Evaluation of heterogeneous metal-organic framework organocatalysts prepared by postsynthetic modification

Abstract

A metal-organic framework (MOF) containing 2-amino-1,4-benzenedicarboxylate (NH(2)-BDC) as a building block is shown to undergo chemical modification with a set of cyclic anhydrides. The modification of the aluminum-based MOF known as MIL-53(Al)-NH(2) (MIL = Material Institut Lavoisier) by these reagents is demonstrated by using a variety of methods, including NMR and electrospray ionization mass spectrometry (ESI-MS), and the structural integrity of the modified MOFs has been confirmed by thermal gravimetric analysis (TGA) and powder X-ray diffraction (PXRD). Reaction with these cyclic anhydrides produces MOFs that display carboxylic acid functional groups within their pores. Furthermore, it is shown that maleic acid functionalized MIL-53(Al)-AMMal can act as a Brønsted acid catalyst and facilitate the methanolysis of several small epoxides. Experiments show that MIL-53(Al)-AMMal acts in a heterogeneous manner and is recyclable with consistent activity over at least three catalytic cycles. The findings presented here demonstrate several important features of covalent postsynthetic modification (PSM) on MOFs, including (1) facile introduction of catalytic functionality using simple organic reagents (e.g., anhydrides); (2) the ability to utilize and recycle organocatalytic MOFs; (3) control of catalytic activity through choice of functional group. The findings clearly illustrate that covalent postsynthetic modification represents a powerful means to access new MOF compounds that serve as organocatalytic materials.

Figure 1

¹H NMR spectra of modified MIL-53(Al)-NH₂ samples digested in HF/d₆-DMSO. Red squares and black circles represent signals of aryl protons in modified and unmodified NH₂-BDC, respectively. All other peaks associated with modifications or solvent are labeled explicitly in red.

Figure 2

¹H NMR of catalytic reaction aliquots employing MIL-53(Al)-AMMal (green), MIL-53(Al)-AM1 (blue), MIL-53(Al)-AMCrot (red), and MIL-53(Al)-NH₂ (black).

Figure 3

¹H NMR spectra of three consecutive reactions of the methanolysis of cis-2,3,-epoxybutane utilizing MIL-53(Al)-AMMal showing essentially quantitative conversion (black), ∼97% conversion (red), and 90% conversion (blue). The MIL was regenerated with HCl between each reaction cycle.

Scheme 1

Synthesis (top left) and structure (top right) of MIL-53(Al)-NH₂. Postsynthetic modification reactions performed on MIL-53-(Al)-NH₂ relevant to this study (bottom).

Scheme 2

Attempted methanolysis of cis-2,3-epoxybutane with carboxylate functionalized MIL-53(Al)-AMMal and MIL-53(Al)-AMSuc. Letters a∼f represent the protons associated with starting material and the ring opening product. Also listed are structural analogues MIL-53(Al)-NH₂, MIL-53(Al)-AM1, MIL-53(Al)-AMCrot that were used for control reactions.