Structural Characterization of a Trinuclear Complex of Platinum(II) Containing Bridging 1-Methylcytosine Ligands. A Combined X-ray and Multinuclear NMR Study

The dinuclear complexes cis-[(PMe3)2Pt(1-MeCy(-H))]2X2 (X = NO3- (1A), ClO4- (1B)), containing NH2-deprotonated 1-methylcytosine (1-MeCy(-H)) ligands, in aqueous or DMSO solution, at 80-degrees-C convert quantitatively into the corresponding trinuclear derivatives cis-[(PMe3)2Pt(1-MeCy(-H))]3X3(2A,B) which have been characterized in the solid state by single-crystal X-ray analysis and in solution by multinuclear (H-1, P-31, Pt-195, C-13, and N-15) NMR spectroscopy. The nitrate salt (2A) crystallizes from ethanol in the triclinic system, space group Pl. The crystals, stable only in the presence of the solvent, have the following cell dimensions: a = 13.542(5) angstrom, b = 17.352(5) angstrom, c = 20.721(6) angstrom, alpha = 113.13(2)degrees, beta = 104.30(3)degrees, gamma = 95.77(3)degrees, Z = 2. The structure, formulated as cis-[(PMe3)2Pt(1-MeCy(-H))]3(NO3)3.EtOH.H2O, was refined to R = 6.3 and R(w) = 6.3. Better structural data were obtained with the perchlorate derivative (2B), cis-[(PMe3)2Pt(1-MeCy(-H))]3(ClO4)3.2H2O, whose structure was refined to R = 4.4 and R(w) =4.9. The colorless prisms formed from aqueous solutions have the following crystallographic data: triclinic system, space group P1BAR, a = 14.634(7) angstrom, b = 14.791(9) angstrom, c = 14.980(9) angstrom, alpha = 101.93(5)degrees, beta = 103.46(5)degrees, gamma = 90.50(4)degrees, Z = 2. In both salts, the molecular structure of the cationic complex contains three cis-(PMe3)2Pt units linked to three 1-methylcytosine anions through the N(3) and N(4) atoms. The resulting 12-membered ring contains the pyrimidinic rings almost perpendicular to the plane defined by the metal atoms. Each platinum has a square-planar coordination with a Pt...Pt distance of 5.2A. The H-1, P-31, and C-13 NMR spectra indicate that the approximate S3 symmetry of the cation is maintained in solution. The compounds 1A and 2A were also characterized by N-15 NMR spectroscopy, at natural abundance. The spectroscopic data, obtained in DMSO-d6 solution, are compared with those of the derivative cis-[(PMe3)2Pt(1-MeCY)2](NO3)2 containing as ligands the neutral N(3)-bonded nucleobase. The dinuclear and trinuclear complexes exhibit very similar N-15 chemical shifts and 2J(PN) coupling constants but remarkably different from those of the mononuclear complex.