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 [
Fe(III)Co(II)(BPBPMP)(mu-OAc)(2)]ClO(4) (1) and [Ga(III)Co(II)(BPBPMP)(mu-OAc)(2)]ClO(4) (2) complexes with the unsymmetrical dinucleating ligand H(2)BPBPMP (2-bis[{(2-pyridyl-methyl)-aminomethyl}-6-{(2-hydroxy-benzyl)-(2-pyridyl-methyl)}-aminomethyl]-4-methylphenol). The previously reported complex [
Fe(III)Zn(II)(BPBPMP)(mu-OAc)(2)]ClO(4) (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 (A) of the [
Fe(III)-O-M(II)]
structural unit for 1 and for related isostructural
Fe(III)M(II) complexes using the
correlation J = -10(7) exp(-6.8d) reveals that this parameter is the major factor that determines the degree of antiferromagnetic coupling in the series [(BPBPMP)
Fe(III)(mu-OAc)(2)M(II)](+) (M(II) = 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)M(III)(mu-OH)Co(II)(H(2)O)] 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 k(H)/k(D)
isotope effects, the kinetic studies suggest a mechanism in which a terminal M(III)-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 Ga(III)-OH terminal group in comparison to
Fe(III)-OH in the
dinuclear M(III)Co(II) species.
