Water soluble manganese(III) and manganese(IV) complexes of tridentate ono ligands

Abstract

Water soluble manganese(III) complexes of sulphonated azophenol and salicylaldimine ligands 3–6, have been isolated: Na 3[Mn(LS) 2]· nH 2O and Na 5[Mn(LSS) 2]· nH 2O ( n = 4 or 5) where H 3LS and H 4LSS 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 (t 2 3e 1) in character (∼5 μ B). The manganese(IV)-manganese(III) couple in water has cyclic voltammetric E 1 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 55Mn 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.