Zn(II) Three-Dimensional Metal-Organic Frameworks Based on 2,5-Diiodoterephthalate and N,N Linkers: Structures and Features of Sorption Behavior

: Two new coordination polymers based on Zn(II) and 2,5-diiodoterephthalate—{[Zn(2,5-I-bdc)bpe}] ( 1 ) and {[Zn(2,5-I-bdc)bpen}] ( 2 )—were synthesized and characterized. Polymers 1 and 2 feature halogen bonding between the I atoms of MOF and DMF guest molecules, which plays a crucial role in structure stabilization. Selectivity of sorption towards different organic substrates was examined.

The latter remark explains why sorption ability and selectivity of MOFs extremely strongly depends not only on the size of the pores/channels and, therefore, the free volume, but also on the features of supramolecular contacts between the organic ligands and guest molecules.Those can be very diverse.Most commonly, hydrogen bond (HB) plays the most important role [32], but there are also other types of non-covalent interactions, such as aromatic•aromatic contacts [33][34][35], halogen (XB) [36,37], chalcogen (ChB) [38,39], pnictogen or tetrel bonds which can also provide a very important contribution to the guest molecule binding [40][41][42].In our opinion, this topic is well worth being investigated further and deeper.
Recently, we focused our attention on iodine-substituted aromatic polycarboxylic acids which, in our opinion, contain several important advantages from the perspective of MOF design.On the one hand, those can serve [43][44][45] as MOF linkers in the same manner that their non-substituted relatives do (there are literally thousands of structurally characterized dicarboxylate-based MOFs with different metals, so we cannot provide an in-depth analysis within this article; however, we can mention several comprehensive reviews by Yahgi et al. and other leading groups in this field [46][47][48][49]).On the other hand, as shown before [50], those can form halogen bonds.Finally, some of those are easily available-for example, 2iodoterephtalic acid (2-I-bdc), which can be prepared from corresponding amino derivative via diazotization [51].Our recent works with 2-I-bdc-based MOFs demonstrated [52] that those indeed can reveal sorption selectivity, as well as I 2 uptake, quite different to MOFs based on corresponding halogen-free ligands.Continuing these studies, we decided to focus on a more iodine-rich ligand-2,5-iodoterephthalic acid (2,5-I-bdc).Herein, we present two new 3D metal-organic frameworks based thereupon-{[Zn(2,5-I-bdc)bpe]} (bpe = 1,2bis(4-pyridyl)ethane (1) and {[Zn(2,5-I-bdc)bpen]} (bpen = 1,2-bis(4-pyridyl)ethylene (2).The compounds were characterized using single crystal and powder X-ray diffraction, as well as by other physical methods (such as thermogravimetric analysis).We also examined their selectivity of sorption towards different mixtures of organic substrates.

Results and Discussion
Complexes 1 and 2 were prepared using the solvothermal method which is widely applied in chemistry of metal-organic frameworks [53,54].In all cases, it yielded single crystals suitable for X-ray diffractometry.As follows from powder X-ray diffractometry data (see Supplementary, Figures S1 and S2), both compounds are formed as single phases.
Interestingly, removal of guest DMF molecule from the pure samples of 1 and 2 (so-called "activation" procedure via presence in acetone for 48 h followed by drying in vacuo; such procedure was successfully used by us before [52]) results in collapse of initial structure and formation of other crystalline phases.However, prolonged keeping of "collapsed" samples in DMF results in regeneration of initial phases of 1 and 2, as follows from powder X-ray diffraction data (Figures 1 and 2), so these MOFs can be regarded as "breathing" ones (similar effects were described for other coordination polymers earlier).Considering the presence of prominent XB between the guest DMF molecules and I atoms of 2,5-bdc (see details below), it can be suggested that this factor plays the key role in structure stabilization.Unfortunately, despite numerous attempts being undertaken, we could not obtain the crystals of "collapsed" phases (neither in the case of 1 nor for 2) suitable for X-ray diffractometry since the "collapse" does not occur via the single crystal-to-single crystal route.

Results and Discussion
Complexes 1 and 2 were prepared using the solvothermal method which is widely applied in chemistry of metal-organic frameworks [53,54].In all cases, it yielded single crystals suitable for X-ray diffractometry.As follows from powder X-ray diffractometry data (see Supplementary, Figures S1 and S2), both compounds are formed as single phases.
Interestingly, removal of guest DMF molecule from the pure samples of 1 and 2 (so-called "activation" procedure via presence in acetone for 48 h followed by drying in vacuo; such procedure was successfully used by us before [52]) results in collapse of initial structure and formation of other crystalline phases.However, prolonged keeping of "collapsed" samples in DMF results in regeneration of initial phases of 1 and 2, as follows from powder X-ray diffraction data (Figures 1 and 2), so these MOFs can be regarded as "breathing" ones (similar effects were described for other coordination polymers earlier).Considering the presence of prominent XB between the guest DMF molecules and I atoms of 2,5-bdc (see details below), it can be suggested that this factor plays the key role in structure stabilization.Unfortunately, despite numerous a empts being undertaken, we could not obtain the crystals of "collapsed" phases (neither in the case of 1 nor for 2) suitable for X-ray diffractometry since the "collapse" does not occur via the single crystal-to-single crystal route.The isostructural 1 and 2 comprise {Zn(I 2 -bdc) 2 L 2 } SBUs (Figure 3, left) that connect with each other in a 3D diamond-like framework (Figure 3, right).The whole structure represents four-fold interpenetration of the frameworks (Figure 4), keeping voids decorated by I atoms.Two crystallographically independent DMF molecules are localized, completely filling the voids; free volume in the absence of the DMF molecules is estimated to be ca.25%.Structures 1 and 2 reveal shortened contacts I•••O (2.98-3.10Å) between the DMF and I 2 -bdc; these values are less than the sum of corresponding Bondi's van der Waals radii (3.50 Å).Via these contacts, each DMF molecule binds with two I atoms.These interactions can be regarded as halogen bonding (XB), and it becomes evident that they are responsible for ordering of guest molecules within the pores.To the best of our knowledge, this is one of the first cases of such ordering or organic molecule achieved due to XB [55,56].In addition, as mentioned above, it is very likely that these interactions play an important role in overall stabilization of both these MOFs.The isostructural 1 and 2 comprise {Zn(I2-bdc)2L2} SBUs (Figure 3, left) that connect with each other in a 3D diamond-like framework (Figure 3, right).The whole structure represents four-fold interpenetration of the frameworks (Figure 4), keeping voids decorated by I atoms.Two crystallographically independent DMF molecules are localized, completely filling the voids; free volume in the absence of the DMF molecules is estimated to be ca.25%.Structures 1 and 2 reveal shortened contacts I•••O (2.98-3.10Å) between the DMF and I2-bdc; these values are less than the sum of corresponding Bondi's van der Waals radii (3.50 Å).Via these contacts, each DMF molecule binds with two I atoms.These interactions can be regarded as halogen bonding (XB), and it becomes evident that they are responsible for ordering of guest molecules within the pores.To the best of our knowledge, this is one of the first cases of such ordering or organic molecule achieved due to XB [55,56].In addition, as mentioned above, it is very likely that these interactions play an important role in overall stabilization of both these MOFs.
To examine the sorption selectivity, we used the approach successfully applied by us earlier [52]: the samples of 1 and 2 were treated to remove the guest solvent molecules and then kept in vapors of bicomponent mixture of organic substrates.After that, the absorbed substrates were extracted by organic solvent and their quantities were measured via comparison of corresponding intensities in 1 H NMR spectra [52].Results are summarized in Table 1 and compared with sorption properties for 2-iodobenzoate-based MOFs reported by us earlier [52].Unfortunately, in all cases, the selectivity for 1 and 2 is lower.The difference between 1 and 2 is especially intriguing considering their high structural similarity; currently, there is no obvious explanation for this phenomenon.Especially discouraging is the low selectivity towards 1,2-dichloroethane (DCE) which is especially interesting in terms of its removal from mixtures and which was expected by us to appear due to possible XB formation.To examine the sorption selectivity, we used the approach successfully applied by us earlier [52]: the samples of 1 and 2 were treated to remove the guest solvent molecules and then kept in vapors of bicomponent mixture of organic substrates.After that, the absorbed substrates were extracted by organic solvent and their quantities were measured via comparison of corresponding intensities in 1 H NMR spectra [52].Results are summarized in Table 1 and compared with sorption properties for 2-iodobenzoate-based MOFs reported by us earlier [52].Unfortunately, in all cases, the selectivity for 1 and 2 is lower.The difference between 1 and 2 is especially intriguing considering their high structural similarity; currently, there is no obvious explanation for this phenomenon.Especially discouraging is the low selectivity towards 1,2-dichloroethane (DCE) which is especially interesting in terms of its removal from mixtures and which was expected by us to appear due to possible XB formation.Results of the TGA experiments for 1 and 2 are presented in SI.In both cases, the first step of decomposition corresponds to the elimination of guest DMF molecules; these processes occur at >120 • C, agreeing well with the literature data.The mass loss matches well with the calculated one.Further heating expectedly results in total decomposition of MOF frameworks.

Experimental Part
The reagents were purchased from commercial sources and used without additional purification.2,5-diiodoterephthalic acid was prepared from p-xylene via the method described earlier [57].

Synthesis of 1
A total of 50 mg (0.16 mmol) of Zn(NO 3 ) 2 •6H 2 O, 70 mg (0.16 mmol) of 2,5-I-bdcH 2 , 30 mg (0.16 mmol) of bpe and 8 mL of DMF were placed into the glass ampoule which was sealed, placed into ultrasonic bath (10 min) and kept at 120 • C for 48 h.Slow cooling resulted in formation of the crystals of 1 on the internal surface of the ampoule.Yield was at 73%.

Synthesis of 2
The procedure was the same as for 1, using bpen instead of bpe, as well as 7 mL of DMF.Yield was at 79%.

X-ray Diffractometry
Crystallographic data and refinement details for 1 and 2 are provided in Table 2. Single crystal XRD data for the compounds were collected with a Bruker D8 Venture diffractometer equipped with a CMOS PHOTON III detector and IµS 3.0 source (Montel mirror optics, λ(MoKα) = 0.71073 Å).Additionally, the cell parameters were checked on a Rigaku XtaLAB Synergy-S (Agilent Technologies) diffractometer with CuKα radiation (λ = 1.54184).Absorption corrections were applied with the use of the SADABS program.The crystal structures were solved using SHELXT [58] and were refined using SHELXL [59] with OLEX2 GUI [60].Atomic displacement parameters for non-hydrogen atoms were refined anisotropically.CCDC 2217437 and 2217438 contain the supplementary data for this paper.These data can be obtained online, free of charge.

Powder X-ray Diffraction (PXRD)
XRD analysis of polycrystals was performed on Shimadzu XRD-7000 diffractometer (CuK-alpha radiation, Ni filter, linear One Sight detector, 0.0143 • 2θ step, 2 s per step).Plotting of PXRD patterns and data treatment was performed using X'Pert Plus software (see supplementary Materials).

Sorption of Organic Substrates
The sorption experiments were conducted according to the procedure reported earlier (interaction of MOF samples with vapors of organic substrates [52]).

Thermogravimetric Analysis (TGA)
Thermogravimetric analyses were carried out on a TG 209 F1 Iris thermobalance (NETZSCH, Germany).The measurements were conducted in a helium flow using the heating rate of 10 • C min −1 , the gas flow rate of 60 mL min −1 , and open Al crucibles.

Conclusions
To conclude, we have demonstrated that 2,5-diiodoterephthalate is a suitable building block for design of porous 3D Zn(II)-based metal-organic frameworks which are porous and reveal sorption properties towards different organic substrates.XRD data confirm that halogen bonding can play the key role in stabilization of the MOF structures; moreover, reversible removal of guests likely results in breathing of the whole framework (this effect deserves further investigation).We believe that the use of a wide range of iodine-substituted arenecarboxylic acids has a great potential in design of metal-organic frameworks.Corresponding experiments are underway in our team.

Figure 3 .
Figure 3. Left: Secondary building unit {Zn(2,5-I-bdc) 2 L 2 } (L = bpe, bpen) in compounds 1 and 2, with the latter as example.Hydrogen atoms are not shown.Shortened intermolecular contacts I•O are highlighted by orange dashed lines.Right: A fragment of diamond-like structures 1 and 2, with the latter as example.Zinc atoms connected via bridging ligands are linked by bold blue lines.

Figure 3 .
Figure 3. Left: Secondary building unit {Zn(2,5-I-bdc)2L2} (L = bpe, bpen) in compounds 1 and 2, with the la er as example.Hydrogen atoms are not shown.Shortened intermolecular contacts I•O are highlighted by orange dashed lines.Right: A fragment of diamond-like structures 1 and 2, with the la er as example.Zinc atoms connected via bridging ligands are linked by bold blue lines.

Figure 4 .
Figure 4. Fragments of crystal packing in compounds 1 and 2, with the latter as example.Hydrogen atoms are not shown.Interpenetrated frameworks are highlighted by different colours; solvate DMF molecules are shown as grey spheres.

Table 2 .
Crystal data and structure refinement for 1 and 2.