Crystal structure of 4′-allyl-4,5,6,7,2′,7′-hexachlorofluorescein allyl ester unknown solvate

In the crystal, tetrameric supramolecular aggregates linked by O—H⋯O hydrogen bonds occur; these further interact with neighboring aggregates through C—Cl⋯π interactions arising from the benzene rings, forming infinite two-dimensional sheets. Each C6Cl4 ring shifts in the direction perpendicular to the two-dimensional sheet, exhibiting a helical chain in which every C6Cl4 ring is utilized as both a donor and an acceptor of Cl⋯π contacts. Thus, these two-dimensional sheets pack in a helical fashion, constructing a three-dimensional network.


Chemical context
Fluorescein derivatives have been widely used in chemical and biological research. The high fluorescence quantum yields and excellent photostability of these dyes make them attractive as fluorescent labels for macromolecules such as proteins (Giepmans et al., 2006) or DNA (Li et al., 1995). Fluorescein derivatives also exhibit tunable optical transitions in the visible range and high molar extinction coefficients, making them suitable for optical laser and dye-sensitized solar cell applications (Pepe et al., 2016). Understanding the properties of these fluorescein derivatives, especially their bonding abilities at certain local environments, is essential for designing and utilizing these compounds. Detailed crystal structure determinations of fluorescein derivatives can reveal their bonding/packing properties, providing valuable insights in directing future molecular engineering design and chemical and biological applications. Until recently, the different forms of fluorescein could only be obtained as microcrystalline powders and the first crystal structure determination of free fluorescein came from powder diffraction data analysis (Tremayne et al., 1997). It was then followed by a number of single crystal X-ray structural analyses of fluorescein derivatives. For several recent examples, see Christianson & Gabbaï (2016), Sezukuri et al. (2016), and Dufresne et al. (2007).
The title compound, 4 0 -allyl-4,5,6,7,2 0 ,7 0 -hexachlorofluorescein allyl ester, is an important intermediate in the ISSN 2056-9890 synthetic route of structurally flexible fluorescein heterodimers that were recently published by us (Wang et al., 2017). Such heterodimers were designed to test the engineering principle of quantum coherences in artificial light-harvesting systems. Herein, we present the crystal structure of the title compound, which reveals the importance of ClÁ Á Á interactions in the solid state.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The structure consists of a xanthene ring system, a perchlorinated phenyl ring and two allyl groups; one is located at the periphery of the xanthene ring while the other is linked to the six-membered ring through the carboxylate linker (atom O6). The phenyl plane inclines from the xanthene plane by about 73 [the C4-C13-C14-C15 torsion angle is 72.7 (3) ]. The unusual unsymmetrical substitution pattern on the xanthene ring of the title compound leads to the possibility of having different tautomers as depicted in the Scheme. Unsymmetrically substituted fluoresceins have previously been reported, but until now all related structural reports showed only their spiroxanthene isomeric forms (Hou et al., 2012;Swamy et al., 2006;Wang et al., 2005), thwarting a direct comparison with this study. While the title compound may exist as a mixture of exchanging tautomers (A) and (B) in solution, the solid-state structure is better described as tautomer (A) based on the bond-length distribution. For example, the bond lengths for C7-O4  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A tetrameric hydrogen-bonded aggregate formed by the title compound: O2-H2Á Á ÁO4 bonds are labeled as 'a'. The assemblage has 4 symmetry.

Figure 1
The molecular structure of the title compound with 40% displacement ellipsoids. H atoms as well as atoms of the disordered allyl groups are shown as spheres of arbitrary radius.
thermodynamically more stable tautomer that may exist in the gas phase, because this form may be stabilized by the formation of tetrameric aggregates through intermolecular O2-H2Á Á ÁO4 bonds as discussed below (Table 1, Fig. 2).

Supramolecular features
In the crystal, the title compound forms tetrameric aggregates linked by O2-H2Á Á ÁO4 hydrogen bonds, as shown in Fig. 2. The allyl groups sit inside the pocket formed by the hydrogen bonds and are not engaged in any particular intermolecular interactions (only one disorder component is shown). The tetrameric aggregates further interact with neighboring aggregates through ClÁ Á Á interactions of dangling C 6 Cl 4 rings forming infinite two-dimensional sheets, as shown in Fig. 3.
Each of the C 6 Cl 4 rings accepts two edge-on ClÁ Á ÁC short contacts from an adjacent C 6 Cl 4 unit [Cl4Á Á ÁC16 = 3.398 (3); Cl5Á Á ÁC18 = 3.333 (3) Å ]. When viewed along the twodimensional sheet located in the ab plane, it may be noted that each -C 6 Cl 4 ring is in fact shifted in the direction perpendicular to the two-dimensional sheet. These C 6 Cl 4 rings thus exhibit a helical chain in which every C 6 Cl 4 ring is utilized as both a donor and an acceptor of ClÁ Á Á contacts. Thus, several layers of the tetrameric aggregates are further packed in a helical manner in the third dimension along the c axis, constructing a three-dimensional network, as shown in Fig. 4.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.38, update May 2017; Groom et al., 2016) indicated that several fluorescein derivatives with halogen substituents on the xanthene ring have been reported (Cody, 1987;Willner et al., 1992;Harrison et al., 2007;Quint et al., 2016). However, there was only one structural report on fluorescein derivatives that contains a tetrachloro-substituted phenyl unit (CCDC refcode KUFTUA; Willner et al., 1992), and there were no structural reports on hexachlorinated fluorescein derivatives. While the hydroxyl groups on the xanthene rings of fluorescein derivatives have been reported to engage in hydrogen bonds (Abrahams et al., 2009), to the best of our knowledge, the tetrameric aggregation motif in this report has not been found previously for fluorescein derivatives.
4,5,6,7,2 0 ,7 0 -Hexachlorofluorescein diallyl ether ester (500 mg) in diphenyl ether (5 ml) was heated in a sealed tube at 443 K under N 2 overnight. The homogeneous mixture was then cooled to room temperature, transferred to a scintillation vial, and diluted with CHCl 3 (5 ml). Red prismatic crystals of the title compound formed slowly from this mixture at room temperature within three months, yield: 52%. This crystalline material contained 0.3 equiv. of diphenyl ether and ca 0.1 equiv of CHCl 3 , as determined by 1 H NMR integration. Note that the quantity of CHCl 3 could be underestimated because of the overly long T 1 relaxation time of the H-CCl 3 proton. The volatile nature of CHCl 3 and the loss in the sampledissolving process could also contribute to underestimation.

Refinement
Crystal data, data collection and structural refinement details are summarized in Table 2. Carbon-bound H atoms were placed in calculated positions (C-H = 0.95-0.98 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2-1.5U eq (C). The H atom of the hydroxyl group was found in a difference-Fourier map and freely refined [O-H = 0.74 (4) Å ]. Most atoms except those of the allyl groups were refined anisotropically. Both allyl groups were found to be disordered and each disorder was individually modeled with the application of appropriate geometric (SADI) restraints or thermal parameters (EADP) constraints. The disorder was modelled over two positions (refined occupancies of 0.5:0.5 and 0.55:0.45). Similar distance soft restraints were used for the allyl groups. Hydrogen atoms were included in idealized positions for structure-factor calculations.
The crystal contained many disordered solvent molecules located in several solvent-accessible voids. 1 H NMR analysis of the crystalline material in MeOD revealed that both Ph 2 O and CHCl 3 are present. The amount of Ph 2 O is quantified to be 0.3 equiv. using the integrals for multiplets at 7.37-7.32 (4H), 7.12-7.07 (2H), and 6.98-6.96 (4H). The amount of CHCl 3 is found to be approximately 0.1 equiv. using the integral for the singlet at 7.90. The amount of the CHCl 3 is most probably underestimated owing to a very long T1 relaxation time of the HCCl 3 proton and its loss in the sample during the dissolving process and crystals transfer. These results guided the disorder modeling of the allyl group pointing into the void as 0.5:0.5. The allyl group inside the void is poorly defined and could not be freely refined. Attempts to model the disordered solvent area were not successful, and the diffuse contribution to scattering was treated by application of   Data collection: APEX3 (Bruker, 2015); cell refinement: SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).