Diethyltin(iv) complexes based on N-salicylidene aliphatic acid hydrazides

Abstract

The coordination capabilities of N-salicylidene mono-, di-, and tricarboxylic aliphatic acid hydrazides with respect to diethyl-substituted tin(IV) have been studied. The structure of the new tin(IV) complexes has been confirmed by ¹H, ¹³C, and ¹¹⁹Sn NMR spectroscopy, IR spectroscopy and high resolution mass-spectrometry. For the first time, it has been shown that upon complexation with Sn(IV), these ligands manifest as di- (in the case of mono(salicylidene) acetic acid hydrazide), tetra- (in the case of bis(salicylidene) malonic, malic and aspartic acid hydrazides) and hexa-anionic (in the case of tris(salicylidene) citric acid hydrazide) ligands. Moreover, despite the presence of several coordination sites and functional groups in the ligands, the tin atom selectively occupies only the ONO coordination site. The study of complexes using UV spectroscopy has shown that during complexation with a metal fragment, a bathochromic shift of all ligand absorption bands is observed. The ability of the complexes to undergo electrochemical transformations was studied using cyclic voltammetry. In all cases, the oxidation and reduction of complexes are irreversible. The oxidation of binuclear tin complexes proceeds in two stages at different potentials, which is probably due to the oxidation of two different “sides” of the complexes independently at different potentials. Density functional theory calculations have shed light on the mechanism and energetics of the keto–enol tautomeric equilibrium in methanol solutions of N-salicylidene aliphatic acid hydrazides.