Structure and characterization of platinum(II) and platinum(IV) complexes with protonated nucleobase ligands

The reaction of H2[PtCl6] · 6H2O and (H3O)[PtCl5(H2O)] · 2(18C6) · 6H2O (18C6 = 18-crown-6) with 9-methylguanine (MeGua) proceeded with the protonation of MeGua forming 9-methylguaninium hexachloroplatinate(IV) dihydrate (MeGuaH)2[PtCl6] · 2H2O (1).The same compound was obtained from the reaction of Na2[PtCl6] with (MeGuaH)Cl.On the other hand, the reaction of guanosine (Guo) with (H3O)[PtCl5(H2O)] · 2(18C6) · 6H2O in methanol at 60 °C proceeded with the cleavage of the glycosidic linkage and with ligand substitution to give a guaninium complex of platinum(IV), [PtCl5(GuaH)] · 1.5(18C6) · H2O (2).Within several weeks in aqueous solution a slow reduction took place yielding the analogous guaninium platinum(II) complex, [PtCl3(GuaH)] · (18C6) · 2Me2CO (3).H2[PtCl6] · 6H2O and guanosine was found to react in water, yielding (GuoH)2[PtCl6] (4) and in ethanol at 50 °C, yielding [PtCl5(GuoH)] · 3H2O (5).Dissolution of complexes 2 and 5 in DMSO resulted in the substitution of the guaninium and guanosinium ligands, respectively, by DMSO forming [PtCl5(DMSO)]−.Reactions of 1-methylcytosine (MeCyt) and cytidine (Cyd) with H2[PtCl6] · 6H2O and(H3O)[PtCl5(H2O)] · 2(18C6) · 6H2O resulted in the formation of hexachloroplatinates with N3 protonated pyrimidine bases as cation (MeCytH)2[PtCl6] · 2H2O (6) and (CydH)2[PtCl6] (7), respectively. Identities of all complexes were confirmed by 1H, 13C and 195Pt NMR spectroscopic investigations, revealing coordination of GuoH+ in complex 5 through N7 whereas GuaH+ in complex 3 may be coordinated through N7 or through N9. Solid state structure of hexachloroplatinate 1 exhibited base pairing of the cations yielding (MeGuaH+)2, whereas in complex 6 non-base-paired MeCytH+ cations were found. In both complexes, a network of hydrogen bonds including the water molecules was found. X-ray diffraction analysis of complex 3 exhibited a guaninium ligand that is coordinated through N9 to platinum and protonated at N1, N3 and N7. In the crystal, these NH groups form hydrogen bonds N–H⋯O to oxygen atoms of crown ether molecules.