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

The reaction of H-2[PtCl6]center dot 6H(2)O and (H3O)[PtCl5(H2O)](2)center dot(18C6)center dot 6H(2)O (18C6 = 18-crown-6) with 9-methylguanine (MeGua) proceeded with the protonation of MeGua forming 9-methylguaninium hexachloroplatinate(IV) dihydrate (MeGuaH)(2)[PtCl6]center dot 2H(2)O (1). The same compound was obtained from the reaction of Na-2[PtCl6] with (MeGuaH)Cl. On the other hand, the reaction of guanosine (Guo) with (H3O)[PtCl5(H2O)](2)(18C6)center dot 6H(2)O in methanol at 60 degrees C proceeded with the cleavage of the glycosidic linkage and with ligand substitution to give a guaninium complex of platinum(IV), [PtCl5(GuaH)]center dot 1.5(18C6)center dot H2O (2). Within several weeks in aqueous solution a slow reduction took place yielding the analogous guaninium platinum(II) complex, [PtCl3(GuaH)]center dot(18C6)center dot 2Me(2)CO (3). H-2[PtCl6]center dot 6H(2)O and guanosine was found to react in water, yielding (GuoH)(2)[PtCl6] (4) and in ethanol at 50 degrees C, yielding [PtCl5(GuoH)]center dot 3H(2)O (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 H-2[PtCl6]center dot 6H(2)O and (H3O)[PtCl5(H2O)]center dot 2(18C6)center dot 6H(2)O resulted in the formation of hexachloroplatinates with N3 protonated pyrimidine bases as cation (MeCytH)(2)[PtCl6]center dot 2H(2)O (6) and (CydH)(2)[PtCl6] (7), respectively. Identities of all complexes were confirmed by H-1, C-13 and Pt-195 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 center dot center dot center dot O to oxygen atoms of crown ether molecules. (C) 2007 Elsevier B.V. All rights reserved.