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: 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 (t₂³e¹) 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.