Ligand control on the synthesis and redox potency of mononuclear manganese-(III) and -(IV) complexes with tridentate ONS co-ordination

Abstract

Trifunctional (ONS) Schiff bases H₂L¹ and H₂L²(H₂L¹= 5-R-salicylaldehyde thiosemicarbazone; H₂L²= 5-R-salicylaldehyde 4-phenylthiosemicarbazone; R = H, Me or Br) furnished manganese(IV) complexes of the type [MnL₂](L = L¹ or L²), whereas H₂L³ and H₂L⁴{H₂L³=S-methyl 3-[(5-R-2-hydroxyphenyl)-methylene]dithiocarbazate; H₂L⁴=S-benzyl 3-[(5-R-2-hydroxyphenyl)methylene]dithiocarbazate; R = H, Me or Br} afforded manganese(III) complexes of the type [MnL(O₂CMe)] or [MnL(acac)](acac = acetyiacetonate, L = L³ or L⁴), when treated with Mn^II(O₂CMe)₂·4H₂O or [Mn^III(acac)₃] in alcoholic media in the air. The room-temperature magnetic moments confirm the corresponding oxidation states in the complexes. The EPR spectra of the manganese(IV) complexes in frozen dimethylformamide (dmf )-methanol solution show weak and strong signals at <g> ≈ 4.0 and ≈ 2.0, respectively, implying a small zero-field splitting. The <g> ≈ 2.0 signal shows hyperfine (due to ⁵⁵Mn) as well as forbidden lines. Cyclic voltammograms of all the complexes scanned in dmf showed reversible to quasi-reversible Mn^IV–Mn^III couples the E°₂₉₈ values of which are significantly affected by the electronic effects of the R′ substituents in the salicyl phenyl ring and the R′ substituents attached to the carbon atom bound to the thiolate functionality of the ligands. The Hammett σ_p values of the R substituents are linearly correlated with the E°₂₉₈ values.