Metal-promoted synthesis of amidines containing the model nucleobases 1-methylcytosine and 9-methyladenine

Abstract

The amidine complexes cis-[L₂PtNHC(R){1-MeCy(-2H)}]NO₃ (R = Me, 1a; Ph, 1b, Me₃C, 1c; Ph₂(H)C, 1d) and cis-[L₂PtNHC(R){9-MeAd(-2H)}]NO₃ (R = Me, 2a; Ph, 2b; Me₃C, 2c; Ph₂(H)C, 2d), are formed when cis-[L₂Pt(μ-OH)]₂(NO₃)₂ (L = PPh₃) reacts with 1-methylcytosine (1-MeCy) and 9-methyladenine (9-MeAd) in solution of MeCN, PhCN, Me₃CCN and Ph₂(H)CCN. Reaction of 1a,b and 2a,b with HCl affords the protonated amidines [NH₂C(R){1-MeCy(-H)}]NO₃ (R = Me, 3a; Ph, 3b) and [NH₂C(R){9-MeAd(-H)}]NO₃ (R = Me, 4a; Ph, 4b) and cis-(PPh₃)₂PtCl₂ in quantitative yield. Treatment of 3b and 4b with NaOH allows the isolation of the neutral benzimidamides NH₂-C(Ph){1-MeCy(-2H)} (5b) and NH₂-C(Ph){9-MeAd(-2H)} (6b). In the solid state 3b shows a planar structure with the hydrogen atom on N(4) cytosine position involved in a strong H-bond with the NO₃⁻ ion. Intermolecular H-bonds between the oxygen of the cytosine ring and one of the H atoms of the amidine-NH₂ group allow the dimerization of the molecule. A detailed analysis of the spectra of 3b in DMF-d₇ at −55 °C indicates the presence of an equilibrium between the species [NH₂C(R){1-MeCy(-H)}]NO₃ and [NH₂C(R){1-MeCy(-H)}]₂(NO₃)₂, exchanging with trace amounts of water at 25 °C. [¹⁵N,¹H]-HMBC experiments for 5b and 6b indicate that the amino tautomer H₂N-C(Ph){nucleobase(-2H)}, is the only detectable in solution and such structure has been confirmed in the solid state. The reaction of 5b and 6b with cis-L₂Pt(ONO₂)₂ (L = PPh₃), in chlorinated solvents, determines the immediate appearance of a pale yellow colour due to the coordination of the neutral amidine, likely in its imino form HNC(Ph){nucleobase(-H)}, to give the adducts cis-[L₂PtNHC(Ph){nucleobase(-H)}]²⁺. In fact, addition of “proton sponge” leads to the immediate deprotonation of the amidine ligand with formation of the starting complexes 1b and 2b.