Crystal structure of 9,9-diethyl-9H-fluorene-2,4,7-tricarbaldehyde

The fluorene skeleton of the title molecule is nearly planar and the crystal structure is composed of molecular layers extending parallel to the 302 plane. A Hirshfeld surface analysis indicated that the most important contributions to the overall surface are from H⋯H, O⋯H and C⋯H interactions.

The title compound, C 20 H 18 O 3 , crystallizes in the space group P2 1 /c with one molecule in the asymmetric unit of the cell. The fluorene skeleton is nearly planar and the crystal structure is composed of molecular layers extending parallel to the (302) plane. Within a layer, one formyl oxygen atom participates in the formation of a C arene -HÁ Á ÁO bond, which is responsible for the formation of an inversion symmetric supramolecular motif of graph set R 2 2 (10). A second oxygen atom is involved in an intramolecular C arene -HÁ Á ÁO hydrogen bond and is further connected with a formyl hydrogen atom of an adjacent molecule. A Hirshfeld surface analysis indicated that the most important contributions to the overall surface are from HÁ Á ÁH (46.9%), OÁ Á ÁH (27.9%) and CÁ Á ÁH (17.8%) interactions.

Chemical context
Compounds featuring a fluorene moiety have been recognized as useful for a broad spectrum of applications, which range from agents for cell imaging, solar cells, organic light-emitting diodes to lasers. Furthermore, fluorene derivatives have the potential to act as artificial receptors for different ionic and neutral substrates in analogy to the known receptors possessing a benzene or biphenyl core, which, for example, are able to complex ammonium ions (Koch et al., 2015;Schulze et al., 2018;Chin et al., 2002;Arunachalam et al., 2010), ion pairs (Stapf et al., 2015) or carbohydrates (Stapf et al., 2020;Kö hler et al., 2020Kö hler et al., , 2021Kaiser et al., 2019;Lippe & Mazik, 2013Amrhein et al., 2016;Amrhein & Mazik, 2021). As a result of the manifold application possibilities of fluorenes, the syntheses of new representatives of this class of compounds are the subject of intensive research (Seidel et al., 2019(Seidel et al., , 2021Seidel & Mazik, 2020;Sicard et al., 2018). Fluorene derivatives bearing halogen, formyl or amino groups are valuable starting materials for a wide range of fluorene-based acyclic and macrocyclic compounds as well as polymers. Recently we have described the efficient one-step synthesis of 9,9-diethyl-9Hfluorene-2,4,7-tricarbaldehyde on the basis of 2,4,7-tris-(bromomethyl)-9,9-diethyl-9H-fluorene (Seidel et al., 2019), which provided a threefold higher yield of the product than the previously known three-step reaction sequence (Yao & Belfield, 2005). In this work we describe the crystal structure of this fluorene derivative bearing three formyl groups.

Supramolecular features
The crystal structure of the title compound is composed of molecular layers extending parallel to the (302) plane. An excerpt of the layer structure showing the mode of hydrogen bonding is depicted in Fig. 2. Within a given layer, the formyl oxygen atom O1 participates in the formation of a C arene -HÁ Á ÁO bond [d(HÁ Á ÁO) 2.59 Å ; Table 1], thus creating an inversion-symmetric supramolecular motif of graph-set R 2 2 (10) (Etter et al., 1990;Bernstein et al., 1995; for examples of other crystal structures including such a ten-membered supra-molecular motif, see Seidel et al., 2021;Stapf et al., 2021). The oxygen atom O2 is connected with the formyl hydrogen H16 of an adjacent molecule [d(HÁ Á ÁO) 2.53 Å ]. The steric requirements of the ethyl groups cause an offset of the molecules of consecutive layers, so that neither hydrogen bonds norarene stacking interactions are observed in the direction of the layer normal. Consequently, the crystal appears to be stabilized by van der Waals forces in the direction of the stacking axis of the molecular layers (Fig. 3).

Figure 2
Packing excerpt of (1) showing selected C-HÁ Á ÁO interactions within one layer of molecules.

Figure 3
Packing excerpt of (1) showing adjacent layers of molecules and selected C-HÁ Á ÁO interactions within the layers. Hydrogen atoms of subunits not involved in intermolecular hydrogen bonding are omitted for clarity.

Figure 1
Perspective view of (1) including the labelling of non-hydrogen atoms. Displacement ellipsoids are drawn at the 50% probability level.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The non-hydrogen atoms were refined anisotropically. All hydrogen atoms were positioned geometrically and allowed to ride on their parent atoms: C-H = 0.95 Å for aryl-H atoms, C-H = 0.99 Å for methylene groups and C-H = 0.98 Å for methyl groups with U iso (H) = 1.5U eq (C) for methyl groups and U iso (H) = 1.2U eq (C) for other hydrogen atoms. The crystal structure of (1) was refined as a two-component twin with an approximate occupancy ratio of 63:37.

Acknowledgements
Open Access Funding by the Publication Fund of the Technische Universitä t Bergakademie Freiberg is gratefully acknowledged. Fingerprint plot of (1) including the contribution of the atomÁ Á Áatom pairs to the overall surface.

Figure 5
Hirshfeld surface for (1) mapped with d norm (front and back views).

sup-2
Acta Cryst. (2021). E77, 1029-1032 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. Refinement. Refined as a two-component twin.