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.

Graphical abstractReactions of guanine-type nucleobases (9-methylguanine, guanosine) with H2[PtCl6] · 6H2O and (H3O)[PtCl5(H2O)] · 2(18C6) · 6H2O (I) may result (i) in the protonation of the nucleobase only, (ii) in ligand substitution with complex formation of the protonated nucleobase, or (iii) in the cleavage of the glycosidic bond of the nucleoside and in complex formation of the protonated nucleobase formed.As an example the formation of a guaninium PtIV complex starting from guanosine is shown and its reduction yielding the requisite PtII complex exhibiting an unprecedented κN9 coordinated guaninium ligand.Figure optionsDownload full-size imageDownload as PowerPoint slide