7-Methylguanine: protonation, formation of linkage isomers with trans-(NH3)2PtII, and base pairing properties

Three protonated forms of 7-methylguanine (7-MeGH, 1) with different counter ions, [7-MeGH2]X (X = NO3, 1a; ClO4, 1b; BF4, 1c) and two PtII complexes, trans-[Pt(NH3)2(7-MeGH-N9)2](ClO4)2 (4) and trans-[Pt(NH3)2(7-MeGH-N9)(7-MeGH-N3)](ClO4)2·3H2O (5) are described and their X-ray crystal structures are reported. 1a–1c form infinite ribbons via pairs of intermolecular hydrogen bonds between N1H⋯O6 and N3⋯N2H2 sites, with anions connecting individual ribbons, thereby generating extended sheets. 4 and 5 do not display unusual features, except that 5 represents a rare case of a bis(nucleobase) complex of PtII in which linkage isomers occur. Unlike in a previously reported compound, [Pt(dien)(7-MeGH-N9)](NO3)(ClO4), the Pt coordination planes and the 7-MeGH planes are not coplanar in 4 and 5. The hydrogen bonding behaviour of 7-MeGH, free and when platinated at N9 (complex 4), was studied in Me2SO-d6. It revealed the following: (i) there is no detectable self-association of 1 in Me2SO solution. (ii) 1 and 1-methylcytosine (1-MeC) form Watson–Crick pairs. (iii) 4 does not self-associate. (iv) 4 associates with 1-MeC in the Watson–Crick fashion. (v) 4 and 1 interact in solution, but no model can be proposed at present. (vi) Remarkable interaction shifts between 4 and 1 occur when NH3 is liberated from trans-(NH3)2PtII to give NH4+ in Me2SO-d6. Feasible models, which imply the presence of deprotonated 7-MeG− species are proposed. Finally, DFT calculations were carried out to qualitatively estimate the effect of 7-MeGH acidity in [Pt(dien)(7-MeGH-N9)]2+ in dependence of the dihedral angle between the Pt coordination plane and the nucleobase.