Synthesis and crystal structures of 5,5′-(propane-2,2-diyl)bis(2-hydroxybenzaldehyde) and 5,5′-(propane-2,2-diyl)bis(2-hydroxyisophthalaldehyde)

The molecule of (1) presents a >C(CH3)2 group that bridges two nearly planar salicylaldehyde groups, each comprising a planar phenyl ring bonded with a hydroxyl and an aldehyde group. Similarly, molecule (2) presents the same bridging group, but it connects two nearly planar appendants, each comprising a phenyl ring bonded with a hydroxyl and two aldehyde groups. Compound (2) exhibits a strong visible luminescence when excited with ultraviolet radiation.


Chemical context
As polymers play an undeniable role in our everyday lives, extensive resources and safety evaluations are devoted toward the development and marketing of the most suitable and effective polymer species for a given application (Andrady & Neal 2009;Fenichell 1996;Teegarden 2004). Bisphenols, salicylaldehydes, and their derivatives have fueled much interest in recent years because they are key precursors for many present and future compounds. Bisphenols typically serve as scaffolds for producing thermoplastics and polymer resins, whereas salicylaldehydes and derivatives are commonly used to synthesize metal-chelating agents for analytical, biological, or material science applications (Lim & Tanski, 2007;Guieu et al., 2012Guieu et al., , 2013Barba & Betanzos, 2007;Vančo et al., 2005;Baisch et al., 2017;Kalinowski & Richardson, 2005, Mounika et al., 2010. As part of our ongoing work on the synthesis and characterization of novel compounds, as well as our effort to eliminate or replace toxic reagents with greener chemicals in the polymer production process, we have synthesized the title compounds, 5,5 0 -(propane-2,2-diyl)bis(2hydroxybenzaldehyde (1) and 5,5 0 -(propane-2,2-diyl)bis(2hydroxyisophthalaldehyde (2). These precursor compounds present a >C(CH 3 ) 2 group that bridges two salicylaldehyde moieties (1) or two phenyl groups with an hydroxyl and two aldehyde appendants (2). The various functional groups in these molecules determine their chemical and physical properties, and the ability to modify them provides the title compounds with a wide versatility and the multifunctionality required for synthesizing safer and better performance materials for future civilian and military applications. For instance, the title compounds may be used for the non-toxic, isocyanate-free synthesis of polyurethanes (Maisonneuve et al., 2015). In addition, (2) is a new, solid-state photoluminescence material that emits radiation in the spectroscopic range between 490 and 590 nm upon ultraviolet light excitation, with potential use as an organic light emitting diode, laser frequency harmonic generator, or photoelectric converter.

Figure 5
Crystal packing of (2) viewed along the b axis showing both the intra-and intermolecular hydrogen bonds (red dashed lines). unlike (1), these interactions result mostly from hydrogen bonding between the phenolic hydrogen atoms and the carboxyl oxygen atoms of adjacent molecules [O5-H5AÁ Á ÁO1 = 2.841 (2) Å ; = 131 (3) ;   (1) and (2) and further discussion. Molecule (3) presents a submolecular structure of the title compounds, as it only lacks the aldehyde groups found in (1) or (2). In contrast, (4) exhibits a pair of salicylaldehyde groups as (1) or (2), except that they are linked by a >CH 2 bridge, instead of a >C(CH 3 ) 2 bridge.

Database survey
Compound (3) crystallizes with three independent molecules in the asymmetric unit. Each molecule presents a pair of planar phenol fragments [r.m.s. deviations = 0.013 (2) and 0.028 (2) Å ; 0.0039 (4) and 0.0078 (5) Å ; and 0.0055 (6) and 0.0039 (3) Å ] subtending dihedral angles of 77.81 (3), 86.15 (4) and 84.34 (4) , respectively, and respective bridge angles of 109.2 (1), 109.5 (1), and 108.1 (1) . In general, both (1) and (2) have similar geometric parameters to (3), although their corresponding phenol groups are less planar than those of (3). This manifestation results most likely because the phenyl groups of the title compounds contain aldehyde groups in addition to the hydroxyl groups. The O atoms of these aldehyde groups participate in hydrogen bonding with the hydroxyl H atoms, thus partially displacing the hydroxyl O atoms away from the phenol planes. A superimposition of the atoms in (1) with the corresponding atoms of one of the three structures of (3) shows that the differences in the atom positions of the two structures are hardly discernible (Fig. 6) [r.m.s. deviation = 0.115 Å ; maximum displacement = 0.217 (2) Å between the O2 atom of (1) and its counterpart of (3)]. An overlay of structure (1) onto either structure two or three of (3) yields comparable results. A similar analysis of structures (2) and (3) yields a r.m.s. deviation of 1.14 Å with maximum displacement of 0.605 (2) Å for the C6 atom of (2) and its counterpart in (3). Again, we obtain comparable results overlaying either structure two or three of (3) onto (1).
Compound (1): A combination of compound (3) (10.0 g, 43.8 mmol, 1.0 equiv.), paraformaldehyde (16.7 g, 556.1 mmol, 12.7 equiv.), and magnesium(II) chloride (35.2 g, 173.1 mmol, 4.0 equiv.) were suspended in tetrahydrofuran (THF, 500 mL), placed under a stream of N 2 , and stirred. Then, triethylamine (49 mL, 351.6 mmol, 8.0 equiv.) was added dropwise to the reaction mixture at ambient temperature and stirred under reflux for 16 h. At the conclusion of the reaction, the mixture was cooled to room temperature before the addition of diethyl ether (500 mL). The organic solution was sequentially extracted with aqueous 1 M HCl (3 Â 500 mL) and water (3 Â 500 mL), dried over Na 2 SO 4 or MgSO4, filtered, and the volatiles were removed under reduced pressure. The solid residue was purified with a series of hexane washes and then dried under vacuum to afford the desired product (1) as a white solid (11.3 g, 39.7 mmol, 91% yield). Slow diffusion of hexanes into a benzene solution saturated with (1) afforded single crystals of (1).
Compound (2) Superimposition of the non-hydrogen atoms of (3) (green) onto the corresponding atoms of (1) (red). under ambient conditions. The reaction mixture was stirred at 403 K for 2.5 h and subsequently cooled to room temperature before aqueous HCl (3M, 150 mL) was added slowly. The reaction mixture was stirred at 383 K for 16 h, cooled to room temperature, and the resulting organic phase extracted with dichloromethane (DCM, 3 Â 150 mL). Then, this organic phase was dried over MgSO 4 , filtered, and the volatiles were removed under reduced pressure. The resulting solid was purified with a series of hexanes washes and dried under vacuum to afford the novel product (2) as a neon yellow solid (9.97 g, 29.3 mmol, 67% yield). Slow evaporation of a DCM solution saturated with (2) afforded single crystals suitable for X-ray diffractometry.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atoms for (1) and most in (2) were refined in a riding-model approximation with C-H = 0.93 or 0.96 Å , U iso (H) = 1.2U eq (C) or 1.5U eq (C methyl ) and O-H = 0.82 Å and U iso (H) = 1.5U eq (O). In (2), atoms H10, H11, H18, and H5A were refined independently with isotropic displacement parameters.

5,5′-(Propane-2,2-diyl)bis(2-hydroxybenzaldehyde) (1)
Crystal data 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.