Structural studies on copper(II) complex containing (Z)-2-(2-Aminothiazol-4-yl)- N-(2-hydroxyethyl)-2-(hydroxyimino) acetamide, A model compound for a cephalosporin antibiotic cefdinir

Abstract

(Z)-2-(2-aminothiazol-4-yl)-N-(2-hydroxyethyl)-2-(hydroxyimino)acetamide (HL) has been employed as a model compound for an orally active cephalosporin antibiotic, Cefdinir (CFDN). A binuclear copper(II) complex Cu 2L 4 (1) has been obtained from an aqueous solution containing CuCl 2 and HL, and its structure determined by means of X-ray crystallography: monoclinic, P2/c, a = 11.954(3) A ̊ , b = 10.661(3) A ̊ , c = 16.969(7) A ̊ , β = 108.13(3)°, Z = 2 . The molecular structure of 1 is a dimeric copper(II) complex consisting of two copper(II) complex units CuL 2, where the two molecules of L (L a and L b) are coordinated to the copper atom through their thiazole and oximate nitrogen atoms. L a is also coordinated to the copper atom of the other CuL 2 unit through the oximate oxygen atom, namely, the two complex units are bound to each other by forming a pair of oximate NO bridges of the two L a molecules. The coordination environment of the copper atom is a distorted trigonal bipyramid. In addition, based on the molecular structure, the coordination property of CFDN to copper(II) in water has been clarified by means of spectrophotometry and titration. Both copper(II) (0.05 mM)-HL (0.1 mM) and copper(II) (0.05 mM)-CFDN (0.1 mM) aqueous solutions gave broad d-d bands at ca. 720 nm above pH 6, corresponding to that observed in the diffuse reflectance spectrum of the crystals of 1. The results indicate that CFDN also forms a binuclear copper(II) complex, Cu 2(CFDN) 4, corresponding to Cu 2L 4 as the main species in water above pH 7. Stability constants are as follows: log β 110 = 11.12, log β 240 = 41.13 for copper(II)-L complexes. The theoretical species distribution diagram as a function of pH for the copper(II) (0.05 mM)-HL (0.1 mM) system shows that Cu 2L 4 exists as the main species above pH 7. The titration results are consistent with the spectral data. The spectral and titration studies indicate that not only HL, but also CFDN forms a binuclear complex Cu 2(CFDN) 4 corresponding to Cu 2L 4 as the main species under the neutral and basic conditions. In addition, the stability constants of CFDN complexes should be as large as those of CuL complexes, although titrations for copper(II)-CFDN systems could not be carried out due to precipitation of copper(II)-CFDN complexes.