A tetra Co(II/III) complex with an open cubane Co 4 O 4 core and square-pyramidal Co(II) and octahedral Co(III) centres: bifunctional electrocatalytic activity towards water splitting at neutral pH.

The reaction of 2,6-diformyl-4-methylphenol, 4-methoxybenzoylhydrazine and Co(OAc) 2 ·4H 2 O in 1 : 2 : 2 mole ratio in methanol under aerobic conditions produced in 61% yield a tetranuclear complex having the molecular formula [Co II Co III (μ-OAc)(μ 3 -OH)(μ-L)] 2 where OAc - and L 3- 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 NO 4 coordination sphere, while each Co(III) centre is in a distorted octahedral NO 5 environment. The four metal atoms and the four bridging O-atoms form an open cubane type Co 4 O 4 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 O 2 and H 2 evolution reactions (OER and HER) in neutral aqueous medium has been explored in detail.