Mononuclear manganese(II) and manganese(III) complexes of N2O donors involving amine and phenolate ligands:: absorption spectra, electrochemistry and crystal structure of [Mn(L3)2](ClO4)

A number of mononuclear manganese(II) and manganese(III) complexes have been synthesized from tridentate N2O aminophenol ligands (HL1-HL5) formed by reduction of corresponding Schiff bases with NaBH4. Three types of tridentate N2O aminophenols have been prepared by reducing with NaBH4 which are (a) Schiff bases obtained by bromo salicylaldehyde reaction with N,N-dimethyl/N,N-diethyl ethylene diamine (HL1, HL2), (b) Schiff bases obtained by condensing salicylaldehyde/bromo salicylaldehyde and picolyl amine (HL3, HL4), (c) pyridine-2-aldehyde and 2-aminophenol (HL5). All the manganese complexes have been prepared by direct addition of manganese perchlorate to the corresponding ligands and were characterized by the combination of i.r., u.v.-vis spectroscopy, magnetic moments and electrochemical studies. The u.v.-vis spectra of all of the manganese(III) complexes show two weak d-d transitions in the 630-520 nm region, which support a distorted octahedral geometry. The electron transfer properties of all of the manganese(III) complexes (1-4 and 6) exhibit mostly similar characteristics consisting two redox couples corresponding to the Mn-III -> Mn-II reductions and Mn-III -> Mn-II oxidations. The electronic effect on the potential has also been studied by changing different substituents in the ligands. In all cases, an electron-donating group stabilizes the higher oxidation state and electron withdrawing group prefers the lower oxidation state. The cyclic voltammogram of [Mn-II stop(L-5)(2)] shows an irreversible oxidation Mn-II -> Mn-III at -0.88 V, followed by another quasi-reversible oxidation Mn-III -> Mn-IV at +0.48 V. The manganese(III) complex (3) [Mn(L-3)(2)]ClO4 has been characterized by X-ray crystallography.