RESUMO
The present work discloses the application of two fluorescent zinc and cadmium complexes (1 and 2) for sensing of Hg(II) ions through a chemodosimetric approach. The ligand under consideration in this work is a N2O donor Schiff base ligand (E)-4-bromo-2-(((2-morpholinoethyl)imino)methyl)phenol (HL), which has been harnessed to generate complexes [Zn3L2(OAc)4] (1) and [Cd3L2(OAc)4] (2). X-ray single crystal diffraction studies unveil the trinuclear skeleton of complexes 1 and 2. Both complexes have been found to be highly fluorescent in nature. However, the quantum efficiency of Zn(II) complex (1) dominates over the Cd(II) analogue (2). The absorption and emission spectroscopic properties of the complexes have been investigated by density functional theory. Complexes 1 and 2 can detect Hg2+ ions selectively by fluorescence quenching, and it is noteworthy to mention that the mechanism of sensing is unique as well as interesting. In the presence of Hg2+ ions, complexes 1 and 2 are transformed to mononuclear mercuric intermediate complex (3) and finally to a trinuclear mercuric complex (4) by hydrolysis. We have successfully trapped the intermediate complex 3, and we characterized it with the aid of X-ray crystallography. Transformation of complexes 1 and 2 to intermediate complex 3 and finally to 4 has been established by UV-vis spectroscopy, fluorescence spectroscopy, ESI-MS spectroscopy, 1H NMR spectroscopy, and X-ray crystallography. The spontaneity of the above conversion is well supported by thermodynamic aspects as reflected from density functional theoretical calculations.
RESUMO
By using a potential tridentate ligand L ((2-piperazine-1-yl-ethyl)-pyridin-2-yl-methylene-amine), a series of group 12 metal complexes namely, [ZnLHCl2][Zn2LCl5]·2H2O (1), [CdL(SCN)2(CH3OH)]n (2), and [Hg(l-pyCO)Cl2] (3), were synthesized and structurally characterized. In all the complexes the piperazine nitrogen of the ligand takes part in coordination and leads to the complexes of group 12 metal ions having structural diversity. The X-ray diffraction analysis of complex 1 indicates for one Zn(ii) ion a geometry in between trigonal bipyramidal/square pyramidal and for the second a distorted tetrahedral sphere. In the polymeric complex 2 the Cd(ii) ion shows a distorted octahedral environment, while in the mononuclear complex 3, where Hg(ii) exhibits a square-pyramidal geometry, an unexpected condensation between the uncoordinated NH piperazine fragment with 2-pyridinecarboxaldehyde was detected. The M-N bond lengths in all the complexes are in accordance with the metal ionic radius. Continuous shape measures through a DFT approach provide the coordination environment around each metal centre that is comparable with the experimental observations. We have also investigated the importance of hydrogen bonding of methanol in the generation of the polymeric Cd complex 2 along with the rearrangement of the tridentate ligand to generate an octahedral complex. The photoluminescence properties of the complexes as well as of the ligand were investigated in solution at ambient temperature. The low quantum yield of the ligand was ascribed due to a very fast photoinduced electron transfer (PET) from the nitrogen lone pair to the conjugated pyridine moiety. Complexation prevents the electron transfer, and consequently an increase in quantum yield was observed in the complexes. Among the three complexes the highest photoluminescence was exhibited by a Zn complex, being lower in Cd and Hg complexes as a consequence of the heavy atom perturbation effect.
