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

MnCl2 . 4H(2)O reacts with benzohydroxamic acid (BHAH(2)) in aqueous/n on-aqueous solution (alkaline) forming manganese(IV) complexes of the type (PPh(4))(2)[Mn(BHA)(3)]. 3H(2)O. Its IR data suggest the hydroxim ato mode of coordination (O, O). This complex in frozen solution exhib its axial spectra with strong zero field splitting giving rise to a g approximate to 4 signal which is more intense than that for g approxim ate to 2. The solid, however, shows rhombic spectra. A probable reason for this difference is assigned. Trifunctional (ONS) Schiff bases H(2 )L(1) and H(2)L(2) (5-R-salicylaldehyde thiosemicarbazone and 5-R-sali cyl aldehyde-4-phenyl thiosemicarbazone, respectively; R = H, Me or Bf ) furnish Mn(IV) complexes of the type [MnL(2)](L = L(1) or L(2)), whe reas H(2)L(3) and H(2)L(4) (H(2)L(3) = S-methyl 3-(5-R-2-hydroxyphenyl ) methylene dithiocarbazate; H(2)L(4) = S-benzyl 3-(5-R-2-hydroxypheny l) methylene dithiocarbazate; R = H, Me or Br) afford manganese(III) c omplexes of the type [MnL(O(2)CMe)] or [MnL(acac)] (acac = acetylaceto nate, L = L(3) Or L(4)), when treated with Mn-II (O(2)CMe)(2) . 4H(2)O or [Mn-III(acac)(3)] in ethanol medium in air. The EPR spectra of the manganese(IV) complexes in frozen dimethyl formamide (dmf)-methanol s olution show weak and strong signals at [g] approximate to 40 and appr oximate to 2.0 respectively, implying a small zero field splitting. Th e [g] approximate to 2.0 shows hyperfine (Mn-55) as well as forbidden lines. Cyclic voltammograms of these complexes scanned in DMF show rev ersible and quasi-reversible Mn-IV-Mn-III couple, the E(298)(0) values of which are significantly affected by the electronic effects of the R-substituents in the salicyl phenyl ring and those attached to the ca rbon atom bound to the thiolate functionality of the ligands. The Hamm ett sigma(P) values of the R-substituents are linearly correlated with the E(298)(0) values.