Crystal packing analysis of in situ cryocrystallized 2,2,2-tri­fluoro­aceto­phenone

Crystals of the liquid compound 2,2,2-tri­fluoro­aceto­phenone (TFAP, C₈H₅F₃O) were obtained using the state-of-art in situ cryocrystallization technique. TFAP crystallizes in the monoclinic space group C2/c, and its crystal structure is mainly stabilized by a set of C—H⋯F, C—H⋯O, F⋯F and F⋯O supra­molecular contacts. The overall mol­ecular arrangement shows the formation of mol­ecular sheets parallel to the bc plane, which are in turn stacked along the a-axis direction. The weak inter­actions have been studied thoroughly, performing both a Hirshfeld surface analysis and theoretical calculations, to obtain the inter­molecular inter­action energies. A structural comparison of this compound with the previously reported substituted analogs was also carried out, showing a qualitative difference in terms of packing behaviour.

1. Chemical context

The use of green, efficient, metal-free and inexpensive catalysts is the desire of every synthetic laboratory. The importance of metal-free catalysts is well known among synthetic chemists. In this class of catalysts, 2,2,2-tri­fluoro­aceto­phenone (TFAP) is well known, because it is cheap and commercially available.

Research work in recent years has shown that TFAP can be used as a green organocatalyst in synthetic procedures, e.g. for the epoxidation of alkenes (Limnios & Kokotos, 2014a), the oxidation of allyl­oximes to form isoxazoline (Triandafillidi & Kokotos, 2017), the oxidation of aliphatic tertiary amines and azines (Limnios & Kokotos, 2014b) and for the synthesis of substituted tetra-hydro­furans (Theodorou & Kokotos, 2017a), indolines and pyrrolidines (Theodorou & Kokotos, 2017b), besides being used for the synthesis of fluorinated polymers (Guzmán-Gutiérrez et al., 2008). Inter­estingly, TFAP has been also used for probing inter­molecular inter­actions involved in the bi-mol­ecular complexes formed on Pt(111) surfaces (Goubert et al., 2011). In fact, TFAP is also an excellent example to study the enanti­oselective hydrogenation on Pt surfaces (Cakl et al., 2011). Keeping in mind both the important applications of this mol­ecule and our work on inter­molecular inter­actions involving organic fluorine, we decided to determine the crystal structure of this compound. It is worth noting that since TFAP is a liquid at room temperature, a crystal structure determination using conventional methods is not feasible; hence, this class of compounds needs special experimental settings. The method for obtaining crystals of these compounds is called the in situ cryocrystallization technique (Boese et al., 2003; Choudhury et al., 2005). In the recent past, we have employed this technique to obtain crystal structures of both organic (Dey et al., 2016a,b) and organometallic liquids (Sirohiwal et al., 2017a). We believe that this study delineates the importance of fluorine-based inter­actions, in addition to other weak inter­actions, which play a role in the crystal packing of TFAP.

2. Computational methodology

All the calculations were performed at the crystal geometry, where hydrogen-atom positions are fixed to their respective neutron values (Allen, 1986). The lattice and inter­molecular inter­action energies were computed using the PIXELC module of the CLP program (Version 12.5.2014; Gavezzotti, 2003, 2011), which partitions the total energy into Coulombic, polarization, dispersion and repulsion energies. For the same purpose, the mol­ecular electron density was computed at the MP2/6-31G (d, p) level of theory using Gaussian09 (Frisch et al., 2009).

3. Structural commentary and supra­molecular features

The single-crystal X-ray diffraction analysis reveals that the title compound crystallizes in the space group C2/c, and confirms the presence of one –COCF₃ functional group attached to the phenyl ring (see Fig. 1). The backbone of the mol­ecule formed by the atoms O1/C1–C8 is essentially planar, with a maximum deviation from the plane of 0.053 (1) Å for C8. In the mol­ecule, two intra­molecular C—H⋯F inter­actions are present, involving C6—H6 and the atoms F1 and F3 (C6—H6⋯F1, 2.48 Å and 115°; C6—H6⋯F3, 2.55 Å and 116°; Table 1). A total of seven mol­ecular pairs are extracted from the crystal packing based on their stabilizing contribution towards the total lattice energy. Their detailed energy decomposition analysis is listed in Table 2. These mol­ecular pairs are associated through various inter­molecular inter­actions involving aromatic C—H groups as donors and C—F and C=O moieties as acceptors. The crystal packing is further stabilized by the presence of π–π stacking and of different types of atom–atom contacts, such as inter­molecular F⋯F, F⋯O, and H⋯H contacts.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C6—H6⋯F1	0.95	2.48	3.004 (2)	115
C6—H6⋯F3	0.95	2.55	3.088 (2)	116
C5—H5⋯F2^i	0.95	2.63	3.522 (2)	156
C4—H4⋯O1^i	0.95	2.74	3.490 (2)	136
C6—H6⋯F2^ii	0.95	2.69	3.614 (2)	163
C6—H6⋯F3^ii	0.95	2.94	3.584 (2)	126
C5—H5⋯F3^ii	0.95	2.98	3.603 (2)	124
C3—H3⋯O1^iii	0.95	2.95	3.882 (2)	166
Symmetry codes: (i) x, y, z-1; (ii) ; (iii) .

Table 2
Stabilization energies (in kJ mol⁻¹) of the individual mol­ecular pairs

CD = centroid–centroid distance.

Motif	Symmetry	CD (Å)	ECoul	EPol	EDisp	ERep	ETot	Possible Inter­actions	Geometry (Å, °)
I	−x + 1, y, −z +	3.731	−5.6	−1.7	−26.2	14.6	−18.8	C7⋯C6	3.6668 (1)
								C1⋯C1	3.6035 (1)
								C2⋯C2	3.5545 (1)
								C8—F3⋯F3—C8	2.8743 (1), 139, 139
II	−x + , −y + , −z + 1	5.470	−3.5	−0.9	−20.4	10.2	−14.5	π–π stacking	3.7869 (1)
								C8—F1⋯C4	3.2425 (1), 134
III	−x + , −y + , −z + 2	5.274	−5.2	−1.5	−12.6	6.7	−12.7	C7—O1⋯F2—C8	3.1436 (1), 100, 96
								C7—O1⋯F1—C8	3.0457, 139, 90
IV	x, y, z + 1	8.360	−6.4	−1.6	−6.8	4.8	−10.0	C4—H4⋯O1	2.75, 134
								C5—H5⋯F2	2.63, 154
V	x, −y, z +	8.524	−1.3	−2.3	−10.0	6.8	−6.9	H3⋯H2	2.40
								C3—H3⋯O1	2.95, 165
VI	x, −y + 1, z +	6.652	−0.7	−0.8	−8.2	3.7	−6.0	C6—H6⋯F2	2.69, 163
								C6—H6⋯F3	2.94, 124
								C8—F1⋯F2—C8	3.1023, 114, 147

Figure 1 Displacement ellipsoid plot of TFAP drawn at the 50% probability level. Weak intra­molecular inter­actions are shown as cyan dotted lines.

The strongest mol­ecular pair I (Fig. 2a), with an inter­action energy of −18.8 kJ mol⁻¹, is formed via mol­ecular stacking inter­actions and inter­molecular type I F⋯F contacts [F3⋯F3, 2.8743 (1) Å and C8—F3⋯F3 139°]. In this case, the dispersion contribution (78%) is more significant in comparison to the electrostatic contribution towards the total stabil­ization of the dimer. The centrosymmetric mol­ecular pair II (Fig. 2b), which is also formed due to π–π stacking, and to inter­molecular F1⋯C4 inter­actions, shows an inter­action energy of −14.5 kJ mol⁻¹ (18% electrostatic and 82% dispersion contribution). Motif III (involving O1 with F1 and F2), with an inter­action energy of −12.7 kJ mol⁻¹, is stabilized via inter­molecular bifurcated F⋯O inter­actions with indiv­idual distances of 3.1436 (1) and 3.0457 Å (Fig. 2c). This shows how inter­molecular F⋯O contacts provide a significant contribution towards the stabilization of the crystal packing, as already investigated in our recent study in terms of the associated nature and energetics (Sirohiwal et al., 2017b).

Figure 2 Mol­ecular pairs (a) I, (b) II, and (c) III with their stabilization energies.

The overall mol­ecular arrangement shows the formation of a mol­ecular sheet parallel to the bc plane (Fig. 3a). This sheet is constructed via the mol­ecular pairs IV (−10.0 kJ mol⁻¹), V (−6.9 KJ mol⁻¹) and VI (−6.0 kJ mol⁻¹). It is inter­esting to note the dominance of the electrostatic (54%) over the dispersion (46%) contribution in case of motif IV, which is not to be found in other motifs. A mol­ecular dimeric chain, associated with motif IV, is formed along the crystallographic c-axis direction, involving inter­molecular C4—H4⋯O1 and C5—H5⋯F2 inter­actions (Table 1). Such dimeric chains are inter­linked alternatively along the b-axis direction either via mol­ecular pairs V (involving C4—H4⋯O1 inter­actions and H⋯H contacts) or VI (involving bifurcated C—H⋯F inter­actions and F⋯F contacts) related by c-glide symmetry. Finally, these parallel mol­ecular sheets are stacked along the a-axis direction (Fig. 3b) via the strongest mol­ecular pairs I. Thus, in the absence of any strong hydrogen bonds, the overall crystal packing is stabilized through weak inter­molecular inter­actions.

Figure 3 Packing network of TFAP showing (a) the mol­ecular sheet formed via weak inter­actions in the bc plane and (b) the mol­ecular stacking of two parallel sheets. Weak inter­actions are shown as cyan dotted lines.

4. Database survey

Most of the substituted TFAPs are also liquid at room temperature and were crystallized via in situ cryocrystallization methods in the absence of OHCD. In particular, the crystal and mol­ecular structures of 4-fluoro TFAP (SIDMAU), 4-chloro TFAP (SIDLUN), 4-bromo TFAP (SIDLOH), 3-bromo TFAP (SIDLEX), and 3-nitro TFAP (SIDLIB) have been obtained and reported (Chopra et al., 2007).

Fig. 4 highlights the similarities and differences of the mol­ecular assemblies for these structures in comparison to unsubstituted TFAP. Inter­estingly, in most of the cases, the mol­ecular sheets are stacked on each other. The supra­molecular assemblies are mainly stabilized via various weak C—H⋯O/F/Cl/Br/N inter­actions and F⋯F, F⋯O, Br⋯O, Br⋯F contacts without the presence of any strong inter­actions. Upon substitution with F, Cl, Br and –NO₂ groups, a mol­ecular chain associated with F⋯F contacts is observed. In particular, in the case of the para-substituted chloro and bromo analogs, the F⋯F chain is quite similar, wherein in the case of the para-substituted fluoro compound, bifurcated F⋯F contacts are present. Finally, in the case of the m-nitro and bromo derivatives, a centrosymmetric, dimeric F⋯F chain is observed.

Figure 4 Mol­ecular assembly in (a) TFAP and substituted TFAPs: (b) 4-fluoro TFAP, (c) 4-chloro TFAP, (d) 4-bromo TFAP, (e) 3-bromo TFAP and (f) 3-nitro TFAP.

5. Hirshfeld surface analysis

The Hirshfeld surface analysis was performed using CrystalExplorer3.3 (Turner et al., 2017) to obtain two-dimensional fingerprint maps (Spackman et al., 2002; McKinnon et al., 2007), which help us to understand the crystalline environment in terms of the contributions of various inter­atomic contacts present in the crystal packing. The 2D fingerprint plots and the decomposed contributions for different atom–atom contacts in unsubstituted TFAP are shown in Fig. 5. It is observed that the contributions for H⋯F (37.4%) and H⋯H (19.0%) contacts is relatively high in comparison to the other inter­atomic contacts. Inter­estingly, in this case, the fluorine atoms present in the –CF₃ group are more involved in the formation of C—H⋯F inter­actions rather than the formation of F··F (6.9%) contacts. The other contacts, namely C⋯H (7.6%), H⋯O (8.4%) and F⋯O (4.0%) also contribute to the overall crystal packing.

Figure 5 Two-dimensional fingerprint plots for TFAP, decomposed into contributions from specific atom–atom contacts.

6. Crystallization, data collection and structure refinement

The compound TFAP was purchased from Sigma–Aldrich and used for the in situ crystallization experiment without any further purification. The detailed procedure of the crystallization process is already discussed in one of our previous reports (Dey et al., 2016a). Good quality crystals (Fig. 6a) were obtained at 200 K using a CO₂ laser scan utilizing an OHCD apparatus. Fig. 6b and c depict the crystal at 110 (2) K inside the Lindemann glass capillary and the corresponding diffraction image, respectively. The crystal data, data collection and details on structure refinement are summarized in Table 3. All non-hydrogen atoms were refined anisotropically and the aromatic hydrogen atoms bonded to C atoms were positioned geometrically and refined using a riding model with U_iso(H) =1.2U_eq(C) and C—H distances of 0.95 Å.

Table 3
Experimental details

Crystal data
Chemical formula	C8H5F3O
Mr	174.12
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	110
a, b, c (Å)	13.8129 (3), 12.6034 (2), 8.3595 (2)
β (°)	90.396 (1)
V (Å^3)	1455.27 (5)
Z	8
Radiation type	Mo Kα
μ (mm^−1)	0.16
Crystal size (mm)	0.30 × 0.30 × 0.30
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2012)
Tmin, Tmax	0.697, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	9958, 1045, 944
Rint	0.014
(sin θ/λ)max (Å^−1)	0.631
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.024, 0.064, 1.08
No. of reflections	1045
No. of parameters	109
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å^−3)	0.19, −0.20
Computer programs: APEX2 and SAINT (Bruker, 2012), SIR92 (Altomare et al., 1994), SHELXL2016/6 (Sheldrick, 2015), Mercury (Macrae et al., 2008), CIFTAB (Sheldrick, 2008) and PLATON (Spek, 2009).

Figure 6 Crystal images at (a) 200 K, (b) 110 K, and (c) the diffraction image at 110 K.

Supporting information

Computing details

Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL2016/6 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: CIFTAB (Sheldrick, 2008) and PLATON (Spek, 2009).

2,2,2-Trifluoroacetophenone

Crystal data

C8H5F3O	F(000) = 704
Mr = 174.12	Dx = 1.589 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 13.8129 (3) Å	Cell parameters from 5549 reflections
b = 12.6034 (2) Å	θ = 2.2–30.2°
c = 8.3595 (2) Å	µ = 0.16 mm^−1
β = 90.396 (1)°	T = 110 K
V = 1455.27 (5) Å^3	Block, colorless
Z = 8	0.30 × 0.30 × 0.30 mm

Data collection

Bruker APEXII CCD diffractometer	944 reflections with I > 2σ(I)
Radiation source: fine focus sealed tube	Rint = 0.014
ω scans	θmax = 26.7°, θmin = 2.2°
Absorption correction: multi-scan (SADABS; Bruker, 2008)	h = −17→17
Tmin = 0.697, Tmax = 0.746	k = −15→15
9958 measured reflections	l = −5→5
1045 independent reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.024	H-atom parameters constrained
wR(F^2) = 0.064	w = 1/[σ^2(Fo^2) + (0.0305P)^2 + 0.8314P] where P = (Fo^2 + 2Fc^2)/3
S = 1.08	(Δ/σ)max < 0.001
1045 reflections	Δρmax = 0.19 e Å^−3
109 parameters	Δρmin = −0.20 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
F1	0.27357 (5)	0.43371 (5)	0.83259 (12)	0.0310 (3)
F2	0.33321 (6)	0.40770 (6)	1.06711 (16)	0.0376 (4)
F3	0.42716 (5)	0.44771 (5)	0.87193 (13)	0.0331 (3)
O1	0.36355 (6)	0.21263 (7)	0.98806 (15)	0.0268 (3)
C4	0.39488 (8)	0.16370 (10)	0.3960 (2)	0.0278 (5)
H4	0.402903	0.138083	0.290060	0.033*
C5	0.38764 (9)	0.27193 (10)	0.4248 (2)	0.0273 (5)
H5	0.390579	0.320406	0.337898	0.033*
C6	0.37625 (8)	0.30955 (9)	0.5781 (2)	0.0223 (5)
H6	0.371252	0.383735	0.596420	0.027*
C1	0.37202 (7)	0.23921 (9)	0.7068 (2)	0.0176 (5)
C7	0.36188 (7)	0.27209 (9)	0.8742 (2)	0.0200 (5)
C8	0.34856 (9)	0.39204 (9)	0.9128 (3)	0.0241 (5)
C3	0.39028 (9)	0.09306 (9)	0.5237 (2)	0.0259 (5)
H3	0.395028	0.018927	0.504787	0.031*
C2	0.37893 (8)	0.12990 (9)	0.6766 (2)	0.0235 (5)
H2	0.375707	0.081052	0.762999	0.028*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
F1	0.0323 (4)	0.0267 (4)	0.0339 (10)	0.0079 (3)	0.0017 (4)	0.0015 (3)
F2	0.0604 (5)	0.0314 (4)	0.0210 (13)	0.0015 (3)	0.0051 (5)	−0.0069 (4)
F3	0.0336 (4)	0.0239 (3)	0.0420 (9)	−0.0065 (3)	0.0030 (4)	−0.0046 (3)
O1	0.0382 (5)	0.0281 (4)	0.0142 (12)	−0.0003 (3)	0.0004 (4)	0.0047 (5)
C4	0.0263 (6)	0.0403 (7)	0.0169 (16)	−0.0026 (5)	0.0001 (6)	−0.0063 (7)
C5	0.0330 (6)	0.0347 (7)	0.0141 (19)	−0.0028 (5)	0.0007 (6)	0.0079 (7)
C6	0.0281 (6)	0.0234 (6)	0.0156 (18)	−0.0001 (4)	0.0004 (6)	0.0031 (6)
C1	0.0193 (5)	0.0217 (5)	0.0118 (17)	−0.0007 (4)	−0.0007 (5)	0.0013 (6)
C7	0.0207 (5)	0.0220 (6)	0.0171 (17)	−0.0011 (4)	−0.0002 (5)	0.0032 (7)
C8	0.0296 (6)	0.0248 (6)	0.018 (2)	−0.0003 (4)	0.0021 (6)	−0.0021 (6)
C3	0.0338 (6)	0.0253 (6)	0.0187 (17)	−0.0013 (4)	0.0001 (6)	−0.0048 (7)
C2	0.0293 (6)	0.0215 (6)	0.0197 (18)	−0.0011 (4)	−0.0012 (6)	0.0026 (6)

Geometric parameters (Å, º)

F1—C8	1.3373 (17)	C6—C1	1.396 (2)
F2—C8	1.324 (2)	C6—H6	0.9500
F3—C8	1.3389 (15)	C1—C2	1.4040 (15)
O1—C7	1.2112 (18)	C1—C7	1.467 (2)
C4—C5	1.3888 (19)	C7—C8	1.5569 (16)
C4—C3	1.392 (2)	C3—C2	1.370 (2)
C4—H4	0.9500	C3—H3	0.9500
C5—C6	1.377 (2)	C2—H2	0.9500
C5—H5	0.9500
C5—C4—C3	119.46 (17)	C1—C7—C8	118.93 (13)
C5—C4—H4	120.3	F2—C8—F1	107.54 (12)
C3—C4—H4	120.3	F2—C8—F3	107.82 (12)
C6—C5—C4	120.53 (15)	F1—C8—F3	107.05 (12)
C6—C5—H5	119.7	F2—C8—C7	111.48 (12)
C4—C5—H5	119.7	F1—C8—C7	111.71 (12)
C5—C6—C1	120.32 (13)	F3—C8—C7	111.03 (11)
C5—C6—H6	119.8	C2—C3—C4	120.33 (13)
C1—C6—H6	119.8	C2—C3—H3	119.8
C6—C1—C2	118.78 (16)	C4—C3—H3	119.8
C6—C1—C7	124.10 (12)	C3—C2—C1	120.58 (14)
C2—C1—C7	117.11 (13)	C3—C2—H2	119.7
O1—C7—C1	124.96 (12)	C1—C2—H2	119.7
O1—C7—C8	116.10 (16)
C3—C4—C5—C6	0.17 (18)	C1—C7—C8—F2	175.26 (9)
C4—C5—C6—C1	0.12 (17)	O1—C7—C8—F1	−125.49 (14)
C5—C6—C1—C2	−0.42 (16)	C1—C7—C8—F1	54.91 (16)
C5—C6—C1—C7	178.79 (10)	O1—C7—C8—F3	115.10 (15)
C6—C1—C7—O1	−175.62 (11)	C1—C7—C8—F3	−64.50 (17)
C2—C1—C7—O1	3.60 (16)	C5—C4—C3—C2	−0.15 (18)
C6—C1—C7—C8	3.94 (15)	C4—C3—C2—C1	−0.15 (17)
C2—C1—C7—C8	−176.84 (10)	C6—C1—C2—C3	0.43 (16)
O1—C7—C8—F2	−5.15 (14)	C7—C1—C2—C3	−178.83 (10)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C6—H6···F1	0.95	2.48	3.004 (2)	115
C6—H6···F3	0.95	2.55	3.088 (2)	116
C5—H5···F2^i	0.95	2.63	3.522 (2)	156
C4—H4···O1^i	0.95	2.74	3.490 (2)	136
C6—H6···F2^ii	0.95	2.69	3.614 (2)	163
C6—H6···F3^ii	0.95	2.94	3.584 (2)	126
C5—H5···F3^ii	0.95	2.98	3.603 (2)	124
C3—H3···O1^iii	0.95	2.95	3.882 (2)	166

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

 

Footnotes

‡Currently at Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany.