Crystal structure of brimonidine hydrogen tartrate, (C₁₁H₁₁BrN₅)(HC₄H₄O₆)

Abstract

The crystal structure of brimonidine hydrogen tartrate has been solved and refined using synchrotron X-ray powder diffraction data and optimized using density functional techniques. Brimonidine hydrogen tartrate crystallizes in space group P2₁ (#4) with a = 7.56032(2), b = 7.35278(2), c = 30.10149(9) Å, β = 90.1992(2)°, V = 1673.312(10) ų, and Z = 4 at 295 K. The crystal structure consists of alternating layers of cations and anions parallel to the ab-plane. Each of the hydrogen tartrate anions is linked to itself by very strong charge-assisted O–H⋯O hydrogen bonds into chains along the a-axis. Each hydroxyl group of each tartrate acts as a donor in an O–H⋯O or O–H⋯N hydrogen bond. One of these is intramolecular, but the other three are intermolecular. These hydrogen bonds link the hydrogen tartrate anions into layers parallel to the ab-plane and also link the anion–cation layers. The protonated N atoms act as donors in N–H⋯O or N–H⋯N hydrogen bonds to the carboxyl groups of the tartrates and to a ring nitrogen atom. These link the cations and anions, as well as providing cation–cation links. The amino N atoms of the cations form N–H⋯O hydrogen bonds to hydroxyl groups of the anions. The powder pattern has been submitted to ICDD for inclusion in the Powder Diffraction File™ (PDF®)

I. INTRODUCTION

Brimonidine tartrate (marketed under the trade names Alphagan and Qoliana) is administered as a dilute aqueous solution used for lowering the intraocular pressure in patients with glaucoma or ocular hypertension. The systematic name (CAS Registry Number 70359-46-5) is 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)quinoxalin-6-amine (2R,3R)-2,3-dihydroxybutanedioic acid. A two-dimensional molecular diagram of brimonidine hydrogen tartrate is shown in Figure 1.

Figure 1. The two-dimensional structure of brimonidine hydrogen tartrate.

Limited powder diffraction data for a novel Form A polymorph of brimonidine tartrate is reported in Japanese patent JP6270603 B2 (Kanazawa, 2018; Kasuma Partners Inc.). A powder pattern for brimonidine tartrate is contained in De Souza et al. (2016); however, the crystal structure was not reported.

This work was carried out as part of a project (Kaduk et al., 2014) to determine the crystal structures of large-volume commercial pharmaceuticals and include high-quality powder diffraction data for them in the Powder Diffraction File (Gates-Rector and Blanton, 2019).

II. EXPERIMENTAL

Brimonidine tartrate was a commercial reagent, purchased from TargetMol (Batch #115202), and was used as received. The white powder was packed into a 1.5 mm diameter Kapton capillary and rotated during the measurement at ~50 Hz. The powder pattern was measured at 295 K at beam line 11-BM (Antao et al., 2008; Lee et al., 2008; Wang et al., 2008) of the Advanced Photon Source at Argonne National Laboratory using a wavelength of 0.459744(2) Å from 0.5 to 40° 2θ with a step size of 0.001° and a counting time of 0.1 s/step. The high-resolution powder diffraction data were collected using 12 silicon crystal analyzers that allow for high angular resolution, high precision, and accurate peak positions. A mixture of silicon (NIST SRM 640c) and alumina (NIST SRM 676a) standards (ratio Al₂O₃:Si = 2:1 by weight) was used to calibrate the instrument and refine the monochromatic wavelength used in the experiment.

The pattern was indexed using N-TREOR (Altomare et al., 2013) on a primitive monoclinic unit cell with a = 7.70622, b = 7.36944, c = 30.43616 Å, β = 91.311°, V = 1728.0 Å³, and Z = 4. The suggested space group was P2₁, which was confirmed by the successful solution and refinement of the structure. A reduced cell search of the Cambridge Structural Database (Groom et al., 2016) with the chemistry C, H, Br, N, and O only, yielded no hits.

The brimonidine molecule was downloaded from PubChem (Kim et al., 2023) as Conformer3D_CID_2435.sdf. It was converted to a *.mol2 file using Mercury (Macrae et al., 2020). The tartrate.mol2 file used in the solution of the structure of varenicline hydrogen tartrate (Kaduk et al., 2021) was used as the tartrate model. The crystal structure was solved using Monte Carlo simulated annealing techniques as implemented in EXPO2014 (Altomare et al., 2013), using two brimonidine and two tartrate molecules as the fragments (37 variables), including a default bump penalty for the non-H atoms.

Examining potential hydrogen bonds in the best solution made it clear that N4 and N30 in the two cations were protonated, and that O57 and O75 in the tartrate anions were also protonated (Figure 2). Hydrogen atoms were added to these N and O atoms in approximate positions. The bulk stoichiometry is a 1:1 salt, and the tartrate anions are monoionized, so the compound is best described as brimonidine hydrogen tartrate.

Figure 2. Potential hydrogen bonds in brimonidine hydrogen tartrate, which suggested the locations of the active hydrogen atoms.

Rietveld refinement was carried out with GSAS-II (Toby and Von Dreele, 2013). Only the 0.8–32.0° portion of the pattern was included in the refinements (d _min = 0.834 Å). The y-coordinate of Br1 was fixed to define the origin. All non-H bond distances and angles were subjected to restraints, based on a Mercury/Mogul Geometry Check (Bruno et al., 2004; Sykes et al., 2011). The Mogul average and standard deviation for each quantity were used as the restraint parameters. The imidazole rings and the fused ring systems in the two independent brimonidine cations were restrained to be planar. The restraints contributed 7.8% to the final χ ². The hydrogen atoms were included in calculated positions, which were recalculated during the refinement using Materials Studio (Dassault, 2022). The two independent Br atoms were refined anisotropically. The U _iso of the C, N, and O atoms were grouped by chemical similarity. The U _iso for the H atoms was fixed at 1.3× the U _iso of the heavy atoms to which they are attached. The peak profiles were described using a uniaxial microstrain model, with 001 as the unique axis.

The final refinement of 198 variables using 31,250 observations and 136 restraints yielded the residuals R _wp = 0.09075 and goodness of fit (GOF) = 1.63. The largest peak (1.59 Å from Br1) and hole (1.43 Å from C37) in the difference Fourier map were 0.51(11) and −0.49(16) eÅ⁻³, respectively. The final Rietveld plot is shown in Figure 3. The largest features in the normalized error plot represent subtle shifts in peak positions and may represent a change to the specimen during the measurement.

Figure 3. The Rietveld plot for the refinement of brimonidine hydrogen tartrate. The blue crosses represent the observed data points, and the green line is the calculated pattern. The cyan curve is the normalized error plot, and the red line is the background curve. The vertical scale has been multiplied by a factor of 20× for 2θ > 17.0̊ and by a factor of 40× for 2θ > 24.0̊.

The crystal structure of brimonidine hydrogen tartrate was optimized (fixed experimental unit cell) with density functional techniques using Vienna Ab initio Simulation Package (VASP) (Kresse and Furthmüller, 1996) through the MedeA graphical interface (Materials Design, 2016). The calculation was carried out on 16 2.4 GHz processors (each with 4 Gb RAM) of a 64-processor HP Proliant DL580 Generation 7 Linux cluster at North Central College. The calculation used the generalized gradient approximation-Perdew-Burke-Ernzerhof (GGA-PBE) functional, a plane wave cutoff energy of 400.0 eV, and a k-point spacing of 0.5 Å⁻¹ leading to a 2 × 2 × 1 mesh, and took ~38 h. Single-point density functional calculations (fixed experimental cell) and population analysis were carried out using CRYSTAL23 (Erba et al., 2023). The basis sets for the H, C, N, and O atoms in the calculation were those of Gatti et al. (1994), and the basis set for Br was that of Peintinger et al. (2013). The calculations were run on a 3.5 GHz personal computer (PC) using 8 k-points and the B3LYP functional and took ~5.1 h.

III. RESULTS AND DISCUSSION

The limited powder data (four peaks) reported by Kanazawa (2018; Table I, Figure 4) makes it difficult to conclude that our material and the “novel polymorph” claimed by Kasuma are the same. The agreement of the third peak in the patent with our pattern is poor, suggesting that the “novel polymorph” claimed there was indeed new. The agreement of our pattern with that reported by De Souza et al. (2016) is quite good (Figure 5), suggesting that the two materials are the same.

TABLE I. Powder pattern of brimonidine tartrate Form A from JP6270603B2

Figure 4. Comparison of this synchrotron powder pattern of brimonidine hydrogen tartrate with peak positions reported in Japanese Patent JP6270603 B2 (Kanazawa, 2018).

Figure 5. Comparison of the synchrotron pattern of brimonidine hydrogen tartrate (black) to that reported by De Souza et al. (2016; green). The literature pattern (measured using Cu K_α radiation) was digitized using UN-SCAN-IT (Silk Scientific, 2013) and converted to the synchrotron wavelength of 0.459744(2) Å using JADE Pro (MDI, 2023). Image generated using JADE Pro (MDI, 2023).

The asymmetric unit (Figure 6) contains two brimonidine cations and two hydrogen tartrate anions, showing that the compound should be properly described as brimonidine hydrogen tartrate. The root-mean-square Cartesian displacements of the non-H atoms in the Rietveld-refined and VASP-optimized cations molecules are 0.241 and 0.240 Å (Figures 7 and 8). The agreement is within the normal range for correct structures (van de Streek and Neumann, 2014). The comparable differences for the tartrate anions are 0.099 and 0.484 Å. The difference in anion 2 is mainly the result of different orientations of the C81–O74–O5 carboxyl group. The remainder of this discussion will emphasize the VASP-optimized structure.

Figure 6. The asymmetric unit of brimonidine hydrogen tartrate, with the atom numbering. The atoms are represented by 50% probability spheroids/ellipsoids. Image generated using Mercury (Macrae et al., 2020).

Figure 7. Comparison of the Rietveld-refined (red) and VASP-optimized (blue) structures of cation 1 in brimonidine hydrogen tartrate. The rms Cartesian displacement is 0.241 Å. Image generated using Mercury (Macrae et al., 2020).

Figure 8. Comparison of the Rietveld-refined (red) and VASP-optimized (blue) structures of cation 2 in brimonidine hydrogen tartrate. The rms Cartesian displacement is 0.240 Å. Image generated using Mercury (Macrae et al., 2020).

Most of the bond distances, bond angles, and torsion angles fall within the normal ranges indicated by a Mercury Mogul Geometry check (Macrae et al., 2020). Exceptions are the C9–N3 distance of 1.351 Å (average = 1.385(11) Å, Z-score = 3.1), the C9–N4 distance of 1.512 Å (average = 1.338(52) Å, Z-score = 3.4), the C37–N30 distance of 1.510 Å (average = 1.338(52) Å, Z-score = 3.4), and the C37–N31 distance of 1.356 Å (average = 1.385(11) Å, Z-score = 3.1). These distances represent the differences between the protonated NH₂ and the N = in the imidazole rings of the cations. The orientations of the hydrogen bonds show that these groups are assigned correctly, and that the rings are not rotated by 180° (which would yield less-reasonable hydrogen bonds). The C10–N3–C9 angle of 117.2° (average = 127.2(24)°, Z-score = 4.2) is flagged as unusual. The torsion angles involving rotation about the C9–C3 and C37–N31 bonds are flagged as unusual. These reflect the orientations of the protonated imidazole rings with respect to the fused ring systems and lie on the tails of 0/180° distributions of similar torsion angles. The protonation of the imidazole rings and the resulting hydrogen bonds to the tartrate anions result in unusual geometries for the brimonidine cations.

The two cations have different conformations (Figure 9). The rms Cartesian displacement is 1.708 Å, and after inverting one of them the difference decreases to 0.861 Å. The main differences after inversion are in the protonated imidazole rings. Quantum chemical geometry optimization of the isolated cations (DFT/B3LYP/6-31G*/water) using Spartan ‘20 (Wavefunction, 2022) indicated that the cation 1 is 2.2 kcal/mol lower in energy. The global minimum-energy conformation of the cation differs only slightly from the observed conformations. The conformations of the isolated tartrate anions differ by 3.3 kcal/mol (tartrate #1 is lower), reflecting the differences in the hydrogen bonding.

Figure 9. Comparison of cation 1 (green) and cation 2 (purple) in brimonidine hydrogen tartrate. Image generated using Mercury (Macrae et al., 2020).

The crystal structure (Figure 10) consists of alternating layers of cations and anions parallel to the ab-plane. The mean planes of the fused ring systems are 7,3,1 and 12,5,–1. The Mercury Aromatics Analyser indicates moderate and weak interactions between the fused ring systems, with distances of 4.54 and 4.70 Å, respectively. Manual analysis indicates two even shorter interactions of 3.50 and 3.60 Å between the C₆ and C₄N₂ rings in adjacent molecules, reflecting the parallel stacking of the rings along the a-axis. Analysis of the contributions to the total crystal energy of the structure using the Forcite module of Materials Studio (Dassault Systèmes, 2022) suggests that angle and torsion distortion terms dominate the intramolecular deformation energy. The intermolecular energy is dominated by electrostatic repulsions.

Figure 10. The crystal structure of brimonidine hydrogen tartrate, viewed down the b-axis. Image generated using Diamond (Crystal Impact, 2023).

Hydrogen bonds are important in the crystal structure (Table II). Each of the hydrogen tartrate anions is linked to itself by very strong charge-assisted O–H⋯O hydrogen bonds into chains along the a-axis. Each hydroxyl group of each tartrate acts as a donor in an O–H⋯O or O–H⋯N hydrogen bond. One of these is intramolecular, but the other three are intermolecular. These hydrogen bonds link the hydrogen tartrate anions into layers parallel to the ab-plane and also link the anion–cation layers. The protonated N atoms N4 and N30 act as donors in N–H⋯O or N–H⋯N hydrogen bonds, to the carboxyl groups of the tartrates and to a ring nitrogen atom. These link the cations and anions, as well as providing cation–cation links. The amino N atoms N3 and H31 form N–H⋯O hydrogen bonds to hydroxyl groups of the anions. The energies of the O–H⋯O hydrogen bonds were calculated using the correlation of Rammohan and Kaduk (2018) and those of the N–H⋯O hydrogen bonds by the correlation of Wheatley and Kaduk (2019). Several C–H⋯O/N hydrogen bonds, both from the cations and anions, also contribute to the lattice energy.

TABLE II. Hydrogen bonds (CRYSTAL23) in brimonidine hydrogen tartrate

^a Intramolecular.

The volume enclosed by the Hirshfeld surface of brimonidine hydrogen tartrate (Figure 11, Hirshfeld, 1977; Spackman et al., 2021) is 825.55 Å³, 98.67% of the unit cell volume. The packing density is thus fairly typical. The only significant close contacts (red in Figure 11) involve the hydrogen bonds. The volume/non-hydrogen atom is smaller than normal, at 15.5 Å³, reflecting the extensive hydrogen bonding.

Figure 11. The Hirshfeld surface of brimonidine hydrogen tartrate. Intermolecular contacts longer than the sums of the van der Waals radii are colored blue, and contacts shorter than the sums of the radii are colored red. Contacts equal to the sums of radii are white. Image generated using CrystalExplorer (Spackman et al., 2021).

The Bravais–Friedel–Donnay–Harker (Bravais, 1866; Friedel, 1907; Donnay and Harker, 1937) morphology suggests that we might expect platy morphology for brimonidine hydrogen tartrate, with {001} as the major faces. A second-order spherical harmonic model was included in the refinement. The texture index was 1.014(0), indicating that the preferred orientation was not significant in this rotated capillary specimen.

IV. DEPOSITED DATA

The powder pattern of brimonidine hydrogen tartrate from this synchrotron data set has been submitted to ICDD for inclusion in the Powder Diffraction File. The Crystallographic Information Framework (CIF) files containing the results of the Rietveld refinement (including the raw data) and the DFT geometry optimization were deposited with the ICDD. The data can be requested at [email protected].