Structures of {Cr2(phen)4[(C6H5)2PO2]2(m - H3O2)}+3 and cis-{Cr(phen)2[(C6H5O)2PO2]2 }+ and Evaluation of the Stability of the Chromium (III) - Phosphate Monodentate Ligation in Aqueous Solution


Alba D.Q. Ferreira (PQ)* and Avi Bino (PQ)‡


* Laboratório de Cristalografia, Estereodinâmica e Modelagem Molecular, Departamento de Química UFSCar, São Carlos (SP). palbadqf@iris.ufscar.br

‡ Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Israel.


Key words: chromium, model complex, X-ray structures.


Adducts of Cr(III)-DNA are known as the final products of the reduction of the carcinogenic Cr (VI) compounds [1-3]. One of the strategies to understand the chemical nature of this interaction is the preparation and crystallographic analysis of structural model compounds. The X-ray structure of a complex with a Cr(III) bond to a phosphodiester which represents one possible model such of these was recently reported [4]. Our interest was to verify the stability of the chromium-phosphate monodentate bond, since the monofunctional ligation is less stable than the chelate bond fashion.

During the preparation and crystallization of the complex cis-{Cr(phen)2[(C6H5O)2PO2](H2O)}2+ (1) in aqueous solution was observed also the formation of a dimmer containing the m-H3O2- ligand. It is known that aqua complexes of chromium trivalent are found in equilibrium, in addition to the dimmer cited above, with the aqua-hydroxyl and dihydroxy forms [5]. Therefore, when the complex 1 was prepared and crystallized in aqueous solution and in the presence of the PF6- ion, occurred the deslocation of this chemical equilibrium, leading to the formation of the less soluble product, the binuclear complex 2, {Cr2(phen)4[(C6H5)2PO2]2(m - H3O2)}+3 (Fig. 1). This complex crystallizes in the triclinic, P space group with two independent molecules in the asymmetric unit . The Cr---Cr’ distance is 5.367(2) Å and the O---O’ m-H3O2- distance is 2.458(9) Å. The Cr-O(P) distance is 1.936(7) Å and the P-O-Cr angle is 147.4°. The bis-diphenylphosphate complex, cis-{Cr(phen)2[(C6H5O)2PO2]2 }+ (3) was prepared in similar conditions of that used for 1 and the crystals were obtained from an organic solvent mixture and its structure was resolved. This complex crystallizes in the triclinic, P space group. The Cr-O(P1) and Cr-O(P2) distances are 1.932(3) Å and 1.908(3) Å; the Cr-O-P1 and Cr-OP2 angles are 137.9(2) ° and 156.5(2)° respectively; the P1---P2 distance is 5.389(1) Å.

The kinetic stability experiments of the monodentate phosphodiester ligand in aqueous solution, consist of keeping the complex at 37°C in HEPES-buffered solution for different reaction times. After that, the solution is adsorbed on to an ion-exchange column. The fraction containing the free ligand is collected and the solvent evaporated. Using 31P NMR analysis and (C6H5O)PO32- as reference for 100% P, the ratio of the (C6H5O)2PO2- free ligand in this fraction is determined. These results for 2

showed that approximately 35 % of the (C

6H5O)2PO2- released after 90 min, and 68 % after 5 hr. The experiment time was 3 days for compound 3 and the free ligand obtained was only 17 %. These results suggest that the bis-phosphate complex 3 was remarkably more stable than 2. One possible explanation is that under these conditions, there is a thermodynamic preference to form one more m-H3O2- bridge and the phosphodiester ligand is removed from the coordination sphere (The recrystallization of 2 in HEPES-buffered solution of pH = 7.4 at 37 °C, leads to the formation of an additional m-H3O2- linkage with the elimination of the phosphatediester ligand which turns to be a counter ion in the crystal of the more stable complex, [Cr(phen)2(m-H3O2)]2((C6H5O)2PO2)2(PF6)2. (4), with O---O’ m-H3O2- distance of 2.549(6) Å. This hydrolytic pathway reaction, with the releasing of (C6H5O)2PO2- by the formation of m-H3O2- ligand, is not available for 3.

Fig 1: Structures of the complexes {Cr2(phen)4[(C6H5)2PO2]2(m - H3O2)}+3 (right) that is in 2·3PF6·3MeCN, (b) {Cr(phen)2[(C6H5)2PO2]2}+ that is in 3·NO3 (left).

References

(1) Shi, X. L., Chiu, A., Chen, C. T., B., Halliwell, B., Castranova, V., Vallyathan, V.(1999) Reduction of chromium(VI) and its relationship to carcinogenesis. J. Toxicol. Environ. Health, Part B-Critical Reviews 2, 87-104.

(2) Anderson, R. A.(1995) Chromium. In Handbook of Metal-Ligand Interactions in Biological Fluids: Bioinorganic Medicine (Berthon, G., Ed.), Deker, Vol. 1, pp 261-265, New York, N.Y.

(3) Katz, S., and Salem, H.(1994) The Biological and Environmental Chemistry of Chromium, VCH publishers.

(4) Ferreira, A. D. Q., Bino, A., and Gibson, D. (1998) Preparation, structure and stability of cis-{Cr(phen)2[OP(O)(OC6H5)2](H2O)}2+ as model for Cr(III)-DNA adducts. Inorg. Chem., 37, 6560-6561.

(5) Ardon, M., and Bino, A. (1987) A new aspect of hydrolisis of metal ions: the hydrogen-oxide bridging ligand m-H3O2- .Struct. Bonding(Berlin) 65, 1-28.

(CAPES, CNPq)