Halogen-bonded adduct of 1,2-di­bromo-1,1,2,2-tetra­fluoro­ethane and 1,4-di­aza­bicyclo­[2.2.2]octa­ne

Halogen bonding is an inter­molecular inter­action capable of being used to direct extended structures. Typical halogen-bonding systems involve a noncovalent inter­action between a Lewis base, such as an amine, as an acceptor and a halogen atom of a halo­fluoro­carbon as a donor. Vapour-phase diffusion of 1,4-di­aza­bicyclo­[2.2.2]octane (DABCO) with 1,2-di­bromo­tetra­fluoro­ethane results in crystals of the 1:1 adduct, C₂Br₂F₄·C₆H₁₂N₂, which crystallizes as an infinite one-dimensional polymeric structure linked by inter­molecular N...Br halogen bonds [2.829 (3) Å], which are 0.57 Å shorter than the sum of the van der Waals radii.

S1. Introduction

Halogen bonding (XB) has become recognised as an inter­molecular inter­action, comparable to hydrogen bonding (Metrangolo et al., 2005), capable of being used to direct extended structures, and there are examples of its application ranging from crystal engineering (Cavallo et al., 2010) to organocatalysis (Kniep et al., 2013). Very recently, halogen bonding has also been used as a method of converting highly volatile organofluorine compounds, which are difficult to handle, into a more easily handled form by halogen-bond adduct formation (Aakeröy et al., 2015).

Prototypical halogen-bonding systems involve a noncovalent inter­action between a Lewis base, such as an amine, as a halogen-bond acceptor and a halogen, most often iodine, of a halo­fluoro­carbon, which acts as a donor (Desiraju et al., 2013). The halide of the fluoro­carbon is able to act in this way because of the distortion of the electron density of the R_f—X bond, caused by the strongly electron-withdrawing R_f group, resulting in an area of reduced electron density on the X atom opposite the C—X bond, called a σ-hole. This linear, or near-linear, arrangement is described as a type I halogen bond (Desiraju & Parthasarathy, 1989).

Because iodine is more readily polarized than bromine, the strength of the halogen-bond inter­action is greater between amines and iodine acceptors than it is in the bromo analogues, and so, typically, C—I···N halogen-bonded inter­actions are frequently stronger than those in C—Br···N systems. However, the C—Br···N halogen-bonded systems are of considerable inter­est because they offer a potential method of trapping and holding small bromo­fluoro­carbons that are volatile and ozone-depleting substances. It was thus of inter­est to investigate the halogen bonding the 1:1 adduct, (I), of 1,4-di­aza­bicyclo­[2.2.2]o­ctane (DABCO) with 1,2-di­bromo­tetra­fluoro­ethane, the structure of which is reported here.

S2. Experimental

1,4-Di­aza­bicyclo­[2.2.2]o­ctane and BrCF₂CF₂Br were obtained from commercial sources and were used without further purification.

S2.1. Synthesis and crystallization

Preparation of the title compound was by vapour diffusion in a sealed system consisting of two concentric glass vials. In the smaller inner vial was placed DABCO (0.1 g), with BrCF₂CF₂Br (0.5 ml) in the outer vial. Crystals suitable for X-ray diffraction studies were formed within 24 h at room temperature on the surface of the inner vial. IR: ν (cm^-1): 2934.9, 2871.0 (C—H), 1149.4, 1097.5 (C—F).

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. Adduct (I) crystallized in the monoclinic space group I2/a, with half a molecule per symmetric unit. H atoms were visible in difference maps and were allowed for as riding atoms, with C—H = 0.97 Å.

S3. Results and discussion

The asymmetric unit of the title adduct, (I), comprises half a molecule of both DABCO and of Br₂C₂F₄. Both the DABCO and Br₂C₂F₄ molecules possess crystallographic C₂ symmetry. The complete structure of (I) is shown in Fig. 1. The bond lengths and angles (Table 2) are largely as expected. DABCO undergoes a phase change at 351 K under atmospheric pressure (Chang & Westrum, 1960; Trowbridge & Westrum, 1963). The low-temperature phase (Sauvajol, 1980), i.e. phase II, data reports N—C and C—C bond lengths of 1.4834 and 1.5355 Å, respectively, and C—N—C angles of 107.29°, which compare with the corresponding [average?] values of 1.471 (5), 1.548 (5) Å and 108.4 (3)° in (I). Similarly, the [average?] parameters obtained for the Br₂C₂F₄ unit here of Br—C = 1.939 (4) Å, C—C = 1.516 (8) Å and C—F = 1.337 (5) Å are comparable to those obtained previously from a neutron diffraction study (Pawley & Whitley, 1988).

The extended structure displays near-linear inter­actions [N···Br—C angle = 175.6 (1)°] between the N atoms of the DABCO molecule with the Br atoms of Br₂C₂F₄, resulting in an extended one-dimensional polymeric structure based on the formation of Br···N contacts (Fig. 2). The N···Br distance of 2.829 (3) Å, is 0.57 Å (16.8%) shorter than the sum of the van der Waals radii for nitro­gen and bromine (3.40 Å). Taken together, the short N···Br distance and co-linear allignment indicate the presence of a type I halogen-bonding inter­action. This is further supported by a reduction in the C—F stretching frequencies (ν_max 1149.4 and 1097.5 cm^-1) in (I), compared with 1158.6 and 1109.8 cm^-1 observed for Br₂C₂F₄.

Inter­estingly, there are no other obvious direction-specific inter­actions in the structure of (I). There are no classical hydrogen bonds formed, indeed the shortest inter­molecular H···F distance is 2.66 Å, and although some short F···F and F···Br inter­actions are observed, these are intra­molecular rather than inter­molecular in nature.

A search of the Cambridge Structural Database (CSD, Version 5.36?; Groom & Allen, 2014) was undertaken of contacts between a tertiary N atom and an organic-bound Br atom less than the sum of their van der Waals radii. Of the 37 hits returned, the C—N···Br distances were found to lie between 2.531 and 3.379 Å, with the average being 3.138 Å. By comparison, searches carried out for the analogous iodine, rather than bromine, system results in a slightly larger number of hits (47), with C—N···I distances in the range 2.715–3.452 Å, and a shorter average distance (2.932 Å), which is consistent with a greater degree of inter­action for the more polarizable iodine centre, in agreement with the current understanding of halogen bonding.

It is noteworthy that of the crystallographically characterized C—N···Br—C adducts, only two of them have DABCO as the halogen-bond acceptor. In the structure of the adduct formed between DABCO and 1,4-di­bromo­tetra­fluoro­benzene (CSD refcode DIVDUI; Cinčić et al., 2008), a one-dimensional polymeric arrangement is also formed, with C—N···Br = 2.894 (2) and 2.910 (2) Å. The shorter of these distances is still significantly longer (20 σ) than is observed in (I), where the halogen-bond distance is 2.829 (3) Å. Whilst in the only reported Br₂C₂F₄ adducts of nitro­gen-containing compounds, namely Me₂NCH₂CH₂NMe₂ (REMBOB; Huang et al., 2006) and 1,4-di­methyl­piperizine (ULOJUA; Chu et al., 2003), the C—N···Br distances are remarkably similar at 2.864 (3) and 2.863 (5) Å, respectively, but both distances are longer than the equivalent inter­action found in (I).

In conclusion, the adduct formed between DABCO and Br₂C₂F₄ results in a method of trapping the volatile (and ozone-depleting) bromo­fluoro­carbon. The resulting crystals adopt a one-dimensional polymeric structure in which the C—N···Br halogen-bond length is shorter than the average for related type I C—N···Br—C halogen-bonded systems, and shorter than found in the two other reported crystal structures of related Br₂C₂F₄ adducts.

Computing details

Data collection: CrysAlis PRO (Agilent, 2014); cell refinement: CrysAlis PRO (Agilent, 2014); data reduction: CrysAlis PRO (Agilent, 2014); program(s) used to solve structure: SHELXS87 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Figures

	Fig. 1. (a) A molecular view of (I), showing the atomic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) -x+1/2, y, -z+1; (ii) -x-1/2, y, -z.]
	Fig. 2. A view of the intermolecular halogen bonds (dashed lines) in the crystal structure of (I).

1,2-Dibromo-1,1,2,2-tetrafluoroethane–1,4-diazabicyclo[2.2.2]octane (1/1)

Crystal data

C2Br2F4·C6H12N2	F(000) = 720
Mr = 372.02	Dx = 1.990 Mg m^−3
Monoclinic, I2/a	Mo Kα radiation, λ = 0.71073 Å
a = 10.9815 (9) Å	Cell parameters from 1070 reflections
b = 10.8697 (10) Å	θ = 3.9–28.5°
c = 11.1525 (9) Å	µ = 6.55 mm^−1
β = 111.135 (9)°	T = 150 K
V = 1241.68 (19) Å^3	Block, colourless
Z = 4	0.2 × 0.13 × 0.07 mm

Data collection

Agilent SuperNova Single Source diffractometer with an Eos detector	1282 independent reflections
Radiation source: SuperNova (Mo) X-ray Source	1028 reflections with I > 2σ(I)
Mirror monochromator	Rint = 0.032
Detector resolution: 8.0714 pixels mm^-1	θmax = 26.5°, θmin = 3.8°
ω scans	h = −8→13
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2014)	k = −5→13
Tmin = 0.159, Tmax = 1.000	l = −13→13
2416 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.037	H-atom parameters constrained
wR(F^2) = 0.069	w = 1/[σ^2(Fo^2) + (0.0193P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.03	(Δ/σ)max = 0.001
1282 reflections	Δρmax = 0.61 e Å^−3
73 parameters	Δρmin = −0.63 e Å^−3
0 restraints

Special details

Experimental. Absorption correction: CrysAlisPro, Agilent Technologies, Version 1.171.37.33 (release 27-03-2014 CrysAlis171 .NET) (compiled Mar 27 2014,17:12:48) Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

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

	x	y	z	Uiso*/Ueq
Br1	0.11014 (3)	0.89688 (4)	0.28496 (3)	0.02625 (16)
F2	0.3393 (2)	0.8020 (3)	0.4401 (2)	0.0557 (9)
F1	0.3359 (2)	1.0005 (3)	0.4381 (2)	0.0601 (9)
N1	−0.1281 (3)	0.8891 (3)	0.0723 (3)	0.0203 (8)
C4	0.2658 (3)	0.9002 (4)	0.4392 (4)	0.0265 (10)
C1	−0.1774 (3)	1.0156 (4)	0.0428 (4)	0.0293 (10)
H1A	−0.1278	1.0585	−0.0007	0.035*
H1B	−0.1658	1.0590	0.1222	0.035*
C2	−0.1466 (3)	0.8249 (4)	−0.0486 (3)	0.0266 (10)
H2A	−0.1155	0.7409	−0.0306	0.032*
H2B	−0.0963	0.8655	−0.0929	0.032*
C3	−0.2066 (3)	0.8244 (4)	0.1358 (3)	0.0285 (10)
H3A	−0.1956	0.8644	0.2168	0.034*
H3B	−0.1763	0.7402	0.1540	0.034*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.0242 (2)	0.0312 (3)	0.0179 (2)	0.00057 (19)	0.00106 (17)	0.00113 (19)
F2	0.0434 (13)	0.082 (3)	0.0311 (14)	0.0343 (16)	0.0011 (12)	−0.0136 (15)
F1	0.0488 (15)	0.080 (3)	0.0357 (14)	−0.0382 (16)	−0.0042 (12)	0.0222 (16)
N1	0.0181 (15)	0.023 (2)	0.0168 (16)	0.0004 (14)	0.0023 (13)	0.0012 (15)
C4	0.0225 (19)	0.028 (3)	0.023 (2)	−0.0032 (19)	0.0008 (17)	0.001 (2)
C1	0.0262 (19)	0.027 (3)	0.027 (2)	−0.005 (2)	−0.0003 (17)	0.001 (2)
C2	0.0222 (19)	0.031 (3)	0.025 (2)	0.0012 (19)	0.0077 (17)	−0.002 (2)
C3	0.0230 (19)	0.036 (3)	0.022 (2)	−0.0017 (19)	0.0028 (17)	0.003 (2)

Geometric parameters (Å, º)

Br1—C4	1.939 (4)	C1—H1A	0.9700
F2—C4	1.336 (4)	C1—H1B	0.9700
F1—C4	1.338 (5)	C2—H2A	0.9700
N1—C1	1.471 (5)	C2—H2B	0.9700
N1—C2	1.466 (5)	C2—C3^ii	1.553 (4)
N1—C3	1.477 (5)	C3—C2^ii	1.553 (4)
C4—C4^i	1.516 (8)	C3—H3A	0.9700
C1—C1^ii	1.535 (6)	C3—H3B	0.9700
C1—N1—C3	108.7 (3)	H1A—C1—H1B	108.1
C2—N1—C1	108.8 (3)	N1—C2—H2A	109.6
C2—N1—C3	107.6 (3)	N1—C2—H2B	109.6
F2—C4—Br1	109.2 (3)	N1—C2—C3^ii	110.2 (3)
F2—C4—F1	107.6 (3)	H2A—C2—H2B	108.1
F2—C4—C4^i	109.0 (3)	C3^ii—C2—H2A	109.6
F1—C4—Br1	109.5 (3)	C3^ii—C2—H2B	109.6
F1—C4—C4^i	109.1 (3)	N1—C3—C2^ii	110.6 (3)
C4^i—C4—Br1	112.4 (3)	N1—C3—H3A	109.5
N1—C1—C1^ii	110.79 (17)	N1—C3—H3B	109.5
N1—C1—H1A	109.5	C2^ii—C3—H3A	109.5
N1—C1—H1B	109.5	C2^ii—C3—H3B	109.5
C1^ii—C1—H1A	109.5	H3A—C3—H3B	108.1
C1^ii—C1—H1B	109.5

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