Intramolecular NH–�€ and �€â€“�€ interactions in ternary cobalt(III) complexes of aromatic aminoacids : a structural and NMR study

The synthesis and characterization of a series of ternary Co(III) complexes which could act as simple model systems for the study of non-covalent interactions involving aromatic residues is reported. Such interactions are important to the discriminatory processes associated with molecular recognition in chemical and biochemical systems, but are relatively poorly understood due to the inherent complexity of the majority of systems. The species reported in this paper are Λ-β₁-[Co(R,R-picchxn)(S-trp)]²⁺ (picchxn=N,N′-di(2-picolyl)-1,2-diaminocyclohexane; TRP=the anion of tryptophan) and Δ-α-, Λ-β₁- and Λ-β₂-[Co(R,R-picchxn)(R-trp)]²⁺. A single-crystal X-ray study of the Δ-α- and Λ-β₂-R-trp complexes has demonstrated that the molecular cation of each adopts a conformation where intramolecular π–π interactions involving the indole group and a pyridyl ring occur in the solid state. 2D-ROESY and ¹H NMR experiments of these diamagnetic complexes have established that these interactions persist in solution, as is also the case with the Λ-β₁-S-trp cation. Except for the Λ-β₂-R-trp species, all possess various combinations of NH–π interactions as well, and these too serve to influence markedly the rotamer distributions in several instances. Equilibrium rotamer populations determined using NMR have enabled the energetics of the system to be evaluated. NMR measurements also show that the rotamer populations are relatively unchanged at elevated temperatures, and that the non-covalent interactions which dictate both the solid state and solution configurations of these complexes persist in a variety of solvents. It is shown for R-trp that combined NH–π and π–π interactions provide the driving force for octahedral inversion of both Λ-β₁ and Λ-β₂ diastereoisomers to the ��-�� form. The unprecedented stabilization of this species is rationalized in terms of a hierarchy of competing non-covalent aromatic interactions.