A novel octadecanuclear copper pyrazo-late phosphonate nanocage with a bowl shaped arrangement of the copper(II) centers in the asymmetric unit is reported . Characterization of intermediates in both solid and solution states aids to propose the Mechanism of such a giant aggregation. Magnetic studies affirm the presence of antiferromagnetic interactions between the adjacent copper(II) centers. Extensive supramolecular interactions result in a framework structure.