Transition metal coordination polymers: Synthesis and catalytic study for hydroxylation of phenol and benzene

New coordination polymers of Ni(II) and Cu(II) of the polymeric salen-type Schiff base ligand derived from the condensation of 5,5′-methylene bis-(salicyaldehyde) with 1,2-diaminopropane yielded N,N′-1,2-propylenebis(5-methylenesalicylidenamine) abbreviated [CH2(H2sal-1,2-pn)]n have been synthesized. Both coordinated polymers with the general formula of [CH2(ML·XDMF)]n, where X = 0, M = Cu; N,N′-1,2-propylenebis(5-methylenesalicylidenaminato)copper(II) and X = 2, M = Ni; N,N′-1,2-propylenebis(5-methylenesalicylidenaminato)nickel(II) have been characterized by elemental analysis, magnetic susceptibility measurements, IR, electronic spectra and thermogravimetric studies. The ligand behaves as a bis-bidentate molecule coordinating through the phenolic oxygen and azomethine nitrogen atoms.These coordinated polymers have been assessed as catalysts for liquid phase hydroxylation of phenol and benzene using H2O2 as an oxidant. The results show a high activity and selectivity of both catalysts toward the formation of diphenols from phenol, and a low activity in the oxidation of benzene. The Cu-based catalyst exhibited higher activity than Ni-based catalyst for hydroxylation of phenol and benzene. The activity and efficiency of H2O2 depends on the reaction parameters viz., temperature, molar ratio of the reactants and the solvent. Concentration of the oxidant and other reaction parameters has been optimised for the maximum oxidation of these substrates. These catalysts can be recovered and reused without notable loss of activity.

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► New Ni(II) and Cu(II) coordination polymers were synthesized and characterized.
► The coordinated polymers is assessed for hydroxylation of phenol and benzene by H2O2.
► The catalysts show high activity and selectivity to catechol for phenol hydroxylation.
► The catalysts can be recovered and reused without significant loss of activity.