Ligand control on the synthesis and redox aspects of mononuclear manganese-(III) and -(IV) complexes with tridentate ONS-coordination

Abstract

MnCl2·4H2O reacts with benzohydroxamic acid (BHAH2) in aqueous/non-aqueous solution (alkaline) forming manganese(IV) complexes of the type (PPh4)2[Mn(BHA)3]·3H2O. Its IR data suggest the hydroximato mode of coordination (O, O). This complex in frozen solution exhibits axial spectra with strong zero field splitting giving rise to ag≈4 signal which is more intense than that forg≈2. The solid, however, shows rhombic spectra. A probable reason for this difference is assigned. Trifunctional (ONS) Schiff bases H2L1 and H2L2 (5-R-salicylaldehyde thiosemicarbazone and 5-R-salicyl aldehyde-4-phenyl thiosemicarbazone, respectively; R=H, Me or Br) furnish Mn(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) afford manganese(III) complexes of the type [MnL(O2CMe)] or [MnL(acac)] (acac=acetylacetonate, L=L3 or L4), when treated with MnII (O2CMe)2·4H2O or [MnIII(acac)3] in ethanol medium in air. The EPR spectra of the manganese(IV) complexes in frozen dimethyl formamide (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 shows hyperfine (55Mn) as well as forbidden lines. Cyclic voltammograms of these complexes scanned in DMF show reversible and quasi-reversible MnIV-MnIII couple, theE2980 values of which are significantly affected by the electronic effects of the R-substituents in the salicyl phenyl ring and those attached to the carbon atom bound to the thiolate functionality of the ligands. The HammettσP values of the R-substituents are linearly correlated with theE2980 values.