Teindl Kaeden, Reid Jolene P, Nichols Eva M
Department of Chemistry, The University of British Columbia Vancouver British Columbia V6T 1Z1 Canada
Chem Sci. 2025 Sep 8. doi: 10.1039/d5sc04700e.
While several studies have investigated the effects of protic secondary coordination sphere (SCS) groups on the kinetics of iron tetraphenylporphyrin (FeTPP) catalysed CO reduction, few have examined how a protic SCS might alter reaction selectivity. Under mildly acidic conditions, FeTPP-based catalysts are selective towards the 2e/2H reduction of CO to CO; however, in the presence of more acidic proton donors, indiscriminate proton transfers often result in parasitic H evolution. This report investigates how SCS amide positioning alters CO H selectivity during CO reduction with a series of four FeTPP isomers bearing SCS amides at varying positions around the porphyrin core: NH donors are placed at either the or position of the aryl porphyrin ring, as well as proximal (closer) or distal (farther away) to the porphyrin plane. In the presence of a conventional, weakly acidic proton source (phenol; p = 29.2 in MeCN), all isomers display the expected high faradaic efficiency (FE) towards CO (FE = 67-85%) along with minimal H evolution (FE = 3-13%). With a significantly stronger acid (3,5-bis(trifluoromethyl)phenol; p = 23.8 in MeCN), H becomes the major product when using the -distal or both isomers (FE = 45-65%) as well as unfunctionalized FeTPP (FE = 78%). Importantly, the -proximal isomer shows dramatically rescued CO selectivity under these unconventionally acidic conditions (FE = 83 ± 4%). These results show how proper SCS placement impacts reaction selectivity during CO reduction, particularly with respect to minimizing indiscriminate proton transfers that lead to undesirable reactivity.
虽然已有多项研究调查了质子性第二配位层（SCS）基团对四苯基卟啉铁（FeTPP）催化的CO还原动力学的影响，但很少有研究考察质子性SCS如何改变反应选择性。在轻度酸性条件下，基于FeTPP的催化剂对CO的2e/2H还原生成CO具有选择性；然而，在存在更强酸性的质子供体时，无差别质子转移常常导致寄生性析氢。本报告研究了SCS酰胺的位置如何在CO还原过程中改变CO/H选择性，该过程使用了一系列四种FeTPP异构体，这些异构体在卟啉核周围的不同位置带有SCS酰胺：NH供体位于芳基卟啉环的β或meso位置，以及靠近或远离卟啉平面。在存在常规的弱酸性质子源（苯酚；在乙腈中的pKa = 29.2）的情况下，所有异构体对CO都表现出预期的高法拉第效率（FE）（FE = 67 - 85%），同时析氢量极少（FE = 3 - 13%）。使用显著更强的酸（3,5 - 双（三氟甲基）苯酚；在乙腈中的pKa = 23.8）时，当使用β - 远端或两个meso异构体（FE = 45 - 65%）以及未官能化的FeTPP（FE = 78%）时，H成为主要产物。重要的是，在这些非传统酸性条件下，β - 近端异构体显示出显著恢复的CO选择性（FE = 83 ± 4%）。这些结果表明了合适的SCS位置在CO还原过程中如何影响反应选择性，特别是在最小化导致不良反应性的无差别质子转移方面。
