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

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.

Partial Text

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).