Binary charge-transfer complexes using pyromellitic acid dianhydride featuring C—H⋯O hydrogen bonds

Four binary charge-transfer complexes were made using pyromellitic acid dianhydride (pmda), all of which show alternating donor and acceptor stacks, which have weak C—H⋯O hydrogen bonds connecting the donor and acceptor molecules.


Chemical context
Crystal engineering, the conception and synthesis of molecular solid state structures, is fundamentally based upon the discernment and subsequent exploitation of intermolecular interactions. Consequently, non-covalent bonding interactions are primarily used to achieve the organization of molecules and ions in the solid state in order to produce materials with desired properties. and this understanding using a variety of intermolecular interactions is at the very heart of crystal engineering. Recently, it has been shown that one can synthesize supramolecular assemblies that contain anywhere from three to six different molecular moieties (Paul et al., 2018). Supramolecular synthesis chiefly uses the hydrogenbond interaction as the most directional of the known intermolecular interactions (Aakerö y & Beatty, 2001). An equally important interaction is that of charge transfer (CT) between an electron-rich -system (donor) and an electron-poorsystem (acceptor) (Herbstein, 2005). Classic donor molecules (polycyclic aromatic hydrocarbons) generally have an electron-rich -system. On the other hand, aromatic hydrocarbons with strongly polarizing groups, such as 1,3,5-trinitrobenzene (TNB), have an electron-poor -system and are classified as the acceptor molecule (Hill et al., 2018a,b). Another common acceptor molecule is pyromellitic acid dianhydride (pmda), which has electron-withdrawing O atoms of the carboxylic acid dianhydride groups. (pmda)Á(pyrene) complexes have been investigated for order-disorder transitions as a function of temperature using infrared and Raman spectroscopy (Isaac et al., 2018), (pmda)Á(naphthalene) has been studied via Raman spectroscopy for having orientational disorder (Macfarlane & Ushioda, 1977), disorder in (pmda)Á(perylene) via computer simulation (Boeyens & Levendis, 1986), and photoconductivity and magentoconductance in pmdaÁ-

Structure Acceptor Cg
Donor Cg CgÁ Á ÁCg Symmetry Operator (I) C1-O1 (Cg3) C4-C6 (Cg6) 3.3724 (2) Àx + 1 2 , y À 1 2 , Àz (II) O1-C10 (Cg5) C11-C19 (Cg14) 3.3193 (5) x, y, z (III) C2-C9 (Cg3) C11-C24 (Cg10) 3.2994 (4) x À 1, y, z (IV) C1-O1 (Cg9) C11-C24 (Cg3) 3.3280 (3) 1 À x, Ày, 1 À z CrystalExplorer 17.5 (Spackman & McKinnon, 2002). Table 2 summarizes the percentages for all combinations of contacts between C, H and O atoms and the relevant fingerprint plots are given in the supporting information. In the paper by Chen et al. (2017), the authors describe that regions of blue and red triangles on the Hirshfeld surface using the shape index as evidence ofinteractions. Fig. 4 shows such surfaces plotted for the pmda molecules in (I)-(IV), and for comparison the shape index of the pmda molecule in its unimolecular crystal structure. The red triangles show concave regions indicative of ring carbons of the stacked molecule above it. Complexes (I)-(IV) display a high number of triangles, which reveals the increased proportion ofstacking observed for the four structures.. The shape index of pmda shows no such pattern [Fig. 4(a)]. This stacking can be quantified by looking at the contribution of the CÁ Á ÁC contacts contained in the fingerprint plots, which vary only slightly from 19.9 to 21.0%. The greatest contribution to the Hirshfeld surfaces are seen in the HÁ Á ÁO contacts, which vary from 48.5 to 58.4%. In comparison, the CÁ Á ÁC contacts only make up 0.2% in pmdaÁ Á Ápmda and the CÁ Á ÁO contacts have the greatest single contribution at 43%. In summary, the introduction of an aromatic polycylic changes the biggest contributor from CÁ Á ÁO in pmda to HÁ Á ÁO in pmda-aromatic polycyclics.

Supramolecular features
Compound (I) crystallizes in the C2/m space group with one quarter of the pmda and naphthalene molecules occupying a twofold axis and a mirror plane, resulting in Z 0 = 0.25 for the asymmetric unit. The donor and acceptor molecules stack along the c-axis direction, and in a checker board fashion along the ab plane [ Fig. 2(a)]. In the direction of the a-axis, there is a symmetrical C4-H4Á Á ÁO2 interaction from both ends of the naphthalene molecule to the oxygen atoms on the pmda [ Fig. 2(b), The molecular Hirshfeld surfaces mapped over shape index for the pmda molecule by itself (PYMDAN) and for the pmda acceptor molecule in charge transfer complexes (I)-(IV).

Figure 3
Packing diagrams for (II)-(IV). The donor molecules are shown in blue or yellow, and the acceptor molecules in green or red.  Table 3 Hydrogen-bond geometry (Å , ) for (I). Symmetry codes: (i) Àx þ 1; y; Àz þ 1; (ii) Àx þ 1 2 ; Ày þ 1 2 ; Àz þ 1. described using graph-set notation (Bernstein et al., 1995). Along the b-axis, there is an additional hydrogen bonded ring, R 2 2 (8), resulting from C3-H3Á Á ÁO1 hydrogen-bond interaction [ Fig. 2 Compound (II) crystallizes in the Pca2 1 space group with two pmda and two fluoranthene molecules in the asymmetric unit. One set of D/A pairs is shown in blue/green, and the second is shown in yellow/red. The separation of the two D/A pairs can be clearly seen in Fig. 3(a). Between the four unique pmda acceptor and fluoranthene donors there are numerous C-HÁ Á ÁO interactions (Table 4). As the fluoranthene has only C and H atoms, it is the molecule that has the most weak hydrogen-bond donor groups (C-H), and the pmda, with six oxygen atoms, has numerous good hydrogen-bond acceptor atoms (O). Fig. 5(a) and 5(b) illustrate four of the hydrogen bonds emanating from the two symmetry-independent fluoranthene molecules, which form a number of hydrogen-bonded rings: R 1 2 (7), R 2 2 (7), R 2 2 (8) and R 3 3 (12).
Compound (III) crystallizes in the P1 space group with both the pmda and 9-methylanthracene in the asymmetric unit. The packing of the structure shows the typical donor-acceptor stacking along the a axis [ Fig. 3(b)] and has the closest centroid-to-centroid distance of all four charge-transfer complexes at 3.2994 (4) Å (Table 1). Perpendicular to the stacking axis, the donor and acceptor molecules form hydrogen-bonded layers using four distinct C-HÁ Á ÁO hydrogen bonds (Table 5). The combination of these individually or in groups results in three types of hydrogen bonded rings, R 2 2 (10), R 3 3 (13) and R 4 4 (24), shown in Fig. 5(c). Compound (IV) crystallizes in the P2 1 /c space group with half a pmda (on a centre of inversion) and one complete 9-ethyl ester anthracene molecule in the asymmetric unit, giving a ratio of one acceptor to two donors. [Fig. 3(c)]. Two donor molecules form a hydrogen-bonded ring dimer [Fig. 5(d)], graph-set R 2 2 (14), via a C21-H21Á Á ÁO4 hydrogen bond Two pmda molecules are connected to the donor via discrete hydrogen bonds C12-H12Á Á ÁO2 and C15-H15Á Á ÁO3 (Table 6).

Figure 5
Hydrogen-bonding diagrams for (II)-(IV). Atom labels correspond to those given in the hydrogen-bonding tables.
we have characterized a further new set of four CT complexes of pmda and aromatic molecules.

Synthesis and crystallization
All chemicals were purchased from commercial sources (Sigma Aldrich) and used as received without further purification. The pyromellitic acid dianhydride charge transfer complexes were prepared in a 10 mL ethanolic solution with a 1:1 stoichiometric ratio of the donor to the acceptor molecule which was then heated and stirred until total dissolution took place (approx. 4 h). The solution was then cooled very slowly and allowed to evaporate to obtain crystals suitable for X-ray diffraction. Detailed masses are as follows: (I): 0.100 g of pyromellitic acid dianhydride and 0.059 g of naphthalene; (II): 0.100 g of pyromellitic acid dianhydride and 0.093 g of fluoranthene; (III): 0.100 g of pyromellitic acid dianhydride and 0.088 g of 9-methylanthracene; and (IV): 0.100 g of pyromellitic acid dianhydride and 0.12 1 g of 9-ethyl ester anthracene.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 7. For all compounds, the C-bound H atoms were geometrically placed (C-H bond lengths of 0.96 (methyl CH 3 ), and 0.95 (Ar-H) Å ) and refined as riding with U iso (H) = 1.2U eq (Ar-C) or U iso (H) = 1.5U eq (methyl-C).  For all structures, data collection: APEX3 (Bruker, 2016); cell refinement: SAINT-Plus (Bruker, 2016); data reduction:

Special details
Experimental. Absorption corrections were made using the program SADABS (Sheldrick, 1996) 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
Experimental. Absorption corrections were made using the program SADABS (Sheldrick, 1996) 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
Experimental. Absorption corrections were made using the program SADABS (Sheldrick, 1996) 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
Experimental. Absorption corrections were made using the program SADABS (Sheldrick, 1996) 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.