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 2 1 Cl, MnL 2 2 Cl) or morpholino pendants (MnL 2 3 Cl, CoL 2 3 ) 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 2 3 Cl and CoL 2 3 , the crowned Schiff base complexes (MnL 2 1 Cl, MnL 2 2 Cl) exhibited higher catalytic activity. The pseudo-first-order rate constant for the PNPP hydrolysis catalyzed by the complex MnL 2 1 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. Four mono-Schiff base complexes of Mn(III) and Co(II) with either benzo-10-aza-crown ether pendants (MnL 2 1 Cl, MnL 2 2 Cl) or morpholino pendants (MnL 2 3 Cl, CoL 2 3 ) 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 2 3 Cl and CoL 2 3 , the crowned Schiff base complexes MnL 2 1 Cl and MnL 2 2 Cl exhibit higher catalytic activity, the catalytic activity of the complexes follows the order Mn(III) > Co(II) under the same ligand.