title: Reactivity of ammonia ligands of the antitumor agent cisplatin: a unique dodecanuclear Pt4,Pd4,Ag4 platform for four cytosine model nucleobases.
abstract: The reaction of a potential mono(nucleobase) model adduct of cisplatin, cis-[Pt(NH(3))(2)(1-MeC-N3)(H(2)O)](2+) (6; 1-MeC: 1-methylcytosine), with the electrophile [Pd(en)(H(2)O)(2)](2+) (en: ethylenediamine) at pH approximately 6 yields a kinetic product X which is likely to be a dinuclear Pt,Pd complex containing 1-MeC(-)-N3,N4 and OH bridges, namely cis-[Pt(NH(3))(2)(1-MeC(-)-N3,N4)(OH)Pd(en)](2+). Upon addition of excess Ag(+) ions, conversion takes place to form a thermodynamic product, which, according to (1)H NMR spectroscopy and X-ray crystallography, is dominated by a mu-NH(2) bridge between the Pt(II) and Pd(II) centers. X-ray crystallography reveals that the compound crystallizes out of solution as a dodecanuclear complex containing four Pt(II), four Pd(II), and four Ag(+) entities: [{Pt(2)(1-MeC(-)-N3,N4)(2)(NH(3))(2)(NH(2))(2)(OH)Pd(2)(en)(2)Ag}(2){Ag(H(2)O)}(2)](NO(3))(10) 6 H(2)O (10) is composed of a roughly planar array of the 12 metal ions, in which the metal ions are interconnected by mu-NH(2) groups (between Pt and Pd centers), mu-OH groups (between pairs of Pt atoms), and metal-metal donor bonds (Pt-->Ag, Pd-->Ag). The four 1-methylcytosinato ligands, which are stacked pairwise, as well as the four NH(3) ligands and parts of the en rings, are approximately perpendicular to the metal plane. Two of the four Ag ions (Ag2, Ag2') of 10 are labile in solution and show the expected behavior of Ag(+) ions in water, that is, they are readily precipitated as AgCl by Cl(-) ions. The resulting pentanuclear complex [Pt(2)Pd(2)Ag(1-MeC(-))(2)(NH(2))(2)(OH)(NH(3))(2)(en)(2)](NO(3))(4)7 H(2)O (11) largely maintains the structural features of one half of 10. The other two Ag(+) ions (Ag1, Ag1') of 10 are remarkably unreactive toward excess NaCl. In fact, the pentanuclear complex [Pt(2)Pd(2)AgCl(1-MeC(-))(2)(NH(2))(2)(OH)(NH(3))(2)(en)(2)](NO(3))(3)4.5 H(2)O (12), obtained from 10 with excess NaCl, displays a Cl(-) anion bound to the Ag center (2.459(3) A) and is thus a rare case of a crystallized "AgCl molecule".
authors.affiliation: Fakultät Chemie, Technische Universität Dortmund, Otto-Hahn-Strasse 6, Dortmund, Germany.
authors.forename: Gunnar
authors.initials: G
authors.lastname: Kampf
authors.forename: Pablo J
authors.initials: PJ
authors.lastname: Sanz Miguel
authors.forename: Marta
authors.initials: M
authors.lastname: Morell Cerdà
authors.forename: Michael
authors.initials: M
authors.lastname: Willermann
authors.forename: Alexandra
authors.initials: A
authors.lastname: Schneider
authors.forename: Bernhard
authors.initials: B
authors.lastname: Lippert
mesh_terms: D000641:Ammonia / Q000737:chemistry*; D000970:Antineoplastic Agents / Q000737:chemistry*; D002945:Cisplatin / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003596:Cytosine / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D010165:Palladium / Q000737:chemistry*; D010984:Platinum / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry; D012834:Silver / Q000737:chemistry*
chemical_list: D000970:Antineoplastic Agents; D008024:Ligands; D011725:Pyridines; D012834:Silver; D010984:Platinum; D010165:Palladium; D000641:Ammonia; D003596:Cytosine; C023666:pyridine; D002945:Cisplatin