Control Interlayer Stacking and Chemical Stability of Two-Dimensional Covalent Organic Frameworks via Steric Tuning

Layer
stacking and chemical stability are crucial for two-dimensional
covalent organic frameworks (2D COFs), but are yet challenging to
gain control. In this work, we demonstrate synthetic control of both
the layer stacking and chemical stability of 2D COFs by managing interlayer
steric hindrance via a multivariate (MTV) approach. By co-condensation
of triamines with and without alkyl substituents (ethyl and isopropyl)
and a di- or trialdehyde, a family of two-, three-, and four-component
2D COFs with AA, AB, or ABC stacking is prepared. The alkyl groups
are periodically appended on the channel walls and their contents,
which can be synthetically tuned by the MTV strategy, control the
stacking model and chemical stability of 2D COFs by maximizing the
total crystal stacking energy and protecting hydrolytically susceptible
backbones through kinetic blocking. Specifically, the COFs with higher
concentration of alkyl substituents adopt AB or ABC stacking, while
lower amount of functionalities leads to the AA stacking. The COFs
bearing high concentration of isopropyl groups represent the first
identified COFs that can retain crystallinity and porosity in boiling
20 M NaOH solution. After postsynthetic metalation with an iridium
complex, the 2,2′-bipyridyl-derived COFs can heterogeneously
catalyze C–H borylation of arenes, whereas the COF with isopropyl
groups exhibits much higher activity than the COFs with ethyl groups
and nonsubstituents due to the increased porosity and chemical stability.
This work underscores the opportunity in using steric hindrance to
tune and control layer stacking, chemical stability and properties
of 2D COFs.

层堆积与化学稳定性是二维共价有机框架（2D covalent organic frameworks, 2D COFs）的两项核心性能，但目前仍难以实现精准调控。本研究通过多元（multivariate, MTV）策略调控层间位阻，成功实现了对2D COFs层堆积模式与化学稳定性的合成调控。通过将带有烷基取代基（乙基与异丙基）和不带取代基的三胺，分别与二醛或三醛进行共缩聚，我们制备了一系列涵盖两组分、三组分及四组分的2D COFs家族，其堆积模式包含AA、AB或ABC型。烷基基团周期性修饰于孔道内壁，其含量可通过MTV策略进行合成调控，通过最大化晶体总堆积能，并借助动力学阻隔保护易水解的骨架，从而实现对2D COFs堆积模式与化学稳定性的调控。具体而言，烷基取代基浓度更高的COFs采用AB或ABC堆积模式，而取代基含量较低的COFs则呈现AA堆积模式。其中，含有高浓度异丙基取代基的COFs是首个被证实可在沸腾的20 mol/L氢氧化钠溶液中保持结晶性与孔隙率的COFs材料。通过与铱配合物进行后合成金属化后，基于2,2'-联吡啶的COFs可作为非均相催化剂催化芳烃的C-H硼化反应；其中，带有异丙基取代基的COFs相较乙基取代基及无取代基的COFs表现出更优异的催化活性，这归因于其更高的孔隙率与化学稳定性。本研究凸显了利用位阻效应调控2D COFs的层堆积、化学稳定性与性能的巨大潜力。