Metallomicellar catalysis: Hydrolysis of p-nitrophenyl picolinate catalyzed by Cu(II) complexes of triazole-based ligands in cationic gemini surfactant micelles

A series of triazole-based ligands with hydroxyl groups, 3,5-bis(hydroxymethyl)-1,2,4-triazole (L1) and N-alkyl(CnH2n+1)-3,5-bis(hydroxy-methyl)-1,2,4-triazole (n = 1 (L2), 10 (L3) and 12 (L4)) were synthesized. Hydrolysis of p-nitrophenyl picolinate (PNPP) catalyzed by Cu(II) complexes of these ligands in micelles of a cationic gemini surfactant, 1,2-ethane bis(dimethyldodecylammonium bromide) (12-2-12), were investigated kinetically at 25 °C. It was found that Cu(II) complexes of these triazole-based ligands, especially ligands L3 and L4, showed effective catalytic activity on the hydrolysis of PNPP. Possible catalytic mechanisms and the effect of molecular structure of the ligands on catalyzed hydrolysis of PNPP were discussed. A binuclear Cu(II) complex was found to be active species in 12-2-12 micelles for catalyzed hydrolysis of PNPP for L1, while mononuclear complexes were found for L2–L4 due to different structures of the ligands. Kinetic study showed that the apparent first-order rate constant for product formation in the metallomicellar phase (k′N)(k′N), the association constant between the substrate and the binary complex (KT), and the association constant between the metal ion and the ligand (KM) increased with an increase in pH value of the system. With an increase in the hydrocarbon chain length of the ligand, kN' and KT increased while KM decreased at constant pH.

Graphical abstractBinuclear Cu(II) complex was found to be active species in micellar solution of a cationic gemini surfactant 12-2-12 for catalyzed hydrolysis of PNPP for L1 (a), while mononuclear complexes were found for L2–L4 (b) due to different structures of the ligands. Figure optionsDownload full-size imageDownload as PowerPoint slide