Postsynthetic modification: a versatile approach toward multifunctional metal-organic frameworks

Abstract

An isoreticular metal-organic framework (IRMOF-3) containing 2-amino-1,4-benzenedicarboxylic acid (NH(2)-BDC) as a building block is shown to undergo chemical modification with a diverse series of anhydrides and isocyanates. The modification of IRMOF-3 by these reagents has been evidenced by using a variety of methods, including NMR and electrospray ionization mass spectrometry, and the structural integrity of the modified MOFs has been confirmed by thermogravimetric analysis, powder X-ray diffraction, and gas sorption analysis. The results show that a variety of functional groups can be introduced onto the MOF including amines, carboxylic acids, and chiral groups. Furthermore, it is shown that tert-butyl-based asymmetric anhydrides can be used to selectively deliver chemical payloads to the IRMOF. Finally, the results demonstrate that at least four different chemical modifications can be performed on IRMOF-3 and that the reaction conditions can be modulated to control the relative abundance of each group. The findings presented here demonstrate several important features of postsynthetic modification on IRMOF-3, including (1) facile introduction of a wide range of functional groups using simple reagents (e.g., anhydrides and isocyanates), (2) the introduction of multiple (as many as four different) substituents into the MOF lattice, and (3) control over reaction conditions to preserve the crystallinity and microporosity of the resultant MOFs. The findings clearly illustrate that postsynthetic modification represents a powerful means to access new MOF compounds with unprecedented chemical complexity, which may serve as the basis of multifunctional materials.

Figure 1

PXRD patterns of anhydride modified IRMOF-3 samples. Modified IRMOF-3 samples were soaked and exchanged with fresh CHCl₃ for 3 days. All modified samples show a PXRD pattern similar to that of the starting material (IRMOF-3, bottom).

Figure 2

¹H NMR spectra of multiple modified IRMOF-3 samples. IRMOF-3 samples modified with decanoic anhydride (blue, 51% conversion), propyl isocyanate (green, 60% conversion), allyl isocyanate (purple, 75% conversion), crotonic anhydride (orange, 100% conversion), and unmodified IRMOF-3 (black) digested in DCl/D₂O and DMSO-d₆. Resonances in the spectra for IRMOF-3-AM9/UR3, IR-MOF-3-AM9/UR3/URAl, and IRMOF-3-AM9/UR3/URAl/AMCrot-a are color-coded corresponding to the top five spectra. The small differences in the positions of some of these resonances is attributed to variations in solution pH after sample digestion. IRMOF-3 resonances appear black in all spectra shown.

Figure 3

LC-UV/MS traces of IRMOF-3-AM9/UR3 (blue), IRMOF-3-AM9/UR3/URAl (red), and IRMOF-3-AM9/UR3/URAl/AMCrot-a (black). The chromatogram from 13 to 16 min is magnified 10-fold.

Figure 4

PXRD patterns of multiple modified IRMOF-3 samples. Modified IRMOF-3 samples were soaked and exchanged with fresh CHCl₃ for 3 days. After the solvent was decanted off, the samples were left drying in the air for 10 min prior to PXRD analysis.

Scheme 1

Postsynthetic Modification Reactions Performed with IRMOF-3

Scheme 2

Reagents Used for Multiple Postsynthetic Modification Reactions with IRMOF-3

Chart 1

Structure of IRMOF-3 (Left) Showing the Disordered Amino Group (Blue) in All Four Possible Positions on the Organic Linkera ^aThe pore diameter of IRMOF-3 is ~9.7Å (ref 59). Schematic representationof IRMOF-3 (right) that is used throughout this manuscript toillustrate postsynthetic modification reactions is also shown.