Coordination polymers based on bridging methylene group as catalysts for the liquid phase hydroxylation of phenol Original Research Article

Reaction between 5,5′-methylene bis(salicylaldehyde) and 1,2-diaminoethane or 1,3-diaminopropane in equimolar ratio leads to the formation of new polymeric chelating ligands abbreviated herein as [CH2H2(salen)]n (I) and [CH2H2(salpn)]n (II). These ligands react with acetates of copper(II), nickel(II) and cobalt(II) to give coordination polymers of the types [CH2{M(salen)·X DMF}]n/[CH2{M(salpn)·X DMF}]n [when ligand=I: X=0, M=Cu (1); X=0, M=Ni (2); X=2, M=Co (3) and when ligand=II: X=0, M=Cu (5); X=2, M=Ni (6); X=2, M=Co (7)]. Treatment of [MoO2(acac)2] with these ligands gives dioxomolybdenum(VI) complexes [CH2{MoO2(salen)}]n (4) and [CH2{MoO2(salpn)}]n (8). All these complexes are insoluble in most coordinating and non-coordinating solvents. The nickel(II) complex [CH2{Ni(salen)}]n and both molybdenum complexes are diamagnetic while others are paramagnetic. IR spectral data confirm the coordination of ligands through the azomethine nitrogen and the phenolic oxygen atoms to the metal ions. Electronic spectral data suggest an octahedral structure for each repeating unit in 3, 4, 6, 7 and 8 while square planar for each repeating unit in 1, 2 and 5. All these complexes, except molybdenum ones, exhibit good catalytic activity towards the hydroxylation of phenol into a mixture of catechol and hydroquinone using H2O2 as an oxidant. Various parameters such as concentration of substrate, catalyst and oxidant, reaction time and volume of the solvent have been taken into consideration for the maximum hydroxylation of phenol.