The Structural Effect of Methyl Substitution on the Binding of Polypyridyl Ru–dppz Complexes to DNA

Polypyridyl ruthenium complexes have been intensively studied and possess photophysical properties that are both interesting and useful. They can act as probes for DNA, with a substantial enhancement in emission when bound, and can induce DNA damage upon photoirradiation. Therefore, the synthesis and characterization of DNA binding of new complexes is an area of intense research activity. While knowledge of how the binding of derivatives compares to that of the parent compound is highly desirable, this information can be difficult to obtain. Here we report the synthesis of three new methylated complexes, [Ru­(TAP)<sub>2</sub>(dppz-10-Me)]­Cl<sub>2</sub>, [Ru­(TAP)<sub>2</sub>(dppz-10,12-Me<sub>2</sub>)]­Cl<sub>2</sub>, and [Ru­(TAP)<sub>2</sub>(dppz-11-Me)]­Cl<sub>2</sub> (TAP = 1,4,5,8-tetraazaphenanthrene; dppz = dipyrido­[3,2-<i>a</i>:2′,3′-<i>c</i>]­phenazine), and examine the consequences for DNA binding through the use of atomic-resolution X-ray crystallography. We find that the methyl groups are located in discrete positions with a complete directional preference. This may help to explain the quenching behavior found in solution for analogous [Ru­(phen)<sub>2</sub>(dppz)]<sup>2+</sup> derivatives.