Ligand-to-ligand electron transfer and temperature induced valence tautomerism in o-dioxolene chelated manganese complexes

Abstract

Three polymeric o-dioxolene chelated manganese(III) complexes, {[Mn III(H 2L 1)(Cl 4Cat) 2][Mn III(Cl 4Cat) 2(H 2O) 2]} ∞ ( 1) (L 1 = N, N′- bis(2-pyridylmethyl)-1,4-butanediamine, Cl 4Cat = tetrachlorocatecholate dianion], {[Mn III(H 2L 1)(Br 4Cat) 2][Mn III(Br 4Cat) 2(H 2O) 2]·4DMF} ∞, ( 2) and {[Mn III(H 2L 2)(Br 4Cat) 2][Mn III(Br 4Cat) 2(DMF) 2]} ∞ ( 3) (L 2 = N, N′- bis(2-pyridylmethyl)-1,6-hexanediamine, Br 4Cat = tetrabromocatecholate dianion) have been synthesized and structures were determined by X-ray crystallography. All the complexes were fully characterized by various spectroscopic techniques and their electronic properties are described. It was found that the simple protonation or deprotonation of the bridging ligand (L 1 or L 2) coordinated to metal–dioxolene chromophore induce a change in the oxidation state of the coordinated dioxolene ligand without affecting the metal oxidation state. As a result, drastic change in the optical absorption properties of the complexes is observed in the visible and near-IR region as the transformation involves semiquinone–catecholate ligands. Moreover, all three complexes undergo thermally induced valence tautomerism in solution. For all the complexes, on increasing the temperature, the intensity of the lower energy Inter Valence Charge Transfer (IVCT) band at about 1930 nm increases with corresponding decrease of 600 nm band with an isosbestic point at 1820 nm due to the formation of mixed valence species Mn II(X 4SQ)(X 4Cat) − from Mn III ( X 4 Cat ) 2 - (X = Cl or Br) by the transfer of one electron from Cat 2− to Mn III center.