Bioinspired 'Fe POT. III''Cd POT. II' and 'Fe POT. III''Hg POT. II' complexes: synthesis, characterization and promiscuous catalytic activity evaluation

Abstract

In this work we report on the synthesis, crystal structure, and physicochemical characterization of the novel dinuclear [FeIIICdII(L)(μ-OAc)2]ClO4·0.5H2O (1) complex containing the unsymmetrical ligand H2L = 2-bis[{(2-pyridyl-methyl)-aminomethyl}-6-{(2-hydroxy-benzyl)-(2-pyridyl-methyl)}-aminomethyl]-4-methylphenol. Also, with this ligand, the tetranuclear [Fe2IIIHg2II(L)2(OH)2](ClO4)2·2CH3OH (2) and [FeIIIHgII(L)(μ-CO3)FeIIIHgII(L)](ClO4)2·H2O (3) complexes were synthesized and fully characterized. It is demonstrated that the precursor [FeIII2HgII2(L)2(OH)2](ClO4)2·2CH3OH (2) can be converted to (3) by the fixation of atmospheric CO2 since the crystal structure of the tetranuclear organometallic complex [FeIIIHgII(L)(μ-CO3)FeIIIHgII(L)](ClO4)2·H2O (3) with an unprecedented {FeIII(μ-Ophenoxo)2(μ-CO3)FeIII} core was obtained through X-ray crystallography. In the reaction 2 → 3 a nucleophilic attack of a FeIII-bound hydroxo group on the CO2 molecule is proposed. In addition, it is also demonstrated that complex (3) can regenerate complex (2) in aqueous/MeOH/NaOH solution. Magnetochemical studies reveal that the FeIII centers in 3 are antiferromagnetically coupled (J = - 7.2 cm-1) and that the FeIII-OR-FeIII angle has no noticeable influence in the exchange coupling. Phosphatase-like activity studies in the hydrolysis of the model substrate bis(2,4-dinitrophenyl) phosphate (2,4-bdnpp) by 1 and 2 show Michaelis–Menten behavior with 1 being ~ 2.5 times more active than 2. In combination with kH/kD isotope effects, the kinetic studies suggest a mechanism in which a terminal FeIII-bound hydroxide is the hydrolysis-initiating nucleophilic catalyst for 1 and 2. Based on the crystal structures of 1 and 3, it is assumed that the relatively long FeIII…HgII distance could be responsible for the lower catalytic effectiveness of 2.