Site-selective recognition of amino acids by Co(III) complexes containing a (N)(O)(3)-type tripodal tetradentate ligand

The bis-N,N-carboxymethy-(S)-phenyl alaninato carbonate cobalt(III) complex , [Co(bcmpa)(CO3)](2-), has been prepared as a simple model that enables th e recognition of an amino acid (Haa) whose coordination behaviours in solut ion have been characterized by electronic absorption (AB), circular dichroi sm (CD) and H-1 NMR spectroscopies. The reaction of the K-2[Co(bcmpa)(CO3)] complex with amino acids (Haa) has predominantly afforded the [Co(bcmpa)(a a)l complex in the trans(N)-configuration mode, rather than in the cis(N)-f orm. By using amino acid derivatives with bulky substituents at their amino or carboxylate sites under a neutral condition, the reactions have been de monstrated to be initiated by coordination of the amino nitrogen site. Inte restingly, the cis(N)-complex, which is isolated as a minor product, isomer izes to the trans(N)-form in the presence of active charcoal under pH 7 in an aqueous solution. The site-selective coordination of Haa to the [Co(bcmp a)(CO3)](2-) complex and the stereoselective isomerization of the [Co(bcmpa )(aa)l complex have been explained to be regulated by weak non-covalent int eractions within the ligands, whose origin has been discussed based on a de tailed examination of the crystal structures of the trans(N)- and cis(N)-K[ Co(bcmpa)(aa)] complexes.