Synthesis, spectroscopic and theoretical studies of two novel tripodal imine-phenol ligands and their complexation with Fe(III)

Two novel tripodal imine-phenol ligands, cis,cis-1,3,5-tris{(2-hydroxybenzilidene)aminomethyl}cyclohexane (TMACHSAL, L1) and of cis,cis-1,3,5-tris{[(2-hydroxyphenyl)ethylidene]aminomethyl}cyclohexane (Me3-TMACHSAL, L2) have been synthesized and characterized by elemental analyses and various spectral (UV–vis, IR and 1H and 13C NMR) data. The complexation reactions of the ligands with H+ and Fe(III) were investigated by potentiometric and spectrophotometric methods at an ionic strength of 0.1 M KCl and 25 ± 1 °C in aqueous medium. Three protonation constants each for ligands L1 and L2 were determined and were used as input data to evaluate the formation constants of the metal complexes. Formations of metal complexes of the types FeLH3, FeLH2, FeLH, FeL and FeLH−1 were depicted in solution. Experimental evidences suggested for a formation of tris(iminophenolate) type metal complex by the ligands. The ligand L1 showed higher affinity towards iron(III) than L2. The pFe value related to L1 (pFe = 20.14) is approximately four units higher than L2 (pFe = 16.41) at pH = 7.4. The structures of the metal complexes were proposed through the molecular mechanics calculation using MM3 force field followed by semi-empirical PM3 method.