Modulation of Nuclearity by Zn(II) and Cd(II) in Their Complexes with a Polytopic Mannich Base Ligand: A Turn-On Luminescence Sensor for Zn(II) and Detection of Nitroaromatic Explosives by Zn(II) Complexes

Reactions of Zn(II) ion with the ligand H2L (H2L = N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)ethylenediamine) in the presence of bridging coligands, chloride, thiocyanate/acetate, or azide/acetate yielded three new dinuclear complexes, [Zn2LCl2(H2O)] (1), [Zn2L(SCN)2(H2O)]·H2O (2), and [Zn2L(N3)(CH3CO2)] (3), whereas Cd(II) ion formed three new tetranuclear complexes, [Cd4L2Cl4]·H2O (4), [Cd4L2(SCN)2(CH3CO2)2]·2H2O (5), and [Cd4L2(N3)2(CH3CO2)2]·3H2O (6) with the same ligand and coligands. The Zn(II) ions are penta-coordinated in all of its complexes 1–3 except one zinc center in complex 3, which is distorted tetrahedral. All four Cd(II) centers in each of its complexes 4 and 5 possess hexa-coordinated distorted octahedral geometry. In complex 6, two Cd(II) centers are hexa-coordinated, and the other two are hepta-coordinated. The deprotonated ligand (L2–) is hexa-/hepta-dentate in the Zn(II) complexes, 1–3 but octa-dentate in the CdII complexes 4–6. The differences in nuclearity and in the coordination modes of the ligands in the resulting complexes have been explained considering the preference for different coordination numbers of these two metal ions. H2L exhibited highly specific, sensitive, and selective turn-on fluorescence sensing properties for the Zn2+ ion. The mechanism of fluorescence enhancement, host guest binding stoichiometry, and binding constant has also been calculated. The detection limit of Zn2+ ion is 7.69 nM, with the binding constant K = 1.508 × 1010 M–2. The produced Zn(II) complexes sense nitroaromatic explosives in solution via a turn-off florescence response. The solution phase sensing mechanism has been studied thoroughly. Remarkably, the limit of detection of picric acid at the ppt level (912, 910, and 896 ppt for complexes 1, 2, and 3 respectively) having a strong quenching constant (KSV) is 8.063 × 104, 7.987 × 104, and 8.51 × 104 M–1 for complex 1, 2, and 3 respectively.