Unsymmetrical Fe^IIICo^II and Ga^IIICo^II Complexes as Chemical Hydrolases: Biomimetic Models for Purple Acid Phosphatases (PAPs)

Abstract

The design and development of suitable biomimetic catalytic systems capable of mimicking the functional properties of enzymes continues to be a challenge for bioinorganic chemists. In this study, we report on the synthesis, X-ray structures, and physicochemical characterization of the novel isostructural [FeIIICoII(BPBPMP)(μ-OAc)2]ClO4 (1) and [GaIIICoII(BPBPMP)(μ-OAc)2]ClO4 (2) complexes with the unsymmetrical dinucleating ligand H2BPBPMP (2-bis[{(2-pyridyl-methyl)-aminomethyl}-6-{(2-hydroxy-benzyl)-(2-pyridyl-methyl)}-aminomethyl]-4-methylphenol). The previously reported complex [FeIIIZnII(BPBPMP)(μ-OAc)2]ClO4 (3) was investigated here by electron paramagnetic resonance for comparison with such studies on 1 and 2. A magneto-structural correlation between the exchange parameter J (cm−1) and the average bond lengh d (Å) of the [FeIII−O−MII] structural unit for 1 and for related isostructural FeIIIMII complexes using the correlation J = −107 exp(−6.8d) reveals that this parameter is the major factor that determines the degree of antiferromagnetic coupling in the series [(BPBPMP)FeIII(μ-OAc)2MII]+ (MII = Mn, Fe, Co, Ni) of complexes. Potentiometric and spectrophotometric titrations along with electronic absorption studies show that, in aqueous solution, complexes 1 and 2 generate the [(HO)MIII(μ-OH)CoII(H2O)] complex as the catalytically active species in diester hydrolysis reactions. Kinetic studies on the hydrolysis of the model substrate bis(2,4-dinitrophenyl)phosphate by 1 and 2 show Michaelis−Menten behavior, with 2 being 35% more active than 1. In combination with kH/kD isotope effects, the kinetic studies suggest a mechanism in which a terminal MIII-bound hydroxide is the hydrolysis-initiating nucleophilic catalyst. In addition, the complexes show maximum catalytic activity in DNA hydrolysis near physiological pH. The modest reactivity difference between 1 and 2 is consistent with the slightly increased nucleophilic character of the GaIII−OH terminal group in comparison to FeIII−OH in the dinuclear MIIICoII species.