co-III, from thermal equilibrium between {(OEB)Co-III} (S = 0) and {(OEB(.))Co-II}, where OEB(.) is the oxidized ligand radical dianion, or from temperature dependent coupling of ligand and metal spins in {(OEB(.))Co-II}. Spectral data indicate that {(OEB)Co] reversibly coordinates pyridine ligands. The manganese(III) complex {(OEB)Mn-III}(2) is dimeric with two helical (OEB)Mn units of like chirality joined by O-Mn bonds to give a dimer of C-2 symmetry. Each manganese ion is five-coordinate, and the magnetic susceptibility is indicative of a high-spin (S = 2), d(4) Mn-III. In pyridine solution this dimer is cleaved to form monomeric five-coordinate {(OEB)Mn-III(py)}. In this complex the maganese ion is coordinated to the four pyrrole nitrogen atoms of the bilindione ligand and to one pyridine ligand.