Carbon Dioxide Insertion into Bridging Iron Hydrides: Kinetic and Mechanistic Studies.
Carbon Dioxide Insertion into Bridging Iron Hydrides: Kinetic and Mechanistic Studies.
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DOI：
10.1002/ejic.201801404
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发表时间：
2019-02
影响因子：
2.3
通讯作者：
Dae Ho Hong;Leslie J. Murray
中科院分区：
文献类型：
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作者：
Dae Ho Hong;Leslie J. Murray
过渡金属氢化物将二氧化碳还原为甲酸是一种潜在的获得活性氯化合物的途径。到目前为止，还没有报道在弱场连接的络合物中插入桥联氢化物到二氧化碳中的动力学研究；这样的中心与催化这一反应的金属酶有关。本文报道了用~1H-核磁共振和温控UV-Vis光谱研究了三(μ-氢化物)三铁簇合物(II/II/II)与CO_2的反应动力学。该簇合物由三(β-二酮亚胺)环(1)担载。我们发现，在298K时，1与二氧化碳在甲苯中反应，生成甲酸单酯(1-CO2)和甲酸盐络合物(1-2CO2)，最终在高温下生成三甲酸盐物种(1-3CO2)。298K时，生成1-CO2的二级速率常数为8.4(3)×10-4M-1·S-1，H/D动力学同位素效应为1.08(9)，∆H‡为11(1)kcal·mol-1，∆S‡为-3(1)×10cal·mol-1·K-1。这些参数表明，在速率控制步骤之前，CO2与铁中心的配位不配位，而Fe-H键断裂配位。
The reduction of CO2 to formic acid by transition metal hydrides is a potential pathway to access reactive C1 compounds. To date, no kinetic study has been reported for insertion of a bridging hydride in a weak-field ligated complex into CO2; such centers have relevance to metalloenzymes that catalyze this reaction. Herein, we report the kinetic study of the reaction of a tri(μ-hydride)triiron(II/II/II) cluster supported by a tris(β-diketimine) cyclophane (1) with CO2 monitored by 1H-NMR and temperature-controlled UV-vis spectroscopy. We found that 1 reacts with CO2 to traverse the reported monoformate (1-CO 2 ) and a diformate complex (1-2CO 2 ) at 298 K in toluene, and ultimately yields the triformate species (1-3CO 2 ) at elevated temperature. The second order rate constant, H/D kinetic isotope effect, ∆H ‡,and ∆S ‡for formation of 1-CO 2 were determined as 8.4(3)×10-4 M-1·s-1, 1.08(9), 11(1) kcal·mol-1, and -3(1)×10 cal·mol-1·K-1, respectively at 298 K. These parameters suggest that CO2 coordination to the iron centers does not coordinate prior to the rate controlling step whereas Fe-H bond cleavage does.