The structure of Λ-β1-[Co(R,R-picchxn)(S-tyr)]Br2 · 3.5H2O and solution configuration of Λ-β1-[Co(R,R-picchxn)(aromatic S-aminoacidate)]2+ complexes: model systems for the investigation of non-covalent aromatic interactions

Abstract

Intramolecular non-covalent interactions involving aromatic residues in the ternary species Λ-β1-[Co(R,R-picchxn)(S-aa)]2+ (aa=Tyr, OMe-tyr or Phe) have been investigated. Such interactions are important to discriminatory processes associated with molecular recognition in chemical and biochemical systems. The single-crystal X-ray study of Λ-β1-[Co(R,R-picchxn)(S-tyr)]Br2·3.5H2O demonstrates the influence of intramolecular π–π and bifurcated NH–π interactions in determining the molecular conformation of the complex cation in the solid state. The Co(III) complexes synthesised are diamagnetic, and have been fully assigned in solution using multidimensional NMR techniques. Remarkably, the solid state conformation observed for Λ-β1-[Co(R,R-picchxn)(S-tyr)]2+ has been shown to predominate in solutions of all the complexes, as evidenced by appropriate rOe correlations. 1H NMR measurements carried out in order to determine equilibrium rotamer distributions confirm the dominance of this conformer in solution. NMR measurements also show that rotamer populations are relatively unchanged at elevated temperatures and in a variety of solvents. The results of this detailed study, which demonstrate the significance of non-covalent interactions involving aromatic residues to the determination of the molecular conformation, serve to highlight the suitability of these simple ternary Co(III) complexes to act as models for such interactions.