A Unique Helicate Comprised of Four Cytosine Nucleobases and Four Metal Entities (PtII, PtII, AuIII, AuI). Implications for the Interactions of Linearly Coordinated Metal Ions with ­Nucleotide Duplexes

Abstract Abstract . Reaction of trans ‐[Pt(CH 3 NH 2 ) 2 (1‐MeC‐ N3 ) 2 ] X 2 (1‐MeC = 1‐methylcytosine; X = NO 3 – or ClO 4 – ) with Na[AuCl 4 ] in strongly alkaline aqueous solution yields an asymmetrical double helicate containing two central Pt II as well as Au III and Au I atoms, four 1‐methylcytosinato bridges (1‐MeC – ‐ N3, N4 ), two terminal methylamine and two bridging methylamido ligands, [Au 2 Pt 2 (CH 3 NH 2 ) 2 (CH 3 NH) 2 (1‐MeC – ) 4 ]Cl 2 · 2H 2 O ( 2 ). The four 1‐MeC – ligands, which are pairwise oriented in head‐head fashion, are deprotonated at their N4 positions and cross‐linked by the gold ions. While one of the gold ions, Au I , adopts a linear coordination arrangement, the second, Au III , is square‐planar and completes its coordination arrangement through two methylamido ligands, which behave as bridging species between Au III and Pt. As a consequence of deprotonation of the original methylamine ligands, the nitrogen atoms of the CH 3 NH – bridges have become chiral. The four heavy metals form a diamond‐shaped plane, with an intramolecular Au III ··· Au I distance of 3.053(4) Å. On the basis of the structure of cation 2 a new model is proposed how linear transition metal ions (M) might interact with (natural or artificial) nucleobases (L) within a double helix to produce compact M 4 L 4 adducts.