Accessing Benzylic Amine and Azide Chemical Handles in Canonical Metal-Organic Frameworks

Abstract

Postsynthetic modification (PSM) is a valuable tool that has significantly widened the scope of functionalizing metal-organic frameworks (MOFs). While many PSM reactions have been reported, a persistent bottleneck is the availability of organic linkers with readily modified functional groups. Herein, an azidomethyl-appended benzene dicarboxylic acid (H2bdc-CH2N3) was synthesized and introduced into both IRMOF-1 (IRMOF-1-CH2N3) and UiO-66 (UiO-66-CH2N3) structure types. The pendant alkyl azide groups in these MOFs were amenable to PSM with alkynes via copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) “click” reactions. PSM was more limited with UiO-66-CH2N3 because of the restricted pore size but could be improved by reducing the mole percent of H2bdc-CH2N3 in a multivariate (i.e., mixed ligand) approach with 1,4-benzene dicarboxylic acid (H2bdc). Importantly, the azidomethyl groups in UiO-66-CH2N3 could be reduced using trimethylphosphine and water via Staudinger reduction to primary benzylic amines. Powder X-ray diffraction and surface area measurements showed that aminomethyl-appended UiO-66 (UiO-66-CH2NH2) retained crystallinity and porosity. This study reports two long pursued and difficult to achieve functional groups on the canonical H2bdc ligand (benzyl azide and benzyl amine) that can serve as versatile handles for PSM for the many MOFs that utilize H2bdc as an organic building block.