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

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 (1L2–) 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 CuI concentration is observed with incorporation of defective linkers, correlating with the elevated isosteric heat of adsorption (ΔHads). Overall, this approach offers valuable insights into the creation and evolution of CUSs within MOFs through the integration of defective linkers.


S1. Chemicals
All chemicals were commercially available.
Yield: 1.52 g, 48 % of theory.In inert atmosphere 2.75 g (1 eq, 0.020 mol) of 3-aminobenzoic acid and 7.50 ml (5 eq, 8.41 g, 0.100 mol) of 2-methyl-1,3,4-oxadiazole were stirred in 30 ml of toluene at 90 °C for 48 h.The crude product was filtered off, then refluxed with a mixture of water, ethanol and methanol (1:1:1; v/v/v) and filtered while hot.The product was dried on air to yield a white solid.

Solvothermal synthesis:
A stainless steel autoclave with Teflon inset (PARR) was loaded with H2 1 L, H 2 L, copper chloride dihydrate and 10 ml solvent (DMF:EtOH), sealed and the reaction mixture was heated within 1 h up to 120 °C.The temperature was kept on a constant level for 5 h, then the autoclave was cooled slowly to room temperature during a period of 60 h.The naming convention used in this manuscript describes DEMOF samples based on the molar ratio of linkers they contain within their framework, such as DEMOF_2.9% and DEMOF_7.0%(see Sections S8 and S9 for determination of the percentages of incorporated linkers).In order to have defined conditions, the MOF samples were solvent exchanged with MeOH and stored in glass vials under MeOH.For measurements, the solvent was removed and the sample was activated in vacuum.
The results of the single crystal X-ray diffraction analysis were reported by Kobalz et al. 2 Formula: C22H14N6O8Cu2, Molecular Mass: 617.47 g mol -1 Space group P21/c (No.

S5.2 Strain effect
Micro-strain broadening 3 effects in the PXRD patterns of the synthesized MOFs were considered by extracting the ε₀ and β(FWHM(Strain)) values using TOPAS (Bruker AXS) software 4 from the broadest peak identified in PXRD.
PXRD data are influenced by defects such as displacements, vacancies, interstitials, substitutions, and related effects.Microstrain can be conceived by considering two extreme values of the lattice spacing d, namely d + Δd and d -Δd, where ₀ = Δd/d represents the relative "mean" deviation (more precise: 50% probability of the undistorted state).We considered the most intense and highly broadened peak around 2Θ = 9.4⁰ for the microstrain determination.

S6. FTIR Spectroscopy
The infrared spectra were recorded with a Bruker Vertex 80V FTIR spectrometer utilizing OPUS 6.0 (Bruker) for analysis.All samples were prepared as KBr pellets from 2 mg of MOF sample and 200 mg of KBr.For further analysis, the samples were measured before and after treatment at 100 °C for removal of water molecules.Normalization and baseline correction were done after spectra acquisition.
The bands at 473 and 726 cm -1 are assigned to Cu-O bending and stretching vibrations, respectively.This agrees well with references, 5,6 where those vibrations are assigned to the Cu-O group.The band around 1450 cm -1 belongs to combination of benzene ring stretching and deformation modes.Bands between 1300-1500 cm -1 are attributed to symmetric stretching vibrations and 1500-1700 cm -1 to antisymmetric stretching vibrations of the carboxylate groups.The band at 675-814 cm -1 is probably the aromatic out of plane (oop) bending vibration for aromatic C-H.Two emerging peaks at 560 cm -1 and 1000 cm -1 , attributed to (CCC) in-plane bending and twisting ring modes 78 , are not significant in ReMOF but become well distinct after 2 L -incorporation.

S7. Raman Spectroscopy
The Raman spectra (Figure 2(b), main manuscript) were acquired on a confocal modular Raman measurement setup (S&I Spectroscopy & Imaging GmbH, Warstein, Germany) equipped with a 532 nm excitation laser (Cobolt AB, Solna, Sweden) with a power of 100 mW.For Raman measurements, the sample was positioned on an IX71 epifluorescence upright microscope (Olympus Corporation, Tokyo Japan), which was equipped with a LUCPlanFI 40-fold objective (NA 0.6, Olympus Corporation, Tokyo Japan) for focusing the laser beam onto the sample.The incident (excitation) laser power on the sample was approx 5-6 mW, controlled using a ND (Neutral-density) filter wheel (Thorlabs INC, NJ, USA).The scattered light passed an Andor Kymera 193i Spectrograph (Oxford Instruments, Abingdon, United Kingdom) with an entrance slit of 100 μm and a grating of 1800 lines/mm and was detected via an Andor Newton 1024 x 255 CCD camera (Oxford Instruments, Abingdon, United Kingdom).
An acquisition time of 1 s was used for all measurements.The spectra were collected in the range of 190 to 1900 cm -1 with a spectral resolution of 3 cm -1 .VistaControl V4.2 Build 12596 (S&I Spectroscopy & Imaging GmbH, Warstein, Germany) was used as interface and recording software 9 .
According to the spectrum, Cu 2+ ion-related vibrational modes cause peaks up to 600 cm -1 .Cu-Cu dimer moieties are responsible for the stretching mode that causes the peaks from 190 to 270 cm -1 .An insignificant peak at 400-490 cm -1 is attributed to the Cu-O stretching mode of the coordinated carboxylate bridges.The C-H bending vibration modes are seen at 756 cm -1 and 866 cm -1 , respectively.The symmetric stretching mode of C=C moieties can be seen at about 1012 cm -1 .The vibrational mode of carboxylate groups is responsible for the region of 1400 to 1600 cm -1 .The peaks around 1389 cm -1 belong to C-N moieties 10 of the triazole group.All the Raman spectra in the current study very well match with those of HKUST-1 type MOFs. 7

S8. NMR Spectroscopy
An Avance DPX-300 NMR spectrometer (Bruker) was used to record the solution phase 1 H NMR spectra (300 MHz).For determining the ratio 1 L 2-: 1 L -in the DEMOFs [Cu2( 1 L(1-x) 2 Lx)2], the MOFs are digested using deuterated digestion media (1M NaOH in D2O) prior to 1 H NMR analysis.The solvent D2O was used as internal lock, the spectra were referenced to D2O 4.79 ppm (reference peak is omitted for the better visualization of spectra).Data acquisition was performed with a delay time of 2 to 2.5 s.Liquid solution 1 H NMR spectroscopy allows for the quantitative analysis and elucidation of the organic components such as the linker, modulator, and pore filling fluids (as a molar ratio with other organic components).
Before digestion, all samples were thoroughly washed with DMF: EtOH and subsequently went through solvent exchange and evacuation up to 7 days, i.e., the samples are fully activated without any solvent inside the pore.For determination of the ratio We determined the molar fraction of 2 L -within the dissolved linker mixture of 1 L 2-and 2 L -by following the procedure described in a previous paper 11 : I and I are the integrated 1 H NMR intensities of specific protons of 1 L 2-and 2 L -in the 1 H NMR spectra, N1 and N2 are the corresponding numbers of equivalent hydrogen atoms.Peaks around 2.30 (s, 3H, CH3-triazole) from 2 L -and 2.35 (s, 3H, CH3-triazole) from 1 L -have been utilized for this integration.
Table S2: Peak integration and ratios of the amount of 2 L -to that of total linkers in the obtained DEMOFs.
Column: ET 250/8/4 NUCLEOSIL, 5 NH2, 250 mm, 5 µm particles Calibration curve: To get the appropriate calibration curves and the defined amounts of 2 L -in a particular DEMOFs sample, the absorption of organic linkers associated with 6 different concentrations was measured and calibrated by determining the respective peak area.
The DEMOFs were synthesized with 8%, 12%,16%, 20% and 24% of defective linker (H 2 L) in the reaction mixture.For sample preparation, 5 mg of the DEMOF were dissolved in 3 ml 0.5M NaOH (aq) under sonication, the solution was filtered, diluted, and samples of the stock solution (in ACN: H2O, 50:50) were analysed by injecting 5 µL of the respective solution into an NH2 based column.Separation of 1 L 2-and 2 L -is done by hydrophilic interaction chromatography (HILIC) under isocratic conditions. 19ble S3: HPLC analysis of digested MOFs.

S10. Simultaneous Thermal Analysis (TG-MS)
The TG-MS analyses were carried out using corundum crucibles on a thermobalance STA 449 F1 Jupiter (Netzsch) 12 coupled to an Aeolos QMS 403C mass spectrometer.The sample was heated at a rate of 10 K min -1 up to 900 °C under constant flow of Argon (99.999 %).All samples were Solvent exchange with methanol to achieve complete solvent exchange.Prior to each measurement the sample was evacuated at room temperature in the instrument, however, the crucibles had contact to air when transferred to the sample holder in the instrument.This might lead to adsorption of minor amounts of water, resulting in an initial weight loss.
Initial Mass Loss: Before the decomposition phase, there is a minor mass loss which might correspond to the desorption of weakly bound molecules such as water or residual solvents.The ReMOF sample begins to decompose at 289.7 °C (onset temperature) and this decomposition process concludes at 313.9 °C.The total mass change during this phase suggests decomposition of the organic components.DEMOF_2.9% shows a similar pattern, with decomposition starting at a slightly lower temperature of 284.6 °C and ending at 312.7 °C.DEMOF_7.0%starts decomposing at an even lower temperature of 279.8 °C and ends at 293.1 °C, indicating that 2 L -incorporation affects its thermal stability.
The decomposing temperature range in each sample is critical as it provides insight into the thermal stability.The lower onset temperatures in the DEMOFs sample compared to the parent ReMOF could be due to a higher defect concentration, which leads to lower thermal stability.Similar results were also noticed by Fang et al. 11 in defect engineered HKUST-1 MOFs with varied linker incorporation.Increasing the defective linker concentration leads to decreasing onset temperatures compared to parent HKUST-1 MOF.S11.X-ray Photoelectron Spectroscopy (XPS) X-ray Photoelectron Spectroscopy: In order to distinguish between different oxidation states.Cu I and Cu II , in the ReMOF and DEMOF samples, Instrumentation details: X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Thermo Fisher Scientific K Alpha+ XPS system (Thermo Fisher Scientific Instruments, UK).For the measurements, monochromatic Al-Kα radiation was utilized, generated in a sealed X-ray tube with a beam current of 6 mA and an acceleration voltage of 12 kV.Instrument calibration was verified using the Ag 3d peak at 352 eV.The spot size for analysis on the sample was 400 micrometers.Binding energies were referenced to the C1s peak at 284.8 eV.
All spectra were calibrated with respect to adventitious carbon observed at 285 eV.A survey scan was performed on each sample before the elemental scans in order to identify every element that was present.Using the CASA XPS tool 13 , the XPS spectra were deconvoluted using a Gaussian Lorentzian mix function with smooth linear background subtraction.
In all the XPS survey scans for ReMOF and DEMOF samples, the spectra display the distinctive peaks from copper, Cu LMM Auger, oxygen, nitrogen, and carbon for each sample.The accompanying Cu LMM Auger peaks at 916.5 eV, typical for Cu I -Cu II pairs and at 917.2 eV 18 , typical for Cu II -Cu II species, provide additional evidence that two different types of dinuclear units are present in Cu LMM Auger spectra.

S12. Electron Paramagnetic Resonance (EPR)
Sample Preparation: Approximately 25 mg of as-synthesized ReMOF and DEMOF samples in methanol were placed inside the quartz tube (inner diameter 3.8 mm).EPR studies for all ReMOF and DEMOFs were carried out at solvated state, i.e., in contact to methanol.
cw EPR Measurements: A series of X-band and Q-band cw EPR measurements were conducted using the commercial spectrometers EMXmicro and EMX 10-40 (Bruker), respectively.The X-band cw EPR spectra were measured at temperatures ranging from T = 10 K to T = 160 K by means of a Bruker EMXmicro spectrometer fitted with a Bruker ER4119HS cylindrical cavity and the He cryostat ESR900 (Oxford instruments) while in case of Q band measurements, the spectrometer was fitted with a cylindrical cavity and an CF935 cryostat (Oxford Instruments) with a working temperature range between T = 20 K and 295 K.The microwave power and modulation amplitude were adjusted such that minimum signal distortion can be obtained.EPR spectral analysis was performed the Easyspin numerical simulation package for MATLAB 17 .

S13. Gas Adsorption, Isosteric Heat of adsorption
High resolution low-pressure isotherms up to 1 bar were measured at 24 K and 67 K to 87 K using a Belsorp max (Microtrac MRB) equipped with a closed cycle helium cryostat.Before the measurement the sample (50-100 mg) was activated overnight in dynamic vacuum.The measurement was conducted in the pressure range p/p0 = 10 -5 -1.Data evaluation was performed using BEL Master 6.3.0.0 software.Helium gas of 99.999% purity was used for determination of the dead volume after each measurement and H2, CO2 gas of high purity 99.999% was used for physisorption.
Pore Size Distribution: The pore size distribution was calculated based on CO2 adsorption isotherms at 298 K, p max = 1 bar using a Belsorp max G (Microtrac, Version: 1.1.0)equipped with a thermostat.BELmaster version 7.3.2.0 software with computer simulation method (HK: Horvath and Kawazoe method) 21 was utilized for calculating the pore size distribution.CO2 gas of high purity 99.999% used for physisorption.Parameter used Interpolated Curve  Figure S21 demonstrates that, while the desorption process is quick, adsorption needs a long time (about 1 h) for equilibration.Thus, desorption isotherms were employed in the context of this study for additional analysis.
Determination of the heat of desorption  (= - ) was carried out using Clausius-Clapeyron approach 18 .ReMOF and DEMOF samples exhibit Langmuir-Freundlich-type isotherms under the aforementioned conditions, which can be fitted using the Langmuir- Freundlich equation ( 1),  = . . .(1)  where n is the amount of adsorbed material (loading) in mmol g -1 , p denotes the pressure in kPa, a denotes the maximum loading in mmol g -1 , b denotes the affinity constant (1/kPa c ), and c denotes the heterogeneity exponent, the multiplication of bp c should be dimensionless.We have calculated the respective pressures for respective temperatures T consecutively at the same loading n by plugging in the respective values for a, b and c derived from the Langmuir-Freundlich fit, then compared the pressures at isosteric conditions, that is at the same uptake of the adsorbate.The fitting routine leads to a continuous sequence of loading n vs pressure p data pairs.Following the fitting process, the isotherms for n vs p data sets with the same loading n are evaluated at each temperature.This method is known as "isosteric" approach, which relies on the Clausius-Clapeyron equation (2) given below with R = ideal gas constant (R = 8.314 J mol -1 K -1 ).
Plotting ln (p) against 1/T for the isosteric adsorptions, that is, given equal n at the three temperatures, yields the isosteric enthalpy of adsorption using the aforementioned equation ( 2) (see Figures S26-S28) As a result, for various uptakes, ln(p) is plotted against the reciprocal temperature 1/T.The slope of the linear fit to this data for each uptake is proportional to the isosteric heat of adsorption.
Fitting of isotherms using Freundlich-Langmuir fit.The data depicted in Figures S22 and S30 (top) reveal a progressive increase in H2 and CO2 gas adsorption capacities transitioning from ReMOF to DEMOF samples.For both cases, CO2 and H2, this pattern correlates with an enhancement of gas adsorption due to stronger adsorption site at low pressure and also due to enhanced porosity across the samples at higher pressure due to higher defect concentrations.To delve deeper into the porosity characteristics, Horvath-Kawazoe (HK) analysis 21 using CO2 gas was employed.This approach is particularly suited for studying ultramicroporous MOFs, which cannot be accurately probed using other adsorbates like N2 at 77 K due to their larger size.CO2, being a smaller molecule, serves well for assessing the ultramicroporous nature of these materials.A noticeable hysteresis observed across all samples is indicative of the flexible nature of the MOF structure, a phenomenon previously reported in studies involving high-pressure CO2 gas adsorption analysis. 2 Figure S30 (bottom), several key points emerge regarding pore volume and width: The ReMOF exhibits the least total pore volume, quantified at 4.0•10 −3 cm³/g.The DEMOF_2.9% displays a modestly greater pore volume of 5.4•10 −3 cm³/g, underscoring that even minimal defect incorporation can appreciably influence porosity.DEMOF_7.0%presents a markedly higher pore volume, 7.4•10 −3 cm³/g, denoting an increase in porosity correlating with the rise in defect levels.In contrast to the pore volumes, the average pore width values determined for ReMOF, DEMOF_2.9% and DEMOF_7.0%do not show a significant change.

S14. Scanning Electron Microscopy (SEM) Images
SEM images were captured using a Phenom Pharos G2 Desktop FEG-SEM Tabletop field emission gun scanning electron microscope for high quality imaging.
Acceleration voltages: Default: 15 kV Detector: Back scattered electron detector (standard) Scanning Electron Microscopy (SEM) was employed to examine the effect of defect engineering on the morphology of ReMOF and DEMOF samples, respectively, particularly after the evaporation of the solvent.The observed particle size varies between 50 μm and 10 μm in length, 4 μm to 7 μm in breadth.The crystallites exhibit a rough texture and numerous cracks, breaking of bigger crystals into smaller particles in the case of ReMOF and DEMOF_2.9% (Figures S31-S33) suggests that the framework structure undergoes significant changes in response to solvent evaporation under vacuum.However, we observed that with increasing defective linker concentration the crystallite sizes increase in length, however, their breadth decreases and also the morphology changes from block (ReMOF) to rod shape (DEMOF_7.0%).
The increase in length (Figures S31-S33), ReMOF (14-6.6 μm), DEMOF_2.9% (34-6.4μm), DEMOF_7.0%(41-4.8μm)) and decrease in breadth of the DEMOFs as the defect percentage increases suggests that defects promote growth in one dimension while inhibiting it in others.This could be due to the defects altering the surface energies of different crystal facets, affecting growth rates, or selectively stabilizing certain growth directions.CHN elemental analyses were carried out on a VARIO EL analyzer (Elementar).The copper content was determined by ICP-OES analyses performed on an Optima 8000 instrument (Perkin Elmer).For sample preparation, the MOFs were dissolved in nitric acid.All the samples were solvent exchanged with MeOH and evacuated before analysis.

Figure S2 .
Figure S2.Fragment of the crystal structure of ReMOF (as synthesized) showing the coordination of the ligand 1 L 2-and the paddle wheel unit as a six-fold connecting node.

)
represents the full width at half maximum of the line profile component related to the microstrain broadening.

Figure S6 .
Figure S6.Variation of  and

Figure S18 .
Figure S18.Temperature dependence of X-band cw EPR spectra for ReMOF and DEMOF samples.All fitted data of these samples according to Bleaney-Bowers equation is also indicated.

Figure S19 .
Figure S19.Q-band cw EPR spectra for ReMOF sample recorded at 110 K.The red line represents the simulated spectrum for species A. M indicates the signal of a mononuclear Cu II species.

Table S1 :
Molar ratios for the synthesis of the ReMOF [ Colour change in DEMOFs with increasing content of 2 L - 1 L 2-: 2 L -in the MOFs, 20 mg of the mixed linker MOFs was dissolved in 0.7 ml of a digestive medium containing NaOH (1M) in D2O in a test-tube.The test-tube was sealed, kept for sonication for 2 h and then left to digest for 24 hours.The inorganic component of the ReMOF and DEMOFs precipitates as CuO, which was removed by filtration using a Whatman filter paper (glass microfiber).The organic portion of the MOF (linkers and solvent if still left after activation) was transferred to an NMR tube.The molar ratios between the protons can be determined by integration of the corresponding NMR signals.Specific 1 H NMR peaks of 2 L -after digestion are located at δ/ppm = 8.480 (s, 1H, triazole-H); 7.92 (dt, 1H, m-benzoate-H); 7.76 (t, 1H, m-benzoate -H); 7.52-7.57(ddd, 1H, m-benzoate -H); 7.43-7.45(dt, 1H, m-benzoate -H); 2.30 (s, 3H, CH3-triazole).