Mono-Schiff base complexes with aza-crown ether or morpholino pendants as synthetic hydrolases for p-nitrophenyl picolinate cleavage

Abstract

Mono-Schiff base complexes of Co(II) and Mn(III) with either benzo-10-aza-crown ether pendants (MnL ¹₂ Cl, MnL ²₂ Cl) or morpholino pendants (MnL ³₂ Cl, CoL ³₂ ) have been employed as models for hydrolase enzymes by studying the kinetics of their hydrolysis reactions with p-nitrophenyl picolinate (PNPP). A kinetic model of PNPP cleavage catalyzed by these complexes is proposed. The effects of complex structures and reaction temperature on the rate of catalytic PNPP hydrolysis have also been examined. The rate increases with pH of the buffer solution; all four complexes exhibited high activity in the catalytic PNPP hydrolysis. Compared with the crown-free analogs MnL ³₂ Cl and CoL ³₂ , the crowned Schiff base complexes (MnL ¹₂ Cl, MnL ²₂ Cl) exhibited higher catalytic activity. The pseudo-first-order rate constant for the PNPP hydrolysis catalyzed by the complex MnL ¹₂ Cl, containing a benzo-10-aza-crown ether pendant, is 1.84 × 10³-fold higher than that of spontaneous hydrolysis of PNPP at pH 7.60, 25 °C, [S] = 2.0 × 10⁻⁴ mol dm⁻³.

Graphical abstract

Four mono-Schiff base complexes of Mn(III) and Co(II) with either benzo-10-aza-crown ether pendants (MnL ¹₂ Cl, MnL ²₂ Cl) or morpholino pendants (MnL ³₂ Cl, CoL ³₂ ) have been employed as models for hydrolase enzymes by studying the kinetics of their hydrolysis reactions with p-nitrophenyl picolinate (PNPP). The effects of ligand structure and reaction temperature on PNPP catalytic hydrolysis have been also discussed. All four complexes exhibited high activity in the catalytic PNPP hydrolysis. Compared with the crown-free analogy MnL ³₂ Cl and CoL ³₂ , the crowned Schiff base complexes MnL ¹₂ Cl and MnL ²₂ Cl exhibit higher catalytic activity, the catalytic activity of the complexes follows the order Mn(III) > Co(II) under the same ligand.