MONONUCLEAR AND DINUCLEAR PALLADIUM(II) COMPLEXES OF URACIL AND THYMINE MODEL NUCLEOBASES AND THE X-RAY STRUCTURE OF [(BPY)PD(1-MET)2PD(BPY)](NO3)2.5.5H2O (HEAD HEAD)

Several mono- and dinuclear complexes of Pd(II) with the model nucleobase anions of 1-methyluracil (1-MeU) and 1-methyl-thymine (1-MeT) have been prepared and isolated: (en)Pd(l-MeT)2-4H2O (la), (en)Pd(l-MeU)2-5H2O (lb), (bpy)Pd(l-MeT)2-4.5H2O (2a), (bpy)Pd(l-MeU)2-4.5H2O (2b), (phen)Pd(l-MeT)2-6.5H2O (3), trans-(NH3)2Pd(l-MeT)2(4a), trans-(NH3)2Pd(l-MeU)2(4b), [(bpy)Pd(l-MeT)2Pd(bpy)](NO3)2-5.5H2O (5a), [(bpy)Pd(l-MeT)2Pd(bpy)](C1O4)2·4H2O (5b), [(bpy)Pd(l-MeT)(H2O)]C1O4-H2O (6), and [(phen)Pd(l-MeT)2Pd(phen)](NO3)2·4.5H2O (7). IR, Raman, and1H NMR spectroscopic data and the crystal structure of 5a are reported: triclinic space group PI, a = 13.085 (4) Å, b = 14.901 (4) Å, c = 12.600 (3) Å, ± = 112.61 (2)°, ² = 113.47 (2)°, γ = 64.33 (2)°, V= 1963 (2) Å3, Z = 2, R = 0.034, and Rw= 0.037 for 4854 independent reflections with F02≫ 2σ (F02) and 547 parameters. The two 1-MeT ligands coordinate to Pd through N(3) and O(4) and are arranged head-to-head, with an intramolecular Pd Pd separation of 2.848 (1) Å. 5a was obtained both by reaction of 2a with [(bpy)Pd(H2O)2](NO3)2and by simple acidification (HNO3, [Pd(H2O)4](NO3)2) of 2a. According to1H NMR spectroscopy, 5a exists in solution (D2O or D2O/(CD3)2CO) in an equilibrium of several species, two of which dominate and are proposed to be two dinuclear species in slow equilibrium. A mononuclear species (6) converts quantitatively to the dinuclear head-head complex 5b, which exists in solution in the same equilibrium as 5a. Attempts to oxidize 5a or 5b with either Ce(IV), concentrated HNO3, or Cl2to a mixed-valence dinuclear species or a dipalladium(III) species failed. © 1990, American Chemical Society. All rights reserved.