Coordination Modes of 9-Methyladenine in cis-Platinum(II) Complexes with Dimethyl(phenyl)phosphanes as Ancillary Ligands - Synthesis and Characterization of cis-[PtL_2(9-MeAd)_2](NO_3)_2, cis-[PtL_2{9-MeAd(-H)}]_3(NO_3)_3, and cis-[L_2Pt{9-MeAd(-H)}P

摘要

Treatment of 9-methyladenine (9-MeAd) with cis-[PtL_2(NO_3)_2] (1) (L=PMe_2Ph) in a 2:1 molar ratio generated the bis(adduct) cis-[PtL_2(9-MeAd)_2](NO_3)_2 (2), which was isolated and fully characterized by multinuclear (~1H, ~31P, ~13C, ~195Pt and ~15N) NMR analysis, which showed that the two nu-cleobases are selectively coordinated through the N1 atom. Small amounts of mono(adduct) cis-[PtL_2(S)(9-MeAd)]~(2+) (3) (S=solvent) and of a diplatinated species cis-(L_2Pt(S){9-MeAd(-H)}PtL_2]~(3+) (4) are formed in DMSO solution when 9-MeAd is present in smaller quantities than 1. Complex 3 is platinated at N1, with a solvent molecule representing the fourth ligand around the metal center. Complex 4 contains an adenine molecule deprotonated and p9latinated at N1, N6, N7, with two cis-L_2Pt units bonded to nitrogen atom N1 and to nitrogen atoms N6 and N7, respectively. With increasing rel-ative concentration of the nucleobase, both complexes 3 and 4 progressively convert into the bis(adduct) 2, the only species detectable in solution when the Ad/Pt molar ratio is 2:1. The trinuclear compound cis-[L_2Pt{9-MeAd(-H)}]_3(NO_3)_3 (5) (l=pmE_2Ph), containing an NH_2-deprotonated nucleobase bridging the metal centers through the N1 and N6 atoms, is quantitatively formed when the dinuclear hydroxo complex cis-[Pt(mu-OH)L_2]_2(NO_3)_2 (6) reacts with 9-MeAd in CH_3CN solution. The isolated complex was fully characterized by multinuclear NMR spectroscopy and mass spectrometry. It appears to be stable in solution in CH_3CN and chlorinated solvents, whereas in DMSO it partially converts into a new species, probably the dinuclear analog cis-[PtL_2(9-MeAd(-H)}]_2(NO_3)_2, in which the adenine maintains its coor-dination mode. At equilibrium the trinuclear/dinuclear species molar ratio is 20:1. Through the addition of a stoichi-ometric amount of nitrate 1 to a DMSO solution of 5 we were able to generate the diplatinated compound 4 in high yield. Complex 4 displays a new coordination mode for the adenin-ate ion, with N1 bonded to one platinum atom whereas N6 and N7 are chelated to a second one.