Crystal structure of ribociclib hydrogen succinate, (C 23 H 31 N 8 O)(HC 4 H 4 O 4 )

The crystal structure of ribociclib hydrogen succinate (commonly referred to as ribociclib succinate) has been solved and re ﬁ ned using synchrotron X-ray powder diffraction data, and optimized using density functional theory techniques. Ribociclib hydrogen succinate crystallizes in space group P-1 (#2) with a = 6.52215(4), b = 12.67120(16), c = 18.16978(33) Å, α = 74.0855(8), β = 82.0814(4), γ = 88.6943(1)°, V = 1430.112(6) Å 3 , and Z = 2 at 295 K. The crystal structure consists of alternating layers of cations and anions parallel to the ab -plane. The protonated N in each ribociclib cation acts as a donor in two strong N – H ⋯ O hydrogen bonds to two different succinate anions. Strong O – H ⋯ O hydrogen bonds link the hydrogen succinate anions into chains parallel to the a -axis. N – H ⋯ N hydrogen bonds link the cations into dimers, with a graph set R2,2(8) . The result is a three-dimensional hydrogen bond network. The powder pattern has been submitted to ICDD for inclusion in the Powder Diffraction File ™ (PDF®) © The Author

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
Ribociclib succinate was a commercial reagent, purchased from TargetMol (Batch #144117), 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 beamline 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 per step.The high-resolution powder diffraction data were collected using twelve 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 Al 2 O 3 :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 both N-TREOR (Altomare et al., 2013) and DICVOL14 (Louër and Boultif, 2014) through the PreDICT interface (Blanton et al., 2019) on a primitive triclinic unit cell with a = 6.5192, b = 12.6781, c = 18.1671Å, α = 74.105,β = 81.996,γ = 88.675°,V = 1429.95Å 3 , and Z = 2.The space group was assumed to be P-1, which was confirmed by successful solution and refinement of the structure.A reduced cell search of the Cambridge Structural Database (Groom et al., 2016) yielded no hits.
The ribociclib molecule was downloaded from PubChem (Kim et al., 2023) as Conformer3D_CID_44631912.sdf.It was converted to a *.mol2 file using Mercury (Macrae et al., 2020).A succinate anion was built using Spartan '20 (Wavefunction, 2022) and saved as a .mol2file.The crystal structure was solved using Monte Carlo simulated annealing techniques as implemented in DASH (David et al., 2006), using ribociclib and succinate molecules as the fragments.Examination of close intermolecular contacts (potential hydrogen bonds) indicated that N7 and O70 were protonated, and thus, the compound is correctly named ribociclib hydrogen succinate.
Rietveld refinement was carried out with GSAS-II (Toby and Von Dreele, 2013).Only the 1.0-25.0°portion of the pattern was included in the refinements (d min = 1.062Å).The region 1.58-1.89°2θ,which contained a moderately sharp peak from the Kapton capillary, was excluded.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 pyridine ring and the fused ring system were restrained to be     planar.The restraints contributed 4.1% 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 U iso of the C, N, and O atoms were grouped by chemical similarity.The U iso for the H atoms were 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 100 as the unique axis.
The final refinement of 159 variables using 23 731 observations and 104 restraints yielded the residuals R wp = 0.12521 and GOF = 1.52.The largest peak (0.09 Å from C10) and hole (1.61 Å from N3) in the difference Fourier map were 0.57(10) and −0.46(10) eÅ −3 , respectively.The final Rietveld plot is shown in Figure 2. The largest features in the normalized error plot represent a subtle error in peak shapes, the result of using a simple profile model.
The crystal structure of ribociclib hydrogen succinate was optimized (fixed experimental unit cell) with density functional techniques using 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 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 ∼8.7 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).The calculations were run on a 3.5 GHz PC using 8 k-points and the B3LYP functional, and took ∼3.6 h.

III. RESULTS AND DISCUSSION
The agreement between the powder pattern of ribociclib hydrogen succinate of this study and the pattern reported by Calienni et al. (2012) is good enough to conclude that the material in this study is the same as that patented by Novartis (Figure 3).The intensities in the patent pattern differ from those observed here, probably indicating some preferred orientation and/or granularity.There are also a few weak extra peaks, perhaps indicating the presence of an impurity.
The asymmetric unit (Figure 4) contains one ribociclib cation and one hydrogen succinate anion, showing that the compound is properly described as ribociclib hydrogen succinate.The root-mean-square (rms) Cartesian displacement of  the non-H atoms in the Rietveld-refined and VASP-optimized cation is 0.088 Å (Figure 5) and in the succinate anion is 0.102 Å (Figure 6).The agreement is within the normal range for correct structures (van de Streek and Neumann, 2014), and provides confirmation that the structure is correct.The remainder of this discussion will emphasize the VASP-optimized structure.
Almost all 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).The torsion angles involving rotation about the C65-C66 bond in the succinate anion are flagged as unusual.They lie on long tails of peaked distributions, and indicate that the succinate anion conformation is unusual.
Quantum chemical geometry optimization of the isolated cation (DFT/B3LYP/6-31G*/water) using Spartan '20 (Wavefunction, 2022) indicated that the solid-state conformation is 6.6 kcal mol −1 higher in energy than a local minimum, which is the global minimum-energy conformation.A similar optimization indicated that the solid-state conformation of the anion is 6.0 kcal mol −1 higher in energy than a local minimum, which has different orientations of the carboxyl groups.The global minimum-energy conformation of the anion has a similar backbone, but again different orientations of the carboxyl groups.Apparently, the hydrogen bonding affects the solid-state conformation.
The crystal structure (Figure 7) consists of alternating layers of cations and anions parallel to the ab-plane.As discussed below, hydrogen bonding is important in the structure.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 distortion terms are the most important contributors to the intramolecular energy, as expected from a fused ring system.The intermolecular energy is dominated by electrostatic attractions, which in the force-field analysis include hydrogen bonds.The hydrogen bonds are better analyzed using the results of the DFT calculation.Hydrogen bonds (Table I) are important in the structure.The protonated N7 in each ribociclib acts as a donor in two strong N-H⋯O hydrogen bonds to two different succinate anions (Figure 8).Strong O-H⋯O hydrogen bonds link the hydrogen succinate anions into chains parallel to the a-axis.N-H⋯N hydrogen bonds link the cations into dimers, with a graph set (Etter, 1990;Bernstein et al., 1995;Shields et al., 2000) R2,2(8) (Figure 9).The result is a threedimensional hydrogen bond network.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 using the correlation of Wheatley and Kaduk (2019).A variety of C-H⋯O, C-H⋯N, and C-H⋯C hydrogen bonds link the cations and anions, also contributing to the lattice energy.
The volume enclosed by the Hirshfeld surface of ribociclib hydrogen succinate (Figure 10;Hirshfeld, 1977;Spackman et al., 2021) is 704.73Å 3 , 98.55% of the unit cell volume.The packing density is thus fairly typical.The only significant close contacts (red in Figure 10) involve the hydrogen bonds.The volume/non-hydrogen atom is typical, at 17.9 Å 3 .
The Bravais-Friedel-Donnay-Harker (Bravais, 1866;Friedel, 1907;Donnay and Harker, 1937) morphology suggests that we might expect elongated morphology for ribociclib hydrogen succinate, with <100> as the long axis, or platy morphology with {001} as the major faces.A fourthorder spherical harmonic model was included in the refinement.The texture index was 1.0066(1), indicating that the preferred orientation was not significant in this rotated capillary specimen.

IV. DEPOSITED DATA
The powder pattern of ribociclib hydrogen succinate 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 pdj@icdd.com

Figure 2 .
Figure 2. The Rietveld plot for the refinement of ribociclib hydrogen succinate.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 > 9.5°.

Figure 4 .
Figure 4.The asymmetric unit of ribociclib hydrogen succinate, with the atom numbering.The atoms are represented by 50% probability spheroids.Image generated using Mercury (Macrae et al., 2020).

Figure 6 .
Figure 6.Comparison of the Rietveld-refined (red) and VASP-optimized (blue) structures of the hydrogen succinate anion in ribociclib hydrogen succinate.The rms Cartesian displacement is 0.102 Å. Image generated using Mercury (Macrae et al., 2020).

Figure 7 .
Figure7.The crystal structure of ribociclib hydrogen succinate, viewed down the a-axis.Image generated using Diamond (Crystal Impact, 2023).

Figure 8 .
Figure 8.The hydrogen bonds (light blue dotted lines) of the ribociclib cation connect to two succinate anions.The dotted red lines indicate other hydrogen bonds.Image generated using Mercury (Macrae et al., 2020).

Figure 10 .
Figure 10.The Hirshfeld surface of ribociclib hydrogen succinate.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).
a Intramolecular.