Manganese (IV) and manganese (V) complexes with N-(2-hydroxyphenyl)-salicylamides

Abstract

N-(2-Hydroxyphenyl)salicylamide (H₃L¹) and its homologues with a substituent on the 2-hydroxyphenyl moiety (5-Me, H₃L²; 5-Cl, H₃L³; 4-NO₂H₃L⁴; 5-NO₂, H₃L⁵) form manganese(IV) complexes of general composition K₂[MnL₂](L = L¹—L⁵) when treated with manganese(II) ion in methanolic KOH solution in air. They were characterized by their magnetic moments at room temperature (µ_eff·= 3.67–4.06), molar conductances in methanol (150–170 Ω^–1 cm² mol^–1), and e.s.r. spectra. X-Band e.s.r. spectra of the complexes in frozen acetonitrile solution showed signals at ca. 2.0 and 4.0 which were assigned to the g_∥ and g_⊥ components, respectively, of Kramer's doublets (M_s=+½ and –½) derived from a d³ electronic configuration. Intense absorptions in the region 16 000–20 000 cm^–1 were attributed to ligand-to-metal charge-transfer bands. Cyclic voltammograms of the complexes determined in CH₂Cl₂(solubilized with 1,4,7,10,13,16-hexaoxacyclo-octadecane) showed three irreversible couples which were assigned to Mn^VI/Mn^V, Mn^V/Mn^IV, and Mn^IV/Mn^III redox processes based on controlled-potential electrolyses. The redox potential of each process was significantly affected by the electronic nature of the substituents, and the higher oxidation states of manganese were stabilized when an electron-releasing group was attached to the ligand. Chemical oxidation of K₂[MnL²₂], with Ce^IV in acetonitrile led to the isolation of a novel manganese(V) complex of composition K[MnL²₂], which was characterized by its magnetic moment (µ_eff·= 2.83) corresponding to two unpaired electrons.