Crystal structures of 2-acetyl-4-ethynylphenol and 2-acetyl-4-(3-hydroxy-3-methylbut-1-yn-1-yl)phenol

Crystal structures of two 4-substituted derivatives of 2-acetylphenol are discussed comparatively with reference to modes of hydrogen bonding.


Chemical context
2-Acetylphenol and its derivatives are well known for their efficiency in the complexation of transition metal ions (Weber, 1977;Duckworth & Stephenson, 1969;Ali et al., 2005). Such molecules, endowed with a 2-acetylphenol moiety, have been used as molecular linkers for the construction of coordination polymers and related porous framework structures (Hü bscher et al., 2013;Gü nthel et al., 2015) that are the subject of great topical interest (MacGillivray, 2010;Furukawa et al., 2013;Eddaoudi et al., 2015). A corresponding linker design features a structure with terminal chelating 2-acetylphenol units attached to a linear central segment. In the course of the synthesis of respective linkers, the 2-acetylphenol derivatives (I) and (II), being substituted acetylenically in the 4-position, are important intermediates (Hü bscher et al., 2013). However, these compounds are not only of experimental preparative relevance but also show interesting structures in the crystalline state, as discussed in the present communication. ISSN 2056-9890

Structural commentary
The crystal structures of the title compounds (I) and (II), crystallize in the space groups P1 and P2 1 /c, respectively. Perspective views of the molecules are depicted in Fig. 1. In (I) the asymmetric part of the unit cell contains one molecule (Fig. 1a). As a result of the presence of an intramolecular O-HÁ Á ÁO hydrogen bond, the molecule has an almost planar geometry with largest atomic distances from the mean plane being À0.034 (1) Å for atom C5 and 0.069 (1) Å for atom O1. Because of substituent effects, the bond distances within the aromatic ring of the molecule deviate significantly from those observed in the polymorphous structures of ethynylbenzene (Dziubek et al. 2007;Thakur et al. 2010). Compound (II) crystallizes with three independent and conformationally nonequivalent molecules in the asymmetric unit. The molecules differ in their geometries around the dimethylhydroxymethyl structural element. These differences are expressed by the torsion angle along the atomic sequences C ethynyl -C-O-H which are 72.1 (2) and 83.9 (2) (gauche) for molecules 1 and 3 and 173.0 (2) (anti) for molecule 2 (Fig. 1b). The ethynyl segment of the molecules also deviates from linearity, possibly because of packing forces and intermolecular interactions.

Supramolecular features
Infinite strands of C-HÁ Á ÁO hydrogen-bonded molecules [d(HÁ Á ÁO) 2.28 Å ] (Desiraju & Steiner, 1999) running along [101] represent the basic supramolecular aggregates of the crystal structure of (I). Within a given strand, the acetylenic hydrogen acts as a donor and the acyl oxygen as an acceptor site ( Fig. 2  Perspective view of the molecular structure of the title compounds, (a) (I) and (b) (II), with the atom labelling. Displacement parameters are drawn at the 50% probability level.

Figure 2
A partial view of the crystal packing of compound (I). Hydrogen bonds are shown as dashed lines (see Table 1), and O atoms as red circles. Table 1 Hydrogen-bond geometry (Å , ) for (I).

D-HÁ
As depicted in Fig. 2, the crystal of (I) lacksarene stacking (Martinez & Iverson, 2012 (101). The molecules pack with the dimethylhydroxymethyl groups assembled in layered structure domains, separated by the non-polar parts of the molecules (Fig. 3).

Synthesis and crystallization
Compounds (I) and (II) were synthesized following a literature procedure (Hü bscher et al., 2013). This involves the reaction of 2-acetyl-4-bromophenol with 2-methylbut-3-yn-2ol (MEBYNOL) using a Sonogashira-Hagihara coupling process to give (II). A deblocking reaction of (II) under basic conditions yielded (I). Crystals of (I) and (II), suitable for X-ray diffraction analysis, were obtained from solutions of nhexane/ethyl acetate (3:1, v/v) and cyclohexane, respectively, upon slow evaporation of the solvents at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were placed geometrically in idealized positions and allowed to ride on their parent atoms: O-H = 0.84 and C-H = 0.95-98 Å with U iso (H) = 1.5U eq (C-methyl and O) and 1.2U eq (C) for other H atoms.

Figure 3
The crystal packing of compound (II), viewed along the c axis. Hydrogen bonds are shown as dashed lines (see Table 2) and C-bound H atoms have been omitted for clarity. Computer programs: APEX2 and SAINT (Bruker, 2008), SHELXS97 and SHELXTL (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and ORTEP-3 for Windows (Farrugia, 2012). For both compounds, data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).  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.

(I) 2-Acetyl-4-ethynylphenol
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) x y z U iso */U eq O1 1.41916 (10) 0.74175 (12) 0.41955 (9) 0.0302 (2)   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.28 e Å −3 Δρ min = −0.22 e Å −3 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.