Cu(II) and Ni(II) complexes based on monofunctional and dendrimeric pyrrole-imine ligands: Applications in catalytic liquid phase hydroxylation of phenol

New bis(pyrrolide-imine) copper(II) and Ni(II) complexes C1 and C2 [{ (C3H7)-NCH (C4H3N)}2Cu], [{(C3H7)-NCH-(C4H3N)}2Ni] as well as the bimetallic dendrimeric (pyrrolide-imine) copper(II) and nickel(II) complexes C3 and C4, [DAB-{(NCH-C4H3N)4}Cu2], [DAB-{(NCH-C4H3N)4}Ni2] (DAB = G1-polypropyleneimine dendrimer with a diaminobutane core) were prepared in good yields. The structure and composition of the complexes were confirmed by a combination of analytical techniques. These complexes were investigated as catalysts in the hydroxylation of phenol in aqueous media in the pH range of 2–6 for the mononuclear complexes, C1 and C2 while the bimetallic systems, C3 and C4 were studied over the pH range 2–8. H2O2 was used as the oxidant under an oxygen atmosphere. The copper systems generally showed higher activity as compared to their nickel analogues. Catechol was the predominant product followed by hydroquinone with small amounts of para-benzoquinone. The nickel complexes showed better selectivity for catechol. The pH of the reaction medium also plays a role in both activity and selectivity with pH 3 being optimal for activity and pH 6 for selectivity to catechol.