Synthesis, structures and fluorescence of nickel, zinc and cadmium complexes with the N,N,O-tridentate Schiff base N-2-pyridylmethylidene-2-hydroxy-phenylamine
Graphical abstract
Three new Schiff base complexes [Ni(HL)(L)] · (ClO4) · 0.16(H2O) (1), [ZnLZn(OOCCH3)4ZnL] (2) and [Cd2(L)2(OCH3CO)2(H2O)2] (3) have been synthesised using the Schiff base ligand and the corresponding metal ions. The Schiff base has been prepared by mixing pyridine-2-carboxaldehyde and 2-aminophenol and was isolated prior to the reaction with metal ions. Complex 1 is mononuclear, 2 is trinuclear and 3 is diphenoxo-bridged dimer. 3 forms infinite 1D chains through hydrogen bonding interactions. Only 2 and 3 exhibit strong fluorescence emission bands at 635 and 630 nm, respectively.
Introduction
Schiff base ligands have been extensively studied in coordination chemistry mainly due to their facile syntheses, easily tunable steric, electronic properties and good solubility in common solvents [1]. Transition metal complexes with oxygen and nitrogen donor Schiff bases are of particular interest [2] because of their ability to possess unusual configurations, be structurally labile and their sensitivity to molecular environments [3]. Schiff base ligands have proven to be very effective in constructing supramolecular architectures such as coordination polymers, double helixes, and triple helicates [4]. Schiff bases can accommodate different metal centres involving various coordination modes allowing successful synthesis of homo- and heterometallic complexes with varied stereochemistry [5]. This feature is employed for modelling active sites in biological systems [6]. Additionally, they have wide applications in fields such as antibacterial, antiviral, antifungal agents [7], homogeneous or heterogeneous catalysis [8] and magnetism [9]. Schiff bases are potential anticancer drugs [10] and when administered as their metal complexes, the anticancer activity of these complexes is enhanced in comparison to the free ligand [11]. It has been shown that Schiff base complexes derived from 4-hydroxysalicylaldehyde and amines have strong anticancer activity, e.g., against Ehrlich ascites carcinoma (EAC) [12]. The π-system in a Schiff base often imposes geometrical constraints as well as affecting electronic structure [13].
Schiff bases derived from a large number of carbonyl compounds and amines have been used [3], however, the studies on their optical properties, such as fluorescence, are rare. The variety of possible Schiff-base metal complexes with a wide choice of ligands, and coordination environments, has prompted us to undertake research in this area. As a part of our continuing work on Schiff base complexes [5], [14], [15], nickel(II), zinc(II) and cadmium(II) complexes with notable photo-activity were prepared and structurally characterised. The chemistry of nickel Schiff base complexes has a strong role in bioinorganic chemistry and redox enzyme systems [6](a), [6](b). Morrow and Kolasa reported the cleavage of plasmid DNA by square planar nickel-salen [bis-(salicylidene)ethylenediamine] in the presence of either magnesium mono peroxypthalic acid (MPPA) or iodosulbenzene [16]. Zinc is an important transition metal in biological systems. Zinc-containing carboxylate-bridged complexes [2] have varied structural motifs in hydrolytic metalloenzymes, such as phosphatases and aminopeptidases. The catalytic role of Zn comprises Lewis acid activation of the substrate, generation of a reactive nucleophile (Zn–OH) and stabilisation of the leaving group [17]. There is substantial interest in the coordination chemistry of cadmium complexes because of the toxic environmental impact of cadmium. The mobilisation and immobilisation of cadmium in the environment, in organisms, and in some technical processes (such as in ligand exchange chromatography) have been shown to depend significantly on the complexation of the metal centre by chelating nitrogen donor ligands [18].
In this paper, we describe the preparation and structures of nickel(II), zinc(II) and cadmium(II) complexes with the related Schiff base ligand N-2-pyridylmethylidene-2-hydroxy-phenylamine displaying strong fluorescent emission at room temperature.
Section snippets
Physical techniques
The infrared spectra of the complexes were recorded on a Perkin–Elmer RX 1 FT-IR spectrophotometer with a KBr disc. Elemental analyses were carried out using a Perkin–Elmer 2400 II elemental analyser. Electrochemical study for complex 1 was performed on a CH 600A cyclic voltammeter instrument in methanol, with tetrabutylammonium perchlorate as the supporting electrolyte. 1H NMR spectral measurements were carried out on a Bruker FT300 MHz spectrometer with TMS as an internal reference. Magnetic
[Ni(HL)(L)] · (ClO4) · 0.16(H2O) (1)
In the molecular structure of 1, there are three crystallographically independent molecules in the asymmetric unit (atoms labeled A, B and C) along with three perchlorate anions and half of one water molecule, which is shown in Fig. 1. Selected bond distances and angles are listed in Table 2. Despite several attempts, we could not obtain better data on 1 so the structure of 1 will not be discussed in great detail. The nickel complexes associate as hydrogen bonded dimers involving phenolic
Conclusion
The Schiff base ligand HL reacts with nickel(II), zinc(II) and cadmium(II) to form three types of complexes. On reaction with nickel(II) ions, HL forms six-coordinate octahedral complex with 1:2 metal:ligand stoichiometry. In the case of 2, the two terminal zinc ions are coordinated with the tridentate Schiff base ligand along with the bridging bidentate acetate groups to form an intermediate trigonal bipyramidal-square pyramidal structure while the central zinc ion resides on a centre of
Supplementary material
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 234749, 265683 and 234750 for 1, 2 and 3, respectively. Copies of this information may be obtained free of charge from The Director, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
Dr. Bappaditya Bag, Department of Chemistry, Jadavpur University, Kolkata 700 032, India, is gratefully acknowledged for his valuable suggestion in the field of the synthesis of the Schiff base ligand. We thank Dr. Alex Slavin, St. Andrews University for data collection on 1. Our thanks is extended to Prof. L. Dahlenburg, Institut für Anorganische Chemie, Universität Erlangen-Nünberg, Egerland Strasse-1, 91058, Erlangen, Germany for the structural study of 3.
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