Coordination Modes of 9‐Methyladenine in cis‐Platinum(II) Complexes with Dimethyl(phenyl)phosphanes as Ancillary Ligands − Synthesis and Characterization of cis‐[PtL2(9‐MeAd)2](NO3)2, cis‐[PtL2{9‐MeAd(−H)}]3(NO3)3, and cis‐[L2Pt{9‐MeAd(−H)}PtL2](NO3)3

Abstract

AbstractTreatment of 9‐methyladenine (9‐MeAd) with cis‐[PtL2(NO3)2] (1) (L = PMe2Ph) in a 2:1 molar ratio generated the bis(adduct) cis‐[PtL2(9‐MeAd)2](NO3)2 (2), which was isolated and fully characterized by multinuclear (1H, 31P, 13C, 195Pt and 15N) NMR analysis, which showed that the two nucleobases are selectively coordinated through the N1 atom. Small amounts of a mono(adduct) cis‐[PtL2(S)(9‐MeAd)]2+ (3) (S = solvent) and of a diplatinated species cis‐[L2Pt(S){9‐MeAd(−H)}PtL2]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 platinated at N1,N6,N7, with two cis‐L2Pt units bonded to nitrogen atom N1 and to nitrogen atoms N6 and N7, respectively. With increasing relative 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‐[L2Pt{9‐MeAd(−H)}]3(NO3)3 (5) (L = PMe2Ph), containing an NH2‐deprotonated nucleobase bridging the metal centers through the N1 and N6 atoms, is quantitatively formed when the dinuclear hydroxo complex cis‐[Pt(μ‐OH)L2]2(NO3)2 (6) reacts with 9‐MeAd in CH3CN solution. The isolated complex was fully characterized by multinuclear NMR spectroscopy and mass spectrometry. It appears to be stable in solution in CH3CN and chlorinated solvents, whereas in DMSO it partially converts into a new species, probably the dinuclear analog cis‐[PtL2{9‐MeAd(−H)}]2(NO3)2, in which the adenine maintains its coordination mode. At equilibrium the trinuclear/dinuclear species molar ratio is 20:1. Through the addition of a stoichiometric 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 adeninate ion, with N1 bonded to one platinum atom whereas N6 and N7 are chelated to a second one. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)