Transition metal complexes with thiosemicarbazide-based ligands, part 47: Synthesis, physicochemical and voltammetric characterization of iron(III) complexes with pyridoxal semi-, thiosemi- and S-meth

Abstract

The reaction of warm EtOH solutions of FeX 3 . nH 2 O (X = Cl, NO 3 ) with tridentate ONX (X = O, S, N) pyridoxal semi-, thiosemi-and S-methylisothiosemicabazones (H 2 L 1 , H 2 L 2 , H 2 L 3 , respectively) yielded high-spin octahedral mono-and bis(ligand) complexes of the formula. H 2 O. The compounds were characterized by elemental analysis, conductometric and magnetochemical measurements, IR and UV-Vis spectra. Besides, a detailed voltammetric study of the complexes was carried out in DMF solution in the presence of several supporting electrolytes, to characterize the nature of the electrode processes and solution equilibria.

Study of Neutral Fe(III) Complexes of Pyridoxal-N-Substituted Thiosemicarbazone with Desolvation-Induced Spin-State Transformation above Room Temperature

“…As it is known already from pyruvic acid X-semicarbazone and salicylaldehyde X-semicarbazone (X ) S, Se), 6,7,19,37,38 this neutral-type of compound is obtained by simply reacting metal salts and the ligands all dissolved in methanol, ethanol, or a mixture of each with water. Adding a base (soft or strong) leads in general to the anionic form of the complex; 20,39,40 i.e., the ligand is attached to the metal ion of its doubly deprotonated form. Contrary to this expectation, it was not possible to obtain the complex with the ligand being twice deprotonated with pyridoxal-Nsubstituted thiosemicarbazone hydrochloride, because of the strong stability of the intermediate compound so formed.…”

“…Despite the good structural determination of the molecular compound, its synthesis involves some intriguing features: first, because the starting material was a Co II salt and, second, the pH range referred to did not correspond with the literature, in view of the deprotonation sequence, on pyridoxal-N-substituted thiosemicarbazone. [11][12][13] In addition, literature presents metal complexes of various terdentate X-semicarbazones (X ) S, Se), [14][15][16][17][18][19][20] with the ligand attached to the central metal ion in two different deprotonated forms, but the structural features of the complexes remain unclear because they have not been made explicit. In any case, the reason put forward to explain the unusual nature of the inequivalency of the ligands is the different tautomeric forms one can find the chelate in in solution, but the pH at which those compounds are formed is often overlooked.…”

Tuning of the charge in octahedral ferric complexes based on pyridoxal-N-substituted thiosemicarbazone ligands

“…The same charge of the metal complex is obtained with its iron(III) chloride salts but the ligated mono(ligand) is of its neutral form. 28 On the other hand, the addition of concentrated ammonia at room temperature to a solution containing pyridoxal-N-substituted thiosemicarbazone hydrochloride on constant stirring leads to the neutralisation of HCl and the deprotonation of the chelate in solution. Thus, dropwise addition of a solution containing the appropriate iron(III) salt results in dark green solution (the pH of the reaction mixture should not be below 10.40 or exceed 10.70), followed by filtration and slow evaporation at room temperature, gives the analytically pure paramagnetic complex in good yield.…”

“…Subsequently, a mixture of H 2 thpy•HCl (0.0454 mol) and triethylamine (0.0089 mol) in dichloromethane (200 mL) was stirred at RT for 24 h under an inert atmosphere of dry nitrogen. The resulting yellow precipitate was filtered off under vacuum, washed with dichloromethane (3 ¥ 20 mL) and recrystallized from water-ethanol solution to give the final product (10.87 (41), 120 (12), 77 (23) and 60 (16).…”

“…It is well known that depending on the appropriate coordination atoms (O, N, X, where X = N, O, S, Se), the ligand can bind to the connectors (metal ions) either in its neutral, monoanionic or dianionic form. 11, 12 This also derives from the fact that in solution, thiosemicarbazone consists of an equilibrium mixture of thione and thiol tautomers. In this regard, our attention is directed to the design of pyridoxal semi-, thiosemi-, and isothiosemicarbazones, which can adopt the abovementioned forms and is especially crucial to the preparation of desirable compound by selecting the suitable synthetic procedure.…”