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.