A custom-designed bifunctional ligand was used to connect an in situ formed Co-3(OH) cluster affording a porous metal organic framework, which represents the first case of ncb-type networks constructed from a single kind of ditopic ligand. Noticeably, the activated MOF shows high volumetric C2H2 uptake and excellent adsorption selectivity for C2H2/CO2 separation at room temperature with a low sorption heat.