Solea Atena B, Sudittapong Burin, Taylor Christopher G P, Ward Michael D
Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK.
Dalton Trans. 2022 Aug 2;51(30):11277-11285. doi: 10.1039/d2dt01713j.
In this work we compare and contrast the hydrolysis of two different aromatic esters using an octanuclear cubic Co coordination cage host as the catalyst. Diacetyl fluorescein (DAF) is too large to bind inside the cage cavity, but in aqueous solution it interacts with the exterior surface of the cage a hydrophobic interaction with = 1.5(2) × 10 M. This is sufficient to bring it into close proximity to the layer of hydroxide ions which also surrounds the 16+ cage surface even at modest pH values, accelerating the hydrolysis of DAF to fluorescein with / (the rate acceleration for that fraction of DAF in contact with the cage surface in the equilibrium) ≈50. This is far smaller than many known examples of catalysis inside a cage cavity, but at the exterior surface it is potentially general with no cavity-imposed size/shape limitations for guest binding. In contrast 4-nitrophenyl acetate (4NPA) binds inside the cage cavity with = 3.5(3) × 10 M and as such is surrounded in solution by the hydroxide ions which accumulate around the cage surface. However its hydrolysis is actually inhibited: either because of a geometrically unfavourable geometry of the bound substrate which makes it inaccessible to surface-bound hydroxide, or because the necessary volume expansion/geometry change associated with formation of a tetrahedral intermediate cannot be accommodated inside the cavity. Any 4NPA that is free in solution as part of the equilibrium undergoes catalysed hydrolysis at the cage exterior surface in the same way as DAF, but the effect is limited by the low affinity of 4NPA for the exterior surface. We conclude that exterior-surface catalysis can be effective and potentially general; and that cavity-binding of guests can result in negative, rather than positive, catalysis.
在这项工作中，我们使用一个八核立方钴配位笼主体作为催化剂，比较和对比了两种不同芳香酯的水解情况。二乙酰荧光素（DAF）太大，无法结合在笼腔内，但在水溶液中，它与笼的外表面相互作用——疏水相互作用常数(K = 1.5(2)×10 M)。这足以使其与即使在适度pH值下也围绕着带(16+)电荷笼表面的氢氧根离子层紧密靠近，加速DAF水解为荧光素，(k_{obs}/k_{uncat})（处于平衡状态下与笼表面接触的那部分DAF的速率加速）约为50。这比许多已知的笼腔内催化例子小得多，但在笼外表面，它可能具有普遍性，对客体结合没有因腔而产生的尺寸/形状限制。相比之下，乙酸对硝基苯酯（4NPA）以(K = 3.5(3)×10 M)的常数结合在笼腔内，因此在溶液中被聚集在笼表面周围的氢氧根离子所包围。然而，它的水解实际上受到抑制：要么是因为结合底物的几何结构不利，使得表面结合的氢氧根无法接近它，要么是因为与形成四面体中间体相关的必要体积膨胀/几何形状变化无法在腔内得到容纳。作为平衡一部分而在溶液中游离的任何4NPA，都以与DAF相同的方式在笼外表面发生催化水解，但这种效应受到4NPA对笼外表面低亲和力的限制。我们得出结论，外表面催化可以是有效的且可能具有普遍性；并且客体的腔结合可能导致负催化，而不是正催化。
