Synthesis, crystal structure, redox property and theoretical studies of a pyrrole containing cobalt(III) Schiff base compound

A new mononuclear cobalt(III) complex with a pentadentate Schiff base ligand derived from the condensation of N-(3-aminopropyl)-propane-1,3-diamine with pyrrole-2-aldehyde has been synthesized and structurally characterized by X-ray crystallography. Structural analysis shows that the geometry of cobalt(III) ion is a distorted octahedron in which triamine part of the pentadentate ligand occupies the meridional positions, while two pyrrole nitrogen atoms are in cis disposition. Despite the potentially interesting similarities of pyrrole-based ligands to porphyrin systems, the present complex is inactive towards the oxidation of o-aminophenol, while analogous systems with pyridine containing ligands have proven to be efficient functional models for phenoxazinone synthase. Although the electrospray ionization mass spectral study clearly indicates the formation of a complex-substrate aggregate, the inability towards oxidation of o-aminophenol by the present complex can be explained electrochemically, where strong electron donating ability of the deprotonated pyrrole moiety reduces the possibility of reduction of cobalt(III) to cobalt(II).