Fernando R. Xavier

Unsymmetrical FeIIICoII and GaIIICoII Complexes as Chemical Hydrolases: Biomimetic Models for Purple Acid Phosphatases (PAPs)

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.

Bioinspired FeIIICdII and FeIIIHgII complexes: Synthesis, characterization and promiscuous catalytic activity evaluation

In this work we report on the synthesis, crystal structure, and physicochemical characterization of the novel dinuclear [Fe(III)Cd(II)(L)(μ-OAc)(2)]ClO(4)·0.5H(2)O (1) complex containing the unsymmetrical ligand H(2)L=2-bis[{(2-pyridyl-methyl)-aminomethyl}-6-{(2-hydroxy-benzyl)-(2-pyridyl-methyl)}-aminomethyl]-4-methylphenol. Also, with this ligand, the tetranuclear [Fe(2)(III)Hg(2)(II)(L)(2)(OH)(2)](ClO(4))(2)·2CH(3)OH (2) and [Fe(III)Hg(II)(L)(μ-CO(3))Fe(III)Hg(II)(L)](ClO(4))(2)·H(2)O (3) complexes were synthesized and fully characterized. It is demonstrated that the precursor [Fe(III)(2)Hg(II)(2)(L)(2)(OH)(2)](ClO(4))(2)·2CH(3)OH (2) can be converted to (3) by the fixation of atmospheric CO(2) since the crystal structure of the tetranuclear organometallic complex [Fe(III)Hg(II)(L)(μ-CO(3))Fe(III)Hg(II)(L)](ClO(4))(2)·H(2)O (3) with an unprecedented {Fe(III)(μ-O(phenoxo))(2)(μ-CO(3))Fe(III)} core was obtained through X-ray crystallography. In the reaction 2→3 a nucleophilic attack of a Fe(III)-bound hydroxo group on the CO(2) 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 Fe(III) centers in 3 are antiferromagnetically coupled (J=-7.2cm(-1)) and that the Fe(III)-OR-Fe(III) 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 k(H)/k(D) isotope effects, the kinetic studies suggest a mechanism in which a terminal Fe(III)-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 Fe(III···)Hg(II) distance could be responsible for the lower catalytic effectiveness of 2.

Electronic and interfacial behavior of gemini metallosurfactants with copper(II)/pseudohalide cascade cores

In this paper we discuss the newly synthesized binuclear species [Cu2(L(PY18))2(μ1,1-N3)2(N3)2] (1) and [Cu2(L(PY18))2(μ1,3-SCN)2(NCS)2] (2), as obtained from the monometallic precursor [Cu(L(PY18))Br2]. These gemini metallosurfactants incorporate metal/anion cascade cores and are investigated by experimental and theoretical methods. Diagnostic IR stretches support the presence of μ1,1-bridged (end-on, 2075 cm(-1)) azide groups in 1 and μ1,3-bridged (end-to-end, 2117 cm(-1)) thiocyanate groups in 2. Anion-to-copper LMCT electronic processes at 390 and 440 nm for 1 and at 415 nm for 2 reinforce the nature of the metal/anion cascade cores. Both species are redox-active, magnetically uncoupled due to poor orbital overlap, and robust in the presence of strongly coordinating solvents. At the air-water interface, 1 and 2 yield Langmuir films with high collapse pressures of ca. 60 mN m(-1). Domain formation is considerably less extensive than that observed for the related monometallic precursor and the average molecular areas are in good agreement with their modeled molecular size. The resulting Langmuir-Blodgett films are isolated on silica substrates and investigated using IR-reflectance/absorbance spectroscopy.

A Synthetic Pathway for an Unsymmetrical N5O2 Heptadentate Ligand and Its Heterodinuclear Iron(III)Zinc(II) Complex: A Biomimetic Model for the Purple Acid Phosphatases

One major field of interest in bioinorganic chemistry is the design and synthesis of inorganic compounds with low molecular mass, showing structural, spectroscopic, and reactivity properties that mimic enzymes, such as purple acid phosphatases (PAPs). In this study, the unsymmetrical heptadentate ligand 2‐[(4,7‐diisopropyl‐1,4,7‐triazacyclonon‐1‐yl)methyl]‐6‐{[(2‐hydroxybenzyl)(pyridin‐2‐ylmethyl)‐amino]methyl}‐4‐methylphenol (H2L) and its first mixed‐valence complex [FeIIIZnII(L)(μ‐OAc)2]ClO4 (1) were synthesized. Physical and chemical measurements (crystal structure, conductometry, IR and UV/VIS spectroscopy, and electrochemistry) were performed for 1, and these properties are compared with those presented by the kbPAPs active sites. Potentiometric titration studies of 1 have confirmed its acid/base properties that are crucial for the understanding of the phosphodiester and DNA catalytic cleavage in future studies.