cis-Bis(phosphine)platinum(II) complexes with pyrimidyl nucleobases. Synthesis, characterization, and crystal structures of cis-(1-methylthyminato-N3)(N,N-dimethylformamide-O)(1,1'-bis(diphenylphosphino)ferrocene)platinum(II) tetrafluoroborate-dichloromethane, [(dppf)Pt(1-MeTy(-H))(DMF)]BF4.cntdot.CH2Cl2, and cis-(1-methylthyminato-N3)(1-methylcytosine-N3)(1,1'-bis(diphenylphosphino)ferrocene)platinum(II) tetrafluoroborate, [(dppf)Pt(1-MeTy(-H))(1-MeCy)]BF4

The dinuclear complex [(dppf)Pt(mu-OH)]2(BF4)2, where dppf is 1,1'-bis(diphenylphosphino)ferrocene, reacts with 1-methylthymine (1-MeTy), in dimethylformamide, dimethyl sulfoxide, or acetonitrile, to give the mononuclear complex [(dppf)Pt(1-MeTy(-H))(S)]+. The dimethylformamide adduct (S = DMF), [(dppf)Pt(1-MeTy(-H))(DMF)]BF4.CH2Cl2 (1), has been characterized by single-crystal X-ray analysis. The complex crystallizes in the orthorhombic system, space group P2(1)2(1)2(1), with a = 13.492 (3) angstrom, b = 14.063 (3) angstrom, c = 23.906 (4) angstrom, and Z = 4. The structure was solved by heavy-atom methods and refined by least-squares techniques to R = 0.078 for 2949 unique data (I > 3-sigma(I)). In the cationic unit, the ligand geometry around Pt is distorted square planar, the chelating bis(phosphine) dppf, the N(3)-bonded 1-methylthyminate and the O-bonded DMF ligands defining the coordination sphere of the metal ion. The 1-MeTy(-H) ring forms an angle of 104.8-degrees with the ligand square plane, while the DMF mean plane is at 73.0-degrees to the same plane. Addition of 1-methylcytosine (1-MeCy) to 1 affords the corresponding adduca [(dppf)Pt(1-MeTy(-H))(1-MeCy)]+ (2) as a mixture of two isomeric forms (2a,b). In solution at room temperature 2a extensively converts in 2b (90% in DMF) in several hours. Crystals of 2 are obtained from a chloroform solution of the two isomers and have been characterized by single-crystal X-ray analysis. They crystallize in the monoclinic system, space group C2/c, with a = 17.821 (5) angstrom, b = 21.718 (7) angstrom, c = 13.814 (3) angstrom, beta = 113.6 (2)-degrees, and Z = 4. The structure was solved by heavy-atom methods and refined by least-squares techniques to R = 0.060 for 2657 unique data (I > 3-sigma(I)). In the cationic complex [(dppf)Pt(1-MeTy(-H))(1-MeCy)]+ the platinum atom is coordinated by the chelated bis(phosphine), by the deprotonated 1-MeTy, and by the neutral 1-MeCy ligands. In 2 both the nucleobases are platinated at the N(3) site with their rings being perpendicular to the PtP2 plane. The structures in solution of the two isomers 2a and 2b are discussed on the basis of their H-1 and P-31 NMR spectra. Isomer 2b contains the neutral cytosine ligand coordinated to the platinum through the monodeprotonated exocyclic amino group, with the proton switched to the N(3) position. The conversion of 2a into 2b is consistent with the migration of the platinum from the N(3) to N(4) site of 1-MeCy indicating that the thermodynamically favored adduct of this neutral ligand is its imino oxo tautomeric form. In addition, the characterization of the species [(dppf)Pt(1-MeCy(-H))]+, obtained from [(dppf)Pt(mu-OH)]2(BF4)2 and 1-MeCy in acetonitrile, is described. On the basis of H-1 and P-31 NMR spectra, the complex appears to contain the deprotonated 1-MeCy chelated to the (dppf)Pt2+ moiety through its N(3) and N(4) donor atoms.