SYNTHESIS, CHARACTERISATION AND FLUORESCENCE PROPERTIES OF SUBSTITUTED IMIDAZOLES AND BRIDGED BIS-PHENOL LIGANDS AND MAGNETIC BEHAVIOUR OF THEIR METAL COMPLEXES

Abstract

Increasing release of harmful metal(II)/metal(III) into the environment has led to a search for ligands with high sensitive and selective abilities to detect these ions. Studies on metal(II) are known but there is need for ligands that can sense its presence at low concentration. Also, studies on metal(III) are limited due to its poor coordination with ligands. Information on the use of multi-donor imidazole derivatives as well as pentadentate S-/O-bridged bis-phenol imines, with metal-chelating ability, as sensors are scarce. Therefore, the aim of this study was to synthesise and characterise structurally diverse imidazoles, imidazole-imines, bridged bis-imine ligands, their metal complexes, and investigate the magnetic and sensing properties of these materials. Nitro- and bis-imidazoles were prepared by one-pot reaction of appropriate aldehyde, 1,2- dicarbonyls, aniline (nitrogen-phenyl variants) and ammonium-acetate dissolved in aceticacid and refluxed at 120oC for 5 hrs. Nitrogen-methyl variants were prepared by methylation of the nitrogen-hydrogen imidazoles, using methyl-iodide and potassium carbonate. Imidazole-amines were prepared by catalytic hydrogenation of the nitroimidazoles for 2 hrs. Tridentate-imidazole-imines were prepared by condensation of salicylaldehyde/ pyridine-2-carboxaldehyde with appropriate imidazole-amines, while the pentadentate S-/O-bridged-imines were prepared by condensation of salicylaldehyde/its derivatives with bis(2-aminophenyl)sulphide/bis(2-aminophenyl)ether. Metal complexes were prepared by reacting the imines with cobalt acetate and copper acetate, separately. The compounds were characterised by elemental analysis, Mass, NMR and IR spectroscopies, X-ray crystallography and magnetic measurements. Sensing properties of the ligands were determined using fluorescence measurements to detect metal(II) and metal(III). Nine nitro-imidazoles (N1 – N9), seven bis-imidazoles (BI1 – BI7), nine imidazole-amines (A1 – A9), nine nitrogen-nitrogen-oxygen imines (I1 – I9), eight pentadentate-imines (H2S1- 7 – H2O1) and forty-three metal complexes were obtained. Elemental and mass analysis of some representatives: BI1 [C,(81.99%), H(5.13%), N(10.66%); m/z = 515.22], I9 [C(83.10%), H(4.78%), N(8.61%); m/z = 489], H2O1 [C(76.13%), H(4.88%), N(6.80%);vii m/z = 408], Co2S52 [C(39.31%), H(1.71%), N(3.48%), S(4.03%); m/z = 1616.5] agreed with proposed molecular formula C36H27.5N4O0.75, C34H23N3O, C26H20N2O3, and C52H28Br8Co2N4O4S2, respectively. In pentadentate-imines, phenanthrene and naphthalene substitutions resulted in downfield-shift of OHphenolic, while p-methyl/bromo substitution resulted in upfield-shift. Ligands exhibited C=Nimine bands around 1591-1617 cm-1 and OHphenol bands around 3371-3383 cm-1. In the complexes, these bands shifted to 1577-1605 cm-1 and disappeared, respectively, suggesting coordination through Nimine and Ophenol atoms. Distorted tetrahedral/square-planar, trigonal-bipyramidal and octahedral geometries were observed in the complexes. The magnetic susceptibility (cm3Kmol-1) for the dinuclear Co(II) [4.09-5.20], dinuclear Cu(II) [0.82-0.86] and trinuclear Cu(II) [1.18-1.21] complexes were larger than expected [3.75, 0.75 and 1.125, respectively], indicating orbital contribution. Extension of π-conjugation at positions 4 and 5 of the imidazole resulted in higher quantum-yields (4-10 folds), while substitution at the nitrogen-hydrogen position resulted in lower quantum-yields (4-10 folds). Among the imidazole-imines, I9 exhibited the best selectivity for Zn2+ with Limit of Detection (LOD) 4.45 nM. Donor-acceptor capabilities (O-H···N), in the pentadentate-imines, enabled excited state intramolecular proton transfer behaviour and H2O1 exhibited best selectivity for Al3+ with LOD 5.48 nM. Structures of substituted imidazoles and bridged bis-phenol imines with their cobalt(II) and copper(II) complexes were established. Structural variation aided different magnetic properties and excellent detection for aluminium(III) and zinc(II).