Complexes of Platinum(II) Containing Neutral and Deprotonated 9-Methyladenine. Synthesis, X-ray Structures, and NMR Studies on the Cyclic Trimer cis-[L2Pt{9-MeAd(−H)}]3(NO3)3 and the Dinuclear cis-[L2Pt(ONO2){9-MeAd(−H)}PtL2](NO3)2 (L = PMePh2)

The dinuclear hydroxo complex cis-[L2Pt(í-OH)]2(NO3)2 (L ) PMePh2, 1), in CH2Cl2, CH3CN, or DMF solution,deprotonates the NH2 group of 9-methyladenine (9-MeAd) to give the complex cis-[L2Pt{9-MeAd(-H)}]3(NO3)3, 2, which was isolated in good yield. The X-ray structure shows that the nucleobase binds symmetrically the metal centers through the N(1),N(6) atoms forming a cyclic trimer with Pt---Pt distances in the range 5.202(1)-5.382(1) Å. Dissolution of 2 in DMSO or DMF determines the partial (or total) dissociation of the cyclic structure to form several fragments. A multinuclear NMR analysis of the resulting mixture supports the presence of the mononuclear species cis-[L2Pt{9-MeAd(-H)}]+, 3, in which the deprotonated nucleobase chelates the metal center with the N(6),N(7) atoms. Addition of a stoichiometric amount of the nitrato complex cis-[L2Pt(ONO2)2] (L ) PMePh2, 4) to a DMSO or DMF solution of 2 affords quantitatively the diplatinated compound cis-[L2Pt(ONO2){9-MeAd(-H)}-PtL2](NO3)2, 5. The single-crystal X-ray analysis shows that the adenine behaves as a tridentate ligand bridgingtwo cis-L2Pt units at the N(1) and N(6),N(7) sites, respectively [Pt(1)-N(1) ) 2.109(5) Å, Pt(2)-N(6) ) 2.095(7) Å, Pt(2)-N(7) ) 2.126(7) Å]. The N(1)-bonded metal center completes the coordination sphere through an oxygen atom of a nitrate group, and its coordination plane is arranged orthogonally with respect the second one. The Pt-O distance [2.109(5) Å] is similar to those found in the nitrato complex 4 [2.110 Å, average]. The related complex cis-[{L2Pt(ONO2)}2(9-MeAd)](NO3)2, 6, containing the neutral adenine platinated at the N(1),N(7) atoms, was isolated and its stability in solution investigated by NMR spectroscopy. In DMSO, 6 undergoes decomposition forming a mixture of the species 4, 5, and the adenine mono- and bis-adducts cis-[L2Pt(9-MeAd)(DMSO)]2+, 7, and cis-[L2-Pt(9-MeAd)2]2+, 8, respectively. This last complex, quantitatively formed upon addition of 9-MeAd (Pt/adenine (1:2) to the mixture, was also isolated and characterized.