Metal-stabilized rare tautomers of nucleobases. 8. Promotion of rare cytosine tautomer upon complex formation with (dien)M2+ (M=Pt, Pd)

Reactions of (dien)M2+ (M=Pt, Pd) with cytosine (CH) lead to mixtures of linkage isomers with N3 and N1 bonded metal entities as well as N3,N1 bridged species, as demonstrated by 1H NMR spectroscopy. The N3 linkage isomer is preferred over the N1 linkage isomer throughout the pH range studied (2.5–9), but the ratio does not parallel the equilibrium constant between the aminooxo tautomer (I) (proton at N1) and the aminooxo tautomer (II) (proton at N3), which has been estimated to be close to 1000 in solution. Rather, the minor tautomer (II) is complexed to a much higher extent. This finding is potentially significant for any metal–nucleobase interaction in that it suggests that metal complexes of rare tautomers may be formed more readily than generally assumed. X-ray data of four (CHN3) complexes as well as of the first example of a (CN1,N3) bridged compound are reported.