Crystal structure and C-H⋯F hydrogen bonding in the fluorinated bis-benzoxazine: 3,3'-(ethane-1,2-di-yl)bis-(6-fluoro-3,4-di-hydro-2H-1,3-benzoxazine).

The title fluorinated bis-benzoxazine, C18H18F2N2O2, crystallizes with one half-mol-ecule in the asymmetric unit, which is completed by inversion symmetry. The fused oxazine ring adopts an approximately half-chair conformation. The two benzoxazine rings are oriented anti to one another around the central C-C bond. The dominant inter-molecular inter-action in the crystal structure is a C-H⋯F hydrogen bond between the F atoms and the axial H atoms of the OCH2N methyl-ene group in the oxazine rings of neighbouring mol-ecules. C-H⋯π contacts further stabilize the crystal packing.

Chemical context

Even though benzoxazines have been known for more than 60 years, a cursory look at the literature cited in relation to the polybenzoxazines in recent years reveals increasing inter­est in polybenzoxazine chemistry (Demir et al., 2013 ▸). Mannich condensation of a phenol and a primary amine with formaldehyde is perhaps the best synthetic route widely employed for the preparation of a variety of benzoxazine monomers. Mono-functional benzoxazines with one oxazine ring yield linear polymers, while bi- and polyfunctional benzoxazines produce cross-linked polymers. As a result, many kinds of benzoxazine monomers, including both mono-benzoxazines and bis-benzoxazines, have been synthesized. For composite applications, bifunctional benzoxazines are important as they produce fillers with good adhesion properties that in turn give high modulus composite materials (Santhosh-Kumar & Reghunadhan-Nair, 2014 ▸).

Much work in our group has been directed at the synthesis of a wide variety of bis-benzoxazines from ethyl­endi­amine, formaldehyde and phenols in the molar ratio of 1:4:2 using a conventional method and solvent-free conditions (Rivera et al., 1989 ▸). Recently, we have also investigated the crystal structures of several bis-benzoxazines namely 3,3′-(ethane-1,2-di­yl)bis­(6-substituted-3,4-di­hydro-2H-1,3-benzoxazine) derivatives (Rivera et al., 2010 ▸, 2011 ▸, 2012a ▸,b ▸). These were prepared to determine whether replacement of the substit­uents at the para position of the phenol affects the mol­ecular conformation and possible supra­molecular aggregation. In this context, the title compound is a model for studying non-conventional mol­ecular inter­actions where the halogen atom may act as a hydrogen-bond acceptor. Although debate has surrounded the role of fluorine as a hydrogen-bond (HB) acceptor, the presence of such weak mol­ecular inter­actions in the solid state has been the subject of both theoretical and spectroscopic studies (Dalvit & Vulpetti, 2016 ▸). However, to the best of our knowledge, there are few examples of X-ray studies. On the other hand, polymers containing fluorinated aromatic systems often exhibit exceptional thermal stability and show good water-repellent properties (Su & Chang, 2003 ▸). Therefore we report herein the crystal structure of 3,3′-(ethane-1,2-di­yl)bis­(6-fluoro-3,4-di­hydro-2H-1,3-benzoxazine) (I), which is a very good candidate as a monomer for the investigation of the polymerization of fluorine-containing bis-benzoxazine monomers.

Structural commentary

The mol­ecular structure of the title compound is illustrated in Fig. 1 ▸. The asymmetric unit contains one-half of the formula unit; a centre of inversion located at the mid-point of the central C1—C1i bond generates the other half of the bis-benzoxazine compound [symmetry code: (i) 1 − x, 1 − y, 1 − z]. Bond lengths in the benzoxazine moiety in (I) are within normal ranges and are comparable to those found in related structures (Rivera et al., 2012a ▸,b ▸, 2011 ▸; Chen & Wu, 2007 ▸).

Figure 1

The mol­ecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Atoms labelled with the suffix A are generated using the symmetry operator (−x + 1, −y + 1, −z + 1).

The fused six-membered heterocyclic rings exist in an approximately half-chair conformation, characterized by a puckering amplitude Q = 0.4913 (15) Å, and θ = 52.03 (17)° and φ = 98.3 (2)°, with C2 and N1 displaced from the mean plane by −0.299 (2) and 0.331 (1) Å, respectively. The C1—C1A bond is in an axial position with a C5—N1—C1—C1A torsion angle of 75.45 (18)°. The two benzoxazine rings are oriented anti to one another about the central C1—C1A bond, with an N1—C1—C1A—N1A torsion angle of 180.0 (2)°.

Supra­molecular features

The packing of title compound is dominated by C2—H2A⋯F1 hydrogen bonds (Table 1 ▸), that connect the mol­ecules into a sheet structure, Fig. 2 ▸. Symmetry dictates that both F atoms are involved in these hydrogen bonds. The crystal structure also features two weak C—H⋯π inter­actions (Table 1 ▸), as indicated in PLATON (Spek, 2009 ▸), with C—H⋯Cg distances of 3.527 (2) and 3.577 (2) Å and with C—H⋯Cg angles of 126 and 129°, respectively.

Table 1

Hydrogen-bond geometry (Å, °)

Cg2 is the centroid of the C3/C4/C6/C7/C8/C9 ring

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C2—H2A⋯F1i	0.99	2.44	3.300 (2)	145
C6—H6⋯Cg2ii	0.95	2.88	3.527 (2)	126
C9—H9⋯Cg2iii	0.95	2.90	3.577 (2)	129

Symmetry codes: (i) ; (ii) ; (iii) .

Figure 2

Packing diagram for title compound, viewed along the b axis. C—H⋯F and C—F⋯π contacts are drawn as dashed lines.

Database survey

A database search yielded four comparable structures, 3,3′-(ethane-1,2-diyl)bis(6-methyl-3,4-di­hydro-2H-1,3-benzoxazine) (AXAKAM; Rivera et al., 2011 ▸), 3,3′-ethyl­enebis(3,4-di­hydro-6-chloro-2H-1,3-benzoxazine) (NUQKAM; Rivera et al., 2010 ▸), 3,3′-(ethane-1,2-di­yl)-bis­(6-meth­oxy-3,4-di­hydro-2H-1,3-benzoxazine) monohydrate (QEDDOU; Rivera et al., 2012b ▸), and 3,3′-(ethane-1,2-diyl)bis(3,4-di­hydro-2H-1,3-benz­oxazine) (SAGPUN; Rivera et al., 2012a ▸).

Synthesis and crystallization

The title compound was synthesized according to the literature procedure (Rivera et al.,1989 ▸), and single crystals were obtained by slow evaporation from an ethyl acetate/benzene 1:3 solvent mixture at room temperature.

Refinement details

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. All H atoms were located in difference electron-density maps. C-bound H atoms were fixed geometrically (C—H = 0.95 or 0.99 Å) and refined using a riding-model approximation, with U iso(H) set to 1.2U eq of the parent atom.

Table 2

Experimental details

Crystal data
Chemical formula	C18H18F2N2O2
M r	332.34
Crystal system, space group	Monoclinic, P21/c
Temperature (K)	173
a, b, c (Å)	7.0242 (6), 6.2316 (6), 17.1574 (15)
β (°)	91.473 (7)
V (Å3)	750.77 (12)
Z	2
Radiation type	Mo Kα
μ (mm−1)	0.11
Crystal size (mm)	0.17 × 0.13 × 0.04
Data collection
Diffractometer	Stoe IPDS II two-circle
Absorption correction	Multi-scan (X-AREA; Stoe & Cie, 2001 ▸)
T min, T max	0.362, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	7746, 1532, 1285
R int	0.036
(sin θ/λ)max (Å−1)	0.625
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.041, 0.101, 1.08
No. of reflections	1532
No. of parameters	109
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.18, −0.16

Computer programs: X-AREA (Stoe & Cie, 2001 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸) and XP in SHELXTL-Plus (Sheldrick, 2008 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989016015243/sj5506sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989016015243/sj5506Isup2.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989016015243/sj5506Isup3.cml

CCDC reference: 1507056

Additional supporting information: crystallographic information; 3D view; checkCIF report

C18H18F2N2O2	F(000) = 348
Mr = 332.34	Dx = 1.470 Mg m−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 7.0242 (6) Å	Cell parameters from 7746 reflections
b = 6.2316 (6) Å	θ = 3.5–27.8°
c = 17.1574 (15) Å	µ = 0.11 mm−1
β = 91.473 (7)°	T = 173 K
V = 750.77 (12) Å3	Plate, colourless
Z = 2	0.17 × 0.13 × 0.04 mm

Stoe IPDS II two-circle diffractometer	1285 reflections with I > 2σ(I)
Radiation source: Genix 3D IµS microfocus X-ray source	Rint = 0.036
ω scans	θmax = 26.4°, θmin = 3.5°
Absorption correction: multi-scan (X-AREA; Stoe & Cie, 2001)	h = −8→8
Tmin = 0.362, Tmax = 1.000	k = −7→7
7746 measured reflections	l = −20→21
1532 independent reflections

Refinement on F2	0 restraints
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.041	H-atom parameters constrained
wR(F2) = 0.101	w = 1/[σ2(Fo2) + (0.0538P)2 + 0.1464P] where P = (Fo2 + 2Fc2)/3
S = 1.08	(Δ/σ)max < 0.001
1532 reflections	Δρmax = 0.18 e Å−3
109 parameters	Δρmin = −0.16 e Å−3

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.

	x	y	z	Uiso*/Ueq
F1	0.80087 (14)	0.60837 (18)	0.86796 (5)	0.0408 (3)
O1	0.69940 (16)	0.06550 (17)	0.61834 (6)	0.0293 (3)
N1	0.73058 (17)	0.3605 (2)	0.52754 (7)	0.0256 (3)
C1	0.5232 (2)	0.3961 (2)	0.52148 (8)	0.0260 (3)
H1A	0.4706	0.4020	0.5744	0.031*
H1B	0.4624	0.2743	0.4934	0.031*
C2	0.7756 (2)	0.1410 (3)	0.54504 (9)	0.0294 (4)
H2A	0.9158	0.1242	0.5471	0.035*
H2B	0.7249	0.0493	0.5022	0.035*
C3	0.72790 (19)	0.2069 (2)	0.67944 (8)	0.0237 (3)
C4	0.79289 (19)	0.4168 (2)	0.66810 (8)	0.0228 (3)
C5	0.8283 (2)	0.4965 (3)	0.58624 (8)	0.0267 (3)
H5A	0.7820	0.6459	0.5809	0.032*
H5B	0.9668	0.4961	0.5770	0.032*
C6	0.81932 (19)	0.5507 (3)	0.73252 (8)	0.0250 (3)
H6	0.8647	0.6931	0.7263	0.030*
C7	0.7786 (2)	0.4732 (3)	0.80519 (8)	0.0273 (3)
C8	0.7114 (2)	0.2675 (3)	0.81752 (9)	0.0293 (4)
H8	0.6837	0.2193	0.8685	0.035*
C9	0.6854 (2)	0.1337 (3)	0.75374 (9)	0.0269 (3)
H9	0.6386	−0.0079	0.7606	0.032*

	U11	U22	U33	U12	U13	U23
F1	0.0442 (6)	0.0526 (7)	0.0255 (5)	−0.0053 (5)	0.0007 (4)	−0.0122 (4)
O1	0.0393 (6)	0.0227 (6)	0.0258 (6)	−0.0001 (4)	0.0008 (4)	−0.0013 (4)
N1	0.0251 (6)	0.0299 (7)	0.0217 (6)	0.0008 (5)	0.0008 (4)	0.0007 (5)
C1	0.0248 (7)	0.0289 (8)	0.0243 (7)	−0.0015 (6)	0.0006 (5)	0.0018 (6)
C2	0.0338 (8)	0.0310 (8)	0.0233 (7)	0.0046 (6)	0.0025 (6)	−0.0032 (6)
C3	0.0227 (7)	0.0253 (8)	0.0231 (7)	0.0034 (6)	−0.0003 (5)	0.0000 (6)
C4	0.0202 (6)	0.0264 (8)	0.0220 (7)	0.0014 (5)	0.0008 (5)	0.0018 (6)
C5	0.0273 (7)	0.0296 (8)	0.0232 (7)	−0.0033 (6)	0.0000 (5)	0.0032 (6)
C6	0.0210 (6)	0.0269 (8)	0.0271 (7)	−0.0004 (5)	0.0000 (5)	0.0003 (6)
C7	0.0245 (7)	0.0351 (9)	0.0223 (7)	0.0020 (6)	−0.0019 (5)	−0.0057 (6)
C8	0.0262 (7)	0.0399 (9)	0.0218 (7)	0.0026 (6)	0.0036 (6)	0.0059 (7)
C9	0.0252 (7)	0.0269 (8)	0.0287 (8)	0.0014 (6)	0.0024 (6)	0.0060 (6)

F1—C7	1.3731 (17)	C3—C4	1.401 (2)
O1—C3	1.3803 (18)	C4—C6	1.393 (2)
O1—C2	1.4574 (18)	C4—C5	1.5161 (19)
N1—C2	1.434 (2)	C5—H5A	0.9900
N1—C5	1.4721 (19)	C5—H5B	0.9900
N1—C1	1.4746 (18)	C6—C7	1.374 (2)
C1—C1i	1.522 (3)	C6—H6	0.9500
C1—H1A	0.9900	C7—C8	1.384 (2)
C1—H1B	0.9900	C8—C9	1.384 (2)
C2—H2A	0.9900	C8—H8	0.9500
C2—H2B	0.9900	C9—H9	0.9500
C3—C9	1.393 (2)
C3—O1—C2	113.57 (12)	C6—C4—C5	121.16 (14)
C2—N1—C5	108.03 (12)	C3—C4—C5	119.75 (13)
C2—N1—C1	111.72 (12)	N1—C5—C4	111.15 (12)
C5—N1—C1	113.84 (12)	N1—C5—H5A	109.4
N1—C1—C1i	111.15 (14)	C4—C5—H5A	109.4
N1—C1—H1A	109.4	N1—C5—H5B	109.4
C1i—C1—H1A	109.4	C4—C5—H5B	109.4
N1—C1—H1B	109.4	H5A—C5—H5B	108.0
C1i—C1—H1B	109.4	C7—C6—C4	118.88 (15)
H1A—C1—H1B	108.0	C7—C6—H6	120.6
N1—C2—O1	113.88 (12)	C4—C6—H6	120.6
N1—C2—H2A	108.8	F1—C7—C6	118.28 (15)
O1—C2—H2A	108.8	F1—C7—C8	118.74 (13)
N1—C2—H2B	108.8	C6—C7—C8	122.95 (14)
O1—C2—H2B	108.8	C7—C8—C9	118.40 (13)
H2A—C2—H2B	107.7	C7—C8—H8	120.8
O1—C3—C9	117.09 (14)	C9—C8—H8	120.8
O1—C3—C4	122.16 (13)	C8—C9—C3	119.93 (15)
C9—C3—C4	120.75 (14)	C8—C9—H9	120.0
C6—C4—C3	119.07 (13)	C3—C9—H9	120.0
C2—N1—C1—C1i	−161.85 (15)	C1—N1—C5—C4	75.31 (15)
C5—N1—C1—C1i	75.45 (18)	C6—C4—C5—N1	−160.16 (12)
C5—N1—C2—O1	65.92 (15)	C3—C4—C5—N1	18.20 (18)
C1—N1—C2—O1	−60.03 (16)	C3—C4—C6—C7	−0.8 (2)
C3—O1—C2—N1	−45.71 (17)	C5—C4—C6—C7	177.62 (13)
C2—O1—C3—C9	−170.60 (12)	C4—C6—C7—F1	−178.47 (12)
C2—O1—C3—C4	10.67 (19)	C4—C6—C7—C8	−0.3 (2)
O1—C3—C4—C6	−179.71 (12)	F1—C7—C8—C9	178.65 (13)
C9—C3—C4—C6	1.6 (2)	C6—C7—C8—C9	0.5 (2)
O1—C3—C4—C5	1.9 (2)	C7—C8—C9—C3	0.4 (2)
C9—C3—C4—C5	−176.79 (13)	O1—C3—C9—C8	179.82 (13)
C2—N1—C5—C4	−49.39 (15)	C4—C3—C9—C8	−1.4 (2)

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2A···F1ii	0.99	2.44	3.300 (2)	145
C6—H6···Cg2iii	0.95	2.88	3.527 (2)	126
C9—H9···Cg2iv	0.95	2.90	3.577 (2)	129