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

Three polymeric o-dioxolene chelated manganese(III) complexes, {[MnIII(H2L1)(Cl4Cat)2][MnIII(Cl4Cat)2(H2O)2]}∞ (1) (L1 = N,N′-bis(2-pyridylmethyl)-1,4-butanediamine, Cl4Cat = tetrachlorocatecholate dianion], {[MnIII(H2L1)(Br4Cat)2][MnIII(Br4Cat)2(H2O)2]·4DMF}∞, (2) and {[MnIII(H2L2)(Br4Cat)2][MnIII(Br4Cat)2(DMF)2]}∞ (3) (L2 = N,N′-bis(2-pyridylmethyl)-1,6-hexanediamine, Br4Cat = 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 (L1 or L2) 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 MnII(X4SQ)(X4Cat)− from MnIII(X4Cat)2- (X = Cl or Br) by the transfer of one electron from Cat2− to MnIII center.

Three polymeric manganese(III) complexes with dioxolene ligands have been synthesized and characterized. It has been demonstrated that the simple protonation or deprotonation of a remote site of metal–dioxolene chromophore can induce a change in the oxidation state of the coordinated dioxolene ligand whereby a drastic change in the optical absorption properties of the complex occur. Magnetic and electrochemical properties of them have also been described.Figure optionsDownload as PowerPoint slideResearch highlights
► Three new polymeric manganese(III) complexes with dioxolene ligads have been synthesized and structurally characterized.
► Influence of protonation or deprotonation of the bridging ancillary ligands over the oxidation state of coordinate dioxolene ligands has been examined.
► All the complexes undergo thermally induced valence tautomerism in solution.
► Electrochemical and magnetic properties of them were also reported.