Crystal structure of [{Cu(tren)}₂(cytosinato)]·(ClO₄)₃·0.5H₂O (tren = tris(2-aminoethyl)amine) and an overview on intra-molecular interligand interactions affecting metal binding sites of cytosine

Treatment of cytosine with tris(2-aminoethyl)amine (tren) and Cu(ClO₄)₂·6H₂O under alkaline conditions gave a ternary tren–Cu²⁺–cytosine complex, [{Cu(tren)}₂(cytosinato)]·(ClO₄)₃·0.5H₂O, whose crystal structure was determined by X-ray diffraction. In the structure of the [{Cu(tren)}₂(cytosinato)]³⁺cation, two tren-capped square-pyramidal Cu²⁺ions bind to a cytosinate anion, one through the deprotonated N(1) of cytosine with the formation of an intra-molecular interligand hydrogen bond between the keto substituent O(2) of the base and the amino group of tren and the other through both the ring nitrogen N(3) and the exocyclic O(2) of the base, forming a four-membered chelate-ring. An overview on metal bonding properties of N(1)-unsubstituted cytosine observed in so far reported crystal structures is given, emphasizing the significance of intra-molecular interligand interaction as a factor that affects metal-binding sites of cytosine.