Halogen-bonded halide networks from chiral neutral spacers

Abstract

Chiral, ditopic, halogen bond donor molecules are prepared from the reaction of C₆F₅I with three different enantiopure chiral diols, namely, (R,R)-2,3-butanediol, (R,R)-hydrobenzoin and S-binaphthol, with displacement of the fluorine atom para to the iodine atom in C₆F₅I, to give (R,R)-1, (R,R)-2 and (S)-3, respectively. Chiral, halogen-bonded networks with halide anions are investigated upon co-crystallisation of (R,R)-1 with (Et₃S⁺, I⁻), (Et₄N⁺, Br⁻), (n-Bu₄N⁺, Br⁻) and (n-Pe₄N⁺, Br⁻). In the first three salts with 1 : 1 stoichiometry, the halide anions are coordinated by two iodine atoms with short I⋯X⁻ (X⁻ = I⁻, Br⁻) distances and acute (75–80°) I⋯X⁻⋯I angles, leading to the formation of chains, eventually organized into helices around twofold screw axes as in [(R,R)-1]·[Bu₄NBr]. Co-crystallisation with tetrapentylammonium bromide affords a 2 : 1 stoichiometry salt, [(R,R)-1]₂·[Pe₄NBr], with a fourfold coordination around the Br⁻ anion and the formation of a square lattice network built out of interconnected helices.

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Introduction

In the context of crystal engineering, the controlled formation of helical crystalline structures remains an attractive challenge as such structures play a crucial role in biological¹ as well as organic or inorganic systems.² Accordingly, different intermolecular interactions such as metal coordination, hydrogen bonding³ or π–π interactions⁴ have been used to generate such helical supramolecular assemblies. Besides, halogen bonding^5–7 has also been recently established as a powerful intermolecular interaction⁸ for the elaboration of complex supramolecular architectures.⁹ Halogen bonding describes an attractive interaction between a Lewis base acting as a halogen bond acceptor and a polarizable covalently bound halogen atom acting as a halogen bond donor through the so-called σ-hole, which develops on the halogen atom, in the prolongation of the C–X bond. This interaction is thus essentially of electrostatic nature albeit charge-transfer contributions start to play a role in the strongest halogen bonds.¹⁰ For example, halide anions (Cl⁻, Br⁻, I⁻) act as very efficient halogen bond acceptors in solution¹¹ as in the solid state.¹² Many examples of extended anionic networks were reported incorporating neutral iodinated spacers as halogen bond donors,¹³ (Scheme 1) with two, three or four halogen atoms, such as diiodoacetylene or tetraiodoethylene.^14,15 Tetrahedral CBr₄ is known to form diamond-type structures.¹⁶ Many other ditopic and linear molecules were used, such as α,ω-diiodoperfluoroalkanes or 1,4-diiodoperfluorobenzene.¹⁷ Honeycomb lattices were isolated with tritopic spacers such as 1,3,5-triiodotrifluorobenzene,¹⁸ or an extended tris(iodoacetylene) analog;¹⁹ thus the possibility is opened for interpenetrated halide networks.²⁰ Besides halide anions, one can also mention thiocyanate,²¹ perchlorate²² or halometallate anions.²³

Scheme 1 Examples of di-, tri-, and tetratopic neutral molecules acting as halogen bond donors toward halide anions.

By comparison with other intermolecular interactions mentioned above, the use of halogen bonding as a crystal engineering tool in the formation of chiral, eventually helical structures has been only rarely mentioned. An earlier beautiful example is provided by the resolution of a racemic 1,2-dibromohexafluoropropane through its crystallization with (−)-sparteine hydrobromide into supramolecular helices stabilised by C–Br⋯Br⁻ halogen bond interactions.²⁴ Halogen bonds were also mentioned to favour the bundling of hydrogen-bonded chiral helices made out of halogen-substituted carboxylate salts of chiral amines.^3b More recently, a chiral iodinated tetrathiafulvalene has been electro-crystallised in the presence of chloride or bromide anions to afford infinite halogen-bonded chains in the chiral C222 space group.²⁵ In other reported examples of such networks built out of neutral di- or tetratopic halogen bond donors, the chirality or helical supramolecular organisation is most often derived from the presence of helices of opposite chirality organized around 2₁ screw axes,^3,26 or from the spontaneous resolution of halogen-bonded systems into a conglomerate of both enantiomeric crystals,²⁷ not only in 3D crystals but also on 2D monolayers grown on graphite.²⁸ Note also the formation of a chiral nematic phase from halogen-bonded, bent-core achiral mesogen.²⁹ In order to favour the formation of intrinsically chiral structures without relying only on serendipity, we decided to investigate neutral iodinated spacers with a built-in chirality, taking advantage of the availability of numerous chiral molecules in their enantiopure form. From this chiral pool, we selected appropriate precursors easily transformable into ditopic halogen bond donors and explored their ability to crystallise with halide anions as halogen bond acceptors toward the formation of helical halogen-bonded anionic chains.

Results and discussion

Neutral chiral spacers

Our synthetic approach to the different chiral spacers described here is based on a reaction path described earlier for achiral diols.^30,31 The S^NAr between oxygen nucleophiles and iodopentafluorobenzene displaces the fluorine atom located para to the iodine atom, affording a variety of telechelic α,ω-di-(2,3,5,6-tetrafluoro-4-iodophenyl) derivatives. It was also demonstrated that despite the presence of a +M alkoxy substituent in the para position to iodine, these resulting derivatives were still effective as halogen bond donors toward neutral pyridine derivatives. We have accordingly expanded the scope of this reaction to three commercially available chiral enantiopure diols, namely, (R,R)-2,3-butanediol, (R,R)-hydrobenzoin and S-binaphthol, as described in Scheme 2. The reaction proceeds without solvent at high temperatures (160 °C), with Cs₂CO₃ as base, and provides the three enantiopure compounds in good to excellent yields (60–85%).

Scheme 2 Syntheses of the three enantiopure neutral halogen bond donor molecules.

Besides the usual characterization (see the Experimental section), the three neutral molecules were also investigated by single-crystal X-ray diffraction. Repeated recrystallizations of (R,R)-1 in different solvents did not afford any good-quality crystals. (R,R)-2 crystallises in the orthorhombic system, chiral space group P2₁2₁2₁, with one molecule in the general position (Fig. 1a). The central C_Ph–C–C–C_Ph part adopts a gauche conformation, while the iodine atoms are not engaged in any specific intermolecular interactions. The binaphthol derivative (S)-3 crystallises in the orthorhombic system, space group P2₁ca, also with one molecule in the general position (Fig. 1b). Note also the almost face-to-face π–π interactions between the electron-deficient iodotetrafluorophenyl and the electron-rich naphthol rings. Such π–π interactions between aryl and fluoroaryl rings are well documented as a strongly stabilizing tool in crystal engineering.^32,33 It should be stressed also here that in the two structures, the two iodine atoms are almost at opposite ends of the molecules, giving them a close-to-linear ditopic character.

Fig. 1 (a) Molecular structure of (R,R)-2. (b) Molecular structure of (S)-3.

Halogen-bonded halide salts

Halogen-bonded networks involving halide anions as XB acceptors were successfully isolated with the butanediol derivative (R,R)-1. Good-quality crystals were obtained with four different alkyl sulfonium and ammonium salts, namely, (Et₃S⁺, I⁻), (Et₄N⁺, Br⁻), (n-Bu₄N⁺, Br⁻) and (n-Pe₄N⁺, Br⁻), from the smallest triethylsulfonium to the largest tetrapentylammonium cation. They will be described successively, with specific attention to their halogen bond characteristics as well as their supramolecular organization.

[(R,R)-1]·(Et₃S⁺, I⁻) crystallises in the orthorhombic system, space group P22₁2₁, with the Et₃S⁺ cation in the general position while two iodide anions and two (R,R)-1 molecules are located on twofold axes, hence the resulting 1 : 1 stoichiometry. Considering the rotation along the central C–C bond, the two organic molecules each adopt an anti conformation, which gives them a V-shape (Fig. 2), while the pyramidal SEt₃⁺ cation is disordered on two positions. The layered structure (Fig. 3) shows an alternation along c of cationic (SEt₃⁺) and halogen-bonded [(R,R)-1]·I⁻ anionic slabs. A projection view of one of these layers (Fig. 4) shows the very acute I⋯I⁻⋯I angles (74–76°) adopted by this structure. Such acute angles are not uncommon^9–17,34 and are probably also stabilized by the existence of important dispersion forces between the iodine atoms coming into contact.

Fig. 2 Different conformations adopted by (R,R)-1 in the four different halide salts.

Fig. 3 Projection view along the a axis of the unit cell of [(R,R)-1]·(Et₃SI) showing the segregation between the disordered SEt₃⁺ cations and the complex anionic layers.

Fig. 4 Projection view along c of one anionic layer in [(R,R)-1]·(Et₃SI) showing the XB interactions between the iodine atoms I(1) and I(2) of [(R,R)-1] and the iodide anions I(3) and I(4).

Geometrical characteristics of the I⋯I⁻ halogen bonds collected in Table 1 confirm the important shortening when compared with the sum of van der Waals (I) and ionic (I⁻) radii, as well as the strong directionality of the interactions.

Table 1 Geometric characteristics of the halogen bonds in the three salts with [(R,R)-1]. The reduction parameter was calculated relative to the sum of the van der Waals radius of iodine (1.98 Å) and the ionic radius of bromide (1.82 Å) or iodide (2.06 Å) anions

Interaction	X	I⋯X^− (Å)	Reduction parameter	C–I⋯X^− (°)	I⋯X^−⋯I (°)
[(R,R)-1]·Et3SI
I(1)⋯I(4)	I	3.484(1)	0.862	178.2(2)	75.59(1)
I(2)⋯I(3)	I	3.475(1)	0.860	173.1(2)	74.43(1)
[(R,R)-1]·Et4NBr:
I(1)⋯Br(1)	Br	3.298(6)	0.868	173.2(4)	80.25(5)
I(2)⋯Br(1)	Br	3.297(6)	0.867	176.1(4)
I(3)⋯Br(2)	Br	3.266(6)	0.859	177.6(4)	79.73(5)
I(4)⋯Br(2)	Br	3.250(6)	0.855	179.4(4)
[(R,R)-1]·Bu4NBr:
I(1)⋯Br(1)	Br	3.214(1)	0.846	174.1(4)	144.37(5)
I(2)⋯Br(1)	Br	3.181(1)	0.837	173.4(3)
I(3)⋯Br(2)	Br	3.273(2)	0.861	175.4(3)	150.24(5)
I(4)⋯Br(2)	Br	3.232(2)	0.850	172.7(3)
[(R,R)-1]·Pe4NBr:
I(1)⋯Br(1)	Br	3.319(1)	0.873	174.3(3)	77.67(2), 120.80(2)
I(2)⋯Br(2)	Br	3.301(1)	0.869	175.5(3)	78.08(2), 122.74(2)

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[(R,R)-1]·(Et₄N⁺,Br⁻) crystallises in the monoclinic system, space group P2₁, with two crystallographically independent molecules (R,R)-1 and two crystallographically independent Et₄NBr salts. A projection view of the unit cell along b (Fig. 5) again shows a segregation of halogen-bonded [(R,R)-1]·Br⁻ anionic layers separated from each other along the (a + c) direction by the Et₄N⁺ cation.

Fig. 5 Projection view along b of the unit cell of [(R,R)-1]·(Et₄NBr).

Within one anionic layer (Fig. 6) the neutral molecules adopt the anti conformation as above (see Fig. 2) and are linked to the Br⁻ anion to form, by translation, halogen-bonded chains running along the b-axis with the same acute I⋯Br⁻⋯I angles. The two crystallographically independent chains are slightly different, with shorter and more linear interactions with the Br(2) anion (see Table 1).

Fig. 6 Projection view of one anionic layer in [(R,R)-1]·Et₄NBr showing the XB interactions (pink dotted lines) with the bromide anions.

The crystal structure of the 1 : 1 adduct with Bu₄NBr offers another structural organization, associated now with a gauche conformation (see Fig. 2) along the central C–C axis of [(R,R)-1]. Indeed, [(R,R)-1]·Bu₄NBr crystallises in the orthorhombic system, space group P2₁2₁2₁, again with two crystallographically independent molecules and two crystallographically independent Bu₄NBr salts. The salt organizes into mixed layers incorporating the [(R,R)-1]·Br⁻ halogen-bonded chains and the Bu₄N⁺ cations (Fig. 7). The gauche conformation of the [(R,R)-1] molecule favours now an extended linear shape and results in a close to linear coordination around the Br⁻ anions, with I⋯Br⁻⋯I angles of 144.37(5)° and 150.24(5)° around Br(1) and Br(2), respectively. The XB chains then develop along the twofold screw axis running along c, providing the first example within these series of a helical shape given to these chains from the combination of the neutral chiral iodinated molecules and the halogen bonding to the anions.

Fig. 7 Projection view along b of the unit cell in [(R,R)-1]·Bu₄NBr showing the XB interactions (pink dotted lines) with the bromide anions. The dark blue spheres are located at the nitrogen positions of the Bu₄N⁺ cations.

Finally, the tetrapentylammonium bromide salt crystallises in the orthorhombic P222 space group with two crystallographically independent bromide anions and two crystallographically independent cations, all of them on 222 positions, while two crystallographically independent neutral XB donor molecules (R,R)-1 are located on a twofold axis, hence the formulation [(R,R)-1]₂·[Pe₄NBr], that is now two di-iodinated molecules (R,R)-1 for one bromide anion. The two (R,R)-1 molecules adopt an anti conformation with the now characteristic V shape (Fig. 2). As a consequence of this 2 : 1 stoichiometry, each anion is now surrounded by four iodine atoms, forming a distorted tetrahedron with I⋯Br⁻⋯I angles [around Br(1)] of 77.67(2)° and 120.80(2)° (see Table 1). The halogen-bonded network develops into layers alternating along the c axis. A projection view of one such layer (Fig. 8) shows a topological square lattice structure within the (a, b) plane. These layers can also be described as parallel sets of helices interconnected through the bromide anion by halogen bonding. As shown in Fig. 9, both “red” and “blue” helices running along a have the same left-handed (M) helicity, while those running along b have the same but right-handed (P) helicity. Note that the helicity observed here does not find its origin in the presence of a crystallographic 2₁ (or higher level) screw axis, but in the molecular chirality brought by the enantiopure iodinated linker.

Fig. 8 Projection view along c of one halogen-bonded plane in [(R,R)-1]₂·[Pe₄NBr]. The tetrapentylammonium cations and hydrogen atoms have been omitted for clarity.

Fig. 9 Projection views along c of one halogen-bonded plane in [(R,R)-1]₂·[Pe₄NBr] highlighting the supramolecular helices developing along a (left-handed helices) or along b (right-handed helices). The tetrapentylammonium cations and hydrogen atoms have been omitted for clarity.

In conclusion, we have prepared original chiral, halogen bond donor, neutral molecules bearing two iodine atoms. Their conformational flexibility allows them, when co-crystallized with halide anions, to act as linear or bent bidentate spacers, allowing for the formation of chains with the Et₃S⁺, Et₄N⁺ and Bu₄N⁺ cations, with the halide anion coordinated by two iodine atoms from two different molecules. In the tetrapentylammonium salt, a different stoichiometry with two linkers for one bromide anion leads to a fourfold coordination around the halide, which connects helical structures running along two perpendicular directions within layers. Such anionic polymeric networks, particularly those exhibiting supramolecular helical organisation, are now being investigated as a counter-ion in the electrocrystallization of π-type donor molecules such as tetrathiafulvalene derivatives,^14b,c in order to favour the formation of chiral conducting salts,³⁵ of current strong interest for the observation of rare magneto-chiral anisotropy effects.³⁶

Experimental section

(1R,2R)-1,2-Dimethyl-1,2-bis(2,3,5,6-tetrafluoro-4-iodophenoxy)ethane

(R,R)-1. Pentafluoroiodobenzene (7.1 g, 24.1 mmol) is added to a mixture of (R,R)-2,3-butanediol (0.52 g, 5.8 mmol) and Cs₂CO₃ (3.6 g, 11.0 mmol), and the whole paste is heated without solvent under Ar at 160° for 20 h. After dilution with hexane and filtration on Celite®, the concentrated solution is purified by chromatography on silica gel with hexane as the eluent. Recrystallization from hexane afforded (R,R)-1 as white crystals (2.23 g). Yield: 61%. M.p. 116 °C. ¹H NMR (CDCl₃, TMS, ppm) δ: 1.43 (d, 3H, CH₃), 4.60 (q, 1H, CH). ¹⁹F NMR (CDCl₃, ppm) δ: −121.1, −153.4. ¹³C NMR (d₆-DMSO, ppm) δ: 15.43, 67.91, 82.90, 135.99, 139.05, 142.34, 145.14, 148.30. Elem. anal. calcd. for C₁₆H₈F₈I₂O₂: C, 30.12; H, 1.26%. Found: C, 30.25; H, 1.28%.

(1R,2R)-1,2-Diphenyl-1,2-bis(2,3,5,6-tetrafluoro-4-iodophenoxy)ethane

(R,R)-2 is prepared as above from (R,R)-hydrobenzoin (0.51 g, 2.4 mmol), Cs₂CO₃ (1.54 g, 4.7 mmol) and C₆F₅I (3 mL, 22.4 mmol) and purified by chromatography on silica gel with CH₂Cl₂/hexane (1 : 3) as the eluent. Recrystallization from hexane affords (R,R)-2 as white crystals (1.48 g). Yield: 82%. M.p. 118 °C. ¹H NMR (CDCl₃, TMS, ppm) δ: 5.61 (s, 2H, CH), 7.09–7.21 (m, 10H, Ar). ¹⁹F NMR (CDCl₃, ppm) δ: −121.4, −152.2. ¹³C NMR (CDCl₃, ppm) δ: 77.36, 88.84, 127.80, 128.53, 139.37, 134.75. Elem. anal. calcd. for C₂₆H₁₂F₈I₂O₂: C, 40.97; H, 1.59%. Found: 41.24; H, 1.66%.

(S)-2,2′-Bis(2,3,5,6-tetrafluoro-4-iodophenoxy)-1,1′-binaphthyl

(S)-3 is prepared as above from S-binaphthol (0.3 g, 1.0 mmol), Cs₂CO₃ (0.70 g, 2.1 mmol) and pentafluoroiodobenzene (3.2 g, 10.9 mmol). Silica gel chromatography and recrystallization from hexane afforded (S)-3 as white crystals (0.55 g). Yield: 86%. M.p. 168 °C. ¹H NMR (CDCl₃, TMS, ppm) δ: 7.15 (d, 2H, Ar), 7.25–7.4 (m, 6H, Ar), 7.83 (d, 2H, Ar), 7.92 (d, 2H, Ar). ¹⁹F NMR (CDCl₃, ppm) δ: −121.1, −150.9. ¹³C NMR (CDCl₃, ppm) δ: 77.36, 117.65, 118.92, 125.29, 125.62, 127.34, 128.27, 130.55, 130.69, 133.44. Elem. anal. calcd. for C₃₂H₁₂F₈I₂O₂: C; 46.07; H, 1.45. Found: C, 46.79; H, 1.67.

X-ray diffraction studies

Colourless crystals of the four halide salts with (R,R)-1 were obtained as small rods by slow evaporation of mixed solutions, as detailed below. [(R,R)-1]·Et₃SI: a solution of (R,R)-1 (10.8 mg, 16.9 μmol) in a 1 : 1 CH₂Cl₂/hexane solution (1.5 mL) is mixed with a solution of Et₃SI (4.4 mg , 17.9 μmol) in the same solvent mixture (1.5 mL). [(R,R)-1]·Et₄NBr: as above with Et₄NBr (7.0 mg, 22.8 μmol) in pure CH₂Cl₂. [(R,R)-1]·Bu₄NBr: as above with (n-Bu)₄NBr (7.0 mg, 21.7 μmol) in a 1 : 3 CH₂Cl₂/hexane solution. [(R,R)-1]₂·Pe₄NBr: as above with (n-Pe)₄NBr (9.5 mg, 25 μmol) in hexane solution. When collected at low temperature, single crystals were taken in a loop in oil and put directly under a N₂ stream at 150 K. Otherwise, they were glued at the end of a glass tip. Data were collected using a Bruker SMART II diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Structures were solved by direct methods (SHELXS-97, SIR97)³⁷ and refined (SHELXL-97) by full-matrix least-squares methods³⁸ as implemented in the WinGX software package.³⁹ Absorption corrections were applied. Hydrogen atoms were introduced at calculated positions (riding model), included in structure factor calculations, and not refined. Restraints were introduced for the refinement of the disordered Et₃S⁺ cation in [(R,R)-1]·Et₃SI as well as for the ends of the pentyl substituents in the Pe₄N⁺ cation in [(R,R)-1]₂·Pe₄NBr. Crystallographic data are summarized in Table 2. The CIF files for each structure can be retrieved from the Cambridge Crystallographic Data Centre (CCDC) with deposition numbers CCDC 1025195–1025200 for (R,R)-2, (S)-3, [(R,R)-1]·Et₃SI, [(R,R)-1]·Et₄NBr, [(R,R)-1]·Bu₄NBr and [(R,R)-1]₂·Pe₄NBr, respectively.

Table 2 Crystallographic data

Compound	(R,R)-2	(S)-3	[(R,R)-1]·Et3SI	[(R,R)-1]·Et4NBr	[(R,R)-1]·Bu4NBr	[(R,R)-1]2·Pe4NBr
Formula	C26H12F8I2O2	C32H12F8I2O2	C22H23F8I3O2S	C48H56Br2F16I4N2O4	C32H44BrF8I2NO2	C52H60BrF16I4NO4
FW (g mol^−1)	762.16	834.22	884.16	1696.37	960.39	1654.51
Crystal color	Colourless	Colourless	Colourless	Colourless	Colourless	Colourless
Crystal size (mm)	0.87, 0.43, 0.38	0.26, 0.16, 0.03	0.31, 0.14, 0.12	0.40, 0.38, 0.23	0.42, 0.28, 0.17	0.27, 0.11, 0.01
Crystal system	Orthorhombic	Orthorhombic	Orthorhombic	Monoclinic	Orthorhombic	Orthorhombic
Space group	P212121	P21ca	P22121	P21	P212121	P222
T (K)	293(2)	293(2)	150(2)	293(2)	150(2)	150(2)
a (Å)	8.0352(2)	8.9564(3)	11.5706(4)	13.2765(2)	12.0074(10)	9.9519(3)
b (Å)	16.1215(3)	13.3787(4)	15.5144(5)	15.9187(2)	15.2998(13)	10.6977(3)
c (Å)	20.0537(4)	23.5539(7)	16.0571(6)	14.6917(2)	41.115(3)	30.4347(10)
α (°)	90.00	90.00	90.00	90.00	90.00	90.00
β (°)	90.00	90.00	90.00	102.437(8)	90.00	90.00
γ (°)	90.00	90.00	90.00	90.00	90.00	90.00
V (Å^3)	2597.75(10)	2822.35(15)	2882.42(17)	3032.15(7)	7553.4(11)	3240.15(17)
Z	4	4	4	2	8	2
D calc (g cm^−3)	1.949	1.963	2.037	1.858	1.689	1.696
μ (mm^−1)	2.501	2.312	3.395	3.463	2.791	2.625
Total refls.	10 107	25 892	25 659	58 725	37 334	14 071
Abs. corr.	Multi-scan	Multi-scan	Multi-scan	Multi-scan	Multi-scan	Multi-scan
T min, Tmax	0.286, 0.387	0.648, 0.933	0.571, 0.665	0.273, 0.451	0.407, 0.622	0.714, 0.974
Unique refls.	5860	6435	6604	13 706	17 353	6357
R int	0.040	0.0602	0.0301	0.0370	0.0495	0.0265
Unique refls. (I > 2σ(I))	5226	3973	5063	8986	14 397	5040
Refined parameter	343	397	339	685	812	335
R 1 (I > 2σ(I))	0.0475	0.0433	0.0518	0.0367	0.0823	0.0568
wR2 (all data)	0.1269	0.1089	0.1641	0.0888	0.1856	0.1711
Flack parameter	0.06(3)	0.03(2)	0.17(7)	0.00(2)	0.10(2)	0.13(4)
Goodness of fit	1.069	1.021	1.134	1.128	1.172	1.162
Res. dens (e Å^−3)	1.131, −0.821	0.465, −0.653	0.937, −1.616	0.899, −0.979	1.508, −3.200	1.339, −1.240

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Acknowledgements

We thank the CDIFX (Rennes) for access to the X-ray diffractometers.

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Footnote

† CCDC 1025195–1025200. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ce01935k

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