10-Phenyl-10H-phenoxazine-4,6-diol tetrahydrofuran monosolvate

In the crystalline state of the title solvate, C18H13NO3·C4H8O, hydrogen-bonding interactions between hydroxyl groups on a phenoxazine backbone and the tetrahydrofuran solvent are observed that suggest the ability for this compound to act as a chelating ligand.

In the crystalline state of the title solvate, C 18 H 13 NO 3 ÁC 4 H 8 O, hydrogen-bonding interactions between hydroxyl groups on a phenoxazine backbone and the tetrahydrofuran solvent are observed that suggest the ability for this compound to act as a chelating ligand. The OÁ Á ÁO donor-acceptor distances for this hydrogen bonding are 2.7729 (15) and 2.7447 (15) Å . The three-ring backbone of the phenoxazine bends out of planarity by 18.92 (3) , as computed using mean planes that encompass each half of the three-ring structure, with the central N and O atoms forming the line of flexion. In the crystal, astacking arrangement exists between inversion-related molecules, with a centroid-tocentroid distance of 3.6355 (11) Å . In the disordered tetrahydrofuran solvate, all atoms except oxygen were modeled over two positions, with occupancies of 0.511 (8) and 0.489 (8).

Structure description
Phenoxazine-based metal complexes have been reported as catalysts in hydroformylation reactions (van der Veen et al., 2000;Verheyen et al., 2019), C-H bond arylations (Li et al., 2016), and aryl chloride cross-couplings (Zhang et al., 2014). One of the most notable phenoxazine ligands is NiXantPhos (Fig. 1). The key to their utility lies in the ability of the ligand to chelate a metal center using the central oxygen atom (O1 in the reported case) alongside the functional groups at the 4 and 6 positions. As best as we can tell, there is not yet a report of a phenoxazine ligand with hydroxyl functional groups at these same positions offering the same ability to chelate.

data reports
The reported compound consists of a 10-phenyl-10Hphenoxazine backbone with two hydroxyl moieties at the 4 and 6 positions of the phenoxazine ring, and this structure was obtained as a tetrahydrofuran solvate (Fig. 2). Similar to the other reported phenoxazine-based ligands, the phenoxazine fused ring system is not planar, with flexion at O1 and N1 resulting in an 18.92 (3) deviation from planarity, as computed using mean planes that encompass each half of the three-ring structure (i.e., atoms C7-C12/N1/O1 and C13-C18/ N1/O1). The plane of the N-phenyl group is nearly perpendicular to the phenoxazine ring structure, with a dihedral angle of 89.14 (6) between the mean plane of the phenyl ring and the plane defined by N1,C10,C11.
This compound was crystallized from a solution of toluene and tetrahydrofuran, and the resulting structure solution shows a single tetrahydrofuran molecule with its oxygen atom accepting two hydrogen-bonding interaction from the phenoxazine hydroxyl groups. The interactions between O4 in the tetrahydrofuran solvent with the O2 and O3 hydroxyl groups mimic a structure that a deprotonated, dianionic form of the title compound might adopt upon complexation with a metal ion. The interaction between these molecules could be classified according to the Jeffrey model as 'moderate, mostly electrostatic' (Jeffrey, 1997) with donor-acceptor distances of 2.7729 (15) Å (O4Á Á ÁO2) and 2.7447 (15) Å (O4Á Á ÁO3) (Fig. 3, Table 1).

Figure 2
Ellipsoid plot (50%) of the title solvate. The minor component of disorder for the THF solvent molecule is omitted for the sake of clarity.

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One other important supramolecular feature is astacking interaction between inversion-related (1 À x, 1 À y, 1 À z) molecules (Fig. 4). One of the peripheral arene rings lies over the same ring in a neighboring molecule. The centroid-to-centroid separation between these two rings is 3.6355 (11) Å .

Synthesis and crystallization
10-Phenyl-10H-phenoxazine was synthesized according to literature procedures (Liu et al., 2014). With this compound in hand, an anhydrous deprotonation was performed on 39 mmol of starting material using 2.2 equivalents of n-butyllithium (2.5 M in hexanes) and N,N,N 0 ,N 0 -tetramethylethane-1,2-diamine in diethyl ether solvent at 273 K. The solution was allowed to warm to room temperature and then stirred overnight. The reaction mixture of the lithiated 10-phenyl-10H-phenoxazine was then cooled again to 273 K and cannulated into a stirred solution of diethyl ether and four equivalents of trimethyl borate. The solution was allowed to warm to room temperature and then stirred overnight. This initial procedure was modeled after the one reported for the synthesis of NiXant-Phos (van der Veen et al., 2009).
The lemon-yellow reaction mixture was evaporated on a rotary evaporator to dryness. The solids were redissolved in $500 ml of methanol and stirred until homogenous. A solution of methanol and six equivalents of urea hydrogen peroxide was prepared and then added dropwise at 273 K to the reaction mixture. The reaction darkened considerably, to a deep red. After stirring overnight, the reaction mixture was concentrated to one quarter of its initial volume using a rotary evaporator before being diluted with $400 ml of distilled water. The solution pH was adjusted using hydrochloric acid until it was neutral to slightly acidic (pH 4-6 indicated by pH paper). At this point the reaction mixture contained a considerable amount of solid that was identified as the 10phenyl-10H-phenoxazine-4,6-diol, so the reaction mixture was filtered to obtain this crude brown-red solid. The subsequent procedure is modeled after the hydroxylation described by Gupta et al. (2016).
The compound was then purified using silica gel column chromatography with a solvent mixture of 15% ethyl acetate in hexanes as an eluent. The eluent polarity was increased by increasing the concentration of ethyl acetate up to 35% over the course of the procedure. The final compound was obtained in 50% yield.
Single crystals suitable for X-ray analysis were obtained using a vapor diffusion method. A small portion of the title compound was dissolved in tetrahydrofuran and transferred to a small cylindrical vial that fitted fully into a standard 20 ml scintillation vial. The volume around the small vial was then filled with toluene until it reached approximately half the capacity of the remaining volume. The 20 ml vial was capped with the internal vial uncapped to allow for vapors diffusion. In this embodiment, the tetrahydrofuran solvent will evaporate and dissolve into the toluene solution, concentrating the title compound in the tetrahydrofuran vial.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The positions of the two hydroxyl H atoms involved in hydrogen bonding, H2 and H3, were refined from difference-map peaks as proof of their correct assignment. The tetrahydrofuran molecule was found to be disordered, and all atoms except for oxygen were modeled across two positions. Due to the positioning of the disordered parts, SHELXL commands EADP and SAME were used to ensure a stable refinement. Disordered part occupancies refined to 0.511 (8) and 0.489 (8).

Funding information
Funding for this research was provided by: program manager Dr Imre Gyuk through the US Department of Energy, Office of Electricity, and Davidson College Faculty Study and Research (grant to ACW, CHB, DT). Sandia National Laboratories is a multi-mission laboratory managed and  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.