Structural Characterization, Solution Studies, and DFT Calculations on a Series of Binuclear Gold(III) Oxo Complexes: Relationships to Biological Properties

A series of structurally related oxo-bridged binuclear gold(III) compounds, [Au2(mu-O)2(N;N)2](PF6)2, where N;N is 2,2'-bipyridine or a substituted 2,2'-bipyridine, have recently been shown to exhibit appreciable stability under physiological-like conditions and to manifest important antiproliferative effects toward selected human tumor cell lines (J. Med. Chem. 2006, 49, 5524). The crystal structures of four members of this series, namely, [Au2(mu-O)2(bipy)2](PF6)2, cis-[Au2(mu-O)2(6-Mebipy)2](PF6)2, trans-[Au2(mu-O)2(6-oXylbipy)2](PF6)2, and [Au2(mu-O)2(6,6'-Me2bipy)2](PF6)2, have been solved here and the respective structural parameters comparatively analyzed. Remarkably, all of the compounds contain a common structural motif consisting of a Au2O2 "diamond core" linked to two bipyridine ligands in a roughly planar arrangement. Interestingly, introduction of different kinds of alkyl or aryl substituents on the 6 (and 6') position(s) of the bipyridine ligand leads to small structural changes that nonetheless greatly affect the reactivity of the metal centers. The chemical behavior of these compounds in solution has been studied in detail, focusing in particular on the electrochemical properties. Some initial correlations among the structural parameters, the chemical behavior in solution, and the known cytotoxic effects of these compounds are proposed. Notably, we have found that the 6,6'-dimethyl-2,2'-bipyridine derivative, which showed the largest structural deviations with respect to the model compound [Au2(mu-O)2(bipy)2](PF6)2, also had the highest oxidizing power, the least thermal stability, and the greatest cytotoxic activity. The positive correlation that exists between the oxidizing power and the antiproliferative effects seems to be of particular interest. Moreover, the electronic structures of these compounds were extensively analyzed using DFT methods, and the effects of the various substituents on reactivity were predicted; overall, very good agreement between theoretical expectations and experimental data was achieved. In turn, theoretical predictions offer interesting hints for the design of new, more active binuclear gold(III) compounds.