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Dinitrosyl Iron Complex [K-18-crown-6-ether][(NO)2Fe(MePyrCO2)]: Intermediate for Capture and Reduction of Carbon Dioxide
Y.-T. Tseng, W.-M. Ching, W.-F. Liaw*, and T.-T. Lu* 
Continued efforts are made for the utilization of CO2 as a C1 feedstock for regeneration of valuable chemicals and fuels. Mechanistic study of molecular (electro‐/photo‐)catalysts disclosed that initial step for CO2 activation involves either nucleophilic insertion or direct reduction of CO2. In this study, nucleophilic activation of CO2 by complex [(NO)2Fe(μ‐MePyr)2Fe(NO)2]2− (2, MePyr=3‐methylpyrazolate) results in the formation of CO2‐captured complex [(NO)2Fe(MePyrCO2)]− (2‐CO2, MePyrCO2=3‐methyl‐pyrazole‐1‐carboxylate). Single‐crystal structure, spectroscopic, reactivity, and computational study unravels 2‐CO2 as a unique intermediate for reductive transformation of CO2 promoted by Ca2+. Moreover, sequential reaction of 2 with CO2, Ca(OTf)2, and KC8 established a synthetic cycle, 2 → 2‐CO2 → [(NO)2Fe(μ‐MePyr)2Fe(NO)2] (1) → 2, for selective conversion of CO2 into oxalate. Presumably, characterization of the unprecedented intermediate 2‐CO2 may open an avenue for systematic evaluation of the effects of alternative Lewis acids on reduction of CO2.

在此研究中,清大醫工所魯才德實驗室、清大化學廖文峰實驗室、荊偉民博士(現職高師大化學系)以及本中心詹丁山博士共同合作,嘗試開發雙核雙亞硝基鐵錯合物[K-18-crown-6-ether]2[(NO)2Fe(-MePyr)2Fe(NO)2],利用其[Fe(NO)2]單元體所具有的單電子氧化還原能力,搭配具有親核特性的吡唑配位基,可以將二氧化碳捕捉形成[K-18-crown-6-ether][(NO)2Fe(MePyrCO2)]中間體。經由進一步與二價鈣離子反應,可將所捕捉的二氧化碳轉換成草酸鈣。為了瞭解中間體捕捉及活化二氧化碳的機制,本研究使用X光吸收光譜,證實中間體中的{Fe(NO)2}10中心,乃利用吡唑配位基的親核攻擊能力捕捉二氧化碳,並無氧化還原反應的參與。然而,中間體[K-18-crown-6-ether] [(NO)2Fe(MePyrCO2)]進一步與二價鈣離子反應的過程中,鈣離子誘導了{Fe(NO)2}10中心提供一個電子將二氧化還原,並透過自由基耦合的機制形成草酸根,伴隨錯合物[(NO)2Fe(-MePyr)2Fe(NO)2]的生成。本研究使用TLS 16A1光束線。