Ligand control on the synthesis and redox potency of mononuclear manganese-(III) and -(IV) complexes with tridentate ONS co-ordination
摘要:
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 = acetylacetonate, 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] almost-equal-to 4.0 and almost-equal-to 2.0, respectively, implying a small zero-field splitting. The [g] almost-equal-to 2.0 signal shows hyperfine (due to Mn-55) 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 E298-degrees 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 sigma(p) values of the R substituents are linearly correlated with the E298-degrees values.