Cu(II) complexes of cytosine and 1-methylcytosine with bromide: old motifs and new structures*
The preparation from aqueous solutions and crystal structures of the compounds trans-[Cu(1-Mecyt)2Br2] (1), trans-[Cu(cyt)2Br2] (2), [Cu(cyt)4]Br2·2H2O (3), and [Cu(cyt)Br3][Hcyt] (4) (where 1-Mecyt = 1-Methylcytosine and cyt = cytosine) is reported. Compounds 1 and 2 are made of trans-CuL2Br2 molecules [L = 1-Mecyt (1) or cyt (2)]. In 1, the coordination geometry around the central Cu(II) is distorted square pyramidal, while in 2 no such distortion is observed. Two solid-state polymorphs of 1 have been isolated, 1a and 1b, both of which contain no crystallization solvent and thus only differ in the relative disposition of vicinal [Cu(1-Mecyt)2Br2] units through space. As far as we know, 1 is the first example of a 1:2 Cu(II) complex with 1-Mecyt showing a trans arrangement of the ligands. The obtaining of 2 is also worth of note, as this species was previously reported as elusive. Compound 3 comprises tetra-cytosine-Cu(II) cations and bromides, these latter building, together with water molecules, a H-bonded tetrameric [Ow2Br2] motif. Compound 4 is constituted by [Cu(cyt)Br3]− anions and Hcyt+ cations, the cytosinium cations being in the outer metal coordination sphere. Likely, a pH-mediated equilibrium between 2 and 4 exists, 4 being only observable in traces as the neutral 1:2 complex with two metal–cyt direct bonds (2) is the thermodynamically favored product.
