X‐ray Crystal Structures Show DNA Stacking Advantage of Terminal Nitrile Substitution in Ru‐dppz Complexes

The new complexes [Ru(TAP)2(11‐CN‐dppz)]2+, [Ru(TAP)2(11‐Br‐dppz)]2+and [Ru(TAP)2(11,12‐diCN‐dppz)]2+ are reported. The addition of nitrile substituents to the dppz ligand of the DNA photo‐oxidising complex [Ru(TAP)2(dppz)]2+ promote π‐stacking interactions and ordered binding to DNA, as shown by X‐ray crystallography. The structure of Λ‐[Ru(TAP)2(11‐CN‐dppz)]2+ with the DNA duplex d(TCGGCGCCGA)2 shows, for the first time with this class of complex, a closed intercalation cavity with an AT base pair at the terminus. The structure obtained is compared to that formed with the 11‐Br and 11,12‐dinitrile derivatives, highlighting the stabilization of syn guanine by this enantiomer when the terminal base pair is GC. In contrast the AT base pair has the normal Watson–Crick orientation, highlighting the difference in charge distribution between the two purine bases and the complementarity of the dppz–purine interaction. The asymmetry of the cavity highlights the importance of the purine–dppz–purine stacking interaction.