Crystal structures and Hirshfeld surface analyses of two new tetrakis-substituted pyrazines and a degredation product

The crystal structures of two new tetrakis-substituted pyrazine compounds, 1,1′,1′′,1′′′-(pyrazine-2,3,5,6-tetrayl)tetrakis(N,N-dimethylmethanamine) and N,N′,N′′,N′′′-[pyrazine-2,3,5,6tetrayltetrakis(methylene)]tetrakis(N-methylaniline), and a degredation product, N,N′-[(6-phenyl-6,7-dihydro-5H-pyrrolo[3,4-b]pyrazine-2,3-diyl)bis(methylene)]bis(N-methylaniline), are described and have been analysed by Hirshfeld surface analysis.


Chemical context
Tetrakis-substituted pyrazines, which are potential bistridentate ligands, have been used in coordination chemistry since the 1980 0 s, to form not only mononuclear and binuclear complexes but also multi-dimensional coordination polymers. A search of the Cambridge Structural Database (CSD, Version 5.41, last update November 2019; Groom et al., 2016) reveals that the principal tetrakis-substituted pyrazine ligands that have been used are 2,3,5,6-tetrakis(pyridin-2-yl)pyrazine, which was first synthesized by Goodwin & Lions (1959), and 2,3,5,6-pyrazinetetracarboxylic acid, which was first synthesized by Wolff at the end of the 19th century (Wolff, 1887(Wolff, , 1893. Since then the coordination chemistry of only a small number of tetrakis-substituted pyrazines has been studied, for example tetrakis(aminomethyl)pyrazine (Ferigo et al., 1994) and, more recently, the new ligand 2,3,5,6-tetrakis(4-carboxyphenyl) pyrazine, which has been shown to be extremely successful in forming metal-organic frameworks (Jiang et al., 2017;Wang et al., 2019).

Structural commentary
The molecular structure of compound (I) is illustrated in Fig. 1. The molecule possesses inversion symmetry with the pyrazine ring being located about a center of symmetry. The adjacent dimethylmethanamine substituents, in positions 2,3 (and 5,6), are directed above and below the plane of the pyrazine ring. There is a short intramolecular C3-H3AÁ Á ÁN3 i contact on either side of the molecule [symmetry code: (i) Àx, Ày, Àz 1 ], linking the two dimethylmethanamine substituents ( Fig. 1 and Table 1).
The molecular structure of compound (II) is illustrated in Fig. 2. This molecule also possesses inversion symmetry with the pyrazine ring being located about a center of symmetry. Again the adjacent methylaniline substituents, in positions 2,3 (and 5,6), are directed above and below the plane of the pyrazine ring. Rings C4-C9 and C12-C17 are inclined to the pyrazine ring by 63.62 (10) and 86.83 (10) , respectively, and to each other by 78.28 (11) . There are short intramolecular C5-H5Á Á ÁN1 contacts on either side of the molecule invol-ving a methylaniline ring and the adjacent pyrazine N atom, and the methylaniline substituents in positions 2,6 (and 3,5) research communications Acta Cryst. (2020 A view of the molecular structure of compound (II), with atom labelling [symmetry code: (i) Àx + 1, Ày + 1, Àz + 2]. Displacement ellipsoids are drawn at the 30% probability level. Intramolecular C-HÁ Á ÁN interactions (Table 2) are shown as dashed lines and the intramolecular C-HÁ Á Á interactions (Table 2) as red dashed arrows.

Supramolecular features
In the crystal of (I), there are no significant intermolecular interactions present (Fig. 4).
In the crystal of (II), molecules are linked by a pair of C-HÁ Á Á interactions, forming chains that propagate along the [001] direction (Fig. 5 and Table 2).

Figure 4
A view along the a axis of the crystal packing of compound (I).

Figure 5
A view along the a axis of the crystal packing of compound (II). The C3-H3AÁ Á Á interactions (

Hirshfeld surface analysis and two-dimensional fingerprint plots
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). The Hirshfeld surfaces are colour-mapped with the normalized contact distance, d norm , 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 the title compounds, mapped over d norm , are given in Fig. 7. It is evident from Figs. 7a and 7b that there are no contact distances shorter than the sum of the van der Waals radii in the crystals of either compounds (I) or (II). For compound (III) (Fig. 7c), two small red spots indicate the presence of weak CÁ Á ÁH contacts (see Table 3).
The two-dimensional fingerprint plots for the title compounds are given in Fig. 8. They reveal, as expected, that the principal contributions to the overall surface involve HÁ Á ÁH contacts at 87.9% for (I) (Fig. 8a), 68.6% for (II) (Fig. 8b), and 63.3% for (III) (Fig. 8c). The second most important contribution to the HS for compound (I) is from the NÁ Á ÁH/HÁ Á ÁN contacts at 8.0%; for compounds (II) and (III) the second most significant contributions are from the CÁ Á ÁH/ HÁ Á ÁC contacts at 26.3 and 27.4%, respectively. For compound (I), the third most important contribution to the HS is from the CÁ Á ÁH/HÁ Á ÁC contacts at 4.0%, while for compounds (II) and (III) it is from the NÁ Á ÁH/HÁ Á ÁN contacts at 2.6 and 5.7%, respectively. All other atomÁ Á Áatom contacts contribute less that 2% to the HS for all three compounds.

Synthesis and crystallization
Synthesis of 1,1 0 0 0 ,1 0 0 00 0 0 ,1 0 0 00 0 00 0 0 -(pyrazine-2,3,5,6-tetrayl) tetrakis(N,Ndimethylmethanamine) (I): A large excess of dimethyl amine hydrochloride in water was neutralized with NaOH in an ice bath. Me 2 NH formed in situ as a gas and was directly condensed in a round-bottom flask in an acetone/liquid N 2 bath at about 213 K using a weak vacuum. Once a sufficient quantity of liquid amine had formed, a solution of 2,3,5,6-tetrakis(bromomethyl)pyrazine (0.4530 g, 1 mmol) in 50 ml of CH 2 Cl 2 was added dropwise at low temperature (ca 243 K). The reaction was left for about 4 h, allowing the temperature rise to RT. The excess amine was allowed to evaporate off before the solvent was gassed off. The residue obtained was dissolved in 40 ml of MeOH and passed through a resin column (15 g of Dowex 1 X8) previously charged with OH À ions in order to exchange the HBr molecules, still attached to the ligand, by H 2 O molecules. About 150 ml were used as eluent. Solvent evaporation yielded 0.27 g (87%) of a light-yellow powder of compound (I). Colourless block-like crystals were obtained by slow diffusion of hexane into a solution of the ligand in dichloromethane. A solution of 2,3,5,6-tetrakis(bromomethyl)pyrazine (0.4530 g, 1 mmol) in 35 ml of CH 3 CN was added dropwise to a suspension of N-methylaniline (1.2 ml, 10 mmol) and Na 2 CO 3 (5.3 g, 50 mmol) in 25 ml of CH 3 CN. The colour changed immediately from light to deep yellow. The mixture was refluxed for ca 2 h, followed by TLC and then cooled to RT. The white precipitate (NaBr and excess Na 2 CO 3 ) was filtered off and the filtrate was evaporated under vacuum. The residue was dissolved in hexane and the insoluble yellow powder obtained was recovered, washed with more hexane and then dried to yield 0.335 g (60%) of compound (II). Palegreenish-yellow block-like crystals were obtained by slow evaporation of a CDCl 3 solution of (II) in an NMR tube.    (Macrae et al., 2020); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015), PLATON (Spek, 2020) and publCIF (Westrip, 2010). Δρ min = −0.12 e Å −3 Extinction correction: (SHELXL2018/3; Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.043 (7) 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.

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.