LIGAND CONTROL ON THE SYNTHESIS AND REDOX POTENCY OF MONONUCLEAR MANGANESE-(III) AND MANGANESE-(IV) COMPLEXES WITH TRIDENTATE ONS COORDINATION

Trifunctional (ONS) Schiff bases H2L1 and H2L2 (H2L1 = 5-R-salicylalde hyde thiosemicarbazone; H2L2 = 5-R-salicylaldehyde 4-phenylthiosemicar bazone; R = H, Me or Br) furnished manganese(IV) complexes of the type [MnL2] (L = L1 or L2), whereas H2L3 and H2L4{H2L3 = S-methyl -[(5-R-2 -hydroxyphenyl)-methylene]dithiocarbazate; H2L4 = S-benzyl 3-[(5-R-2-h ydroxyphenyl)methylene]dithiocarbazate; R = H, Me or Br} afforded mang anese(III) complexes of the type [MnL(O2CMe)] or [MnL(acac)] (acac = a cetylacetonate, L = L3 or L4), when treated with Mn(II)(O2CMe)2.4H2O o r [Mn(III)(acac)3] in alcoholic media in the air. The room-temperature magnetic moments confirm the corresponding oxidation states in the co mplexes. The EPR spectra of the manganese(IV) complexes in frozen dime thylformamide (dmf)-methanol solution show weak and strong signals at [g] almost-equal-to 4.0 and almost-equal-to 2.0, respectively, implyin g a small zero-field splitting. The [g] almost-equal-to 2.0 signal sho ws hyperfine (due to Mn-55) as well as forbidden lines. Cyclic voltamm ograms 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 substituent s in the salicyl phenyl ring and the R' substituents attached to the c arbon atom bound to the thiolate functionality of the ligands. The Ham mett sigma(p) values of the R substituents are linearly correlated wit h the E298-degrees values.