A Unique Helicate Comprised of Four Cytosine Nucleobases and Four Metal Entities (Pt~(II), Pt~(II), Au~(III), Au~I). Implications for the Interactions of Linearly Coordinated Metal Ions with Nucleotide Duplexes

摘要

Reaction of trans-[Pt(CH_3NH_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, [Au2Pt2(CH_3NH_2)_2(CH3NH)2(1- MeC~-)_4]Cl_2·2H_2O (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 squareplanar 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_3NH~- 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) ?. 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_4L_4 adducts.