Synthesis of Chiral MOF-74 Frameworks by Post-Synthetic Modification by Using an Amino Acid

Abstract

The synthesis of chiral metal-organic frameworks (MOFs) is highly relevant for asymmetric heterogenous catalysis, yet very challenging. Chiral MOFs with MOF-74 topology were synthesised by using post-synthetic modification with proline. Vibrational circular dichroism studies demonstrate that proline is the source of chirality. The solvents used in the synthesis play a key role in tuning the loading of proline and its interaction with the MOF-74 framework. In N,N'-dimethylformamide, proline coordinates monodentate to the Zn²⁺ ions within the MOF-74 framework, whereas it is only weakly bound to the framework when using methanol as solvent. Introducing chirality within the MOF-74 framework also leads to the formation of defects, with both the organic linker and metal ions missing from the framework. The formation of defects combined with the coordination of DMF and proline within the framework leads to a pore blocking effect. This is confirmed by adsorption studies and testing of the chiral MOFs in the asymmetric aldol reaction between acetone and para-nitrobenzaldehyde.

Keywords: chiral induction; chirality; defects; metal-organic frameworks; post-synthetic modifications; vibrational circular dichroism.

Figure 1

The left‐ (M or L) and right‐handed (P or D) helical SBU rods of Zn‐MOF‐74.

Figure 2

The tautomeric species of l‐ or d‐Pro and their expected coordination to the SBUs of MOF‐74.

Figure 3

View along the c axis of the cell of Zn‐MOF‐74 fully loaded with proline coordinated in a monodentate manner.

Figure 4

PXRD patterns of the as‐synthesised Zn‐MOF‐74 (black) and Zn‐MOF‐74‐l‐Pro synthesised in MeOH (blue) and DMF (red). The calculated PXRD pattern of Zn‐MOF‐74 (orange) was obtained from literature. ^[53] .

Figure 5

TGA (continuous line) and DSC (dotted line) curves of Zn‐MOF‐74 (black) and Zn‐MOF‐74‐l‐Pro synthesised in MeOH (blue) and DMF (red).

Figure 6

FTIR spectra of l‐Pro (green) and Zn‐MOF‐74‐l‐Pro (red) (top) and VCD spectra of the Zn‐MOF‐74‐l‐Pro (red) and Zn‐MOF‐74‐d‐Pro (blue) (bottom), all MOF materials being synthesised in DMF.

Figure 7

SEM images of Zn‐MOF‐74‐l‐Pro (top) and Zn‐MOF‐74‐d‐Pro (bottom).

Figure 8

N₂ adsorption isotherms measured at 77 K for Zn‐MOF‐74 (orange), Zn‐MOF‐74‐l‐Pro synthesised in MeOH (blue) and Zn‐MOF‐74‐l‐Pro synthesised in DMF (red).

Scheme 1

The asymmetric aldol reaction between acetone and 4‐nitrobenzaldehyde (pNBA) to obtain the chiral β‐aldol product. The reaction can also undergo water elimination to form the α,β‐unsaturated product.

Figure 9

Enantioselectivity towards the (R)‐β‐aldol product in the asymmetric aldol reaction between acetone and pNBA by using 30 mol % loading of Zn‐MOF‐74‐l‐Pro in 4:1, v/v, THF/acetone (black) or DMF/acetone (grey). Literature reported ee values for using 10 mol % loading of CMIL‐1. ^[27] 5 mol % loading of [Zn(l‐Pro)₂] (blue) was tested in H₂O. ^[65]