WATER-SOLUBLE MANGANESE(III) AND MANGANESE(IV) COMPLEXES OF TRIDENTATE ONO LIGANDS

Water soluble manganese(III) complexes of sulphonated azophenol and salicylaldimine ligands 3-6, have been isolated: Na3[Mn(LS)2].nH2O and Na5[Mn(LSS)2]. nH2O (n=4 or 5) where H3LS and H4LSS represent mono- and di-sulphonated ligands, respectively. The coordination sphere of the complexes are of the type Mn(ONO)2 where the O and N atoms are of phenolic and azo/azomethine type. The complexes are uniformly high-spin (t2(3)e1) in character (approximately 5 mu(B)). The manganese(IV)-manganese(III) couple in water has cyclic voltammetric E1/2 values in the range 0.30-0.50 V vs SCE and it is estimated that the potentials increase by 70 mV for each sulphonate substitution. The manganese(IV) complexes [Mn(LS)2]2- and [Mn(LSS)2]4- have been quantitatively generated in solution both chemically and electrochemically. Their X-band EPR spectra consist of a strong resonance near g = 2. In frozen water-ethylene glycol glass (77 K) the g = 2 resonance shows Mn-55 hyperfine structure (A, 94-97 G). Forbidden lines are also resolved and with their help the axial zero-field splitting parameter D is estimated to be 0.012 cm-1. It is small compared to the X-band quantum, 0.31 cm-1.