2.1 Materials and instrumentation

Organic solvents, dichloromethane (Carlo Erba, 99.5% purity), toluene (Acros, 99.99%) and n-pentane (Carlo Erba, 99% purity) were refluxed over appropriate dessicants, distilled, and saturated with argon prior to use. Chemicals were purchased from Acros Organics and Fluka, and used without further purification. The starting compound [n-Bu₂Sn(OH)(H₂O)(CF₃SO₃)]₂ (1) was synthesized from n-Bu₂SnO (Acros, 98% purity) and trifluoromethanesulfonic acid (Fluka, 98% purity) according to a published method [2a]. The ¹H, ¹⁹F, ¹¹⁹Sn{¹H}, and ¹³C{¹H} NMR experiments were run at 298 K on a Bruker Avance 300 spectrometer (at the Plateforme d’Analyses Chimiques et de Synthèse Moléculaire de l’Université de Bourgogne – PACSMUB), at 300.131, 282.404, 111.910, and 75.475 MHz, respectively, and calibrated with Me₄Si, trifluoromethylbenzene or Me₄Sn as an internal standard. Chemical shift δ values are given in ppm. IR spectra were recorded on a Bruker Vector 22 equipped with a Specac Golden Gate™ ATR device. Melting points were determined in capillaries on an Electrothermal 9100 apparatus. Elemental analyses (C, H, N) were performed at the Institut de Chimie Moléculaire de l’Université de Bourgogne, Dijon.

2.2 Isolation of [C₁₃H₁₀N·H₂O][CF₃SO₃] (2) and [C₁₃H₁₀N][CF₃SO₃] (3)

Three molar equivalents of acridine (C₁₃H₉N, Acros, 98% purity) (0.237 g, 1.322 mmol) were added to a colourless solution of [n-Bu₂Sn(OH)(H₂O)(CF₃SO₃)]₂ (1) (0.360 g, 0.441 mmol) in dichloromethane (15 mL) at room temperature. After stirring for two hours, fresh toluene (10 mL), used as co-solvent for crystallization, was then introduced to the resulting orange solution. After few days at room temperature, colourless single-crystals (squares and rods) were first obtained and attributed according to their IR fingerprints to oxo-cluster 5 which crystallized in fact in two polymorphic forms, 5a (square) and 5b (rod). After separation, two new types of coloured crystals, yellow needles and yellow squares, were grown from the mother-liquor by slow evaporation. The crystals were then filtered off, washed with portions of n-pentane, dried in air at room temperature, collected separately and finally characterized as salt 2 and 3, respectively.

[C₁₃H₁₀N·H₂O][CF₃SO₃] (2) (mp 202 °C). ¹H NMR (CD₂Cl₂): δ 15.94 (brs, 1H, AcrH), 9.59 (s, 1H, AcrH), 8.70 (d, J = 8.9 Hz, 2H, AcrH), 8.38 (d, J = 8.5 Hz, 2H, AcrH), 8.30 (m, 2H, AcrH), 7.94 (m, 2H, AcrH), 1.56 (brs, 2H, H₂O). ¹⁹F NMR (CD₂Cl₂): δ –78.97 (s, CF₃SO₃⁻). ¹³C{¹H} NMR (CD₂Cl₂): δ 148.5, 140.4, 138.4, 129.7, 128.8, 126.5, 121.0 (q, ¹J_C-F = 319 Hz, CF₃SO₃⁻), 120.8. Anal. Calc. For C₁₄H₁₂F₃NO₄S (347.31): C, 48.41; H, 3.48; N, 4.03. Found: C, 48.30; H, 3.29; N, 4.30%. Main IR bands (ν, cm⁻¹): 3387, 3088, 3048, 2522, 2052, 1643, 1486, 1467, 1402, 1382, 1321, 1246, 1224, 1166, 1152, 1143, 1025, 902, 862, 825, 778, 743, 675.

[C₁₃H₁₀N][CF₃SO₃] (3) (mp 208 °C). ¹H NMR (CD₂Cl₂): δ 15.82 (brs, 1H, AcrH), 9.62 (s, 1H, AcrH), 8.65 (d, J = 8.9 Hz, 2H, AcrH), 8.38 (d, J = 8.5 Hz, 2H, AcrH), 8.27 (m, 2H, AcrH), 7.92 (m, 2H, AcrH), ¹⁹F NMR (CD₂Cl₂): δ –78.94 (s, CF₃SO₃⁻).¹³C{¹H} NMR (CD₂Cl₂): δ 148.5, 140.4, 138.4, 129.7, 128.8, 126.5, 121.0 (q, ¹J_C-F = 319 Hz, CF₃SO₃⁻), 120.8. Anal. Calc. For C₁₄H₁₀F₃NO₃S (329.29): C, 51.06; H, 3.06; N, 4.25. Found: C, 51.18; H, 2.96; N, 4.27%. Main IR bands (ν, cm⁻¹): 3090, 3048, 3011, 2916, 2888, 2837, 2777, 1647, 1626, 1587, 1481, 1464, 1425, 1401, 1284, 1221, 1156, 1026, 776, 742, 674, 632, 601, 572, 515.

2.3 Isolation of {([C₁₃H₁₀N][CF₃SO₃])₂·C₁₄H₁₀} (4)

Three molar equivalents of anthracene (C₁₄H₁₀, Acros, 99% purity) (0.466 g, 2.607 mmol) and acridine (C₁₃H₉N, Acros, 98% purity) (0.467 g, 2.607 mmol) were successively added to a colourless solution of [n-Bu₂Sn(OH)(H₂O)(CF₃SO₃)]₂ (1) (0.709 g, 0.869 mmol) in dichloromethane (25 mL) at room temperature. After stirring for two hours, fresh toluene (15 mL), used as co-solvent for crystallization, was then introduced to the resulting dark red solution. After few days at room temperature, colourless square single-crystals were first obtained, corresponding to organotin(IV) complex 5a. After separation, dark red square single crystals were grown from the mother liquor by slow evaporation. The crystals were filtered off, washed with portions of n-pentane, dried in air at room temperature, and finally characterized as salt {([C₁₃H₁₀N][CF₃SO₃])₂·C₁₄H₁₀} 4 (mp 178 °C). ¹H NMR (CD₂Cl₂): δ 15.88 (brs, 2H, AcrH), 9.59 (s, 2H, AcrH), 8.69 (d, J = 8.9 Hz, 4H, AcrH), 8.47 (s, 2H, Ant), 8.37 (d, J = 8.5 Hz, 4H, AcrH), 8.29 (m, 4H, AcrH), 8.05 (m, 4H, Ant), 7.94 (m, 4H, AcrH), 7.50 (m, 4H, Ant). ¹⁹F NMR (CD₂Cl₂): δ –78.94 (s, CF₃SO₃⁻). ¹³C{¹H} NMR (CD₂Cl₂): δ 148.5 (AcrH), 140.4 (AcrH), 138.4 (AcrH), 132.0 (Ant), 129.7 (AcrH), 128.8 (AcrH), 128.4 (Ant), 126.5 (AcrH), 126.4 (Ant), 125.7 (Ant), 121.0 (q, ¹J_C-F = 319 Hz, CF₃SO₃⁻), 120.7 (AcrH). Anal. Calc. For C₄₂H₃₀F₆N₂O₆S₂ (836.82): C, 60.28; H, 3.61; N, 3.35. Found: C, 59.90; H, 3.16; N, 3.52%. Main IR bands (ν, cm⁻¹): 3302, 3257, 3208, 3148, 3103, 3059, 3024, 2989, 2948, 2872, 1650, 1476, 1427, 1257, 1224, 1154, 1028, 873, 790, 757, 727, 618, 574, 516.

2.4 X-Ray diffraction analysis

Diffraction data were collected from suitable crystals on a Bruker Nonius ApexII CCD (Mo Kα radiation, λ = 0.71073 Å). The structures were solved using Charge Flipping Algorithm Methods (SUPERFLIP) [12] and refined with full-matrix least-squares methods based on F² (SHELX-97) [13] with the aid of the WINGX program (14]. All non-hydrogen atoms were refined with anisotropic thermal parameters and hydrogen atoms were included in their calculated positions and refined with a riding model. Programs used for the representation of the molecular and crystal structures: ORTEP [15], DIAMOND [16], and MERCURY [17]. Crystallographic data and structures refinement details or 2, 3 and 4 are summarized in Table 1.

Crystallographic data for the structures reported in this paper have been deposited at the Cambridge Crystallographic Data Centre, CCDC, No. CCDC 891927 for 2, No. CCDC 891928 for 3, and No. CCDC 891929 for 4. Copies of the data may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: int Code +44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).

3.1 Isolation and crystal structures of acridinium salts 2 and 3

The strategy recently developed for the design of phenazinium-based stacks [4b,11] was applied to acridine. Thus, the reaction of the dimeric hydroxo di-n-butylstannane trifluoromethanesulfonato complex [n-Bu₂Sn(μ-OH)(H₂O)((¹-O₃SCF₃)]₂ (1) with acridine (C₁₃H₉N, Acr) (1:3 molar ratio) in dichloromethane at room temperature led to colourless crystals of the tetra-n-butyldistannoxane trifluoromethanesulfonato cluster (5). Compound 5 could be isolated in two polymorphic forms 5a and 5b which crystallized in two distinct crystal systems (triclinic- P1¯ and monoclinic-P2₁/n, respectively), and with specific crystal habits (square and rod) that easily facilitate their visual identification [4b]. According to the infrared fingerprints recorded on crystals, and which are also characteristic for each polymorph (shown in the Supporting Information, Fig. S1), we can unambiguously conclude that both structures of 5 are formed successively during the reaction. Moreover, the monitoring of the reaction by ¹¹⁹Sn{¹H} spectroscopy showed, after addition of three molar equivalents of acridine to a CD₂Cl₂ solution of 1, the predominant formation of the pair of the characteristic resonances of the distannoxane species 5a at δ = –135 and −143 ppm (Fig. S2). Upon crystallization of 5a and 5b, and separation by filtration, two new types of yellow crystals were grown together from the mother liquor. They were collected as needles and plates, and were identified as the acridinium trifluoromethanesulfonate salts [C₁₃H₁₀N]⁺[CF₃SO₃]^–, 2 and 3, respectively. From a mechanistic point of view, the removal of protons from 1 by acridine molecules leads to the in situ formation of acridinium cations (AcrH), and promotes the rearrangement of organotin species into tetranuclear oxo-cluster 5. This subsequently led to the release of CF₃SO₃⁻, which ultimately compensates the positive charge of AcrH.

The difference between the acridinium salts 2 and 3 comes exclusively from the presence of a water molecule in the crystal lattice of one of the two salts. Indeed, the infrared spectrum recorded from 2 exhibits the characteristic vibration bands of the trifluoromethasulfonate anions (in the stretching region between 1000 and 1300 cm⁻¹ [18], and also observed for 3), and a broad elongation band at 3387 cm⁻¹. This band is a strong evidence of the presence of water. It is not present in the spectrum of 3. It can be attributed to the σ(OH) vibration. The ¹H NMR spectra of 2 and 3 corroborate this observation (IR and NMR spectra are shown in the Supporting Information). The X-ray crystallographic analysis of 2 confirms the hydrogen bond occurring between the NH group of the acridium cation (AcrH) and one molecule of water [NH⋯O=2.6803 Å and 179.4°]. The acridinium and water molecules are located in the same mirror plane. Furthermore, the water molecule is also in hydrogen bonding interaction with one oxygen atom of the trifluoromethanesulfonate anion [OH⋯O=2.7552 Å and 177.3°]. An ORTEP view of 2 is shown in Fig. 1a together with selected bond distances and angles. Interestingly, and from a supramolecular point of view, the water molecule in each [C₁₃H₁₀N·H₂O][CF₃SO₃] unit is also implicated in a third hydrogen-bonding interaction, with an identical distance that previously [OH⋯O=2.7552 Å], with an oxygen atom of an adjacent trifluoromethanesulfonate anion, leading to the formation of an one-dimensional infinite polymer chain propagating along the b axis, and which consists of H2O⋯CF3SO3 building blocks as repeating unit. The AcrH cations are orthogonally positioned along this chain, parallel to plane (010) and in an isotactic arrangement. As a result of the self-assembly induced by π-π interactions between anti-parallel acridinium rings, two polymeric chains are fitted into each other. It leads to a 2-D architecture which can be compared to a ladder structure. The construction of the resulting supramolecular edifice is summarized in Scheme 3 and a view of the crystal packing is shown in Fig. 1b. The interplanar and the centroid-to-centroid distances between parallel AcrH are 3.35 and 4.35 Å, respectively. The difference in these two distances indicates that acridinium rings are strongly slipped. The slip angle (angle between the normal to the planes and the centroid–centroid vector) is 39.75°, corresponding to a slippage distance of 2.79 Å. Typically for such interactions, the interplanar distance between the arene planes is commonly found around 3.3 to 3.8 Å [19]. Interestingly, the organization of the crystal packing described for 2 is comparable to ionic supramolecular architectures reported in the past by Wolf and Mei, and prepared from acridine and dicarboxylic acids [20].

Salt 3 corresponds to the dehydrated form of 2. Initially, both salts crystallized together from the same batch but they are only distinguished by their respective shape (needles for 2 and plates for 3). However, the conversion of 2 into 3 occurs when molecular sieves were added to a dichloromethane solution of 2 or alternatively when 2 is heated at the solid state under vacuum at 100 °C for 1 hour (Scheme 2, pathway (ii)). On the basis on an X-ray crystallographic analysis, the molecular structure of 3 can be described as constituted by an acridium cation, AcrH [C₁₃H₁₀N]⁺, and by a trifluoromethanesulfonate anion [CF₃SO₃]⁻ in hydrogen bonding interaction with the NH group of the cation [NH⋯O=2.7432 Å and 169.34°]. An ORTEP representation of 3 is shown in Fig. 2a.

From a supramolecular point of view, acridinium rings of 3 are self-assembled in antiparallel arrangement along the a-axis, and through π-π interactions leading to columnar oblique stacks in which the rings emerge tilted by 22.6° with respect to the (100) plane. Two values of interplanar distances, alternating, have been determined between AcrH: 3.218 Å [centroid-to-centroid distance = 4.584 Å, implicating a slippage of 3.26 Å (slip angle 45.35°)] and 3.349 Å [centroid-to-centroid distance = 3.698 Å, implicating a slippage of 1.56 Å (slip angle 25.03°)]. Additionally, short intramolecular contacts exist also between fluorine and oxygen atoms of trifluoromethanesulfonate anions and the nearest neighbour molecules of acridinium, through O⋯HC and F⋯HC interactions, and thus uniting the columns, to each others. A short F⋯F distance (2.910(2) Å), slightly lower than the sums of the van der Waals radii (2.94), is also observed. A view of the crystal packing of 3 in the unit cell is reported in Fig. 2b.

3.2 Isolation and crystal structures of acridinium salt 4

In previous studies, we reported the incorporation of neutral polyaromatic molecules, e.g. phenazine and anthracene, between two phenazinium salts, leading to unusual sandwich-type structures [4b,11]. A similar methodology was used in the case of acridine. When an equimolar mixture of acridine and anthracene reacts with organotin(IV) complex 1, nice red-blood square crystals were grown in addition to colourless crystals corresponding to organotin(IV) cluster 5. The X-ray crystallographic analysis performed on suitable red crystals, indeed, revealed the expected sandwich-type structure and determined as {([C₁₃H₁₀N][CF₃SO₃])₂·C₁₄H₁₀} (4) resulting from the intercalation of a neutral molecule of anthracene between two acridiniunium trifluoromethanesulfonate salt molecules via slipped parallel π-π aromatic interactions. The micro-analytical data corroborates the formulation reported for 4. The average inter-planar distance between the aromatic rings is 3.424 Å. The two rings are slipped by 1.52 Å (slip angle equal to 23.90°) and the ring-centroid to ring-centroid distances are equal to 3.74 Å. One of the oxygen atoms of the trifluoromethanesulfonate anion is engaged in an intermolecular hydrogen bonding interaction with the NH function of the acridinium cation [N–H⋯O=2.7743 Å and 169.06°]. As in the case of 3, additional short intramolecular contacts are observed between fluorine and oxygen atoms of trifluoromethanesulfonate anions and surrounding molecules of anthracene or acridinium, through O⋯HC and F⋯HC interactions. An ORTEP representation and the crystal packing of salt 4 are reported in Fig. 3a and b, respectively.

Furthermore, the π-π stacking occurring between the acridinium and anthracene rings brings the CF₃ groups of trifluoromethanesulfonate anions in close proximity, in a face-to-face orientation. The resulting F⋯F distance (2.892(2) Å), shorter than that observed in the crystal packing of 3, can be described as a type I conformation according to the Desiraju's classification (C–F⋯F=95.58 and 120.95°, angle dihedral 74°) [21,22]. Two fluorine atoms of each trifluoromethanesulfonate anions are involved in such an interaction. A MERCURY representation focusing on this contact is depicted in Fig. 4. Synthetically, the sandwich structure of 4 can be also obtained mixing two equivalents of 3 with one equivalent of anthracene in dichloromethane solution (Scheme 2, pathway (iii)).

Thus, the three types of supramolecular stacks described in this study and built from acridinium trifluoromethanesulfonates result from the simultaneous involvement of different types of intermolecular interactions: π-π aromatic interactions, hydrogen bonds that can be qualified as strong and weak [23], and also F⋯F contacts. All modes of interaction which are implicated in the crystal packing of 2, 3 and 4 are listed in Tables 2 and 3. In the three architectures, acridinium cations are directly involved in π-π aromatic stacking and in strong hydrogen bond interactions (N–H⋯O), with water molecules (2) or F₃CSO₃⁻ anions (3 and 4). According to us, these interactions can be considered as the main driving forces leading to the reported self-assembled edifices. In addition, the trifluoromethanesulfonates anions through their fluorine atoms and their oxygen atoms non implicated in strong intermolecular interactions strengthen the cohesion of the crystal packing building bridges between aromatic stacks via the formation of secondary (weak) C–H⋯O and C–H⋯F hydrogen bonding interactions and short F⋯F contacts. However, these weaker interactions can be also considered, owing to their orientation, as one of the factors responsible for the slippage, more or less marked (2.79 Å for 2, 1.52 Å for 4), observed between the acridinium rings in π-π interaction.