Crystal structure of a host–guest complex between mephedrone hydrochloride and a tetraphosphonate cavitand

The molecular recognition properties of the tetraphosphonate cavitand Tiiii[C3H7,CH3,C6H5] towards mephedrone hydrochloride, an illicit drug belonging to the amphetamine family, have been analysed in the solid state through the detailed analysis of the crystal and molecular structure of the resulting supramolecular compound, and in solution via NMR studies.


Chemical context
Mephedrone (2-methylamino-1-p-tolylpropan-1-one), often abbreviated as 4-MMC, the acronym of 4-methyl methcathinone, is a synthetic drug belonging to the family of methamphetamines known for its stimulant effects (Winstock et al., 2010;Morris, 2010;Wood et al., 2010). It can be considered a 'designer drug', that is, a compound resulting from the chemical modification of an existing drug, which in this case is cathinone, a natural alkaloid found in the plant Catha edulis. As a result of the major impact these substances have on human health and social security, it is extremely important to have sensitive, selective and fast methods to identify them as a class, independently from all the synthetic modifications that can be devised to market them and to bypass the legal restrictions to which the parent compounds are subjected. Among the existing analytical methods used to detect 4-MMC in human biological samples or in different media (water, mixtures of powders, etc), solid-phase extraction (SPE) and liquid chromatography combined with mass spectrometry (LC/MS) are the most common, as can be seen from the ISSN 2056-9890 extended literature which has been published on the subject in the past few years (Kolmonen et al., 2009;Singh et al., 2010;Santali et al., 2011;Frison et al., 2011;Strano-Rossi et al., 2012;Power et al., 2012;Perera et al., 2012;Lua et al., 2012;Vircks & Mulligan, 2012;Concheiro et al., 2013;Mayer et al., 2013;Mwenesongole et al., 2013;Pedersen et al., 2013;Kanu et al., 2013;Strano-Rossi et al., 2014;de Castro et al., 2014;Mercolini et al., 2016;Salomone et al., 2016;Fontanals et al., 2017;Lendoiro et al., 2017;Mercieca et al., 2018;Robin et al., 2018). Recently, the group of Professor Dalcanale has reported a new method to detect methamphetamine salts with extremely high selectivity in water, using cavitand-grafted silicon microcantilevers (Biavardi et al., 2014); more precisely, MDMA (methylenedioxymethamphetamine), cocaine, amphetamine, and 3-fluoromethamphetamine hydrochlorides have been successfully detected in this way. This method takes advantage of the ability shown by tetraphosphonate cavitands to selectively recognize the + NH 2 -CH 3 group ( + NHR-CH 3 in the case of cocaine) common to all the above-mentioned drug salts through the concomitant formation of CH 3 Á Á Á interactions and hydrogen bonding. Indeed, resorcinarene-based cavitands (Cram, 1983;Cram & Cram, 1994) decorated at the upper rim with phosphonate groups or quinoxaline moieties have long been exploited for their molecular recognition properties towards charged and neutral molecules (Dutasta, 2004;Vachon et al., 2011;Melegari et al., 2013;Pinalli et al., 2016;Tudisco et al., 2016;Trzciń ski et al., 2017;Pinalli et al., 2018;Wu et al., 2012;Clé ment et al., 2015). In order to further assess the recognition properties of tetraphosphonate cavitands towards quaternary ammonium salts of social interest, the supramolecular complex between Tiiii[C 3 H 7 , CH 3 , C 6 H 5 ] and mephedrone hydrochloride is herein reported and analysed, both in the solid state through the detailed analysis of its crystal and molecular structure, and in solution via NMR studies.

Structural commentary
The host-guest complex (I) of general formula (C 11 H 16 NO)@Tiiii[C 3 H 7 , CH 3 , C 6 H 5 ]ClÁCH 3 OH crystallizes in the monoclinic space group P21/c; its molecular structure is shown in Fig. 1. It consists of a 1:1 inclusion compound between mephedrone hydrochloride and a resorcinarenebased tetraphosphonate cavitand with the four P O groups bridging the upper rim all pointing inwards the aromatic cavity. At the lower rim, four propyl chains are present, one of which is disordered over two equivalent positions with occupancy factors of 0.5. For each supramolecular complex, one lattice methanol molecule is present, disordered over two positions with occupancy factors of 0.665 (6) and 0.335 (6) (see Fig. 3). The mephedrone cation (C 11 H 16 NO) + , which is protonated at the nitrogen atom N1, is located inside the cavity through the formation of two strong, charge-assisted N-HÁ Á ÁO hydrogen bonds involving the P O groups at the upper rim as acceptors (N1-H1AÁ Á ÁO3A and N1-H1BÁ Á ÁO3B, see Fig. 2 and Table 1 for the detailed geometrical parameters). The methyl group C1 directly bonded to the amino moiety is located inside the basic cavity, stabilized via a cationÁ Á Á interaction involving the C1-H1D moiety and the aromatic ring C1B-C6B [C1-H1DÁ Á ÁCg1, 3.672 (7) Å and 145.1 , where Cg1 is the centroid of the benzene ring]. According to the electrostatic model, the term 'cationÁ Á Á' is more appropriate than 'C-HÁ Á Á' to describe the interactions of N-methylammonium ions (Dougherty, 2013).] Further stabilization is provided by three C-H guest Á Á ÁO P host hydrogen bonds ( Fig. 2 and Table 1). The distance of C1 from the mean plane passing through the methylene atoms C8A, C8B, C8C and C8D of the lower rim is 3.001 (5) Å , which gives a measure of how deeply the guest is inserted inside the cavity (see also the discussion in Section 5).
The chloride anion is located between the alkyl legs of the cavitand, with a Cl1Á Á ÁN1 distance of 7.097 (5) Å , forming numerous C-HÁ Á ÁCl interactions with the aromatic and methylenic hydrogen atoms of the lower rim (see Table 1), as well as a hydrogen bond with the O2S-H2S group of the methanol molecule of occupancy factor 0.665 (6) [O2S-H2SÁ Á ÁCl1, 3.105 (5) Å and 168.5 ]. Moreover, the O1S atom from the other methanol fraction accepts a hydrogen bond from the methyl group C3 of the mephedrone guest [C3-H3BÁ Á ÁO1S, 3.51 (2) Å and 164.3 ].

Figure 2
Left: view of the main host-guest supramolecular interactions shown as blue and green dotted lines. Only relevant H atoms are shown, while the alkyl chains, the chloride anion and the methanol lattice molecules have been omitted for clarity. Right: side view of the host-guest complex.

Figure 3
Supramolecular interactions (blue dotted lines) involving the chloride anion (represented as a green sphere) and the disordered methanol lattice molecules.

Studies in solution
In solution, complexation was observed both via phosphorous and proton NMR spectroscopy following the shift of the 31 P signals of the Tiiii[C 3 H 7 , CH 3 , Ph] host and the shift of the + N-CH 3 protons of the mephedrone hydrochloride guest. The titration was performed in deuterated methanol at 253 K, in order to be under slow chemical exchange in the NMR time scale and better observe the complexation event. The NMR tube was filled with 0.4 mL of a deuterated methanol solution containing the cavitand (7.5 mM concentration). The mephedrone hydrochloride titrant solution was prepared by dissolving the guest in 0.1 mL of deuterated methanol (31 mM). Two portions (0.5 eq., 48.5 mL) of the titrant were added by syringe to the NMR tube. During the titration, the phosphorous singlet of the cavitand shifted downfield, from 8.70 (free host) to 11.14 ppm upon addition of one equivalent of the guest ( Fig. 7a and 7c View of the packing of (I) along the b-axis direction, mediated by C14B-H14BÁ Á ÁCl À interactions (blue lines). The C and H atoms highlighted in purple are in general positions, while the chloride anion is at the symmetry position Àx, 1 2 + y, 3 2 À z.

Figure 5
View of the packing of (I) mediated by C-HÁ Á ÁO and C-HÁ Á Á interactions between the guests and adjacent cavitands. Symmetry code: (i) 1 À x, 1 2 + y, dipole interactions between the + N-CH 3 and the phosphonate groups at the upper rim. The addition of 0.5 eq. of guest caused the appearance of two phosphorous signals at 8.74 and 11.14 ppm related to the free host and to the complex, respectively (Fig. 7b).
In the proton NMR, after the addition of 0.5 equivalent of mephedrone hydrochloride the diagnostic upfield shift of the guest + N-CH 3 signals was observed, as expected for the shielding effect caused by its inclusion in the aromatic cavity of the host (Fig. 8b). After the addition of one equivalent of guest, the + N-CH 3 singlet appeared still shifted upfield but broadened (Fig. 8c).

Database survey
As already discussed in Section 1, tetraphosphonate cavitands of general formula Tiiii[R, R1, R2] (where R, R1 and R2 are the substituents at the lower rim, on the four benzene rings of the cavity, and on the phosphonate groups, respectively; Pinalli et al., 2004), are excellent receptors for molecular recognition of neutral and charged guests because of the presence of P O groups that act as hydrogen-bond acceptors, and of the aromatic cavity that allows the formation of C-HÁ Á Á interactions. The substituent R at the lower rim can be modified to tune the solubility of the host, to enhance the crystallization process, or to graft the cavity on different surfaces, but does not play any significant role in the recognition process, if not that of interacting with the anionic counterpart of a positively charged guest. A search in the Cambridge Structural Database (Version 5.38, update August 2018; Groom et al., 2016) for a tetraphosphonate scaffold without limitations on R, R1 and R2 yielded 82 hits, with the most populated class (44 hits) being the one of general formula Tiiii[H, CH 3 , CH 3 ]. The substitution of the alkyl chains with hydrogen atoms favours the formation of crystals, albeit lowering the solubility of the macrocycle, and the methyl group on the phosphonate moiety generates less steric hindrance than a phenyl one. Besides these general considerations, the most interesting structural comparisons with the title compound are to be made with supramolecular complexes in which the guests are: (i) the zwitterionic species 1,1-dicyano-2-(dicyanomethyl)-3-(dicyanomethylene)-4,4-bis- The full two-dimensional fingerprint plot (a) and those delineated into HÁ Á ÁH (b), CÁ Á ÁH/HÁ Á ÁC (c), OÁ Á ÁH/HÁ Á ÁO (d) and ClÁ Á ÁH/HÁ Á ÁCl (e) contacts for (I).    Biavardi et al., 2014). A molecular sketch of the guests is reported in Fig. 9. In the case of KEGNIV, the positive charge of the zwitterionic species is localized on the N,N-dimethylanilino rings, in particular on the NMe 2 moiety, and that has been demonstrated by the supramolecular complex formed with Tiiii in which the guest enters the cavity with the positive fragment to form ion-dipole interactions with the P O groups. Ephedrine and pseudoephedrine are complexed by the cavitand via a set of supramolecular contacts very similar to those present in the title compound, that is, hydrogen bonding involving the -NH 2 + fragment as donor and the phosphonate groups as acceptors, and cationÁ Á Á interactions. The distance of the carbon atom of the methyl group interacting with the cavity from the mean plane passing through the methylene atoms C8A, C8B, C8C and C8D of the lower rim (the labelling is the same as in Fig. 2) is 3.023 (4) Å for ephedrine, 3.202 (3) Å for the sterically hindered pseudoephedrine and 3.001 (5) Å for (I). This value is of 3.122 (2), 4.104 (4), 2.853 (3) and 2.983 (5) Å for MDMA, cocaine, amphetamine and 3-fluoromethamphetamine hydrochloride, respectively, all in good agreement with that of the title compound (cocaine is less included inside the cavity because of its bulky substituents).

Synthesis and crystallization
1 H NMR spectra were obtained using a Bruker AMX-400 (400 MHz) spectrometer. All chemical shifts () were reported in ppm relative to the proton resonances resulting from incomplete deuteration of the NMR solvents. 31 P NMR spectra were obtained using a Bruker AMX-400 (162 MHz) spectrometer. All chemical shifts () were recorded in ppm relative to external 85% H 3 PO 4 at 0.00 ppm. The cavitand Tiiii[C 3 H 7 , CH 3 , C 6 H 5 ] was prepared following published procedures (Biavardi et al., 2008). Mephedrone hydrochloride in its racemic form was purchased from SALAR SpA (Italy) and used as received without further purification.
(C 11 H 16 NO)@Tiiii[C 3 H 7 , CH 3 , C 6 H 5 ]ClÁCH 3 OH was obtained by mixing a methanol solution of Tiiii[C 3 H 7 , CH 3 , C 6 H 5 ] (1 eq.) with a dichloromethane solution of C 11 H 16 NOCl (1 eq.). The mixture was left to evaporate to yield colourless single crystals of the 1:1 complex which were suitable for X-ray diffraction analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms bound to C, N and O were placed in calculated positions and refined isotropically using a riding model with C-H ranging from 0.95 to 1.00 Å , N-H = 0.91 Å , O-H = 0.98 Å and Uiso(H) set to 1.2-1.5Ueq(C/N/O). For each cavitand:guest complex, a methanol solvent molecule was located in the difference-Fourier map, disordered over two positions with occupancy factors of 0.665 (6) and 0.335 (6). One of the four alkyl chains of the cavitand was also found to be disordered over two equivalent positions with occupancy factors of 0.5, and the relative carbon Molecular sketch of the different guests described in the Database survey. Computer programs: APEX2 and SAINT (Bruker, 2008), SIR97 (Altomare et al., 1999), SHELXL2014/7 (Sheldrick, 2015), Mercury (Macrae et al., 2006), WinGX (Farrugia, 2012), PARST (Nardelli, 1995) and publCIF (Westrip, 2010 Data collection: APEXII (Bruker, 2008); cell refinement: APEXII (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 2012), PARST (Nardelli, 1995) and publCIF (Westrip, 2010).  (8)  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.