SYNTHESIS AND CHARACTERIZATION OF NICKEL(II) COMPLEXES OF PURINE AND PYRIMIDINE-BASES - CRYSTAL AND MOLECULAR-STRUCTURE OF TRANS-BIS(CYTOSINE-O2)BIS(ETHYLENEDIAMINE)NICKEL(II) BIS(TETRAPHENYLBORATE) - AN UNUSUAL METAL-BINDING MODE OF CYTOSINE

Three nucleobase complexes [Ni(en)2(Ade)]Cl2 (1), [Ni(en)2(Hyp)]Cl.0.5CH3OH.H2O (2) and [Ni(en)2(Cyt)2][B(C6H5)4]2 (3) (en = ethylenediamine; Ade = adenine; Hyp = hypoxanthine; Cyt = cytosine) have been prepared by reaction of Ni(en)2Cl2 and the corresponding purine or pyrimidine base in a methanol (1,2) or an ethanol-water (3) medium. The cytosine complex was obtained in the form of suitable crystals for X-ray crystallography. The complex crystallizes in the monoclinic space group P2(1)/a with a = 18.985 (4) angstrom, b = 9.685 (2) angstrom, c = 16.010 (4) angstrom, beta = 113.23 (3)degrees, V = 2705 (2), and Z = 2. The structure was refined by a full-matrix least-squares technique on the basis of 2090 observed reflections (I greater-than-or-equal-to 2.5-sigma(I)) to a final R value of 0.061. Contrary to what could be expected, both unsubstituted cytosine bases are bound in the trans position exclusively via O(2) to the nickel atom. The octahedral coordination of Ni(II) is completed by four nitrogen atoms of two ethylenediamine molecules. Metal binding sites of the nucleobases in the three complexes are discussed in terms of their IR and electronic spectra and magnetic properties. Thermogravimetric data are in agreement with the stoichiometry and formulas proposed.