AFFINITY OF THE IMINOOXO TAUTOMER ANION OF 1-METHYLCYTOSINE IN TRANS-[PT(NH3)(2)(1-MEC-N4)(2)](2+) FOR HETEROMETALS

Reaction of a Pt-11 complex containing two 1-methylcytosine (1-MeC) nu cleobases bound through the exocyclic amino group N4, trans-[Pt(NH3)(2 )(1-MeC-N4)(2)](NO3)(2) (1), with the heterometal species [(dien)Pd](2 +) or Hg2+ gives s-[(NH3)(2)Pt{(N4-1-MeC--N3)Pd(dien)}(2)](ClO4)(4) . 2H(2)O (3) and trans-[(NH3)(2)Pt(N4-1-MeC--N3)(2)Hg](NO3)(2) . 2H(2)O (4), respectively, The heterometal(s) is (are) bound through the N3 po sitions of the two cytosine rings. Compound 1 contains the nucleobase in the form of its rare iminooxo tautomer. In the solid-state structur e of 1, the two nucleobases display a syn orientation between Pt and t he endocyclic N3 position, whereas in compound 3 they adopt an anti co nformation. In both compounds the cytosine bases are in a head-to-tail orientation. In the bimetallic complex 4 however. the 1-methylcytosin e ligands are head-to-head and syn with the two nucleobases acting as chelating ligands. The Pt-Hg distance in 4 is quite short (2.7498(6) A ngstrom), suggesting a weak bonding interaction, In the case of 3 the Pt-Pd distance (5.13 A) is too long for any interaction, While I-I-bon d formation between the iminooxo tautomer of 1-MeC in 1 with free 1-Me C and likewise between the deprotonated form trans-[Pt(NH3)(2)-(1-MeC- -N4)(2)] (2) and free 9-ethylguanine (9-EtGH) is possible only if the cytosine bases are in an anti orientation, there is no indication for such H-bonding patterns from H-1 NMR studies.