Bis phenylene flattened 13-membered tetraamide macrocyclic ligand (TAML) for square planar cobalt(III).

The preparation, characterization, and evaluation of a cobalt(III) complex with 13-membered tetraamide macrocyclic ligand (TAML) is described. This is a square-planar (X-ray) = 1 paramagnetic (H NMR) compound, which becomes an = 0 diamagnetic octahedral species in excess d-pyridine. Its one-electron oxidation at an electrode is fully reversible with the lowest value (0.66 V SCE) among all investigated Co TAML complexes. The oxidation results in a neutral blue species which is consistent with a Co/radical-cation ligand. The ease of oxidation is likely due to the two benzene rings incorporated in the ligand structure (whereas there is just one in many other Co TAMLs). The oxidized neutral species are unexpectedly EPR silent, presumably due to the π-stacking aggregation. However, they display eight-line hyperfine patterns in the presence of excess of 4--butylpyridine or 4--butyl isonitrile. The EPR spectra are more consistent with the Co/radical-cation ligand formulation rather than with a Co complex. Attempts to synthesize a similar vanadium complex under the same conditions as for cobalt using [VO(OCHMe)] were not successful. TAML-free decavanadate was isolated instead.