First report of Calcium sensor (Cover Page, Dalton Trans.)

A novel metal directed discrete self-assembly, Na2O[Cu12Zn4(m-OH)8(H4btp)8](ClO4)8·26H2O (1) employing a flexible aminoalcohol ligand,2,2′-(propane-1,3-diyldiimino)bis-[2-(hydroxylmethyl)propane-1,3-diol] (H6btp) and mixed metal ions (CuII and ZnII) was synthesized in order to explore it as a multifunctional material. The molecular cluster is characterized by spectral, analytical techniques, single crystal X-ray and magnetic studies. Crystallography reveals the presence of eight peripheral CuII centers in octahedral environment while four interior CuII and ZnII centers are present as tetrahedral system. The ligand is coordinated in dianionic form (H4btp2-) with copper and zinc metal ions through various bridging modes. Magnetic data suggest presence of strong antiferromagnetic coupling between the twelve Cu(II) centers (C = 4.31 cm3Kmol-1 and θ = −120.44 K). Interestingly, the eight ligands present in the system have eight free hydroxyl groups at the periphery of the cluster which could be employed in molecular recognition with the analytes under investigation. One of the sodium cations of the lattice sodium oxide [Na2O or Na+(Na+O2-)] is encapsulated in the cavity of {Cu12Zn4} and held through electrostatic interactions with oxygen atoms forming Na{Cu12Zn4} core. Taking advantage of this entrapped cation, we have used the present assembly in sensing of various cations of S-block. Interestingly, the cluster shows sensitive and selective sensing ability towards Ca2+ ion through cation exchange. The selectivity of the Ca2+ can be rationalized in terms of the identical size of the sodium and calcium ions and HSAB principle. Hard sodium is exchanged with the hard Ca2+ ion which effectively interacts with hard oxygen donor giving rise to stable hard-hard interaction. More over the Na{Cu12Zn4} cluster shows ultimate selectivity towards adsorption of cationic dyes specifically methylene blue (MB) via non-covalent interactions. A plenty of the free O‒H groups in the cluster gives rise to the existence of O‒H⸱⸱⸱π interaction with dye for enhanced adsorption. Thus, the present report is first example of any heterometallic coordination compound (in contrast to the complex systems like polymeric frameworks/nano composites) with discriminating sensing of Ca2+ ion with ultra-low detection limit (4 nM) and with high efficiency to selectively adsorb and separate methylene blue.