Exploring Defect-Engineered Metal-Organic Frameworks with 1,2,4-Triazolyl Isophthalate and Benzoate Linkers

Abstract

Synthesis and characterization of DEMOFs (defect-engineered metal-organic frameworks) with coordinatively unsaturated sites (CUSs) for gas adsorption, catalysis, and separation are reported. We use the mixed-linker approach to introduce defects in Cu₂-paddle wheel units of MOFs [Cu₂(Me-trz-ia)₂] by replacing up to 7% of the 3-methyl-triazolyl isophthalate linker (¹L^2-) with the "defective linker" 3-methyl-triazolyl m-benzoate (²L^-), causing uncoordinated equatorial sites. PXRD of DEMOFs shows broadened reflections; IR and Raman analysis demonstrates only marginal changes as compared to the regular MOF (ReMOF, without a defective linker). The concentration of the integrated defective linker in DEMOFs is determined by ¹H NMR and HPLC, while PXRD patterns reveal that DEMOFs maintain phase purity and crystallinity. Combined XPS (X-ray photoelectron spectroscopy) and cw EPR (continuous wave electron paramagnetic resonance) spectroscopy analyses provide insights into the local structure of defective sites and charge balance, suggesting the presence of two types of defects. Notably, an increase in Cu^I concentration is observed with incorporation of defective linkers, correlating with the elevated isosteric heat of adsorption (ΔH_ads). Overall, this approach offers valuable insights into the creation and evolution of CUSs within MOFs through the integration of defective linkers.

Scheme 1. The Strategy Implemented for the Development of DEMOFs is Based on Partial Replacement of the Regular Linker ¹L^2– by the “Defective Linker” ²L^– Lacking One Carboxylate Group

The diagram focuses on Cu–Cu paddlewheel unit changes due to ²L^– incorporation. Color coding: Cu^I, brown; Cu^II, turquoise; C, gray; O, red; and N, blue. Additional structural specifics are excluded for the sake of simplicity.

Figure 1

PXRD pattern (r.t., Cu–K_α1 radiation) of solvent-exchanged ReMOF and DEMOF samples. The simulated pattern is based on single crystal data measured at 180 K.

Figure 2

(a) FTIR spectra of ReMOF and DEMOF samples (δ: bending, ν_sym: symmetric stretching, ν_asym: antisymmetric stretching, τ: twisting) and (b) Raman spectra of ReMOF and DEMOF samples with vibrational frequency assignments.

Figure 3

Molar ratio x = ¹L^2–/(¹L^2– + ²L^–) in the [Cu₂(¹L_(1–x)²L_x)₂] DEMOF as determined by integration of ¹H NMR signals (black) and by HPLC (red) vs H²L/(H₂¹L + H²L) in the reaction mixture for synthesis.

Figure 4

Cu 2p_3/2 XPS spectra of the ReMOF and DEMOF samples. The line shape analysis and copper species quantification were performed using CASA XPS software.

Figure 5

X-band cw EPR spectra of the ReMOF [Cu₂(¹L)₂] measured at 160 K and its spectral simulation of [Cu₂(¹L)₂] by considering contributions of two different species, species A with S = 1 and species B with S = 1/2.

Figure 6

X-band cw EPR spectra of the S = 1/2 section (uncoupled Cu^II species) of the [Cu₂(¹L_1–x²L_x)₂] MOF series measured at 10 K. The simulated spectrum in red is the total contribution of B1 and B2 species with the corresponding weight provided in the small insets.

Figure 7

EPR spectra of the ReMOF (pink) in comparison to DEMOF_2.9% (orange) and DEMOF_7.0% (blue) recorded at X-band frequency and T = 160 K. All spectra were normalized to the intensity of species A for better comparison. The inset shows the comparison of signal intensities of the noncoupled Cu^II (B1, B2) from the abovementioned samples.

Figure 8

ΔH_ads obtained using Freundlich–Langmuir fits and Clausius–Clapeyron equation to the H₂ desorption isotherms on ReMOF and DEMOF samples at 67, 77, and 87 K.