Synthesis of a Chiral 3,6T22-Zn-MOF with a T-Shaped Bifunctional Pyrazole-Isophthalate Ligand Following the Principles of the Supramolecular Building Layer Approach

The metal–organic framework (MOF) [Zn(Isa-az-tmpz)]·~1–1.5 DMF with the novel T-shaped bifunctional linker 5-(2-(1,3,5-trimethyl-1H-pyrazol-4-yl)azo)isophthalate (Isa-az-tmpz) was obtained as a conglomerate of crystals with varying degrees of enantiomeric excess in the chiral tetragonal space groups P43212 or P41212. A topological analysis of the compound resulted in the rare 3,6T22-topology, deviating from the expected rtl-topology, which has been found before in pyrazolate-isophthalate-functionalized MOFs using the supramolecular building layer (SBL) approach. 3,6T22-[Zn(Isa-az-tmpz)]·~1–1.5 DMF is a potentially porous, three-dimensional structure with DMF molecules included in the corrugated channels along the a and b-axis of the as synthesized material. The small trigonal cross-section of about 6 × 4 Å (considering the van der Waals surface) prevents the access of N2 and Ar under cryogenic conditions. After activation, only smaller H2 (at 87 K) and CO2 (at 195 K) are allowed for gas uptakes of 2 mmol g–1 and 5.4 mmol g–1, respectively, in the ultramicroporous material, for which a BET surface area of 496 m2·g–1 was calculated from CO2 adsorption. Thermogravimetric analysis of the compound shows a thermal stability of up to 400 °C.


Introduction
Metal-organic frameworks (MOFs) are a much-studied topic with a wide variety of potential applications, interesting properties and topologies [1][2][3]. There are many factors that can influence the synthesis, growth and the structure of MOFs. One approach to designing MOFs with a certain structure and underlying net is the 'supramolecular building layer approach', formulated by Guillerm et al. [4]. Within this approach, multiple ways were rationalized to obtain MOFs with certain topologies through the inter-connection of twodimensional nets with accessible perpendicular bridging sites. One of the best-known strategies is the use of 4,4 -bipyridine derivates to connect carboxylate-based paddlewheel clusters along the axial open metal sites, thereby effectively turning the tetragonal 4-c (fourconnected) nodes into octahedral 6-c-nodes [5][6][7][8]. Seki et al. were among the first to utilize this strategy using copper (II) terephthalate and triethylenediamine as a pillaring ligand to synthesize a mixed-ligand MOF with a pcu topology [9]. The success of this strategy led to the synthesis and use of new bifunctional ligands, wherein the dicarboxylate group constructs a two-dimensional net while a N-heterocycle functions as an axial pillaring unit, unifying the mixed-ligand approach. This strategy has been termed 'ligand-to-axial pillaring' by Eubank et al. based on the utilization of T-shaped bifunctional ligands that function as a 3-c node [4,10]. Using this strategy, a plethora of 3,6-c connected MOFs, with topologies such as apo (α-PO2), [10] eea (based on the Kagomé-lattice (kgm)) [10][11][12], pyr (pyrite) [12] and rtl (rutile) [10,[12][13][14], have been synthesized.
The asymmetric unit consisted of one Zn(II) ion and one fully deprotonated Isa-aztmpz 2ligand, as well as a strongly disordered DMF molecule which was removed with the SQUEEZE function in PLATON [24,25]. Each Zn(II) ion had a square-pyramidal coordination with Zn-O bond lengths between 2.024(2) and 2.046(2) Å in equatorial positions and a similarly long Zn-N bond of 2.039(2) Å in axial position. Two of these Zn(II) ions form a paddlewheel cluster connected through four isophthalate and two pyrazole units of six different linker molecules to a 3D network (Figure 2a). The isophthalate and pyrazole-ring plane are tilted by about 20 • with respect to each other. The asymmetric unit consisted of one Zn(II) ion and one fully deprotonated Isa-aztmpz 2-ligand, as well as a strongly disordered DMF molecule which was removed with the SQUEEZE function in PLATON [24,25]. Each Zn(II) ion had a square-pyramidal coordination with Zn-O bond lengths between 2.024(2) and 2.046(2) Å in equatorial positions and a similarly long Zn-N bond of 2.039(2) Å in axial position. Two of these Zn(II) ions form a paddlewheel cluster connected through four isophthalate and two pyrazole units of six different linker molecules to a 3D network (Figure 2a). The isophthalate and pyrazole-ring plane are tilted by about 20° with respect to each other.
Paddlewheel clusters are commonly found in Zn-MOFs, with Zn-HKUST-1 and SDU-1 being two examples [26,27]. These paddlewheel clusters can exhibit symmetries up to D4h depending on the symmetries of the ligand and the molecules or ligands in axial positions. Due to the low symmetry of Isa-az-tmpz 2-, the clusters in the network [Zn(Isaaz-tmpz)] have a reduced C2 symmetry without an inversion center or mirror faces ( Figure  2b).
Each paddlewheel cluster was connected to six ligand molecules (Figure 2b), which in turn were linked to two further paddlewheel clusters to form a 3D-network ( Figure 2c). This 3D-network can be separated into 2D-chains, consisting of the paddlewheel units interconnected through the isophthalate functionality of the ligand. As a result, these form a left-handed 43 or a right-handed 41 helix along the crystallographic 43 or 41 axis, respectively, which was colinear with the c-axis (Figure 2d). The compound crystallizes in the chiral, enantiomorphic tetragonal space groups P43212 or P41212 as a conglomerate of crystals, with varying degrees of enantiomeric excess. The four investigated crystals (Table 1) were refined as inversion twins with ratios of about 7:3 and 6:4 or 4:6 and 2:8 of both enantiomorphic forms [28,29]. Thus, the individual investigated crystals were not enantiopure (homochiral) but only of enantiomeric excess. The formation of fourfold helices in isophthalate-MOFs was also seen in [Al(OH)(isophthalate)] Paddlewheel clusters are commonly found in Zn-MOFs, with Zn-HKUST-1 and SDU-1 being two examples [26,27]. These paddlewheel clusters can exhibit symmetries up to D 4h depending on the symmetries of the ligand and the molecules or ligands in axial positions. Due to the low symmetry of Isa-az-tmpz 2-, the clusters in the network [Zn(Isa-az-tmpz)] have a reduced C 2 symmetry without an inversion center or mirror faces (Figure 2b).
Each paddlewheel cluster was connected to six ligand molecules (Figure 2b), which in turn were linked to two further paddlewheel clusters to form a 3D-network ( Figure 2c). This 3D-network can be separated into 2D-chains, consisting of the paddlewheel units interconnected through the isophthalate functionality of the ligand. As a result, these form a left-handed 4 3 or a right-handed 4 1 helix along the crystallographic 4 3 or 4 1 axis, respectively, which was colinear with the c-axis (Figure 2d).
The compound crystallizes in the chiral, enantiomorphic tetragonal space groups P4 3 2 1 2 or P4 1 2 1 2 as a conglomerate of crystals, with varying degrees of enantiomeric excess. The four investigated crystals (Table 1) were refined as inversion twins with ratios of about 7:3 and 6:4 or 4:6 and 2:8 of both enantiomorphic forms [28,29]. Thus, the individual investigated crystals were not enantiopure (homochiral) but only of enantiomeric excess. The formation of fourfold helices in isophthalate-MOFs was also seen in [Al(OH)(isophthalate)] (CAU-10-H, including benzene-functionalized derivatives) [30][31][32].  A packing analysis of the network [Zn(Isa-az-tmpz)] shows the absence of π-π and C-H···π interactions within the structure [37]. The steric constraints of the three methyl groups on the pyrazolyl ring prevent such π-π interactions between the aromatic rings of the ligand. The network [Zn(Isa-az-tmpz)] follows the basic principles of the 'ligand-toaxial pillaring' strategy of the SBL approach consisting of a paddlewheel cluster, which can be described as an octahedral 6-c node and a T-shaped ligand, which works as a trigonal 3-c node. Deviating from the SBL approach, the topological analysis of the structure with the program ToposPro [38,39] and the Topcryst database [40,41] yielded the rare chiral 3,6T22-topology as the underlying net, instead of the expected rtl-topology ( Figure 3).  Figure S13b,c). In [Zn(Isa-az-tmpz)]·~1-1.5 DMF the dinuclear paddlewheel SBU gave rise to a tetragonally distorted, elongated octahedron ( Figure S13d). In G⊂Cd(L)2, [Fe2M(Bptc)] and [Zn(Isa-az-tmpz)] the 3c-linker node also was asymmetric, as it had short and long bonds to the SBU ( Figure S13a,c,d). Only the Tatab 3-linker was trigonal symmetric ( Figure S13b). The Bptc 3-linker was also a T-shaped linker like Isa-aztmpz 2-, albeit with a tricarboxylate donor set. To the best of our knowledge, this was the first work that shows that this topology can be achieved using a T-shaped bifunctional pyrazole-dicarboxylate ligand following the 'ligand-to-axial pillaring' approach.
The solvent-depleted 3D network [Zn(Isa-az-tmpz)] has potential porosity from the identical perpendicular corrugated channel systems along the a-and b-axes with trigonal cross-selections of about 6 × 4 Å , which could only accommodate a sphere of about 3 Å diameter (considering the van der Waals surface) ( Figure 4). A solvent accessible volume (SAV) of 1661 Å 3 or 39 vol% out of the unit cell volume of 4223 Å 3 was calculated with PLATON [24] for the solvent-depleted structure. The SAV of 1661 Å 3 calculates into a specific pore volume of 0.34 cm 3 g -1 according to (SAV  NA)/(Z  Masym unit); (NA = Avogadro's constant: 6.022·10 23 mol -1 , Z = number of asymmetric formula units, Masym unit = molecular weight of asymmetric formula unit in g mol -1 ; see Table S1).  3 (O 2 C-) 6 } SBU was a trigonal prism, which also represents a distortion from an octahedron ( Figure  S13b,c). In [Zn(Isa-az-tmpz)]·~1-1.5 DMF the dinuclear paddlewheel SBU gave rise to a tetragonally distorted, elongated octahedron ( Figure S13d). In G⊂Cd(L) 2 , [Fe 2 M(Bptc)] and [Zn(Isa-az-tmpz)] the 3c-linker node also was asymmetric, as it had short and long bonds to the SBU ( Figure S13a,c,d). Only the Tatab 3linker was trigonal symmetric ( Figure S13b). The Bptc 3linker was also a T-shaped linker like Isa-az-tmpz 2-, albeit with a tricarboxylate donor set. To the best of our knowledge, this was the first work that shows that this topology can be achieved using a T-shaped bifunctional pyrazole-dicarboxylate ligand following the 'ligand-to-axial pillaring' approach.
The solvent-depleted 3D network [Zn(Isa-az-tmpz)] has potential porosity from the identical perpendicular corrugated channel systems along the a-and b-axes with trigonal cross-selections of about 6 × 4 Å, which could only accommodate a sphere of about 3 Å diameter (considering the van der Waals surface) ( Figure 4). A solvent accessible volume (SAV) of 1661 Å 3 or 39 vol% out of the unit cell volume of 4223 Å 3 was calculated with PLATON [24] for the solvent-depleted structure. The SAV of 1661 Å 3 calculates into a specific pore volume of 0.34 cm 3 g -1 according to (SAV × N A )/(Z × M asym unit ); (N A = Avogadro's constant: 6.022·10 23 mol -1 , Z = number of asymmetric formula units, M asym unit = molecular weight of asymmetric formula unit in g mol -1 ; see Table S1).  The representative nature of the selected crystal from [Zn(Isa-az-tmpz)]·~1-1.5 DMF and the phase-purity of the bulk material was confirmed by a positive match between the simulated powder X-ray diffraction (PXRD) pattern and experimental pattern of the as-synthesized material (Figure 5a,b). For the prospective gas sorption studies, DMF was exchanged with acetone, and afterwards the material has been dried with supercritical CO2. At this stage, no phase change or loss of crystallinity in the bulk material could be observed with PXRD (Figure 5c,d). The solvent exchange does not influence the general structure of the MOF, as the peak positions remained unchanged. However, the difference in electron density in the pores from the solvent exchange can affect the peak intensities [46]. The representative nature of the selected crystal from [Zn(Isa-az-tmpz)]·~1-1.5 DMF and the phase-purity of the bulk material was confirmed by a positive match between the simulated powder X-ray diffraction (PXRD) pattern and experimental pattern of the as-synthesized material (Figure 5a,b). For the prospective gas sorption studies, DMF was exchanged with acetone, and afterwards the material has been dried with supercritical CO 2 . At this stage, no phase change or loss of crystallinity in the bulk material could be observed with PXRD (Figure 5c,d). The solvent exchange does not influence the general structure of the MOF, as the peak positions remained unchanged. However, the difference in electron density in the pores from the solvent exchange can affect the peak intensities [46]. Thermal analysis of the supercritically dried (sc-dried) sample with TGA showed only a slightly decreased mass loss compared to the acetone-washed sample. While in one sample, the solvent could nearly be completely removed according to TGA (Figure S8), in another sample about 15 mass% of DMF up to 250 °C remained in both the acetone- Thermal analysis of the supercritically dried (sc-dried) sample with TGA showed only a slightly decreased mass loss compared to the acetone-washed sample. While in one sample, the solvent could nearly be completely removed according to TGA (Figure S8), in another sample about 15 mass% of DMF up to 250 • C remained in both the acetonewashed and the subsequently sc-dried sample ( Figure S9). Hence, the sc-dried material was additionally heated for 3 h at 120 • C under high vacuum to further activate the sample before measurement, which yielded a material with low crystallinity (Figure 5e). This indicates a partial collapse of the 3D network structure during solvent removal under too harsh conditions. Following a volumetric nitrogen sorption experiment at 77 K, no gas uptake could be seen. A sorption measurement with the sc-dried sample, activated under high vacuum at room temperature, had a similarly low N 2 uptake ( Figure S10).
Due to the low nitrogen uptake and small channel size of solvent-depleted [Zn(Isaaz-tmpz)], sorption experiments at 87 K for argon (Ar) and hydrogen (H 2 ) and at 195 K for carbon dioxide were collected for the sc-dried and additionally heated material (3 h, 120 • C). While the Ar sorption measurement showed low gas uptake ( Figure S11), the H 2 sorption yielded an isotherm similar to type I(b) isotherms for microporous materials, with a total uptake of 2 mmol·g -1 ( Figure 6) [47]. The CO 2 sorption experiment showed the microporous nature of the material by also providing a type I(b) isotherm and a significant CO 2 uptake, from which a Langmuir surface area of 588 m 2 ·g -1 and a BET surface area of 496 m 2 ·g -1 was calculated. Assuming the validity of the Gurvich rule, the division of (specific CO 2 amount adsorbed in g g -1 with the CO 2 saturation pressure at 195 K of 1.00 bar)/(density of liquid CO 2 adsorbate with ρ CO2 (195 K) = 1.08 g cm -3 ) gave a pore volume of 0.22 cm 3 g -1 (the uptake of 120 cm 3 g -1 at STP at 1 bar is 5.4 mmol g -1 or 2.4 g g -1 ) [48]. The difference in gas uptake between N 2 , Ar, H 2 and CO 2 correlates with the kinetic diameters (3.64, 3.40, 2.89 and 3.30 Å, respectively), with the cryogenic temperatures for N 2 (77 K) and Ar (87 K) and the ultramicroporous (<7 Å pore size) nature of the framework. The diffusion of N 2 molecules and Ar atoms into small pores was then very slow, while kinetic inhibition was less severe for smaller H 2 molecules and for CO 2 at 195 K.

Materials and Characterization
All reagents were obtained from commercial sources and used without further purification. C, H, N analyses were executed on a vario MICRO cube from Elementar Analysentechnik. 1 H-NMR and 13 C-NMR spectra were measured on a Bruker Avance III-300. IR-spectra were recorded on a Bruker Tensor 37 IR spectrometer equipped with an attenuated total reflection (ATR) unit (Platinum ATR-QL, Diamond). Electrospray ionization-mass spectra (ESI-MS) were measured on a Finnigan LCQ Deca Thermoquest in acetone, electron ionization-mass spectra (EI-MS) on a TSQ 7000 Finnigan MAT. Thermogravimetric analysis was executed on a Netzsch TG 209 F3 Tarsus in the range from 30 °C to 600 °C with a heating rate of 5 K min -1 under a nitrogen atmosphere. Powder X-ray

Materials and Characterization
All reagents were obtained from commercial sources and used without further purification. C, H, N analyses were executed on a vario MICRO cube from Elementar Analysentechnik. 1 H-NMR and 13 C-NMR spectra were measured on a Bruker Avance III-300. IR-spectra were recorded on a Bruker Tensor 37 IR spectrometer equipped with an attenuated total reflection (ATR) unit (Platinum ATR-QL, Diamond). Electrospray ionization-mass spectra (ESI-MS) were measured on a Finnigan LCQ Deca Thermoquest in acetone, electron ionization-mass spectra (EI-MS) on a TSQ 7000 Finnigan MAT. Thermogravimetric analysis was executed on a Netzsch TG 209 F3 Tarsus in the range from 30 • C to 600 • C with a heating rate of 5 K min -1 under a nitrogen atmosphere. Powder X-ray diffraction (PXRD) patterns were measured on a Bruker D2 Phaser powder diffractometer with a flat silicon, low background sample holder, at 30 kV, 10 mA with Cu-Kα radiation (λ = 1.5418 Å). The most intense reflection in each diffractogram was normalized to 1. The simulated PXRD pattern has been calculated with MERCURY software [23]. Supercritical drying was carried out on a Leica EMPCD 300 over 99 exchange cycles with CO 2 . Adsorption data for N 2 at 77 K (liquid nitrogen bath) was collected on a Quantachrome NOVA 4000 gas adsorption analyzer. Additional sorption experiments for Ar and H 2 at 87 K (Quantachrome CRYOCOOLER) and CO 2 at 195 K (Quantachrome CRYOCOOLER), respectively, were conducted on a Quantachrome Autosorb iQ MP. The supercritically dried sample was used and outgassed before the gas sorption measurements were taken, either at room temperature or at 120 • C for a minimum of 3 h.

Single Crystal X-ray Diffraction
Suitable crystals were carefully selected under a polarized-light microscope, covered in protective oil and mounted on a cryo-loop.
For crystals 1b and 2b, the single crystal diffraction data were collected using a Rigaku XtaLAB Synergy S four circle diffractometer with a Hybrid Pixel Array Detector and a PhotonJet X-ray source for Cu-Kα radiation (λ = 1.54184 Å), with a multilayer mirror monochromator. Data collection took place at 100.0 ± 0.1 K using ω-scans. Data reduction and absorption correction were performed with CrysAlisPro 1.171.41.105a [52].
Structure analysis and refinement: The structures were solved by direct methods (SHELXT-2015), full-matrix least-squares refinements on F 2 were executed using the SHELXL-2017/1 program package [53,54]. All hydrogen atoms were positioned geometrically (with C-H = 0.95 Å for aromatic CH and C-H = 0.98 Å for CH 3 ) and refined using riding models (AFIX 43 and 137 with U iso(H) = 1.2 U eq (CH) and 1.5 U eq (CH 3 )).
Highly disordered solvent molecules were either masked with the SQUEEZE option in PLATON [24,25] (crystals 1a and 2a) or by using the solvent mask feature as implemented in OLEX 2.1.3 [55] (crystals 1b and 2b). Crystal data and details on the structure refinement are provided in Table 1. Details about selected bond distances and angles are provided in Table S2 in the supporting information. Graphics were drawn with the program DIAMOND [56].
The crystallographic data (excluding structure factors) for the structures were deposited with the Cambridge Crystallographic Data Centre (CCDC-numbers 2192050-2192053) and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

Scheme 2.
Reaction scheme for the synthesis of H2Isa-az-tmpz.

Synthesis of [Zn(Isa-az-tmpz)]·~1-1.5 DMF
In a Pyrex tube, 9.7 mg (0.032 mmol) of H 2 Isa-az-tmpz and 16.8 mg (0.064 mmol) of Zn(NO 3 ) 2 ·4 H 2 O were dissolved in 2 mL of DMF. The tube with the yellow solution was placed into a preheated oven at 80 • C for 72 h to obtain yellow block-shaped crystals. The mother liquor was then exchanged against fresh DMF (2 mL) to prevent further growth and stored at RT until single crystal analysis.
On a larger scale, 100 mg (0.33 mmol) of H 2 Isa-az-tmpz dissolved in 10 mL of DMF was added to a solution of 170 mg (0.65 mmol) Zn(NO 3 ) 2 ·4 H 2 O in 10 mL of DMF. The same temperature program used for the single crystal synthesis was applied. The obtained as-synthesized polycrystalline yellow material was washed with DMF (10 mL). The solvent was replaced once a day for three days followed by acetone (10 mL), which also had the solvent replaced once a day for three days. Subsequently, the sample was activated via supercritical drying with CO 2

Conclusions
We presented the synthesis of the MOF [Zn(Isa-az-tmpz)]·~1-1.5 DMF with the rare chiral 3,6T22-topology through the reaction of zinc nitrate with the newly synthesized T-shaped linker 5-(2-(1,3,5-trimethyl-1H-pyrazol-4-yl)azo)isophthalic acid (H 2 Isa-az-tmpz) in DMF at elevated temperatures. Even though the deprotonated form Isa-az-tmpz 2fulfills the general principles of the 'ligand-to-axial pillaring' strategy of the supramolecular building layer (SBL) approach, leading to a 3,6-c connected MOF, the resulting topology does not follow the SBL approach and cannot be described through this. Contrary to the related coordination polymer rtl-[Zn(HIsa-az-dmpz)], which could not be activated to its porous material, the porosity for the ultramicroporous MOF (pore diameters less than 7 Å) 3,6T22-[Zn(Isa-az-tmpz)]·~1-1.5 DMF could be assessed with H 2 at 87 K and with CO 2 at 195 K but not with N 2 and Ar under the cryogenic conditions of 77 and 87 K, respectively. Overall, this work can be considered a starting point to obtain chiral and potentially porous MOFs using bifunctional and T-shaped pyrazole-carboxylate ligands. Chiral MOFs are promising materials for enantioselective adsorption, separation and catalysis [20][21][22].