CRYSTALLOGRAPHIC EVIDENCE OF PREFERRED N(9)-COORDINATION OF XANTHINE - STRUCTURES OF CU(II)(XAN)2(NO3)2.2H2O, CU(II)(XAN)2CL2.2H2O, AND ZN(II)(XAN)2CL2 (XAN = XANTHINE)

The reaction of the oxopurine base xanthine (xan) with the respective metal chlorides has resulted in the formation of compounds of the composition Cu(xan)2Cl2.2H2O and Zn(xan)2Cl2. Cu(xan)2(NO3)2.2H2O has been prepared from aqueous solution using an excess of Cu(NO3)2.3H2O. Crystal data: Cu(xan)2(NO3)2.2H2O, a = 7.111 (2) angstrom, b = 7.528 (1) angstrom, c = 8.888 (1) angstrom, alpha = 108.63 (1)-degrees, beta = 98.29 (2)-degrees, gamma = 102.39 (2)-degrees, V = 428.6 (3) angstrom3, Z = 1, space group P1BAR; Cu(xan)2Cl2.2H2O, a = 3.647 (5) angstrom, b = 9.849 (2) angstrom, c = 12.763 (4) angstrom, alpha = 72.41 (2)-degrees, beta = 81.07 (6)-degrees, gamma = 82.93 (6)-degrees, V = 430.3 (7) angstrom3, Z = 1, Space group P1BAR; Zn(xan)2Cl2, a = 14.090 (7) angstrom, b = 5.456 (3) angstrom, c = 20.397 (16) angstrom, beta = 105.82 (5)-degrees, V = 1509 (3) angstrom3, Z = 4, space group C2/c. The structures of all three compounds were refined to final R-values of 0.036, 0.051, and 0.056, respectively. In Cu(xan)2(NO3)2.2H2O, the copper ion is situated in the center of a 4 + 2 elongated octahedron including two N(9)-coordinating monodentate xanthine ligands, two water molecules in the equatorial plane, and two nitrate groups in the apical positions. The monomeric structure is stabilized by intramolecular bifurcated hydrogen bonding contacts of the type N-H...O forming an indirect chelate complex. The copper complex Cu(xan)2Cl2.2H2O exhibits a square planar centrosymmetric coordination of copper by two chloride ions and two xanthines monodentately binding through N(9). The two water molecules are located at disordered positions. In the monomeric zinc complex Zn(xan)2Cl2, the metal ion is situated in the center of a markedly distorted ZnN2Cl2 tetrahedron including two N(9)-coordinating xanthine ligands and two chloride ions. An indirect chelate complex is formed by an intramolecular hydrogen bond of the type N-H...Cl. A review of all structure types of metal xanthine complexes established by X-ray crystallography is given. The crystallographic evidence of preferred N(9)-coordination of neutral xanthine presented here supports those structural models for the oxidation of xanthine catalyzed by xanthine oxidase that suggest intermediate coordination of the molybdenum center of the enzyme through N(9) and not through N(7) of xanthine.