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.

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 [CoIICoIII(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.

Densely Packed Lanthanide Cubane Based 3D Metal-Organic Frameworks for Efficient Magnetic Refrigeration and Slow Magnetic Relaxation.

Two isostructural densely packed squarato-bridged lanthanide-based 3D metal-organic frameworks (MOFs) [Ln5(µ3-OH)5(µ3-O)(CO3)2(HCO2)2(C4O4)(H2O)2] [Ln = Gd (1) and Dy (2)] show giant cryogenic magnetic refrigeration (for 1) and slow magnetic relaxation (for 2). The structural analyses reveal the presence of a self-assembled crown-shaped building unit with a cubane-based rectangular moiety that leads to a special array of metal centers in 3D space in the complexes. Magnetic investigations confirm that complex 1 exhibits one of the largest cryogenic magnetocaloric effects among the molecular magnetic refrigerant materials reported so far (-ΔSm = 64.0 J kg(-1) K(-1) for ΔH = 9 T at 3 K). The cryogenic cooling effect (of 1) is also quite comparable with that of the commercially used magnetic refrigerant gadolinium-gallium garnet, whereas for complex 2, slow relaxation of magnetization was observed below 10 K.