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Characterization of Mononuclear Non-heme Iron(III)-Superoxo Complex with a Five-Azole Ligand Set

投稿者: Yuma Morimoto
Angewandte Chemie International Edition by Frédéric Oddon, Yosuke Chiba, Jun Nakazawa, Takehiro Ohta, Takashi Ogura, Shiro Hikichi  /  28d  //  keep unread  //  hide  //  preview

Abstract

Reaction of O2 with a high-spin mononuclear iron(II) complex supported by a five-azole donor set yields the corresponding mononuclear non-heme iron(III)–superoxo species, which was characterized by UV/Vis spectroscopy and resonance Raman spectroscopy. 1H NMR analysis reveals diamagnetic nature of the superoxo complex arising from antiferromagnetic coupling between the spins on the low-spin iron(III) and superoxide. This superoxo species reacts with H-atom donating reagents to give a low-spin iron(III)–hydroperoxo species showing characteristic UV/Vis, resonance Raman, and EPR spectra.
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Reaction of O2 with a mononuclear high-spin iron(II) complex supported by a five-azole donor set yields the corresponding mononuclear non-heme low-spin iron(III)–superoxo species. This species reacts with hydrogen-atom-donating reagents to give a low-spin iron(III)–hydroperoxo species.
引地先生の研究グループからの鉄-スーパーオキソ種の報告です。ブレオマイシンのモデル化合物のよいモデルとのことです。トリスピラゾリルボレート（通称Tp）タイプのリガンドとイミダゾールからなる二座配位子を鉄に持たせることで、これまでTpで見れなかったスーパーオキソの安定化に成功しています。（結合長を見ると、ピラゾールよりイミダゾールの方が強く配位しているようです。参考：http://medchem4410.seesaa.net/article/179719815.html）なぜこのリガンドの時にはダイメライズしてエンドオンのパーオキソにならないのでしょうか？生成する化学種は、ロースピンでNMRで見れます。
基質の酸化能に関しては、TEMPO系の化合物で、少しO–HボンドのつよいAZADOL(O–H: 72.6 kcal mol-1)まで酸化できますが、C–Hの酸化能は低いようで、BNAHを酸化できません(67.9 kcal mol-1)。ちなみに、近年台湾のW.-Z. Leeらによって報告されたハイスピンの鉄(III)-スーパーオキソ錯体は、ジヒドロアントラセン(C–H: 76 kcal mol-1)も酸化できるようです( J. Am. Chem. Soc. 2014, 136, 10846 )。
求核性もほとんど持っておらず、アルデヒド、アシルハライドへの攻撃もしません。これは、スーパーオキソのスピンが鉄中心のスピンと強くカップリングしているために、酸素上の電子密度が低いためかもしれない、としています。
TEMPO系の基質のバリエーションを付けるために、AZADOL(http://www.wako-chem.co.jp/siyaku/product/chemical/AZADOL/index.htm)を始めとするN-OH系の試薬は持っておいても良いかもしれません。



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