An ice-like water hexa­mer with symmetry in the hydrogen-bonded structure of 2,3,5,6-tetra­fluoro-1,4-bis­(imidazol-1-ylmeth­yl)benzene dihydrate

An ice-like hexa­meric water cluster, stabilized by the flexible bis-imidazolyl compound 2,3,5,6-tetra­fluoro-1,4-bis­(imidazol-1-ylmeth­yl)benzene (Fbix), is found in the trigonal R crystal structure of the title compound, C₁₄H₁₀F₄N₄·2H₂O or Fbix·2H₂O. The Fbix mol­ecule lies about an inversion centre with one water mol­ecule in the asymmetric unit in a general position. A cyclic chair-like hexa­meric water cluster with symmetry is generated with a hydrogen-bonded O...O distance within the hexa­mer of 2.786 (3) Å. The Fbix mol­ecule adopts a trans conformation, where the imidazole ring makes a dihedral angle of 70.24 (11)° with the central tetra­fluorinated aromatic ring. Each water hexa­mer is connected by six Fbix mol­ecules through inter­molecular O—H...N hydrogen bonds [N...O = 2.868 (3) Å] to yield a three-dimensional supra­molecular network with primitive cubic (pcu) topology. Large voids in each single pcu network lead to fourfold inter­penetrated aggregates of Fbix·2H₂O.

S1. Comment

In recent years there have been many systematic and successful studies of the crystal engineering of organic crystals, and evidence has been published demonstrating the molecule recognitions of hydrogen-bonding and/or other noncolvalent interactions (Desiraju, 1995; Hosseini, 2005). In the field of water chemistry, intense efforts have focused on unravelling the structural morphologies of water aggregates to improve understanding of the nature of water–water interactions in bulk water or ice (Ludwig, 2001). Small water clusters, (H₂O)_n where n = 3–8, have been extensively studied and structurally characterized; numerous water clusters including tetramers (Zhang, Tian et al., 2007; Xu et al., 2008), pentamers (Ma et al., 2004), hexamers (Ye et al., 2004; Mukhopadhyay & Bernal, 2005; Siddiqui et al., 2008) and octamers (Doedens et al., 2002) reveal various conformations in the crystal hosts in the solid state. Among these, the water hexamer is of great interest since it represents the smallest possible unit that can exhibit some of the properties found in bulk water (Gregory et al., 1997). Moreover, the water hexamer behaves as the transition from two-dimensional to three-dimensional structures by adopting ring (chair and boat), book, bag, cage, and prism with nearly equal energy by the energetic discrimination of 0.7 kcal.mol^-1 (Ludwig, 2000, 2001). This realization has prompted considerable attention on the structural characterization of water hexamers trapped in hydrate clathrates (Moorthy et al., 2002; Mukhopadhyay & Bernal, 2005).

Imidazole-containing ligands such as 1,4-bis(imidazol-1-yl-methyl)benzene (bix) (Hoskins et al., 1997a,b; Fan et al., 2005), 1,1-(1,4-butanediyl)bis(imidazole) (bbix) (Ma et al., 2000; Duncan et al., 1996), 1,3,5-tris(imidazole-1-yl-methyl)benzene (tib) (Liu & Tong, 2002) and related species have been used to generate a rich variety of metal–organic architectures with interesting structural topologies and potential properties, as well as many hydrogen-bonding aggregates (Aakeröy et al., 2005, 2006; Zhang, Gembicky et al., 2007; Xu et al., 2008). Considering the weak intermolecular forces among fluorinated compounds, Shreeve and co-workers have reported three novel coordination polymers constructed from transition metals with 2,3,5,6-tetrafluoro-1,4-bis(imidazol-1-yl-methyl)benzene (Fbix) (Gao et al., 2006). In the process of exploring the structural diversity and recognition pattern of the Fbix building block, we isolated its dihydrate, Fbix.2H₂O, (I). Herein, we describe the formation and structural features of the cyclic water hexamer with an ice-like chair conformation and its assembly into a cubic supramolecular network through the O—H···N linkages with the Fbix crystal host.

The asymmetric unit of (I) comprises one Fbix on an inversion centre at (1/3,1/6,2/3) and one lattice water in a general position near the unit-cell origin; a perspective view with the atom-numbering scheme is shown in Fig. 1. The Fbix molecule adopts a trans conformation with a dihedral angle of 70.24 (11)° between the terminal imidazole ring and the central tetrafluorinated benzene plane. The R-3 space-group symmetry generates a hexameric cluster of water with -3 symmetry and a perfect chair form of the O atoms (Fig. 2). Within the chair-like hexameric cluster, each water molecule is simultaneously involved in interactions with symmetry-related water molecules (see Table 1) leading to a head-to-tail R₆⁶(12) hydrogen-bonding pattern (Etter, 1990). The hydrogen-bonded O···O distance within the water hexamer [2.786 (3) Å], is comparable with the value in ice I_h (2.759 Å) at 183 K (Eisenberg & Kauzmann, 1969) and shorter than the value in liquid water (2.85 Å) (Narten et al., 1982). The O—O—O—O torsion angles in the hexamer are ±63.54 (2)° and the O—O—O angle [107.95 (7)°] is close to the corresponding value of 109.3° for the preferred tetrahedral geometry in hexagonal ice. By way of contrast, a similar water hexamer crystallizing in the same R-3 space group as (I) has an almost ideal planar system (Moorthy, et al., 2002) with an O···O distance of 2.906 (5) Å, an O—O—O bond angle of 120.0 (2)° and O—O—O—O torsion angles of ±3.0 (2)°.

In the present system, the other H atom of each water molecule is connected to the Fbix moiety via an O1—H1A···N2 hydrogen bond (see Table 1). From the viewpoint of topology, each discrete cyclic water hexamer, acting as a six-connected node, is linked by the bi-connected Fbix spacer to generate a three-dimensional supramolecular network (Fig. 3). The topology is as follows: the motif of the water hexamer connected to six Fbix linkers is represented to join other six hexamers through the O—H···N interactions, which results in a six-connected net with the Schläfli symbol 4¹².6³ (that represents primitive cubic topology), the net being somewhat offset. Because of the large void in each single net with the adjoining hexameric centre-to-centre distance separated by the Fbix linker [19.858 (11) Å], the final fourfold interpenetrated structure of this type is realized through a translation vector [1/3,-1/3,-1/3] of circa 11.10 Å according to the calculation in TOPOS (Blatov, 2004), as illustrated in Fig. 4. All the independent nets are related by this single vector and the whole fourfold interpenetrated array is generated by translating the primitive single net three times, which has a PIC (primitive interpenetration cell) with the unit-cell vectors [4/3,-4/3,-4/3], [0,1,0] and [1,0,0] (Blatov, 2004).

Related literature

For background information on Imidazole-containing ligands, see: Duncan et al. (1996); Fan et al. (2005); Hoskins et al. (1997ab); Liu et al. (2002); Ma et al. (2000). Gao et al. (2006) describes the synthesis of the title compound.

For related literature, see: Sheldrick (2008).

S2. Experimental

The compound Fbix was synthesized according to the literature method described by Gao et al. (2006), through the reaction of 1,4-bis-(chloromethyl)-2,3,5,6-tetrafluorobenzene with extensive imidazole in methanol solution (yield circa 60% on the basis of the former). Colourless block single crystals of the hydrate, (I), were obtained by recrystallizing the products from aqueous solution at room temperature.

S3. Refinement

All H atoms bound to C atoms were assigned to calculated positions, with C—H = 0.93 (aromatic) and 0.97 Å (methylene) and refined using a riding model, with U_iso(H) = 1.2U_eq(C). The H atoms of water were firstly located in a difference Fourier map and then refined as riding with the restraints of O—H = 0.82 Å and H···H = 1.43 Å [U_iso(H) = 1.5U_eq(O)].

Computing details

Data collection: APEX2 (Bruker, 2007); cell refinement: APEX2 and SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 2005); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of the title compound, with the atom-numbering scheme [symmetry code: (A) -x + 2/3, -y + 1/3, -z + 4/3]. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii. Hydrogen bonds are shown as dashed lines.
	Fig. 2. Representation of the chair conformation of the water hexamer [symmetry codes: (A) x - y , x, -z; (B) -y, x - y, z; (C) -x, -y, -z; (D) -x + y, -x, z; and (E) y, -x + y, -z].
	Fig. 3. Self-assembly of the water hexamers and Fbix molecules by O—H···N interactions into an extended three-dimensional supramolecular arrangement.
	Fig. 4. A stereoview of the formation of the fourfold interpenetrated network. Each ball represents an R6^6(12) water hexamer.

(I)

Crystal data

C14H10F4N4·2H2O	Dx = 1.486 Mg m^−3
Mr = 346.29	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3	Cell parameters from 5189 reflections
Hall symbol: -R 3	θ = 3.1–32.2°
a = 17.753 (10) Å	µ = 0.13 mm^−1
c = 12.756 (7) Å	T = 297 K
V = 3482 (3) Å^3	Block, colorless
Z = 9	0.24 × 0.22 × 0.22 mm
F(000) = 1602

Data collection

Bruker APEXII CCD diffractometer	1518 independent reflections
Radiation source: fine-focus sealed tube	1327 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.018
ϕ and ω scans	θmax = 25.9°, θmin = 2.1°
Absorption correction: multi-scan (SADABS; Sheldrick, 2003)	h = −17→21
Tmin = 0.969, Tmax = 0.971	k = −18→21
8266 measured reflections	l = −15→6

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.042	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.134	H-atom parameters constrained
S = 1.05	w = 1/[σ^2(Fo^2) + (0.0865P)^2 + 1.7362P] where P = (Fo^2 + 2Fc^2)/3
1518 reflections	(Δ/σ)max < 0.001
109 parameters	Δρmax = 0.40 e Å^−3
0 restraints	Δρmin = −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.

Refinement. Refinement of F^2^ against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F^2^, conventional R-factors R are based on F, with F set to zero for negative F^2^. The threshold expression of F^2^ > σ(F^2^) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F^2^ are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
O1	0.16893 (10)	0.07977 (9)	0.03938 (10)	0.0743 (4)
H1A	0.1886	0.0955	0.0986	0.111*
H1B	0.1396	0.0978	0.0109	0.111*
C1	0.18731 (11)	0.16251 (11)	0.42122 (15)	0.0578 (4)
H1	0.1638	0.1663	0.4848	0.069*
C2	0.14391 (14)	0.12350 (12)	0.33282 (18)	0.0715 (6)
H2	0.0837	0.0952	0.3256	0.086*
C3	0.27711 (14)	0.17490 (13)	0.29756 (13)	0.0639 (5)
H3	0.3287	0.1902	0.2624	0.077*
C4	0.34753 (10)	0.24816 (10)	0.46431 (11)	0.0502 (4)
H4A	0.3522	0.3044	0.4751	0.060*
H4B	0.4001	0.2576	0.4294	0.060*
C5	0.34011 (9)	0.20595 (9)	0.56866 (10)	0.0434 (4)
C6	0.37128 (9)	0.14992 (10)	0.58466 (11)	0.0454 (4)
C7	0.30183 (10)	0.22127 (10)	0.65303 (11)	0.0458 (4)
F1	0.40904 (7)	0.13179 (7)	0.50588 (8)	0.0648 (3)
F2	0.27087 (7)	0.27585 (7)	0.64270 (8)	0.0638 (3)
N1	0.27302 (8)	0.19521 (8)	0.39768 (10)	0.0480 (4)
N2	0.20007 (14)	0.13113 (12)	0.25527 (14)	0.0776 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0931 (10)	0.0857 (10)	0.0558 (8)	0.0535 (8)	−0.0088 (6)	−0.0084 (6)
C1	0.0526 (9)	0.0532 (9)	0.0623 (10)	0.0224 (7)	−0.0069 (7)	0.0043 (7)
C2	0.0675 (12)	0.0525 (9)	0.0875 (14)	0.0247 (9)	−0.0329 (10)	−0.0023 (9)
C3	0.0868 (13)	0.0758 (11)	0.0440 (9)	0.0518 (11)	−0.0165 (8)	−0.0104 (7)
C4	0.0486 (8)	0.0545 (9)	0.0397 (8)	0.0201 (7)	−0.0055 (6)	0.0028 (6)
C5	0.0389 (7)	0.0493 (8)	0.0361 (7)	0.0177 (6)	−0.0060 (5)	−0.0017 (5)
C6	0.0426 (7)	0.0555 (8)	0.0360 (7)	0.0230 (6)	−0.0025 (5)	−0.0060 (6)
C7	0.0444 (8)	0.0494 (8)	0.0452 (8)	0.0247 (6)	−0.0065 (6)	−0.0034 (6)
F1	0.0764 (7)	0.0877 (7)	0.0445 (6)	0.0517 (6)	0.0086 (4)	−0.0024 (4)
F2	0.0775 (7)	0.0752 (7)	0.0594 (6)	0.0536 (6)	−0.0013 (5)	0.0022 (5)
N1	0.0547 (8)	0.0517 (7)	0.0398 (7)	0.0282 (6)	−0.0101 (5)	−0.0017 (5)
N2	0.1045 (14)	0.0774 (11)	0.0632 (10)	0.0547 (10)	−0.0401 (10)	−0.0225 (8)

Geometric parameters (Å, º)

O1—H1A	0.8202	C4—N1	1.4533 (19)
O1—H1B	0.8203	C4—C5	1.501 (2)
C1—C2	1.346 (3)	C4—H4A	0.9700
C1—N1	1.364 (2)	C4—H4B	0.9700
C1—H1	0.9300	C5—C7	1.371 (2)
C2—N2	1.362 (3)	C5—C6	1.374 (2)
C2—H2	0.9300	C6—F1	1.3336 (17)
C3—N2	1.305 (3)	C6—C7^i	1.370 (2)
C3—N1	1.339 (2)	C7—F2	1.3383 (18)
C3—H3	0.9300	C7—C6^i	1.370 (2)
H1A—O1—H1B	121.3	H4A—C4—H4B	107.9
C2—C1—N1	105.33 (18)	C7—C5—C6	116.49 (13)
C2—C1—H1	127.3	C7—C5—C4	121.72 (14)
N1—C1—H1	127.3	C6—C5—C4	121.79 (13)
C1—C2—N2	110.79 (18)	F1—C6—C7^i	118.78 (14)
C1—C2—H2	124.6	F1—C6—C5	119.72 (13)
N2—C2—H2	124.6	C7^i—C6—C5	121.50 (14)
N2—C3—N1	111.90 (19)	F2—C7—C6^i	118.63 (14)
N2—C3—H3	124.1	F2—C7—C5	119.36 (13)
N1—C3—H3	124.1	C6^i—C7—C5	122.01 (14)
N1—C4—C5	111.81 (12)	C3—N1—C1	107.14 (14)
N1—C4—H4A	109.3	C3—N1—C4	125.28 (15)
C5—C4—H4A	109.3	C1—N1—C4	127.42 (14)
N1—C4—H4B	109.3	C3—N2—C2	104.84 (16)
C5—C4—H4B	109.3
N1—C1—C2—N2	−0.1 (2)	C4—C5—C7—C6^i	−179.55 (13)
N1—C4—C5—C7	−90.79 (17)	N2—C3—N1—C1	−0.3 (2)
N1—C4—C5—C6	89.18 (17)	N2—C3—N1—C4	−175.97 (15)
C7—C5—C6—F1	179.76 (13)	C2—C1—N1—C3	0.22 (19)
C4—C5—C6—F1	−0.2 (2)	C2—C1—N1—C4	175.78 (14)
C7—C5—C6—C7^i	−0.5 (2)	C5—C4—N1—C3	−131.29 (16)
C4—C5—C6—C7^i	179.55 (13)	C5—C4—N1—C1	53.9 (2)
C6—C5—C7—F2	179.42 (13)	N1—C3—N2—C2	0.2 (2)
C4—C5—C7—F2	−0.6 (2)	C1—C2—N2—C3	−0.1 (2)
C6—C5—C7—C6^i	0.5 (2)

Symmetry code: (i) −x+2/3, −y+1/3, −z+4/3.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···N2	0.82	2.08	2.867 (3)	162
O1—H1B···O1^ii	0.82	1.98	2.786 (3)	166
C4—H4B···O1^iii	0.97	2.47	3.404 (3)	161

Symmetry codes: (ii) x−y, x, −z; (iii) −x+2/3, −y+1/3, −z+1/3.