A new tetrakis-substituted pyrazine carboxylic acid, 3,3′,3′′,3′′′-{[pyrazine-2,3,5,6-tetrayltetrakis(methylene)]tetrakis(sulfanediyl)}tetrapropionic acid: crystal structures of two triclinic polymorphs and of two potassium–organic frameworks

The crystal structures of two triclinic polymorphs of a new tetrakis-substituted pyrazine carboxylic acid, 3,3′,3′′,3′′′-[(pyrazine-2,3,5,6-tetrayltetrakis(methylene))tetrakis(sulfanediyl)]tetrapropionic acid, are reported, together with the crystal structures of two potassium-organic frameworks.

Reaction of H 4 L1 with Hg(NO 3 ) 2 in the presence a 1 M potassium acetate buffer led to the formation of colourless crystals that proved to be a potassium-organic framework (KH 3 L1); see Fig. 6. The asymmetric unit consists of half a mono-deprotonated ligand molecule located about an inversion center, and half a potassium ion located on an inversion center. The carboxy H atom is disordered by symmetry. The K + ion is linked to the O atoms of the acid groups and has a coordination number of eight (KO 8 ) and a distorted dodecahedral geometry (Fig. 7a). The KÁ Á ÁO bond lengths vary between 2.682 (2) and 3.069 (3) Å (Table 2). Interestingly, here there is a significant difference between the KÁ Á ÁO(C O) and KÁ Á ÁO(O À ) distances: 2.6823 (2) and 2.828 (2) Å compared to 3.056 (3) and 3.069 (3) Å , respectively.
Reaction of H 4 L1 with Zn(NO 3 ) 2 in the presence of a 1 M potassium acetate buffer led to the formation of colourless crystals that proved to be a dipotassium-organic framework (K 2 H 2 L1); see Fig. 8. The asymmetric unit consists of half a dideprotonated ligand molecule located about an inversion center, and two half potassium ions located on inversion centers. The K + ions are linked to the O atoms of the acid groups and both K + ions have a coordination number of six (KO 6 ) and have edge-sharing bipyramidal geometries. The K + ions are bridged by atoms O1 and O3, forming chains propagating along the b-axis direction (Fig. 7b). The KÁ Á ÁO bond lengths vary between 2.6682 (12) and 2.8099 (14) Å (Table 3). Here, the difference between the KÁ Á ÁO(C O) and KÁ Á ÁO(O À ) bond lengths is much less significant (Table 3).
In the crystal of H 4 L1_B, molecules are linked by pairs of O-HÁ Á ÁO hydrogen bonds, forming chains propagating along the c-axis direction and enclosing R 2 2 (8) loops ( Fig. 10 and Table 5). There are no other significant directional contacts present in the crystal.

Figure 9
A view along the a axis of the crystal packing of H 4 L1_A. The hydrogen bonds are shown as dashed lines (see Table 4).

Hirshfeld surface analysis and two-dimensional fingerprint plots for H 4 L1_A, and H 4 L1_B
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) were performed with Crystal-Explorer17 (Turner et al., 2017) following the protocol of Tiekink and collaborators (Tan et al., 2019). The Hirshfeld surfaces are colour-mapped with the normalized contact distance, d norm , varying from red (distances shorter than the sum of the van der Waals radii) through white to blue (distances longer than the sum of the van der Waals radii). The Hirshfeld surfaces (HS) of H 4 L1_A, and H 4 L1_B mapped over d norm are given in Fig. 13. The most significant short contacts in the crystal structures of the two polymorphs are given in Table 8. The large red spots in Fig. 13a and b concern the O-HÁ Á ÁO hydrogen bonds in the crystal structures of both compounds.

Figure 12
A view along the b axis of the crystal packing of complex K 2 H 2 L1. For clarity, the H atoms have been omitted.

Energies frameworks for H 4 L1_A, and H 4 L1_B
The colour-coded interaction mappings within a radius of 6 Å of a central reference molecule for H 4 L1_A, and H 4 L1_B, are given in Fig. 15. Full details of the various contributions to the total energy (E tot ) are also included there; see Tan et al. (2019) for an explanation of the various parameters.
A comparison of the energy frameworks calculated for H 4 L1_A, and H 4 L1_B, showing the electrostatic potential forces (E ele ), the dispersion forces (E dis ) and the total energy diagrams (E tot ), are shown in Fig. 16. The energies were obtained by using the wave function at the HF/3-21G level of theory. The cylindrical radii are proportional to the relative strength of the corresponding energies (Turner et al., 2017;Tan et al., 2019). They have been adjusted to the same scale factor of 80 with a cut-off value of 5 kJ mol À1 within a radius of 6 Å of a central reference molecule. It can be seen that for both polymorphs the major contribution to the intermolecular interactions is from electrostatic potential forces (E ele ), reflecting the presence of the classical O-HÁ Á ÁO hydrogen bonds.

Figure 13
The Hirshfeld surfaces of compounds (a) H 4 L1_A and (b) H 4 L1_B, mapped over d norm in the colour ranges of À0.7146 to 1.2167 and À0.6847 to 1.3548 au., respectively.

Figure 15
The colour-coded interaction mappings within a radius of 6 Å of a central reference molecule for (a) H 4 L1_A and (b) H 4 L1_B.

Figure 16
The energy frameworks calculated for (a) H 4 L1_A and (b) H 4 L1_B, both viewed along the b-axis direction, showing the electrostatic potential forces (E ele ), the dispersion forces (E dis ) and the total energy diagrams (E tot ).
Potassium salts of carboxylic acids are relatively common. A search for potassium salts of purely organic carboxylic acids and excluding hydrates, yielded over 200 hits. The potassium salt of pztca has been reported, viz. catena-[( 4 -3,5,6-tricarboxypyrazine-2-carboxylato)potassium] (CSD refcode UBUPAK; Masci et al., 2010). The structure of UBUPAK is that of a potassium-organic framework (Fig. 17a). The asymmetric unit consists of half a mono-deprotonated ligand molecule located about an inversion center, and half a potassium ion. The carboxy H atom is disordered by symmetry, similar to the situation in the structure of KH 3 L1.
The structure of MUMPIW is that of a potassium-organic framework (Fig. 17b), with the KÁ Á ÁO bonds lengths varying from 2.8197 (14) to 3.0449 (15) Å . The K + ion has a coordination number of seven (KO 6 N) and has an edge-sharing pentagonal antiprism geometry, forming chains (Fig. 17b). This structure can be compared to that of K 2 H 2 L1 where the two independent K + ions, each with a coordination number of six (KO 6 ), have edge-sharing bipyramidal geometries, also forming chains ( Fig. 7b and 12).

Synthesis and crystallization
The synthesis and crystal structure of the reagent tetra-2,3,5,6bromomethyl-pyrazine (TBr) have been reported (Ferigo et al., 1994;Assoumatine & Stoeckli-Evans, 2014  Mercaptopropionic acid (1.8795 g, 1.77 mol, 4 eq) was dissolved in 50 ml THF. A minimum amount of water (a few ml) was added to dissolve 1.4166 g (3.54 mol, 8 eq) of NaOH. The volume of the mixture was increased to 100 ml by adding THF and the reaction was stirred under reflux for 1 h. Then TBr (2 g, 4.42 mol, 1 eq) dissolved in 50 ml THF was added dropwise using an addition funnel. The mixture was stirred under reflux for 6 h. After drying under vacuum, the residue was dissolved in 50 ml of deionized water, and HCl puriss. was added dropwise until a clearly acid pH was obtained. This mixture was stirred at room temperature for 1-2 h. The yellow precipitate that formed was filtered off and washed with a minimum amount of water and then CHCl 3 . It was then dried under vacuum conditions. Recrystallization carried out with methanol gave pale-yellow crystals of H 4 L1 (yield 88%, m.p. 466 K) that X-ray diffraction analysis indicated to be triclinic polymorph H 4 L1_A.
The presence of terephthalic acid in an equimolar quantity with H 4 L1 in methanol gave colourless crystals of rather poor quality. However, X-ray diffraction analysis indicated that a second triclinic (P1) polymorph, H 4 L1_B, had been obtained.
Spectroscopic and elemental analyses: Zn(NO 3 ) 2 (28.4 mg, 0.109 mmol, 2 eq) and H 4 L1 (30 mg, 0.054 mmol, 1eq) were mixed together in 20 ml of a 1M potassium acetate buffer. The mixture was left at 323 K under stirring and nitrogen for 1 h. The mixture was then filtered and left to evaporate in air for 6 weeks. Colourless plate-like crystals were obtained, which proved to be a dipotassium-organic framework.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 10.
For H 4 L1_A, KH 3 L1 and K 2 H 2 L1, the various -CO 2 H H atoms were located in difference-Fourier maps and freely refined. For H 4 L1_B, the -CO 2 H H atoms were difficult to locate, probably due to the poor quality of the crystal and the disorder in the side chain (atoms C8/C8B, C9/C9B, C10/C10B, O3/O3B, O4/O4B; Fig. 4b). They were therefore included in calculated positions assuming the formation of carboxylic acid dimers; O-H = 0.82 Å and refined as riding with U iso (H) = 1.5U eq (O).
As in the K + salt of pyrazine tetracarboxylic acid (UBUPAK; Masci et al., 2010), the carboxy H atom in KH 3 L1 is disordered by symmetry, hence the H atom on O3 was given an occupancy factor of 0.5 to balance the charges.
For all four compounds, the C-bound H atoms were included in calculated positions and treated as riding on their parent C atom with C-H = 0.97 Å and U iso (H) = 1.2U eq (C).
For H 4 L1_A and H 4 L1_B, the alert _diffrn_reflns_ point_group_measured_fraction_full value (0.94 and 0.93, respectively) below minimum (0.95) was given. For H 4 L1_A it involves 131 random reflections out of a total of 2180, viz. 6.0%, while for H 4 L1_B it involves 158 random reflections out of a total of 2184, viz. 7.2%.
For H 4 L1_A, H 4 L1_B and K 2 H 2 L1 the multiplicity of reflections was 2 or less and so an empirical absorption correction was applied.   structures, software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015), PLATON (Spek, 2020) and publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.35 e Å −3 Δρ min = −0.28 e Å −3 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.