Gabapentin Coordination Networks: Mechanochemical Synthesis and

Article pubs.acs.org/crystal

Gabapentin Coordination Networks: Mechanochemical Synthesis and Behavior under Shelf Conditions Sílvia Quaresma,† Vânia André,*,† Alexandra M. M. Antunes,† Luís Cunha-Silva,‡ and M. Teresa Duarte*,† †

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal REQUIMTE & Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal

‡

S Supporting Information *

ABSTRACT: Active pharmaceutical ingredients (API) coordination complexes and networks present a promising pathway for developing new bioinspired materials. In the present study, we report several coordination networks of gabapentin with Y(III), Mn(II), and several lanthanides (LnCl3), Ln = La3+, Ce3+, Nd3+, Er3+ obtained by mechanosynthesis. To the best of our knowledge, these are among the first coordination networks of pharmaceuticals involving lanthanides. These novel compounds proved to be unstable under shelf conditions, are thermally stable until water release at approximately 80 °C, and decompose above 200−250 °C. The coordination networks obtained present different structural architectures based on mono-, di-, tri-, and hexa-metallic centers (herein called monomers, dimers, trimers, and hexamers), and also a one-dimensional polymeric chain was obtained. Gabapentin chelation modes are the same in most of the networks, adopting three typical geometries: the bidentate coordination − chelation, mode I; the bridge coordination, mode II, and the “bidentate-bridge” coordination, mode III. NMR studies show that the compounds have different behavior in solution, where a single coordination mode seems to be present.

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INTRODUCTION Extended metal−ligand networks with metal nodes and bridging organic ligands such as coordination networks, porous coordination networks (PCNs), porous coordination polymers (PCPs), and metal−organic frameworks (MOFs) have attracted great attention in the last few years.1−5 MOFs have generated a large interest owing to their versatile architectures6 and their promising applications in ion exchange, adsorption and gas storage7−12 separation processes,13 heterogeneous catalysis,14,15 polymerization reactions,16,17 luminescence,18 nonlinear optics,19 magnetism,20 drug carrier and delivery,21−24 and more recently as contrast agents for magnetic resonance imaging (MRI)25 and in other biomedical applications.21 The use of porous solids for biomedical applications requires a biological friendly composition, making compulsory the use of metals and linkers with acceptable toxicity.4 Most probably, the best approach to use these porous solids in biomedical applications, such as drug delivery, consists of introducing the therapeutic molecule directly as a linker to avoid unwanted effects such as low drug-storage capacity, too rapid delivery, and solid degradation. With this approach, no large pores are required, and the release of the drug molecule is achieved directly through the degradation of the solid, without any side effects arising from the release of a nonactive ligand.26,27 For their use as contrasting agents, a possible approach consists of linking the therapeutic molecule to a paramagnetic active metal such as Mn(II) or lanthanides (Ln), for instance, Gd(III). MRI is a powerful and noninvasive technique based on the detection of nuclear spin reorientations in a magnetic field and is used to differentiate diseased tissues from normal tissues © XXXX American Chemical Society

based on their varied NMR water proton signals, which occurs as a result of different water densities or nuclear relaxation rates.3,25 Combining bioactivity with imaging in these materials leads to their potential application as theranostics, that allow following drug delivery within the body through the imaging properties of the porous solid.3,27 Gabapentin (1-(aminomethyl)cyclohexane acetic acid, GBP) is a neuroleptic drug that is usually prescribed with other medications for the prevention of seizure, the treatment of mood disorders, anxiety, and tardive dyskinesia,28−35 as well as for the treatment of neuropathic pain.36 It has been widely used in polymorphic studies and screenings of multicomponent crystal forms (co-crystals and molecular salts) involving different carboxylic acid derivatives.37−41 It is also known to coordinate to metals such as Zn and Cu.42 With the aim of obtaining new coordination materials involving active pharmaceutical ingredients (API) as linkers and active paramagnetic metals, we report here the mechanochemical synthesis and structural characterization of coordination networks obtained by reacting GBP with Y(III), Mn(II), and several lanthanides (LnCl3), Ln = La3+, Ce3+, Nd3+, and Er3+. These compounds are unstable to moisture under shelf conditions, leading to several coordination networks that display a wide variety of structural architectures. Gabapentin adopts a common chelation pattern in the majority of the compounds obtained. Received: August 5, 2013 Revised: September 18, 2013

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dx.doi.org/10.1021/cg401187x | Cryst. Growth Des. XXXX, XXX, XXX−XXX

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then attempted in water, ethanol, acetonitrile, acetone, and chloroform. Single crystals of GBP:Mn1, [MnGBP2(H2O)2Cl2], were grown from the methanol solution after three weeks. GBP:Mn2, GBP:Mn3, and GBP:Mn4. The GBP:Mn1 powder has shown to be unstable evolving under shelf conditions to GBP:Mn2, after one week, to GBP:Mn3 and after two weeks and to GBP:Mn4 after one month. The powder diffraction pattern for GBP:Mn4 has shown that this form is stable for at least two months. GBP:Mn5. An equimolar mixture containing 0.2321 g (1.3554 mmol) of gabapentin and 0.2682 g (1.3552 mmol) of MnCl2·4H2O was manually ground together in an agate mortar for 5 min. A powder, compound GBP:Mn5, was obtained and analyzed by PXRD. The new product is formed without any traces of the reagents used. Recrystallization was then attempted in water, ethanol, acetonitrile, acetone, and chloroform. Single crystals of GBP:Mn5, [Mn3GBP4 (H2O)2Cl6], were grown from the water solution after three weeks. GBP:Mn6. After one month, single crystals of GBP:Mn6, [MnGBP2(H2O)2Cl2], were grown from the methanol solution in which the recrystallization of GBP:Mn5 was attempted. X-ray Crystallography. Single-Crystal X-ray Diffraction (SCXRD). Bruker AXS-KAPPA APEX II diffractometer with graphitemonochromated radiation (Mo Kα, λ = 0.71069 Å) at 150 K. The X-ray generator was operated at 50 kV and 30 mA, and the X-ray data collection was monitored by the APEX243 program. All data were corrected for Lorentzian, polarization, and absorption effects using SAINT43 and SADABS44 programs. Crystals suitable for X-ray diffraction study were mounted on a loop with Fomblin protective oil. SIR9745 and SHELXS-97 were used for structure solution, and SHELXL-9746 was used for full matrix least-squares refinement on F2. These three programs are included in the package of programs WINGX-Version 1.80.05.47 A full-matrix least-squares refinement was used for the non-hydrogen atoms with anisotropic thermal parameters. All the hydrogens were inserted in idealized positions and allowed to refine riding in the parent carbon atom, except the nitrogen and water hydrogen atoms that were not placed due to the impossibility of finding them in the electron density map. MERCURY 3.048 was used for packing diagrams. PLATON49 was used for hydrogen bond interactions. Table 1 summarizes crystallographic details for GBP:Er1, GBP:Nd2, GBP:Y2, GBP:Y3, GBP:Ce1, GBP:Ce2, GBP:La2, GBP:Mn1, GBP:Mn5, and GBP:Mn6. Powder X-ray Diffraction (PXRD). Data were collected in a D8 Advance Bruker AXS θ-2θ diffractometer, with a copper radiation source (Cu Kα, λ = 1.5406 Å) and a secondary monochromator, operated at 40 kV and 30 mA. The program Mercury 3.048 was used for calculation of X-ray powder patterns on the basis of the single crystal structure determinations. The identity of single crystals and the bulk material obtained from solution and grinding experiments was always verified by comparison of the calculated and observed X-ray powder diffraction patterns. Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). Combined TG-DSC measurements were carried out on a SETARAM TG-DTA 92 thermobalance under nitrogen flow with a heating rate of 10 °C·min−1. The samples weights were in the range of 5−10 mg. Solubility Studies by High Performance Liquid Chromatography (HPLC). were carried out in a Dionex system equipped with an Ultimate 3000 pump and a photodiode detector (DAD, Ultimate 3000). A reverse phase column RP-18e (Luna C18(2), 250 × 4.6 mm, 5 μm) from Phenomenex was used with a flow of 1 mL·min−1. The selected program consisted on a 32 min linear gradient of 5−70% of acetonitrile in 1% of aqueous formic acid, followed by a 8 min linear gradient of 100% acetonitrile, with a flow of 1 mL·min−1. Solution NMR. 1H NMR spectra of GBP:Ce1, GBP:Nd1, GBP:Y1, GBP:La1, GBP:Mn1, GBP:Er1, and GBP:Er2 complexes were recorded on Bruker Avance III 500 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) operating at 500 MHz.13C NMR spectra were recorded on the same instrument, operating 125.77 MHz. Chemical shifts are reported in ppm. The 1H NMR and 13C NMR spectra were recorded in D2O solution at room temperature.

EXPERIMENTAL SECTION

Synthesis. All reagents and solvents were acquired from SigmaAldrich and used without modification. The stoichiometric ratio of the starting reagents has been shown to influence only the reaction products obtained with Mn; the remaing were consistently the same. Synthesis of GBP:Er Complexes. GBP:Er1. 0.2260 g (1.3198 mmol) of gabapentin and 0.3041 g (0.7967 mmol) of ErCl3·6H2O were manually ground together in an agate mortar for 10 min. A wet powder, compound GBP:Er1, was obtained and analyzed by powder X-ray diffraction (PXRD). No traces of reagents were detected. The sample was left under shelf conditions, and after two weeks single crystals of GBP:Er1, [Er2GBP6(H2O)2Cl2]Cl4·8H2O, suitable for single crystal X-ray diffraction (SCXRD) had formed in the rim of the powder sample. GBP:Er2. The powder sample of GBP:Er1 was left under shelf conditions, and after two months the power diffraction pattern showed that it had evolved to a different form, GBP:Er2, which is maintained for several months. Synthesis of GBP:Nd Complexes. GBP:Nd1. 0.2442 g (1.4261 mmol) of gabapentin and 0.2554 g (0.7120 mmol) of NdCl3·6H2O were manually ground together in an agate mortar for 5 min. A wet powder, compound GBP:Nd1, was obtained and analyzed by PXRD and was shown to be a new product without any traces from reagents. GBP:Nd2. The powder sample of GBP:Nd1 was left under shelf conditions, and after two weeks single crystals of GBP:Nd2, [Nd2GBP6(H2O)2Cl2]Cl4.8H2O, suitable for SCXRD formed in the rim of the powder sample. Synthesis of GBP:Y Complexes. GBP:Y1. 0.3010 g (1.8103 mmol) of gabapentin and 0.2661 g (0.8722 mmol) of YCl3·7H2O were manually ground together in an agate mortar for 10 min. A wet powder, compound GBP:Y1, was obtained and analyzed by PXRD, showing that the product is new and without any trace from the reagents used. Despite all the efforts, we were not able to grow single crystals suitable for SCXRD. GBP:Y2. The powder sample of GBP:Y1 was left under shelf conditions, and after two weeks some very small crystals of GBP:Y2, [Y2GBP6(H2O)2Cl2]Cl4·8H2O, suitable for SCXRD had formed in the rim of the powder sample. GBP:Y3. Recrystallization of GBP:Y1 in a water/ethanol solution with seeding was carried out and single crystals of GBP:Y3, [Y3GBP8(H2O)5]Cl9·10H2O, suitable for SCXRD were grown after one month. Synthesis of GBP:La Complexes. GBP:La1. 0.2774 g (1.6199 mmol) of gabapentin and 0.362 g (0.9758 mmol) of LaCl3·7H2O were manually ground together in an agate mortar for 10 min. A wet powder, compound GBP:La1, was obtained and analyzed by PXDR and was shown to be a new product without any traces of the reagents. GBP:La2. The powder sample of GBP:La1 was left under shelf conditions, and after three months the moisture in the sample had increased and some small crystals of GBP:La2, [La3GBP10(H2O)4]Cl9· 12H2O, suitable for SCXRD formed in the viscous bulk (see Figure S6, Supporting Information). Synthesis of GBP:Ce Complexes. GBP:Ce1. 0.2313g (1.3507 mmol) of gabapentin and 0.2627 g (0.7051 mmol) of CeCl3·6H2O were manually ground together in an agate mortar for 5 min. A wet powder, compound GBP:Ce1, was obtained and analyzed by PXRD and was also shown to be free of any reagent residuals. Recrystallization was attempted in water, ethanol, acetonitrile, acetone and chloroform. Single crystals of GBP:Ce1, [Ce6GBP16(H2O)10Cl4]Cl14·20H2O, were grown from the water solution after two months. GBP:Ce2. The powder sample of GBP:Ce1 was left under shelf conditions, and after one month some very small crystals of GBP:Ce2, [Ce3GBP10(H2O)4]Cl9·12H2O, suitable for SCXRD formed in the rim of the powder sample. Synthesis of GBP:Mn Complexes. GBP:Mn1. A 2:1 mixture containing 0.3160 g (1.8454 mmol) of gabapentin and 0.1849 g (0.9343 mmol) of MnCl2·4H2O were manually ground together in an agate mortar for 10 min. A powder, compound GBP:Mn1, was obtained and analyzed by PXRD. No trace of reagents was detected. Recrystallization was B


GBP:Nd2

GBP:Y2

GBP:Y3

GBP:Ce1

GBP:Ce2

GBP:La2

GBP:Mn1

GBP:Mn5

GBP:Mn6

chemical formula [Er2GBP6(H2O)2 [Nd2GBP6(H2O)2 [Y2GBP6(H2O)2Cl2] [Y3GBP8(H2O)5] [Ce6GBP16(H2O) [Ce3GBP10(H2O)4] [La3GBP10(H2O)4] [MnGBP2(H2O)2Cl2] [Mn3GBP4(H2O)2Cl6] [MnGBP2(H2O)2Cl2] Cl2]Cl4·8H2O Cl2]Cl4 ·8H2O Cl4·8H2O Cl9·10H2O 10Cl4]Cl14· Cl9·12H2O Cl9·12H2O 20H2O formula weight 1716.49 1670.45 1559.79 4326.95 4650.23 2673.50 2673.90 494.26 1082.37 494.26 temperature (K) 150 150 150 150 150 150 150 150 150 150 wavelength (Å) 0.71069 0.71069 0.71009 0.71069 0.71069 0.71069 0.71069 0.71069 0.71069 0.71069 crystal form, color plate, colorless plate, colorless plate, colorless block, colorless block, colorless block, colorless plate, colorless plate, colorless needle, colorless plate, colorless crystal size (mm) 0.03 × 0.025 × 0.05 × 0.03 × 0.01 0.06 × 0.03 × 0.025 0.03 × 0.01 × 0.18 × 0.06 × 0.18 × 0.06 × 0.02 0.02 × 0.02 × 0.01 0.5× 0.2 × 0.2 0.5× 0.2 × 0.2 0.3× 0.3 × 0.2 0.01 0.01 0.02 crystal system triclinic triclinic triclinic triclinic triclinic monoclinic monoclinic monoclinic monoclinic monoclinic P21 C2/c C2/c C2/c P1̅ C2/c P1̅ P1̅ P1̅ P1̅ space group 11.4370(6) 11.5150(10) 11.4430(15) 13.138(2) 14.1581(5) 35.1640(7) 35.1400(8) 36.627 (10) 36.627 (10) 5.927(6) a (Å) b (Å) 13.0290(7) 13.0920(16) 13.0220(19) 16.849(3) 18.1097(6) 12.2270(8) 12.186(2) 6.3860(16) 6.3860(16) 31.84(3) 13.6880(7) 13.6880(13) 13.682(2) 24.074(3) 21.740(6) 29.8690(6) 29.8470(10) 20.670(4) 20.670(4) 6.549(7) c (Å) 96.288(3) 96.563(7) 96.398(8) 77.338(7) 102.173(5) 90.00 90.00 90.00 90.00 90.00 α (Å) β (°) 109.889(3) 109.958(6) 109.892(8) 80.725(8) 91.760(6) 107.908(4) 107.908(7) 93.424(16) 93.424(16) 112.87(2) γ (°) 99.060(3) 99.020(7) 98.990(7) 75.877(6) 109.364(3) 90.00 90.00 90.00 90.00 90.00 V (Å3) 1864.65(17) 1897.9(4) 1863.6(5) 5009.6(13) 5109.9(14) 12213.7(9) 12162(2) 4826(2) 4826(2) 1139(2) Z 1 1 1 1 1 4 4 4 4 2 d (mg·cm−3) 1.529 1.462 1.390 1.434 1.511 1.454 1.460 1.490 1.490 1.442 2.519 1.632 1.834 2.038 1.623 1.372 1.309 1.159 1.159 0.849 μ (mm−1) F(000) 864 848 806 2228 2350 5444 5448 2228 2228 514 1.28−29.990 1.97−26.66 1.97−26.66 2.40−28.42 2.40−25.52 1.60−26.41 2.21−25.96 2.51−25.42 1.92−28.02 1.61−27.94 θ range (°) 27359/8928 22497/999 14793/6678 45707/18839 65823/20386 48146/11221 97353/15185 23394/4839 23394/4839 11956/6012 reflections collected/unique Rint 00746 0.0845 0.1437 0.1449 0.0700 0.1114 0.0540 0.3174 0.3174 0.0964 GOF 0.939 0.981 0.871 0.857 1.045 1.088 1.222 1.034 1.034 0.916 final R indices [I > R1 = 0.0366, wR2 R1 = 0.0575, wR2 = R1 = 0.0756, wR2 = R1 = 0.0729, wR2 R1 = 0.0665, wR2 R1 = 0.0795, wR2 = R1 = 0.0741, wR2 = R1 = 0.1436, wR2 = R1 = 0.1436, wR2 = R1 = 0.0630, wR2 = 2σ(I)] 0.1078 0.0908 0.1676 01637 0.3299 0.3299 0.1284 = 0.0774 = 0.1382 = 0.1608 indices all data R1 = 0.0591, wR2 R1 = 0.1049, wR2 = R1 = 0.1916, wR2 = R1 = 0.2249, wR2 R1 = 0.1064, wR2 R1 = 0.1367, wR2 = R1 = 0.0891, wR2 = R1 = 0.2624, wR2 = R1 = 0.2624, wR2 = R1 = 0.1224, wR2 = 0.1192 0.1127 0.1816 0.1679 0.3749 0.3749 0.1583 = 0.0935 = 0.1679 = 0.1833

GBP:Er1

Table 1. Crystallographic Details for GBP:Er1, GBP:Nd2, GBP:Y2, GBP:Y3, GBP:Ce1, GBP:Ce2, GBP:La2, GBP:Mn1, GBP:Mn5, and GBP:Mn6

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Figure 1. Schematic view of the compounds obtained by mechanochemical synthesis with GBP and Ce, La, Er, Nd, Y, and Mn. Compounds represented with the same color (except black) are isostructural (t = time and Recx = recrystallization). Whenever possible resonance assignments were based on the analysis of coupling patterns, including the 13C−1H coupling profiles obtained in bidimensional heteronuclear multiple bond correlation (HMBC), heteronuclear single quantum coherence (HSQC) and correlation spectroscopy (COSY) experiments, performed with standard pulse programs. GBP 1H NMR (D2O) δ: 2.97 (2H, s, H1″), 2.39 (2H, s, H2), 1.46 (6H, sl, H2′/H6′ + H3′ + H5′), 1.36−1.34 (4H, m, H2′/H6′ + H4′); 13 C NMR (D2O) δ: 180.7 (C1), 48.3 (C1″), 45.9 (C2), 34.2 (C1′), 33.3 (C2′ + C6′), 25.2 (C4′), 20.8 (C3′+C5′). GBP:Nd1 1H NMR (D2O) δ: 4.07 (2H, s, H2), 3.60 (2H, s, H1″), 2.02 (2H, sl, H2′/H6′), 1.83−1.79 (4H, m, H2′/H6′ + H3′/H5′), 1.70−1.69 (2H, m, H3′/H5′), 1.58 (2H, m, H4′); 13C NMR (D2O) δ: 154.6 (C1), 62.8 (C2), 48.6 (C1″), 34.8 (C1′), 33.9 (C2′ + C6′), 25.4 (C4′), 21.2 (C3′ + C5′). GBP:Y1 1H NMR (D2O) δ: 3.00 (2H, s, H1″), 2.40 (2H, s, H2), 1.46 (6H, sl, H2′/H6′ + H3′ + H5′), 1.37−1.35 (4H, m, H2′/H6′ + H4′); 13C NMR (D2O) δ: 182.4 (C1), 48.0 (C1″), 45.4 (C2), 34.3 (C1′), 33.2 (C2′ + C6′), 25.1 (C4′), 20.8 (C3′+C5′). GBP:La1 1H NMR (D2O) δ: 3.00 (2H, s, H1″), 2.45 (2H, s, H2), 1.48 (6H, sl, H2′/H6′ + H3′ + H5′), 1.39−1.38 (4H, m, H2′/H6′ + H4′); 13C NMR (D2O) δ: 182.0 (C1), 48.3 (C1″), 46.1 (C2), 34.3 (C1′), 33.3 (C2′ + C6′), 25.2 (C4′), 20.8 (C3′+C5′). GBP:Ce1 1H NMR (D2O) δ: 3.23 (2H, s, H1″), 2.92 (2H, s, H2), 1.69−167 (2H, sl, H2′/H6′), 1.61−1.50 (6H, m, H2′/H6′ + H3′ + H5′), 1.45−1.43 (2H, m, H4′); 13C NMR (D2O) δ: 180.7 (C1), 48.4 (C1″), 47.8 (C2), 34.5 (C1′), 33.5 (C2′ + C6′), 25.3 (C4′), 20.9 (C3′ + C5′). GBP:Er1 13C NMR (D2O) δ: 210.8 (C1), 47.5 (C1″), 35.0 (C1′), 33.2 (C2′ + C6′), 25.4 (C4′), 21.0 (C3′ + C5′), 18.8 (C2). GBP:Er2 13C NMR (D2O) δ: 213.7 (C1), 47.5 (C1″), 35.1 (C1′), 33.2 (C2′ + C6′), 25.5 (C4′), 21.1 (C3′ + C5′), 15.0 (C2). GBP:Mn1 13C NMR (D2O) δ: 213.7 (C1), 47.0 (C1″), 35.1 (C1′), 33.2 (C2′ + C6′), 23.8 (C4′), 19.6 (C3′ + C5′).

Nd(III), Y(III), La(III), Ce(III), and Mn(II) salts (Figure 1). Structural characterization using PXRD and SCXRD analysis shows that the coumpounds evolve with time and are unstable to moisture. Some of the novel compounds with Nd, Er, and Y are isostrucutural: GBP:Er1 = GBP:Nd2 = GBP:Y2, and GBP:Er2 = GBP:Nd1 (Figure 1). Also with La and Ce some isostructural networks were attained: GBP:La1 = GBP:Ce1, and GBP:La2 = GBP:Ce2 (Figure 1). Some of the samples were impossible to characterized by SCXRD, despite all the efforts to obtain single crystals that could reproduced the bulk. In the following section, we describe the results obtained by X-ray diffraction techniques. DSC, TGA, and HPLC studies is illustrated and discussed. A detailed structural description is presented and discussed. PXRD Data Analysis. Two different GBP:Er complexes were isolated, GBP:Er1 and GBP:Er2 (Figure 2), the first one converting into the latter after two months under shelf conditions. Despite all the efforts, it was only possible to grow crystals suitable for SCXRD for GBP:Er1 complex that was shown to be [Er2GBP6(H2O)2Cl2]Cl4·8H2O. The simulated powder diffraction pattern confirmed that the bulk of the material is GBP:Er1 (Figure S1, Supporting Information). A starting complex isolated from the reaction of gabapentin with neodymium, GBP:Nd1, evolved to a second form, GBP:Nd2, from which suitable crystals for SCXRD formed as [Nd2GBP6(H2O)2Cl2]Cl4·8H2O. Interestingly, GBP:Nd2 is isostructural with GBP:Er1, and the powder patterns of GBP:Nd1 and GBP:Er2 indicate that they are also isostrucutural (Figure 2). Time seems to promote the stabililization into opposite directions: the most unstable GBP:Er1 is isostructural with the most stable GBP:Nd2 (Figure S2, Supporting Information). As for yttrium, three different GBP:Y networks were obtained: the starting form GBP:Y1 yielded GBP:Y2 crystals after a few

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RESULTS AND DISCUSSION A series of new coordination complexes and networks were obtained by manually grinding together GBP with Er(III), D



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GBP:Er and GBP:Nd, in these experiments the evolution under shelf conditions has led to identically stable networks. In the case of manganese, starting from a 2:1 mixture of GBP and Mn, an initial complex was obtained, GBP:Mn1, that converted to GBP:Mn2 after one week, to GBP:Mn3 after two weeks, and to GBP:Mn4 after one month. Despite all efforts, it was only possible to grow crystals suitable for SCXRD for GBP:Mn1 that was identified as cis-[MnGBP2(H2O)2Cl2] (Figure 4).

Figure 2. Calculated PXRD patterns obtained from SCXRD data of GBP:Y3, GBP:Er1, GBP:Nd2, and GBP:Y2, compared with the experimental diffractograms obtained for GBP:Y1, GBP:Er1, GBP:Er2 and GBP:Nd1, showing that GBP:Er1, GBP:Nd2, and GBP:Y2 are isostructural as well as GBP:Er2 and GBP:Nd1. It can also be ascertained that the single crystal structure obtained for GBP:Er1 represents the bulk product.

weeks under shelf conditions, while its recrystallization in a blend of water and ethanol yielded GBP:Y3 (Figure 2). GBP:Y2 corresponds to a [Y2GBP6(H2O)2Cl2]Cl4·8H2O dimeric complex, while GBP:Y3 is one-dimensional (1D) network based on [Y3GBP8(H2O)5]Cl9·10H2O. Furthermore GBP:Y2 has proved to be isostructural with GBP:Er1 and GBP:Nd2 (Figure 2). Comparing the powder diffraction patterns, we can state that the starting material GBP:Y1, from which it is impossible to grow single crystals suitable for SCXRD, is a completely different form with no evidence of the presence of GBP:Y2 or GBP:Y3 (Figure S3, Supporting Information). With lanthanum, also the starting complex GBP:La1 yielded single crystals of a second form GBP:La2, [La3GBP10(H2O)4]Cl9·12H2O, under shelf conditions (Figures 3 and S4).

Figure 4. Calculated PXRD patterns obtained from SCXRD data of GBP:Mn1, compared with the experimental diffractograms obtained for GBP:Mn1, GBP:Mn2, GBP:Mn3, and GBP:Mn4.

Grinding equimolar mixtures of GBP and Mn results in complex GBP:Mn5 that when recrystallized in methanol led to GBP:Mn6, and in this case it was possible to determine the crystal structure for both forms. GBP:Mn5 has shown to be a trimeric network [Mn3GBP4μ-Cl4(H2O)2Cl2] that evolved into GBP:Mn6, trans-[MnGBP2(H2O)2Cl2], during recrystallization (Figure 5). No experimental powder diffraction pattern was collected for GBP:Mn6 as only very few crystals were grown.

Figure 3. Calculated PXRD patterns obtained from SCXRD data of GBP:Ce2, GBP:La2, and GBP:Ce1, compared with the experimental diffractograms obtained for GBP:Ce1 and GBP:La1. Confirming that compounds are isostructural.

Figure 5. Calculated PXRD patterns obtained from SCXRD data of GBP:Mn6 and GBP:Mn5 compared with the experimental diffractograms obtained for GBP:Mn5.

In the case of cerium, once again two different complexes were isolated: GBP:Ce1 and GBP:Ce2, but in this case it was possible to determine the crystal structure for both products showing that we have an hexametric network [Ce6GBP16(H2O)10Cl4]Cl14·20H2O in GBP:Ce1 and a trimeric one [Ce3GBP10(H2O)4]Cl9·12H2O for GBP:Ce2. Comparing the powder diffractograms, it is possible to conclude that GBP:Ce1 is isostructural with GBP:La1, and GBP:Ce2 is isostructural with GBP:La2 (Figure 3). In contrast to what happens with

Structural Elucidation of the GBP Complexes and Networks. Gabapentin is known to exist in its zwitterionic form,40,50,51 and it behaves as such in all the molecular structures reported herein. This was proved by the C−O distances and charge balance, since the location of the three hydrogen atoms around the N atoms was not possible to be accurately determined. Furthermore, excluding GBP:Mn1 and GBP:Mn6 complexes, GBP coordinates to the metal/lanthanide using at least one of this three coordination modes (Figure 6a,b): the E



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bidentate coordination − chelation, mode I (purple); the bridge coordination, mode II (green); and the “bidentatebridge” coordination, mode III (orange). All the compounds (Figure 6c) present a GBP intramolecular hydrogen bond

Figure 8. (a) Detailed supramolecular packing of GBP:Er1 showing that chloride ions (pink) and water (red) forming “cubic-like” bridges between dimers; (b) detailed supramolecular packing in a view along a showing that the dimers grow through N−H···Cl···Ow−H···OGBP. H atoms were omitted for clarity.

Isostructural GBP:Ce1 and GBP:La1 Networks, [Ln6GBP16(H2O)10Cl4]Cl14·20H2O. In compound GBP:Ce1, that proved to be isostructural to GBP:La1 by PXRD, GBP presents the referred three binding modes, but in this case the networks are based on hexamers where once again the chelating mode I of GBP ends the repeating unit. In this network, Ce ions have different coordination numbers and geometries: Ce1 and Ce3 have a CN 9, presenting a distorted trigonal prismatic geometry, while Ce2 has CN 8, displaying a distorted octahedral geometry (Figure 9a). It is also possible to observe that the structure packs in a similar way to the previously discussed, with the chloride ions and water molecules lying in channels between the hexameric moieties (Figure 9b). The networks grow along the a axis through interactions between chloride ions, water, and GBP (Figure 9b) and along the c axis through N−H···O and N−H···Cl contacts (Figure 9c). Isostructural GBP:Ce2 and GBP:La2 Networks, [Ln3GBP10(H2O)4]Cl9·12H2O. Isostructural compounds GBP:Ce2 and GBP:La2 form trimers with gabapentin exhibiting the three coordination modes. In these networks, terminal Ln ions display a distorted bicapped square antiprismatic coordination geometry (CN 10), obtained through the binding modes I, II, and III completed by the coordination to two water molecules, while a distorted octahedral geometry (CN 8) around the Ln central ion is achieved through modes II and III. Similarly to the previous structures, chelating mode I acts as the ending of the repeating unit (Figure 10a). The packing arrangement is again based on the hydrogen bonding system between the trimers unities and the chloride ions and water molecules (Figure 10b). The networks grow along both the a and c axis as depicted in Figure 10b and S7. Compound GBP:Y3, [Y3GBP8(H2O)5]Cl9·10H2O. For compound GBP:Y3 an infinite 1D chain based on [Y3GBP8(H2O)5]n growing along c is obtained, wherein GBP does not present chelation mode I. The asymmetric unit presents three independent Y3+ ions, where the full distorted square antiprismatic (CN 8) is attained using binding modes II and III of GBP and water molecules that complete the metal coordination sphere (Figure 11a). Once again water molecules and chloride ions lye in the space between the GBP:Y chains (Figure 11b). Compound GBP:Mn1, cis-[MnGBP2(H2O)2Cl2]. Complex GBP:Mn1 presents two GBP molecules coordinating to Mn through only one of the oxygen atoms of the carboxylate group. Water molecules and chloride ions fulfill the metal coordination

Figure 6. (a) Scheme presenting the binding modes; (b) asymmetric unit in GBP:La2, where the three different coordination modes are illustrated: mode I (purple), mode II (green), and mode III (orange). GBP moieties are only fully shown once for each mode; (c) view of GBP:Mn1, illustrating the intramolecular interaction of GBP (blue dashed line) is illustrated. H atoms were omitted for clarity.

resembling GBP polymorphic form IV;40,51 the only exception is found in 1D network GBP:Y3. Isostructural GBP:Er1, GBP:Nd2, and GBP:Y2 Networks, [Ln2GBP6(H2O)2Cl2]Cl4·8H2O. In these isostructural networks, gabapentin presents the three binding modes, giving rise to a dimeric complex, in which the metal/Ln center exhibits a coordination number (CN) 9, in a distorted tricapped trigonal prismatic geometry, and the chelating mode I of GBP ends the repeating unit (Figure 7a). Dimeric units form chains through

Figure 7. (a) View of GBP:Er1 dimer. Water molecules (red) and chloride ions (pink) fulfill the metal coordination sphere; (b) simplified packing in a view along c, showing the alternated layers of chloride ions (pink) and water molecules (red) lying in the space between the chains. H atoms were omitted for clarity.

hydrogen bonding with chloride ions and water molecules (Figure 7b). The networks grow along the c axis through hydrogen bonds forming “cubic-like” bridges between dimers (Figure 8a) and along the b axis through hydrogen interactions of the type N−H···Cl···Ow−H···OGBP (Figures 8b and S5). F



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Figure 11. (a) View of GBP:Y3 where we can observe the infinite 1D chain based on Y1−Y2−Y3−Y3−Y2−Y1−Y1−Y2−Y3 sequences. Water molecules (red) complete the coordination of the metal centers. GBP moieties are only fully shown once for each mode; (b) simplified view of the layered packing along b, showing the 1D infinite chains and the space left for chloride ions (pink) and water molecules (red). H atoms were omitted for clarity. Figure 9. (a) View of GBP:Ce1 hexamer, where the coordination of Ce3+ ions alternates as Ce1, Ce2, Ce3, Ce3, Ce2, and Ce1. Chloride ions (pink) and water molecules (red) complete the coordination environments. GBP moieties are only fully shown once for each mode; (b) view along b of the supramolecular packing, showing the chloride ions (pink) and water (red) alternated layers; (c) detailed supramolecular packing showing that the hexamers grow through N−H···O (black dashed line) and N−H···Cl (red dashed line). H atoms were omitted for clarity.

Compound GBP:Mn5, [Mn3GBP4μ-Cl4(H2O)2Cl2]. In compound GBP:Mn5, the network is based in a trimeric metallic fragment with two terminal GBP molecules coordinated to Mn through one of the oxygen of the carboxylate group and two GBP molecules coordinated to Mn through binding mode II. Chloride ions bridge the two metallic centers and fulfill the metal coordination sphere of the terminal Mn along with water molecules. All the metallic centers exhibit an octahedral geometry (CN 6) (Figure 13a). In this compound, the network grows along c through Ow− H···Cl interactions, forming tapes of hydrogen-bonded trimers (Figure 13b). There are no contacts between sequential tapes where GBP molecules align in a head-to-head fashion (see Figure S9, Supporting Information). Compound GBP:Mn6, trans-[MnGBP2(H2O)2Cl2]. As in GBP:Mn1, GBP:Mn6 is a complex with two GBP coordinated to Mn through one of the oxygen of the carboxylate group. Water molecules and chloride ions fulfill the metal coordination sphere in a trans configuration, exhibiting an octahedral geometry (CN 6) (Figure 14a). This network grows along a through N−H···O hydrogen bonds between gabapentin, and also N−H···Cl and Ow−H···Cl interactions (Figure 14b), forming chains of hydrogen-bonded monomers (see Figure S10).

Figure 10. (a) View of GBP:La2 trimer. Water molecules (red) fulfill the metal coordination needs. GBP moieties are only fully shown once for each mode. (b) Simplified packing in a view along b depicting the chains of the trimeric repeating units and water molecules (red) and chloride ions (pink) lying in the space between chains. H atoms were omitted for clarity.

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sphere in a cis configuration, completing the slightly distorted octahedral coordination (CN 6) polyhedra (Figure 12a). The supramolecular arrangement shows tapes forming along b through N−H···O and OW−H···Cl interactions (Figure 12b). These tapes have no contact between them, and gabapentin packs in a head-to-head fashion (see Figure S8, Supporting Information).

GENERAL STRUCTURAL CONSIDERATIONS The analysis of the coordination distances show that for mode II both M−O distances are very similar, while in mode I both distances usually differ by 0.1−0.2 Å (Table S15, Supporting Information). In mode III, which somehow resembles both modes I and II, we can depict different M−O distances, that for G



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Figure 12. (a) View of GBP:Mn1 monomer. Water molecules (red) and chloride ions (pink) fulfill the metal coordination sphere. H atoms and intramolecular interactions were omitted for clarity. (b) Simplified packing in a view along a showing that the network grows along b through N−H···O between gabapentin and OW−H···Cl interactions. H atoms were omitted for clarity.

Figure 14. (a) View of GBP:Mn6 monomer. Water molecules (red) and chloride ions (pink) fulfill the metal coordination sphere; (b) simplified packing showing that the network grows along a through N−H···O interactions between gabapentin, as well as N−H···Cl and Ow−H···Cl. H atoms were omitted for clarity.

Scheme 1. Detailed Scheme of Mode III, Illustrating the a, b, and c Distances

shows that the distances follow the trend: Mode II < Mode IIIc < Mode IIIa ≅ Mode I < Mode IIIb. The analysis of the distances should also take into account the metal used, particularly its atomic radii, where as expected smaller radii correspond to smaller M−O distances (Mn < Y < Er < Nd < Ce < La). All the distances and angles are within the expected values based on a search in the Cambridge Structural Database52 (see Tables S12−14, Supporting Information). A deepest insight of the structural parameters shows that, except in the isostructural GBP:Ce2 and GBP:La2, the intramolecular interaction promotes shorter M−O coordination distances, either when considering coordination modes I, II, or III, where both carboxylate oxygens are linked to the metal, or when looking at the manganese structures, where the coordination is established only through the oxygen used in the intramolecular hydrogen bond. This conclusion is reinforced by the fact that GBP:Zn and GBP:Cu complexes previously reported by Braga and co-workers42 present a similar behavior. In general the coordination spheres present distorted geometries due to the constraints imposed by the coordination modes, especially in modes I and III where the bidentate chelation inflicts major distortions. Thermal Characterization. The thermal characterization of these complexes is not straightforward due to their high

Figure 13. (a) View of GBP:Mn5 trimer where the green represent the binding mode II of GBP. Water molecules (red) and chloride ions (pink) fulfill the metal coordination needs; (b) simplified packing showing that the network grows along c through interactions Ow−H··· Cl between the water molecules and chloride ions in the coordination sphere, forming chains of hydrogen-bonded trimers. H atoms were omitted for clarity.

a better understanding will be divided into a, b, and c (Scheme 1). As previously described, most of the complexes present at least one of the coordination modes I, II, and III. An overall comparison of the different modes in all the metals studied H



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Figure 15. 1H NMR spectra recorded in D2O at room temperature of GBP, and complexes GBP:Ce1, GBP:Nd1, GBP:Y1, GBP:La1, GBP:Mn1, GBP:Er1, and GBP:Er2. The resonance assignment of GBP unit is also presented.

of complexes when compared with GBP are observed in carbons C1 and C2, which is consistent with the carboxylate group as the coordination site (see Supporting Information for further details).

hygroscopicity, in addition to their high hydration level and water coordinated to the metal. Ideally, moisture water should be the first to be released then the crystallization water and the water coordinated to the metal should be the last; however, we are aware that these phenomena may overlap. Indeed, DSC and TGA data for most of these coordination complexes show the moisture being released until approximately 80−90 °C, temperature after which both crystallization and coordinated water are released. Above 200−250 °C melting/decomposition are detected (see Supporting Information for further details). Solubility Studies by HPLC Analysis. Solubilities of the all the novel compounds in PBS buffer solutions (0.1 M, pH = 7.4) were studied by HPLC with UV/vis detector. All the measurements indicate that the bulk materials present a much lower solubility than GBP, even though we were unable to quantify it, due to the instability of the compounds. Solution NMR. Whereas the 1H NMR spectra of the diamagnetic GBP:La1 and GBP:Y1 complexes exhibit only subtle differences in comparison with GBP spectrum (Figure 15), for the paramagnetic complexes GBP:Nd1 and GBP:Ce1 a marked downfield shift of H2 and H1″ protons was clearly observed. Additionally, these two paramagnetic complexes exhibit two distinct signals for protons H3′ and H5′ of cyclohexyl unit, clear evidence of their magnetic anisotropy. However, the large line broadening observed on the 1H NMR spectra of Er and Mn complexes, caused by the paramagnetic effect on the relaxation times, precluded any further structural information by this technique. Taking into consideration that the paramagnetic effect on the relaxation times causes a line broadening on 13C NMR spectra considerable smaller than in the 1H NMR spectra, the 13C NMR chemical shifts were only used to monitor the complexation of GBP. These suggest that the compounds in solution present only one type of coordination to the metal, which contrasts with the different coordination modes observed in the solid-state structures determined by X-ray diffraction. Nonetheless, the most noticeable differences between the carbon resonances

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CONCLUSIONS New complexes of GBP coordinated to lanthanides, Y(III) and Mn(II) were obtained, showing the good affinity of gabapentin to react with these metals. All the solids were obtained using mechanochemistry, a “green” and environment-friendly synthetic technique. The majority of the complexes reported are highly hygroscopic and have been shown to evolve with time under shelf conditions. No conclusions could be drawn concerning the most probable reaction pathways as some of the starting materials were isostructural with the final ones, as in the case of Er and Nd. Y and Nd starting compounds evolve under shelf conditions to analogous final compounds, while when the Y initial compound is recrystallized in a water/ethanol solution a new 1D network chain was obtained. La and Ce evolve similarly from compounds based on hexa-metallic centers to a trimetallic based network, the latter with a higher water content. As for the Mn, different products were obtained depending on the initial reagents ratio. Nevertheless, the initial product obtained from a 2:1 ratio was similar to the final product obtained from a 1:1 ratio. The common feature for all the networks was the coordination modes of GBP: the bidentate coordination − chelation, mode I; the “bridge”coordination, mode II; and the “bidentate-bridge” coordination, mode III. For these different coordination modes, it is possible to say that mode II is the one with the smallest distances, while distance b of mode III is the longest, the intramolecular hydrogen bond promoting shorter bond distances. Exceptions were detected in complexes GBP:Mn1 and GBP:Mn6, in which the coordination to the metal is achieved by the carboxylate oxygen involved in the intramolecular interaction. I



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DSC and TGA studies show that moisture is released until approximately 80−90 °C, the temperature after which both crystallization and coordinated waters are released. Above 200−250 °C, melting/decomposition is detected. Solution NMR studies suggest that these complexes adopt distinct structures in the solid state and in solution, and preliminary solubility tests indicate that the bulk materials have a much lower solubility than GBP. Further diffraction studies are underway in an attempt to characterize the powder samples. Instability of these networks to moisture and time precluded further studies and their possible application in biological media. The results obtained encouraged us to apply the same synthetic techniques in the development of new coordination networks and MOFs that can lead us to obtain new biomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

Further details on the characterization of these compounds are given. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallograpic information files (CIF) are accessible for the ten crystal structures; they have also been deposited at the CCDC with the reference numbers 900275− 900278 and 953484−953489.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge funding to Fundaçaõ para a Ciência e a Tecnologia (PTDC/CTM-BPC/122447/2010, Projeto Estratégico PEST-OE/QUI/UI0100/2013, RECI/QEQ-QIN70189/2012 and SFRH/BPD/78854/2011). Professor M. Matilde Marques, Shrika G. Harjivan, and David M. A. Novais are acknowledged for the solubility studies and Dr. Auguste Fernandes for the TGA and DSC analysis.

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Gabapentin Coordination Networks: Mechanochemical Synthesis and

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