Platinum(II)-Mediated Coupling Reactions of Acetonitrile with the Exocyclic Nitrogen of 9-Methyladenine and 1-Methylcytosine. Synthesis, NMR Characterization, and X-ray Structures of New Azametallacycle Complexes

Abstract

The hydroxo complex cis-[L2Pt(μ-OH)]2(NO3)2, (L = PMePh2, 1a), in CH3CN solution, deprotonates the NH2 group of 9-methyladenine (9-MeAd) to give the cyclic trinuclear species cis-[L2Pt{9-MeAd(−H)}]3(NO3)3, (L = PMePh2, 2a), in which the nucleobase binds the metal centers through the N(1), N(6) atoms. In solution at room temperature, 2a slowly reacts with the solvent to form quantitatively the mononuclear azametallacycle cis-[L2PtNHC(Me){9-MeAd(−2H)}]NO3 (L = PMePh2, 3a), containing as anionic ligand the deprotonated form of molecule N-(9-methyl-1,9-dihydro-purin-6-ylidene)-acetamidine. In the same experimental conditions, the hydroxo complex with PPh3 (1b) forms immediately the insertion product 3b. Single-crystal X-ray analyses of 3a and 3b show the coordination of the platinum cation at the N(1) site of the purine moiety and to the N atom of the inserted acetonitrile, whereas the exocyclic amino nitrogen binds the carbon atom of the same CN group. The resulting six-membered ring is slightly distorted from planarity, with carbon−nitrogen bond distances for the inserted nitrile typical of a double bond [C(3)−N(2) = 1.292(7) Å in 3a and 1.279(11) Å in 3b], while the remaining CN bonds of the metallocycle are in the range of 1.335(8)−1.397(10) Å. A detailed multinuclear 1H, 31P, 13C, and 15N NMR study shows that the nitrogen atom of the inserted acetonitrile molecule binds a proton suggesting for 3a,b an imino structure in solution. In DMSO and chlorinated solvents, 3a slowly releases the nitrile reforming the trinuclear species 2a, whereas 3b forms the mononuclear derivative cis-[L2Pt{9-MeAd(−H)}]NO3 (L = PPh3, 4b), in which the adeninate ion chelates the metal center through the N(6) and N(7) atoms. Complex 4b is quantitatively obtained when 1b reacts with 9-MeAd in DMSO and can be easily isolated if the reaction is carried out in CH2Cl2. In CH3CN solution, at room temperature, 4b slowly converts into 3b indicating that the insertion of acetonitrile is a reversible process. A similar metal-mediated coupling reaction occurs when 1a,b react with 1-methylcytosine (1-MeCy) in CH3CN. The resulting complexes, cis-[L2PtNHC(Me){1-MeCy(−2H)}]NO3, (L = PMePh2, 5a and PPh3, 5b), contain the deprotonated form of the ligand N-(1-methyl-2-oxo-2,3-dihydro-1H-pyrimidin-4-ylidene)-acetamidine. The X-ray analysis of 5a shows the coordination of the metal at the N(3) site of the pyrimidine cycle and to the nitrogen atom of the acetonitrile, with features of the six-membered metallocycle only slightly different from those found in 3a and 3b. In CD3CN/CH313CN solution complexes 5a,b undergo exchange of the inserted nitrile, while in DMSO or chlorinated solvents they irreversibly release CH3CN to form species not yet fully characterized. No insertion of CH3CN occurs when the hydroxo complexes are stabilized by PMe3 and PMe2Ph