Nucleobase rotation about bonds with transition metal ions has been intensively studied for purine complexes of d s-a2Pt(II) (with a = N H 3 or amine) [1][2][3][4][5][6][7][8][9]. Results obtained from these studies are considered meaningful with regard to the second step (chelate closure) of reactions of the antitum or agent d s-a2PtCl2 with DNA. Favorable or unfa vorable interactions between the am(m)ine li gands at the platinum atom and exocyclic groups of the nucleobases in the vicinity of the metal co ordination site are crucial as far as rotational b ar riers are concerned [5,7,8]. Although this aspect has been studied in detail for /V7-platinated p u rines only, it is of even greater significance to N3-platinated pyrimidine nucleobases. Qualitatively, we have dem onstrated in many instances that binding of a second metal ion to a TVJ-platinated pyrimidine base (uracil, thymine, cytosine) has a We have now applied the very same principle to a related system, namely to fra«s-[a2Pt(l-M eC -N3)2]2+ (1-MeC = 1-methylcytosine) for the following reasons: First, the aspect of nucleobase rotation has hardly been studied in complexes of trans-a2Pt(II) [13]; second, we have recently been able to characterize a series of heteronuclear com plexes of this compound with the two (deproto nated) nucleobases arranged head-head [14][15][16]; third, we found that removal of the heterom etal by a suitable nucleophile can be sufficiently fast, and subsequent ligand rotation sufficiently slow, to permit crystallization of the minor rotam er (headhead) of rra/7s-[a2Pt(l-MeC-/V3)2]2+ from aqueous solution. To the best of our knowledge, this is the

1,3-Dimethyluracil (1,3-DimeU) reacts with trans-[(CH(3)NH(2))(2)Pt(H(2)O)(2)](+) to give trans-[(CH(3)NH(2))(2)Pt(1,3-DimeU-C5)(H(2)O)]X (X = NO(3)(-), 1a, ClO(4)(-), 1b) and subsequently with NaCl to give trans-(CH(3)NH(2))(2)Pt(1,3-DimeU-C5)Cl (2) or with NH(3) to yield trans-[(CH(3)NH(2))(2)Pt(1,3-DimeU-C5)(NH(3))]ClO(4) (3). In a similar way, (dien)Pt(II) forms [dienPt(1,3-DimeU-C5)](+) (4). Reactions leading to formation of 1 and 4 are slow, taking days. In contrast, Hg(CH(3)COO)(2) reacts fast with 1,3-DimeU to give (1,3-DimeU-C5)Hg(CH(3)COO) (5). Both 1-methyluracil (1-MeUH) and uridine (urdH) react with (dien)Pt(II) initially at N(3) and subsequently with either (dien)Pt(II) or Hg(CH(3)COO)(2) also at C(5) to give the diplatinated species 7 and 9 or the mixed PtHg complex 8. C(5) binding of either Pt(II) or Hg(II) is evident from coupling of uracil-H(6) with either (195)Pt or (199)Hg nuclei and (3)J values of 47-74 Hz (for Pt compounds) and 185-197 Hz (for Hg compounds). J values of Pt compounds are influenced both by the ligands trans to the uracil C(5) position and by the number of metal entities bound to a uracil ring. Both 2 and 5 were X-ray structurally characterized. 2: monoclinic system, space group P2(1)/c, a = 15.736(6) Å, b = 11.481(6) Å, c = 25.655 (10) Å, beta = 145.55(3) degrees, V = 2621.9(28) Å(3), Z = 4. 5: monoclinic system, space group P2(1)/c, a = 4.905(2) Å, b = 18.451(6) Å, c = 11.801(5) Å, beta = 94.47(3) degrees, V = 1064.77(72) Å(3), Z = 4.

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