移行金属ベースの水素原子移転反応における基底状態エントロピーの傾向
Elizabeth A Mader1, Virginia W Manner, Todd F Markle
1Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA.
Journal of the American Chemical Society
|March 12, 2009
まとめ
移行金属複合体は,オキシルラジカルによる水素原子移転 (HAT) 反応の間に,著しい負のエントロピー変化を示します. これは有機分子と対照的であり,HAT熱力学における金属中心の重要性を強調しています.
科学分野:
- 協調化化学について
- 熱化学は熱化学である.
- 化学動力学 化学動力学
背景:
- 水素原子移転 (HAT) は化学における基本的な反応機構である.
- エントロピーを含む熱力学的原動力を理解することは,反応予測に不可欠です.
- 以前の研究では,有機系におけるHAT反応のエントロピーの変化がほぼゼロであることをしばしば仮定していた.
研究 の 目的:
- H原子ドナーとして移行金属複合体を含むHAT反応の熱化学的性質を調査する.
- これらの反応における基底状態エントロピーの変化 (ΔS°(HAT)) を定量化します.
- 金属ベースのHATにおけるエントロピーの貢献を有機系におけるエントロピーの貢献と比較する.
主な方法:
- 移行金属複合体の合成と特徴付け: [Fe (II) (H2bip) ]3+2+, [Fe (II) (H2bim) ]3+2+, [Co (II) (H2bim) ]3+2+, Ru (II) (acac) 2 (py-imH) など.
- 熱力学的パラメータを決定するための溶液均衡測定.
- 反応エンタルピーとエントロピーを定量的に確認するためのカロリメトリック研究.
主要な成果:
- 移行金属複合体 [Fe (II) (H2bip) ] 3 2+, [Fe (II) (H2bim) ] 3 2+, [Co (II) (H2bim) ] 3 2+は,TEMPOまたはtBu3PhO*と反応した際,有意な負の ΔS° (HAT) 値 (-30から-41cal mol−1 K−1) を示した.
- ルテニウム複合体Ru ((II) ((acac) 2 ((py-imH) は,より小さな正の ΔS° ((HAT) (4.9 cal mol−1 K−1) を示した.
- カロリメトリーは[Fe (II) (H2bip) ]3+2+TEMPOに対する反応のエンタルピーを確認し,TEMPOH+tBu3PhO*に対する ΔH° (HAT) = -11.2 ± 0.5 kcal mol−1 を提供した.
結論:
- ΔS°(HAT) ≈0の仮定は,移行金属ベースのHAT反応では無効である.
- このΔS°{HAT}の大きさは,関連する金属中心の電子移転エントロピー (ΔS°{ET}) の傾向と相関しています.
- 金属中心の影響を受ける振動エントロピーの貢献は,金属ベースのHAT反応において重要であり,HATとET熱力学を結びつける.
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