Novel Polydentate Macroacyclic Schiff Base Ligands Based on 2,6-Diformylphenol

Abstract

[1+2] Condensation of 2,6-diformylphenol (1) with ethyl-2-[(2-aminophenyl) IntroductionDirected synthesis of polydentate organic compounds with various donor centers followed by metal complexes formation, their structures' definition and evaluation of regioselective coordination factors play an important role in design of artificial models of biologically important objects [1,2] and searching for compounds with required physicochemical and applied properties, [3][4][5] e.g. magnetic, [6,7] catalytic, [8][9][10] medicobiological, [11] etc. Previously we have reported on macroacyclic polydentate ligands via diethyl-2-[(2-aminophenyl)-aminomethylidene]malonate or ethyl-2-[(2-aminophenyl)aminomethylidene]-3-oxo-3-(polyfluoro)alkylpropionates with thiophene-2,5-dicarboxaldehyde. [12,13] It was shown the structure of their coordination centers depends on the symmetry of 1,3-dicarbonyl fragment. These compounds were expected to form complexes with 3d metal ions; however our attempts to isolate stable chelates were unsuccessful because of sulfur atom in thiophene cycle seems to have the low donor ability for the formation of strong donor-acceptor bond with metal ion. [14] Unlike thiophene-2,5-dicarboxaldehyde 2,6-diformylphenol have a hydroxy group that can provide the covalent bonding with metal ions. It is worth noting that 2,6-diformylphenol and its derivatives are widely used in the design of polydentate ligands which can form polynuclear complex systems having magnetic interaction between the metal centers. Due to the study of stereochemical, electronic, magnetic, catalytic, spectroscopic and biological properties one can suggest for them some important applications. [15][16][17][18] In this paper we describe the synthesis, coordination properties and tuberculostatic activity of new macroacyclic Schiff base ligands based on 2,6-diformylphenol 1 and ethyl-2-[(2-aminophenyl)aminomethylidene]-1,3-dicarbonyl compounds 2a-d. Results and DiscussionIt was found the reaction of dialdehyde 1 with amines 2a-d in ethanol at room temperature proceeds regioselectively and gives [1+2] condensation productsdiazomethines 3a-d in good yields (Scheme 1). Structures of 3a-d were characterized by IR, 1 H, 19 F NMR spectroscopy and elemental analysis.An important point is that compounds 3a-d have labile hydrogen atom and therefore they can be involved in keto-enol and amine-imine tautomerism. The imino-enone, imino-enol, and/or amino-enone tautomers are possible for their structure. Furthermore, amino-enone tautomers of 3-oxoesters derivatives 3c,d can exhibit Z,E-isomerism because of different position of non-equivalent substituents relative to C=C bond. ). In this case absorption bands are either doublet or broadened. Based on the IR spectra analogy we suggested for compounds 3c,d E,Eisomeric structure in which bis(amino-enone) fragments stabilized with IMHB are in trans-position relative to aromatic spacer similarly to described earlier [13] thiophene derivatives.According to the 1 H and 19 F NMR spectral data azomethines 3c,d in C...