Double-pillared cobalt Pacman complexes: synthesis, structures and oxygen reduction catalysis

Abstract

The syntheses and structures of binuclear cobalt complexes of a double-pillared cofacial Schiff-base pyrrole macrocycle (L) were determined and their activity as catalysts for the oxygen reduction reaction evaluated. The new binuclear cobalt complex, [Co₂(L)], 1 was formed in good yield using a salt-elimination method and was characterised as adopting a cofacial structure in solution by NMR spectroscopy and as its THF and pyridine solvates in the solid state by X-ray crystallography. Using a variety of spectroscopic techniques, this complex was found to react reversibly with dioxygen to form a new paramagnetic complex. Furthermore, the new aqua-hydroxy double salt [Co₂(μ-H₃O₂)(py)₂(L)][BF₄] 2 was characterised by X-ray crystallography. In acidified benzonitrile solution, 1 behaves as a catalyst for the selective four-electron reduction of dioxygen to water and showed a large improvement in efficacy compared to its o-phenylene Schiff-base analogues.