Results and Discussion

1-[bis(4-fluorophenyl)methyl]piperazine (1) is an intermediate for the preparation of flunarizine, 1-[bis(4-fluorophenyl)methyl]-4-[(2E)-3-phenylprop-2-en-1-yl]piperazine, which is a calcium channel blocker [1, 2] and has been used for treatment of migraine and seems to be a little less effective than propranolol [3].

The crystal structures of some mono-salts of 1 (always protonated at N34 nitrogen atom) have been reported: 2-(2-phenylethyl)benzoate [4] and picrate [5], as well as the structures of the unsubstituted free base, 1-benzhydrylpiperazine [6] and some of its derivatives’ di-salts, e.g. 1-benzhydryl-4-(2-benzoyl-ethyl)piperazinium tetrachlorocuprate(ii) hydrate [7] or 1-diphenylmethyl-4-ethylpiperazine-1,4-diium dichloride [8]. Here we present the first crystal structure that contains the dication of 1, namely 1-[bis(4-fluorophenyl)methyl]piperazine-1,4-diium dimaleate (1dim, Scheme 1). No phase transition has been observed between room temperature and 100 K.

Scheme 1
scheme 1

Molecular formula of the salt 1dim

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Molecular Structure

The asymmetric part of the unit cell of 1dim contains one dication and two maleate anions (Fig. 1). The cation is protonated at both nitrogen atoms of piperazine ring, the positions of appropriate hydrogen atoms were found in the difference Fourier maps and successfully refined without constraints. The piperazinium ring is in the chair conformation, and the small values of asymmetry parameters (which measure the deviations from the ideal symmetry of a given conformation [9]) show that the distortions are relatively unimportant. Table 1 lists some relevant geometrical parameters. The hydrogen atom at N31 is in an axial position (C33–C32–N31–H31 torsion angle is −62.6(11)° and C35–C36–N31–H31 61.4(11)°.

Fig. 1
figure 1

Anisotropic ellipsoid representation of the molecular complex of 1dim_100, together with atom labeling scheme [14]. The ellipsoids are drawn at 50 % probability level, hydrogen atoms are depicted as spheres with arbitrary radii, hydrogen bonds are shown as dashed lines

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Table 1 Selected geometrical parameters (Å, °) with s.u.’s in parentheses
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In both maleate anions strong intramolecular hydrogen bonds are observed (O···O distances of 2.4184(12) and 2.4414(13) Å). Such bonds, enforced by cis conformation of the double bond (C–C=C–C torsion angles are 1.0(2)° and 0.7(2)° in anions A and B respectively) were also observed in other maleates. For instance, the imidazolium hydrogen maleate and calcium di(hydrogen maleate) were studied by the difference X-N (X-ray vs. neutron) method [10]. Figure 2 shows the distribution of the O···O distances and O–H···O angles for 169 hits found in the Cambridge Structural Database [11]; ver.5.33 of Nov. 2011, updated Feb. 2012; search criteria: organic, R < 0.075, absolute value of the C–C–C–C torsion angle less than 30°, O···O contact less than 3 Å).

Fig. 2
figure 2

The distribution of O···O distances (left) and O–H···O angles (right) in maleate anions found in the Cambridge Structural Database

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In the structure of 1dim the details of intramolecular hydrogen bonds in two symmetry-independent molecules are different, as far as the standard X-ray diffraction can determine this. The hydrogen bond in the anion A is almost symmetrical (O–H and H···O distances are 1.15(2) and 1.27(2) Å while in the anion B it is better localized near one of the O atoms (appropriate distances are 0.99(2) and 1.45(2) Å). The reasonability of these results is somehow confirmed by the difference electron density plots, calculated after removing the hydrogen atom (Fig. 3a, b), where the results for additional data collection at room temperature (CCDC-870693) are also shown for comparison.

Fig. 3
figure 3

The plot of the residual electron density across the O···O direction of the anions A (left) and B (right) at room temperature (top) and at 100 K (bottom). The hydrogen atoms were removed, the contours are drawn every 0.1e Å−3 Maximum and minimum values of the electron density are 0.53 and −0.19e Å−3

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Interestingly, these differences seem to be correlated with the geometrical features of both anions. Anion A (with more “symmetrical” hydrogen bond) is almost ideally planar, maximum deviation from the mean plane calculated for all non-hydrogen atoms is only 0.0373(8) Å and the COO groups make the dihedral angles of 2.43(18)° and 1.44(20)° with perfectly planar (within 00035(8) Å) C–C=C–C fragment. In contrast, anion B is significantly twisted: the dihedral angles between COO groups and planar (0.0024(9) Å) carbon skeleton are as large as 9.4(2)° and 12.54(15)°.

Crystal Packing

In the crystal, the “building block” consists of a dication and two anions connected by the strong, linear hydrogen bonds (cf. Fig. 1; Table 2) that involve two N–H groups of cation (different nitrogen atoms) and two oxygen atoms from two anions, not involved in intramolecular hydrogen bonding. These blocks are connected by means of another N–H···O hydrogen bond and quite strong C–H···O bond into the infinite chains along y direction (Fig. 4; Table 2).

Table 2 Short contact data (Å, °); CgA denotes the middlepoint of the ring C11···C16, and CgB – of the ring C21···C26
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Fig. 4
figure 4

A ribbon of the cations and anions as seen approximately along x direction (y across, z down) [14]; hydrogen bonds are shown as dashed lines

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This figure shows that the anion A connects two cations by means of N–H···O hydrogen bonds, while anion B by one N–H···O and one C–H···O interactions. These chains are then connected by weaker contacts (C–H···O, C–H···π and C–H···F) into three-dimensional crystal structure (Fig. 5; Table 2).

Fig. 5
figure 5

Crystal packing of 1dim as seen along x-direction; hydrogen bonds and weak interactions (see text) are shown as dashed lines [14]

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Experimental

4,4′-Difluorobenzhydryl piperazine (2.88 g, 0.01 mol) in 10 ml of benzene was mixed with maleic acid (1.16 g, 0.01 mol) in 10 ml of toluene. Both the solutions were mixed and stirred in a beaker at 333 K for 30 min. The mixture was kept aside for a day at room temperature. The formed salt was filtered and dried in a vacuum desiccator over phosphorous pentoxide. The compound was recrystallized from a mixture of (1:1) ethanol and acetonitrile by slow evaporation (m.p.: 395–398 K).

X-ray diffraction data were collected at 130(1) K by the ω-scan technique, on an Agilent Technologies four-circle SuperNova diffractometer equipped with Atlas detector [12] equipped with Nova microfocus Cu Kα radiation source (λ = 1.54178 Å). The temperature was controlled with an Oxford Instruments Cryosystem device. The data were corrected for Lorentz-polarization effects as well as for absorption (multiscan) [12]. Accurate unit-cell parameters were determined by a least-squares fit of and 9393 reflections of highest intensity, chosen from the whole experiment. The calculations were mainly performed within the WinGX program system [13]. The structures were solved with SIR92 [14] and refined with the full-matrix least-squares procedure on F2 by SHELXL97 [15]. Scattering factors incorporated in SHELXL97 were used. The function \( \Upsigma {\text{w}}(\mid {\text{F}}_{\text{o}} \mid^{2} - \mid {\text{F}}_{\text{o}} \mid^{2} )^{2} \) was minimized, with w−1=[σ2(Fo)2 + (0.0411·P)2 + 1.544·P], where P = [Max (F 2o , 0) + 2F 2c ]/3. All non-hydrogen atoms were refined anisotropically; all hydrogen atoms were found in difference Fourier maps and isotropically refined. Relevant crystal data are listed in Table 3, together with refinement details.

Table 3 Crystallographic data and refinement details
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Crystallographic data (excluding structure factors) for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, No. 870694. Additionally, data for room temperature structure have been also deposited as CCDC 870693. Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Fax: +44(1223)336-033, e-mail:deposit@ccdc.cam.ac.uk, or www: www.ccdc.cam.ac.uk.