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 quanti tatively 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. T he crystals, stable only in the presence of the solvent, have the foll owing 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)deg rees, 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 d erivative (2B), cis-[(PMe3)2Pt(1-MeCy(-H))]3(ClO4)3.2H2O, whose struct ure was refined to R = 4.4 and R(w) =4.9. The colorless prisms formed from aqueous solutions have the following crystallographic data: tricl inic 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 mole cular structure of the cationic complex contains three cis-(PMe3)2Pt u nits 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 pla tinum 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 an d 2A were also characterized by N-15 NMR spectroscopy, at natural abun dance. The spectroscopic data, obtained in DMSO-d6 solution, are compa red with those of the derivative cis-[(PMe3)2Pt(1-MeCY)2](NO3)2 contai ning 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 mono nuclear complex.