INTERMOLECULAR INTERACTIONS IN CHIRAL-AT-METAL COMPLEXES - CHIRALITY CHANGES AT THE CRYSTALLIZATION PROCESS F.J. Lahoz and R. Atencio. Departamento de Química Inorgánica, Instituto de Ciencia de Materiales de Aragón, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain.

In the course of our studies on transition metal complexes with chiral metal centers we have been surprised by the occurrence of epimerization processes (chirality changes at the metal center) associated to the crystallization process. Furthermore, in some cases, the ratio between diastereoisomers observed in solution is dramatically altered upon crystallization, with the unique presence, in the solid state, of the minor-in-solution diastereoisomer. Here we present a detailed analysis of the intra and inter-molecular interactions of these chiral-at-metal complexes in a search of the factors that control the different stability of diastereoisomers in solution and in the solid state.

The studied complexes are new mononuclear neutral or monocationic species of the type [Cp*M(aa)L], containing Rh or Ir as chiral metal centers. They all present a bonded C₅Me₅ group (Cp*) as a fixed ligand to restrict the synthetic processes. The asymmetry of the complexes is related to the N,O-quelate coordination of a chiral a-aminoacidate (aa = L-prolinate, L-valinate, etc.). The third ligand L varies among different neutral or anionic N, P or C-donor groups.

The crystal structures have been investigated by means of empirical packing potential calculations, van der Waals volume analysis and computer graphics. In general, the packing of these complexes seems to be fundamentally controlled by the steric effects associated to the C₅Me₅ groups. In most of the cases, a columnar arrangement of the molecules around an helical axis has been observed, always with an external and parallel-to-the-axis disposition of the Cp* rings. No significant hydrogen bonding has been detected.

Additionally, the influence of the different metal center has been considered and a comparison between two analogous complexes [Cp*M(L-valinate)(PPh₃)] (M = Rh or Ir), only differing at the metal, has been carried out. Interestingly, both crystalline organizations show a peculiar packing based on layers formed by an intercrossing of the phenyl groups of the PPh₃ ligands, surprisingly in an analogous way to those observed in the free ligand or in its oxide.