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).
