Pendant Acid-Base Groups in Molecular Catalysts: H-Bond Promoters or Proton Relays? Mechanisms of the Conversion of CO2 to CO by Electrogenerated Iron(0)Porphyrins Bearing Prepositioned Phenol Functionalities
Pendant Acid-Base Groups in Molecular Catalysts: H-Bond Promoters or Proton Relays? Mechanisms of the Conversion of CO2 to CO by Electrogenerated Iron(0)Porphyrins Bearing Prepositioned Phenol Functionalities
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DOI：
10.1021/ja506193v
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发表时间：
2014-08-20
影响因子：
15
通讯作者：
Saveant, Jean-Michel
中科院分区：
文献类型：
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作者：
Costentin, Cyrille;Passard, Guillaume;Saveant, Jean-Michel
两个四苯基铁的衍生物在两个相对的苯基上具有预定位的酚性官能团，在Fe-O氧化态下电化学生成时，对CO2还原为CO具有显著的催化作用。在一种情况下，另外两个苯基上存在相同的取代基，而在另一种情况下，另两个苯基是全氟化的。它们被用作侧链酸碱基团在分子催化中的可能作用的例子。密度泛函理论计算证实，催化剂分子中预置的苯酚基团通过氢键对初始的(FeCO2)-C-0加合物起到很强的稳定作用。这一积极因素部分地被由于同样的稳定而产生的必要性所抵消，即注入额外的电子来触发催化。由于初始(FeCO_2)-C-0加合物的预质子化，第二次电子转移所需的电势与通过将CO_2加到Fe-0络合物上而产生加合物的电势不是很远。质子化步骤包括内部酚基和由此由添加的苯酚产生的酚氧离子的再质子化。因此，预置的苯酚基团既起到氢键稳定剂的作用，又起到高浓度质子供体的作用。它们在第二电子转移步骤中起着相同的作用，第二电子转移步骤与二氧化碳的两个C-O键之一的断裂和质子转移一起关闭催化环路。同样值得注意的是，通过添加苯酚进行的再质子化与其他三个事件是一致的。
Two derivatives of iron tetraphenylporphyrin bearing prepositioned phenolic functionalities on two of the opposed phenyl groups prove to be remarkable catalysts for the reduction of CO2 to CO when generated electrochemically at the Fe-0 oxidation state. In one case, the same substituents are present on the two other phenyls, whereas in the other the two other phenyls are perfluorinated. They are taken as examples of the possible role of pendant acid base groups in molecular catalysis. The prepositioned phenol groups incorporated into the catalyst molecule induce strong stabilization of the initial (FeCO2)-C-0 adduct through H-bonding, confirmed by DFT calculations. This positive factor is partly counterbalanced by the necessity, resulting from the same stabilization, to inject an additional electron to trigger catalysis. Thanks to the preprotonation of the initial (FeCO2)-C-0, adduct, the potential required for this second electron transfer is not very distant from the potential at which the adduct is generated by addition of CO2 to the Fe-0 complex. The protonation step involves an internal phenolic group and the reprotonation of the phenoxide ion thus generated by added phenol. The prepositioned phenol groups thus play both the role of H-bonding stabilizers and high-concentration proton donors. They play the same role in the second electron transfer step which closes the catalytic loop concertedly with the breaking of one of the two C-O bonds of CO2 and with proton transfer. It is also remarkable that reprotonation by added phenol is concerted with the three other events.