Synthesis, spectral and catalytic activity of some Manganese(II) bis-benzimidazole diamide complexes

Article abstract of DOI:10.1016/j.saa.2011.05.024

Four Mn(II) complexes bound to a neutral bis-benzimidazole diamide ligand N,N′-bis(2-methyl benzimidazolyl 2,2′-oxy-diethanamide) (GBOA) have been synthesized and characterized. Anionic ligand associated with the complexes varies as Cl^- CH₃COO^-, SCN^- and ClO₄^-. X-ray structure of one of the complexes [Mn(GBOA)₂(H₂O)₂]Cl₂·4H ₂O was solved and shows that the Mn(II) ion is hexacoordinate. Two equatorial positions are occupied by benzimidazole imine nitrogen atoms while the other two sites are occupied by amide carbonyl oxygens. The imine nitrogen and carbonyl oxygens are bound to Mn(II) by different arms of the two ligands while axial sites are occupied by two water molecules. Two Cl^- anions are outside the coordination sphere and form an extensive 3D H-bonded network. Axially distorted octahedral geometry is confirmed for all the four complexes by low temperature EPR spectroscopy. Distortion parameter D was found to be similar for [Mn(GBOA)₂(H₂O)₂]Cl ₂·4H₂O and [Mn(GBOA)₂(H ₂O)₂]·(CH₃COO)₂·H ₂O. Cyclic voltammograms have been obtained for all the four complexes and E_1/2 values are dependant on the anionic ligand being in the coordination sphere or outside. [Mn(GBOA)₂(H ₂O)₂]Cl₂·4H₂O and [Mn(GBOA)₂(H₂O)₂]·(CH ₃COO)₂·H₂O carry out the selective oxidation of N-benzyldimethylamine, and 1-methyl-pyrollidine to their respective carbonyl products with catalytic efficiency of 35-50%.