Two highly unsymmetrical tetradentate (N3O) Schiff base copper(II) complexes: Template synthesis, structural characterization, magnetic and computational studies
Graphical abstract
Crystallographic analysis of two new copper(II) complexes, [Cu2(L1)2](ClO4)2 (1) and [Cu(L2)(ClO4)] (2) of highly unsymmetrical tetradentate (N3O) Schiff base ligands show that complex 1 is doubly phenoxo-O bridged while complex 2 is mononuclear. Cryomagnetic susceptibility measurements (5–300 K) on complex 1 reveal a distinct antiferromagnetic interaction with J = −23.57 cm−1, which is substantiated by a DFT calculation (J = −27.6 cm−1) using the B3LYP functional.
Introduction
The impetus for the study of the chemistry of dinuclear copper(II) complexes has come from different areas of interest which include mainly (i) homogeneous catalysis [1], [2], [3], [4], (ii) their use as model systems for the study of mechanisms of magnetic exchange [5], [6], [7], [8], [9], [10], (iii) as speculative models for the active copper site in several copper proteins [11], [12], [13], [14], [15], [16], [17], [18], [19] and (iv) to bind and activate small molecules [1], [2], [3], [4], [20]. The metal–metal separation and the number, nature and disposition of donor atoms allow the study of systematic changes in physical and chemical properties that depend on the binuclearity of the system [5], [6], [7], [8], [9], [10]. In this respect, DFT calculations provide an important means to interpret the magnetic interaction exhibited by such species [5], [6], [7], [8], [9], [10], [17], [18], [19]. Metal complexes of unsymmetrical tetradentate Schiff base ligands [21], [22], [23], [24], where the metal atom is in an unsymmetrical ligand environment, have generated tremendous interest because of their relevance towards many biological systems. The systematic investigation of these complexes may shed light on the structural and functional role of these systems. The syntheses of such unsymmetrical Schiff base ligands are generally carried out in two ways: (a) under the conditions of high dilution, leading to 1:1 condensation of diamines and aliphatic or aromatic carbonyl compounds and (b) template synthesis in the presence of the desired metal ion; here free ligands may or may not be obtained through demetallation.
Again, heterogeneous catalysts are thought to be superior to homogeneous ones, because these are easier to handle and separate, and in suitable cases they can be reused. The heterogeneous catalysts may be obtained by immobilization of the ligand to an organic polymer or inorganic support with [25] or without [26], [27], [28], [29], [30] covalent bonding. Transition metals, which are present at the surface of the highly ordered mesoporous silica (HMS) material with a very high surface area, particularly show excellent catalytic activity in liquid phase partial oxidation reactions in the presence of tert-butyl hydroperoxide (TBHP) as the oxidant [31], [32], [33].
In continuation of our earlier studies [34], we report here the crystal structures of two copper(II) complexes with two unsymmetrical ligands prepared by using the template method (Scheme 1), in which one of the ligands has a simple azomethine group while the other has a methyl group on the azomethine carbon. It is interesting to note that the presence of a methyl group on the azomethine carbon leads to the formation of phenoxo-bridged dinuclear complex, while [Cu(L2)(ClO4)], complex 2, remains as a mononuclear complex, in which there are two independent copper(II) complexes in the asymmetric unit. The magnetic interaction between the two copper(II) centers in [Cu2(L1)2](ClO4)2, complex 1, has been investigated and interpreted by DFT calculations. In addition, the soluble metal complex 1 was immobilized over hexagonal mesoporous silica (HMS) to get a heterogeneous catalyst that was used in the catalytic epoxidation of cyclohexene and styrene in presence of TBHP as an oxidant.
Section snippets
Materials and methods
Materials such as bis-(3-aminopropyl)amine (Aldrich), salicylaldehyde (E-Merck, India), 2-hydroxyacetophenone (Aldrich) were reagent grade and used as received.
Syntheses
Caution! Since the perchlorate salts are potentially explosive, only small amounts of the materials should be handled, and with care.
Syntheses
The two copper(II) complexes of highly unsymmetrical ligands were prepared by using a template synthetic procedure. In a typical method, equimolar quantities of bis-(3-aminopropylamine) and Cu(ClO4)2·6H2O were added in a 100 ml anhydrous MeOH, while an equimolar amount of either salicyldehyde or o-hydroxyaceto-phenone dissolved in 100 ml MeOH was added dropwise with constant stirring. After 30 min the solutions were refluxed for 2 h to get brown solutions, which were filtered and kept at ambient
Conclusion
In this paper, we report two new copper(II) complexes of highly unsymmetrical Schiff base ligands prepared by the template method under high dilution conditions. Complex 1 bearing a Me-group on the azomethine carbon atom is dinuclear, while complex 2, bearing a H atom on the azomethine carbon atom is mononuclear, exhibiting two closely related Cu-complexes in the asymmetric center. DFT calculations based on the single crystal X-ray structures have been used to model the exchange process,
Acknowledgements
Financial support from DST (Ref.No.SR/S1/IC-35/2006) and CSIR (Ref No. 01(7329)/07/EMR-II), New Delhi, India is gratefully acknowledged. Thanks are due to Dr. Sudip Mukhopadhyay and Mr. Basab Chattopadhyay, Department of SSP, IACS, India, for magnetic measurements.
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