A Versatile Synthetic Approach to Rhodium(III) Diimine Metallointercalators:  Condensation of o-Quinones with Coordinated cis-Ammines

Abstract

A new route to the preparation of rhodium(III) diimine complexes which bind DNA by intercalation was developed by condensation of coordinated ammine ligands with o-quinones. Starting from cis-[Rh(L∧L)_2(NH_3)_2]^(3+) (L∧L:  2,2‘-bipyridine or 1,10-phenanthroline) and the appropriate quinones the ligands 9,10-phenanthrenequinone diimine (phi) and 5,6-chrysenequinone diimine (chrysi) were introduced in high yields. The reactions are completed within hours at ambient temperature in MeCN/water mixtures, 0.1 M in NaOH. Experiments with enantiomerically pure Δ-[Rh(phen)_2(NH_3)_2]^(3+) and 9,10-phenanthrenequinone showed that the configuration at the metal center is retained during the course of the reaction. Condensation of rhodium(III) tetraammine starting material with 9,10-phenanthrenequinone allows the selective introduction of one or two phi ligands, depending on the reaction conditions. The ability to incorporate specifically only one phi ligand makes this a promising approach for the synthesis of tris(heteroleptic) coordination compounds, and one example of such a complex, [Rh(phen)(phi)(chrysi)]^(3+), is provided.