Synthesis, thermodynamic, photophysical and DFT studies of some trivalent metal chelates of a hexadentate tripodal hydroxyquinolinate-based ligand

Photophysical Studies of a Catechol Based Polyfunctional Dipodal Chelator: Application for Optical Probe for Selective Detection of Fe(III)

A novel catechol based dipodal fluorescent chelator N,N'-bis[3-[(E)-(2,3-dihydroxyphenyl)methyleneamino]propyl]propanediamide(MPC), has been developed and its photophysical behaviour was studied by experimental (UV-VIS and fluorescence) and DFT method. The design of the molecule has been inspired from the naturally occurring siderophore enterobactin, a catechol based chelator with amide linkage, that shows an excellent binding efficiency towards Fe(III). The dipodal molecule (MPC) presented here, carries two catechol pendant binding moieties linked to the malonate central unit through propylene spacers by amide linkage. MPC showed good selectivity for Fe(III) at 10^-4 M concentration in aqueous medium amongst the biologically and environmentally important metal ions chosen viz., Na(I), K(I), Al(III), Cr(III), Fe(III), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II), by demonstrating a remarkable quenching in the fluorescent emission from 262 a.u. to 55 a.u. at λ_max = 477 nm. Also, the pre-organized assembled ligand favored an efficient Fe (III) encapsulation through coordination by imine nitrogen and catecholate oxygen donors. High formation constant (log β = 31.3) for 1:1 metal-ligand complex evaluated by both potentiometric and spectrophotometric methods, established the strong binding efficiency of the ligand for Fe(III) metal ion. The binding stoichiometry in the complex was also confirmed from Stern -Volmer and Hill Plot analysis. Further investigation on the emission behavior of MPC in a completely DMSO system explored its suitability for extensive applications in the areas such as, metallurgy, material science, iron contamination remedial in the materials etc.. DFT studies suggest that the ligand displays a U-shaped geometry with a parallel π-stacking and the hydrogen bond between two arms. The experimental infrared, electronic, fluorescence, ¹H nmr, ¹³C nmr spectra were correlated with the theoretical results. The nature of electronic transitions were identified from the TDDFT calculation. The ligand forms a hexa-coordinated complex with six Fe-O bonds extending an orthorhombic geometry due distortion from a regular octahedron.