Synthesis, characterization, and spectroscopic studies of tetradentate Schiff base chromium(III) complexes
Graphical abstract
Eight chromium(III) complexes of tetradentate Schiff bases have been prepared and characterized by elemental analysis, m.p., IR, molar conductivity and electronic spectra. The free ligands were also characterized by 1H and 13C NMR spectra. The IR and electronic spectra of the free ligand and the complexes are compared and discussed.
Introduction
The chemistry of Schiff bases is an area of increasing interest. Metal complexes with these bases have numerous applications, such as, in the treatment of cancer [1], as antibactericide agents [2], [3], as antivirus agents [4], [5], [6], as fungicide agents [7], [8] and for other biological properties [9], [10], [11]. Several applications have been related for these complexes in chemical analysis [12], absorption and transport of oxygen [13], in pesticides [14] and heterogeneous and homogeneous catalysis for oxidation and polymerization of organic compounds [15], [16], [17], [18].
Chromium with +3 oxidation state, under physiological conditions, is neither an oxidizing nor a reducing agent, however, chromium(III) due its d3 electronic configuration forms stable and substationally inert metal complexes [19]. Complexes of chromium(III) are much less cytotoxic than chromium(VI) to cultured human cells [20]. Chromium(III) is an essential nutrient that is involved in the glucose tolerance factor (GTF) in maintenance of normal carbohydrate and lipid metabolism [21], [22]. Insufficient dietary intake of chromium is linked to increase risk factors associated with Type II diabetes and cardiovascular diseases. Schiff base complexes of chromium(III) such as N,N′-ethylene bis(salicylidene-iminate)diaquochromium(III)chloride, [Cr(salen)(H2O)2]Cl used as a new model of GTF, was also shown to reduce the symptoms of diabetes like, hyperglycemia and cholesterol in diabetic rats [23], [24].
Chromium–salen complexes are well-known catalysts, both in heterogeneous and homogeneous systems. Other applications of theses complexes are reported, such as, the stereoselective alkene epoxidations [25], [26], [27], [28], alcohol oxidations [29], the asymmetric addition of organometallic reagents to aldehydes [30], [31] and asymmetric hetero Diels–Alder reactions [32], [33]. Recently, the interaction of DNA with complexes [Cr(Schiff base)(OH2)2]ClO4 [34], [35], was reported. These complexes have been shown to bring about apoptosis in lymphocyte cell cultures [36]. The (salen)chromium(III)/DMAP catalyst system is used in the fixation of CO2 with epoxides to form carbonates [37].
As part of the ongoing investigations [38] by our group of metal–salen catalyzed oxidations and of insulin-mimetic complexes [39] we prepared complexes of chromium–salen. In this paper we describe chromium(III) complexes of the tetradentate Schiff bases, N,N′-bis(salicylidene)ethylene-diamine, H2L1; N,N′-bis(5-bromosalicylidene)ethylenediamine, H2L2; N,N′-bis(5-chlorosalicylidene)ethylenediamine, H2L3; N,N′-bis(5-methoxysalicylidene)ethylenediamine, H2L4; N,N′-bis(5-nitrosalicylidene)ethylenediamine, H2L5; N,N′-bis(3,5-diclorosalicylidene)ethylenediamine, H2L6; N,N′-bis(3,5-dibromo-salicylidene)ethylenediamine, H2L7; and N,N′-bis(3,5-diiodosalicylidene)ethylenediamine, H2L8. A structural representation of the ligands, their abbreviations, and numbering system of the carbons for 13C NMR are given in Fig. 1.
Section snippets
Reagents
Ethylenediamine, 99%; salicylaldehyde, 98%; 5-bromosalicylaldehyde, 98%; 5-chlorosalicylaldehyde, 98%; 5-methoxyosalicylaldehyde, 98%; 5-nitrosalicylaldehyde, 99%; 3,5-dibromosalicylaldehyde, 98%; 3,5-dichlorosalicylaldehyde, 99%; 3,5-diiodosalicylaldehyde, 97%; (methyl sulfoxide)-d6, 99.9 atom% D, DMSO-d6; chloroform-d 99.8 atom% D, CDCl3 (Aldrich Chemical Co.) and CrCl3 · 6H2O (Riedel-de-Haën Inc.) were used as received. All other solvents and reagents were obtained commercially and were used
Results and discussion
The complexes of chromium(III) are stable in air, both in solution and in the crystalline state. The solids are various shades of brown. The yields of the purified complexes varied from 13.7% to 53.7%. The variation is due to differences in the solubility of the starting ligands and the final complexes. The complexes showed magnetic moment in the range of 3.5–4.2 BM which corresponds to three unpaired electrons [44].
The molar conductance of the complexes was an aid for proposing their formulas.
Conclusions
A series of symmetrical tetradentate Schiff bases containing the N2O2 donor set and their corresponding chromium(III) complexes have been synthesized and spectrally characterized. The same synthetic procedure gives two series of complexes depending on the amount and rate of addition of potassium carbonate used to deprotonate the Schiff base. The initial mass spectra study indicates that for the two series of complexes in DMSO that the same species, [Cr(Ln)(DMSO)2]+ is present.
Acknowledgements
Financial support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnológico, CNPq, and the Fundação de Amparo à Pesquisa do Estado de São Paulo, FAPESP, is gratefully acknowledged. We thank FAPESP for providing an undergraduate scholarship to P.E.A. and the CNPq for graduate scholarships to M.P. dos S. and S.R. We thank Prof. Alzir A. Batista for the use of the magnetic susceptibility balance and Dr. Gustavo Von Poelhsitz for his help with these measurements.
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