Metalated Nucleobase Quartets: Dimerization of a Metal-Modified Guanine, Cytosine Pair of <i>trans</i>-(NH₃)₂Pt^II and Formation of CH···N Hydrogen Bonds

The X-ray crystal structures of two complexes of the composition trans-[Pt(NH3)2(9-EtG-N7)(1-MeC-N3‘)]X·nH2O (1) with 9-EtG = 9-ethylguaninate and 1-MeC = 1-methylcytosine are reported. 1b (X = picrate, n = 1) crystallizes to produce a dimetalated base quartet, held together by H-bonding interactions between pairs of cations. This feature essentially corresponds to the solution structure previously proposed by us on the basis of 1H NMR and ESI-MS, with a H-bonding interaction between the aromatic H5‘ proton of 1-MeC and the deprotonated N1 position of 9-EtG. 1c (X = trifluoromethanesulfonate, n = 0) crystallizes in a radically different fashion as a consequence of nucleobase rotation about the Pt−N bond, leading to a reversed Hoogsteen arrangement without any intracomplex H bonding between the two bases. In the solid-state structure of 1b short intermolecular H bonds exist between the exocyclic NH2 group of 1-MeC and O6 of 9-EtG (2.715(7) Å). Considerably longer intra- (N4‘(1-MeC)···O6(9-EtG), 3.229(6) Å) and intermolecular (C5‘(1-MeC)···N1(9-EtG), 3.548(7) Å) H bonds are primarily a consequence of considerable base twisting, presumably caused by stacking between the guanine residues and the picrate anions. In DMSO-d6 solution, the cyclic base quartet structure is favored, regardless of the nature of the anion X (X = picrate, trifluoromethanesulfonate, perchlorate, nitrate). An association constant KD = 44.1 ± 3.2 M-1 for the dimerization has been determined.