Functionalization of 3D covalent organic frameworks using monofunctional boronic acids

Co-crystallizing a monomer capable of forming a three-dimensional covalent organic framework (3D COF) with a truncated analog represents a robust strategy to functionalize the pores of these crystalline polymer networks. Here we elaborate this approach by demonstrating that monofunctional arylboronic acids serve as effective truncation/functionalization agents for COF-102, a boroxine-linked 3D network derived from the dehydration of a tetrahedral tetrakis(boronic acid) monomer. The COF-102 network forms under typical solvothermal conditions, even in the presence of a large excess of 4-tolylboronic acid, which is incorporated into the polymer's boroxine linkages up to a maximum loading level of ca. 33 mol%. This finding indicates the maximum truncation level for the COF-102 network and suggests that framework crystallization is irreversible. At high feed ratios of the monofunctional boronic acid, the isolated COF-102-tolyl powders are initially contaminated by significant amounts of tris(4-tolyl)boroxine, which is removed through a solution-based activation process to provide COF-102-tolyl samples with high functionalization density, long-range order, and permanent porosity. We also demonstrate the generality of this truncation study by evaluating several other readily available arylboronic acids, each of which are incorporated into the COF similarly. Together these findings demonstrate the simplicity and generality of this truncation/functionalization approach, as well as its fundamental limits.