1983 Volume 56 Issue 12 Pages 3608-3614
The kinetics for isomerization between ΔL-β2- and ΛL-β2-diastereoisomers of [Co(α-Me-sal2en)(L-aa)], where L-aa represents L-phe, L-met, L-ile, L-pro, N-Bz–L-ala, and N-Me–L-ala, were studied in methanol. The activation enthalpies (ΔH\ eweq/kJ mol−1) and the activation entropies (ΔS\ eweq/J K−1 mol−1) from the ΔL-β2-isomer (unstable form) to the ΛL-β2-isomer (stable form) were determined to be 86.6 and −9.6 for L-phe, 92.5 and +6.7 for L-met, 87.5 and −8.0 for L-ile, 100.4 and +28.0 for N-Bz-L-ala, and 102.9 and +18.4 for N-Me-L-ala, respectively. For L-procomplex, the ΔL-β2-isomer is the stable form, and the enthalpy and entropy for isomerization from ΛL-β2- to ΔL-β2- isomers were 96.2 and 0.0. The isomerization was catalyzed by acids but not by bases, and was much faster than the substitution of coordinated L-amino acid. The H–D exchange of N–H proton of N-Me–L-ala-complex proceeded with almost the same rate as the isomerization. The isomerization showed strong solvent dependency; it was fast in a hydrogen-bonding solvent, but was quite slow in a weak- or nonhydrogen-bonding solvent. On the basis of these facts, proton-assisted and/or hydrogen-bonding solvent-assisted intramolecular mechanism via an intermediate is proposed, in which the Schiff base takes a planar configuration and the amino acid coordinates as a unidentate ligand with its carboxylate group.
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