Weak intramolecular interactions in perchlorate salts of Λ-β 1-[Co( R, R-picchxn)( R-aa)] 2+ [picchxn= N, N′-di(2-picolyl)-1,2-diaminocyclohexane; aa=phenylalaninato(1−), tyrosinato(1−)] and their diastereoisomeric equilibration in solution

Abstract

Single-crystal X-ray structures and high-resolution solution NMR studies of Λ-β 1-[Co( R, R-picchxn)( R-phe)](ClO 4) 2 · H 2O and Λ-β 1-[Co( R, R-picchxn)( R-tyr)](ClO 4) 2 · 2H 2O (pheH=phenylalanine, tyrH=tyrosine) are reported. In the former, the aromatic side group of the phenylalanine ligand is extended, as is found in related halide salts. In the tyrosine analogue, the aromatic ring adopts a conformation such that a weak intramolecular NH–π interaction with a NH group of the tetradentate ligand is stabilized. The interaction is rather weak and is not particularly favoured in solution, as revealed by 1H NMR. The β 1 complex of phenylalanine is not the thermodynamically stable diastereoisomer. Equilibration experiments give a mixture of Λ-β 1, Λ-β 2 and Δ-α diastereoisomers in D 2O solution, the α diastereoisomer being more stable than either of the β complexes by about 1 kJ mol −1 at 298.2 K. Intramolecular π–π and NH–π interactions are responsible for the stabilization of the α complex, demonstrating the significance of the cooperative effects of such interactions. The ternary cobalt complexes described in this study could act as simple model systems for investigating the discriminatory effects of analogous weak interactions that occur in a complex biological setting.