{'fraction_non_alphanumeric': 0.035929219437707716, 'fraction_numerical': 0.021288062516841823, 'mean_word_length': 4.881669307976757, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Decisive Role of Spin-States and Spin-Coupling in Dictating Selective O2 Adsorption in Chromium(II) Metal-Organic Framework

Coordinatively unsaturated Chromium (II)-based Cr3[(Cr4Cl)3(BTT)8]2 (Cr-BTT; BTT=1,3,5-benzenetristetrazolate) metal-organic framework (MOF) has been shown to exhibit exceptional selectivity towards adsorption of O2 over N2/H2. Using periodic Density Functional Theory (DFT) calculations, we attempt to decipher the origin of this puzzling selectivity. By computing and analyzing the magnetic exchange coupling, binding energies, the partial density of states (pDOS), and adsorption isotherms for the pristine and gas bound MOFs [(Cr4(X)4Cl)3(BTT)8] 3(X=O2, N2, and H2), we unequivocally establish the role of spin-states and spin-coupling in controlling the gas selectivity. The computed geometries and gas adsorption isotherms are consistent with the earlier experiments. The O2 binding to the MOF follows an electron-transfer mechanism resulting in a Cr(III) superoxo species (O2 ) with a very strong antiferromagnetic coupling between the two centers, while N2/H2 found to weakly interact with the metal centre and hence only slightly perturb the associated coupling constants. Although the gas bound and unbound MOFs have S =0 ground state (GS), the nature of spin configurations and the associated magnetic exchanges are dramatically different. The binding energy and the number of oxygen molecules that can favorably bind to the Cr center were found to vary with respect to the spin-state with a significant energy margin (47.6 kJ/mol). This study offers a hitherto unknown strategy of utilizing spin-state/spin-couplings to control gas adsorption selectivity in MOFs.


Introduction
Metal Organic Frameworks (MOFs), owing to their fascinating properties such as structural and functional diversity, tunable pore size, and enormous surface area, have tremendous applications in gas storage 1 , molecular separation 2 , sensors 3 , and catalysis. 4 Though zeolites, covalent organic frameworks, and complex hydrides are available for gas separation applications 5 , MOF based separative systems have attracted wide attention due to their high selectivity as a result of their extensive chemical diversity and porous nature in comparison to other classical adsorbate systems. 6 The microporous nature of the MOFs enables systematic immobilization of the functional materials and are envisaged as a practical promise for gas purification and separation applications at an industrial scale. Whilst several MOFs based on first-row transition metals are known for switchable magnetic characteristics under the influence of guest molecule 7 , temperature 8 , and pressure, 9 there are only a few reports on high selectivity towards oxygen. The selective adsorption of oxygen over other gases that generate high purity oxygen (> 99%) has applications in healthcare, oxy-fuel combustion 10 , ozone generation, semiconductors and other industries. This is a challenging task, as the separation of O2 from the N2 mixture is a hassle due to the similarity in their physical properties 11 , such as kinetic diameter, polarizability, boiling point, etc. The current industry standard is cryogenic separation, which demands high pressure, temperature and large infrastructure that escalate production costs. Thus, an alternative separation technique based on porous material is in high demand. Long et al. recently reported Cr3[(Cr4Cl)3(BTT)8]2 (BTT 3-=1,3,5-benzenetristetrazolate) 12 MOF, which exhibits very high selectivity for oxygen over hydrogen and nitrogen. Despite the structural similarity of this MOF to other metal ion MOFs reported, this shows high selectivity towards O2 compared to N2 and is currently one among the best MOFs for O2 selective separations. 12 While multiple factors can influence gas adsorption in MOFs, the origin of very high selectivity for O2 is perplexing because structurally similar metal ion MOFs are unselective. Though the role of spin state and spin coupling in selective gas binding is not well studied in MOFs, there are reports on the magnetic separation of small molecules. 13 The spin states-based selectivity of gas in MOF 14 needs to be addressed in detail.

Computational details
All calculations were carried out using periodic Density Functional Methods 15  approach with a kinetic energy cutoff of 400 Ry. The Grimme's D2 correction (DFT-D2) was used to account for the dispersion interactions. We have also performed calculations using D3 dispersion correction, which yields very similar geometries, binding energies, and J values (see Table S1). The boundary conditions a=b=c, which is set as 18.6695 Å. 17 Our earlier studies on [Mn19] cluster reveal that the combination of PBE with plane-wave basis sets yields J values which are similar to the estimate obtained from B3LYP employing Gaussian type basis set. The PBE functional with the plane-wave basis set is shown to offer less delocalization, perhaps due to confinement, and this improves the numerical estimate of J compared to hybrid B3LYP functional. This benchmark established by Ruiz et al. and also by us earlier offers confidence in the methodology chosen. [17][18] Adsorption isotherms were simulated through the Grand Canonical Monte Carlo technique as implemented in the RASPA software. 19 The Lennard−Jones (LJ) parameters for the considered framework atoms were taken from Universal Force Field (UFF), 20 , and Lorentz-Berthelot mixing rules are used to calculate the cross interactions. Both N2 and O2 molecules were modeled using the TraPPE force field. 21 We used 50,000 equilibration cycles and 50,000 production cycles for each simulation and considered insertion, deletion, translation, and rotation moves. Since the chemisorption of O2 cannot be modeled using the GCMC technique, we considered initial framework structures with the different number of O2 molecules chemisorbed, and the corresponding weight of O2 is also considered for plotting the O2 loading in addition to the loading obtained from the GCMC simulations.

Results and Discussion
In this work, we have undertaken detailed periodic DFT calculations 16 to decipher the origin of the selectivity observed starting from the X-ray structure reported 12 (Figure 1(a)). This methodology has proven to yield very good geometries for open-shell MOFs and has shown to yield a good numerical estimate of exchange coupling constants (J). 22 We have optimized the geometry of  Table S2). Computed O-O stretching frequencies on a [(Cr4Cl)3(BTT)8] 3model is consistent with experiments (1206 cm -1 vs. 1193 cm -1 ). 12 For the unbound MOF, the antiferromagnetic J2 is the strongest, followed by J1 and weakly ferromagnetic J3, and this trend is in line with the available magneto-structural correlations. [24][25] The pseudo-Jahn-Teller elongation along the Cr-Cl bond results in dominant dz 2 |pz|dz 2 overlap leading to a strong antiferromagnetic coupling. The J3 interaction is mediated via longer tetrazolate bridges, and hence it is relatively weak.     This large exothermicity computed reveals the favorable formation of end-on Cr(III)-O2 superoxo species. 12 The BE reported by us for a well characterized Cr(III)-O2 superoxo species [Cr(14-TMC)(OO)(Cl)] + is -77 kJ/mol, which is significantly smaller compared to MOFs. 23,26 In both cases, the -Cl ligand is found to be in the trans position, unveiling the importance of -donor ligands in the stability of such transient species. 27 To understand the role of spin-state in gasbinding, we computed the BE for various spin-ladders estimated from the exchange interactions.
If a higher excited S =0 state is considered, the BE is found to differ by a few kJ/mol (see Figure   3). For the high-spin (HS) state, the BE is relatively unfavorable (-87.9 kJ/mol per site with respect to the GS), and this is 47.6 kJ/mol (per site) higher compared to the GS, suggesting strong dependency of binding energy with respect to the spin states. The O2 adsorption isotherm experimentally reported reveals that it is extraordinarily steep, reaching 7.01 wt% at 0.2 bar and saturates at 7.56 wt% at 1 bar, which is smaller than 11.3 wt% expected if all Cr ions are bound to O2. This approximately translates to O2 binding in eight Cr centers among the 12 ions present in our adapted unit cell. In order to check the difference in BE with respect to the number of oxygen molecules to see if a cooperative binding is present, we estimated BE by varying the number of O2 molecules from 2 (leaving ten chromium sites vacant) to 12. As we go from 2 to 8 (~6.3 kJ per site), BE becomes larger and larger, with the largest per site BE noted for eight oxygen molecules (-139.4 kJ/mol). The addition of further oxygen molecules tends to decrease the BE (2.5 kJ/mol per site), suggesting that 10 or 12 oxygen (full saturation point) binding is not very favorable, and this is consistent with experiments. The addition of oxygen beyond eight units tends to increase the steric crowding at the tetrameric binding sites and also demands oxidation of all the Cr II to Cr III at the tetrameric unit cell. Both of which has a larger energy penalty, diminishing the binding energy.
For the HS state, different behavior is noted, with the most favorable BE found for six O2 molecules with a much steeper energy penalty for additional oxygen bindings (12.8 kJ/mol per site). Again, spin-state was found to dictate not only the BE preferences but also the number of oxygen molecules that can bind to the MOF in a favorable manner (see Figure 4c). This suggests that switching of the spin-state/exchange coupling is therefore expected to influence the selectivity considerably. To further understand the binding of oxygen, we have simulated the adsorption isotherms at 298 K and 0-4 bar using the RASPA suite (see Figure 4  Å. This reveals a partial formation of Cr(III)-N2 - species at N-N~1.34 Å, with an energy penalty of ~150 kJ/mol (per site), revealing the reason for very high selectivity towards O2 (see Figure   4d). Unlike O2, the isotherm obtained for N2 shows a similar binding trend for HS and S=0 GS, suggesting that spin-state dependency in N2 is absent (Figure 4(b)). A similar trend is also visible for the H2 binding, unveiling the reason for the very high selectivity towards O2 for this MOF.

Conclusions
To this end, using periodic DFT calculations, we have shown that spin-state/exchange interaction plays a pivotal role in dictating gas adsorption selectivity in Cr MOF. Among the O2/N2/H2 gases studied, the electron transfer mechanism adopted by O2 leads to its strong spin-state dependent binding while the other two gases do not exhibit such drastic variations. As the O2/N2 selective adsorption/desorption process has high commercial value, this study offers a hitherto unknown strategy of utilizing spin-state/spin-couplings switching to control gas adsorption selectivity in MOFs.

Conflicts of interest
There are no conflicts to declare.
TOC text: The importance of spin-state and spin-coupling in gas binding selectivity in MOFs are uncovered using periodic DFT calculations.