A New Di-μ-sulfate Complex as a Model of Purple Acid Phosphatase-Sulfate Complexes

Reportamos neste trabalho, a síntese, a estrutura cristalina e as propriedades espectroscópicas e eletroquímicas do complexo NH 4[Fe2 (bbpmp)(μ-SO4)2] 1 onde bbpmp é o ânion do ligante 2,6-bis[(2-hidróxibenzil)(2-metilpiridil)-amino-metil]-4-metilfenol. Este representa o primeiro exemplo de um complexo contendo a unidade estrutural ( μ-fenolato)(μ-SO4)2 e um ligante N,Odoador de relevância, como um análogo sintético para os derivados sulfatos das fosfatasas ácidas púrpuras.


Introduction
The interaction of the active site of purple acid phosphatases (PAPs) with sulfate and other perturbants has been described 1,2 .This oxyanion is known to be able to interact with the reduced enzymes, which in the presence of air form the oxidized PAPox-sulfate complexes.In a recent report, Witzel 3 has shown that the addition of SO4 2-to the oxidized form (PAPox, λ max = 558 nm) leads to the immediate formation of an enzyme-sulfate complex (PAPox-sulfate) with an absorption maximum at 546 nm.In order to gain more information about the binding mode of PAPs with small bridging oxyanions we describe here the synthesis, crystal structure and properties of a novel Fe2 III synthetic analogue which contains the Fe III (λ-SO4)2Fe III unit with a biologically relevant N,O-donor dinucleating ligand.This work is a continuation of a wide research program for the preparation and characterization of iron complexes with bioinorganic interest [4][5][6][7][8] .
All the H atoms were placed at geometrically calculated positions except those of the NH4+ ion that were not found.It was assigned to them an isotropic temperature factor of 1.3 times the isotropic temperature factor of the atom to which they were attached; µ = 0.756 mm -1 ; 14944 mesured reflections with 7942 unique reflections; 1678 with I > 3σ (I); 241 least-square parameters; R = 0.0826 (Rw = 0.0976).
The oxidation states of the iron centers in 1 are supported by the Mössbauer spectrum at 115 K and the following parameters: isomer shift (relative to metallic iron with the source at room temperature) δ, 0.51 mm/s and quadrupole splitting, ∆EQ, 0.87 mm/s which indicates the presence of high-spin Fe2 III centers 13 .The magnetic data for a powder sample of 1 were collected in the temperature range of 5.1 to 300 K and indicate a weak antiferromagnetic coupling interaction for the two Fe 3 ions in the complex.The data were fitted by using the expression for the molar susceptibility vs. temperature from the spin-exchange Hamiltonian H = -2JS1S2 (S1 = S2 = 5/2) 14 and the following parameters: g = 2.00 (fixed); % imp = 5.5; θ = -3.5 K; TIP = 400 x 10 -6 cm 3 / mol; J = -6.4cm -1 .This J value is very similar to those detected for the complexes [Fe2 III (bbpmp)(CH3COO)2]ClO4.H2O 5,7 and [Fe2 III (bbpmp)(O2P(OPh)2)2]ClO4.H2O 10 , despite the structural differences detected in these complexes.
The electrochemical properties of 1 were investigated by cyclic voltammetry in acetonitrile with [Bu4N][PF6] as the supporting electrolyte.A quasi-reversible wave is observed at -1.29 V vs. Fc/Fc which is ascribed to the Fe2 III /Fe II Fe III redox couple.The corresponding couple in 2, is observed to occurs at -0.57V vs. Fc + /Fc 5,7 and, as expected, the substitution of two acetate by sulfate groups, shifts the redox couple to a more negative potential.
We have synthesized and characterized 1 to serve as a synthetic analogue for PAPox-sulfate complexes but, to our knowledge, there are very few informations in the literature on the corresponding PAPox complexes to make further comparisons 1,3 .On the other hand, due to the presence of two terminal phenolate groups in 1 and based on the redox potential reported for uteroferrin (E o' = -0.03V vs. Fc/Fc at pH 5) 15 , one should expect a less negative redox potential for the PAPo-sulfate complex compared to 1.
Finally, further preparative, structural and physicochemical studies on the XO4 2-(X = Cr, Mo) complexes are in progress in our laboratory, and will be the subject of a full paper.

Supplementary Material
The following tables are available from the authors on request: complete table of crystal data, positional parameters, bond distances, bond angles, hydrogen atoms coordinates, displacement parameters (12 pages), and list of observed and calculated structure factors (79 pages).