Syntheses, structures, and properties of Co(III) complexes derived from polypyridine ligands containing one carboxamido nitrogen donor

Four cobalt(III) complexes containing the polypyridine pentadentate ligands N,N-bis(2-pyridylmethyl)amine-N′-ethyl-2-pyridine-2-carboxamide (PaPy3H), N,N-bis(2-pyridylmethyl)amine-N′-[1-(2-pyridylethyl)acetamide (MePcPy3H), and N,N-bis(2-pyridylmethyl)amine-N′-(2-pyridylmethyl)acetamide (PcPy3H), have been synthesized. All three ligands bind the Co(III) center in the same fashion with the exception of loss of conjugation between the carboxamide moiety and the pyridine ring in the latter two. The structures of [(PaPy3)Co(OH)][(PaPy3)Co(H2O)](ClO4)3 · 3H2O (1), [(PaPy3)Co(NO2)](ClO4) · 2MeCN (2), [(MePcPy3)Co(MeCN)](ClO4)2 · 0.5MeCN (3), and [(PcPy3)Co(Cl)](ClO4) · 2MeCN (4) have been determined. These ligands with strong-field carboxamido N donor stabilize the +3 oxidation state of the Co center as demonstrated by the facile oxidation of the corresponding Co(II) complexes (prepared in situ) by H2O2, [Fe(Cp)2](BF4), or nitric oxide (NO). The Co–Namido bond distances of 1–4 lie in the narrow range of 1.853–1.898 Å. 1H NMR spectra of these complexes confirm the low-spin d6 ground states of the metal centers.

Graphical abstractCo(III) complexes of a set of polypyridine ligands with one carboxamide group have been synthesized and structurally characterized. The corresponding Co(II) species are unstable and readily converted to the Co(III) complexes via oxidation with dioxygen, nitric oxide, hydrogen peroxide, and ferrocenium salts.Figure optionsDownload full-size imageDownload as PowerPoint slide