Planar versus non-planar: The important role of weak C—H⋯O hydrogen bonds in the crystal structure of 5-methyl­salicyl­aldehyde

Baisch, U.; Scicluna, M.C.; Näther, C.; Vella-Zarb, L.

doi:10.1107/S2056989017000238

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Volume 73| Part 2| February 2017| Pages 155-158

https://doi.org/10.1107/S2056989017000238

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Planar versus non-planar: The important role of weak C—H⋯O hydrogen bonds in the crystal structure of 5-methylsalicylaldehyde

Ulrich Baisch,^a Marie Christine Scicluna,^a Christian Näther ^b and Liana Vella-Zarb ^a ^*

^aDepartment of Chemistry, University of Malta, Msida, MSD 2080, Malta, and ^bAnorganische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str 2, 24118 Kiel, Germany
^*Correspondence e-mail: liana.vella-zarb@um.edu.mt

Edited by A. J. Lough, University of Toronto, Canada (Received 17 November 2016; accepted 5 January 2017; online 13 January 2017)

The crystal structure of 5-methylsalicylaldehyde (5-MSA; systematic name 2-hydroxy-5-methylbenzaldehyde), C₈H₈O₂, was discovered to be a textbook example of the drastic structural changes caused by just a few weak C—H⋯O interactions due to the additional methylation of the aromatic ring compared to salicylaldehyde SA. This weak intermolecular hydrogen bonding is observed between aromatic or methyl carbon donor atoms and hydroxyl or aldehyde acceptor oxygen atoms with d(D⋯A) = 3.4801 (18) and 3.499 (11) Å. The molecule shows a distorted geometry of the aromatic ring with elongated bonds in the vicinity of substituted aldehyde and hydroxyl carbon atoms. The methyl hydrogen atoms are disordered over two sets of sites with occupancies of 0.69 (2) and 0.31 (2).

Keywords: crystal structure; 5-MSA; organic; salicylic acid; hydrogen bonds.

CCDC reference: 1525796

1. Chemical context

Salicylaldehydes form an important and widely used group of compounds in the pharmaceutical and agrochemical industry (Kirchner et al., 2011 ). They have a functional role as metabolites in eukaryotic plants and as nematicides (Caboni et al., 2013 ; Kim et al., 2008 ). As part of a series of co-crystallization experiments in which the title compound was used as a coformer, single-crystals of 5-methylated salicylaldehyde (5-MSA) were obtained and characterized by single-crystal X-ray diffraction. Its crystal structure is reported herein and compared to the unsubstituted form of salicylaldehyde (SA) [Kirchner et al. (2011); refcode YADJOE in the Cambridge Structural Database (Groom et al., 2016 )]. Even though 5-MSA carries just one additional methyl group compared to the latter, a very large difference in melting point is observed. Whereas SA is a liquid at room temperature, 5-MSA is a crystalline solid with a melting point of 328–330 K.

2. Structural commentary

The molecular structure of 5-MSA features a benzene ring (C1–C6), carrying a hydroxyl substituent at position 1, which is bound intramolecularly to the aldehyde group at the ortho position by a fairly strong hydrogen-bond interaction with d(D⋯A) = 2.6260 (17) Å (Fig. 1). In the aromatic ring, the adjacent hydroxyl and aldehyde groups, as well as the methylated C4 atom, lead to a distortion of its geometry, expressed by the slight increase in the C1—C2, C2—C3 and C4—C5 bond lengths to 1.4028 (18) Å, 1.4001 (18) Å, and 1.398 (2) Å, respectively. The other bonds of the ring lie within the expected range, exhibiting the usual lengths of aromatic carbon–carbon bonds [C3—C4 = 1.3781 (19) Å, C5—C6 = 1.377 (2) Å and C1—C6 1.3879 (19) Å]. This affects the corresponding bond angle C3—C4—C5 in the ring, which is 117.16 (13)°. The distance of atom C2 from the aldehyde carbon atom C7 is 1.4507 (18) Å and the deviation from the mean plane defined by the aldehyde and the aromatic ring can be established by the torsion angles C7—C2—C3—C4 [−177.15 (12)°] and C7—C2—C1—C6 [176.64 (11)°]. A similar distortion is observed at torsion angles C7—C2—C1—O9 [−3.38 (18)°] and C1—C2—C7—O8 [2.7 (2)°]. This particular geometry may facilitate the intramolecular O9—H9⋯O8 hydrogen bond [d(O9⋯O8) = 2.6260 (17) Å; O9—H9⋯O8 = 152 (2)°;Table 1]. In comparison, the corresponding hydrogen-bonding interaction in SA has d(D⋯A) = 2.6231 (17) Å and an angle of 156°. The benzene ring in 5-MSA also carries a methyl substituent at the 5-meta position, with a C4—C10 bond length of 1.505 (2) Å.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C10—H10E⋯O8ⁱ	0.98	2.60	3.499 (2)	152
O9—H9⋯O8	0.94 (3)	1.77 (3)	2.6260 (17)	151 (2)
C5—H5⋯O8ⁱⁱ	0.974 (18)	2.607 (18)	3.4801 (18)	149.3 (13)
C6—H6⋯O9ⁱⁱⁱ	0.989 (16)	2.599 (17)	3.4053 (18)	138.7 (12)
Symmetry codes: (i) $[x+1, -y+{\script{1\over 2}}, z+{\script{1\over 2}}]$ ; (ii) x+1, y, z; (iii) -x+1, -y+1, -z-1.

Figure 1
The molecular structure of 5-MSA showing the labeling scheme and anisotropic displacement ellipsoids drawn at the 50% probability level using DIAMOND (Brandenburg, 1999

). The dashed line indicates the intramolecular hydrogen bond.

3. Supramolecular features

The large difference in melting point between SA and 5-MSA is unequivocally related to the different way the two molecules pack in the crystal lattice. Layers of SA molecules are arranged in almost perfect sheets, resulting in a layered structure roughly along the a axis. The distance between these layers of molecules can be analysed by the distance between the centroids (Cg) of the phenyl rings with d(Cg⋯Cg) = 3.7838 (11) Å (Figs. 2 and 3). No intermolecular hydrogen-bonding interactions can be detected in the range d(D⋯A) = 2.5–3.5 Å.

Figure 2
The crystal packing (DIAMOND; Brandenburg, 1999

) of SA viewed along the a axis. π-stacking interactions are indicated by blue dashed lines.

Figure 3
The crystal packing (DIAMOND; Brandenburg, 1999

) of SA viewed along the b axis. π-stacking interactions are indicated by blue dashed lines.

The 5-MSA molecules do show some interesting intermolecular interactions (Steiner, 2002 ) in the same range [d(D⋯A) = 2.5–3.5 Å] apart from van der Waals interactions. Three C—H⋯O interactions are present between either aromatic or methyl C atoms and aldehyde or alcohol oxygen atoms: two close to 3.5 Å with C10⋯O8 = 3.499 (2) Å and C5⋯O8 = 3.4801 (18) Å and corresponding C—H⋯O angles of 152 and 149.3 (13)°, respectively. The third and shortest interaction, has a C6⋯O9 distance of 3.4053 (18) Å and an angle of 138.7 (12)° (Table 1). The latter results in a R₂²(8) ring, a graph set very often observed in the centrosymmetric structures of aromatic acids and aldehydes due to the occurrence of inversion centres between molecules (Fig. 4). In this manner, pairs of molecules are connected to each other by weak intermolecular interactions.

Figure 4
The crystal packing (DIAMOND; Brandenburg, 1999

) of 5-MSA viewed along the a axis. Hydrogen-bonding interactions are shown as blue dashed lines.

The most significant consequence of the additional interactions compared to SA, however, can be seen in the distances between the phenyl rings and the geometry of how they are arranged towards each other. There are two distances between the centroids of the phenyl rings, one within significance range, the other one slightly above, with d(Cg⋯Cg) = 3.7539 (11) and 4.7456 (13) Å, respectively. This results in a deviation from the usually expected herringbone or completely planar arrangement of planar molecules. Wavy layers of molecules are formed instead, whereby the 5-MSA molecules form columns in which the methyl groups are oriented in opposite directions layer-by-layer along the a axis (Figs. 5 and 6).

Figure 5
The crystal packing (DIAMOND; Brandenburg, 1999

) of 5-MSA viewed along the c axis. π-stacking interactions are indicated by blue dashed lines drawn between the centroids of the aromatic rings.

Figure 6
The crystal packing (DIAMOND; Brandenburg, 1999

) of 5-MSA viewed along the a axis. π-stacking interactions are indicated by blue dashed lines drawn between centroids of the aromatic ring.

The stronger π-stacking of the aromatic rings combined with the additional weak intermolecular interactions provides a logical explanation for the difference in melting points between SA and 5-MSA and is a perfect textbook example of the drastic structural changes caused by just a few weak C—H⋯O interactions due to an additional methylation of the aromatic ring.

4. Synthesis and crystallization

The title compound, together with a catalytic volume of ethanol solvent, was ground in a mortar and pestle into a dried powder, which was then dissolved in 1.5 mL of the solvent and allowed to crystallize. Single crystals of suitable quality were selected directly from the dried crystalline precipitate.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The structure solution was not straightforward. A first attempt to solve the structure in space group P2₁/c was unsuccessful. The structure solution was carried out in P1 and then transformation using PLATON (Spek, 2009 ) to the correct space group P2₁/c took place. The hydrogen atoms of the methyl substituent show disorder with an occupancy of 0.69 (2) at positions H10A, H10B, H10C and 0.31 (2) at positions H10D, H10E, H10F. They were included at idealized positions riding on the parent carbon atom, with isotropic displacement parameters U_i_so(H) = 1.5U_eq(CH₃). Refinement of the corresponding site-occupation factors of the methyl-group hydrogen atoms was carried out using a free variable so that their sum is unity. All other hydrogen atoms were located individually in a difference-Fourier map and refined isotropically.

Table 2
Experimental details

Crystal data
Chemical formula	C₈H₈O₂
M_r	136.14
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	170
a, b, c (Å)	8.3676 (17), 13.088 (3), 6.4867 (13)
β (°)	106.30 (3)
V (Å³)	681.8 (3)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	0.10
Crystal size (mm)	0.2 × 0.15 × 0.1

Data collection
Diffractometer	STOE IPDS2
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	7980, 1617, 1276
R_int	0.049
(sin θ/λ)_max (Å⁻¹)	0.658

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.045, 0.118, 1.07
No. of reflections	1617
No. of parameters	112
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.15, −0.11
Computer programs: X-AREA (Stoe & Cie, 2002 ), SHELXS (Sheldrick, 2008 ), SHELXL (Sheldrick, 2015 ), OLEX2 (Dolomanov et al., 2009 ) and DIAMOND (Brandenburg, 1999 ).

Supporting information

CCDC reference: 1525796

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989017000238/lh5831sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989017000238/lh5831Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2056989017000238/lh5831Isup3.cml

checkCIF report

Computing details top

Data collection: X-AREA (Stoe & Cie, 2002); cell refinement: X-AREA (Stoe & Cie, 2002); data reduction: X-AREA (Stoe & Cie, 2002); program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009) and DIAMOND (Brandenburg, 1999); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

2-Hydroxy-5-methylbenzaldehyde top

Crystal data top

C₈H₈O₂	F(000) = 288
M_r = 136.14	D_x = 1.326 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
a = 8.3676 (17) Å	Cell parameters from 2429 reflections
b = 13.088 (3) Å	θ = 1.5–28.5°
c = 6.4867 (13) Å	µ = 0.10 mm⁻¹
β = 106.30 (3)°	T = 170 K
V = 681.8 (3) Å³	Block, clear colourless
Z = 4	0.2 × 0.15 × 0.1 mm

Data collection top

STOE IPDS-2 diffractometer	R_int = 0.049
Graphite monochromator	θ_max = 27.9°, θ_min = 3.0°
ω scans	h = −10→10
7980 measured reflections	k = −17→17
1617 independent reflections	l = −8→8
1276 reflections with I > 2σ(I)

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	Hydrogen site location: mixed
R[F² > 2σ(F²)] = 0.045	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.118	w = 1/[σ²(F_o²) + (0.0511P)² + 0.1028P] where P = (F_o² + 2F_c²)/3
S = 1.07	(Δ/σ)_max < 0.001
1617 reflections	Δρ_max = 0.15 e Å⁻³
112 parameters	Δρ_min = −0.11 e Å⁻³

Special details top

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. Chicken Wire Problem: first structure solution in P1 then transformation using Platon to P2(1)/c (Brandenburg, 1999)'

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
O9	0.33041 (14)	0.39946 (8)	−0.40519 (16)	0.0531 (3)
O8	0.15789 (12)	0.35357 (8)	−0.13645 (19)	0.0578 (3)
C4	0.75281 (16)	0.37298 (10)	0.1197 (2)	0.0447 (3)
C6	0.62263 (18)	0.41085 (10)	−0.2573 (2)	0.0462 (3)
C2	0.45246 (15)	0.36322 (9)	−0.0290 (2)	0.0388 (3)
C5	0.76193 (17)	0.40142 (10)	−0.0845 (2)	0.0469 (3)
C10	0.90763 (19)	0.36471 (13)	0.3055 (3)	0.0618 (4)
H10A	1.0054	0.3807	0.2564	0.074*	0.361 (18)
H10B	0.9008	0.4130	0.4182	0.074*	0.361 (18)
H10C	0.9175	0.2950	0.3630	0.074*	0.361 (18)
H10D	0.8771	0.3451	0.4353	0.074*	0.639 (18)
H10E	0.9817	0.3128	0.2735	0.074*	0.639 (18)
H10F	0.9649	0.4308	0.3288	0.074*	0.639 (18)
C3	0.59671 (17)	0.35444 (9)	0.1431 (2)	0.0419 (3)
C7	0.29005 (17)	0.34759 (10)	0.0048 (2)	0.0471 (3)
C1	0.46618 (16)	0.39116 (9)	−0.2321 (2)	0.0409 (3)
H9	0.240 (3)	0.3860 (18)	−0.351 (4)	0.101 (8)*
H3	0.5831 (18)	0.3354 (12)	0.278 (2)	0.046 (4)*
H5	0.871 (2)	0.4169 (13)	−0.104 (3)	0.063 (5)*
H6	0.630 (2)	0.4329 (12)	−0.400 (3)	0.057 (4)*
H7	0.293 (2)	0.3328 (12)	0.158 (3)	0.054 (4)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O9	0.0495 (6)	0.0571 (6)	0.0463 (5)	−0.0023 (4)	0.0031 (5)	0.0024 (4)
O8	0.0387 (5)	0.0562 (6)	0.0768 (7)	−0.0045 (4)	0.0135 (5)	−0.0033 (5)
C4	0.0394 (6)	0.0362 (6)	0.0552 (8)	0.0030 (5)	0.0080 (6)	−0.0040 (5)
C6	0.0508 (8)	0.0433 (7)	0.0488 (7)	−0.0015 (5)	0.0211 (6)	−0.0014 (5)
C2	0.0394 (6)	0.0321 (6)	0.0453 (7)	−0.0007 (4)	0.0124 (5)	−0.0025 (4)
C5	0.0390 (7)	0.0419 (7)	0.0630 (8)	−0.0004 (5)	0.0198 (6)	−0.0050 (6)
C10	0.0456 (8)	0.0586 (9)	0.0708 (10)	0.0051 (7)	−0.0008 (7)	−0.0018 (7)
C3	0.0461 (7)	0.0366 (6)	0.0427 (7)	0.0008 (5)	0.0120 (5)	0.0003 (5)
C7	0.0437 (7)	0.0411 (7)	0.0586 (8)	−0.0032 (5)	0.0175 (6)	−0.0023 (6)
C1	0.0420 (7)	0.0362 (6)	0.0429 (7)	0.0001 (5)	0.0090 (5)	−0.0019 (5)

Geometric parameters (Å, º) top

O9—C1	1.3589 (16)	C2—C1	1.4028 (18)
O9—H9	0.93 (3)	C5—H5	0.974 (18)
O8—C7	1.2248 (18)	C10—H10A	0.9800
C4—C5	1.398 (2)	C10—H10B	0.9800
C4—C10	1.505 (2)	C10—H10C	0.9800
C4—C3	1.3781 (19)	C10—H10D	0.9800
C6—C5	1.377 (2)	C10—H10E	0.9800
C6—C1	1.3879 (19)	C10—H10F	0.9800
C6—H6	0.989 (16)	C3—H3	0.950 (15)
C2—C3	1.4001 (18)	C7—H7	1.005 (17)
C2—C7	1.4507 (18)

C1—O9—H9	104.4 (15)	H10A—C10—H10E	56.3
C5—C4—C10	120.93 (13)	H10A—C10—H10F	56.3
C3—C4—C5	117.16 (13)	H10B—C10—H10C	109.5
C3—C4—C10	121.91 (13)	H10B—C10—H10D	56.3
C5—C6—C1	119.91 (13)	H10B—C10—H10E	141.1
C5—C6—H6	121.7 (9)	H10B—C10—H10F	56.3
C1—C6—H6	118.4 (9)	H10C—C10—H10D	56.3
C3—C2—C7	120.15 (12)	H10C—C10—H10E	56.3
C3—C2—C1	119.40 (12)	H10C—C10—H10F	141.1
C1—C2—C7	120.41 (12)	H10D—C10—H10E	109.5
C4—C5—H5	118.7 (10)	H10D—C10—H10F	109.5
C6—C5—C4	122.41 (13)	H10E—C10—H10F	109.5
C6—C5—H5	118.9 (10)	C4—C3—C2	121.97 (12)
C4—C10—H10A	109.5	C4—C3—H3	120.7 (9)
C4—C10—H10B	109.5	C2—C3—H3	117.3 (9)
C4—C10—H10C	109.5	O8—C7—C2	124.41 (14)
C4—C10—H10D	109.5	O8—C7—H7	121.1 (9)
C4—C10—H10E	109.5	C2—C7—H7	114.5 (9)
C4—C10—H10F	109.5	O9—C1—C6	119.05 (12)
H10A—C10—H10B	109.5	O9—C1—C2	121.79 (12)
H10A—C10—H10C	109.5	C6—C1—C2	119.16 (12)
H10A—C10—H10D	141.1

C5—C4—C3—C2	0.11 (18)	C3—C2—C1—C6	−0.87 (18)
C5—C6—C1—O9	−179.07 (12)	C7—C2—C3—C4	−177.15 (12)
C5—C6—C1—C2	0.91 (19)	C7—C2—C1—O9	−3.38 (18)
C10—C4—C5—C6	−179.24 (13)	C7—C2—C1—C6	176.64 (11)
C10—C4—C3—C2	179.27 (12)	C1—C6—C5—C4	−0.4 (2)
C3—C4—C5—C6	−0.08 (19)	C1—C2—C3—C4	0.36 (18)
C3—C2—C7—O8	−179.83 (13)	C1—C2—C7—O8	2.7 (2)
C3—C2—C1—O9	179.11 (11)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C10—H10E···O8ⁱ	0.98	2.60	3.499 (2)	152
O9—H9···O8	0.94 (3)	1.77 (3)	2.6260 (17)	151 (2)
C5—H5···O8ⁱⁱ	0.974 (18)	2.607 (18)	3.4801 (18)	149.3 (13)
C6—H6···O9ⁱⁱⁱ	0.989 (16)	2.599 (17)	3.4053 (18)	138.7 (12)

Symmetry codes: (i) x+1, −y+1/2, z+1/2; (ii) x+1, y, z; (iii) −x+1, −y+1, −z−1.

Acknowledgements

The research work disclosed in this publication is partially funded by the Endeavour Scholarship Scheme (Malta). Scholarships are part-financed by the European Union – European Social Fund (ESF) – Operational Programme II – Cohesion Policy 2014–2020 `Investing in human capital to create more opportunities and promote the well-being of society'.

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