Introduction

Planar cobalt coordination complexes with a nuclearity bigger than seven are rarely described in literature.[i],[ii] Transition metal polynuclear complexes are mostly molecular oxide-bridged assemblies of metals, enclosed by organic envelope, which keeps them zero-dimensional, and inhibits from polymerizing. Apart from oxide bridges halides, pseudohalides and hydroxides have also been reported. The organic moieties in the shell with arms and functional groups bind to metal ions to form a coordination complex.[iii] Due to fascinating structural and functional applications such as electronic, optical, fluorescent, catalytic and magnetism coordination cluster species have undergone remarkable development.[iv] For the synthesis of such metal complexes, the choice of ligands as bridges or terminal functionalities with the metal ions have to be considered. Although accurate prediction of design or exact crystal structure is not yet possible however, various reaction parameters such as type of metal and ligand, metal-to-ligand molar ratio, ligand denticity, pH and temperature, counter ions and presence of solvent molecules plays critical roles in the structure formation.[v] Solvothermal strategy for the high nuclearity inorganic complexes has recently been used extensively. This method not only affords the possibility to generate unusual structural motifs which are otherwise impossible to get via traditional methods but also provides a one-pot synthesis for the construction of metal- organic coordinated complexes through crystal engineering.[vi] For instance, two high nuclearity complexes i.e {Mn16} and {Mn19} were synthesized via solvothermal method carried out at 120 °C in methanol. It is reported that high temperature and pressure have a profound role to synthesize these coordination clusters.[vii]
A few cobalt-based coordination complexes have been reported including {Co4} molecular squares and cubanes,[viii] {Co5} square pyramid,[ix] and {Co8} systems[x]. Disk-like hepta- {Co7}, dodeca- {Co12} and tridecanuclear {Co13} coordination clusters using cobalt as metal center and different ligands are also reported.2, [xi],[xii] [xiii] In these two structures ({Co7} & {Co13}), the core structures are related to one another, in which the central cobalt ion is surrounded by a Co6-hexagon while {Co13} is the vertex-sharing oligomer of {Co7}. To the best of our knowledge, the {Co10} disk-like core structure has not been reported yet. Recently, our group reported an horseshoe-like {Co5} coordination cluster, [CoII3CoIII2(Hbda)2(bda)2(ib)6].[xiv]
Herein, we have explored that by reacting this {Co5} coordination cluster under solvothermal condition in DMSO solvent, the missing disk-like decanuclear homovalent {Co10} coordination cluster, [Co10(OH)2(bda)6(ib)6] can be obtained. Interestingly, all CoIII ions of the {Co5} precursor are reduced to CoII in the {Co10} coordination cluster. This novel {Co10} structure represents the missing link between the planar {Co7} and {Co13} structures, as shown by X-ray crystallography.