Interligand interactions affecting specific metal bonding to nucleic acid bases: the tetrakis(μ-trifluoroacetamidato)dirhodium(II)–cytosine system. Crystal structures of [Rh2(CF3CONH)4(cytosine)] and [Rh2(CF3CONH)4(1-methylcytosine)2]·2H2O

The reaction of tetrakis(μ-trifluoroacetamidato)dirhodium(II) with cytosine and with 1-methylcytosine in methanolic aqueous solution gave [Rh2(CF3CONH)4(cytosine)] (1) and [Rh2(CF3CONH)4(1-methylcytosine)2]·2H2O (2), respectively, structures of which have been determined by X-ray diffraction. In the cytosine complex 1, a cytosine molecule bridges between the axial positions of the different [Rh2(CF3CONH)4] nuclei, each of which has a crystallographic centre of symmetry at the midpoint of the Rh–Rh bond, through the ring nitrogen N(3) and the exocyclic keto substituent O(2), thereby producing a one-dimensional polymeric adduct. In the 1-methylcytosine complex 2, two 1-methylcytosine molecules occupy the axial positions of the [Rh2(CF3CONH)4] nucleus through N(3), forming a discrete structure. In 1 and 2, the Rh–N(3) axial bonding is accompanied by the formation of two intramolecular hydrogen bonds, one between N(4) of the base and the amidato oxygen and the other between O(2) of the base and the amidato NH. The importance of interligand interactions, associated with the steric metal environment, as factor determining metal binding site on cytosine is emphasised.