Two metal–organic frameworks based on Sr2+ and 1,2,4,5-tetrakis(4-carboxyphenyl)benzene linkers

Two structurally different metal–organic frameworks based on Sr2+ ions and 1,2,4,5-tetrakis(4-carboxyphenyl)benzene linkers have been synthesized solvothermally in different solvent systems, viz. poly[[μ12-4,4′,4′′,4′′′-(benzene-1,2,4,5-tetrayl)tetrabenzoato](dimethylformamide)distrontium(II)], [Sr2(C34H18O8)(C3H7NO)2] n , and poly[tetraaqua[μ2-4,5-bis(4-carboxyphenyl)-4,4′-(benzene-1,2-diyl)dibenzoato]tristrontium(II)], [Sr3(C34H20O8)2(H2O)4]. The differences are noted between the crystal structures and coordination modes of these two MOFs, which are responsible for their semiconductor properties, where structural control over the bandgap is desirable.


Chemical context
Porous crystalline networks based on metal ion-coordinated organic ligands, known as metal-organic frameworks (MOFs), have been an object of extensive studies for the past two decades. Such interest in these materials can be attributed to their fascinating properties and potential applications in a wide range of areas -from luminescent lighting and sensing to gas storage, to semiconductors (Kreno et al. 2012;Zhou et al. 2012;Furukawa et al. 2013;Gassensmith et al. 2014). Their intrinsically unlimited structural and compositional diversity allows the design of structures with virtually any desirable properties. Belonging to the class of coordination compounds, MOFs naturally tend to work particularly well when synthesized with transition-metal-ion centers, yet they still suffer from several drawbacks, namely the decreased stability, toxicity and relatively high cost of manufacture. In recent years, a new class of alkaline-metal-based MOFs has arisen, providing a solution for the aforementioned problems. Abundant in Earth's crust and generally non-toxic, ions of Ca, Sr and Ba, for example, have been reported to provide a structurally rich array of compounds with increased stability and unique properties (Kundu et al. 2012). Strontium to date has been a more 'exotic' choice in MOF design, with very few structures synthesized and studied. Still, several reports have recently indicated the possibility of Sr-MOF design, which yields structures with unique luminescent (Jia et al. 2017) and semiconducting (Usman et al. 2015) properties, the latter being relatively rare for MOFs and of great interest. ISSN 2056-9890 In this work two metal-organic complexes have been synthesized from strontium nitrate as metal ion source and 1,2,4,5-tetrakis(4-carboxyphenyl)benzene as linker under slightly different synthetic conditions (see Synthesis and crystallization). For reference purposes these are labeled as MOF1 and MOF2, for the dimethylformamide (DMF) and non-DMF containing products, poly [[ 12 -4,4 0 ,4 00 ,4 000 -(benzene-1,2,4,5-tetrayl)tetrabenzoato](dimethylformamide)distrontium(II)] and poly[tetraaqua[ 2 -4,5-bis(4-carboxyphenyl)-4,4 0 -(benzene-1,2-diyl)dibenzoato]tristrontium(II)], respectively.  Tables 1 and 2. In both complexes, an Sr atom with an O 7 coordination set is present; however, in MOF2 the asymmetric unit contains two Sr atoms, one seven-and the other eight-coordinated. In MOF1, the O 7 set comprises six O atoms belonging to the carboxyl groups of the ligands (O1-O4) and one atom (O5) belonging to a DMF molecule. In MOF2, the seven-coordinated Sr atom is surrounded by five oxygens of carboxyl groups (O4-O7, O9) and two oxygens of water molecules (O8 and O10). The other Sr atom coordinates eight oxygen atoms somewhat similarly: two from water (O8) and six from the carboxyl groups of the ligands (O5-O7). The multidentate nature of the 1,2,4,5-tetrakis(4-carboxyphenyl)benzene ligand, together with the high coordination number of the Sr atom, results in an interesting structure for both complexes.

Structural commentary
The coordination environments of the Sr ions for both complexes are presented in Fig. 2. It can be seen that in MOF1 all available oxygen atoms are coordinated to a metal center, thus all carboxyl groups in the ligands participate in the coordination.

Supramolecular features
. The packing of MOF1 is shown in Fig. 3. While the abundance of carboxyl groups in the ligand provides a lot of potential for hydrogen-bonding sites, only MOF2 exhibits such interactions (Table 3). Four inequivalent hydrogen bonds of the type O-HÁ Á ÁO are found in the crystal packing ( Fig. 4), which are likely to contribute to additional structural stability compared to MOF1, which is lacking these or any other specific interactions. That said, three out of the four hydrogen bonds in MOF2 stabilize the water molecule rather than the crystal structure directly

Database survey
No entries were found in the Cambridge Structural Database (CSD version 5.40, update of September 2019; Groom et al., 2016) for metal-organic frameworks with the same metalligand combination as in the title compounds. For MOFs based on the title ligand, shown in the scheme below, and different metal ions, the search yielded eleven matches, among which ions of such metals as Cu, Mg, Zn, Co and Bi were present. The crystal structure of the pure ligand (ZARXOI; Hisaki et al. 2017), shown below, was also found during the search.

Figure 3
A view along the a axis of MOF1. Large channel-like pores are occupied by the DMF solvent molecules.

Figure 4
A view along the a axis of MOF2.
The dihedral angles between the phenyl rings and the central benzene moiety in the ligand are nearly equal: two pairs of 52.66 (18) and two pairs of 51.05 (18) . In MOF1, the pairwise equality of these angles is conserved; however, both sets of phenyl rings experience significant twists, being 38.08 (11) and 57.88 (11) , respectively, for each pair. In MOF2, an even larger difference is observed, with dihedral angles of 47.44 (8), 60.17 (8), 60.49 (8) and 70.64 (8) being found between the rings.

Synthesis and crystallization
MOF1 was synthesized as follows. Strontium nitrate (0.0212 g, 0.1 mmol), and 1,2,4,5-tetrakis(4-carboxyphenyl) benzene (0.0558 g, 0.1 mmol) were measured, placed in a beaker and dissolved in a mixture of DMF (3 mL) and water (3 mL). The solution was stirred, transferred to a Teflon-lined autoclave and sealed in a reactor, which was placed in the oven at 393 K for 120 h. The autoclave was removed from the oven and allowed to cool to room temperature.
The procedure for MOF2 differed slightly. The same amounts of the metal precursor and ligand were placed in a beaker and dissolved in a mixture of ethanol (3 mL) and water (3 mL). The solution was stirred, transferred to a Teflon-lined autoclave and sealed in a reactor, which was placed in the oven at 393 K for 120 h. The autoclave was removed from the oven and allowed to cool to room temperature.
After each synthesis, the white crystals of the products were washed with methanol and collected by means of vacuum

Figure 5
Pawley fit for MOF2. The initial parameters were taken from the cif file.

Figure 6
Pawley fit for the bulk solid obtained in the synthesis of MOF1. The thick blue lines indicate the crystalline phase for MOF2 while the thick black lines indicate the crystalline phase for MOF1. The initial parameters were taken from the cif files for both MOFs.
filtration into a capped vial. An important aspect of this study is the demonstrated possibility of structural control over Srbased MOFs via slight changes in the synthesis conditions. This may be particularly important for semiconducting MOFs, where a structurally tuned bandgap may be desirable.

Powder X-ray diffraction
In order to identify any potential byproducts or starting materials within the bulk material of MOF2, PXRD was conducted using a conventional Bragg-Brentano PXRD instrument. A Pawley fit shows only one crystalline phase (Fig. 5), and this crystalline phase corresponds to the desired product as it has similar lattice parameters to the single crystal with only a minor increase of 7 Å 3 of the total unit-cell volume from the single crystal to bulk solid at RT. The resulting lattice parameters for MOF2 from PXRD are a = 9.274 (1), b = 11.391 (1), c = 19.274 (3) Å , = 80.38 (1), = 82.04 (1), = 86.11 (1) , V = 1986.3 Å 3 . Unfortunately, in the case of MOF1, an analysis by PXRD reveals the phases for MOF1 and MOF2 in the same bulk material (Fig. 6), as in order to do a Pawley fit for this sample both structures are needed. It is possible that for the bulk solid of MOF1 other additional impurities are present as a few peaks below 10 were not indexed for either MOF1 or MOF2 (Fig. 6).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. All C-bound H atoms were positioned geometrically (C-H = 0.95-0.98 Å ) and refined using a riding model, U iso (H) = 1.2U eq (C). All O-bound H atoms were found from difference Fourier maps and freely refined. For MOF2, it was not possible to localize the H atoms at O3 and O6.

Funding information
Funding for this research was provided by: National Science Foundation, PREM (award No. DMR-1523611; award No. DMR-2122108). T min = 0.63, T max = 0.75 91152 measured reflections 5387 independent reflections 4453 reflections with I > 2σ(I) where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.94 e Å −3 Δρ min = −0.79 e Å −3 Special details Experimental. Crystal suitable for X-ray structure determination was selected under the polarizing microscope, covered with Paratone oil and mounted on a goniometer head using Mitegen cryoloop. Experiment was performed at the low temperature. QUINN software was used to calculate optimal data collection strategy. Data were collected till resolution of 0.71 A and were truncated with XPREP till actual observed resolution. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. The systematic absences in the diffraction data were consistent for the stated space group. The position of almost all non-hydrogen atoms were found by direct methods. The remaining atoms were located in an alternating series of least-squares cycles on difference Fourier map. All non-hydrogen atoms were refined in full-matrix anisotropic approximation. All hydrogen atoms were placed in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients. Final results were tested with CHECKCIF routine and all A-warnings (if any) were addressed on the very top of this file.

Poly[tetraaqua{µ 2 -4,4′-[4,5-bis(4-carboxyphenyl)benzene-1,2-diyl]dibenzoato}tristrontium(II)] (MOF2)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.