Date Published: December 01, 2018
Publisher: International Union of Crystallography
Author(s): Tania N. Hill, Andreas Lemmerer.
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.
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 hydrogen-bond interaction as the most directional of the known intermolecular interactions (Aakeröy & Beatty, 2001 ▸). An equally important interaction is that of charge transfer (CT) between an electron-rich π-system (donor) and an electron-poor π-system (acceptor) (Herbstein, 2005 ▸). Classic donor molecules (polycyclic aromatic hydrocarbons) generally have an electron-rich π-system. On the other hand, aromatic hydrocarbons with strongly polarizing groups, such as 1,3,5-trinitrobenzene (TNB), have an electron-poor π-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)·(pyrene) complexes have been investigated for order–disorder transitions as a function of temperature using infrared and Raman spectroscopy (Isaac et al., 2018 ▸), (pmda)·(naphthalene) has been studied via Raman spectroscopy for having orientational disorder (Macfarlane & Ushioda, 1977 ▸), disorder in (pmda)·(perylene) via computer simulation (Boeyens & Levendis, 1986 ▸), and photoconductivity and magentoconductance in pmda·(pyrene) (Kato et al., 2017 ▸). To this end, we have synthesized four new charge-transfer co-crystals that show no disorder: (pmda)·(naphthalene) (I), (pmda)·(fluoranthene) (II), (pmda)·(9-methylanthracene) (III), and (pmda)2·(9-ethyl ester anthracene) (IV).
The asymmetric units and atom-labelling schemes are shown in Fig. 1 ▸, together with their displacement ellipsoids, for all charge-transfer complexes. As a result of the strong polarizing effect of the carboxylic acid dianhydride groups, pmda has an electron-poor π-system and functions as an acceptor. On the other side, the donor molecules comprising polycyclic aromatic hydrocarbons have an electron-rich π-system. The packing of the molecules of the four complexes follows a donor (D) acceptor (A) π–π interaction, which is the major driving force in the formation of these complexes, as seen in Figs. 2 ▸ and 3 ▸ (donor molecules shown in blue/yellow and acceptor in green/red), resulting in a general face-to-face π-stacking, with Table 1 ▸ summarizing the closest centroid–centroid distances between the pmda acceptor and aromatic donor systems. The intermolecular interactions of the D⋯A stacks can be quantified using Hirshfeld surface analysis as well as the resulting fingerprint plots using the programme 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 of π–π interactions. 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 π stacked molecule above it. Complexes (I)–(IV) display a high number of triangles, which reveals the increased proportion of π–π stacking observed for the four structures.. The shape index of pmda shows no such pattern [Fig. 4 ▸(a)]. This π stacking can be quantified by looking at the contribution of the C⋯C 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 H⋯O contacts, which vary from 48.5 to 58.4%. In comparison, the C⋯C contacts only make up 0.2% in pmda⋯pmda and the C⋯O contacts have the greatest single contribution at 43%. In summary, the introduction of an aromatic polycylic changes the biggest contributor from C⋯O in pmda to H⋯O in pmda-aromatic polycyclics.
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′ = 0.25 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—H4⋯O2 interaction from both ends of the naphthalene molecule to the oxygen atoms on the pmda [Fig. 2 ▸(b), Table 3 ▸]. As a result of the mirror plane symmetry, this results in a very symmetrical (5) ring as described using graph-set notation (Bernstein et al., 1995 ▸). Along the b-axis, there is an additional hydrogen bonded ring, (8), resulting from C3—H3⋯O1 hydrogen-bond interaction [Fig. 2 ▸(b)].
A database survey in the Cambridge Structural Database (CSD, Version 5.39; November 2017 update; Groom et al., 2016 ▸) was undertaken for any structures containing the pmda moiety. A total of 26 complexes were found, four showing polymorphism [BECNUS02 (Karl et al., 1982a ▸) and BECNUS10 (Bugarovskaya et al., 1982 ▸); DURZAR and DURZAR01 (Stezowski et al., 1986 ▸); NAPYMA01 (Le Bars-Combe et al., 1979 ▸) and NAPYMA12 (Le Bars-Combe et al., 1981 ▸); PYRPMA04 (Herbstein et al., 1994 ▸) and PYRPMA11 (Kato et al., 2017 ▸)] and one showing stoichiometric variation [VILFEB and VILFIF (Bulgarovskaya et al., 1989 ▸)].
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.
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 CH3), and 0.95 (Ar—H) Å) and refined as riding with Uiso(H) = 1.2Ueq(Ar-C) or Uiso(H) = 1.5Ueq(methyl-C).