Evaluating the Robustness of Metal-Organic Frameworks for Synthetic Chemistry

Abstract

Metal-organic frameworks (MOFs) are emerging as sustainable reagents and catalysts with promising applications in synthetic chemistry. Although the hydrothermal stabilities of MOFs have been well studied, their robustness toward various reagents, including acids, bases, nucleophiles, electrophiles, oxidants, and reductants, remains poorly characterized. As such, heterogeneous platforms for promising catalysts are generally identified on an ad hoc basis and have largely been limited to carboxylate frameworks to date. To address these limitations, here we systematically characterize the robustness of 17 representative carboxylate, salicylate, and azolate MOFs toward 30 conditions representing the scope of synthetic organic chemistry. Specifically, analysis of the full width at half-maximum of powder X-ray diffraction patterns, as well as infrared spectroscopy, 77 K N₂ adsorption measurements, and scanning electron microscopy in select cases are employed to appraise framework degradation and dissolution under a range of representative conditions. Our studies demonstrate that azolate MOFs, such as Fe₂(bdp)₃ (bdp^2- = 4,4'-(1,4-phenylene)bis(pyrazolate)), generally possess excellent chemical stabilities under myriad conditions. In addition, we find that carboxylate and salicylate frameworks possess complementary stabilities, with carboxylate MOFs possessing superior robustness toward acids, electrophiles, and oxidants, and salicylate MOFs demonstrating improved robustness toward bases, nucleophiles, and reductants. The guidelines provided herein should facilitate the rational design of robust frameworks for applications in synthetic chemistry and guide the development of new strategies for the postsynthetic modification of MOFs as well.

Keywords: catalysis; metal−organic frameworks; robustness.; stability.

Figure 1.

Carboxylate metal–organic frameworks studied as part of this work, including MIL-100 (Fe, Cr) or M₃O(OH)(btc)₂ (M = Fe, Cr), MOF-808 or Zr₆O₄(OH)₄(btc)₂(HCO₂)₆ (btc³⁻ = benzene-1,3,5-tricarboxylate), UIO-66 or Zr₆O₄(OH)₄(bdc)₆ (bdc²⁻ = benzene-1,4-dicarboxylate), UIO-67 or Zr₆O₄(OH)₄(bpdc)₆ (bpdc²⁻ = 1,1′-biphenyl-4,4′-dicarboxylate), and PCN-128 or Zr₆O₆(OH)₂(etbptc)₂(CF₃CO₂)₂(H₂O)₂ (etbptc⁴⁻ = 4′,4′′′,4′′′′′,4′′′′′′′-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1′-biphenyl]-4-carboxylate))). Gray, white, red, orange, pale blue, and bright green spheres represent carbon, hydrogen, oxygen, iron/chromium, zirconium, and fluorine, respectively.

Figure 2.

Salicylate metal–organic frameworks studied as part of this work, including MOF-74 or M₂(dobdc) (M = Mg, Ni; dobdc⁴⁻ = 2,5-dioxidobenzene-1,4-dicarboxylate), Ni₂(m-dobdc) (m-dobdc⁴⁻ = 4,6-dioxidobenzene-1,3-dicarboxylate), Mg₂(dobpdc) (dobpdc⁴⁻ = 4,4′-dioxido-1,1′-biphenyl-3,3′-dicarboxylate), and Mg₂(dotpdc) (dotpdc⁴⁻ = 4,4′′-dioxido-[1,1′:4′,1′′-terphenyl]-3,3′′-dicarboxylate). Gray, white, red, dark green, and black spheres represent carbon, hydrogen, oxygen, magnesium, and nickel, respectively.

Figure 3.

Azolate metal–organic frameworks studied as part of this work, including ZIF-8 or Zn(2mim)₂ (2mim⁻ = 2-methylimidazolate), Ni₂Cl₂(btdd) (btdd²⁻ = bis(1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin)), Ni₃(btp)₂ (btp³⁻ = 4,4′,4′′-(benzene-1,3,5-triyl)tris(pyrazolate)), Zn(bdp), Ni(bdp), and Fe₂(bdp)₃ (bdp²⁻ = 4,4′-(1,4-phenylene)bis(pyrazolate)). Gray, white, dark blue, sky blue, black, and orange spheres represent carbon, hydrogen, nitrogen, zinc, nickel, and iron, respectively.

Figure 4.

Proposed mechanism for the decomposition of MOF-74 frameworks in air via quinone formation.