A tetra Co(ii/iii) complex with an open cubane Co4O4 core and square-pyramidal Co(ii) and octahedral Co(iii) centres: bifunctional electrocatalytic activity towards water splitting at neutral pH

Abstract

The reaction of 2,6-diformyl-4-methylphenol, 4-methoxybenzoylhydrazine and Co(OAc)₂·4H₂O in 1 : 2 : 2 mole ratio in methanol under aerobic conditions produced in 61% yield a tetranuclear complex having the molecular formula [Co^IICo^III(μ-OAc)(μ₃-OH)(μ-L)]₂ where OAc⁻ and L³⁻ represent acetate and N′,N′′-(5-methyl-2-oxido-1,3-phenylene)bis(methan-1-yl-1-ylidene)bis(4-methoxybenzoylhydrazonate), respectively. The elemental analysis and the mass spectrometric data confirmed the molecular formula of the complex. It is electrically non-conducting and paramagnetic. The complex crystallized as acetonitrile solvate. The X-ray structure shows that each Co(II) centre has a distorted square-pyramidal NO₄ coordination sphere, while each Co(III) centre is in a distorted octahedral NO₅ environment. The four metal atoms and the four bridging O-atoms form an open cubane type Co₄O₄ motif. In the crystal lattice, self-assembly of the solvated complex via intermolecular O–H⋯O interaction leads to a two-dimensional network structure. The infrared and electronic spectroscopic features of the complex are consistent with its molecular structure. Cryomagnetic measurements together with theoretical calculations suggest the presence of easy-axis anisotropy for the square-pyramidal Co(II) centres. The complex is redox-active and displays metal centred oxidation and reduction responses on the anodic and cathodic sides, respectively, of the Ag/AgCl electrode. Bifunctional heterogeneous electrocatalytic activity of the complex towards O₂ and H₂ evolution reactions (OER and HER) in neutral aqueous medium has been explored in detail.