The stereochemistry and reactivity of metal-Schiff base complexes. VI. The kinetics and mechanism of isomerization between .DELTA.L-.BETA.2- and .LAMBDA.L-.BETA.2-diastereoisomers of N,N'-ethylenebis(.ALPHA.-methylsalicylideneaminato)cobalt(III) complexes

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概要

• 論文の詳細を見る

• The kinetics for isomerization between Δ<SUB>L</SUB>-β<SUB>2</SUB>- and <I>Λ</I><SUB>L</SUB>-β<SUB>2</SUB>-diastereoisomers of [Co(α-Me-sal<SUB>2</SUB>en)(L-aa)], where L-aa represents L-phe, L-met, L-ile, L-pro, <I>N</I>-Bz–L-ala, and <I>N</I>-Me–L-ala, were studied in methanol. The activation enthalpies (Δ<I>H</I><SUP>\neweq</SUP>/kJ mol<SUP>−1</SUP>) and the activation entropies (Δ<I>S</I><SUP>\neweq</SUP>/J K<SUP>−1</SUP> mol<SUP>−1</SUP>) from the Δ<SUB>L</SUB>-β<SUB>2</SUB>-isomer (unstable form) to the <I>Λ</I><SUB>L</SUB>-β<SUB>2</SUB>-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 <I>N</I>-Bz-L-ala, and 102.9 and +18.4 for <I>N</I>-Me-L-ala, respectively. For L-procomplex, the Δ<SUB>L</SUB>-β<SUB>2</SUB>-isomer is the stable form, and the enthalpy and entropy for isomerization from <I>Λ</I><SUB>L</SUB>-β<SUB>2</SUB>- to Δ<SUB>L</SUB>-β<SUB>2</SUB>- 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 <I>N</I>-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 <I>via</I> 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.

• 公益社団法人 日本化学会の論文

著者

• Fujii Yuki

  Department Of Chemistry Faculty Of Science Ibaraki University

• Takahashi Sozo

  Department of Chemistry, Ibaraki Technical College

• Matsufuru Makoto

  Department of Chemistry, Faculty of Science, Ibaraki University

• Kobayashi Takemaru

  Department of Chemistry, Faculty of Science, Ibaraki University

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